Prof. Dr.-Ing. Viktor Scherer

Department of Energy Plant Technology
Ruhr-Universität Bochum

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  • Adaptation of the Chemical Percolation Devolatilization Model for Low Temperature Pyrolysis in a Fluidized Bed Reactor
    Pielsticker, S. and Ontyd, C. and Kreitzberg, T. and Hatzfeld, O. and Schiemann, M. and Scherer, V. and Kneer, R.
    Combustion Science and Technology 194 (2022)
    In the present study, the CPD model originally developed based on predictions from heated grid (HGR) and entrained flow (EFR) experiments, has been adapted to analyze pyrolysis kinetics in a small-scale fluidized bed reactor. Impacts of particle feed, particle heat up as well as tar cracking reactions in the gas phase are considered. Furthermore, an optimized solver structure allows a time step independent solution and enables the use of implicit methods. A comparison with experimental results is undertaken for pulverized Rhenish lignite fuel particles in the temperature range from 673 to 973 K in N2 atmosphere. The comparison between simulated and experimentally derived volatile release rates reveals a good agreement, indicating that the high temperature derived kinetic parameters from HRG and EFR experiments can be extrapolated to lower temperatures. Nevertheless, discrepancies in the tar to light gas ratio occur with the proposed model implementation. © 2022 Taylor & Francis Group, LLC.
    view abstract10.1080/00102202.2019.1682433
  • Combustion of Straw Pellets in an Agitated Fuel Bed: Experiments and DEM/CFD Simulations
    Buss, F. and Wirtz, S. and Scherer, V.
    Combustion Science and Technology 194 (2022)
    The influence of bed agitation during the combustion of biomass pellets was investigated experimentally and numerically. In the experiments, a bulk of straw pellets was burnt in a batch-operated reactor. The reactor allows for air staging and mixing of the fuel bed by vertically moveable mixing elements. The primary to secondary air ratio was varied and the reactor was operated either in the agitated (moving mixing elements) or the static mode (mixing elements at rest). The overall mass of the bulk was measured continuously during the combustion process. The results show a significant increase of the mass loss rate by almost 60% when the bed was agitated compared to the static case. Samples of the residual material of the pellets reveal a totally different amount of molten and agglomerated ash particles for the different operational conditions. Decreased primary to secondary air ratios as well as agitation of the fuel bed did lead to less agglomeration of the ash. The Discrete Element Method (DEM) was coupled with a Computational Fluid Dynamics (CFD) simulation. Coupled DEM/CFD simulations of the batch reactor were performed to get access to bulk internal data of the solid material and the fluid phase. Simulations identified that a reduced amount of ash exposed to the volatile flame through agitation of the fuel bed was the main reason for minimized ash agglomeration. © 2019 Taylor & Francis Group, LLC.
    view abstract10.1080/00102202.2019.1678844
  • Mixing and segregation of spheres of three different sizes on a batch stoker grate: Experiments and discrete element simulation
    Hilse, N. and Kriegeskorte, M. and Illana, E. and Wirtz, S. and Scherer, V.
    Powder Technology 400 (2022)
    Mixing and segregation in a tri-disperse granular assembly of polyoxymethylene (POM) spheres induced by the moveable stoking bars of a generic batch grate system are examined. Each particle size class features a separate colour. Stroke bar velocity and stroke length are varied. Different moving modes of the bars are analysed. Optically transparent walls of the grate allow for the localization of the visible particles. Based on the visible particle positions a segregation index is calculated. The initial arrangement of the particles in the experiments, which exhibits small statistical differences introduced by the grate filling procedure, has an influence on the progression of the segregation index. The experiments are compared with discrete element (DEM) simulations employing an in-house DEM code. Experiments are in good general agreement with the simulations. The particle rearrangement during bar movement is characterized by an initially fast mixing on short time-scales and a slow process to reach a final state of segregation. These two processes are influenced by the penetration depth of the bars into the bed and the specific movement mode. Three modes predominantly showed segregation in the direction of bar movement, whereas two modes showed large-scale spatial particle rearrangement. Two moving modes show bridging at the beginning of the experiments, an effect that could be reproduced by the DEM simulations. The influence of the modes and their specific parameters on segregation indices, a mixing rate and a segregation efficiency are discussed in detail. © 2022 Elsevier B.V.
    view abstract10.1016/j.powtec.2022.117258
  • Adjoint-based sensitivity analysis of char combustion surface reaction kinetics
    Hassan, A. and Sayadi, T. and Schiemann, M. and Scherer, V.
    Fuel 287 (2021)
    Simulations of solid particle combustion rely on models to approximate the reactions on the surface of the particle and in the surrounding gas. These models, in turn, depend on many model parameters, which are determined, most commonly, by experiments and contain a certain level of uncertainty. It is therefore essential to correctly determine the sensitivities of measured quantities of interest, with respect to the existing model parameters. This study, concentrates on a one-step model describing the heterogeneous reaction on the surface of a char particle, and in particular, the surface model presented by Schiemann et al. [24]. Adjoint-based methods are then employed to extract sensitivities of various quantities of interests, including the burning rate and total heat release, with respect to surface model parameters, such as, the activation energy, and the pre-exponential factor. The variation of these sensitivities are then assessed as the particle size and free stream composition are varied. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.119503
  • Analysis of wood pellet degradation characteristics based on single particle impact tests
    Jägers, J. and Spatz, P. and Wirtz, S. and Scherer, V.
    Powder Technology 378 (2021)
    The knowledge of size reduction of wood pellets during pneumatic conveying is important to achieve failure-free system operation and to set-up adequate quality assurance processes. In the present study, experiments are performed with a single particle impact test facility. A stereoscopic high-speed camera set allows 3D-particle tracking and visual analysis of the particles' degradation properties. Based on the data obtained, empirical correlations for the statistical description of the pellets' breakage behaviour are presented depending on particle length, impact velocity and collision angle. These relationships are expressed mathematically by two key functions: the so-called selection function (breakage probability) and the breakage function (fragment size distribution). As expected, higher impact velocities lead to more damage, especially at normal collisions (90°) due to the maximum change of momentum. Furthermore, smaller particles tend to be more breakage resistant as they contain less impurities and cracks. Finally, an outlook on the influence of pellet quality and target material on the particles' degradation behaviour is given. Here, pellets with higher durability tend to break at higher impact loads and into larger fragments. In addition, a softer target material (e.g. HDPE) causes less particle breakage than e.g. steel. © 2020 Elsevier B.V.
    view abstract10.1016/j.powtec.2020.10.017
  • Calibration and validation of a comprehensive kinetic model of coal conversion in inert, air and oxy-fuel conditions using data from multiple test rigs
    Debiagi, P. and Ontyd, C. and Pielsticker, S. and Schiemann, M. and Faravelli, T. and Kneer, R. and Hasse, C. and Scherer, V.
    Fuel 290 (2021)
    This work presents detailed information on pyrolysis and char oxidation for a high-volatile Colombian bituminous coal. The investigation includes experiments at low and high particle heating rates, performed in a thermogravimetric analyzer (TGA), a drop-tube reactor (DTR), a flat-flame burner (FFB) and a fluidized-bed reactor (FBR). The TGA and DTR data were used when developing and calibrating the kinetic model for the conversion of coal in air and oxy-fuel atmospheres, while the FFB and FBR data were used to validate the resulting mechanism. The proposed model is an updated version of the CRECK-S-C model from the Politecnico di Milano (PoliMi), consisting of a fuel characterization step, coupled with a multi-step kinetic mechanism based on reference coals. Both the devolatilization and heterogeneous char reactions are accounted for and interconnected seamlessly. Key reactions were introduced and the existing reactions were calibrated to account for the particularities of this fuel and the effects of the abundant CO2 concentration in the reactors. The importance of successive gas-phase reactions was observed and a gas-phase kinetic model was coupled to properly simulate such conditions. The resulting model is applied to simulate and systematically evaluate the experimental findings, highlighting the model's features and limitations. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.119682
  • Catalytic influence of mineral compounds on the reactivity of cellulose-derived char in O2-, CO2-, and H2O-containing atmospheres
    Pflieger, C. and Lotz, K. and Hilse, N. and Berger, C.M. and Schiemann, M. and Debiagi, P. and Hasse, C. and Scherer, V. and Muhler, M.
    Fuel 287 (2021)
    The catalytic effects of mineral compounds on the conversion of a biomass-derived char in air- and oxyfuel-related atmospheres were investigated by thermogravimetric analysis at atmospheric pressure. The applied char originated from the hydrothermal carbonization (HTC) of cellulose followed by pyrolysis at 1073 K and subsequent mixing with 20 wt% of minerals by grinding to achieve tight contact. The reactivities of the mineral-loaded HTC chars were evaluated based on isothermal experiments in O2-, CO2-, and H2O-containing atmospheres as a function of their composition applying a magnetic suspension balance. The reactivity sequence K2CO3 > Na2CO3 ≫ Fe2O3 > CaO > MgO ≥ mineral-free was derived consistently for char oxidation in O2/inert as well as for char gasification in diluted H2O and CO2 mixtures. In addition to this qualitative assessment, the kinetic experiments were first modelled based on a simple global nth-order power-law rate expression. Then, the more complex Carbon Burnout Kinetics (CBK/G) model and the PoliMi model were applied. All three modeling approaches enabled a systematic quantification of the catalytic effects and led to a comparable lowering in the apparent activation energy. In combination with the kinetic parameters determined for the mineral-free char, the lowered apparent activation energies specific for the applied mineral and atmosphere facilitate the implementation of catalytic effects on the conversion of biomass-derived char into combustion models. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.119584
  • Comprehensive Data Set of Single Particle Combustion under Oxy-fuel Conditions, Part I: Measurement Technique
    Vorobiev, N. and Valentiner, S. and Schiemann, M. and Scherer, V.
    Combustion Science and Technology 193 (2021)
    An improved experimental methodology is presented that provides combustion data of single pulverized coal and biomass particles with a high level of detail. This is the first part of a two-article series. A stereoscopic imaging system based on four intensified CCD-cameras is calibrated for in-situ measurements of temperature, size, shape, and velocity of solid fuel particles in the diameter range of 30–300 µm. An elaborate approach for 3D shape reconstruction from orthogonal projections of single particles shows significantly improved accuracy, which is validated against particle samples being collected with a suction probe. The close-meshed combination of imaging pyrometry and shadowgraphy is a further novelty. The parallel application of both techniques provides results for both self-luminous “hot” (imaging pyrometry) and “cold” (shadowgraphy) particles. This enhances information on particle ignition (particles switch from cold to hot) and duration of char burn-out (particles switch from hot to cold). Selected experimental results are presented which demonstrate the informative power of data sets formed by this approach. Torrefied miscanthus is burned in a laminar flow reactor. The particle size and aspect ratio are evaluated for the in-situ measuring method as well as for the collected samples. The results show a good correlation between both analysis routines, indicating the progress in solid fuel characterization by the improved optical technique in combination with particle sampling. © 2020 Taylor & Francis Group, LLC.
    view abstract10.1080/00102202.2020.1743696
  • Comprehensive Data Set of Single Particle Combustion under Oxy-fuel Conditions, Part II: Data Set
    Vorobiev, N. and Valentiner, S. and Schiemann, M. and Scherer, V.
    Combustion Science and Technology 193 (2021)
    The analysis of the current literature on the subject of detailed single-particle measurements revealed that, despite the fact that numerous experimental investigations are reported, the database for the calibration of predictive burnout models is insufficient. To close this gap, a test rig for the optical investigation of the burning behavior of pulverized fuel particles was put into operation and introduced in the first part of this two-article series. In the second part, the results of a measurement campaign, on a high-volatile bituminous coal, and torrefied Miscanthus in eight oxy-fuel atmospheres are presented. The experimental data contains profiles of particle temperature, size, shape, burnout progress over residence time and, thus, provides a sound basis for the calibration of char burnout models. The combination of chosen optical techniques enables distinguishing between burning and cold particles, the latter being non-ignited or already burnt out. This information is important for the analysis of particle size distributions. In addition to optical measurements, partially reacted solid samples were extracted from the reactor. Besides the proximate and elemental analysis, also the porosity of the samples was determined at several burnout levels. Interestingly, the evolution of particle porosity shows atmosphere-independent trends. © 2020 Taylor & Francis Group, LLC.
    view abstract10.1080/00102202.2020.1754207
  • DEM-CFD simulation of wood pellet degradation by particle-wall impact during pneumatic conveying
    Jägers, J. and Brömmer, M. and Illana, E. and Wirtz, S. and Scherer, V.
    Powder Technology 391 (2021)
    A numerical degradation model based on two key-functions, the so-called selection and breakage functions, is developed and implemented into a coupled DEM-CFD approach for investigating wood pellet degradation and fines formation during pneumatic conveying by particle-wall impact. The influence of operating conditions like air flow, product flow and solids loading ratio as well as of pipe component geometry on degradation is examined. Detailed insights into the flow field and particle motion are obtained. The inter-particle and particle-wall collisions are statistically analysed since they are the major cause of particle degradation. Apart from operating conditions, the bend radius is varied in four steps, supplemented by a 90°-Elbow as the worst-case scenario and a straight pipe section for reference. Increasing air flow rates, and thus higher particle velocities, as well as decreasing pellet mass flows and smaller bend radii result in progressive particle degradation. Particle-wall collisions turned out to be the major reason for particle breakage. Numerical results are compared to experimental data and show good agreement. © 2021 Elsevier B.V.
    view abstract10.1016/j.powtec.2021.06.037
  • Experimental analysis of the flight design effect on the temperature distribution in rotary kilns
    Seidenbecher, J. and Herz, F. and Meitzner, C. and Specht, E. and Wirtz, S. and Scherer, V. and Liu, X.
    Chemical Engineering Science 240 (2021)
    An experimental parameter study in an indirectly heated rotary drum (L = 1.76 m, D = 0.5 m) with L-shaped flights was conducted. The influence of the flight design parameters as the flight length ratio (0.375–2.0) and the number of flights (6–18) on the temperature-time profile and the temperature drop are analyzed. Therefore, the rotary drum is filled with glass beads as reference material and heated in batch operation while contact heat transfer is the main heat transfer mechanism. After reaching an upper temperature of 330 °C, the system is cooled by forced convection as ambient air is sucked through the drum. Type k-thermocouples in three axial and six radial positions are used to measure the time evolution of the temperature. The thermocouples are mounted to a flight, such that they rotate with the drum and also capture the circumferential temperature distribution. © 2021 Elsevier Ltd
    view abstract10.1016/j.ces.2021.116652
  • Experimental determination of walnut shell pyrolysis kinetics in N2 and CO2 via thermogravimetric analysis, fluidized bed and drop tube reactors
    Ontyd, C. and Pielsticker, S. and Yildiz, C. and Schiemann, M. and Hatzfeld, O. and Ströhle, J. and Epple, B. and Kneer, R. and Scherer, V.
    Fuel 287 (2021)
    A thermogravimetric analyzer (TGA), a fluidized bed reactor (FBR) and a drop tube reactor (DTR) are used to study the effect of reactor type, heating rate and temperature on the pyrolysis of pulverized walnut shell particles in N2 and in CO2. These setups cover a temperature range of 400–1300 K with heating rates of 10−1 to 105 K s−1. The single first-order model in combination with an Arrhenius approach is used to describe the pyrolysis reaction. Derived activation energies for all setups show similar values (Ea,TGA = 71.96 kJ mol−1, Ea,FBR = 68.60 kJ mol−1 and Ea,DTR = 60.83 kJ mol−1), while an increase in the reactor temperature tend to lower the activation energy. Pyrolysis gas compositions in FBR and DTR reveal consistent trends towards lower H2O and higher CO contents with increasing reactor temperature. To evaluate the impact of CO2 on the solid conversion, TGA measurements in CO2 are used to determine gasification kinetics (Ea,g = 214.1 kJ mol−1, Ag = 71.96 s−1). CFD simulations using these kinetics in CO2 drop tube experiments let assume that the changed thermophysical properties of the gas and not the gasification reaction lead to the observed stronger conversion in CO2 compared to N2. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.119313
  • Experimental Evaluation of a Novel Sensor-Based Sorting Approach Featuring Predictive Real-Time Multiobject Tracking
    Maier, G. and Pfaff, F. and Pieper, C. and Gruna, R. and Noack, B. and Kruggel-Emden, H. and Langle, T. and Hanebeck, U.D. and Wirtz, S. and Scherer, V. and Beyerer, J.
    IEEE Transactions on Industrial Electronics 68 (2021)
    Sensor-based sorting is a machine vision application that has found industrial application in various fields. An accept-or-reject task is executed by separating a material stream into two fractions. Current systems use line-scanning sensors, which is convenient as the material is perceived during transportation. However, line-scanning sensors yield a single observation of each object and no information about their movement. Due to a delay between localization and separation, assumptions regarding the location and point in time for separation need to be made based on the prior localization. Hence, it is necessary to ensure that all objects are transported at uniform velocities. This is often a complex and costly solution. In this article, we propose a new method for reliably separating particles at nonuniform velocities. The problem is transferred from a mechanical to an algorithmic level. Our novel advanced image processing approach includes equipping the sorter with an area-scan camera in combination with a real-time multiobject tracking system, which enables predictions of the location of individual objects for separation. For the experimental validation of our approach, we present a modular sorting system, which allows comparing sorting results using a line-scan and area-scan camera. Results show that our approach performs reliable separation and hence increases sorting efficiency. © 1982-2012 IEEE.
    view abstract10.1109/TIE.2020.2970643
  • Investigation on flow dynamics and temperatures of solid fuel particles in a gas-assisted oxy-fuel combustion chamber
    Schneider, H. and Valentiner, S. and Vorobiev, N. and Böhm, B. and Schiemann, M. and Scherer, V. and Kneer, R. and Dreizler, A.
    Fuel 286 (2021)
    Flow dynamics and temperatures of solid fuel particles strongly influence flame stabilization, local heat release and fuel conversion inside pulverized solid fuel combustors. To investigate these phenomena, experiments are carried out under well-controlled inflow and boundary conditions inside a gas-assisted, swirled oxy-fuel combustion chamber. Flow fields of small particles that represent the gas phase velocity are determined in the near-burner region by PIV using a particle separation algorithm. Trajectories of large solid fuel particles are evaluated in a two-dimensional plane using a combined high-speed PIV/PTV approach. Particle temperatures and particles sizes are measured at different levels downstream the burner exit to reveal different stages of combustion. Therefore, a two-color pyrometer is used that dissolve single particles to achieve local particle temperature and particle size distributions. Two oxy-fuel operation conditions with an oxygen fraction of 33%V and a reference operation point in air are investigated within this study. In the flow fields of the gas phase the impact of the atmosphere is clearly visible in the spatial expansion of the internal recirculation area. Regions of high slip velocities and high heat release could be identified by analyzing particle trajectories in terms of direction, velocity and acceleration. Residence times of small and large particles are estimated from the flow fields. Significantly larger residence times are observed for large particles which leads to higher burn out rates in the near-burner region. Furthermore, particle temperature measurements reveal similar particle temperatures for the investigated oxy-fuel and air operation conditions. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.119424
  • Numerical investigation of the impact of coating layers on RDF combustion and clinker properties in rotary cement kilns
    Pieper, C. and Wirtz, S. and Schaefer, S. and Scherer, V.
    Fuel 283 (2021)
    The formation of regions of solid coating, where agglomerated clinker material adheres to the refractory lining of the kiln wall, is very common during cement clinker production. While a thin coating layer protects the refractory lining, strong deposit formation can impair the material flow through the kiln. In this study, the impact of these coating layers on the clinker production process within a rotary kiln is investigated with CFD simulations. The fuel injected at the main burner is a mixture of pulverized coal and refuse derived fuel (RDF). Advanced models were developed to accurately describe the trajectories and thermal conversion of non-spherical RDF particles in the gas phase. These models are based on a detailed fuel analysis of major RDF fractions. A blocked-off region approach is used to consider different coating profiles within the simulation domain. The thermochemical processes in the clinker bed of the kiln are approximated with a one-dimensional model that calculates heat and mass exchange with the gas phase, the incorporation of fuel ashes into the bed and the chemical-mineralogical reactions of the clinker. The blocked-off region approach is also employed to account for the clinker bed geometry in the kiln, which greatly depends on the considered coating profile. Two cases, one with a thin and evenly distributed coating profile and one with a thick and locally concentrated coating, are simulated. The resulting impact on RDF conversion, gas phase properties and clinker phase formation are assessed and compared to a reference case without any coating. Results show that the insulation effect of a thin coating profile increases the gas phase temperature in the kiln and helps to reduce the free lime content of the final clinker product. In the case of heavy coating, a temperature shift towards the solid material inlet of the kiln occurs, which outweighs the beneficial insulation effect of the coating in the sintering zone and leads to lower local gas phase temperatures. In combination with reduced clinker residence times, this results in a slight increase of the free lime content in the clinker. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.118951
  • Oscillating combustion—primary measure to reduce nitrogen oxide in a grate furnace–experiments and simulations
    Gehrmann, H.-J. and Jaeger, B. and Wirtz, S. and Scherer, V. and Aleksandrov, K. and Hauser, M. and Stapf, D. and Pollmeier, G. and Danz, P.
    Processes 9 (2021)
    The emission from industries and the mobility sector is under strong legal regulations in many countries worldwide. In Germany, the amendment to the 17th BlmSchV (Federal pollution control ordinance), which has been in force for waste incineration plants since 2013, has given rise to a new limit for nitrogen oxides of 150 mg/m3 as the daily mean level from 2019 on. A similar focus is on biomass-fired plants. According to the MCP (medium combustion plant) guideline of the EU, as a consequence, existing plants are required to either increase their consumption of ammonia water for nitrogen oxide reduction (SNCR process) or back fit SCR catalysts as secondary measures, which is a costly procedure. This paper presents a novel two-stage process in which an oscillating supply of secondary air allows nitrogen oxides to be reduced by approx. 50% at a good burnout level, which may obviate the need for secondary measures. Besides experimental investigations in a fixed bed reactor, CFD simulations confirm a high potential for reduction of nitrogen oxides. Together with the company POLZENITH, this process is under development for scale-up in a biomass incineration plant as a next step. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/pr9122210
  • Radiative heat transfer with a blocked-off approach for application in the discrete element method
    Jaeger, B. and Schlag, M. and Scherer, V. and Wirtz, S. and Schiemann, M.
    Powder Technology 392 (2021)
    Combining the Discrete Ordinates method with the so-called blocked-off approach to calculate radiative heat transfer is investigated for a prospective application in DEM/CFD simulations. Therefore, a single cubic DEM object is located within a prismatic enclosure. The temperature of all walls of the enclosure and of the surface of the DEM object are set to fixed values. The three-dimensional radiative transport is calculated and the heat transfer onto the individual surfaces of the DEM object is determined. A conventional CFD simulation employing a body-conformal mesh at the surface of the embedded DEM object, serves as a reference, an “exact” solution. The results obtained are compared to corresponding solutions with a blocked-off approach, where the shape of the DEM object and the required boundary conditions are enforced within the discretized intensity field while suppressing the solution process in the region obstructed by the object. Mesh-resolution and orientation of the cube in the enclosure are varied and two computationally cheap methods to obtain the radiative fluxes onto the embedded cube surfaces are employed. The heat fluxes onto the cube's surfaces are generally overestimated. This is mainly due to the “artificial” increase of the cube's surface area by the blocked-off approach. The introduction of a simple surface area scaling factor improves the results significantly. The deviation in radiative heat from the exact solution depends on the cube's orientation in the prismatic box and on the position of the actual surface relative to the closest mesh line. The approximation deviates about 1.4% to 3.1% in radiative heat to the cube, values which seem to be acceptable. © 2021 Elsevier B.V.
    view abstract10.1016/j.powtec.2021.07.010
  • Temperature analysis in flighted rotary drums and the influence of operating parameters
    Seidenbecher, J. and Herz, F. and Meitzner, C. and Specht, E. and Wirtz, S. and Scherer, V. and Liu, X.
    Chemical Engineering Science 229 (2021)
    Flights in rotary drums are used to improve the mixing and the heat transfer. They lift particles out of the solid bed to shower them as curtains through the gas phase of the drum. The number of particles and their distribution are influenced by several operational parameters. An indirectly heated flighted rotary drum was designed and constructed to conduct experiments relating transverse particle motion to heat transfer. Batch experiments divided into heating and cooling periods were performed with glass beads as reference material. The axial, circumferential and radial temperature distributions were measured. It was found that increasing the rotational speed as well as the volumetric airflow rate leads to quick temperature changes, while an increase in the filling degree results in a gradual decrease of the temperature drop during cooling. © 2020 Elsevier Ltd
    view abstract10.1016/j.ces.2020.115972
  • An automated and continuous method for the optical measurement of wood pellet size distribution and the gravimetric determination of fines
    Jägers, J. and Wirtz, S. and Scherer, V.
    Powder Technology 367 (2020)
    This paper presents an automated, continuous method to determine wood pellet size distributions of larger and, hence, representative samples based on 2D-digital image analysis. The analysis routine developed combines several image processing steps to detect and separate the respective particles. The test unit designed allows length determination of up to 140 kg of pellets with a rate of 10.2 kg/h without any manual intervention. Pellets with a diameter up to 10 mm and a maximum length of about 50 mm can be analysed. In addition, the amount of fines is determined gravimetrically. Verification tests confirm an average standard deviation of 0.42% for the effective pellet length. First measurements of various pellet types show significant differences in their initial size distributions. The method developed provides a tool, which allows a more precise quality control of wood pellet size distribution. © 2020 Elsevier B.V.
    view abstract10.1016/j.powtec.2020.04.023
  • An observer for partially obstructed wood particles in industrial drying processes
    Berner, M.O. and Scherer, V. and Mönnigmann, M.
    Computers and Chemical Engineering 141 (2020)
    In order for biomass drying processes to be efficient, it is crucial to achieve the target residual water content within a close margin, since more conservative drying would result in a waste of energy. A method for a reliable estimation of the water content is therefore of obvious importance. Ideally, such a method does not require any expensive sensors. We show reduced order models and extended Kalman filters can be combined to reliably determine the water content and temperature of wood particles based on only surface temperature measurements. The proposed observer works reliably if measurements are only available for parts of a particle face. It can therefore still be applied if particle surfaces are partially obstructed, which is a prerequisite for use in industrial processes and units, such as rotary dryers. The extended Kalman filter uses a reduced order model that is obtained by applying proper orthogonal decomposition and Galerkin projection to coupled PDEs that model heat conduction and water diffusion in anisotropic particles. In contrast to the original PDE simulation model, the reduced model and the filter based on it are suitable for real time computations and monitoring. © 2020 The Authors
    view abstract10.1016/j.compchemeng.2020.107013
  • Controllability analysis and optimal control of biomass drying with reduced order models
    Berner, M.O. and Scherer, V. and Mönnigmann, M.
    Journal of Process Control 89 (2020)
    Complex industrial processes such as the drying of combustible biomass can be modeled with partial differential equations. Due to their complexity, it is not straightforward to use these models for the analysis of system properties or for solving optimal control problems. We show reduced order models can be derived and used for these purposes for industrial drying processes. © 2020
    view abstract10.1016/j.jprocont.2020.03.002
  • Effect of O2/CO2 atmospheres on coal fragmentation
    Bareschino, P. and Urciuolo, M. and Scherer, V. and Chirone, R. and Senneca, O.
    Fuel 267 (2020)
    Recently, a single particle pyrolysis-combustion fragmentation model has been developed (Senneca et al., 2013, 2017) [1,2] to predict the propensity of coal particles to fragment under a wide range of heating conditions as a consequence of mechanical failure of the particle. Stress inside the particle arises from thermal shock, associated to particles’ heat up, as well as from overpressure generated by volatiles release upon devolatilization. The model is now used to calculate the propensity of coal particles to undergo fragmentation in the early stages of oxy-combustion, with gaseous atmospheres of 5–30% O2 in CO2 in entrained flow and fluidized beds reactors. Accordingly particles size of 0.1–10 mm are assumed, temperatures of 1123 and 2073 K, heating rates of 100 and 10,000 K/s. Results show that under entrained flow reactor conditions the particles break in the first 20–30 ms, producing a bimodal particle-size distribution. Under fluidized bed conditions, the particles undergo explosive fragmentation after 1–2 s, before pyrolysis is complete, generating broad particle size distribution. In both cases fragmentation occurs over short timescales compared to char combustion and gasification. Operative conditions where fragmentation occurs before or in parallel with char combustion or gasification are inferred by comparing on an Arrhenius plot the timescale of fragmentation and heterogeneous reactions for a larger array of operating conditions. The figure reveals that for high reaction temperatures, more reactive coals, larger particles size, gasification reactions can have an important role and maybe enhance porosity and percolative fragmentation. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.117145
  • Experimental analysis of wood pellet degradation during pneumatic conveying processes
    Jägers, J. and Wirtz, S. and Scherer, V. and Behr, M.
    Powder Technology 359 (2020)
    The size reduction of wood pellets during pneumatic transport in a laboratory test rig is investigated. For the quantification of pellet breakage and attrition, the length distribution of each bulk sample is measured in combination with the gravimetrical determination of the amount of fines before and after every conveying step. In laboratory tests a variation of pipe elements (bend radii, couplings, pipe reducer or cyclone separator), air volume and product mass flows and pellet quality is investigated. Additionally, particle velocities are determined with a stereoscopic high-speed camera set. Results demonstrate the strong dependence of wood pellet degradation on operating conditions and selection of pipe components. Increasing air volume flow and, thus, higher particle velocities induce particle size reduction, whereas increasing pellet mass flow has the opposite effect, although the influence is weak. Increasing pipe length or decreasing bend radius leads to progressive formation of fines. © 2019 Elsevier B.V.
    view abstract10.1016/j.powtec.2019.10.004
  • Extension of the thermal annealing concepts developed for coal combustion to conversion of lignocellulosic biomass
    Senneca, O. and Ontyd, C. and Cerciello, F. and Schiemann, M. and Scherer, V.
    Energy and Fuels 34 (2020)
    During the lifetime of coal and biomass particles in a reactor, severe changes in the carbonaceous structure occur. In the early stages of heat treatment, transformations at both the structural and chemical level are dramatic and well-recognized in the literature under the name of pyrolysis. Further heat treatment, even in parallel with heterogeneous reaction, produces less evident changes but is still very impactful on the char reactivity in the late stages of burnoff, which are recognized in the literature under the name of thermal annealing. Thermal annealing of biomass has often been neglected and underestimated. In the present work, a large experimental campaign has been carried out to measure the effects of heat treatment on the reactivity of a lignocellulosic material, namely, walnut shells, and, for comparison, a bituminous coal. The campaign included experiments in a thermogravimetric analyzer, fixed bed reactor, drop tube reactor, heated strip reactor, and flat flame burner, spanning over a very wide range of temperatures (700−2300 K), heating rates (0.1−105 K/s), and residence times (0.02−20 000 s). Results provide a unique set of data useful for testing pyrolysis−annealing models on lignocellulosic materials. Thermal annealing is very relevant also for lignocellulosic biomass and reduces the reactivity of char up to 1 order of magnitude. However, the distinction between pyrolysis and thermal annealing is made complex by the presence of multiple components with different inertia to thermal treatment. The applicability of the concepts developed for thermal annealing of coal to lignocellulosic biomass is therefore open to discussion. It is observed that the boarder line between the stage of pyrolysis and the stage of thermal annealing can be reasonably set when approximately 70−80% of the (ASTM) volatile matter content has been released. The early stages of thermal annealing, involving aromatization and graphitization of the carbon structure, occur in parallel with pyrolysis tails. © 2020 American Chemical Society
    view abstract10.1021/acs.energyfuels.9b03172
  • Impact of coating layers in rotary cement kilns: Numerical investigation with a blocked-off region approach for radiation and momentum
    Wirtz, S. and Pieper, C. and Buss, F. and Schiemann, M. and Schaefer, S. and Scherer, V.
    Thermal Science and Engineering Progress 15 (2020)
    In this study CFD simulations of an industrial scale rotary kiln for cement clinker production are conducted. A solid layer of agglomerated clinker material, which adheres to the kiln wall and forms a stable coating of variable thickness during kiln operation, is considered in the simulations of the furnace. During operation of the kiln the thickness of the coating layer is unknown, but the routinely measured temperature profile along the kiln shell is an indicator for the local layer thickness. Therefore, a first estimate of the initially unknown thickness of the coating layer is calculated by a one-dimensional heat transfer model, based on the temperature profile along the kiln shell, and introduced into the CFD simulations. As the process conditions within the furnace and, thus, the heat transfer to the kiln wall and the resulting coating thickness change over time, an adaptive modelling approach is required to describe the solid coating region in the CFD simulations. A method to represent the geometry of the coating layer as a blocked-off region for momentum is presented and extended for radiation. The results are compared to the conventional approach where the coating layer is represented by a highly resolved CFD mesh following the wall contour. Two generic temperature profiles of the kiln shell are assumed and used to simulate the corresponding coating profile in the furnace. The effect on the process conditions within the kiln are assessed and compared to a reference case without coating regions. Results show that the blocked-off approach employed is suitable to model the additional solid boundary profile in the furnace. The coating regions are found to have a significant impact on the process conditions within the kiln, especially if the coating layers are located towards the burner end of the kiln. © 2019 Elsevier Ltd
    view abstract10.1016/j.tsep.2019.100429
  • Insights on the role of primary and secondary tar reactions in soot inception during fast pyrolysis of coal
    Apicella, B. and Russo, C. and Cerciello, F. and Stanzione, F. and Ciajolo, A. and Scherer, V. and Senneca, O.
    Fuel 275 (2020)
    In the present work fast pyrolysis of coal in N2 and CO2 atmospheres was studied in a drop tube reactor (DTR) and in a heated strip reactor (HSR). In the DTR the volatiles generated by coal pyrolysis were entrained in a hot gas stream and were collected at the reactor outlet by tar traps. In the HSR, the volatiles were ejected from the hot coal particles into a cool environment and the condensable species, including primary tar, deposited and/or condensed on a glass bridge located above the heated strip. The composition of tars produced in the two reactors was compared to study the role of gas tar reactions in soot inception, and reference compounds for each class of tar species produced were identified. In the DTR the formation and growth of polycyclic aromatic hydrocarbons (PAH) were found higher than in the HSR. Soot formation occurred only in the DTR, being negligible in the HSR. It was concluded that the hot gas environment of the DTR favours secondary tar reactions, formation of PAH and eventually soot, while in the HSR this path was prevented due to prompt cooling down of volatiles. The presence of large concentration of CO2 in the pyrolysis atmospheres further promoted formation of heavy PAH and soot in the DTR, but not in the HSR, where the cooler environment limits soot-CO2 reactions in the gas phase. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.117957
  • Interaction of the combustion of refuse derived fuel with the clinker bed in rotary cement kilns: A numerical study
    Pieper, C. and Liedmann, B. and Wirtz, S. and Scherer, V. and Bodendiek, N. and Schaefer, S.
    Fuel 266 (2020)
    This paper presents CFD simulations of an industrial scale rotary kiln for cement clinker production. The fuel for the kiln flame is a mixture of pulverized coal and a Refuse Derived Fuel (RDF). Advanced models were developed to appropriately describe the thermal conversion characteristics and aerodynamics of non-spherical RDF particles. The models are based on detailed fuel analyses (e.g. flight and combustion characteristics, physical and chemical fuel properties) of major RDF fractions, like plastic foils, 3D plastic particles, paper & cardboard and textiles. The processes in the clinker within the kiln are approximated using a simplified one-dimensional model that calculates heat and mass exchange with the gas phase and the resulting chemical-mineralogical reactions in the solid bed. Calculation results of the one-dimensional model are compared to measurements obtained from a semi-industrial laboratory rotary kiln. Two cases, one with 100% lignite and one where 50% of the fuel heat input is substituted with RDF, are simulated. Based on the simulation results, the shift of flame shape and fuel conversion as well as the resulting effects on the clinker phase transition are analyzed and discussed. Results show that co-combustion of RDF can lead to lower gas and clinker temperatures in the sintering zone, which can affect the clinker properties. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.117048
  • On the agglomeration tendency of carbonaceous fuels in fluidized beds
    Urciuolo, M. and Solimene, R. and Ammendola, P. and Krusch, S. and Scherer, V. and Salatino, P. and Chirone, R. and Senneca, O.
    Fuel 277 (2020)
    Single particle pyrolysis and combustion experiments have been carried out in a lab scale fluidized bed reactor at temperatures of 600–850 °C. The behavior of three different fuels is compared: a bituminous coal (Auguste Victoria), a typical bitumen used in the cement industry, a carbon rich solid waste from the refinery industry, characterized by a very high content of metals. The bituminous coal and the refinery waste particles, during the pyrolysis stage, produce interesting carbon-sand aggregates. The outer shell of these aggregates is constituted by quartz sand particles embedded in a carbon matrix. The aggregates are hollow inside. The size of the cavity is comparable with that of the original coal particles, while the outer shell is larger. The increase of particle size due to aggregate formation slows down the combustion rate. For bitumen, no carbon-sand aggregates are observed. The relations between the fuel properties and aggregates formation are discussed, in particular the chemical composition and the pyrolysis kinetics are examined. It is concluded that heavy/tarry species formed in the early pyrolysis stages are most likely responsible for the capture of the sand particles and formation of aggregates. © 2020 Elsevier Ltd
    view abstract10.1016/j.fuel.2020.118187
  • Predictive tracking with improved motion models for optical belt sorting Prädiktives Tracking mit verbesserten Bewegungsmodellen für die optische Schüttgutsortierung
    Pfaff, F. and Pieper, C. and Maier, G. and Noack, B. and Gruna, R. and Kruggel-Emden, H. and Hanebeck, U.D. and Wirtz, S. and Scherer, V. and Längle, T. and Beyerer, J.
    At-Automatisierungstechnik 68 (2020)
    Optical belt sorters are a versatile means to sort bulk materials. In previous work, we presented a novel design of an optical belt sorter, which includes an area scan camera instead of a line scan camera. Line scan cameras, which are well-established in optical belt sorting, only allow for a single observation of each particle. Using multitarget tracking, the data of the area scan camera can be used to derive a part of the trajectory of each particle. The knowledge of the trajectories can be used to generate accurate predictions as to when and where each particle passes the separation mechanism. Accurate predictions are key to achieve high quality sorting results. The accuracy of the trajectories and the predictions heavily depends on the motion model used. In an evaluation based on a simulation that provides us with ground truth trajectories, we previously identified a bias in the temporal component of the prediction. In this paper, we analyze the simulation-based ground truth data of the motion of different bulk materials and derive models specifically tailored to the generation of accurate predictions for particles traveling on a conveyor belt. The derived models are evaluated using simulation data involving three different bulk materials. The evaluation shows that the constant velocity model and constant acceleration model can be outperformed by utilizing the similarities in the motion behavior of particles of the same type. © 2020 Walter de Gruyter GmbH, Berlin/Boston 2020.
    view abstract10.1515/auto-2019-0134
  • Simulation of a reacting agitated bed of straw pellets by a resolved coupled DEM/CFD method using a blocked-off approach
    Buss, F. and Wirtz, S. and Scherer, V.
    International Journal of Thermal Sciences 152 (2020)
    A resolved coupled DEM/CFD approach including a so called “blocked-off” method is presented which allows a continuous transition from the dense reacting pellet bed, with a CFD mesh resolving the voids between the pellets, to the freeboard above the pellet bed with a volatile gas flame present, where much coarser meshes are sufficient. The approach is tested and evaluated simulating the processes in a static bed and in a mechanically agitated bed of straw pellets. The pellets are approximated as spherocyclinders and treated as thermally thick objects. Momentum, radiation, heat and mass exchange at the interfaces to the pellets are calculated using wall functions, eliminating the requirement of shape specific correlations. Thermochemical source terms are determined by modelling conduction, drying, devolatilization and char burn-out for each fuel object. Local adaption of the effective turbulent viscosity, based on a Very Large Eddy model, accounts for different flow regimes and mesh densities in the pellet bed and the freeboard. Converted pellet mass is transferred to the passing fluid. Flame temperature and species distribution are determined from resulting mass fractions in the CFD domain and provide boundary conditions for thermochemical calculations of the pellets. First results show advantages of the approach, especially for the simulation of agitated fuel beds, by resolving the dynamic interaction between the gas phase and the pellets. This allows to interpret the difference in the reaction progress of static and agitated fuel beds which is difficult to assess by experiments. © 2020 Elsevier Masson SAS
    view abstract10.1016/j.ijthermalsci.2020.106332
  • A correlation between char emissivity and temperature
    Schiemann, M. and Gronarz, T. and Graeser, P. and Gorewoda, J. and Kneer, R. and Scherer, V.
    Fuel 256 (2019)
    The radiative behaviour of coal and char particles is an important input parameter for simulations of coal combustion and gasification processes, as those typically feature elevated reactor temperatures. For consideration of radiative heat transfer, accurate knowledge of particle emissivity is a pre-requisite. Combining theoretical considerations and experimental data from literature, a temperature dependent relation for char emissivity is provided. © 2019 Elsevier Ltd
    view abstract10.1016/j.fuel.2019.115889
  • Assessment of coal pyrolysis kinetics for Barracuda or Ansys Fluent
    Krusch, S. and Scherer, V. and Solimene, R. and Senneca, O.
    Energy Procedia 158 (2019)
    In the present work different methodologies have been used to analyze pyrolysis of a German bituminous coal "Auguste Victoria" with the aim to obtain pyrolysis kinetic submodels suitable for combustion models developed under Barracuda or other CFD environment such as Ansys Fluent. Intrinsic kinetics have been obtained by conventional TGA analysis at low heating rate (2-20 K/min) and with small particle size (in the order of 0.1 mm) and described by a two-step pyrolysis model. Additionally, the rates of pyrolysis were measured under fluidized bed conditions for larger particle size fractions using a method based on the time-resolved measurement of pressure in the freeboard induced by volatiles release and by a flow restriction at the exhaust. Results of thermogravimetric experiments have been worked out to obtain submodels of coal pyrolysis, comparing and discussing alternative reaction networks of different complexity. However, in fluidized bed experiments, using larger particle sizes, transport limitations become important. As expected, for this regime the reactions rates become lower the larger the particle size, reducing the relevance of intrinsic kinetic submodels. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.
    view abstract10.1016/j.egypro.2019.01.459
  • Assessment of combustion rates of coal chars for oxy-combustion applications
    Senneca, O. and Vorobiev, N. and Wütscher, A. and Cerciello, F. and Heuer, S. and Wedler, C. and Span, R. and Schiemann, M. and Muhler, M. and Scherer, V.
    Fuel 238 (2019)
    A drop tube reactor with high heating rates typical of pulverized boilers (>104 K/s) has been used to carry out experiments with coal in different atmospheres: N2, CO2, O2/N2 and O2/CO2. The reactor wall temperature was set at 1573 K and the particles’ residence time was kept below 130 ms. In O2/N2 and O2/CO2 atmospheres coal pyrolysis was complete and additional char conversion occurred. The degree of char conversion increased with oxygen concentration values but was further enhanced by the presence of carbon dioxide, suggesting a positive contribution of CO2 to the overall rate of conversion. Chemico-physical and structural analysis of chars revealed internal burning under regime II conditions and highlighted that the presence of CO2 favors the formation of lactones in the chars. In N2 and CO2 atmospheres the pyrolysis stage was completed, but char conversion was negligible. The combustion stage of the N2 and CO2 chars was investigated in a second stage by thermogravimetric (TG) analysis (in regime I conditions) and in a flat flame burner (in regime II conditions) to separate atmospheric effects on char formation from those on char combustion. In TG, the CO2 chars resulted to be less reactive then the N2 chars, but in the flat flame burner, the experimental rate of carbon conversion of the N2 char and the CO2 char were similar. The TG results were worked out to estimate the intrinsic kinetics of the N2 and CO2 chars towards oxygen, carbon dioxide and O2/CO2 mixtures. Kinetic rate expressions were extrapolated to regime II conditions after consideration of mass transfer limitations. Notably, the kinetic model developed for the CO2-char matched the observed rate of char (oxy-) combustion well, whereas the kinetic model of the N2-char overpredicted the reaction rate. © 2018 Elsevier Ltd
    view abstract10.1016/j.fuel.2018.10.093
  • Differences in formation and oxidation of Colombian coal chars in air and oxy-fuel atmospheres
    Debiagi, P.E.A. and Heuer, S. and Ontyd, C. and Schiemann, M. and Faravelli, T. and Hasse, C. and Scherer, V.
    Chemical Engineering Transactions 74 (2019)
    The increasing interest towards more efficient and clean technologies, specially paying attention to CO2 neutral processes is encouraging the investigation of coal thermochemical conversion under oxy-fuel atmospheres (i.e. without N2). The process offers many advantages such as easy separation of the CO2 produced and low NOX/SOX emissions. While coal conversion in air is already well understood, full understanding of the influences of CO2-rich atmospheres is still required. A series of experiments in thermogravimetric analyser, drop-tube reactor and flat-flame burner were performed using a mid-range bituminous coal (Colombian coal) to understand the differences in the chars obtained after the pyrolysis step. Comparing the pyrolysis in N2 and CO2 atmospheres, significant differences were observed in the resulting chemical (composition) and physical properties of the chars, whereas mass loss was very similar for short residence times (< 130 ms). Afterwards, the chars obtained were submitted to oxidation and gasification, under several different operating conditions, in order to evaluate the difference in reactivity of these chars. Chars obtained under CO2 atmospheres revealed a lower reactivity, despite their higher surface area. These aspects cannot be explained and captured by a model which does not focus on some important details. In this paper, the results obtained in these experiments are summarized and discussed on the point of view of the POLIMI modelling approach for thermochemical conversion of solids. This model offers several advantages, such as being flexible to improvements, requiring simple experimental data of the fuel and offers an all-in-one solution for describing the kinetics of the whole process. The developments accounted for a wide range of experimental data, which allowed its calibration for several fuels, mostly in air combustion. It was first developed to describe the pyrolysis step, and later char oxidation/gasification was included in a simplified approach. The detailed mechanism of homogeneous gas phase reactions of the volatiles is also coupled. In order to extend the predictive capabilities of the model for oxy-fuel conditions, dedicated experiments must be considered for future improvements. In this work these missing effects are discussed, identifying the main necessary improvements, allowing this model to be extended and applied also for the designing of reactors that use oxy-fuel technologies. Copyright © 2019, AIDIC Servizi S.r.l.
    view abstract10.3303/CET1974026
  • Emittance of typical ash minerals: Influence of particle size and mixtures of carbonates and sulfates
    Gorewoda, J. and Maas, P. and Scherer, V.
    Energy Procedia 158 (2019)
    The current paper presents measurements of the normal spectral and total emittances of mixtures of different powdery material, all typical constituents of coal ashes. Measurement of sample surface temperature, a key parameter for the calculation of emittance, is carried out by thermocouples and by two-colour pyrometry. Two size fractions of 0 to 32 µm and 125 to 160 µm were examined. The influence of particle size on emittance is investigated by measurement of mixtures of different size fractions of SiO2. In addition, mixtures of CaCO3, CaSO4, K2SO4, Fe2O3 are examined. Measurements were made from 500 up to 1000 °C. The spectra were recorded from 1.6 µm to 12 µm, the major wavelength range for radiative heat transfer in boilers. The results show that the larger the particle size, the higher the emittance. In carbonate/sulfate mixtures absorption bands are visible in the spectra. Fe2O3 shows no absorption bands and seems to dominate the spectra when mixed with carbonates or sulfates. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.
    view abstract10.1016/j.egypro.2019.01.420
  • Evolution of coal char porosity from CO2-pyrolysis experiments
    Heuer, S. and Wedler, C. and Ontyd, C. and Schiemann, M. and Span, R. and Richter, M. and Scherer, V.
    Fuel 253 (2019)
    Pyrolysis experiments on a high volatile bituminous Columbian coal were performed in a laminar drop tube reactor at T = 1300 K and 1475 K. Measurements in CO2 were carried out at different residence times up to 150 ms, and the data were complemented by end-point measurements in N2 at approximately 165 ms. These low residence times are typical for the duration of pyrolysis in pulverized coal flames. Mass loss has been determined by solid sampling based on the ash tracer method, and the evolution of porosity was evaluated. Pyrolysis mass loss kinetics were determined based on a single first order reaction and a competing two-step reaction model with distributed activation energies. The particle temperature and residence time needed for the determination of the kinetics were derived by CFD simulations. Results indicate that, despite the low residence time selected, the influence of the Boudouard reaction on mass loss and, hence, evolution of porosity cannot be neglected. In general, porosity increases with increasing residence time and progressing mass loss and porosity is influenced by the both, the release of volatiles and the contribution of gasification. © 2019 Elsevier Ltd
    view abstract10.1016/j.fuel.2019.05.071
  • Fragmentation of pulverized coal in a laminar drop tube reactor: Experiments and model
    Senneca, O. and Heuer, S. and Bareschino, P. and Urciuolo, M. and Pepe, F. and Schiemann, M. and Chirone, R. and Scherer, V.
    Proceedings of the Combustion Institute 37 (2019)
    Fragmentation during pulverized coal particles conversion shifts the particle size distribution of the fuel towards smaller particle sizes, affecting both conversion rates and heat release. After pyrolysis of a high volatiles Colombian coal in CO2 atmosphere in a drop tube reactor at 1573 K, solid carbonaceous particles of different size, from 100 μm of the particle feed down to the nanometric size, have been observed. A fragmentation model has been used to predict the fate of Colombian coal particles under the experimental conditions of the drop tube experiment and predict the particle size distribution (PSD). Model and experimental results are in very good agreement and indicate that in the DTR experiment the coal underwent almost complete pyrolysis and that fragmentation generated a 36 wt% population of particles with size close to 30 μm. The close match between the PSDs obtained from experiments and from the fragmentation model is an important novelty. It demonstrates that fragmentation occurs not only under fluidized bed conditions but also under the conditions of pulverized coal combustion. Experimentalists are warned against the fact that the fine particulate sampled at the outlet of laminar flow reactors and boilers is not always composed of soot only. Char fragments can be misidentified as soot. The implementation of fragmentation submodels in pulverized fuel combustion and gasification codes is highly recommended. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.08.057
  • Investigation of a liquid air energy storage (LAES) system with different cryogenic heat storage devices
    Hüttermann, L. and Span, R. and Maas, P. and Scherer, V.
    Energy Procedia 158 (2019)
    Liquid air energy storage (LAES) is a large-scale storage technology, which is using liquefied air as storage medium. Comparable to pumped hydro (PHES) and compressed air energy storage (CAES), LAES is charged with excess electricity from the grid and discharged, when the electricity demand is high. Working as a buffer for the electric grid, the availability and integrability of fluctuating renewable energy sources can be improved by LAES. In the charging process, ambient air is liquefied with an adopted Claude respectively Kapitza process. Compression heat is stored in a hot thermal energy storage device (HTES); a cold thermal energy storage device (CTES) is used as heat sink at cryogenic temperature to significantly improve the efficiency of the liquefaction. In the discharging process, liquid air is pressurized, heated up to ambient temperature by the CTES, superheated by the HTES, and expanded in an air expander for electricity generation. The CTES is used to recycle an exergy flow at cryogenic temperature from the discharging to the charging process. Since the round trip efficiency of the LAES strongly depends on this exergy flow, two different types of CTES are compared within this work. The liquid cold thermal energy storage device (LCTES) is based on a multi-tank storage system using propane and methanol, the direct cold thermal energy storage device (DCTES) is a packed bed storage system with direct contact between the fluid and the solid storage material. In this work, a comparison and an exergetic investigation of both systems is presented. The significant influence of the exergetic efficiency of the CTES and other technical aspects are worked out. Additionally, the influence of the pressure on the liquefaction and discharging process, and on the round trip efficiency is investigated. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.
    view abstract10.1016/j.egypro.2019.01.776
  • Real-time multitarget tracking for sensor-based sorting: A new implementation of the auction algorithm for graphics processing units
    Maier, G. and Pfaff, F. and Wagner, M. and Pieper, C. and Gruna, R. and Noack, B. and Kruggel-Emden, H. and Längle, T. and Hanebeck, U.D. and Wirtz, S. and Scherer, V. and Beyerer, J.
    Journal of Real-Time Image Processing 16 (2019)
    Utilizing parallel algorithms is an established way of increasing performance in systems that are bound to real-time restrictions. Sensor-based sorting is a machine vision application for which firm real-time requirements need to be respected in order to reliably remove potentially harmful entities from a material feed. Recently, employing a predictive tracking approach using multitarget tracking in order to decrease the error in the physical separation in optical sorting has been proposed. For implementations that use hard associations between measurements and tracks, a linear assignment problem has to be solved for each frame recorded by a camera. The auction algorithm can be utilized for this purpose, which also has the advantage of being well suited for parallel architectures. In this paper, an improved implementation of this algorithm for a graphics processing unit (GPU) is presented. The resulting algorithm is implemented in both an OpenCL and a CUDA based environment. By using an optimized data structure, the presented algorithm outperforms recently proposed implementations in terms of speed while retaining the quality of output of the algorithm. Furthermore, memory requirements are significantly decreased, which is important for embedded systems. Experimental results are provided for two different GPUs and six datasets. It is shown that the proposed approach is of particular interest for applications dealing with comparatively large problem sizes. © 2017, The Author(s).
    view abstract10.1007/s11554-017-0735-y
  • A comparison of the torrefaction behavior of wood, miscanthus and palm kernel shells: Measurements on single particles with geometries of technical relevance
    Becker, A. and Scherer, V.
    Fuel 224 (2018)
    A torrefaction test rig was designed to investigate large single biomass particles up to characteristic sizes of 25 mm, typical for industrial reactors. Time-resolved mass loss for such particles is measured with a magnetic suspension balance at well-defined torrefaction conditions (temperature, residence time, gas atmosphere). This paper comprises the results of woody and non-woody biomass: pine, a coniferous, and beech, a deciduous, wood, palm kernel shells and miscanthus. Influence of process temperature (240 to 320 °C), residence time (up to 1 h) and type of solid biomass on time-resolved mass loss is presented. Additional tests with oxygen in the process gas (0–15 vol%), typical for industrial torrefaction systems, are carried out for selected samples of beech wood. The differences in torrefaction behaviour of bark, sap- and heartwood of pine are evaluated. Finally, it is shown that the torrefaction reactor developed allows to derive kinetic parameters for mass loss. At temperatures up to 300 °C the mass loss for palm kern shells is highest followed by miscanthus, and pine. By examining pine, as an example, it is shown that heartwood is significantly more reactive than sapwood and bark. Finally, it is demonstrated, that for the particle sizes considered here heat and mass transfer limitations can be neglected for the determination of torrefaction kinetics. Kinetic data agree well with data from literature. © 2018 Elsevier Ltd
    view abstract10.1016/j.fuel.2018.01.095
  • An Alternative Approach for the Estimation of Biochar Yields
    Weber, K. and Heuer, S. and Quicker, P. and Li, T. and Løvås, T. and Scherer, V.
    Energy and Fuels 32 (2018)
    In this study, a novel approach for the determination of the solid mass yield from slow pyrolysis based on a comparison of the volatile matter contents of feedstock and char is presented. The approach was tested with experimental data from the literature and our own measurements. For these experiments, gravimetric data are available to determine the mass yield. The proposed method was compared with conventional ash-based calculations and the gravimetric determination of the yield. It was shown that the new approach not only does perform significantly better than ash-based methods but also approximates the real mass yield of slow pyrolysis under atmospheric pressure quite accurately. These findings may indicate that secondary char formation does not contribute significantly to the mass yield of biomass pyrolysis under conditions found in practical production processes (low heating rates, atmospheric pressure, and medium temperatures). © 2018 American Chemical Society.
    view abstract10.1021/acs.energyfuels.8b01825
  • Effects of CO2 enriched atmosphere on chars from walnut shells pyrolysis in a drop tube reactor
    Senneca, O. and Cerciello, F. and Cortese, L. and Heuer, S. and Schiemann, M. and Scherer, V.
    Fuel 229 (2018)
    A laminar drop tube reactor (DTR) was used to perform fast pyrolysis of walnut shells, a ligno-cellulosic biomass sample, in nitrogen and carbon dioxide atmospheres. The DTR reached the temperature of 1300 °C and the heating rate of 104-105 °C/s. Char samples collected at different residence times along the reactor were characterized by ultimate and proximate analysis and by SEM. Char combustion reactivity was then measured by non-isothermal thermogravimetric analysis (TGA) in air. The analyses show that at residence times of 66 ms pyrolysis in N2 is not complete, whereas it is complete in CO2. For residence times of 115 ms the differences between samples produced in N2 and CO2 atmospheres level off. The derivative thermogravimetric (DTG) curves of the char combustion show the existence of multiple peaks. Notably, early combustion peaks progressively fade in the chars collected at increasing reactor residence time, confirming the completion of pyrolysis. A kinetic model of char combustion is proposed which includes multiple parallel reactions. © 2018 Elsevier Ltd
    view abstract10.1016/j.fuel.2018.04.152
  • Influence of stoking on the combustion of beech wood particles of different shape in an agitated bed
    Buß, F. and Wirtz, S. and Scherer, V.
    Experimental Thermal and Fluid Science 95 (2018)
    Mass loss of beech wood particles during combustion was measured in a batch reactor which allows stoking of the fuel bed. The test rig allows air staging, where primary air is flowing through the fuel bed and secondary air is added in the freeboard above the bed. The bed is ignited by radiation from electrically heated walls. The shape of the biomass particles (spheres, cylinders and cubes) and the primary to secondary air mass flow ratio were varied. Influence of stoking has been assessed by determining a mixing index for the top particle layer. Results show the general influence of stoking on the mass loss rate of the bulk in different combustion regimes. Stoking delays volatile ignition above the bed but accelerates fuel bed mass loss during combustion. Increased radiative heat flux from the flame into the bed when the bed is stoked was identified as the main reason for accelerated mass loss. Particle shape influences bulk mixing which is reflected in the combustion behavior. In particular, ignition of the volatiles is delayed by increased bulk mixing. © 2018 Elsevier Inc.
    view abstract10.1016/j.expthermflusci.2018.01.009
  • Lithium as energy carrier: CFD simulations of LI combustion in a 100 MW slag tap furnace
    Maas, P. and Schiemann, M. and Scherer, V. and Fischer, P. and Taroata, D. and Schmid, G.
    Applied Energy 227 (2018)
    Metal combustion is currently under discussion as a possible basis for a closed energy loop. One potential metal with several benefits for such a process is lithium. While the reaction products in conventional combustion processes are gaseous, the reaction products of lithium combustion are solid (Li2CO3, Li2O) and, hence, easy to capture and to recycle. The current paper describes the lay-out and optimization of a 100 MWth lithium slag tap furnace by computational fluid dynamics (CFD) using CO2 as oxidizer for the lithium. ANSYS Fluent has been extended by two lithium combustion models developed by the authors. The first reference model is one-step model directly converting Li to Li2CO3, neglecting the intermediate species Li2O. The second extended model is a two-step model considering Li2O as intermediate species. Simulations were carried out using a fixed geometry of the slag tab, varying the injection angle of gas and lithium spray and the CO2-Li ratio with respect to the lithium conversion level and lithium product capture efficiency. The simulations show that a high capture efficiency of lithium combustion products is possible when a large injection angle is used. The conversion level is highly dependent on injection angle, CO2-Li ratio and the Li combustion model used. While the conversion level of the reference model is inherently limited and lies between 84 and 87.6%, the extended model predicts significantly higher conversion levels in the order of 96.7–99.2% which would be needed for industrial application. © 2017 Elsevier Ltd
    view abstract10.1016/j.apenergy.2017.09.041
  • Motion-based material characterization in sensor-based sorting
    Maier, G. and Pfaff, F. and Becker, F. and Pieper, C. and Gruna, R. and Noack, B. and Kruggel-Emden, H. and Längle, T. and Hanebeck, U.D. and Wirtz, S. and Scherer, V. and Beyerer, J.
    Technisches Messen 85 (2018)
    Sensor-based sorting provides state-of-the-art solutions for sorting cohesive, granular materials. Typically, involved sensors, illumination, implementation of data analysis and other components are designed and chosen according to the sorting task at hand. A common property of conventional systems is the utilization of scanning sensors. However, the usage of area-scan cameras has recently been proposed. When observing objects at multiple time points, the corresponding paths can be reconstructed by using multiobject tracking. This in turn allows to accurately estimate the point in time and position at which any object will reach the separation stage of the optical sorter and hence contributes to decreasing the error in physical separation. In this paper, it is proposed to further exploit motion information for the purpose of material characterization. By deriving suitable features from the motion information, we show that high classification performance is obtained for an exemplary classification task. The approach therefore contributes towards decreasing the detection error of sorting systems. © 2017 Walter de Gruyter Berlin/Boston.
    view abstract10.1515/teme-2017-0063
  • Numerical analysis of wet plastic particle separation using a coupled DEM-SPH method
    Markauskas, D. and Kruggel-Emden, H. and Scherer, V.
    Powder Technology 325 (2018)
    The separation of different kind of plastic particles is required in the process of waste recycling. For the separation, drum processes with liquid can be used. The separation is based on the principle that particles either sink or float in a liquid depending on their densities. In this study, this process is numerically analysed for the separation of polyethylene terephthalate (PET) from polypropylene (PP) particles. The discrete element method coupled with the smoothed particle hydrodynamics method (DEM-SPH) is used for modelling purposes. The employment of the SPH for the modelling of the liquid exploits the strong side of this meshless method, namely, the relative ease in modelling large movements of the fluid with free surfaces and moving boundaries. This theoretical model is presented, and verification tests are performed, where a dam-break problem is considered as an example. Simulations of the plastic particle separation in the rotating drum are performed thereafter. The influences of the different operational and design parameters, such as the rotational velocity, feed rate, and number of lifters on the resultant purity of the plastic are estimated. It is expected that, in the future, the performed analysis will allow the numerical optimisation of drum separation processes. © 2017 Elsevier B.V.
    view abstract10.1016/j.powtec.2017.11.021
  • Numerical modelling of an optical belt sorter using a DEM–CFD approach coupled with particle tracking and comparison with experiments
    Pieper, C. and Pfaff, F. and Maier, G. and Kruggel-Emden, H. and Wirtz, S. and Noack, B. and Gruna, R. and Scherer, V. and Hanebeck, U.D. and Längle, T. and Beyerer, J.
    Powder Technology 340 (2018)
    State-of-the-art optical sorting systems suffer from delays between the particle detection and separation stage, during which the material movement is not accounted for. Commonly line scan cameras, using simple assumptions to predict the future particle movement, are employed. In this study, a novel prediction approach is presented, where an area scan camera records the particle movement over multiple time steps and a tracking algorithm is used to reconstruct the corresponding paths to determine the time and position at which the material reaches the separation stage. In order to assess the benefit of such a model at different operating parameters, an automated optical belt sorter is numerically modelled and coupled with the tracking procedure. The Discrete Element Method (DEM) is used to describe the particle–particle as well as particle–wall interactions, while the air nozzles required for deflecting undesired material fractions are described with Computational Fluid Dynamics (CFD). The accuracy of the employed numerical approach is ensured by comparing the separation results of a predefined sorting task with experimental investigations. The quality of the aforementioned prediction models is compared when utilizing different belt lengths, nozzle activation durations, particle types, sampling frequencies and detection windows. Results show that the numerical model of the optical belt sorter is able to accurately describe the sorting system and is suitable for detailed investigation of various operational parameters. The proposed tracking prediction model was found to be superior to the common line scan camera method in all investigated scenarios. Its advantage is especially profound when difficult sorting conditions, e.g. short conveyor belt lengths or uncooperative moving bulk solids, apply. © 2018 Elsevier B.V.
    view abstract10.1016/j.powtec.2018.09.003
  • Pyrolysis and Thermal Annealing of Coal and Biomass in CO2-Rich Atmospheres
    Senneca, O. and Apicella, B. and Russo, C. and Cerciello, F. and Salatino, P. and Heuer, S. and Wütscher, A. and Schiemann, M. and Muhler, M. and Scherer, V.
    Energy and Fuels 32 (2018)
    The high temperatures and heating rates typical of PF are known to induce thermal annealing of char and loss of its reactivity. Several authors investigated this effect for coals in inert atmospheres, while little is known about the effects of CO2-rich atmospheres, typical of oxy-combustion and gasification, on the course of thermal annealing. Thermal annealing of biomass has been scarcely investigated in the literature; however, available studies reported that also biomass can suffer from thermo-deactivation. The present study aims to provide further insight on thermal annealing of biomass in the context of gasification and oxy-combustion. A lignin-rich biomass (walnut shells) has been heat-treated in a heated strip reactor at temperatures of 1573-2073 K with a holding time of 3 s using atmospheres of either N2 or CO2. Similar experiments have been performed with a high volatile bituminous coal (Colombian coal) used as reference. The char samples have been analyzed by thermogravimetric analysis and Raman spectroscopy. Results have been further compared with those reported in previous studies where heat treatment of the same fuels were performed in fixed bed, fluidized bed, and drop tube reactors at lower temperature or shorter holding time. Two remarkable results have been obtained: (1) Loss of reactivity by thermal annealing and structural reorganization follow similar pathways for coal and biomass. (2) The effect of CO2 on pyrolysis and thermal annealing is non-monotonic along with heat treatment: in the early instances of heat treatment (T = 1573 K, t < 0.1 s), CO2 fosters pyrolysis and thermal annealing, increasing structural ordering. At longer holding times (T > 1573 K, t > 1 s), instead, CO2 somewhat hampers thermal annealing. © 2018 American Chemical Society.
    view abstract10.1021/acs.energyfuels.8b02417
  • The convective heat transfer coefficient in flighted rotary drums
    Seidenbecher, J. and Meitzner, C. and Herz, F. and Wirtz, S. and Berndt, A. and Scherer, V.
    International Heat Transfer Conference 2018-August (2018)
    Rotary drums are used in industrial applications for the thermal treatment of granular materials. Additional flights lift particles out of the bulk bed to shower them as curtains in the airborne phase of the drum, resulting in an improved heat transfer. The amount of particles and their distribution in the airborne phase is influenced by operational, design and bulk material parameters, which was researched in previous studies about transverse particle motion. In order to analyze the convective heat transfer, experiments were carried out in an indirectly heated (Pel = 21 kW) flighted rotary drum (D = 0.5 m, L = 1.76 m). The temperature distribution inside the drum is measured using k-type thermocouples at different axial and radial positions. After the drum and the test material (4 mm glass beads) are heated to the set temperature in batch mode, ambient air is sucked through the rotary drum to cool the system. During both phases the temperature profiles for the gas, the granular material, the flights and the drum wall are measured. The contact heat transfer coefficient is calculated for the heating phase. The convective heat transfer coefficient between gas and particles in the airborne phase (curtains) is analyzed in the cooling phase based on the energy balance of the granular material. The operational parameters rotational speed (1 - 8 rpm), air volume flow (100 - 800 Nm3/h) as well as filling degree (10, 20, 30 %) are varied and their effect on the convective heat transfer is discussed. © 2018 International Heat Transfer Conference. All rights reserved.
    view abstract10.1615/ihtc16.tpm.022104
  • The influence of operating parameters on the temperature distribution in flighted rotary drums
    Seidenbecher, J. and Herz, F. and Specht, E. and Wirtz, S. and Berndt, A. and Scherer, V.
    Proceedings of the Thermal and Fluids Engineering Summer Conference 2018-March (2018)
    Rotary drums are used in many industrial applications for heat and mass transfer between gas and solid particles. Additional flights are installed to improve the mixing of adhesive products and especially to increase the exchange surface between gas and solids. The flights are mounted on the inside of the drum and lift particles out of the solid bed to shower them as curtains in the passing gas phase of the drum, where heat and mass transfer is very intense. The amount of particles and their distribution are influenced by operating parameters (rotational speed, filling degree, air volume flow), which were researched in a previous study about transverse particle motion. In order to couple the transverse particle motion to the heat transfer, heat transfer experiments were carried out. Therefore, an indirectly heated flighted rotary drum was designed and constructed. Batch experiments, which were divided into heating and cooling process, with glass beads as reference material were performed. The temperature distribution inside the drum was measured using thermocouples at different axial and radial positions in the cylindrical drum. The operating parameters rotational speed, filling degree and air volume flow were varied to investigate their influence on the temperature distribution inside the drum. It was found that increasing the rotational speed as well as the air volume flow leads to faster changing temperatures. Meanwhile, increasing the filling degree results in slower temperature drop rates. © 2018 Begell House Inc.. All rights reserved.
    view abstract10.1615/TFEC2018.tfh.020944
  • Transient DEM-CFD simulation of solid and fluid flow in a three dimensional blast furnace model
    Bambauer, F. and Wirtz, S. and Scherer, V. and Bartusch, H.
    Powder Technology 334 (2018)
    A numerical investigation of gas and solid flow as well as the liquid accumulation in a scaled Blast Furnace (BF) model was carried out using the discrete element method (DEM) coupled with computational fluid dynamics (CFD). This three dimensional simulation considers the interaction of the solid flow of a layered burden column with the counter flowing process gas. The influence of the cohesive zone is modeled by a material dependent permeability, the molten iron and the slag are considered as one additional liquid phase in the hearth. The method used is compared with a two dimensional numerical slot model presented in former publications by Zhou et al. and then applied to a different three dimensional setup. The comparison of the models exhibits considerable differences concerning the general multiphase flow behaviour. Moreover, it is shown that three dimensional models are required to correctly resolve the actual spatial flow structure and its influence on the shape of the coke free region in the hearth. Finally, the three dimensional model is used to show the influence of the liquid discharge on the stress distribution along the walls. © 2017 Elsevier B.V.
    view abstract10.1016/j.powtec.2018.04.062
  • 3D-DEM-CFD simulation of heat and mass transfer, gas combustion and calcination in an intermittent operating lime shaft kiln
    Krause, B. and Liedmann, B. and Wiese, J. and Bucher, P. and Wirtz, S. and Piringer, H. and Scherer, V.
    International Journal of Thermal Sciences 117 (2017)
    This work presents DEM-CFD simulations of the transient processes occurring in an industrial scale PFR-kiln. DEM allows the numerical simulation of the moving and reacting limestone bed in the kiln and is coupled with a 3-dimensional CFD simulation describing the interstitial gas phase. A PFR-kiln consists of two vertical shafts and a connecting crossover channel. The two shafts periodically switch their function at regular intervals of about 15 min. While one shaft calcines the product in parallel flow with gas temperatures above 900 °C, the other preheats the stones in counter flow. The model has been applied to an industrial PFR-kiln of 18 m height. A realistic particle size distribution of the limestone with particles ranging from 50 to 90 mm has been set. Methane combustion provides the heat for calcination and is simulated by a two-step mechanism. Simulation results show a nearly uniform temperature distribution in the calcination zone but significant inner particle temperature gradients. The calcination degree depends on the particle location within the kiln and decreases towards the outer kiln walls. Measured and simulated temperatures are compared. Maximum temperature values as well as its characteristic oscillation induced by the periodic kiln operation could be reproduced by the simulation, especially keeping in mind the difficulties of thermocouple measurements under the harsh conditions in industrial reality. © 2017 Elsevier Masson SAS
    view abstract10.1016/j.ijthermalsci.2017.03.017
  • An approach to model the thermal conversion and flight behaviour of Refuse Derived Fuel
    Liedmann, B. and Arnold, W. and Krüger, B. and Becker, A. and Krusch, S. and Wirtz, S. and Scherer, V.
    Fuel 200 (2017)
    The current paper presents a simplified approach which allows the CFD simulation of Refuse Derived Fuel (RDF) combustion. The starting point is the subdivision of a real RDF into characteristic fuel fractions by sorting. Each of the fractions was analyzed concerning elemental composition, heating value, proximate analysis as well as size and shape. The flight behavior of the RDF fractions has been characterized in a drop-shaft. A stereoscopic camera system was used to derive drag and lift coefficients. In addition, a single particle combustion reactor has been used to measure the duration of the relevant combustion phases like volatile combustion or char burn-out. A model calculating the particle trajectories based on the measured drag and lift frequency distributions has been developed. For combustion modelling the RDF has been subdivided into devolatilizing and char forming fractions and into fractions which are converting through a melting and decomposition process. For both types of materials combustion models have been formulated. Intra particle temperature gradients are accounted for. A change of particle shape during combustion is considered using sphericity as a model parameter. The models have finally been introduced into FLUENT by user defined functions. Comparison with drop-shaft measurements and a single particle combustion reactor show that the models formulated can statistically describe the motion and conversion behaviour of RDF with sufficient accuracy. As an example of application, the models were finally used for the CFD simulation of the furnaces of a 612 MW(e) RDF co-fired coal power plant. The results indicate an overall slower reaction rate of RDF compared to coal, resulting in a total conversion of RDF of 83%. © 2017 Elsevier Ltd
    view abstract10.1016/j.fuel.2017.03.069
  • CFD Simulation of a 100 MWth Lithium Combustion Slag Tap Furnace as a basis for an Energy Storage Process
    Maas, P. and Schiemann, M. and Scherer, V. and Fischer, P. and Taroata, D. and Schmid, G.
    Energy Procedia 105 (2017)
    Lithium combustion has been discussed as a possible basis for a closed energy loop. While the reaction products in conventional combustion processes are gaseous, the reaction products of lithium combustion are solid (Li2CO3, Li2O) and hence easy to capture and to recycle. The current paper describes the lay-out and optimization of a 100 MWth lithium slag tap furnace by computational fluid dynamics (CFD) using CO2 as oxidizer for the lithium. ANSYS Fluent has been extended by a lithium combustion model developed by the authors. Simulations with five different gas and particle injection angles and three atmospheres with different CO2-fuel ratios were conducted to investigate the lithium conversion level and separation efficiency. The simulations show, that a high separation efficiency of lithium combustion products is possible when a large injection angle is used. The conversion level on the other hand is highly dependent on both injection angle and CO2-fuel ratio and lies between 84 and 87.6%. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.egypro.2017.03.830
  • Comparative ignition tests of coal under oxy-fuel conditions in a standardized laboratory test rig
    Becker, A. and Schiemann, M. and Scherer, V. and Shaddix, C. and Haxter, D. and Mayer, J.
    Fuel 208 (2017)
    Ignitability is important to characterize pulverized coal combustion, as it is directly related to flame stability. The current work describes a practical test rig for rapid laboratory analysis of pulverized coal cloud ignition properties. The system has been designed for conventional coal combustion conditions using air as the oxidant. In the current work, the measurement principle of the device is described and its adaption to and applicability for oxy-fuel combustion tests is demonstrated. Four coals with different rank were measured in air and in oxy-fuel atmospheres containing 20–35 vol% O2 in CO2. The major influencing factors for the investigated samples were found to be the coal rank and the gas-phase oxygen concentration, while a minor influence of particle size was observed. © 2017 Elsevier Ltd
    view abstract10.1016/j.fuel.2017.06.129
  • Comparison of pyrolysis test rigs for oxy-fuel conditions
    Pielsticker, S. and Heuer, S. and Senneca, O. and Cerciello, F. and Salatino, P. and Cortese, L. and Gövert, B. and Hatzfeld, O. and Schiemann, M. and Scherer, V. and Kneer, R.
    Fuel Processing Technology 156 (2017)
    In oxy-combustion, coal particles undergo devolatilization in CO2 enriched atmospheres. Besides the well-known influence of thermal conditions, the composition of the pyrolysis atmosphere may also have important effects on the formation and properties of pyrolysis products. In an international collaboration, researchers from three institutions from Aachen, Bochum and Naples carried out pyrolysis experiments with a medium rank coal in a fixed bed, fluidized bed and drop tube reactor, substituting N2 with CO2. The goal of the current study was to investigate the influence of increased CO2 concentrations on the pyrolysis products (tar, gas and solids) when different heating rates, temperature and residence times are applied. Pyrolysis products were analyzed by several techniques to highlight differences in structure and chemical composition. At low heating rates and temperature, the differences between N2 and CO2 pyrolysis products were marginal. A CO2 rich atmosphere, instead, impacted severely the properties of pyrolysis products under the fast heating-short residence time conditions typical of drop tube reactors. Upon prolonged exposure to severe treatment differences apparently leveled off. © 2016 Elsevier B.V.
    view abstract10.1016/j.fuproc.2016.10.010
  • Editorial to the Proceedings of 11th European Conference on Industrial Furnaces and Boilers, INFUB-11, 18-21 April 2017
    Scherer, V. and Fricker, N. and Reis, A.
    Energy Procedia 120 (2017)
    view abstract10.1016/j.egypro.2017.07.150
  • Improving multitarget tracking using orientation estimates for sorting bulk materials
    Pfaff, F. and Kurz, G. and Pieper, C. and Maier, G. and Noack, B. and Kruggel-Emden, H. and Gruna, R. and Hanebeck, U.D. and Wirtz, S. and Scherer, V. and Langle, T. and Beyerer, J.
    IEEE International Conference on Multisensor Fusion and Integration for Intelligent Systems 2017-November (2017)
    Optical belt sorters can be used to sort a large variety of bulk materials. By the use of sophisticated algorithms, the performance of the complex machinery can be further improved. Recently, we have proposed an extension to industrial optical belt sorters that involves tracking the individual particles on the belt using an area scan camera. If the estimated behavior of the particles matches the true behavior, the reliability of the separation process can be improved. The approach relies on multitarget tracking using hard association decisions between the tracks and the measurements. In this paper, we propose to include the orientation in the assessment of the compatibility of a track and a measurement. This allows us to achieve more reliable associations, facilitating a higher accuracy of the tracking results. © 2017 IEEE.
    view abstract10.1109/MFI.2017.8170379
  • Key parameters for the radiative emittance of ashes of solid fuels
    Gorewoda, J. and Scherer, V.
    Energy Procedia 120 (2017)
    The knowledge of the emittance of solid fuel ashes is important for the radiative energy balance in boilers and, hence, crucial for their design. This paper summarizes the most important effects on emittance based on own experiments and makes references to literature. Experimental results will be presented on the spectral emittance of typical minerals (SiO2, CaCO3, MgCO3, SrCO3, CaSO4, MgSO4, Fe2O3) contained in solid fuels ashes, extended by exemplary measurements on natural ashes (coal). The normal emittance is measured in a temperature range between 500 and 1000 °C in the wavelength range from 1.6 to 12 μm in a radiation test rig. The influence of physical surface structure and chemical-mineralogical composition on emittance is discussed. The results show that sizes of ash particles influences emittance. Emittance is increasing with particle size. Surface sintering as well as Fe in the ash also increases emittance. Surface fusion can either increase or decrease emittance based on ash composition. Sulfates and carbonates, typical for ashes under oxyfuel conditions, show characteristic spectral emittance bands. These bands vanish when the sulfates and carbonates being converted to the corresponding oxides at elevated temperatures. These characteristic bands can also be detected in natural ashes which consist of a variety of mineral components. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.egypro.2017.07.163
  • Normal Radiative Emittance of Coal Ash Sulfates in the Context of Oxyfuel Combustion
    Gorewoda, J. and Scherer, V.
    Energy and Fuels 31 (2017)
    Oxyfuel ashes are supposed to form more sulfates than ashes from air-fired systems. This can be caused by the increased SO2 concentrations due to intensive flue gas recirculation in oxyfuel systems. Therefore, we investigated the spectral emittance characteristics of typical mineral sulfates in coal ashes, namely Mg and Ca sulfates. The samples were prepared in powder form. Two particle size fractions were examined (x < 32 μm and 125 < x < 160 μm). The powders were investigated concerning their temperature-dependent normal emittance in a radiation test rig. Spectral measurements by a Fourier transform infrared spectrometer in the temperature range from 500 to 1000 °C were carried out. The results reveal that Ca and Mg sulfates show characteristic S-O absorption bands in the wavelength regions from 3 to 4 μm, from 4.5 to 6 μm, and from 8 to 9.5 μm. MgSO4 transforms to MgO at around 930 °C. The total emittance of the oxide is significantly reduced by Δε = 0.15 compared to the sulfate. The small size fractions MgSO4 and CaSO4 undergo sintering when being heated, which influences emittance. An increase of total emittance up to a value of Δε = 0.08 is detected for CaSO4. Finally, it is shown that emittance increases with particle size (Δε in total emittance approximately = 0.1). © 2017 American Chemical Society.
    view abstract10.1021/acs.energyfuels.6b02866
  • Numerical investigation of optical sorting using the discrete element method
    Pieper, C. and Kruggel-Emden, H. and Wirtz, S. and Scherer, V. and Pfaff, F. and Noack, B. and Hanebeck, U.D. and Maier, G. and Gruna, R. and Längle, T. and Beyerer, J.
    Springer Proceedings in Physics 188 (2017)
    Automated optical sorting systems are important devices in the growing field of bulk solids handling. The initial sorter calibration and the precise optical sorting of many materials is still very time consuming and difficult. A numerical model of an automated optical belt sorter is presented in this study. The sorter and particle interaction is described with the Discrete Element Method (DEM) while the separation phase is considered in a post processing step. Different operating parameters and their influence on sorting quality are investigated. In addition, two models for detecting and predicting the particle movement between the detection point and the separation step are presented and compared, namely a conventional line scan camera model and a new approach combining an area scan camera model with particle tracking. © Springer Science+Business Media Singapore 2017.
    view abstract10.1007/978-981-10-1926-5_115
  • Numerical Study on the Influence of Operational Settings on Refuse Derived Fuel Co-firing in Cement Rotary Kilns
    Liedmann, B. and Wirtz, S. and Scherer, V. and Krüger, B.
    Energy Procedia 120 (2017)
    Cement production in rotary kilns requires large amounts of thermal energy, which is provided by combustion of different fuels. Substitution of fossil fuels by refuse derived fuels (RDF) can minimize production costs and reduce CO2 emissions, but often causes displacement of the sintering zone, impacts flame stability and cement quality. The current paper briefly introduces our numerical approach which describes particle motion and combustion characteristics of typical non-spherical RDF particles. By using these models in CFD simulations, a case study is presented. Fuel properties, primary- and secondary air settings and fuel feed location for a generic rotary kiln of industrial scale are varied to show the effects of operational settings on co-firing of RDF. Shift of flame shape and location as well as particle burnout are analyzed. Based on the information generated, optimized operational settings are identified and discussed. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.egypro.2017.07.176
  • POD and Galerkin-based reduction of a wood chip drying model
    Oliver Berner, M. and Sudbrock, F. and Scherer, V. and Mönnigmann, M.
    IFAC-PapersOnLine 50 (2017)
    The drying of wood chips can be modeled with coupled PDEs that describe water diffusion and heat conduction. Models of this type are coupled to discrete element codes that simulate the motion of wood chips in industrial rotary dryers in state-of-the-art solvers. Because many wood chips need to be modeled, the computational effort quickly becomes prohibitive. Reduced order models for the drying process of wood chips are obviously of interest in this context. We apply proper orthogonal decomposition and Galerkin projection to a model of a wood chip drying process. Gauss’ theorem is applied in the Galerkin projection step so that the boundary conditions appear explicitly in the reduced model. Our computational experiments indicate the wood chip drying process may be controlled with the ambient conditions in the rotary dryer. © 2017
    view abstract10.1016/j.ifacol.2017.08.696
  • Pulverized coal ignition testing under air-fired conditions using the Zelkowski method: Comparison of coals of different rank and provenience
    Becker, A. and Schiemann, M. and Scherer, V. and Haxter, D. and Mayer, J.
    Energy Procedia 120 (2017)
    Ignitability, which is directly related to flame stability, is an important factor in pulverized coal combustion. In the current work, a laboratory test rig for comparative measurement of coal ignition properties is described. It has been designed for standard coal combustion conditions using air as oxidant. The measurement principle is described on the basis of experimental data gained from four "standard coals" of different rank and provenience. The results show the general tendencies known for the influence of coal rank. Higher volatile content in coal leads to lower ignition temperature. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.egypro.2017.07.162
  • Quantification of the influence of parameters determining radiative heat transfer in an oxy-fuel operated boiler
    Gronarz, T. and Schulze, J. and Laemmerhold, M. and Graeser, P. and Gorewoda, J. and Kez, V. and Habermehl, M. and Schiemann, M. and Ströhle, J. and Epple, B. and Scherer, V. and Kneer, R.
    Fuel Processing Technology 157 (2017)
    Radiative heat transfer is a very important heat transfer mechanism in pulverized coal combustion. To identify the influence of parameters determining radiatve heat transfer and to give recommendations on the required accuracy of corresponding submodels, a 3D-periodic oxy-fuel pulverized coal combustion test case is investigated. Measurement values determined by the authors or elaborate submodels are applied for each parameter and compared to simplified models or empirical constants. To investigate the interaction between particle radiation and the strong spectral dependence of gas radiation in oxy-fuel scenarios, a comparison between spectrally averaged and spectrally resolved calculations performed. To the best knowledge of the authors, for the first time the contribution of the parameters determining radiative heat transfer are quantified and compared in one comprehensive study. The results indicate a strong influence of coal particle emissivity and scattering phase function as well as the projected particle surface on the radiative source term. For the wall heat flux, the largest influences were found for ash and coal particle emissivity, projected particle surface and the scattering phase function. Additionally, the difference between coal particle and gas temperature was found to have a significant influence on wall heat flux. A comparison of spectrally averaged to spectrally resolved results and the corresponding models for gas radiation (WSGGM and SNBM) yielded similar trends for the influence of each parameter. Thus, based on the models and parameters involved in this study, a spectrally averaged approach seems to be of sufficient accuracy to describe radiative heat transfer in oxy-fuel combustion systems. © 2016 Elsevier B.V.
    view abstract10.1016/j.fuproc.2016.11.012
  • Real-time motion prediction using the chromatic offset of line scan cameras
    Pfaff, F. and Maier, G. and Aristov, M. and Noack, B. and Gruna, R. and Hanebeck, U.D. and Längle, T. and Beyerer, J. and Pieper, C. and Kruggel-Emden, H. and Wirtz, S. and Scherer, V.
    At-Automatisierungstechnik 65 (2017)
    State-of-the-art optical belt sorters commonly employ line scan cameras and use simple assumptions to predict each particle's movement, which is required for the separation process. Previously, we have equipped an experimental optical belt sorter with an area scan camera and were able to show that tracking the particles of the bulk material results in an improvement of the predictions and thus also the sorting process. In this paper, we use the slight gap between the sensor lines of an RGB line scan camera to derive information about the particles' movements in real-time. This approach allows improving the predictions in optical belt sorters without necessitating any hardware modifications. © 2017 Walter de Gruyter Berlin/Boston.
    view abstract10.1515/auto-2017-0009
  • Resolved simulations of single char particle combustion in a laminar flow field
    Farazi, S. and Sadr, M. and Kang, S. and Schiemann, M. and Vorobiev, N. and Scherer, V. and Pitsch, H.
    Fuel 201 (2017)
    The aim of this work is to study spatially and chemically resolved particle combustion cases to understand chemical and laminar transport processes and to support model development. In the present study, the combustion process of a single char particle located in air or oxy-fuel atmosphere composed of oxygen, carbon dioxide, and steam is investigated. Char burnout is represented in highly resolved numerical simulations including a detailed description of the surface and the gas phase chemistry. At the solid-gas interface, heat and mass fluxes due to the surface reactions involving carbon oxidation and gasification are considered. The model is validated based on experimental results for char burnout phase in a flat flame burner. We perform a comprehensive set of fully resolved reactive 2-D simulations by varying particle size, relative velocity, diluent, and oxygen composition in the surrounding gas. The simulation results are discussed regarding the CO2 and N2 content of the atmosphere highlighting the effects of oxy-fuel combustion. Furthermore, the impact of the particle flow motion on the flame that forms around the char particle is investigated by varying relative Reynolds number with particle size and relative slip velocity. © 2016 Elsevier Ltd
    view abstract10.1016/j.fuel.2016.11.011
  • Separation and characterization of carbonaceous particulate (soot and char) produced from fast pyrolysis of coal in inert and CO2 atmospheres
    Apicella, B. and Senneca, O. and Russo, C. and Heuer, S. and Cortese, L. and Cerciello, F. and Scherer, V. and Schiemann, M. and Ciajolo, A.
    Fuel 201 (2017)
    In a previous work [Heuer et al., 2016] a large production of a fluffy carbon-rich material was observed to accompany the char formed during the early stages of a medium rank (bituminous) coal pyrolysis in a drop tube furnace (1573 K, residence times < 130 ms). This peculiar material was found to be much more abundantly formed in CO2 than in N2 flow. SEM analysis showed that it contains a large portion of submicron soot-like particles mixed with particles of tenths of microns in size with the typical char morphology. The present work reports on the separation of the two differently sized fractions produced in CO2 and N2 flow and their subsequent analysis. The separation was performed dispersing the material in ethanol by ultrasonic mixing, followed by settling, and decanting to produce top and bottom products enriched in the fine and coarse particle fractions, respectively. The procedure was repeated several times and the size separation effectiveness was checked by SEM and laser granulometry sizing. Thermogravimetry, elemental and spectroscopic analysis were applied to the coarse and fine fractions to provide insights on their structural features. The fine soot particulate was almost ash-free, suggesting that its formation occurs in the gas phase, as typically soot does, while the coarse fraction presented significant residues of coal inorganic matter typical of char. Both fine and coarse particulate resulted less reactive, and somewhat smaller in size, when produced in CO2 in comparison to N2/Ar pyrolysis conditions. Their lower reactivity is associated with higher aromaticity and structural order as well as with a lower presence of hydrogen and aliphatic functionalities. © 2016 Elsevier Ltd
    view abstract10.1016/j.fuel.2016.11.049
  • Simulating municipal solid waste incineration with a DEM/CFD method – Influences of waste properties, grate and furnace design
    Wissing, F. and Wirtz, S. and Scherer, V.
    Fuel 206 (2017)
    The current work presents an initial approach of using a particle based method (Discrete Element Method, DEM) to simulate municipal solid waste (MSW) incineration on grates. Therefore, an in-house DEM code has been coupled with FLUENT. Models have been formulated for drying, volatile release and char conversion. The volatiles released are converted in the furnace above the waste bed which has been calculated with FLUENT. A comparison of simulations with measurements in an existing MSW incineration plant of CO2, O2, H2O and temperatures above the bed is presented. Agreement is fair considering the measurement uncertainties and the complexity of the process. In a sensitivity study concerning the influence of waste composition (heating value, number of waste fractions), waste particle size distribution and radiative flux onto the waste bed on conversion has been carried out. Finally, the most common grate systems, backward stoking, forward stoking and roller grates have been compared briefly, including different types of furnace geometries above the bed. The results demonstrate that the approach developed gives new insight into the complex interaction of waste movement, waste conversion and gas phase combustion above the bed which can't be obtained with other approaches, like continuum models for the waste bed. © 2017
    view abstract10.1016/j.fuel.2017.06.037
  • Simulation of Reacting Moving Granular Material in Furnaces and Boilers An Overview on the Capabilities of the Discrete Element Method
    Scherer, V. and Wirtz, S. and Krause, B. and Wissing, F.
    Energy Procedia 120 (2017)
    A brief overview is given on the capabilities and on the current limitations of the Discrete Element Method (DEM) coupled with Computational Fluid Mechanics (CFD) to simulate chemical reacting moving granular material. An approach to resolve the internal transport and reaction phenomena in particles of complex geometry is presented. Heat and mass transfer from and to the particles are accounted for as well as heat transfer between particles and between particles and a surrounding gas phase including radiation. Gas phase reactions outside the particle interact with inner particle processes. Examples will be shown to demonstrate the capabilities of DEM/CFD coupling. These examples are an industrial scale lime shaft kilns, the simulation of a domestic pellet stove and a grate firing system for the incineration of municipal waste. The advantages of a DEM/CFD approach will be highlighted but also the still existing drawbacks and limitations are discussed. The paper ends with an outlook on necessary developments to make DEM/CFD a standard engineering tool for chemically reacting granular material. © 2017 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.egypro.2017.07.154
  • Thermal bed mixing in rotary drums for different operational parameters
    Nafsun, A.I. and Herz, F. and Specht, E. and Komossa, H. and Wirtz, S. and Scherer, V. and Liu, X.
    Chemical Engineering Science 160 (2017)
    The transversal thermal bed mixing was experimentally investigated in a batch rotary drum with a diameter of 0.6 m and a length of 0.45 m. The drum was filled with two fractions of granular material with different thermal conditions and the mixing temperature in the solid bed was measured with thermocouples located at different bed height. Quartz sand with a mean particle diameter of dP=0.2 mm was used as test material. The operating parameters, rotational speed and filling degree of the drum were varied in the range of n=1–6 rpm and F=10–20% respectively, whereas the influence on thermal mixing time was evaluated. The thermal mixing behavior was shown in terms of time constant, number of bed rotation, peak time and mixing number. Thermal mixing time decreases with higher rotational speed and lower filling degree. Comparison between experimental data and penetration model shows good agreement for low rotational speeds. © 2016 Elsevier Ltd
    view abstract10.1016/j.ces.2016.11.005
  • A review on lithium combustion
    Schiemann, M. and Bergthorson, J. and Fischer, P. and Scherer, V. and Taroata, D. and Schmid, G.
    Applied Energy 162 (2016)
    Lithium combustion has been studied for several decades, with a primary focus on safety issues, such as lithium fires resulting from spills in nuclear reactors. Several studies have also considered the use of lithium as a fuel within propellants, or within propulsion systems that burn lithium in the atmospheric "air" of other planets. Lithium safety has typically been investigated through combustion of molten pieces of lithium or within pool fires. For propulsion applications, experiments were carried out using packed beds of lithium particles.A novel approach that has recently been proposed is the use of lithium as a recyclable fuel, or energy carrier that can compactly store renewable energy. In this scheme, lithium is burned with air, or power-plant exhaust, to generate heat for thermal power systems when power is needed. The solid-phase combustion products would be collected and recycled, via electrolysis, back into elemental lithium when excess renewable power is available.This paper summarizes the existing knowledge on lithium combustion. It presents the available findings on lithium combustion for large single pieces of lithium, on pool fires, reaction in packed beds, as well as the combustion of sub-mm sized particles and droplets which are needed for the use of lithium as an energy carrier. The combustion reactions of lithium with O2, H2O, CO2 and N2 are discussed. Modelling of lithium-particle combustion is at the early stages of development and available results are discussed. © 2015.
    view abstract10.1016/j.apenergy.2015.10.172
  • Coupled DEM–CFD simulation of drying wood chips in a rotary drum – Baffle design and model reduction
    Scherer, V. and Mönnigmann, M. and Berner, M.O. and Sudbrock, F.
    Fuel 184 (2016)
    In this contribution coupled DEM–CFD simulations of convective drying of wood chips in a baffled laboratory rotary dryer are presented. Due to the anisotropy of the biogenous (fibrous) material a three-dimensional spatial resolution of inner particle transport processes within the DEM code has been incorporated. The drying law is based on a diffusion approach. The simulations show that L-shaped baffles lead to higher drying rates than straight baffles. L-shaped baffles lead to a more even distribution of the particles across the cross-section of the drum where a larger amount of the wood chips are located in regions of hot, unsaturated air. However, the assumption of anisotropic transport properties within the wood chips and the subsequent solving of the associated differential equation of the three dimensionally resolved particle requires high computational effort. Therefore, the second purpose of the paper is to propose a model reduction method for single particle DEM-models based on proper orthogonal decomposition to reduce computation times. Specifically, we assess the impact of these methods on the computational complexity of the single particle models. The results show that even with a basic implementation a considerable reduction can be achieved on the single particle level. While our results only apply to the specific example treated here, it is evident that the effect of model reduction grows with grid size. © 2016 Elsevier Ltd
    view abstract10.1016/j.fuel.2016.05.054
  • DEM/CFD modeling of the fuel conversion in a pellet stove
    Wiese, J. and Wissing, F. and Höhner, D. and Wirtz, S. and Scherer, V. and Ley, U. and Behr, H.M.
    Fuel Processing Technology 152 (2016)
    This study presents an approach to simulate wood pellet combustion in domestic heating systems. Until now the challenge of such simulations lies in the description of the movement and conversion of the solid fuel while interacting with the surrounding gas phase. To tackle this problem the discrete element method (DEM), a method which allows simultaneous tracking of all individual reacting particles, was combined with computational fluid dynamics (CFD). The pellet shape is represented by polyhedrals to approximate their cylindrical geometry. Pellet length size distribution has been taken into account. Particles are allowed to shrink during the combustion process. Heating, drying, pyrolysis and char combustion are accounted for by a three-dimensional resolution of the particle. Radiative transfer among pellets is calculated based on a surface triangulation of each particle. The typical batchwise feeding of the fuel in a pellet stove has been taken into account in the simulations. Furthermore, experimental results from a 13 kW domestic wood pellet stove are compared with simulations. The simulations reveal the details of the unsteady release of water vapor, pyrolysis gases and combustion products. The non-uniform geometrical distribution of the pellets on the burner grate resulting from non-symmetric fuel feeding leads to a deflection of the gas flame above the pellet bed to one side of the combustion chamber. This is also reflected in the measurements. The trend of the gas phase temperature above the pellet bed as well as the mean CO emissions as a function of stoichiometry are well represented by the simulations, however, further room for improvement is left. © 2016 Elsevier B.V.
    view abstract10.1016/j.fuproc.2016.06.005
  • Devolatilization and volatiles reaction of individual coal particles in the context of FGM tabulated chemistry
    Knappstein, R. and Kuenne, G. and Ketelheun, A. and Köser, J. and Becker, L. and Heuer, S. and Schiemann, M. and Scherer, V. and Dreizler, A. and Sadiki, A. and Janicka, J.
    Combustion and Flame 169 (2016)
    The method of Flamelet Generated Manifolds (FGM) is coupled with a coal devolatilization model to perform transient simulations of a well-defined single coal particle combustion experiment for the first time. The gas phase chemistry is mapped onto a three-dimensional manifold controlled by the mixture fraction, a reaction progress variable and the enthalpy. A simulation of an electrically heated inert pyrolysis reactor is performed in order to evaluate transferability and applicability of experimentally obtained devolatilization kinetic parameters to Large Eddy Simulation (LES) codes for combustion configurations. Finally, the FGM modeling approach is applied to a premixed flat flame configuration, in which the coal particles cross a laminar flame front and are exposed to the hot gases. Numerical results regarding the volatiles reaction are compared to experimental findings. Particle and gas phase states are studied. Overall, good agreement between numerical results and experimental findings regarding the volatiles ignition range could be observed. © 2016 The Combustion Institute.
    view abstract10.1016/j.combustflame.2016.04.014
  • Effects of CO2 on submicronic carbon particulate (soot) formed during coal pyrolysis in a drop tube reactor
    Senneca, O. and Apicella, B. and Heuer, S. and Schiemann, M. and Scherer, V. and Stanzione, F. and Ciajolo, A. and Russo, C.
    Combustion and Flame 172 (2016)
    In oxycombustion and gasification processes coal pyrolysis occurs in CO2-rich atmospheres. The present work investigates the effect of such conditions on the quantity and quality of the submicronic carbon particulate produced. Pyrolysis experiments were carried out in either N2 or CO2 atmospheres in a laminar drop tube reactor, with wall temperatures of 1573 K, heating rates of 104–105 K/s and residence times below 130 ms, so as to reproduce pyrolysis conditions comparable to those of pulverized coal-fired boilers. The carbon particulate sampled in the reactor was found to have bimodal distribution in the micronic and submicronic ranges. A method based on solvent extraction was applied to carbon particulate for separating the two modes and determining the relative mass contribution of micronic and submicronic fractions. In CO2 atmosphere the amount of submicronic fraction of carbon particulate, referred to as soot, was found to be up to four times as much as upon N2 experiments. Beside the larger formation of soot, relevant differences in terms of combustion reactivity, size distribution and chemical structure of the residual carbon particulate produced in CO2 environment in respect to N2 environment were observed by means of a large array of techniques including thermogravimetry, microscopy (SEM+EDX), FT-IR, UV–visible and Raman spectroscopy along with XRD and XPS techniques. © 2016 The Combustion Institute
    view abstract10.1016/j.combustflame.2016.07.023
  • Effects of oxy-fuel conditions on the products of pyrolysis in a drop tube reactor
    Heuer, S. and Senneca, O. and Wütscher, A. and Düdder, H. and Schiemann, M. and Muhler, M. and Scherer, V.
    Fuel Processing Technology 150 (2016)
    The goal of the current study was to investigate the influence of increased CO2 concentrations in oxy-fuel combustion on the products of coal devolatilization (gas, tar, soot and char). Experiments have been carried out in a laminar drop tube reactor (DTR) at conditions comparable to pulverized coal-fired boilers, in particular at a temperature of 1573 K and heating rate of 104–105 K/s. Atmospheres of N2, Ar, and CO2 as well as with O2/N2 and O2/CO2 mixtures (oxidizing oxy-fuel conditions) were applied. The work focuses on the early stages of reaction of coal particles in a pulverized combustor, therefore, a residence time of 120 ms was chosen, which assured the completion of pyrolysis while limiting the progress of char combustion and gasification. Gaseous, liquid and solid pyrolysis residues were extracted and analyzed by a multitude of techniques. A remarkable result is the effect that CO2 has on the solid products of pyrolysis. A much larger production of soot is observed in CO2 conditions over Ar or N2 conditions (3:1). The combustion reactivity of both soot and char produced in CO2 is lower than that of the corresponding samples produced in Ar or N2 atmosphere. Differences in reactivity couple with differences in the C[sbnd]O complexes residing on the surface and measured by XPS. The effect of CO2 on gaseous products is to increase the concentration of acetylene, while abating most other hydrocarbon species. When experiments are carried out in air and oxy-fuel atmospheres, soot and tar are consumed by combustion. Differences among chars are observed which can be mostly related to the attainment of different extents of burn out. In the oxy-fuel experiments, lower NO and NO2 and higher N2O concentrations are found in the gas compared to air experiments. © 2016 Elsevier B.V.
    view abstract10.1016/j.fuproc.2016.04.034
  • Energetic and economic evaluation of membrane-based carbon capture routes for power plant processes
    Maas, P. and Nauels, N. and Zhao, L. and Markewitz, P. and Scherer, V. and Modigell, M. and Stolten, D. and Hake, J.-F.
    International Journal of Greenhouse Gas Control 44 (2016)
    The application of CCS technology involves considerable efficiency losses and significant additional investments. The aim is therefore to reduce these efficiency losses and to cut costs. Against this background, membrane-based carbon capture routes for the post-combustion, oxyfuel and pre-combustion technology lines will be analyzed in the following for hard-coal-fired power plants. To the best knowledge of the authors, this paper is the first one comparing membrane based capture routes on common technical and economic boundary conditions. The post-combustion process involves a cascade arrangement of polymer membranes. In the optimum case, the efficiency losses for this concept amount to 9.6 percentage points. In comparison, efficiency losses for the other two membrane-based concepts, i.e. oxyfuel (oxygen transport membrane (OTM) with vacuum pump) and pre-combustion (water-gas shift reactor-WGSMR), are considerably lower (5.3/5.5 percentage points). The main goal of this paper is to assess levelized cost of electricity (LCOE) for the process routes under consideration and their sensitivity on CO2 allowance costs, yearly operating hours, membrane costs and membrane lifetime. The specific investment costs for the capture plants are 2410€/kWh (oxyfuel), 2572€/kWh (post-combustion) and 2660€/kWh (pre-combustion). This is 66% (post-combustion), 55% (oxyfuel) and 33% (pre-combustion) above the specific investment costs for the corresponding reference case without carbon capture. Allowance prices in a range from €20 (pre-combustion) to €39 (post-combustion) per tonne of CO2 would be necessary to compensate for the additional investments. Since it can be assumed that the membranes have a limited lifetime, the influence on electricity generation costs was calculated for different lifetimes. The results show that a technical service life of more than 3 years does not have a significant impact on generation costs. This applies to all the technological concepts investigated. In terms of LCOE and CO2 avoidance costs (€/tCO2) it turns out that oxyfuel and pre-combustion based membrane power plants are favorable compared to the post-combustion route. However, it has to be kept in mind that the uncertainty in membrane costs are higher for the oxyfuel membranes (ceramic oxygen transport membranes) and the pre-combustion membranes (microporous ceramic membranes) compared to the polymeric post-combustion membranes which already have achieved a commercial level. © 2015.
    view abstract10.1016/j.ijggc.2015.11.018
  • Experimentation for char combustion kinetics measurements: Bias from char preparation
    Vorobiev, N. and Geier, M. and Schiemann, M. and Scherer, V.
    Fuel Processing Technology 151 (2016)
    As an alternative to direct use in char burnout kinetics experiments, coal and other solid fuels may be devolatilized and converted to char in a separate step in order to eliminate the influence of volatile release and combustion on measured char conversion properties. In this study, the effects of char preparation conditions on char burnout characteristics during pulverized coal combustion are investigated. Untreated particles of a Colombian high-volatile bituminous coal as well as three chars from that coal, which were externally produced in three different reactors, were tested in a combustion-driven entrained flow reactor. The reactivities of the chars were quantified as pre-exponential factors for commonly employed single-film burnout models, which were estimated from optically measured temperatures and sizes of individual burning particles. The results indicate that char production with devolatilization under heating rates greater than 2 × 104 K/s yields suitable materials for experimental research on char consumption kinetics. The characteristics of high-heating-rate production methods appear to affect char reactivities only in the early stages of the consumption process (less than 20-25% conversion of the initial char mass). © 2016 Elsevier B.V.
    view abstract10.1016/j.fuproc.2016.05.005
  • Fast multitarget tracking via strategy switching for sensor-based sorting
    Maier, G. and Pfaff, F. and Pieper, C. and Gruna, R. and Noack, B. and Kruggel-Emden, H. and Langle, T. and Hanebeck, U.D. and Wirtz, S. and Scherer, V. and Beyerer, J.
    IEEE International Conference on Multisensor Fusion and Integration for Intelligent Systems (2016)
    State-of-the-art sensor-based sorting systems provide solutions to sort various products according to quality aspects. Such systems face the challenge of an existing delay between perception and separation of the material. To reliably predict an object's position when reaching the separation stage, information regarding its movement needs to be derived. Multitarget tracking offers approaches through which this can be achieved. However, processing time is typically limited since the sorting decision for each object needs to be derived sufficiently early before it reaches the separation stage. In this paper, an approach for multitarget tracking in sensor-based sorting is proposed which supports establishing an upper bound regarding processing time required for solving the measurement to track association problem. To demonstrate the success of the proposed method, experiments are conducted for data-sets obtained via simulation of a sorting system. This way, it is possible to not only demonstrate the impact on required runtime but also on the quality of the association. © 2016 IEEE.
    view abstract10.1109/MFI.2016.7849538
  • Heat Transfer Experiments in a Rotary Drum for a Variety of Granular Materials
    Nafsun, A.I. and Herz, F. and Specht, E. and Scherer, V. and Wirtz, S.
    Experimental Heat Transfer 29 (2016)
    The influence of material properties on the contact heat transfer coefficient between the covered wall surface and the solid bed was investigated. The contact heat transfer coefficients were calculated from the measured radial and circumferential temperature profiles. Experiments were carried out with six different materials, including steel spheres, animal powder, cement clinker, quartz sand, glass beads, and expanded clay. The rotational speeds were varied from 1 to 6 rpm to evaluate the influence of rotational speed on the contact heat transfer coefficient. The measured contact heat transfer coefficients were compared with four models from the literature. © 2016 Taylor & Francis Group, LLC.
    view abstract10.1080/08916152.2015.1036180
  • Improving optical sorting of bulk materials using sophisticated motion models
    Pfaff, F. and Pieper, C. and Maier, G. and Noack, B. and Kruggel-Emden, H. and Gruna, R. and Hanebeck, U.D. and Wirtz, S. and Scherer, V. and Längle, T. and Beyerer, J.
    Technisches Messen 83 (2016)
    Visual properties are powerful features to reliably classify bulk materials, thereby allowing to detect defect or low quality particles. Optical belt sorters are an established technology to sort based on these properties, but they suffer from delays between the simultaneous classification and localization step and the subsequent separation step. Therefore, accurate models to predict the particles' motions are a necessity to bridge this gap. In this paper, we explicate our concept to use sophisticated simulations to derive accurate models and optimize the flow of bulk solids via adjustments of the sorter design. This allows us to improve overall sorting accuracy and cost efficiency. Lastly, initial results are presented. © 2016 Walter de Gruyter Berlin/Boston.
    view abstract10.1515/teme-2015-0108
  • Influence of carbonate decomposition on normal spectral radiative emittance in the context of oxyfuel combustion
    Gorewoda, J. and Scherer, V.
    Energy and Fuels 30 (2016)
    To investigate whether the radiative properties of carbonate rich ash layers in oxyfuel combustion systems might be influenced by the carbonate decomposition to the corresponding oxide, the emittance of Sr, Mg, and Ca carbonates is examined. In addition “synthetic coal ashes” were produced from mixtures of CaCO3 and SiO2 as well as Fe2O3 and SiO2. The mixture ratios of the minerals were varied. All samples were prepared from powders with known particle size fractions of x < 32 μm and 125 < 160 μm. The powders were investigated for their temperature-dependent normal emittance in a radiation test rig by FT-IR spectroscopy in the temperature range from 500 to 1000 °C. The results reveal that the phase transformation from the carbonate to the corresponding oxide has a significant influence on spectral emittance. Whereas the carbonates show characteristic peaks in spectral emittance around 4 μm which stem from the infrared active CO3 group, these peaks vanish after transformation to the oxide. For CaCO3, the most prominent carbonate in typical coal ashes, the emittance of the oxide is significantly lower than for the carbonate. Such a behavior in terms of total and spectral emittance has also been detected, for example, examining a Ca rich Rhenish lignite. Emittance increases with particle size for all samples. An enrichment of SiO2 with Fe2O3 leads to an increase in emittance. © 2016 American Chemical Society
    view abstract10.1021/acs.energyfuels.6b01398
  • Numerical modeling of an automated optical belt sorter using the Discrete Element Method
    Pieper, C. and Maier, G. and Pfaff, F. and Kruggel-Emden, H. and Wirtz, S. and Gruna, R. and Noack, B. and Scherer, V. and Längle, T. and Beyerer, J. and Hanebeck, U.D.
    Powder Technology 301 (2016)
    Optical sorters are important devices in the processing and handling of the globally growing material streams. The precise optical sorting of many bulk solids is still difficult due to the great technical effort necessary for transport and flow control. In this study, particle separation with an automated optical belt sorter is modeled numerically. The Discrete Element Method (DEM) is used to model the sorter and calculate the particle movement as well as particle – particle and particle – wall interactions. The particle ejection stage with air valves is described with the help of a MATLAB script utilizing particle movement information obtained with the DEM. Two models for predicting the particle movement between the detection and separation phase are implemented and compared. In the first model, it is assumed that the particles are moving with belt velocity and without any cross movements and a conventional line scan camera is used for particle detection. In the second model, a more sophisticated approach is employed where the particle motion is predicted with an area scan camera combined with a tracking algorithm. In addition, the influence of different operating parameters like particle shape or conveyor belt length on the separation quality of the system is investigated. Results show that numerical simulations can offer detailed insight into the operation performance of optical sorters and help to optimize operating parameters. The area scan camera approach was found to be superior to the standard line scan camera model in almost all investigated categories. © 2016 Elsevier B.V.
    view abstract10.1016/j.powtec.2016.07.018
  • Oxidation characteristics of a cellulose-derived hydrochar in thermogravimetric and laminar flow burner experiments
    Düdder, H. and Wütscher, A. and Vorobiev, N. and Schiemann, M. and Scherer, V. and Muhler, M.
    Fuel Processing Technology 148 (2016)
    Synthetic lignite was prepared by hydrothermal carbonization, in which minerals typical for coal ashes were incorporated during hydrothermal carbonization allowing us to study the catalytic effect of ash components on the oxidation rate of the fuel. Chemically leached lignite using hydrochloric acid was applied as reference material. Combustion experiments were performed by thermogravimetric analysis under chemically controlled conditions and in a laminar flow reactor under pore-diffusion limitation. The investigation of the hydrochar in the laminar flow reactor provided gas temperatures and heating rates typical for pulverized coal combustion. The combustion rate of the chemically leached lignite was found to be comparable to the hydrochar. Furthermore, the catalytic effect of incorporated iron oxide was detected in both combustion experiments. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.fuproc.2016.02.027
  • Performance improvements of polydisperse DEM simulations using a Loose Octree approach
    Stein, G. and Wirtz, S. and Scherer, V.
    Advances in Parallel Computing 27 (2016)
    Numerical simulations using the Discrete Element Method (DEM) are used at LEAT in the context of several important, energy related particulate flow systems. The focus of these investigations is the understanding of the heat and mass transfer processes on the micro-scale and the prediction of the related macroscopic behaviour. Most of the currently available DEM implementations, especially if the required number of particles is large, only allow for small variations in particle size if the computational effort must be kept within reasonable bounds. This is contrary to the actual requirements of many technically relevant processes where a broad size spectrum must be considered. Parallel processing helps to ease this situation to a certain degree, but the ongoing search for algorithmic improvements has not yet accomplished a definitive solution. The process of neighbourhood detection, which is required to identify the partners of the pairwise interactions determining momentum fluxes among the particles and between particles and surrounding walls is one common bottleneck. Besides the commonly used Linked-Cell method, hierarchically structured "background" meshes or octrees were proposed in the past and applied in various implementations. A new variant of the octree approach is presented and its performance with respect to particle number, particle size distribution and parallelisation is analysed and compared to conventional approaches. In order to obtain a realistic analysis, for a given code in a typical hardware environment (small engineering companies or university institutes), the benchmark addresses the technical application of particle movement in a section of a rotary drum. © 2016 The authors and IOS Press.
    view abstract10.3233/978-1-61499-621-7-53
  • Simulation-based evaluation of predictive tracking for sorting bulk materials
    Pfaff, F. and Pieper, C. and Maier, G. and Noack, B. and Kruggel-Emden, H. and Gruna, R. and Hanebeck, U.D. and Wirtz, S. and Scherer, V. and Langle, T. and Beyerer, J.
    IEEE International Conference on Multisensor Fusion and Integration for Intelligent Systems (2016)
    Multitarget tracking problems arise in many real-world applications. The performance of the utilized algorithm strongly depends both on how the data association problem is handled and on the suitability of the motion models employed. Especially the motion models can be hard to validate. Previously, we have proposed to use multitarget tracking to improve optical belt sorters. In this paper, we evaluate both the suitability of our model and the tracking and then of our entire system incorporating the image processing component via the use of highly realistic numerical simulations. We first assess the model using noise-free measurements generated by the simulation and then evaluate the entire system by using synthetically generated image data. © 2016 IEEE.
    view abstract10.1109/MFI.2016.7849539
  • Sorption measurements for determining surface effects and structure of solid fuels
    Seibel, C. and Wedler, C. and Vorobiev, N. and Schiemann, M. and Scherer, V. and Span, R. and Fieback, T.M.
    Fuel Processing Technology 153 (2016)
    Novel experimental results on temperature dependent diffusion of CO2 inside porous char particles are provided as well as corresponding data on adsorption of oxygen and carbon dioxide. For this purpose, different chars from a Colombian coal were generated either in a flat flame burner (FFB) under realistic conditions for pulverized coal combustion with heating rates in the order 104–105 K/s or in a thermogravimetric analyser (TGA) at low heating rates and inert conditions (Ar). The chars produced are used for kinetic adsorption measurements with a suspension balance to determine temperature dependent diffusion coefficients for CO2 up to 160 °C. Based on these data the resistance factor for Knudsen diffusion, which describes the influence of the inner particle morphology on gas diffusion, was determined. The results indicate that the diffusion coefficients of the chars converge to the same value with rising temperature, ending in a Knudsen diffusion dominated regime. Furthermore, adsorption measurements for O2 and CO2 were conducted up to temperatures of 150 °C and 450 °C, respectively, on coal chars for the first time. Based on the pure component results, multicomponent adsorption has been predicted based on the well-known multi component IAST model. The results indicate that individual species selectivity changes with temperature. © 2016 Elsevier B.V.
    view abstract10.1016/j.fuproc.2016.08.004
  • A numerical model of the combustion of single lithium particles with CO2
    Fischer, P. and Schiemann, M. and Scherer, V. and Maas, P. and Schmid, G. and Taroata, D.
    Fuel 160 (2015)
    Abstract A numerical model for the ignition and combustion of lithium particles (dp = 20-250 μm) in pure CO2 atmosphere was developed and implemented in ANSYS Fluent's "discrete phase model". The combustion model is based on experimental findings gained in a laminar flow reactor: the experiments indicate two reaction mechanisms: An initial high temperature above gas-phase combustion (>2500 K) with a reaction zone apart from the particle surface ("stand-of flame") followed by a surface reaction at lower temperature (1500-1800 K). As reaction kinetics is only available for the surface reaction, a theoretical approach was established to calculate duration and mass conversion of the gas-phase reaction. The complete model includes inert heating, lithium melting and the reaction steps described above and enables the complete calculation of single particle or droplet combustion of lithium. The results of the numerical simulation were compared to experiments conducted in a laminar flow reactor. As the numerical results show, the predicted combustion behavior is in good agreement with the experimental results. © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.fuel.2015.07.033
  • A study on the influence of particle shape on the mechanical interactions of granular media in a hopper using the Discrete Element Method
    Höhner, D. and Wirtz, S. and Scherer, V.
    Powder Technology 278 (2015)
    In this study experimental and numerical investigations with the Discrete Element Method (DEM) on the mechanical interactions of particles with varying sphericity and aspect ratio in a rectangular hopper are conducted. In the DEM the test particles are approximated by four commonly used approximation schemes. A decrease of particle sphericity or an increase of the aspect ratio results in an more uneven, intermittend particle flow and overall lower discharge rate. It was deducted from the measurement results that changing these geometric particle properties elevates the shear strength of the particle bed and, hence, has a significant influence on the discharge properties of a hopper. Simulation results are in good general agreement with the experiments and thus demonstrate the adequacy of the DEM to predict the mechanical interactions in granular media consisting of non-spherical particles. The results presented in this study show only a minor influence of the method used to approximate particle shape within the DEM. Obviously the discharge characteristics are much stronger related to macroscopic geometric parameters than the fine scale resolution of particle geometry. © 2015 Elsevier B.V.
    view abstract10.1016/j.powtec.2015.02.046
  • Characterization of single coal particle combustion within oxygen-enriched environments using high-speed OH-PLIF
    Köser, J. and Becker, L.G. and Vorobiev, N. and Schiemann, M. and Scherer, V. and Böhm, B. and Dreizler, A.
    Applied Physics B: Lasers and Optics 121 (2015)
    This work presents first-of-its-kind high-speed planar laser-induced fluorescence measurements of the hydroxyl radical in the boundary layer of single coal particles. Experiments were performed in a laminar flow reactor providing an oxygen-enriched exhaust gas environment at elevated temperatures. Single coal particles in a sieve fraction of 90–125 µm and a significant amount of volatiles (36 wt%) were injected along the burner’s centerline. Coherent anti-Stokes Raman spectroscopy measurements were taken to characterize the gas-phase temperature. Time-resolved imaging of the OH distribution at 10 kHz allowed identifying reaction and post-flame zones and gave access to the temporal evolution of burning coal particles. During volatile combustion, a symmetric diffusion flame was observed around the particle starting from a distance of ~150 µm from the particle surface. For subsequent char combustion, this distance decreased and the highest OH signals appeared close to the particle surface. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-015-6253-3
  • Convective drying of agitated silica gel and beech wood particle beds—experiments and transient DEM-CFD simulations
    Sudbrock, F. and Kruggel-Emden, H. and Wirtz, S. and Scherer, V.
    Drying Technology 33 (2015)
    With coupled discrete element (DEM)-computational fluid dynamics (CFD) simulations, drying processes can be simultaneously described on the system scale while resolving detailed sub processes on the particle scale. In this contribution, DEM-CFD simulations are used to analyze the transient heat and mass transfer in mechanically agitated particle beds during drying. Results are compared to convective batch-drying experiments with silica gel and beech wood spheres and mixing effects are studied in detail. A good agreement with the measurements of both single-particle and particle bed drying is achieved by resolving heat and moisture transport three-dimensionally inside each particle. © 2015 Taylor & Francis Group, LLC.
    view abstract10.1080/07373937.2015.1026982
  • Coupled three dimensional DEM-CFD simulation of a lime shaft kiln-Calcination, particle movement and gas phase flow field
    Krause, B. and Liedmann, B. and Wiese, J. and Wirtz, S. and Scherer, V.
    Chemical Engineering Science 134 (2015)
    In lime shaft kilns the limestone is heated in counterflow with the flue gas from multiple burners, commonly fired by fossil or alternative fuels. After preheating, the limestone enters the calcination zone with gas temperatures above approximately 900. °C. To achieve high thermal efficiencies and calcination degrees, a homogeneous gas flow through the packed bed is essential. Spatial pressure differences and energy sinks and sources resulting from the calcination process and the fuel combustion make it even more difficult to predict the actual three-dimensional temperature and flow distribution in the kiln. Furthermore the particle size distribution is important for the system design, because limestone with larger size needs significantly higher residence time in the calcination zone for full conversion. This work presents the application of a novel 3-dimensional particle-mechanics based, numerical tool on an industrial scale. The tool allows the simulation of a moving and reacting limestone bed in a shaft kiln coupled with the 3-dimensional CFD-simulation describing the interstitial gas phase. Convective heat transfer between gas phase and particles, radiative and contact heat transfer between particles as well as calcination reaction are accounted for. With this combined approach the calcination of the limestone bed can be simultaneously described on the system scale while resolving detailed thermo-chemical processes on the particle scale. © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.ces.2015.06.002
  • Experimental characterization of the combustion of single lithium particles with CO2
    Fischer, P. and Schiemann, M. and Scherer, V. and Maas, P. and Schmid, G. and Taroata, D.
    Fuel 153 (2015)
    Combustion and temperature measurement of single lithium particles (dp < 250 μm) with CO2 was carried out in a laminar flow reactor. An imaging two-color pyrometer system was used to measure particle and flame size as well as combustion temperatures. The results indicate two different combustion phenomena, which have been identified in literature before: Gas-phase reaction at temperatures above 2500 K and surface reaction of lithium with CO2 at temperatures between 1500 and 1800 K. In addition, a sampling probe was utilized to extract burning particles from the reactor. The extracted probes were analyzed concerning their constituents using X-ray diffraction analysis and their shape and surface with scanning electron microscopy. The results showed lithium carbonate as main reaction product and a relatively smooth surface of the particles after burn-out. Combining the experimental findings, a single particle combustion model was suggested and apparent reaction kinetics was determined. © 2015 Elsevier Ltd.All rights reserved.
    view abstract10.1016/j.fuel.2015.02.098
  • Heat transfer in indirect heated rotary drums filled with monodisperse spheres: Comparison of experiments with DEM simulations
    Komossa, H. and Wirtz, S. and Scherer, V. and Herz, F. and Specht, E.
    Powder Technology 286 (2015)
    Numerical simulations with the discrete element method (DEM) and corresponding experimental investigations were carried out to understand and to quantify the heat transfer in indirect heated rotating drums. Monodisperse glass spheres (diameter 2. mm) were used and the bulk movement was kept within the rolling motion mode (rotational speed between 1 and 9. rpm). The focus is on the heat transfer between the covered wall and the particles in contact with this wall, as well as between the particles on the free bed surface and the adjacent fluid. Radiative heat transfer has been neglected due to the low maximum temperature within the system (474. K). Effective heat transfer coefficients for the heat fluxes mentioned were derived from the DEM simulations, considering the actual particle velocities on the free bed surface and on the wall.The particle movement and the heat transfer resulting from the simulations show good agreement with the experiments in general and thus allow the calculation of the effective heat transfer coefficients for the range of parameters considered in the current study. © 2015 Published by Elsevier B.V.
    view abstract10.1016/j.powtec.2015.07.022
  • Measurement of drag coefficients of non-spherical particles with a camera-based method
    Krueger, B. and Wirtz, S. and Scherer, V.
    Powder Technology 278 (2015)
    The current paper presents a novel experimental set-up which allows the automated determination of the drag coefficients of relatively large particles with complex shape. Typical examples of such types of particles are waste derived fuel (RDF) particles which are non-spherical and have a size up to a few centimeters. In contrast to conventional fossil fuel particles, where the particles may be considered as material points during the calculation of particle tracks in a reacting flow field, the spatial extent of RDF-particles and their lack of sphericity lead to pronounced self-induced movement and associated variations in the drag-coefficients.The experiments are based on a drop shaft equipped with two digital cameras. This allows to obtain time resolved stereo image sequences from which the settling velocity of particles, the self-induced velocity fluctuations and the corresponding drag and lift coefficients can be derived. As the system is automated, a large number of particles can be examined and statistical information on the distribution of drag coefficients can be obtained.In this publication, the methodology of these drop shaft measurements and their evaluation will be presented. Additionally drag coefficients of isometric spherical and non-spherical particle geometries (spheres, cubes, square plates and circular disks) were measured and compared with known correlations for drag coefficients. Probability density functions for the properties of typical RDF particles will be presented to highlight the potential of the new set-up. © 2015 Elsevier B.V.
    view abstract10.1016/j.powtec.2015.03.024
  • Stereoscopic pyrometer for char combustion characterization
    Schiemann, M. and Vorobiev, N. and Scherer, V.
    Applied Optics 54 (2015)
    For many pulverized fuels, especially coal and biomass, char combustion is the time determining step. Based on intensified ICCD cameras, a novel setup has been developed to study pulverized fuel combustion, mainly in a laminar flow reactor. For char burning characterization, the typical measurement parameters are particle temperature, size, and velocity. The working principle of the camera setup is introduced and its capabilities are discussed by examination of coal particle combustion under CO2-enriched, so-called oxy-fuel atmospheres with varying O2 content. © 2015 Optical Society of America.
    view abstract10.1364/AO.54.001097
  • A study on the influence of particle shape and shape approximation on particle mechanics in a rotating drum using the discrete element method
    Höhner, D. and Wirtz, S. and Scherer, V.
    Powder Technology 253 (2014)
    In this study experimental and numerical investigations with the discrete element method (DEM) on the mechanical interactions of spheres and polyhedral dices in a rotating drum are conducted. In DEM the dices are approximated by polyhedra and smoothed polyhedra respectively and hence allow examining the influence of sharply-edged and smooth particle geometries on the mechanical behavior. Simulation results are in good general agreement with the experiments and hence demonstrate the adequacy of DEM as well as polyhedral and smoothed polyhedral approximation schemes to simulate non-spherical particle geometries. It was observed that an increase of particle angularity leads to an increase of the dynamic angle of repose. On the other hand, while spheres mix faster than the polyhedral dices, no significant difference in the mixing behaviors of the dices can be observed. © 2013 Elsevier B.V.
    view abstract10.1016/j.powtec.2013.11.023
  • Combustion of lithium particles: Optical measurement methodology and initial results
    Schiemann, M. and Fischer, P. and Scherer, V. and Schmid, G. and Taroata, D.
    Chemical Engineering and Technology 37 (2014)
    Combustion examinations on the single-grain level were carried out in order to get further fundamental insight into the ignition and combustion of lithium particles. Combustion of solid lithium particles in a defined size fraction was analyzed in a laminar-flow reactor. The exhaust gases of a methane-air flame provided the reactants O2, CO2, N2, and H2O for the lithium conversion. Two different atmospheres at various temperatures were investigated. A high-speed camera system measured size and radiation intensity of burning particles. The results indicate that two different combustion phenomena occurred in lithium combustion. The first was identified as a homogeneously enveloping flame around the lithium particle and the second as a reaction zone next to the particle surface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ceat.201400011
  • Determination of char combustion kinetics parameters: Comparison of point detector and imaging-based particle-sizing pyrometry
    Schiemann, M. and Geier, M. and Shaddix, C.R. and Vorobiev, N. and Scherer, V.
    Review of Scientific Instruments 85 (2014)
    In this study, the char burnout characteristics of two German coals (a lignite and a high-volatile bituminous coal) were investigated using two different experimental configurations and optical techniques in two distinct laboratories for measurement of temperature and size of burning particles. The optical diagnostic hardware is quite different in the two systems, but both perform two-color pyrometry and optical sizing measurements on individual particles burning in isolation from each other in high-temperature laminar flows to characterize the char consumption kinetics. The performance of the specialized systems is compared for two different combustion atmospheres (with 6.6 and 12 vol.% O2) and gas temperatures between 1700 and 1800 K. The measured particle temperatures and diameters are converted to char burning rate parameters for several residence times during the course of the particles' burnout. The results confirm that comparable results are obtained with the two configurations, although higher levels of variability in the measured data were observed in the imaging-based pyrometer setup. Corresponding uncertainties in kinetics parameters were larger, and appear to be more sensitive to systematic measurement errors when lower oxygen contents are used in the experiments. Consequently, burnout experiments in environments with sufficiently high O 2 contents may be used to measure reliable char burning kinetics rates. Based on simulation results for the two coals, O2 concentrations in the range 10%-30% are recommended for kinetic rate measurements on 100 μm particles. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4890438
  • Economic evaluation of pre-combustion CO2-capture in IGCC power plants by porous ceramic membranes
    Franz, J. and Maas, P. and Scherer, V.
    Applied Energy 130 (2014)
    Pre-combustion-carbon-capture is one of the three main routes for the mitigation of CO2-emissions by fossil fueled power plants. Based on the data of a detailed technical evaluation of CO2-capture by porous ceramic membranes (CM) and ceramic membrane reactors (WGSMR) in an Integrated-Gasification-Combined-Cycle (IGCC) power plant this paper focuses on the economic effects of CO2-abatement. First the results of the process simulations are presented briefly. The analysis is based on a comparison with a reference IGCC without CO2-capture (dry syngas cooling, bituminous coal, efficiency of 47.4%). In addition, as a second reference, an IGCC process with CO2 removal based on standard Selexol-scrubbing is taken into account. The most promising technology for CO2-capture by membranes in IGCC applications is the combination of a water gas shift reactor and a H2-selective membrane into one water gas shift membrane reactor. For the WGSRM-case efficiency losses can be limited to about 6%-points (including losses for CO2 compression) for a CO2 separation degree of 90%. This is a severe reduction of the efficiency loss compared to Selexol (10.3% points) or IGCC-CM (8.6% points). The economic evaluation is based on a detailed analysis of investment and operational costs. Parameters like membrane costs and lifetime, costs of CO2-certificates and annual operating hours are taken into account. The purpose of these evaluations is to identify the minimum cost of electricity for the different capture cases for the variation of the boundary conditions. Fixing 90% CO2 separation the analysis identifies clearly that the economic minimum for cost of electricity and maximum thermodynamic efficiencies do not coincide. The cost of electricity for the reference case was 67€/MWh and for the WGSMR integration with 90% CO2 separation 57€/MWh, if certificate costs of 30€/tCO2, membrane costs of 300€/m2 and 8000 operating hours/year are considered. Further studies on the sensitivity of cost of electricity on the technical and commercial boundary conditions will be presented. © 2014 Elsevier Ltd.
    view abstract10.1016/j.apenergy.2014.02.021
  • Influence of chemical composition and physical structure on normal radiant emittance characteristics of ash deposits
    Greffrath, F. and Gorewoda, J. and Schiemann, M. and Scherer, V.
    Fuel 134 (2014)
    Ash residues that arise during combustion of solid fuels form deposits on heat exchanger surfaces which hinder the heat transfer to the working fluid steam. The thermal conductivity and - especially within the combustion chamber - the optical properties of the deposits determine the transferred amount of heat. Thus, knowledge of the optical properties, here represented by the normal emittance, are crucial for the design and operation of a steam generator. Since the emittance is dependent on both the physical structure of the surface as well as the mineralogical composition, both parameters are subject to the current investigation. In the work presented the spectral normal emittances of mineral samples as well as their dependence on both the chemical composition and physical structure were investigated experimentally. In the course of the research the degree of complexity was increased gradually. Starting with pure quartz sand the influence of the surface structure on emittance has been investigated. By mixing quartz and hematite powder the influence of chemical enrichment with this typical ash component has been investigated. Measurements with real coal ashes complete the experimental program. Additionally, the latter samples were undergone thermal treatment to investigate the effect of particle agglomeration and melting. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.fuel.2014.05.047
  • Lignite fired igcc with ceramic membranes for CO2 separation
    Maas, P. and Scherer, V.
    Energy Procedia 63 (2014)
    In this paper pre-combustion CO2-capture via porous ceramic membranes in lignite fired IGCC power plants is investigated. Four different cases were configured with Aspen Plus and Epsilon, including a reference case without carbon capture and three cases with carbon capture. The capture technologies were a porous ceramic membrane and a ceramic membrane reactor with simultaneous CO2 separation and CO-shift. Two different water gas-shift configurations were combined with the membrane reactor. Sensitivity analyses of membrane area and permeation pressure were done for the capture cases to investigate the influence on membrane and power plant performance and identify the optimum conditions from an energetic viewpoint. All capture concepts showed capture rates over 97.5 % and the achievable efficiency losses lay between 6.8 and 9.4 %-points. © 2014 The Authors. Published by Elsevier Ltd.
    view abstract10.1016/j.egypro.2014.11.212
  • Reaction products in the combustion of the high energy density storage material lithium with carbon dioxide and nitrogen
    Kellermann, R. and Taroata, D. and Schiemann, M. and Eckert, H. and Fischer, P. and Scherer, V. and Hock, R. and Schmid, G.
    Prehospital and Disaster Medicine 1644 (2014)
    In this work, electrochemically recyclable lithium is analyzed as high energy density, large scale storage material for stranded renewable energy in a closed loop. The strongly exothermic reaction of lithium with carbon dioxide (CO2) yields thermal energy directly comparable to the combustion of coal or methane in an oxygen containing atmosphere. The thermal level of the reaction is sufficient for re-electrification in a thermal power plant compatible process. The reaction of single lithium particles, avoiding particle-particle interactions, is compared to the combustion of atomized lithium spray in a CO2 containing atmosphere. Particle temperatures of up to 4000K were found for the reaction of single lithium particles in a CO2, nitrogen (N2), oxygen (O2) and steam gas mixture. Furthermore the combustion of atomized lithium spray in both dry CO2 atmosphere and CO2/steam gas mixture was analyzed. The identified solid reaction products are lithium carbonate, lithium oxide and lithium hydroxide. The formation of carbon monoxide (CO) as gaseous reaction product is demonstrated. Carbon monoxide is a valuable by-product, which could be converted to methanol or gasoline using hydrogen. Copyright © Materials Research Society 2014.
    view abstract10.1557/opl.2014.314
  • Reaction products in the combustion of the high energy density storage material lithium with carbon dioxide and nitrogen
    Kellermann, R. and Taroata, D. and Schiemann, M. and Eckert, H. and Fischer, P. and Scherer, V. and Hock, R. and Schmid, G.
    Materials Research Society Symposium Proceedings 1644 (2014)
    In this work, electrochemically recyclable lithium is analyzed as high energy density, large scale storage material for stranded renewable energy in a closed loop. The strongly exothermic reaction of lithium with carbon dioxide (CO2) yields thermal energy directly comparable to the combustion of coal or methane in an oxygen containing atmosphere. The thermal level of the reaction is sufficient for re-electrification in a thermal power plant compatible process. The reaction of single lithium particles, avoiding particle-particle interactions, is compared to the combustion of atomized lithium spray in a CO2 containing atmosphere. Particle temperatures of up to 4000K were found for the reaction of single lithium particles in a CO2, nitrogen (N2), oxygen (O2) and steam gas mixture. Furthermore the combustion of atomized lithium spray in both dry CO2 atmosphere and CO2/steam gas mixture was analyzed. The identified solid reaction products are lithium carbonate, lithium oxide and lithium hydroxide. The formation of carbon monoxide (CO) as gaseous reaction product is demonstrated. Carbon monoxide is a valuable by-product, which could be converted to methanol or gasoline using hydrogen. Copyright © 2014 Materials Research Society.
    view abstract10.1557/opl.2014.314
  • The contact heat transfer in rotary drums in dependence on the particle size ratio
    Nafsun, A.I. and Herz, F. and Specht, E. and Scherer, V. and Wirtz, S. and Komossa, H.
    Proceedings of the 15th International Heat Transfer Conference, IHTC 2014 (2014)
    An experimental study of the heat transfer from the inner drum wall to the covered surface of the solid bed has been done in an indirectly heated rotary drum with a diameter of D=0.6 m and a length of L=0.45 m. Temperature of the gas, the drum wall and the solid bed were measured using 16 k-type thermocouples assembled on both a rotating and a fixed rod that attached inside the drum. Hence, the radial and circumferential temperature profiles were measured, and the contact heat transfer coefficients were calculated from these temperature profiles. Experiments have been carried out using monodisperse and bidisperse solids beds with different particle size ratios to see the effect on the contact heat transfer coefficient. Two different materials, glass beads and steel spheres, have been used in these experiments. These materials differ greatly in their thermo physical properties (density, heat capacity and thermal conductivity). The operating parameters, rotational speed and filling degree, are varied in the range n=1.6 rpm and F=10.20% to see their influence on the contact heat transfer coefficient in addition to the particle size ratio and thermo physical properties.
    view abstract10.1615/ihtc15.mfp.008844
  • The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane
    Düdder, H. and Kähler, K. and Krause, B. and Mette, K. and Kühl, S. and Behrens, M. and Scherer, V. and Muhler, M.
    Catalysis Science and Technology 4 (2014)
    Highly stable Ni catalysts with varying Ni contents up to 50 mol% originating from hydrotalcite-like precursors were applied in the dry reforming of methane at 800 and 900 °C. The integral specific rate of methane conversion determined after 10 h on stream was 3.8 mmol s-1 g cat -1 at 900 °C. Due to the outstanding high activity, a catalyst mass of just 10 mg had to be used to avoid operating the reaction in thermodynamic equilibrium. The resulting WHSV was as high as 1.44 × 106 ml gcat -1 h-1. The observed axial temperature distribution with a pronounced cold spot was analyzed by computational fluid dynamics simulations to verify the strong influence of this highly endothermic reaction. Transmission electron microscopy and temperature-programmed oxidation experiments were used to probe the formation of different carbon species, which was found to depend on the catalyst composition and the reaction temperature. Among the formed carbon species, multi-walled carbon nanofibers were detrimental to the long-term stability at 800 °C, whereas their formation was suppressed at 900 °C. The formation of graphitic carbon at 900 °C originating from methane pyrolysis played a minor role. The methane conversion after 100 h of dry reforming at 900 °C compared to the initial one amounted to 98% for the 25 mol% Ni catalyst. The oxidative regeneration of the catalyst was achieved in the isothermal mode using only carbon dioxide in the feed. © the Partner Organisations 2014.
    view abstract10.1039/c4cy00409d
  • Transversal bed motion in rotating drums using spherical particles: Comparison of experiments with DEM simulations
    Komossa, H. and Wirtz, S. and Scherer, V. and Herz, F. and Specht, E.
    Powder Technology 264 (2014)
    Experimental investigations and numerical simulations with the discrete element method (DEM) were carried out to improve the understanding of the movement and mechanical interaction of particles in rotary kilns. The focus is on the bed movement in the rolling motion mode with rotational speeds varying between 3 and 15. rpm. Characteristic parameters for this system are the Froude number, the dynamic angle of repose, the thickness of the active layer and the particle velocity on the bed surface and at the wall. These parameters were measured in rotating drum experiments, computed in DEM simulations and compared with each other. When available, analytical and/or semi-empirical macroscopic models were included in the comparison. Both the DEM simulations and the analytical macroscopic models show good agreement with the experiments in general. The wide range of parameters considered, like drum diameter, particle diameter and rotational speeds, provide a comprehensive reference data set. © 2014 Elsevier B.V.
    view abstract10.1016/j.powtec.2014.05.021
  • An experimental investigation of mixing of wood pellets on a forward acting grate in discontinuous operation
    Kruggel-Emden, H. and Wirtz, S. and Scherer, V.
    Powder Technology 233 (2013)
    The knowledge about mixing and the transport efficiency of non-spherical particles on grates is needed for the improvement of design and adaption of operational parameters in grate combustion systems. To gain insight into details of the transport of non-spherical particles experimental investigations with a scale model of a forward acting grate are performed in discontinuous operation using wood pellets as bed material. Wood pellets are an important source of biomass within an energy market increasingly relying on renewable sources. Different motion patterns, grate operational conditions and pellet types are investigated. A comparison to spherical particles is performed. The wood pellets as well as the spherical particles are dyed in different colors to distinguish between different layers applied vertically and horizontally on the grate. The particle motion and the mixing are monitored by image analysis from the top and the side of the grate through transparent walls. In addition the discharged particle mass is recorded. The particle mass, the stroke length as well as the particle shape and the motion pattern have a strong influence on the mixing. The stroke velocity only has minor effect on mixing and the amount of particles discharged from the grate. The results obtained under well established boundary conditions are suitable for use as reference data in the verification of particle based simulation approaches as, e.g. the discrete element method. © 2012 Elsevier B.V.
    view abstract10.1016/j.powtec.2012.08.029
  • Coupled DEM/CFD simulation of heat transfer in a generic grate system agitated by bars
    Rickelt, S. and Sudbrock, F. and Wirtz, S. and Scherer, V.
    Powder Technology 249 (2013)
    Coupled DEM/CFD simulations are compared to experiments with agitated spherical particles (10. mm in diameter) on a generic grate and exposed to a transient heat-up by a hot air flow. A video camera and a thermographic system monitor the experiments. Because the particles show Biot numbers >. 1 the radial temperature distribution within the particle has been calculated by solving the unsteady one-dimensional differential equation for heat conduction in the DEM simulations. A visual comparison of the heat-up of the particles and particle transport within the bed shows a good agreement between simulations and experiments. In addition, local minimum, maximum and average particle surface temperatures in different layers of the particle bed are calculated. Experimentally measured temperature gradients over the particle bed height are in very good agreement to DEM/CFD simulations. © 2013 Elsevier B.V.
    view abstract10.1016/j.powtec.2013.08.043
  • Experimental and numerical investigation on the influence of particle shape and shape approximation on hopper discharge using the discrete element method
    Höhner, D. and Wirtz, S. and Scherer, V.
    Powder Technology 235 (2013)
    In this study experimental and numerical investigations with the discrete element method (DEM) on the discharge of spheres and polyhedral dices from a hopper are conducted. In DEM the dices are approximated by polyhedra and smoothed polyhedra respectively and hence allow examining the influence of sharply-edged and smooth particle geometries on the discharge properties. Simulation results are in good general agreement with the experiments and hence demonstrate the adequacy of DEM as well as polyhedral and smoothed polyhedral approximation schemes to simulate non-spherical particle geometries. Compared to spheres the dices exhibit an increased flow resistance and readiness to form pile-ups at the bottom walls of the hopper. Both phenomena are better approximated using polyhedral approximations of the dices, showcasing the influence of the selected shape approximation scheme on the numerical results. © 2012 Elsevier B.V.
    view abstract10.1016/j.powtec.2012.11.004
  • Investigating the influence of sweep gas on CO2/N2 membranes for post-combustion capture
    Franz, J. and Schiebahn, S. and Zhao, L. and Riensche, E. and Scherer, V. and Stolten, D.
    International Journal of Greenhouse Gas Control 13 (2013)
    CO2 partial pressure difference is the driving force for CO2-selective membranes and is emerging as the critical issue, when applying CO2-selective membranes for post-combustion capture. Using sweep gas can be an option to increase the driving force for CO2 permeation without changing the total pressure. Hence, in contrast to the application of compressors and vacuum pumps, the usage of sweep gas achieves separation without extra energy consumption. Nevertheless, it has the inherent feature of diluting the permeate. In this paper, two membrane concepts using air as sweep gas for post-combustion capture were investigated. The influence of the sweep gas on membrane area and the degree of CO2 separation was explored. An energetic and economic analysis was carried out comparing this concept with the cascade membrane concept without sweep gas. The technical route using sweep gas makes it possible to achieve an efficiency loss of 3.8%-points with a 70% degree of CO2 separation for a 600MW reference power plant. © 2012 Elsevier Ltd.
    view abstract10.1016/j.ijggc.2012.12.008
  • Spray roasting of iron chloride FeCl2: Numerical modelling of industrial scale reactors
    Schiemann, M. and Wirtz, S. and Scherer, V. and Bärhold, F.
    Powder Technology 245 (2013)
    Iron chloride solutions are a waste product in steel pickling plants. A technique to recover the spent solutions is a spray roasting process, where the spent solution is sprayed into a hot reaction atmosphere and solid iron oxide particles are formed. The particle history in spray roasting reactors has an important influence on the efficiency of the recovery process and on the quality of the by-product Fe2O3. The iron oxide underlays strong quality demands for further processing. The particle quality is influenced by the plant design and operation parameters. To investigate the influence of those properties on the iron oxide produced in the spray roasting process, a model for CFD simulations has been developed. It describes the particle formation and chemical reaction of the iron chloride solution in the spray roasting reactor in a simplified way suitable for CFD simulations. Simulations of two different industrial reactor configurations show the capability of the model to predict the influence of geometric variations on the composition of the resulting iron oxide. © 2013 Elsevier B.V.
    view abstract10.1016/j.powtec.2013.04.034
  • Transition to Renewable Energy Systems
    Stolten, D. and Scherer, V.
    Transition to Renewable Energy Systems (2013)
    In this ready reference, top academic researchers, industry players and government officers join forces to develop commercial concepts for the transition from current nuclear or fossil fuel-based energy to renewable energy systems within a limited time span. They take into account the latest science and technology, including an analysis of the feasibility and impact on the environment, economy and society. In so doing, they discuss such complex topics as electrical and gas grids, fossil power plants and energy storage technologies. The contributions also include robust, conceivable and breakthrough technologies that will be viable and implementable by 2020. © 2013 Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved.
    view abstract10.1002/9783527673872
  • A CFD model for thermal conversion of thermally thick biomass particles
    Mehrabian, R. and Zahirovic, S. and Scharler, R. and Obernberger, I. and Kleditzsch, S. and Wirtz, S. and Scherer, V. and Lu, H. and Baxter, L.L.
    Fuel Processing Technology 95 (2012)
    A one-dimensional model for the thermal conversion of thermally thick biomass particles is developed for the simulation of the fuel bed of biomass grate furnaces. The model can be applied for cylindrical and spherical particles. The particle is divided into four layers corresponding to the main stages of biomass thermal conversion. The energy and mass conservation equations are solved for each layer. The reactions are assigned to the boundaries. The model can predict the intra-particle temperature gradient, the particle mass loss rate as well as the time-dependent variations of particle size and density, as the most essential features of particle thermal conversion. When simulating the fuel bed of a biomass grate furnace, the particle model has to be numerically efficient. By reducing the number of variables and considering the lowest possible number of grid points inside the particle, a reasonable calculation time of less than 1 min for each particle is achieved. Comparisons between the results predicted by the model and by the measurements have been performed for different particle sizes, shapes and moisture contents during the pyrolysis and combustion in a single-particle reactor. The results of the model are in good agreement with experimental data which implies that the simplifications do not impair the model accuracy. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.fuproc.2011.11.021
  • A model for the devolatilisation of large thermoplastic particles under co-firing conditions
    Bluhm-Drenhaus, T. and Becker, A. and Wirtz, S. and Scherer, V.
    Fuel 101 (2012)
    Thermoplastic materials are utilised in large quantities for packaging purposes and thus make up a significant share of municipal solid waste. Due to their high calorific value, plastic solid waste (PSW) is suitable for co-firing as a refuse derived fuel in high temperature processes such as industrial furnaces and boilers. In order to achieve a high substitution ratio of fossil fuels and a high degree of burnout, a characterisation of the PSW properties and a prediction of the decomposition behaviour are crucial for process optimisation. However, for PSW particles e.g. polyethylene in the size range of several millimetres combustion models are scarcely published nor are they available in commercial simulation packages. Therefore a simplified devolatilisation model is presented in this paper, transferring findings from the fire protection literature to combustion modelling. The model is capable of calculating the conversion rate of thermoplastic particles under co-firing conditions, considering aspects like particle size, particle shape and related heat and mass transfer phenomena. The modelling results are compared to laboratory experiments in a bench scale reactor and an extrapolation to co-firing conditions is carried out for particles of generic shape. © 2012 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.fuel.2012.05.041
  • A numerical study on the influence of particle shape on hopper discharge within the polyhedral and multi-sphere discrete element method
    Höhner, D. and Wirtz, S. and Scherer, V.
    Powder Technology 226 (2012)
    In this study 3D DEM-simulations of hopper discharge using non-cohesive, monodisperse spherical and polyhedral particles as well as particle shapes generated by the multi-sphere method are carried out. For this purpose an overview of the Common Plane algorithm for contact detection between polyhedral particles is given and an important refinement of the contact point definition is presented. In the hopper the effect of increasing particle angularity on the flow properties is investigated. Moreover, three different hopper designs are chosen, to further examine the influence of hopper angle and hopper opening size on the flow properties in combination with varying particle shapes. It is demonstrated that particles with an increasing angularity reduce the mass flow rate from the hopper and in case of the flat bottom hopper (α = 0°) increase the residual quantity after discharge. In all simulations significant differences between polyhedral and clustered particles were observed, which indicates that the type of particle shape approximation is a parameter that has to be considered in DEM-simulations of hopper discharge. © 2012 Elsevier B.V.
    view abstract10.1016/j.powtec.2012.03.041
  • CCS separation techniques - Review on existing technologies and developments [CCS-Abscheidetechniken: Stand der Technik und Entwicklungen]
    Scherer, V. and Stolten, D. and Franz, J. and Riensche, E.
    Chemie-Ingenieur-Technik 84 (2012)
    The following contribution summarizes the existing technologies for CO 2 separation in coal power plants and gives an outlook on recent and upcoming developments. In addition to the well-known processes also the potentials and drawbacks of membrane based separation techniques are discussed. Additionally chemical and carbonate looping - where an oxygen- or CO 2 carrier is circulated - complete the overview. For processes based on established technologies an efficiency loss of 10 %-points is to be expected, already accounting for the compression to pipeline pressure. Recent developments promise efficiency losses between 5 and 10 %-points. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cite.201100233
  • Experimental and numerical investigation of the bulk behavior of wood pellets on a model type grate
    Kruggel-Emden, H. and Sudbrock, F. and Wirtz, S. and Scherer, V.
    Granular Matter 14 (2012)
    Transport and storage equipment for wood pellets can in theory be readily improved through simulation approaches like the discrete element method. However, scientific investigations verifying the applicability of the discrete element method in case of non-spherical particles are still limited. The sensitivity of simulations on the size and shape approximation of particles and mixing and segregation behavior are not well studied. These issues are addressed in the current paper. For amodel type grate system experimental and numerical investigations of the mixing behavior of wood pellets under different motion patterns are performed. Results indicate that the discrete element method is well capable of representing experimentally obtained results. Altering the representation of the size distribution has impact on the segregation behavior, but does not strongly impact the overall mixing tendencies. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/s10035-012-0376-6
  • Experimental and numerical investigation on the discharge of wood pellets from a hopper with the discrete element method
    Höhner, D. and Wirtz, S. and Scherer, V.
    ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 1 (2012)
    In this study hopper discharge experiments with wood pellets were conducted. The experimental bulk density, flow behavior and discharge rate were compared to corresponding 3- dimensional discrete element simulations with both multisphere and polyhedral approximations of the pellet geometry. Additionally a numerical sensitivity analysis for the particlewall friction was made in order to evaluate the influence of this parameter on hopper discharge in the context of different particle geometries. In the past comparisons of experimentally and numerically obtained results demonstrated the adequacy of the discrete element method for predicting the general discharge behavior of a hopper. Nevertheless, in this study, comparing two different particle shape-approximations, significant differences in terms of bulk density, discharge rate, flow profile and dependency on the particle-wall friction coefficient between both investigated particle-shape approximation schemes could be observed. As a result, particle shape-representation must be considered a significant parameter in DEM-simulations. Copyright © 2012 by ASME.
    view abstract10.1115/IMECE2012-87406
  • Influence of particle diameter and material properties on mixing of monodisperse spheres on a grate: Experiments and discrete element simulation
    Simsek, E. and Sudbrock, F. and Wirtz, S. and Scherer, V.
    Powder Technology 221 (2012)
    The Discrete Element Method (DEM) can help to obtain detailed information on the mixing process within a bed of fuel particles on grate firing systems. In a first study it was shown by comparing experiments on a generic grate with DEM simulations that for a granulate consisting of monodisperse spheres the influence of grate stroke length and bar velocity can be reproduced. Prior to tackling particle assemblies which are polydisperse and consist of particles of complex shape it has to be shown that the influence of particle diameter and material properties of the particles is described correctly by DEM simulations. Thus experiments with monodisperse spheres of three different diameters (5, 10, 20. mm) and two materials (plastic, wood) are compared to DEM simulations. The mixing process is measured at the front side wall and at the top surface of the particle bed and quantified by image analysis. Both visual observation and mixing parameter results attest a very good agreement between experiments and simulations. The influence of particle diameter and material properties measured in the experiments can be reproduced by DEM simulations. © 2011 Elsevier B.V.
    view abstract10.1016/j.powtec.2011.12.051
  • Spray roasting of iron chloride FeCl 2: Laboratory scale experiments and a model for numerical simulation
    Schiemann, M. and Wirtz, S. and Scherer, V. and Bärhold, F.
    Powder Technology 228 (2012)
    Iron chloride solutions are a waste product in steel pickling plants. A technique to recover the spent solutions is the so-called spray roasting process, where the spent solution is sprayed into a hot reaction atmosphere and solid iron oxide particles are formed. The particle formation in spray roasting reactors has important influence on the efficiency of the recovery process and on the quality of the desired by-product Fe 2O 3. A laboratory reactor was designed to investigate the particle formation. Experiments were carried out covering the predominant conditions in spray roasting reactors. The results offer valuable insight into the particle formation process, providing data on the surface structure of the Fe 2O 3 particles formed and on the progress of chemical conversion. Based on these results, a simplified model applicable to CFD-modelling of spray roasting reactors has been developed. Simulations of particle trajectories in the laboratory reactor are presented to show the capabilities of the model. © 2012 Elsevier B.V.
    view abstract10.1016/j.powtec.2012.05.037
  • Comparison of the multi-sphere and polyhedral approach to simulate non-spherical particles within the discrete element method: Influence on temporal force evolution for multiple contacts
    Höhner, D. and Wirtz, S. and Kruggel-Emden, H. and Scherer, V.
    Powder Technology 208 (2011)
    In this paper a multi-sphere as well as a polyhedral approach is investigated as method of shape-approximation within the discrete element method. The two approaches are compared against each other using an ellipsoidal particle impacting on a flat wall as a reference scenario. In general it is shown that there is a non-negligible effect of shape approximation on the temporal force evolution in normal and tangential direction. The occurrence of multiple contacts between the particles when approximated by a multi-sphere or a polyhedral approach is detected as a main source of differences between the exact solution and the solution of the approximated particles. A method to account for these effects is provided in order to increase the accuracy when complex particle shapes are used. Moreover the effect of shape-approximation accuracy on the accuracy of the results is investigated. For both approximation procedures higher accuracy levels in terms of particle shape representation do not automatically lead to more accurate results for normal and tangential forces in the investigated collision scenarios. It will be shown that the effects of a more accurate ellipsoid-surface approximation and higher numbers of contact points influence the quality of the results in opposite directions. © 2011 Elsevier B.V.
    view abstract10.1016/j.powtec.2011.01.003
  • Discrete element analysis of experiments on mixing and stoking of monodisperse spheres on a grate
    Sudbrock, F. and Simsek, E. and Rickelt, S. and Wirtz, S. and Scherer, V.
    Powder Technology 208 (2011)
    Understanding the details of the mixing and stoking process on grate firing systems is crucial for the optimization of the combustion process in waste or biomass incineration plants. The Discrete Element Method (DEM) can help to obtain further information on the mixing process within a bed of fuel particles. Especially the influence of a change in operational parameters can be examined avoiding large experimental effort. In the current paper five simulations for a generic grate are compared with the corresponding experiments. The experiments were carried out throughout an anterior parameter study on mixing and stoking on a grate [Sudbrock F.; Simsek E.; Wirtz S.; Scherer V.: "An experimental analysis of the influence of operational parameters on mixing and stoking of a monodisperse granulate on a grate", Powder Technology 198, Issue 1, 29-37, 2010] [19]. The system considered is equipped with vertically moving bars which induce stoking. In a first approach monodisperse plastic spheres are used. The grate is encased by a transparent polycarbonate housing which provides optical access to the movement of the particles in the wall planes. The mixing process is measured and quantified by image analysis of the front wall of the grate. The mixing behaviour of the particle assembly observed in experiments and simulation appears to be very similar indicating that DEM is able to predict the particle mixing in the bed. In order to quantify the visual observations the mixing behaviour has been evaluated by different mixing parameters. They are compared in dependence of the number of strokes of the grate bars. A good agreement between measurements and simulations could be observed. © 2010 Elsevier B.V.
    view abstract10.1016/j.powtec.2010.12.008
  • Heat and Mass Flow Control in an Interconnected Multiphase CFD Model for Chemical Looping Combustion
    Kruggel-Emden, H. and Scherer, V.
    Chemical Engineering and Technology 34 (2011)
    Chemical looping combustion is a feasible option for carbon capture from fossil fuels. Within the process, the oxygen necessary for combustion is provided by a solid carrier material which alternately undergoes oxidation and reduction reactions. Features of the process are that the oxidation reaction of the particulate carrier in the air reactor is strongly exothermic and that the conversion of both oxidation and reduction reactions has to be in balance for stable operation. Simulations of the transient behavior of chemical looping combustion systems are possible through multiphase CFD. To allow for the modeling of chemical looping at steady state, cooling of the reactors and mass flow between fuel and air reactor must be adequately adjusted. Therefore, an interconnected multiphase CFD model was extended by an adjustment control. In this extended modeling framework variations of the operational load, control set points and carrier materials were performed. These simulations allow detailed insight into the dynamic behavior of chemical looping systems. An interconnected multiphase CFD model was extended by a temperature and mass flow control. For this purpose, PI controllers were used and their control parameters had to be obtained. The extended CFD framework was then used to model load changes, variation of set points for temperature and degree of conversion and changes in carrier material. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ceat.201100068
  • Impact of ceramic membranes for CO2 separation on IGCC power plant performance
    Franz, J. and Scherer, V.
    Energy Procedia 4 (2011)
    The potential of carbon capture from coal gasification power plants by H2 selective ceramic membranes is investigated. Detailed models of a reference power plant and three different carbon capture concepts were setup with Aspen Plus and Ebsilon. Parameter variations were performed to investigate the influence of membrane characteristics and power plant specific boundary conditions on the performance of the capture concepts. For ceramic membranes with a selectivity of H2 versus N2 and CO2 of 500 the results showed that for a sour CO-shift and sweet CO-shift efficiency losses of 9.07 and 9.43% points are feasible, respectively, while separating about 97% of the CO2 with a purity of 95%. A ceramic membrane reactor concept with simultaneous CO2 separation and CO-shift was the third carbon capture concept investigated. This concept achieves separation degrees of 96.6% and purities above 95% with an efficiency loss of 6.7% points. © 2011 Published by Elsevier Ltd.
    view abstract10.1016/j.egypro.2011.01.100
  • A coupled fluid dynamic-discrete element simulation of heat and mass transfer in a lime shaft kiln
    Bluhm-Drenhaus, T. and Simsek, E. and Wirtz, S. and Scherer, V.
    Chemical Engineering Science 65 (2010)
    In the present study heat and mass transfer related to the chemical conversion of limestone to quicklime in a shaft kiln are investigated by means of a coupled numerical scheme for gas and solid phase transport. The three-dimensional transport of mass, momentum and energy in the gas phase is modelled by computational fluid dynamics (CFD), while a discrete element method (DEM) is employed for the mechanical movement and the conversion reactions of the solid material. The DEM simulation readily describes the mechanical and thermal particle-to-particle interactions of a large number of differently sized particles. Novel aspects addressed in this work are the simultaneous effects of inner particle heat-conduction and pore-diffusion of the gaseous product of the calcination reaction (CO2) modelled by a shrinking core approach. Simulations of laboratory scale experiments of single reacting spheres show good agreement with the measured conversion rates. Simulations of an idealised vertical shaft kiln including pressure drop calculations demonstrate the suitability of the proposed approach for the modelling of industrial scale systems. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ces.2010.01.015
  • An evaluation of CO2 and H2 selective polymeric membranes for CO2 separation in IGCC processes
    Franz, J. and Scherer, V.
    Journal of Membrane Science 359 (2010)
    The potential of CO2 and H2 selective polymeric membranes to separate CO2 from integrated coal gasification combined cycle power plants is examined. As a reference power plant a modified version of the Puertollano gasification plant has been defined. A detailed ASPEN model for the reference case has been developed. For the CO2 selective membranes single membrane module concepts as well as cascade concepts have been examined. The results for H2 and CO2 selective membranes show that with membranes of state of the art (CO2/H2 selectivity 15.5, H2/CO2 selectivity 5.91) the current requirements concerning CO2 purity and CO2 separation degree cannot be fulfilled. A CO2/H2 selectivity of 150 for a single CO2 selective membrane would be needed to obtain power plant efficiency losses below 10% points with separation degrees above 85%. For a cascade concept the CO2/H2 selectivity needed would be in the order of 60 to achieve the same values. For H2 selective membranes with a H2/CO2 selectivity of 50 separation degrees of 85% at efficiency losses below 10% points can be reached. © 2010 Elsevier B.V.
    view abstract10.1016/j.memsci.2010.01.047
  • An experimental analysis on mixing and stoking of monodisperse spheres on a grate
    Sudbrock, F. and Simsek, E. and Wirtz, S. and Scherer, V.
    Powder Technology 198 (2010)
    A better understanding of the mixing and stoking process is crucial for an optimization of the combustion process on grate firing systems. Thus experimental studies were carried out to analyse the response of a particle assembly on varying grate operational parameters. To reduce the number of variables which affect the system a generic grate design was chosen and a material of monodisperse spheres was selected. The grate system applied uses vertically moving parallel bars to induce mixing. Different patterns of bar motion were created by linking the bars in groups of uniform movement. A transparent polycarbonate side wall gives optical access to front layer of spheres. The mixing process was measured and quantified by image analysis of this visible layer. When applying a constant number of bar strokes it is found that the mixing performance is independent of the bar velocity. However, mixing performance increases nearly linearly with the stroke length. It turned out that specific "movement patterns" could be identified which show improved mixing behaviour. The results provided here may also be used for comparison with simulations of the particle mixing with the Discrete Element Method (DEM). © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.powtec.2009.10.011
  • Coupled DEM-CFD simulation of pneumatically conveyed granular media
    Sturm, M. and Wirtz, S. and Scherer, V. and Denecke, J.
    Chemical Engineering and Technology 33 (2010)
    A DEM-CFD coupling for the simulation of gas-solid flows was successfully implemented and simulations were performed for the application to industrial-scale pneumatic conveying. Therefore, all particle collisions and phase interactions were considered and porosity determination was optimized. The aim of this work is to show the applicability of the presented simulation model to the different regimes of pneumatic conveying systems. As a first test case a dense vertical pneumatic conveying system was chosen and an individual plug was investigated in detail. Variations of the conveying air velocity were also considered. As a second test case dilute conveying in a horizontal-to-vertical pipe bend was simulated. The occurrence of roping and the reduction of particle velocity is of high interest for the design of specific pneumatic systems. It is shown that both regimes can be captured reasonably well and the results are rich in details. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ceat.201000162
  • Coupled Discrete Element (DEM) - Continuous Fluid (CFD) method for the application of pneumatic conveyed granular media
    Sturm, M. and Wirtz, S. and Scherer, V. and Denecke, J.
    American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP 4 (2010)
    A Coupled Discrete Element - Continuous Fluid Method is developed, which allows the numerical simulation of the behaviour of pneumatic conveyed granular media. While the fluid is simulated by computational fluid dynamics, the soft-sphere discrete element approach is used for the particle system. A "four-way coupling" is implemented to account for phase interaction on both sides and for the collisions among particles and with the pipe walls. The purpose of this work is to show the applicability of the presented simulation model to dense pneumatic conveying systems. Unfortunately, only few complete experimental data are available in this field. In this work a pneumatic conveying system for dense phase regime with a vertical pipe of 2 m height and a diameter of 50 mm is investigated. Periodic conditions are applied to both phases, the fluid and the particle system. The particles, which leave the pipe at one end, are inserted back at the other end of the pipe, so that a much larger simulation domain is represented. After an initial set up and some intermediate time steps a stable mode is reached and one single plug forms, which is then investigated in detail. The pressure distribution along the plug which coincides with the porosity distribution, the length of the plug and its velocity are important parameters which are examined and compared to experimental investigations. Variations of the conveying air velocity and their influence are also considered. Copyright © 2009 by ASME.
    view abstract10.1115/PVP2009-77240
  • energy conversion

  • granular materials

  • laminar flow

  • numerical methods

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