Particle Resolved Thermo-Chemical Conversion of Pulverized Coal Clusters.

Particle clusters form in the turbulent flow of boilers and furnaces when injecting pulverized coal. We study the difference in conversion behavior between a pul verized coal cluster and a single char particle. The simulations are done using a detailed particle model in OpenFOAM.The detailed particle model is capable of spatially resolv ing char particles and solving transport equations for heat and mass inside the particles. Furthermore, a surface chemistry model is employed for modeling gas-solid reactions in side the pores of the particles. The char particles are coupled to the surrounding bulk fluid phase using explicit Robert-Neuman coupling. 2-D simulations are carried out for a cluster of 16 particles and a single particle. Both simulations use the same initial con ditions, boundary conditions and particle properties. The results indicate that a lack of oxygen penetration toward the center of the particle cluster alters the thermo-chemical conversion behavior when compared to the single particle. On average this leads to a decrease of the thermo-chemical conversion rates for particles in the cluster, resulting in a reduction of solid carbon consumption compared to the same number of single particles

The effect of turbulence on the conversion of coal under blast furnace raceway conditions

dynamics (CFD) can be used to analyze the process virtually and thus improve its performance. Different reducing agents can be used to (partially) substitute the coke and consequently reduce overall emissions. To analyze different reducing agents effectively using CFD, their conversion process has to be modeled accurately. Under certain conditions, coal particles can cluster as the result of turbulence effects, which further reduces the mass transfer to the coal surface and consequently the conversion rate. We analyze the effect of turbulence under blast furnace raceway conditions on the conversion of coal particles and on the overall burnout. The model is applied in RANS to polydisperse particle systems and this is then compared to the simplified monodisperse assumption. Additionally, the model is extended by adding gasification reactions. Overall, we find that the turbulent effects on coal conversion are significant under blast furnace raceway conditions and should be considered in further simulations. Furthermore, we show that an a-priori assessment is difficult because the analysis via averaged quantities is impractical due to a strong variation of conditions in the furnace. Therefore, the effects of turbulence need to be correlated to the regions of conversion. © 2022 The Author(s)The effect of turbulence on the conversion of coal under blast furnace raceway conditionspublishedVersio

Modellierung von reaktiven Mehrphasenströmungen: thermochemische Konversion in der Raceway-Zone von Hochöfen

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersKumulative Dissertation aus acht ArtikelnEine effiziente und ressourcenschonende Industrie ist die Voraussetzung für eine nachhaltige Gesellschaft. Es bedarf unermesslicher Forschungs- und Entwicklungsanstrengungen, um diesem emissionsarmen, auf Recycling basierendem Wirtschaftssystem näher zu kommen. Ein solches System, wäre ein wichtiger Baustein zur Sicherung des Überlebens der menschlichen Spezies auf der Erde. Momentane Entwicklungs- und Optimierungsanstrengungen basieren hauptsächlich auf ökonomischen Überlegungen. Ohne sorgfältig überlegte und mutige politische Entscheidungen, wird das reine Streben nach wirtschaftlicher Effizienz auf lange Sicht nicht ausreichen um einen gesunden Planeten an die nächsten Generationen weiter zu geben.Ausgehend von Experiment-getriebener Forschung mit Unterstützung von händischen Berechnungen, entwickelte sich die Forschung in den letzten Jahren zu einem integrierten Prozess, welcher Experimente und Modellierung verbindet. Dank der immer höheren Rechenleistungen, werden in der Forschung immer komplexere Berechnungen, Modellierungen oder Simulationen bis hin zu virtuellen Experimenten durchgeführt. Diese Experimente verwenden sorgfältig validierte Modelle, um die zugrunde liegenden Physik eines realen Problems zu untersuchen. Zu Beginn wurden nur kleine, akademische Probleme modelliert, während heutzutage räumlich aufgelöste Berechnungen von industriellen Prozessen in der Entwicklung und Optimierung immer wichtiger werden.Die numerische Strömungsmechanik (CFD) hat unter Beweis gestellt, dass sie ein wichtiges Werkzeug für die Unterstützung bei der Erforschung und Entwicklung von industriellen Prozessen ist. Die Simulation von großtechnischen Prozessen beruht zum Teil auf Modellen, da es derzeit selbst auf den größten Großrechnern noch nicht möglich ist, alle notwendigen Größen- und Zeitskalen aufzulösen. Im Endeffekt benötigen aber alle Modelle Eingabeparameter, welche üblicherweise durch Experimente bestimmt werden. Deshalb können selbst die besten Modelle keine physikalisch richtigen Ergebnisse liefern, wenn die Eingabeparameter von falschen oder unpassenden Experimenten stammen.Der Hochofenprozess, die wesentliche Roheisenproduktionsmethode, wird seit Jahrhunderten kontinuierlich verbessert. Momentan wird versucht, die Effizienz des Hochofens durch Einblasen von Kohlenstoff- bzw. Energieträgern, sogenannte Alternative Reducing Agents (ARAs) in die Wirbelzone zu steigern. Das direkte Messen des thermo-chemischen Umwandlungsprozesses von ARAs ist technisch sehr aufwendig. Zusätzlich können Versuchsläufe am Hochofen schwerwiegende Betriebsstörungen auslösen. Die CFD Modellierung des ARA Injektionsprozesses ermöglicht es, potentielle neue, nachhaltige ARAs durch virtuelle Experimente zu bewerten. Dazu wird einumfassendes Simulationsmodell benötigt, welches Mehrphasenströmungen sowie homo- und heterogene thermo-chemische Umwandlungsprozesse beherrschen muss. Die Qualität der chemischen Mechanismen und die verwendeten Kinetikparameter bestimmen die Vertrauenswürdigkeit der Simulationsergebnisse. Folglich müssen themo-chemische Umwandlungsraten neuer ARAs vor den Simulationen experimentell bestimmt werden.Diese Arbeit beschreibt einen geeigneten Ablauf (Versuchsaufbau, Kinetikbestimmung) zur Bestimmung von thermo-chemischen Umsatzkinetiken von ARAs zu ermitteln. Dazu werden zuerst Referenzbedingungen der Wirbelzone definiert und nachfolgend vorhandene Versuchsaufbauten evaluiert. Ein digitaler Zwilling eines ausgewählten Versuchsaufbaus – Sandias Pressurized Entrained Flow Reactor (PEFR) – wird erstellt und, um detaillierte Informationen über die Bedingungen in der Reaktionszone zu erhalten, eine Versuchsreihe modelliert. Zusätzlich wird ein geeignetes Simulationsmodell für die ARA Umsetzung im open-source CFD Code OpenFOAM ® entwickelt.Zusammenfassend kann gesagt werden, dass vorhandene Versuchsaufbauten die benötigten Aufheizraten, welche nötig wären um die Bedingungen in der Wirbelzone zu simulieren, bei erhöhten Drücken nicht erreichen. Zusätzlich weisen die tatsächlichen Bedingungen signifikante räumliche Schwankungen auf und weichen stark vom Sollwert ab. Dies gilt insbesondere für die Temperatur. Da die meisten Methoden zur Bestimmung von kinetischen Parametern von räumlichen Variationen abhängen, ist detailliertes Wissen über diese von enormer Bedeutung. Der Umwandlungsprozess von ARAs hängt auch stark von der Gasphasenchemie ab, weshalb die umfassende Modellierung von chemischen Reaktionen in der Gasphase wichtig für die virtuellen Experimente ist. Im Hochofen verändert sich die Gasphasenchemie von mischungslimitiert in der Nähe der Windformen zu chemischlimitiert im Koksbett. Modelle für die Turbulenz-Chemie Interaktion, welche beide Verbrennungsregime abdecken, sind der Schlüssel zu verbesserten Simulationsergebnissen. Der Einfluss der reaktiven Koksphase auf die thermo-chemische Umwandlung von ARAs bedarf noch weiterer Untersuchungen.An efficient and resource-conserving industry is a prerequisite for a sustainable society. Tremendous research and development efforts are required to progress towards this low-emission, recycling-based economy which is necessary to ensure the survival of the human species on earth. Current industry developments and optimization are primarily for economic reasons. However, without deliberate and bold political decisions, the endeavor for economic efficiency alone will not be sufficient in the long term to pass over a healthy planet earth on to future generations.Research and development has changed considerably in recent decades, from purely experiment-driven (with some calculations by hand) towards an integrated process combining experiments and modeling. Meanwhile, ever-increasing computational power enables using increasingly complex calculations, models, or simulations in contemporary research. This includes the establishment of virtual experiments, which use thoroughly validated models to reveal the underlying physics of a real-world problem. While modeling was first applied to small, academic problems, the spatially resolved modeling of industrial processes is now becoming progressively more important in development and optimization.Computation Fluid Dynamics (CFD) has proven to be a vital tool for supporting the ongoing research and development of industrial processes. The simulation of industrial-scale processes relies on models, since resolving all size and time scales is still impossible even with today’s high-performance computation centers. Ultimately, all models require input parameters, which are usually obtained from experiments. Therefore, even the most sophisticated model fails to provide physically-correct results if the input parameters come from faulty or inappropriate experiments.The blast furnace process, which is the major pig iron production route, has been under continuous optimization for centuries. The current optimization measures focus on replacing coke by injecting carbon and energy carriers – Alternative Reducing Agents (ARAs) – into the raceway zone of the blast furnace. Directly measuring the thermo-chemical ARA conversion process is challenging and experimental trails at blast furnaces can cause severe operational problems. Using CFD to model the ARA injection process enables virtual experiments to evaluate potential sustainable ARAs. A comprehensive modeling framework is required for these simulations, which needs to master multi-phase flow and homo- and heterogeneous thermo-chemical conversion processes. The quality of chemical kinetic conversion mechanisms and the employed kinetic parameters determine the reliability of the simulation results. Thus, thermo-chemical conversion rates for new ARAs must be determined experimentally prior to the virtual experiments. The current work presented aims to identify a suitable work-flow (experimental setup, extraction procedure) to determine the thermo-chemical conversion kinetic of ARAs. To accomplish this, a screening of existing experimental equipment is carried out based on previously-defined reference raceway conditions. Furthermore, a digital twin of a selected experimental setup – Sandia’s Pressurized Entrained Flow Reactor (PEFR) – is established and a set of experiments is modeled to obtain detailed information about the conversion conditions in the reactive zone. Furthermore, an accompanying modeling framework for the ARA conversion was developed based on the open-source CFD toolbox, OpenFOAM ® . The emphasis of the implemented framework is its applicability to thermo-chemical conversion modeling.Summarizing these activities indicates that currently available experimental equipment fails to achieve the required heating rates under elevated pressure to reproduce raceway conditions. Furthermore, the actual conditions in the reaction zone are subject to significant spatial variations and substantial deviations from the set point. This is particularly true for temperature. Extracting reliable kinetic parameters depends on a detailed knowledge of the spatial variations, since most extraction/fitting approaches are sensitive to them. The ARA conversion process is strongly affected by gas phase chemistry; thus, comprehensively modeling homogeneous chemistry is important for the virtual experiments. Gas phase chemistry changes from mixing dominated in the vicinity of the tuyeres to chemistry dominated towards the dense coke bed. Turbulence-chemistry interaction models capable of both combustion regimes are the key for improved modeling results. The influence of the reactive coke phase on the thermo-chemical conversion of the ARAs needs further investigation.32

Non-Isothermal Effectiveness Factors in Coal Combustion

Thermo-chemical conversion of porous solids like coal is often modeled using generic approaches capable to reproduce chemical kinetic and mass transfer limitations of the conversion rate. The effectiveness factor (η) is used to describe the intraparticle mass transfer limitations in these models. Since a non-linear second order reaction-diffusion equation has to be solved to obtain the effectiveness factors for exothermic reactions, analytic solutions for the isothermal case are often used instead of solving the reaction diffusion equation. For highly exothermic reactions, this approach might underestimates the conversion rates. Therefore, this work investigates the influence of non‑isothermal effectiveness factors on conversion rates under coal combustion conditions.The results indicate that non-isothermal effectiveness factors cause a change in the conversion characteristics from progressive core to shrinking particle. Moreover, the conversion rates might be significantly under-estimated by the isothermal effectiveness factors for the investigated conditions.Österreichische Forschungsförderungsgesellschaft mbH (FFG)MoV2-(01) page 1MoV2-(01) page 5

COMBINING AN IMPLICIT SOLUTION WITH AN EXPLICIT CORRECTOR STEP FOR THE SOLUTION OF THE CONTINUITY EQUATIONS IN A TWO-FLUID SOLVER

To model two-phase flows in industrial applications, for example the raceway zone in a blast furnace, an Eulerian two-fluid model is usually the method of choice. It has proven to predict the behavior of gas-solid flows well and has a justifiable computational demand. Although, it is already widely used, there are still some deficiencies which arise from the averaged equations. Especially the continuity equation needs some special care compared to single phase flows. The consistency and boundedness need to be ensured, which is not straightforward. One widely used approach to target this problem is to use the relative velocities in the continuity equation. A drawback is, that this modified equation is non-linear in the phase fraction and therefore needs to be solved iteratively if solved implicitly. We propose to solve the discretized equation by combining an implicit solution step with (an) explicit corrector step(s). This new approach was implemented in the open source software OpenFOAM® and compared with the standard implementation. The new algorithm gives good prediction results for several test cases and this implicit approach could lead to larger time steps through better stability of the solution procedure.publishedVersio

COMBINING AN IMPLICIT SOLUTION WITH AN EXPLICIT CORRECTOR STEP FOR THE SOLUTION OF THE CONTINUITY EQUATIONS IN A TWO-FLUID SOLVER

To model two-phase flows in industrial applications, for example the raceway zone in a blast furnace, an Eulerian two-fluid model is usually the method of choice. It has proven to predict the behavior of gas-solid flows well and has a justifiable computational demand. Although, it is already widely used, there are still some deficiencies which arise from the averaged equations. Especially the continuity equation needs some special care compared to single phase flows. The consistency and boundedness need to be ensured, which is not straightforward. One widely used approach to target this problem is to use the relative velocities in the continuity equation. A drawback is, that this modified equation is non-linear in the phase fraction and therefore needs to be solved iteratively if solved implicitly. We propose to solve the discretized equation by combining an implicit solution step with (an) explicit corrector step(s). This new approach was implemented in the open source software OpenFOAM® and compared with the standard implementation. The new algorithm gives good prediction results for several test cases and this implicit approach could lead to larger time steps through better stability of the solution procedure

The Eddy Dissipation Concept—Analysis of Different Fine Structure Treatments for Classical Combustion

The Eddy Dissipation Concept (EDC) is common in modeling turbulent combustion. Several model improvements have been proposed in literature; recent modifications aim to extend its validity to Moderate or Intense Low oxygen Dilution (MILD) conditions. In general, the EDC divides a fluid into a reacting and a non-reacting part. The reacting part is modeled as perfectly stirred reactor (PSR) or plug flow reactor (PFR). EDC theory suggests PSR treatment, while PFR treatment provides numerical advantages. Literature lacks a thorough evaluation of the consequences of employing the PFR fine structure treatment. Therefore, these consequences were evaluated by employing tests to isolate the effects of the EDC variations and fine structure treatment and by conducting a Sandia Flame D modeling study. Species concentration as well as EDC species consumption/production rates were evaluated. The isolated tests revealed an influence of the EDC improvements on the EDC rates, which is prominent at low shares of the reacting fluid. In contrast, PSR and PFR differences increase at large fine fraction shares. The modeling study revealed significant differences in the EDC rates of intermediate species. Summarizing, the PFR fine structure treatment might be chosen for schematic investigations, but for detailed investigations a careful evaluation is necessary

The effect of turbulence on the conversion of coal under blast furnace raceway conditions

dynamics (CFD) can be used to analyze the process virtually and thus improve its performance. Different reducing agents can be used to (partially) substitute the coke and consequently reduce overall emissions. To analyze different reducing agents effectively using CFD, their conversion process has to be modeled accurately. Under certain conditions, coal particles can cluster as the result of turbulence effects, which further reduces the mass transfer to the coal surface and consequently the conversion rate. We analyze the effect of turbulence under blast furnace raceway conditions on the conversion of coal particles and on the overall burnout. The model is applied in RANS to polydisperse particle systems and this is then compared to the simplified monodisperse assumption. Additionally, the model is extended by adding gasification reactions. Overall, we find that the turbulent effects on coal conversion are significant under blast furnace raceway conditions and should be considered in further simulations. Furthermore, we show that an a-priori assessment is difficult because the analysis via averaged quantities is impractical due to a strong variation of conditions in the furnace. Therefore, the effects of turbulence need to be correlated to the regions of conversion. © 2022 The Author(s

The effect of turbulence on the conversion of coal under blast furnace raceway conditions

dynamics (CFD) can be used to analyze the process virtually and thus improve its performance. Different reducing agents can be used to (partially) substitute the coke and consequently reduce overall emissions. To analyze different reducing agents effectively using CFD, their conversion process has to be modeled accurately. Under certain conditions, coal particles can cluster as the result of turbulence effects, which further reduces the mass transfer to the coal surface and consequently the conversion rate. We analyze the effect of turbulence under blast furnace raceway conditions on the conversion of coal particles and on the overall burnout. The model is applied in RANS to polydisperse particle systems and this is then compared to the simplified monodisperse assumption. Additionally, the model is extended by adding gasification reactions. Overall, we find that the turbulent effects on coal conversion are significant under blast furnace raceway conditions and should be considered in further simulations. Furthermore, we show that an a-priori assessment is difficult because the analysis via averaged quantities is impractical due to a strong variation of conditions in the furnace. Therefore, the effects of turbulence need to be correlated to the regions of conversion. © 2022 The Author(s