The Experts below are selected from a list of 309 Experts worldwide ranked by ideXlab platform
Leonardo Tognotti - One of the best experts on this subject based on the ideXlab platform.
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Characterization of high temperature rapid Char Oxidation of raw and torrefied biomass fuels
Fuel, 2015Co-Authors: Jun Li, Giorgio Bonvicini, Weihong Yang, Leonardo Tognotti, Enrico BiaginiAbstract:The promising properties of torrefied biomass provide a valid co-firing option for large percentage biomass utilization in existing coal-fired boilers. Torrefied biomass is expected to have a better combustion stability than raw biomass and similar to that of coal. The present work will Characterizes the Oxidation properties of torrefied biomass Char and compare with that of raw biomass Char. The studied two Chars are produced from raw and torrefied biomass in an Isothermal Plug Flow Reactor (IPFR) at high temperature and high heating rate, a sufficient residence time is applied for the completion of the high temperature devolatilization. Char Oxidation tests are carried out in the IPFR by varying temperature, oxygen concentration and residence time. The reactivity of two studied Chars are analyzed and compared with referenced biomass Char and coal Char, and the impact of torrefaction on Char reactivity is also discussed in this paper. Finally, the Char Oxidation kinetic parameters are determined using a parameter optimization method, and the obtained kinetics are examined by comparing the experimental and predicted mass conversions.
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Char Oxidation of torrefied biomass at high temperatures and high heating rates
Energy Procedia, 2014Co-Authors: Jun Li, Giorgio Bonvicini, Xiaolei Zhang, Weihong Yang, Leonardo TognottiAbstract:The Char Oxidation of a torrefied biomass and its parent material was carried out in an isothermal plug flow reactor (IPFR), which is able to rapidly heat the biomass particles to a maximum temperature of 1400°C at a heating rate of 10;4; °C/s, similar to the real conditions found in power plant furnaces. During each Char Oxidation test, the residues of biomass particles were collected and analyzed to determine the weight loss based on the ash tracer method. According to the experimental results, it can be concluded that Chars produced from a torrefied biomass are less reactive than the ones produced, under the same conditions, from its raw material. The apparent kinetics of the torrefied biomass and its parent material are determined by minimizing the difference between the modeled and the experimental results. The predicted weight loss during Char Oxidation, using the determined kinetics, agrees well with experimental results.
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comparison of devolatilization Char Oxidation and direct Oxidation of solid fuels at low heating rate
Energy & Fuels, 2006Co-Authors: Enrico Biagini, Leonardo TognottiAbstract:Fundamental information can be obtained by comparing the behavior of different classes of solid fuels (low- and high-volatile matter coals, biomass fuels, residues, wastes, and plastics) in oxidizing and inert thermogravimetric runs. Different mechanisms can be recognized and attributed to devolatilization, Char Oxidation, and direct Oxidation. The direct Oxidation of the fuel can be favored with respect to the traditional devolatilization/Char Oxidation scheme depending on the conditions used and the Characteristics of the fuel. Oxygen enhances the thermal devolatilization of the fuel and the reactivity of the produced Char. The reactivity of all fuels are quantified by defining Characteristic temperatures of each mechanism and providing kinetics for all steps. Also, the effect of the operating conditions are evaluated. Results obtained in this work represent a significant set of data on very different solid fuels, useful for modeling purposes, for practical system handling (grate furnaces or fixed-bed combustors, operating in conditions similar to this study), for safety of fuel stockpiles, and for investigations on ignition and Char reactivity.
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Comparison of Devolatilization/Char Oxidation and Direct Oxidation of Solid Fuels at Low Heating Rate
Energy & Fuels, 2006Co-Authors: Enrico Biagini, Leonardo TognottiAbstract:Fundamental information can be obtained by comparing the behavior of different classes of solid fuels (low- and high-volatile matter coals, biomass fuels, residues, wastes, and plastics) in oxidizing and inert thermogravimetric runs. Different mechanisms can be recognized and attributed to devolatilization, Char Oxidation, and direct Oxidation. The direct Oxidation of the fuel can be favored with respect to the traditional devolatilization/Char Oxidation scheme depending on the conditions used and the Characteristics of the fuel. Oxygen enhances the thermal devolatilization of the fuel and the reactivity of the produced Char. The reactivity of all fuels are quantified by defining Characteristic temperatures of each mechanism and providing kinetics for all steps. Also, the effect of the operating conditions are evaluated. Results obtained in this work represent a significant set of data on very different solid fuels, useful for modeling purposes, for practical system handling (grate furnaces or fixed-bed combustors, operating in conditions similar to this study), for safety of fuel stockpiles, and for investigations on ignition and Char reactivity.
Christopher R Shaddix - One of the best experts on this subject based on the ideXlab platform.
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numerical assessment of tognotti determination of co2 co production ratio during Char Oxidation
Combustion and Flame, 2013Co-Authors: Christopher R Shaddix, F. Holzleithner, Manfred Geier, Brian S HaynesAbstract:Abstract A review of the experimental investigations of the CO 2 /CO production ratio during the high temperature Oxidation of carbon reveals a wide variation in this critical parameter for determining the Char combustion temperature and burning rate. Of the studies that have been performed, the experiment used by Tognotti et al. [5] , with small, laser-heated, electrodynamically levitated Spherocarb particles in cool surroundings, appears to be the most promising for giving accurate results. Proper interpretation of the results from Tognotti’s study requires assumptions of kinetically controlled combustion behavior, negligible CO conversion either within the particle pores or in the boundary layer, and a uniform particle temperature. To evaluate whether the Tognotti data in fact fulfill these assumptions, we have employed a detailed model of porous particle combustion to simulate the Tognotti experiments. The model results indicate that particle temperatures were uniform and there was negligible Oxidation of CO either within the particle or in the particle boundary layer over the range of particle temperatures that was used to determine the Tognotti CO 2 /CO production ratio correlations. On the other hand, the model results show that O 2 diffusional resistance became important for temperatures greater than 1050 K in the Tognotti experiments. However, because of the low sensitivity of the observed CO 2 /CO production ratio to the local oxygen concentration, computational analysis also shows that the influence of this Zone II combustion behavior on the measured CO 2 /CO production ratio is quite minor. Therefore, it appears that the empirical correlation derived by Tognotti et al. [5] to describe the CO 2 /CO production ratio during high temperature Char Oxidation is credible, though its temperature range of empirical validation is limited to less than 1250 K.
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Numerical assessment of Tognotti determination of CO2/CO production ratio during Char Oxidation
Combustion and Flame, 2013Co-Authors: Christopher R Shaddix, F. Holzleithner, Manfred Geier, Brian S HaynesAbstract:Abstract A review of the experimental investigations of the CO 2 /CO production ratio during the high temperature Oxidation of carbon reveals a wide variation in this critical parameter for determining the Char combustion temperature and burning rate. Of the studies that have been performed, the experiment used by Tognotti et al. [5] , with small, laser-heated, electrodynamically levitated Spherocarb particles in cool surroundings, appears to be the most promising for giving accurate results. Proper interpretation of the results from Tognotti’s study requires assumptions of kinetically controlled combustion behavior, negligible CO conversion either within the particle pores or in the boundary layer, and a uniform particle temperature. To evaluate whether the Tognotti data in fact fulfill these assumptions, we have employed a detailed model of porous particle combustion to simulate the Tognotti experiments. The model results indicate that particle temperatures were uniform and there was negligible Oxidation of CO either within the particle or in the particle boundary layer over the range of particle temperatures that was used to determine the Tognotti CO 2 /CO production ratio correlations. On the other hand, the model results show that O 2 diffusional resistance became important for temperatures greater than 1050 K in the Tognotti experiments. However, because of the low sensitivity of the observed CO 2 /CO production ratio to the local oxygen concentration, computational analysis also shows that the influence of this Zone II combustion behavior on the measured CO 2 /CO production ratio is quite minor. Therefore, it appears that the empirical correlation derived by Tognotti et al. [5] to describe the CO 2 /CO production ratio during high temperature Char Oxidation is credible, though its temperature range of empirical validation is limited to less than 1250 K.
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A mechanistic Char Oxidation model consistent with observed CO2/CO production ratios
Proceedings of the Combustion Institute, 2013Co-Authors: Manfred Geier, Christopher R Shaddix, F. HolzleithnerAbstract:Reliable prediction of Char conversion, heat release, and particle temperature during heterogeneous Char Oxidation relies upon quantitative calculation of the CO2/CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more important in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of Char combustion have been calibrated against the temperature-dependence of the CO2/CO production ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO2/CO production ratio. A suitable 5-step heterogeneous reaction mechanism is developed and its rate parameters fit to match CO2/CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions Characteristic of pressurized, oxy-fuel, and conventional high-temperature Char combustion, for which essentially no experimental information on the CO2/CO production ratio is available. The results suggest that the CO2/CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO2/CO production ratio results, an analytical procedure for calculating the CO2/CO production ratio is presented. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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a mechanistic Char Oxidation model consistent with observed co2 co production ratios
Proceedings of the Combustion Institute, 2013Co-Authors: Manfred Geier, Christopher R Shaddix, F. HolzleithnerAbstract:Abstract Reliable prediction of Char conversion, heat release, and particle temperature during heterogeneous Char Oxidation relies upon quantitative calculation of the CO 2 /CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more important in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of Char combustion have been calibrated against the temperature-dependence of the CO 2 /CO production ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO 2 /CO production ratio. A suitable 5-step heterogeneous reaction mechanism is developed and its rate parameters fit to match CO 2 /CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions Characteristic of pressurized, oxy-fuel, and conventional high-temperature Char combustion, for which essentially no experimental information on the CO 2 /CO production ratio is available. The results suggest that the CO 2 /CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO 2 /CO production ratio results, an analytical procedure for calculating the CO 2 /CO production ratio is presented.
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A mechanistic Char Oxidation model consistent with observed CO2/CO production ratios
Proceedings of the Combustion Institute, 2012Co-Authors: Manfred Geier, Christopher R Shaddix, F. HolzleithnerAbstract:Abstract Reliable prediction of Char conversion, heat release, and particle temperature during heterogeneous Char Oxidation relies upon quantitative calculation of the CO 2 /CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more important in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of Char combustion have been calibrated against the temperature-dependence of the CO 2 /CO production ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO 2 /CO production ratio. A suitable 5-step heterogeneous reaction mechanism is developed and its rate parameters fit to match CO 2 /CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions Characteristic of pressurized, oxy-fuel, and conventional high-temperature Char combustion, for which essentially no experimental information on the CO 2 /CO production ratio is available. The results suggest that the CO 2 /CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO 2 /CO production ratio results, an analytical procedure for calculating the CO 2 /CO production ratio is presented.
Thomas H Fletcher - One of the best experts on this subject based on the ideXlab platform.
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The Use of Two Mixture Fractions to Treat Coal Combustion Products in Turbulent Pulverized-Coal Flames
Combustion Science and Technology, 2020Co-Authors: Daniel V. Flores, Thomas H FletcherAbstract:Abstract Previous coal combustion models using assumed-shape PDF's to treat turbulence-chemistry interactions have used only one progress variable to treat products from coal reactions. This assumes that the products of all coal reactions have the same composition. However, the composition of the combustion products of coal particles is known to vary with burnout, especially between devolatilization and Char Oxidation. In this work, two progress variables were implemented which distinguish between the products of devolatilization and those of Char Oxidation. This new approach requires as input the specified volatile content and elemental release during devolatilization. The values for these parameters were estimated based on elemental release data obtained in flat-flame burners. Predictions of the new and the old approaches for the major variables of the field were not appreciably different. However, NO pollutant predictions of the new method were, in general, better than those of the old method, particul...
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investigation of an iron based additive on coal pyrolysis and Char Oxidation at high heating rates
Fuel Processing Technology, 2011Co-Authors: John M Sowa, Thomas H FletcherAbstract:Abstract Iron-based catalysts have been shown to enhance coal pyrolysis and Char Oxidation at low to moderate temperatures and heating rates (
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A Two mixture fraction approach for modeling turbulent combustion of coal volatiles and Char Oxidation products
Coal science and technology, 2007Co-Authors: Daniel V. Flores, Thomas H FletcherAbstract:Publisher Summary This chapter discusses one mixture fraction that distinguishes between primary and secondary air streams and two progress variables that are used to treat mixing between oxidizers and pulverized-coal combustion products. The source terms of Char off-gases and volatiles to the gas phase are obtained from the Lagrangian treatments of particle trajectories and reactions. The Chemical Percolation Devolatilization (CPD) model is used for devolatilization rates and Char-Oxidation rates. The elemental composition of the volatiles is calculated by mass balance from the volatiles yield and the elemental composition of the Char. Predicted temperatures and major gas species concentrations between the one and two coal mixture fraction approaches are not very different. There is a significant need for detailed experimental data regarding the yield and composition of volatiles versus Char as a function of coal type and operating conditions. A better understanding of the conversion of Char nitrogen to NO as a function of coal type and operating conditions is also needed.
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modeling high pressure Char Oxidation using langmuir kinetics with an effectiveness factor
Proceedings of the Combustion Institute, 2000Co-Authors: Jianhui Hong, William C Hecker, Thomas H FletcherAbstract:The global nth order rate equation has been criticized for lack of theoretical basis and has been shown to be inadequate for modeling Char Oxidation rates as a function of total gas pressure. The simple Langmuir rate equation is believed to have more potential for modeling high pressure Char Oxidation. The intrinsic Langmuir rate equation is applied to graphite flake Oxidation data and agrees well with reaction rates at three temperatures over the entire range of oxygen pressure (1–64 atm). It also explains the change of reaction order with temperature. In this work, the intrinsic Langmuir rate equation is combined with (1) an effectiveness factor to account for pore diffusion effects and (2) a random pore structure model to calculate effective diffusivity. The resulting model is able to predict the reaction rates of large (ca. 8 mm) coal Char particles as a function of gas velocity, total pressure, oxygen partial pressure, oxygen mole fraction, initial particle size, and gas temperature. This approach is also able to correlate the particle burnouts of pulverized (70 lm) coal Char particles in a drop tube reactor as a function of total pressure, oxygen mole fraction, gas and wall temperatures, and residence time. The ability of the model to correlate data over wide range of temperature and pressure is promising.
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Char Oxidation conversion and reaction rate processes
1994Co-Authors: Lee K Smith, Douglas L Smoot, Thomas H Fletcher, Ronald J PugmireAbstract:The heterogeneous carbon Oxidation and Char gasification step is the second process to occur in the utilization of coal, and proceeds simultaneously or after devolatilization, depending on reaction conditions (Saito et al., 1991). The time required for the combustion of a Char particle can be several orders of magnitude larger than that for devolatilization, ranging from 30 ms to over an hour, and is often the rate-determining step in the overall combustion of pulverized fuels (Essenhigh, 1981; I. Smith, 1982; Smoot and Smith, 1985). The processes of Char Oxidation are no less complex than those of devolatilization. The chemical structure of the coal does not control the reaction processes to the same extent as devolatilization, but, due to the high temperatures generally associated with Char Oxidation, pore diffusion and external diffusion often play a pronounced role. Thus, the physical structure of coal, including pore structure, surface area, particle size, and inorganic content, is important in understanding and modeling Char Oxidation processes. Intrinsic reactivity of Char refers to the chemical reaction on the pore walls, after diffusion of gas through the pores inside the Char. When intrinsic reactivity is the ratecontrolling process, oxygen migrates toward the center of the Char particle, the particle size remains nearly constant during combustion, and the particle density decreases with conversion. However, if the reaction is fast, typically at high temperature, oxygen diffusion is the dominant process; the oxygen is consumed as it reaches the particle surface, and the density of the particle is near constant while the particle size decreases in a shrinking core mode.
Jun Li - One of the best experts on this subject based on the ideXlab platform.
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Characterization of high temperature rapid Char Oxidation of raw and torrefied biomass fuels
Fuel, 2015Co-Authors: Jun Li, Giorgio Bonvicini, Weihong Yang, Leonardo Tognotti, Enrico BiaginiAbstract:The promising properties of torrefied biomass provide a valid co-firing option for large percentage biomass utilization in existing coal-fired boilers. Torrefied biomass is expected to have a better combustion stability than raw biomass and similar to that of coal. The present work will Characterizes the Oxidation properties of torrefied biomass Char and compare with that of raw biomass Char. The studied two Chars are produced from raw and torrefied biomass in an Isothermal Plug Flow Reactor (IPFR) at high temperature and high heating rate, a sufficient residence time is applied for the completion of the high temperature devolatilization. Char Oxidation tests are carried out in the IPFR by varying temperature, oxygen concentration and residence time. The reactivity of two studied Chars are analyzed and compared with referenced biomass Char and coal Char, and the impact of torrefaction on Char reactivity is also discussed in this paper. Finally, the Char Oxidation kinetic parameters are determined using a parameter optimization method, and the obtained kinetics are examined by comparing the experimental and predicted mass conversions.
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Char Oxidation of torrefied biomass at high temperatures and high heating rates
Energy Procedia, 2014Co-Authors: Jun Li, Giorgio Bonvicini, Xiaolei Zhang, Weihong Yang, Leonardo TognottiAbstract:The Char Oxidation of a torrefied biomass and its parent material was carried out in an isothermal plug flow reactor (IPFR), which is able to rapidly heat the biomass particles to a maximum temperature of 1400°C at a heating rate of 10;4; °C/s, similar to the real conditions found in power plant furnaces. During each Char Oxidation test, the residues of biomass particles were collected and analyzed to determine the weight loss based on the ash tracer method. According to the experimental results, it can be concluded that Chars produced from a torrefied biomass are less reactive than the ones produced, under the same conditions, from its raw material. The apparent kinetics of the torrefied biomass and its parent material are determined by minimizing the difference between the modeled and the experimental results. The predicted weight loss during Char Oxidation, using the determined kinetics, agrees well with experimental results.
F. Holzleithner - One of the best experts on this subject based on the ideXlab platform.
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numerical assessment of tognotti determination of co2 co production ratio during Char Oxidation
Combustion and Flame, 2013Co-Authors: Christopher R Shaddix, F. Holzleithner, Manfred Geier, Brian S HaynesAbstract:Abstract A review of the experimental investigations of the CO 2 /CO production ratio during the high temperature Oxidation of carbon reveals a wide variation in this critical parameter for determining the Char combustion temperature and burning rate. Of the studies that have been performed, the experiment used by Tognotti et al. [5] , with small, laser-heated, electrodynamically levitated Spherocarb particles in cool surroundings, appears to be the most promising for giving accurate results. Proper interpretation of the results from Tognotti’s study requires assumptions of kinetically controlled combustion behavior, negligible CO conversion either within the particle pores or in the boundary layer, and a uniform particle temperature. To evaluate whether the Tognotti data in fact fulfill these assumptions, we have employed a detailed model of porous particle combustion to simulate the Tognotti experiments. The model results indicate that particle temperatures were uniform and there was negligible Oxidation of CO either within the particle or in the particle boundary layer over the range of particle temperatures that was used to determine the Tognotti CO 2 /CO production ratio correlations. On the other hand, the model results show that O 2 diffusional resistance became important for temperatures greater than 1050 K in the Tognotti experiments. However, because of the low sensitivity of the observed CO 2 /CO production ratio to the local oxygen concentration, computational analysis also shows that the influence of this Zone II combustion behavior on the measured CO 2 /CO production ratio is quite minor. Therefore, it appears that the empirical correlation derived by Tognotti et al. [5] to describe the CO 2 /CO production ratio during high temperature Char Oxidation is credible, though its temperature range of empirical validation is limited to less than 1250 K.
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Numerical assessment of Tognotti determination of CO2/CO production ratio during Char Oxidation
Combustion and Flame, 2013Co-Authors: Christopher R Shaddix, F. Holzleithner, Manfred Geier, Brian S HaynesAbstract:Abstract A review of the experimental investigations of the CO 2 /CO production ratio during the high temperature Oxidation of carbon reveals a wide variation in this critical parameter for determining the Char combustion temperature and burning rate. Of the studies that have been performed, the experiment used by Tognotti et al. [5] , with small, laser-heated, electrodynamically levitated Spherocarb particles in cool surroundings, appears to be the most promising for giving accurate results. Proper interpretation of the results from Tognotti’s study requires assumptions of kinetically controlled combustion behavior, negligible CO conversion either within the particle pores or in the boundary layer, and a uniform particle temperature. To evaluate whether the Tognotti data in fact fulfill these assumptions, we have employed a detailed model of porous particle combustion to simulate the Tognotti experiments. The model results indicate that particle temperatures were uniform and there was negligible Oxidation of CO either within the particle or in the particle boundary layer over the range of particle temperatures that was used to determine the Tognotti CO 2 /CO production ratio correlations. On the other hand, the model results show that O 2 diffusional resistance became important for temperatures greater than 1050 K in the Tognotti experiments. However, because of the low sensitivity of the observed CO 2 /CO production ratio to the local oxygen concentration, computational analysis also shows that the influence of this Zone II combustion behavior on the measured CO 2 /CO production ratio is quite minor. Therefore, it appears that the empirical correlation derived by Tognotti et al. [5] to describe the CO 2 /CO production ratio during high temperature Char Oxidation is credible, though its temperature range of empirical validation is limited to less than 1250 K.
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A mechanistic Char Oxidation model consistent with observed CO2/CO production ratios
Proceedings of the Combustion Institute, 2013Co-Authors: Manfred Geier, Christopher R Shaddix, F. HolzleithnerAbstract:Reliable prediction of Char conversion, heat release, and particle temperature during heterogeneous Char Oxidation relies upon quantitative calculation of the CO2/CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more important in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of Char combustion have been calibrated against the temperature-dependence of the CO2/CO production ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO2/CO production ratio. A suitable 5-step heterogeneous reaction mechanism is developed and its rate parameters fit to match CO2/CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions Characteristic of pressurized, oxy-fuel, and conventional high-temperature Char combustion, for which essentially no experimental information on the CO2/CO production ratio is available. The results suggest that the CO2/CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO2/CO production ratio results, an analytical procedure for calculating the CO2/CO production ratio is presented. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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a mechanistic Char Oxidation model consistent with observed co2 co production ratios
Proceedings of the Combustion Institute, 2013Co-Authors: Manfred Geier, Christopher R Shaddix, F. HolzleithnerAbstract:Abstract Reliable prediction of Char conversion, heat release, and particle temperature during heterogeneous Char Oxidation relies upon quantitative calculation of the CO 2 /CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more important in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of Char combustion have been calibrated against the temperature-dependence of the CO 2 /CO production ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO 2 /CO production ratio. A suitable 5-step heterogeneous reaction mechanism is developed and its rate parameters fit to match CO 2 /CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions Characteristic of pressurized, oxy-fuel, and conventional high-temperature Char combustion, for which essentially no experimental information on the CO 2 /CO production ratio is available. The results suggest that the CO 2 /CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO 2 /CO production ratio results, an analytical procedure for calculating the CO 2 /CO production ratio is presented.
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A mechanistic Char Oxidation model consistent with observed CO2/CO production ratios
Proceedings of the Combustion Institute, 2012Co-Authors: Manfred Geier, Christopher R Shaddix, F. HolzleithnerAbstract:Abstract Reliable prediction of Char conversion, heat release, and particle temperature during heterogeneous Char Oxidation relies upon quantitative calculation of the CO 2 /CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more important in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of Char combustion have been calibrated against the temperature-dependence of the CO 2 /CO production ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO 2 /CO production ratio. A suitable 5-step heterogeneous reaction mechanism is developed and its rate parameters fit to match CO 2 /CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions Characteristic of pressurized, oxy-fuel, and conventional high-temperature Char combustion, for which essentially no experimental information on the CO 2 /CO production ratio is available. The results suggest that the CO 2 /CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO 2 /CO production ratio results, an analytical procedure for calculating the CO 2 /CO production ratio is presented.