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Brian S Haynes - One of the best experts on this subject based on the ideXlab platform.
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the effect of bulk gas diffusivity on apparent pulverized coal Char combustion kinetics
Proceedings of the Combustion Institute, 2019Co-Authors: Christopher R Shaddix, Cristina Gonzalotirado, Ethan S Hecht, Brian S HaynesAbstract:Abstract Apparent Char kinetic rates are commonly used to predict pulverized coal Char burning rates. These kinetic rates quantify the Char burning rate based on the temperature of the Particle and the oxygen concentration at the external Particle surface, inherently neglecting the impact of variations in the internal diffusion rate and penetration of oxygen. To investigate the impact of bulk gas diffusivity on these phenomena during Zone II burning conditions, experimental measurements were performed of Char Particle combustion temperature and burnout for a subbituminous coal burning in an optical entrained flow reactor with helium and nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in cooler Char combustion temperatures than in equivalent N2 environments. Measured Char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given Char combustion temperature. To augment the experimental measurements, detailed Particle simulations of the experimental conditions were conducted with the SKIPPY code. These simulations also showed a 60% higher burning rate in the helium environments for a given Char Particle combustion temperature. To differentiate the effect of enhanced diffusion through the external boundary layer from the effect of enhanced diffusion through the Particle, additional SKIPPY simulations were conducted under selected conditions in N2 and He environments for which the temperature and concentrations of reactants (oxygen and steam) were identical on the external Char surface. Under these conditions, which yield matching apparent Char burning rates, the computed Char burning rate for He was 50% larger, demonstrating the potential for significant errors with the apparent kinetics approach. However, for specific application to oxy-fuel combustion in CO2 environments, these results suggest the error to be as low as 3% when applying apparent Char burning rates from nitrogen environments.
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effect of co2 and steam gasification reactions on the oxy combustion of pulverized coal Char
Combustion and Flame, 2012Co-Authors: Manfred Geier, Christopher R Shaddix, Ethan S Hecht, Alejandro Molina, Brian S HaynesAbstract:Abstract For oxy-combustion with flue gas recirculation, elevated levels of CO2 and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal Char on the Char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal Char, we computed the Char consumption Characteristics for a range of CO2 and H2O reaction rate coefficients for a 100 μm coal Char Particle reacting in environments of varying O2, H2O, and CO2 concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the Char Particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of Char oxidation and the radiant emission from burning Char Particles. However, the overall effect of the combined steam and CO2 gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on Char combustion in oxygen-enriched environments, due to the higher Char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O2 concentration) and as the Particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in Particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.
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effect of co2 gasification reaction on oxy combustion of pulverized coal Char
Proceedings of the Combustion Institute, 2011Co-Authors: Ethan S Hecht, Christopher R Shaddix, Alejandro Molina, Brian S HaynesAbstract:Abstract For oxy-combustion with flue gas recirculation, as is commonly employed, it is recognized that elevated CO 2 levels affect radiant transport, the heat capacity of the gas, and other gas transport properties. A topic of widespread speculation has concerned the effect of the CO 2 gasification reaction with coal Char on the Char burning rate. To give clarity to the likely impact of this reaction on the oxy-fuel combustion of pulverized coal Char, the Surface Kinetics in Porous Particles (SKIPPY) code was employed for a range of potential CO 2 reaction rates for a high-volatile bituminous coal Char Particle (130 μm diameter) reacting in several O 2 concentration environments. The effects of boundary layer chemistry are also examined in this analysis. Under oxygen-enriched conditions, boundary layer reactions (converting CO to CO 2 , with concomitant heat release) are shown to increase the Char Particle temperature and burning rate, while decreasing the O 2 concentration at the Particle surface. The CO 2 gasification reaction acts to reduce the Char Particle temperature (because of the reaction endothermicity) and thereby reduces the rate of Char oxidation. Interestingly, the presence of the CO 2 gasification reaction increases the Char conversion rate for combustion at low O 2 concentrations, but decreases Char conversion for combustion at high O 2 concentrations. These calculations give new insight into the complexity of the effects from the CO 2 gasification reaction and should help improve the understanding of experimentally measured oxy-fuel Char combustion and burnout trends in the literature.
Adel F Sarofim - One of the best experts on this subject based on the ideXlab platform.
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modeling of ignition and co oxidation in the boundary layer of a single Char Particle
Energy & Fuels, 1996Co-Authors: Shakti Goel, John P Longwell, Adel F SarofimAbstract:A model is developed for CO oxidation in the boundary layer of a single Char Particle. The model includes Char oxidation and a 56 reaction gas phase kinetic scheme which is coupled with the diffusive properties of the 12 species involved in the CO oxidation. The model is compared with the experimental data of Tognotti and co-workers. The temperature reached on ignition, the resulting CO 2 /CO ratio, and the effect of changing water vapor concentrations are well described. Studies on the effect of water concentration show that significant CO oxidation at low temperatures requires a high surrounding water concentration. The presence of water vapor or hydrogen is found to be necessary to obtain a high degree of CO oxidation at low temperatures; however, there is a minimum temperature below which significant CO oxidation in the boundary layer does not occur irrespective of how high the water concentration is. In addition, CO oxidation is negligible even at a surface temperature as high as 2500 K when water and hydrogen are absent. The model has also been applied to predict the effects of changing parameters. Catalytic acceleration of the rate of carbon oxidation, for example by the addition of calcium, leads to both a high Particle temperature overshoot and a significant increase in CO oxidation over the Particle surface.
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oxidation rates of a single Char Particle in an electrodynamic balance
Combustion and Flame, 1993Co-Authors: Adel F Sarofim, M Damore, Leonardo TognottiAbstract:Abstract The changes in reaction rate of a carbon Char with conversion in the temperature range 500–1200 K are followed by using an electrodynamic balance (EDB). This device allows, in a temperature range wider than in other apparatus, in situ measurements of mass, diameter, density, surface area, rate of reaction, and temperature for a single, suspended submillimeter Particle as the reaction takes place. A synthetic Char (Spherocarb) has been used because of its low ash and volatile matter contents and its spherical shape. The results on oxidation kinetics obtained in an EDB are compared with low-temperature reaction rate data, obtained by conventional thermogravimetric apparatus on Spherocarb, and with the data obtained in an entrained-flow reactor at temperatures typical of pulverized-fuels combustors. The correlation between all the data in the temperature range 500–2300 K is examined. EDB data can be combined with entrained-flow data to depict a sharp change in reaction regime from a chemical kinetic to an internal diffusion control. A simple Thiele analysis, considering different classes of pores, yields results that suggest pores in the 100–1000 nm range are responsible for diffusional constraints that cause the kinetic data to depart from the chemical regime.
Ethan S Hecht - One of the best experts on this subject based on the ideXlab platform.
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the effect of bulk gas diffusivity on apparent pulverized coal Char combustion kinetics
Proceedings of the Combustion Institute, 2019Co-Authors: Christopher R Shaddix, Cristina Gonzalotirado, Ethan S Hecht, Brian S HaynesAbstract:Abstract Apparent Char kinetic rates are commonly used to predict pulverized coal Char burning rates. These kinetic rates quantify the Char burning rate based on the temperature of the Particle and the oxygen concentration at the external Particle surface, inherently neglecting the impact of variations in the internal diffusion rate and penetration of oxygen. To investigate the impact of bulk gas diffusivity on these phenomena during Zone II burning conditions, experimental measurements were performed of Char Particle combustion temperature and burnout for a subbituminous coal burning in an optical entrained flow reactor with helium and nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in cooler Char combustion temperatures than in equivalent N2 environments. Measured Char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given Char combustion temperature. To augment the experimental measurements, detailed Particle simulations of the experimental conditions were conducted with the SKIPPY code. These simulations also showed a 60% higher burning rate in the helium environments for a given Char Particle combustion temperature. To differentiate the effect of enhanced diffusion through the external boundary layer from the effect of enhanced diffusion through the Particle, additional SKIPPY simulations were conducted under selected conditions in N2 and He environments for which the temperature and concentrations of reactants (oxygen and steam) were identical on the external Char surface. Under these conditions, which yield matching apparent Char burning rates, the computed Char burning rate for He was 50% larger, demonstrating the potential for significant errors with the apparent kinetics approach. However, for specific application to oxy-fuel combustion in CO2 environments, these results suggest the error to be as low as 3% when applying apparent Char burning rates from nitrogen environments.
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effect of co2 and steam gasification reactions on the oxy combustion of pulverized coal Char
Combustion and Flame, 2012Co-Authors: Manfred Geier, Christopher R Shaddix, Ethan S Hecht, Alejandro Molina, Brian S HaynesAbstract:Abstract For oxy-combustion with flue gas recirculation, elevated levels of CO2 and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal Char on the Char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal Char, we computed the Char consumption Characteristics for a range of CO2 and H2O reaction rate coefficients for a 100 μm coal Char Particle reacting in environments of varying O2, H2O, and CO2 concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the Char Particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of Char oxidation and the radiant emission from burning Char Particles. However, the overall effect of the combined steam and CO2 gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on Char combustion in oxygen-enriched environments, due to the higher Char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O2 concentration) and as the Particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in Particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.
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effect of co2 gasification reaction on oxy combustion of pulverized coal Char
Proceedings of the Combustion Institute, 2011Co-Authors: Ethan S Hecht, Christopher R Shaddix, Alejandro Molina, Brian S HaynesAbstract:Abstract For oxy-combustion with flue gas recirculation, as is commonly employed, it is recognized that elevated CO 2 levels affect radiant transport, the heat capacity of the gas, and other gas transport properties. A topic of widespread speculation has concerned the effect of the CO 2 gasification reaction with coal Char on the Char burning rate. To give clarity to the likely impact of this reaction on the oxy-fuel combustion of pulverized coal Char, the Surface Kinetics in Porous Particles (SKIPPY) code was employed for a range of potential CO 2 reaction rates for a high-volatile bituminous coal Char Particle (130 μm diameter) reacting in several O 2 concentration environments. The effects of boundary layer chemistry are also examined in this analysis. Under oxygen-enriched conditions, boundary layer reactions (converting CO to CO 2 , with concomitant heat release) are shown to increase the Char Particle temperature and burning rate, while decreasing the O 2 concentration at the Particle surface. The CO 2 gasification reaction acts to reduce the Char Particle temperature (because of the reaction endothermicity) and thereby reduces the rate of Char oxidation. Interestingly, the presence of the CO 2 gasification reaction increases the Char conversion rate for combustion at low O 2 concentrations, but decreases Char conversion for combustion at high O 2 concentrations. These calculations give new insight into the complexity of the effects from the CO 2 gasification reaction and should help improve the understanding of experimentally measured oxy-fuel Char combustion and burnout trends in the literature.
Yuan Chen - One of the best experts on this subject based on the ideXlab platform.
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modeling of a single Char Particle burning in oxygen enriched o2 n2 and o2 co2 environment with single film model
Fuel, 2016Co-Authors: Yuan Chen, Changdong ShengAbstract:Abstract The present work aims to clarify the influences of gas thermal properties and CO 2 gasification on a single Char Particle burning during O 2 /CO 2 pulverized coal combustion based on the comparison between O 2 /N 2 and O 2 /CO 2 combustion under O 2 -enriched conditions. A mathematical model was developed to describe the Char Particle burning in O 2 /CO 2 atmosphere through extending the model for O 2 /N 2 combustion by including CO 2 gasification. The model incorporated a single-film approach, assuming no homogenous reaction in Particle boundary layer. The gas properties, thermal conductivity and diffusivities, were modeled in detail to consider the transport mechanisms in the boundary layer and, particularly, the impact of gas dissociations due to high temperature O 2 -enriched combustion. The model was validated with literature data, indicating its applicability to represent Char burning behavior at least at O 2 molar fractions of 2 /N 2 and O 2 /CO 2 combustion. It is demonstrated that, when replacing N 2 by CO 2 , the change of gas thermal properties accounts for the most part of the decrease in Char burning temperature, while CO 2 gasification is responsible for 25–33% of the temperature decrease, corroborating the importance of Char-CO 2 gasification.
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modeling of a single Char Particle burning in oxygen enriched o2 n2 and o2 co2 environment
International Symposium on Coal Combustion, 2013Co-Authors: Yuan Chen, C D ShengAbstract:The combustion Characteristics of pulverized coal in O2/CO2 were reported considerably different from those in conventional air combustion. Properly describing the combustion behaviors of coal Particles at elevated O2 levels in both oxy-firing and air firing is essential for the design and simulation of the advance combustion systems. The objective of the present work is to describe the rate-limit step of coal conversion, i.e., the combustion process of Char Particles with modeling approach. In order to understand the reasons to cause the differences when replacing N2 with CO2 in the oxidant stream, a fundamental model has been developed to describe in detail the combustion of a single pulverized Char Particle at elevated O2 mole fractions in either N2 or CO2 balance gas. The model includes Char oxidation and a kinetic scheme which is coupled with the diffusive properties within the Particle and in the boundary layer. The main features of the model include: (1) The properties e.g., thermal conductivity and diffusivities, of the gaseous species in the gas mixture are modeled in detail; (2) The model considers the distributions of carbon reactivity and structure, temperature and gas concentrations within the Particle and their evolutions during the conversion process; (3) Thermal annealing and its impact on the evolution of carbon reactivity are considered; (4) The model describes the intra-Particle reactions and diffusion as well as the reactions, particularly CO oxidation, and diffusion outside the Particle. The model is validated with the comparisons of Particle surface temperature between the calculation results and the experimental data of Char Particle combustion reported in the literature. The oxygen concentration involved covers from normal concentration to up to 100% for both O2/CO2 and O2/N2 combustion. Model results represent that coal Particles burned at lower mean temperatures in O2/CO2 than in O2/N2 environments at analogous oxygen mole fractions. Additionally, the model has also been applied to numerically investigate the effects of Char property and combustion condition parameters on the combustion process so as to explore the applicability of model in prediction Char conversion process in practice.
Christopher R Shaddix - One of the best experts on this subject based on the ideXlab platform.
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the effect of bulk gas diffusivity on apparent pulverized coal Char combustion kinetics
Proceedings of the Combustion Institute, 2019Co-Authors: Christopher R Shaddix, Cristina Gonzalotirado, Ethan S Hecht, Brian S HaynesAbstract:Abstract Apparent Char kinetic rates are commonly used to predict pulverized coal Char burning rates. These kinetic rates quantify the Char burning rate based on the temperature of the Particle and the oxygen concentration at the external Particle surface, inherently neglecting the impact of variations in the internal diffusion rate and penetration of oxygen. To investigate the impact of bulk gas diffusivity on these phenomena during Zone II burning conditions, experimental measurements were performed of Char Particle combustion temperature and burnout for a subbituminous coal burning in an optical entrained flow reactor with helium and nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in cooler Char combustion temperatures than in equivalent N2 environments. Measured Char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given Char combustion temperature. To augment the experimental measurements, detailed Particle simulations of the experimental conditions were conducted with the SKIPPY code. These simulations also showed a 60% higher burning rate in the helium environments for a given Char Particle combustion temperature. To differentiate the effect of enhanced diffusion through the external boundary layer from the effect of enhanced diffusion through the Particle, additional SKIPPY simulations were conducted under selected conditions in N2 and He environments for which the temperature and concentrations of reactants (oxygen and steam) were identical on the external Char surface. Under these conditions, which yield matching apparent Char burning rates, the computed Char burning rate for He was 50% larger, demonstrating the potential for significant errors with the apparent kinetics approach. However, for specific application to oxy-fuel combustion in CO2 environments, these results suggest the error to be as low as 3% when applying apparent Char burning rates from nitrogen environments.
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effect of co2 gasification reaction on Char Particle combustion in oxy fuel conditions
Fuel, 2014Co-Authors: Sangmin Choi, Christopher R Shaddix, Manfred GeierAbstract:CO2 gasification of coal Char may play an important role in oxy-combustion environments with flue gas recirculation (FGR), but its effect on the overall reaction rate has not been clearly understood. To give clarity to the likely impact of CO2 gasification on the oxy-combustion of pulverized coal Chars, burnout simulations of coal Char Particles were carried out, adopting apparent Char reactivity and a single-film model that includes the Stefan flow effect on mass and energy transfer. Three oxygen concentrations (21%, 30%, and 5% O2), representing air, oxy-fuel, and oxygen-deficient combustion environments were simulated. A new experimental approach was used to directly measure the CO2 gasification rate of a subbituminous coal Char at high temperatures and atmospheric pressure. The measured gasification rate is somewhat higher than previous measurements. The simulation results show that the endothermic gasification reaction reduces the Char Particle temperature and thereby reduces the oxidation rates. However, due to the contribution of the direct gasification reaction on carbon consumption, the Char burnout time and the carbon consumption were improved. The gasification reaction has a greater influence on the Char burnout time and the relative carbon consumption in an oxygen-deficient environment and on the drop of Particle temperature in an oxygen-enriched environment (for a given gas temperature). In addition, the influence of the gasification reaction on Char combustion increases as the gas temperature increases and as the Particle size increases. Further, it was observed that the impact of the gasification reaction is dependent on the presumed kinetic rate, which highlights the importance of using reliable kinetic parameters in simulations. Based on the present results, it is important to include the gasification reaction by CO2 when simulating Char combustion in oxy-fuel combustion environments.
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Effect of CO2gasification reaction on Char Particle combustion in oxy-fuel conditions
Fuel, 2014Co-Authors: Daehee Kim, Sangmin Choi, Christopher R Shaddix, Manfred GeierAbstract:CO2gasification of coal Char may play an important role in oxy-combustion environments with flue gas recirculation (FGR), but its effect on the overall reaction rate has not been clearly understood. To give clarity to the likely impact of CO2gasification on the oxy-combustion of pulverized coal Chars, burnout simulations of coal Char Particles were carried out, adopting apparent Char reactivity and a single-film model that includes the Stefan flow effect on mass and energy transfer. Three oxygen concentrations (21%, 30%, and 5% O2), representing air, oxy-fuel, and oxygen-deficient combustion environments were simulated. A new experimental approach was used to directly measure the CO2gasification rate of a subbituminous coal Char at high temperatures and atmospheric pressure. The measured gasification rate is somewhat higher than previous measurements. The simulation results show that the endothermic gasification reaction reduces the Char Particle temperature and thereby reduces the oxidation rates. However, due to the contribution of the direct gasification reaction on carbon consumption, the Char burnout time and the carbon consumption were improved. The gasification reaction has a greater influence on the Char burnout time and the relative carbon consumption in an oxygen-deficient environment and on the drop of Particle temperature in an oxygen-enriched environment (for a given gas temperature). In addition, the influence of the gasification reaction on Char combustion increases as the gas temperature increases and as the Particle size increases. Further, it was observed that the impact of the gasification reaction is dependent on the presumed kinetic rate, which highlights the importance of using reliable kinetic parameters in simulations. Based on the present results, it is important to include the gasification reaction by CO2when simulating Char combustion in oxy-fuel combustion environments. © 2013 Published by Elsevier Ltd.
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effect of co2 and steam gasification reactions on the oxy combustion of pulverized coal Char
Combustion and Flame, 2012Co-Authors: Manfred Geier, Christopher R Shaddix, Ethan S Hecht, Alejandro Molina, Brian S HaynesAbstract:Abstract For oxy-combustion with flue gas recirculation, elevated levels of CO2 and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal Char on the Char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal Char, we computed the Char consumption Characteristics for a range of CO2 and H2O reaction rate coefficients for a 100 μm coal Char Particle reacting in environments of varying O2, H2O, and CO2 concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the Char Particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of Char oxidation and the radiant emission from burning Char Particles. However, the overall effect of the combined steam and CO2 gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on Char combustion in oxygen-enriched environments, due to the higher Char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O2 concentration) and as the Particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in Particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.
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effect of co2 gasification reaction on oxy combustion of pulverized coal Char
Proceedings of the Combustion Institute, 2011Co-Authors: Ethan S Hecht, Christopher R Shaddix, Alejandro Molina, Brian S HaynesAbstract:Abstract For oxy-combustion with flue gas recirculation, as is commonly employed, it is recognized that elevated CO 2 levels affect radiant transport, the heat capacity of the gas, and other gas transport properties. A topic of widespread speculation has concerned the effect of the CO 2 gasification reaction with coal Char on the Char burning rate. To give clarity to the likely impact of this reaction on the oxy-fuel combustion of pulverized coal Char, the Surface Kinetics in Porous Particles (SKIPPY) code was employed for a range of potential CO 2 reaction rates for a high-volatile bituminous coal Char Particle (130 μm diameter) reacting in several O 2 concentration environments. The effects of boundary layer chemistry are also examined in this analysis. Under oxygen-enriched conditions, boundary layer reactions (converting CO to CO 2 , with concomitant heat release) are shown to increase the Char Particle temperature and burning rate, while decreasing the O 2 concentration at the Particle surface. The CO 2 gasification reaction acts to reduce the Char Particle temperature (because of the reaction endothermicity) and thereby reduces the rate of Char oxidation. Interestingly, the presence of the CO 2 gasification reaction increases the Char conversion rate for combustion at low O 2 concentrations, but decreases Char conversion for combustion at high O 2 concentrations. These calculations give new insight into the complexity of the effects from the CO 2 gasification reaction and should help improve the understanding of experimentally measured oxy-fuel Char combustion and burnout trends in the literature.