Radulovic, PT
1995
An Improved Model for Fixed-Bed Coal Combustion and Gasification: Sensitivity Analysis and Applications
Ghani, M.U.; Radulovic, P.T. and Smoot, L.D.
Fuel, 74:1213-1226, 1995. Funded by ACERC and US Department of Energy/Morgantown Energy Technology Center.
Detailed sensitivity analysis and applications of an improved, comprehensive, one-dimensional model for combustion and gasification of coal in fixed beds, FBED-1, are presented. The effects of the devolatilization, oxidation and gasification submodels on the model predictions are discussed. The product gas compositions predicted by various options for gas-phase chemistry are shown. The effects of five model parameters and on operational variable on the predictions of the model are also presented. The sensitivity analysis presented is quantitative since the boundary conditions for both the feed coal and the feed gas streams are satisfied exactly. The utility of the model as a design and analysis tool is demonstrated by simulating two gasifiers: and METC medium-pressure gasifier, and a PyGas high -pressure staged gasifier. Submodels and areas that need further improvements are identified; among these are submodels for large-particle devolatilization, oxidation and gasification and a robust solution method suitable for stiff, highly non-linear problems. Additional features that should be implemented to develop a model for general industrial applications are also identified. These include provisions for additions and withdrawals of gases at multiple locations and options for different flow configurations.
1994
Combustion and Gasification of Coals in Fixed-Beds
Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
Prog. Energy Combustion Science, 19:505-586, 1993. Funded by US Department of Energy/Morgantown Energy Technology Center through Advanced Fuel Research Co. and ACERC.
Fixed-bed processes are commercially used for the combustion and conversion of coal for generation of power or production of gaseous or liquid products. Coal particle sizes in fixed-bed processes are typically in the mm to cm diameter range, being much larger than in most other coal processes. This review provides a broad treatment of the technology and the science related to fixed-bed systems. Commercialized and developmental fixed-bed combustion and gasification processes are explored, including countercurrent, concurrent, and crosscurrent configurations. Ongoing demonstrations in the U.S. Clean Coal Technology program are included. Physical and chemical rate processes occurring in fixed-bed combustion are summarized, with emphasis on coal devolatilization and char oxidation. Mechanisms, rate data and models of these steps are considered with emphasis on large particles. Heat and mass transfer processes, solid flows, bed voidage, tar production and gas phase reactions were also considered. Modeling of fixed-bed processes is also reviewed. Features and assumptions of a large number of one- and two-dimensional fixed-bed combustion and gasification models are summarized while the details of a recent model from this laboratory are presented and compared with data. Research needs are also discussed.
An Improved Model for Fixed-Bed Coal Combustion and Gasification
Radulovic, P.T.; Ghani, M.U. and Smoot, L.D.
Fuel, 1994 (in press). Funded by US Department of Energy/Morgantown Energy Technology Center and ACERC.
An improved one-dimensional model for countercurrent oxidation and gasification of coal in fixed beds has been developed. The model incorporates an advanced devolatilization submodel that can predict the evolution rates and the yields of individual gas species and tar. A split, back-and-forth, shooting methods is implemented to exactly satisfy the boundary conditions for both the feed coal and the feed gas streams. An option to switch between equilibrium and non-equilibrium gas phase composition has been added. The model predictions are compared with the experimental data for two coals; a Jetson bituminous coal and a Rosebud subbituminous coal. An illustrative simulation for an atmospheric, air-blown, dry ash, Wellman-Galusha gasifier, fired with the Jetson bituminous coal, is presented. Areas that need additional improvements are identified.
Fossil-Fuel Conversion - Measurement and Modeling
Solomon, P.R.; Serio, M.A.; Hamblen, D.G.; Smoot, L.D.; Brewster, B.S. and Radulovic, P.T.
Proceedings of the Coal-Fired Power Systems 94 - Advances in IGCC and PFBC Review Meeting, Morgantown, West Virginia, June 1994. Funded by US Department of Energy/Morgantown Energy Technology Center.
The main objective of this program is to understand the chemical and physical mechanisms in coal conversion processes and incorporate this technology for the purposes of development, evaluation in advanced coal conversion devices. To accomplish this objective, this program will: 1) provide critical data on the physical and chemical processes in fossil fuel gasifiers and combustors; 2) further develop a set of comprehensive codes; and 3) apply these codes to model various types of combustors and gasifiers (fixed-bed transport reactor, and fluidized-bed for coal and gas turbines for natural gas).
To expand the utilization of coal, it is necessary to reduce the technical and economic risks inherent in operating a feedstock which is highly variable and which sometimes exhibits unexpected and unwanted behavior. Reducing the risks can be achieved by establishing the technology to predict a coal's behavior in a process. This program is creating this predictive capability by merging technology developed at Advanced Fuel Research, Inc. (AFR) in predicting coal devolatilization behavior with technology developed at Brigham Young University (BYU) in comprehensive computer codes for modeling of entrained-bed and fixed-bed reactors and technology developed at the U.S. DOE-METC in comprehensive computer codes for fluidized-bed reactors. These advanced technologies will be further developed to provide: 1) a fixed-bed model capable of predicting combustion and gasification of large coal particles, 2) a transport reactor model, 3) a model for lean premixed combustion of natural gas, and 4) an improved fluidized-bed code with an advanced coal devolatilization chemistry submodel.
1993
Comprehensive Modeling
Brewster, B.S.; Hill, S.C.; Radulovic, P.T. and Smoot, L.D.
Chapter 8, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.
This chapter treats comprehensive modeling of combustion and gasification systems. Entrained, fluidized and fixed bed models are considered. Single and multidimensional models are reviewed. Developing comprehensive computer models to help design combustors and gasifiers for clean and efficient utilization of coal and other fossil fuels is a primary objective of ACERC. Such models provide not only a framework for effectively integrating combustion-related technology from a wide array of disciplines, but a vehicle for transferring this technology to industry. In order to be useful, these models must satisfy at least three criteria: First, the input and output must be easily accessible (user-friendly graphics must play a role here). Second, the computer algorithms must be robust and computationally efficient. And third, the models must be thoroughly evaluated to demonstrate applicability to industrial processes and to justify confidence in their predictions. Developing and implementing user-friendly, robust, efficient, applicable, accurate models requires significant, on-going effort that is reviewed herein.
Coal Processes and Technologies
Radulovic, P.T. and Smoot, L.D.
Chapter 1, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.
This chapter discusses current coal combustion and gasification processes and technologies, with emphasis on clean and efficient use. Entrained, fluidized and fixed beds together with MHD generation and fuel cell cycles are treated. Coal is the world's most abundant fuel. Most of the coal presently being consumed is by direct combustion of finely pulverized coal in large-scale utility furnaces for generation of electric power, and this is likely to remain the way through the end of this century. However, many other processes for the conversion of coal into other products or for the direct combustion of coal are being developed and demonstrated, including various coal combustion and gasification processes. Several other processes and technologies such as underground coal gasification, magnetohydrodynamic generators, and fuel cells are also being developed, as discussed herein. Increasing the use of coal presents many technical problems, particularly in protecting environment while maintaining or increasing efficiency. In order to solve these problems and increase the use of coal, the USA and many other countries in the world are supporting research and development of clean coal technologies that are summarized in this chapter.
Modeling of Coal Conversion Processes in Fixed Beds
Ghani, M.U.; Radulovic, P.T. and Smoot, L.D.
American Chemical Society, Division of Fuel Chemistry, 38: 1358-1369, 1993. Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC.
An advanced, one-dimensional fixed-bed coal gasification and combustion model is presented. The model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on functional groups and depolymerization, vaporization and cross-linking, oxidation and gasification of char, and partial equilibrium in the gas phase. The model is described by 191 highly non-linear, coupled, first order differential equations. Due to the countercurrent nature of the gas and solids flow the system of equations constitutes a split-boundary value problem that is solved by converting it to an initial value problem. This paper presents a split back-and-forth shooting technique that exactly satisfies conditions at both the upper and the lower boundary and provides significant improvements in the predictions. Comparisons of the predicted and experimental results for an atmospheric, air-blown Wellman-Galusha gasifier fired with Jetson bituminous coal are presented.
User's Manual for FBED-1: Fixed Bed Coal Combustion and Gasification Model with a Generalized Coal Devolatilization Submodel (FG-DVC)
Ghani, M.U.; Hobbs, M.L.; Radulovic, P.T.; Smoot, L.D.; Hamblen, D.G. and Zho, Y.
US Department of Energy/Morgantown Energy Technology Center/Advanced Fuel Research/Brigham Young University Final Contract Report, Vol. III, 1993. Funded by US Department of Energy and Morgantown Energy Technology Center.
A generalized, one-dimensional, heterogeneous, steady state, fixed-bed model for gasification and combustion of coal is presented. The model, referred to as FBED-1, is a design and analysis tool that can be used to simulate a variety of fixed or moving bed gasification, combustion, and devolatilization processes. The model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on chemical functional group composition, depolymerization, vaporization and crosslinking, oxidation and gasification of char, and partial equilibrium in the gas phase. The conservation equations and boundary conditions are formulated for gas and solid overall continuity, gas and solid energy equations, and gas and solid species or elemental continuity equations. Plug flow is assumed in both the solid and the gas phase with variable axial velocities. Gas phase pressure drop is calculated with the Ergun equation for packed beds. Large coal particle devolatilization is allowed to occur simultaneously with char oxidation and gasification. A generalized, coal devolatilization submodel, FG-DFC, is an important part of the model. Shell progressive or ash segregation, shrinking core char submodel describes oxidation and gasification. Turbulence is not treated formally in the slowly moving bed with low gas velocity, but is included implicitly through model correlations such as the effective heat transfer coefficient. A split, back-and-forth iteration and a Livermore solver for ordinary differential equations, LSODE, are used to solve a highly non-linear, stiff system of differential governing equations. Model formulation and solution method are presented, along with user and implementation guides and a sample problem.
1992
Modeling Fixed-Bed Coal Gasifiers
Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
AIChE Journal, 38(5):681-702, 1992. Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC.
A one-dimensional model of countercurrent fixed-bed coal gasification has been developed, and results have been compared to experimental data from commercial-scale gasifiers. The steady-state model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on chemical functional group composition, oxidation and gasification of char, and partial equilibrium in the gas phase. Generalized treatment of gas-phase chemistry and accounting for variable bed void fraction were necessary to predict realistic axial temperature and pressure profiles in an atmospheric fixed-bed gasifier. Model evaluation includes sensitivity of axial temperature profiles to model options, model parameters and operational parameters. Model predictions agree reasonably well with experimental temperature and pressure profile data for gasification of eight coal types ranging from lignite to bituminous. The relative importance of char oxidation resistances to bulk film diffusion, ash diffusion, and chemical reaction is identified.
Prediction of Effluent Compositions for Fixed-Bed Coal Gasifiers
Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
Fuel, 71(10):1177-1194, 1992. Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC
1990
Fixed-Bed Coal Gasification Modeling
Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
Twenty-third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1990. Funded by Morgantown Energy Technology Center through Advanced Fuel Research Co.
A one-dimensional model of countercurrent, fixed-bed gasification has been developed and predictions have been compared to experimental data obtained from a large-scale gasifier. The study-state model considers separate gas and solid temperatures, partial equilibrium in the gas phase, variable bed void fraction, coal devolatilization based on chemical functional group composition, oxidation and gasification of residual char with an ash layer, and axially variable solid and gas flow rates. Predictions are compared to experimental data from an atmospheric, dry-ash Wellman-Galusha gasifier for carbon conversion, effluent gas composition and temperatures, and axial profiles of temperature and pressure for a high volatile bituminous coal. The relative importance of the char oxidation resistances, bulk film diffusion, ash diffusion and surface reaction, are identified. For the cases examined, chemical resistance dominates in the cool regions at the bottom and top of the reactor while ash diffusion resistance competes with chemical resistance through most of the reactor. The importance of adequate treatment of devolatilization, gas phase chemistry, and variable bed void fraction is identified.
An accurate initial estimate of the effluent composition and temperature from a two-zone, partial equilibrium submodel was essential for efficient solution of this highly nonlinear fix-bed model. This initial estimate considers devolatilization, partial equilibrium of volatile gases, treatment of a large number of gas phase species, and tar production with potential for recirculation of effluent products. It has been shown that the submodel is adequate by itself for reliable predictions of effluent gas compositions. Effluent gas estimates from the submodel compared favorably to measured effluent temperatures and compositions from a high-pressure, dry-ash Lurgi gasifier in Westfield, Scotland for four American coals.
The importance of treating various chemical and physical processes in fixed-bed gasifiers with sufficient detail has been addressed with emphasis on coal devolatilization, char oxidation, gas phase chemistry, and bed void fraction. Calculations have shown that devolatilization in fixed-bed reactors is not an instantaneous process but is an intimate part of the overall fixed-bed process. Similarly, oxidation and gasification do not occur in separate zones, but simultaneously in certain regions of the reactor bed. Competition between endothermic gasification reactions and exothermic oxidation is evident in broad predicted and measured temperature peaks. Detailed gas phase chemistry was necessary to predict the features of temperature and concentration profiles. Variable bed void fraction was also necessary to accurately predict pressure drop, varying bed velocity, and temperature and concentration profiles.
Radulovic, PT
1995
An Improved Model for Fixed-Bed Coal Combustion and Gasification: Sensitivity Analysis and Applications
Ghani, M.U.; Radulovic, P.T. and Smoot, L.D.
Fuel, 74:1213-1226, 1995. Funded by ACERC and US Department of Energy/Morgantown Energy Technology Center.
Detailed sensitivity analysis and applications of an improved, comprehensive, one-dimensional model for combustion and gasification of coal in fixed beds, FBED-1, are presented. The effects of the devolatilization, oxidation and gasification submodels on the model predictions are discussed. The product gas compositions predicted by various options for gas-phase chemistry are shown. The effects of five model parameters and on operational variable on the predictions of the model are also presented. The sensitivity analysis presented is quantitative since the boundary conditions for both the feed coal and the feed gas streams are satisfied exactly. The utility of the model as a design and analysis tool is demonstrated by simulating two gasifiers: and METC medium-pressure gasifier, and a PyGas high -pressure staged gasifier. Submodels and areas that need further improvements are identified; among these are submodels for large-particle devolatilization, oxidation and gasification and a robust solution method suitable for stiff, highly non-linear problems. Additional features that should be implemented to develop a model for general industrial applications are also identified. These include provisions for additions and withdrawals of gases at multiple locations and options for different flow configurations.
1994
Combustion and Gasification of Coals in Fixed-Beds
Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
Prog. Energy Combustion Science, 19:505-586, 1993. Funded by US Department of Energy/Morgantown Energy Technology Center through Advanced Fuel Research Co. and ACERC.
Fixed-bed processes are commercially used for the combustion and conversion of coal for generation of power or production of gaseous or liquid products. Coal particle sizes in fixed-bed processes are typically in the mm to cm diameter range, being much larger than in most other coal processes. This review provides a broad treatment of the technology and the science related to fixed-bed systems. Commercialized and developmental fixed-bed combustion and gasification processes are explored, including countercurrent, concurrent, and crosscurrent configurations. Ongoing demonstrations in the U.S. Clean Coal Technology program are included. Physical and chemical rate processes occurring in fixed-bed combustion are summarized, with emphasis on coal devolatilization and char oxidation. Mechanisms, rate data and models of these steps are considered with emphasis on large particles. Heat and mass transfer processes, solid flows, bed voidage, tar production and gas phase reactions were also considered. Modeling of fixed-bed processes is also reviewed. Features and assumptions of a large number of one- and two-dimensional fixed-bed combustion and gasification models are summarized while the details of a recent model from this laboratory are presented and compared with data. Research needs are also discussed.
An Improved Model for Fixed-Bed Coal Combustion and Gasification
Radulovic, P.T.; Ghani, M.U. and Smoot, L.D.
Fuel, 1994 (in press). Funded by US Department of Energy/Morgantown Energy Technology Center and ACERC.
An improved one-dimensional model for countercurrent oxidation and gasification of coal in fixed beds has been developed. The model incorporates an advanced devolatilization submodel that can predict the evolution rates and the yields of individual gas species and tar. A split, back-and-forth, shooting methods is implemented to exactly satisfy the boundary conditions for both the feed coal and the feed gas streams. An option to switch between equilibrium and non-equilibrium gas phase composition has been added. The model predictions are compared with the experimental data for two coals; a Jetson bituminous coal and a Rosebud subbituminous coal. An illustrative simulation for an atmospheric, air-blown, dry ash, Wellman-Galusha gasifier, fired with the Jetson bituminous coal, is presented. Areas that need additional improvements are identified.
Fossil-Fuel Conversion - Measurement and Modeling
Solomon, P.R.; Serio, M.A.; Hamblen, D.G.; Smoot, L.D.; Brewster, B.S. and Radulovic, P.T.
Proceedings of the Coal-Fired Power Systems 94 - Advances in IGCC and PFBC Review Meeting, Morgantown, West Virginia, June 1994. Funded by US Department of Energy/Morgantown Energy Technology Center.
The main objective of this program is to understand the chemical and physical mechanisms in coal conversion processes and incorporate this technology for the purposes of development, evaluation in advanced coal conversion devices. To accomplish this objective, this program will: 1) provide critical data on the physical and chemical processes in fossil fuel gasifiers and combustors; 2) further develop a set of comprehensive codes; and 3) apply these codes to model various types of combustors and gasifiers (fixed-bed transport reactor, and fluidized-bed for coal and gas turbines for natural gas).
To expand the utilization of coal, it is necessary to reduce the technical and economic risks inherent in operating a feedstock which is highly variable and which sometimes exhibits unexpected and unwanted behavior. Reducing the risks can be achieved by establishing the technology to predict a coal's behavior in a process. This program is creating this predictive capability by merging technology developed at Advanced Fuel Research, Inc. (AFR) in predicting coal devolatilization behavior with technology developed at Brigham Young University (BYU) in comprehensive computer codes for modeling of entrained-bed and fixed-bed reactors and technology developed at the U.S. DOE-METC in comprehensive computer codes for fluidized-bed reactors. These advanced technologies will be further developed to provide: 1) a fixed-bed model capable of predicting combustion and gasification of large coal particles, 2) a transport reactor model, 3) a model for lean premixed combustion of natural gas, and 4) an improved fluidized-bed code with an advanced coal devolatilization chemistry submodel.
1993
Comprehensive Modeling
Brewster, B.S.; Hill, S.C.; Radulovic, P.T. and Smoot, L.D.
Chapter 8, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.
This chapter treats comprehensive modeling of combustion and gasification systems. Entrained, fluidized and fixed bed models are considered. Single and multidimensional models are reviewed. Developing comprehensive computer models to help design combustors and gasifiers for clean and efficient utilization of coal and other fossil fuels is a primary objective of ACERC. Such models provide not only a framework for effectively integrating combustion-related technology from a wide array of disciplines, but a vehicle for transferring this technology to industry. In order to be useful, these models must satisfy at least three criteria: First, the input and output must be easily accessible (user-friendly graphics must play a role here). Second, the computer algorithms must be robust and computationally efficient. And third, the models must be thoroughly evaluated to demonstrate applicability to industrial processes and to justify confidence in their predictions. Developing and implementing user-friendly, robust, efficient, applicable, accurate models requires significant, on-going effort that is reviewed herein.
Coal Processes and Technologies
Radulovic, P.T. and Smoot, L.D.
Chapter 1, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.
This chapter discusses current coal combustion and gasification processes and technologies, with emphasis on clean and efficient use. Entrained, fluidized and fixed beds together with MHD generation and fuel cell cycles are treated. Coal is the world's most abundant fuel. Most of the coal presently being consumed is by direct combustion of finely pulverized coal in large-scale utility furnaces for generation of electric power, and this is likely to remain the way through the end of this century. However, many other processes for the conversion of coal into other products or for the direct combustion of coal are being developed and demonstrated, including various coal combustion and gasification processes. Several other processes and technologies such as underground coal gasification, magnetohydrodynamic generators, and fuel cells are also being developed, as discussed herein. Increasing the use of coal presents many technical problems, particularly in protecting environment while maintaining or increasing efficiency. In order to solve these problems and increase the use of coal, the USA and many other countries in the world are supporting research and development of clean coal technologies that are summarized in this chapter.
Modeling of Coal Conversion Processes in Fixed Beds
Ghani, M.U.; Radulovic, P.T. and Smoot, L.D.
American Chemical Society, Division of Fuel Chemistry, 38: 1358-1369, 1993. Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC.
An advanced, one-dimensional fixed-bed coal gasification and combustion model is presented. The model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on functional groups and depolymerization, vaporization and cross-linking, oxidation and gasification of char, and partial equilibrium in the gas phase. The model is described by 191 highly non-linear, coupled, first order differential equations. Due to the countercurrent nature of the gas and solids flow the system of equations constitutes a split-boundary value problem that is solved by converting it to an initial value problem. This paper presents a split back-and-forth shooting technique that exactly satisfies conditions at both the upper and the lower boundary and provides significant improvements in the predictions. Comparisons of the predicted and experimental results for an atmospheric, air-blown Wellman-Galusha gasifier fired with Jetson bituminous coal are presented.
User's Manual for FBED-1: Fixed Bed Coal Combustion and Gasification Model with a Generalized Coal Devolatilization Submodel (FG-DVC)
Ghani, M.U.; Hobbs, M.L.; Radulovic, P.T.; Smoot, L.D.; Hamblen, D.G. and Zho, Y.
US Department of Energy/Morgantown Energy Technology Center/Advanced Fuel Research/Brigham Young University Final Contract Report, Vol. III, 1993. Funded by US Department of Energy and Morgantown Energy Technology Center.
A generalized, one-dimensional, heterogeneous, steady state, fixed-bed model for gasification and combustion of coal is presented. The model, referred to as FBED-1, is a design and analysis tool that can be used to simulate a variety of fixed or moving bed gasification, combustion, and devolatilization processes. The model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on chemical functional group composition, depolymerization, vaporization and crosslinking, oxidation and gasification of char, and partial equilibrium in the gas phase. The conservation equations and boundary conditions are formulated for gas and solid overall continuity, gas and solid energy equations, and gas and solid species or elemental continuity equations. Plug flow is assumed in both the solid and the gas phase with variable axial velocities. Gas phase pressure drop is calculated with the Ergun equation for packed beds. Large coal particle devolatilization is allowed to occur simultaneously with char oxidation and gasification. A generalized, coal devolatilization submodel, FG-DFC, is an important part of the model. Shell progressive or ash segregation, shrinking core char submodel describes oxidation and gasification. Turbulence is not treated formally in the slowly moving bed with low gas velocity, but is included implicitly through model correlations such as the effective heat transfer coefficient. A split, back-and-forth iteration and a Livermore solver for ordinary differential equations, LSODE, are used to solve a highly non-linear, stiff system of differential governing equations. Model formulation and solution method are presented, along with user and implementation guides and a sample problem.
1992
Modeling Fixed-Bed Coal Gasifiers
Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
AIChE Journal, 38(5):681-702, 1992. Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC.
A one-dimensional model of countercurrent fixed-bed coal gasification has been developed, and results have been compared to experimental data from commercial-scale gasifiers. The steady-state model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on chemical functional group composition, oxidation and gasification of char, and partial equilibrium in the gas phase. Generalized treatment of gas-phase chemistry and accounting for variable bed void fraction were necessary to predict realistic axial temperature and pressure profiles in an atmospheric fixed-bed gasifier. Model evaluation includes sensitivity of axial temperature profiles to model options, model parameters and operational parameters. Model predictions agree reasonably well with experimental temperature and pressure profile data for gasification of eight coal types ranging from lignite to bituminous. The relative importance of char oxidation resistances to bulk film diffusion, ash diffusion, and chemical reaction is identified.
Prediction of Effluent Compositions for Fixed-Bed Coal Gasifiers
Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
Fuel, 71(10):1177-1194, 1992. Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC
1990
Fixed-Bed Coal Gasification Modeling
Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
Twenty-third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1990. Funded by Morgantown Energy Technology Center through Advanced Fuel Research Co.
A one-dimensional model of countercurrent, fixed-bed gasification has been developed and predictions have been compared to experimental data obtained from a large-scale gasifier. The study-state model considers separate gas and solid temperatures, partial equilibrium in the gas phase, variable bed void fraction, coal devolatilization based on chemical functional group composition, oxidation and gasification of residual char with an ash layer, and axially variable solid and gas flow rates. Predictions are compared to experimental data from an atmospheric, dry-ash Wellman-Galusha gasifier for carbon conversion, effluent gas composition and temperatures, and axial profiles of temperature and pressure for a high volatile bituminous coal. The relative importance of the char oxidation resistances, bulk film diffusion, ash diffusion and surface reaction, are identified. For the cases examined, chemical resistance dominates in the cool regions at the bottom and top of the reactor while ash diffusion resistance competes with chemical resistance through most of the reactor. The importance of adequate treatment of devolatilization, gas phase chemistry, and variable bed void fraction is identified.
An accurate initial estimate of the effluent composition and temperature from a two-zone, partial equilibrium submodel was essential for efficient solution of this highly nonlinear fix-bed model. This initial estimate considers devolatilization, partial equilibrium of volatile gases, treatment of a large number of gas phase species, and tar production with potential for recirculation of effluent products. It has been shown that the submodel is adequate by itself for reliable predictions of effluent gas compositions. Effluent gas estimates from the submodel compared favorably to measured effluent temperatures and compositions from a high-pressure, dry-ash Lurgi gasifier in Westfield, Scotland for four American coals.
The importance of treating various chemical and physical processes in fixed-bed gasifiers with sufficient detail has been addressed with emphasis on coal devolatilization, char oxidation, gas phase chemistry, and bed void fraction. Calculations have shown that devolatilization in fixed-bed reactors is not an instantaneous process but is an intimate part of the overall fixed-bed process. Similarly, oxidation and gasification do not occur in separate zones, but simultaneously in certain regions of the reactor bed. Competition between endothermic gasification reactions and exothermic oxidation is evident in broad predicted and measured temperature peaks. Detailed gas phase chemistry was necessary to predict the features of temperature and concentration profiles. Variable bed void fraction was also necessary to accurately predict pressure drop, varying bed velocity, and temperature and concentration profiles.