Hobbs, ML
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.
1993
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
Algebraic, Multi-Zoned Radiation Model for a Two-Zoned Zero-Dimensional Cylindrical Furnace
Hobbs, M.L. and Smith, P.J.
Fuel, 69, 1990. Funded by Utah Power & Light and Tennessee Valley Authority.
A simple two-zone zero-dimensional combustion model that estimates the influence of impurities in the fuel on the radiative energy transport has previously been developed based on an overall energy balance coupled with a multi-zoned radiation model. This paper presents the equations of the model, illustrates the method of calculating the radiative exchange areas for the two-zone system, and presents predictions for pulverized-coal and fluidized-bed combustion. The model predicts thermal performance as a function of coal input and furnace operational parameters, steam mass flow rates, and superheated steam temperatures leading to the high-pressure turbine. Two wall ash deposit parameters, thermal conductivity and maximum deposit thickness, have been determined by a sensitivity analysis to be critical to furnace performance. These parameters have been obtained experimentally by others. The predictions from the two-zone model have been compared with predictions from an earlier single-zone model. The general trends from both models were the same, although the two-zone model predictions were closer to expected values.
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.
1989
Algebraic, Multi-Zoned Radiation Model for a Two-Zoned Zero-Dimensional Cylindrical Furnace
Hobbs, M.L. and Smith, P.J.
Accepted for publication in Fuel, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates) and Utah Power & Light.
A simple two-zone zero-dimensional combustion model which estimates the influence of impurities in the fuel on the radiative energy transport has been developed based on an overall energy balance coupled with a multi-zoned radiation model. This paper presents the equations of the model, illustrates the method of calculating the radiative exchange areas for the two-zone system, and presents predictions for pulverized-coal and fluidized-bed combustion. The model predicts thermal performance as a function of coal input and furnace operational parameters, steam mass flow rates, and superheated steam temperatures leading to the high-pressure turbine. Two wall ash deposit parameters, thermal conductivity, and maximum deposit thickness, have been determined by a sensitivity analysis to be critical to furnace performance. Others have obtained these parameters experimentally. The predictions from the two-zone model have been compared to predictions from an earlier single-zone model. The general trends from both models were the same, although the two-zone model predictions were closer to expected values.
The foundation to describe coal-specific conversion behavior will be AFR's Functional Group (FG) and Devolatilization, Vaporization, and Cross linking (DVC) models, developed under previous and on-going METC sponsored programs. These models have demonstrated the capability to describe the time dependent evolution of individual gas species, and the amount and characteristics of tar and char. The combined FG-DVC model will be integrated with Brigham Young University's comprehensive two-dimensional reactor model, PCGC-2, which is currently the most widely used reactor simulation for combustion or gasification. The program includes: i) validation of the submodels by comparison with laboratory data obtained in this program, ii) extensive validation of the modified comprehensive code by comparison of predicted results with data from bench-scale and process scale investigations of gasification, mild gasification and combustion of coal or coal-derived products in heat engines, and iii) development of well documented user friendly software applicable to a "workstation" environment.
Success in this program will be a major step in improving the predictive capabilities for coal conversion processes including: demonstrated accuracy and reliability and a generalized "first principles" treatment of coals based on readily obtained composition data.
Hobbs, ML
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.
1993
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
Algebraic, Multi-Zoned Radiation Model for a Two-Zoned Zero-Dimensional Cylindrical Furnace
Hobbs, M.L. and Smith, P.J.
Fuel, 69, 1990. Funded by Utah Power & Light and Tennessee Valley Authority.
A simple two-zone zero-dimensional combustion model that estimates the influence of impurities in the fuel on the radiative energy transport has previously been developed based on an overall energy balance coupled with a multi-zoned radiation model. This paper presents the equations of the model, illustrates the method of calculating the radiative exchange areas for the two-zone system, and presents predictions for pulverized-coal and fluidized-bed combustion. The model predicts thermal performance as a function of coal input and furnace operational parameters, steam mass flow rates, and superheated steam temperatures leading to the high-pressure turbine. Two wall ash deposit parameters, thermal conductivity and maximum deposit thickness, have been determined by a sensitivity analysis to be critical to furnace performance. These parameters have been obtained experimentally by others. The predictions from the two-zone model have been compared with predictions from an earlier single-zone model. The general trends from both models were the same, although the two-zone model predictions were closer to expected values.
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.
1989
Algebraic, Multi-Zoned Radiation Model for a Two-Zoned Zero-Dimensional Cylindrical Furnace
Hobbs, M.L. and Smith, P.J.
Accepted for publication in Fuel, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates) and Utah Power & Light.
A simple two-zone zero-dimensional combustion model which estimates the influence of impurities in the fuel on the radiative energy transport has been developed based on an overall energy balance coupled with a multi-zoned radiation model. This paper presents the equations of the model, illustrates the method of calculating the radiative exchange areas for the two-zone system, and presents predictions for pulverized-coal and fluidized-bed combustion. The model predicts thermal performance as a function of coal input and furnace operational parameters, steam mass flow rates, and superheated steam temperatures leading to the high-pressure turbine. Two wall ash deposit parameters, thermal conductivity, and maximum deposit thickness, have been determined by a sensitivity analysis to be critical to furnace performance. Others have obtained these parameters experimentally. The predictions from the two-zone model have been compared to predictions from an earlier single-zone model. The general trends from both models were the same, although the two-zone model predictions were closer to expected values.
The foundation to describe coal-specific conversion behavior will be AFR's Functional Group (FG) and Devolatilization, Vaporization, and Cross linking (DVC) models, developed under previous and on-going METC sponsored programs. These models have demonstrated the capability to describe the time dependent evolution of individual gas species, and the amount and characteristics of tar and char. The combined FG-DVC model will be integrated with Brigham Young University's comprehensive two-dimensional reactor model, PCGC-2, which is currently the most widely used reactor simulation for combustion or gasification. The program includes: i) validation of the submodels by comparison with laboratory data obtained in this program, ii) extensive validation of the modified comprehensive code by comparison of predicted results with data from bench-scale and process scale investigations of gasification, mild gasification and combustion of coal or coal-derived products in heat engines, and iii) development of well documented user friendly software applicable to a "workstation" environment.
Success in this program will be a major step in improving the predictive capabilities for coal conversion processes including: demonstrated accuracy and reliability and a generalized "first principles" treatment of coals based on readily obtained composition data.