Baxter, LL
2000
Fossil Fuel Power Stations-Coal Utilization
Smoot, L.D. and Baxter, L.L.
Chapter in the Encyclopedia of Physical Sciences and Technology, John R. Wiley and Sons, New York (in press), 2000.
Contact: Smoot
1996
Nitrogen Release During Coal Combustion
Baxter, L.L.; Reginald, E.M.; Fletcher, T.H. and Hurt, R.H.
Energy & Fuels, 10:188-196. Funded by US Department of Energy/Sandia National Laboratories.
Experiments in entrained flow reactors at combustion temperatures are performed to resolve the rank dependence of nitrogen release on an elemental basis for a suite of 15 U.S. coals ranging from lignite to low-volatile bituminous. Data were obtained as a function of particle conversion, with overall mass loss up to 99% on a dry, ash-free basis. Nitrogen release rates are presented relative to both carbon loss and overall mass loss. During devolatilization, fractional nitrogen release from low-rank coals is much slower than fractional mass release and noticeably slower than fractional carbon release. As coal rank increases, fractional nitrogen release rate relative to that of carbon and mass increases, with fractional nitrogen release rates exceeding fractional mass and fractional carbon release rates during devolatilization for high-rank (low-volatile bituminous) coals. At the onset of combustion, nitrogen loss rates increase significantly. For all coals investigated, fractional nitrogen loss rates relative to those of mass and carbon pass through a maximum during the earliest stages of oxidation. The mechanism for generating this maximum is postulated to involve nascent thermal rupture of nitrogen-containing compounds and possible preferential oxidation of nitrogen sites. During later stages of oxidation, the fractional loss rate of nitrogen approaches that of carbon for all coals. Changes in the relative release rates of nitrogen compared to those of both overall mass and carbon during all stages of combustion are attributed to a combination of the chemical structure of coals, temperature histories during combustion, and char chemistry.
1994
Radiative Heat Transfer in Pulverized-Coal Fired Boilers - Development of the Absorptive/Reflective Character of Initial Ash Deposit
Richards, G.H.; Harb, J.N.; Baxter, L.L.; Bhattacharya, S.; Gupta, R.P. and Wall, T.F.
25th Symposium (International on Combustion, 1994 (in press). (Proceedings of the 25th Symposium (International) on Combustion, Irvine, CA, August 1994). Funded by Australian Research Council, US Department of Energy (Pittsburgh Energy Technology Center) and ACERC.
Emission Fourier transform infrared (FTIR) spectroscopy data provide in situ,. Time-resolved, spectral emissivity measurements for ash deposits generated from two U.S. Powder River Basin coals. The first 3 h of deposit growth on a tube in a cross flow in a pilot-scale furnace detail the development of surface emissivity with time. Measured emissivities vary significantly with wavelength, indicating the influence of the physical properties and chemical composition of the deposit. At long wavelengths (>7 µm), emissions features exhibit characteristics of silica, sulfates, and silicates. The spectral emissivity measured in this region approaches a steady value due to an increase in deposit thickness and the size of particles in the deposit. In contrast, deposits are not opaque at shorter wavelengths where the measured emissivity is influenced by the properties of the underlying metal surface. Theoretical predictions of the emissivity of a particulate layer were performed, and results are compared to the measured values. The theory adequately predicts the general features of spectral variation of the emissivity. The predicted trends in emissivity with particle size and deposit composition are also consistent with experimental observations. Total (Planck-weighted) emissivities are calculated from the measured spectral values for the deposits at the tube temperatures. They increase with time from the clean tube value (0.2-0.3) to values typical of deposits formed from western U.S. coals (0.45-0.55). Calculated total absorptivities are found to be lower than the corresponding emissivities.
1993
In Situ, Real-Time Characterization of Coal Ash Deposits Using Fourier Transform Infrared Emission Spectroscopy
Baxter, L.L.; Richards, G.H.; Ottesen, D.K. and Harb, J.N.
Energy & Fuels, 7 (6):755-760, 1993. (Also presented at the Annual Advanced Combustion Engineering Research Center Conference, Park City, UT, March 1993). Funded by ACERC.
In situ Fourier transform infrared (FTIR) emission spectroscopy is used to identify the presence of silica, sulfates, and silicates as a function of time in coal ash deposits generated in Sandia's multifuel combustor, a pilot-scale reactor. Ash deposits are formed on a cylindrical tube in cross flow under experimental conditions that correspond to convection pass (fouling) regions of a commercial coal-fired boiler. The gas temperature, gas composition, particle loading, and extent of particle reaction in the combustor are typical of commercial boiler operation. The major classes of inorganic species deposited on the tube, including silicates and sulfates, are identified using the FTIR emission spectroscopy technique. Post mortem X-ray diffraction and conventional infrared absorption and reflectance analyses on the same deposits are used to corroborate the in situ FTIR emission data. The deposit composition from a western coal changes significantly as a function of both deposition time and combustion conditions. The observed changes include formation of sulfates and silicates. Such changes have implications for deposit properties such as tenacity and strength; the FTIR emission diagnostic shows promise as a method for monitoring such changes in practical systems.
Ash Deposits, Coal Blends and the Thermal Performance of Furnaces
Wall, T.F.; Baxter, L.L. and Harb, J.N.
Proceedings of the Engineering Foundation Conference on Coal Blending and Switching of Western Low-Sulfur Coals, Snowbird, UT, September 1993. Funded by Australian Research Council, US Department of Energy, Pittsburgh Energy Technology Center and ACERC.
The character of fireside ash deposits depends on the processes by which deposits are formed and subsequent reactions within the deposit and with furnace gases. The properties influencing furnace heat transfer, absorptivity for radiative transfer and thermal conductivity for conductive transfer are shown from many measurements to depend on this character. Illustrative trends in these properties as deposits mature and grow are presented together with their effect on furnace exit temperature and efficiency. The reflective character of initial deposits from particular coals is then considered with predictions and measurements of the spectral character of such deposits, during the first three hours of growth, using on-line FTIR spectroscopy.
1988-1986
Treatment of Coal Devolatilization in Comprehensive Combustion Modeling
Brewster, B.S.; Baxter, L.L. and Smoot, L.D.
Energy & Fuels, 2, (4), 362-370, 1988. 9 pgs. Funded by Morgantown Energy Technology Center through subcontract from Advanced Fuel Research Co.
Comprehensive combustion codes typically use simple empirical models to predict weight loss associated with coal devolatilization. Individual evolved species are not taken into account nor are the individual products of heterogeneous char reaction. The effects of all particle reactions are lumped into a single overall rate of weight loss, and coal off gas composition and heating value are assumed constant. More detailed devolatilization models that consider the evolution of individual species and predict both rate and composition of the volatiles are now available. These models use general kinetic parameters for each coal constituent that are nearly independent of rank. Such models provide a basis for predicting composition and heating value of the volatiles as a function of burnout and reactor conditions for a wide range of coals. This paper presents a generalized theory based on the existing coal gas mixture fraction model, which allows the variation of off gas composition and heating value to be taken into account in comprehensive code predictions. Results are presented for a swirling combustion case. Results illustrating code sensitivity to several thermal parameters affecting devolatilization and to turbulent fluctuations are also presented. This publication also relates directly to Thrust Area 4.
Coal Combustion Modeling: Particulate and Gas Phase Heat Transfer Effects
Smith, P.J.; Baxter, L.L. and Jamaluddin, A.S.
AIChE Conference, 1988, New Orleans, LA. 15 pgs. All internal funding.
Heterogeneous heat transfer aspects strongly influence the performance of practical coal combustion systems since many of the subprocesses within the flame are highly temperature sensitive, and since the purpose of most furnaces is to extract energy from the flame. Coal combustion simulation or computer modeling permits investigation of the effect of various heat transfer mechanisms within flames on the many other simultaneous processes of turbulent fluid mechanics, coal conversion, gaseous reaction, etc. This paper examines two aspects of heat transfer on pulverized coal combustion processes: 1) the particle dominated radiation process and 2) the gas phase dominated turbulent convection processes. Comprehensive furnace modeling is used to study practical furnaces and the mechanisms are elucidated by comparing predicted results with experimental data from several coal and gas fired furnaces.
A Study of Turbulent Particle Dispersion in Pulverized Coal Combustion and Gasification
Smith, P.J. and Baxter, L.L.
Western States Section, 1986, The Combustion Institute, Banff, Canada. 20 pgs. Funding source US Department of Energy, Brigham Young University and Electric Power Research Institute.
The location of the coal particles in pulverized coal combustion and gasification processes is a dominant factor in determining flame stability, combustion/gasification efficiency and pollutant concentrations. Experimental and predicted data show that particles do not mix at the same rate as the gas phase and that this difference in turbulent mixing is important to the coal reaction process. The environment around a given particle during heat up, devolatilization and heterogeneous reaction controls the overall combustion process. In flows of interest to pulverized coal combustion, the particle dispersion is dominated by the random fluctuations of the gas phase turbulence as opposed to the mean drag on the particle. A mixed Eulerian-Lagrangian mathematical model for predicting mean trajectories of expected values for ensembles of representative particles is presented, including the turbulent dispersion effects. The mean turbulent velocity is modeled as a Fickian diffusion process from the man number density as calculated from the Eulerian part of the formation. This model is discussed and compared to alternate procedures. Applications of the model are shown for particle dispersion in non-reacting flow and pulverized coal combustion. An evaluation by comparison with experimental data is presented. Conclusions regarding the significance of turbulent particle dispersion are discussed.
Turbulent Dispersion of Particles
Baxter, L.L. and Smith, P.J.
Western States Section, 1988, The Combustion Institute, Salt Lake City, UT. 21 pgs. Funded by ACERC consortium: Babcock & Wilcox, Combustion Engineering, Consol, Electric Power Research Institute, Empire State Electrical Energy Research Corp., Foster Wheeler, Pittsburgh Energy Technology Center, Tennessee Valley Authority, and Utah Power & Light.
An approach to describing the transport of particles in turbulent flows is presented which is derivable from first principles. The approach is independent of the particular turbulence model used. It is valid in applications ranging from entrained flow dispersion to fluidized bed applications. It involves no adjustable parameters and requires a description of the turbulence in terms of the mean square velocities and Lagrangian autocorrelation functions.
The model has been rigorously evaluated by comparison to exact solutions, accurate alternative approaches, and experimental data. The model is able to reproduce the exact solutions both analytically and numerically, predict results which are both more accurate and precise than those of the leading alternative models, and predict experimental data within its inherent error under a range of different conditions.
The incorporation of the model into a comprehensive combustor simulator is also demonstrated. The model is found to be more efficient and robust, both in terms of the particle dispersion portion of the comprehensive code and in terms of the overall code performance. Predictions using this description of turbulent particle dispersion yield results that agree in detail with previous predictions using a calibrated particle dispersion model.
Optimization of Coal Devolatilization Model Parameters for Comprehensive Gasification and Combustion Models
Baxter, L.L.; Smith, P.J. and Smoot, L.D.
Western States Section, 1986, The Combustion Institute, Banff, Canada. 17 pgs. Funded by ACERC Consortium: Babcock & Wilcox, Combustion Engineering, Consol, Electric Power Research Institute, Empire State Electrical Energy Research Corp., Foster Wheeler, Pittsburgh Energy Technology Center, Tennessee Valley Authority, and Utah Power & Light.
Parameters for empirical devolatilization models are recommended for lignite and bituminous coals. These parameters were generated by combining optimization algorithms with rigorous particle and flow field modeling of experimental facilities.
PCGC-2, a comprehensive, axisymmetric, gasification and combustion code developed at the Brigham Young University Combustion Laboratory, was used to predict the velocity, temperature, and radiation fields of experimental apparatus used to collect devolatilization data. The effect of changing devolatilization models on these fields was neglected due to very low particle loadings. These fields were subsequently entered into a rigorous particle reaction model and the particle response to them was predicted.
A particle model predicted the trajectories, temperature, and reaction rates of coal particles in the flow fields described above. The particle model and its results are discussed. The particle model was coupled with an optimization algorithm to determine optimum parameters for the two-reaction devolatilization model.
Generalized reduced gradient (GRG) and sequential quadratic programming (SQP) methods were used in performing the optimization with OPTDES, an optimization program developed at Brigham Young University. The model parameters were optimized over six coal heat-up rates for each coal type. Statistical analyses of the residual sum of squares for the optimized two-reaction model revealed that there was no significant lack of fit of the data compared to the inherent variability of the data.
The Effects of Interfacial Conditions on Mass and Heat Transfer
Baxter, L.L. and Smith, P.J.
Western States Section, 1987, The Combustion Institute, Honolulu, HI. 3 pgs. Funded by ACERC Consortium: Babcock & Wilcox, Combustion Engineering, Consol, Electric Power Research Institute, Empire State Electrical Energy Research Corp., Foster Wheeler, Pittsburgh Energy Technology Center, Tennessee Valley Authority, and Utah Power & Light.
Classical assumptions about equilibrium conditions at phase interfaces during heat and/or mass transfer are shown to be approximations that can lead to errors in combustion environments. Specific illustrations involving droplet vaporization are given which show the nature and magnitude of the errors.
A method of relaxing these assumptions is discussed. Equations are developed from fundamental principles of physical chemistry to describe the actual conditions at interfaces. The corrections to the equilibrium assumptions depend on the rates of mass/heat transfer and physical constants that describe specific molecular behavior.
Computations illustrating the use of the nonequilibrium assumption reveal both the potentially large error associated with assuming equilibrium at the interfaces and the sensitivity of the predictions to the molecular parameters.
Revised User's Manual: 87-PCGC-2
Smoot, L.D.; Smith, P.J.; Brewster, B.S. and Baxter, L.L.
Advanced Combustion Engineering Research Center, 1988. Funded by US Department of Energy, Electric Power Research Institute, Consortium, and ACERC.
A two-dimensional, steady state model for describing a variety of reactive and non-reactive flows, including pulverized coal combustion and gasification, is presented. Recent code revisions and additions are described. The model, referred to as 87-PCGC-2, is applicable to cylindrical axi-symmetric systems. Turbulence is accounted for in both the fluid mechanics equations and the combustion scheme. Radiation from gases, wall and particles is taken into account using either a flux method or discrete ordinates methods. The particle phase is modeled in a Lagrangian framework, such that mean paths of particle groups are followed. Several multi-step coal devolatilization schemes are included along with a heterogeneous reaction scheme that allows for both diffusion and chemical reaction. Major gas-phase reactions are modeled assuming local instantaneous equilibrium, and thus the reaction rates are limited by the turbulent rate of mixing. A NOx finite rate chemistry submodel is included which integrates chemical kinetics and the statistics of the turbulence. The gas phase is described by elliptic partial differential equations that are solved by an iterative line-by-line technique. Under-relaxation is used to achieve numerical stability. The generalized nature of the model allows for calculation of isothermal fluid mechanics/gaseous combustion, droplet combustion, particulate combustion and various mixtures of the above, including combustion of coal-water and coal-oil slurries. Both combustion and gasification environments are permissible. User information and theory are presented, along with same problems.
Baxter, LL
2000
Fossil Fuel Power Stations-Coal Utilization
Smoot, L.D. and Baxter, L.L.
Chapter in the Encyclopedia of Physical Sciences and Technology, John R. Wiley and Sons, New York (in press), 2000.
Contact: Smoot
1996
Nitrogen Release During Coal Combustion
Baxter, L.L.; Reginald, E.M.; Fletcher, T.H. and Hurt, R.H.
Energy & Fuels, 10:188-196. Funded by US Department of Energy/Sandia National Laboratories.
Experiments in entrained flow reactors at combustion temperatures are performed to resolve the rank dependence of nitrogen release on an elemental basis for a suite of 15 U.S. coals ranging from lignite to low-volatile bituminous. Data were obtained as a function of particle conversion, with overall mass loss up to 99% on a dry, ash-free basis. Nitrogen release rates are presented relative to both carbon loss and overall mass loss. During devolatilization, fractional nitrogen release from low-rank coals is much slower than fractional mass release and noticeably slower than fractional carbon release. As coal rank increases, fractional nitrogen release rate relative to that of carbon and mass increases, with fractional nitrogen release rates exceeding fractional mass and fractional carbon release rates during devolatilization for high-rank (low-volatile bituminous) coals. At the onset of combustion, nitrogen loss rates increase significantly. For all coals investigated, fractional nitrogen loss rates relative to those of mass and carbon pass through a maximum during the earliest stages of oxidation. The mechanism for generating this maximum is postulated to involve nascent thermal rupture of nitrogen-containing compounds and possible preferential oxidation of nitrogen sites. During later stages of oxidation, the fractional loss rate of nitrogen approaches that of carbon for all coals. Changes in the relative release rates of nitrogen compared to those of both overall mass and carbon during all stages of combustion are attributed to a combination of the chemical structure of coals, temperature histories during combustion, and char chemistry.
1994
Radiative Heat Transfer in Pulverized-Coal Fired Boilers - Development of the Absorptive/Reflective Character of Initial Ash Deposit
Richards, G.H.; Harb, J.N.; Baxter, L.L.; Bhattacharya, S.; Gupta, R.P. and Wall, T.F.
25th Symposium (International on Combustion, 1994 (in press). (Proceedings of the 25th Symposium (International) on Combustion, Irvine, CA, August 1994). Funded by Australian Research Council, US Department of Energy (Pittsburgh Energy Technology Center) and ACERC.
Emission Fourier transform infrared (FTIR) spectroscopy data provide in situ,. Time-resolved, spectral emissivity measurements for ash deposits generated from two U.S. Powder River Basin coals. The first 3 h of deposit growth on a tube in a cross flow in a pilot-scale furnace detail the development of surface emissivity with time. Measured emissivities vary significantly with wavelength, indicating the influence of the physical properties and chemical composition of the deposit. At long wavelengths (>7 µm), emissions features exhibit characteristics of silica, sulfates, and silicates. The spectral emissivity measured in this region approaches a steady value due to an increase in deposit thickness and the size of particles in the deposit. In contrast, deposits are not opaque at shorter wavelengths where the measured emissivity is influenced by the properties of the underlying metal surface. Theoretical predictions of the emissivity of a particulate layer were performed, and results are compared to the measured values. The theory adequately predicts the general features of spectral variation of the emissivity. The predicted trends in emissivity with particle size and deposit composition are also consistent with experimental observations. Total (Planck-weighted) emissivities are calculated from the measured spectral values for the deposits at the tube temperatures. They increase with time from the clean tube value (0.2-0.3) to values typical of deposits formed from western U.S. coals (0.45-0.55). Calculated total absorptivities are found to be lower than the corresponding emissivities.
1993
In Situ, Real-Time Characterization of Coal Ash Deposits Using Fourier Transform Infrared Emission Spectroscopy
Baxter, L.L.; Richards, G.H.; Ottesen, D.K. and Harb, J.N.
Energy & Fuels, 7 (6):755-760, 1993. (Also presented at the Annual Advanced Combustion Engineering Research Center Conference, Park City, UT, March 1993). Funded by ACERC.
In situ Fourier transform infrared (FTIR) emission spectroscopy is used to identify the presence of silica, sulfates, and silicates as a function of time in coal ash deposits generated in Sandia's multifuel combustor, a pilot-scale reactor. Ash deposits are formed on a cylindrical tube in cross flow under experimental conditions that correspond to convection pass (fouling) regions of a commercial coal-fired boiler. The gas temperature, gas composition, particle loading, and extent of particle reaction in the combustor are typical of commercial boiler operation. The major classes of inorganic species deposited on the tube, including silicates and sulfates, are identified using the FTIR emission spectroscopy technique. Post mortem X-ray diffraction and conventional infrared absorption and reflectance analyses on the same deposits are used to corroborate the in situ FTIR emission data. The deposit composition from a western coal changes significantly as a function of both deposition time and combustion conditions. The observed changes include formation of sulfates and silicates. Such changes have implications for deposit properties such as tenacity and strength; the FTIR emission diagnostic shows promise as a method for monitoring such changes in practical systems.
Ash Deposits, Coal Blends and the Thermal Performance of Furnaces
Wall, T.F.; Baxter, L.L. and Harb, J.N.
Proceedings of the Engineering Foundation Conference on Coal Blending and Switching of Western Low-Sulfur Coals, Snowbird, UT, September 1993. Funded by Australian Research Council, US Department of Energy, Pittsburgh Energy Technology Center and ACERC.
The character of fireside ash deposits depends on the processes by which deposits are formed and subsequent reactions within the deposit and with furnace gases. The properties influencing furnace heat transfer, absorptivity for radiative transfer and thermal conductivity for conductive transfer are shown from many measurements to depend on this character. Illustrative trends in these properties as deposits mature and grow are presented together with their effect on furnace exit temperature and efficiency. The reflective character of initial deposits from particular coals is then considered with predictions and measurements of the spectral character of such deposits, during the first three hours of growth, using on-line FTIR spectroscopy.
1988-1986
Treatment of Coal Devolatilization in Comprehensive Combustion Modeling
Brewster, B.S.; Baxter, L.L. and Smoot, L.D.
Energy & Fuels, 2, (4), 362-370, 1988. 9 pgs. Funded by Morgantown Energy Technology Center through subcontract from Advanced Fuel Research Co.
Comprehensive combustion codes typically use simple empirical models to predict weight loss associated with coal devolatilization. Individual evolved species are not taken into account nor are the individual products of heterogeneous char reaction. The effects of all particle reactions are lumped into a single overall rate of weight loss, and coal off gas composition and heating value are assumed constant. More detailed devolatilization models that consider the evolution of individual species and predict both rate and composition of the volatiles are now available. These models use general kinetic parameters for each coal constituent that are nearly independent of rank. Such models provide a basis for predicting composition and heating value of the volatiles as a function of burnout and reactor conditions for a wide range of coals. This paper presents a generalized theory based on the existing coal gas mixture fraction model, which allows the variation of off gas composition and heating value to be taken into account in comprehensive code predictions. Results are presented for a swirling combustion case. Results illustrating code sensitivity to several thermal parameters affecting devolatilization and to turbulent fluctuations are also presented. This publication also relates directly to Thrust Area 4.
Coal Combustion Modeling: Particulate and Gas Phase Heat Transfer Effects
Smith, P.J.; Baxter, L.L. and Jamaluddin, A.S.
AIChE Conference, 1988, New Orleans, LA. 15 pgs. All internal funding.
Heterogeneous heat transfer aspects strongly influence the performance of practical coal combustion systems since many of the subprocesses within the flame are highly temperature sensitive, and since the purpose of most furnaces is to extract energy from the flame. Coal combustion simulation or computer modeling permits investigation of the effect of various heat transfer mechanisms within flames on the many other simultaneous processes of turbulent fluid mechanics, coal conversion, gaseous reaction, etc. This paper examines two aspects of heat transfer on pulverized coal combustion processes: 1) the particle dominated radiation process and 2) the gas phase dominated turbulent convection processes. Comprehensive furnace modeling is used to study practical furnaces and the mechanisms are elucidated by comparing predicted results with experimental data from several coal and gas fired furnaces.
A Study of Turbulent Particle Dispersion in Pulverized Coal Combustion and Gasification
Smith, P.J. and Baxter, L.L.
Western States Section, 1986, The Combustion Institute, Banff, Canada. 20 pgs. Funding source US Department of Energy, Brigham Young University and Electric Power Research Institute.
The location of the coal particles in pulverized coal combustion and gasification processes is a dominant factor in determining flame stability, combustion/gasification efficiency and pollutant concentrations. Experimental and predicted data show that particles do not mix at the same rate as the gas phase and that this difference in turbulent mixing is important to the coal reaction process. The environment around a given particle during heat up, devolatilization and heterogeneous reaction controls the overall combustion process. In flows of interest to pulverized coal combustion, the particle dispersion is dominated by the random fluctuations of the gas phase turbulence as opposed to the mean drag on the particle. A mixed Eulerian-Lagrangian mathematical model for predicting mean trajectories of expected values for ensembles of representative particles is presented, including the turbulent dispersion effects. The mean turbulent velocity is modeled as a Fickian diffusion process from the man number density as calculated from the Eulerian part of the formation. This model is discussed and compared to alternate procedures. Applications of the model are shown for particle dispersion in non-reacting flow and pulverized coal combustion. An evaluation by comparison with experimental data is presented. Conclusions regarding the significance of turbulent particle dispersion are discussed.
Turbulent Dispersion of Particles
Baxter, L.L. and Smith, P.J.
Western States Section, 1988, The Combustion Institute, Salt Lake City, UT. 21 pgs. Funded by ACERC consortium: Babcock & Wilcox, Combustion Engineering, Consol, Electric Power Research Institute, Empire State Electrical Energy Research Corp., Foster Wheeler, Pittsburgh Energy Technology Center, Tennessee Valley Authority, and Utah Power & Light.
An approach to describing the transport of particles in turbulent flows is presented which is derivable from first principles. The approach is independent of the particular turbulence model used. It is valid in applications ranging from entrained flow dispersion to fluidized bed applications. It involves no adjustable parameters and requires a description of the turbulence in terms of the mean square velocities and Lagrangian autocorrelation functions.
The model has been rigorously evaluated by comparison to exact solutions, accurate alternative approaches, and experimental data. The model is able to reproduce the exact solutions both analytically and numerically, predict results which are both more accurate and precise than those of the leading alternative models, and predict experimental data within its inherent error under a range of different conditions.
The incorporation of the model into a comprehensive combustor simulator is also demonstrated. The model is found to be more efficient and robust, both in terms of the particle dispersion portion of the comprehensive code and in terms of the overall code performance. Predictions using this description of turbulent particle dispersion yield results that agree in detail with previous predictions using a calibrated particle dispersion model.
Optimization of Coal Devolatilization Model Parameters for Comprehensive Gasification and Combustion Models
Baxter, L.L.; Smith, P.J. and Smoot, L.D.
Western States Section, 1986, The Combustion Institute, Banff, Canada. 17 pgs. Funded by ACERC Consortium: Babcock & Wilcox, Combustion Engineering, Consol, Electric Power Research Institute, Empire State Electrical Energy Research Corp., Foster Wheeler, Pittsburgh Energy Technology Center, Tennessee Valley Authority, and Utah Power & Light.
Parameters for empirical devolatilization models are recommended for lignite and bituminous coals. These parameters were generated by combining optimization algorithms with rigorous particle and flow field modeling of experimental facilities.
PCGC-2, a comprehensive, axisymmetric, gasification and combustion code developed at the Brigham Young University Combustion Laboratory, was used to predict the velocity, temperature, and radiation fields of experimental apparatus used to collect devolatilization data. The effect of changing devolatilization models on these fields was neglected due to very low particle loadings. These fields were subsequently entered into a rigorous particle reaction model and the particle response to them was predicted.
A particle model predicted the trajectories, temperature, and reaction rates of coal particles in the flow fields described above. The particle model and its results are discussed. The particle model was coupled with an optimization algorithm to determine optimum parameters for the two-reaction devolatilization model.
Generalized reduced gradient (GRG) and sequential quadratic programming (SQP) methods were used in performing the optimization with OPTDES, an optimization program developed at Brigham Young University. The model parameters were optimized over six coal heat-up rates for each coal type. Statistical analyses of the residual sum of squares for the optimized two-reaction model revealed that there was no significant lack of fit of the data compared to the inherent variability of the data.
The Effects of Interfacial Conditions on Mass and Heat Transfer
Baxter, L.L. and Smith, P.J.
Western States Section, 1987, The Combustion Institute, Honolulu, HI. 3 pgs. Funded by ACERC Consortium: Babcock & Wilcox, Combustion Engineering, Consol, Electric Power Research Institute, Empire State Electrical Energy Research Corp., Foster Wheeler, Pittsburgh Energy Technology Center, Tennessee Valley Authority, and Utah Power & Light.
Classical assumptions about equilibrium conditions at phase interfaces during heat and/or mass transfer are shown to be approximations that can lead to errors in combustion environments. Specific illustrations involving droplet vaporization are given which show the nature and magnitude of the errors.
A method of relaxing these assumptions is discussed. Equations are developed from fundamental principles of physical chemistry to describe the actual conditions at interfaces. The corrections to the equilibrium assumptions depend on the rates of mass/heat transfer and physical constants that describe specific molecular behavior.
Computations illustrating the use of the nonequilibrium assumption reveal both the potentially large error associated with assuming equilibrium at the interfaces and the sensitivity of the predictions to the molecular parameters.
Revised User's Manual: 87-PCGC-2
Smoot, L.D.; Smith, P.J.; Brewster, B.S. and Baxter, L.L.
Advanced Combustion Engineering Research Center, 1988. Funded by US Department of Energy, Electric Power Research Institute, Consortium, and ACERC.
A two-dimensional, steady state model for describing a variety of reactive and non-reactive flows, including pulverized coal combustion and gasification, is presented. Recent code revisions and additions are described. The model, referred to as 87-PCGC-2, is applicable to cylindrical axi-symmetric systems. Turbulence is accounted for in both the fluid mechanics equations and the combustion scheme. Radiation from gases, wall and particles is taken into account using either a flux method or discrete ordinates methods. The particle phase is modeled in a Lagrangian framework, such that mean paths of particle groups are followed. Several multi-step coal devolatilization schemes are included along with a heterogeneous reaction scheme that allows for both diffusion and chemical reaction. Major gas-phase reactions are modeled assuming local instantaneous equilibrium, and thus the reaction rates are limited by the turbulent rate of mixing. A NOx finite rate chemistry submodel is included which integrates chemical kinetics and the statistics of the turbulence. The gas phase is described by elliptic partial differential equations that are solved by an iterative line-by-line technique. Under-relaxation is used to achieve numerical stability. The generalized nature of the model allows for calculation of isothermal fluid mechanics/gaseous combustion, droplet combustion, particulate combustion and various mixtures of the above, including combustion of coal-water and coal-oil slurries. Both combustion and gasification environments are permissible. User information and theory are presented, along with same problems.