Hill, SC
2000
PCGC-3
Hill, S.C.; Eaton, A.M. and Smoot, L.D.
(Book chapter, pp. 95-160) in Computational Fluid Dynamics in Combustion, CRC Press. New York, 2000.
Contact: Smoot
Prediction of Nitric Oxide Destruction by Advanced Reburning
Xu, H.; Smoot, L.D.; Tree, D.R. and Hill, S.C.
Energy & Fuel. Accepted December 2000.
Contact: Tree
1999
Computational Model for NOx Reduction by Advanced Reburning
Xu, H.; Smoot, L.D. and Hill, S.C.
Energy & Fuels, 13, 411-420 (1999).
Advanced reburning is a NOx reduction process wherein injection of a hydrocarbon fuel such as natural gas downstream of the combustion zone is followed by injection of a nitrogen-containing species such as ammonia. The authors recently reported a seven-step, 11-species reduced mechanism for NO reduction by advanced reburning processes. However, inclusion of even a seven-step reduced mechanism into a CFD code for turbulent combustion leads to substantial computational demands. IN this work, the authors have further simplified the kinetic mechanism. A simpler four-step, eight-species reduced mechanism for NO reduction by advanced reburning has been developed from a 312-step, 50-species full mechanism through the use of a systematic reduction method. The four-step reduced mechanism is in good agreement with the full mechanism for most laminar flow cases. It also agrees qualitatively with three sets of experimental data, which show the influences of temperature, CO concentration, O2 concentration, and the ratio (NH3/NO)in. It can be applied for coal-, gas-, and oil-fired combustion. The four-step reaction sequence has been integrated into a comprehensive CFD combustion code for turbulent combustion, PCGC-3. The method of integration is described. Several computations are reported with the combined code to demonstrate the predictive behavior of the advance reburning mechanism in turbulent, pulverized coal combustion. The model calculations show the effects of temperature and concentrations of CO, O2, and NH3 on NO reduction.
Computational Model for NOx Reduction by Advanced Reburning
Xu, H.; Smoot, L.D. and Hill, S.C.
Energy & Fuels, 13:411-420, 1999.
Advanced reburning is a NOx reduction process wherein injection of a hydrocarbon fuel such as natural gas downstream of the combustion zone is followed by injection of a nitrogen-containing species such as ammonia. The authors recently reported a seven-step, 11-species reduced mechanism for NO reduction by advanced reburning processes. However, inclusion of even a seven-step reduced mechanism into a CFD code for turbulent combustion leads to substantial computational demands. In this work, the authors have further simplified the kinetic mechanism. A simpler four-step, eight-species reduced mechanism for NO reduction by advanced reburning has been developed from a 312-step, 50-species full mechanism through the use of a systematic reduction method. The four-step reduced mechanism is in good agreement with the full mechanism for most laminar flow cases. It also agrees qualitatively with three sets of experimental data, which show the influences of temperature, CO concentration, O2 concentration, and the ratio (NH3/NO)in. It can be applied for coal-, gas-, and oil-fired combustion. The four-step reaction sequence has been integrated into a comprehensive CFD combustion code for turbulent combustion, PCGC-3. The method of integration is described. Several computations are reported with the combined code to demonstrate the predictive behavior of the advance reburning mechanism in turbulent, pulverized coal combustion. The model calculations show the effects of temperature and concentrations of CO, O2, and NH3 on NO reduction.
Components, Formulations, Solutions, Evaluation, and Application of Comprehensive Combustion Mode
Eaton, A.M.; Smoot, L.D.; Hill, S.C. and Eatough, C.N.
Progress in Energy and Combustion Science, 25:387-436 (1999).
Development and application of comprehensive, multidimensional, computational combustion models are increasing at a significant pace across the world. While once confined to specialized research computer codes, these combustion models are becoming more readily accessible as features in commercially available computational fluid dynamics (CFD) computer codes. Simulations made with such computer codes offer great potential for use in analyzing, designing, retrofitting, and optimizing the performance of fossil-fuel combustion and conversion systems.
The purpose of this paper is to provide an overview of comprehensive combustion modeling technology as applied to fossil-fuel combustion processes. This overview is divided into three main parts. First, a brief review of the state-of-the-art of the various components or submodels that are required in a comprehensive combustion model is presented. These submodels embody mathematical and numerical representations of the fundamental principles that characterize the physico-chemical phenomena of interest. The submodel review is limited to those required for characterizing non-premixed, gaseous and pulverized coal gasification and combustion processes. A summary of the submodels that are available in representative computer codes is also presented.
Second, the kinds of data required to evaluate and validate the predictions of comprehensive combustion codes are considered. To be viewed with confidence, code simulations must have been rigorously evaluated and validated by comparison with appropriate experimental data, preferably from a variety of combustor geometries at various geometric scales. Three sets of validation data are discussed in detail. Two sets are from the highly instrumented, pilot-scale combustor called the controlled profile reactor (CPR) (one natural gas-fired and one coal-fired), and the other set is for a full-scale, corner-fired 85 MWe utility boiler.
Third, representative applications of comprehensive combustion models are summarized, and three sets of model simulations are compared with experimental data. The model simulations for the three test cases were made using two commonly used, CFD-based computer codes with comprehensive combustion model features, PCGC-3 and FLUENT 4.4. In addition to the standard version of FLUENT, predictions were also made with a version of FLUENT incorporating advanced submodels for coal reactions and NO pollutant formation.
1998
NOx Control Through Reburning
Smoot, L.D.; Hill, S.C. and Xu, H.
Prog. Energy Combust. Sci., 24:385-408 (1998)
Reburning is a process whereby a hydrocarbon fuel is injected immediately downstream of the combustion zone to establish a fuel-rich zone in order to convert nitric oxide to HCN. The reburning fuel can be gaseous (e.g., natural gas), solid (e.g., coal char or wood) or liquid (e.g., residual oil). Typically, the amount of reburning fuel used is 10-30% of the total fuel. This technology is practiced commercially with nitric oxide reduction levels of 35-65%, depending on the type and scale of the boiler or combustion, the primary and reburning fuels and other variables. Current research and development are suggesting several advanced reburning concepts including injection of ammonia or urea aft of the reburning fuel injection. Nitric oxide reductions of over 90% are anticipated. In this mini-review, a review of reburning technologies, measurements and mechanisms is presented. Predictive methods for reburning are also discussed. Recent work on reburning, including development of a global reburning reaction rate, is summarized, and results of application of a comprehensive combustion model to reburning measurements are summarized.
A Reduced Kinetic model for NOx Reduction by Advanced Reburning
Xu, H.; Smoot, L.D. and Hill, S.C.
Energy & Fuels, 12:1278-1289, 1998.
Advanced reburning technology, which makes use of natural gas injection followed by ammonia injection, has proven to be an effective method for the removal of up to 85-95% of the NO in pulverized, coal-fired furnaces. This paper reports the development of a seven-step, 11-species reduced mechanism for the prediction of nitric oxide concentrations using advanced reburning from a 312-step, 50-species full mechanism. The derivation of the reduced mechanism is described, including the selection of the full mechanism, the development of the skeletal mechanism, and the selection of steady-state species. The predictions of the seven-step reduced mechanism are in good agreement with those of the full mechanism over a wide range of parameters, applicable to coal-based, gas-based, and oil-based combustion cases. Comparisons with three independent sets of experimental laminar data indicate that the reduced mechanism correctly predicts the observed trends, including the effects of temperature, the ratio of (NH3/NO)in, and concentrations of CO, CO2, O2, and H2O on NO reduction. The observed effects of CO on NH3 slip were also reliably predicted. Mechanistic consideration provides an explanation for the roles of the important radicals and species. Also, parametric studies of the effects of CO2 and H2O have been performed with the reduced mechanism. A maximum in NO reduction exists, which strongly depends on the concentrations of NOin, CO, and O2, the ratio of (NH3/NO)in, and temperature.
1997
NOx Control Through Reburning (A Review)
Smoot, L.D.; Hill, S.C. and Xu, H.
Progress in Energy and Combustion Science, (in press), 1997. Funded by ACERC.
Reburning is a process whereby a hydrocarbon fuel is injected immediately downstream of the combustion zone to establish a fuel-rich zone in order to convert nitric oxide to HCN. The reburning fuel can be gaseous (e.g., natural gas), solid (e.g., coal char or wood) or liquid (e.g., residual oil. Typically, the amount of reburning fuel used is 10-30% of the total fuel. This technology is practiced commercially with nitric oxide reduction levels of 35-65%, depending on the type of scale of the boiler or combustion, the primary and reburning fuels and other variables. Current research and development are suggesting several advanced reburning concepts including injection of ammonia or urea aft of the reburning fuel injection. Nitric oxide reductions of over 90% are anticipated. In this mini-review, a review of reburning technologies, measurements and mechanisms is presented. Predictive methods for reburning are also discussed. Recent work on reburning, including development of a global reburning reaction rate, is summarized, and results of application of a comprehensive combustion model to reburning measurements are summarized.
A Reduced Kinetic Model for NOx Reduction by Advanced Reburning
Xu, H.; Smoot, L.D. and Hill, S.C.
1997 Fall Meeting of the Western States Section/Combustion Institute, Diamond Bar, California, October 23-24, 1997. Funded by ACERC.
Advanced reburning technology, which makes use of natural gas injection followed by ammonia injection, has proved to be and effective method for removal up to 85-95% of NO in pulverized, coal-fired furnaces. This paper reports the development of a 7-step, 11-species reduced mechanism for the prediction of nitric oxide concentrations for advanced reburning from a 312-step 50-species full mechanisms. The derivation of the reduced model is described in detail, including the selection of the full mechanism, the development of the skeletal mechanism and the selection of steady-state species. The prediction of the 7 step reduced mechanism are in good agreement with those of the full mechanism over a wide range of parameters, applicable to coal-based, gas-based, and oil-based combustion cases. Comparisons with three independent sets of experimental laminar data indicate that the reduced model correctly predicts observed trends, and H2O on NO reduction. The observed effects of CO on NH3 slip were also reliably predicted. Mechanistic considerations explain the roles of important radicals and species. Also, parametric studies of effects of CO2 and H2O have been performed with reduced mechanism. A maximum NO reduction exists, which strongly depends on the concentrations of CO, CO2, (NH3/NO)in, and temperature.
1996
Global Rate Expression for Nitric Oxide Reburning. Part 2
Chen, W.; Smoot, L.D.; Hill, S.C. and Fletcher, T.H.
Energy & Fuels, 10(5):1046-1052, 1996. Funded by ACERC.
An investigation of a global reburning-NO reaction, SigmaijCiHj + NO --> HCN + . . ., which is a reduction pathway of nitric oxide (NO) by reaction with gaseous hydrocarbons, was conducted. The global reburning-NO rate expression was deduced from a combination of elemental reactions. The global rate expression and its rate constants were then determined by correlating predicted species profiles from simple hydrocarbon flames. This global reburning-NO rate constant can be expressed as 2.7 x 106 exp(-18,800/RT) (gmol/cm³s). This expression and constants are applicable to atmospheric pressure with an equivalence ratio rang of 1.2-2.08 for light hydrocarbon reburning ghases (CH4 and C2H4).
Application of Comprehensive Combustion Modeling to Practical Combustion Systems
Hill, S.C. and Smoot, L.D.
Proceedings at the Thirteenth Annual International Pittsburgh Coal Conference, Pittsburgh, Pennsylvania, September 3-7, 1996. Funded by ACERC.
Comprehensive combustion modeling is becoming an indispensable tool for the design and optimization of practical combustion systems. Comprehensive combustion modeling involves the numerical solution of partial differential equations that describe the physical processes occurring in combustion systems. This paper discusses the information required to create such a model of practical combustion systems and the computational and personnel resources required to apply the model. The procedures to create the model, solve the problem and analyze the results are also described. The information that can be obtained from comprehensive combustion simulations is discussed, and typical applications and some limitations of these simulations are also described. The paper also presents the results of several applications of practical combustion systems compared with experimental data to illustrate potential applications of combustion modeling.
1994
Application of a Comprehensive Combustion Code to Simulate NO Pollutant Formation in a Utility-Scale Furnace
Hill, S.C. and Cannon, J.N.
Proceedings of the Joint AFRC/JFRC Pacific Rim International Conference on Environmental Control of Combustion Processes, Maui, HI, October 1994. Funded by New York State Electrical & Gas Corp. and Empire State Electrical Energy Research Corp.
A comprehensive combustion code, PCGC-3, is used to simulate the flow, combustion, and NO pollutant formation processes in an 85 MWe coal-fired utility boiler. The code is used to predict NO emissions from the boiler under various operating conditions. The conditions tested in this study are: over-fire air, % excess air, and burner tilt. Code predictions are compared with effluent NO measurements made in this boiler. These comparisons show good agreement between model predictions for some observed trends, and demonstrate that the computer code is a useful tool that can provide insights into boiler operation. Comparisons that do not show the correct trend suggest that a finer grid resolution is required to correctly predict some trends.
1993
Development and Application of an Acid Rain Precursor Model for Practical Furnaces
Smoot, L.D.; Boardman, R.D.; Brewster, B.S.; Hill, S.C. and Foli, A.K.
Energy & Fuels, 7 (6):786-795, 1993. Funded by ACERC.
Control of emissions of sulfur (SO2, SO3, H2S) and nitrogen (NO, NO2, N2O, HCN, NH3) pollutants from fossil-fuel-fired furnaces and gasifiers remains a vital worldwide requirement as the utilization of fossil fuels continues to increase. Development and refinement of a predictive model for these acid rain precursors (MARP) has reached the point where this technology can contribute to acid rain control. In this paper, model foundations and recent developments are summarized, including formation of thermal and fuel NOx and sorbent capture of sulfur oxides. The method includes global formation, capture, and destruction processes in turbulent, reacting, particle-laden flows. This submodel has been combined with comprehensive, generalized combustion models (PCGC-2, PCGC-3) that provide the required local properties for the combustion or gasification processes. The submodel has been applied to NOx formation in a full-scale (85 MWe), corner-fired utility boiler, where recent in situ NOx measurements were made, with variations in coal feedstock quality (including fuel N percentage) load-level and percentage excess air. Predictions are also made for in situ sorbent capture of sulfur pollutants in both combustion (fuel-lean, SO2), and gasification (fuel-rich, H2S) laboratory-scale reactors. Limitations of MARP are identified and work to improve the submodel is outlined.
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.
Overview of ACERC Comprehensive Model Development
Fletcher, T.H. and Hill, S.C.
Energy & Fuels, 7, (6):870-873, 1993. Funded by ACERC.
A major objective of the Advanced Combustion Engineering Research Center (ACERC) is the development of comprehensive combustion models to help in the solution of critical national combustion problems. Computer models incorporate research and technology results from center projects and from external research programs. The synergistic integration of scientific knowledge that is expected from the NSF engineering research centers is demonstrated to a great extent at ACERC by the development of these software tools. The transfer of technology from ACERC to industry is also accomplished in part by the implementation of the models at industrial firms. The effort to develop such products requires significant integration and development, together with fundamental research. The development of comprehensive models also produces personnel and technology able to help address the challenge of synergistic cross-linkage among thrust areas within ACERC and provides an important means of transferring this technology to industry. This article is an overview of the purpose, accomplishments and goals of research at ACERC in comprehensive modeling. (Thrust Area 5.)
A Comprehensive Three-Dimensional Model for Simulation of Combustion Systems: PCGC-3
Hill, S.C. and Smoot, L.D.
Energy & Fuels, 7 (6):874-883, 1993. Funded by ACERC.
A generalized, three-dimensional combustion model has been developed to simulate large-scale, stead-state, gaseous and particle-laden, reacting and nonreacting systems. The model, which is based on an earlier two-dimensional model, has been applied to turbulent, combustion systems, including pulverized-coal systems. It uses an Eulerian framework for the gas phase and a Lagrangian framework for the particles. The code assumes equilibrium gas-phase chemistry and couples the turbulent flow field with the chemical reactions by integrating the equations over a probability density function. The model uses advanced numerics and a differencing scheme capable of solving the large computational meshes required to simulate practical furnaces. Convective and radiative heat transfer are also modeled. Radiative heat transfer is modeled using the discrete ordinates method. The model has been evaluated by comparison of predictions with experimental data from a large-scale 85-MEe coal-fired utility boiler. The data include furnace profile measurements obtained with intrusive and laser-based optical probes. These comparisons show qualitative agreement of model predictions with observed trends, and indicate that the model can be used to provide insights into boiler operation.
Parametric Sensitivity Study of a CFD-Based Coal Combustion Model
Smith, J.D.; Smith, P.J. and Hill, S.C.
AIChE Journal, 39, (10):1668-1679, 1993. Funded by Combustion Laboratory Consortium through Brigham Young University.
Parametric sensitivity of a two-dimensional pulverized-fuel (PF) combustion model is studied extensively for the effect of parametric uncertainty on model predictions. Results show that error in coal devolatilization/oxidation parameters has the dominant effect on predicted burnout, NOx formation, local gas temperature, and coal-gas mixture fraction. Uncertainty in the turbulent particle dispersion parameters appears to have a secondary effect, while error in the particle-gas radiation parameters has little impact on model predictions. Regions of the computational domain exhibiting sensitivity to specific parameters are identified. Specific parameter sensitivity implies the relative importance of various mechanisms in the overall process. Turbulent particle dispersion seems to be important early in the reactor with kinetic processes dominating at and following the predicted ignition point. Radiation appears to be of minor importance. These results indicate the need for a better method of predicting the overall volatiles yield and further understanding of the devolatilization/oxidation mechanism and its role in the overall PF combustion process. The study provides fundamental direction for future comprehensive model development and focuses on experimental work to better quantify critical input parameters.
1992
Measurements and Predictions of Coal Combustion in a Utility Furnace
Hill, S.C.; Cannon, J.N. and Smoot, L.D.
International Chemical Recovery Conference, Seattle, WA, June 1992. (Also presented at The Effects of Coal Quality on Power Plants, San Diego, CA, August 1992, and the Ninth Annual Pittsburgh Coal Conference, Pittsburgh, PA, August 1992). Funded by ACERC.
A new generalized 3-dimensional combustion model has been developed to simulate large-scale, steady state, particle laden reacting and non-reacting systems. The model, which is based on an earlier 2-dimensional model has been applied to turbulent, pulverized-coal combustion systems. It uses an Eulerian framework for the gas phase and a Lagrangian framework for the particles. The code assumes equilibrium gas-phase chemistry, and couples the turbulent flow field with the chemical reactions by integrating the equations over a probability density function. The model uses advanced numerics and a differencing scheme capable of solving the large computational meshes required to simulate practical furnaces. The model has been evaluated by comparison of predictions with new experimental data from a large-scale, 80 MWe coal-fired utility boiler. The data include furnace profile measurements obtained with intrusive and laser-based optical probes. These comparisons show qualitative agreement of model predictions with observed trends, and indicate that the model can be used to provide insights into boiler operation. The comparisons also indicate the further evaluation, concepts for improvement of some sub models are required, and provide direction for future model development.
1991
Detailed Model for Practical Pulverized Coal Furnaces and Gasifiers, Volume II, User Manual for 1990 Version Pulverized Coal Gasification and Combustion 3-Dimensional (90-PCGC-3), Final Report
Smith, P.J.; Smoot, L.D.; Hill, S.C. and Eaton, A.M.
Advanced Combustion Engineering Research Center, 1991. Funded by US Department of Energy, Consortium and ACERC.
The theoretical foundations, numerical approach and a guide for users of 90-PCGC-3 are presented. 90-PCGC-3 is a generalized, three-dimensional, steady state model that can be used to predict the behavior of a variety of reactive and non-reactive (isothermal) fluid flows. The model solves the Navier-Stokes equations in three-dimensional, Cartesian coordinates. Turbulence is accounted for in both the fluid mechanics equations and the combustion solution scheme. Major gas-phase reactions are modeled assuming local instantaneous equilibrium, and thus the reaction rates are limited by the turbulent rate of mixing. 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.
1987-1986
Parametric Sensitivity Study of an Entrained-Flow Pulverized-Fuel Combustion Model
Smith, J.D.; Smith, P.J. and Hill, S.C.
Submitted for publication to Computer and Chem.Engineering, 1987. 35 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 extensive parametric sensitivity study of a two-dimensional pulverized-fuel (pf) combustion model has been performed to investigate the effect of parametric uncertainty on model predictions. Results illustrate the dominant effect that error in coal devolatilization/oxidation parameters has on predicted burnout, Nox formation, local gas temperature, and coal-gas mixture fraction. Uncertainty in the turbulent particle dispersion parameters appear to have a secondary effect, while error in the particle-gas radiation parameters seems to have little impact on model predictions. Regions of the computational domain exhibiting sensitivity to specific parameters are identified. Specific parameter sensitivity implies the relative importance of various mechanisms in the overall process. Turbulent particle dispersion seems to be important early in the reactor with kinetic processes dominating at and following the predicted ignition point. Radiation appears to be of minor importance for the case under investigation. These results provide unique insight into the general pf combustion process. Specifically, they emphasize the need for a better methodology of predicting the overall volatiles yield. This implies additional understanding of the devolatilization/oxidation mechanism and its role in the overall pf combustion process. Although the results presented here are case specific, they provide unique insight into the overall pf combustion process and illustrate the effects that parameter uncertainty has on model predictions. This information provides fundamental direction for future comprehensive model development and focuses attention on pertinent experimental work to better quantify critical input parameters.
Effects of Swirling Flow on Nitrogen Oxide Concentration in Pulverized Coal Combustors
Smith, P.J.; Smoot, L.D. and Hill, S.C.
AIChE Journal, 32, (11), 1917-1919, 1986. 3 pgs. Funded by US Department of Energy.
Nitrogen-containing pollutants from pulverized coal conversion processes have been of concern for several years, and reviews of this subject have been published (Wendt, 1980; Chen et al., 1981). Swirl of the secondary stream is one technique of fuel-air contacting that causes significant changes in NO concentrations. In some cases, increased swirl increases NO emissions (Heap et al., 1973b; Brown et al., 1977; Pershing and Wendt, 1979). In other cases, increased swirl initially decreases NO emissions (Harding et al., 1982; Asay et al., 1983). Thus, the observed effects of swirl are varied, and indicate that additional parameters influence the NO emissions. In this note a model, described in detail elsewhere (Smith and Smoot, 1981; Smith et al., 1981, 1982; Hill et al., 1984), was used for interpreting effects of inlet stream swirl and velocity on NO emissions during combustion of pulverized coal.
Hill, SC
2000
PCGC-3
Hill, S.C.; Eaton, A.M. and Smoot, L.D.
(Book chapter, pp. 95-160) in Computational Fluid Dynamics in Combustion, CRC Press. New York, 2000.
Contact: Smoot
Prediction of Nitric Oxide Destruction by Advanced Reburning
Xu, H.; Smoot, L.D.; Tree, D.R. and Hill, S.C.
Energy & Fuel. Accepted December 2000.
Contact: Tree
1999
Computational Model for NOx Reduction by Advanced Reburning
Xu, H.; Smoot, L.D. and Hill, S.C.
Energy & Fuels, 13, 411-420 (1999).
Advanced reburning is a NOx reduction process wherein injection of a hydrocarbon fuel such as natural gas downstream of the combustion zone is followed by injection of a nitrogen-containing species such as ammonia. The authors recently reported a seven-step, 11-species reduced mechanism for NO reduction by advanced reburning processes. However, inclusion of even a seven-step reduced mechanism into a CFD code for turbulent combustion leads to substantial computational demands. IN this work, the authors have further simplified the kinetic mechanism. A simpler four-step, eight-species reduced mechanism for NO reduction by advanced reburning has been developed from a 312-step, 50-species full mechanism through the use of a systematic reduction method. The four-step reduced mechanism is in good agreement with the full mechanism for most laminar flow cases. It also agrees qualitatively with three sets of experimental data, which show the influences of temperature, CO concentration, O2 concentration, and the ratio (NH3/NO)in. It can be applied for coal-, gas-, and oil-fired combustion. The four-step reaction sequence has been integrated into a comprehensive CFD combustion code for turbulent combustion, PCGC-3. The method of integration is described. Several computations are reported with the combined code to demonstrate the predictive behavior of the advance reburning mechanism in turbulent, pulverized coal combustion. The model calculations show the effects of temperature and concentrations of CO, O2, and NH3 on NO reduction.
Computational Model for NOx Reduction by Advanced Reburning
Xu, H.; Smoot, L.D. and Hill, S.C.
Energy & Fuels, 13:411-420, 1999.
Advanced reburning is a NOx reduction process wherein injection of a hydrocarbon fuel such as natural gas downstream of the combustion zone is followed by injection of a nitrogen-containing species such as ammonia. The authors recently reported a seven-step, 11-species reduced mechanism for NO reduction by advanced reburning processes. However, inclusion of even a seven-step reduced mechanism into a CFD code for turbulent combustion leads to substantial computational demands. In this work, the authors have further simplified the kinetic mechanism. A simpler four-step, eight-species reduced mechanism for NO reduction by advanced reburning has been developed from a 312-step, 50-species full mechanism through the use of a systematic reduction method. The four-step reduced mechanism is in good agreement with the full mechanism for most laminar flow cases. It also agrees qualitatively with three sets of experimental data, which show the influences of temperature, CO concentration, O2 concentration, and the ratio (NH3/NO)in. It can be applied for coal-, gas-, and oil-fired combustion. The four-step reaction sequence has been integrated into a comprehensive CFD combustion code for turbulent combustion, PCGC-3. The method of integration is described. Several computations are reported with the combined code to demonstrate the predictive behavior of the advance reburning mechanism in turbulent, pulverized coal combustion. The model calculations show the effects of temperature and concentrations of CO, O2, and NH3 on NO reduction.
Components, Formulations, Solutions, Evaluation, and Application of Comprehensive Combustion Mode
Eaton, A.M.; Smoot, L.D.; Hill, S.C. and Eatough, C.N.
Progress in Energy and Combustion Science, 25:387-436 (1999).
Development and application of comprehensive, multidimensional, computational combustion models are increasing at a significant pace across the world. While once confined to specialized research computer codes, these combustion models are becoming more readily accessible as features in commercially available computational fluid dynamics (CFD) computer codes. Simulations made with such computer codes offer great potential for use in analyzing, designing, retrofitting, and optimizing the performance of fossil-fuel combustion and conversion systems.
The purpose of this paper is to provide an overview of comprehensive combustion modeling technology as applied to fossil-fuel combustion processes. This overview is divided into three main parts. First, a brief review of the state-of-the-art of the various components or submodels that are required in a comprehensive combustion model is presented. These submodels embody mathematical and numerical representations of the fundamental principles that characterize the physico-chemical phenomena of interest. The submodel review is limited to those required for characterizing non-premixed, gaseous and pulverized coal gasification and combustion processes. A summary of the submodels that are available in representative computer codes is also presented.
Second, the kinds of data required to evaluate and validate the predictions of comprehensive combustion codes are considered. To be viewed with confidence, code simulations must have been rigorously evaluated and validated by comparison with appropriate experimental data, preferably from a variety of combustor geometries at various geometric scales. Three sets of validation data are discussed in detail. Two sets are from the highly instrumented, pilot-scale combustor called the controlled profile reactor (CPR) (one natural gas-fired and one coal-fired), and the other set is for a full-scale, corner-fired 85 MWe utility boiler.
Third, representative applications of comprehensive combustion models are summarized, and three sets of model simulations are compared with experimental data. The model simulations for the three test cases were made using two commonly used, CFD-based computer codes with comprehensive combustion model features, PCGC-3 and FLUENT 4.4. In addition to the standard version of FLUENT, predictions were also made with a version of FLUENT incorporating advanced submodels for coal reactions and NO pollutant formation.
1998
NOx Control Through Reburning
Smoot, L.D.; Hill, S.C. and Xu, H.
Prog. Energy Combust. Sci., 24:385-408 (1998)
Reburning is a process whereby a hydrocarbon fuel is injected immediately downstream of the combustion zone to establish a fuel-rich zone in order to convert nitric oxide to HCN. The reburning fuel can be gaseous (e.g., natural gas), solid (e.g., coal char or wood) or liquid (e.g., residual oil). Typically, the amount of reburning fuel used is 10-30% of the total fuel. This technology is practiced commercially with nitric oxide reduction levels of 35-65%, depending on the type and scale of the boiler or combustion, the primary and reburning fuels and other variables. Current research and development are suggesting several advanced reburning concepts including injection of ammonia or urea aft of the reburning fuel injection. Nitric oxide reductions of over 90% are anticipated. In this mini-review, a review of reburning technologies, measurements and mechanisms is presented. Predictive methods for reburning are also discussed. Recent work on reburning, including development of a global reburning reaction rate, is summarized, and results of application of a comprehensive combustion model to reburning measurements are summarized.
A Reduced Kinetic model for NOx Reduction by Advanced Reburning
Xu, H.; Smoot, L.D. and Hill, S.C.
Energy & Fuels, 12:1278-1289, 1998.
Advanced reburning technology, which makes use of natural gas injection followed by ammonia injection, has proven to be an effective method for the removal of up to 85-95% of the NO in pulverized, coal-fired furnaces. This paper reports the development of a seven-step, 11-species reduced mechanism for the prediction of nitric oxide concentrations using advanced reburning from a 312-step, 50-species full mechanism. The derivation of the reduced mechanism is described, including the selection of the full mechanism, the development of the skeletal mechanism, and the selection of steady-state species. The predictions of the seven-step reduced mechanism are in good agreement with those of the full mechanism over a wide range of parameters, applicable to coal-based, gas-based, and oil-based combustion cases. Comparisons with three independent sets of experimental laminar data indicate that the reduced mechanism correctly predicts the observed trends, including the effects of temperature, the ratio of (NH3/NO)in, and concentrations of CO, CO2, O2, and H2O on NO reduction. The observed effects of CO on NH3 slip were also reliably predicted. Mechanistic consideration provides an explanation for the roles of the important radicals and species. Also, parametric studies of the effects of CO2 and H2O have been performed with the reduced mechanism. A maximum in NO reduction exists, which strongly depends on the concentrations of NOin, CO, and O2, the ratio of (NH3/NO)in, and temperature.
1997
NOx Control Through Reburning (A Review)
Smoot, L.D.; Hill, S.C. and Xu, H.
Progress in Energy and Combustion Science, (in press), 1997. Funded by ACERC.
Reburning is a process whereby a hydrocarbon fuel is injected immediately downstream of the combustion zone to establish a fuel-rich zone in order to convert nitric oxide to HCN. The reburning fuel can be gaseous (e.g., natural gas), solid (e.g., coal char or wood) or liquid (e.g., residual oil. Typically, the amount of reburning fuel used is 10-30% of the total fuel. This technology is practiced commercially with nitric oxide reduction levels of 35-65%, depending on the type of scale of the boiler or combustion, the primary and reburning fuels and other variables. Current research and development are suggesting several advanced reburning concepts including injection of ammonia or urea aft of the reburning fuel injection. Nitric oxide reductions of over 90% are anticipated. In this mini-review, a review of reburning technologies, measurements and mechanisms is presented. Predictive methods for reburning are also discussed. Recent work on reburning, including development of a global reburning reaction rate, is summarized, and results of application of a comprehensive combustion model to reburning measurements are summarized.
A Reduced Kinetic Model for NOx Reduction by Advanced Reburning
Xu, H.; Smoot, L.D. and Hill, S.C.
1997 Fall Meeting of the Western States Section/Combustion Institute, Diamond Bar, California, October 23-24, 1997. Funded by ACERC.
Advanced reburning technology, which makes use of natural gas injection followed by ammonia injection, has proved to be and effective method for removal up to 85-95% of NO in pulverized, coal-fired furnaces. This paper reports the development of a 7-step, 11-species reduced mechanism for the prediction of nitric oxide concentrations for advanced reburning from a 312-step 50-species full mechanisms. The derivation of the reduced model is described in detail, including the selection of the full mechanism, the development of the skeletal mechanism and the selection of steady-state species. The prediction of the 7 step reduced mechanism are in good agreement with those of the full mechanism over a wide range of parameters, applicable to coal-based, gas-based, and oil-based combustion cases. Comparisons with three independent sets of experimental laminar data indicate that the reduced model correctly predicts observed trends, and H2O on NO reduction. The observed effects of CO on NH3 slip were also reliably predicted. Mechanistic considerations explain the roles of important radicals and species. Also, parametric studies of effects of CO2 and H2O have been performed with reduced mechanism. A maximum NO reduction exists, which strongly depends on the concentrations of CO, CO2, (NH3/NO)in, and temperature.
1996
Global Rate Expression for Nitric Oxide Reburning. Part 2
Chen, W.; Smoot, L.D.; Hill, S.C. and Fletcher, T.H.
Energy & Fuels, 10(5):1046-1052, 1996. Funded by ACERC.
An investigation of a global reburning-NO reaction, SigmaijCiHj + NO --> HCN + . . ., which is a reduction pathway of nitric oxide (NO) by reaction with gaseous hydrocarbons, was conducted. The global reburning-NO rate expression was deduced from a combination of elemental reactions. The global rate expression and its rate constants were then determined by correlating predicted species profiles from simple hydrocarbon flames. This global reburning-NO rate constant can be expressed as 2.7 x 106 exp(-18,800/RT) (gmol/cm³s). This expression and constants are applicable to atmospheric pressure with an equivalence ratio rang of 1.2-2.08 for light hydrocarbon reburning ghases (CH4 and C2H4).
Application of Comprehensive Combustion Modeling to Practical Combustion Systems
Hill, S.C. and Smoot, L.D.
Proceedings at the Thirteenth Annual International Pittsburgh Coal Conference, Pittsburgh, Pennsylvania, September 3-7, 1996. Funded by ACERC.
Comprehensive combustion modeling is becoming an indispensable tool for the design and optimization of practical combustion systems. Comprehensive combustion modeling involves the numerical solution of partial differential equations that describe the physical processes occurring in combustion systems. This paper discusses the information required to create such a model of practical combustion systems and the computational and personnel resources required to apply the model. The procedures to create the model, solve the problem and analyze the results are also described. The information that can be obtained from comprehensive combustion simulations is discussed, and typical applications and some limitations of these simulations are also described. The paper also presents the results of several applications of practical combustion systems compared with experimental data to illustrate potential applications of combustion modeling.
1994
Application of a Comprehensive Combustion Code to Simulate NO Pollutant Formation in a Utility-Scale Furnace
Hill, S.C. and Cannon, J.N.
Proceedings of the Joint AFRC/JFRC Pacific Rim International Conference on Environmental Control of Combustion Processes, Maui, HI, October 1994. Funded by New York State Electrical & Gas Corp. and Empire State Electrical Energy Research Corp.
A comprehensive combustion code, PCGC-3, is used to simulate the flow, combustion, and NO pollutant formation processes in an 85 MWe coal-fired utility boiler. The code is used to predict NO emissions from the boiler under various operating conditions. The conditions tested in this study are: over-fire air, % excess air, and burner tilt. Code predictions are compared with effluent NO measurements made in this boiler. These comparisons show good agreement between model predictions for some observed trends, and demonstrate that the computer code is a useful tool that can provide insights into boiler operation. Comparisons that do not show the correct trend suggest that a finer grid resolution is required to correctly predict some trends.
1993
Development and Application of an Acid Rain Precursor Model for Practical Furnaces
Smoot, L.D.; Boardman, R.D.; Brewster, B.S.; Hill, S.C. and Foli, A.K.
Energy & Fuels, 7 (6):786-795, 1993. Funded by ACERC.
Control of emissions of sulfur (SO2, SO3, H2S) and nitrogen (NO, NO2, N2O, HCN, NH3) pollutants from fossil-fuel-fired furnaces and gasifiers remains a vital worldwide requirement as the utilization of fossil fuels continues to increase. Development and refinement of a predictive model for these acid rain precursors (MARP) has reached the point where this technology can contribute to acid rain control. In this paper, model foundations and recent developments are summarized, including formation of thermal and fuel NOx and sorbent capture of sulfur oxides. The method includes global formation, capture, and destruction processes in turbulent, reacting, particle-laden flows. This submodel has been combined with comprehensive, generalized combustion models (PCGC-2, PCGC-3) that provide the required local properties for the combustion or gasification processes. The submodel has been applied to NOx formation in a full-scale (85 MWe), corner-fired utility boiler, where recent in situ NOx measurements were made, with variations in coal feedstock quality (including fuel N percentage) load-level and percentage excess air. Predictions are also made for in situ sorbent capture of sulfur pollutants in both combustion (fuel-lean, SO2), and gasification (fuel-rich, H2S) laboratory-scale reactors. Limitations of MARP are identified and work to improve the submodel is outlined.
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.
Overview of ACERC Comprehensive Model Development
Fletcher, T.H. and Hill, S.C.
Energy & Fuels, 7, (6):870-873, 1993. Funded by ACERC.
A major objective of the Advanced Combustion Engineering Research Center (ACERC) is the development of comprehensive combustion models to help in the solution of critical national combustion problems. Computer models incorporate research and technology results from center projects and from external research programs. The synergistic integration of scientific knowledge that is expected from the NSF engineering research centers is demonstrated to a great extent at ACERC by the development of these software tools. The transfer of technology from ACERC to industry is also accomplished in part by the implementation of the models at industrial firms. The effort to develop such products requires significant integration and development, together with fundamental research. The development of comprehensive models also produces personnel and technology able to help address the challenge of synergistic cross-linkage among thrust areas within ACERC and provides an important means of transferring this technology to industry. This article is an overview of the purpose, accomplishments and goals of research at ACERC in comprehensive modeling. (Thrust Area 5.)
A Comprehensive Three-Dimensional Model for Simulation of Combustion Systems: PCGC-3
Hill, S.C. and Smoot, L.D.
Energy & Fuels, 7 (6):874-883, 1993. Funded by ACERC.
A generalized, three-dimensional combustion model has been developed to simulate large-scale, stead-state, gaseous and particle-laden, reacting and nonreacting systems. The model, which is based on an earlier two-dimensional model, has been applied to turbulent, combustion systems, including pulverized-coal systems. It uses an Eulerian framework for the gas phase and a Lagrangian framework for the particles. The code assumes equilibrium gas-phase chemistry and couples the turbulent flow field with the chemical reactions by integrating the equations over a probability density function. The model uses advanced numerics and a differencing scheme capable of solving the large computational meshes required to simulate practical furnaces. Convective and radiative heat transfer are also modeled. Radiative heat transfer is modeled using the discrete ordinates method. The model has been evaluated by comparison of predictions with experimental data from a large-scale 85-MEe coal-fired utility boiler. The data include furnace profile measurements obtained with intrusive and laser-based optical probes. These comparisons show qualitative agreement of model predictions with observed trends, and indicate that the model can be used to provide insights into boiler operation.
Parametric Sensitivity Study of a CFD-Based Coal Combustion Model
Smith, J.D.; Smith, P.J. and Hill, S.C.
AIChE Journal, 39, (10):1668-1679, 1993. Funded by Combustion Laboratory Consortium through Brigham Young University.
Parametric sensitivity of a two-dimensional pulverized-fuel (PF) combustion model is studied extensively for the effect of parametric uncertainty on model predictions. Results show that error in coal devolatilization/oxidation parameters has the dominant effect on predicted burnout, NOx formation, local gas temperature, and coal-gas mixture fraction. Uncertainty in the turbulent particle dispersion parameters appears to have a secondary effect, while error in the particle-gas radiation parameters has little impact on model predictions. Regions of the computational domain exhibiting sensitivity to specific parameters are identified. Specific parameter sensitivity implies the relative importance of various mechanisms in the overall process. Turbulent particle dispersion seems to be important early in the reactor with kinetic processes dominating at and following the predicted ignition point. Radiation appears to be of minor importance. These results indicate the need for a better method of predicting the overall volatiles yield and further understanding of the devolatilization/oxidation mechanism and its role in the overall PF combustion process. The study provides fundamental direction for future comprehensive model development and focuses on experimental work to better quantify critical input parameters.
1992
Measurements and Predictions of Coal Combustion in a Utility Furnace
Hill, S.C.; Cannon, J.N. and Smoot, L.D.
International Chemical Recovery Conference, Seattle, WA, June 1992. (Also presented at The Effects of Coal Quality on Power Plants, San Diego, CA, August 1992, and the Ninth Annual Pittsburgh Coal Conference, Pittsburgh, PA, August 1992). Funded by ACERC.
A new generalized 3-dimensional combustion model has been developed to simulate large-scale, steady state, particle laden reacting and non-reacting systems. The model, which is based on an earlier 2-dimensional model has been applied to turbulent, pulverized-coal combustion systems. It uses an Eulerian framework for the gas phase and a Lagrangian framework for the particles. The code assumes equilibrium gas-phase chemistry, and couples the turbulent flow field with the chemical reactions by integrating the equations over a probability density function. The model uses advanced numerics and a differencing scheme capable of solving the large computational meshes required to simulate practical furnaces. The model has been evaluated by comparison of predictions with new experimental data from a large-scale, 80 MWe coal-fired utility boiler. The data include furnace profile measurements obtained with intrusive and laser-based optical probes. These comparisons show qualitative agreement of model predictions with observed trends, and indicate that the model can be used to provide insights into boiler operation. The comparisons also indicate the further evaluation, concepts for improvement of some sub models are required, and provide direction for future model development.
1991
Detailed Model for Practical Pulverized Coal Furnaces and Gasifiers, Volume II, User Manual for 1990 Version Pulverized Coal Gasification and Combustion 3-Dimensional (90-PCGC-3), Final Report
Smith, P.J.; Smoot, L.D.; Hill, S.C. and Eaton, A.M.
Advanced Combustion Engineering Research Center, 1991. Funded by US Department of Energy, Consortium and ACERC.
The theoretical foundations, numerical approach and a guide for users of 90-PCGC-3 are presented. 90-PCGC-3 is a generalized, three-dimensional, steady state model that can be used to predict the behavior of a variety of reactive and non-reactive (isothermal) fluid flows. The model solves the Navier-Stokes equations in three-dimensional, Cartesian coordinates. Turbulence is accounted for in both the fluid mechanics equations and the combustion solution scheme. Major gas-phase reactions are modeled assuming local instantaneous equilibrium, and thus the reaction rates are limited by the turbulent rate of mixing. 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.
1987-1986
Parametric Sensitivity Study of an Entrained-Flow Pulverized-Fuel Combustion Model
Smith, J.D.; Smith, P.J. and Hill, S.C.
Submitted for publication to Computer and Chem.Engineering, 1987. 35 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 extensive parametric sensitivity study of a two-dimensional pulverized-fuel (pf) combustion model has been performed to investigate the effect of parametric uncertainty on model predictions. Results illustrate the dominant effect that error in coal devolatilization/oxidation parameters has on predicted burnout, Nox formation, local gas temperature, and coal-gas mixture fraction. Uncertainty in the turbulent particle dispersion parameters appear to have a secondary effect, while error in the particle-gas radiation parameters seems to have little impact on model predictions. Regions of the computational domain exhibiting sensitivity to specific parameters are identified. Specific parameter sensitivity implies the relative importance of various mechanisms in the overall process. Turbulent particle dispersion seems to be important early in the reactor with kinetic processes dominating at and following the predicted ignition point. Radiation appears to be of minor importance for the case under investigation. These results provide unique insight into the general pf combustion process. Specifically, they emphasize the need for a better methodology of predicting the overall volatiles yield. This implies additional understanding of the devolatilization/oxidation mechanism and its role in the overall pf combustion process. Although the results presented here are case specific, they provide unique insight into the overall pf combustion process and illustrate the effects that parameter uncertainty has on model predictions. This information provides fundamental direction for future comprehensive model development and focuses attention on pertinent experimental work to better quantify critical input parameters.
Effects of Swirling Flow on Nitrogen Oxide Concentration in Pulverized Coal Combustors
Smith, P.J.; Smoot, L.D. and Hill, S.C.
AIChE Journal, 32, (11), 1917-1919, 1986. 3 pgs. Funded by US Department of Energy.
Nitrogen-containing pollutants from pulverized coal conversion processes have been of concern for several years, and reviews of this subject have been published (Wendt, 1980; Chen et al., 1981). Swirl of the secondary stream is one technique of fuel-air contacting that causes significant changes in NO concentrations. In some cases, increased swirl increases NO emissions (Heap et al., 1973b; Brown et al., 1977; Pershing and Wendt, 1979). In other cases, increased swirl initially decreases NO emissions (Harding et al., 1982; Asay et al., 1983). Thus, the observed effects of swirl are varied, and indicate that additional parameters influence the NO emissions. In this note a model, described in detail elsewhere (Smith and Smoot, 1981; Smith et al., 1981, 1982; Hill et al., 1984), was used for interpreting effects of inlet stream swirl and velocity on NO emissions during combustion of pulverized coal.