Smoot, LD

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

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

Kinetics of High-Pressure Char Oxidation

Sawaya, R.J.; Allen, J.W.; Hecker, W.C.; Fletcher, T.H. and Smoot, L.D.
ACS Preprint, Div. Fuel Chem., 44, pp xx (Aug 1999).

The kinetics of char oxidation at atmospheric pressure have been much studied and are fairly well agreed upon. However, the kinetics of char oxidation at elevated pressures have not been studied to any significant extent, and standard kinetic models which work at low pressure do not work at high pressure. This paper reports the results of a study to determine the high-pressure kinetics of char oxidation for Pittsburgh #8 char under Zone I conditions. Rate data were obtained for total pressures from one to 64 atmospheres and oxygen mole fractions between 0.03 and 0.40. Temperature dependencies as well as oxygen partial pressure dependencies were determined and the suitability of using various Langmuir-Hinschellwood expressions to fit the data were explored.

PDF Modeling of Lean Premixed Combustion Using In Situ Tabulated Chemistry

Cannon, S.M.; Brewster, B.S. and Smoot, L.D.
Combustion & Flame, 119:233-252 (1999).

The velocity-composition probability density function (pdf) model coupled with a k-?-based mean flow computational fluid dynamics (CFD) model was used to describe the turbulent fluid flow, heat transfer, chemistry, and their interactions in a bluff-body, lean, premixed, methane-air combustor. Measured data [1,2] including velocity, temperature, and chemical species concentrations were used to evaluate the model. The chemistry calculations were performed with an in situ look-up tabulation method [3]. A reduced, 5-step chemical mechanism [4] for describing fuel oxidation, CO, and NO chemistry was used in the model. NO formation from thermal, N2O-intermediate, and prompt pathways was included in the 5-step mechanism. An axisymmetric, unstructured grid was used for solving the Eulerian, mean flow equations and the vertices were used to store mean statistics for solving the Lagrangian, fluid particle equations. Predicted velocity and composition mean statistics were compared to measurements in the bluff-body combustor for a lean equivalence ratio of 0.59. The predictions of major species matched measured and calculated equilibrium values in the recirculation zone. Comparisons of mean CO throughout the combustor were always within an order of magnitude and showed marked improvements over past predictions. Maximum discrepancies between measured and predicted NO concentrations were between 5 and 7 ppm (~50%). The accessed composition space in this turbulent combustion simulation represented the values of species mole fraction and enthalpy for each fluid particle at each time step and was found to lie in a relatively small, uniquely shaped region that was dictated by the mixing, reaction, and heat transfer in the combustor. This accessed composition region was obtained in situ and required about 35 megabytes of storage once a steady state was reached. This memory requirement was more than three orders of magnitude less than would be needed in a standard, a priori table. The in situ tabulation approach allowed for technically correct and efficient chemical kinetic calculations by using the 5-step mechanism in this pdf-method-based, multidimensional combustor model.

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.

Stochastic Modeling of CO and NO in Premixed Methane Combustion

Cannon, S.M.; Brewster, B.S. and Smoot, L.D.
Combustion & Flame, 113:135-146 (1998).

The ability to use reduced CH4-air chemical mechanisms to predict CO and NO emissions in premixed turbulent combustion has been evaluated in a Partially Stirred Reactor (PaSR) model. CO emissions were described with reduced 4-, 5-, and 9-step mechanisms and a detailed 276-step mechanism. NO emissions from thermal, N2O-intermediate, and prompt pathways were included in the 5-, 9-, and 276-step mechanisms. Molecular mixing was described with a deterministic, Interaction-by Exchange-with-the-Mean (IEM) submodel. Random selection and replacement (without repetition) of fluid particles were used to simulate through-flow. The evolution of mean and root mean square (rms) temperature, CO, and NO in the PaSR was accurately described with the 9-step mechanism over a wide range in mixing frequency and equivalence ratio. Also, the 9-step mechanism provided accurate instantaneous reaction rates and concentrations for a broad region of the accessed composition space in the PaSR. The 5-step mechanism performed less reliably than the 9-step mechanism at phi = 1.0 but performed similarly to the 9-step mechanism at phi = 0.65. The 4-step mechanism underpredicted mean CO values and overpredicted instantaneous temperature reaction rates, most likely due to its inferior parent mechanism, partial equilibrium assumption for OH, and unallowed dissociation of neglected radical species. The detailed and reduced mechanism predictions of the accessed composition space in the PaSR covered only a small fraction of the allowable composition space, thus facilitating the use of an efficient in situ chemical look-up table for multidimensional, pdf-method calculations.

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.

Stochastic Modeling of CO and NO in Premixed Methane Combustion

Cannon, S.M.; Brewster, B.S. and Smoot, L.D.
Combustion & Flame, (in press), 1997. Funded by ACERC.

The ability to use reduced CH4-air chemical mechanisms to predict CO and NO emission in premixed turbulent combustion has been evaluated in a Partially Stirred Reactor (PaSR) model. CO emissions were described with reduced 4-, 5-, and 9-step mechanisms and a detailed 276-step mechanism. NO emissions from thermal N2O-intermediate and prompt pathways were included in the 5-, 9- and 276-step mechanisms. Molecular mixing was described with a deterministic, Interaction-by-Exchange-with-the-Mean (IEM) submodel. Random selection and replacement (without repetition) of fluid particles was used to simulate through-flow. The evolution of mean and rms temperature, CO, and NO in the PaSR was accurately described with the 9-step mechanisms over a wide range in mixing frequency and equivalence ratio. Also, the 9-step mechanism provided accurate instantaneous reaction rates and concentrations for a broad region of the accessed composition space in the PaSR. The 5-step mechanism performed less reliably than the 9-step mechanism at phi = 1.0 but performed similarly to the 9-step mechanism at phi = 0.65. The 4-step mechanism underpredicted mean CO values and overpredicted instantaneous temperature reaction rates, most likely due to its inferior parent mechanism, partial equilibrium assumption for OH, and unallowed dissociation of neglected radical species. The detailed reduced mechanism predictions of the accessed composition space in the PaSR covered only a small fraction of the allowable composition space, thus facilitating the use of an efficient, in situ chemical look-up table in multi-dimensional, pdf-method calculations.

Comprehensive Model for Lean Premixed Combustion in Industrial Gas Turbines - Part I. Validation

Cannon, S.M.; Brewster, B.S.; Smoot, L.D.; Murray, R. and Hedman, P.O.
Presented at the Spring Meeting of the Western States Section/The Combustion Institute, Sandia National Laboratories, Livermore, California, April 14-15, 1997. Funded by US Department of Energy.

The velocity composition pdf model coupled with a mean flow CFD model was used to describe the turbulent fluid flow, heat transfer, chemistry, and their interactions in a swirling, lean premixed, methane-air combustor for which laser-based measurements of mean velocity and temperature were made. A flame was stabilized in this axi-symmetric, lab-scale, gas-turbine combustor (LSGTC. A reduced, 5-step chemical mechanism, for describing fuel oxidation and NO chemistry, was used in this LSGTC model. NO emissions from thermal, N2)-intermediate, and prompt pathways were described with this 5-step mechanism. The chemistry calculations were performed efficiently with and in-situ look-up table. An axi-symmetric, unstructured grid consisting of 2283 vertices and 4302 triangular elements was used for solving the Eulerian, mean flow equations and the vertices were used to store mean statistics for solving the Lagrangian, fluid particle (~310,000 fluid particles) equations. Predicted velocity and composition statistics were compared to measurements in the LSGTC for lean equivalence ratios of 0.8 and 0.65. The comparisons of predicted mean velocity and temperature were reasonable good throughout the combustor. The location and magnitude of peak axial velocity was well represented by the model at near inlet regions, through the negative mean axial velocity in the internal recirculation zone was over-predicted. The predicted maximum mean temperature and the penetration zone of the cold unburned fluid were in reasonable agreement with the experimental data. Correct trends in CO and NO with equivalence ration were predicted with the model. The in situ tabulation method was used to represent the chemical kinetics in this axi-symmetric combustor without requiring significant CPU time and memory. The model is currently being applied to simulate 3-dimensional, gas-turbine combustor geometries and is described in a companion paper.

Comprehensive Model for Lean Premixed Combustion in Industrial Gas Turbines - Part II. Application

Meng, F.L.; Brewster, B.S. and Smoot, L.D.
Presented at the Spring Meeting of the Western States Section/The Combustion Institute, Sandia National Laboratories, Livermore, California, April 14-15, 1997. Funded by US Department of Energy.

A new comprehensive COmbustion Simulation MOdel for Gas Turbines (COSMO/GT) has been developed for simulating modern gas turbine combustors. The model includes the capability of simulating lean premixed combustion of methane (or natural gas) and air, and uses and unstructured-grid flow solver to accommodate geometrical complexity. In our earlier paper we extended the velocity-composition PDF approach to an unstructured grid platform for modeling two-dimensional, axisymmetric, lean premixed turbulent combustion in a lab-scale gas turbine combustor. In this paper, the extension of this PDF approach to a three-dimensional, unstructured grid is reported. The turbulence/chemistry interaction is modeled using the velocity-composition, Monte-Carlo PDF approach coupled with a five-step kinetic mechanism of methane and air for calculating CO and NO emissions. In order to increase the calculation speed of the PDF algorithm, in situ tabulation for chemical reaction and a zonal search method for locating particle positions are used. Validation of this model for and axisymmetric, lab-scale gas turbine combustor is described in a companion paper. Application of this model has been initiated by modeling lean, premixed combustion of natural gas and air in three-dimensional gas turbine combustors.

A Decade of Combustion Research

Smoot, L.D.
Prog. Energy Combust. Sci., 23:203-32 (1997). Funded in part by ACERC

The Advanced Combustion Engineering Research Center (ACERC) at Brigham Young University and the University of Utah, in cooperation with other universities, 37 industrial members, and six governmental members, has marked a decade of combustion research. The review emphasizes the contributions of ACERC over this past decade. While the state-of-the-art relating to fossil fuel combustion is discussed, the paper does not treat the substantial contributions of other researchers to recent advances in combustion science and technology. The mission of ACERC has been to develop advanced combustion technology through fundamental engineering and scientific research and educational programs aimed at the solution of critical national combustion problems. These programs have been designed to contribute to the Center's focus on the clean and efficient use of fossil fuels and waste materials, particularly coal and other low-quality fuels. The average annual ACERC budget over this decade has been about $4 million, with about 40% form NSF (ERC), 25% from industry, 20% from other federal and state grants, and 15% from participation universities. ACERC has 38,000 square feet of administrative, computational and laboratory space at the two universities. Research equipment exceeds a value of $16 million. The team of about 140 researchers typically includes 30 faculty and professionals from nine academic departments, 10 post-doctoral associates, and 100 students at doctoral, masters and bachelor levels. Since ACERC was initiated in May of 1986, five books and over 700 journal and conference manuscripts have been publishes. Basic experimental research has provided insights, parameters and submodels toward development of comprehensive combustion models for industrial use. Eight software products from ACERC have been licensed to industries. Six new companies can trace their origins, in part, to ACERC. New NOX control concepts have been developed under an advanced pulverized coal system development program of the Department of Energy. New work in coal structure, coal reaction processes and rates, methods of acid rain control, turbulent reacting flows, fuel minerals behavior, and fuel and waste conversion processes has given new insights into complex combustion processes, while new combustion modeling software products for large furnaces, gasifiers and rotary kilns are being used by industry. ACERC has also built a unique set of small, highly-instrumented, pilot-scale test-bed facilities which allow industrial and academic researchers to characterize the combustion of fuels and wastes in high-temperature furnaces, rotary kilns, fixed beds, fluidized beds (both bubbling and circulating), stokers, and gasifiers. Many of the Center's significant accomplishments can be categorized into five areas: (1) comprehensive combustion model development, (2) combustion submodel development, (3) pollutant emission, (4) air toxics, and (5) advanced combustion system testing. This overview highlights selected research accomplishments of ACERC during the past decade.

1996

Devolatilization of Large Coal Particles at High Pressure

Eatough, C.N. and Smoot, L.D.
Fuel, 75(3):1601-1606, 1996. Funded by ACERC.

Devolatilization times of large (0.1 and 0.2g) Utah hvBb and North Dakota lignite coal particles, in the range 15-30 s, were measured in the air at 101 and 507 kPa, at air temperatures of 900 and 1200 K in a connective flow reactor. Visual observations indicated infrequent heterogeneous ignition of the lignite prior to devolatilization and occasional explosion of bituminous coal particles during devolatilization. Devolatilization times were correlated with the temperature, pressure and particle size. Power-law exponents for tests at 101 kPa and 900 K were determined to be 2.5 for Utah hvBb coal and 2.2 for North Dakota lignite. At 507 kPa and 900 K, exponents decreased to 1.6 for both Utah and North Dakota lignite.

Model Comparisons with Drop Tube Combustion Data for Various Devolatilization Submodels

Brewster, B.S.; Smoot, L.D. and Barthelson, S.H.
Energy & Fuels, 9:870-878, 1996. Funded by ACERC.

Predictions of a two-dimensional, axisymmetric combustion model, using various devolatilization submodel options, are compared with new experimental data from a near-laminar, drop-tube furnace. Included in the devolatilization submodels that were teste are the commonly used empirical one-and two-step models and a chemical, coal network model with parameters based on coal structure. The goals of this work were to evaluate the latter approach as compared with the simple, empirical approach usually used in such calculations and to assess the role of turbulence in a near-laminar reacting flow. Comparisons were made for carbon conversion, radially averaged oxygen and near-effluent NOX concentrations, for a range of coal types and equivalence ratios. The predictions quantify an ignition delay that is consistent with the measurements. Computations with the fundamental, chemical devolatilization submodel gave superior predictions of mass loss when the coal type was within the interpolation range of the submodel parameter database. Accuracy declined significantly when the coal type was outside the interpolation range. Inclusion of the effects of turbulence was required to account for the observations. Near effluent NO predictions with the chemical submodel agreed with measured NOX values to within an average of about 20 percent.

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).

A Computational Method for Determining Global Fuel-NO Rate Expressions. Part 1

Chen, W.; Smoot, L.D.; Fletcher, T.H. and Boardman, R.D.
Energy & Fuels, 10(5):1036-1045, 1996. Funded by ACERC.

Global chemical reaction rates used in the modeling of NOx formation in comprehensive combustion codes have traditionally been obtained trough correlation of experimental data. In this paper, a computational approach for obtaining global rates is presented. Several premixed flames were simulated, and sensitivity analysis of species concentration profiles was used to suggest global pathways in fuel-nitrogen conversion to NO. Based on these analyses, the global reaction rates were formulated. The predicted species concentration profiles and their derivatives were then used in the determination of the global rate constants. The correlation of rate constants for the two fuel-NO global rates (HCN + NO N2 + . . . and HCN + O2 NO + . . .) are discussed. Comparisons of the computed global rate constants with those rate constants with those deduced from experimental data show good agreement. The global rates provide practical kinetics for simulating nitrogen pollutant chemistry in complex flames.

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.

Modeling of Industrial Gas Turbine Combustors

Meng, F.L.; Farmer, J.R.; Brewster, B.S. and Smoot, L.D.
Proceedings of the Fall 1996 Meeting of the Western States Section / The Combustion Institute, The University of Southern California, Los Angeles, California, October 28-29, 1996. Funded by ACERC.

A new model has been developed for simulating modern gas turbine combustors. The new model includes the capability of simulating lean, premixed combustion of methane (or natural gas) and air, and uses and unstructured-grid flow solver to accommodate geometrical complexity. The set of incompressible, steady state, Navier-Stokes equations is solved using a co-located, equal-order, control volume finite element method. The convection term is treated using the mass-weighted, skew upwind scheme, and the diffusion, pressure gradient, and source terms are interpolated linearly in each element. Turbulence is modeled using the kappa-epsilon model. Convective and radiative heat losses are modeled using a wall function method and a discrete ordinates method, respectively. The interaction between turbulence and chemistry is modeled using the velocity-composition Monte-Carlo PDF approach, coupled with a multiple-step reaction mechanism for methane and air. Validation of the code has been initiated by modeling lean, premixed combustion (Phi = 0.8) of natural gas and air in a laboratory-scale, gas turbine combustor with a simple, two-step kinetic mechanism for CH4-O2. Comparison with detailed measurements is forthcoming. Application to industrial gas turbine combustor components is also underway.

Prediction of CO and NOx in Lean Premixed Turbulent Combustion

Cannon, S.M.; Brewster, B.S. and Smoot L.D.
Proceedings of the Fall 1996 Meeting of the Western States Section/The Combustion Institute, The University of Southern California, Los Angeles, California, October 28-29, 1996. Funded by ACERC.

The ability to use reduced CH4-air chemical mechanisms to predict CO and NOx emissions in lean premixed turbulent combustion has been evaluated in a Partially Stirred Reactor (PaSR) model. CO emissions were described with mathematically reduced 4-, 5- and 9-step mechanism and a detailed 276-step mechanism. NOx emission form thermal, N2O-intermediate, and prompt pathways were described with the 5-, and 9-step reduced mechanisms provided accurate instantaneous reaction rate calculations for a broad region of the accessed composition space in the PaSR. The 4-step mechanism and the partial equilibrium assumption for OH. Practicality of using the 5- and 9-step mechanisms in industrial, 3-dimensional calculations may require the use of a novel, in situ look-up table.

1995

An Improved Model for Fixed-Bed Coal Combustion and Gasification: Sensitivity Analysis and Applications

Ghani, M.U.; Radulovic, P.T. and Smoot, L.D.
Fuel, 74:1213-1226, 1995. Funded by ACERC and US Department of Energy/Morgantown Energy Technology Center.

Detailed sensitivity analysis and applications of an improved, comprehensive, one-dimensional model for combustion and gasification of coal in fixed beds, FBED-1, are presented. The effects of the devolatilization, oxidation and gasification submodels on the model predictions are discussed. The product gas compositions predicted by various options for gas-phase chemistry are shown. The effects of five model parameters and on operational variable on the predictions of the model are also presented. The sensitivity analysis presented is quantitative since the boundary conditions for both the feed coal and the feed gas streams are satisfied exactly. The utility of the model as a design and analysis tool is demonstrated by simulating two gasifiers: and METC medium-pressure gasifier, and a PyGas high -pressure staged gasifier. Submodels and areas that need further improvements are identified; among these are submodels for large-particle devolatilization, oxidation and gasification and a robust solution method suitable for stiff, highly non-linear problems. Additional features that should be implemented to develop a model for general industrial applications are also identified. These include provisions for additions and withdrawals of gases at multiple locations and options for different flow configurations.

1994

The Structure and Reaction Process of Coal

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Plenum Press Co., New York, 1994. Funded by ACERC.

This book characterizes the properties and reaction rates of the eleven U.S. coals selected emphasis by ACERC. Eight of the eleven comprise the Argonne National Laboratory's Premium Coal Sample Bank. The book features the comprehensive measurement of organic and inorganic components of the coal structure by advanced methods (SEM, NMR, GC, porosimetry, Pycnometry, X-ray, and MS). The book features the measurement of coal devolatilization and char oxidation rates by advanced, optical methods and correlative relationships between the structure and reaction processes.

Rates of Oxidation of Millimeter-Sized Char Particles: Simple Experiments

Blackham, A.U.; Smoot, L.D. and Yousefi, P.
Fuel, 73:602-612, 1994. Funded by US Department of Energy/Morgantown Energy Technology Center through Advanced Fuel Research Co. and ACERC.

Rates of oxidation of 5-10 mm particles of chars from six coals at various temperatures were measured in air at ambient pressure in simple devices: a muffle furnace, a Meker burner, and a heated ceramic tube. The chars were first prepared from the coals in the Meker burner at comparable temperatures. As well as coal type and oxidation temperature, initial char particle steps of several minutes for periods up to 1 h. The cube root of particles mass decreased linearly with increasing time in all tests. Ash layers formed and usually remained in place around the particle. Average mass reactivities increased with decreasing initial particle mass. With decreasing furnace temperature, char reactivity decreased at the lower temperatures. Two or four closely spaced char particles burned much more slowly than single particles of the same size. Correlative equations are consistent with the data, elucidating the roles of kinetic reaction and oxygen diffusion.

Effect of Pressure on Oxidation Rates of MM-Sized Char

Bateman, K.J.; Smoot, L.D.; Germane, G.J.; Blackham, A.U. and Eatough, C.N.
Fuel, 1994 (in press). Funded by US Department of Energy/Morgantown Energy Technology Center and ACERC.

Mass loss and burnout ties of large (five and eight millimeter diameter) char particles at pressures between 101 to 760 kPa have been measured in a newly designed and constructed high-pressure reactor. A cantilever balance attachment was fitted to the reactor to measure instantaneous particle mass while an optical pyrometer measured particle temperature continuously. The process was also videotaped at 1/30 s frame speed. Sixty-two combustion experiments produced burning and oxidation times for two sizes of Utah bituminous (HVBB) coal and North Dakota Lignite (L) at 101, 507, 760 kPa total pressure. The reactor air temperatures were about 900 or 1200 K while the airflow Reynolds Number was varied by a factor of two. Coal particles were placed in a platinum-wire basket inside the reactor at the end of the balance beam. The oxidation process was recorded by computer and on videotape, while continuous char oxidation rates were measured to burnout. An ash layer accumulated around the particles, and receded as the char was consumed. In all of the tests, including the elevated pressure tests, a linear decrease in the cube root of char mass with time was observed during char oxidation until near the end of burnout. Changes in air velocity had little effect on oxidation times while either increasing gas temperature or increasing pressure from 101 kPa to 507 kPa reduced oxidation times by about one-quarter. Further increase in pressure caused no further reduction in burn time. Pairs of nearly equally sized particles of coal had oxidation times similar to single particles that had a mass equal to the sum of the pairs.

A Facility for High-Pressure Measurement of Reaction Rates of MM-Sized Coal

Bateman, K.J.; Germane, G.J.; Smoot, L.D. and Eatough, C.N.
Energy & Fuels, 1994 (in press). Funded by US Department of Energy/Morgantown Energy Technology Center and ACERC.

A study was undertaken to design, construct, characterize, and demonstrate a new facility for determination of reaction rates of large coal particles at elevated pressures. A cantilever balance attachment (CBA) was designed, fabricated, and utilized in conjunction with the existing High Pressure Controlled Profile (HPCP) reactor. Large particle (8mm diameter) combustion experiments of Utah HVBB coal at both atmospheric and elevated pressures were performed to demonstrate the facility's capabilities. Measurements were obtained of particle mass loss rate and surface temperature coupled with a video record for visual observation.

Combustion and Gasification of Coals in Fixed-Beds

Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
Prog. Energy Combustion Science, 19:505-586, 1993. Funded by US Department of Energy/Morgantown Energy Technology Center through Advanced Fuel Research Co. and ACERC.

Fixed-bed processes are commercially used for the combustion and conversion of coal for generation of power or production of gaseous or liquid products. Coal particle sizes in fixed-bed processes are typically in the mm to cm diameter range, being much larger than in most other coal processes. This review provides a broad treatment of the technology and the science related to fixed-bed systems. Commercialized and developmental fixed-bed combustion and gasification processes are explored, including countercurrent, concurrent, and crosscurrent configurations. Ongoing demonstrations in the U.S. Clean Coal Technology program are included. Physical and chemical rate processes occurring in fixed-bed combustion are summarized, with emphasis on coal devolatilization and char oxidation. Mechanisms, rate data and models of these steps are considered with emphasis on large particles. Heat and mass transfer processes, solid flows, bed voidage, tar production and gas phase reactions were also considered. Modeling of fixed-bed processes is also reviewed. Features and assumptions of a large number of one- and two-dimensional fixed-bed combustion and gasification models are summarized while the details of a recent model from this laboratory are presented and compared with data. Research needs are also discussed.

An Improved Model for Fixed-Bed Coal Combustion and Gasification

Radulovic, P.T.; Ghani, M.U. and Smoot, L.D.
Fuel, 1994 (in press). Funded by US Department of Energy/Morgantown Energy Technology Center and ACERC.

An improved one-dimensional model for countercurrent oxidation and gasification of coal in fixed beds has been developed. The model incorporates an advanced devolatilization submodel that can predict the evolution rates and the yields of individual gas species and tar. A split, back-and-forth, shooting methods is implemented to exactly satisfy the boundary conditions for both the feed coal and the feed gas streams. An option to switch between equilibrium and non-equilibrium gas phase composition has been added. The model predictions are compared with the experimental data for two coals; a Jetson bituminous coal and a Rosebud subbituminous coal. An illustrative simulation for an atmospheric, air-blown, dry ash, Wellman-Galusha gasifier, fired with the Jetson bituminous coal, is presented. Areas that need additional improvements are identified.

Fossil-Fuel Conversion - Measurement and Modeling

Solomon, P.R.; Serio, M.A.; Hamblen, D.G.; Smoot, L.D.; Brewster, B.S. and Radulovic, P.T.
Proceedings of the Coal-Fired Power Systems 94 - Advances in IGCC and PFBC Review Meeting, Morgantown, West Virginia, June 1994. Funded by US Department of Energy/Morgantown Energy Technology Center.

The main objective of this program is to understand the chemical and physical mechanisms in coal conversion processes and incorporate this technology for the purposes of development, evaluation in advanced coal conversion devices. To accomplish this objective, this program will: 1) provide critical data on the physical and chemical processes in fossil fuel gasifiers and combustors; 2) further develop a set of comprehensive codes; and 3) apply these codes to model various types of combustors and gasifiers (fixed-bed transport reactor, and fluidized-bed for coal and gas turbines for natural gas).

To expand the utilization of coal, it is necessary to reduce the technical and economic risks inherent in operating a feedstock which is highly variable and which sometimes exhibits unexpected and unwanted behavior. Reducing the risks can be achieved by establishing the technology to predict a coal's behavior in a process. This program is creating this predictive capability by merging technology developed at Advanced Fuel Research, Inc. (AFR) in predicting coal devolatilization behavior with technology developed at Brigham Young University (BYU) in comprehensive computer codes for modeling of entrained-bed and fixed-bed reactors and technology developed at the U.S. DOE-METC in comprehensive computer codes for fluidized-bed reactors. These advanced technologies will be further developed to provide: 1) a fixed-bed model capable of predicting combustion and gasification of large coal particles, 2) a transport reactor model, 3) a model for lean premixed combustion of natural gas, and 4) an improved fluidized-bed code with an advanced coal devolatilization chemistry submodel.

1993

Coal Characteristics, Structure, and Reaction Rates

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Chapter 3, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

The purposes of this chapter are to document, correlate, synthesize, integrate and relate the structural characterization and reaction rates of the suite of ACERC coals. The focus has been on research projects sponsored by ACERC. However, related research work outside of ACERC has also been considered. This chapter (1) reviews the selection of the suite of research coals, (2) reviews the origin of coal which gives rise to the various structural moieties in coal, (3) reviews coal characterization programs and documents the structure and characteristics of the research coals, (4) reviews coal reaction mechanisms, (5) explores the relationships of coal structure to devolatilization and char oxidation reaction rates, and (6) considers the models being developed which predict reaction characteristics based on structurally dependent parameters. Research programs in the field are still very active, the models are still in the formative states, reaction rates for the selected research coals are being measured, and the reaction processes for these coals have yet to be fully explored using the structurally dependent models. Further results will undoubtedly be forthcoming. This chapter is a condensed version of a larger work to characterize the structure and conversion processes of the research coals.

The Structure and Reaction Processes of Coal

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Plenum Publishing Corp., The Netherlands, 1993 (in press). Funded by ACERC.

This new ACERC book documents and integrates the current understanding of the organic and inorganic structure of coal and its reaction processes. Work in ACERC forms the foundation while the book attempts to include pertinent worldwide results. The book cites more than eight hundred references, almost all within the past decade, while the large majority are from various researchers around the world.

Eleven U.S. coals of various rank are emphasized in the book. These commonly used and highly characterized eleven coals form the research coals for ACERC and include all eight coals of the Argonne National Laboratory's Premium Coal Sample Bank. Altogether, the book contains six chapters. After an introduction, Chapter 2 documents the selection and characteristics of the suite of eleven coals, and relates them to various national databases. Chapter 3 deals with the geochemical history of coal and its macromolecular structure. Chapter 4 describes advanced analytical methods for measuring organic and inorganic structure of coal and documents results for the eleven coals. Chapters 5 and 6 treat the reaction processes of coals and chars. Recent model developments that relate fuel structure to yields and reaction rates are presented and compared to rate and yield data. Important measurements from the coal suite and other coals are reported and related to coal structure.

Laying a foundation for the future, this book has been written at a time when progress in this area is dramatic. The authors acknowledge that new results will be published at a rapid rate. What we have sought to accomplish through the writing of this manuscript is to promote increasing cooperative focus regarding the understanding of coal structure and its reaction and conversion processes. From this perspective, the book is thought to be the first of its kind.

The Structure and Reaction Processes of Coal

Smith, L.K.; Smoot, L.D.; Fletcher, T.H. and Pugmire, R.J.
Plenum Press Co., New York, 1993 (in press). Funded by ACERC.

This book characterizes the properties and reaction rates of the eleven US coals selected for emphasis by ACERC. Eight of the eleven comprise the Argonne National Laboratory's Premium Coal Sample Bank. The book features the comprehensive measurement of organic and inorganic components of the coal structure by advanced methods (SEM, NMR, GC, porosimetry, pycnometry, X-ray, and MS). The book features the measurement of coal devolatilization and char oxidation rates by advanced, optical methods and correlative relationships between the structure and reaction processes.

Rates of Millimeter-Sized Char Particle Oxidation: Simple Experiments

Blackham, A.U.; Smoot, L.D. and Yousefi, P.
Fuel, 1993 (in press). Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC.

Oxidation rates of large (5-10 mm) coal particles are required in the description of fixed- and fluidized-bed combustion, gasification and mild gasification processes. Yet, very little has been published regarding these rates. In this study, rates of oxidation of chars for six coals at various temperatures were measured in simple devices in air at ambient pressure: in a muffle furnace, a Meker burner, and a heated ceramic tube. Chars were first prepared from the coals in the Meker burner at comparable temperatures. Test variables were coal type, oxidation temperature, initial char particle mass and number of particles. Char particles were oxidized in incremental steps, each over several minutes for time periods up to one hour. The cube root of particle mass declined linearly with time in all tests. Ash layers formed and usually remained in place around the coal particle. Average mass reactivities increased with decreasing initial char particle mass. Decreasing furnace temperature decreased char reactivity at the lower temperatures. Two or four char particles, closely spaced, burned at much slower rates than single particles of the same size. Correlative methods are consistent with the data, which elucidate the roles of kinetic reaction and oxygen diffusion.

Ash Formation and Deposition

Benson, S.A.; Jones, M.L. and Harb, J.N.
Chapter 4, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

This chapter discusses the fundamental and applied aspects of coal ash formation and deposition. Ash deposition on heat-transfer surfaces has been examined for many years, resulting in voluminous literature on the subject. However, a precise and quantitative knowledge of the chemical and physical transformations of the inorganic components in coal during combustion has not been obtained due to the inability to quantitatively determine the inorganic composition of coal and to understand the complexity of the processes involved. The status of predictive methods for the fate of the inorganic constituents during combustion as a function of coal composition and combustion conditions is discussed herein. The composition of the coal ash produced under ASTM ashing conditions is used in most methods as an approximate guide to predict the behavior of inorganic constituents of a specific coal during combustion. This ashing technique can be used to predict average properties of the ash; however, examination of fly ash shows that many different types of particles are present, each having its own composition and probably its own melting behavior. Therefore, the behavior of individual fly ash particles may be very different from the predicted for the average ash composition. The extent of ash-related problems depends upon the quantity and association of inorganic constituents in the coal, the combustion conditions, and the system geometry. The inorganic constituents are distributed within the coal matrix in several forms, including organically associated inorganic elements; coal-bound, included minerals; and coal-free, excluded minerals. The primary mineral groups that are found in all coals consist of clay minerals, carbonates, sulfides, oxides, and quartz. However, the specific types of inorganic components present depend upon the rank of the coal and the environment in which the coal was formed.

Pollutant Formation and Control

Boardman, R.D. and Smoot, L.D.
Chapter 6, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

By far the most striking problem associated with human consumption of fossil fuels is the control of air pollutants. The inexorable trend for increasing power demand, both by industrially established countries and developing nations, will inevitably lead to increased production and utilization of fossil fuels. The combustion of fossil fuels produces both primary and secondary pollutants. Primary pollutants include all species in the combustor exhaust gases that are considered contaminants to the environment. The major primary pollutants include CO, hydrocarbons, sulfur-containing compounds, nitrogen oxides, particulate materials, various trace metals, and even CO2 that reached pollutant status with increasing concern for global atmospheric warming or the "greenhouse" effect. Secondary pollutants are defined as environmentally detrimental species that are formed in the atmosphere as a consequence of precursor combustion emissions. The list of secondary pollutants includes particulate matter and aerosols that accumulate in the size range of 0.1-10 µm diameter, NO2, O3, and other photochemical oxidants, and acid vapors. The connection between combustion-generated pollutants and airborne toxins, acid rain, visibility degradation, the greenhouse effect, and stratospheric ozone depletion is well established. The detrimental impact of these contaminants on ecosystems in the biosphere and stratosphere has been the impetus of stricter standards around the world. This chapter emphasizes the formation and control of nitrogen oxides (referred to as NOx pollutants) and sulfur-containing pollutants (referred to as SO pollutants) in which temperature stationary combustors. The focus is on the formation of nitrogen oxides since they can be effectively controlled in the combustion chamber. An understanding of nitrogen chemistry and furnace fluid dynamics is imperative to optimizing in-situ nitrogen oxide control schemes. A brief review on the development of mathematical tools used to predict nitrogen and sulfur oxide formation during combustion of fossil fuels will also be presented, In addition, an overview and comparison of NOx and sulfur pollutant abatement strategies is given.

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.

Comparison of Measurements and Predictions of Flame Structure and Thermal NOx, in a Swirling, Natural Gas Diffusion Flame

Boardman, R.D.; Eatough, C.N.; Germane, G.J. and Smoot, L.D.
Combustion Science and Technology, 20: 1-18, 1993. (Previously presented at the First International Conference on Combustion Technologies For a Clean Environment, Vilamoura, Portugal, September 1991). Funded by Morgantown Energy Technology Center.

A combined thermal and fuel nitric oxide submodel has recently been added to a generalized, 2-dimensional pulverized coal gasification and combustion model (PCGC-2). This model is applicable to reacting and non-reacting gaseous and particle-laden flows. The thermal NO model is based on the extended Zel'dovich mechanism. To perform an evaluation of the NOx submodel, combustion measurements of gas velocities, temperatures, and species concentrations were made in a laboratory-scale, experimental reactor with a 150 kW natural gas flame at an equivalence ratio of 1.05 and a secondary-air swirl number of 1.5. Combustion measurements of velocities and major species concentrations show generally good agreement with predicted values. Gas temperature measurements closely match predictions in the recovery region but fail to show predicted high temperature in the annular region. This study provides an evaluation of a comprehensive combustion model and the NOx submodel that can be useful as a design tool to provide pollutant formation trends in applied systems as combustion parameters are varied.

Turbulent Reacting Flows

McMurtry, P.A. and Queiroz, M.
Chapter 7, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

This chapter summarizes current technologies and developments for treating reacting turbulent flows. Accurately predicting the effects of turbulence on combustion processes ranks among the most challenging problems in the engineering sciences. Since most combustion occurs in a turbulent environment, effects of turbulence must be treated in a realistic manner in any predictive method. In addition to the effects of turbulence on the chemical species transport, density changes resulting from exothermic chemical reactions can alter the structure and development of the flow field. As such, the fluid mechanics, thermodynamics, and chemistry are strongly coupled, making the description of the turbulent combustion process extremely difficult. Moreover, other complications often arise. Among these are the effects of multiple phases, an inherent aspect of entrained solids and liquids. This chapter illustrates some of the characteristics of turbulent flows and outlines some of the methods used to treat turbulence in reacting flows, while pointing out the need for improved capabilities in this field of study. First, general background on turbulent flows is provided. The most popular approaches used to model turbulence in reactive systems for engineering applications are discussed, along with a few newly introduced techniques. Some of the complications that arise in multi-phase turbulent flow are also discussed. More computationally intensive numerical approaches, used primarily in fundamental research applications, are presented. These methods include Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). Direct simulation results for simplified flow configurations are discussed in light of the information they have provided concerning the physics of turbulent reacting flows.

Radiative Heat Transfer

Mengüç, M.P. and Webb, B.W.
Chapter 5, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

This chapter presents methods and data for reacting radiative heat transfer in coal combusting systems. The key factors to be considered in pulverized coal combustion have been outlined as 1) turbulent fluid mechanics, 2) gaseous turbulent combustion, 3) particle dispersion, 4) heterogeneous char reactions, 5) radiation heat transfer, 6) coal devolatilization, 7) ash/slag formation, and 8) pollutant formation. In most global coal combustion prediction methodologies, each of these facets is modeled separately, and then coupled with the others in a global prediction scheme. An intimate coupling exists between these different phenomena in a heterogeneous combustion system. The radiation transport lies at the very heart of this coupling, particularly since it is the dominant mode of heat transfer even in moderately scaled pulverized coal combustion systems. In this chapter, we concentrate on the fundamentals of radiation transfer and its application to pulverized-coal combustion systems.

Comprehensive Modeling

Brewster, B.S.; Hill, S.C.; Radulovic, P.T. and Smoot, L.D.
Chapter 8, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

This chapter treats comprehensive modeling of combustion and gasification systems. Entrained, fluidized and fixed bed models are considered. Single and multidimensional models are reviewed. Developing comprehensive computer models to help design combustors and gasifiers for clean and efficient utilization of coal and other fossil fuels is a primary objective of ACERC. Such models provide not only a framework for effectively integrating combustion-related technology from a wide array of disciplines, but a vehicle for transferring this technology to industry. In order to be useful, these models must satisfy at least three criteria: First, the input and output must be easily accessible (user-friendly graphics must play a role here). Second, the computer algorithms must be robust and computationally efficient. And third, the models must be thoroughly evaluated to demonstrate applicability to industrial processes and to justify confidence in their predictions. Developing and implementing user-friendly, robust, efficient, applicable, accurate models requires significant, on-going effort that is reviewed herein.

Coal Processes and Technologies

Radulovic, P.T. and Smoot, L.D.
Chapter 1, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

This chapter discusses current coal combustion and gasification processes and technologies, with emphasis on clean and efficient use. Entrained, fluidized and fixed beds together with MHD generation and fuel cell cycles are treated. Coal is the world's most abundant fuel. Most of the coal presently being consumed is by direct combustion of finely pulverized coal in large-scale utility furnaces for generation of electric power, and this is likely to remain the way through the end of this century. However, many other processes for the conversion of coal into other products or for the direct combustion of coal are being developed and demonstrated, including various coal combustion and gasification processes. Several other processes and technologies such as underground coal gasification, magnetohydrodynamic generators, and fuel cells are also being developed, as discussed herein. Increasing the use of coal presents many technical problems, particularly in protecting environment while maintaining or increasing efficiency. In order to solve these problems and increase the use of coal, the USA and many other countries in the world are supporting research and development of clean coal technologies that are summarized in this chapter.

Structure of a Near-Laminar Coal Jet Flame

Brewster, B.S. and Smoot, L.D.
Energy & Fuels, 7 (6):884-890, 1993. Funded by US Department of Energy.

Flame data from a near-laminar coal jet have been compared with model predictions. Inclusion of gas turbulence with laminarization was necessary for adequately predicting the upper-flame and postflame regions and for predicting particle dispersion. Dispersion of gas and particles was insensitive to inlet turbulence intensity. Gas buoyancy induced radially inward flow that opposed particle dispersion. Gas temperature was predicted too high near the coal nozzle, perhaps due to neglecting finite-rate mixing of volatiles with the bulk gas and chemical kinetics effects. Single-particle burning effects were important in the flame zone, as evidenced by the sensitivity of particle temperature to direct enthalpy feedback from volatiles combustion. Particle burnout was insensitive to enthalpy feedback, heterogeneous CO2 formation, and chemistry/turbulence interaction.

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.

Modeling of Coal Conversion Processes in Fixed Beds

Ghani, M.U.; Radulovic, P.T. and Smoot, L.D.
American Chemical Society, Division of Fuel Chemistry, 38: 1358-1369, 1993. Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC.

An advanced, one-dimensional fixed-bed coal gasification and combustion model is presented. The model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on functional groups and depolymerization, vaporization and cross-linking, oxidation and gasification of char, and partial equilibrium in the gas phase. The model is described by 191 highly non-linear, coupled, first order differential equations. Due to the countercurrent nature of the gas and solids flow the system of equations constitutes a split-boundary value problem that is solved by converting it to an initial value problem. This paper presents a split back-and-forth shooting technique that exactly satisfies conditions at both the upper and the lower boundary and provides significant improvements in the predictions. Comparisons of the predicted and experimental results for an atmospheric, air-blown Wellman-Galusha gasifier fired with Jetson bituminous coal are presented.

User's Manual for 93-PCGC-2:Pulverized Coal Gasification and Combustion Model (2-Dimensional) with Generalized Coal Reactions Submodel FG-DVC

Brewster, B.S.; Boardman, R.D.; Huque, Z.; Berrondo, S.K.; Eaton, A.M.; Smoot, L.D.; Zhao, Y.; Solomon, P.R.; Hamblen, D.G.; Serio, M.A.; Charpenay, S.; Best, P.E. and Yu, Z.-Z.
US Department of Energy/Morgantown Energy Technology Center/Advanced Fuel Research/Brigham Young University Final Contract Report, Vol. II, 1993. Funded by US Department of Energy and Morgantown Energy Technology Center.

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 93-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, walls, and particles is taken into account using a discrete ordinates method. The particle phase is modeled in a Lagrangian framework, such that mean paths of particle groups are followed. A new coal-general devolatilization submodel (FG-DVC) with coal swelling and char reactivity submodels has been added. The heterogeneous reaction scheme 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 thermal and fuel NOx finite rate chemistry submodel is included which integrates chemical kinetics and the statistics of the turbulence. A sorbent injection submodel with sulfur capture is included. 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. Both combustion and gasification environments are permissible. User information and theory are presented, along with sample problems.

Measurement and Modeling of Advanced Coal Conversion Processes

Solomon, P.R.; Hamblen, D.G.; Serio, M.A.; Smoot, L.D. and Brewster, B.S.
US Department of Energy/Morgantown Energy Technology Center/Advanced Fuel Research/Brigham Young University Final Contract Report, Vol. I, 1993. (Also presented at the Coal-fired power systems 93 Conference, Morgantown, WV, June 1993.) Funded by US Department of Energy and Morgantown Energy Technology Center. (This report is available from Advanced Fuel Research, Inc.)

This project included research in the following areas: (1) fundamental high-pressure reaction rate data; (2) large particle oxidation at high pressures; (3) SOx-NOx submodel development; (3) integration of advanced submodels into entrained-flow code, with evaluation and documentation; (4) comprehensive fixed-bed modeling, review, development, evaluation and implementation; (5) generalized fuels feedstock submodel; (6) application of generalized pulverized coal comprehensive code and (7) application of fixed-bed code.

Measurement and Modeling of Advanced Coal Conversion Processes

Solomon, P.R.; Hamblen, D.G.; Serio, M.A.; Smoot, L.D. and Brewster, B.S.
US Department of Energy/Morgantown Energy Technology Center/Advanced Fuel Research/Brigham Young University Final Contract Report, Vol. I, 1993. (Also presented at the Coal-fired power systems 93 Conference, Morgantown, WV, June 1993.) Funded by US Department of Energy and Morgantown Energy Technology Center. (This report is available from Advanced Fuel Research, Inc.)

This project included research in the following areas: (1) fundamental high-pressure reaction rate data; (2) large particle oxidation at high pressures; (3) SOx-NOx submodel development; (3) integration of advanced submodels into entrained-flow code, with evaluation and documentation; (4) comprehensive fixed-bed modeling, review, development, evaluation and implementation; (5) generalized fuels feedstock submodel; (6) application of generalized pulverized coal comprehensive code and (7) application of fixed-bed code.

Process Data and Strategies

Germane, G.J.; Eatough, C.N. and Cannon, J.N.
Chapter 2, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.

This chapter documents the measurement methods and multidimensional data for evaluation of combustion models. Data are reported for several scales from laboratory to full-scale furnaces. The design of advanced combustion systems and processes for gas, liquid and solid fossil fuels can be greatly enhanced by the utilization of verified predictive and interpretive combustion models. Development of an accurate three-dimensional model applicable to non-reacting and reacting flow systems, and specifically coal combustion and entrained flow gasification, is a primary research initiative of ACERC, and is also being pursued in several other countries. Once the code, with appropriate submodels, has been completed, it is necessary to make comparisons of code predictions to data from turbulent flames in reactors that embody various aspects of turbulent combustion of coal, oil, gas or slurry fuels. Consequently, data from a range of different-sized facilities are necessary in order to adequately demonstrate the adequacy of the code predictions, and to establish the degree of precision that the code can give in making predictions for industrial furnaces. Such detailed data gives new insights into combustion processes and strategies. The detailed measurements possible in the laboratory-scale facilities complement the coarser or sparser measurements of three-dimensional flow patterns and flame heat transfer characteristics obtained in industrial and utility furnaces.

Role of Combustion Research in the Fossil Fuel Industry

Smoot, L.D.
Energy & Fuels, 7 (6):689-699, 1993. Funded by ACERC.

The use of fossil fuels currently dominates worldwide energy production. While there are many alternatives to the use of coal, oil, natural gas, and other fossil fuels, published projections show that fossil fuels will continue to provide the bulk of the world's expanding energy needs in the foreseeable future. Given its vast reserves and the rapidly developing clean-coal technologies, coal is projected to assume an increasingly important role. Yet increasing use of these fossil fuels presents many challenges of worldwide significance, including control of acid rain, emissions of toxic compounds and trace metals, particulate emissions, and carbon oxide emission. Further, with the expanding world energy needs, every-increasing efficiencies for generation of power and industrial heat are essential. Accomplishments in cleaner and more efficient use of fossil fuels have been substantial. This paper specifically examines the role of fundamental and applied research toward these new developments. Six specific commercial applications of new technology based on prior research are examined: (1) increasing efficiency of utility boilers, (2) reduction in carbon carryover in pulverized coal boilers, (3) coal selection for minimum fouling tendencies, (4) SOx removal through sorbent injection, (5) low NOx burners in large furnaces, and (6) mild gasification of coal. In each specific case, the vital role of research in the current commercial practice is examined and discussed. The rapidly developing new technology of combustion modeling is also explored. Its state of development is summarized and examples of commercial application are illustrated. Results of a survey among several international groups active in comprehensive combustion modeling show a rapidly developing level of application of this technology to industrial needs. The future industrial role of this technology is also assessed. On the basis of this foundation of research need and accomplishment in the fossil energy industry, the role and research program of the Advanced Combustion Engineering Research Center, on whose annual conference this special publication of Energy and Fuels is based, is summarized. The Center's focus on clean and efficient use of fossil energy is identified, and the research program in six related thrust areas is outlined. Recent general research progress is identified.

1992

Comparison of Measurements and Predictions of Flame Structure and Thermal NOx, in a Swirling, Natural Gas Diffusion Flame

Boardman, R.D.; Eatough, C.N.; Germane, G.J. and Smoot, L.D.
Combustion Science and Technology, 1992 (in press). (Previously presented at the First International Conference on Combustion Technologies For a Clean Environment, Vilamoura, Portugal, September 1991.) Funded by Morgantown Energy Technology Center through subcontract from Advanced Fuel Research and ACERC.

A combined thermal and fuel nitric oxide submodel has recently been added to a generalized, 2-dimensional pulverized coal gasification and combustion model (PCGC-2). This model is applicable to reacting and non-reacting gaseous and particle-laden flows. The thermal NO model is based on the extended Zel'dovich mechanism. To perform an evaluation of the NOx submodel, combustion measurements of gas velocities, temperatures, and species concentrations were made in a laboratory-scale, experimental reactor with a 150 kW natural gas flame at an equivalence ratio of 1.05 and a secondary-air swirl number of 1.5. Combustion measurements of velocities and major species concentrations show generally good agreement with predicted values. Gas temperature measurements closely match predictions in the recovery region but fail to show predicted high temperature in the annular region. This study provides an evaluation of a comprehensive combustion model and the NOx submodel that can be useful as a design tool to provide pollutant formation trends in applied systems as combustion parameters are varied.

Modeling Fixed-Bed Coal Gasifiers

Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
AIChE Journal, 38(5):681-702, 1992. Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC.

A one-dimensional model of countercurrent fixed-bed coal gasification has been developed, and results have been compared to experimental data from commercial-scale gasifiers. The steady-state model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on chemical functional group composition, oxidation and gasification of char, and partial equilibrium in the gas phase. Generalized treatment of gas-phase chemistry and accounting for variable bed void fraction were necessary to predict realistic axial temperature and pressure profiles in an atmospheric fixed-bed gasifier. Model evaluation includes sensitivity of axial temperature profiles to model options, model parameters and operational parameters. Model predictions agree reasonably well with experimental temperature and pressure profile data for gasification of eight coal types ranging from lignite to bituminous. The relative importance of char oxidation resistances to bulk film diffusion, ash diffusion, and chemical reaction is identified.

Modeling Sorbent Injection and Sulfur Capture in Pulverized Coal Combustion

Boardman, R.D.; Brewster, B.S.; Huque, Z.; Smoot, L.D. and Silcox, G.D.
Air Toxic Reduction and Combustion Modeling, 15:1-9, 1992. (Also presented at the ASME International Joint Power Generation Conference, Atlanta, GA, October 1992). Funded by Advanced Fuel Research and ACERC.

A computer model has been developed for predicting mixing and reactions of injected sorbent particles in pulverized coal combustors and gasifiers. A shrinking-core, grain model was used for sulfation. The model accounts for the effects of surface area, pore diffusion, diffusion through the product layer, chemical reaction, and reduction of the pore volume due to grain swelling. The submodel was evaluated for a fuel-lean case and for a fuel-rich case. Predictions were compared with limited experimental data (for the fuel-rich case). The results illustrate the model's capability for predicting the effectiveness of sulfur capture. The importance of sorbent particle properties was also investigated parametrically, and model limitations were identified.

Prediction of Effluent Compositions for Fixed-Bed Coal Gasifiers

Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
Fuel, 71(10):1177-1194, 1992. Funded by US Department of Energy, Morgantown Energy Technology Center and ACERC.

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.

Comprehensive Fixed-Bed Modeling, Review, Development, Evaluation, and Implementation

Solomon, P.R.; Hamblen, D.G.; Serio, M.A.; Smoot, L.D. and Brewster, B.S.
21st, 22nd, and 23rd Quarterly Reports for the US Department of Energy, 1992. Funded by US Department of Energy and Morgantown Energy Technology Center.

The overall objective of this program is the development of predictive capability for the design, scale up, simulation, control and feedstock evaluation in advanced coal conversion devices. This technology is important to reduce the technical and economic risks inherent in utilizing coal, a feedstock whose variable and often unexpected behavior presents a significant challenge. This program is merging significant advances made at Advanced Fuel Research, Inc. (AFR) in measuring and quantitatively describing the mechanisms in coal conversion behavior, with technology being developed at Brigham Young University (BYU) in comprehensive computer codes for mechanistic modeling of entrained-bed gasification. Additional capabilities in predicting pollutant formation is being implemented and the technology was expanded to fixed-bed reactors. The foundation to describe coal-specific conversion behavior is AFR's Functional Group (FG) and Devolatilization, Vaporization, and Crosslinking (DVC) models, developed under previous and on-going METC sponsored programs. These models have demonstrated the capability to describe the time dependent evolution of individual gas species, and the amount and characteristics of tar and char. The combined FG-DVC model has been integrated with BYU's comprehensive two-dimensional reactor mode, PCGC-2, which is a widely used reactor simulation for combustion or gasification. The program includes: 1) validation of the submodels by comparison with laboratory data obtained in this program, 2) extensive validation of the modified comprehensive code by comparison of predicted results with data from bench-scale and process scale investigations of gasification, mild gasification and combustion of coal or coal-derived products in heat engines, and 3) development of well documented user friendly software applicable to a "workstation" environment.

The Sensitivity of Entrained-Flow Coal Gasification Diffusion Burners to Changes in Geometry

Sowa, W.A.; Hedman, P.O.; Smoot, L.D. and Richards, D.O.
Fuel, 71(5):593-604, 1992. Funded by US Department of Energy, Morgantown Energy Technology Center.

Three axisymmetric diffusion flame burners were designed and installed on a laboratory-scale, downfired, entrained-flow, coal gasifier operated at pressures up to 560 kPa. Each burner was studied by varying reactor pressure, oxygen/coal ratio and steam/coal ratio. The gasifier performance was assessed by collecting space-resolved gas and char samples in the reaction chamber and analyzing them for carbon conversion, gas composition (CO, CO2, H2, H20 and CH4) and cold gas efficiency. Burner geometry affected carbon conversion, gas composition and cold gas efficiency. Each burner had unique flame structural characteristics that resulted in burner-unique trends with reactor pressure, oxygen/coal ratio and steam/coal ratio. At 560 kPa, diffusion flame burner performance approached premixed flame performance. The results from this study suggest that it might be possible to design a diffusion burner that outperforms a fuel-oxidant premixing burner for some operating conditions due to its flame structure and its characteristic energy transfer to the chamber. Performance characteristics of diffusion burners correlated with system pressure, oxygen/coal ratio or steam/coal ratio cannot be generalized into trends representative of all diffusion flame burners.

Char Oxidation at Elevated Pressures

Monson, C.R.; Germane, G.J.; Blackham, A.U. and Smoot, L.D.
Fall Meeting of Western States Section/The Combustion Institute, Berkely, CA, October 1992. Funded by US Department of Energy/Morgantown Energy Technology Center through Advanced Fuel Research and ACERC.

Most of the coal currently being consumed is combusted at atmospheric pressure in utility furnaces, but several other processes are also being used and developed for either the direct combustion of coal or its conversion into other products. Many of these other processes, including coal gasification, operate at elevated pressure. While a great deal of research has been conducted on coal and char combustion at atmospheric pressure, elevated pressure char oxidation has largely been ignored. This paper describes the results obtained from char oxidation experiments at atmospheric and elevated pressures.

The experiments were carried out in a high-pressure, electrically heated drop tube reactor. A particle imaging system provided in situ, simultaneous measurement of individual particle temperature, size and velocity. Approximately 100 oxidation experiments were performed with two sizes (70 and 40 µm) of Utah and Pittsburgh bituminous coal chars at 1, 5, 10, and 15 atm total pressure. Reactor temperatures were varied between 1000 and 1500K with 5 to 21% oxygen in the bulk gas, resulting in average particle temperatures ranging from 1400 to 2100K and burnouts from 15 to 96%. Independently determined particle temperature and overall reaction rate allowed an internal check of the data consistency and provided insight into the products of combustion. Results from atmospheric pressure tests were shown to be consistent with results obtained by other researchers using the same coal. The chars burned in a reducing density and diameter mode in an intermediate regime between the kinetic and pore diffusion zones, irrespective to total pressure. Significant CO2 formation occurred at the particle surface at particle temperatures below about 1800K over the entire pressure range. Particle temperatures were strongly dependent on the oxygen and total pressures; increasing oxygen pressure at constant total pressure resulted in substantial increases in particle temperature, while increasing the total pressure at constant oxygen pressure led to substantial decreases in particle temperature. Increasing total pressure from 1 to 5 atm in an environment of constant gas composition led to modest increases in the reaction rate coefficients (based on the nth order rate equation) showed a large pressure dependence; both the activation energy and frequency factor decreased with increasing pressure. The results suggested that the empirical nth order rate equation is not valid at elevated pressures.

1991

Relation Between Reactivity and Structure for Coals and Chars

Wells, W.F. and Smoot, L.D.
Fuel, 70:454-458, 1991. Funded by Pittsburgh Energy Technology Center.

Causal relationships were determined for the chemical and physical properties and reaction characteristics of three coals and two types of chars derived from these coals. The reactivities of the virgin coals and extract chars correlated well with the fuel rank. The higher proximate volatiles contents in these prepared fuels resulted in reactivities higher than those measured for the low volatile pyrolysis chars. The reactivities of the pyrolysis chars were strongly correlated with fuel properties, whose relative order of importance was: catalytic elements > porosity > hydrogen types > cluster size.

Comparison of Measurements and Predictions of Flame Structure and Thermal NOx, in a Swirling, Natural Gas Diffusion Flame

Boardman, R.D.; Eatough, C.N.; Germane, G.J. and Smoot, L.D.
First International Conference on Combustion Technologies For a Clean Environment, Vilamoura, Portugal, September 1991. Funded by Morgantown Energy Technology Center through subcontract from Advanced Fuel Research Co.

A combined thermal and fuel nitric oxide submodel has recently been added to a generalized, 2-dimensional pulverized coal gasification and combustion model (PCGC-2). This model is applicable to reacting and non-reacting gaseous and particle-laden flows. The thermal NO model is based on the extended Zel'dovich mechanism. To perform an evaluation of the NOx submodel, combustion measurements of gas velocities, temperatures, and species concentrations were made in a laboratory-scale, experimental reactor with a 150 kW natural gas flame at an equivalence ratio of 1.05 and a secondary-air swirl number of 1.5. Combustion measurements of velocities and major species concentrations show generally good agreement with predicted values. Gas temperature measurements closely match predictions in the recovery region but fail to show predicted high temperature in the annular region. This study provides an evaluation of a comprehensive combustion model and the NOx submodel that can be useful as a design tool to provide pollutant formation trends in applied systems as combustion parameters are varied.

Fossil Fuel Combustion: A Source Book

Smoot, L.D.
Coal and Char Combustion, Chapter 10
(W. Bartok and A.F. Sarofim, eds.), John Wiley and Sons, Inc., 1991. Funded by Exxon Corp.

A comprehensive summary of 128 pages that treats reaction processes involving coal, char and other solid fossil fuels. Contents include coal processes and properties, fuel ignition, devolatilization, and oxidation. Practical flames are classified and their characteristics are discussed. Computerized models of these flames are reviewed and applied to industrial flames. The work cites 234 technical references, including 39 that report original combustion research work conducted at Brigham Young University.

Measurements and Modeling of Advanced Coal Conversion Processes: Fixed-Bed Coal Gasification Modeling

Solomon, P.R.; Hamblen, D.G.; Serio, M.A.; Smoot, L.D. and Brewster, B.S.
Contractors Review Meeting, Morgantown, WV, August 1991. Funded by Morgantown Energy Technology Center.

The overall objective of this program is to understand the chemical and physical mechanisms in coal conversion processes and incorporate this knowledge in computer-aided reactor engineering technology for the purposes of development, evaluation, design, scale up, simulation, control and feedstock evaluation in advanced coal conversion devices. To accomplish this objective, the study will: establish the mechanisms and rates of basic steps in coal conversion processes, incorporate this information into comprehensive computer models for coal conversion processes, evaluate these models, and apply them to gasification, mild gasification and combustion in heat engines.

Measurement and Modeling of Advanced Coal Conversion Processes

Solomon, P.R.; Hamblen, D.G.; Serio, M.A.; Smoot, L.D. and Brewster, B.S.
5th Annual Report for the US Department of Energy, 1991. Funded by Morgantown Energy Technology Center.

The overall objective of this program is the development of predictive capability for the design, scale up, simulation, control and feedstock evaluation in advanced coal conversion devices. This technology is important to reduce the technical and economic risks inherent in utilizing coal a feedstock whose variable and often unexpected behavior presents a significant challenge. This program will merge significant advances made at Advanced Fuel Research, Inc. (AFR) in measuring and quantitatively describing the mechanisms in coal conversion behavior, with technology being developed at Brigham Young University (BYU) in comprehensive computer codes for mechanistic modeling of entrained-bed gasification. Additional capabilities in predicting pollutant formation will be implemented and the technology will be expanded to fixed-bed reactors.

The foundation to describe coal-specific conversion behavior is AFR's Functional Group (FG) and Devolatilization, Vaporization, and Crosslinking (DVC) models, developed under previous and on-going METC sponsored programs. These models have demonstrated the capability to describe the time dependent evolution of individual gas species, and the amount and characteristics of tar and char. The combined FG-DVC model will be integrated with BYU's comprehensive two-dimensional reactor model, PCGC-2, which is currently the most widely used reactor simulation for combustion or gasification. The program includes: i) validation of the submodels by comparison with laboratory data obtained in this program, ii) extensive validation of the modified comprehensive code by comparison of predicted results with data from bench-scale and process scale investigations of gasification, mild gasification and combustion of coal or coal-derived products in heat engines, and iii) development of well documented user friendly software applicable to a "workstation" environment. Success in this program will be a major step in improving the predictive capabilities for coal conversion processes including: demonstrated accuracy and reliability and a generalized "first principles" treatment of coals based on readily obtained composition data. The progress during the fifth year of the program is summarized in the document.

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.

Modeling and Experimental Research in the Advanced Combustion Engineering Research Center

Smoot, L.D.
Proceedings of the 2nd International Symposium on Coal Combustion Science and Technology, 58-72, Beijing, China, October 1991. Funded by ACERC.

This invited paper describes an integrated, interdisciplinary research program at the Advanced Combustion Engineering Research Center in the United States. The program focuses on combustion research related to clean and efficient use of low-quality fossil fuels, particularly coal. About forty on-going experimental and modeling research projects in six thrust areas among 125 investigators are noted. Some specific projects in which the author participates are summarized, and results from two of these projects are presented and discussed. The first is development of a generalized NOx/SOx submodel for inclusion in a comprehensive combustion code, PCGC-2. The NOx model links, for the first time, thermal and fuel NOx with turbulence fluctuations. Thermal NOx measurements from an advanced, controlled-profile laboratory reactor agree generally with predictions and also support a key assumption used to estimate the radial concentration. Methods for predicting capture of SOx by limestone sorbents in coal combustion are outlined. Development and evaluation of an advanced, top-fed, counterflow, one-dimensional fixed bed gasification model is also summarized. Key assumptions and model parameters are identified. The model is based on solution of mass and energy balances for coal particles and gas with different axial temperature distributions. Pressure distribution varies with void fraction axially. Predictions of axial temperature and pressure variation compare well with reported measurements for several coals in a commercial atmospheric gasifier.

1990

Relation Between Reactivity and Structure for Coals and Chars

Wells, W.F. and Smoot, L.D.
Fuel, 1990 (In press). Funded by Pittsburgh Energy Technology Center.

Causal relationships were determined for the chemical and physical properties and reaction characteristics of three coals and two types of chars derived from these coals. The reactivities of the virgin coals and extract chars correlated well with the fuel rank. The higher proximate volatiles contents in these prepared fuels resulted in reactivities higher than those measured for the low volatile pyrolysis chars. The reactivities of the pyrolysis chars were strongly correlated with fuel properties, whose relative order of importance was: catalytic elements > porosity > hydrogen types > cluster size.

Characteristics of Commonly-Used U.S.Coals: Towards a Set of Standard Research Coals

Smith, K.L. and Smoot, L.D.
Prog. Energy Combust. Science, 16, 1-53, 1990. Funded by ACERC.

This review summarizes the selection and characterization of a set of coals commonly used in research programs in the United States. These coals have been selected from available U.S. coal databases. Organizations that provide coal samples and/or coal characterization data include the following: (1) Pennsylvania State University, which has characterized many of the Nation's coal resources, as documented in the Penn State Coal Data Base operated by the Energy and Fuels Research Center, (2) the coal sample suite used in the Direct Utilization-Advanced Research and Technology Development program managed by the Pittsburgh Energy Technology Center, and (3) Argonne National Laboratory's Premium Coal Sample Program. The selection of eleven coals from these national banks provides a standard suite of coals for the Advanced Combustion Engineering Research Center of Brigham Young University and the University of Utah. These standard coals were selected according to the following criteria: (1) representative of a variety of characteristics, ranks and properties, (2) available analyses of chemical and physical properties with wide property variations among coal types and ranks, (3) availability from major producing seams, (4) future production expected, (5) wide geographical distribution within the U.S., (6) used in previous combustion research work, (7) common to existing prominent coal banks, and (8) availability of small, controlled samples. Information about the general aspects of coal characterization is summarized. Experimental data on the physical and chemical properties of these coals are documented, and the selected coals are related to the coal data banks. Major combustion research studies where these coals have been or are being used are referenced. General use of these well-characterized coals will help coordinate and integrate a national research effort in coal combustion and conversion.

Structure of a Near-Laminar Coal Jet Flame

Brewster, B.S.; Smoot, L.D.; Solomon, P.R. and Markham, J.R.
Tenth Annual Gasification and Gas Stream Cleanup Systems Contractors Review Meeting, Morgantown, WV, 1990. (Also Presented at the Western States Section/The Combustion Institute, San Diego, CA, 1990). Funded by Morgantown Energy Technology Center and Advanced Fuel Research Co.

An advanced 2-D model for pulverized-coal combustion has been modified and applied to a laminar coal flame in a transparent wall reactor. Modifications were made to allow for the up-fired flow configuration, laminarization, and gas buoyancy. A laminarization extension to the k- turbulence model was incorporated. Particle dispersion is sensitive to laminarization and to the value of turbulent particle Schmidt number. Predicted particle velocity and residence time are sensitive to the inclusion of gas buoyancy, which increases the velocity in the center of the reactor and induces a radial, inward flow. Model predictions have been compared with flame measurements to evaluate the comprehensive model that incorporates an advanced devolatilization submodel. Predicted velocities of burning particles agree well with values determined from particle streaks on video recording. Good agreement was also obtained between measured and predicted particle burnout. Discrepancies between measured and predicted particle and gas temperature may be due to neglecting heterogeneous formation of CO2 and the variation of char reactivity with extent of burnout. Discrepancies between predicted bulk gas temperature and measured CO2 gas temperature in the ignition zone can also be explained by the fact that the combustion energy first heats the CO2 that subsequently heats the other gases. Soot decays more slowly than predicted from equilibrium concentrations of condensed carbon.

Fixed-Bed Coal Gasification Modeling

Hobbs, M.L.; Radulovic, P.T. and Smoot, L.D.
Twenty-third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1990. Funded by Morgantown Energy Technology Center through Advanced Fuel Research Co.

A one-dimensional model of countercurrent, fixed-bed gasification has been developed and predictions have been compared to experimental data obtained from a large-scale gasifier. The study-state model considers separate gas and solid temperatures, partial equilibrium in the gas phase, variable bed void fraction, coal devolatilization based on chemical functional group composition, oxidation and gasification of residual char with an ash layer, and axially variable solid and gas flow rates. Predictions are compared to experimental data from an atmospheric, dry-ash Wellman-Galusha gasifier for carbon conversion, effluent gas composition and temperatures, and axial profiles of temperature and pressure for a high volatile bituminous coal. The relative importance of the char oxidation resistances, bulk film diffusion, ash diffusion and surface reaction, are identified. For the cases examined, chemical resistance dominates in the cool regions at the bottom and top of the reactor while ash diffusion resistance competes with chemical resistance through most of the reactor. The importance of adequate treatment of devolatilization, gas phase chemistry, and variable bed void fraction is identified.

An accurate initial estimate of the effluent composition and temperature from a two-zone, partial equilibrium submodel was essential for efficient solution of this highly nonlinear fix-bed model. This initial estimate considers devolatilization, partial equilibrium of volatile gases, treatment of a large number of gas phase species, and tar production with potential for recirculation of effluent products. It has been shown that the submodel is adequate by itself for reliable predictions of effluent gas compositions. Effluent gas estimates from the submodel compared favorably to measured effluent temperatures and compositions from a high-pressure, dry-ash Lurgi gasifier in Westfield, Scotland for four American coals.

The importance of treating various chemical and physical processes in fixed-bed gasifiers with sufficient detail has been addressed with emphasis on coal devolatilization, char oxidation, gas phase chemistry, and bed void fraction. Calculations have shown that devolatilization in fixed-bed reactors is not an instantaneous process but is an intimate part of the overall fixed-bed process. Similarly, oxidation and gasification do not occur in separate zones, but simultaneously in certain regions of the reactor bed. Competition between endothermic gasification reactions and exothermic oxidation is evident in broad predicted and measured temperature peaks. Detailed gas phase chemistry was necessary to predict the features of temperature and concentration profiles. Variable bed void fraction was also necessary to accurately predict pressure drop, varying bed velocity, and temperature and concentration profiles.

1989

Characteristics of Commonly-Used U.S. Coals--Towards a Set of Standard Research Coals

Smith, K.L. and Smoot, L.D.
Accepted for publication in Prog. Energy Combust. Sci., 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

This review summarizes the selection and characterization of a set of coals commonly used in research programs in the United States. These coals have been selected from available U.S. coal data bases. Organizations that provide coal samples and/or coal characterization data include the following: (1) Pennsylvania State University, which has characterized many of the Nation's coal resources, as documented in the Penn State Coal Data Base operated by the Energy and Fuels Research Center, (2) the coal sample suite used in the Direct Utilization-Advanced Research and Technology Development program managed by the Pittsburgh Energy Technology Center, and (3) Argonne National Laboratory's Premium Coal Sample Program. The selection of eleven coals from these national banks provides a standard suite of coals for the Advanced Combustion Engineering Research Center of Brigham Young University and the University of Utah. These standard coals were selected according to the following criteria: (1) representative of a variety of characteristics, ranks and properties, (2) available analyses of chemical and physical properties with wide property variations among coal types and ranks, (3) availability from major producing seams, (4) future production expected, (5) wide geographical distribution within the U.S., (6) used in previous combustion research work, (7) common to existing prominent coal banks, and (8) availability of small, controlled samples. Information about the general aspects of coal characterization is summarized. Experimental data on the physical and chemical properties of these coals are documented, and the selected coals are related to the coal data banks. Major combustion research studies where these coals have been or are being used are referenced. General use of these well-characterized coals will help coordinate and integrate a national research effort in coal combustion and conversion.

Oxidation Rate Measurements of Coals and Derived Chars

Wells, W.F.; Hyde, W.D.; Cope, R.F.; Smoot, L.D.; Hecker, W.C. and Bartholomew, C.H.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

To aid in the development and provide validation data to the char oxidation submodel, measurements of oxidation rates at high and low temperatures are being collected for chars prepared from five select ACERC coals. Chars are prepared using a flat flame burner and a recently constructed inert atmosphere drop-tube reactor heated using an inductively coupled plasma. The drop-tube reactor is also used to obtain reaction rates at high temperatures; low temperature rates were measured in a TGA. Chemical and physical properties of the fuel were measured. Multivariate statistics are used to correlate fuel properties to reaction rates.

Surface Properties and Pore Structure of ACERC Coals and Chars

White, W.E.; Bartholomew, C.H.; Thornock, D.; Wells, W.F.; Hecker, W.C. and Smoot, L.D.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Results of an ongoing collaborative study of the surface properties and pore structure of a suite of 11 coals selected for comprehensive study by ACERC are reported. The principal objective is to correlate the surface, pore, and chemical properties of coals and chars with their rates of combustion. Surface areas, pore volumes, pore size distributions, and solid densities were measured for Pittsburgh No. 8, Wyodak, Beulah Zap Lignite, Lower Wilcox, Dietz and a Utah Scofield coal and for chars derived from these coals. Surface areas, pore volumes and pore size distributions were measured using nitrogen and carbon dioxide adsorptions, mercury porosimetry and NMR spin-lattice relaxation measurements for samples saturated with water vapor. Solid densities were obtained using helium displacement. The results indicate that chars have larger surface areas and pores volumes relative to coals. New mesopores are created and micropore volume increases during devolatilization. Large fractions of the internal pore volume of coals are not penetrated by nitrogen molecules during adsorption, but are penetrated by carbon dioxide, suggesting that a fraction of the pore volume is microporous, or involves blocked pores. By using several techniques for measuring surface properties (e.g. N2 and CO2 adsorption isotherms, NMR,etc.), the pore structure of coals and chars can be defined more accurately, and char oxidation models can be evaluated with more understanding.

Measurement and Prediction of Thermal and Fuel NOx

Boardman, R.D.; Smoot, L.D. and Brewster, B.S.
Western States Section, The Combustion Institute, Livermore, California, 1989. Funded by US Department of Energy through subcontract from Advanced Fuel Research Co., and ACERC (National Science Foundation and Associates and Affiliates).

A generalized NOx model is being developed to predict nitric oxide formation in practical combustors. The NOx model incorporates an extended global fuel-NO mechanism and the modified Zeldovich mechanism to predict thermal NO formation. The importance of coupling turbulence with the chemical kinetics for practical combustors is addressed. Thermal NO data for a turbulent gaseous diffusion flame (in a laboratory-scale furnace) are presented. The model is being validated using these previously unpublished data and other pulverized-coal combustion and gasification data from the literature.

Coal and Char Combustion

Smoot, L.D.
Chapter 10, Handbook of Combustion Theory, Bartok, W. and Sarofim, A. (Eds.), John Wiley and Sons, New York, New York (In Press), 1989. Funded by Exxon and Brigham Young University.

This chapter deals with reaction processes involving coal, char, and other solid fossil fuels. Properties and uses of these fossil fuels are treated; reaction processes of coal particles are also considered and modeled. Then these results are applied to the description of practical coal processes.

The key objectives of this chapter are the following:

1. Review the existing and potential uses of coal and the processes most commonly applied.

2. Identify the general chemical and physical properties of coal, emphasizing the complexity and variability of these natural materials.

3. Summarize major issues being addressed in the increasing uses of coal and other solid fossil fuels.

4. Characterize effects of key variables such as coal type, particle size, heating rate, temperature, pressure, and oxidizer type on coal particle reaction rate.

5. Outline useful existing methods for modeling of coal particle reactions, including devolatilization and heterogeneous oxidation processes.

6. Identify the nature and controlling processes of practical coal dust flames in various coal processes.

7. Outline general methods for modeling of coal reaction processes, and illustrate by application of a one-dimensional model.

The entire area of coal reaction processes is very extensive. This field of study includes or interacts with such topics as (1) the origin and geologic nature of coal; (2) the chemical and physical properties and classification of coal; (3) the relationship of coal to other solid and solid-derived fossil fuels, such as oil shale or solvent-refined coal; (5) thermal devolatilization of coal and its dependence on coal type, particle size, heating rate, temperature, etc.; (6) the nature and chemical composition of coal volatiles and their dependence on coal type, heating rate, temperature, etc.; (7) the chemical reaction of coal volatiles in the gas phase, including formation of soot and cracking of hydrocarbons; (8) the formation of char during devolatilization, including swelling, softening, cracking, and formation of internal surfaces; (9) the reaction of char particles, including oxidizer diffusional processes internal and external to the particle, effects of volatiles transpiration, surface reaction, and product diffusion; (10) formation and control of a variety of pollutant species, including oxides of nitrogen and their precursors, oxides of carbon, potentially carcinogenic hydrocarbons, carbon dioxide, volatile trace metals, and small particulates; (11) radiative processes of coal and its solid products (i.e., soot, ash, slag, and char) and gaseous products (e.g., CO2 and H2O); (12) formation of ash and slag particles, their change in particle sizes, and their control and removal; (13) interaction of particles with walls and surfaces, including formation of ash or slag layer; (14) particle-gas interactions including convective and radiative heat transfer, reactant and product diffusion, and particle motion in the turbulent gas media; (15) design and optimization of coal reaction processes.

In this chapter, only solid fossil fuels are considered, and emphasis is placed on finely pulverized coal reaction processes. This form of coal is dominant in existing coal processes. Less treatment is given to processing of larger coal particles. Reactions of coal processes are considered in some detail.

Detailed Model for Practical Pulverized Coal Furnaces and Gasifiers

Smith, P.J. and Smoot, L.D.
AR&TD Contractor's Meeting, Morgantown, West Virginia, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates) and ACERC Consortium: Babcock & Wilcox, Combustion Engineering, Consol, Electric Power Research Institute, Empire State Electrical Energy Research Corp., Foster Wheeler, Pittsburgh Energy Technology Center, and Utah Power & Light.

This project was jointly funded by a consortium of eight different industrial firms and governmental agencies of which PETC is a part. The objective of the project was to improve and extend a generalized two-dimensional, pulverized coal combustion and gasification code for application to large-scale practical configurations. The work initiated in this four-year project is being continued with NSF and private sponsorship under the Advanced Combustion Engineering Research Center. Over the past year the consortium project tasks were significantly scaled down from the preceding three years and focused on the evaluation of model and submodel development, integration of the pertinent algorithms into the 3-D model, and evaluation of the integrated code. This paper will report on work accomplished on this development over the last year and summarize the state of development of the 3-D modeling effort.

Three tasks were originally outlined for this four-year project. Task 1 - extend an available 2-D model to three-dimensional, large-scale furnace and gasifier configurations. Task 2 - evaluate the 3-D model a: through a statistical sensitivity analysis to determine parameters most influential on predictions, and b) through review, selection, documentation and comparison of a set of available data relating to practical, large-scale furnaces and gasifiers with model predictions. Task 3 - improve or develop submodel equations and/or data for the 3-D model.

All submodel work on task 3 was completed in the first three years of the contract and included the development of a new turbulent particle-dispersion model, a new two and three dimensional discrete ordinates model for radiative transfer, the generalization of the devolatilization submodel to include all known devolatilization models, advancements in the char reaction submodel, a new submodel for interface conditions in multi-phase reaction systems (i.e. coal or oil combustion), the incorporation of chemical kinetically limited gaseous reactions of CO - CO2, and the development of a crude but complete framework for incorporating mineral matter transformations and deposition in the combustion chamber. All of this work on this contract has been previously reported.

During this year work has been accomplished towards the incorporation of advanced numerical algorithms and submodels into the first-generation 3-D comprehensive coal combustion code. The code has been assembled for arbitrary geometries in Cartesian and cylindrical coordinate systems. It includes turbulent mixing with eddy diffusivity, mixing-length and k-e turbulence models. It incorporates dispersed particulate phases in an Eulerian description. The gas phase chemistry couples turbulent mixing-limited equilibrium reactions for all major species. Over the last year an ongoing evaluation has occurred to quantify three levels of error: numerical and algorithmic errors, submodel errors, and overall model or data comparison errors. Each of these areas of evaluation is presented in the paper.

A computer graphics package has been developed for the display of three-dimensional combustion data. The package is based on the FIGS graphics industry standard and thus will run on many different hardware platforms. The package allows menu driven access to all computed variables from the 3-D code and takes advantage of color capabilities to cue the user and display the results. A computer graphics pre-processor has been developed to facilitate the creation of the large set of input required for an industrial scale furnace. The complex three-dimensional mesh can be easily defined with menu driven color graphics input.

Calculations have been performed for pilot scale combustion furnaces including wall-fired and corner-fired units. Comparisons have been made with data for non-reacting fluid dynamics, gaseous combustion and coal combustion. Analysis of model performance has been completed for a variety of model options. Analyses of numerical accuracy and model robustness have shown that the formulation and implementation of this code to be more accurate, more computationally efficient and more robust than previous models from this and other laboratories.

Effects of Pressure and Coal Rank on Carbon Conversion in an Entrained-Coal Gasifier

Cope, R.F.; Smoot, L.D. and Hedman, P.O.
Fuel, 68, 806-808, 1989. Funded by US Department of Energy (Morgantown Energy Technology Center).

Elevated-pressure gasification tests were completed with North Dakota lignite, Wyoming subbituminous and Illinois No. 6 bituminous coals. Carbon conversion values obtained in theses tests were compared with those obtained with the same three coals at atmospheric pressure. Increased pressure produced greater increases in carbon conversion as coal rank decreased.

Measurement and Interpretation of Explosion Bomb Tests for Low Rank Coal

Cannon, J.N.; Kramer, S.K.; Smoot, L.D. and Dahn, C.J.
Western States Section, The Combustion Institute, Pullman, Washington, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates) and US Department of Energy.

Twenty-liter bomb tests have become a common method to evaluate safety implications involving pulverized coal (p.c.) in coalmines, coal power plant pulverizers and other equipment. However, only limited data are available for low rank coals. Further, some published bomb test results show more variation than is acceptable to many researchers which clouds credibility of the data. Often, significant variables are not controlled or reported adequately. The objective of this paper is to provide new data for low rank coals and illuminate the variables and effects that are significant.

Subbituminous coal from the Decker, Montana open pit mine was obtained from the mine face and immediately sealed in double-wall, plastic bags under nitrogen. Fifteen sets of 20-liter bomb tests with this coal were conducted to determine the effects of particle size, moisture, oxidation age and dust concentration on explosion characteristics. Mean particle size ranged from 3 to 50 mm; moisture ranged from 3 to 21%; low temperature oxidation age varied from mine-face fresh to 10 days. Dust concentrations ranged from 0.025 to 0.875 gm/liter. The test samples were obtained, stored, ground, classified and analyzed at this laboratory. The bomb tests were conducted by Safety Consulting Engineers Inc., of Chicago and included 20-liter bomb pressure-time traces. Maximum pressure varied between 95 and 135 psi and maximum pressure rise rate ranged from 2000 to 7000 psi/sec. The influence of coal sample storage length was examined through duplicate tests. Multivariate statistical regressions are used to extract information from the noted tests, clarifying the estimates of error for the data. Concentration and particle size have greater influence than moisture while oxidation age has very little influence.

The transient combustion processes during bomb tests are identified and the effects of the test variables are interpreted in light of these processes. Burning velocities are also estimated from various available theories and compared. Analytical methods show that the maximum pressure in the bomb is related to fuel concentration, fuel heating value and molecular weight of the product gases. Experimental results indicate that convective and radiant heat losses to the container wall and incomplete combustion significantly lower maximum pressure compared to predictions. In addition, initial turbulence in the bomb prior to ignition has a significant influence on observations. Requirements for controlling these variables in order to obtain consistent and repeatable test data from standard bomb tests are noted. The implication of the laboratory test results to full-scale explosions is also noted.

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.

Surface and Pore Properties of ANL and PETC Coals

Bartholomew, C.H.; White, W.E.; Thornock, D.; Wells, W.F.; Hecker, W.C.; Smoot, L.D.; Smith, D.M. and Williams, F.L.
Preprint ACS Fuels Chem. Divl., 1988, Los Angeles. 9 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

Surface areas, pore volumes, pore size distributions, and solid densities were measured for three ANL coals (Pittsburgh No. 8, Wyodak, and Beulah Zap Lignite), two PETC coals (Lower Wilcox, and Dietz) and a Utah Scofield coal and for chars derived from these coals. Surface areas were measured using nitrogen and carbon dioxide adsorptions; pore volumes were determined using nitrogen adsorption, mercury porosimetry, and NMR spin-lattice relaxation measurements of samples saturated with water. Solid densities were obtained using helium displacement. The results indicated that chars have larger surface areas and pores relative to coals; large fractions of the internal surfaces of coals are not penetrated by nitrogen molecules but are penetrated by carbon dioxide suggesting that the pores are mostly smaller than 1 NM.

Sulfur Pollutant Formation During Coal Combustion

Zaugg, S.D.; Blackham, A.U.; Hedman, P.O. and Smoot, L.D.
Submitted to Fuel, 1988. Funded by Electric Power Research Institute.

A laboratory-scale pulverized coal combustor was used to determine the effects of secondary air swirl, stoichiometric ratio (O2/fuel), and coal type on the formation and reaction of sulfur pollutants (SO2, H2S, COs and CS2). Detailed local measurements within the reactor were obtained by analyzing solid-liquid-gas samples collected with a water-quenched probe. Increasing the stoichiometric ratio increased sulfur conversion and SO2 levels, and decreased H2S, COs, and CS2 levels. Swirl of secondary combustion air had a pronounced effect on the distribution of sulfur species formed at an O2-coal stoichiometric ratio of 0.87, but had very little effect at stoichiometric ratios of 0.57 and 1.17. Combustion of a bituminous coal produced more SO2 and less H2S, COs, and CS2 compared to a subbituminous coal.

Lignite Slurry Spray Characterization and Combustion Studies

Eatough, C.N.; Rawlins, D.C.; Germane, G.J. and Smoot, L.D.
Western States Section, The Combustion Institute, Dana Point, California, 1988. Funded by US Department of Energy (Morgantown Energy Technology Center) and ACERC (National Science Foundation Associates and Affiliates).

Lignite slurry atomization and combustion characteristics were studied using two atomizers, one developed at Brigham Young University (laboratory nozzle) and the other a Parker-Hannifin Model 6840610 M3 atomizer (commercial nozzle). These nozzles were used because of the significantly different spray patterns produced by each. In these cold-flow studies, it was found that the laboratory nozzle produced a solid cone type spray pattern with the highest mass flux near the spray center line. The commercial nozzle has a hollow cone spray pattern with a larger spray angle. Atomization studies were performed with these nozzles to determine the effect of atomizing air to slurry mass flow ratio (A/S) on particle/droplet size and velocity, and slurry spray mass distribution. These measurements were then used to study the effect of particle/droplet size and velocity, and spray mass distribution on carbon burnout in a laboratory scale reactor using hot-water dried lignite slurry as a fuel. Both the laboratory and commercial nozzles follow the same trends for mean droplet size and droplet velocity with variation in A/S. As expected, mean droplet size decreased with A/S and velocity increased with A/S. Spray angle decreased for the laboratory nozzle but increased for the commercial nozzle with increase in A/S.

Analysis of combustion data indicates an expected strong dependence of burnout on particle/droplet size. Burnout increased markedly as particle/droplet size decreased. Burnout was also affected by the mass distribution of the slurry spray. Large spray angles directed slurry to the relatively cool reactor walls resulting in lower burnout values. Burnout values from both nozzles followed the same trends with regard to droplet size. Burnout increased with decreasing mean droplet size to about 50 mm, which corresponded closely with the coal particle size in the slurry which has a mean diameter of about 40 mm. A mean droplet diameter larger than about 80 mm with a 300 mm top size could not sustain combustion in the laboratory reactor.

A combustion map of burnout values was made using the laboratory nozzle at an A/S of 0.7, swirl number of 1.5 and SR of 1.1.

Predicted Effects of Coal Volatiles Composition in Turbulent Flames

Brewster, B.S. and Smoot, L.D.
Western States Section, 1988, The Combustion Institute, Salt Lake City, UT. 22 pgs. Funded by Morgantown Energy Technology Center through subcontract from Advanced Fuel Research Co.

The predicted effects of turbulence on the mean gas properties in coal combustors have been shown previously to be significant (Smith and Fletcher, 1986; Brewster, et al., 1988). These effects have been incorporated in comprehensive models by the statistical, coal-gas mixture fraction method (Smoot and Smith, 1985), which is based on the scalar approach for gas diffusion flames. Previously, only a single progress variable has been used for tracking coal off gas, including all material originating in the coal and transferring to the gas during the reaction process. Simple weight-loss models for devolatilization have been adequate for this approach that assumes constant off gas composition. With recent technological advances, the incorporation of detailed devolatilization models into comprehensive codes has become a practical consideration. However, detailed models that predict varying off gas composition require improved and/or extended methods of accounting for the effects of turbulence/chemistry interactions for successful incorporation.

In a recent paper, Brewster et al. (1988) presented a generalized formulation of the coal-gas mixture fraction method which allows for an arbitrary number of progress variables, thus allowing each element, or group of elements that evolve at similar rates, to be tracked independently. Independence of the progress variables was assumed, although it would not be difficult to include correlation coefficients if they were known. Results were presented for a slightly fuel-lean, swirling combustion case to illustrate the method. Two progress variables were used to separately track coal volatiles and char off gas, and significant decreases in the size of the fuel-rich region and in coal burnout were noted. These differences were attributed to the enriched oxygen content and (apparently) decreased heating value of the early off gas in the case of two progress variables. In this paper, additional results with multiple progress variables tracking coal off gas are reported and discussed. Calculations using an alternative method of calculating char off gas enthalpy agree qualitatively with previous observations of the effects of tracking volatiles and char off gas separately in combustion. Similar calculations in an oxygen-brown, fuel-rich gasification case show relatively little effect. Even less effect is observed when hydrogen from the coal is tracked separately and all of the other elements are lumped together. The effect of tracking volatiles separately decreased when ultimate volatile yield was increased from 40 to 80 percent, and noticeable differences existed with 50 percent heat loss (uniform) from the reactor compared to no differences when the reactor was assumed to be adiabatic. See also 1-88-J09 which relates to this Thrust Area.

Measurement and Prediction of Entrained-Flow Gasification Processes

Brown, B.W.; Smoot, L.D.; Smith, P.J. and Hedman, P.O.
AIChE Journal, 34, 3, 435-446, 1988. 12 pgs. Funding source by Morgantown Energy Technology Center.

A detailed mathematical model is used to predict local and effluent properties within an axisymmetric, entrained-flow gasifier. Laboratory experiments were conducted to provide local properties for four coal types from a gasifier operating at near-atmospheric pressure. Effects of selected model parameters and test variables were examined and compared with measurements in most cases. The comparison of predictions and measurements provides the first evaluation of capabilities and limitations of a comprehensive model for entrained-flow gasifiers.

Numerical Model of a Moving-Bed Coal Reactor

Yi, S.C.; Smoot, L.D. and Brewster, B.S.
Western States Section, 1988, The Combustion Institute, Salt Lake City, UT. 27 pgs. Funded by Morgantown Energy Technology Center through subcontract from Advanced Fuel Research Co.

A literature review of existing models for moving-bed coal gasifiers and combustors was conducted, and three available 2-D codes were installed and tested. Predictions and sensitivity analyses of the 2-D code developed by Washington University (Bhattacharya et al., 1986) were performed. Based on the review, the proposed features of an advanced model incorporating detailed coal chemistry submodels were identified. One major difference between the proposed model and the existing models is that the proposed model will have separate gas and solids temperatures. As a foundation for developing the advanced model, equations were formulated for an improved model incorporating separate gas and solids temperatures, but not incorporating the detailed coal reaction chemistry submodels or detailed compositions for bed hydrodynamics. A preliminary review of effective transport properties for fixed beds was also completed for the advanced model.

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.

The Sensitivity of Entrained-Flow Coal Gasification Burners to Changers in Inlet Boundary Conditions

Sowa, W.A.; Hedman, P.O. and Smoot, L.D.
Submitted to Fuel, 1988. 25 pgs. Funded by Morgantown Energy Technology Center.

The impact of diffusion flame burner geometry on entrained flow coal gasification was studied. Three diffusion flame burners were designed and installed on a laboratory-scale, downfired, entrained-flow, coal gasifier operated at pressures up to 560 kPa. Each burner was studied by varying reactor pressure, oxygen/coal ratio, and steam/coal ratio. Gasifier performance was assessed by collecting space-resolved gas and char samples in the reaction chamber and analyzing them for carbon conversion, gas composition (CO, CO2, H2, H2O, and CH4), and cold gas efficiency. Burner geometry affected carbon conversion, gas composition, and cold gas efficiency. Each burner had unique flame structural characteristics that resulted in burner-unique trends with reactor pressure, oxygen/coal ratio, and steam/coal ratio.

Effects of Pressure and Coal Rank on Carbon Conversion in an Entrained-Coal Gasifier

Cope, R.F.; Smoot, L.D. and Hedman, P.O.
Accepted for publication Fuel, 1988. 9 pgs. Funded by Morgantown Energy Technology Center.

Between 1946 and 1962, the U.S. Bureau of Mines developed and operated five different atmospheric pressure and elevated pressure entrained-coal gasifiers. From 1963 to 1982, Bituminous Coal Research, Inc. and others developed and operated the Bi-Gas pressurized two-stage entrained-coal gasifier. These programs demonstrated the feasibility of gasifying US coals in entrained gasifiers, and investigated the effects of such operating variables as pressure and coal rank. A number of other entrained coal gasification processes have demonstrated the ability to operate at elevated pressure with several coals of different rank (e.g. Texaco, Shell and Mountain Fuel Resources), but little research has been performed with these processes to determine how carbon conversion is affected by coal rank at elevated pressures.

Sebastion, Strimbeck et al., and Brown et al. reported that carbon conversion increased as coal rank decreased in atmospheric pressure entrained-coal gasification. Preliminary tests in the BI-Gas study show a similar coal rank effect at elevated pressures, but simultaneous changes in other operating variables (i.e. coal, oxygen and steam feed rates) obscure the combined effects of pressure and coal rank on carbon conversion. In a limited investigation of pressure effects in a laboratory gasifier (using bituminous coal only), McIntosh and Coates observed a 10 to 20 percent increase in exit carbon conversion as operating pressure was increased from 1050 to 2320 kPa. Studies conducted by Azuhata et al., showed that for a diffusion flame and an O2/coal ratio greater than 0.8, increasing the operating pressure form 100 to 500 kPa produced a 10-15 percent increase in the carbon conversion of Utah bituminous coal. A similar pressure effect was not observed in the Azuhata et al. premixed flame data. The objective of this study was to determine the combined effects of coal rank and elevated operating pressure on carbon conversion, in entrained-coal gasification.

Low Rank Coal-Water Fuel Combustion in a Laboratory Scale Furnace

Rawlins, D.C.; Germane, G.J. and Smoot, L.D.
Accepted for publication in Combustion and Flame, 1988. Funded by US Department of Energy.

A detailed study of hot-water dried lignite slurry combustion and the formation of nitrogen-containing pollutants was performed in a vertical, laboratory-scale combustor. Space-resolved local measurements of solid and gaseous combustion products were obtained from throughout the combustion zone using a stainless steel, water-quenched sample probe. Coal burnout (daf) of greater than 99% was achieved without supplementary fuel support, in an estimated residence time of 1.4s. Flame stability was strongly affected by the atomized droplet size, which is controlled by the atomizing air to slurry mass ration (A/S). For A/S greater than 0.7, coal burnout was relatively insensitive to further increases in A/S, yet burnout decreased rapidly as A/S was decreased. Nitric oxide (NO) emissions were not affected greatly by changes in A/S. Decreasing stoichiometric ration (SR) to about 0.8, caused coal burnout to decrease from about 98% to 94% and NO emissions to decrease from around 600 PPM to less than 100 PPM Changes in secondary air swirl number from 0 to 4.25 had little or no effect on coal burnout or NO emissions for a SR of 1.1 and an A/S of 0.75. At low A/S (0.24), high secondary air swirl was required in order to stabilize the slurry flame. Reactor mapping tests showed rapid mixing between the slurry and the combustion air. CO was found only near the slurry inlet at a maximum concentration of 0.3%. No other fuel-rich species were detected in measurable quantities.

The Impact of Diffusion Flame Injectors on Entrained Coal Gasification

Sowa, W.A.; Hedman, P.O. and Smoot, L.D.
Western States Section, 1988, The Combustion Institute, Salt Lake City, UT. Funded by Morgantown Energy Technology Center.

The impact of diffusion flame burner geometry on entrained flow coal gasification was studied. Three diffusion flame burners were designed and installed on a laboratory-scale, downfired, entrained-flow, coal gasifier operated at pressures up to 560 kPa. Each burner was studied by varying reactor pressure, oxygen/coal ratio and steam/coal ratio. Gasifier performance was assessed by collecting space resolved gas and char samples in the reaction chamber and analyzing them for carbon conversion, gas compositions (CO, CO2, H2, H2O, and CH4), and cold gas efficiency. Burner geometry was found to significantly affect carbon conversion, gas compositions, and cold gas efficiency. Each burner had unique flame structure characteristics that resulted in burner-unique trends with reactor pressure, oxygen/coal ratio and steam/coal ratio.

A Comparison of Combustion Characteristics Between Lignite-Water Slurry and Pulverized Lignite

Rawlins, D.C.; Smoot, L.D. and Germane, G.J.
Western States Section, 1988, The Combustion Institute, Salt Lake City, UT. 27 pgs. Funded by the Morgantown Energy Technology Center.

Experiments of the combustion of hot-water dried lignite slurry and its parent, pulverized coal have been performed in a laboratory-scale combustor. The operating parameter that had the greatest effect on flame location for lignite slurry combustion was the slurry mean droplet diameter. A stable flame could not be maintained with large droplet sizes. The air blast of the slurry-atomizing nozzle caused the mixing of the primary and secondary streams to be much more rapid for slurry combustion than for pulverized coal. Due to this rapid mixing, fuel-rich products of combustion were only observed in trace quantities near the top of the combustion zone during slurry combustion; however, with pulverized coal combustion, significant concentrations persisted throughout the combustor. Secondary air swirl number had the greatest effect on the pulverized lignite flame location. A minimum in nitrogen oxide (NO) concentration was observed during the pulverized coal combustion as swirl was increased. Secondary air swirl, however, had only a negligible effect on coal burnout and NO emissions for slurry combustion. A five-fold increase in the primary air velocity more than doubled NO concentrations at the exit plane. Changing the primary air velocity through the slurry atomizer (by changing the air mass flow rate) did not affect NO emissions during slurry combustion. Changes in the water concentration within the combustion system did not affect combustion performance with pulverized coal. Thus, NO emissions are more strongly controlled by the mixing of the fuel with the secondary air than by flame temperature reduction caused by water added to the combustion system.

Explosion Limits and Flame Speed For a Low-Rank Coal

Kramer, S.K.; Cannon, J.N. and Smoot, L.D.
Western States Section, 1988, The Combustion Institute, Salt lake City, UT. 16 pgs. Funded by US Department of Energy, Morgantown Energy Technology Center and Brigham Young University.

Explosion limits and flame propagation rates for a low-rank coal have been determined in an explosion bomb and are being studied in a steady flame device. The Decker, Montana subbituminous coal was obtained at the mine and stored in a nitrogen atmosphere. Minimum ignition energy and minimum dust cloud auto-ignition temperature were significantly influenced by sample moisture and particle size. However, the minimum explosive concentration, which was less than half that of the reference Pittsburgh bituminous coal dust, was not strongly affected by size or moisture content. Maximum pressure rise rate increased over three times with decreasing particle size and moisture content while the maximum pressure rise varied by only 20%. Ageing of the coal through low temperature surface oxidation of the sample had little effect on any of the ignition or explosion parameters. When compared to the reference Pittsburgh bituminous coal dust through the use of standard explosion indices, the dry coal is much more explosive while the wet coal is roughly equivalent. A well-insulated, one-dimensional, steady flow facility has been designed and constructed to measure the premixed, laminar flame speed of the sample coal. Preliminary tests have been made with a methane-supported coal dust flame to demonstrate in-situ measurement of species, species concentration and temperature with the coherent anti-Stokes Raman spectroscopy (CARS) system.

Particle Size Dependence of Coal Char Reactivity

Wells, W.F. and Smoot, L.D.
Combustion and Flame, 68, 81-83, 1987. 3 pgs. Funded by the Pittsburgh Energy Technology Center.

Studies of the reactivity of coal chars are limited in both scope and number, particularly for process chars from US coals at high temperatures. Pioneering work in coal char combustion was conducted by Field (1). He studied the effects on particle size on reactivity for a laboratory char prepared from a British coal. Smith (2) reviewed the extensive work performed by himself and coworkers. They have investigated the effects of size variation on reactivity for a wide variety of Australian coals. The work on US chars is more limited. Among those that have been reported is the study of Goetz et al. (3) for chars derived from Illinois bituminous, Wyoming subbituminous, and Texas lignite coals. The particle size distribution for these chars was very wide (-200 +400 mesh). Chen et al. (4) studied the ignition temperature for several process coal chars which were derived from US coals, one of which was also examined by this study (FMS COED), but they did not conduct high temperature reactivity measurements. Gomez and Vastola (5) studied char from a Wyoming subbituminous coal with particle sizes from 850 to 1000 mm. Due to the large particle size the reaction mechanism could be different than that for much smaller particles (6). Young et al. (7) have presented preliminary data for a North Dakota lignite char, but have not presented any data concerning effect of particle size on reactivity. This brief communication presents new results on the dependence of char reactivity at high temperature on particle diameter for five process chars and interprets these data and companion results of a previous study (8) as they relate to reaction zone.

Selection and Characteristics of ACERC Coals

Smith, L. and Smoot, L.D.
ACERC Report, 1987. Funded by ACERC (National Science Foundation and Associates and Affiliates).

This report, Selection and Characteristics of Standard ACERC Coals, summarizes the selection and characterization of a standard set of eleven coals to be used in research programs of the Advanced Combustion Engineering Research Center (ACERC). The report also documents the experimental data on the physical and chemical properties of these coals, relates these coals to other coal data banks, identifies major studies where these coals have been or are being used and documents selected references where the coals have been tested in combustion systems. Also included in the report is information about the general aspects of coal characterization. The standard coals were selected according to the following set of criteria: (1) available chemical and physical properties with wide property variations among coal types and ranks, (2) obtained from major producing seams, (3) future production expected, (4) wide geographical distribution within the US, and (5) used in previous work. The suite of eleven ACERC coals was selected because of their general availability and past record of use both nationally and in labs related to ACERC. National centers that provide coal samples and/or coal characterization data include the following: (1) Pennsylvania State University's program aimed at characterizing the Nation's coal resources, including the Penn State Coal Sample Bank and the Penn State Coal Data Base operated by the Energy and Fuels Research Center, (2) the coal sample suite used in the Direct Utilization-Advanced Research and Technology Development program managed by the Pittsburgh Energy Technology Center, and (3) Argonne National Laboratory's Premium Coal Sample Program. These sources were a key element in the selection of coals for ACERC study. The selection of the eleven Standard ACERC Coals relied mostly on standard coals that were already specified and in use by these major coal banks nation-wide. Thus, a source of pristine coal samples is available for most of the Standard ACERC Coals. This suite of well-selected coals will help coordinate the large experimental research effort of ACERC. This report provides a foundation of information on the properties and characteristics of ACERC coals and will facilitate their use in the Center. The report has been published in expandable form so that the information can be updated periodically as needed.

Prediction of Nitric Oxide in Advanced Combustion Systems

Boardman, R.D. and Smoot, L.D.
AlChE J., 1987. 14 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.

A computer model to predict nitric oxide (NO) concentrations has been applied to advanced-concept, pulverized-coal systems and evaluated by comparing model predictions with experimental data. Specifically, the effects of pressure, stoichiometric ratio, coal moisture content, particle size, and swirling and non-swirling diffusion flames (Hill et al., 1984; Smith et al., 1986).

The NO model is a subcomponent of a general combustion code that provides theoretical predictions for the temperature, velocity, major species, and other properties at local points throughout turbulent, combusting flow fields (Smoot and Smith, 1985). In the NO model, fuel nitrogen release from the coal is assumed to occur at a rate proportional to total coal weight loss. The volatile nitrogen is assumed to be instantaneously converted to HCN. NO is formed by oxidation of the HCN and is competitively reduced to N2 by reaction with HCN. Global rate expressions for these reactions were measured by the Soete (1985). The model also accounts for the destruction of NO by heterogeneous interaction with char using a rate expression from Levy et al., (1981). All rate parameters are used as reported except for the information of HO. For this rate equation, the pre-exponential factor was increased by a factor of ten. This value is still in within the range of experimental error and was used because it yielded better results. Effects of turbulence on the gaseous reactions are accounted for through use of a joint probability calculation of fluctuating gaseous and coal off-gas mixture fractions. These two progress variables are sufficient to track turbulent temperature and gas concentration variations (Smith et al., 1982). Experimental data sources for model comparisons were selected for axi-symmetric combustion exponents that investigated the variation of key test variables (e.g., pressure or stoichiometric ratio).

Release and Reaction of Fuel-Nitrogen in a High-Pressure Entrained-Coal Gasifier

Nichols, K.M.; Hedman, P.O. and Smoot, L.D.
Fuel, 66, 1257-1263, 1987. 7 pgs. Funded by Morgantown Energy Technology Center.

Effects of pressure, flame type and coal feed rate on fuel-nitrogen release and nitrogen pollutant formation were examined in a laboratory scale, entrained-coal gasifier. A Utah, high-volatile bituminous coal was used. With a water-quenched probe, gas-particulate samples were collected for oxygen-coal mass ratios from 0.6 to 1.1, pressures of 1, 4.9 and 10.4 atm and coal feed rates of 25 and 35 kg·h-1. Two injector types were utilized; one produced a diffusion flame, the other a premixed flame. Fuel-nitrogen release from the coal showed little dependence on oxygen-coal ratio, pressure or coal feed rate. Values at the gasifier exit averaged 83% for the diffusion flame and 92% for the premixed flame.

Fuel-nitrogen release, mostly during devolatilization, exceeded fuel-carbon release by . 10% for the premixed flame and . 30% for the diffusion flame, depending on oxygen-coal mass ratio. Over 50% of the released fuel-nitrogen formed N2, with significant amounts of NH3 and HCN, and smaller amounts of NO. Increased pressure at constant mass feed rates caused sharp decreases in effluent NO concentrations (to near zero) for both flame types which was explained by a combination of increased residence time and increased homogeneous NO decay rate. Elevated pressure also increased the effluent NH3 and decreased HCN concentrations for the diffusion flame whereas the more complete mixing of the premixed flame resulted in lower NH3 and HCN levels, and higher N2 levels. In general, nitrogen species concentrations were not largely affected by coal feed rate, though increased coal feed rate decreased NH3 levels somewhat. From these observations, together with observations from other investigators; possible explanations are postulated.

Further Evaluation of a Nitric Oxide Model

Boardman, R.D. and Smoot, L.D.
Western States Section, 1987, The Combustion Institute, Provo, UT. 25 pgs. Funded by Morgantown Energy Technology Center through subcontract from Advanced Fuel Research Co.

Further verification of a predictive model for nitric oxide formation during turbulent combustion of coal containing fuels has been conducted. Computations for pulverized coal combustion in CO2-02 mixtures of various percents have been completed. The predicted NO concentrations compare favorably with experimental measurements. Simulations were also completed for entrained-flow gasification in a laboratory-scale combustor. Again, reasonable agreement is demonstrated by comparing laboratory NO maps to predicted NO concentrations. The effects of pressure on NO concentrations were reliably predicted. Calculations were also completed for air-staged combustion in a one-dimensional, laboratory-scale reactor. In general, the trend of decreasing primary zone stoichiometric ratio and variation in staging air location were correctly predicted. The simplified global mechanism expressions of the NO model appear to sufficiently account for the formation and competing destruction of NO in both fuel-lean and fuel-rich environments for different reactor systems and conditions.

Sulfur Species Formation in a High-pressure Entrained-Coal Gasifier

Nichols, K.M.; Hedman, P.O.; Smoot, L.D. and Blackham, A.U.
Western States Section, 1987, The Combustion Institute, Provo, UT. 16 pgs. Funded by Morgantown Energy Technology Center.

This work summarizes several observations concerning the effects of pressure and oxygen-to-coal mass ratio on the fate of coal-sulfur during entrained gasification. A high-volatile bituminous coal was pulverized to a mass mean of near 50 mm. The coal was gasified with oxygen in a laboratory-scale entrained-flow gasifier. Test pressures were atmospheric (1.0 ATM, 101 kPa), 4.9 ATM (500 kPa), and 10.4 ATM (1050 kPa). Oxygen-to-coal mass ratios between 0.6 and 1.1 were investigated. Gas-particulate samples were collected with a water-quenched probe from the gasifier chamber effluent stream. Measurements were made of the sulfur retained in the char particles and of the concentrations of H2S, SO2, COS and CS2 in the product gas. Conversion of sulfur to the gas phase was observed to decrease with increasing pressure, possibly through sulfur captured by char. Changing pressure caused a change in the distribution of gas phase sulfur species. At higher pressure, the proportions of SO2 and CS2 decreased, and the proportion of H2S increased. This redistribution with increasing pressure is not predicted by equilibrium calculations, nor was it observed in learner (less particle laden) combustion environments. This suggests the importance of char in determining the fate of the coal-sulfur during gasification. Increasing oxygen-to-coal mass ratio increased sulfur conversion, SO2 concentration, and COs concentration, while it decreased H2S and CS2 concentrations.

Comprehensive Modeling of Combustion Systems

Smoot, L.D. and Smith, P.J.
ASME/JSME Thermal Engineering Joint Conference, 1987, Honolulu, Hawaii. 13 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).

This paper provides a brief review of comprehensive modeling of combustion systems. The focus is on continuous, subsonic flow systems. Supersonic flows and intermittent or unsteady flows with combustion were not considered. Where condensed phases are present, the focus is further placed on entrained systems where particulate and droplet collisions or interactions through motion can be neglected. Gaseous, liquid, solid and slurry fuels were considered. A brief review of related literature is included. Model foundations, elements (submodels), requirements, and potential uses are summarized. Recent technical work of the authors or colleagues in turbulent fluid mechanics of swirling flows, radiation, and gas-phase reactions is summarized. Example predictions for selected cases are shown from several investigators, mostly with comparisons to measured properties. Future directions, research needs and plans are also identified.

Controlling Mechanisms in Gasification of Pulverized Coal

Smoot, L.D. and Brown, B.W.
Fuel, 66, 1249-1256, 1987. 8 pgs. Funded by Morgantown Energy Technology Center.

Controlling mechanisms in the fuel-rich reaction of pulverized coal with oxygen at atmospheric pressure were investigated through analysis of experimental data and by comparison with predictions of a comprehensive model. Gasification data were obtained for four coal types at various oxygen-steam-coal ratios and effects of coal feed rate, particle size and flame type (premixed, diffusion) were also determined. The results show that coal particle heat-up and devolatilization occur very rapidly (<60-80 ms) near the coal inlet with up to 70% of the coal consumed. Coal volatiles and oxygen are rapidly consumed through gas-phase reaction, producing high gas temperature and high CO2 concentrations. Addition of steam plays little role in the coal reaction process, while residual char (typically containing 20-30% of the carbon) is consumed less rapidly (>200 ms) through surface reaction with CO2 and H2O, and possibly O2 at the onset. This general reaction process varies little among the coal types examined. The surface reactions are controlled in high-temperature regions through oxidizer diffusion to the char surface; however, as the gasifier temperature declines through heat loss and endothermic reaction, heterogeneous char-oxidizer reaction near the particle external surface become more important, giving rise to some dependence on coal type.

The Sensitivity of Entrained-Flow Coal Gasification Diffusion Burners to Changes in Geometry

Sowa, W.A.; Hedman, P.O. and Smoot, L.D.
Western States Conference, 1987. 25 pgs. Funded by Morgantown Energy Technology Center.

The impact of diffusion flame burner geometry on entrained flow coal gasification was studied. Three diffusion flame burners were designed and installed on a laboratory-scale, downfield, entrained-flow, coal gasifier operated at pressures up to 560 kPa. Each burner was studied by varying reactor pressure, oxygen/coal ratio and steam/coal ratio. Gasifier performance was assessed by collecting space resolved gas and char samples in the reaction chamber and analyzing them for carbon conversion, gas compositions (CO, CO2, H2, H2O, and CH4), and cold gas efficiency. Burner geometry was found to significantly affect carbon conversion, gas compositions, and cold gas efficiency. Each burner had unique flame structure characteristics which resulted in burner-unique trends with reactor pressure, oxygen/coal ratio and steam/coal ratio.

The Behavior of Chlorine in Kentucky and Illinois Coals During Combustion and Its Effects on Ash Deposits

Sowa, W.A.; Hedman, P.O.; Smoot, L.D. and Blackham, A.U.
Western States Section, 1986, The Combustion Institute, Tucson, AZ. Also accepted for publication in Fuel, 1988. 34 pgs. Funded by Tennessee Valley Authority.

Ash deposition tests were performed in a modified pulverized coal combustor with four different coals: low chlorine Kentucky No. 9, and Kentucky No. 11, and high chlorine Illinois No. 5 and Illinois No. 6. The amount of coal available for testing differed markedly between coal types ranging from 100-1000 kg. per coal type. Several repeated one-hour combustion tests were performed for all four coals. Each firing consumed 15-25 kg. of coal. Ash deposition tests provided samples from simulated waterwall and superheater probes, and from an exhaust cyclone and a water-quenched char sample probe. Measured physical properties included, ash chemical analyses, proximate and elemental analyses of both raw coal and ash deposits, ash fusion temperature tests, ash sintering temperature tests, ash shear and compressive strength analyses, and ash thermal conductivity and emittance. Chlorine was found to release quickly from the coal to the gas phase. Gas phase chlorine was found to release quickly from the coal to the gas phase. Gas phase chlorine condensed and concentrated on the waterwall collection surfaces. The amount of chlorine that condensed onto the ash collection surfaces was dependent on the temperature of the collection surface. The colder surfaces had the highest chlorine concentrations. Corrosion of the stainless steel test surfaces was observed during the combustion tests with the Illinois coals. The carbon and chlorine conversion rate from the char appeared to be equal for carbon conversion levels above 65%. Ash fusion temperature, ash sintering temperature, emittance, thermal conductivity, shear strength and compressive strength measurements which were performed on samples from the waterwall and superheater probes showed no observable differences between the four coal types tested. The one-hour firings were probably too short for the ash deposits to reflect the influence of metal corrosion on the measured physical properties. Emittance, ash sintering temperature, compressive strength and shear strength were dependent on sample location.

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.