L. Douglas Smoot

Modeling of nitrogen oxides formation and destruction in combustion systems

Abstract The formation of nitrogen oxides (NO X ) in combustion systems is a significant pollutant source in the environment, and the control of NO X emissions is a world-wide concern as the utilization of fossil fuels continues to increase. In addition, the use of alternative fuels, which are typically of lower quality, tends to worsen the problem. Advances in the science of NO X reactions, mathematical modeling, and increased performance of computer systems have made comprehensive modeling of NO X formation and destruction a valuable tool to provide insights and understanding of the NO X reaction processes in combustion systems. This technology has the potential to enhance the application of various combustion techniques used to reduce NO X emissions from practical combustion systems. This paper presents a review of modeling of NO X reactions in combustion systems, with an emphasis on coal-fired systems, including current NO X control technologies, NO X reaction processes, and techniques to calculate chemical kinetics in turbulent flames. Models of NO X formation in combustion systems are reviewed. Comparisons of measured and predicted values of NO X concentrations are shown for several full-scale and laboratory-scale systems. Applications of NO X models for developing technologies, in order to reduce NO X emissions from combustion systems are also reported, including the use of over-fire air, swirling combustion air streams, fuel type, burner tilt angle, use of reburning fuels, and other methods.

Modeling of swirl in turbulent flow systems

Abstract The standard κ-ϵ equations and other turbulence models are evaluated with respect to their applicability in swirling, recirculating flows. The turbulence models are formulated on the basis of two separate viewpoints. The first perspective assumes that an isotropic eddy viscosity and the modified Boussinesq hypothesis adequately describe the stress distributions, and that the source of predictive error is a consequence of the modeled terms in the κ-ϵ equations. Both stabilizing and destabilizing Richardson number corrections are incorporated to investigate this line of reasoning. A second viewpoint proposes that the eddy viscosity approach is inherently inadequate and that a redistribution of the stress magnitudes is necessary. Investigation of higher-order closure is pursued on the level of an algebraic stress closure. Various turbulence model predictions are compared with experimental data from a variety of isothermal, confined studies. Supportive swirl comparisons are also performed for a laminar flow case, as well as reacting flow cases. Parallel predictions or contributions from other sources are also consulted where appropriate. Predictive accuracy was found to be a partial function of inlet boundary conditions and numerical diffusion. Despite prediction sensitivity to inlet conditions and numerics, the data comparisons delineate the relative advantages and disadvantages of the various modifications. Possible research avenues in the area of computational modeling of strongly swirling, recirculating flows are reviewed and discussed.

Prediction of propagating methane-air flames☆

Abstract The kinetics and propagation of laminar methane-air flames were studied using a one-dimensional, flame propagation model. The model is based on a numerical, unsteady-state solution of transformed species and energy conservation equations using explicit techniques for diffusion terms and linearized, implicit techniques for kinetic terms. A methaneoxygen kinetic mechanism consisting of 28 elementary reactions was postulated and used in the flame model. Flame velocity, flame thickness, temperature profile and concentration profiles of 13 species were predicted for a series of methane-air flames. The effects of pressure, methane concentration, initial temperature, rate constants, and transport coefficients were investigated. Many of the model predictions were compared with experimental data, and agreement was generally very good. The concentrations of the radicals H, OH, and O were major factors in the propagation of methane-air flames. The relative importance of each of the 28 reactions was examined; five were found to be negligible, while several were shown to be important in determination of propagation velocity and flame characteristics.

Char oxidation at elevated pressures

Abstract Approximately 100 char oxidation experiments were performed at atmospheric and elevated pressures, 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 1500 K with 5% to 21% oxygen in the bulk gas, resulting in average particle temperatures ranging from 1400 to 2100 K and burnoff from 15% to 96% (daf). Independently determined particle temperature and overall reaction rate allowed an internal check on the data consistency and provided insight into the products of combustion. The chars burned in a reducing density and reducing diameter mode in an intermediate regime between the kinetic and pore diffusion zones, irrespective of total pressure. Significant surface CO 2 formation occurred at particle temperatures below about 1800 K 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; the rate decreased with further increases in pressure. Results indicate that ambient pressure global model kinetic parameters cannot be accurately extrapolated to elevated pressures. The apparent reaction rate coefficients (based on the partial pressure form of the n th-order rate equation) showed significant pressure dependence, since both the activation energy and frequency factor decreased with increasing pressure. This suggests that the empirical n th-order rate equation is not valid over the range of pressures encountered in the experiments. However, simulations indicate that the global model can be used to model elevated pressure char oxidation provided pressure-dependent kinetic parameters are used.

Modeling of coal-combustion processes

Abstract Modeling of coal-reaction processes has received significant emphasis over the past decade. Yet, model development has not reached the point where significant use is made in process development for coal utilization. One of the key recommendations from a panel of industrial and university professionals to the U.S. Department of Energy on research needs for coal utilization was that “Development of combustion models is needed to the point where they will find application in the management and control of practical systems.” The purpose of this publication is to present a review and evaluation of the development of coal-reaction models. Coal-combustion models are classified and potential uses identified. Then, the state of development of models for fixed (or slowly moving) beds, fluidized beds and suspended (or entrained) beds is presented. Six fixed-bed models, ten fluidized-bed models and thirteen suspended-bed models are reviewed and compared. Selected comparisons of model predictions with measurements are shown and research needs are noted. Emphasis is placed on suspended-bed models.

Pulverized-coal combustion research at Brigham Young University

Abstract This review paper presents a comprehensive summary and analysis of research work conducted principally at the Brigham Young University Combustion Laboratory over the past decade. An attempt is made to set forth the philosophy and foundations for the laboratory research, which include commitments to both measurement and modeling of complex combustion processes. Recent work has emphasized pulverized-coal processes. The review paper reports past results, together with some recent, previously unpublished results, and provides an integration and summary of key findings. Information from independent investigators is also included or referenced where comparisons are made or where similar work provided clarification. Among the most significant results include the following: (1) measurement of local properties (e.g. velocity, concentration, temperature, mixing rate) from reacting and nonreacting gaseous and particle-laden flows for a variety of test conditions. These data provide a basis for determining controlling processes and for evaluating comprehensive coal reaction models; (2) development and application of comprehensive pulverized-coal combustion models for premixed and diffusion flames; (3) evaluation of combustion models by extensive comparisons with detailed profile data from this and other laboratories. Research needs and concerns in each area of activity are also identified, and future possible laboratory directions are considered.

An improved model for fixed bed coal combustion and gasification

Abstract An improved one-dimensional model for countercurrent oxidation and gasification of coal in fixed or slowly moving 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 method is implemented to satisfy exactly 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 yields improved predictions for product gas composition and product tar flow rate. 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, airblown, dry ash, Wellman-Galusha gasifier, fired with the Jetson bituminous coal, is presented. Areas which need additional improvements are identified.

Turbulent gaseous combustion part I: Local species concentration measurements

Abstract Experimental measurements were made of fuel mixture fraction profiles and species concentration profiles (O 2 , N 2 , CH 4 , CO, CO 2 , H 2 O, and tracer Ar) in turbulent natural gas diffusion flames. Twenty-eight tests were performed in an axisymmetric combustor with coaxial feed of fuel and air. The test variables included type and arrangement of sample probe, combustor wall temperature, and air feed temperature. Gas samples were obtained with both water-quench and water-cooled probes. Direct water-quench probes were more effective in quenching reactions inside the sampling probe than water-cooled probes; however, some of the CO 2 in the gas samples was dissolved in the quench water. A 350 K change in the wall temperature had a negligible effect on gas mixing rates, but did alter the local gas species distributions somewhat. More complete combustion was achieved with elevated air feed temperatures, but more complete mixing was obtained at lower air temperatures. The results provide a data base for evaluating predictive computer models.

One-Dimensional Model for Pulverized Coal Combustion and Gasification

Abstract A one-dimensional model has been developed for pulverized coal combustors and gasifiers. The model describes the response of a coal particle system to its thermal, chemical and physical environment. Moisture vaporization, coal devolatilization, heterogeneous char oxidation, gas particle interchange, radiation, gas phase oxidation, primary and secondary stream mixing, and heat losses are considered. A predictor-corrector solution technique was used to solve the ordinary non-linear differential equations. Several combustor and gasifier predictions are shown. The model predictions are compared with experimental data. The effects of particle size and distribution are shown to be particularly important. Significant rate controlling processes include initial particle heat-up and char surface reaction.

Model for pulverized coal-fired reactors

An axisymmetric, elliptic model has been developed for analysis of confined, turbulent, coal-laden diffusion flames. The scheme is Eulerian for gases and Lagrangian for particles. The approach emphasizes the turbulent fluid mechanics of the mixing-limited gas phase reaction processes. The two-equation (k−∈) turbulence model is used for closure. Particle drag and turbulent particle diffusion are also modeled. Gaseous combustion is modeled with a probability density function for the mixture fraction. Fluctuations in mixing of inlet streams and coal off-gas are considered. Coal pyrolysis and oxidation reaction processes are assumed to be slow with respect to the turbulent time scale. Particle and gas radiation are incorporated by a flux method. Predictions emphasize the importance of turbulent particle dispersion.