ACERC
Abstracts - 1989 |
ACERC
Abstracts - 1989 |
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Thrust Area 5: Comprehensive Model Development |
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:
Review the existing and potential uses of coal and the processes most commonly applied.
Identify the general chemical and physical properties of coal, emphasizing the complexity and variability of these natural materials.
Summarize major issues being addressed in the increasing uses of coal and other solid fossil fuels.
Characterize effects of key variables such as coal type, particle size, heating rate, temperature, pressure, and oxidizer type on coal particle reaction rate.
Outline useful existing methods for modeling of coal particle reactions, including devolatilization and heterogeneous oxidation processes.
Identify the nature and controlling processes of practical coal dust flames in various coal processes.
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.
Smith, P.J.; Sowa, W.A.
and Hedman, P.O.
Accepted for publication in Combustion and Flame, 1989. Funded by ACERC
(National Science Foundation and Associates and Affiliates) and Brigham Young
University.
A new design methodology is presented which allows for the use of comprehensive coal combustion codes in design applications and provides a priori information on the cost of the optimization. A statistical response surface methodology is used to determine appropriate sample points from the design space at which the computation for the comprehensive code are performed. Statistical regression analysis is used to provide interpolating functions for the optimization process. The optimum design point is then checked with a final comprehensive code calculation. The technique is demonstrated with simple examples for design of two injectors for an entrained coal gasifier and of a burner for a pulverized coal combustor. The three designs demonstrate the method as well as showing significantly different optima for different configurations. The importance of specifying operating conditions independently for different injectors or burners is demonstrated.
Bai, D.
Research Report, Dept. of Mathematics, Utah State University, 1989. Funded
by ACERC (National Science Foundation and Associates and Affiliates), the State
of Utah, and US Department of Energy.
As part of our effort in applying multigrid solvers to combustion modeling, we have developed a multigrid solver for a simplified 3-D combustion model. With this solver we were able to reduce the error on a 65 x 65 x 65 grid within 181 CPU minutes on a Convex C120 computer. The details of the algorithms, including the distributive relaxation and the results of numerical experiments are presented in this paper.
Hobbs, M.L. and Smith, P.J.
Accepted for publication in Fuel, 1989. Funded by ACERC (National Science
Foundation and Associates and Affiliates) and Utah Power & Light.
A simple two-zone zero-dimensional combustion model which estimates the influence of impurities in the fuel on the radiative energy transport has been developed based on an overall energy balance coupled with a multi-zoned radiation model. This paper presents the equations of the model, illustrates the method of calculating the radiative exchange areas for the two-zone system, and presents predictions for pulverized-coal and fluidized-bed combustion. The model predicts thermal performance as a function of coal input and furnace operational parameters, steam mass flow rates, and superheated steam temperatures leading to the high-pressure turbine. Two wall ash deposit parameters, thermal conductivity, and maximum deposit thickness, have been determined by a sensitivity analysis to be critical to furnace performance. Others have obtained these parameters experimentally. The predictions from the two-zone model have been compared to predictions from an earlier single-zone model. The general trends from both models were the same, although the two-zone model predictions were closer to expected values.
The foundation to describe coal-specific conversion behavior will be AFR's Functional Group (FG) and Devolatilization, Vaporization, and Cross linking (DVC) models, developed under previous and on-going METC sponsored programs. These models have demonstrated the capability to describe the time dependent evolution of individual gas species, and the amount and characteristics of tar and char. The combined FG-DVC model will be integrated with Brigham Young University's comprehensive two-dimensional reactor model, PCGC-2, which is currently the most widely used reactor simulation for combustion or gasification. The program includes: i) validation of the submodels by comparison with laboratory data obtained in this program, ii) extensive validation of the modified comprehensive code by comparison of predicted results with data from bench-scale and process scale investigations of gasification, mild gasification and combustion of coal or coal-derived products in heat engines, and iii) development of well documented user friendly software applicable to a "workstation" environment.
Success in this program will be a major step in improving the predictive capabilities for coal conversion processes including: demonstrated accuracy and reliability and a generalized "first principles" treatment of coals based on readily obtained composition data.
Fife, P.; Hastings, S. and
Lu, C.
Accepted for publication in SIAM J. Appl. Math., 1989. Funded by National
Science Foundation.
A full analysis is given for planar steady flames under the following model chemistry:
A + B —> 2B
2B —> Products
The analysis is asymptotic, based on large activation energy of the first reaction, and the smallness of a certain other parameter. This is followed by a mathematically rigorous analysis justifying the asymptotics. Of especial interest is the regime where the first reversible reaction is in partial equilibrium. Parameter values leading to that regime are delineated. The forward first reaction can be either exothermic or endothermic.
Smith, P.J. and Gillis,
P.A.
1st International Congress on Toxic Combustion Byproduct: Formation and Control,
Los Angeles, California, 1989. Funded by ACERC (National Science Foundation
and Associates and Affiliates) and Brigham Young University.
Mathematical model simulations are presented for pilot plant and laboratory test furnaces to quantify the predictability and accuracy of combustion by-product formation and destruction. The roles of numerical accuracy and of closely coupled physico-chemical processes are explored. Capabilities of 3-D furnace models to describe global, site-specific flow and fine scale mesh resolution is shown to be an important consideration in simulating flow patterns in utility boiler geometries, even for relatively simple configurations. Fabricated exact numerical solutions for the coupled particle differential equation set are shown to be useful in identifying algorithmic and coding errors in these large mathematical models. The coupling between local heat transfer and other physico-chemical processes occurring in coal combustion applications is emphasized. Differences of 50-70% are shown to be obtained if particle-gas convective/conductive heat exchange and gas radiation are ignored or if turbulent fluctuations are not accounted for. This variability is shown to have a large impact than the chemical kinetic reaction rate of CO to CO2. The importance of advancing chemistry submodels and their coupling to the turbulent fluid mechanics for more accurate predictability of combustion by-products is emphasized.
Gillis, P.A. and Smith,
P.J.
Western States Section, The Combustion Institute, Livermore, California,
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.
In recent years, advances in computer technology have revolutionized the methods used to analyze complex flow phenomena. Many practical flows, which were previously expensive and difficult to model, are now gradually yielding their mysteries through the power of today's computers. This process is aided by the continuing development of efficient numerical algorithms and further research into the modeling of physical phenomena that are not well understood, such as turbulence. Several three-dimensional comprehensive combustion models have appeared in the literature over the past three years. Nearly all these models employ similar numerical techniques, variations of the TEACH method. A review of these codes has revealed that they use coarse grid structures and typically lack validation through comparison with experimental data. The purpose of this paper is to describe and demonstrate a three-dimensional CFD (Computational Fluid Dynamics) code that has been designed for industrial geometries. This code includes new numerical techniques, has been validated with data comparisons in several industrial furnaces, and will be used to establish the grid resolution required to obtain grid independent solutions.
The CFD model was designed to be a fundamental element in a larger comprehensive pulverized-coal combustion code for utility boilers. The model handles Cartesian and cylindrical coordinate systems and allows for irregular grid spacing in order to efficiently model the large scale disparities encountered in industrial furnaces. The model includes the option of employing different variations of the SIMPLE algorithm to couple the Navier-Stokes equations. Coupling options include the SIMPLE, SIMPLER, SIMPLEC, SIMPLEST, and various combinations of these algorithms. First-order finite differencing is performed with a combined central and revised upwind scheme that has been adapted to handle grid irregularities. The resulting finite-difference coefficient matrices are solved with a vectorized relaxed Thomas algorithm. Several turbulence models have been incorporated into the code, include the k-e turbulence model. Significant portions of the model have been vectorized to improve code performance.
The emphasis of this paper is on evaluating model performance. Experimental data has been obtained from Consolidation Coal Company for a wall-fired pilot-scale furnace with four swirled burners. Combustion Engineering has also provided a large collection of velocity data from a tangentially fired furnace. Comparisons will be made between predicted and experiment velocities for both configurations. These comparisons will be performed in different locations within each furnace and model performance will be evaluated. Additional numerical experiments were performed to determine the grid resolution needed to achieve grid independent solutions. Recommendations will be made detailing the grid resolution needed to accurately model the major types of industrial furnaces. The different coupling algorithm options employed in the model will also be evaluated for both robustness and efficiency. Additional comparisons between the TEACH method and the model's numerical techniques will be made.
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.
Sikorski, K.
North American Transputer Users Group Meeting, Salt Lake City, Utah,
1989. Funded by University of Utah.
We survey our recent experience with using parallel computers for modeling seismic wave propagation in the Salt Lake Basin. The computers under consideration are:
Distributed memory transputer array (we experimented with 16 to 96 processor machines, having from 0.25 to 2 Megabytes of memory per node), and
The Stellar GS 1000 graphics super workstation.
We have implemented a second-order finite difference algorithm with the first-order absorbing boundary conditions for the acoustic case and a staggered grid fourth-order finite difference algorithm for the elastic simulation. Our results suggest a linear speed-up with the increase in the number of nodes.
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