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FBED-1 was developed at Brigham Young University (BYU) and FG-DVC was developed at Advanced Fuel Research (AFR) principally with funding form the U.S. Department of Energy's Morgantown Energy Technology Center.
OBJECTIVES
An improved model for fixed bed coal combustion and gasification, 96-FBED-1, is being developed based on the existing ACERC model, 93-FBED-1. Many advanced features have been introduced through the existing model, but some limitations remain. The new model will remove the main limitations by: (1) including provisions for countercurrent, cocurrent, and possible crosscurrent flows as well as for the additions and withdrawals of gases at multiple locations in the bed; (2) incorporating advanced submodels for coal devolatilization, oxidation, and gasification processes for large coal particles at high pressures; and (3) incorporating an efficient and robust numerical method appropriate for stiff, non-linear problems.
DESCRIPTION
FBED-1 is a generalized, one-dimensional, heterogeneous, steady-state analysis and design tool that can be used to simulate a variety of fixed or moving bed gasification, combustion, and devolatilization processes. The 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 variable bed void fraction are incorporated to predict realistic axial temperature and pressure profiles.
The conservation equations and related boundary conditions for mass and energy form the foundation of the model. These equations are formed for gas and solid overall mass continuity, gas and solid overal energy conservation, and gas and solid species or elemental mass continuity. The source terms in the continuity and energy equations are described by physical and chemical submodels. Plug flow is assumed in both the solid and the gas phase with variable axial velocities. Gas phase pressure is calculated with the Ergun-type equation for packed beds. Large coal particle devolatilization is allowed to occur simultaneously with char oxidation and gasification. Turbulence is not treated formally in the slowly moving bed with low gas velocity, but is included implicitly through model correlation such as the effective heat transfer coefficient .
A zero-dimensional submodel, FBED-0, is embedded into FBED-1 to provide initial estimates of effluent temperature and composition. FBED-0 is two-zone, well-mixed, partial equilibrium model, that can also be used as a stand-alone code.
NUMERICAL METHOD
FBED-1 is comprised of a set of 44 highly non-linear, stiff, and coupled ordinary differential equations. This set of equations will be solved using a full back-and-forth iteration strategy, in conjunction with the LSODE (Livermore Solver for Ordinary Differential Equations) integration package. Implementation of this strategy, which satisfies both the gas and the solid phase boundary conditions, is expected to enhance convergence and improve robustness over the method currently in use.
PARTICLE PHASE REACTIONS
In FBED-1, coal particles are considered to have four components: moisture, volatiles, char and ash. The particle phase reactions include drying, devolatilization, combustion and gasification. Submodels for each of these processes are provided.
DRYING AND
DEVOLATILIZATION
Drying is assumed to be diffusion limited and is described simply through vaporization of moisture from the particle surface. Devolatilization is described by assuming that the organic part of the coal particle is composed of various functional groups: carboxyl, hydroxyl, ether, nitrogen, etc. The functional group, depolymerization, vaporization and crosslinking submodel, FG-DVC, is used to determine the devolatilization rates and yields. The FG portion of the submodel describes the evolution of functional groups to the gas phase. The evolution of tar is governed by the DVC portion of the submodel. The functional group evolution and the bond breaking kinetic rates follow Arrhenius type dependence on temperature with distributed activation energies; the kinectic parameters are coal rank specific.
Intraparticle heat and mass transport processes, which have significant effects on large particle devolatilization will be modeled, with pressure effects accounted for. The full FG-DVC model will be used as a pre-processor to provide input for a reduced FG-DVC. The reduced FG-DVC, which is numerically much more efficient, will be integrated in the FBED-1 comprehensive code. Gas phase decomposition of tar to light gaseous species is taken into account by a distributed activation energy submodel.
OXIDATION AND GASIFICATION
Oxidation and gasification reactions consume the char and act on all functional groups. Three gasification agents are considered: carbon dioxide, steam, and hydrogen. The oxidation and the gasification kinetic rates follow Arrhenius type dependence on temperature; the kinetic parameters are coal rank specific.
The existing models for oxidation and gasification processes use kinetics for small particles at atmospheric pressure. Incorporation of kinetic data for large coal particles at high pressures, being measured at ACERC, will significantly improve predictions.
GAS PHASE REACTIONS
Gas temperature and composition are determined by assuming all species to be in thermal equilibrium and total or partial chemical equilibrium. Partial equilibrium refers to a gaseous mixture where at least one species is held out of chemical equilibrium. Three options are provided: total equilibrium, partial equilibrium where tar is held out of chemical equilibrium, and partial equilibrium where all gases below a user specified temperature are assumed to be nonreactive.
Full kinetic treatment of gas phase reactions at high pressures is under consideration. Such treatment may be possible by incorporating in FBED-1 two codes from Sandia National Laboratories in Livermore: CHEMKIN-II and CHEMKIN-RG.
HEAT AND MASS TRANSFER
Heat and mass transfer processes in fixed or moving bed gasifiers are affected by complex solids flow and chemical reactions. Coarsely crushed coal settles while undergoing heating, drying, devolatilization, gasification and combustion. Coal particles change in diameter, shape, and porosity. Non-ideal behavior may result from coal bridges, gas bubbles, and channels. Variable void fraction may also change heat and mass transfer characteristics. Correlations for solid-to-gas heat and mass transfer coefficients, obtained for non-reactive fixed beds, are corrected by empirical factors to account for these effects. Correlations for solid-to-wall and gas-to-wall heat transfer coefficients are simlarly corrected. All solid phase and gas phase transport properties are considered to be functions of both temperature and composition.
USER REQUIREMENTS
FBED-1 is a stand-alone computer code that represents the state-of-
the-art in fixed or moving bed combustion, gasification, and devolatilization modeling. Its use is recommended to technical specialists working in research, development, design, and operation of these processes. The preparation of input data is straightforward and the user's manual provides a sample problem that is also supplied with the computer code. The output is available in both tabular and graphical forms; third party software is available for graphical data reduction.
SYSTEM REQUIREMENTS
Every effort has been made to maintain ANSI FORTRAN 77 coding standards in the development of FBED-1. As such, the code can be ported to several different computer platforms. Simulations can be readily performed with acceptable computation times on both mainframe computers and more powerful workstations.
AVAILABILITY AND USER SUPPORT
General release of 96-FBED-1 is planned for Fall of 1996. In addition to source code, a user manual detailing code theory and use will be prepared for distribution with FBED-1. Participation in a user's group and workshops on training in use of the code will also be available.
For more information about code availability, licensing fees, or to obtain a copy of the code, contact:
Research and development are being actively pursued to improve existing submodels in FBED-1 and to add additional features. These include: generalized fixed bed configurations; improved submodels for pollutant formation and destruction; improved submodels for large particle combustion, gasification, and devolatilization.
CODE DEVELOPMENT HISTORY AND CREDITS
Principal financial support for this work was provided by the U.S. Department of Energy's Morgantown Energy Technology Center under subcontract from Advanced Fuel Research, Inc., East Hartford, Connecticut. Additional support was received from the National Science Foundation's Engineering Centers Division, the State of Utah and 42 U.S. industries, which are all participants in ACERC.
Code development, integration, evaluation, documentation and demonstration were conducted at BYU/ACERC. Key contributors include Dr. James R. Farmer, Dr. M. Usman Ghani, Dr. Predrag T. Radulovic, principal investigator and Prof. L. Douglas Smoot, project director.
AFR's principal contribution to this software product was the development of a coal devolatilization model and code called FG-DVC, which is included in FBED-1. They also provided insight and assistance in code integration, evaluation, and documentation. Major participants at AFR were Dr. Peter R. Solomon, project director, Drs. Michael A. Serio and David G. Hamblen, principal investigators, and Mr. Yuxin Zhao.
FURTHER INFORMATION
For additional technical information, several publications are available describing the application and development of FBED-1. A sampling include the following:
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