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Show mixing of the CO and air. Large amounts of CO can affect the mixture temperature and consequently the heat transfer. We need to understand formation and reduction of CO t o minimize environmental impact, and we need to understand basic formation/reduction mechanisms to optimize combustion processes. Studies so far have not addressed formation and oxidation of CO for practical furnace geometries . The existing technology base for predicting CO combustion has relied on equilibrium chemistry calculations or experimentally-based correlations for CO concentration as a function of equivalence ratio (Smoot and Smith, 1985). These methods are appropriate for standard applications of PC combustion for which fuel-lean conditions effect large reductions in CO concentration and where CO has little impact on combustion and heat transfer. However, new applications involving systems that operate with fuel-rich conditions necessitate that we improve our capabilities for predicting CO combustion. One such way is through the use of computational methods. Computational methods in fluid flow, heat transfer, and pollutant processes have been advanced to the point that such methods are being applied to the simulation of complex, yet practical, combustion systems. The simulation of a pulverized fuel combustion system involves modeling a number of complex, simultaneous, interdependent processes, and must account for fluid flow, turbulence, particle transport, combustion, radiation heat transfer, and pollutant formation. Recent studies by Richter and Fleischhans (1976), Lockwood, et al., (1980), Smoot and Smith (1985), Fiveland, et al., (1984), and Fiveland and Wessel (1988) have employed more sophisticated techniques and demonstrated promise in predicting PC combustor performance. In this paper, a three-dimensional model is presented for assessing PC combustion with finite-rate CO oxidation in practical furnace geometries. Results are presented for a pilot-scale furnace (1.76 MW scale) fired with PC from a low-NOx burner with overfire air injected in the upper furnace to produce staged combustion. Results show promise of assisting engineers and designers in the optimization and design of combustion equipment. OVERVIEW OF MODELING APPROACH PC combustion may be described by innumerable elementary reactions involving hundreds of species. Recent success in modeling this process has been achieved by identifying reaction mechanisms which are important in determining the local heat release and balance of major species within the furnaoe. The model, which is described here, is based on a global description of the heterogeneous and homogeneous chemical kinetics of coal combustion, and on the interacting processes of turbulent flow and heat transfer. Features of the model essential for predicting chemical kinetics of coal combustion are presented here. A more general description of the numerical model for predicting flow, combustion, and heat transfer is given elsewhere (Fiveland and Wessel, 1988). The heterogeneous reactions of coal devolatilization and char oxidation are characterized by a total fuel evolution rate. This permits use of a single fuel mixture fraction for modeling the homogeneous chemistry and simplifies calculations. Predictions using a single fuel mixture fraction have been shown to be reasonably accurate for most coals. However, the use of 5 |
