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Abstract
Previous coal combustion models using assumed-shape PDF's to treat turbulence-chemistry interactions have used only one progress variable to treat products from coal reactions. This assumes that the products of all coal reactions have the same composition. However, the composition of the combustion products of coal particles is known to vary with burnout, especially between devolatilization and char oxidation. In this work, two progress variables were implemented which distinguish between the products of devolatilization and those of char oxidation. This new approach requires as input the specified volatile content and elemental release during devolatilization. The values for these parameters were estimated based on elemental release data obtained in flat-flame burners. Predictions of the new and the old approaches for the major variables of the field were not appreciably different. However, NO pollutant predictions of the new method were, in general, better than those of the old method, particularly at downstream locations.
The new two-progress variable method is currently limited by the scientific understanding of nitrogen release during coal devolatilization and char oxidation; predictions should improve as better fundamental models of nitrogen release are developed.