A Two Mixture Fraction Approach for Modeling Turbulent Combustion

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. In addition, this new approach neglects turbulent fluctuations on char combustion products. Required input parameters are the elemental composition of char and volatiles, together with the volatile yield. Calculations are presented for an axisymmetric (hypothetical) case using both the previous and new models. Significant differences are shown in predicted values of gas temperature, CO2 and NO near the burner. 1