OCR Text |
Show It is assumed that all three reactions proceed independently of each other, and that the char consumption rate is the sum of individual rates. The diffusion rate of oxygen to the surface may be related to the laminar diffusion coefficient. Kinetic rates for the above three reactions were taken from Stickler et al(9). Heterogeneous char combustion is assumed to take place at the outer surface, resulting in the removal of material from the surface at constant particle density. Devolatilization and blowing out of gaseous products hinders diffusion of gaseous species to the surface. It is therefore assumed that char oxidation does not commence until the devolatilization is complete. 2.2.3 Gaseous Combustion Model The original version of the three-dimensional code incorporated a two step model, in which differential transport equations governing the evolution of the mixture fraction, f, the mass fraction of unburned fuel, mfu ' and the mass fraction of CO, mCO ' were solved(4). Concentrations of other reactants (including O2) as well as equilibrium combustion products (mC02 ' mH2 , mN2 , etc) were obtained via algebraically formulated balance expressions. The following global mechanisms are postulated to represent the combustion of the two different kinds of volatiles and solids representative of coal combustion. The volatiles are assumed to combust to yield CO and H2 initially. 6CO + 3H2 7 |