| OCR Text |
Show Sh = n 6(m h ) (10) Particle Mass. Coal particles enter the combustor whereupon the volatiles are released and the remaining char undergoes oxidation with 0 H^O, and C0„. In each case, the mass of the particle changes along its trajectory due to devolitalization and char oxidation. The decrease in total mass can be represented, in general, as: J™. -K = -(Kd + KC) (11) where K is a rate constant characteristic of devolatilization and char oxidation. Computation of the change in mass with equations over a time step is performed by subdividing the time step and integrating the continuity equation over that time step. The source of fuel mass contributed to the gas phase for this cell from a stream of particles becomes an integration over the time step: = n/(Kd S' = n / (Kd ++ Kc) dt (12) m At where the parameter n is the particle number flow rate for the particle stream. The total source, S' , is found by summing over all such particle streams in the cell. In this study, all of the gaseous products are assumed to have a fixed composition, thus, SI - S . a) Devolatilization For coal, the use of a single, first-order kinetic expression for pyrolysis is inadequate. Several (19)(20)(21) . . . ,«. , researchers have suggested multiple reaction (21) rates. We follow the proposal of Ubhayakar, et al. in which two first-order reaction rates are expressed as: d K! coa 1 + oj gas i + (1 - of) char; i = 1, 2 (13) -10- |
