OCR Text |
Show where s, and s2 are char solids residuals, V, and V2 are evolved pyrolysis products of approximate elemental ratio C6H6 and C6H,2 and a, and a2 are stoichiometric constants. The rate of disappearance of dry, ash free (OAF) coal is given by: dmOAF dt = where k, and k2 are Arrhenius constants and mOAF is the instantaneous mass of coal. The constants are dependent on coal type and rank. Available experimental data on the ultimate and proximate analysis may be used to approximate the stoichiometric constants and the composition of the volatiles. The devolatilization of the coal particle is assumed to take place without change in the particle radius, and thus leaves behind a less dense, porous particle. 2.2.2 Char Gasification Model Char gasification may be treated as a three stage process consisting of (i) diffusion of gaseous species to the particle surface, (2) surface reaction, and (3) diffusion of products away from the surface. The overall process may be diffusion limited or kinetically limited, depending on the gas phase conditions, particle size and temperature, and the particle pore structure. The model adopted was an extended version of that of Field(7), where surface diffusion and chemical reactions are both accounted for. Three different surface reactions are included: C 1 k3 + 2 O2 CO ~ C + CO2 k4 ~ 2CO C + H2O k5 ---.. CO + H2 6 |