| OCR Text |
Show 2.4 Particle-Phase Reactions Particle Continuity Equations The reacting particle is composed of raw coal, char and ash. The continuity equations for each coal component are as follows: d dt (cxc,J = rc,i ( 8 ) -ddt (CXh ,~. ) = rh, i ( 9 ) (10) where i represents particle size and subscripts c, h, and a denote coal, char and ash, respectively. The raw coal portion of coal undergoes devolatilization to form volatile products and char by reactions of the form: (raw coal) i····· · Yi (volatiles) + (I - Yi ) (char) (11) The char reacts heterogeneously after the oxidizer diffuses to the particle surface. The char-oxidation reactions can be expressed in the form: (char) + (oxidizer) k ...... (gaseous products) k ( 12 ) The model suggested by Kobayashi et ale [5] is employed for the devolatilization process and the model suggested by Smoot and Smith[6] is employed for the char-oxidation process. Particle Energy Equation To obtain the particle temperature, the energy conservation equation is solved. Following Smoot and Smith [6], the heat of the reaction is fully assigned to the gas phase. The particle energy equation includes contributions from radiation heat transfer to the particle and the convective heat transfer to the gas phase. The energy equation in the Lagrangian form is ddt (cxphp) = qx - qc - m~p ( 13) The last term in Eqn.13 represents the loss in enthalpy due to change in mass of the particle. 2.5 Radiation Model The six-flux radiation model [e.g.,8] is employed to provide the radiative heat transfer rates in the particle and gas-phase energy 4 |
