OCR Text |
Show calculated by solving the ratio of the average oxygen mass flux at the surface of an oblate (ellipsoidal) particle to the spherical particle with the same surface area (Grow, 1990) : _ Sh,llipmd l.d 0 = ~S K (d1 dL\ .. (16) - + arcsin( 4"¥m*ii <+i ft & where d and L are ellipsoidal particle's minor and major axes respectively, and e is the eccentricity. Coal Char Combustion The surface reaction for coal combustion is controlled by kinetic rate, defined by: dmP _r^2 n 0.5 P- = - T T D ; P O 0 5 R 2 <17> dt where R2=C2Qxp(-E/RTp) (18) where A2 and E are assumed to be 0.917 s'1 and 1.08e+8 J/Kg mol respectively (Mitchell et al., 1992). The particle heat balance during the surface reaction is given by : mpCP^ = hAp(Tm-Tp)-fi*^H„+epAMd4 R-Tt) (19) dt dt where Hreac is the heat released by the surface reaction, and fh is fraction of heat absorbed by the particle. In this case, only the portion of heat released (1- fh) during the surface reaction appears as a heat source in the gas phase energy equation. Ash Heating Without Any Mass Exchange After combustion, a simple heat balance is used to relate the inert ash particle's temperature (Tp) to the convective heat transfer and the absorption/emission of radiation at the surface as in Eq. (8). As the trajectory of the particle is computed, the heat, mass, and momentum gained or lost by the particle stream is transferred to continuous phase in the subsequent calculation. The momentum transfer from the continuous phase to the discrete phase is computed by taking the change in momentum of a particle when it enters and leaves the control volume, as: 18/iCoRe PpDp' 24 __ j.isa»i_^.,) + lr m 4, where At is the integration time step. This momentum exchange appears as a momentum sink in the continuous phase momentum balance in the subsequent calculation. The heat transfer from the continuous phase to the discrete phase is computed by taking the change in thermal energy of a particle as it enters and leaves the control volume. The heat exchange in all of the above laws except surface combustion is calculated as: |