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Show ____ + ____ (7) Re Re2 where a,= 1.222. a2. =-3.889 and a3 =29.17 for Reynolds number less than 100. CD = „, + - ^ + - - D ' Re Re" Heat and Mass Transfer Calculations The heat and mass transfer from/to the particles is described in a sequence of 4 stages, the first stage accounts inert heating of the particle to 400 K which is followed by the devolatilization, the 2nd stage. This is followed by the char oxidation and concluded by inert heating of ash. Initial Heating of the Particle During the first stage, a simple heat balance (without any mass transfer) is used to relate the particle temperature (Tp) to the convective heat transfer and the absorption/emission of radiation at the particle surface: dTp dt mPCP^ = hAp(T„-Tp) + epApcr(ei-T;) (8) where mp, Tp, Ap, ep, are the mass, temperature, surface area, and emissivity of the particle, respectively. 0 R is the radiation temperature defined by (I74a)1/4 where I is the radiation intensity. The above equation assumes the negligible internal resistance to heat transfer, i.e., the particle is at uniform temperature throughout. The Eq.(8) is integrated in time using an approximate linearized form that assumes the particle temperature changes slowly from one time step to the next. Finally, the heat loss or gained by the particle as it traverses through each computational cell appears as a source or sink of heat in the subsequent calculations of the continuous phase energy equation. Biomass Devolatilization A single kinetic rate devolatilization model is used to predict volatile yield from the biomass, and the two competing rate model is used to predict volatile yield from the coal. The single kinetic rate devolatilization model assumes that the rate of devolatilization is first order dependent on the amount of volatiles remaining in the particle: -^ = *K-(i-Ao-/.oKo) (9) where m p is the particle mass at any time (t), k is the kinetic rate, fvo, and fw0 are the fraction of volatiles and moisture initially present in the particle, and nipo is the initial particle mass. The kinetic rate, k, is defined by input of an Arrhenius type pre-exponential factor and activation energy expression: k = A{exp(-E/RTP) (10) where A| and E are assumed to be 3120000 s'1 and 7.4 e+7 J/Kg mol, respectively, which are based on the burnout data from the S N L Multifuel Combustor. |