| OCR Text |
Show - 3 - atmosphere by using a small sample with a thermobalance and by using a slow heating rate. However, the sample weight may have an effect on the yield of volatiles due to secondary char reactions /8/. When considering a small single particle, the asymptotic yield may be greater than for a small sample. For this reason, in order to obtain the real asymptotic yield, the laminar flow reactor with nitrogen flow is better means for finding the curve e(T) for different coals. Fig. 1. The dependence of the 1 M/Mo asymptotic yield of volatiles e(T) on the temperature. ot------r-..:...------1~ a 800 TEMPERATURE The pyrolysis is limited by diffusion processes and the kinetics has an insignificant role for large particles, about d>O.3 rnrn. However, the curve e(T) has a significant effect on the pyrolysis of large particles. For large particles the transient mass/initial dry mass of the particle is uniquely defined by the curve e(T) during the devolatilization stage of the combustion, if the pyrolysis is limited by heat transport rather than mass transport. Then m(t)/mo = e(T(t)), -if the spatial temperature differences inside the particle are insignificant. For particles large enough to have a significant spatial temperature distribution inside the particle, the corresponding equation is 3 R m(t)/mo = R3 { e(T(t,r))r 2 dr ( 1 ) for a spherical particle. The transient temperature of the particle, T(t), can be calculated with a model. The effect of a possible flame can be taken into account. The escape |
