OCR Text |
Show 2.1 .6 the temperature is not taken into account, the correlation (12) may even give pyrolysis rates that are geater than "infinite", since the driving force is {p(T)-pf} , where pf is the future local equiliblium density that is based on the future temperature (which the local point cannot "forecast"). A physically more reasonable driving force is {p (T)-pf(T)} , in which, instead of pf, the equiliblium density pf(T) of the present temperature T is used. The dependence of the equiliblium volumetric mass on the temperature is found experimentally for different materials by using the thermogravimetric method. A small powder sample is gradually heated, the temperature is slowly increased. The temperature of the sample and the dimensionless mass e(T) = mass/initial mass are measured. Then the (local) equilibrium density can be expressed as pf(T) = p0e(T), where Po is the initial (dry) density of the material. (In this model the effect of shrinkage or swelling is neglected, the true density is obtained multiplying by the coefficient of shrinkage.) Some measured equiliblium curves for wood and peat are presented in Fig.1 . The measured equilibrium curves e(T) for different fuels is taken as the basis for the calculation in the present model for larger particles. It is assumed that the pyrolysis is controlled by the diffusion of heat and that the kinetics has a negligible effect. We assume that the rate of the pyrolysis is infinite, but the equiliblium dependence pf(T) = p0e(T) is taken into account. Then the local solid density is uniquely defined by the local temperature according to the measured equiliblium curve, p(T) = p0e(T) (13) and the local generation of volatiles or the local sink of the solid matter can be expressed as p = - p0e'(T) || , e'(T) = ^|^- (14) |