| OCR Text |
Show or even larger than the ignition temperature of the char (see for example figure 8). Trials to explain this behaviour by the assumption, that the upper part of the particles (in a mono-layer of particles) or the upper particles (in the case of a multi-layer of particles) is less effectively heated by the grid than the lower one(s), failed since (1) the unburat fraction does not depend systematically on the mass of the sample , nor decreases systematically upon passing from a multi-layer to a mono-layer of particles, and (2) since, once the combustion reaction has started, the heat losses from the particles to the grid are less important for the upper particles or the upper part of the particles than for the lower (part of the) particles. In the particular case of "whole coal" ignition at temperatures below the char extinction temperature (taken here in the sense given by the Semonov/Frank-Kamenetskii theory of ignition) the extinction of coal before complete combustion of the carbon may better be attributed to the effect of progressive devolatilization. This may be explained on hand 11,12 of the thermal ignition theory (- ' - ) when applied to a pvrolysing solid particle. Schematically this is represented on figure 15 , showing the heat released per unit time by the oxidation (Q_) compared to the heat absorbed by the surrounding gases and the grid (Q..) as a function of the solid temperature T . For simplification it is assumed that the grid temp-s erature (T ) is identical to the temperature of the surrounding gas and that heat losses occur onlv bv conduction (i.e. Q_versus T is represen- U . s ted as a straight line the slope of which is equal to the heat conductivity constant). Three heat release curves Q- are represented corresponding to three different values of the pyrolysis advancement \ : curve {\ = 1) corresponds to the case of totally devolatilized char, whereas curve (i- 0) corresponds to undevolatilized coal. Depending on whether the pre-exponential part of the rate constants of coal and char are identical or not, the corresponding Q_ curves may or may not merge at very high temperature (obviously the latter case is represented on the schema). 7-28 |
