OCR Text |
Show dV * ^T = kv(V -V) (1) dr kv = Av exp(-Ey I RTp) (2) Integration of equation (1) gives the fractional volatile yield as a function of time: _ = l-exp(-MO (3) V v 0 The total volatile yield (V*) is often measured at low heating rates (proximate analysis) where the volatiles undergo secondary reactions (cracking, polymerisation). For bituminous coals, volatile yields are much greater at higher temperatures, and depend strongly on heating rate, the ratio commonly being denoted by "R". Consequently, a two reaction devolatilisation mechanism is required such as that developed and quantified by Kobayashi et al.4 m„ + m mr ^ = J{(a,k, +a2k2)exp[j(kl + k2)</t]}</t (4) where kx and k^ represent competing rates that may control the devolatilization over different temperature ranges: *, = ^exp(£, / RTp) (5) k2 = A2exp(E2 I RTp) (6) The rate parameters for k, are determined from the proximate analysis (analogous to equation(2)) while those for k2 are determined from high temperature pyrolysis. A key factor in determining the rate of devolatilisation is the particle temperature, Tp. CFD codes commonly include particle heat up by convective heating from product gases and radiative heating. A n accurate prediction of Tp is essential, and this is discussed in more detail in Section 2.1. In most computational codes the "volatiles" are represented as a single species (e.g. heptane) for which the combustion rate (steps 2 and 3) is controlled by mixing with oxidant. However, this can result in inaccuracies in terms of stoichiometry, ignition, flame temperature etc. In addition, the influence of tars on the rate of volatile combustion and soot formation is excluded. This is one area which is highlighted for future developments in C F D codes5. Typical CFD codes also allow for swelling of coal particles upon devolatilisation, in which case the increase in coal particle radius as a function of devolatilisation is a required input parameter. 2 |