OCR Text |
Show 2.2 PHYSICAL MODELS This section will discuss specifically the details of the coal combustion models incorporated in the code, as the remaining physical, models have been extensively documented in the literature(S). At present, the treatment of particle radiation is somewhat simplistic, in that the particles are assumed to be black bodies exchanging radiation with combustor walls. 2.2.' Pulverized Coal Combustion Model Due to the extremely complex physicochemical processes that accompany coal combustion in practical combu stor configurations, the coal particle combustion model is designed to address key, rate-controlling steps. Initially, the pulverized coal size distribution is approximated by a pre-specified number of size ranges and the combustion of the mean sized particle in each range is then examined. Coal particle combustion is generally assumed to be a two-stage process consisting initially of particle devolatilization or pyrolysis, followed by char oxidation. The model adopted to describe the pyrolysis is that of Kobayashi et al(8), which proposes the use of two competing first order reactions to describe particle mass loss during pyrolysis. One reaction has a low activation energy and is thus of significance at lower temperatures (around 600 0 k) and the other has a higher activation energy and is thus important at higher temperatures (above '400 0 k). The process is described in detail by Stickler et al(6). Briefly it may be written: OAF Coal a, s, + ('-a,) V, OAF Coal 5 |