REACTIVITY DISTRIBUTIONS AND EXTINCTION PHENOMENA IN COAL CHAR COMBUSTION

Based on in situ optical measurements and off-line analyses for four coals, the basic features of single-particle pulverized coal char combustion have been elucidated as a function of carbon conversion. Two regimes can be clearly defined: one at low carbon conversion, where the reacting particle populations have properties that are nearly time invariant and a second regime at higher carbon conversion where the distribution properties change dramatically. At low carbon conversion, there is a broad distribution of single-particle combustion rates, reflecting the heterogeneity in the parent fuel. Particle-to-particle reactivity differences are shown to be the primary cause of the broad temperature distribution for Pocahontas coal char. At high carbon conversion, carbon-rich particles can be distinguished statistically from inorganic-rich particles by in situ measurement of their spectral emissive factors at 800 nm. In each case where char carbon conversion proceeds past 50-60%, many particles are observed to undergo large temperature decreases resulting from a loss of reactivity, referred to as near-extinction events. Near-extinction is generally observed to occur before large changes are observed in the particle optical properties, suggesting that deactivation occurs when the particles are still carbon-rich. Plots of particle temperature vs emissive factor conveniently illustrate and summarize the process of char particle combustion to high conversion. These plots reveal two distinct stages in the combustion lifetime of a char particle: (1) a rapid combustion stage at low carbon conversion, followed by (2) a deactivation and near-extinction at roughly constant optical properties, initiating a final burnout stage that occurs slowly and at low temperatures. The two-stage nature of the char combustion process significantly lengthens the time required to achieve high carbon conversion, and the existence of two stages cannot be predicted by conversion-independent kinetic models. More realistic char oxidation models are needed that account for fuel heterogeneity and conversion-dependence effects.