The combustion of individual particles of various coal types.

摘要

This modeling study characterizes the initial stages of pulverized fuel (p.f.) firing of various types of coal, at the level of individual particles. Noncondensible gas, tars, and soot, if present, from different coal types are distinguished by different evolution rates, elemental compositions, average molecular weights, and transport properties.; The combustion enthalpies, flame temperatures, and stoichiometric requirements of the envelope flames are based on thermochemical equilibrium among 12 species, including primary dissociation fragments. Fuel accumulation between the particle and flame is also accounted for. Tar condensation into soot is studied in two limits: (1) frozen secondary pyrolysis in which no soot formed, and (2) infinitely fast secondary pyrolysis in which tar condenses into soot instantly at the particle surface. The product of primary devolatilization are radically transformed by secondary pyrolysis after they are expelled into hot gases. Distributions of noncondensible gases from primary devolatilization are reduced to only 3 fuel species, H{dollar}sb2{dollar} and {dollar}rm Csb2Hsb2,{dollar} and CO, while tars are converted into soot. The model developed represents the limiting scenario of infinitely-fast secondary pyrolysis followed by combustion of gaseous fuels and soot in a flame sheet, either on or around the particle. Extensions also develop separate limiting behavior for instantaneous soot oxidation in envelope flames and for frozen soot oxidation chemistry. Comparisons among predicted and observed flame lifetimes and maximum flame standoffs select the most realistic modeling scenarios.; Soot is almost 1000 times more efficient than the host particle in radiating energy into the surroundings, and dissipates up to 90% of the radiation during the initial stages of combustion. Soot radiation cools flame temperatures by up to 400 K, reducing the differences among flame temperatures for diverse coal types in 8% O{dollar}sb2{dollar} to only 100 K around 2200 K. At such temperatures, water/gas shift equilibrium determines the distribution of combustion products, and the energy carried away by intermediates becomes negligible. For envelope flames around 100 {dollar}mu{dollar}m particles of all coal types, about 60% of the heat of combustion is fedback to the particle, and one-third is conducted or radiated into the surroundings. But for attached flames on smaller particles, more than 90% is retained by the particle. (Abstract shortened by UMI.)