THE IMPACT OF SOOT ON THE COMBUSTION CHARACTERISTICS OF COAL PARTICLES OF VARIOUS TYPES

The products of coal devolatilization are radically transformed by sec ondary pyrolysis after they are expelled into hot gases until only H-2 , C2H2, CO, CO2, H2O and soot remain. The coal combustion model develo ped in this article represents the limiting scenario of infinitely fas t secondary pyrolysis followed by combustion of gaseous fuels and soot in flame sheets, either on or around individual coal particles. Soot and gases from different coal types are distinguished by different evo lution rates, elemental compositions, and transport properties. Thermo phoresis and radiation are accounted for in the transport analysis. Th is study also develops separate limiting behavior for instantaneous so ot oxidation in envelope flames and for frozen soot oxidation chemistr y. Comparisons among predicted and observed flame lifetimes and maximu m flame standoffs select the most realistic modeling scenarios. Radiat ion from soot into the surroundings dictates the overall impact of sec ondary pyrolysis on macroscopic combustion characteristics. Soot is al most 1000 times more efficient than its host particle in radiating ene rgy, and dissipates up to 90% of the radiation during the initial stag es of combustion. Soot radiation cools flame temperatures by up to 300 K, reducing the differences among flame temperatures for diverse coal types in 8% O2 to only 100 K around 2200 K. At such temperatures, wat er/gas shift equilibrium determines the distribution of combustion pro ducts, and the energy carried away by intermediates becomes negligible . For envelope flames around 100 mum particles of all coal types, abou t 60% of the heat of combustion is fedback to the particle, and one-th ird is conducted or radiated into the surroundings. But for attached f lames on smaller particles more than 90% is retained by the particle.