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Published:
Jun 30, 2006Article Details
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Gasification Kinetics of Ultrafine Coal Char Oxidation in Flame Reactions
Abstract
Pulverized coal combustion is exceedingly complex with a vast number of variables. In order to avoid the complications associated with the burning of coal char, a controlled study has been made of the oxidation of fine graphite in laminar methane-air flame as a contribution to the understanding of the more complex phenomena of the gasification kinetics of the char. The burning of methane is equivalent to the burning of volatiles and graphite oxidation is similar to that of coal char. Aspects of this have been simulated in experimental and computational studies of 4pm diameter graphite particle burning in methane -air flames at a pressure of 0.160 atm. The low pressure not only ensured good spatial resolution of the flame structure but it also ensured that the graphite reaction rate was chemically controlled. Comparable values from prediction and measurements were observed and were explainable in terms of the presence of reactive flame radicals 0, OH, and H in controlling oxidation aided by the catalytic radicals attack by molecular oxygen. The primary oxidation product from the kinetics is CO. This is simultaneously coupled to a detailed multi-step gas phase kinetic mechanism in the oxidation for CH4 -air flames. For lean flames, the available CO in the presence of the reactive radicals enhances reactions with the effect of enhancing the associated heat release rate. This contributes to the observed enhanced burning velocities in these flames. For rich flames, because of the limited amounts of 02, the catalytic effect is impaired and thus reduces the char oxidation rate and thus reduces the burning velocity of the propagating flame.
