Secondary pyrolysis and combustion of coal volatiles

Abstract

Combustion of volatiles in a pulverized coal flame involves the most complex combustible mixtures encountered in any utilization technology. This laboratory study aims to clarify the macroscopic combustion characteristics of coal volatiles, especially the way that burning velocities increase as tar is converted into soot during secondary pyrolysis. Mixtures of the noncondensible fuel compounds released under simulated p. f. firing conditions are generated in a novel coal flow reactor which independently regulates the extent of secondary pyrolysis. After tars, soot, and char particles are filtered out, the fuel mixtures are blended with oxygen and ignited in a combustion bomb. Laminar burning velocities are assigned from transient pressure measurements during flame propagation. Burning velocities are reported for (fuel) equivalence ratios from 0.5 to 1.5 at 2 dilution ratios for noncondensible volatiles from bituminous and subbituminous coals. Burning velocities triple as the extent of tar conversion into soot increases from 50 to 100%. This tendency is consistent with conversion of all of the oxygen and hydrogen in tar into H 2 and CO, and of all light hydrocarbons into acetylene. The burning velocities for volatiles from the bituminous coal are two-and-one-half times greater than those for the volatiles from the subbituminous coal, consistent with the abundance of oxygen in the subbituminous coal, and its correspondingly higher yield of CO and lower yields of H 2 and hydrocarbons.