COMBUSTION OF THE NONCONDENSABLE VOLATILES FROM VARIOUS COALS

In pulverized coal flames, the products of primary devolatilization ar e radically transformed by secondary chemistry after they escape into the gas phase, but before they burn. This laboratory study aims to cla rify the macroscopic combustion characteristics of coal volatiles, esp ecially the way that burning rates increase as tar is converted into s oot during secondary pyrolysis. Mixtures of the noncondensible fuel co mpounds released under simulated pulverized fuel firing conditions are generated in a novel coal flow reactor that independently regulates t he extent of secondary pyrolysis. After tars, soot, and char particles are filtered out, the fuel mixtures are blended with oxygen and ignit ed in a combustion bomb. Laminar burning velocities are assigned from transient pressure measurements during flame propagation. Burning velo cities are reported for (fuel) equivalence ratios from 0.5 to 1.5 at t wo dilution ratios for noncondensible volatiles from four coals repres enting ranks from subbituminous through low volatile bituminous. For a ll coal types, burning velocities triple as the extent of tar conversi on into soot increases from 50 to 100%. This tendency is consistent wi th conversion of all of the oxygen and hydrogen in tar into H2 and CO, and of all light hydrocarbons into acetylene. The burning velocities for volatiles from the bituminous coal are two-and-one-half times grea ter than those for the volatiles from the subbituminous coal, consiste nt with the abundance of oxygen in the subbituminous coal, and its cor respondingly higher yield of CO and lower yields of H2 and hydrocarbon s. A correlation for this database is developed from the thermal theor y of premixed laminar flame propagation and a generic skeletal mechani sm for hydrocarbon combustion. This scaling is also used to assign nom inal burning rates from the measured burning velocities. The rates are up to an order of magnitude faster than burning rates of soot. Nomina l burning rates also gauge the combined influences of coal loading, ra nk-dependent yields and heating values, and extent of secondary pyroly sis.