Secondary pyrolysis and combustion of coal volatiles.

摘要

Under rapid heating, pulverized coal releases half of its mass as noncondensibles and high molecular weight tars. The tars are pyrolysed to soot, drastically altering the composition of the noncondensibles. Subsequent combustion of the soot and the fuel species in the noncondensibles causes half the heat release and pollutant formation, and governs the stability of the pulverized coal flame. Even approximate rates for combustion of the volatiles are unknown, but they can be estimated from measurements of the laminar burning velocity.; Laminar burning velocities were measured in a constant volume combustion bomb for the volatiles from an Ill. #6 bituminous and a Dietz subbituminous coal. The volatiles were generated in a radiant flow reactor which had previously been thoroughly characterized for both primary devolatilization and secondary pyrolysis. For volatiles from the Ill. #6 coal, burning velocities triple as the last half of the tar is converted to soot, reaching 150 cm/s at an equivalence ratio of 0.7 in air. The impact of secondary pyrolysis is the same at an increased dilution level. A similar impact of secondary pyrolysis is observed for the volatiles from the Dietz coal, but their burning velocities are about half those from the Ill. #6 for the same conditions.; These tendencies are well described by the following correlation: {dollar}{dollar}rm Ssb{lcub}u{rcub}sp2 = CXsb{lcub}O2{rcub}(Xsb{lcub}H2{rcub} + alpha Xsb{lcub}CH4{rcub})exp({lcub}-{rcub}E/RTsb{lcub}f{rcub}){dollar}{dollar}where the variations of the fuels are represented by the mole fractions of CH{dollar}sb4{dollar} and H{dollar}sb2{dollar}. The equivalence and dilution ratios determine {dollar}rm Xsb{lcub}O2{rcub}{dollar} and the flame temperatures. This equation successfully correlates all of the data for a single coal over wide ranges of both extent of secondary pyrolysis and equivalence ratio. However correlation of the volatiles from the two coals requires different fitting parameters. A modified equation, {dollar}{dollar}rm Ssb{lcub}u{rcub}sp2 = CXsb{lcub}O2{rcub}Xsb{lcub}H2{rcub}Xsb{lcub}CH4{rcub}spalpha Xsb{lcub}CO{rcub}spbeta exp({lcub}-{rcub}E/RTsb{lcub}f{rcub}){dollar}{dollar}successfully correlates all of the data for both coals with a single set of fitting parameters. For the Ill. #6 and Dietz coals of this study, at a reactant temperature and pressure of 430 K and 1 atm, fuel equivalence ratios from 0.4 to 1.1, soot fractions from.5 to 1, and (N{dollar}sb2{dollar}+CO{dollar}sb2{dollar}+H{dollar}sb2{dollar}O)/O{dollar}sb2{dollar} ratios from 3.8 to 6.8, C = 2.7 {dollar}times{dollar} 10{dollar}sp4{dollar} cm{dollar}sp2{dollar}/s{dollar}sp2{dollar}, {dollar}alpha{dollar} = {dollar}-{dollar}1.46, {dollar}beta{dollar} = {dollar}-{dollar}2.69, and E = 52.5 kcal/mole.