Experimental and numerical study on combustion of secondary pyrolysis products from various coals.

摘要

In burning coal suspensions, non-uniform mixing and particle dispersion promote secondary pyrolysis and soot formation. The changes in volatile compositions affect volatiles combustion, which is responsible for flame propagation and pollutant formation during pulverized coal combustion. The lack of kinetic data on volatiles combustion is attributed to the complexity of fuel compositions and its close association with devolatilization and char oxidation, which cause difficulties in the isolation of volatiles combustion. This work attempts to determine global combustion rates of noncondensible volatiles by measuring laminar burning velocities.; A radiant coal flow reactor generates combustible mixtures of noncondensibles, which are combusted in a constant volume combustion bomb to generate laminar burning velocities for the noncondensibles from Pit. #8 bituminous and Low. Kit. lv bituminous coals. The laminar burning velocities of noncondensible volatiles increase with the extent of secondary pyrolysis for both coals because the molar yields of H{dollar}sb2{dollar} and C{dollar}sb2{dollar}H{dollar}sb2{dollar} increase during the conversion of tar into soot. For noncondensibles from Pit. #8 coal, the burning velocity at a fuel equivalence ratio of 1.2 and (N{dollar}sb2{dollar} + CO{dollar}sb2{dollar} + H{dollar}sb2{dollar}O):O{dollar}sb2{dollar} = 6.8, is quadrupled from 55 to 215 cm/s as the extent of sooting increases from 0.66 to 1.0. The combustion in air shows a similar variation.; A correlation is developed using the equivalent concentrations of H{dollar}sb2{dollar} and CO with the adjustment of O{dollar}sb2{dollar} concentrations to convert hydrocarbons into CO: {dollar}{dollar}rm Ssb{lcub}u{rcub}sp2 = Cxisb{lcub}Osb2{rcub}(xisb{lcub}Hsb2{rcub} - alphaxisb{lcub}CO{rcub})exp({lcub}-{rcub}{lcub}Eover RTsb{lcub}f{rcub}{rcub}){dollar}{dollar}A single set of parameters describes the variation of burning velocities on the extent of sooting and coal types for four coals including Ill. #6 bituminous and Dietz subbituminous coals. Correlation parameters are C = 13.2 {dollar}times{dollar} 10{dollar}sp8{dollar} cm{dollar}sp2{dollar}/s{dollar}sp2{dollar}, {dollar}alpha{dollar} = 1.0, and E = 27.4 kcal/mole.; Numerical predictions of burning velocities with sensitivity analyses are performed using a detailed C{dollar}sb1{dollar}/C{dollar}sb2{dollar} oxidation mechanism. A systematic method is implemented to convert higher carbon number hydrocarbons into equivalent C{dollar}sb2{dollar} hydrocarbons. The experimental data are corrected for stretch effects. The mechanism quantitatively predicts the variation of burning velocities for noncondensibles from four coals. Sensitivity analyses reveal the reactions sensitive to the extent of sooting and coal type.