Burning velocities of multicomponent organic fuel mixtures derived from various coals

Abstract

In pulverized coal flames, nonuniform mixing and particle dispersion ensure that the products of primary devolatilization are transformed by secondary pyrolysis they burn. This laboratory study shows how the rates of volatiles combustion change during the course of secondary pyrolysis for a spectrum of coal types. Volatiles mixtures are generated in a novel coal flow reactor that independently regulates the extent of secondary pyrolysis. After tars, soot, and char particles are filtered out, oxygen and nitrogen are added to prepare combustible mixtures that have specified fuel equivalence and dilution ratios. Flames are propagated through the mixtures in a combustion bomb to acquire the pressure histories that define laminar burning velocities. Burning velocities of noncondensible volatiles from 4 coals representing ranks from subbituminous to low volatile bituminous are reported for fuel equivalence ratios from 0.4 to 1.5, at two dilution ratios and two unburned gas temperatures. For all coal types, burning velocities triple as the extent of secondary pyrolysis increases from 50 to 100%, in rough proportion to variations in the H 2 levels. This tendency is consistent with conversion of the oxygen and hydrogen in tar into CO and H 2, and of light hydrocarbons into acetylene. For complete secondary pyrolysis, the burning velocities of volatiles from higher rank coals approach the comparable values for hydrogen combustion. But this tendency is not seen with lower rank coals because hydrogen makes smaller contributions to their heating values. Increasing contributions from hydrogen explain the impact of increasing extents of secondary pyrolysis, and different proportions of CO and H 2 determine the variation of burning velocities among different coal ranks. This database is used to develop a correlation based on a scaling from the thermal theory of laminar flame propagation and only two pseudo-components: the actual H 2 and CO levels plus the amounts associated with instantaneous partial oxidation of all hydrocarbons. Provided that flame temperatures are based on actual mixture compositions, this strategy closely correlates a database that covers virtually the entire spectrum of coal types, the last half of secondary pyrolysis, the full range of equivalence ratio encountered in practice, plus two dilution ratios and two unburned gas temperatures. The same scaling is used to assign nominal burning rates from the measured burning velocities, which are found to be up to an order of magnitude faster than burning rates of soot, as expected.