BURNING VELOCITIES OF MULTICOMPONENT ORGANIC FUEL MIXTURES DERIVED FROM VARIOUS COALS

In pulverized coal flames, nonuniform mixing and particle dispersion e nsure that the products of primary devolatilization are transformed by secondary pyrolysis before they burn. This laboratory study shows how the rates of volatiles combustion change during the course of seconda ry pyrolysis for a spectrum of coal types. Volatiles mixtures are gene rated in a novel coal flow reactor that independently regulates the ex tent of secondary pyrolysis. After tars, soot, and char particles are filtered out, oxygen and nitrogen are added to prepare combustible mix tures that have specified fuel equivalence and dilution ratios. Flames are propagated through the mixtures in a combustion bomb to acquire t he 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 e quivalence ratios from 0.4 to 1.5, at two dilution ratios and two unbu rned gas temperatures. For all coal types, burning velocities triple a s the extent of secondary pyrolysis increases from 50 to 100%, in roug h proportion to variations in the H-2 levels. This tendency is consist ent with conversion of the oxygen and hydrogen in tar into CO and H-2, and of light hydrocarbons into acetylene. For complete secondary pyro lysis, the burning velocities of volatiles from higher rank coals appr oach the comparable values for hydrogen combustion. But this tendency is not seen with lower rank coals because hydrogen makes smaller contr ibutions to their heating values. Increasing contributions from hydrog en explain the impact of increasing extents of secondary pyrolysis, an d different proportions of CO and H-2 determine the variation of burni ng velocities among different coal ranks. This database is used to dev elop a correlation based on a scaling from the thermal theory of lamin ar flame propagation and only two pseudo-components: the actual H-2 an d CO levels plus the amounts associated with instantaneous partial oxi dation of all hydrocarbons. Provided that flame temperatures are based on actual mixture compositions, this strategy closely correlates a da tabase that covers virtually the entire spectrum of coal types, the la st half of secondary pyrolysis, the full range of equivalence ratio en countered in practice, plus two dilution ratios and two unburned gas t emperatures. The same scaling is used to assign nominal burning rates from the measured burning velocities, which are found to be up to an o rder of magnitude faster than burning rates of soot, as expected.