ON THE SURVIVABILITY AND PYROSYNTHESIS OF PAH DURING COMBUSTION OF PULVERIZED COAL AND TIRE CRUMB

Results are presented on the emissions of semivolatile polycyclic arom atic hydrocarbons (PAH) from the combustion of a pulverized bituminous coal and ground waste automobile tires. Streams of fuel particles wer e injected at steady-state steady-flow conditions, and burned inside a n isothermal drop-tube furnace, in air, at a gas temperature and gas r esidence time of 1150 degrees C and 0.75 s, respectively. Combustion o ccurred under either very fuel-lean conditions (bulk equivalence ratio , phi < 0.5) or substantially fuel-rich conditions (phi = 1.6-1.9). Em issions from fuel pyrolysis, in the absence of oxygen, were also exami ned. The survivability of the fuel-PAHs during combustion/pyrolysis wa s assessed by examining the reactants (fuels) and the products of thei r oxidation/pyrolysis. The PAH species in the effluent of combustion w ere: 1) qualitatively compared with indigenous PAH constituents of the input fuels, and 2) quantitatively contrasted with known amounts of d euterium-labeled PAH standards, which were adsorbed on the input fuels . No PAHs were detected in the effluent of combustion of either fuel u nder sufficiently fuel-lean conditions, e.g., phi < 0.5. This indicate d that the PAH constituents of the input fuels, either indigenous or a dsorbed, as well as those formed by pyrosynthesis in either the diffus ion volatile flames or during the heterogeneous oxidation of the chars were destroyed. Significant amounts of PAHs were detected in the effl uent of the combustion of both fuels under sufficiently fuel-rich cond itions, e.g., phi > 1.6 and, especially, under pyrolytic conditions in N-2. These PAHs were mostly attributed to pyrosynthesis since none of the deuterated PAHs, adsorbed on the fuels, survived the combustion p rocess. Small amounts of the labeled compounds, however, survived unde r purely pyrolytic conditions. These results were confirmed with separ ate experiments, where deuterium-labeled PAH standards were adsorbed o n highly porous calcium/magnesium oxide or mullite particles. Again, s mall amounts of some PAHs survived in high-temperature pyrolytic condi tions, but none in oxidative environments. These observations suggest that pyrosynthesis is the major contributing mechanism to the PAH emis sions from the combustion of these fuels. Survivability of parent PAHs may be a minor mechanism at very high equivalence ratios. Finally, bo th fuels were mixed with powders of calcium magnesium acetate (CMA), c alcium carbonate (CaCO3), and calcium oxide (CaO) all of which are kno wn sulfur reduction agents, at a molar Ca/S ratio of 1. Combustion of the fuels mixed with CMA or CaCO3 generated enhanced amounts of PAHs, while combustion with CaO had no effect on the PAH emissions. (C) 1997 by The Combustion Institute.