On the survivability and pyrosynthesis of PAH during combustion of pulverized coal and tire crumb

Abstract

Results are presented on the emissions of semivolatile polycyclic aromatic hydrocarbons (PAH) from the combustion of a pulverized bituminous coal and ground waste automobile tires. Streams of fuel particles were injected at steady-state steady-flow conditions, and burned inside an isothermal drop-tube furnace, in air, at a gas temperature and gas residence time of 1150°C and 0.75 s, respectively. Combustion occurred under either very fuel-lean conditions (bulk equivalence ratio, φ < 0.5) or substantially fuel-rich conditions (φ = 1.6–1.9). Emissions from fuel pyrolysis, in the absence of oxygen, were also examined. The survivability of the fuel-PAHs during combustion/pyrolysis was assessed by examining the reactants (fuels) and the products of their oxidation/pyrolysis. The PAH species in the effluent of combustion were: 1) qualitatively compared with indigenous PAH constituents of the input fuels, and 2) quantitatively contrasted with known amounts of deuterium-labeled PAH standards, which were absorbed on the input fuels. No PAHs were detected in the effluent of combustion of either fuel under sufficiently fuel-lean conditions, e.g., φ < 0.5. This indicated that the PAH constituents of the input fuels, either indigenous or adsorbed, as well as those formed by pyrosynthesis in either the diffusion volatile flames or during the heterogeneous oxidation of the chars were destroyed. Significant amounts of PAHs were detected in the effluent of the combustion of both fuels under sufficiently fuel-rich conditions, e.g., φ > 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 process. Small amounts of the labeled compounds, however, survived under purely pyrolytic conditions. These results were confirmed with separate experiments, where deuterium-labeled PAH standards were adsorbed on highly porous calcium/magnesium oxide or mullite particles. Again, small amounts of some PAHs survived in high-temperature pyrolytic conditions, but none in oxidative environments. These observations suggest that pyrosynthesis is the major contributing mechanism to the PAH emissions from the combustion of these fuels. Survivability of parent PAHs may be a minor mechanism at very high equivalence ratios. Finally, both fuels were mixed with powders of calcium magnesium acetate (CMA), calcium carbonate (CaCO 3), and calcium oxide (CaO), all of which are known sulfur reduction agents, at a molar Ca/S ratio of 1. Combustion of the fuels mixed with CMA or CaCO 3 generated enhanced amounts of PAHs, while combustion with CaO had no effect on the PAH emissions.