Char- and Aerosol-associated Polycyclic Aromatic Compounds from Coal Pyrolysis: Relationship Between Particle Size and Surface Composition

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Abstract
In order to investigate the relationship between solid size and polycyclic aromatic compound (PAC) amount and composition, we have pyrolyzed 45–53-μm particles of a high-volatility bituminous coal at 1100–1500 K in a laminar flow, drop-tube furnace. Aerodynamically separated in a cascade impactor, three size cuts of the solid products have been collected for determination of PAC and solid carbon mass yields. We have also examined differences in PAC composition of the three size cuts by employing a liquid chromatographic technique that separates PAC according to their number of fused aromatic rings. Soot yields are found to increase with increasing temperature; char and PAC yields decrease. Yields of the most volatile PAC decay particularly rapidly with increasing temperature. For all pyrolysis conditions investigated, > 90% of the PAC is associated with the soot (the smaller two size cuts), but the proportion associated with the larger particles grows as the total amount of PAC lessens. For all experiments, the larger particles exhibit enrichment in the lighter species, and this enrichment becomes most pronounced when there is the least amount of PAC. The experimental results show qualitative consistency with a model of simultaneous coagulation and condensation. However, if the effective condensation diameter for the soot is the primary particle size, the amounts of PAC associated with the char, relative to those associated with the soot, are much larger than predictions from mass transfer-controlled condensation theory. Various secondary effects—surface curvature, particle terminal velocity, and volume fraction size dependency—help account for the direction of the observed trends but not their magnitude. The higher than predicted ratios of char-associated to soot-associated PAC suggest that the governing dimension for mass transfer to the soot is the agglomerate size.