Pyrolysis-induced changes in the ring number composition of polycyclic aromatic compounds from a high volatile bituminous coal

Abstract

Combining a high pressure liquid chromatographic separation by ring number and a mass proportional means of detection, we have monitored the pyrolytic behavior of nonpolar coalderived polycyclic aromatic compounds (PAC) at temperatures of 1100–1500 K and average gas residence times of .25 to .75 sec. PAC composition with respect to ring number correlates well with total PAC yield (or conversion) and depends on temperature or time only as much as these variables determine conversion. 2- and 3-ring aromatics decay the most rapidly with increases in temperature or time; 4- and 5-ring PAC emerge as the most stable species. As a whole, PAC of 6 to 10 rings exhibit general insensitivity of pyrolysis conditions up to an intermediate level of conversion, after which they too decay with increasing temperature or time. Ring number analyses on substituted and unsubstituted PAC fractions of the coal pyrolysis samples reveal that for each ring number group, the substituted PAC decay more quickly than the unsubstituted PAC. The departure of product concentrations from equilibrium levels indicates kinetic control. Our experimental results show consistency with the free radical mechanism of Badger et al. , which can account for the general ring buildup we observe in the low to moderate PAC conversion regime and the ever increasing degree of condensation in the remaining PAC as conversion continues. Empirical findings of others on the pyrolysis of model compounds as well as theory-based ease-of-radical-formation parameters explain both the preferential loss of substituted PAC and the survival of the highly condensed unsubstituted PAC that we observe experimentally. In contrast to premixed flame systems where PAC and soot grow by the addition of acetylene, aromatic species appear, to be primarily responsible for the growth of the PAC and soot in our coal pyrolysis experiments.