| Tar samples generated from two types of coal have been chemically characterized to quantify and compare the trends in various organic compounds classes over the entire range of secondary pyrolysis. The polarity distributions of the tars have been measured using gravity flow column chromatography, the fused ring number distributions of the polycyclic aromatic compounds (PAC) in the tars have been quantified using high pressure liquid chromatography, and the fused ring number distributions of the nitrogen-containing PAC (NPAC) have been examined using gas chromatography with a nitrogen-specific detector.; Throughout secondary pyrolysis, the polarity distributions of coal tars show that the less-polar toluene fraction contains the largest amount of tar compounds, and is a major contributor to soot formation. During the early stages of secondary pyrolysis, the changes in the polarity distributions reflect mass transformation from the more polar to less polar solvent fractions. The characteristics of both the polarity and ring number distributions at the early stages of secondary pyrolysis reflect the features of the parent coals.; In the middle stages of secondary pyrolysis, the polarity distributions show an abrupt increase in the heptane fraction, which occurs concurrently with a depletion of smaller ring compounds, an increase in the 4- and 5-ring PAC, and a buildup of NPAC species. Polymerization, thermal dissociation, and soot growth via acetylene addition become more prominent during this stage.; By the final stages of secondary pyrolysis, the concentrations in both the solvent fractions and PAC ring groups become greatly depleted. Both coal tars attain similar compositions in polarity and aromatic fused ring numbers, showing that the original coal structure becomes much less influential by the end of secondary pyrolysis. NPAC appear to be especially persistent among PAC species, in particular the 2-ring NPAC, which remains the most abundant of all NPAC ring groups throughout secondary pyrolysis. The higher stability of NPAC suggests their potential presence in coal pyrolysis emissions, which is of substantial health concern. |
