STRUCTURE AND COMPOSITION OF COAL TARS: AN ATTEMPT TO CORRELATE MOLECULAR STRUCTURE WITH INCREASING MOLECULAR MASS

Near-burner-zone combustion rates in pulverized-fuel (PF) combustors depend primarily on the amount and the structure/composition of volatiles released from the fuel. Mathematical simulations of PF flames often assume that volatiles combustion takes place at rates similar to those of light hydrocarbons. However, tars constitute 60-70% of volatiles released by most power station coals; tar molecules are far larger and oxidize more slowly. The problem is usually ignored. This article describes the characterization of structures and compositions of molecules in a low-temperature coal tar, typical of material combusted in the near-burner zone. Several techniques have been used in combination. The procedure relies on the estimation of molecular mass distributions by size exclusion chromatography coupled with bulk structural characterization by several established techniques (e.g., C-13-NMR, UV-fluorescence spectrometry). Sample fractionation by polarity and molecular mass have been found necessary for enhancing the resolution of the analytical tools, as well as for allowing meaningful correlations to be established between changing molecular mass ranges of successive fractions of the tar and the structural features of each fraction. The approach, described in some detail, extends the range of molecular masses amenable to examination to levels far above ceilings imposed by limitations of conventional GC, GC-MS, and probe-MS. The distribution of structures and molecular masses found in the tar suggests reaction pathways for the formation of soot during PF combustion.