Transformations of Coal-Derived Soot at Elevated Temperature

Abstract

Coal pyrolysis experiments were performed in the post-flame region of a CH 4 /H 2 /air flat-flame burner operating under fuel-rich conditions, where the temperature and gas compositions were similar to those found in the near-burner region of an industrial pulverized coal-fired furnace. Volatiles released from the coal particles formed a cloud of soot particles surrounding a centrally fed coal/char particle stream. Soot samples were collected from the cloud at different residence times using a water-cooled, nitrogen-quenched probe. The soot samples were then analyzed for their elemental compositions of carbon, hydrogen, nitrogen, sulfur, and (by difference) oxygen plus inorganic matter. Soot from three parent coals (Pittsburgh #8, Illinois #6, and Utah Hiawatha) and two gaseous hydrocarbon fuels (propane and acetylene) were investigated at temperatures of 1650, 1800, and 1900 K. The results reveal that the yield of coal-derived soot decreases with increasing reactor temperature, even though the total volatiles yield increased only slightly with temperature. The coal-derived soot yield at each reactor temperature condition also increased slightly with residence time. The carbon content in the coal-derived soot decreased with increasing particle residence time (at a given reactor temperature) and with increasing reactor temperature (at a given residence time) for all three coals. Carbon content remained constant with residence time for the gaseous hydrocarbon-fuel-derived soot. It is suggested that the observed decrease in coal-derived soot yield with increasing temperature is due to reactions of radical species from the flame with the soot precursors (i.e., the tar molecules). The slight increase in coal-derived soot yield with increasing residence time is due to attachment of light gas species such as acetylene which are richer in hydrogen than the local soot particles. The different behavior of soot from coal and the gaseous hydrocarbon fuels is explained in terms of their different chemical structures; coal-derived soot molecules have more aliphatic attachments and heteroatoms than soot from acetylene or propane. Carbon/hydrogen ratios in the soot samples were observed to be significantly different for the different soot types depending on parent fuel.