Nitrogen-containing polycyclic aromatic compounds in sooting flames

Abstract

The production of nitrogen-containing polycyclic aromatic compounds (NPAC) in benzene/methane flames doped with organic nitrogen and in coal combustion was studied in small-scale equipment. A mixture of benzene vapor and methane doped with either pyrrole or pyridine was burned in a Meker burner, modified to allow oxygen to mix with the fuel before ignition. A bituminous coal was burned with oxygen in a fixed-bed combustor. Soot and associated organic compounds were sampled at several heights above the Meker burner and above the coal bed. The compounds were analyzed by gas chromatography with and without a nitrogen specific flame ionization detector. Selected runs with benzene/methane flames were also conducted with sulfur-containing fuel additives, namely carbon disulfide or thiophene. None of the fuel additives exerted any discernible effect on the quantities of polycyclic aromatic hydrocarbons (PAH) or soot formed in the benzene/methane flames. Cyanonaphthalenes, cyanoacenaphthylenes, and cyanophenanthrenes were found in significant quantities in association with soot formed in the nitrogen-doped benzene/methane flames, and in the coal combustor. The nitrogen source in these compounds was determined to be fuel rather than atmospheric nitrogen. Significant levels of other unidentified NPAC were also found in both combustion systems, but the number of such species was much larger in the coal combustion products. At a fuel equivalence ratio of 4.5, and with one percent nitrogen by weight in the fuel, each of the identified compounds was found in concentrations of 5–25 ng/cm3 of combustion gases, depending upon the compound.