A carbon-14 tracer study of the relative fractions of various fuel carbons in soot

Abstract

The effect of fuel hydrocarbon structure on soot emissions was studied using a carbon-14 isotope tracer technique. A diesel engine or a laminar wick diffusion flame generated radioactive soot from a #2 diesel fuel containing 14Chydrocarbons. For the same diesel fuel, the individual soot yields of several carbon atom types were determined. For the diesel engine, the soot yield from aromatic carbons was about a factor of 1.5 greater than that from nonaromatic carbons. For the wick flame, the soot yield from aromatic carbons was about a factor of 2.0 greater than that from nonaromatic carbons. Interestingly, these differences in soot yield between aromatic and nonaromatic carbons within the same fuel are too small to explain the differences in soot yield between aromatic and nonaromatic fuels. This is consistent with the hypothesis that aromatic carbons increase soot emissions by greatly increasing the number of nascent particles while contributing only slightly more than other carbons to the final soot mass. It also shows that variations in fuel hydrocarbon structure have a minimal effect on the soot particle growth process, which produces most of the soot mass. The wick flame was set to give a soot yield that was 20 times greater than that of the diesel engine; yet, the carbon-14 concentrations in the soot were similar. This is expected if the formation of soot growth species from the original fuel molecules is independent of the amount of soot ultimately formed from the growth species. Also, the soot yield of a given carbon atom type was unaffected by the molecular weight of the hydrocarbon molecule that contained it. So for #2 diesel fuel, soot is produced equally from all points along the fuel's distillation curve (molecular weight range) even though heavier fuels make more soot.