Sooting tendencies of oxygenated hydrocarbons in laboratory-scale flames.

Sooting tendencies have been measured for 186 oxygenated and 89 regular hydrocarbons under controlled laboratory conditions. The test compounds include alcohols, ethers, aldehydes, ketones, esters, alkanes, alkenes, and cycloalkanes ranging in size from methanol to isododecane. Sooting tendency was characterized with a new method based on measuring particle concentrations in methane/air nonpremixed flames when 1000 ppm of each test compound was added to the fuel. This method offers high precision and high sensitivity to the direct chemical effects of the additive. The results provide a wide-ranging yet detailed quantitative picture of how fuel oxygen affects soot formation, which will be useful for optimizing the soot-reducing benefits of oxygenated renewable fuels. The measured sooting tendencies of 1-alcohols are similar to those of n-alkanes with the same number of carbon atoms, while those of secondary alcohols are slightly higher. Aldehydes and ketones soot the same as n-alkanes with one less carbon atom. The sooting tendencies of esters depend strongly on molecular structure and increase in this order: methyl and ethyl esters < carboxylic acids, propyl esters, and n-alkanes < butyl and pentyl esters. The high sooting tendencies of the secondary alcohols and higher esters suggest that four-center and six-center reaction pathways are important.