Predicting the propensity to soot of hydrocarbons and oxygenated molecules by means of structural group contribution factors derived from the processing of unified sooting indexes

To limit the emission of particulate pollutants at the exhaust of combustion processes, we must improve our understanding of soot formation mechanisms while identifying promising low-sooting additives to be blended with conventional fuels. In order to characterize the sooting behavior of hydrocarbons, different sooting indexes (SI) have been developed including the threshold soot index (TSI), the oxygen extended sooting index (OESI), the yield sooting index (YSI) and the fuel equivalent sooting index (FESI). By processing data derived from the use of such indicators, one can assign sooting contribution factors to each structural group composing a given molecule based on regressions against measured SI. Results that have been reported using such a methodology, however, remain difficult to compare due to the presence of wide varied index scales and uncertainties. Moreover, many studies consider relatively limited panels of molecules, while the statistical relevance of group increment models strongly depends on the number and variety of species considered. To overcome this issue, SI values drawn from 15 studies representing more than 700 experimental points were unified on a same numeric scale. This statistical sample integrates new OESI and FESI measurements conducted with hydrocarbons and oxygenated molecules, including furanic species. Developing the most comprehensive group increment model ever built allowed characterizing the sooting tendency of 93 structural groups entering into the composition of the main classes of compounds encountered in practical fuels. While satisfactorily reproducing the SI values from the database, the model also captures well the impacts of the number of C atoms, the unsaturation degree, the chain branching or the presence of oxygenated groups in soot formation. The sooting propensity of non-oxygenated compounds was found to increase in the following order: alkanes < alkenes < alkynes < aromatics against esters < aldehydes < ketones < 1-alcohols < 2-alcohols for oxygenated molecules and tetrahydrofuran < furan - 2,3-dihydrofuran 2,5-dihydrofuran < 2-methyltetrahydrofuran < 2-methylfuran < 2,5-dimethylfuran for furanic species. The proposed model was finally shown capable of properly simulating the sooting propensity of a gasoline surrogate, as well as its behavior towards the addition of oxygenated additives during both lab-scale and engine tests.

Automated summary

To control particulate emissions during combustion processes, understanding soot formation mechanisms and finding low-sooting additives are necessary. Various sooting indexes have been developed and a group increment model has been built to characterize the behavior of hydrocarbons. Factors such as the number of carbon atoms, unsaturation, and presence of oxygen groups play crucial roles in soot formation.