Soot abatement using fuel additives

by Wu, Juntao.

A study of the mechanism of soot abatement by oxygen-containing additives was conducted by both experimental and modeling methods. The overall technical objective is to develop a fundamental understanding of the complex roles of oxygen-containing additives in the processes which lead to particulate matter emissions. Two classes of compounds were investigated - oxygenates and nitro-alkanes. Experiments were performed on a premixed ethylene/air flame with two oxygenated additives - ethanol and dimethyl ether (DME). The experiments were conducted at two equivalence ratios, ? = 2.34 and ? = 2.64, and two levels of ethanol or DME, 5% and 10% oxygen in mass of the fuel, were added to the fuel at each equivalence ratio. The experimental results show that both ethanol and DME reduced aromatic species and soot, and they were more effective at ? = 2.34 than at ? = 2.64. The comparison between ethanol and DME shows that, at ? = 2.34, DME is more effective than ethanol on PAH and soot reduction; however, at ? = 2.64, there is no detectable difference between these two additives on the aromatic species and soot suppression. A chemical model, Howard-DME-Ethanol (HDE) mechanism, was used to investigate the chemical processes leading to the abatement of aromatic species and soot by ethanol and DME. The analysis shows that these reduction effects result from the removal of carbon from the pathway to aromatic species formation. DME is more iv effective than ethanol in reducing soot precursors, because more carbon in DME is removed from the participation of aromatic formation than that in ethanol. This difference is due to the molecular structure difference between ethanol and DME. Screening studies of the nitro-alkanes showed them to be effective in reducing soot. The primary mechanism of soot reduction was hypothesized to be linked to NO2, so a modeling study of NO2 addition to premixed flames was undertaken. A chemical model, Howard-NO2 (HN) mechanism, was used to investigate the chemical processes leading to the effect of NO2 on aromatic species and soot reduction. The mechanism analysis shows that the addition of NO2 increases the level of OH radicals in the flame, through reactions of NO2+H?NO+OH, HO2+NO?NO2+OH. Then the increased OH decreases the level of H2 by reactions H2+OH?H2O+H. The lower level of H2 increases the reaction rate of H2CCCH+H?C3H2+H2, and results a lower level of H2CCCH. Since all C6H6 comes from H2CCCH through reaction 2H2CCCH?C6H6, low level H2CCCH leads to a reduction of C6H6. As a key element during the aromatic species growth, low level of C6H6 leads to suppression of overall aromatic species formation and growth. v