Effect of Unsaturated Bond on NOx and PAH Formation in n-Heptane and 1-Heptene Triple Flames

Abstract

Various engine and shock tube studies have observed increased NOx emissions from the combustion of biodiesels relative to regular diesel and linked them to the degree of unsaturation or the number of double bonds in the molecular structure of long-chain biodiesel fuels. We report herein a numerical investigation on the structure and emission characteristics of triple flames burning n-heptane and 1-heptene fuels, which represent, respectively, the hydrocarbon side chain of the saturated (methyl octanoate) and unsaturated (methyl octenoate) biodiesel surrogates. Our objective is to examine the effect of unsaturated (double) bond on NOx and soot emissions in a flame environment containing regions of lean premixed, rich premixed, and nonpremixed combustion. A validated detailed kinetic model with 198 species and 4932 reactions was used to simulate triple flames in a counterflow configuration with different levels of premixing and strain rates. Results indicate that although the global structures of n-heptane and 1-heptene triple flames are quite similar, there are significant differences with respect to NOx and polycyclic aromatic hydrocarbon (PAH) emissions from these flames. The NOx production rates in the rich premixed, lean premixed, and nonpremixed zones are higher in 1-heptene flames than in n-heptane flames, and the differences become more pronounced as the level of premixing is increased. The NOx formed through the prompt, thermal, N2O, and NNH mechanisms is also higher in 1-heptene flames. NOx formation in the rich premixed zone is primarily due to the prompt NO, that in the nonpremixed zone is through the thermal NO, and that in the lean premixed zone is due to the NNH and N2O routes. The PAH species are mainly formed in the rich premixed zone, and their emissions are significantly higher in 1-heptene flames than in n-heptane flames. The reaction pathway analysis indicated that the dominant path for benzene formation involves the recombination of two propargyl (C3H3) radicals, and the presence of the double bond in 1-heptene provides a significant route for its production through the formation of C3H5. This path is not favored in the oxidation of n-heptane, as it decomposes directly to smaller alkyl radicals. Whereas the NOx and PAH emissions decrease with the increase in strain rate, they are consistently higher in 1-heptne flames than in n-heptane flames, irrespective of the strain rate.