Experimental and mechanistic investigation of benzene formation during atmospheric pressure flow reactor oxidation of n-hexane, n-nonane, and n-dodecane below 1200 K

Abstract

The reaction kinetics of three n-alkanes: hexane, nonane, and dodecane are investigated both experimentally and numerically through speciation data obtained in a high temperature flow reactor. The measurements are performed in the temperature range of 750–1200 K for three different equivalence ratios (Φ = 0.5, 1.0, and 2.0) and major combustion products, intermediates such as olefins, oxygenates, and a soot precursor benzene have been measured.Formation of benzene is emphasized in this work at intermediate flame temperatures (in this case below 1200 K). Even though this temperature regime has its importance to technical combustors (e.g. at aero engines), the regime is often not studied except a few flow reactor studies. From the three fuels studied, one can see that the benzene formation increases apparently with fuel stoichiometry as well as with increase in carbon number. Unlike at flame conditions, where propargyl recombination plays an important role in benzene formation, at lower flame temperatures (< 1200 K) the benzene formation is a result of reactions through small hydrocarbon species occurring from an even (C2 + C4) route. The benzene formation reactions present in the reaction model of current work are n-C4H5 + C2H3 = C6H6 + H2, n-C4H5 + C2H2 = C6H6 + H, i-C4H5 + C2H4 → C6H6 + H + H2, i-C4H5 + C2H2 = C6H6 + H, and i-C4H5 + C2H = C6H6. Analysis of the mechanism at conditions of present work shows that the benzene formation is dominated by the reactions of i-C4H5 + C2H4 → C6H6 + H + H2 and i-C4H5 + C2H2 = C6H6 + H, their relative importance being dependent on the temperatures at which C2H4 and C2H2 is formed. The predictions of the speciation data are also compared with prominent literature reaction models (JetSurF 2.0, POLIMI, and RWTH). It appears that absence of either or both of these two reactions in JetSurF 2.0 and RWTH models leads to noticeably lesser contribution to benzene formation giving maximum deviation observed with the measurements. For the DLR and POLIMI models where benzene predictions are close to the experiments, these two reactions are present.As a consequence a detailed in-house reaction model, which was already extensively validated against global combustion characteristics, has been tested against the measured speciation data. The model succeeds in reproducing all the measured species and is in good agreement with the measurements. This study identifies the major paths during the oxidation of all three fuels studied and provides valuable database and insight into the product spectrum and prediction of soot precursors.