Kinetic Modeling of the Mutual Oxidation of NO and Larger Alkanes at Low Temperature

Abstract

A detailed chemical kinetic modeling investigation of n-butane and n-pentane oxidation interaction with NO to NO2 conversion is presented. The model was validated against experiments obtained in an atmospheric-pressure, quartz flow reactor, which was used to examine the NO oxidation behavior for the temperature range of 600−1100 K and residence times of 0.16−1.46 s. Probe measurement of the species concentrations was performed in the flow reactor using a mixture NO (20 ppm)/air/hydrocarbon (10 ppm). In the chemical kinetic calculation, the time evolution of NO, NO2, hydrocarbons, and reaction intermediates were evaluated using two mechanisms of oxidation of n-butane and n-pentane, coupled with a nitrogen oxide submechanism for all temperatures. The model of the reaction of hydrocarbon uses the same set of parameters previously developed and validated by the DCPR for the oxidation of alkanes, without any specific adjustment. The reactions of coupling between the alkane oxidation and the nitrous compounds were added. The model reproduces the temperature dependence of the conversion of the reactants and the species concentration profiles versus the residence time in the reactor. The results show a strong coupling between the conversion of hydrocarbon in the low-temperature range and the NO-to-NO2 conversion. Both hydrocarbon-fuel oxidation systems are accelerated in the blends, in comparison to undoped or pure hydrocarbon oxidation systems. The NO + •HO2 = NO2 + •OH and alkylperoxy + NO = alkyloxy + NO2 reactions have a major role in converting NO to NO2 at the lower temperatures. The same reactions strongly accelerate the oxidation of hydrocarbon by converting •HO2 and alkylperoxy to •OH. Above 900 K, the decrease in NO2 concentration is attributed to HONO formation and reaction of NO2 with small radicals. Calculations with various initial concentrations of NO and n-pentane were performed and analyzed for main reaction channels and most sensitive reactions. The coupling between many mechanisms involving equilibriums such as R + O2, NO + •OH, or NO2 + •CH3 explain the very complex behavior of the mixtures versus temperature. Ignition delay times have been calculated for a mixture of n-pentane/air with and without NO. Calculated results for conditions representative of engines show the importance of NO kinetics in new combustion modes.