Effects of Bath Gas and NOx Addition on n-Pentane Low-Temperature Oxidation in a Jet-Stirred Reactor

The oxidation of n-pentane (C5H12) in different bath gases (He, Ar, and CO2) and in Ar with NO2 or NO addition has been studied in a jet-stirred reactor at 107 kPa, temperatures between 500 and 1100 K, with a fixed residence time of 2.0 s, under stoichiometric conditions. Four different quantification diagnostics were used: gas chromatography, a chemiluminescence NOx analyzer, continuous wave cavity ring-down spectroscopy, and Fourier transform infrared spectroscopy. The results showed that the onset temperature of the fuel reactivity was the same (575 K) regardless of the type of bath gases. Although the low-temperature fuel oxidation window was not affected by the type of bath gas, the n-pentane conversion was slightly larger when diluted in Ar through the negative temperature coefficient (NTC) region (625-725 K). Above 800 K, the reactivity according to the diluent was in the order CO2 > Ar > He. In the presence of NO2 or NO, it was found that the consumption rate of n-pentane exhibited a different trend below 700 K. The presence of NO2 did not modify the fuel conversion below 675 K. On the contrary, NO addition increased the onset temperature of the fuel reactivity by 75 K and almost no NTC zone was observed. This clearly indicated that NO addition inhibited n-pentane oxidation below 675 K. Above 700 K, n-pentane conversion was promoted by the presence of both NOx additives. The intermediate species HONO was quantified, and a search for HCN and CH3NO2 species was also attempted. A new detailed kinetic mechanism was developed, which allowed a good prediction of the experimental data. Reaction rate and sensitivity analyses were conducted to illustrate the different kinetic regimes induced by the NOx addition. The inhibition by NO of the n-pentane oxidation below 675 K can be explained by its direct reaction with C5H11O2 radicals disfavoring the classical promoting channels via isomerizations, second O-2 addition, and formation of ketohydroperoxides, the well-known branching agents during alkane oxidation. With respect to NO2 addition, the major consumption route is via NO2 + CH3 = NO + CH3O, which is not directly related to the direct fuel consumption. HONO formation mainly derives from NO2 reacting with CHiO (i = 2, 3). The reaction, HONO + M = OH + NO + M, is one of the most sensitive reactions for HONO depletion.