Temperature dependence of NO to NO 2 conversion by n-butane and n-pentane oxidation

Abstract

An experimental and detailed chemical kinetic modeling investigation of the temperature-dependentrole of n-butane and n-pentane oxidation on the NO to NO2 conversion is presented. An atmospheric pressure, quartz flow reactor was used to examine the NO oxidation to NO2 behavior for the 600 to 1100 K temperature range and residence times from 0.16 to 1.46 s. In the experiment, probe measurement of the species concentrations was performed in the flow reactor using a mixture of 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 a n-butane oxidation model coupled with a nitrogen oxides submechanism for all temperatures. The detailed chemical kinetic model consisted of 897 reactions and 158 species. The experimental results show n-pentane promoting the NO to NO2 conversion to a greater extent thann-butane for the entire temperature range. This may be explained by n-pentane oxidation exhibiting a vigorous chain-branched hydroperoxy-pentylperoxy radical isomerization kinetic system more so than found in n-butane. Kinetic calculations performed on the n-butane oxidation system revealed that the NO to NO2 conversion is strongly temperature dependent. The NO+HO2=NO2+OH and alkylperoxy +NO=alkyloxy+NO2 reactions play an important role in converting NO to NO2 at the lower temperatures in this study. However, as the temperature increases toward 800–900 K, the butyl+O2 and hydroperoxy-butyl+O2 network of reactions undergoes reaction reversal and allows other reaction channels to be accessed which heavily promotes NO to NO2 conversion. Above 900 K, the decrease in NO2 concentration is attributed to NO2+HO2=HONO+O2, HONO(+M)=NO+OH(+M), and NO2+O=NO+O2 destruction reactions. Consequently, the change of HO2 formation with temperature plays the most important role for the temperature dependence of the NO to NO2 conversion.