Low temperature oxidation of methane: the influence of nitrogen oxides

An experimental investigation of methane oxidation in the presence of NO an d NO2 has been made in an isothermal plug-flow reactor at 750-1250K. The te mperature for on-set of oxidation was lowered by 250 K in the presence of N O or NO2 at residence times of 200 ms. At shorter residence times (140 ms) this enhancement effect is reduced for NO but maintained for NO2. Furthermo re two temperature regimes of oxidation separated by an intermediate regime where only little oxidation takes place exist at residence times of 140 ms , if NO is the only nitrogen oxide initially present. The results were expl ained by the competition between three reaction paths from CH3 to CH2O. A d irect high temperature path (A), a two-step NO2 enhanced low temperature pa th (B) and a slow three step NO enhanced path (C), which may produce NO2 to activate path B. The negative temperature coefficient behaviour was explai ned by a competition between paths (A) and (C). A previously published reac tion set was modified to take these reaction patterns into account. The processes discussed here presumably occur to some extent in the exhaust of natural gas engines, but conditions may be modified, either to ensure a n enhanced activity to oxidize methane in the exhaust, or alternatively to decrease the activity to reduce the production of unwanted intermediates su ch as formaldehyde.