Low temperature oxidation of methane: the influence of nitrogen oxides

An experimental investigation of methane oxidation in the presence of NO and NO 2 has been made in an isothermal plug-flow reactor at 750-1250K. The temperature for on-set of oxidation was lowered by 250 K in the presence of NO or NO 2 at residence times of 200 ms. At shorter residence times (140 ms) this enhancement effect is reduced for NO but maintained for NO 2 . Furthermore two temperature regimes of oxidation separated by an intermediate regime where only little oxidation lakes place exist at residence times of 140 ms, if NO is the only nitrogen oxide initially present. The results were explained by the competition between three reaction paths from CH 3 to CH 2 O. A direct high temperature path (A), a two-step NO 2 enhanced low temperature path (B) and a slow three step NO enhanced path (C), which may produce NO 2 to activate path B. The negative temperature coefricient behaviour was explained by a competition between paths (A) and (C). A previously published reaction 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 an enhanced activity to oxidize methane in the exhaust, or alternatively to decrease the activity 10 reduce the production of unwanted intermediates such as formaldehyde.