Low temperature oxidation of methane: the influence of nitrogen oxides

An experimental investigation of methane oxidation in the presence of NO and NO2 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 NO2 at residence times of 200 ms. At shorter residence times (140 ms) this enhancement effect is reduced for NO but maintained for NO2. Furthermore 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 explained by the competition between three reaction paths from CH3 to CH2O. A direct high temperature path (A), a two-step NO2 enhanced low temperature path (B) and a slow three step NO enhanced path (C), which may produce NO2 to activate path B. The negative temperature coefficient 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 to reduce the production of unwanted intermediates such as formaldehyde.