The reaction of NO and NH 3 in the presence of excess O 2 has been investigated using 15N-isotopic labeling techniques under selective noncatalytic reduction (SNCR) conditions. Experiments were conducted with and without added H 2. The results show that 14N 15N is always the predominant nitrogen-containing product. Conversion of NH 3 to 14NO is a prerequisite for the formation of both 14N 2 and 14N 2O. Experiments conducted in the presence of added H 2 result in higher selectivities to N 2O; at temperatures above the maximum NO conversion, a large proportion (43%) of the nitrous oxide is 14N 2O. In general, the experimentally determined isotopic product distributions were in good agreement with model predictions based on accepted detailed kinetic schemes for the SNCR process. However, under some conditions, the model predicted significant concentrations of 15N 2 and 15N 2O, but these doubly labeled species were not observed experimentally. Measured concentrations of 15N 2O never exceeded the detection limit (0.05 ppm). Sensitivity and rate-of-progress analyses show that these doubly labeled 15N species are produced via channels in which the first steps are reactions of the NNH species with H radicals. An NNH lifetime of less than 10 -8s decreased the predicted amounts of these species. However, this lifetime would imply that H atoms should be produced in kinetic studies of the NH 2 + NO reaction, but this has not been observed. Alternately, the kinetics of the reaction involving the N 2H i species should be reviewed. The source of HCN produced from the propane-promoted SNCR reaction has also been investigated using labeling techniques. Both HC 14N and HC 15N, in similar concentrations, are produced from an equimolar mixture of 15NO and 14NH 3, under conditions in which ammonia oxidation to NO is insignificant. The results therefore imply that both NO-propane and NH 3-propane interactions are important in HCN formation under competitive conditions.
|Number of pages||10|
|Journal||Symposium (International) on Combustion|
|Publication status||Published - 1996|