Exploring the Effect of Different Reactivity Promoters on the Oxidation of Ammonia in a Jet-Stirred Reactor.

The low reactivity of ammonia (NH 3 ) is the main barrier to applying neat NH 3 as fuel in technical applications, such as internal combustion engines and gas turbines. Introducing combustion promoters as additives in NH 3 -based fuel can be a feasible solution. In this work, the oxidation of ammonia by adding different reactivity promoters, i.e., hydrogen (H 2 ), methane (CH 4 ), and methanol (CH 3 OH), was investigated in a jet-stirred reactor (JSR) at temperatures between 700 and 1200 K and at a pressure of 1 bar. The effect of ozone (O 3 ) was also studied, starting from an extremely low temperature (450 K). Species mole fraction profiles as a function of the temperature were measured by molecular-beam mass spectrometry (MBMS). With the help of the promoters, NH 3 consumption can be triggered at lower temperatures than in the neat NH 3 case. CH 3 OH has the most prominent effect on enhancing the reactivity, followed by H 2 and CH 4 . Furthermore, two-stage NH 3 consumption was observed in NH 3 /CH 3 OH blends, whereas no such phenomenon was found by adding H 2 or CH 4 . The mechanism constructed in this work can reasonably reproduce the promoting effect of the additives on NH 3 oxidation. The cyanide chemistry is validated by the measurement of HCN and HNCO. The reaction CH 2 O + NH 2 ⇄ HCO + NH 3 is responsible for the underestimation of CH 2 O in NH 3 /CH 4 fuel blends. The discrepancies observed in the modeling of NH 3 fuel blends are mainly due to the deviations in the neat NH 3 case. The total rate coefficient and the branching ratio of NH 2 + HO 2 are still controversial. The high branching fraction of the chain-propagating channel NH 2 + HO 2 ⇄ H 2 NO + OH improves the model performance under low-pressure JSR conditions for neat NH 3 but overestimates the reactivity for NH 3 fuel blends. Based on this mechanism, the reaction pathway and rate of production analyses were conducted. The HONO-related reaction routine was found to be activated uniquely by adding CH 3 OH, which enhances the reactivity most significantly. It was observed from the experiment that adding ozone to the oxidant can effectively initiate NH 3 consumption at temperatures below 450 K but unexpectedly inhibit the NH 3 consumption at temperatures higher than 900 K. The preliminary mechanism reveals that adding the elementary reactions between NH 3 -related species and O 3 is effective for improving the model performance, but their rate coefficients have to be refined.