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

The low reactivity of ammonia (NH) is the main barrier to applying neat NH as fuel in technical applications, such as internal combustion engines and gas turbines. Introducing combustion promoters as additives in NH-based fuel can be a feasible solution. In this work, the oxidation of ammonia by adding different reactivity promoters, i.e., hydrogen (H), methane (CH), and methanol (CHOH), 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) 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 consumption can be triggered at lower temperatures than in the neat NH case. CHOH has the most prominent effect on enhancing the reactivity, followed by H and CH. Furthermore, two-stage NH consumption was observed in NH/CHOH blends, whereas no such phenomenon was found by adding H or CH. The mechanism constructed in this work can reasonably reproduce the promoting effect of the additives on NH oxidation. The cyanide chemistry is validated by the measurement of HCN and HNCO. The reaction CHO + NH ⇄ HCO + NH is responsible for the underestimation of CHO in NH/CH fuel blends. The discrepancies observed in the modeling of NH fuel blends are mainly due to the deviations in the neat NH case. The total rate coefficient and the branching ratio of NH + HO are still controversial. The high branching fraction of the chain-propagating channel NH + HO ⇄ HNO + OH improves the model performance under low-pressure JSR conditions for neat NH but overestimates the reactivity for NH 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 CHOH, which enhances the reactivity most significantly. It was observed from the experiment that adding ozone to the oxidant can effectively initiate NH consumption at temperatures below 450 K but unexpectedly inhibit the NH consumption at temperatures higher than 900 K. The preliminary mechanism reveals that adding the elementary reactions between NH-related species and O is effective for improving the model performance, but their rate coefficients have to be refined.