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

Abstract

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 lowpressure 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.

Combustion in a Sustainable World: From Molecules to Processes

“…In-situ laser diagnostic methods for the quantitative detection of HCN, a toxic byproduct of NO x formation reactions during the combustion of nitrogenous fuels, were developed and refined by Stuhr and Friedrichs, who applied mid-infrared frequency modulation spectroscopy to enable time-resolved, sensitive measurements of this species in shock tube experiments . For the technical application of ammonia as a combustion fuel, its low reactivity must be enhanced by the addition of promoters in the fuel mixture. , The reactivity and oxidation reactions of ammonia have been analyzed in the presence of such blending compounds as hydrogen, methane, methanol, and ozone in jet-stirred and flow reactor as well as shock tube experiments, providing valuable data sets for model improvement. , Efficient use of ammonia as a hydrogen carrier requires effective cracking procedures to release hydrogen on demand. Bang et al investigated a plasma-assisted process to lower the cracking temperature and performed kinetics experiments using a temperature-controlled dielectric barrier discharge reactor .…”

“…35 For the technical application of ammonia as a combustion fuel, its low reactivity must be enhanced by the addition of promoters in the fuel mixture. 36,37 The reactivity and oxidation reactions of ammonia have been analyzed in the presence of such blending compounds as hydrogen, methane, methanol, and ozone in jet-stirred and flow reactor as well as shock tube experiments, providing valuable data sets for model improvement. 36,37 Efficient use of ammonia as a hydrogen carrier requires effective cracking procedures to release hydrogen on demand.…”

“…36,37 The reactivity and oxidation reactions of ammonia have been analyzed in the presence of such blending compounds as hydrogen, methane, methanol, and ozone in jet-stirred and flow reactor as well as shock tube experiments, providing valuable data sets for model improvement. 36,37 Efficient use of ammonia as a hydrogen carrier requires effective cracking procedures to release hydrogen on demand. Bang et al investigated a plasma-assisted process to lower the cracking temperature and performed kinetics experiments using a temperature-controlled dielectric barrier discharge reactor.…”

Development of Ammonia Reaction Kinetic Mechanism under Engine-Relevant Conditions

“…It is found that the Okafor model overestimated the ignition delay time, while the Konnov model underestimates it. The He model shows good prediction accuracy under low pressure but underestimates the simulation values under a high pressure and low equivalence ratio as well as stoichiometric conditions. Since the Bertolino model was an improved version based on the Stagni model, the simulation results of both had small differences and both underestimated the ignition delay time.…”

“…(a–d) Comparison of predicted results and experimental values of the species profile of different mechanisms.…”