Research on the impact of nitromethane on the combustion mechanism of ammonia/methanol blends

Ammonia/methanol co-combustion is considered an effective liquid-liquid blending strategy to enhance the combustion performance of ammonia. However, both methanol and ammonia have high latent heats of vaporization, which necessitate significant heat absorption during the vaporization process. This often results in excessively low ambient temperatures before the ignition of the mixture, negatively affecting low-temperature ignition and combustion. To improve the combustion characteristics of ammonia/methanol blends, this study proposes the addition of nitromethane, forming a ternary blend of ammonia/methanol/nitromethane to enhance fuel performance. To evaluate the impact of nitromethane on the combustion mechanism of ammonia/methanol blends, this study utilizes synchronous vacuum ultraviolet photoionization mass spectrometry to analyze the oxidation reactions of the ammonia/methanol/nitromethane blends. Based on the Brequigny model, cross-reactions involving C-N bonds and reactions related to nitromethane were incorporated for model modification, resulting in the newly modified model, termed A-M. Pathway and sensitivity analyses, as well as ignition delay time simulations, were conducted to further understand the combustion process. The results indicate that the addition of nitromethane to the ammonia/methanol blend lowers the initial reaction temperature from 860 K to 740 K and increases nitrogen oxide (NO_x) concentrations at 1050 K. At 800 K, nitromethane reduces the conversion of NH₂ to NH₃, thereby enhancing ammonia consumption and altering the NO_x consumption pathway. Furthermore, at 1020 K, 98.6 % of H₂NO reacts with H to form NH₂, which is a crucial species in ammonia regeneration. Additionally, at 1020 K, 90.8 % of nitromethane decomposes through the reaction CH₃NO₂(+M) = CH₃ + NO₂(+M), contributing to increased NO_x emissions. Moreover, the incorporation of nitromethane significantly reduces the ignition delay time of ammonia/methanol blends, demonstrating its potential to improve the overall combustion performance of these mixtures.