An experimental and modeling study on auto-ignition kinetics of ammonia/methanol mixtures at intermediate temperature and high pressure

A rapid compression machine (RCM) has been applied to measure the ignition delay times of NH₃/CH₃OH mixtures covering pressures of 20 and 40 bar, equivalence ratios of 0.5, 1.0 and 2.0, and temperatures between 845 and 1100 K. The measurements show that the NH₃/CH₃OH mixtures become more reactive with increasing methanol addition. Addition of merely 1% (molar basis) of CH₃OH to NH₃ lowers the ignition temperature around 100 K at 40 bar in comparison to pure NH₃. The ignition delay is a complex function of fuel mixture and stoichiometry. For the 1% CH₃OH mixture, the leaner mixtures are more reactive, while the reverse trend is found for mixtures with 5%, 20% and pure CH₃OH. Analysis of the pressure profiles shows three distinct ignition modes for NH₃/CH₃OH mixtures, facilitated by the pre-ignition heat release from NH₃ consumption. A detailed mechanism for ignition of NH₃/CH₃OH fuel blends has been developed, capable of reproducing the ignition behavior of mixtures with reasonable accuracy. A subset for amine / methanol interactions was established, with rate constants for the key reaction between NH₂ and CH₃OH calculated from ab initio theory. A sensitivity analysis indicates that the critical reactions during the auto-ignition process vary with the CH₃OH mole fraction in the fuel. The ammonia chemistry, namely NH₂ + NO, NH₂ + NO₂ and NH₂ + HO₂, is dominant for the mixture with 1% CH₃OH, while the reactions related to CH₃OH and H₂O₂ are more important for the 20% CH₃OH mixture. The interaction between ammonia and methanol shows a more prominent effect on auto-ignition for mixtures with 5% CH₃OH in fuel as compared to those with 1% and 20% CH₃OH. According to the modeling results, methanol addition is found to enrich the O/H radical pool, consuming ammonia and promoting auto-ignition through different reaction pathways.