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.