Chemical mechanism of ammonia-methanol combustion and chemical reaction kinetics analysis for different methanol blends

Abstract

As a carbon-free fuel, ammonia is considered as an attractive alternative fuel for internal combustion engines. However, ammonia has the problems of high ignition temperature and NOx emissions while using in engines. Methanol as liquid fuel is easy to store and transport. Ammonia-methanol blended fuel is probably-one of the most appropriate methods to solve the problem. However, there are few studies of chemical mechanism for the combustion of ammonia/methanol dual fuel. Moreover, it is important to study ammonia-methanol combustion by chemical reaction path analysis. In this paper, a chemical reaction mechanism of ammonia-methanol blends with 60 species and 399 reactions was developed. This mechanism can accurately predict the laminar burning velocity (LBV) and ignition delay time (IDT) in a wide range of equivalent ratios (Φ) and methanol blends. By using this chemical reaction mechanism, the effects of methanol addition on ammonia combustion and emissions at different equivalent ratios were numerically studied. Results showed that the addition of methanol significantly improves the chemical reactivity of ammonia, and blending a small amount of methanol can greatly reduce the IDT. However, the accumulation of radical HO2 caused by ammonia production enhances the fuel NOx path of NH2 → H2NO, which, together with high temperature and oxygen enrichment, leads to a NOx peak of 12500 ppm at equivalent ratio of 0.8. However, with the increase of equivalent ratio, NOx emission will decrease, which is only 2500 ppm at equivalent ratio of 1.4. Due to the existence of cross reaction of NH2 + CH3OH = NH3 + CH2OH and NH2 + CH3OH = NH3 + CH3O between ammonia and methanol, NH2 is difficult to participate in the thermal DeNOx process in the premixed combustion process, resulting in high NOx emissions. In addition, it is also found that after equivalent ratio of 1.1, H will replace OH as the most important chain propagation carrier.