Study on the Ignition Characteristics of Ammonia Blended with C1–C4 Small-Molecule Alkanes

With increasingly stringent greenhouse gas emission regulations, carbon emissions from marine engines have become a major concern, driving the shipping industry to actively explore efficient and clean alternative fuels. Among the various candidates, ammonia has attracted considerable attention in recent years due to its carbon-free nature and potential as a high-quality clean fuel. However, its practical application in marine engines is constrained by several inherent drawbacks, including a high auto-ignition temperature, low flame propagation speed, and low calorific value. Blending ammonia with natural gas has been demonstrated as an effective strategy to enhance its ignition performance. In this study, the ignition characteristics of NH3/C1–C4 alkane mixed fuels were systematically investigated using numerical simulations. Rate of production (ROP) analysis, reaction pathway analysis, and other kinetic evaluation methods were employed to elucidate the underlying ignition mechanisms. The results reveal that blending NH3 with C1–C4 alkanes significantly shortens the ignition delay time. When XCH ≥ 30%, at high initial temperatures, the ignition-promoting effect is most pronounced for NH3/C2H6 mixtures. In contrast, under low temperature conditions, ignition performance progressively improves with increasing carbon chain length of the blended alkane fuel. The ignition delay time across different operating conditions is primarily governed by highly reactive radicals, including O, H, and OH. Elevating the initial temperature, pressure, and blending ratio promotes the earlier formation of these key radicals and increases their production rates. ROP analysis of OH radicals indicates that reaction R10 (O2 + H ⇌ OH + O) contributes most significantly to OH generation. Furthermore, reaction pathway analysis of NH3 shows that at lower initial temperatures, NH3 dehydrogenation is dominated by reactions with OH radicals. At higher temperatures, a greater fraction of NH3 participates in NO reduction reactions, thereby decreasing the proportion of NH3 involved in dehydrogenation pathways.