High-temperature auto-ignition characteristics of NH3-H2-CH4

To extend the application range of the carbon -free fuel ammonia (NH3), the ignition delay time (IDT) of NH3/H2/ CH4 ternary fuel at high temperatures was measured using the reflected shock waves and the luminescence characteristics of fuel excited hydroxyl (OH*) on a shock tube (ST) test platform. Experimental data and available literature were used to comprehensively validate the IDT for eight models. Experimental investigations, combined with simulation methods, were used to compare and study the ignition characteristics of various fuel/air mixtures, including NH3, CH4, NH3/CH4, NH3/H2, NH3/H2/CH4 at 80 % Ar dilution, T = 1200-2200 K, p = 0.14, 0.5 MPa, and phi = 1.0. The effects of the activator ratio (H2/CH4 = 1:4, 1:1, 4:1) and the proportion of mixed activator in the fuel ((H2 + CH4)/fuel = 10-50 %) on the IDT of the NH3/H2/CH4 ternary fuel mixture were focused on. The results showed that under the same (H2 + CH4)/fuel conditions, the IDT curves (Arrhenius relationship curve between IDT and 1/T) of each H2/CH4 ratio converge around a point at higher temperatures and exhibit a "high temperature convergence" shape. In addition, increasing the (H2 + CH4)/fuel ratio (with constant H2/CH4) leads to a nonlinear shortening of the IDT. The curves are approximatively parallel. Therefore, the IDT range can be controlled within different temperature ranges by adjusting the proportions of NH3, H2, and CH4 in the mixture. Under experimental conditions, N70H6C24 (70 % NH3, 6 % H2, 24 % CH4) exhibits an IDT curve most similar to pure CH4 and is a feasible alternative fuel for CH4. Finally, the reason for the difference between the two kinetic models in predicting results such as IDT and negative pressure coefficient (NPC) were thoroughly analyzed. The ignition mechanism of multi -component mixed fuels was elucidated using various methods, including species time history analysis, reaction pathway analysis, sensitivity analysis, and the rate of production (ROP) analysis of fuel and small molecule radicals.