Chemical insights into the two-stage ignition behavior of NH3/H2 mixtures in an RCM

Abstract

This study investigated the two-stage ignition behavior of NH3/H2 mixtures using a rapid compression machine (RCM). The ignition delay times and first-stage ignition delay times of NH3/H2 blends were measured at temperatures ranging from 881 to 1127 K, pressures of 15 and 25 bar, and equivalence ratios (ϕ) of 1.0 and 1.5. Experimental results showed that H2 had a significant promotional effect on the ignition of NH3, and two-stage ignition behavior was observed in some test mixtures. Time-resolved species concentrations were recorded using the gas chromatography (GC) method during the single- and two-stage ignition process. Species evolution suggested that the consumption of NH3 and H2 separated during the two-stage ignition process. The oxidation of H2 primarily occurred at the first-stage ignition point, while NH3 oxidation occurred at the total ignition point. A kinetic model was developed to predict the two-stage ignition behavior of NH3/H2 and the species profiles. Model analysis showed that H2 played a key role in initiating the oxidation process and contributed to the early heat release. When H2 content was high (e.g. 50%), its oxidation led to the simultaneous total oxidation of NH3, resulting in single-stage ignition characteristics. However, when a small amount of H2 was present (e.g. 10%), its oxidation only partially consumed NH3, leading to the two-stage ignition behavior. Sensitivity analyses indicated that the co-oxidation of fuels during the first-stage ignition was primarily dominated by H2 oxidation chemistry, while NH3 oxidation became dominant during the following total ignition stage as H2 was completely consumed. Additional model simulations revealed that the ignition behavior of NH3/H2 mixtures is strongly influenced by temperature and pressure. A distinct threshold for temperature and pressure was identified, demarcating the transition between single-stage and two-stage ignition phenomena. Moreover, the fraction of H2 in the mixture had a significant impact on the ignition behavior, with higher fractions leading to increased intensity of the first-stage ignition and closer proximity to the total ignition point. However, when the H2 fraction exceeds a certain threshold, two-stage ignition behavior disappears.