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

This study investigated the two-stage ignition behavior of NH 3 /H 2 mixtures using a rapid compression machine (RCM). The ignition delay times and first-stage ignition delay times of NH 3 /H 2 blends were measured at temperatures ranging from 881 to 1127 K, pressures of 15 and 25 bar, and equivalence ratios ( & phi;) of 1.0 and 1.5. Experimental results showed that H 2 had a significant promotional effect on the ignition of NH 3 , 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 NH 3 and H 2 separated during the two-stage ignition process. The oxidation of H 2 primarily occurred at the first-stage ignition point, while NH 3 oxidation occurred at the total ignition point. A kinetic model was developed to predict the twostage ignition behavior of NH 3 /H 2 and the species profiles. Model analysis showed that H 2 played a key role in initiating the oxidation process and contributed to the early heat release. When H 2 content was high (e.g. 50%), its oxidation led to the simultaneous total oxidation of NH 3 , resulting in single-stage ignition characteristics. However, when a small amount of H 2 was present (e.g. 10%), its oxidation only partially consumed NH 3 , 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 H 2 oxidation chemistry, while NH 3 oxidation became dominant during the following total ignition stage as H 2 was completely consumed. Additional model simulations revealed that the ignition behavior of NH 3 /H 2 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 H 2 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 H 2 fraction exceeds a certain threshold, two-stage ignition behavior disappears.& COPY; 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.