Two-stage ignition in a dual-oxidizer system: Experimental and kinetic study of H2–N2O–O2 mixtures in an RCM

Nitrous oxide (N 2 O) is a potent greenhouse gas, and its combustion kinetic mechanisms are vital for developing carbon-neutral heat engines. Autoignition characteristics of H 2 –N 2 O and H 2 –N 2 O–O 2 mixtures were investigated using an RCM at 30 bar and 906–1036 K, with O 2 mole fractions of 0%–1.0% and equivalence ratios of 1.0 and 1.5. Two-stage ignition was observed under specific temperature and O 2 concentration conditions. The ignition behavior shifted from single-stage to two-stage and back to single-stage as O 2 content increased. Time-resolved species concentration profiles measured by a gas chromatography (GC) fast sampling system showed a notable correlation between O 2 consumption and the ignition behavior of H 2 –N 2 O–O 2 mixtures. A modified kinetic model incorporating an HNNO sub-mechanism was developed, which satisfactorily predicted the ignition delay times and species evolution under all tested conditions. Kinetic analyses revealed that the first-stage ignition of H 2 –N 2 O–O 2 mixtures is mainly controlled by H 2 –O 2 chemistry dominated by HO 2 /H 2 O 2 pathways, while the subsequent ignition process is governed by H 2 –N 2 O chemistry. The HNNO pathway plays a critical role in the nitrogen chemistry of the system: during the first-stage ignition period, NO is primarily produced via the HNNO pathway reaction tHNNO + H = NH 2 + NO. The produced NO further accelerates the conversion of HO 2 to OH radicals and thus promotes the first-stage ignition. Further analysis uncovered that the two-stage ignition of H 2 –N 2 O–O 2 mixtures only occurs above 20 bar, below 1050 K and O 2 mole fraction ≤ 0.375%. Under these conditions, O 2 is primarily consumed via H + O 2 (+M) = HO 2 (+M) to form HO 2 radicals. Since the H 2 –N 2 O system lacks effective reaction pathways to directly react with HO 2 , O 2 consumption cannot simultaneously drive the H 2 –N 2 O reaction. Consequently, the ignition controlled by H 2 –O 2 chemistry is kinetically relatively independent in the H 2 –N 2 O–O 2 system. At low O 2 concentrations, the ignition driven by the H 2 –O 2 reactions is insufficient to trigger global ignition of the H 2 –N 2 O–O 2 mixture, resulting in two-stage ignition.