Exploring NH3 and NOx Interaction Chemistry With CH4 and C2H4 at Moderate Temperatures and Various Pressures

The oxidation of CH 4 /C 2 H 4 /NH 3 /NO/NO 2 gas mixtures was studied aiming to explore the homogenous chemistry of exhaust gas from lean-operated natural gas engine. With respect to this goal, experiments were carried out with a laminar flow reactor under engine-relevant (diluted and lean) conditions over the temperature range of 600–1400 K. Four gas mixtures were designed to evaluate the effects of NO/NO 2 ratio (1, 4) and pressure (0.04 and 1.0 atm) on the interaction chemistry of NH 3 /NO x with CH 4 and C 2 H 4 . By using synchrotron vacuum ultraviolet photoionization mass spectrometry, fingerprint products for revealing interaction chemistry were identified and quantified, e.g., nitrogenous and oxygenated intermediates. The experimental results show that the NO concentrations are significantly affected by adding CH 4 /C 2 H 4 , changing NO/NO 2 ratio and pressure. Besides, the promotion of DeNO x reactions and narrower temperature windows of NO reduction are unexpectedly observed in the presence of CH 4 /C 2 H 4 . To interpret the experimental observations, a detailed kinetic model was developed by integrating hydrocarbons/NH 3 /NO x interaction chemistry. Rate of production and sensitivity analyses indicate that the active radical pool is enriched and additional chain-branching pathways regarding NO/NO 2 interconversion are activated with the addition of hydrocarbons. In the presence of both CH 4 and C 2 H 4 , reaction C 2 H 3 + O 2 = CH 2 CHO + O was demonstrated as a crucial reaction that drives the reactivity of CH 4 /C 2 H 4 /NH 3 /NO/NO 2 mixture. This is explained by the production of CH 2 CHO, whose dissociation generates CH 2 O and ultimately leads to the abundant production of active OH via the reaction sequence CH 2 O → HCO → HO 2 → OH. The conversion kinetics of hydrocarbons, NO and NH 3 under different NO/NO 2 ratios and pressure, as well as the formation kinetics of oxygenated and nitrogenous intermediates was also analyzed in this work.