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

Abstract

The oxidation of CH4/C2H4/NH3/NO/NO2 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/NO2 ratio (1, 4) and pressure (0.04 and 1.0 atm) on the interaction chemistry of NH3/NOx with CH4 and C2H4. 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 CH4/C2H4, changing NO/NO2 ratio and pressure. Besides, the promotion of DeNOx reactions and narrower temperature windows of NO reduction are unexpectedly observed in the presence of CH4/C2H4. To interpret the experimental observations, a detailed kinetic model was developed by integrating hydrocarbons/NH3/NOx interaction chemistry. Rate of production and sensitivity analyses indicate that the active radical pool is enriched and additional chain-branching pathways regarding NO/NO2 interconversion are activated with the addition of hydrocarbons. In the presence of both CH4 and C2H4, reaction C2H3 + O2 = CH2CHO + O was demonstrated as a crucial reaction that drives the reactivity of CH4/C2H4/NH3/NO/NO2 mixture. This is explained by the production of CH2CHO, whose dissociation generates CH2O and ultimately leads to the abundant production of active OH via the reaction sequence CH2O → HCO → HO2 → OH. The conversion kinetics of hydrocarbons, NO and NH3 under different NO/NO2 ratios and pressure, as well as the formation kinetics of oxygenated and nitrogenous intermediates was also analyzed in this work.