Nitromethane as a nitric oxide precursor for studying high-temperature interactions between ammonia and nitric oxide in a shock tube

Abstract

Ammonia (NH3) is a promising carbon-free fuel for carbon neutrality that has recently received increasing attention. However, NOx emissions are one of the main problems for NH3 or NH3-blended combustion. This study used nitromethane (CH3NO2) as a nitric oxide precursor to study NH3/NO interactions at high temperatures. The temperature, CO, CO2, NO, and NH3 time-histories were measured simultaneously during the pyrolysis and oxidation (equivalence ratios ϕ = 2.0 and 1.0) of CH3NO2 and CH3NO2/NH3 mixtures behind reflected shock waves using mid-infrared laser absorption spectroscopy to analyze the NH3/NO interactions. The measurements were used to validate and evaluate seven kinetic models which gave very different predictions for the different mixtures, among which the Glarborg et al. (2018) and Shang et al. (2020) models agreed best with the temperature, NO, and NH3 measurements. The measured CO and CO2 time-histories are shown in the Supplementary Material. Both mechanisms accurately capture the temperature plateau while the main difference is the temperature increase during oxidation. The NO formation rate and plateau concentrations during CH3NO2 pyrolysis are better predicted by the Shang model than by the Glarborg model. Both mechanisms capture the NO formation rate for CH3NO2 oxidation with ϕ = 2.0 but underestimate it for ϕ = 1.0. With NH3, the Glarborg model better predicts the NO removal during CH3NO2/NH3 pyrolysis. However, both mechanisms underestimate the NO formation rate and maximum but overestimate the NO removal rate and NH3 consumption for CH3NO2/NH3 oxidation. Rate-of-production and sensitivity analyses show that: for the NO formation, CH3 + NO2 = CH3O + NO and NO2 + H = NO + OH are the primary pathways and CH3NO2 (+ M) = CH3 + NO2 (+ M) is the most sensitive reaction. NH3 + OH = NH2 + H2O contributes the most to the NH3 consumption during CH3NO2/NH3 pyrolysis, while NH3 + H = NH2 + H2 and NH3 + O = NH2 + OH are increasingly important at oxidation condition. NH2 + NO = N2 + H2O and NH2 + NO = NNH + OH are the most important reactions for the NO reduction with NH3 existing. The rate constants of some key reactions were revised to improve the predictions.