On the key kinetic interactions between NOx and unsaturated hydrocarbons: H-atom abstraction from C3-C7 alkynes and dienes by NO2

TL;DR

This study provides detailed quantum chemistry calculations of NO2 reacting with C3-C7 alkynes and dienes, revealing their significant role in combustion chemistry and improving predictive models for autoignition.

Contribution

It offers the first comprehensive quantum chemical analysis of H atom abstraction by NO2 from alkynes and dienes, including rate coefficients and branching ratios, enhancing combustion models.

Findings

Updated reaction rate parameters increase model reactivity predictions.

H atom abstraction by NO2 dominates over OH in fuel consumption.

Reaction pathways significantly influence autoignition characteristics.

Abstract

An adequate understanding of NOx interacting chemistry is a prerequisite for a smoother transition to carbon lean and carbon free fuels such as ammonia and hydrogen. In this regard, this study presents a comprehensive study on the H atom abstraction by NO2 from C3 to C7 alkynes and dienes forming 3 HNO2 isomers (i.e., TRANS HONO, HNO2, and CIS HONO), encompassing 8 hydrocarbons and 24 reactions. Through a combination of high level quantum chemistry computation, the rate coefficients for all studied reactions, over a temperature range from 298 to 2000 K, are computed based on Transition State Theory using the Master Equation System Solver program with considering unsymmetric tunneling corrections. Comprehensive analysis of branching ratios elucidates the diversity and similarities between different species, different HNO2 isomers, and different abstraction sites. Incorporating the calculated rate parameters into a recent chemistry model reveals the significant influences of this type of reaction on model performance, where the updated model is consistently more reactive for all the alkynes and dienes studied in predicting autoignition characteristics. Sensitivity and flux analyses are further conducted, through which the importance of H atom abstractions by NO2 is highlighted. With the updated rate parameters, the branching ratios in fuel consumption clearly shifts towards H atom abstractions by NO2 while away from H atom abstractions by OH. The obtained results emphasize the need for adequately representing these kinetics in new alkyne and diene chemistry models to be developed by using the rate parameters determined in this study, and call for future efforts to experimentally investigate NO2 blending effects on alkynes and dienes.