The impact of hydrogen addition and OH concentration on NO emissions in high-pressure NH3/air combustion

Abstract

The addition of hydrogen to ammonia enhances combustion stability and widens the flame limits. However, it also brings the drawback of increased nitrogen oxide (NO) emissions. Therefore, this study employs a chemical kinetics simulation system to investigate the effects of OH concentration and hydrogen addition on NO emissions in a laminar premixed hydrogen/ammonia/air swirling flame at high pressures. The research reveals a distinct positive linear correlation between NO emissions and the peak mole fraction of OH radicals. Within the range of 1.0≤φ ≤ 1.1 and 0≤XH2≤0.5, as hydrogen content (0≤XH2≤0.5) increases, the promoting effect of max(OH) on NO emissions continuously intensifies. Pressure, on the other hand, has virtually no impact on their correlation.Furthermore,the analysis of production rates, sensitivity, and reaction pathways indicates that NH3 primarily transforms into NO through three pathways: NH3→NH2(NH)→HNO→NO, NH3→NH2→NH→NO, and NH3→NH2→NH→N→NO. The HNO intermediate pathway is the major route for NO generation, with OH radicals significantly promoting NO production via the NH→HNO intermediate pathway. This theoretically validates the correlation between max(OH) and NO emissions. Data analysis using MATLAB yields the relationship between max(OH) and NO emissions, providing a theoretical basis for the practical application of OH chemiluminescence detection. Additionally, NNH proves to be a crucial radical in ammonia oxidation chemistry and is involved in the NO formation process in reaction R69 NNH + ONH + NO. This study sheds light on the complex interplay between OH, hydrogen content, and NO emissions, providing valuable insights into combustion dynamics and emissions control.