Numerical study of laminar premixed ammonia/air flames enriched with nitrogen oxides

Abstract

The interactions between NH₃ and nitrogen oxides (including N₂O and NO_x) offer unique insights into ammonia’s combustion chemistry via active nitrogen–nitrogen coupling mechanisms. An in-depth understanding of these interactions is essential for the development of kinetic models, emission control, and strategies for enhancing combustion via modified oxidizers. This study presents a comprehensive investigation of laminar premixed ammonia/air flames enriched with N₂O or NO_xvia one-dimensional simulations. The results demonstrate that all three nitrogen oxide species significantly enhance flame propagation, with NO showing the strongest enhancement at low concentrations, followed by NO₂ and N₂O. However, with increasing NO addition, the flame speed attains a maximum at $x_a\approx 0.6$ and then gradually declines with further enrichment. Such non-monotonic trends are not seen with N₂O or NO₂ addition. A decoupling analysis is further employed to isolate oxy-enrichment, thermal, chemical, and transport effects. For all three cases, the thermal effect is the dominant contributor. Interestingly, NO exhibits dual chemical behavior: it promotes combustion at low concentrations but inhibits it at high levels. The key reactions are also identified with the rate of progress and sensitivity analyses. Meanwhile, the correlations between the simulated laminar flame speeds and the theoretical expressions are examined over different addition ratios. Besides, pollutant emissions are also examined, showing increased NO levels with N₂O or NO_x addition. The chemical effects have a more pronounced impact on NO emissions than on flame speed. At the end, the flammability limits are determined where the radiative heat losses are modeled via the optically thin model. The results indicated a significant widening of the flammable range, with N₂O or NO_x addition. These findings provide critical insights into nitrogen–nitrogen coupling mechanisms and offer pathways for enhanced ammonia combustion technologies in carbon-free energy systems.

Novelty and significance

Understanding the dual role of nitrogen oxides as both pollutants and active participants in ammonia combustion remains a critical gap in low-carbon energy research. This study addresses this gap by offering a systematic quantification of oxy-enrichment, thermal, chemical, and transport effects induced by N₂O, and NO_x addition, in modifying flame speed and pollutant emissions of ammonia/air flames. It reveals a non-monotonic response of flame speed to NO addition ratio, which is not seen for N₂O or NO₂ additions. Another novelty lies in the quantification of correlations between simulated flame speeds and theoretical expressions, providing insight into the validity and limitations of simplified predictive models in ammonia systems. Moreover, this work presents the first estimations of flammability limit for NH₃/N₂O/NO_x mixtures at both 1 atm and 5 atm. Together, these contributions advance fundamental understanding of NH₃-N₂O and NH₃-NO_x interactions, supporting the development of ammonia-fueled systems.