On the extinction and burning limits of stretched premixed ammonia flames at elevated pressures

Abstract

In this study, one-dimensional detailed simulations of stretched premixed counterflow flames are conducted to investigate the flame extinction, bifurcations, and burning limits of NH₃/air and NH₃/H₂/air mixtures at elevated pressures up to 25 atm. For the NH₃/air mixtures, the incorporation of radiative heat loss results in a left weak flame branch at low strain rates, which exists only within a narrow strain rate range. A regime diagram is proposed to illustrate sustainable strain rate ranges of normal and weak flames at different equivalence ratios. Kinetic analyses show that the ammonia oxidation in the weak flames is governed by H₂NO at the lean side and N₂H_x at the rich side. Nonetheless, for the normal flames, the H₂NO, NH_i, and N₂H_x pathways are significant under lean, near stoichiometric, and rich conditions, respectively. Furthermore, the influence of hydrogen additions on ammonia flames is investigated. The results show that hydrogen addition, e.g., $\ge$ 0.1 by volume, leads to the formation of a stable right weak flame branch, with the H₂NO sub-chemistry being the dominant oxidization mechanism. Then, the flame bifurcation and extinction behaviors at different equivalence ratios are summarized in a regime diagram with five critical strain rates. It is shown that the low-temperature H₂NO route extends the rich burning limit from 3.45 to 5.68 for the NH₃/H₂/air mixture at a hydrogen mole fraction of 0.1. In the end, the burning limits are determined as a function of hydrogen molar fraction and pressure, and the hydrogen addition significantly expands the burning limits both on the lean and rich side. These findings provide valuable insights into the flammability of ammonia/hydrogen flames and the underlying oxidation mechanisms subject to elevated pressure conditions.

Novelty and Significance Statement

This study fills a noticeable gap in the literature by providing a comprehensive regime diagram of extinction and burning limits for NH₃/air and NH₃/H₂/air flames under varying mixture composition conditions. A key novelty lies in exploring NH₃/H₂ combustion behaviors under elevated pressures up to 25 atm, revealing distinct flame bifurcations and weak flame branches induced by radiative heat loss—phenomena not captured for NH₃/H₂ mixtures previously. The work also highlights those most relevant ammonia oxidation paths under different temperature and composition conditions, which is beneficial for future mechanism development. It further clarifies the influence of hydrogen enrichment and pressure on the burning limits and discusses their relationship with fundamental flammability limits derived from unstretched planar flames, which were poorly explored previously. While the simulation framework dates back to the 90s, its application to ammonia-based flames at high pressure remains novel. Therefore, these findings offer meaningful contributions to the development of ammonia-based combustion systems, supporting broader decarbonization efforts.