Optimized global reaction mechanism for H2+NH3+N2 mixtures

Abstract

Hydrogen and ammonia are playing an increasingly vital role in combustion technologies. Recently, some detailed reaction mechanisms (DRMs) can predict reliable laminar burning velocities (LBVs). Meanwhile, despite their broad applicability in practical combustion engineering, global reaction mechanisms (GRMs) suitable for ammonia and its mixtures with hydrogen or nitrogen have yet to be reported. This study aims to address this gap by investigating new GRMs. Five DRMs were used to calculate the LBVs, and their average values were employed as a reference. First, a single-step GRM was developed for hydrogen (H2 + 0.5O2 → H2O), and another single-step GRM was developed for ammonia (NH3 + 0.75O2 → 0.5N2 + 1.5H2O). However, their combined GRM did not provide reasonable predictions of LBVs for the H2+NH3 mixtures. Therefore, a 4-step GRM was developed consisting of the single-step hydrogen and three additional reactions, i.e., an endothermic ammonia decomposition (NH3 + 0.5O2 → NO + 3H), an exothermic NO reduction (NO + 2H → 0.5N2 + H2O), and an exothermic H recombination (H + H → H2). In conclusion, this 4-step GRM and its revisions could predict reasonable LBVs, flame temperatures, and primary product compositions for mixtures of hydrogen, ammonia, and cracked ammonia gas over a wide range of temperatures (300–600 K) and pressures (1–5 atm).