Effects of Reynolds number and ammonia fraction on combustion characteristics of premixed ammonia-hydrogen-air swirling flames

Abstract

Several studies have investigated the effects of equivalence ratio and blending ratio in constant power cases for ammonia-based fuels as ammonia is being investigated extensively as an alternative zero-carbon fuel in recent years. However, in many fields, such as power generation, it is common to operate practical equipment at constant Reynolds number (Re) conditions. To that extent, this study investigates the impact of Re (4000 ≤ Re ≤ 7000), equivalence ratio (0.6 ≤ Φ ≤ 1.45), and ammonia volume ratio (0.55 ≤ XNH3 ≤ 0.9) on the combustion characteristics of NH3/H2 swirling flames. To supplement the experimental work, a previously developed Chemical Reactor Network (CRN) has been used to identify the reaction pathways affecting NOx emissions through reaction analysis. Sampled gas analysis (NO, NO2, N2O, NH3) at the exhaust was reported for the first time at a wide range of equivalence ratios and XNH3 at different Re. At constant Re = 6000, NO and NO2 peaked at around Φ = 0.8 – 0.9, while N2O became substantial at Φ ≤ 0.8 and unburnt ammonia became significant at Φ ≥ 1.05. The chemiluminescence images suggested that at Re = 6000, as XNH3 or Φ increases, the NH2* region expands, and reactions occur further downstream. Furthermore, with decreasing XNH3 at Re = 6000 and Φ = 0.9, the system becomes partially abundant in H2 and H but deficient in O2, suppressing the generation of O and OH radicals, and ultimately also suppressing HNO generation. This leads to the N/NH system reactions becoming dominant and increased NO consumption. NO emissions increased steadily with increasing Re at Φ = 0.8, while at stoichiometry, NO emissions remained somewhat unchanged with changing Re. Findings from this study will aid further in the development of carbon-free zero-emissions systems.