Chemical kinetic analysis on influence of hydrogen enrichment on the combustion characteristics of ammonia air using newly proposed reaction model

Abstract

Summary In recent years, ammonia has shown great potential for future green fuel. However, ammonia combustion is a challenging task due to its narrow flammability limit, low flame speed, and slow kinetics compared with conventional hydrocarbon fuels. The blending of ammonia with higher reactive fuel such as hydrogen seems a novel alternative to overcome these difficulties. So, a comprehensive study of NH3/H2/air combustion characteristics is necessary. In the literature, numerous ammonia reaction models can be found, but most of them failed to demonstrate good combustion characteristics with NH3/H2/air mixtures. A new NH3/H2/air reaction model is suggested or proposed in the present work by referring to the previously available literature. The newly proposed reaction model consists of 32 species and 259 reactions. Model validation for laminar flame speed and ignition delay time at different operating conditions are performed in this study using the newly proposed reaction mechanism. Also, an extensive chemical kinetic modeling for heat release rate, flame speed and OH sensitivity analysis, reaction pathway analysis, and NOx emission characteristics of NH3/H2/air mixture is studied. The results show that the proposed reaction model closely follows the experimental trends, and lesser inconsistency is observed than the other reaction models compared in this study. All referenced reaction models used in the present work for comparison shows large discrepancies with experimental results of NH3/H2/air combustion, especially more discrepancies are observed at higher hydrogen enrichment conditions. Hydrogen enrichment positively affects the heat release rate intensity and flame speed of NH3/air mixture. Through the addition of hydrogen, the burning velocity of ammonia can be increased up to the level of hydrocarbon fuels, and the combination of ammonia and hydrogen blend can be used as an effective industrial fuel.

Effect of Intermediate Radicals on NOx and De‐NOx Characteristics of NH3/H2/Air Flames at High Pressure

“…The relation of O/H radical to NO x production in ammonia flames have been widely discussed in various reports 5, 8, 10, 17, 18. However, less literature is published about the role of nitrogen‐based intermediate radicals such as NH 2 , HNO, and NH on NO x formation or consumption in NH 3 /H 2 /air flames at higher pressure.…”

“…The total NO x consists of NO, NO 2 , and N 2 O. Previous studies [5,8,15] suggest that in ammonia flames the NO significantly contributes to the total NO x production compared to N 2 O and NO 2 . Therefore, the NO 2 and N 2 O analysis can be neglected.…”

“…Several other studies [5,7,8,11] also demonstrated that the recombination reactions of N 2 H x (x = 2, 3, and 4) are crucial for understanding NH 3 oxidation chemistry at low-intermediate temperatures and high pressure. In combustion, nitrogen oxides are formed as either a result of oxidation of chemically fuel-bound nitrogen or in a high-temperature environment by fixation of nitrogen in the combustion air.…”

“…Kobayashi et al [7] compared the NO emission characteristics of CH 4 /air and NH 3 /air flames at peak heat release rate and highlighted that the ammonia/air flames produce higher NO compared to methane/air. Previous studies [8][9][10] reported that the hydrogen addition to NH 3 /air flames increases NO x production and positively affects the heat release rate, adiabatic flame temperature, and laminar burning velocity of the premixed mixture. A recent study carried out by Li et al [10] and Singh et al [5,8] concluded that NO production in ammonia combustion is strongly influenced by fuel-bound nitrogen, and intermediate radicals such as OH, HNO, NH 2 , and NNH play a significant role in NO production and reduction chemistry.…”

Understanding of MILD Combustion Characteristics of NH3 Air Flames in N2 And H2O Steam Diluted Environment at Atmospheric Pressure

“…The world is under energy transition due to thecontinuous rise in global energy consumption, fossil fuel depletion, and growing concerns over environmental problems. According to studies, the combustion of fossil fuels supplies more than 80 % of world energy 1 . Hence, the existing combustion systems need an extensive transformation imperative for sustainable energy supply and emissions reduction.…”