Unraveling Pressure Effects in Laminar Flame Propagation of Ammonia: A Comparative Study with Hydrogen, Methane, and Ammonia/Hydrogen

As a promising zero-carbon fuel, ammonia (NH3) has attracted great attention in combustion research. Understanding the pressure effects in the laminar flame propagation of NH3 is crucial for its practical applications in gas turbines, internal combustion engines, boilers, and industrial furnaces. In combination with new measurements in a high-pressure constant-volume cylindrical combustion vessel and experimental data in the literature, the pressure effects in the laminar flame propagation of NH3 were explored in this work and compared to those of hydrogen (H-2), methane (CH4), and NH3/H-2. A kinetic model for the combustion of NH3, H-2, CH4, and NH3/H-2 was developed to simulate the laminar burning velocities of these fuels and reproduce the pressure effects in their laminar flame propagation. The pressure-dependent coefficients of laminar burning velocities of fuels are found in the general order of H-2 < NH3 < CH4 similar to NH3/H-2 under the investigated conditions, which reveals the reverse trend of the global reaction order in their combustion. Modeling analysis was performed to provide mechanistic explanations to the order of observed pressure effects. It is concluded that, in the laminar flame propagation of NH3 and H-2, the most important pressure-dependent reaction is H + O-2 (+M) = HO2 (+M), while a chain-termination reaction NH2 + HO2 = NH3 + O-2 can convert HO2 to O-2 in the laminar flame propagation of NH3 and leads to the enhanced pressure effects compared to that of H-2. In the laminar flame propagation of CH4, both H + O-2 (+M) = HO2 (+M) and CH3 + H (+M) = CH4 (+M) contribute to the pressure effects, which explains its greater pressure effects than those of H-2 and NH3, especially under rich conditions. In the laminar flame propagation of NH3/H-2, a synergistic effect between NH3 and H-2 occurs at the reaction NH2 + HO2 = NH3 + O-2 as a result of the simultaneously abundant production of NH2 and HO2, which further enhances the role of H + O-2 (+M) = HO2 (+M) in the laminar flame propagation of NH3/H-2. This explains the interesting enhancement in its pressure effects, which can even become comparable to those of CH4.