Measurement and modelling of the laminar burning velocity of methane-ammonia-air flames at high pressures using a reduced reaction mechanism

Ammonia and blends of ammonia with methane are gaining increased interest as fuels for gas turbine applications and hence optimized reduced reaction mechanisms for the fuels are required for the development of combustors. However, there is a scarcity of measured data on the laminar burning velocity of the fuels for optimizing and validating reaction mechanisms especially at high pressures. In this study, an extensive set of measurements of the unstretched laminar burning velocity and Markstein lengths of CH₄–NH₃-air flames at high pressures are reported for the first time and an optimized reduced reaction mechanism is proposed. The experiments were conducted in a constant volume chamber for various ammonia heat fractions in the fuel ranging from 0 to 0.30, equivalence ratios ranging from 0.7 to 1.3, and mixture pressures ranging from 0.10 to 0.50 MPa. The reduced reaction mechanism was developed from a detailed reaction mechanism for CH₄–NH₃-air flames by Okafor et al., and optimized against the present measurements and data in the literature. It is shown that the reduced mechanism models the unstretched laminar burning velocity of NH₃/air and CH₄–NH₃-air flames with high fidelity at all studied conditions. It also models satisfactorily NH₃, NO and CO concentration in CH₄–NH₃–O₂–N₂ oxidation in a laminar flow reactor. Furthermore, the reduced mechanism is demonstrated to predict NO, OH, and NH profiles in a premixed stagnation NH₃-air flame satisfactorily. The experimental measurements were also used to validate selected detailed reaction mechanisms. It was found that the significant over-prediction of NO production from NH₃ oxidation by GRI Mech 3.0 is primarily due to the influence of the reaction NH+H₂O[dbnd]HNO+H₂, which in fact may not be important in fuel NO chemistry.