Measurement of the laminar burning velocity and kinetics study of the importance of the hydrogen recovery mechanism of ammonia/hydrogen/air premixed flames

The application of ammonia (NH3) blended with hydrogen (H2) as a fuel in combustion systems is a practical approach to decarbonise the energy sector, and the combustion of the fuel at rich conditions is relevant in emissions control through rich-lean combustion. However, the chemistry of rich NH3/H2 flames at high pressure, and the interaction between NH3 and H2 still need to be clarified. Therefore, the present study focuses on the chemical kinetics of NH3/H2/air flames at rich conditions and elevated pressures. To validate chemical kinetics in the literature, the laminar burning velocity of NH3/H2/Air premixed flames were measured at 0.1 and 0.5 MPa and equivalence ratios up to 1.8. The results show that the seven kinetics mechanisms studied could not satisfactorily predict the measurements at fuel-rich conditions and elevated pressure. The kinetics mechanism by Han et al. was optimized, leading to a new detailed kinetics, which can be reduced to 26 species and 119 reactions and satisfactorily predicts the present measurements and those in the literature. Analysis of the chemistry of NH3/H2 flames using the new mechanism shows NH3 and H2 kinetics are strongly coupled through a H2 decomposition/recovery mechanism, here named H2 recovery mechanism, which is important in modelling the burning velocity of the flame at fuel-rich conditions. The burned gas Markstein length was also extracted from the measured flame speed and its behaviour was studied using theoretical correlations.