Uniqueness and similarity in flame propagation of pre-dissociated NH3 + air and NH3 + H2 + air mixtures: An experimental and modelling study

Ammonia (NH3) has attracted significant attention as a promising hydrogen carrier and a carbon-free alternative fuel. Partial dissociation could convert part of ammonia to H2 and N2 before injecting the fuel into a combustor, thus overcoming the low reactivity and high NOx emission problems during the NH3 combustion. The pre -dissociated NH3 + air mixture has unburnt species NH3, H2, O2, and N2, the same as more widely investi-gated NH3 + H2 + air flames, while similarities or differences between these two types of flames have not yet been investigated. In the present work, the laminar burning velocities of pre-dissociated NH3 + air flames at 1 atm and an initial temperature of 298 K have been measured and compared to the scarce data from the literature. Experiments were carried out using the heat flux method at varied dissociation ratio gamma and equivalence ratio phi. Kinetic simulations were also performed using six recently published or updated mechanisms, while none of the tested mechanisms can accurately reproduce the present results for the pre-dissociated NH3 + air flames over the whole range of the covered conditions, even for those predicting well the NH3 + H2 + air flames. To understand this deficiency, flame temperatures for the two fuel systems were examined, as well as in-depth sensitivity an-alyses were carried out. Similar conditions between the pre-dissociated NH3 + air and the NH3 + H2 + air flames were found, and a new approach to identifying inconsistent experimental data obtained using the same exper-imental setup was also suggested and discussed.

Automated summary

Ammonia is a promising hydrogen carrier and alternative fuel. Partial dissociation of ammonia can overcome the reactivity and emission problems during combustion. The laminar burning velocities of pre-dissociated ammonia + air flames were measured and compared to literature data. However, existing mechanisms cannot accurately predict the experimental results.