Experimental and Kinetic Investigation of Stoichiometric to Rich NH3/H2/Air Flames in a Swirl and Bluff-Body Stabilized Burner

Ammonia (NH3) is an inorganic substance considered as a promising fuel for power sector decarbonization. As a result of the absence of carbon in its structure, ammonia is capable of producing energy with zero CO2 emissions when burned. However, the combustion of NH3 presents several challenges as a result of its low reactivity and low flame speed as well as the formation of large quantities of nitrogen oxides (NOx) and frequent ammonia slip. A suggested solution for gas turbines is the use of rich-to-lean approaches, with a fuel-rich first-stage combustion, which mitigates NOx formation, followed by a lean phase for the oxidation of the remaining reactants, improving combustion efficiency. To help assess this concept, the present work investigates experimentally and computationally the combustion of ammonia/air mixtures, enriched by hydrogen (H-2) for enhanced burning characteristics, in a swirl and bluff-body stabilized burner, at stoichiometric to fuel-rich conditions. Stability tests were performed for a fixed thermal input (2.8 kW), and flames of fuel/air equivalence ratios of 1.0, 1.1, and 1.2, with molar fractions of ammonia in fuel of 0.7 and 0.8, were studied. Temperature profiles along the combustor axis were measured, and flue gas measurements for NOx emissions and unburned NH3 and H-2 concentrations were performed for the six studied flames. Computational simulations were performed using a chemical reactor network coupled with recent kinetic mechanisms to compare species trends and further understand the NOx formation and NH3 conversion into hydrogen, through rate of production analyses. It was found that the present laboratory combustor performed well in terms of flame stability, also generating low levels of NOx emissions in all fuel-rich conditions. H-2 was detected in high concentrations in the flue gas, partially originated from ammonia dissociation, and is followed by high unburned ammonia emissions. Both H-2 and NH3 emissions increase with the equivalence ratio. A secondary, spontaneous diffusion flame was observed above the combustor, proving that the flue gas may subsequently be burned. Higher fractions of hydrogen in the fuel generate more unburned ammonia but also higher H-2 concentrations in the flue gas. The predictions based on a reactor network model coupled with the evaluated kinetic mechanisms presented good agreement with the experimentally observed species trends and fair agreement with species values.