Characteristics of NH3/H2/air flames in a combustor fired by a swirl and bluff-body stabilized burner

Abstract

Combustion of ammonia (NH3) presents a number of challenges due to its low reactivity, poor flame stability, very low flame speed and high NOx emissions in a range of burning conditions. Therefore, its application in combustion systems requires device modifications, either by adapting existing systems or developing new ones. In this work, a novel combustor fired by a swirl and bluff-body stabilized burner was designed and tested. Initially, flame stability diagrams were established for NH3/H2 fuel mixtures. Then, emissions of NOx and unburnt NH3 were measured for a range of equivalence ratios and NH3 mole fractions in the fuel mixture. In addition, in-flame data for O2 and NOx concentrations and gas temperature were measured for five combustor operating conditions. A parallel mathematical modelling exercise was performed to obtain a qualitative knowledge of the burner aerodynamics and assist the interpretation of the experimental results. The predicted aerodynamic pattern suggests the establishment of an extended central recirculation zone that allows long residence times in the near burner region (NBR) that are critical to ensure stable combustion of the NH3/H2/air mixtures. The measurements revealed that the burner presents a wide range of stable flames for xNH3 between 0.6 and 0.8, showing the positive effects of adding relatively small amounts of H2 to the fuel mixture. The in-flame data show that the flame initiates close to the burner inlet and presents two main reaction zones. NOx is formed in the NBR and is consumed afterwards, through selective non-catalytic reduction reactions. Overall, the present laboratory combustor performs well in terms of flame stability and both NOx and NH3 emissions, and it is foreseen that industrial combustors based on the present design would be able to ensure flame stability and NOx and NH3 emissions control with simple complementary systems.