Experimental and kinetic modelling investigation on NO, CO and NH3 emissions from NH3/CH4/air premixed flames

Ammonia (NH3) is regarded as one of the most viable alternatives to produce carbon-free energy. However, its low flammability and high NOx emissions associated with its combustion inhibit the implementation of pure NH3 as a fuel. Dual-fuel approaches with NH3 and more reactive fuels have been proposed; however, information on pollutant emissions is still scarce. The present work focuses on quantifying the gaseous pollutant emissions from the combustion of mixtures of NH3 and CH4 as a function of the equivalence ratio and amount of NH3 in the fuel mixture. A premixed laminar burner was fired with various mixtures of NH3/CH4 in air with equivalence ratios of 0.8, 0.9 and 1, and a fixed thermal input of 300 W. The NH3 molar fraction in the fuel mixture was varied from 0 (pure CH4) up to 0.7. Gas temperatures along the burner axis and emissions of NOx, CO and NH3 were measured for all conditions. The results were compared with kinetic simulations using recent chemical kinetic mechanism models. The experimental results showed that the NOx concentration initially increases as the fraction of NH3 in the fuel mixture rises up to 0.5, decreasing afterwards. Furthermore, NOx emissions decrease as the equivalence ratio is reduced towards fuel-lean conditions. CO and NH3 emissions are fairly low indicating complete combustion of CH4 and NH3. The chemical kinetic mechanisms considered showed fair agreement with the experimental trends, with the NOx and CO emissions being over-predicted for most conditions.