Effects of Premixed and Diffusion Conditions on the Iso-Octane/ammonia and Iso-Octane/ethanol Flames

The combustion engines fueled by liquid hydrocarbon fuel significantly release emissions such as CO, CO₂, and soot. Renewable fuels such as zero-carbon fuel and biofuel mitigate these emissions of IC engines when used with liquid hydrocarbon fuel. Previously, ammonia and ethanol were used in IC engines and also studied fundamentally, but these fuels were tested separately. In the current study, a comparative effect of these renewable fuels on the emission characteristics of hydrocarbon fuel was made on premixed and diffusion flame conditions. A kinetic mechanism was prepared by adding the sub-mechanisms of ammonia combustion, soot precursors formation and nitrogen oxide emissions to the detailed mechanism of iso-octane flame. The combustion conditions of premixed flow flame, counterflow flame and coflow flame were adopted. The Arrhenius model with soot surface reaction pathways was used in the premixed flow flame and counterflow flame analysis. The Moss-Brooke model with soot surface reaction pathways was added in OpenFOAM Solver to study the soot formation in normal and inverse coflow flames. The results tell that a higher reduction in peak combustion temperature was observed in premixed flame instead of diffusion flame thanks to alternative fuels. Ethanol greatly reduced the peak temperature of the premixed flame rather than ammonia. The impact of carbon-free fuel on total reduction in CO and CO₂ emissions was higher than that of biofuel at premixed and diffusion flame conditions. Ethanol impact on total reduction in benzene formation was stronger than ammonia at premixed and normal coflow flames. Pyrene was reduced highly by ammonia rather than ethanol at diffusion flame conditions. The study suggests that biofuel can be used from 0% to 90% in iso-octane fuel, and better results may be achieved when its fraction is ≥ 70%. In contrast, the use of ammonia with iso-octane may be better if its fraction is < 70% because a higher reduction in burning velocity will occur with higher NO₂ emissions at NH₃ ≥70%.