NH ₃ and H ₂ Impact on Combustion and Emission Characteristics of i-C ₈ H ₁₈ Flame under Premixed and Diffusion Conditions

Soot and carbon dioxide released from internal combustion engines became the key issues when using fossil fuels. Ammonia and hydrogen having zero-carbon species can reduce carbon-related emissions and enhance the reliance on renewable fuels. A comparative study of ammonia and hydrogen impact on combustion and emission characteristics of iso-octane flame was performed under different combustion conditions. Arrhenius equation, soot surface reactions, and modified kinetic mechanism were used to study the flame growth, soot nucleation, and surface growth rates. The results show that hydrogen increased the temperature about 20.74 K and 59.30 K, whereas ammonia reduced it about 82.17 K and 66.03 K at premixed and counterflow conditions, respectively. The flame speed of iso-octane was increased 43.83 cm/s by hydrogen and decreased 34.36 cm/s by ammonia. A reduction in CH₂O caused a reduction in CO and CO₂ emissions. Ammonia impact on CO reduction was stronger than hydrogen under premixed and counterflow flame conditions. In contrast, a higher reduction in CO₂ emission was noticed when adding hydrogen instead of ammonia adopting counterflow flame conditions. The diluted fuel reduced the fraction of small-chain hydrocarbon species in iso-octane combustion, resulting in a reduction of the soot precursors, soot particle number density, and soot volume fractions. NO emission decreased in premixed flame and increased in counterflow flame when adding hydrogen. Ammonia enhanced NO emissions, and its impact was greater on counterflow flame rather than premixed combustion. The current study suggested that ammonia dilution in iso-octane should be ≤ 50% because a higher reduction in soot and carbon dioxide emissions can be achieved with a small reduction in burning velocity and increment in NO emission. However, the hydrogen contents > 50% in iso-octane fuel significantly reduced the emission and increased the burning velocity.