Promoting effect of hydrogen compared with hydrocarbons on laminar flame speeds of ammonia

A comprehensive model for ammonia blends with hydrogen, syngas, methane, ethane and propane, named P-NUIG, was proposed in this work. This model undergoes rigorous experimental validation across a wide array of conditions, promoters, and combustion characteristics. Additionally, this paper compares the promotional effects of different fuels, namely hydrogen, syngas, methane, ethane, and propane, on the laminar flame speed of ammonia at full blending ratios (0–90 vol%). Our findings reveal that these promoters exhibit a non-linear enhancement on ammonia's laminar flame speed. Specifically, hydrogen and syngas demonstrate an initial gradual increase, followed by a rapid acceleration as the blending ratio escalates. Conversely, for larger alkanes, the laminar flame speed experiences a swift initial rise and then a gradual increase thereafter. This phenomenon can be categorized into three distinct regions: the ammonia chemistry-dominant region, the transition region, and the promoter chemistry-dominant region. Notably, an intersection point is identified at approximately X_P = 40%. This indicates that when X_P is less than 40%, the enhancement of ammonia's laminar flame speed by larger alkanes surpasses that of syngas and hydrogen. However, when X_P exceeds 40%, the enhancement by larger alkanes becomes weaker compared to syngas and hydrogen. Thermal effect analysis indicated that the adiabatic temperature of ammonia mixed with larger alkanes follows the similar pattern as the laminar flame speed versus X_P. Additionally, kinetic analysis uncovers a linear relationship between laminar flame speed and Y_{max(H + OH)} regardless of fuel type. This relationship is attributed to the chemical structure of the fuels, where H atoms are more readily generated during the oxidation of larger alkanes. Consequently, the laminar flame speed of NH₃ responds more swiftly to the incorporation of alkanes.