Effect of Equivalence Ratio on Pollutant Formation in CH₄O/H₂/NH₃ Blend Combustion

This paper investigates the effect of equivalence ratio on pollutant formation characteristics of CH₄O/H₂/NH₃ ternary fuel combustion and analyzes the pollutant formation mechanisms of CO, CO₂, and NO_X at the molecular level. It was found that lowering the equivalence ratio accelerates the decomposition of CH₄O, H₂, and NH₃ in general. The fastest rate of consumption of each fuel was found at φ = 0.33, while the rates of CH₄O and NH₃ decomposition were similar for the φ = 0.66 and φ = 0.4. CO shows an inverted U-shaped trend with time, and peaks at φ = 0.5. The rate and amount of CO₂ formation are inversely proportional to the equivalence ratio. The effect of equivalence ratio on CO₂ is obvious when φ > 0.5. NO₂ is the main component of NO_X. When φ < 0.66, NO_X shows a continuous increasing trend, while when φ ≥ 0.66, NO_X shows an increasing and then stabilizing trend. Reaction path analysis showed that intermediates such as CH₃ and CH₄ were added to the CH₄O to CH₂O conversion stage as the equivalence ratio decreased with φ ≥ 0.5. New pathways, CH₄O→CH₃→CH₂O and CH₄O→CH₃→CH₄→CH₂O, were added. At φ ≤ 0.5, new intermediates CHO₂ and CH₂O₂ were added to the CH₂O to CO₂ conversion stage, and new pathways are added: CH₂O→CO→CHO₂→CO₂, CH₂O→CO→CO₂, CH₂O→CHO→CO→CHO₂→CO₂, and CH₂O→CH₂O₂→CO₂. The reduction in the number of radical reactions required for the conversion of NH₃ to NO from five to two directly contributes to the large amount of NO_X formation. Equivalent ratios from 1 to 0.33 corresponded to 12%, 21.4%, 34%, 46.95%, and 48.86% of NO₂ remaining, respectively. This is due to the fact that as the equivalence ratio decreases, more O₂ collides to form OH and some of the O₂ is directly involved in the reaction forming NO₂.