The effects of NH3 pre-cracking and initial temperature on the intrinsic instability and NOx emissions of NH3/bio-syngas/air premixed flames

The study of the combustion characteristics of NH₃/bio-syngas/air under NH₃ partial cracking and elevated initial temperatures can enhance its feasibility as a practical fuel. The effects of NH₃ cracking rates (ζ) and initial temperature (T₀) on the laminar burning velocity (S_L), instability, and NO emissions of NH₃/bio-syngas/air premixed flames under different equivalence ratios are investigated. The results indicate that increasing ζ and T₀ enhances the S_L of the premixed flame, with ζ having a more pronounced effect on combustion enhancement. Virtual gas analysis reveals that pre-cracking primarily strengthens combustion through chemical effect. An increase in ζ significantly shifts the peak S_L towards the fuel-rich region, while at any T₀, the peak S_L consistently occurs around Φ = 1.1. Increasing ζ and T₀ reduces the critical radius (r_c) and the critical Peclet number (Pe_c) of the premixed fuel, with r_c decreasing more rapidly when ζ is below 30 %. The dimensionless growth rate (∑) increases with the rise in ζ and T₀, consistently remaining positive, indicating an unstable state. Additionally, ∑ varies more significantly with T₀ when T₀ is below 450 K. When ζ is below 60 %, the NO mole fraction increases with the increase in ζ. However, at ζ = 80 %, the NO mole fraction is lower than at ζ = 40 %. Increasing T₀ continually increases the NO mole fraction. Analysis of the NH₃ reaction pathways indicates that NH_i (i = 0, 1, 2) is closely related to the NO → N₂ reduction reactions.