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 (4) and initial temperature (T0) on the laminar burning velocity (SL), instability, and NO emissions of NH3/bio-syngas/air premixed flames under different equivalence ratios are investigated. The results indicate that increasing 4 and T0 enhances the SL of the premixed flame, with 4 having a more pronounced effect on combustion enhancement. Virtual gas analysis reveals that pre-cracking primarily strengthens combustion through chemical effect. An increase in 4 significantly shifts the peak SL towards the fuel-rich region, while at any T0, the peak SL consistently occurs around Phi = 1.1. Increasing 4 and T0 reduces the critical radius (rc) and the critical Peclet number (Pec) of the premixed fuel, with rc decreasing more rapidly when 4 is below 30 %. The dimensionless growth rate (& sum;) increases with the rise in 4 and T0, consistently remaining positive, indicating an unstable state. Additionally, & sum; varies more significantly with T0 when T0 is below 450 K. When 4 is below 60 %, the NO mole fraction increases with the increase in 4. However, at 4 = 80 %, the NO mole fraction is lower than at 4 = 40 %. Increasing T0 continually increases the NO mole fraction. Analysis of the NH3 reaction pathways indicates that NHi (i = 0, 1, 2) is closely related to the NO -> N2 reduction reactions.