Asymmetric flue gas recirculation distribution for simultaneous NO and CO reduction in a biomass chain-grate furnace: CFD optimization and experimental validation

Biomass chain grate boilers face a persistent challenge in achieving the simultaneous mitigation of NO and CO emissions. In this study, the distribution ratio of flue gas recirculation (FGR) between the secondary air streams at the front and rear arches was systematically optimized to meet stringent emission targets (NO<150 mg m −3 and CO < 200 mg m −3 , normalized to 9% O 2 ). A validated computational fluid dynamics model was coupled with a central composite design and response surface methodology. Second-order regression models for NO and CO were established with coefficients of determination of 0.9695 and 0.9951, respectively. NO decreases monotonically with increasing total FGR, dominated by a linear effect, whereas CO exhibits a non-monotonic trend due to a significant negative quadratic term. The feasible operating region satisfying both emission constraints is identified as A ∈ [15%, 30%] and B ≥ Bₘᵢₙ(A), where A and B denote the FGR fractions in the front- and rear-arch secondary air, respectively. At the representative operating point (A = 30%, B = 20%), predicted emissions are 146.8 mg m −3 for NO and 169.8 mg m −3 for CO, validated by additional simulations and experiments with deviations below 10%. Mechanistic analysis reveals that a high FGR ratio in the front arch suppresses thermal NO formation through temperature reduction and oxygen dilution, while a moderate FGR ratio in the rear arch activates the water-gas shift reaction via H 2 O enrichment, compensating for OH radical quenching and limiting CO accumulation. The proposed asymmetric distribution strategy offers a practical, hardware-free solution for existing boilers to comply with stringent emission standards.