Effect of high-humidity air on toxic gas generation from flexible polyurethane foam combustion and its simplified modeling

• Investigated the impact of high humidity on FPUF combustion and toxic gas generation. • High humidity significantly reduces CO by 31 %, slightly decreases HCN by 5 %. • Developed an improved simplified gas-phase reaction model for accurate prediction of CO, CO₂, and HCN. Flexible polyurethane foam (FPUF) is widely used in indoor decoration and furniture, but its combustion releases toxic gases that pose severe risks to occupants. The influence of high-humidity air on FPUF combustion and the generation of toxic gases remains poorly understood. In this study, combustion experiments and numerical simulations were conducted under dry and high-humidity air conditions to investigate the effects of equivalence ratio ( ϕ ) and humidity on CO, CO₂, HCN, and NO formation. An improved simplified gas-phase reaction model was developed to capture these effects. Results indicate that CO and HCN concentrations increase with ϕ , whereas CO₂ and NO decrease. Under high-humidity conditions, CO production decreases significantly (∼31 %), HCN shows only a minor reduction (∼5 %), and NO increases markedly (∼1.4 times). Mechanistic analysis reveals that water vapor enhances OH radical concentration, promoting CO oxidation but having limited impact on HCN consumption, thereby amplifying NO formation. Numerical simulations demonstrate that the proposed simplified model accurately predicts CO, CO₂, and HCN, outperforming existing models, although NO predictions still show some deviation. These findings clarify the role of high-humidity air in FPUF toxic gas generation and provide a reference for fire toxicity assessment and numerical modeling.