Effects of sub-atmospheric pressure on appearance and pollutant formation of inverse diffusion flame within a confined space

Gas-fired boilers operating at high-altitude regions often suffer from inadequate output, decreased thermal efficiency, and excessive NO_x emissions. The effect of sub-atmospheric pressure on flame appearance and pollutant formation is the main reason for those problems, and thus needs to be clarified particularly under furnace combustion conditions with a fixed excess air coefficient. Inverse diffusion is a widely employed fuel–air configuration in burners of gas-fired boilers, and therefore the flame appearance, CO generation, and NO generation were experimentally investigated in this paper by adopting a low-pressure quartz tube reactor. Results show that the flame is elongated from reducing pressure under fuel-lean conditions, mainly due to the reduced oxygen mass concentration and the elevated jet velocity. Under fuel-rich combustion conditions, however, the flame is shorted at sub-atmospheric pressure from the suppressed soot formation. The reduced pressure leads to an increase in the global strain rate, making the flame more prone to uplift. With decreasing pressure, the increased air–fuel mixing and flame length facilitate the gas burnout, thus decreasing CO generation. The sub-atmospheric pressure could evidently reduce the NO generation under fuel-rich conditions, but slightly increase it under fuel-lean conditions. Under fuel-lean conditions, the NO major pathways (prompt, thermal, NNH, and N₂O) are promoted which leads to an increase in NO generation with decreasing pressure. Under fuel-rich conditions, however, NO formation is suppressed from the decreased rate of reaction $\text{N2}+\text{CH}\leftrightarrow \text{HCN}+\text{N}$.