Effects of secondary air distribution in primary combustion zone on combustion and NOx emissions of a large-scale down-fired boiler with air staging

A new air-staged and low-NOx emission combustion technology has been applied to a 300 MWe anthracite- and down-fired boiler with swirl burners. To achieve the optimum air distribution in the primary combustion zone and further reduce NOx emissions, full-scale industrial experiments varying the ratios between burner secondary air ratio (Rbsa) and staged air ratio (Rsa) (i.e., Rbsa/Rsa ratios of 42.9/17.8, 47.1/13.5 and 51.5/9.3), on the premise that the overfire air (OFA) ratio was approximately 20%, were performed to evaluate the overall performance of retrofitted boiler. The improved ignition of coal/air flow was at a distance of 0.8-1.4 m to the burner outlet with increased Rbsa/Rsa ratio. Compared with the boiler before retrofit, under Rbsa/Rsa ratios of 47.1/13.5 and 51.5/9.3, the flue gas temperature in the primary combustion zone of the retrofitted boiler increased slightly. Oxygen and carbon monoxide concentrations in the near-sidewall region revealed the furnace flame fullness varied under different Rbsa/Rsa ratios. With increased the air staged level in the lower furnace, NOx emissions decreased continually and the carbon content in fly ash did not necessarily increased. Under the optimum Rbsa/Rsa ratio of 47.1/13.5, the NOx reduction efficiency was approximately 46% with increasing boiler thermal efficiency slightly.