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

Abstract A new air-staged and low-NO x emission combustion technology has been applied to a 300 MW e anthracite- and down-fired boiler with swirl burners. To achieve the optimum air distribution in the primary combustion zone and further reduce NO x emissions, full-scale industrial experiments varying the ratios between burner secondary air ratio (R bsa) and staged air ratio (R sa) (i.e., R bsa / R sa 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 R bsa / R sa ratio. Compared with the boiler before retrofit, under R bsa / R sa 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 R bsa / R sa ratios. With increased the air staged level in the lower furnace, NO x emissions decreased continually and the carbon content in fly ash did not necessarily increased. Under the optimum R bsa / R sa ratio of 47.1/13.5, the NO x reduction efficiency was approximately 46% with increasing boiler thermal efficiency slightly. Highlights • Low-NO x combustion technology was applied to a down-fired boiler with swirl burners. • As NO x emissions increased, carbon content in fly ash did not necessarily decreased. • NO x reduction of 46% was achieved without negative effects at the optimal condition. • Coal ignition and flue gas temperature in the primary combustion zone were improved. • Furnace flame fullness under different operation conditions was revealed. [ABSTRACT FROM AUTHOR]