Analysis of influence of air distribution on the reduction of NOx emissions in a 300-MWe tangentially lean coal-fired boiler

Abstract

Retrofitting of low-NOx combustion system in boilers is one of the main strategies for coal-fired units to reduce NOx. Compared with bituminous coal-fired boilers, there is a big difficulty for low volatile coal boilers to retrofit. In addition, the tertiary air in tangentially lean coal-fired boiler will affect mixing of coal and air, combustion atmosphere and combustion temperature, these have significant influences on both coal combustion process and NOx formation. Therefore, optimal tertiary air arrangement modes should be explored at retrofitting project design to get low-NOx emissions and high combustion efficiency. In this paper, based on a 300-MWe tangentially lean coal-fired boiler, the influences of tertiary air arrangement on combustion parameters were studied by means of numerical simulation. Then slag, fly ash carbon content, NOx, furnace outlet temperature, and other parameters were measured at hot states. The results show that when tertiary air at the bottom of the furnace, the fire center is elevated and resulting in a lower burnout rate, the NOx generated early in tertiary air has a longer reducing zone so that the NOx formation is relatively lower. When tertiary air in the middle of primary combustion zone, the combustion temperature decreases, the NOx formation is moderate for the opposite function of weaker reducing atmosphere and lower temperature and the coal burns out well because a good mixture of coal and air in primary combustion zone. As tertiary air is above primary combustion zone, the burnout rate and NOx emission fall off with tertiary air location moving up higher. In addition, the simulation results were highly consistent with the hot tests. The findings of this study provide guidance for the tertiary air arrangement of tangentially lean coal-fired boilers with low-NOx combustion system.