Influence of tertiary air arrangement on NO_x emissions in a 300 MW tangentially lean coal-fired boiler

Retrofitting of Low-NO_x combustion system in boilers is one of the main strategies for coal-fired units to reduce NO_x emissions.The air classification combustion technology has been widely used in bituminous coal-fired boilers because of its mature technology and low cost.However,with the further increase of coal/air ratio,the reduction of NO_x decreases and the content of unburned carbon increases significantly.Compared with bituminous coal fired boilers,there is a big difficulty for low volatile coal boilers to retrofit.The tertiary air of tangentially lean coal-fired boilers will affect the mixing of air and coal,combustion atmosphere and temperature,which will have a significant effect on the combustion process of pulverized coal and the formation of NO_x . If only air classification technology is used,it will not meet the emission standard of NO_x .Therefore,the modes of optimal tertiary air arrangement should be explored at retrofitting project design to get low NO_x emissions and high combustion efficiency.In this paper,based on a 300 MW tangentially lean coal-fired boiler,the influences of tertiary air arrangement on combustion characteristics of the boiler were studied by means of numerical simulation.The results show that when the tertiary air is injected at the bottom of the furnace,the pulverized coal in the primary and tertiary air of the lower part rapidly ignites and burns,the temperature rises,and the flame center moves upward.The NO_x reduction zone becomes longer,and the NO_x concentration at the furnace outlet is the lowest,at 405 mg/Nm~3.The downward movement of the tertiary air causes less oxygen in the upper and middle part of the main combustion zone of the furnace,resulting in a decrease in the burnout rate and insufficient of the pulverized coal and the air.When the tertiary air is injected in the middle of primary combustion zone,the combustion temperature of the furnace decreases due to the lower temperature of the tertiary air,which inhibits the generation of thermal NO_x to some extent,and reduces the emission of NO_x at the outlet of the furnace.The injection of tertiary air increases the excess air ratio in the main combustion zone,which is beneficial to the full combustion of the coal powder and the improvement of burnout rate. As the tertiary air is injected above primary combustion zone,with the increase of the position of the tertiary air,the overall combustion burnout area of the tertiary air pulverized coal moves up,and the temperature near the furnace arch increases in turn.The upward movement of the tertiary air increases the length of the reduction zone of NO_x ,and the higher the position of the tertiary air vent is,the lower the NO_x concentration at the furnace outlet is. The upward movement of the tertiary air causes the residence time of the third pulverized coal powder in the furnace to become shorter,resulting in insufficient combustion,and the carbon content of the fly ash is increased and the burn-out rate is reduced.In addition,the carbon content of fly ash and slag,flue gas temperature at furnace outlet and NO_x concentration were measured on site.The simulated and measured results of NO_x emission concentration were 445 and 448 mg/Nm~3,respectively.The numerical simulation results are in good agreement with the measured results.