Study On Secondary Air Dynamic Optimization Of Tangential Coal-fired Boiler Under Deep Peak Regulating Condition

Tangentially fired pulverized coal boilers have been widely used in thermal generating units because of their wide adaptability,high combustion efficiency and easy operation.In recent years,with the rapid development of clean energy such as wind energy and solar energy,thermal power generation systems need to carry out deep and rapid peak load to improve the power grid’s absorption capacity of new energy.When the thermal power unit is running at 30%～40%rated load,it usually reduces the load by adjusting the amount of fuel and air introduced into the boiler.Therefore,the ratio of air volume becomes the key to affect the combustion stability of furnace and pollutant emission.Taking a 350MW supercritical tangential combustion pulverized coal boiler in a thermal power plant in northwest China as the research object,this paper conducted numerical simulation research on the secondary air system and furnace of the boiler,aiming at understanding the distribution law of boiler air volume and the influence of air distribution on the combustion process and pollutant emission in the furnace,and proposed an optimization scheme for throttle opening.It provides reference for the normal operation of boiler unit.Firstly,according to the actual boiler structure and size,the geometric models of the secondary air chamber of the burner nozzle and the horizontal branch pipe,the complete secondary air duct and the whole furnace are established respectively,the resistance composition of the secondary air system is analyzed,and the influencing factors of the secondary air distribution in actual operation are pointed out,so as to establish the relationship between the secondary air duct and the bellows,the secondary air nozzle and the secondary air damper and the air volume distribution.Secondly,a gas turbulent flow model is established for the burner nozzle and the secondary damper in the horizontal branch pipe where the burner nozzle is located as a separate calculation domain.The flow field distribution of fluid flowing through the secondary damper with different opening in the secondary air horizontal branch pipe is studied.The resistance characteristics of the damper and the influence of the burner nozzle shape on the resistance are quantitatively analyzed.The nonlinear relationship between wind speed,damper opening and resistance is obtained.The results show that the opening of the damper has little effect on the velocity distribution of the nozzle section of the burner,and the nozzle resistance of the burner with different structure is different and increases with the increase of the velocity.With the increase of the baffle opening,the resistance of the damper gradually decreases,and the smaller the damper opening,the greater the disturbance to the secondary air flow field and the stronger the adjustment effect.When the damper opening degree is greater than 60%,the resistance gradually becomes stable,and the adjustment effect of the secondary damper on the resistance is poor.Thirdly,the gas-solid flow model of the secondary air system is established with the opening of the damper plate as the means of air distribution adjustment,and the opening of the secondary damper is optimized.It is explored that when the air distribution mode is"equal"air distribution,"positive tower"air distribution,"inverted tower"air distribution,"drum waist"air distribution and"shrink waist"air distribution,the air distribution is 100%open.And the air volume distribution rule in the secondary air system when the load changes to 75%THA,50%THA,40%THA.The results show that adjusting the opening of the damper can effectively reduce the unevenness of the air duct.On this basis,the optimization scheme of damper opening is given.Then,the factors affecting the deposition of fly ash particles(fly ash particle size and load)in the wind box are numerically simulated,and the deposition of fly ash particles at the bottom of the big bellows is obtained.The results show that the particle quantity at the lowest burner nozzle is the largest.The fly ash particles with small particle size follow the airflow better,while the fly ash particles with large particle size are more likely to deposit at the bottom of the secondary bellows because of the influence of gravity.The particle deposition amount of horns 2 and 3 near the air preheater is greater than that of horns 1 and 4 away from the air preheater.With the decrease of the load,the low-speed zone in the secondary air system increases,and the deposition of fly ash particles at the bottom of the bellows becomes more serious.Finally,the gas-solid flow model and combustion model of the furnace are established.Based on the above numerical simulation results,the numerical simulation of pulverized coal combustion under varying load of the boiler is carried out.The influence of the opening combination of 5 air distribution modes and different burner arrangement on combustion characteristics and pollutant emission characteristics of boiler under typical load(75%THA)and 50%load were analyzed.The results show that under 75%load,the secondary air distribution mode of"drum waist"has better ignition and combustion performance of pulverized coal,and relatively low pollutant emission.At 50%load,due to the higher temperature in the main combustion zone of the lower pulverized coal nozzle,better combustion performance,and the lowest NO_X emission concentration at the furnace outlet,it is suitable to close the lower burner when the boiler is running at half load.