| Down-fired boiler has unique furnace structure and air distribution mode,which is widely used to burn low volatile coal.Due to the recent changes in China's coal market,bituminous coal prices have dropped in some areas.In order to reduce NOx emissions and improve economy,some down-fired boilers begin to blend bituminous coal,while down-fired boilers are specially designed for burning anthracite and lean coal.Practice shows that blending bituminous coal will bring some problems,including slagging,excessive temperature of SCR inlet flue gas,increase of desuperheated water and so on.In order to master the flow and combustion characteristics of 600 MW down-fired boiler blended with bituminous coal,the parameters such as coal quality,burner ignition point,furnace temperature,SCR inlet flue gas temperature and exhaust gas temperature are measured and studied by means of industrial measurements.Then the effects of different burner structures and staged air downward angles on the flow and combustion characteristics of the furnace are studied by means of numerical simulation.The industrial measurements results show that the ash-free volatile fraction of the original lean coal is 17.05%,which is 20.17%lower than that of bituminous coal.The ignition curve of the burner under 580MW load fluctuates at first,then rises rapidly,start to ignite at 2.72m,and the temperature of the upper part of the lower furnace is 47?higher than that of the lower part of the lower furnace.Under high load,partial measuring points of SCR inlet flue gas temperature was over-heated,which reached the maximum blending ratio of 16.7%.According to the industrial test results,it is found that the high volatile content of bituminous coal leads to more intense combustion of pulverized coal and the upper flame center of lower furnace,which is the reason for the excessive temperature of flue gas at SCR inlet.On the basis of industrial measurements results,the optimization scheme of burner is put forward,including eliminating circular secondary air cyclone blade and central air,coaxial primary air duct and circular secondary air duct,and arranging near the central side of furnace.The numerical simulation results show that compared with the original burner structure,the downrush depth of air flow on the arch of the new structure burner is larger,the ignition distance is extended by 0.34 m,the flame center is lower,the outlet temperature of the furnace is lowered by 61?,which can improve the phenomenon of overheating of gas temperature at SCR inlet,the emission of NOx is reduced by 40 mg/m3?6%O2?,the content of fly ash combustible material is reduced by 1.11%,and the oxygen content at the outlet of the furnace is reduced by 0.42%.On the basis of the new burner structure,the numerical simulation results of different staged air downward angles show that,except for the staged air downward angle of 60 degrees,the flow field in the cold ash hopper area is relatively stable.With the increase of staged air downward angle,the NOx emission decreases first and then increases.When the staged air downward angle increases from 0 degrees to 30 degrees,the combustible content of fly as h decreases from 3.84%to 3.35%,the staged air downward angle increased to 45degrees,and the combustible content of fly ash basically remained unchanged.When the staged air downward angle increased to 60 degrees,the combustible content of fly ash increased to 3.44%.The oxygen content at the furnace outlet changed in the same way as the combustible content of fly ash,and the temperatu re at the furnace outlet changed little under different staged air downward angles.Based on the above analysis results,the optimized new structure burner can effectively improve the problem of overheating of gas temperature at SCR Inlet,and the NOx emission and fly ash combustible content at the furnace outlet are also reduced slightly.At the same time,the staged air downward angle is recommend ed to be 30 degrees,and the boiler achieves better performance. |
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