Efficiency-improving Reform For SCR Flue Gas Denitrification System Of 600MW Coal-fired Units And Research On The Capture Of Fly Ash

The nitrogen oxides emitted by coal-fired power plants as a harmful gas seriously endanger human health and atmospheric environment.At present,my country requires the NO_x emission standard of coal-fired power plants to be reduced from 100mg/m³ to 50mg/m³.In the face of increasingly stringent nitrogen oxide emission requirements,domestic power plants have vigorously promoted environmental protection to meet the ultra-low emission requirements of nitrogen oxides.This paper takes the SCR flue gas denitration system of a 600MW coal-fired unit as the research object,and analyzes and solves the problem of catalyst ash wear,low denitration efficiency,high ammonia escape and air preheater plugging corrosion during the ultra-low-emission operation process through the combination of field test and numerical simulation.First of all,the unit’s denitration system was tested in cold state and hot state,and the problems in the unit’s denitration system were preliminarily understood.Secondly,based on the Realizable k-εturbulence model,the internal flow field of the flue from the economizer outlet to the air preheater section was simulated.The results show that there are multiple expansion cross-sections and large expansion angles in the horizontal section of the economizer exit of the original structure economizer.The irrational structure of the flue causes the distribution of the flue flow field in the horizontal expansion section to be extremely uneven.The existence of low-speed vortex zone and high-speed zone reduce the uniformity of the flow field upstream of ammonia injection,which will affect the mixing of ammonia and flue gas.Comparing the simulation results before and after the optimization of the internal deflector of the flue,it can be found that the use of multiple groups and small angles of the deflector arrangement at the changing cross section of the flue can effectively improve the uniformity of the flow field,reduce the resistance of the flue gas flow.It also can improve the uniformity of the fly ash concentration distribution upstream of the first layer of catalyst and reduce the ash accumulation and wear of the catalyst.Afterwards,based on the Species Transport model and the Laminar Finite-Rate laminar finite rate chemical reaction model,the distribution of ammonia and nitrogen components in the denitration system and the reaction of ammonia and nitrogen were simulated.Due to the small number of nozzles and the large nozzle diameter,the vortex static mixing ammonia injection device has poor mixing effect of ammonia and gas."H"zoned ammonia injection device has a large number of nozzles and a large number of zones.The two-dimensional ammonia injection flow adjustment is conducive to the uniform distribution of ammonia gas,and the downstream circular static mixer is conducive to the secondary mixing and diffusion of ammonia gas.The flue gas downstream of the spin ammonia injection device forms a counterclockwise tangential vortex,and the formation of the rotating vortex can enhance the turbulence of the flue gas turbulence,effectively promote the mixing between the components,and achieve the purpose of uniform distribution of ammonia and nitrogen.Under600MW operating conditions,the denitration efficiency of the three ammonia injection devices when the NH₃/NO_x molar ratio is equal to 0.9 is calculated as 79.5%,88.2%,and 88.7%,and the ammonia slip is 3.87ppm,0.026ppm,and 0.021ppm,respectively.The latter two types of ammonia injection devices can effectively enhance the mixing of ammonia gas and flue gas,improve the uniformity of the ammonia nitrogen distribution upstream of the catalyst,and thus improve the denitration efficiency and reduce ammonia escape.In addition,this thesis also simulates the movement trajectory of fly ash particles through the Discreate Random Walk Model and calculates the fly ash trapping efficiency of the ash bucket.The research results show that the small-diameter fly ash particles follow the flue gas flow well and are evenly distributed inside the flue.The large-diameter fly ash particles are greatly affected by inertia and gravity,and are mainly concentrated in the middle and lower parts of the horizontal flue.The capture effect of the ash hopper on large-diameter fly ash particles is significantly better than that of small-diameter fly ash particles.The ash hopper has a good capture effect on the fly ash particles incident on the front side of the economizer outlet.However,the gray bucket could not capture the fly ash particles incident on the the rear side of the economizer outlet and fly ash particles are almost all escaped.In order to improve the fly ash collection capacity of the economizer ash hopper and denitration ash hopper,reduce the catalytic wear and blockage,the two ash hoppers were optimized.The horizontal flue inlet was equipped with a"V"-shaped folding plate and move the denitration ash hopper below the shaft flue.After optimization,the fly ash capture rate of the economizer ash hopper and denitration ash hopper increased by 2.05%and 2.91%respectively;the capture efficiency of the large-diameter fly ash particles and the average diameter fly ash particles increased by 8.11%and4.99%respectively.This thesis focuses on the optimization study of the flue gas denitration system from three aspects:flow field distribution,component distribution,and fly ash capture.These research work can provide technical reference for the optimization of ultra-low emission optimization of coal-fired units.