Simulation Optimization And Its Application Of The SCR Denitrification System

The emission of NO_x from coal-fired power plants is one of the significant precursors for the long-term large-scale haze in China.In order to deal with the increasingly severe atmospheric pollution problems,the government of China enacted the ultra-low emission standards with a limited NO_x emission of 50mg/Nm~3.Therefore,the high-efficiency denitrification technology of coal-fired power plants has become the key technology to win‘the battle of China's blue sky'.Selective Catalytic Reduction(SCR)denitrification technology is widely used in coal-fired power plants due to its high denitrification efficiency,mature technology,and simple structure.However,because of the complexity of the engineering conditions?Such problems of uneven velocity distribution,insufficient mixing of the reaction gas,and catalyst breakage were discorvered.These problems will seriously affect the denitrification performance.Therefore,practical investigations and tests were conducted on SCR reactors in a coal-fired power plant.The CFD simulations were carried out and catalyst breakage causes were discussed,and an optimized retrofit scheme was proposed to tackle the catalyst breakage problem.Firstly,the engineering investigation and practical testing was carried out.From which,the uneven temperature field,velocity field and NO_x concentration field were found,also the heavily worn problem in the approximately 1m near the front wall and in the back at the catalyst layer were found.The factors above can lead to a reduction in the denitrification efficiency of the SCR system.Furthermore,it also could be found that the physical and chemical properties of the catalyst have not changed apparently.So,the catalyst erosion was mainly caused by the physical erosion due to the uneven flow field.Secondly,the flow field in the practical SCR reactor was simulated.It was found that the flow velocity near the front and back walls at the layer of catalyst in the reactor was relatively high.It is similar to the concentration of fly ash particles which was more densely distributed in the front and back wall areas.Compared with the results of practical investigation,this could be a verification of the accuracy of the simulation.Also it could be confirmed that the reason of the severe catalyst wear was the high velocity fields and fly ash concentration fields in the location of around 1 m to the front wall and near the back wall.By analyzing the uniformity of the flow field before and after each flow guide elements in the reactor,it was figured out that the baffle above the catalyst is the key element affecting the flow velocity and the fly ash concentration distribution.On this basis,a series of the different arrangement angle(8°,13.5°,20°,25°)and the overall arrangement angle(from 16°to 20°)of the upper baffle were simulated.The conclusion could be brought:the angle of the front wall deflector should be increased,and the angle of the back wall deflector should be reduced.In this method the problem of fly ash enrichment of the front wall and back wall.Finally,an optimized scheme of the baffle located above the catalyst layer was proposed:including a 25°of increased angle of the baffle near front wall,and a decreased angle of the baffle of 8°near the back wall,also an addition of an arch on the top of the SCR reactor near the back wall.The simulation results shown that the scheme can achieve a better velocity distribution and DPM concentration distribution,which means it could concentrate more gas and particles to the middle of the SCR reactor.Moreover,the optimized scheme met the relevant standard of RSD of the velocity magnitude,the velocity vector deviation,the ratio of ammonia nitrogen,and the temperature.The retrofit was carried out because of its low renovation difficulty and high feasibility,and the denitrification efficiency after retrofit could stabled at a value of 98%and the pressure drop did not increase significantly.After 6 months'operation,there was no obvious wear.