Simulation Study On Flue Gas Flow Characteristics And Ammonia Injection Optimization In SCR Updraft Flue For High Ash Fue

With the continuous improvement of environmental protection requirements,domestic power plants have generally carried out ultra-low emission transformation,but due to the variable coal quality and the large fluctuation of unit operating conditions and other factors,the actual SCR denitration system still has many deficiencies and large room for improvement.In this paper,the SCR denitration system of 660 MW unit in a power plant was studied.The numerical simulation method was used to study the SCR denitration system from three aspects:flue gas flow characteristics,factors affecting denitration performance of catalyst and optimization scheme of precise ammonia injection.Firstly,Ansys fluent 2021 R2 software was used to establish a three-dimensional numerical model of SCR denitration system.Based on the grid independence and the accuracy verification of the results,the influence of ammonia injection characteristics on the flue gas flow law under different partition schemes was studied by using the MATLAB software call model.The results show that:Compared with the flue,the size of ammonia injection grid is relatively small,so the measure of optimizing the flow field in SCR by adding flue components will not be ineffective due to the effect of ammonia injection velocity.The use of a relatively broad ammonia spray grid partition scheme will lead to a decline in the accurate control of ammonia concentration distribution in a specific area at the entrance of the SCR reactor,but at the same time,the ammonia concentration distribution does not appear too concentrated due to a relatively wide distribution area.Although a smaller partition scheme for ammonia injection grid can enhance the accurate control ability of ammonia concentration distribution in a specific area at the entrance of the SCR reactor,it is also easy to cause excessive concentration of local ammonia,so it is more difficult to control precise ammonia injection under this scheme.Secondly,the numerical model of SCR catalyst single-channel denitration was established on the MATLAB platform by using the self-programming method,and the algorithm for generating the inlet boundary conditions was designed by combining the Monte Carlo principle.The denitration performance of SCR catalyst was studied with the inlet parameters,structural parameters and inlet distribution uniformity as variables.The results showed that the honeycomb catalyst had the best denitration performance under the same conditions in terms of geometric structure,The denitration performance of plate and corrugated plate catalysts is relatively close;Higher denitration efficiency and lower ammonia escape rate can be obtained by properly extending the catalyst length;Increasing the pitch of catalyst will reduce the denitration performance of catalyst,but will improve the blockage of catalyst channel.The appropriate pitch of catalyst should be comprehensively considered according to the actual situation.In terms of inlet parameters,too high inlet speed will shorten the residence time of flue gas on the catalyst surface,reduce the denitration efficiency and increase the ammonia escape rate;Increasing the inlet NO concentration will only improve the whole denitration process after the reaction temperature reaches 600 K.Increasing the inlet ammonia nitrogen ratio can improve the denitration efficiency,but will lead to an increase in the ammonia escape rate near the optimal reaction temperature.It is recommended to limit the inlet ammonia nitrogen ratio to less than 0.9.In terms of inlet distribution uniformity,when the inlet velocity is greater than 1.5 m/s,the velocity deviation has a greater impact on the denitration efficiency and ammonia escape rate;A large temperature deviation will significantly reduce the denitration efficiency.Only when the temperature deviation is greater than 10%,the temperature deviation will become the main factor affecting the denitration efficiency;The deviation of ammonia-nitrogen ratio has little effect on denitration performance,but it will significantly affect the ammonia escape rate,and the larger the deviation of ammonia-nitrogen ratio distribution,the more obvious the increase of ammonia escape rate.Finally,through the establishment of the cyclic oscillation particle swarm optimization algorithm,combined with the neural network model of SCR ammonia injection characteristics and the multi-objective optimization model,the multi-objective prediction and optimization model of SCR ammonia injection characteristics was established.The model can solve the difficult problem of control of precise ammonia injection under a wide range of unit operating conditions.The results show that:The established particle swarm optimization algorithm with cyclic oscillation changing inertia weight is superior to LINWPSO,LDDWPSO,NLDWPSO,RWPSO,CCWPSO and other five kinds of particle swarm optimization algorithms in three aspects of stability,convergence rate and convergence accuracy.Based on the actual operation data,the neural network model of ammonia spraying characteristics of SCR can accurately predict the distribution of NO concentration and NH3 at the exit of SCR denitration system.Through calculation,the mean square error between the predicted value and the actual value in the test set is 25.98%,the mean absolute error is 3.81%,and the root mean square error is 5.10%,showing high accuracy.Based on the cyclic oscillation particle swarm optimization algorithm,combined with the neural network model of ammonia injection characteristics of SCR and the multi-objective optimization model,the multi-objective prediction and optimization model of ammonia injection characteristics of SCR denitration system was established,which could ensure the denitration efficiency and reduce ammonia escape at the same time.After optimizing the ammonia injection quantity of SCR system through the model,the average ammonia injection quantity was reduced by 1.65 kg/KWh.The NOx concentration at the outlet of SCR system was reduced by 3.83 mg/Nm³ on average.The NH₃ concentration at the outlet of SCR system was decreased by 0.47 mg/Nm³ on average,and the effect was more obvious.