An Intelligent Control Technology For The Oriented Injection Of Ammonia Into SCR System And Its Application

A large number of nitrogen oxides(NO_x)were discharged by the coal-fired power plants annually,and the selective catalytic reduction(SCR)technology had been widely adopted in the process of flue gas denitrification.A retrofit project of ultra-low emission of NO_x was being forced to proceed for coal-fired power plants in China since 2014.The common problem of excessive ammonia injection can be easily found in the SCR system that had been retrofitted,resulting in the increase of ammonia escape at the outlet of the SCR system,furtherly leading to a series of unfavorable operation problems such as the corrosion of the cold end of the air preheater and the increase in power consumption of induced draft fan.Consequently,the risk of forced shutdown of the plant was increased,which seriously affected the safe operation of the plant.Focusing on the above common problems,an intelligent control technology for the oriented injection of ammonia into SCR system and its application was investigated in order to improve the safety,economy and stable operation of the plant.The anticipatory control technology of total ammonia injection based on multiple input single output(MISO)predictive control module was put forward.The large lag characteristic of the SCR system was quantitatively analyzed,and the key influencing parameters of inlet NO_x concentration could be locked via the method of multiple linear regression.The MISO predictive control module,which is suitable for accurate prediction of the inlet NO_x concentration,was established,and its online self-learning and self-updating were realized.The prediction results of a 660MW coal-fired power plant showed that the root mean square error(RMSE)and the mean absolute percentage error(MAPE)between the predicted NO_x concentration and the measured value was separately about 5.52mg·Nm^-3,1.12%when the MISO predictive control module was applied.It was found that the uniformity of flue gas velocity and concentration within the SCR system would both be improved by the reasonable arrangement of guiding plate.However,the uniformity of flue gas velocity distribution was improved to a greater extent.Accordingly,a manual tuning technique of ammonia injection branch-pipes in SCR system was proposed aimed at optimizing the NO_x/NH₃ mixing ratio,which would definitely benefit the the uniformity of concentration.The weight coefficient of each branch-pipe can be quantitatively determined with regard to the continuous distribution characteristics of flue gas and the structure of the ammonia injection grid(AIG)system,and then the manual valves installed on the AIG branch-pipe can be accordingly adjusted based on the weight characteristics.The proposed technology has been applied on a 660MW coal-fired power plant,exhibiting that the ammonia consumption rate(ACR)was reduced by about 4.33%.The weight sensitivity of AIG branch-pipes was further analyzed on basis of the manual tuning technique above mentioned,and an intelligent control technology of AIG branch-pipes was proposed.Three off-line methods for obtaining the distribution of the NO_x flux within the cross-section area in front of AIG were formed combined with the typical AIG system.Weight of NO_x flux in the sub-zone corresponding to each of the AIG branch-pipes was analyzed and the sensitivity of which against the plant power load was figured out.A number of?critical?AIG branch-pipes were determined with regard to the weight sensitivity analysis,and the valves installed on the?critical?branch-pipes were changed to be controlled by the automatic valves.The simulation results showed that the ACR of the SCR system can be reduced by about 7.99%and the standard deviation of the outlet NO_x concentration distribution can be reduced by about 64.66%when the intelligent control technology equipped with the distributed NO_x flux online measurement was applied,comparing with the uniform ammonia injection strategy.The software of the intelligent control technology for the oriented injection of ammonia was developed according to the anticipatory control technology of total ammonia injection and the intelligent control technology of AIG branch-pipes.The engineering application of this software was realized relying on a plug-in controller.The engineering application results showed that the average fixed value control deviation of outlet NO_x concentration was about 2.27 mg Nm^-3 during three consecutive days after optimization.The average ACR of SCR system was found to be reduced by 6.44%compared to that under previous control system,and was 10.62%lower than that of the unapplied system.The maximum standard deviation of the outlet NO_x concentration distribution can be reduced by about 83.08%,and the rising rate of the flue gas resistance of the air preheater was slown down by39.18%compared to that unapplied side.The total ammonia injection can be controlled in advance and the intelligent control of AIG branch-pipes can be achieved with the help of the intelligent control technology fromed above,providing the reliable technical support for the safety,economy and stable operation of retrofitted coal-fired power plant.The intelligent ammonia injection scheme fromed above was considered to be an international advanced technology by China Electricity Council,and it was selected into the?Catalogue of Advanced pollution Prevention and Control techniques of 2018(Field of Air pollution Prevention and Control)?issued by Ministry of Ecology and Environment of the People's Republic of China.