Research And Application On Control Methods Of SCR-DeNO_x System In The Thermal Power Units

Recently,the problems of global environmental are becoming increasingly severe and not optimistic and most regions in China also appear long-lasting fog and haze weather,which is a serious threat to the natural environment and human health.The emission of atmospheric pollutants in thermal power generation process is the main cause of fog and haze weather,of which nitrogen oxide,as one of air pollutants,results in formation of fog and haze accompanied by acid rain and photochemical smog.Therefore,a series of policies and measures have been put forward to control the pollution of nitrogen oxide.At present,most SCR-DeNOx systems in thermal power plants in China still adopt PID controllers which have effective control performance when the systems operate in stable conditions or when the load changes little.However,the system load always changes due to the non-linearity,large delay and time-varying of SCR-DeNOx systems,so the use of PID controller can not achieve satisfactory control quality in the actual operation,resulting in too much or too little ammonia injection with ammonia escaping or inadequate reactions and other issues which lead to a large fluctuation of the concentration of nitrogen oxide in the export flue gas,exceed the emission limit and endanger the environment and human health.In this paper,an multiple model control method with Fractional Order Proportional Integral Differential based on Internal Model Control(IMC-FOPID)controllers applied is proposed for SCR-DeNOx system under varying operating conditions.The fluctuation of system under varying operating conditions is overcame by using the multiple model control method.This control method can improve the dynamic characteristics of the system.Then,an improved scheme based on disturbance observer is presented based on the multiple model control strategy to deal with the disturbance such as the fluctuation of gas flow and NOx concentration.And the disturbance is observed by disturbance observer and then is compensated.The system with disturbance observer enhance the capability of disturbance rejection.Then,in order to ensure the stability and robustness of SCR-DeNOx system,the robust gain scheduling controller based on linear matrix inequation(LMI)is designed and applied to the DE denitration system.The simulation shows that the system still has satisfactory stability and robustness when system works in varying operating conditions.Finally,aiming at restraining the internal disturbance and external disturbance,the active disturbance rejection control(ADRC)method is introduced to overcome the total disturbance in system.The simulation results indicates that the ADRC is better than the traditional cascade PID control in set point tracking and disturbance rejection.The active disturbance rejection controller can still maintain satisfactory control performance with load varying.Several control methods proposed and used in this paper can not only improve the control quality of SCR-DeNOx system,but also provide more abundant control ideas for the control and optimization for SCR-DeNOx system in the actual industry.