Optimization Of Ammonia Control Strategy For SCR System In Coal-fired Power Plants

In response to national environmental policies and reducing NOx emissions from the thermal power industry,Selective Catalytic Reduction(SCR)technology is accepted by most power plants with its unique advantages.In the complex SCR denitration system,the ammonia control system plays a very important role.The control effect of ammonia spray amount has a direct impact on the NO_X emission value,production cost and smooth operation of downstream equipment.The basic control principle of the ammonia spray amount is to compare the actual measured value of the NO_X concentration at the outlet with the set value to obtain a deviation signal between the two values.The controller performs calculation based on the deviation signal,sends a control signal and transmits it to the actuator.The actuator controls the ammonia flow rate by adjusting the opening degree of the ammonia injection valve to ensure that the NO_X concentration value at the outlet is within the target range.At present,there are three main types of ammonia spray amount control strategies for coal-fired power plants in SCR reactors:fixed molar ratio control,fixed outlet NO_X concentration control,and cascade composite control.The SCR ammonia spray amount control system of a power plant is selected as the object model.Firstly,the simulation experiment of cascade control is carried out by using MATLAB software.The result shows that oscillation occurs during the adjustment process in the cascade control mode.It has a high overshoot,long adjustment time and poor control quality.Then,based on the cascade control mode,the Smith predictive controller is added to the system to compensate the delay link.Tests show that Smith predictor control has significantly improved the control quality.The system no longer oscillates,the response speed is fast,the overshoot is greatly reduced,and the adjustment time is short.However,Smith predictor control also has defects:it has a high dependence on the exact matching of the research model,is sensitive to the changes of the model parameters,and has poor robustness.Finally,according to the problems existing in Smith predictor control method,the fuzzy control theory is applied to the ammonia spray control system to form a fuzzy Smith predictor control system.The robustness of the system is enhanced,and the traditional control method is effectively improved.