Abstract: Regarding the delayed response of SCR DeNO_x system due to the high inertia under large perturbations, and other problems in the operation of SCR ammonia injection control system after ultra-low emission retrofit, the SCR Control⁺ system model with state prediction algorithm is proposed based on the data accumulation of multiple denitrification optimization projects. Through the analysis of dominant factors, the state variables influencing the NO_x concentration are selected, and the change of the NO_x concentration is accurately and quickly predicted by designing the state variable predictive control module. The state variable predictive control is used to overcome the influence of delay and input perturbation, and the cascade feedback control is applied to ensure the steady state index of the control system. The practical application in a 660 MW unit shows that the DeNO_x control system based on SCR Control⁺ model can effectively maintain the control performance of SCR DeNO_x system during large fluctuations of SCR inlet NO_x concentrations under substantial load variations and only little deviations are observed from the setting values. The system can realize fast, effective and stable control over the denitrification under all loading conditions, which meets the control requirements for ultra low emissions of NO_x and ammonia escape.

Key words: ultra low emission, SCR De-NOx, control model, ammonia injection control, state estimation