Decentralized nonlinear control of 300MWe circulating fluidized boiler power unit

The dynamics of large scale circulating fluidized bed (CFB) boiler power plant shows the behaves of large time delay, complex coupling and nonlinearities, which brings difficulties for controller design to improve the plant performance. This paper firstly introduces a well-established and validated nonlinear model of a 300MWe CFB power plant, and then the model is simplified through analysis and approximation which could show clearly the reason of difficulties associated with controller design. It is found that the coupling and nonlinearity mainly come from the opening of main steam governing valve under various load conditions, and it is hard to completely decouple the multivariable system. To avoid the difficulties of decoupling, a decentralized PID control system is finally proposed to control the unit power and main steam pressure after nonlinear model transformation. The simulation results show that the control system has improved dynamic performance in load reference tracking for a wide range of load changes, and also good disturbance rejection to the coal quality variation. The analysis and control in this paper provide the first step to move forward for future control quality improvement and advanced control strategy study.