Constrained control of a once-through boiler with recirculation

There is an increasing need to operate power plants at low load for longer periods of time. When a once-through boiler operates at a sufficiently low load, recirculation is introduced, significantly altering the control structure. This paper illustrates the possibilities for using constrained control to obtain optimal load gradients in the recirculation mode. A model predictive controller is designed for a simulation model with good results. It is also shown how the feed water flow can be used as an extra control signal.