Optimal control for fast boiler start-up based on a nonlinear model and considering the thermal stress on thick-walled components

The basis for control optimization is a nonlinear model for a high-pressure bypass and a drum-type boiler consisting of furnace, economizer, evaporator, superheaters, attemperator, and thick-walled components such as boiler drum and superheater header. After the mathematical formulation of the boiler start-up optimization problem and the cost function, including the related hard constraints (e.g. control bounds and thermal stress limits), the optimal reference and input control is calculated using successive quadratic programming techniques. Results of the optimization process are optimal reference values for the boiler firing rate, steam pressure and steam temperature, and the positioning of the control valves for attemperator and high-pressure bypass during the start-up phase. The control structure applied minimizes both the fuel consumption and start-up time of the boiler and is similar to a model-following control application.