Optimal Set-Point Scheduling in a Boiler-Turbine System

A 32nd order boiler-turbine model is developed to solve the optimal set-point scheduling problem for main and hot reheat steam conditions in a 235 MW gas fired electric generating plant. The model has 7 states in the process and 25 in the control system. The optimization strategy uses control vector parameterization in which the boiler controller set-point function generators are modified to produce suboptimum signals. The performance index consists of the total energy input to the system from the fuel and boiler feedpump. Constraints in the process variables are monitored by critical alarm limits, and hoop stresses in the heat exchanger tubes are estimated as well. The optimization problem is solved for steady state and dynamic conditions in the 75 to 235 MW load range. Savings in fuel between 0.7 to 4.0% are predicted by computer simulations in the normal operating load range of the unit. Preliminary plant tests indicate these estimated savings may be realistic. The boiler-turbine model and optimization strategy illustrate the potential use of mathematical models for power plants.