Control of steam bottoming cycles using nonlinear input and output transformations for feedforward disturbance rejection

Abstract

In this paper we analyze the control problem for a steam cycle that produces power by recovering waste-heat from gas turbines on an offshore installation. The waste-heat recovery unit is based on once-through steam generator technology. The main disturbances are large variations of the gas turbine exhaust gas flowrate and temperature. We analyze the effect of these disturbances on the operation of the steam cycle, specifically for the superheated steam pressure and temperature. We compare the performance of different decentralized control strategies based on standard PID-controllers and nonlinear feedforward. We consider floating pressure and constant pressure operation strategies. For steam temperature control we implement feedback, and feedback in combination with nonlinear input and output transformations for feedforward disturbance rejection. These transformations are based on the steady-state energy balance on the waste heat recovery unit, while for simulation purposes we use a high-fidelity dynamic model of the bottoming cycle designed to minimize the weight and volume. The outcome from this work can be used to propose control strategies for coordinating a combined cycle (gas turbine with a steam bottoming cycle) that can operate with large and rapid changes in power demand.