Control structures for heat delivery in compact bottoming cycles for heat and power production

Compact designs of combined cycle power plants based on gas turbines and steam bottoming cycle (CCGTs) are deemed as a promising technology for increasing energy efficiency and reducing greenhouse gas emissions of offshore oil and gas production facilities. The control of such systems can be challenging due to the need for operational flexibility regarding production of power and heat to satisfy the corresponding demands, and it differs from traditional onshore designs in dynamic characteristics and requirements. In this work, we propose and evaluate the performance of control structures for compact steam bottoming cycles with combined heat and power production, focusing on the solutions for satisfying heat demands and their effect on power production. The proposed control structures are based on the different prioritization of operational objectives and constraints, using simple control elements to switch between operating regions. The control structures were evaluated under different disturbances on the gas turbine loads and on the heat demand. It is shown that controlling the intermediate pressure in the steam turbine, which serves as source of steam for heat production, is necessary for achieving the heat demand objectives. We also show that sudden disturbances on the heat demand heavily impact the power production, and it is desirable that such disturbances happen on a ramp-like manner. Overall, we highlight how near-optimal operation with satisfaction of constraints can be achieved with the use of well-designed, simple control structures.