Process Control Strategy of Amine-Based Post-Combustion CO2 Capture Systems

Abstract

CO₂ removal with alkanolamine solvent is the most advanced technology to be deployed on a large scale; the CO₂ capture pilot plants from worldwide had already demonstrated the feasibility of this technology. The main drawback of the implementation of CO₂ capture unit is the high energy penalty associated with CO₂ absorption process. For power plants equipped with carbon capture, the load-following operation of the power block will also affect the post-combustion CO₂ capture unit. The implementation of a control strategy is necessary for the flexible operation of CO₂ capture process during the periods of fluctuation in the plant loading. The main objective of the control system is to assure the efficiency of the process EP, which means a high CO₂ capture rate CC% with minimum requirements of energy for regeneration of the solvent. The desired values for these operating goals are situated in the range of 80 – 90 % for CC and 3 – 4 MJ/kgCO₂ for EP. Another objective of the control strategy is to maintain a constant molar ratio between inlet CO₂ flow and MEA flow introduced in the absorption process (to assure enough solvent flow to capture at least 80 % of the CO₂ and not wasting it). The molar ratio can be controlled either by solvent flow rate or by solvent concentration, using a ratio controller. EP index is controlled indirectly through the CC parameter, CC is controlled by the reboiler heat duty, molar ratio is controlled by solvent flow rate while temperature and level in the buffer tank are controlled by thermal agent flow rate, respectively fresh MEA flow rate introduced into the buffer tank. The designed control system merges ratio, feedforward and feedback decentralised control. The simulation results showed that the control strategy proposed can handle the fluctuations in the upstream power plant load and the imposed set-point changes, providing flexibility to the operation of the process.