(165d) A Multivariable Model-Based Control Framework for a Post-Combustion CO2 Capture Plant

The future of fossil-fired power plants involves the reduction of Carbon dioxide (CO₂) emissions through the application of CO₂ capture and sequestration (CCS) processes. One of the most feasible CCS strategies to be integrated with different power plants, i.e., Integrated Gasification Combined Cycle (IGCC) and coal-fired combustion power plants, is the post-combustion CO₂ capture process using Mono-Ethanol-Amine (MEA) absorption. Therefore, the integration of CO₂ capture process and power plants will increase the need to design advanced model-based control systems to overcome the process interactions and maintain dynamic operability of power plant and CCS unit. Almost all the control schemes that have been recently proposed for the CO₂ capture process are focused on decentralized multi-loop control schemes [1-4]. A comprehensive model-based controller for the complete CO₂ capture process has not been addressed in the open literature. To the authors’ knowledge, only one study have evaluated the controllability of different operational regions in a post-combustion CO₂ capture plant using 2x2 model-based multivariable controller [5].

In this study, an advanced multi-variable Model Predictive Control (MPC) scheme has been designed to evaluate the controllability of a post-combustion CO₂ capture unit in the presence of disturbances that may appear in the actual operation due to the plant’s interactions with coal-fired power plants. The plant’s dynamic model and the MPC-based control system were implemented in Aspen HYSYS and MATLAB, respectively. For the MPC-based control scheme, an instant client-server data communication link between the process simulator and the control framework was designed by the means of component objective model programing. The performance of the proposed MPC strategy for the CO₂ capture plant was evaluated under various scenarios and compared against the performance obtained by a decentralized multi-loop control scheme. The controllability study have shown that the proposed MPC-based control scheme rejected the disturbances faster with less deviation in the set points and did not violate the process constraints compared to the decentralized control scheme. In order to provide insight into the operation of a CO₂ capture plant, the proposed MPC-based control strategy and the CO₂ capture plant were embedded within a dynamic multi-objective optimization framework to search for the optimal scheduling of CO₂ capture in the presence of sinusoidal disturbances in the flue-gas flow rate. Two different scenarios were considered in the optimal scheduling problem: i) low CO₂ emission scenario where total CO₂ capture is reduced by 2.3%; ii) high electricity generation scenario on which the reboiler unit was only allowed to extract 37% less energy than that used for the base case scenario. The insight gained from the optimal scheduling study will support the design of optimal operating policies for the integration of CO₂ capture to coal-fired power plants by considering the dynamic performance of the system in closed-loop.

 References

[1]	Thanita Nittaya, Peter L. Douglas, Eric Croiset, Luis A. Ricardez-Sandoval, "Dynamic modelling and control of MEA absorption processes for CO2 capture from power plants," Fuel , 2014;116:672–691.
[2]	Panahi M, Skogestad S. , "Economically efficient operation of CO2 capturing process part I: self-optimizing procedure for selecting the best controlled variables," Chem Eng Process , 2011;50(3):247–53.
[3]	Lawal A, Wang M, Stephenson P, Koumpouras G, Yeung H. , "Dynamic modelling and analysis of post-combustion CO2 chemical absorption process for coal- fired power plants," Fuel, 2010;89(10):2791–801.
[4]	Lin Y, Pan T,Wong DS, Jang S, Chi Y, Yeh C., "Plantwide control of CO2 capture by absorption and stripping using monoethanolamine solution," Ind Eng Chem Res, 2011;50(3):1338–45.
[5]	Panahi M, Skogestad S., "Economically efficient operation of CO2 capturing process part II: control layer," Chem Eng Process, 2012;52:112–24.