An integrated real-time optimization, control, and estimation scheme for post-combustion CO2 capture

Abstract

This study presents a novel operational scheme for post-combustion CO2 capture (PCC) plants downstream from fuel-fired power plants. The approach is comprised of real-time optimization (RTO), nonlinear model predictive control (NMPC), and moving horizon estimation (MHE) layers. These layers are integrated to operate the system economically via a new economic function that accounts for the most significant economic aspects of PCC, including the carbon economy, energy, chemical, and utility costs. The proposed approach was employed on the case study of an MEA-based PCC absorber section, which uses a mechanistic process model to provide an accurate representation of the system. The NMPC layer is novel in its ability to enable flexible control of the plant by manipulating fresh material streams to impact CO2 capture and the MHE layer is the first to provide accurate system estimates to the controller with realistically accessible measurements. The proposed scheme was subjected to a cofiring scenario, whereby the switching between two fuels (i.e., biomass and coal) is reflected in the flue gas composition. In this scenario, a ∼19% steady-state cost improvement is observed with respect to the pre-disturbance cost. Moreover, the MHE was shown to cause an acceptable ∼0.5% of performance loss in the process economics through its effect on the NMPC. The scheme was also subjected to a ±20% diurnal variation in power plant load through steps in the flue gas flowrate and was found to provide consistent steady-state economic improvements (from ∼12% cost improvement to ∼17% loss abatement) for each of the disturbances observed. Furthermore, a price variation scenario highlighted the operational dependence of the system upon changes in economic incentives via the prices. When compared to a ‘no RTO’ case, the scheme was found to yield economic improvement ranging from ∼3% to ∼14% depending on the pricing. All scenarios in the case study displayed steady-state cost savings that exceeded the energy penalty imposed on the power plant by the PCC plant. This suggests the proposed scheme is an effective framework for the economic operation of a general class of PCC plants (i.e., with different solvents, process designs and control schemes, etc.) and can help enable the viability of PCC for the continued use of fuel-firing.