Efficient decentralized control of the post combustion CO2 capture plant for flexible operation against influent flue gas disturbances

Abstract

The increase and accumulation of the CO2 greenhouse gases emissions raises the society’s concern and has become a stringent environmental problem. Flue gases released by thermal power plants are important contributors to these emissions and the scientific research community firmly engaged to find efficient carbon capture solutions. The post-combustion CO2 capture using amines is a leading technology, striving to continuously increase its efficiency and reduce its associated costs. Achieving these aims is progressively dependent on the flexibility of the carbon capture plant to cope with changes in its operation due to the power plant variable loading. Control systems have the potential and are responsible for fulfilling this task. The paper presents the design and assesses the performance of a 4 × 4 proposed decentralized control system intended to reject the most challenging CO2 influent flow rate and concentration disturbances emerging from the power plant, but also to satisfy the setpoint tracking ability demand. A previously developed complex model for the absorber and desorber columns was extended with a buffer tank and a cross heat exchanger meant to reduce the interacting effects between the columns and support the control system efficiency. The model was then scaled-up with the aim of building a novel simulation tool for an industrial scale carbon capture plant. Carbon Capture rate and Energy Performance indices have been selected as main metrics for evaluating the capture plant performance. Selection of the controlled and manipulated variables was based on a process analysis and the pairing of the control loops was conducted according to the Relative Gain Array matrix. The control system disturbance rejection performance was investigated considering the influent flow rate and CO2 concentration major disturbances, either with positive and negative slope ramp signals or with periodic changes and considering different testing scenarios. Setpoint tracking of the Carbon Capture rate ability was also examined. Simulation results revealed the capability of the decentralized control system to successfully reject disturbances, demonstrating zero steady state offset, reduced overshoot and short settling time control performance. Setpoint tracking also proved to be efficient. The synergic contribution of all control loops succeeded to achieve flexible control against the action of both ramp, constant or periodic disturbances. The proposed decentralized control system showed promising perspectives for the industrial implementation.