Evaluation of control strategies in CO2 capture unit

Abstract

Abstract Post Combustion CO2 Capture process (PCC) is a popular technology for regulating CO2 emissions from Coal-based power generation plants. The process uses Mono Ethanol Amine (MEA) solvent to absorb CO2 from Flue gas. The CO2 rich solvent is regenerated in a stripping tower with kettle reboiler using process steam as heating media. The lean MEA solution is recirculated back to absorption column, and the recovered CO2 is taken for further processing depending on the nature of demand. The PCC unit control set-up has challenges in coping up with process upsets caused by variations in power demand resulting in lower operating rate for the power plant. The rapid reduction of flue gas flow rate to the absorber leads to a decreased load on the regeneration stripping tower, necessitating steep reduction of the steam flow to the reboiler. The problem is caused by the dynamic response rate of the regenerator-reboiler combine, which must match the fast-dynamics of the flue gas feed rate. The process upset in the absorber-stripper system takes time to settle down, leading to CO2 slip from the PCC unit. The loss of CO2 is not only an economic concern but an environmental issue as well. This study intends to observe this problem in a simulated environment and evaluate options to ensure faster stabilization of the MEA absorber-regenerator system; to minimize loss of CO2 during changes in production rate of the power plant. The study involves developing a high-fidelity dynamic simulation model of the MEA absorber-stripper unit using 1st principles unit-op models and rigorous thermodynamics on a commercial dynamic simulator platform. Once the steady state conditions and disturbance caused in the system due to plant turn-down have been successfully replicated; the model is used to study the impact of process upsets on key operating parameters like solvent flow rate to absorber, regenerator bottom and top temperature, steam flow-rate and temperature of the reboiler, CO2 slip from the absorber and the key parameter of overall CO2 capture by the system. The next step is evaluation of various control schemes and operating strategies in their effectiveness of controlling the key process parameters viz. lean solvent flow rate to absorber, ratio of solvent flow to feed gas rate and regenerator bottom / reboiler temperature. In the final step the optimal control scheme and operating strategy to be followed by plant operators for the absorber-stripper system is identified. The selection is made based on the capability of the strategy to quickly react to the process upsets caused by reduction in power plant production rate, and to establish steady state operations in the least amount of time; reducing CO2 loss during the process transients.