Written by Eni Oko (PhD Research Student, University of Hull). The author is grateful to the UKCCSRC for the opportunity to participate in this meeting through its ECR meeting fund
The presentation was given by Dr Evgenia Mechleri as part of the Process Engineering Technical Session at the UKCCSRC Biannual in Cambridge 2-3 April 2014. Dr Mechleri is a Research Associate at the Chemical Engineering Department of Imperial College, London. She completed a PhD in Chemical Engineering from the National Technical University of Athens (NTUA). Her research is funded by ESPRC under the Gas-FACTS (Gas-Future Advanced Capture Technology Options) Project (EP/J020788/1).
The study is driven by the realisation that post-combustion CO2 capture plants have limited capacity to be turned up and down to meet changing load demand in upstream power plants. Many approaches are reported in literature for meeting this objective. Dr Mechleri preferred investigating options related to developing better and robust control strategies for the process. This choice is justified on the basis of its cost and efficiency benefits. Control of post-combustion CO2 capture has been reasonably reported in literature. In these studies, the key control loop for maintaining capture involves a single PID controller loop for maintaining capture level. The capture level set point is maintained by manipulating the solvent flowrate with other loops maintaining the lean solvent temperature among others.
Initial study carried out by Dr Mechleri using Aspen HYSYS tool show that such controller configuration requires longer settling time to achieve stable conditions in capture level after a disturbance is introduced. More capability for load variability is expected from fossil power plants in the future as more and more renewable sources become available in the grid. On this basis, she predicts that such single loop PID controllers will not be able to maintain capture level since the power plant load level will predictably change more frequently. This necessitated the search for a new control strategy.
Using the same Aspen HYSYS, Dr Mechleri has successfully developed a cascade control loop for maintaining capture level by regulating the solvent flowrate as the power plant load changes. The controller comprises of two layers, namely master and slave controller. The capture level set-point is fixed and is used alongside the gas-liquid flow difference to drive the lower slave controller which regulates the lean flowrate. The results show that cascade control are more efficient in stabilizing the system than single loop PID controller with settling times several times lower. In the future, she expects to take the research forward by using MPC approach for control of the post-combustion CO2 capture.