Exergy-based control strategy selection for flue gas recycle in oxy-fuel combustion plant

Abstract

Control system design is one of the key elements which need to be studied before commercial implementation of oxy-fuel power plants. Among others, the control strategy for flue gas recycle process should be firstly considered as it is one of the major differences between oxy-fuel combustion and traditional power plants. In this paper, a dynamic model combined with exergy analysis was firstly proposed to design the control system and evaluate the performance of potential control schemes for flue gas recycle process. The dynamic model had been extensively validated using static and dynamic data from a 3MWth oxy-fuel combustion facility. Based on the dynamic model, the characteristics of the flue gas recycle system was investigated and two possible control configurations, RR (recycle valve coupled with recycle fan) and SR (stack valve combined with recycle fan), were proposed. The control performances of the two candidates were tested in three typical types of disturbances usually occurred in the operation and further evaluated from the perspective of exergy efficiency. It was shown that both control loops could maintain the target variables (flue gas recycle ratio and recycled flue gas pressure) stable in the disturbances, while the total exergy destruction of flue gas recycle system in RR is 2.4%, 1.7% and 0.6% higher than that in SR during the disturbance tests, respectively. The exergy-based control strategy selection method proposed in this paper provides good insight to obtain the optimum control method for other subsystems in the power plant.