Modelling and dynamic simulation of a supercritical, oxy combustion circulating fluidized bed power plant concept-Firing mode switching case

Dynamic process simulation provides a tool to evaluate operational issues of a new process concept before the plant construction. This paper studies a carbon capture and storage (CCS) capable power plant concept with a model including a supercritical once-through CFB boiler with gas and water steam sides, a turbine island, an interface from the air separation unit (ASU) and the control system to manage typical operational transients. Switching between the air and oxy firing modes is one of the key operations in oxy combustion processes. The selected mode switching strategy uses simultaneous linear ramps for the mass flows of the primary and secondary air, oxygen, and recirculated flue gas. The results show that the firing mode can be successfully switched within 25-37min. The flue gas path difference between the air-firing and oxy-firing modes due to the flue gas recirculation causes significant differences in dynamic behaviour. The simulations emphasize importance of good control and coordination of the gas flows. Feedback control of the flue gas and/or oxidants O2 content during the mode switching is suggested to improve robustness against disturbances, for example, in oxygen delivery, flow measurements, fuel feeding and combustion.