To achieve the UK’s ambitious target of reducing the greenhouse gas emissions by 80% by 2050, the UK’s conventional power plants must be operated in a more flexible manner in terms of high efficiency, using alternative fuels such as biomass and integrating technologies such as energy storage and carbon capture and storage. Currently the most advanced solid fuel-based conventional power generation is the ultra-supercritical (USC) steam cycle combined with Circulating Fluidised Bed (CFB) combustion technology. USC/CFB has several advantages over pulverised coal (PC)-based USC power generation, especially regarding fuel flexibility. There are still many fundamental research and technical challenges such as the hydrodynamic behaviour when coal/biomass co-firing is used as flexible feedstock. In this study, a steady state model of power plant system using CFB technology is established using the commercial software Aspen Plus. Unlike the traditional way in which a fluidised bed is discretised into a series of reactors, a compact and more realistic module FluidBed is explored to simulate an industrial 600 MW USC/CFB power plant for the first time. The hydrodynamic behaviour inside the CFB boiler and external recirculation loop including external heat exchangers has been investigated. Ash mass balance model is constructed considering various particle size distribution of coal, ash and additional bed materials. The fluidization quality has accordingly evaluated by a number of factors when coal and/or biomass is used as the fuel feed. The simulation results such as the bed pressure drop, bubble size and velocity, bed voidage etc. have provided useful guidance on the flexible operation of a USC/CFB power plant system.