A detailed analysis of the effect of coal rank on the size and configuration of circulating fluidised-bed (CFB) boilers is presented. These data, the first in published literature, would help boiler designers and users to appreciate the extent of the fuel flexibility of CFB boilers. These boilers are known for their fuel flexibility; they allow the burning of non-design fuels, but with some performance penalty. The extent of this penalty is apparent from the difference in size and configuration of furnaces designed for various fuels. Unlike conventional pulverised-coal (PC) boilers, CFB boilers do not have a data base on the impact of fuel parameters on the furnace size. The present work used an Expert system (CFBCAD (R)) to investigate the effect of 57 coals of various ranks. On the basis of this study, a general guide for the design of CFB boilers for various fuel specifications was developed.