A circulating fluidised bed boiler runs at 840 to 900 °C like a fluidized bed, but at gas velocities of roughly 4 to 9 m/s that carry the bed material up through the furnace. A cyclone separates the solids and returns them to the base through a loop seal, so the fuel circulates many times and burns out completely. There are no tubes immersed in the bed; the membrane water walls and the convective pass do the work. Limestone in the circulating solids captures sulphur at high efficiency, and fuel flexibility extends to high-ash coal, lignite, petcoke, biomass and waste-derived fuels. The design is generally the economical choice above about 75 to 100 TPH of steam.