In an atmospheric bubbling fluidised bed boiler, air blown through a bed of inert material and fuel keeps the bed fluidised and burning at 800 to 900 °C, below the ash softening point. Heat transfer tubes sit in the bed and in the convective passes above it. The low combustion temperature keeps thermal NOx down, and limestone fed to the bed captures sulphur in place, so flue gas desulphurisation can be omitted. The design is far less sensitive to fuel and ash properties than a conventional stoker or pulverised fuel boiler.