A fluid bed boiler and combustion method are provided having a first fast bed combustion zone followed by a second slow bed combustion zone. Coal, as a typical fuel, and limestone are fed to the first zone, where the coal undergoes combustion and sulfur is captured by the limestone. The solids which flow from the top of the first combustion zone are separated and introduced into a low velocity "slow" bubbling fluid bed in which arrays of heat exchange tubes are immersed. Additional heat transfer to water can be achieved by employing water walls for the first fast bed combustion zone.