and a bottom fluidization air distributor plate 16 fluidization air is supplied through air plenum 18 which is connected to an independently controllable fluidization air source 20 the contents of container 12 for the combustion bed are a combination of bed material such as sand or other granular material known in the art reference numeral 22 and fuel for example coal reference numeral 24 fuel is inputted into container 12 by means conventional and known in the art and which are not shown in fig1 container 12 is constructed to retain the bed material 22 and fuel 24 during combustion and over the range of fluidization air pressure levels for the fluidized combustion bed in fig1 container 12 is cylindrical in shape a second annular container 26 surrounds container 12 annular container 26 has cylindrical vertical walls 28 and an annular air distributor plate 30 which extends between vertical walls 28 and vertical walls 14 of container 12 a second air plenum 32 channels fluidization air from an independent separately controllable fluidization air source 34 into annular container 26 which contains only bed material 36 sand or other granular material both container 12 and annular container 26 open to a common exhaust outlet 38 air exiting the heat transfer bed serves as secondary air for combustion by mixing.