An apparatus for extracting energy from fiber introduced as a fuel into a vertically elongated combustion chamber having a suspension burner capable of projecting a flame down the axis of the combustion chamber with a heat collection surface located radially from the flame and below the burner, and an exhaust opening located below the flame and below at least a portion of the heat collection surface, in which the combusted fiber yields a mixture containing hot flue gas and molten ash above the exhaust opening and heat transfers therefrom to the heat collection surface prior to any substantial contact of ash to a surface of the combustion chamber, to yield a mixture containing warm flue gas and non-molten ash, that is thereafter cooled to yield a mixture containing cool flue gas and non-molten ash.