A method for extracting energy from biomass depleted of at least some carbohydrate, at least some oil, or both by a) introducing the biomass into a vertically elongated combustion chamber having i) at least one suspension burner at the top of the combustion chamber which is capable of projecting a flame down the axis of the combustion chamber, ii) a heat transfer apparatus having at least a portion of a heat collection surface located radially from the flame and below the burner, and iii) an exhaust opening located below the flame and below at least a portion of the heat collection surface; b) combusting the biomass to yield a mixture containing hot flue gas and molten ash above the exhaust opening; c) transferring heat from the hot flue gas to at least a portion of the heat collection surface substantially by radiation 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 and having a lower molten ash content than the mixture yielded by the combusting step; and d) cooling the warm flue gas with cold gas, to yield a mixture containing cool flue gas and non-molten ash and having a lower molten ash content than the mixture yielded by the transferring step.