discussed below the temperature of the flue gas decreases and each higher elevation retort chamber is maintained at a lower temperature than the physically lower retort chambers the flue gas eventually exits from the staged retort through a flue cap 44 the combustion tube 40 is located within a heating chamber or combustion compartment 46 which can be seen in fig1 and 4 the carbonaceous material which is introduced into the combustion tube 40 is the carbonaceous residue remaining from the last retort chamber located above it the residue is very hot and when exposed to an oxygen carrying gas burns to produce a significant amount of heat the combustion tube 40 is provided with one or more lower apertures 48 through which the oxygen carrying gas enters and contacts the carbonaceous residue the combustion tube 40 is also provided with one or more upper apertures 50 through which the hot flue gas created by combustion exits and circulates upwardly in close proximity to the retort chamber walls in the preferred embodiment the combustion tube has one upper aperture 50 running for nearly its entire length so that it is open at the top to facilitate a quick and complete burning of the flammable materials remaining in the carbonaceous residue the open.