On the main page under 2.05 it is written:
A Large Drum Retort. Use a drum with a fairly tight lid. Place it on a stand over the hearth, and perforate the bottom of the drum to use the volatile gasses to fire the retort. The alternative is to run a piece of perforated pipe from the top of the drum to the firebox underneath.
However, this may or may not be an entirely sound adaptation, in that the heat provided by the flare comes too late in the process to be of much use. The counter to this opinion is that maybe using the flared heat is useful in that makes up for a lack of mass and insulation in the wall of the retort.
From Sean K. Berry:
I keep trying to say this. Pyrolysis of biomass has two distinct thermodynamic stages. The first is "endothermic", where input of external heat is required in order to bring the temperature of the feedstock up high enough, so that the second "exothermic" stage can begin. This requires heating the biomass up through about ~250-300C. In the "exothermic" stage, the pyrolyzing biomass is being converted into both charcoal and liberated gases, {H2, CO, CH4, CO2, H2O}. Additionally (this is the important point I have been trying to make), in the exothermic stage, there is no need to continue to supply external heat. The reaction itself provides enough heat to be self-sustaining at that point, as long as there is a supply of oxygen. The supply of oxygen is limited to prevent the char/feedstock bed from bursting into flames, ie. complete combustion (complete oxidation).
The biomass begins to eject flammable gases {H2, CO, CH4} instead of complete combustion gases {CO2, H2O} because the oxygen is limited.
But, there is plenty of heat. Adding heat will not accelerate this process, nor prevent the consumption of more charcoal Carbon. The only mechanisms that can do that are things like throttling back the inflow of oxygen, or removing the charcoal/biomass from the pyrolysis zone (ie. suffocate and cool the charcoal/feedstock).
This means that wrapping the gases back around to heat the pyrolysis zone is pretty much useless. The gases could be used to pre-heat or dry a cool wet feedstock, but this really needs a slower heating rate to do well, and combustion of the flammable component of the pyrolysis gases is much hotter than needed for this purpose. There are better uses for the chemical energy found in these gases and/or the quality of "sensible" heat that can be generated by combusting these gases.
Capturing the liberated pyrolysis gases is the best way to capture some of the "bio-energy" that is made available via pyrolysis of biomass.
Once pyrolysis and liberation of these gases is occurring, then throwing it back into the reactor would be as much a waste as FLARING it off above the reactor. Finding a real "use" for the energy is a better approach, I think.
I suggest that for your pyrolysis to be self-sustaining once past the exothermic stage, a reasonably-insulated container is required. The old-fashioned cover of soil would have provided this, but a tin metal drum will not. Thus the burning of the exhaust gases is a sort of substitute for the lack of insulation.
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