The simplicity of the design of the version 1 retort allowed us to start making charcoal very quickly, but the toll of high heat on the components, the inefficient use of fuel, and the inconsistency of the finished batches left much to be desired.
The two areas of weakness in the first design were the loss of heat and the inclusion of oxygen into the retort. The heat loss was mainly out the single side walls of the retort chamber and up the chimney due to combustion occurring too late in the flow. The increasing oxygen problem was due to heat damage to the lower lid and support bars that opened larger gaps into the retort chamber each time the unit was fired. Heat damage was worse than expected because so much forced air was required to get the retort up to temperature due to the heat loss issues.
Version 2 is a slightly more sophisticated design partly inspired by the work of Baja Rob and Biochar Costa Rica, though they focus more on controlled low temperature burns and extraction of other by-products for small-scale farm applications.
The design uses three nested cylinders, the central to contain the charcoal wood, the second as a combustion chamber, and the outside to add an insulating air layer. The central cylinder is fairly well sealed except for a pipe out the bottom with a valve to direct the wood gas into the combustion chamber. The major improvements are that the retort chamber has a tighter seal and a much larger surface area exposed to the heat. Another addition is a simple heat shield to protect the bottom of the retort chamber from direct flame.
The retort chamber was made from two 16 gal. drums (though 20 would be ideal), the combustion chamber from a 55 gal. drum, the insulation jacket from welded sheet steel (though it could also be made from a 55 gal. drum with a section welded in from another drum), and the lid and heat shield from scraps of sheet steel. All piping is 2″/50mm steel fittings commonly available. The valve (fabricated for a forge blower by Lester of Dragonfly Iron) was intended to control the steam output near the beginning of the cycle but is more useful in sealing off oxygen during cooling.
The first run used some reclaimed scraps of old growth fir that had already been reclaimed for another project via Demxx and provided by Peter and Denise. The finished charcoal was very clean without any white ash which indicates reasonable isolation from oxygen and the inside of the retort chamber was very sooty and free of tar which indicates a reduction environment with high burn temperatures.
After the success of the test run, three retort loads of cedar scraps from Dave Bull’s mill on Chatsworth provided a good stock of charcoal for the next few weeks. Thanks, Dave and Doug!
Retort chamber loaded with wood:
Sealed for firing:
Combustion chamber and insulating jacket added:
Chimney and lid:
Wood gas valve and output:
Lid doubles as drying rack for fuel wood:
Running on wood gas, near the end of the cycle:
The finished batch of charcoal:
Second run with a batch of cedar scraps:
Before and after comparison:
A cut-away illustration of the retort construction and functioning:
Interesting oxide and scale on the insulating cylinder:
Read more about the charcoal making process and why we do it: How Charcoal is Made
Read some background information on fuel alternatives: Sustainable ‘Smithing?
A better compact charcoal kiln design: Charcoal Kiln V.3
A larger traditional charcoal kiln design: Charcoal Kiln V.4
Follow the charcoal making progress: All posts tagged Charcoal