Charcoal Retort Day 4

The weather was great and today’s burn was like a party with everyone out to help chop wood, fuel the fire, and roast marshmallows.

The official version 1.0 of the charcoal retort was based on the 0.9 design but incorporated a better airflow design via a grate and used bricks rather than logs as a structural base. The two major improvements over version 0.9 are that the air will be drawn through the lower fuel rather than over it, and that the closer seal around the base of the drum will allow much better control of the air draft and the wood gas.

Right after light-up, breaking in the new brick base and scrap iron air grate
Building a charcoal retort.

To prepare the new base, I dug a trench, lined the sides with bricks and placed an angle iron grate over it and a clay pipe leading in from ground level to act as a tuyère. Above the level of the grate, two rings of bricks supported around the outside by stones form a nice fire pit. The gaps are mortared with a mud and ash mix except for a few removable bricks above and below to serve for fueling and ash clean out respectively.

The subterranean clay tuyère providing draft-drawn and hand-cranked air to the grate.
Building a charcoal retort.

We loaded the drum with the leftovers from the previous burn and some construction cut offs from the guys at Pacific Ridge Homes and sealed it loosely with the bottom lid and angle iron.

Mostly reno scraps from Chris and his crew, the fuel wood was very dry this time. With the enhanced draft from the hand cranked blower, starting up was quick and easy. The blower and grate helped get up to operating temperature much quicker, but air had to be used sparingly as it was easy to go over-pressure and lose heat and wood gas out the side intakes.

Running over-pressure with the hand-cranked blower
Building a charcoal retort.

After about an hour, the retort was up to heat and there was time for a bit of “bush ‘smithing” with the old anvil and some small round stock. The lower combustion chamber was hot enough for low temperature forging, and by the end of the burn, the angle iron supports had a couple of inches of bow in them as well.

Running a bit smoky after a recent re-fueling
Building a charcoal retort.

Back up to heat with a clean burn and a good yellow-white glow
Building a charcoal retort.

After a couple of hours, there was a large amount of wood gas being produced and there was ample combustion all the way up the chimney and out to the air for several feet. After this, we stopped adding external fuel and let the burning wood gas supply heat to the retort. At this stage Neido~chan was busy using a red-hot iron rod for some wood-burning art.

Up to four foot high burn-off flares for a few minutes during the external fuel wood to wood gas transition
Building a charcoal retort.

Over the last hour, the coals in the lower combustion chamber slowly burned off and the wood gas took over as fuel. The final half hour was entirely fueled by wood gas streaming out around the bottom lid and igniting as it traveled into the upper combustion chamber. This was the original goal and a very welcome sign. The centre pipe of the upper combustion chamber was glowing at a low red from the bottom almost to the top. When the wood gas stopped burning, we sealed all of the lower holes and openings with soil and put a steel plate over the top of the upper combustion chamber to exclude as much oxygen as possible while it cooled for the night.

Wood gas flame being drawn inward and up the rocket stove combustion chamber
Building a charcoal retort.

The next morning when we opened the retort, there was a much higher percentage of completed charcoal than the first burn. Approximately three fourths of the retort contents were ready and the remaining fourth was more than halfway to completion. We will set it aside and reprocess it with the next batch. The charcoal was sorted, chopped, and stored in about an hour, ready for the next project in the forge.

Chopping and sorting the charcoal on a beautiful west coast winter morning
Building a charcoal retort.

The yield from the retort, sorted, chopped, and ready to forge with
Building a charcoal retort.

The brick base and air grate system proved to be significant improvements over the prototypes and this system was much more efficient in many ways. The one remaining area of improvement that was discussed after the first tests would be to insulate the outside of the drum in order to contain the heat better.

Marshmallow Cloud Dragon

A perfectly carbonized-but-not-consumed marshmallow, left in the retort right before sealing at the end of the burn
Building a charcoal retort.

Find out about the construction and operation of our new and improved charcoal kiln: Charcoal Kiln V.3