Charcoal Retort

I have had a couple of attempts making charcoal using 44 gallon drums and they have worked okay, but needing a new project after buying a mig welder, I decided to make a retort.

The theory of a retort is that you heat the wood to be made into charcoal up to a point where it starts expelling all of the wood gas and impurities, which is then fed back into the fire to keep the process going.

After reading up and watching a number of videos on rocket stoves and retorts, I decided to make one with the idea of combining the two into one rocket stove powered charcoal retort.

Step 1: Obtain and Flush the Cylinder

I started out with an expired 90kg propane tank purchased from a scrap dealer and a length of 100mm box section. The box section was the dearest part as the scrap dealer did not have this in stock, therefore I had to buy it new.

The first thing and most important thing to do was to remove the tap valve and fill the cylinder with water to ensure all the gas was removed before I started cutting and welding it. I filled the cylinder up until it overflowed to ensure all gas was gone.

Once the water had been emptied, I cut the top off the cylinder with an angle grinder.

Step 2: Make the Rocket Stove

To make the rocket stove I cut the piece of box section off on a 45 degree angle and welded it back together to form an L shape. I made the small section 100mm shorter so I could physically fit it into the cylinder and later I would weld the 100mm back on, once the box section is in place. If not done like this you cannot fit the stove into the cylinder and have enough poking out the front.

The longer part of the stove was made to length so it sits lower than the top of the cylinder.

Step 3: Weld the Rocket Stove Into the Cylinder and Add the Chimney

Next I cut out two 100x100mm holes in the cylinder. One in the top for the chimney and one in the bottom (front) for the rocket stove to sit. I cut these with an angle grinder and welded the rocket stove in place. I then welded 100mm more box section onto the front, so I had a decent amount out the front to hold the wood being burnt.

I welded a 50mm flat bar around the chimney top (lid) box section to seal off the chimney as the top sits on cylinder and internal box section to ensure a snug and aligned fit of the chimney section.

I also welded a piece of 50mm flat around the top of the cylinder to create a seal when the top fits on.

Step 4: Add the Retort Vent and Paint - Ready for Trial

I cut a hole in the top front of the cylinder and one in the top of the rocket stove and then welded some 25mm tubing and angle between the two points. This enables the wood gas to feed back into the fire once the charcoal wood heats up and produces this gas.

I drilled a small hole and mounted a BBQ temperature gauge into the cylinder also so I could monitor the internal temperature.

At this point I painted the whole thing with heat paint.

Step 5: Firing It All Up for the First Time

Once all painted, I loaded it half full with Tea Tree (manuka) to do a trial run. I fitted the top on with some force (hammer) as it was a tight fit. I needed to line up the chimney box section to get the top to go down and lock in place.

There was a small gap around the edges to the lid, so I used mud to fill this, as I had seen this done on other charcoal making videos and this seemed to work well.

I lit the fire and sat down to watch as I would need to keep feeding wood into the rocket stove.

The temperature increased quite fast and the rocket stove roared.

Step 6: Once It Got to Temperature It Started to Self Feed

Once the unit heated up to over 200 degrees Celsius, it started to put out visible gas which was sucked back into the fire and I assume burned. It also put out some sort of tar which was also burnt.

Once it reached 300 degrees Celsius the gas coming from retort was clearly flammable and the fire started to roar.

I stopped feeding wood in at this point as it had become self feeding with the wood gas.

I kept watching it for about an hour after this as every now and again it would do what i could only describe as "cough" and blow itself out, so I would start it again with a gas torch and it would be off again.

I decided to keep a few smaller bits of wood burning after this happened a few times and that seemed to fix the problem as it would relight if it coughed itself out.

Step 7: The End Result

The next day once it had cooled down, I removed the top from the retort with a hammer as it was locked on tight and emptied the charcoal out.

I tried all different sizes of wood in this first run and the larger pieces had not fully turned into charcoal but all of the smaller pieces had converted well.

I hope this experiment gives others some ideas and I would like to hear any recommendations to enhance this design.

I really wanted to prove the theory worked... which it definitely did.