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Picture of 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

Picture of Obtain and flush the cylinder
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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.

windoz4 months ago
I think you did a good job on this project, but I did notice your amperage on your welding unit was probably set a bit too low based on the welds.
Kuzmanic (author)  windoz4 months ago
Thanks for the tip, the main problem was i has a disposable argoshield bottle that had run out and i didnt realise so most of the welds were with no gas.
windoz Kuzmanic4 months ago
Your stick was melting okay, but the workpiece wasn't quite hot enough. Sticks with flux don't require a gas shield as they make their own.
JosephA1864 months ago
FOR what do you use the charcoal for?
Kuzmanic (author)  JosephA1864 months ago
BBQ cooking.
ubosnugo4 months ago
Great idea! I think you are really onto something with this concept.
Have you thought about insulation with stone wool or something similar?
Kuzmanic (author)  ubosnugo4 months ago
I saw one that you sat on a fire that was a 44 gallon drum and they wrapped that to keep the heat in. This is why i chose a vessel with a smaller diameter as i thought this would not be needed. Grteat sugestion though may try.
Hey buddy, good work!
I know this design.
I have built 3 variations of it so far.
It can make perfect charcoal.
It can also break your heart!

To stop the extinguishing of the flame you need a "turbolator", like the one in the video, which is basically a disc almost cut through from either side then twisted to offset the 2 halfs. It could be anything really, It disrupts the airflow and somehow magically stops the puffing and chuffing.
Insulation, never mind glass wool or anything else of that sort, it will eventually go up in flames and you get to breath the nasty, which is plain nasty.
You need ceramic wool or the affordable way, which I did, was to make a box around it and fill with loose vermiculite.

Your timber feed stock for charcoal and fuel must be super dry. I live in Scotland, its damp here a lot, this is one of my biggest challenges.

Secondary air. Figure out a way to introduce secondary air, this I believe is a major stumbling block of mine so far.

Taz above said it.... Bigger diameter burn chamber.
James Hookway, the inventor of this design states no smaller than 6" or 150mm diameter burn chamber for the rocket stove. Mine is 125mm as that is what I had, but it bugs me to this day if this is the root of my charcoal woes..

Mind you, best charcoal I ever got out of this was with a cookie flue setup, I had 2 x 90 degree elbows as I was redirecting it out of a window in a stone building, maybe that acted as a baffle, but omg, see when this thing really, really gets going, you will bow your head to forces far greater than yourself....

Thats all I can think of just now, but please feel free to hit me up if you want to talk turkey some more..

Good luck!!!

Really like the temp guage btw, Nice!


Kuzmanic (author)  PaPa RaMs4 months ago
Thanks for your feedback i will look at making a few changes.
DaveH_NE4 months ago
Wood that has dried one year works best. This gives enough time for most of the moisture to evaporate but still retain the volatile oils. The moisture left in the wood has to be heated up to create a gas, which is not flammable. Consequently, the moisture is robbing heat which can be better utilized to volatilize the oils which you want to burn to create a hotter fire. A hotter fire makes better charcoal.

Cut your wood into smaller lengths, that will enable the oils to get out of the wood easier. I generally cut their length to a few inches.

You may want to consider an angled feed ramp so your heating wood will be self feeding. Still provide a horizontal tube for clean out and fire breathing. Use the angle ramp for retort access then the tar can drip closer to the heart of the fire.

Nice job!
brysown14 months ago
Great Project & helpful comments too, “green” wood will produce the black stuff & it is flammable. Not sure if it built up in your burn chamber but could cause unwanted results. In regular wood burning stoves it can cause chimney fires
Nice piece of kiwi ingenuity!
bpark10004 months ago
You are adding heat to the charcoal only around the central tube. You need to insulate the outside surface of the tank to prevent heat loss. You could also speed the process by lighting up the lower pieces of charcoal wood, loading up the rest, and feeding some air into the charcoal chamber while you are firing up the rocket stove. Then cut off the air to the charcoal chamber and finish.
Another technique is to load up the charcoal chamber, and light up the top layer just before you put on and seal the lid. Feed air into the bottom of the charcoal chamber. As the fire burns down inside, the upper layers are protected from oxygen by the burning layers below. As soon as you see fire at the bottom, you cut off all air and allow to cool. The bottom layer will not be fully converted, so use this for the next run.
obillo4 months ago
Pretty impressive, but I'm sorry you had to buy the box section. Don't know where you live but here in NYC and I presume other dense urban areas one of the commonest--and most ignored--street-find materials is the mattress frame. They're made of steel angle iron and run 6 ft long and more. Should be easily welded into box sections.Think it would work?
That's a lot of welding. Maybe a piece of pipe would work, and you can get 90 degree sections too.
eddand4 months ago
nice job done on the welding and construction of this piece too.
eddand4 months ago
there are a few things about this design that make it less efficient for producing charcoal. the most critical one is that the rocket stove is designed to produce maximum heat out of the top of the stack pipe not the side of the pipe.this is why you got an incomplete burn for biochar. the fire that is cooking/pyrolysing the wood must have a long residency time to transfer heat to the compartment that is pyrolysing the material being turned into biochar. perhaps you could lay the machine on its side there by slowing the exhaust gases out the chimney, connect the wood gas tubes to collect the gas from the top of the horizontal tank to several locations spread along the bottom of the outside tank spreading the secondary combustion heat more evenly along the tank. the unit could still be upright to be loaded with wood then laid down perhaps in a cradle to be burned. you will still need to experiment with the size and dryness! of the pieces loaded on the outside /primary burn container and the biochar pyrolysing container. good luck in your development process.
DavidJ4984 months ago
You're making a bio-char retort. Here are photos of my bio-char ghetto setup.
Ghetto Bio-char - 1.jpgGhetto Bio-char - 2.jpgGhetto Bio-char - 3.jpg
itzsnitz4 months ago
I suspect the fire coughed out due to insufficient O2 for combustion. An orifice at the end of the return tube will reduce fuel flow and should help. Perhaps weld a short coupling to the end, and then add a pipe plug with a small hole drilled in it; this way you can change the pipe cap and can experiment with different hole sizes to find the right orifice diameter. Definitely use a lot of anti-seize on those threads if you ever want to remove it. Also, adding a few holes in the side of the square tube near the end will increase the open area allowing for more fresh air draw; if you do this with say one 1/4" and one 1/2" hole it will also whistle letting you know when combustion is dying down. Adding mineral wool insulation will help with the conversion of larger pieces, but realistically I think the only way to get those larger pieces to complete the process will be a second burn. I would recommend just starting another fire in the rocket tube with more raw material once the gas discharge rate gets too low to sustain the fire. A second heat cycle should not damage the completed charcoal but should help get the larger pieces through the conversion process. Alternatively, once you unload the unit, toss the incomplete pieces back end at the bottom with fresh smaller wood on top and run it again; that should force the larger pieces from the previous burn to complete their conversion.
Taz-Hood4 months ago
Wonderful proof-of-concept; WELL DONE! I'm admittedly ignorant, so I wonder what increasing the diameter of the return tube would do. Also, would there be any advantage to making the return tube removable/replaceable to be able to clean out tar and experiment with its diameter? Two things I don't understand: 1.) Do you have to feed wood through the air intake? Could there be a separate "door" through which you could introduce larger pieces of wood? 2.) Given the high temperatures you achieved, can you speculate as to why the larger pieces weren't completely carbonized? Would it just be a matter of letting the stove run longer? ANYWAY, please forgive me if my questions are stupid or obvious. You've done a fantastic job with this Instructable, and I thank you very much for sharing it. I'm in awe of your knowledge and abilities. With kind regards, Taz
AndrewG3284 months ago
Well done! Perhaps the gas gets a bit too rich when the heat spikes. May be a blow off valve and control mech to regulate the gas flow. Awesome build though. That welder probably just paid for itself.
interesting perspective.. yea this could work too.

<thinking outloud>
The 1st video seemed to catch this 'glitch'. If I remember the roar kind of changed right before it 'popped'.

Perhaps a resonance - a pulse of high/low pressure. So yea, the front of the high pressure could be rich, followed by lean where the now burning gas expands into & 'poof' blows out the flame?
</thinking outloud>

I wonder if a baffle would help.Extending the opening a bit can tune it to not back draft and pull smoke out of it and suddenly blast it with air and put it out. Get some box scrap large enough to slip over the end to experiment with the right length. Now I want to make one! Great project!
Agree and also suggest varying the intake opening. Maybe a chunk of AL dryer vent hose could help tuning?

Beautiful design & job! Hearing the thing roar was a pleasure! Bravo