There are a heap of ways on Youtube to make Biochar . One good one was a fellow who digs a trench to make it in and I will have to try this one day . I'm not much of a digger so I opted for an above ground solution using a 44 Gallon drum of any kind and an old large gas /propane bottle ,a cut off wheel ,grinder and a welder. This method is producing very good biochar and takes about half a day all up .

I am extraordinarily pleased because I have been meaning to do this for a long time and I have purposely kept all my timber offcuts for 40 years so they would not be wasted. Now the good timber goes in the "Pot" and its heated by burning old rotten and broken fence posts and rotten pallet wood that I have also stored outside.

The big plus for me is it is said that Biochar stays in the ground for a thousand years and adds greatly to soil health and fertility. The black soils of South America (Terra Pretta)are claimed to be due to Indian use of fire remains in their gardens.

Supplies

Wiki Terra Preta

Step 1: Making the Outside Barrel

Now beware , people cutting gas and fuel bottles have been badly hurt and even killed so how you ensure the gas bottle is safe to cut is your responsibility. What method you use is also your choice from grinders to cut off wheels to plasma cutters or oxy accetylene . You are the one taking the risk so be sure its safe .

Take any 44 gallon drum and cut a flap down the bottom about 6 inches by 9 inches . Cut 3 sides so it can be bent up on the 4th side by a hit with the hammer. Cut the lid off it any way you can . Some people use a sharpened piece of 1/4 inch by 2 inch steel flat bar and hammer it from the back all round the top. My DaD showed me how to do that. You can cut the bottom hole with it too if you wish . I prefer a disk cutter these days but the steel chisel method is good for a starter because it usually makes no sparks so it doesn't ignite anything that was in the drum beforehand. .

Drop the cutout, if it will go, down to the bottom of the drum. It just insulates it a bit more. You could even put a bit of insulation under it if you got real keen. Secure a bit of corrugated tin or shed siding big enough to go over the top . Once the fire is away this will sit on top of the drum to slow the air flow.

Step 2: Make the Inner Retort.

Using a full size gas bottle cut through about 8 inches from the bottom weld and cut right round to separate it . Cut another ring of 2 inches width from the same offcut piece . Keep that for the lid.

My method here was to buy an already cut gas bottle so I was then able to safely mould it to my needs.

I had a square of boiler plate in about 3/16ths big enough to cut down for a lid . Any suitable sized piece of steel will do over 1/8th inch thick.

Cut the hoop of 2 inch you cut from the drum across to break it and place it round the outside of your retort . Shape it a bit with a hammer to fit round the outside and you will have a gap left now . Find a suitable piece of 2 inch strap and weld it in to complete the outer hoop. When you have fashioned this to a suitable tolerance place it round the retort and upend the lot so you can use a plasma cutter or cut off wheel to cut around outside the hoop . Then weld the hoop to your lid in a few places . There can be gaps . That is where some gasses will escape. I drilled about 6 of 3/8ths holes down the bottom of my retort so the expelled gasses would also feed the fire and assist with heating the retort.

Now You need some legs on the bottom of it to suspend it above your fire so I cut 3 pieces of 1 inch gal pipe about 10 inches long and crushed one end with a big hammer so I could conveniently weld them to the sides of the retort with a little bit of an angle. I welded some scrap on the feet and then some steel mesh a couple of inches up from the floor so the air could get in under the fire.

Step 3: Filling the Retort

I had a lot of small scraps of 2 by 3 and 2 by 1 all about 2 to 3 inches long so I chopped them down the middle and just threw them in the retort until it was full. Give it a good shake to settle the timber pieces down as far as possible and add more until you can get no more in .Charcoal burners of old, stack the timber side by side upright to get as much into a given space .

Put the lid on and seat it down a bit . As the timber inside cooks it will gradually fall downwards .

Star a fire under the retort now . I cheat and use good tinder and a gas starter to get it going . When it is burning well throw a few pieces of timber down the sides around the retort making sure you can sit the tin on the top of the barrel. When the fire is going strongly put the tin on top to slow down the fires draw.

Step 4: Tips and Bits

I place a few bits of solid hardwood on top of the retort lid . I usually find they reduce to a nice biochar too and when I wet down the fire to stop it I get all the unburnt biochar from the fire and all the ash . It all helps. I am burning the fire for about 3 hours at present but I splurged and bought some ceramic insulation that I will eventually wrap round my pizza oven when I build it . I placed them round the outside of the 44 gal drum with a cover sheet to hold them there and that seemed to speed up the burn.

When the biochar is cooking it expells ignitable gas and on mine with the holes down the bottom I can hear a roar start and when I look in there is an ignited jet burning downwards out of each hole.

When the roar stops I lift the tin and wet everything down to save the biochar and ash there too. Generally I leave it a few hours to cool down before opening the retort.

I use a garden mulcher/shredder to break up the biochar to a finer tilth . The feed port of the mulcher/shredder can be opened somewhat to make the feed in quicker ,see pics , but make sure the opening is angled down to stop pieces being thrown back out.

I attach a bag with masking tape over the outlet and slightly wet down the biochar first to reduce dust . I also wear a mask . A good use for old corona virus masks. Its supposed to be carcinogenic but then what isn't?

We have been living around fires for millions of years and we are still here.

Step 5: Putting It on the Garden

The pictures show you what can be achieved this way. Spread it out and dig it in the vegie and fruit gardens.

Not much I can make that will survive a thousand years so this is very pleasing to me and the garden seems to like it too. Please give me some feedback if you do this . Always looking for improvements.

Oh extra tip for fruit tree growers. That orange rust that kills fruit trees can be stopped with sheeps' lanolin thinned in metho (alchohol ) . You know it as lanocoat , or lanolin spray but thin it with more metho and spray all affected areas with a hand sprayer bottle.