I've been experimenting with making my own version of Terra Preta ever since I first saw the BBC documentary The Secret of El Dorado about ten years ago. I started by burning piles of tree limbs and then putting out the fires to create charcoal, which worked but wasn't an ideal way to make bio char.
Then, I discovered the TLUD (top lit up draft) method of making bio char, and I was able to make a higher quality bio-char in a more efficient way. The pie-in-the-sky plan is that by using a TLUD, I can scale the process up and eventually have a small field of terra preta instead of just a large garden of terra preta.
To start, I found a 55 gal barrel with a removable lid (pull out the plastic sealer around the inside of the lid or it will smoke like a son-of-a-gun until it burns away).
The next thing I did was drill a bunch of 3/8" holes in the bottom. After I drilled the holes, I drove a tapered punch into each hole to flare them inward so it looked like a barrel with a bunch of bullet holes. Besides making it look like I had shot all those holes by practicing my quick draw, I thought it might stiffen up the bottom and keep the bottom from warping or burning out as quick.
After I found another lid and a piece of 8" diameter stovepipe about 4 foot long, I started making my chimney by plopping the stove pipe on top of the lid and marking out an eight inch circle before using a grinder with a cut-off wheel to make eight pie cuts. Make sure to cut a little past the 8" diameter circle (I'll explain later)
After bending those cuts up (it might take a few times bending them and unbending them to get the pipe to fit right), the chimney can be screwed to a few of the sharp-pointy-bent-up parts. Those eight little slits are needed to feed air to the chimney to get the afterburner effect when you actually start making bio-char with this thing so don't forget to include them.
Besides a few bricks to put the barrel on if you want to get all fancy or a few sticks to put the barrel on if you want to go all minimalist with your design, that's basically all here is to making my version of a TLUD.
When it comes time to make some bio-char, I put the barrel on three bricks or sticks, fill it up with wood, light a fire on top, and put the chimney on. It smokes a little at first, then flames start up the chimney, and it roars to life for about an hour. After about an hour of so, the chimney is taken off, the air is blocked off by throwing dirt all around the bottom, and the lid is put back on tight (I usually throw some concrete blocks on top of the lid to make sure it is tight).
The next day, I end up with some more bio-char to add to the garden. A little muss and no fuss, and I'm on my way to creating some terra preta that will last for hundreds or thousands of years.
Next time, I'll show the actual burn process.