Simple & Effective Method of Biochar Production
Dig the Pit:
For your first burn I recommend starting with a smaller hole. A 3ft wide X 2ft deep hole is a good place to start. This can be scaled up as desired. Josiah Hunt’s pit is ~8ft wide and makes A LOT of char:
Josiah Hunt of PacificBiochar.com used this pit for commercial production
Determine in which zone you will locate the pit. Consider where you have the most carbonaceous biomass suitable for biochar feedstock. When determining the location keep in mind the need to process and transport the product. Remember, you can always fill in the hole and plant a tree there.
Plot out a circle using a simple compass. This is your digging guide. To make the compass, first cut your string to length (the radius of your hole, for example: 3ft hole = 1.5ft string).
Stick one nail in the ground where you would like the center of the pit to be. Tie one end of your string to the nail. Tie the other end of the string to the second nail. The distance between the two nails should equal your desired circle’s radius.
Taking the loose nail in your hand, scratch a complete circle in the ground with the string taught, keeping the nail vertical. You should come back to where you started.
I used two tent stakes and some pink mason line for this compass. My first one was done with two sticks and a hay string.
Now you can begin digging the hole. Dig around the circle first. Try to keep the blade of the shovel at the same angle for each cut.
Now just dig down into a nice cone shape, maintaining the same angle as all your initial cuts around the circle. As you dig, pile the dirt in a semicircular mound on the backside of the pit, leaving enough space to let you walk all the way around it.
Note the circular base at the bottom of the pit. It doesn’t come to a sharp point. It’s about 1ft diameter down there (see diagram below).
These dimensions establish a good wall angle of ~63 Degrees. This doesn’t have to be hyper-accurate, but I once dug a pit that was too wide, too short, with a slope too shallow and had a hard time achieving a proper burn.
Collect and Process Feedstock:
Feedstock can be any flammable carbonaceous biomass. Twigs, branches, rice hulls, untreated construction waste, bones, wood chips, etc. I have only used branches and twigs from trimmed trees but am eager to try wood chips if I locate an abundant source. Josiah Hunt had a local and abundant source of sawmill scrap which was ideal.
According to the designers of the Kon-Tiki kiln, once your ember bed is hot enough “the massive heat released during pyrolysis is thus used as drying energy and wet biomass with a water content of over 50% can be carbonized.” That said, dry material burns faster and with less smoke.
Process the feedstock so that it is a suitable size for your pit. Sort it out in piles nearby. Have your feedstock ready so you don’t have to scramble for it while you have a raging fire. Be prepared.
Use the largest pieces in the middle of the burn to better ensure complete charring. Save some small stuff for the end of the burn to finish off the top.
How much do you need? A lot. It will surprise you how much wood the pit can eat. I overfilled an 8 cu ft wheelbarrow with fallen branches twice and still was a little short at the end of this burn.