Concerning volume. In my previous biochar post, I summarized that my two rectangular lined pits were producing around 1.3 cubic yards of char per firing. Since then, I’ve been told that a simple earthen pit could produce as many as 5 yd3 in a single firing! Let’s calculate how large that would be.
A round pit with a flat bottom is basically a truncated cone, which is like this but upside down:
5 cubic yards is 135 cubic feet. To be able to step down into the pit, I don’t imagine it being much deeper than 3′ deep (my current pits are around 2′ deep, and it’s quite a step to get in/out of them). Considering a truncated cone 3′ deep, with 4′ diameter at the bottom, and a 7′ diameter at the top, that’s only 73 ft3. That isn’t even considering compaction (char settles, so that as you walk on it and unload, it packs down, so the entire pit is not full of char). To get to 5 yd3 = 135 ft3, you’d need a pit 3′ deep, 5′ bottom diameter, and 10′ top diameter! That’s a huge area to build a fire in.
What if we kept the current approach of a lined pit, but allow for sloped sides, and deeper, and wider? The sloped sides would allow for better aeration, allowing us to go deeper. The shape would be like this, upside down:
With a depth of 2.5′, a bottom of 3×3′, and a top of 5×5′, that’s… only around 31 ft3, and considering compaction, probably more like 25 ft3. That’s not much better than my current “big” pit of 20 ft3, while being deeper to step into, and a much larger area to have to cover with soil.
There’s just no way around it. To get a lot of char, you need a really massive pit, regardless of shape, or whether it’s dirt or bricks.
Meanwhile, concerning price, I haven’t actually found anyone on the Island of Hawaii selling bulk char. Retail, all I’ve found is at Farm Supply Coop in Hilo, as of July, they were carrying cubic-foot bags of Black Owl biochar for $42 (!) The price is similar on Black Owl’s website. At this price, the 34.6 ft3 of char that my pits are producing would be $1453, which is crazy. I don’t think there are many people willing/able to pay so much. Currently, I’m only charging $30 for a 10-gallon (1.33 ft3) bag for the 1/2″-minus char that I screen by hand. This screened char is around 2/3 of the total output, but our own farm needs a lot, so it’s more like 1/3 leftover to sell (11.5 ft3), which is only around 8 bags per burn.