Coming next weekend! I guarantee the actual video is less exciting than the trailer, but it is much more edifying! This video will just be an introduction to the idea, and the benefits of frankentreeing. I will certainly put together a much more technical video in the future.
Below is my second fast motion video on the two simple biochar methods I've been experimenting with. A few notes...
Fuels: I suspect that pieces larger than about 3 inches are better either split down or charred by another method, and chips might be better done in a TLUD or some such device. I haven't tried either in the trench though, so that's just speculation. I doubt that large wood will char well in the trench because it takes so long to char all the way through, but chips might be just fine if fed pretty constantly in thin layers. As long as everything you're putting in turns to charcoal and you're not getting a lot of ashes or a lot of smoke with it, you're doing well. I've done green and dry wood. Dry is better of course. I think the jury is still out on green wood. The one I did mostly with pretty green wood was a very hot, large pit and the wood was brushy allowing for the ingress for large amounts of air. It was still pretty sluggish and I'd certainly tend to let the stuff dry for a summer first if possible.
Wood size and shape: It's hard to say without actually measuring things, but the trench seems to have a very efficient conversion ratio (wood to charcoal with low ash production) if the material is of a nature that it can be laid thickly and flat onto the coals, and of course if it is tended adequately. This method takes a little more effort than last week's open burn, since you have to dig a hole, but it handles certain materials better. I've done a number of these now and have found that they don't handle tangly brush all that well. I did pretty good with douglas fir limbs, but not with oak, madrone and similar branches that have a lot of twigs poking in every direction. The fir limbs are pretty linear and stack into the trench closely enough to get by. If the fuel doesn't lay into the trench well, it will allow too much oxygen to reach the coals and result in more ash formation. So, really tangly stuff that takes up a lot of volume of space might be better burned in an open pile, or reduced in size and shape to fit into the trench better. When I take trees down, I typically limb up the 2 1/2" to 3" branches and larger, setting them aside, while anything smaller is brush for burning. So, I'll usually end up with a pile of each, larger stuff with little to no brush for the firewood pile or the trench, and small tangly stuff for the open pile method.
Trench v.s. Pit Shape: You could just dig a round pit, and that might be good for small wood, and especially small chunks like lumber cutoffs, but the long shape allows burning of long wood without cutting it up which can be a huge savings if long wood is what you've got. It of course works fine for short wood too. I haven't done a burn in just one end of the trench, but it seems like it should work fine as long as the open end of the fire has wood added to it same as the top. If nothing else, the trench could be blocked with dirt of bricks for small burns of short stuff.
Other options: FYI, this is based on the Japanese Cone Kiln concept, and you can also do it in other containers, like a webber BBQ for micro scale (be sure to seal any holes on the bottom). I think you could also use an old wheel barrow body. You can see Backyard Biochar for more on the cone kilns and other simple methods.
The burn requires maintenance, but not constant maintenance, so bring a book or a project to work on.
Summarizing:Between this and the open burn pile method in last week's video, a lot of wood types that most people have access to can be charred easily, with a minimum of preparations, planning or technology. Considering the simplicity and low inputs of these two methods, there seems little reason not to turn the woody resources that accumulate around a homestead, or even backyard, into charcoal instead of ash. That is my main message here. You could of course turn smaller stuff into compost, but since charcoal can serve some of the purposes of organic matter in the soil (nutrient reservoir, microbe housing, moisture holding and soil texture changes) but permanently with a one time applications, it's worth considering charring it in your webber for a while, or some of it at least. It's not a complete substitute for organic matter, but it should, in theory, help you get more out of the organic matter and other nutrients you add to the soil in the future. I add some organic matter to the soil, but I consider the most important the plant roots that are left behind after every crop.
Anyway, that's more than enough said. Most of what you need to know is in the video...