making biochar: methods pros and cons

Mike Farmer wrote:The kids and I did a small campfire biochar burn today.  

Mike, you had me at "the kids and I." If they understand a bit of this and see it as normal, I think you have planted a seed that will bear wonderful fruit.

Douglas Alpenstock wrote:

Mike Farmer wrote:The kids and I did a small campfire biochar burn today.  

Mike, you had me at "the kids and I." If they understand a bit of this and see it as normal, I think you have planted a seed that will bear wonderful fruit.

My son, who is 12, is big into biochar. We talked today about using some in the chicken coop, inoculating some before mixing it with compost to plant some new grass, and putting some in the compost pile.

My daughter, who's only 7, is mostly in it for the marshmallows.  

I gathered up conifer limbs 2.5 to 3" at the thick end,  about 8ft long and all almost the same shape with a single gentle curve and no branching.

Then stacked them into a uniform triangular pile about 2.5' across at the base and 2' high (and 8' long) on some flat dirt. So just one limb at the top.

Then set the thin end on fire, and raked out and quenched embers as they became available.  It took about 2.5 hours to finish but it was a bonfire social and I was happy with the result.

I ended up with a little more than half a yard of charcoal that went to uniform half inch minus pieces under a single footstep. Perfecto

Only problem was that I got all sweaty trying to rake out embers even though I spent a total of 10 or so minutes actually raking.  Need a 10 ft handle on that rake!  At max fire intensity the comfortable distance for people hanging out was about 12' on  a 40 f evening.  It got hot

2 problems w/ the pit method.  # 1, you have to get the stuff out, meaning you'll likely scoop up some dirt/sand in your char.  If you're looking for pure char for a filter that's a problem.  #2, pour a pool of water over it, it's still gonna turn to ash later unless you've got something to cover it completely.  What happens is, you have to put out the entire fire or eventually the heats dries off the coals and they keep going.  Put out the fire, come back in 3 hours and poof, red coals again.  You'll still wind up w/ char, just a whole lot less.

Survival gardener site recently shared a video of traditional charcoal making in Spain here and I was quite impressed by their techniques.

I tried it out myself this morningwhen it was calm. Usually when the smaller branches done burning, I would quench with water, leaving lots of large ones unfinished for the next round. This time I poked around to move the unfinished stuffs to the surface in a corner and poured down a thick layer of chopped oak leaves over the red glowing char. The pile kept on burning for extra 10 minutes and there was a lot less unburnt logs left. I stirred the pile around and let the remaining heat char the leaves too. I cooled down any remaining hot charcoal and used less than 10% water I normally did with a hot pile.

Salute to the traditional wisdom!

I tried a few more times with my barrel-in-a-barrel design (to be clear, not a retort with an inner, inverted, un-perforated barrel; but rather an inner fuel chamber with up-flow ventilation from the annular space between barrels) and this just does not work most of the time. So I set the inner barrel aside and burned with only the outer barrel - now featuring some air holes (8 holes with total area of about three square inches per hole - four at the bottom of the side and four about mid-barrel. I continued to use the three-foot chimney (about 10-inch diameter). This configuration burns furiously and the bottom of the barrel reached >400C in about thirty minutes.

This single-barrel system seems to burn quite more of the fuel mass - noticably less biochar remained and I quenched the fire when the bottom of the barrel exceeded 400C. There were a few small pieces of partially-burned wood remaining. I think the variability of outcome is going to be attributable to variation in fuel going forward; bigger (thicker) pieces of wood just take longer. The pyrolysis rate seems to be around 3/4-inch or one inch per hour in a barrel fire - maybe I can get a better estimate of that with practice. So a scrap of nominal one-inch plank will be reduced to carbon in a burn, but a scrap of 2x4 often will retain some solidity. Cut branches behave similarly. Of course, partly-carbonized pieces can be set aside for a future burn. So FWIW, this concludes my reporting on my (failed) dual-shell burner.

I started using the single barrel TLUD because I just couldn't make enough char with the container within container/retort.  One of the important ideas that I realized is that when you drench the fire affects how much char you get.  At first, I tried to follow the advice in this video, which was about the least helpful guidance in it.  

https://duckduckgo.com/?t=ffab&q=warmheart+biochar&atb=v401-1&iax=videos&ia=videos&iai=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DYIbGkmt1VdE

Then I started empirically measuring to see what gave me more char and more ash.  I decided that when the flame had dropped to 5-8 inches above the char was when I got the most char out of it.

I eventually switched to a larger solid chimney and riveted it together, and I just estimated on turning up the lid into the chimney.  All of those adaptations worked much better.

John S
PDX OR

May Lotito wrote:Survival gardener site recently shared a video of traditional charcoal making in Spain here and I was quite impressed by their techniques.

Going by the amount of smoke in the video, what they're making is more of a fuel/cooking grade charcoal but the stuff that comes out of the first phase of the burn will be decent biochar. If what you want is the best possible outcome for the soil and the climate, you aim for the least smoke possible. The best biochar has all the volatile stuff driven out of the pores by high heat and in a good burn that gets consumed by the flames.

The early part where they kept putting brush on the pile was a good flame cap method. If you keep going in that direction and skip the smoky part where the leaves get heaped on and mixed in, and quench with water or dirt at that point instead, you'll get a better result. Copious smoke is an indicator that the process temperature is too low and what comes out will be only partially "baked" -- the pores will be clogged with tars and resinous ick and it won't do much good in soil.