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.

While biochar was not the original purpose of my burn, it quickly became a welcome end product

Camp Fire Biochar

I have been assisting my neighbor reclaim their yard and part of that has been dealing with a bunch of old cut up maple that was taking up the back yard. They had a large section of the tree come down that actually laid on their back deck. They got the branch cut up but then it sat. I do not have a wood burning stove and neither do they so I settled on a backyard campfire.

The coal bed got so deep and so hot it started breaking the large rocks that delineate the fire ring. I quenched with a garden hose for what felt like half an hour. It was incredible!

Great response to a particular situation, Timothy.   As my grandpa never said, "If life gives you lemons, make hay while the sun shines."

John S
PDX OR

We started making biochar over the past year and then a YouTube channel wanted to do a video with us explaining how we made our kiln. Check out the link for information on how to make yours
 

One really nice thing about showing all of these different methods is that you can apply the best method to your situation.  I can't make an open fire at my place.  I live in the suburbs.  They've already called the fire department on me a few times. The fire department realizes what I"m doing and it's not only that they aren't worried. They're impressed and want to learn about it.  Some people can make open fires on their acreage.  

I can get unlimited wood for free. Not everyone can do that.  I have to prune my orchard to balance out the sunlight and plants.  Most of it is already the right size for a TLUD barrel.  

Some people need more biochar for their acreage, some need less.  Some don't want to haul wood all the way around their acreage. Makes sense.  My yard is fairly small, so it's not an issue.  

Some are mostly into cooking and making fires.  I use the heat to dry the wood just before putting it into the TLUD barrel.  

Some like to burn in the winter, because it is dry there. I can't, because it is way too wet here in the winter and the days are too short to get that done.

The beauty of everyone showing what works for them is that each of us can apply these concepts to our individual lifestyles and make it work the best for us.

JohN S
PDX OR

My son was retrieving some char from a yard fireplace-thing fire they had so the kids could roast marshmellos. He had quenched it so it wouldn't all burn to ash, but that's all the intervention he did. It wasn't "for" biochar, but he wants to use the char, since he has it.
Emptying the fireplace after dousing it, he got a slurry of ash and char. I was recommending separating the two. He asked me what harm there would be in putting the ash in the soil or the compost along with the char. I had no answer.
Please advise us.

It depends on your soil.  If you already have soil that is highly alkaline, I would be wary of adding a lot of ash.  You could add it to your compost, or your chicken area.  If you have highly acidic soil, I wouldn't worry about it.

I do think it's a better idea to douse it at the time of max char before it turns to ash, in general.

John S
PDX OR

Great thread!  I notice that most of the posts here are debating the pit burn method vs top-lit updraft retort (TLUD) kilns.  The pit method is simple and certainly the cheapest method - it costs nothing - but it necessitates digging the char out of your pit once complete, which is a deal-breaker for me.  The TLUD is less efficient in terms of combustible materials, since nearly half of your wood generates heat only but no char.  However, the TLUD promises to be more efficient in terms of time, as once you have a proven kiln and methodology you can set it and forget it.  You don't even have to quench at the end, just come back the next day and collect your dry, cooled char.  This intrigues me, and I may experiment with a TLUD in the future.

In the meantime, I have pursued a third option: a simple and cheap homemade take on the Kon-Tiki kiln.  Like a name brand Kon-Tiki kiln, mine has a solid bottom and solid sides; you build the fire up in layers, but all air flows from the top down.  I have built and operated mine fairly faithfully to this man's design and method: Farm Life Australia

I've only burnt char once, but I would call that one burn a success.  My observations:
- In the video, he is using 44-gallon drums (?)  I used 55-gallon drums, which is a standard size here in North America.
- I actually use two identical 55-gallon kilns side by side.  Both can be tended easily enough at one time, doubling the output per man-hour invested in each burn.
- Where I live, this design of kiln is easy to build with minimal capital investment.  The only expense, besides a few metal-cutting disks for your angle grinder, are the steel drums themselves.  I was able to obtain mine, used, for $10 each.  Finding ones with the solid top and bungs, as opposed to a clamp-down removable lid, was somewhat difficult.  Still, I obtained mine within a 1-hour drive.
- At first, burn a pilot fire with kindling and the bottom (larger) bung open.  (Note in the video that the opening cut into the "top" of the kiln, as laid on the drum's side, is exactly opposite the larger bung, which is closest to the ground.)  The extra airflow from the open bunghole will facilitate the burn.  This pilot fire is meant only to heat up the kiln.
- Once hot, reclose the bung hole.  I used a pair of tongs to manipulate the bung.  It isn't easy to get that close to the hot, radiating kiln, but it is possible if one is careful!  Then start layering additional fuel.  With all airflow now restricted to top-down, the kiln is ready to make char.
- Unlike a TLUD, this style kiln requires continual, though not constant, maintenance.  On average, I found myself adding wood and poking and prodding for about 20 minutes at a time, then taking 20 minutes to rest until the next layer.  This varies with the size of wood you're burning; larger wood, longer rest between layers.
- What he says in the video is true: so long as the fuel is well seasoned - and you really don't want to try burning green wood in this type of biochar kiln (ask me how I know this) - it is amazing how little smoke is produced.  This is because...
- The burn is hot!  The BTUs it puts out are staggering.  Next time, I will wear a face mask and a thermal apron to facilitate tending the fire without getting heat stroke, to which I thought I might succumb last time!  It was also a very hot day to begin with.  Poor timing on my part.
- To fill a 55-gallon kiln (or two, in my case) with char will take 4-5 hours burn time.  This includes a solid 30 minutes  to fill each barrel with quenching water using a garden hose.  Sadly, this is one part of the process that does require double the time for my two side-by-side kilns, unless you have two people wielding two garden hoses at once.
- In the video, he doesn't show how he drains the kiln after quenching, but describes using a syphon (only because his kiln has a busted bung).  Instead, when setting up your kiln, do so on slightly sloping ground and point the end with the bungs downhill.  Then, draining the kiln is simple: unscrew the bottom bung and 95% of the water drains out quickly.
- Perhaps I'm just a noob.  And as alluded to above, I had an unfortunate encounter half-way through my first burn with some unseasoned logs that nearly sent the burn off of the rails: they smoked to high heaven and seriously compromised the intensity of the fire for a while.  Still, I was surprised at how many incompletely-charred logs I was left with.  I still made huge amounts of good char, but there remains a nice little pile of too-heavy logs to add to my next burn.  I hope to improve on this: next time, I will split all logs into smaller diameters to avoid a repeat of these results.  I didn't think any of the logs were too large, but apparently it pays to be conservative.

Good post, Matthew.  Some TLUDs are retorts, and thus less efficient. My TLUD is not a retort. I couldn't make enough char that way. I just fill up the 55 gallon barrel and light it from the top.  I use the heat to super dry the wood on the chimney shelf before it goes into the fire.

JOhn S
PDX OR

John Suavecito wrote:Some TLUDs are retorts, and thus less efficient. My TLUD is not a retort.

Yeah, took me a while to figure out that difference reading through all of these posts.  Sorry for oversimplifying the situation.