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.

I have just made a lot of biochar with the pit method.  It all started with digging out a large bush and its roots.  And finally burning what I could not cut out.  Leaving me a "pit".  I have read over the years about biochar but decided it was to much work..
But looking at the hole in the ground decided to give it a try.  It worked so well. I made a second pit.  A friend who heats with wood said I could have some of his wood as the city has been giving him green ash trees that they had to remove.
I ended up burning over 2 cords of wood and old lumber.
 If you have been puting off trying to make char give the pit method a try.⁰

Great example! I waited for a few years until I could see a method that I could really do.  I can't do the pit method, because I live in the suburbs, and the burning pit would be way too close to houses.  Once I found a method that could work for me, I've been going steadily all that time. The more examples we see, the more people will say, "Hey, I could do that!"

John S
PDX OR

We have had the help of our local Fire Safe Council crew in making shaded fuel breaks around and below our structures, which are also along a strategically important corridor to protecting old growth, redwoods, nearby towns, and private forest interests. We have had five biochar burns this winter, consuming most of the 3 acres of brushy ladder fuels relatively close to decent place for the Ring of Fire Kiln. Standard burn piles were made on steep slopes too far to haul brush to the kiln reasonably efficiently. The crew cut and dragged it into a ring around the kiln as I fed it. Once the fire got ripping, it chewed up even green, wet brush with minimal smoke if at all careful in feeding. Putting fuels around and not on top of the flame helps reduce smoke a lot. The 3acres that have been converted to shaded fuel breaks still have the larger, healthier trees, any snags not risking a healthy large tree, and mosaic patches of diverse understory plants where they pose minimal risk of spreading fire. Much of the native vegetation will grow back.

We produced between 200gal-400gal of char per burn (some went most of a day, others just a couple hours). I estimate 1500gal+ total since December. What I plan to use in the garden, food forest, or putting back where it came, I inoculated with compost extract, fish hydrolysate, kelp, azomite and a little of my homemade oyster shell and weeds imbued apple cider vinegar. One batch I only inoculated with compost extract, as I intend to use this for filtration and decontamination purposes, like between my driveway and shop and our pond, above our water source, or off roadways going into ponds and creeks. I inoculate on the back half of quenching, once it is cool enough to walk on or touch. Char is further inoculated by being used at 5% of compost piles in progress, in chicken bedding woodchips, or mixed with 2 parts finished compost.

Pictures below include burns, shaded fuel breaks (finished and in progress), and uses of the biochar like in compost, bird bedding, pond and road runoff filtration.

Fantastic example, Ben!  Although we live in the same general region, your land is so different than mine!   I could never make biochar like that, but there are many people on this forum, and I'm glad they get to see some examples that they could use.  I think that the cooperative purchase of a large kiln will inspire people too.

John S
PDX OR

I was at a local non-profit farm where I'm on the board today doing some odds and ends. I knew their fire pit was a bit full of char, so was planning to clean it out. Folks had put more wood and sticks in there, and it's been damp lately. I started a fire to clean it out, but it was obvious that if I was going to burn through the residual char I'd be there a while.

So, instead, I found a metal bucket and filled it with water. I scooped out hot char and quenched it. Once it was cold out and fully saturated, I repeated the process. Probably did about 10-12 of those small metal buckets full to get the char in the fire pit down to a reasonable level.

The wet char then got added to the compost chicken dome to get charged in advance of the compost harvest in about a month. The compost/char will get worked into garden beds across the property.

As I was wrapping up, it started to rain pretty hard, so I  felt extra safe adding the char to the chicken system since everything was going to continue to get a good soaking at least through tomorrow AM.

Maybe a non-traditional way of making char, but it worked and stacked functions in that it cleaned up one area to soak up nutrients in another (chicken compost system) to eventually share those nutrients with a third space (garden beds)

That sounds very efficient in an unusual sort of way!

John S
PDX OR

John Suavecito wrote:That sounds very efficient in an unusual sort of way!

John S
PDX OR

Yes, the "harvesting" was really quick. The 10-12 buckets probably took me maybe 30 min of going back and forth between water source, fire pit, and chicken run in a little circle.

I'm sure this mixed a bit more ash in than many other methods, but up here in the northeast our soil is very acidic so getting it into the mix will help even out the soil a bit, while adding plenty minerals.

I was back at the farm today planting some hops rhizomes around the chicken composter, and dug around a bit in the compost to expose some worms for the chickens. The little piles of biochar I had dumped in a week before had completely mixed in by the chickens...in less than a week you'd never know the piles were there. Pretty cool.

The plan is to harvest a bunch of the compost (and inoculated char) in a few weeks and put it on some garden beds.

The one thing that's not been addressed is exactly what the biochar is to be used for, or the species of abundantly available feedstock, which might help determine which method is used.

All true charcoal can be considered bio-char, but not all bio-char can be considered charcoal. This is specifically relevant to any "bio-char" that will be used for personal care, food preparation, or ingestion.
The same is true for using bio-char in the production of adhesives, pitches, or tars.

Additionally, if used for gardens, the pH will vary widely, depending on which species/,type of wood feedstock burned, and ANY type of Ivy used, which hasn't fully completed the combustion process can leach toxins into your food.
Worse yet, if poison ivy, poison oak, poison sumac, etc., are contained within any batch which you use for personal care, or human consumption can cause severely adverse reactions.
Even just burning those poisonous plants can cause respiratory ailments; some, which can cause long term health effects.

So, it's not as "cut and dry" as some folks might suggest, or think.

I use retorts because I have an abundance of conifers (pine, cedar, for) which is great for the use in adhesives, tars, and pitches, and to use as the sacrificial burn wood.

I also have fruit trees, Oak, Hickory, Manzanita, Molene, Lavender, Blackberry, etc. which are great for charcoal grills, soaps, scrubs, toothpaste, and as a fertilizer supplement, and harden use.

If you use the white ash, this can also be of concern for producing "clays, bricks, Adobe, Cob etc., or producing "pressed" bricks, pressed Stones which aren't"fired," to use in construction.

Soft sappy woods work better for use in Cob, some Adobe, and some rammed earth processes because the saps have binding properties and as opposed to hardwood.

Conversely, hardwoods produce better results for mortars, concretes, stone, bricks, and clays that can be "fired" later.
Hardwood must be used if you plan to make "water glass."

It might sound as if I'm knit picking, because you can just burn a pile of wood, and use the bio char as a deep surface garden supplement, and all will be good.
But, you can also end up at the ER because you burned a 6' tall pile of poison ivy laced brush after you inhaled the toxic smoke.

Please be mindful, and do some research. It might appear to be a lot of work, but like anything else; the effort put forth, will reflect the results achieved.

Plus, you might find a new product to make, that could be extremely valuable to you in particular. For me, that happened to be a wood finish. I use a lot of Cedar, Pine, and Fir, for outdoor furniture, decks, stairs, etc., and mixing homemade pine pitch/tar, with boiled linseed, Manzanita, and blackberry oils, creates a wood finish that's virtually waterproof, and can preserves wood for decades with two applications, or centuries if applied semi annually, or (if applied correctly) to a depth that reaches the core.

Making biochar, can be a life changing process that can be used to create all sorts of things; that are not only less expensive, but more versatile than their "store bought," counterparts.

I have made many small batches at a time using basically soup cans filled and jammed together. Then they were put into the
woodstove after most of the heating fire had burned down. 1 can has the open end fluted with a HVAC like crimp tool and the
other paired can with a nail hole poked in the bottom for escaping gases. Some were filled with humanure. (poo, TP and fine
wood chip bedding and sometimes with added sprinkles of wood ash to help reduce smell) Some would get filled with bits of
wood sticks and some with cardboard shredded with a heavy duty paper shredder. I did that a couple years ago (garbage bag
full) but didn't get around to it this last heating season.

I figure the humanure type and wood stick types make chunks of char appropriate in size for use to be bio-charged and used
that way for gardening. But I'm not so sure how well the shredded cardboard will work for that because the cardboard char is
rather small, thin and light. But I'll tell you what!!! That cardboard char sure works great for sprinkling a coffee can full in the
chicken coops to knock down any stink.  I've not checked yet to see the results though using the cardboard char from the coop
cleanouts in the compost pile. I'm guessing it may just break apart too finely to be of lasting biochar utility. I'm still going to use
the shredded cardboard char in the coops anyway, regardless of composting results. Even some of the shredded cardboard not
put through pyrolysis may get used as nest bedding. That stuff should compost easily.

Leigh Tate wrote:I've recently started reading about making biochar. There seems to be quite a few ways of making it, and I'm curious as to why folks prefer one method to another, and what they view as the pros and cons of the various methods. Anybody care to share?

Simple drum kiln it’s pretty popular  cuz it’s cheap and easy to build.

All these posts are SO helpful, thank yall for sharing! I'm hoping to make some biochar using the pit method today, we'll see how it goes! 🤞🏻 I'm hoping eventually to find ways to use the other things it produces, like heat- to be honest biochar systems intimidate me a little because they seem so complex, (almost like redstone systems in Minecraft lol) I need to grow more confident building things, which sounds like a silly thing to be intimidated by, but it is true! Anyway, thanks to everyone who took time to post their systems!

Many of you have amazing ideas! My wife's workplace will be making biochar later this year so I'll capture some photos and share. I believe she said something about using a culvert to contain the fire and build up the heat.

Matthew Nistico wrote:...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.

UPDATE: After about a half dozen total burns with the two-kiln setup described above, I have a few notes to add.

First of all, the video I originally linked is sadly no longer available.  Here are two new good videos with info on both fabricating and burning basically the same kiln as I described above.  Only note that he cut his open panel out of the opposite side of the barrel than I did; i.e. on top of the larger bung, as opposed to opposite the larger bung, as I did.  No idea why.  Thus, on his design the smaller bung is on the bottom, near to the ground when burning the kiln, which seems to me less useful.

My updated notes:
- I really do like this kiln design.  For minimal capital investment in time and money, I have two simple, durable kilns that have given me multiple good burns and show no sign of crapping out yet.  Surely I will eventually degrade the steel of the barrels and need to replace them, but that looks to be a long ways off.
- Very little smoke is produced using well-seasoned wood.  Once you learn a good rhythm for tending the kilns, and so long as you burn on a mild or even a cool day, running the burn is fairly easy, pleasant, and safe.  No serious smoke inhalation.  Little risk of fire escaping the kilns, even with minimal supervision.  Only once have I had a spark ignite some nearby (8' away) leaves on the ground.  Fortunately I ALWAYS have a garden hose pressurized on hand, and the escaped fire was extinguished in seconds.
- You can skip the step I outlined above involving removing the bottom bung during the pilot burn to provide extra O2 while heating up the kiln.  It works well enough with the bung in, and then you don't have to burn your face off bending down close to re-insert the bung while the kiln is hot.
- As each layer of fuel burns to the desired level of char - blackened wood with a good frosting of white ash; you'll get the feel for it - it is time to add the next layer.  First, rake the coals flat, keeping any larger chunks toward the center of the kiln where the heat is most concentrated.  Then add an armful of small kindling.  As soon as this flares up and the heat is really pouring out of the kiln, throw your larger fuel logs on top to completely cover the burning kindling.  At first they may seem to extinguish the flames from the kindling, but it will reignite, reliably lighting the larger pieces.  This is your rhythm, repeating about every 40 minutes in total, layer after layer.  I find this is the most reliable and efficient way to keep the burn going smoothly, get new wood burning quickly, and prevent most of the wood from burning down too far to ash.  About halfway through each layer's burn, poke the logs a little, repositioning them to keep the largest/least burnt logs toward the center.  The goal is for all logs to burn at an even pace.  Early in the day, pay closer attention to ensure that new wood catches fire quickly and burns evenly.  Later in the day, when the kilns are half full of hot, pyrolyzing coals, it's easy and reliable and less attention is required after a layer is added.
- I have had much better success once I learnt to be conservative with my log sizes.  2.5" maximum diameter.  Anything larger I put through the log splitter before burning.  This way I assure that 1) each layer burns at an even pace; and 2) there is only a tiny portion of not-fully-charred pieces for the reject pile when I empty the kilns.  These are much improved results compared to my first burn.
- The burns go more slowly than I first reported.  Filling both kilns nearly to the top with char will consistently take a good 7 hours or so, from ignition to final quench.  I've only once stopped the day because my kilns were too full to add a new layer; every other time I ended the burn because I'd run out of daylight.  And it would pretty much take just as long were I burning only one kiln at a time, instead of two.
- When quenching the kilns, no matter whether half full or totally full of char, always add water until it reaches the tippy top of the kiln.  You aren't just spraying down the char, you are soaking it.  Drain it the following day.  Anything less, and you risk it reigniting during the night and burning down to ash.  I've not experienced reignition, but I believe it can happen; the amount of heat in a full kiln at the end of a burn day is incredible.
- In my newly linked videos, he doesn't illustrate how to empty the kiln after draining it.  It is quite simple: remove the brick chocks and then role the barrel onto its side, dumping some of the char onto a large sheet of metal or plastic.  I bought a sheet of corrugated PVC like this:
This provides a good surface on which to sort through the char, crunching the larger pieces with your hands, looking for incompletely-charred rejects.  Then pick up the sheet, bend it into a trough, and tip the end into a 5-gallon bucket.  No shoveling required.