making biochar: methods pros and cons

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?

I like the method where you start with a small fire, and when the coals start to turn white you put more wood on, and you just keep covering up the new coals as they turn white until you either run out of wood or the fire gets too big/hot. Then you spray it with water and put it out completely. If done in a flat spot you can then drive your car back and forth over it to crush it.

Plusses with this method are that you can make huge batches, and you don't need a kiln or a hole in the ground. Minuses are that it's an open fire, and it takes a lot of hot busy work to add wood to the fire as it burns, and you have to quench it at the end.

I myself am a pit man. But I also have an excavator, so pits start to seem like a logical solution to a lot of problems. Case-in-point: my poison oak pit.

The advantages for making biochar in a pit are: 1) that it contains the fire really well. Even when the wind picks up, or you are working close to the fire, the heat and flames stay contained. It feels a bit safer than a pile burning on the ground, and it is rarely unbearably hot work like open burning often seems to be. 2) It is surprisingly easy to put out the coals on the top when you are done. Dont get complacent, though, a lot of heat remains deeper in the pit for a long time, and it will relight on its own pretty easily if you dont deal with it sooner rather than later. It takes about as long to hose down all the char and bag it up as it does to do the actual burn (a couple hours each) 3) Pits scale up (if you have the right tools). My pit is about 10 feet long and 4 feet deep. I need a pile of brush about 6 feet high, 10 wide and 20 long to make a complete batch. Maybe more. It makes about a quarter of a ton of biochar in one go. 4) Toddlers love playing in pits. The soot washes right off.

The downsides: when you wet it down it gets really heavy, so digging it out is a lot of work. I spray water and stir it around to both ensure it is fully out and to cut down on the dust. And the big drawback; if you dont have a hulking diesel digging machine, it is a lot of work to dig a pit that big. It will last forever, though, and just imagine how much carbon could be captured if every farm had a biochar pit instead of a big burn pile that they let burn down to ash.

One up for trenches/pits! Best way to get rid of waste branches and get biochar at the same time. I tried a couple of other methods (bonfire covered with soil/turf once it burns properly, covered oil drum) but these methods have distinct drawbacks. They take a lot more time (both active work and total burn time) per quantity of biochar harvested, and they produce a lot of nasty-smelling smoke that definitely contains toxic stuff. I have never tried a retort-type method, seems overly complicated for turning "waste" into char. However, I'd really like to try making a mini retort-type system for use in an indoor stove/RMH (something like this:   [youtube]https://www.youtube.com/watch?v=jxBUqk2M3Y8)[/youtube]   That way you both get the char and the heat from the burning syngas. Don't know how large a percentage of your total fuel you could get away with charring in a system like that, though. I suppose once it gets going (and if you change the boxes often) you could probably char more than you burn?

I use a number of methods, but retort is my favorite.  

Pros:

It makes the highest quality charcoal for me.

I load the retort, start the fire, and I don't have to do anything else.  I have a lot to do, and limited time, so methods that you have to babysit and continue to stock don't work for me.  Inevitably, I get busy and by the time I get back to add more wood, my charcoal is burned into ash, or, I have to keep watch and keep adding wood and it eats up many hours of time.  With the retort, I start the fire and don't even have to look at it again until the next day.  By then I have high quality charcoal with the smallest possible time invested.

No quenching.  That was a big one for me at my new land when I didn't have water available, and now that I do, it's just easier if you don't have to do it.

Cons:

I can't make the large amounts that can be made with a pit or trench.  The caveat to that is that if you figure my total time invested, I think I probably get more charcoal per time with a retort than any other method.  That doesn't change the fact that I don't get huge amounts at one time, and I would like to.

I can't use long branches.  This normally doesn't affect me because I use scrap wood for making charcoal, but for cleaning up larger areas, it would be nice to be able to char longer pieces.

Eino Kenttä wrote:
I have never tried a retort-type method, seems overly complicated for turning "waste" into char.

I admit to not really understanding this.  I just fill a 35 gal barrel with wood, put it upside down into a 55 gal barrel, fill the gap between barrels with wood, and light it.  That's all there is to it.  

I have to confess here that I'm not real clear on the various labels used for biochar makers. There seems to be quite a bit of crossover as to what is labeled what. I was thinking a retort kiln was something else, but I don't know that I'm correct! My husband made one like Trace describes, a top-lit updraft TLUD, but I've seen that paired with the terms kiln, retort, and gasifier. I have no idea which one is "correct."



Here's my analysis of our TLUD (from a novice point of view):

PROS:

- The unit was inexpensive to make (but also see CONS). Ours was free because we had all the parts (55-gal steel drum, 30-gal steel drum, and some old ductwork.)
- Kiln construction took a couple of hours and required no welding.
- To make biochar, the only labor involved is filling the barrels, lighting the fire, and later removing the biochar after it cools.
- Once it's going, it's self-maintaining and requires no monitoring.
- I like knowing that we're burning the gases rather than releasing them into the environment.
- It's portable.

CONS:

- The inner 30-gallon drum would be expensive to replace. We happened to have one, but no one sells them locally and they are several hundred dollars on the internet.
- The gap between the two barrels for fuel wood is narrow, so it took some experimenting to get a thorough burn.
- Only makes fairly small amounts.
- Barrels will eventually have to be replaced.

I use mini-retorts in the wood stove. My retorts are made from empty soup cans.

Cons:
- I can only make small amounts of charcoal at a time.
- Taking out the finished cans and loading fresh ones in can cause more smoke to escape into the house than some members of my household are comfortable with, so I have to wait until they're away from the house long enough to be worth it. That doesn't happen nearly often enough.

Pros:
- The retorts are cheap, and easy to make.
- This method works with nearly any feedstock, not just wood. This is important, because my most plentiful feedstocks come from crop waste, and don't burn hot enough to cook themselves.
- I can use the heat from the burning gasses to heat my house.

But the biggest pro is that, right now, this is the only method available to me.

I use pyramid kilns, true TLUD stoves (not the barrel-in-barrel TLUD retort) and retorts depending on my feedstock and or goals.

TLUDs allow me to use the process energy in a productive way, either to heat my house, for on demand hot water or to heat up my retort without burning any biomass down to ashes for optimal efficiency.

Pyramid kilns are great for brush and tree prunings.

Retorts are great for wood vinegar production, bone char, novelty items such as charred animal skulls or specialty char such as high grade hemp biochar which I use in toothpaste or for internal use.

   Eino Kenttä wrote:

   I have never tried a retort-type method, seems overly complicated for turning "waste" into char.



I admit to not really understanding this.  I just fill a 35 gal barrel with wood, put it upside down into a 55 gal barrel, fill the gap between barrels with wood, and light it.  That's all there is to it.  

Well, admittedly that doesn't sound too tricky. Only thing is, our place is about 3km of partially steep terrain from the nearest road, so getting the barrels there would be a bit of a pain. (Will have to get one there for building a RMH eventually, but it would be good to minimize the carrying.) Whereas for making a trench I only need a spade. Also, I'm not overly enthusiastic about cutting up waste branches in small pieces to fit in a retort. I suppose all I'm trying to say is that I find the trench method convenient, and it gives a lot of char, so for larger-scale burns I think I'll stick to it. However, if I ever have a couple barrels handy I might try this, for experience if nothing else.

I wonder how the numbers work out on total percentage of the carbon retained in the biochar, retort vs trench? I suppose retort wins by a lot if you only count the feedstock that actually turns into char, but if you count the fuel as well? My guess would be retort wins anyways... Anyone aware of any research done on this? Or have a hunch? Sorry if this has been answered elsewhere...

The 30 gallon steel barrel can be hard to source.
If I build this kind of "retort inside of a TLUD"  kiln, I will use a water heater tank as my inner barrel.
Defunct water heaters are usually offered up free to scrappers, and can also be dourced at some plumbing supply houses
They are taller than a 55 gallon barrel is , but you will be cutting one end off anyway, so cut it to fit.
You could also leave it as long as possible and use a second barrel or part of one to contain it.

A lot of the guys at the driveonwood forum use some variation of a TLUD,  and they will often include a layer of rockwool insulation, to make the  process more efficient.

I would like to try a few variations, like a TLUD that is a pit with air piped into the bottom and a lid that has a chimney
Alternatively it could be a barrel TLUD that is placed into the pit for burning
The point of this is to lower the kilns exhaust to a height that is ergonomic, so the heat can be used for something.

Another approach is to use a dryer drum since it will usually be wider and shorter than a 55 gallon barrel.

I also want to explore using pocket rockets to make charcoal.
Water in the bottom and a grate above that, for self quenching.

I wonder how the numbers work out on total percentage of the carbon retained in the biochar, retort vs trench? I suppose retort wins by a lot if you only count the feedstock that actually turns into char, but if you count the fuel as well? My guess would be retort wins anyways... Anyone aware of any research done on this? Or have a hunch? Sorry if this has been answered elsewhere...

The 30(35) gallon in 55 gallon barrel in barrel method is highly inefficient because 20-25 gallons of biomass are burned down to ashes in an attempt to pyrolize 30-35 gallons of material and its not uncommon for people to fail to achieve full pyrolysis b cause this method is tricky to get right consistently.

With a pit or trench burn only a minor portion of the feedstock turns to ashes so it's much more efficient.

And like you mentioned cutting branches to fit inside the barrels is a lot of work as well which further adds to the inefficiency.


More sophisticated retorts however can achieve the same level of efficiency or even go beyond that of a pit or trench method when wither they are heated with a TLUD gasifier like I'm doing or when multiple retort chambers are connected in a unit and the waste heat from one burn is used to start the next one in a continuous cycle like the unit they have at Living Web Farms.

I have a 55 gallon drum with a chimney and I make the biochar in a TLUD way. It looks like yours, Leigh Tate.

I live in a suburb and I have to burn in my driveway, because I only have .2 acres.  
Pros                                                                     Cons
Almost free to build                                           Wood has to be chopped up ( I have an orchard, so a lot of the wood is the right size anyway)
Makes much more biochar than a retort         I haven't figured out how to use the heat for anything else efficiently
Doesn't make much ash or smoke
Makes a good amount for my land

This was the video that convinced me that I could really do this. I mostly follow their plan, but have some minor adjustments.





John S
PDX OR

I have only made biochar incidentally, but it worked reasonably well.

I made a big campfire for cooking. And quenched it before it burned out completely.

Ended up with decent amount of black coals on several occasions.

Pros: Byproduct of cookfire.

Cons: Lots of ash.

John Suavecito wrote:This was the video that convinced me that I could really do this. I mostly follow their plan, but have some minor adjustments.

Thank you for the video, John. Very interesting and helping me better understand the process.

For me, one of the big appeals of some sort of retort system is that it doesn't need as much monitoring to stop the burn at the right point. Like William, however, I find the 30-gallon drums hard to come-by, so alternatives for that are important.

Martijn Macaopino wrote:

I wonder how the numbers work out on total percentage of the carbon retained in the biochar, retort vs trench? I suppose retort wins by a lot if you only count the feedstock that actually turns into char, but if you count the fuel as well? My guess would be retort wins anyways... Anyone aware of any research done on this? Or have a hunch? Sorry if this has been answered elsewhere..."

The 30(35) gallon in 55 gallon barrel in barrel method is highly inefficient because 20-25 gallons of biomass are burned down to ashes in an attempt to pyrolize 30-35 gallons of material and its not uncommon for people to fail to achieve full pyrolysis because this method is tricky to get right consistently.

With a pit or trench burn only a minor portion of the feedstock turns to ashes so it's much more efficient.

And like you mentioned cutting branches to fit inside the barrels is a lot of work as well which further adds to the inefficiency.

One thing that's coming to mind as I read all these interesting comments, is that what's "best" will depend in part on one's source materials and what's acceptable in terms of the time and work necessary to use the system. For example, we have volumes of waste and deadfall wood here, so fuel isn't an issue. But I can see that it would be for someone who has to source it.

One thing I don't see addressed often with the open methods, is the release of the wood gases into the atmosphere, as opposed to burning them with a gasifier type of biochar maker. How much are they adding to the greenhouse / pollution problem? Or are they? It seems that ought to be part of the efficiency equation.

Leigh Tate wrote:
One thing that's coming to mind as I read all these interesting comments, is that what's "best" will depend in part on one's source materials and what's acceptable in terms of the time and work necessary to use the system. For example, we have volumes of waste and deadfall wood here, so fuel isn't an issue. But I can see that it would be for someone who has to source it.

One thing I don't see addressed often with the open methods, is the release of the wood gases into the atmosphere, as opposed to burning them with a gasifier type of biochar maker. How much are they adding to the greenhouse / pollution problem? Or are they? It seems that ought to be part of the efficiency equation.

Feedstock is indeed the main determining factor when it comes to stove selection.

With open methods all the woodgas is burned.
That's another reason why I prefer it over the barrel in barrel retort, if you look around on YouTube you'll see way too many videos of people demonstrating the barrel in barrel method to be a huge smoking mess. Which it shouldn't be if it's done right but the difficulty of that method is getting it consistently right, especially if your feedstock isn't absolutely perfect.

One major issue is an uneven burn in the outer barrel or even inside the inner barrel which can cause too much woodgas to be released which is then difficult to supply with enough oxygen for a clean combustion.
Feedstock having too high a moisture content can cause issues with burning the woodgas as well.
An open pit is more forgiving with that once it's up to temperature.

My TLUD system just requires me to monitor while it is burning, which takes between one and two hours.  I like campfires. It's fun.  I also get so much more biochar than in the retorts I've seen and I only have to burn at most once every 2-3 weeks during the drier season.  My 55 gallon drum was free and has lasted for years without visible wear.

The advantage for me is that after burning, the wood is already on my driveway, so I hardly have to move it at all to crush it. I drive over it for about a week in between two panels of plywood.  I can't imagine how difficult it must be to crush the biochar when it is in an open pit, far away from all one's equipment.  It is also near the house where I am going to inoculate it for two weeks, which also seems really difficult in an open pit.  

Everything works together in a convenient system for my land set up.  I understand that it wouldn't work for others in different terrain situations.  

John S
PDX OR

I have a huge amount of small irregular branches  of various degrees of dryness.
Placed in a steel toolbox retort and pyrolyzed in a bonfire, they come out nicely.
My Tluds so far have been rather small and until now, I never considered putting a retort inside one, but I think it should work.
I like the idea of retort + Tlud because even if you fail to stop char in the Tlud from burning to ash, you still will get the char in the retort, plus the retort feed stock needn't be very refined at all.
It also occurs to me that a retort filled with arborist wood chips may fails to pyrolyze fully because the lack of airflow,  yet still be excellent Tlud fuel.

So I am not sure if this is the same as making biochar. I enjoyed the film. I enjoy working with clay soils and find it fun. So hopefully some will find it useful.

The method which stood out to me is the second one. I am wanting to make charcoal from maple wood. I have a hankering for grilled meat cooked over charcoal. mmmhhmm

Just FYI, I have read at Oxford 1000s of research documents, one of which states the optimum temperature to produce biochar is 300 degrees c. This makes the maximum cation sites, which must be quenched immediately with your chosen cations, ca,mg,fe,k otherwise they will be leached from your compost etc and made unavailable for your crops. I produce my biochar using a tin in a wood burning stove any unburnt gases are consumed by the fire and not released into the atmosphere. Hope it is of some help, all the best barney

There has been a lot of research put into biochar over the years and a lot of technical knowledge built up as a result, most of which I have little knowledge of.
So naturally I prefer the low tech solution of pit burning and don't worry if I achieve perfect structure for maximum water or nutrient retention

In my area it reduces the danger of burning on hot dry or windy days(though I still don't burn on excessively windy days)
It is simple and costs nothing to make a reasonably sized pit for the homestead that will produce approximately 4 to 8 50lb feed sacks of finished charcoal
Dousing can be achieved by simply filling the pit until you can see water at the top of the charcoal mound
The pit can be repurchased to burn trash wood to ash and can be placed on a scale line or at least against contour to act as a small scale water harvesting feature when not in use

For charging my char I use it I'm my compost buckets to absorb odors and moisture and then it gets dumped into the chicken compost pipeline after the bucket is full, so in theory it's had plenty of opportunity to charge up by the time it's used in soil

Everybody has to adapt biochar to what they can do at their property and what they can afford.  Even inputs for inoculation are going to differ.  The feed stock (wood, corn cobs, etc. ) is going to be different too.  I don't think it is going to be worth it for most people to worry about some kind of theoretically perfect model of biochar.  Listening to others' experiences and adapting our own as we go makes the most sense to me too.

Adding types of garbage might make it more authentic to the original terra preta from Brazil.

JohnS
PDX OR

One of my "projects" is digging a trench along side a long raised bed.
I hope to catch surface water in it.
I plan on filling it with biochar for that very purpose,so perhaps I"ll make the char in the trench.
I wonder if a charcoal making trench in clay soil will fire to any degree?
I suspect not, simply due to the immense thermal inertia of the soil.

When I've done pit burns in soil that has a decent clay fraction, they develop a terra cotta "skin" that is fairly tough. If I use the same pit repeatedly and smush some cob into the cracks, it gets thicker and stronger with each burn and even withstands some moderate scraping with shovels.

Our Wild Rivers Permaculture Guild has gone in on a Ring of Fire biochar kiln: https://wilsonbiochar.com/

We are working on the logistics of getting 10yds of brush together in one place, and dry enough for the first burn before burn bans come into effect. We are also working with our local fire safe council, the forest service, and waste management department to divert woody debris.

A couple questions have come to mind where I do not have any clear cut ideas of how’d be best to proceed. First, how much to return to the site of collection for regeneration of the soil there. The other is how to divide the proceeds of any given burn.

For the first, I am inclined to go with 1/3 of the biochar returned to the sites of collection where feasible (like from an individual property or National Forest site. This is what my uncle, who is a forest policy analyst and ecologist in Minnesota, has said is their baseline organic matter redistribution for biofuels projects.

The second question of dividing proceeds to individuals, I am more unclear. 19 people went in on the kiln, but many others will help with wood collection and burns. How much would be fair for someone who helped pay for it but did not show up for a burn? What about if someone who didnt pay provided their land for wood collection? Or put in grunt labor? Grunt labor plus a truck? How about providing a tractor or dumptruck for hauling wood? Other tasks or cooking for the crew? I have considered a ridiculously complicated share tabulation approach that would undoubtedly get unwieldy and requiring inconvenient and productivity diminishing accounting. What are your thoughts?

In a situation like this, I often try to be transparent.  I devise what I think is a reasonable algorithm for determining the distribution.  Then I allow input.  I accept all positive feedback that will improve the system and incorporate it.  SOmetimes I have to wait until the next cycle to change distribution. Some people will come up with ideas that aren't really practical and be very aggressive that their idea should be it.    Some people will bitch, but you can generally tell who is improving the system and who is emotionally needy or too aggressive.  If you generally continue to be reasonable and transparent, my experience is that people will accept the system and buy into it.  
John S
PDX OR

Thank you for those thoughtful insights John. I should point out that I have no more than 1/19th of the decision making power/responsibility on this, but I will take that largely concurrent thinking to my own to heart in how I approach this. I’ve always liked the divider-chooser method for fair distribution.

Anyone have ideas or advice  on scaling up brush collection regeneratively?

Congrats on the Ring of Fire, Ben. I've been on a few of Kelpie's online meetings recently and she's really inspiring. If you've got access to lots of land that needs thinning and clearing, I'd say to work out a schedule that takes into account seasonal conditions and rostering your colleagues plus (possibly) some willing volunteers.

Go out to each area and do a "gathering and piling day" where you drag everything to a central staging area. Pile everything as loosely as you can in rows or semicircles around where you expect to set up the ring. Give enough room for a tractor to do its thing.

As the piles dry out, you schedule your burns around the weather and fire danger. Ideally, you'll always have a queue of piles waiting to be charred and enough info about local conditions to make good calls about whether to go ahead on a given day. If someone in your group has a bit of land that they don't mind turning into a biochar yard, that could become a depot for locals to drop clean brush and prunings...it's great when they bring it to you.

Good presentation from Living Web Farms (Asheville, NC)  given by Bob Wells, on small scale biochar (and industrial scale biochar).   Shows how to build a biochar retort from 55 gal drums.  Living Web Farms Youtube - Biochar Workshop

I have been using my cob oven to make biochar, as a secondary function to making food. It doesn’t make much or very good biochar, but I also can cook food with that heat. Does anyone else do this?

Leigh Tate wrote:

One thing I don't see addressed often with the open methods, is the release of the wood gases into the atmosphere, as opposed to burning them with a gasifier type of biochar maker. How much are they adding to the greenhouse / pollution problem? Or are they? It seems that ought to be part of the efficiency equation.

This is why we advocate the use of flame cap methods. The active flames and mixing in the combustion zone "eat up" all the nasties in the smoke. Done well, a burn in a pit or a kontiki-type vessel releases very little smoke...just CO2 and water vapour, and that CO2 would have gone back into the atmosphere eventually anyway, because it's part of the cycle. Also, the higher temperatures in the combustion zone are producing better quality biochar, with more porosity and higher fixed carbon ratios.

Dry feedstock, a hot fire, and paying attention to the feedback given by the flames are the main factors here. It's an active process, as opposed to a TLUD or other retort system where you load, light and leave it alone. But personally, I will very happily stand around a fire and I can do other tasks in the vicinity in between feedings.

I am a volunteer with OpenAir Collective (https://openaircollective.cc) - working on methods for CO2 removal. My focus has been on biochar and I have done a few simple experiments which relate to this discussion. I think anyone who works with biochar for a while starts to recognize the distinction between biomass as raw material and biomass as fuel. When you pyrolyze biomass, the pyrolysis gas is a potential resource, which can be burned in the moment to heat more biomass, or possibly turned to other uses. But it is somewhat of a nasty mixture, and burning often seems like a pretty good idea for now.

So among the experiments, I made a little TLUD from a gallon paint can. The chimney was a sheet-metal funnel and cylinder assembled with pop-rivets - cute. I was using shop wood scrap for fuel and the biochar yield averaged about 15% of the original fuel mass in four trial burns. More recently, I decided to upscale to steel barrel size, and finally come to my points in writing:

First on the matter of drums, I devised an inner drum from a full size drum with a little creative side grinder work and drilling - no welding, so I think it is pretty reproducible - a series of photos of the build and burn are here: https://drive.google.com/drive/folders/10t085VjnScTgzg0JschJ0jGu_90Z83gd?usp=sharing

Second, this TLUD does not actually work. There are four 4" x 1" gaps at the top of the outer barrel where air enters, and the top fire burns cheerfully and vigorously for about 30 minutes and makes char. But about the time the fire reaches mid-barrel, it dies. There is no updraft created. I know this because of the fire dying, but also because I check the outer barrel temperature with an infrared thermometer and the bottom of the barrel does not get warm. So I am working on modifications to force a draft but I am shooting in the dark a little. Have others had this difficulty? Did I build it all wrong? I refer experts to the link above which shows my build. I have enlarged the  bottom holes of the inner barrel but this has had no effect, nor has loose-piling the fuel. Air comes in at the top and stays at the top.

My current trial is to close the top inlets and make nozzle inlets lower on the side of the outer drum that will direct air downward and in a circulating pattern in the annular space. I'll comment again once I have a chance to do that test, and I will add more photos to the gallery link cited above.

OK! I made a couple of minor modifications and had a good burn. One lesson I though I had already learned but apparently not: if you are going to make something get really hot, do not do that on a concrete slab. The first three firings of the TLUD were duds and the bottom never got warm. But with the modifications (documented now in the same photo directory in my Google Drive), the bottom of the outer barrel reached a toasty 450C and a barrel-bottom shaped chunk of concrete under the barrel finally exploded - not hurting anybody, but dishing up the barrel bottom about an inch in the center. Very exciting! Very stupid!

No more burns for a while, I am proud owner of 15 gallons of biochar that needs to be broken up, compost-tea infused, and plowed in.

Nice job, Cade! We are all just experimenting and showing our experiments to others.  I learn tons from all of you.  I don't know if you saw, but I had some adjustments that I've made to my TLUD.  First, I put it on fire bricks, so air could come up through the botttom better.  Then I turned the bricks on their sides, so even more air could flow through.  I've also recently found that adding one piece of wood quickly doesn't mess up the fire if it's already roaring.  

I have been doing this many times a year for about 10 years, and I have never had any visible wear on my set up.

Here's what my old set up looks like. My newer one has a wider chimney, which I recommend.

https://permies.com/t/223940/a/219525/New-10-chimney.jpg

This is what the new one looks like, which I highly recommend.
John S
PDX OR

So, what is inside, John? You mention admitting air to the bottom, but I thought part of the concept of TLUD was to force air down from the top and pre-heat it before it enters the bottom. I guess there are lots of ways though. (EDIT: OK, I watched the video earlier in the thread and I saw that simple updraft one-barrel design - certainly more economical and easy to construct.)

I will say that there was a dramatic change in the operation of my device on the last, I suppose proper burn. When the fire burned down close to the modified air inlets and started to be pre-heated, burn spots appeared on the paint of the new drum about where one would expect the heat to be focused. I monitor the temperature with an infrared thermometer because I am not just making biochar, but also reporting experimental results. Here is a brief summary:
10 minutes after ignition: mid-barrel (this is the OUTER barrel - inner barrel must be significantly hotter) 170C, bottom of barrel: 32C (in earlier tests, when I did not achieve updraft, this value never changed).
20 minutes: mid barrel - 240C, bottom - 50C (yay, some heating! This means heat is being carried downward so air is updrafting in the inner barrel and preheating fuel. There is still some faint smoke from the chimney - maybe a little oxygen starved?)
30 minutes: mid barrel - 290C, bottom - 100C (still faint smoke visible against dark background, not visible against blue sky)
40 minutes: mid barrel - 250C, bottom - 300C (I notice unusual amount of radiant heat from the chimney and measure 500C at base of chimney, normally about 400C)
50 minutes: mid barrel - 220C, bottom - 370C (average of three readings, chimney base 450C, flames escaping chimney past 20 minutes maybe I will add some supplemental air vents at the chimney base)
60 minutes: mid-barrel - 270C, bottom - 395C (chimney base 390C - back to what I'd come to expect, I noticed reduced flame in barrel soon afterward)

several minutes later, the concrete under the barrel exploded and ended the experiment, but all the wood was consumed so it was good timing (if such is possible). The high bottom temperature could be indicating hot charcoal fire which is counterproductive. On a subsequent test, I will probably quench the fire when the bottom temperature passes about 360C. If any torrefied wood bits are remaining, I can save them for another batch, or just plow them in like biochar perhaps. An infrared thermometer is a pretty inexpensive and useful tool for a biochar maker, imho. I got mine on Amazon: https://www.amazon.com/gp/product/B07CNW991V/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&th=1 for about $25. Just aim and read (laser is for aiming it).

Hi Cade,
I use a more blue collar approach.  I don't use thermometers or measuring devices. The holes in the bottom are so there is enough air (but not too much) to nurture the burning of the gases off of the wood as it goes up the chimney. The chimney reburns some of the gases so there is hardly any smoke.   I don't use a retort, because I need more biochar than I could have made with one. The only things inside the barrel at the beginning are wood and air.  It is a TLUD- top lit updraft system.  I start at the top with a stick tepee and I use bits of cardboard, etc, to start the fire. It gets going quickly. I add more wood and then put on the chimney when it is really roaring.  Because I want more char than I could get from when I cover it at the beginning, every 5 minutes or so, I very quickly slip another piece of wood in until I think I have added enough.  The fire burns for quite awhile-an hour and a half or so.  When the flame is barely above the coals, say 5-8 inches on average, I rapidly quench the fire with water.  This puffs out the char, so it is a better home to the microbes.  I had to experimentally find that out because the original video I based it on wasn't very clear about when to stop the fire.  I stop it then because I have very little ash and the maximum amount of char. I usually have a couple of small pieces that aren't completely charred, so I just save them and throw them into the next batch.  I have never had damage to the cement below or to any of my equipment.  The bricks are hot for a few minutes as is the equipment, but they cool down after a bit. It works really well for me.
John S
PDX OR

I mostly make my biochar in the fire pit made with recycled concrete blocks. It's about 3 ft in diameter but I can rearrange to change the size if necessary. I simply stack the feed stock vertically to 4- 4.5 ft tall and lit from the top. I mostly use materials less than 2" in diameter as I have other uses for bigger logs.
I quench the fire with water and pick out the unfinished pieces for next burn.
Pros:
No equipment needed
Getting rid of small diameter stuffs that take up lots of room
Fast and clean burning
Little ash

Cons:
Open fire may not be allowed in some other places
Highly depending on weather
Needs close supervision
Use lots of water
Not efficient if the feedstock size is uneven

The kids and I did a small campfire biochar burn today. The snow up here in Rhode Island finally melted enough to get out in the back yard.

We used my trash can kiln in a standard metal ring fire pit.



We filled the barrel with little scraps and off-cut bits of wood. The outside fire was junk wood from the yard, woods, and a few old scraps from the basement and whatnot. In the picture above you can see

After a couple hours in the fire, the flare off stopped burning so I knew the barrel was done.

Obviously a reduction in size in the material, but a nice char all the way through.



After I pulled the barrel out, I was going to let the fire just burn out, as I usually do. Then I realized if I doused it, I'd get another bunch of char that once the ash and nails were sorted out should be useful.

If I was doing a longer fire, I'd have lined up a second barrel to "swap in" when I pulled the first barrel. Hoping to try out that "batch system" as the weather continues to warm up and things dry out.



The kids got to toast marshmallows and the dog ran around the yard, so overall a pretty good afternoon making biochar!