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making biochar: methods pros and cons

 
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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?
 
pollinator
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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.
 
pollinator
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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.
 
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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:  https://www.youtube.com/watch?v=jxBUqk2M3Y8)  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?
 
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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.  



 
Leigh Tate
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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.
 
pollinator
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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.
 
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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.
IMG_20211119_122253.jpg
My latest pyramid kiln, a lightweight 60x60cm kiln for workshops and demonstrations
My latest pyramid kiln, a lightweight 60x60cm kiln for workshops and demonstrations
IMG_20211119_121926.jpg
Indoor TLUD gasifier, stove plans will be available for purchase in a couple days
Indoor TLUD gasifier, stove plans will be available for purchase in a couple days
IMG_20211103_204921.jpg
On demand hot water heater using 3 TLUD gasifiers
On demand hot water heater using 3 TLUD gasifiers
IMG_20211017_124633.jpg
Back of the retort with wood vinegar collection, still loads of improvements to make so plans will be a few more months
Back of the retort with wood vinegar collection, still loads of improvements to make so plans will be a few more months
IMG_20211019_185238.jpg
Inside of the 60liter retort
Inside of the 60liter retort
 
Eino Kenttä
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   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...
 
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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.
 
Martijn Macaopino
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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.
 
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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
 
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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.
 
Leigh Tate
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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.
 
Martijn Macaopino
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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.
 
John Suavecito
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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
 
William Bronson
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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.



 
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