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Containers for making biochar inside a wood burning stove?

 
Robert Alcock
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Location: Cantabria, N Spain
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I'm looking to experiment with biochar in our garden, and to do this, I want to make quantities of finely divided biochar. Also I want to do this as efficiently as possible in terms of effort, mess, emissions (smoke) and also if possible capture the heat from the process.

The best way I have found so far is to use an old metal tin (normally paint or varnish tins) filled with the feedstock (which is mostly wood shavings that we get for free, but could equally be sticks from the garden). I punch a few holes in the lid and put it in the fire when it's nice and hot. The feedstock evaporates, the gas emerges and is burnt, then I take the tin out when it's finished and Bob's your uncle.

I can't make very much at a time because it's hard to find used metal tins that are bigger than 1 litre. But it's a reliable method that works, is no hassle, and over a winter can produce a fairly large amount. Since we light the fire most days for about 3-4 months a year, and we can probably produce about 2 litres worth of biochar a day, that should make a couple of hundred litres over the course of a winter. Theoretically.

The problem I have found, though, is that the tins don't last very long. As in, about a week of intensive use. The intense heat and the wood acids just eat them up. Of course I might be able to find a regular source of used tins, but that isn't really the principle of the thing. Also, I would love to be able to make 5-10 litres at a time (which is about all that would fit inside the stove) instead of just one litre.

I would love to know if anyone else has tried this method of biochar production and if so, what solutions you have found. Is there a kind of biochar-making insert for a woodstove that you can actually buy? Or do you use some other material for the container? Or just keep getting more tins? Any suggestions are more than welcome.

Thanks a lot!
 
Charli Wilson
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Location: Derbyshire, UK
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Not much help I'm afraid but I do the same thing! Old tins with a few small holes in or a not-quite-closed lid. As you do, I get through tins quite quickly- I collect the chocolate tins that float around the office at christmas and use those- but I'll be interested if you do find a better method that produces less burnt out tins.
 
Michael Cox
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One bonfire in the garden, quenched with water at the right moment, gets me about 100 litres of char and takes about 2 hours total.

The scale of production is just so unfavourable in an ordinary wood stove that it doesn't seem worthwhile in comparison. Plus I don't need to find tins to do it in!
 
Robert Alcock
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Location: Cantabria, N Spain
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Thanks for your replies!

Michael, I have also tried the bonfire method. I have found that the result is a mixture of all stages of charring (from uncharred wood to partially charred to ash) and is too coarse, so it would not be any good to add to the composting process, which is the next stage of using the biochar. I think you would need a very big bonfire to make 100 litres of usable char. Also you are producing a lot of smoke and CO2 and wasting the resulting heat. So it doesn't work for me, I'm afraid.
 
Bryant RedHawk
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Hau, Robert; I like stainless steel stock pots, if you want to do this in an oven look for a cheep stainless steel roasting pan with lid. Spring clips like you can find at office supply houses will work for holding the lid in place.

Don't forget that biochar can be just about anything that is carbon based. I do cow chips along with wood pieces, even grass can become biochar if you want to use it, as can straw, leaves, etc.

Using the cook method to derive your biochar is the way it should be done, burning in the open is not going to give the best results, but can be used. Jack Daniels makes charcoal by ricking maple wood and using a misting nozzle to keep the heat in the proper range to make charcoal not a bon fire. It requires at least one person is present at all times to regulate the flames so it doesn't go willy-nilly and make a lot of ash.

 
Landon Sunrich
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Michael Cox wrote:One bonfire in the garden, quenched with water at the right moment, gets me about 100 litres of char and takes about 2 hours total.

The scale of production is just so unfavourable in an ordinary wood stove that it doesn't seem worthwhile in comparison. Plus I don't need to find tins to do it in!


Around here I do this with hemlock bows doing the wet season. The pitch makes them burn supper hot and you just keep layering soaking wet bows on top. All the energy goes to boil off the water and you are left with a substantial amount of char. Of course the energy is wasted with this method.
 
Joseph Lofthouse
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Robert Alcock wrote:Is there a kind of biochar-making insert for a woodstove that you can actually buy? Or do you use some other material for the container?


I use a dutch oven. It ruins the seasoning.... But whatever.
 
Robert Alcock
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Bryant, that's a good suggestion. I will try and find an old stainless steel pressure cooker or similar, that should work fine. Of course it will take longer to get up to temperature.

Joseph, I'm not sure what you mean by using a dutch oven... can you elucidate?
 
Joseph Lofthouse
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Around here, we use the term "Dutch Oven" to refer to a cast iron cooking pot. They are often used for cooking directly in a fire. I use them in my day to day cooking for anything that needs to be fried, steamed, or stir-fried. The term "seasoning" refers to the layer of burnt on oil that makes cast iron cookware non-stick. Here's what one of mine looks like.

 
Bryant RedHawk
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Robert, I make most of my biochar in 55 gallon steel drums these are lidded and have 5-8 holes around the bottom edge that are around 1" in size, I load the drum up with what ever I am going to cook, light a small fire in the bottom and let it get going good then slap the lid on (it has one 1" diameter hole in it) and walk away. The fire doesn't consume but a little of the fuel I am turning into biochar, it makes quite a few pounds per load. I did one oven type production and was told "NEVER AGAIN" by the wife, since then I have moved to the steel drums and the great outdoors.


Just thought I would mention this for an other reference for you.
 
Robert Alcock
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That sounds a very interesting idea, Bryant. Have you got any photos?
 
Bryant RedHawk
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Yes I have some photos but no way to get them posted up here. Sadly I have no internet access at the farm and my office doesn't allow me to do things like up loading photos. We are hoping that next year we will have internet on the farm and when that happens, I will be able to post photos here.
 
Robert Alcock
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Thanks anyway, it sounds pretty clear the way you describe it!
 
Michael Cox
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Location: Kent, UK - Zone 8
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Robert Alcock wrote:
Michael, I have also tried the bonfire method. I have found that the result is a mixture of all stages of charring (from uncharred wood to partially charred to ash) and is too coarse, so it would not be any good to add to the composting process, which is the next stage of using the biochar. I think you would need a very big bonfire to make 100 litres of usable char. Also you are producing a lot of smoke and CO2 and wasting the resulting heat. So it doesn't work for me, I'm afraid.


Managing nearly 9 acres of largely wooded gardens/field/hedges produces a LOT of woody waste, just simply in defending the remaining open spaces from in growth. Anything larger than about 2" diameter goes in the wood pile, smaller gets piled and eventually burned - we don't have the means to use the waste more productively at the moment. On the whole it is pretty much fully charred, light weight and friable... but there is a fair size difference. The variable size is no problem as far as I can see; plant roots colonise the larger pieces just fine and they break down over time in the soil. I don't compost the biochar first but I do spread both compost and biochar on the soil together (topdressing), and also periodically wee on the biochar pile while it ages in the open before using it.

I would prefer to make biochar as a bi-product of some constructive process - cook stoves are the obvious ones - but that is not compatible with our current kitchen arrangements. If I were to try and make char in a stove designed for heating I would probably do it by letting the fire build up a big bed of embers then shovel some of the embers into a sealed heavy metal container (dutch oven as mentioned earlier would work well) or dump it into a bucket of water. No extra process needed here.

Mike
 
William Bronson
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For making it inside a woodstove how about an ammo can with the rubber gasket replaced with a suitable gasket?
 
Bryant RedHawk
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William Bronson wrote: For making it inside a woodstove how about an ammo can with the rubber gasket replaced with a suitable gasket?


That should work nicely. Might not even need a gasket (you do need to let the gasses get out or you will have an explosion).
 
Jack Edmondson
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Location: Central Texas zone 8a, 800 chill hours 28 blessed inches of rain
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Robert,

If your can is not lasting, try putting the container on the fire towards the end of the burn. It sounds like too much heat is causing fabric fatigue. You don't need a lot of heat to burn off the gases and the temp will rise when the gases start to burn. I would suggest trying it on the coals of the waning burn. If you can hold your open palm over the fire for less than 7 seconds, the fire is too hot.

Temperature: High
Thermometer reading: 450° F to 500° F
Hand check: 1 to 2 seconds

Temperature: Medium-high
Thermometer reading: 400° F to 450° F
Hand check: 2 to 4 seconds

Temperature: Medium
Thermometer reading: 350° F to 400° F
Hand check: 4 to 5 seconds

Temperature: Medium-low
Thermometer reading: 300° F to 350° F
Hand check: 6 to 7 seconds

Temperature: Low
Thermometer reading: 250° F to 300° F
Hand check: 7 to 9 seconds
 
Morgan Barker
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How about a large diameter iron pipe with threaded ends and end caps? You could make one(or have it made by a plumber friend, if you have one of those) that is log diameter and the length of your firebox. You might want to make several and load all of them because the mass of the pipe will make for a long cool down period.
 
Jack Edmondson
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Location: Central Texas zone 8a, 800 chill hours 28 blessed inches of rain
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Morgan,

Good idea. Just make sure to vent both caps. Expanding flammable gases trapped inside a rigid enclosure with no vent is a recipe for a bomb. The same vented on only one end is the basic design of rocketry.
 
Morgan Barker
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I was thinking that the drilled holes would work well if done equally spaced in a single row down the length of the pipe so if you did want to char sawdust, the gasses wouldn't have to push through all that material to get to one end.
 
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