Charcoal from sawdust in the open air

[gaudencio]

Hello all (and by all, I mean Essay), a few months ago I posted a little about my thoughts on charcoal and tp, and since then I've been playing with making charcoal from sawdust, since in my experiments it seemed to produce by far the best quality, finest activated charcoal. No grinding and bashing, just a lovely fine powder. However, after trying manifold schemes to create it on a larger scale, I stumbled upon an idea that I would really like some advice on from the experts. Basically, it seems to me the most efficient way to make activated charcoal on a small, garden scale is to burn the very dry sawdust out in the open.

 

Now, obviously that goes against what we all know about charcoal - i.e. it's organic matter heated in the absence of oxygen. However, after reading up about it, it seems wood goes through a number of stages when burning, and low temperatures will only cause it to char, not to burn right through. I admit I don't understand this all fully, which is why I'd like the advice, but I noticed that on lighting a match and leaving it to burn, the end result is what I would call perfect biochar: a light, fluffy, black powder with that distinctive charcoal smell.

 

Thus, it seems to me perfectly possible to turn wood into charcoal in the presence of oxygen, so long as the wood is comparatively thin that it can char right through without needing additional heat (which would turn the outside to ash). Hence, my solution. I experimented with VERY controlled burning of thin layers of sawdust. Really thin, and really controlled. If the flames get too big, it turns to ash. If you keep it controlled, you end up with a fine black powder that looks, smells, feels like charcoal.

 

So my final question: is it charcoal?

[gaudencio]

I just realised I made a big mistake - it's not sawdust. Don't try it with sawdust, it won't work. You have to use planer shavings, from a carpenter who only deals with dried out woods - not wet woods from a lumber yard.

 

Oh, and I also forgot to mention WHY you would do it like this rather than in a barrel. Well, because I can convert a vanload of planer shavings to charcoal in about an hour, with no extra fire needed. No wasted fuel, apart from the gas from a cigarette lighter.

[Essay]

...efficient way to make activated charcoal on a small, garden scale.... So my final question: is it charcoal?

Yes! Wow! I think I can see how that works; each shaving flaring up enough to catch some neighbors--igniting more shavings--but each shaving dying out before it can turn to ash.

Dying out... due to lack of "critical mass" of heat, and lack of oxygen on the underside of the shavings (roughly).

 

It's probably like the char of a burnt wooden matchstick, except thinner and more fragile--I imagine.

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It occurs to me that the bio-oils that one can extract through pyrolysis of "logs" are not being captured with this method of carbonization; though that probably means the air smells wonderful as those shavings are catching and dying.

So as a method to easily make some home-produced biochar it sounds great.

On a commercial scale, I'd like to see something taking advantage of pyrolysis--capturing more carbon.

But for a home garden, that sounds like an cheap & easy replacement (given proper safety!) for the expensive horticultural char.

===

 

I've been wondering if a concentrated-solar-heating pyrolysis unit could be used to make char out of compost.

It would probably work better with just wood shaving.

Certainly the bio-oils derived from fine wood shavings would be preferable--over bio-oils from compost.

===

 

It probably is still a low-temp. char that you're producing--retaining some beneficial oils ...for the wee beasties. Thanks for keeping us posted....

 

p.s. I think it is very high-temp. char that is called "activated-char" --and the lower-temp. char is called "biochar" --if I've figured this out correctly.

[gaudencio]

Hey Essay, glad to know you're still here so my post didn't fall on deaf ears. Actually, I think most of the wood oils are burnt up when doing it in open air. If I understand it correctly, that's what a flame is. Therefore there's not actually that much waste into the atmosphere, since they burn very clean - almost no smoke. Lots of people here burn their crop residues at the end of the season, so it seems to me if only they controlled the burning a bit better, not letting the flames get so high, they could be covering their fields in a thin layer of charcoal every year.

 

As for cooking compost in a solar heater....I've been there on this one, mostly. I did a few very disappointing experiments with compost and other materials and decided planer shavings was the way to go. The reason is that when they come from the carpenter they're so dry that all the hard work is done for you. With compost, you need huge amounts of heat just to drive off all the moisture, and it doesn't cook evenly. Plus, as a gardener, good compost is most precious, so the idea of cooking it doesn't sit well.

 

Give the planer shavings a try if you can get them free from a carpenter. My next plan is to start a huge compost stack with alternate layers: burning the shavings right on the heap every time I have some. That way, over 6 months or so any uncharred shavings will rot down, and all the char fills up with bacterial goodness. Then you're left with biochar enhanced compost that can just be dug straight in the soil.

 

----

 

By the way, you may be right on the "activated" charcoal thing. It was my understanding that activated was the powdered form, but a quick glance at wiki shows I know nothing!

[Essay]

Hey Essay, glad to know you're still here so my post didn't fall on deaf ears. Actually, I think most of the wood oils are burnt up when doing it in open air. If I understand it correctly, that's what a flame is. Therefore there's not actually that much waste into the atmosphere, since they burn very clean - almost no smoke.

So, does it smell really nice?

 

Lots of people here burn their crop residues at the end of the season, so it seems to me if only they controlled the burning a bit better, not letting the flames get so high, they could be covering their fields in a thin layer of charcoal every year.

What a good idea! Maybe some sort of processing could help, though since it's the volatiles in the wood that causes it to flare up, maybe the crop wastes wouldn't burn right without the volatile oil residue. Maybe as a part of "chipping and shaving" some oil could be added. ....but that's sounding too complicated.

 

As for cooking compost in a solar heater....I've been there on this one, mostly. I did a few very disappointing experiments with compost and other materials and decided planer shavings was the way to go. The reason is that when they come from the carpenter they're so dry that all the hard work is done for you. With compost, you need huge amounts of heat just to drive off all the moisture, and it doesn't cook evenly. I can imagine! Thanks for the experience. Plus, as a gardener, good compost is most precious, so the idea of cooking it doesn't sit well. :)

 

Give the planer shavings a try if you can get them free from a carpenter. Yep! Already on my to-do list.

 

My next plan is to start a huge compost stack with alternate layers: burning the shavings right on the heap every time I have some. That way, over 6 months or so any uncharred shavings will rot down, and all the char fills up with bacterial goodness. Then you're left with biochar enhanced compost that can just be dug straight in the soil.

 

Sounds great! I wouldn't be surprised if that is just the way they did it in the Amazon.

 

p.s. I wish I had more experience with composting; but maybe this year. Thanks for sharing yours.

[lemit]

That's a great idea.

 

Since creating biochar requires some dampening, I have a question that might dampen the ideas about the Amazon.

 

The Amazon, like where I grew up in Missouri, is below the lime line. I've started to wonder if the Native people in both places might have created regular burns not to generate biochar so much as to balance the pH of the soil.

 

Do I have the chemistry backwards?

 

--lemit

[Essay]

I'm gonna have to learn about this "Lime Line" of which you speak. :)

 

I've spent too much time in the ivory towers to know of these practical, real-life things.

 

...in the Amazon? Really? Do you have a source about this?

 

~ :(

[lemit]

In the US, the lime line is roughly the 100th meridian, the line between the moist midwest and the dry west. I'm not sure exactly how it works, but I think the moisture probably leaches salts out of the soil and thereby lowers the pH.

 

So, in the midwest, where I have a farm, we have to add lime to the soil to raise the pH and grow crops. In the west, where I live, there are alkali flats because the soil has a high pH.

 

I hope that makes sense to everyone, and I hope it's right.

 

I'm sure other countries have the same phenomenon. Let's see. Don't we have some people from Australia and South Africa? I'm sure, from the little bit of geography I'm learning about those two countries, that they have the same line. Am I right?

 

--lemit

[Philip Small]

I'm sure other countries have the same phenomenon.

 

Lime line in Africa should be near the ustic/udic boundary in this map, with ustic xeric and aridic tending to have free carbonates in the soil profile, whereas free carbonates tend to absent in udic and perudic.

 

Compare to USA Ust* and Ud* soils here.