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Biochar Solar Kiln

 
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I am interested in building a solar kiln to use pyrolysis to bake wood into charcoal.  I want to use a container, possibly a 48-gallon drum, and concentrate solar energy onto the drum.  I would need to be able to control the temperature, ultimately with a thermostat.  I am thinking of placing the drum into a large reflective-metal curved depression.  I need to know how much sheeting, I would need, in relation to the drum, in order to maintain a temperature of 600 to 700-plus degrees.  Is there anyone who can advise or help me with this?  Looking for a design.
 
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Welcome to permies, Jim,
This is a really interesting project.  I wish you much success, because if it works, it could be impressive. Sadly, I don't have the technical experience you are looking for, but I will be following it.
John S
PDX OR
 
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It's an interesting idea. I'm curious why you would want to go this route versus traditional combustion.

Offhand, I think I would start with small scale experiments to test and fine tune the concept. A fresnel lens from a discarded projection television comes to mind.
 
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You are intending to waste all the fuel value of out gassing volatiles?  Or are you going to burn it as part of the process?  The reason for asking is you will need far more heat if you are going to do it pure solar.
 
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I agree that it's a terrific idea to take some of the energy required from the sun. I also agree that if you don't capture and burn the woodgas in some measure, it's extremely wasteful.

My feeling is that it would entail a sealed retort inside a heavily-insulated burn chamber with a couple pressure release valves directing pressurized wood gas down into a burn area below the retort. As it heats, the pressure builds, and opens the valves once there's enough. No explosions, and that woodgas is used to increase the efficiency and capacity of your system, as opposed to being lost to the atmosphere. And, you know, no explosions.

It would also be necessary to account for the liquids released by pyrolysis, and either collect them for use or add a feature by which they can be combusted to increase system efficiency.

I think that a non-biochar solar kiln would be awesome, too, and if you had a rig that operated as the latter with only the sun, and as the former with a combustion chamber and burn of the woodgas, you would end up with a very versatile system.

Great ideas, though. Keep 'em coming, and good luck.

-CK
 
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It would help if you mention C or F after you mention "degrees." If your reflector is very well focused, it could sometimes burn through or melt spots of the metal, so you might need to use high temperature steel or something.
 
Chris Kott
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Rebecca Norman wrote:It would help if you mention C or F after you mention "degrees." If your reflector is very well focused, it could sometimes burn through or melt spots of the metal, so you might need to use high temperature steel or something.



Interesting fact: "degrees Celcius" is actually redundant.

Celcius is a newer usage of the original term centigrade, which literally translates to a hundred gradations, or degrees.

So while a temperature reading in F should be read as "degrees Fahrenheit," a temperature reading in Celcius should read as "such and such a temperature Celcius (or centigrade, if you want to be fancy).

Of course, colloquial usage is king, but if it's all the same to everyone else, I will ignore fahrenheit as I typically do for it's lack of usefulness, and I will continue to omit the extra "degrees" to my Celcius reading.

-CK
 
Douglas Alpenstock
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A couple of threads that may be relevant:

https://permies.com/t/40/1758/charcoal

https://permies.com/t/118371/permaculture-projects/Solar-Glass-Recycler-aka-Fresnel

 
Chris Kott
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Of course, when I wrote the cheeky bit about ignoring fahrenheit for it's uselessness, I neglected to think about baking. When I am baking, degrees fahrenheit is my default. I guess I forgot to upgrade the UI. Oops.

-CK
 
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I've been noodling over this idea for a couple years now. I haven't been able to build a prototype yet, but just from working out designs on paper, I have a few suggestions.

If you want to build one big enough to cook a 48-gallon drum, you're going to end up with a huge installation. Picture lenses the size of buildings! I'd recommend going with a smaller burn chamber.

If you're thinking of a round, dish-shaped lens, you'll need a way to reposition it to keep it pointed at the sun. A trough-style lens, with a long, narrow burn chamber, might work better. That way you don't have to keep moving it around.

You say you're planning to use wood for the feedstock. To be honest, there are much easier ways to turn wood into charcoal. Depending on the size of the pieces, you might find a TLUD or kon-tiki trench works better for your needs. The solar kiln might be a better option for materials that either don't burn easily, or else don't produce enough heat when burning to reach clean charcoal temperatures.

I'm hoping to use a solar kiln to char bean shells from my dry bean crop. They don't burn hot enough to cook themselves like wood does. And my land doesn't produce enough firewood to be worth using it for making charcoal. The design I'm working on now uses a 4" diameter steel pipe for the burn chamber, with an auger to move material through. There's a hopper on one end for the raw feedstock, and at the other end is a dousing chamber. I'm going to start with a trough-style lens made from a 4'x8' piece of plywood covered with reflective material, but I may need to adjust the size of that part. After I get the sizes and angles worked out, and have tested to make sure it works, I'll replace the plywood with something more durable.
 
Jim Highfill
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Thanks for all of the helpful comments.  

I am not sure if a fresnel would work for my project, as it may not be large enough for what I am trying to do.

Ultimately I do plan to burn the methane from outgassing and use it to produce more charcoal. I am not sure if it would be worth it.  Question:  If I produce 40 gallons of charcoal, how much charcoal could I then produce from the methane left over?  Another 40 gallons?  20?  10?  5?  If it’s not enough, a person might as just as well burn off the methane.

The temperature I am talking about is the standard, commonly used Farhrenheit, used by the weatherman and body temperature, e.g. 96 degrees Fahrenheit.

I honestly don’t think a 50 gallon drum would require a reflector the size of a building.  Maybe several feet on each side.  If you have any information that I am wrong, please let me know.  
 
John Suavecito
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Along your way, you can use a lens to dry out wood before burning.  That might be your 2.0 on your way to eventually making it work.
John S
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Jim, biomass pyrolysis is exothermic and in the case of woody matter there is more than sufficient energy in the feedstock to drive the entire process (as long as the wood is dry). So if you're capturing the syngas output of a batch in your solar retort, it will be more than enough to "cook" another batch of equal mass. This is why pit and kontiki methods are so effective: we use that process heat right at the source and dispense with lots of technology to capture, scrub, store, and transport the syngas. Flame cap burns easily achieve 600-700C in the active zone.

So, as John suggests, you could use the solar concentrator to kiln-dry the feedstock on its way into your retort, where the output gas would provide all the heat required beyond startup.

[edit] I just noticed you were aiming for process temps of 600-700F. This won't produce biochar, as it's nowhere near hot enough to drive off the volatile HC compounds and form the aromatic C bonds. What you will get out of that will be torrefied wood. We usually consider 450C to be a minimum for high-quality biochar that does all the good stuff in soil.
 
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haven't read the whole thread, but there's a prototype being built in Burkina Faso and Haiti, I posted about the NGO's design somewhere on permies here.  IF you can't find it I'll dig it up for you.

No need to use a retort IMHO, just capture that gas for later use and be more patient.
 
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I really appreciate all of these helpful and informative comments.

Phil, the information you have on temperatures required is not what I have been told.  My understanding is that you can pyrolize at 650 to 750 degrees F 3 1/2 hours; or you can cook at 350 degrees F for 6 1/2 or 7 hours.  The longer time at lower temperature produces higher quality biochar.  According to the conversion calculator, 450C equals 842F, which is super hot, and I think would be unlikely to produce high quality biochar.  If I am wrong then please give me the information, hopefully along with some sources I can read.  With all of the trouble I will be going through, I want to make sure I do it right.  Thanks.

 
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Any time anyone says that charcoal for biochar needs to be created at X degrees, I take it with a grain of salt.  I have real misgivings that the people that originally made terra preta were overly concerned about reaching some exact temperature.  That isn't to say there isn't a perfect temperature.  There may well be.  What I am saying is that people without a way to precisely measure the temperature of the charcoal they made created terra preta, and people are still trying to replicate it.  Clearly, whatever temperatures they used "worked".  I think making charcoal and turning it into biochar is the "big rock".  Perfect temperature of the fire that made the charcoal, perfect inoculation, perfect ratios of biochar to soil, perfect depth to incorporate the biochar, and on and on, are the "small rocks".  
 
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My google-fu disagrees with you, Jim.

My understanding is that 450 C is about optimal. Lower temperatures don't break down some of the stuff that turns into tars and creosote. It results in lots of sooty smoke, and if there's any plastic contamination, it's well within the range to create dioxins.

Low and slow is good for caramelizing onions and garlic, not so much for incinerating harmful volatiles to leave a porous carbon sponge.

But test it out. Maybe try making a batch each way. You can tell if it's been done right because proper fresh char sounds a bit like porcelain chinking together.

Good luck anyways. Keep us posted.

-CK
 
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Phil Stevens wrote:Jim, biomass pyrolysis is exothermic and in the case of woody matter there is more than sufficient energy in the feedstock to drive the entire process (as long as the wood is dry). So if you're capturing the syngas output of a batch in your solar retort, it will be more than enough to "cook" another batch of equal mass. This is why pit and kontiki methods are so effective: we use that process heat right at the source and dispense with lots of technology to capture, scrub, store, and transport the syngas. Flame cap burns easily achieve 600-700C in the active zone.

So, as John suggests, you could use the solar concentrator to kiln-dry the feedstock on its way into your retort, where the output gas would provide all the heat required beyond startup.

[edit] I just noticed you were aiming for process temps of 600-700F. This won't produce biochar, as it's nowhere near hot enough to drive off the volatile HC compounds and form the aromatic C bonds. What you will get out of that will be torrefied wood. We usually consider 450C to be a minimum for high-quality biochar that does all the good stuff in soil.



I didn't know about the ideal temperature for biochar, my reply was just in reference to making charcoal in general.  I think you could get the same high temperature.

Storing the syngas, I think that could be done in a low-tech way if you're not too picky about the quality of the product.  You can use it for low-intensity uses.  I don't have specific info, but it just seems like a waste of that energy to burn it just to do what the sun would do given more mirrors.  My understanding is that the storing of the gas simply requires bubbling it through water, Bill Mollison wrote something about double-chamber distillers in either Permaculture II or the Yankee Permaculture lectures.
 
Phil Stevens
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OK, first of all 650-700F is a very different animal from 350F. You will get OK results at that temp range but it won't be the greatest quality. 350F simply will not cut it, no matter how long you let it cook.

As far as grains of salt go, there's a pretty good body of research now which goes into a lot of depth on the "little rocks." Some of those little rocks act as keystones in dynamic systems like soils that we're trying to improve, so I'm not quick to dismiss all of them. I've got the massive second edition of Biochar for Environmental Management (Lehmann and Joseph 2015) and in the chapters on physical, structural and macromolecular properties there are dozens of articles cited, plus charts and text describing the correlation between highest treatment temperature (HTT) and attributes like porosity, surface area, and formation of aromatic bonds and graphene structures.

Some of the basic science is good to know, because it has a major effect on how long the biochar will persist before breaking down, and how much good it will do in the meantime in the areas of water retention, aeration, nutrient absorption, and providing microbial habitat. Materials pyrolysed at lower temperatures have lower porosity, less internal surface area, and a general lack of aromatic bonds. They can also have high levels of condensed tars and oils (that's what is clogging up the micropores) which will weather and break down in the soil. In general, for woody feedstocks, a minimum HTT of 300C is regarded as necessary to get biochar that has the desirable qualities for soil application and the proportion of aromatic structures only starts to really kick in at 450C.

Here is a chart from the IBI current biochar standards. The H:C ratio is a measure of how much the hydrocarbons in the original cellulose and lignin have been broken down versus the carbon remaining:



So, what this says to me is that you need 300C (572F) at least if you don't just want a bunch of torrefied wood at the end, and aiming for 450C gets you a far better product if it's attainable in your situation. The reason terra preta works is that wood fires very easily achieve temps of 600-700C and so prehistoric cooking and rubbish fires would have been totally up to the task of making long-lasting and highly effective biochar.

Here is a good online summary of the process, including a description under "Stages of pyrolysis" explaining what happens at different temperatures:

Basic principles of biochar production
 
Joshua Myrvaagnes
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Fascinating.  I suppose it's worth using the retort then to make a limited amount of biochar, then it has an ongoing benefit.

But for basic biochar, the Lytefire website says it bakes at a temperature of 300C.  Lytefire.com I think.  They are partners of the NGO, Gosol, https://gosol.org/.   I imagine you could get even higher temperatures though if you just mirror the bejeezus out of it.  Solar array power plants have much higher temperatures, yes?  My own solar cooker could light a leaf on fire (which was great for entertaining passing children and sharing knowledge about solar cooking in a dramatic way, if not so functional for any other purpose).  It was kind of crappy, marginally portable, 1 m2 dish parabola.  It's not the temperature per se that's the question, it's the amount of energy building up over time, so if you can build the same basic thing only bigger I believe it should work.  

Maybe making less biochar of higher quality (smaller oven space) would also make sense.

I also wonder about a pressure-cooker type situation...very dangerous if it exploded but they have a safety valve, right? some pressure cookers are black on the outside, we had one for the solar cooker.

The embodied energy in the manufacture of glass and that shiny backing they have is something I don't know, but theoretically it's a one-time investment.

Do let us know what you end up discovering in your experiments!
 
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Phil Stevens wrote:Jim, biomass pyrolysis is exothermic and in the case of woody matter there is more than sufficient energy in the feedstock to drive the entire process (as long as the wood is dry). So if you're capturing the syngas output of a batch in your solar retort, it will be more than enough to "cook" another batch of equal mass. This is why pit and kontiki methods are so effective: we use that process heat right at the source and dispense with lots of technology to capture, scrub, store, and transport the syngas. Flame cap burns easily achieve 600-700C in the active zone.

So, as John suggests, you could use the solar concentrator to kiln-dry the feedstock on its way into your retort, where the output gas would provide all the heat required beyond startup.

[edit] I just noticed you were aiming for process temps of 600-700F. This won't produce biochar, as it's nowhere near hot enough to drive off the volatile HC compounds and form the aromatic C bonds. What you will get out of that will be torrefied wood. We usually consider 450C to be a minimum for high-quality biochar that does all the good stuff in soil.



Jim, I was planning to comment earlier, and so glad I waited... Phil's reply is what I wanted to say and more! I also agree that drying the wood would be a more effective way to incorporate solar power into the process, since drying the wood in the kiln/retort uses more time and fuel. (I've been dreaming up such a system myself)
I don't know where you are, but the reliability of solar may be the biggest trouble, having both the intensity and duration to char a load of wood, and how to modulate the temperature? Insulating the retort, and exposing it to concentrated solar are at odds as well.

Is your thought on using solar to make the char "for free, using the power of the sun" rather than burning some more wood? If your kiln is wood fired, you could adjust the temperature easily by adding fuel, and overcome a cloudy weather event.
Have you thought of a use for the waste heat from the process? If you could make use of the heat, you might get a greater return than the wood saved to get the char reaction going.
 
Jim Highfill
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Good information.  Thanks.
 
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I'm genuinely struggling to see what design problem this proposal is hoping to overcome. You are choosing to not use the combustion energy of the wood fuel itself in exchange for using solar, but I still don't understand from what has been written above why you think that is desirable enough to pursue? Do you find that you can't make biochar by combustion for some reason? Does normal combustion have some other problems associated with it that you need to overcome?


The external energy needed for me to make a pit full of biochar is - literally - that of one match. It burns clean and smokeless, and produces fabulous crumbly biochar. You still need to find a way to safely handle the gases produced from your solar kiln, which if they escape to the air, are the very worst set of complex hydrocarbons, smoke particles, carbon monoxide etc...

My portable "pit" is half an oil drum cut down it's length. I take it to where the brash is and just start burning. No complex set up, no need to haul bulky brash to a permanent structure.

I see other design problems as well. You mention keeping a retort at high temperature for 3 or more hours using solar. This is highly dependent on long hours of uninterrupted sunlight, and a lens setup that can accurately track the sun over that time period. Your retort will end up heating very unevenly - one side of the retort will get the direct heat from the solar reflector, while the other will be in shadow and losing heat all the time. To ensure even, complete pyrolysis you will need a mechanism to agitate the contents of the retort. I had similar problems in my old designs, where the heat would be uneven within a drum, one side would be untouched and the other near burned away to ash.

Solar for intense heating has so specific issues. Here in the cool and cloudy UK it would be a complete non-starter. In countries with enough consistent sunlight, those hot sunny days often also coincide with high fire risk days. The components of powerful lens/mirror systems can start fires at a considerable distance unintentionally, especially when conditions are dry and hot.

Overall this feels like a solution looking for a problem to solve. You could spend a lot of time, money and energy building something that works less well than an oild drum cut in half and has a whole host of extra safety and pollution issues.
 
Chris Kott
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One of the biggest developments in solar thermal technology of the past few years has been a project backed by Bill Gates to decarbonize smelting and other heat-based industrial processes. It holds great promise to get industry off fossil fuels.

But they're using desert-based solar tracking arrays. Yes, I think that a trough reflector with a drum at the focal point might work,  I do think that the drum retorts would need to rotate. And still, something would need to be done with the raw wood gas, which this approach has turned from a boost to efficiency to a waste product.

Basically, I think that using solar to kickstart the wood gas and to thereby combust the absolute minimum material is a laudable goal, but for me it would only ever be a secondary boost. Half the year,  it wouldn't even work for me except at noon.

-CK
 
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Hi
Interesting concept.
Mexico has a agave and tomato agro waste plant and in middle east they are ideating a solarpowered biochar for date leaf waste.  

https://www.sciencedirect.com/science/article/pii/S1018363920302440#f0005

https://www.solarpaces.org/designing-solar-furnaces-to-make-biochars-without-air-pollution/

should be helpful
if the input size is small and tempertaure of 350 C can be achieved. with wood solar may not work as required with saw dust it will.
we are working on a rice husk to biochar with solar . we ae focusing on biochar not syngas as we are doing  torrefaction .
after torrefaction do biochar briquettes for industrial use . the end product needs to store all GCV.
as suggested solar  pre dryer is feasible with wood chips
btw we are a agro waste briquettes start up from India .
any guidance / opinion will be helpful
 
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A lot of the responses here suggest using the the energy of the "wood" itself to fuel the process.
I believe Jim stated that his feed stock is ag waste from beans that does not burn hot enough to drive the reaction.

The suggestions that he use solar to dry the feed stock might work, I'm not sure if moisture is the issue.
 
Douglas Alpenstock
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As noted by others, reasonably dry wood provides all the heat needed for making char. No real need for solar input unless the goal is to capture the offgas.

If non-woody, low density feedstock is used, I can see the value of solar in drying the feedstock (required for a TLUD, for example) and as a supplement to combustion of the offgasses, ensuring an adequate temperature for quality char.
 
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