An idea: solar biochar at home
A while back I read about biochar. I think it was not-too-long after I attended HOPE 2020, and it felt like there were a few ideas floating around that pointed towards diverse, local approaches to climate change (solarpunk feels associated with this). I think it was also shortly before or after I read this article in Scientific American, plus countless sources/articles looking at how adding carbon to soil can be a good method of carbon sequestration, while also improving crop yields and reducing disease in plants (which means less fertilizer and pesticide).
Is this worthwhile?
Now, my understanding is that we’re past the point where everyone making small changes (like recycling better, or turning off lights) is going to make much difference in what happens over the next 30-50 years. That might be wrong, but even if it’s true, I think finding ways to let individuals and small organizations make positive, real environmental impacts is worthwhile for a couple of reasons:
- If we are headed for catastrophe, even a 1% improvement in the outcome might be helping a huge number of people.
- I think it’s worthwhile to share ideas and stories about how to bring the world closer to what we want and to let other people build on those ideas and stories – you never know where someone will draw inspiration.
So, probably not actually a useful thing, but the thought is roughly this: can you make biochar (targeted at amending soil, not for use as fuel) at home, safely, with minimal emissions (most methods seem to produce lots of smoke), and without a lot of energy input (minimal fuel/electricity to run the process). I know there are solar grills, so could the sun supply all the heat required to produce biochar?
Is this feasible?
Of course, someone has to be doing this already, right? I looked around a bit, and while I’m not the best at googling I really don’t think there’s evidence of people doing this. I did find this small-scale biochar kiln at biochar.info, which seems like good option, but isn’t something I’d put in my backyard. Can we do better?
Well, first, how reasonable is the idea of using solar power? The kiln described at biochar.info sounds like it generally runs at about 475°C, though elsewhere on the website they list 300-550°C as optimal for soil biochar (Wikipedia agrees, roughly). I know a CSP plant can heat salt to several thousand degrees C, but those tend to be bigger than my backyard. Let’s look at a few solar grills.
- https://www.amazon.com/All-Season-Solar-Cooker-Camper/dp/B074S74FQC answers say the best you’ll get is 325°, maybe 350°F
- https://www.amazon.com/Haines-Solar-Cooker-Dutch-Oven/dp/B07B5KX18D/ similar answers, 250-350°F
- https://www.amazon.com/GOSHENG-Smokeless-Portable-Capacity-Multifunction/dp/B0828S8B3N/ claims it can hit 400°F
- https://www.amazon.com/TFCFL-Diameter-Parabolic-Portable-Camping/dp/B0824XZWCB/ claims it can hit 1000°C!
All of the above use reflection (as opposed to, say, a big lens), and it seems like the 1.5m diameter parabola is the only one that could run anything at the temperature we want, but that implies to me we don’t need a massive solar array, a few square meters might be enough.
Seems like this might be something I could make.
Principles/goals & notes
Some goals to keep in mind:
- Effective. This should actually produce biochar in a way that emits as little carbon as reasonable.
- Simple materials. By simple I mean a couple of things:
- Conceptually simple, such that they can potentially be swapped out for more-local or more-eco-friendly alternatives by people building these (also means describing in layman’s terms the requirements of each material, and maybe how to check those requirements).
- Minimize complex electronics, or things with very precise tolerances. This helps keep cost down, helps with repairability, etc.
- “Eco-friendly” materials. I feel like one of the common mistakes in “eco friendly” products is assuming an essentially infinite lifespan of the product to offset an “eco unfriendly” bill of materials. There’s also different kinds of eco friendly: are you concerned about plastic pollution? Greenhouse gas emissions? Particulate emissions?
- Composed of low-atomic-weight elements, which are more common in the universe and generally easier to extract from the earth.
- Low-energy/low emissions to produce.
- Easy. To put together, to move, to use. People are more likely to use something, to find time for it, if it’s easy.
- Safe. Hundreds of degrees Celsius is dangerous. Focused sunlight is dangerous. Hot gases are dangerous, and some of the specific gases that are produced especially so (CO2, CO, various flammable hydrocarbons). Thought needs to go into how to make each aspect as safe as possible.
And a few additional thoughts on construction:
- Even before I saw the small-scale kiln linked above I was thinking 50-gallon drum with bricks/cinder blocks around it. Both of those feel like conceptually “simple” materials, though cinder blocks, being concrete, have significant negative impacts. Firebrick, which is what they’re using for the lining, however, seems relatively benign?
- I don’t know much about mirrors/lenses, but I assume that if it takes a 1.5m diameter mirror to collect enough sunlight, it’ll take a similarly large lens. A glass lens would be too heavy and expensive. I do remember reading about lenses made from water between flexible plastic (I even found the article) but that was years ago, I assume it’s just a bad idea and water would be way heavy for our size. I do wonder whether a similar thing could be done with a carefully-selected gas, though. Any gasses with a high refraction value?
- More likely, a big mirror is what’s needed. Most of the solar cookers seem to just use any old shiny surface.
- For flat shiny things, like old mirrors or sheet metal salvaged from the trash, it would be good to have a framework for how to maximize the amount of sun caught, and minimized the possibility of focused sun hitting other nearby objects.
- I can imagine using something like a large roll of mylar on a frame to produce a parabola.
- The gasses produced seem like they could actually be a useful source of heat, if the gas could be ignited before the reaction gets to the point where it’s self-sustaining anyway. I think to do that you could have the pressure relief hole on the underside of a drum elevated above some fire brick with a way to ignite it under there… Seems like the disadvantage there would be dangerous gas by foot level rather than up a chimney above your head.
- I also spent some time wondering about pressure relief valves, and their heat tolerance, and whether a buildup of hydrocarbons would make them stop working. I was wondering if you could use the pressure release to trigger an igniter for the afterburner.
Anyway, I think it’s an interesting thing to think about. I doubt I’ll try to build one, but you never know.