How do you make biochar?
This is one of the most popular questions when it comes to biochar. We have synthesised our learnings from the past 3 years and broken it down into the key elements.
There are a number of different ways of making biochar, some of which are better than others. Here, we will focus on two. Making it at home on a small scale and making it in a factory on a large scale.
The fundamentals
First, let’s cover the basics. Biochar can be made from anything that was once alive, but we focus on making it from ‘waste’ woody materials like wood chips. (I use quote marks for the word waste because in nature there is no such thing as waste. Everything just becomes part of another system. Waste is a human concept born out of a lack of imagination.)
How do you turn wood into biochar?
This is called pyrolysis, which means separation by fire. When woody material is heated in a low oxygen environment, gasses escape and smoke is released. Smoke is flammable and can be used to create carbon neutral heat. This heat is used to bring the next batch of wood up to temperature and any waste heat can be used to heat homes, factories or even swimming pools.
Alternatively, the heat can be converted into electricity and fed into the national grid or used on site.
Heating wood in the absence of oxygen is the key to making biochar. Limiting the oxygen prevents most of the carbon from reacting and forming CO2. This is because carbon needs oxygen to complete the chemical reaction and turn into CO2. When we limit oxygen, we prevent the wood from turning to ash.
When the wood has run out of smoke, almost pure carbon is left behind. The high temperatures cause this carbon to form a crystalline structure which is almost impossible for microbes to break down in the soil, therefore it is not biodegradable. But this is a good thing! It’s why biochar is used to fight climate change. It is a permanent form of carbon storage.
What happens to the carbon in the smoke? Is it released into the atmosphere?
As the wood is carbonising, the wood smoke is directed to a combustion zone and mixed with oxygen to create heat, CO2 and water vapour. It is also possible to condense the wood smoke and turn it into a range of chemical products.
How is biochar a form of carbon storage?
Wood that is considered ‘waste’ is essentially a ticking carbon time bomb. Most of the time, it is sent to landfill or incinerated for energy. In landfills, wood produces methane and CO2, and in an incinerator, 100% of the carbon is put back into the atmosphere through complete combustion.
This is why we choose to use ‘waste’ wood, because if we didn’t make it into biochar it would inevitably end up releasing its carbon back into the atmosphere.
Currently, most biochar production sites aren’t making use of the 50% of the carbon that is released when the smoke is combusted. This means, some of the CO2 has a chance to re-enter the carbon cycle for the next tree to make use of it. However it is possible, and probably sensible, to use this ‘waste’ CO2 to replace imported CO2 for use in the greenhouse. We could connect up a biochar machine to a greenhouse and transfer CO2 and heat to help the plants grow. Making greenhouses all the more sustainable!
In summary, making biochar releases roughly 50% of the carbon in the wood in order to turn the remaining 50% into a non-biodegradable form. Trees take it from the air, we take it from the tree and what’s left is made into biochar.
What about making biochar on a small scale?
We’ve covered the basics and touched upon how large scale machines work, but what if we wanted to make biochar on a smaller scale? What if we wanted to make it ourselves, from local woody waste, in our gardens whilst having a BBQ?
Well, here’s how. First, work out if you’re happy to build a kiln from scrap metal and potentially only get about a year’s worth of use out of it (due to the accelerated wear on the thin metal walls of most oil drums). If you have the skills and the tools necessary for the job, go ahead, but be careful because you don’t want to end up creating a smoke machine. Because any smoke produced will outweigh any carbon savings achieved through the biochar. Smoke is methane and methane is 25x worse than carbon dioxide as a greenhouse gas…yikes.
If you want to make your own kiln, this is one of the best resources I have come across. But please be careful… If it gets smokey, put it out with water and try again with new feedstock in a new design. Biochar must only be made in a carbon negative way, otherwise, you’re doing more harm than good for combating climate change.
We’ve made kilns from recycled food cans, paint tins and oil drums, but we thought, what if we could design one that was more reliable and lasted longer than a year? What if we could make biochar more accessible by making it easier for people who didn’t necessarily want to build a kiln themselves. We googled ‘biochar kiln for sale’ and nothing came up. This is when we realised we might be onto something.
9 months, 12 baked bean cans and 3 oil drums later we ended up with this. An easy to use, totally smokeless, garden biochar maker.
How does it work?
You load it up with ‘waste’ wood, here we are using discarded wooden pallets (without paint) but we’ve found that using dry wood chip works best. Fill it up and light it from the top. The goal is to get the fire burning as evenly as possible because if one side burns faster than the other, the delicate balance of air fuel and heat is disrupted and you end up with a smoky fire.
Once it’s up and running, you can now sit back and enjoy the heat of the dancing flames. We designed a cooking grate so you can even cook your dinner whilst sequestering carbon from the atmosphere, pretty cool right?
Any smoke which is generated is quickly burnt off meaning no methane pollution. Smoke is flammable, so by mixing it with air you stop it from escaping.
After about 45 minutes to 1 hour, the flame will go from a big orange one to a small blue one. This is when nearly all of the wood has turned into biochar.
Wood burns twice, from wood to charcoal and from charcoal to ash. You want to stop it half way. Wood smoke burns orange and charcoal burns blue. The change of colour shows you that it’s time to stop the burn.
At this point you need to quench the fire with water. This stops it from turning to ash and creates steam, which in turn, improves the quality of the biochar. When you are sure the fire is out, put your heat proof gloves on, pick up the burn chamber using the handle and tip the biochar into a bucket. Crush it up using a sledge hammer, space or block of wood and then it’s ready to be used!
We keep a stash of biochar to mix it into the garden compost bin, the kitchen compost bin to control odour, or when we want to do the 53 other things you can do with biochar.
There we go. That’s how to make it at home. Now it’s up to you what you do with this information. You can either build your own or give our kiln a go.
Good luck, and don’t hesitate to get in touch. We love talking about biochar so we’d be delighted to talk to anyone who is interested in learning more.
Ultimately we want to see biochar in everyone’s gardens, because if we make enough of it we can sequester 2.2–4.4 billion tonnes of CO2 by 2050 and avoid the worst of climate change. Biochar is no silver bullet but it definitely plays its part in getting us out of this mess. If you’re interested in learning about the other 75 solutions to climate change here’s a great resource.
To learn more, visit our website and download our Free Biochar Guide, watch our How To Make Biochar webinar recording and follow us on Instagram and Twitter.
