Biochar Is Highly Beneficial For Plant Growing & Can Be Easily Produced in UK Gardens, Allotments and Community Gardens.
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In this article I am going to explain How To Make Biochar at home or in a community space (assuming you have permission). In the previous article I explained the science behind biochar, how it can be used and the benefits. I also mentioned a few things to be avoided. The two methods I am going to explain are quite simple once you understand the basic principle. That is the organic matter is heated in a low oxygen environment so that it gives off everything except its carbon as flammable syngas.
The Basic Principle Behind Biochar
Let me explain that in more detail as it took me a while to understand this. In a fire organic matter such as wood burns until all we have left is ash, smoke and a lot of heat. When making biochar the aim is to replace the ash with biochar. We still get a lot of heat but the organic matter doesn’t actually burn as there is no oxygen. It is however heated to very high temperatures, at least 500C is normal. But because we exclude oxygen the feedstock (the organic matter such as wood, hedge trimmings, brewery waste, farmyard manure etc) doesn’t flame. It gives off gases, these are collectively called syngas, and nothing burns as such. this means that instead of ash we get left with the organic materials carbon skeleton .. a product that looks like charcoal but is actually far purer. Low quality charcoal is around 50% carbon. Good quality charcoal might be as much as 70-75% carbon. Biochar is typically 88-92% carbon. That means that it is a network of carbon with very little inclusion, whereas charcoal has the same potential carbon skeleton but it is full of organic material which means it is less porous and cannot achieve what biochar does with its intricate network on minute pores within a microscopic carbon network. If it were a material it would be like the difference between fine lace and a heavy tarpaulin!
Making Biochar In A Hole in the Ground!
Dig a conical shaped hole in the ground, light a fire in the bottom and then add more dry organic material and you have the start of a biochar production unit.
This is how it works. The fire burns up through the heap and it gets hot in the bottom of the cone shaped hole. The fire on top then excludes the oxygen from getting deep downed the bottom of the cone is oxygen depleted. Once the top of the fire is glowing white quench the whole lot with water. The idea is to quench the flames before the whole lot turns to ash.
Rather than digging a hole in the ground you can buy a metal cone in which to build your fire. And if you want to to this on large scale you can do it in a large tank .. say a 1000 gallon / 5000 litre tank and quench the lot by lifting the tank with a tractor mounted loader and dropping it into a big tank of water.
This works but isn’t the most efficient process for making biochar. To begin with some of the feedstock is used to heat the rest of the organic matter and this proportion of the feedstock will turn to ash. Plus you have to judge the best time to wet quench the burn. Too late and too much of the product will have turned to ash. too early and there will be incomplete burn and you ends up with “brown ends” in your biochar.
This technique is sometimes called the flame cap method as the flames on top of the fire caps it and prevents oxygen getting to the base of the fire. To fully quench the flames it takes a lot of water. in the next method I will explain a dry quenching method.
The above is a bit cumbersome and inefficient there are benefits to this type of biochar manufacture. For example if a farmer wants to turn hedge trimmings into biochar this is a great way of doing it as it takes the process to the organic matter rather than having to trailer it back to a central unit. It does however depend on being able to get a tractor and the tanks on site and that isn’t always easy a wet winter!
So far I have explained a biochar making method that uses a huge tank and needs a tractor BUT can be carried out by digging a hole in the ground. The next one also needs special equipment if done on large scale but can also be done on a small scale using a recycled biscuit tin!
The Double Retort Biochar Manufacture Process
In this method the organic matter we are turning into biochar doesn’t burn as it is kept oxygen free during manufacture. This gives much better quality biochar and maximises yields.
The method is easy. Onto an open fire we put a “sealed” tin of organic material. The fire heats the “sealed” tin and the contents turn to biochar.
Now let me fill in the detail of how this works.
The open fire is often inside a container .. something like a BBQ. And the “sealed” container actually has a few holes in it .. otherwise it would explode. The gases (syngas) that are given off by the organic matter in the tin are expelled through the holes and catch fire in the open fire area. This increases the temperature even higher and speeds the process.
Now imagine the BBQ and “sealed” tin on a large scale. for example the “sealed ” tin could be a 40 gallon drum and the BBQ is just big enough for the drum to sit inside.
The photos below are of a Dartmoor Dragon biochar kiln which will produce between 18-25kg of biochar per burn.
I previously mentioned wet quenching of the biochar. In this method dry quenching is used.
The holes in the drum are all along one side .. the “bottom” of the drum as it lies on its side. This jets all the syngas downwards in to the fire pit and increases the temperature. To quench the fire the drum is removed using a device that allows two people to manhandle the drum out of the Dragon. It is then laid down in dry sand so that the holes are blocked by the sand. This prevents the air from entering and the “burn” continuing. If air does enter then the burn continues and the drum will then contain ash and not biochar.
Biochar Feedstock Types
In my previous post I mentioned how the feedstock needs to be “clean”. Free of pollutants such as wood preservative, paint and other contaminants. In machines such as the Dartmoor Dragon the fire in the main retort, the BBQ box, can use contaminated material. Things like old pallets etc., as it is not in direct contact with the material in the drum. The material in the drum can be any sort of organic material. For example Plymouth University evaluated eight different materials in their 2024 GISS project, Conifer, Pine, Ash, Oak, Rhododendron, Willow, Hazel, Agricultural leftovers (such as recycled citrus pruning waste). Other materials people use include woodchip, hedge trimmings, farmyard manure, straw, organic factory waste, etc.
Feedstock Moisture Content
Clearly using wet material is not going to be efficient as loads of heat will be used to drive off the water before the process starts. So ideally use drier materials, under 20% moisture where possible.
Also try to ensure the feedstock is of similar diameter. If you mix large diameter with very small the finer stuff starts turns to ash before the large stuff fully pyrolyses.
Double Retort Process On a Small Scale
On a garden or community scale the process still works. Fill a small tin of feedstock and add it to a BBQ or bonfire. Quench it in a bucket of water. It works.
For me I like using woodchip as I can pack in tightly in a container. it is all of similar size and it is plentiful.
Tawi Stoves For Biochar Production
The Tawi stove, similar to rocket stoves, heat water, can be cooked on AND produce biochar. It isn’t the most efficient way yo produce biochar but it works .. and a you get to heat water to cook food.
Very Efficient Biochar Production
All the above biochar production methods vary in efficiency. The very efficient ones harvest the “waste” heat as an energy source just as raw heart or to make electricity. It is also possible to harvest wood vinegar from the steam given off during the process.
