Today I got my medieval on and had a go at making biochar. This is a skill still practiced by charcoal burners in Europe, although it’s been a dying art for a while now. It’s rediscovery as a potential solution to climate change has caused a resurgence in interest and production of this ancient material as it helps sequester carbon when the charcoal is added to soil.
Now, this sequestration of carbon from a burnt product may sound counter-intuitive as we all know that burning fossil fuels is how we got into our current mess, but stick with me here. Yes, combustion releases carbon dioxide (CO2) gas, one of the major contributing factors in climate change. The combustion process looks like this:
Fuel + O2 (oxygen) +heat => CO2 + H2O (water)
But when I say burning to make charcoal, I’m actually talking not about combustion, but about pyrolysis; a slightly different burning process. The pyrolysis process is different because it aims to exclude oxygen from the burn, thus producing charcoal and the formula looks like this:
Fuel + heat => CO2 + H2O + Carbon (charcoal)
You will note that CO2 is still produced in this process, but because of the concurrent production of the charcoal, which is pure carbon, the amount of gas produced is much less than when fuel is burnt during combustion.
I should probably clarify at this point that when I say fuel, I’m not talking petrol, diesel or gas, I’m talking about solid fuel such as wood or wood chips (but not treated or painted woods), grass, nutshells or whatever else that you have lying around that will burn without chemical contamination of the charcoal and atmosphere.
So, as I said, pyrolysis is the simple secret to making biochar. Achieving pyrolysis is relatively easy, you just need to limit the amount of oxygen reaching your fuel. Traditionally this was done by slathering the bonfire in mud prior to lighting it and using moveable wattle (woven willow, hazel or hawthorn) screens as windbreaks. Being in Australia, there was a scarcity of willow, hawthorn or hazel trees in my immediate vicinity, so I positioned my burn in the lee of our woodshed to help minimise the airflow from the breeze*. I used a 44 gallon drum that had been cut in half lengthways to contain the burn. This is our usual outdoor firepit and has a number of rust holes in the base which I figured would allow enough oxygen to keep the fire burning but not so much that I got a complete combustion reaction happening. By keeping the fuel load below the edges of the barrel, I achieved some further protection of the fire from the breeze.
Due to the length of the barrel, I set three small starter fires, then started loading the bulk of the fuel. I used some old hardwood tree stakes and stacked them in a criss-cross pattern in three layers running along, across and along the barrel. It is important to have your fuel closely stacked to help exclude oxygen from the fire as it burns. I lit the starter fires before laying the final layer of fuel. At that stage the barrel was around one third of it’s depth full of fuel.
Only one of the end starter fires really took hold, so I had to stagger the next step. In order to exclude most of the oxygen from the burn, I didn’t have any handy mud, so I shovelled some old woodchips onto the fire, gradually covering the whole fuel load as more of it caught alight. You can see from the video below what it looked like at this stage.
The easiest way to know your fire is suitably pyrolytic is to make sure that you can’t hear the fire “crackling”, but that you can see some smoke escaping the mulch layer. If you can hear noise, there’s too much oxygen getting to the fire and you should add more woodchips, particularly if you can see flames above the mulch.
I should also mention that as it’s Spring here in Australia, the woodchips (except the very top layer of the pile I collected them from) were quite damp and a bit decomposed. I think this helped to exclude oxygen very efficiently as the small particles could easily sift between the larger fuel sticks and the heat would dry them prior to charring them so I didn’t need to add them as frequently.
The only problem I’ve had is that I started the fire at midday and my recent musings on the history of charcoal burning leads me to recall that a burn often lasted a number of days, during which the fire had to be attended constantly . . . At least I’ll have warmth when the sun goes down!
*The moment I lit the fire, the wind direction changed so that it was no longer the sheltered spot that I had desired, but my fire design (as discussed in the next paragraph) helped to minimise this issue of air flow somewhat anyway.