Further biochar musings

My recent experiment with biochar is working really well as a mulch for the feijoa trees at the farm.

I have been thinking of ways to refine the process to produce maximum charcoal and little or no ash. I should have stayed up all night to watch the previous burn but I didn’t and so lost a fair amount of the charcoal in the process. I’m now curious to see if it’s possible for me to produce charcoal with no ash at all.

It occurred to me that pyrolysis only needs a source of heat to start the process and that, while lighting the fuel is the traditional way of producing that heat, what if I tried “baking” the fuel rather than “grilling” it? If I could make a large-scale pyrolytic oven*, and use solar cooking principles to run it, that would be a much more conservative approach to making biochar from an energy perspective.

The most time consuming part of setting up a biochar production run is definitely stacking the fuel, in my limited experience. It is important to stack your fuel as neatly as possible to exclude air from the fire, particularly if you’re using sticks or logs as fuel. In retrospect, I don’t recommend these large fuel sources as the lumps of charcoal take longer to completely carbonise. I know that my initial burn experiment would have proceeded much more quickly if all of the fuel had been wood chips as they are smaller particles and so heated more quickly than the old tree stakes that I mainly used. However, this is definitely a variable that can be tweaked for future production runs to see how they turn out.

But, back to my solar-powered pyrolytic oven.

Very near our house in Melbourne is an awesome community resource called CERES Community Environment Park. In their energy park they demonstrate different ways to make and use energy. One of the very cool toys they have is a Scheffler Reflector. Ta-DA! Sun energy at extremely high temperatures achieved.

According to Adrian, one of the CERES educators, their reflector can produce temperatures of between 400-500 degrees Celsius at the point of focus. I learnt this little snippet at a site tour and green building seminar I attended there back in September. It was organised by Earthship Australia and was very well attended and received. Sometime in the future I will write a post about my plans for experimenting with Earthship building techniques.

So many exciting and sidetracking thoughts! Back to biochar!

Since the half-barrel of my first burn worked so well, I think containing the next production “bake” in a similar vessel will be ideal. I plan to get hold of a whole barrel with lid and ring seal and paint the outside black with heat resistant paint. It will need a vent at the top to release smoke and steam and possible some vents for air intake at the bottom, although that might be trial number three.

Whether I use sticks as fuel again or not, due to the indirect heat source, I think filling the air pockets in the barrel with wood chips again will help to spread the temperature more evenly and create a better baking environment. I’ll keep you posted how it works out and whether or not I get permission to try it out at CERES!

*On a side note, because pyrolytic ovens turn your crusted on bits into ash, they’re not technically pyrolytic, just REALLY hot. Ash is a by-product of combustion, which requires oxygen. Charcoal is a by-product of pyrolysis, which aims to exclude as much oxygen from the burning process as possible.

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