Hi all,
I'm new to the forum, but have been reading for quite a while researching permaculture. I've been looking into building a small batch biochar retort for my suburban property. I've just read this thread and I wanted to chime in. It is the case that char is left in the ash of incompletely burned wood where, under the ash, it was heated in an oxygen poor environment. This is not the ideal way to produce biochar, but it will work. Break this up and it will benefit your soil (more so when inoculated), the smaller the better, to a point. What you're adding to your is a refuge for microorganisms, so small pieces (small to us) are still large in the micro-world and the smaller you can get it, the more uniformly it can be distributed in the soil.
While the traditional model of charcoal production consists of relatively low temperature heating over a period of several days, research in the US is showing that this is not the ideal way to produce biochar. Ideally biochar is produced in the complete absence of oxygen and at temperatures at or above 450C. This drives off ALL of the volatile compounds and leaves only extremely porous black carbon (the pores store the water, microbes, nutrients, etc...). The more organic compounds left over after pyrolysis, the more likely the char will be to actually repel water. When you crumble it in your hands, the black dust should wash off completely with water alone and should not leave any oily residue. The gasses that come out during pyrolysis should be burned to do some kind of work (heating, cooking, electrical generation, etc...) and not allowed back into the atmosphere. As I said, this is the ideal; I would still put your wood stove char in my garden.
Cheers,
Kevin
