I’m still looking for an easy clean way to make large quantities of biochar for my garden. Ideally I’d like to turn the woody brush that accumulates throughout the year, and that’s too slow to compost, into char for the garden. This will probably take the form of a couple of barrels arranged into a char maker that I burn outside, but that project hasn’t happened yet. Garden time is coming and I would really like to do another char experiment, so necessity is the mother of invention.
My wood stove char maker is the lowest of low tech and it doesn’t make a lot of char, but what I get is made cleanly, and I get heat from combustion of the off-gases. All I do is pack a large tin can with small pieces of wood. I then invert the can into the ashes in the bottom of the wood stove off to one side. I usually do this before I stoke the last fire for the evening. The fire eventually heats the contents of the can sufficiently that it starts driving off the light fraction of the fuel as smoke which is expelled through the ashes out the bottom of the can. These gases burn brightly and cleanly in the stove. After a while the flames stop once all that remains in the can is the less volatile char. Without air, the char in the upside down can remains intact until the next morning when the rest of the stove’s contents have burned up and cooled down.
There are surely some more clever implementations of this basic system that would work better. All you need is a quasi-air-tight canister to hold the fuel to be charred that can be placed in the wood stove fire – but the can works pretty well!
There is always the question of how best to use biochar. I look at biochar as part of long-term soil-building. The scientist in me wants to do controlled experiments, but my gardening experience tempers that enthusiasm when I realize how hard it is to get any two plants to grow similarly in my garden even under theoretically identical conditions. Good experiments require a large number of plants, replicated trials, randomized plots, and other such tricks to separate real effects from natural variability. I don’t have the space for that. Rather, I’m tempted to just make one or two beds that get char and otherwise are treated similarly to the rest of the garden. Then just watch and see how they compare over the years. I know I need to keep adding compost and manure all the time and that I’m always fighting with the natural heavy ground to avoid compaction and maintain a friable well-drained soil. I wonder if char will change any of that. So, I make a can of char at a time until I get a couple of wheelbarrow loads to add to my test beds … and we will see.