Finally after long time setting things up, I was able to invest some time to make a biochar stove here in Pai, Thailand with zero material cost of course, reusing and recycling as usual... This is another small Elsa stove prototype: it works nicely producing perfect biochar in about 20 minutes process with this size, probably enough to get your carbon-negative coffee going.
Instructions and details follow in this post, in the perspective of more extensive demonstrations and workshops here in Thailand.