In search of faster, simpler, and cleaner ways to make biochar in your back yard.
In trying to come up with better ways to make biochar at home I have been doing quite a bit of reading on “gasification” stoves. There are numerous youtube videos and articles on DIY Gasification camp stoves. A gasification stove differs from a normal combustion stove in that it is designed to generate heat at least in part by circulating the wood gas that is released from the fuel, combining the gas with oxygen and injecting it back into the combustion chamber. The result is supposed to be a cleaner more efficient burn with less harmful gasses and particulates exhausted into the environment. In addition, by burning the wood gasses from the fuel and not the wood itself I was intrigued by the possibility of using this kind of stove as a means to make biochar.
Looking at the Youtube videos one thing became readily apparent: Most stoves that are advertised as “gasification stoves” are not really gasification stoves at all. That is, most of them have large primary air holes at the bottom of the burn chamber and small secondary holes at the top. In diagram these stoves look something like this:
While the diagram looks valid, when I built one of these stoves I was disappointed in the results. The stove produced few flames from the top holes, burned barely hot enough to boil a cup of water, and produced quite a bit of smoke. Worst of all, when the burn was finished, most of what remained was ash.
I was pretty sure the problem lay in the design of the stove. Placing the air intake below the primary holes I felt would virtually ensure that the majority of air entering the stove would get sucked into the lower holes and go towards standard combustion. And little if any wood gas Downdraft would be created.
In order to get true gasification – that is, to create a true down draft that would suck wood gas out the bottom holes to combine with fresh air to combust at the top of the burn chamber – it seemed logical that the stove would have to be redesigned such that the air intake was located above the lower holes. By this manner air entering the air intake would be heated from the fire within, travel upward creating suction that would pull wood gas from the lower holes.
To further encourage the upward flow of gas and air I felt that the lower holes should be smallest, the air intake holes slightly larger and the upper holes should be larger still. The concept was to create a chimney effect that would create an upward draft and suction to pull the wood gas through the bottom holes.
In Diagram it would look like this:
I decided to build a number of stoves to test out my theory. The results were quite encouraging. The new paint can stoves burned longer and hotter with less smoke and much less ash produced. Furthermore there were visible flames jetting from the top holes.
The last tweak to this design idea I wanted to try out was to make the outer can into an inverted cone shape. In this manner I hoped to create a Venturi effect to aid the suction of wood gas through the bottom holes. I decided to add an extra can around the whole thing to further insulate the burn chamber and make the resulting combustion that much more complete.
In Diagram the new design looked like this:
At my local junkyard I found two cans that were almost perfect dimension for this experiment. I added a length of stovepipe for the outer can and put the whole thing together.
Worried I may have reduced the holes too much and that the stove would not draw well, at the last minute I decided to add a vent stack to the whole thing. I needn’t have worried. When I lit the stove, the draw was so strong the gasification flame shot up like a torch.
Best of all, the burn produced a substantial amount of char and very little ash!
While this design may not be practical as a camp stove, it shows great promise as a biochar stove. In the coming weeks I will be experimenting at upscaling this design to produce larger amounts of biochar. I will try different hole sizes and size and shape of containers. I will report my findings here.