I’ve been looking into building one of these for a while now. Known as Rocket Stoves, they are a kind of improved biomass stove. In essence they use less fuel to burn longer, hotter and cleaner. There are many designs out there. Some like the sixteen brick rocket stove are simple to build, others like this one
are much more complicated. I like the bucket type linked to on top because it’s portable and insulated (with vermiculite).