Rocket Stoveshave been used for generations but are constantly being improved – even a teenager in Ireland made such good improvements that he won a major Science Award Irish Young Scientist of the Year Awards.
The principle works by super-insulating the surrounds to an L-shaped pipe where small twigs and chips of wood are fed into the lower-shorter end of the pipe. These burn with the hole being semi-covered to provide good air control and the material gasifies in the longer portion. The result is a veritable rocket-type exhaust at the top of the pipe (if the insulation is right). I’ve seen 2 liters of water boil in less than 2 mins on tiny (dry) twigs and slivers of wood.
Winiarski developed the concept in
the early 1980s creating a stove that cleaned up combustion and improved heat transfer
More on Winarski
How Texans do it; U-Tube video/s
Seriously evolved ‘rocket stoves’; http://www.richsoil.com/rocket-stove-mass-heater.jsp