Rocket stoves have been shown to be much cleaner when burning at medium power (Jetter, et al 2012, Agenbroad, 2010). For this reason, a fence located at the back of the combustion chamber in the new DOE funded Sunken Pot Rocket stove allows only 8cm of the tips of the sticks to burn. The outer portion of the stick remains too cold to make more wood gas. The stove is insulated with spirals of stainless and aluminum foil to increase the R-value and to decrease lost energy into the stove body. The riser tube above the combustion chamber is 10cm in diameter and the depth of the fuel magazine is shortened to 11cm.
The hot gases flow upwards past the sides of the pot in a 6mm channel gap (Seen below) and then flow down a larger space to exit out of the chimney which is located below the bottom of the pot. The pot fits tightly in a hole cut into the stove top so no smoke escapes. Allowing only 8cm of four 1cm x 2cm sticks of wood to burn creates a firepower of about 2.7kW. This amount of flue gas can flow unimpeded through the 6mm channel gap resulting in excellent heat transfer efficiency.
The pot sinks into the stove and is surrounded by a 6mm wide channel through which the hot exhaust gasses must pass.
When a well-insulated Rocket combustion chamber (with fence) is combined with an effective heat transfer mechanism the emissions made per liter of food cooked can be substantially reduced. At close to 50% thermal efficiency, less wood is burned to boil and simmer water and cook food. However, would medium power Rocket stoves be acceptable to cooks? Perhaps when the thermal efficiency is close to optimal the stove might cook food fast enough even though the firepower is relatively low?
Stay tuned next time for: The ‘tier 4’ refractory ceramic side feed forced draft rocket stove!
Aprovecho Research Center
PO Box 1175
Cottage Grove, OR 97424, USA