There are two parts to fire. The first part is called pyrolysis. This is the decomposition of condensed fuel into volatile gasses by heat. It is a chemical reaction. When burning biomass, these gasses are simply the smoke that is given off. If this smoke is fed enough oxygen while it is still hot, it combusts with a clear flame. The smoke is actually the fuel for the biomass stove. The stove burner unit itself is called a MIDGE stove. This stands for Modified Inverted Downdraft Gasifier Experiment. A gasifier is a unit that separates the two stages of combustion and burns the smoke separately. Google +"midge stove" or +"gasifier" and you will find a wealth of information about these stoves. The basic principles have been known for centuries.
There is a lot of argument on the forums about whether these MIDGE stoves are "truly" gasifier units. It does not matter a bit whether they are called gasifiers or not. The fact is that they burn biomass in an extremely clean manner, compared to other stove technologies. The burners here at the center are made from used tin cans. They have been in use for almost two years. They are used intermittently now, but were the only source of cooking heat here for over a year. They are still in great shape and safe to use. I modified the design to burn cleaner and to fit the application I had in mind. In the renewable energy house, they will be the only source of cooking heat outside of a solar cooker. Here is my page with full instructions to build one type, the type I use. There are many variations possible. I change mine as I develop better ones.
The critical elements are good air flow and the spacing between the burner can and the internal cowling. A lot of experimenting can be done in those two areas.
They burn wood chips, wood pellets, twigs, dried lama dung, dried rabbit dung, etc. Most small biomass, if fairly dry, will burn in them. Wood pellets burn the longest and cleanest, since they are compressed sawdust. Wood pellets are both recycled waste and renewable, since they come from the waste of milling trees.
The way these burners are used is in a conventional electric stove. I have a stainless steel counter cooktop. I pulled all the electrical stuff out of it, the burners, controls, etc. It was then just an empty shell. I cut up a grey metal shelf unit and reworked it to make the cooktop pan deeper. I made it just deep enough that the top of the burners were an inch below where the electric burners used to be. Then I removed the center supports from the electric burners and they fit nicely to hold pans for cooking over the wood burners. So in essence, I converted the electric stove to a wood stove! This first picture shoes the cooktop with three burners in it. As mentioned on the instructions page, two of the burners are rusty due to weather exposure. The third one is clean. It is the one shown being manufactured on the instructions page, so it has seen little use. This new one has very good air proportions and burns very cleanly. The second picture shows the new burner nestled in the old electric burner hole.
The maple trees in the front are constantly shedding twigs and I dry these twigs and use them for fuel. It works very well. The only sizeable drawback to the burners is that they leave the pans somewhat sooty. It took me awhile to figure out how to keep them clean. It is still a pain, but it is free! And renewable! Sometimes, I have to supplement with wood pellets because of the wet winters here. It takes the twigs some time to dry out and occasionally I run out. But this summer, I intend to harvest a winters supply of twigs and have them ready to go.
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