I’ve got this idea running around in my mind about building a very small, but total (100%) efficient wood stove. The idea would be to use a small firebox like a 10 gal drum, then use 4″ or even smaller pipe for the draft, and just run the pipe all over a wall (this is in a brick garage, so no fire danger) zig-zag back and forth until there was just zero heat left, then the blow it outside with a small fan. Anyone ever try anything like this? I wonder just how small a pipe you could get away with?

In the UK, gas fired domestic boilers (furnaces) often use a balanced flue consisting of a 2″ diameter aluminium pipe running inside a 4″ diameter plastic pipe. The hot exhaust runs through the smaller pipe whilst the fresh inlet air is pre-warmed by running through the annular gap between the pipes. The boiler has a fan to assist the inlet air and runs the combustion chamber at marginally above atmospheric pressure. The problem with woodstoves is the build up of tars – and I have seen a 6″ flue pipe completely blocked by brown gunge as a result of burning green-wood after only 6 months of burning. Small zig-zagging pipes would IMHO just give trouble on raw wood. Possibly think in terms of the masonry stove where a small amount of wood is burnt quickly and yields its heat content to the masonry which then releases it slowly. Personally, I would be in favour of gasifying the wood first and then burning the woodgas in a clean burning stove – as a 2 stage process.

It seems to me, has the right idea; this is essentially a two-stage process. It may be a mistake to talk about burning wood efficiently without first converting it to wood gas, because in doing so in a one step process of the great difficulty of avoiding the coking problem. From the BEF site it appears that a lot of work has been done on gasifying. A number of reports are listed at the BEF site, claiming how to build small wood gas generators. So what publication(s) would you get that shows how to build one, that is if someone has already done the research and development and reduced it to practice? One of these publications, “Construction of a Simplified Wood Gas Generator,” sounds interesting, if the title is really definitive. Having a good wood gas generator the problem of constant feeding of fuel may not be that difficult if the wood is pelletized first, since pellet feeding has apparently been reduced to state of the art. And, correct me if I am wrong, wood gas can be used to fuel most any engine, as was routinely done during WW2. Or, the wood gas can be used in stoves or furnaces, perhaps even to power a gas refrigerator. Should the problem be any more complicated than this, or am I oversimplifying or missing something? For one thing I don’t know how difficult or costly it would be to provide a pelletizer, but if pellets are not the answer, how about chips?

I thought about the difficulties of fuel handling with a wood gasifier a few months back and decided that it would be best to work using a pseudo-batch process. By this I mean having a large hopper of the raw fuel-stuff available and then convert it into wood-gas, charcoal and heat in manageable batches. Charcoal remains at the end of the wood gasification process- and this in its own right can be further reduced using steam to release CO and H2. The energy left in the charcoal could be used directly to boost the mechanical handling of the next batch of fuel. For example, if you have to run a wood chipper for 2 minutes on a 5hp engine to process the next 100 lbs of wood into a form ready to gasify, then you use the energy content of the charcoal by-product to boost the output to the engine. I have thought about fully automatic, rotary turbine systems to not only provide the griunt to process the fuel but turn spare torque into power using a high-speed alternator.