Over the last few months I have been working on a burner for the clean burning of woodchips – so that they may be used as a heat source for a small Stirling engine generator rated at about 500W. My first burner was built using a 2lb coffee tin, fitted with an air nozzle powered from a PC cooling fan. The burner is 3/4 filled with woodchips and lit with some newspaper. The fan soon starts a small fiery cyclone within the confines of the tin, and all the heated air emerges through a 1.5″ diameter hole cut in the lid of the tin.

Recent modifications have included an outer skin, made from an old paint can, which retains the otherwise radiated heat from the sides of the tin, and pre-heats the combustion air. The flame emerging from the aperture is an intense yellow orange flame – similar in nature to that of a propane torch. I have used the flame to heat a Stirling engine, and also to provide heat to initiate a charcoal and woodchip gasifier reaction. I will shortly post some of the pictures to the waste watts members files area.

The woodchips were not very dry – they had been stored under cover, but no attempt was made to artificially dry them to reduce the water content. I wanted to prove that the coffe-can burner could cope with very rough & ready fuel stuffs. The volatile woodgas compounds (creosotes, tars etc) form a flammable gaseous mixture, which characteristically burns with a rich orange flame. Once the volatiles have been driven off, and charcoal remains, then the carbon monoxide emitted from the hot charcoal will burn with a wispy blue flame.

I have tried analyzing the flue gases with an flue analyzer, and see that the CO content in the exhaust varies considerably – up to concentrations greater than 5000 ppm. Anyone contemplating experimenting with burning hot charcoal must be aware of the extreme dangers of carbon monoxide poisoning, and not conduct these sort of tests in an enclosed space such as a garage. This is one step further on from barbeque science, but most barbeques burn with adequate oxygen and do not produce excessive CO.

You’re doing something wrong if you’re getting that much CO in the exhaust. Is this a duplicate of the Reed/Larson IDD stove? The point is to gasify the biomass and then burn *ALL* the CO and other volatiles. Done properly, the only exhaust should be H2O and CO2. Plus NOx, of course. Don’t know if you’re on the stoves list or not — the major emphasis there is designing these small stoves to burn biomass inside peoples homes usually with no chimney whatsoever, since that’s what people in the Third World are doing anyway.

I have chosen to experiment with forced draught systems because I find them easier to control. There are two distinct stages to the burning of a batch of biomass. The first stage is gasification and driving off the volatiles, which produces the characteristic orange flame. With secondary air supplied to the burner, this gas combustion can be made to occur in the burner region – so effectively you have a biomass powered bunser burner.

The second stage is the reduction of the remaining charcoal into CO, with the CO being carried away from the charcoal into the burner zone. Too much oxygen at this stage will cause the char to burn intensely in the fuel container and will probably lead to premature burnout of the tin container. The key to capitalising in on both stages of the burn, is being able to reliably control the air so as to gasify the biomass and then gasify the remaining charcoal.

The coffee tin burner was an easy way to investigate the processes without resorting to elaborate equipment. Someone wrote to me about a similar system he had when a teenager for a garden waste burner – a garbage can sized burner using an old vacuum cleaner (on blow) to force the draught.