How to Make Charcoal
Robert Flanagan, SAFFE, January 30, 2008
I've just been playing around with my natural draft stove to see how easy it would be to use it for cooking and making charcoal.
I fed some extra fuel in the side to show the pyrolysis reaction taking place.
So at the start I light a fire on top of the biomass to build this hot carbon layer, it's during this start up period that we get smoke coming out the flue. But when the whole top layer is on fire (So to speak) and we've got our hot carbon layer then all gasses coming up must pass through this layer and they get broken down and become combustable. The biggest problem is always moisture (biomass should have a moisture content of less then 25%) and typically we get a white smoke but when water passes through hot carbon it reacts to give us H2O+C = H2+CO (Syngas) so we actually burn water along with the woodgas. Gasification allows a small amount of fresh air to pass through the unburned biomass up to the flaming pryolysis zone and this is where get the exothermic reaction to take place. Then the oxygen lean gas travels up until it meets the secondary air where it finally has the air:fuel ratio correct for complete combustion.
The reason I was left with just charcoal at the end of the clip is because of the restricted amount of air passing up through the charcoal. When we have a mix of hot charcoal and unburned biomass the air will always react with the unburned wood first so there is no oxygen left to break down the carbon. Now at the end I could have opened the bottom and side door and the flow of fresh air meeting the hot charcoal would have raised the temperature and reduced the charcoal to ash (But then I'd have no charcoal to play with!)
This is very very different to pyrolysis where the biomass is typically high in moisture (50%) and the oxygen is restricted. Here the moisture content in the gas is way to high to combust so unless the biomass was pre-dried the gas typically has to go through a condenser to produce vinegar and bio-oil, then the non-condensables should be combustible.
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