This page looks at a series of experiments with an old oil fired boiler, which has been converted to make a wood gas generator and charcoal stove.
From the web site:
Some time ago, I managed to find an old oil-fired boiler which was being
scrapped and thought that the boiler would make a convenient test bed for
some wood gasification and charcoal burning experiments.
Various fuels will be tried, including sawmill waste, shredded tree waste, wood
chips, peat and general garden compost.
The intention is to build a multifuel stove with reasonably constant heat output that can be used for domestic heating purposes and for providing a heat source for a Stirling engine driven generator.
The oil fired boiler is basically an 18" diameter pipe made from mild steel. Around the outside is an annular water jacket with a 1.5" gap. The height of the combustion chamber is 26" and the whole unit stands on a mount so that there is about 12" between the chamber and the ground.
A 6" diameter porthole allows access from the front and this is fitted with a 2" diameter glass window so that combustion may be observed. It also gives a convenient access panel for lighting the fuel with the propane torch.
The inside of the boiler was first lined with refractory bricks salvaged from some electric storage heaters. These are very high density and act as a heat store. Once lined, the combustion space was reduced to a 9" diameter pentagonal chamber.
A 9" stainless steel collander bought for a couple of pounds in a cheap kitchen shop, makes a good hearth to burn the charcoal.
A vacuum cleaner motor fitted with a speed controller makes an ideal blower fan. Running on ac mains it generates a very powerful blast, but on 12V dc it gives a gentle fan breeze.
For more, see: http://www.powercubes.com/charcoal.html