Charcoal production
Practical Action
The success of the carbonisation process is the efficiency of a kiln, defined as the mass of charcoal
obtained expressed as a percentage of the mass of wood initially put into the kiln:
Ek = MC/MW
Where Ek = kiln efficiency
MC = mass of charcoal produced
MW = mass of wood put into the kiln
Strictly speaking, this is the recovery efficiency whereas the conversion efficiency includes the
charcoal fines (rejects) that may not be packaged for sale due to their small size. Both efficiencies
are calculated on wet/dry air or oven dry basis. For example, if a piece of wood weighing 100kg has
20kg of free water, then the actual weight of the wood is 80kg. The moisture content is thus:
Moisture content (MC) = Mass of water
x 100%
Mass of wood (dry or wet)
Wet or dry air basis: MC = 20/100 x 100%
= 20%
Oven dry basis:
MC = 20/80 x 100%
= 25%
Thus if a kiln produces, say, 10kg of charcoal, then the kiln conversion efficiency (Ekc) at,
Wet or dry air basis: Ekc = 10/100 x 100%
= 10%
Oven dry basis:
Ekc = 10/80 x 100%
= 12.5%
Now, assuming 5% of the charcoal ends up as fines or dust that cannot be packaged, 0.5kg of
charcoal will remain in the kiln and the kiln recovery efficiency, Ekr, can be calculated as follows:
Wet or dry air basis: Ekr = (10-0.5)/100 x 100%
= 9.5/100 x 100%
= 9.5%
Oven dry basis:
Ekc = (10-0.5)/80 x 100%
= 9.5/80 x 100%
= 11.9%
Normally, kiln efficiencies are based on the simplest conditions, that is, conversion efficacies on dry
air basis, as these are the easiest to measure and calculate. Most small-scale charcoal production
relies on partial combustion of the wood charge to provide the heat necessary for carbonisation hence
yields depend heavily on the moisture content (Stassen, 2002).
Kilns
Traditional charcoal production is an acquired skill. The most critical factor in the efficient
conversion of wood to charcoal is the careful operation of the kiln. Wood must be dried and carefully
stacked to allow an even flow of air through the kiln and sufficient time for reactions to take place. If
kilns are not operated correctly, yields can be half the optimum level.
Traditional kilns
Much charcoal for domestic consumption in developing countries is produced in pit kilns (holes dug
in the ground), or in mound kilns (piles of wood stacked on the ground and covered with soil), by
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