|Boiling Point No. 06 - April 1984 (ITDG, 1984, 20 p.)|
E G K Rao, India
When wood is converted into charcoal, a high grade fuel of greatly improved burning characteristics and calorific content is produced. It is non-smokey, non hygroscopic and is not attacked by insects or fungus. 90% of it comprises of pyrogenic substances.
Traditional charcoal production is an art and calls for a great deal of skill. The pit method of charcoal production results in about 15% to 20% of the charge by weight being converted. Improved methods can raise the yield to about 30%. The mini-charcoal kiln described here (which is a modification of the one used in the traditional Phillipino process, to make coconut shell charcoal) yields over 30% high grade charcoal from an 80 kg charge of firewood.
The mini kiln can be constructed from a surplus oil drum, which must be in good condition without any pin holes. Any drum with an opened seam or tiny perforation should be rejected. One of the ends is removed and 4 holes (about 3 cm diameter) drilled at the remaining (bottom) end. Twelve 13 mm diameter holes are drilled at the sides of the drum at 3 levels: these play an important role in the operation of the kiln. A loose fitting cover with a stub chimney of about 10 cm diameter and 25 cm length is attached to the top plate using a reinforcing flange of 6 mm plate. Two handles are attached to the top cover. Twelve closure discs with handling rings are needed for the 12 side holes.
The drum is placed over a pit 25 cm diameter and 10 cm deep with four rocks or blocks to support the drum level. A 2" length of 2" diameter pipe serves as an air entry to the pit end thence to the drum. After the drum is levelled over the pit with the air pipe in position, the base is sealed using heaped-up soil es shown in Figure 1. Air entry should be only through the pipe.
While charging the drum, some amount of kindling is placed centrally at the bottom over which a 3" diameter bamboo pole about 4' long is temporarily supported, vertically. Pieces of wood are packed around this central pole until the drum is completely filled. After this the central bamboo pole is withdrawn leaving a hollow shaft leading to the kindling material at the floor of the drum.
A kerosene soaked rag is dropped on to the kindling to light the charge after ensuring that all the side holes, except the four at the bottom level, are covered with the cover discs. When the wood starts burning vigorously, the cover is placed on the top and the process of charring commences, from the bottom upwards. The side holes help in monitoring the process, during which heavy dense smoke emerges from the top which clears up when charring of the charge is complete.
The four open side holes are watched and when a bright red glow appears at all four holes the charring of that section is judged complete. The four bottom holes are then closed and the four side holes above it are opened. When the bright red glow appears at these holes they are closed and the holes of the top level are opened. When the top-most set of holes is closed, the smoke emanating from the chimney is watched. When this is clear with only a suggestion of bluish smoke, the entire charge has been converted to charcoal. Now the charge has to be sealed to exclude air and allowed to cool till the temperature has dropped to well below 80ºC. The sides of the drum then would feel very hot to touch, but not scorchingly hot. Sealing of the charge after charring is accomplished by withdrawing the air supply pipe from the pit and firming up the soil seal at the base. A tile or metallic cover is placed over the chimney to block the entry of air from the top.
After the charge has cooled tie change may be removed by removing the cover from the top and tipping the barrel on its side. There may be one or two pieces of wood unconverted at-the bottom Out-the rest of the charge would be rich dark black in colour and yielding good quality charcoal.
The yield for this type of kiln can be up to 32% by weight if a dense, dry wood is used. One person could operate a batch of about 10 oil barrel kilos, producing up to 250 kg of high quality charcoal per day. The major drawback of this type of kiln will be its short lifetime, but where there are cheap oil barrels and a good market for high quality charcoal, it could be a profitable small business.