UHDP and partners have been exploring the use of 200-liter horizontal drum charcoal kilns. According to Kittichai Sumpansinkor, the 200-liter horizontal drum kiln is efficient, easy to construct and appropriate for household charcoal production.
It is able to convert even small branches and farm residues to charcoal, and also yields wood vinegar, a by-product with significant farming applications (see accompanying story).
Below is a breakdown of materials and costs for the UHDP trial unit:
Listed materials for the 200-liter horizontal drum kilns are relatively inexpensive and widely available. ATA states that kiln materials add up to 480 baht (p. 30). According to the Thai language manual Producing Charcoal and Wood Vinegar (การผลิตถ่านและน้ำ ส้มควันไม้), the cost of one unit is 800 baht. Such kilns can reportedly be used up to three years
Constructing and Using 200-Liter Horizontal Drum Kilns
The most basic steps of constructing and using 200-liter horizontal kilns, adapted from Producing Charcoal and Wood Vinegar and UHDP’s 200-Liter Drum Kilns, are offered below. Additional details from the illustrated English language version of UHDP’s document.
- Find and prepare a suitable location for the kiln at least 50 meters from any residence. Such a spot should accessible to fuel wood and be sheltered from sun and rain. It should also be located on ground high enough to avoid flooding. Level an area large enough to accommodate a 200-liter drum kiln.
- Locate kiln materials including:
• 200-liter drum (cut 20 x 20 cm square hole in drum lid and a 4 in. diameter hole in the bottom of the drum).
• Flue pipe and 90º pipe joint.
• Hollowed out green bamboo pole (3-5 m long; 12 cm wide) with which to collect wood vinegar
- Place drum into position. Using corner posts and suitable material for retaining walls (e.g. tile, galvanized sheets), construct the walls on three sides of the kiln. Thoroughly tamp soil or sand between the retaining walls and drum, as well as over the drum. The layer of soil or sand will help insulate heat that is produced in the kiln.
- Arrangement of wood. Place at least 3 pieces of wood, about 3 cm wide and 25 cm long, crossways at the bottom of the kiln to serve as a grate. On top of the grate, arrange the smallest pieces of wood lengthways along the bottom of the kiln. Follow up by stacking larger pieces higher in the kiln until the largest pieces of wood are placed lengthways on top. When arranging pieces of wood into the kiln, place the large ends toward the front opening. Bamboo charcoal is particularly brittle, so in order to prevent shattering, Kittichai Sumpansinkor recommends cutting bamboo poles in half before arranging the shortened segments into two adjacent stacks inside the kiln
- Sealing the front of the kiln. When the drum is full of wood, seal it by placing the modified lid over the front of the drum, positioning the square opening at the bottom. Seal spaces along the edge of the lid with sticky clay or a mixture of clay and burnt rice husks. This crude seal will prevent air that would hinder carbonization (causing wood to be burned into ash) from entering the drum.
- Make an outer vent in front of the square opening in the lid by standing two cement blocks lengthways toward the opening with a space in-between as wide as the opening. Lay two more blocks crossways on top of the two bottom blocks to form the top of the vent. To make the outer vent airtight, use clay or the clay/burnt rice husk mixture to seal the space between the drum lid and the cement blocks as well as spaces between the blocks
- Light a small fire in the cement block outer vent. Gradually insert burning wood from the outer vent into the opening of the kiln. This initial small amount of heat will help drive moisture out from the wood and from within the kiln. UHDP estimates that the time needed to heat up the kiln and drive out moisture from small wood inside will be at least 1 to 2 hours (depending on the amount of moisture and size of wood). No more external heat is needed once the smoke exiting the flue is very thick and white. Such smoke indicates that wood in the kiln is sustaining necessary heat for carbonization
- Collecting wood vinegar. Approximately 30 minutes to 1 hour after you stop feeding fuel into the kiln, if the smoke is yellowish and acrid, close off most of the outer vent with the fifth cement block and seal with sticky clay. A small crack, approximately 3 cm (1.18 in.) wide, should remain open. Extend a hollow green bamboo pole (far end elevated to 45º) from the flue pipe. Wood vinegar can be collected with containers fastened underneath one to two holes, approximately 2 cm (3/4 in.) wide, drilled into the bamboo pole roughly 30 cm from the connection with the flue pipe
- When the smoke exiting the flue is bluish in color, reopen the outer vent about 50 percent for 5 to 10 minutes to flush out heat with fresh air. For charcoal being produced from larger wood, you can open the vent for 20 to 30 minutes. At this point, biomass inside the kiln will have been converted into pure charcoal. However, allowing the vent to remain open longer than necessary can cause the charcoal to become reduced to ashes—so following the purge of hot air inside the kiln, close the outer vent completely with the fifth block and seal with wet, sticky clay. Additionally, stop up the end of the flue with clay. This is the beginning of the cool down period.
- After sealing the flue and the outer vent, shift the soil or sand from on top of the kiln over to the retaining wall to enable heat to dissipate from the kiln. Allow the kiln and contents to cool 12 to 15 hours. Afterward, the finished charcoal can be removed from the kiln for packaging and/or storage.
Time Requirements for Charcoal Production in Drum Kilns
200-liter drum kilns should not be used to carbonize particularly large pieces of wood (more than several inches wide) or wood that is not yet dry, as too much time will be required for the process. Such excessive use will shorten the useable life of the kiln.
According to Wiraphong, using wood that is not yet dry will not only increase the time needed to complete carbonization but will also require a considerable amount of fuel wood to be fed through the outer vent during the pre-carbonization stages. Prior to converting fresh cut wood into charcoal, at least two weeks may be required for adequate drying (p. 38).
According to Kittichai Sumpansinkor, the amount of time needed to carbonize wood also depends upon its type and size. For instance, during initial trials at UHDP, rain tree branches with diameters ranging 4.5 to 5.5 cm (1.8 to 2.2 in.) required an average carbonizing time of 9.4 hours. However, teak branches, 1 to 1.5 cm (0.4 to 0.6 in.) in diameter required only 1.0 to 1.3 hours for charcoal conversion. Somewhat larger teak branches, 1.7 to 2.2 cm in diameter (0.7 to 0.9 in.), required an average of 3.6 hours. Bamboo lengths, 1 to 3.8 cm (0.4 to 1.5 in.) in diameter, required an average of 8.1 hours to carbonize
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Yields and Burn Times from Charcoal Produced from Small Branches in 200-Liter Drum Kilns
Stassen reports that traditional earth kilns generally produce 1 kg (2.2 lb.) of charcoal per 8 to 12 kg (17.6 to 26.4 lb.) or more of wood; only an 8 to 12.5 percent yield. Improved traditional methods, such as earth kilns equipped with chimneys made from metal drums, or small-scale steel and brick kilns, have boosted production to 1 kg of charcoal from 4 to 5 kg (8.8 to 11 lb.) of air-dried wood (for a 20 to 25 percent yield). However, production of 1 kg of charcoal from 6 to 8 kg of wood (13.2 to 17.6 lb.), yielding 12.4 to 16.6 percent, is more common for improved kilns
According to ATA, 200-liter drum kilns produce approximately 12 to 18 kg (26.4 to 39.6 lbs.) of charcoal for each 60 to 80 kg (132 to 176 lbs.) of wood that can be inserted inside a drum kiln per batch. This is a 20 to 22 percent yield of charcoal by weight. Wiraphong reports average yields of 15 to 20 percent using the same method of production.
Click here to view FULL detailed infos in PDF version Charcoal_Production_in_200-Liter_Horizontal_Drum_Kilns
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Here are video clip of a look-like above charcoal kiln also from Thailand :