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Chapter 8 - Metal kilns


8.1. Available designs of transportable metal kilns
8.2. Metal charcoal kiln made from oil drums
8.3. Advantages and disadvantages of transportable metal kilns
8.4. Manufacture of the TPI metal kiln
8.5. The transportation and location of kilns
8.6. Selection and preparation of site
8.7. Preparation of the raw material
8.8. Method of operating the TPI kiln
8.9. Alternative method of operation
8.10. Schedule for commercial operation
8.11. The most common operational faults
8.12. Yields of charcoal
8.13. Working life of transportable metal kilns


The widespread use of cylindrical transportable metal kilns for charcoal production originated in Europe in the 1930's. During the Second World War the technique was further developed by the United Kingdom (U.K.) Forest Products Research Laboratory. Various versions of the original design have been used throughout the United Kingdom. This technology was transferred to developing countries in the late 1960's, notably by activities in the Uganda Forestry Department. For more information see also refs. 7, 8, 9, 10, 17, 18, 25, 34.

8.1. Available designs of transportable metal kilns

The Tropical Products Institute (TPI), a scientific unit of the Overseas Development Administration, has gained considerable experience in operating transportable metal kilns of various designs both in the U.K. and in many developing countries. The Institute has evolved a kiln design which is considered to be optimal in economy of construction, robustness and durability, ease of operation and maximum efficiency and productivity for developing country situations.

The major features of the TPI designed kiln are:

- 3 mm thick sheet steel is used for the fabrication of the bottom section of the kiln; 2 m thick sheet steel is used for the top section and cover.

- The two main sections of the kiln are cylindrical.

- 50 mm angle iron shelves are used to support the top section and cover. These are welded to the inside of the uppermost rims of the two main cylindrical sections.

- The eight inlet/outlet channels positioned under the bottom section of the kiln are open based. A collar is provided around the hole in the top face of each channel to support the chimney during the kiln's operation.

- Four equally spaced steam release ports are provided in the cover of the kiln.

Several deviations from the above features are to be found in other versions of transportable metal kilns. Some kiln manufacturers use thinner gauge metal sheets than that recommended in the TPI design. To ensure the maximum working life of a kiln, especially when operated carelessly, the thickness and type of metal used in the kiln's construction is of prime importance. The lower section of the kiln is subjected to the greatest heat stresses and should be fabricated from 3 mm thick sheet steel. For the upper section and cover a thickness of 2 mm will suffice. The use of Corten 'A' steel in the kiln's construction instead of mild steel will further increase its durability. This special alloy has heat and rust resisting properties. Its basic cost is about 10% higher than that of mild steel, but may be greater where special arrangements for obtaining it are necessary.

A kiln widely used in the forests of the U.K. has a tapering upper section so that the diameter of the cover is considerably smaller than that of the lower section. This reduces the weight of the cover, makes the kiln easier to assembly and adds to the overall rigidity of the upper section without affecting the performance of the kiln. It is not used in the TPI design, however, as a tapered section would be more difficult to construct in small engineering workshops found in developing countries and reduces its mobility on the forest floor.

The details of the seals between sections are critical. One design of metal kiln employs channels instead of shelves as the supports for the upper section and the cover. These channels are welded to the inside of the uppermost rim of both the lower and upper sections; they are filled with sand during assembly to form a seal when the upper section and cover are located into them. However, the channels collect the wood tar which condenses on the kiln walls and the high temperatures attained during the latter stages of carbonization bake the wood tar/sand mixture into a hard concrete, requiring many hours of hard effort to clean the channels after each operation. In addition, the sections often fuse together after cooling and the joints are damaged when metal levers are used to prise the sections apart. A further serious design fault is the use of an overshot cover where a seal is effected by a vertical metal skirt welded to the underside of the cover. Should any leaks at the joint between the cover and the top section be observed after the kiln has been lit, it is extremely difficult to seal these as the overshot cover prevents the operator from loading extra soil or sand into the joint. These problems do not arise with the TPI design as the angle iron shelves used to support the top section and cover do not trap the tar. Also the location of the cover inside the rim of the upper section permits the operator to add extra sand or soil to the joint for sealing as and when required.

Some metal kilns employ square section pipes as inlet/outlet channels. In practice, it has been found more convenient to use the open based channels of the TPI design. This is because the wood tar which condenses in the chimneys during carbonization flows down into the channels where it is baked to a hard pitch by the heat. Where the channels have an open base, most of the tar is absorbed by the soil and any that remains can be easily removed because of free access to the inner surfaces. Also, after prolonged use, the ends of the inlet/outlet channels inserted into the kiln become distorted with intense localised heat generated in that area by the air meeting the charge. Where a square section tube channel is used, the top and bottom face distort inwardly and this seriously restricts the flow of air and exhaust gases through the channel. To restore the original shape is difficult because the inner surfaces of the tube are inaccessible. On the other hand, the distortion of an open based channel can be easily corrected by inverting it and reshaping it with a hammer.

In some designs the chimneys are inserted into the channel through a hole cut in the top surface. The base of each chimney has a small section cut out to allow the smoke to enter from the channel. This practice may restrict the exit of the gases by accident rotation of the chimney and the channels cannot be cleaned easily during the operation of the kiln.

All transportable metal kilns incorporate some reinforcement to protect the kiln during handling. Excessive reinforcement can, however, create problems as even the most robust kiln can be damaged if unloaded carelessly from a truck. When a highly reinforced kiln is damaged considerable effort is needed to return it to its original shape. This can sometimes be achieved using a lifting jack between two lengths of timber across the diameter of the damaged section. The most important reinforcement recommended for metal kilns is the use of a strip of angle iron continuously welded around the outside of the lower rim of the base section. This will give a double thickness of metal at the lower edge of the kiln where heat stresses are most severe. It also provides a flat surface which distributes the weight of the kiln onto the inlet/outlet channels. The angle iron support shelves welded inside the top rim of the base and top section provide more reinforcement and a flat strip welded around the inside of the lower rim of both the top section and cover provide the rest.

Not all kilns incorporate steam release ports in the cover. After the lighting of the kiln the large quantities of steam released during the initial stage of the process must escape. Without steam release ports the cover must be propped open prior to lighting and dropped into place once the charge is burning fiercely. Alternatively the kiln can be overloaded and the natural sinking of the charge provides a sufficient period of time for the steam to escape before the cover settles into its supporting ledge. The obvious hazards presented by both these practices give rise to some anxiety during initial training programmes to operators of limited experience. Steam release ports have been found more acceptable when introducing metal kilns into new areas in the developing world. Another advantage is in the production of charcoal from small dimension raw materials such as scrub and coconut shells. To maintain sufficient gas flow through the charge during the carbonization of these materials it is recommended that the kiln is lit at the top through the steam release ports of the cover.

Finally, some of the kilns commercially available are equipped with hinged flaps and metal caps on the inlet/outlet channels which are used to cut off the air supply to the kiln on sealing. These extras add to the capital cost of the kiln and are found to be completely unnecessary. The hinged flaps become unusable after only a few days because of rust. Sealing can be achieved more effectively using earth or sand. In some designs handles are provided on the outside of the cylindrical sections of the kiln. Although handles are necessary on the cover of the kiln on the main cylindrical sections, they cause problems when rolling these sections from one site to another. It has been found that the kiln can be easily manipulated without the use of handles on the two cylindrical sections.

8.2. Metal charcoal kiln made from oil drums

Charcoal can be produced in kilns manufactured from standard 45 gallon oil drums. This method has been operated successfully using fast burning raw materials such as coconut palm timber, coconut shells and scrub wood. However, when operated with dense hardwoods, complete carbonization is difficult to achieve and the resulting charcoal is likely to have a high volatile content. Even with low density materials the volatile content of the charcoal produced is somewhat high, although this is not a major disadvantage for a local domestic fuel. If the charcoal is to be produced for export, however, the use of proper metal kilns will enable the high quality demanded by the trade to be achieved.

One man can operate a group of up to 10 oil drum units. The process involves a carbonization period of about two to three hours, followed by a cooling period of about three hours. An experienced operator can cycle ten drums twice each day to produce a total output of up to 30 kg of charcoal from each drum. This means that a one man operation, using 10 kilns, can produce 1½ tons of charcoal per 5-day week, if supplied with adequate prepared wood.

Compared with traditional methods of production the conversion efficiency obtained in oil drum kilns is comparatively high with reported yields of up to 23% (dry basis). (8) The main disadvantage of the method is that the raw material must be less than 30 cm long, with a maximum diameter of 5 cm, to achieve satisfactory results. This means a considerable amount of labour in the preparation of the raw material. Also used oil drums are sometimes difficult and expensive to obtain. The drums tend to burn out rather quickly due to the thin metal used end have to be replaced fairly frequently.

Photo. 25. Retort made from oil drums. Note pipe to collect condensate and fire laid in trench. Ghana. Photo Lejeune.

8.3. Advantages and disadvantages of transportable metal kilns

The main advantages of transportable metal kilns compared with the traditional earth pit or clamp method are:

- Raw material and product are in a sealed container giving maximum control of air supply and gas flows during the carbonization process.

- Unskilled personnel can be trained quickly and easily to operate these units.

- Loss of supervision of the process is required compared to the constant attendance necessary with pits and clamps.

- Mean conversion efficiencies of 24% including fines (dry weight basis) can be consistently achieved. Pits and clamps give erratic, often lower yields.

- All of the charcoal produced in the process can be recovered. With traditional methods (pits and mounds) some of the charcoal produced is lost in the ground and that which is recovered is often contaminated with earth and stones.

- Transportable metal kilns, if designed to shed water from the cover, can be operated in areas of high rainfall, providing the site has adequate drainage. Traditional methods of charcoal production are difficult to operate in wet conditions.

- With maximum control of the process a wider variety of raw materials can be carbonised. These include softwood, scrubwood, coconut palm timber and coconut shells.

- The total production cycle using metal kilns takes two to three days.

The disadvantages of using metal kilns compared with the traditional earth pit or clamp method are:

- Initial capital to cover the cost of the manufacture of the kilns must be obtained. Basic mechanical workshop skills and equipment must be available and the steel used in the kiln construction often has to be imported.

- For ease of packing and maximum efficiency some care is needed in the preparation of the raw material. The wood must be cut and/or split to size and seasoned for a period of at least three weeks.

- Transportable metal kilns may prove difficult to move in very hilly terrain, although more gentle slopes can be easily traversed.

- The life span of metal kilns is only two to three years.

The advantages of using transportable metal kilns compared with fixed installations, including kilns manufactured with bricks are:

- Transportable metal kilns can be easily and frequently dismantled and rolled along the forest floor to follow commercial timber extraction, plantation thinning or land clearance operations. This means that the laborious and expensive transportation of wood to a centralised processing site can be avoided.

- The total production cycle for these units is approximately one week, compared to the two to three days for the metal kilns.

The disadvantages of using metal kilns compared with kilns built from locally made bricks are:

- The cost of manufacturing a transportable metal kiln is usually greater than a brick-built kiln of comparable output. This is mainly because of the cost of raw material. Foreign exchange is needed in cases where the steel has to be imported. Sheet metal working skills and a workshop are needed for manufacture and maintenance.

- Because of the higher thermal insulation of the walls of the brick built kiln, less of the wood charged is burnt during the carbonization process and a slightly higher conversion efficiency is usually achieved than that obtained with transportable metal kilns. Brick kilns can carbonise large diameter wood and less cross cutting splitting is needed.

- The recovery of by-products from transportable metal charcoal kilos is not feasible. Where brick-built kilns are used, there are possibilities that the condensable tars may be recovered.

- Management supervision and logistical support is more readily supplied in a centralised processing situation where batteries of static brick-built kilns are in operation.

Photo. 26. Metallic transport table kiln. Photo. TPI

8.4. Manufacture of the TPI metal kiln

The kiln consists of two interlocking cylindrical sections and a conical cover. The cover is provided with four equally spaced steam release ports which may be closed off with plugs as required. The kiln is supported on eight air inlet/outlet channels, arranged radially around the base. During charring, four smoke stacks are fitted onto alternate air channels.

The kiln can be built locally in a simple sheet metal workshop. If available, Corten 'A' sheet steel 3 mm thick should be used instead of mild steel, as its heat and rust resistant properties will extend the working life of the kiln.

8.5. The transportation and location of kilns

The kilns can be easily using a flat-top truck. To transport the kiln in the back of a pick-up vehicle, the two cylindrical sections can be nested together - by inserting the top section into the lower rim of the base section - and rolled onto the back of the vehicle using a wooden ramp. The conical cover can be positioned inside the cylinders and the load should be firmly wedged and tied to prevent rolling.

Care must be taken when unloading the cylindrical sections from vehicles. If the sections are dropped onto their sides, then distortion is likely and difficulties will be encountered when attempts are made to assemble the sections during use. Slight distortion can be tolerated but severe distortion should be corrected using a car jack and two lengths of timber inserted across the diameter of the damaged section.

For economy of labour it is recommended that two or more kilns be operated as a group within reasonable walking distance of each other. This enables operators to unload and load one unit whilst the other kilns are in the carbonization or cooling stage.

To avoid the unnecessary carrying of wood, the kilns should frequently be rolled to new sites adjacent to the wood supply. The individual sections of the kiln can be rolled by two or three men, usually with two men pushing from behind and one man guiding the section from the front. Wooden levers are recommended for tipping the individual sections onto their sides prior to rolling them to a new position. Rolling the sections is far easier than sliding them horizontally, even where distances of only 1 or 2 metres are concerned. The task of manipulating these kilns on the forest floor becomes considerably easier with experience.

8.6. Selection and preparation of site

A well drained and roughly levelled area, approximately 3 metres in diameter, should be chosen in close proximity to the wood supply. Tree stumps and large root systems should be avoided and excessive undergrowth should be removed from the chosen area and the ground made firm by stamping it down. Loose earth or sand should be available close to the site for sealing off the air supply to the kiln during operation. A sandy or loamy soil is preferred and, if not available, a supply of sand should be obtained from a nearby stream for the initial operations. This material can be re-used and will soon increase in volume as charcoal dust and wood ash, produced during successive operations, are incorporated into it.

8.7. Preparation of the raw material

The wood should be felled, cut up and stacked at least three weeks before kilning, if the maximum yield of charcoal is to be obtained. Dry wood needs less charring time and increases the conversion efficiency of the process. The size of wood most suitable is between 450-600 mm long and up to 200 mm in diameter. Branches up to 900 mm long can be included provided their diameters do not exceed 130 mm and the packing density in the kiln is not markedly reduced. Logs with diameters approaching 300 mm may be used provided they are cut into lengths no greater than 300 mm. Wood with a diameter greater than 300 mm should be split before use.

Branches with a diameter of less than 40 mm should not be mixed in the same charge with timber of maximum diameter. This material should be charged with other small to medium size wood. Approximately 7 stacked cubic metres of wood are required to fill the kiln.

Photo. 27. Wood preparation for charcoal. Note short length of wood. Photo TPI

8.8. Method of operating the TPI kiln


8.8.1. Tools required for a 2-3 man operation:
8.8.2. Assembly and loading the kiln
8.8.3. Lighting the kiln.
8.8.4. Reducing the draught
8.8.5. Control of charring
8.8.6. Unloading the kiln
8.8.7. Bagging of charcoal


8.8.1. Tools required for a 2-3 man operation:

Chainsaw or crosscut saw
Shovels or spades (2)
Cutlass
Axe
Wedges (2)
Sledge hammer
Wooden pole or plank
Sieve chute
Sacks
Needle and string
Heat proof gloves

8.8.2. Assembly and loading the kiln

(a) The bottom section of the kiln is rolled onto the prepared site and lowered into its operating position. Using a wooden pole as a lever the eight air inlet/outlet channels are inserted open side down radially underneath the bottom section at equidistant intervals. Equal spacing will be easily achieved if the first four channels are inserted at 90° intervals and the remaining four inserted between them.

Photo. 28. Assembling of metallic kiln. Photo. TPI

The air channel must protrude a minimum of 250 mm into the kiln to prevent overheating of the kiln wall. The supporting collar on the top of the channel must not lean inwards towards the kiln wall, otherwise it will be difficult to position the chimneys once the kiln has been assembled. When the inlet/outlet channels are in position, it is necessary to check that they are completely clear of any obstruction.

(b) The bottom of the kiln is loaded with wood making sure that the ends of the inlet/outlet channels and the spaces between them are not blocked. To achieve this, the charge is supported on 'stringers' which are medium diameter (150 mm) pieces of cordwood arranged radially like the spokes of a wheel.

(c) At each quadrant of the kiln's base dry kindling wood, together with any inflammable waste (paper, sump oil, etc.) is placed between the stringers from the edge of the bottom of the kiln to the centre to provide four lighting points.

Photo. 30. Inflammable waste is placed between the stringers to provide four lighting points. Photo. TPI

(d) A bridge of small/medium diameter wood and brands (incompletely charred wood from a previous firing) is now placed across the stringers over the kindling in the shape of a cross. The base layer of the kiln is completed by bridging the remaining exposed stringers with small/medium diameter wood.

By supporting the first layer of wood above the ground on stringers, air ducts are formed under the charge which will allow the fire to spread more rapidly into the centre of the kiln.

Photo. 31. Wood charge operation. Photo. TPI

(e) The bottom section of the kiln is loaded with successive layers of wood, filling in as many voids as possible and placing the larger diameter timber towards the centre of the kiln. When the bottom section is full and all the joint surfaces of the kiln are scraped clean, the top cylindrical section is rolled alongside. The top section is then pushed up on to the supporting shelf of the base section.

(f) The loading of wood is continued until the charge forms a conical shape above the rim of the top section but, at the same time, will allow the cover to be located into the rim without hindrance. The cover is then rolled alongside the kiln and pushed up onto its supporting shelf. Two experienced men can load the kiln in about two hours.

8.8.3. Lighting the kiln.

(a) After ensuring that all four steam release ports in the cover are open, a flame is applied to the four lighting points. Where there is a prevailing wind, the area of the kiln on the windward side will burn more quickly. To allow for this, the lighting points facing the wind are not lit until the lee side of the kiln is well alight.

Photo. 32. Lighting points. Photo. TPI

(b) The kiln is allowed to burn freely for about 30 minutes until the bottom section at each lighting point becomes so hot that it is unpleasant to stand close to the kiln* During this period copious amounts of steam will be released from the four ports in the cover of the kiln. While this is in progress the joints between the main sections of the kiln are filled with sand or soil and the four smoke stacks are placed into position over the supporting collars of each alternate air channel.

Photo. 33. Joints between the main sections of kiln are filled with sand or soil and smoke stack is placed into position. Photo. TPI

8.8.4. Reducing the draught

As each sector of the kiln reaches the required temperature the spaces between the inlet/outlet channels are covered with sand or soil. When all the spaces between the channels have been covered, the ends of the four channels supporting the smoke stacks are sealed. The steam release ports are now closed so that the smoke is drawn out of the base of the kiln by the four chimneys. When the draught has been reduced air enters the kiln only through the inlet channels from where it flows up through the centre of the charge. The combustion gases are drawn down the outer edge of the kiln and are released through the smoke stacks. As the air and exhaust gases flow in opposite directions, this condition is known as the reverse draught.

Photo. 34. Air and exhaust gases flow in the kiln. Photo TPI

8.8.5. Control of charring

(a) Each chimney should emit a column of thick white smoke 15-30 minutes after promoting the reverse draught. Throughout the period of carbonization it is advisable to ensure that even temperatures are maintained around the circumference of the kiln. Control is easier when kilns are operated in sheltered positions. If there is a strong prevailing wind, the temperature profile across the kiln may become unbalanced and it will then be necessary to partially or completely block one or two of the air inlets on the windward side. When operating in wet conditions, or with freshly felled wet wood in windy conditions, more extreme efforts may be necessary to balance the kiln during the initial stages of carbonization. Under these conditions large quantities of water will evaporate from the soil and wood on the hottest side and condense in the cooler regions of the kiln. This water is likely to quench any fires remaining in the lighting points and will further depress the temperature in these areas.

To correct this situation the air inlets on the hot side of the kiln must be temporarily blocked and the spaces between the inlet/outlet channels on the cooler side uncovered to allow more air to enter this region of the kiln. This action will draw the fire over to the cooler side of the kiln and, once the temperature in this area has been sufficiently increased, the spaces between the channels may be resealed. After this, the normal method of controlling the supply of air to the kiln can be followed.

It is important that a burning kiln is never left unattended when the spaces between the inlet/outlet channels are uncovered as serious damage to the kiln could result.

When wet wood is used or when the kiln is operated under wet conditions, the charring period is likely to be extended up to a total of 48 hours. Because of the increased amount of wood burned internally to drive off the excess moisture, lower yields of charcoal are to be expected.

(b) During charring a certain amount of tar is deposited in the outlet channels and smoke stacks. This tar restricts the exhaust gas flow from the kiln and should be removed when there is a noticeable reduction in the quantity of smoke issuing from any of the stacks. To achieve this the stack is lifted off the supporting collar of the outlet channel using a pair of heat proof gloves or an old sack and any obstruction inside the chimney is removed. At the same time a long stick should be inserted through the channel into the centre of the kiln to ensure that there is no internal restriction.

Some time during the carbonization period (usually 8-10 hours after lighting) the smoke stacks should be moved on to the adjacent air channels to convert air inlets to smoke outlets and vice versa. This creates a more even burn and reduces the formation of ash at the regions where air enters the kiln.

(c) Charring is complete when the colour of the smoke from all chimneys takes on a bluish sings and becomes almost transparent. This normally occurs about 16 to 24 hours after lighting. At this stage the whole surface of the kiln should be very hot (150-200 C) so that a spot of water applied to the wall of the kiln will evaporate immediately with a spitting noise. When this stage is reached the kiln is completely sealed for cooling.

The kiln is sealed by removing the smoke stacks and completely blocking all air channels with soil or sand. If necessary, additional soil or sand is added to the joints of the main sections of the kiln and the steam release ports to ensure that they are fully sealed and that no air may enter. The kiln is allowed to cool for between 16 and 24 hours before opening and unloading. Cooling will be greatly assisted if rain falls.

8.8.6. Unloading the kiln

(a) The kiln must not be opened until the contents are cold and the outside surface of the kiln is cool to the touch. The action of direct sunlight could obscure this and the temperature of the inside of the kiln can be more easily assessed by feeling the surface of the bottom section in an area shaded from the sun. Following this the temperature of the remaining surface of the lower section should be assessed to ensure that no "hot-spots" exist. If the contents of the kiln are still hot after a cooling period of 24 hours, then complete sealing from the outside air has not been effected and efforts must be made to achieve this. Furthermore, if the kiln is opened and part of the charcoal is seen to be still alight, the kiln must be resealed for a further cooling period.

The charcoal must be removed immediately the seal on the kiln is broken even when the contents appear to be completely cold. Any delay could result in localised fires igniting the charcoal and, in addition to this loss, serious damage to the kiln may occur.

(b) During carbonization the wood will have been reduced to about half its original volume and it will be possible to remove the cover and top section once the kiln has cooled, leaving the charcoal in the lower section. The cover is removed with a minimum effort by lifting one side from its supporting shelf and inserting the end of a long branch or plank into the resulting gap. This piece of wood can then be used as a ramp on which the cover is slid gently to the ground. The same method is used to remove the top section of the kiln.

Photo. 35. During carbonization period wood has been reduced to about half its original volume. Photo. TPI

(c) To remove the bottom section the inlet/outlet channels are first removed from one side of the kiln using a lever. By applying the lever to the opposite side, the bottom section can be tipped onto its side leaving the charcoal free to be loaded into sacks. A bucket of water or a quantity of sand or soil should be on hand while unloading the kiln in order to quench any small fires.

8.8.7. Bagging of charcoal

To speed up this operation a sieve chute should be used to separate the large charcoal pieces from the fines and dust. The construction of the sieve chute is shown in figure 11.

It is recommended that the bottom section of the kiln be positioned on the leeward side of the charcoal and used to support the sieve chute. This will not only increase the stability of the sieve but will reduce the amount of dust reaching the operator.

If required a free-standing sieve chute can also be used. Two men can unload it and fill the sacks with charcoal in about an hour.

Fig. 11. Sieve chute for loading charcoal into sacks

Photo. 36. Sieve chute should be used to separate the large charcoal pieces from fines and dust. Photo. TPI

8.9. Alternative method of operation


8.9.1. Loading
8.9.2. Lighting
8.9.3. Reducing the draught


Lighting the kiln from the top is a method which is particularly suitable for the carbonization of small wood or coconut shells, as it ensures a sufficient gas flow through the charge.

8.9.1. Loading

The kiln is loaded as previously described, without kindling between the stringers at the base. The kindling wood is placed instead in a depression 250 mm deep on top of the charge, which is then covered with a final layer of wood.

When carbonising coconut shells the use of stringers is not required. Care must be taken to ensure that the shell material does not block the ends of the inlet/outlet channels inside the kiln. To achieve this, a flat piece of wood (for example, a piece of rib from a dead palm frond) is placed on top of the end of each channel before covering it with shells.

8.9.2. Lighting

The fire is lit at the top through one of the four steam release ports and the charge is allowed to burn with a completely free access of air into the base of the kiln. The smoke will escape through the four ports in the cover. This stage is allowed to continue for about two hours until the whole of the top section of the kiln is too hot to touch with bare hands.

8.9.3. Reducing the draught

When the top section is sufficiently hot the spaces between the inlet/outlet channels are covered with sand or soil and the chimneys are placed in position. The steam release ports are sealed. The reverse draught and control of the supply of air to the kiln are achieved as in the normal method of operation described above.

8.10. Schedule for commercial operation

Two experienced men can operate two transportable metal kilns producing 23 tons of charcoal per week. Depending on local conditions and facilities, assistance may be required in cutting the wood and moving the kilns and, in some cases, the use of a third man, preferably a chain saw operator, may be necessary.

Experience has shown that the most successful commercial ventures involving the use of transportable metal kilns have been those offering maximum incentives to the operators. An example of this is a cooperative run by kiln owner/operators where the members receive most of the proceeds of the sale of the charcoal.

A suggested 5-day week work plan is outlined below. Modifications can be made to this schedule to allow for variations in daily working hours and a 6-day working week. Moreover, if arrangements can be made for someone living near to the production area to undertake a half hour period of light duty to seal the kiln during the weekend, then extra operations can be achieved.

Monday


08.00-10-00

Kiln 1 and Kiln 2

Unload both kilns.

10.00-12.00

Kiln 1


Load kiln with wood.

12.00-13.00

Kiln 1


Light kiln and reduce draught.

13.00-17.00

Kiln 1


Control charring. Change and clean stacks at 16.30.



Kiln 2

Load kiln with wood.

Tuesday

08.00-08.30

Kiln 1


Change and clean stacks.

08.30-11.00



Prepare wood for future operations.

11.00-12.00


Kiln 2

Light kiln and reduce draught.

12.00-17.00


Kiln 2

Control charring. Change and clean stacks at 16.30.


Kiln 1


Shut down kiln when charring is complete.
Prepare wood for future operations.

Wednesday



08.00-08.30


Kiln 2

Change and clean stacks.

08.30-14.00



Prepare wood for future operations.

14.00-15.00

Kiln 1


Unload charcoal from kiln.

15.00-17.00

Kiln 1


Start loading kiln with wood.



Kiln 2

Shut down kiln when charring is complete.

Thursday




08.00-10.00

Kiln 1


Finish loading kiln with wood.

10.00-11.00

Kiln 1


Light kiln and reduce draught.

11.00-13.00


Kiln 2

Unload charcoal from kiln.


Kiln 1


Control charring.

13.00-15.00


Kiln 2

Load kiln with wood.


Kiln 1


Control charring.

15.00-16.00


Kiln 2

Light kiln and reduce draught

16.00-17.00

Kiln 1


Change and clean stacks



Kiln 2

Control charring.

Friday


08.00-09.00

Kiln 1 and Kiln 2

Change and clean stacks.

09.00-13.00

Kiln 1


Shut down kiln when charring is complete.
Prepare wood for future operations.



Kiln 2

Change and clean stacks at 12.30.

13.00-17.00



Prepare wood for future operations.



Kiln 2

Close down kiln when charring is complete.

8.11. The most common operational faults

(a) Failure to insert the inlet/outlet channels sufficiently under the lower rim of the bottom section of the kiln during assembly. The high temperature produced at the inner end of the air channel causes serious damage to the kiln wall if the required distance between the hot zone and the kiln wall is not maintained.

(b) Failure to achieve sufficient gas flow through the system by not removing deposits of tar from the outlet channels and chimneys. This results in low kiln temperatures and prolonged charring periods.

(c) Excessive periods allowed for cooling the kiln which reduces the number of operations possible in the working week.

(d) Reluctance to move the kiln closer to the available wood supply, resulting in a waste of time and effort in carrying the wood to the kiln.

(e) Insufficient supply of wood available in the area adjacent to the kiln for loading immediately the previous operation has been completed.

(f) The practice of allowing large fires to develop next to the surface of the wall of the kiln during the lighting stage. This usually restricts the flow of air under the kiln and prevents the fire spreading quickly to the centre of the charge. It can also cause serious damage to the kiln wall. Once the prepared kindling has been ignited inside the kiln, a maximum flox of air is all that is normally required.

(g) The laborious and time-consuming practice of hand picking the charcoal into sacks instead of using shovels and a sieve. Excessive time spent on unloading the kiln causes a delay in loading and lighting the next operation.

8.12. Yields of charcoal

The weight of charcoal produced in each batch operation of a transportable metal kiln is related to several physical factors. The main factors which contribute towards maximum yields are:

- high timber density
- low moisture content of wood
- dry operating conditions and a dry well-drained site for the kiln
- high packing density of charge obtained with regular size and shape of raw material.

In practice, it is seldom found that all of these conditions can be arranged and consequently the yields and conversion efficiencies reported vary to a considerable degree. (7, 10, 18, 31).

Training programmes by TPI in seven countries gave an average charcoal yield including fines, of 26% on a dry basis. The maximum weight of charcoal obtained from a single operation was observed in Guyana where 1 083 kg of charcoal was produced from 3 852 kg (dry weight) of regular sized billets of a high density hardwood. The moisture content of the wood used was approximately 25% (wet basis). This indicates a conversion efficiency (dry weight basis) of 28.12%. Higher conversion efficiencies have been obtained in the arid coastal region of Ecuador where a 31.40% recovery was observed.

Conversely, the lowest weight of charcoal obtained from a single operation was from soft wood slab wood waste in Sudan. 297 kg of charcoal was produced from an estimated 1 568 kg (dry weight) of timber. This indicates a conversion efficiency of 18.94%. The moisture content of the freshly sawn slab wood was approximately 57% (wet basis).

8.13. Working life of transportable metal kilns

The durability of kilns depends to a large extent on the care and expertise shown by the operators. If the kilns are not operated by the owners but by casual labourers, then the incentive to prevent operational damage is reduce. Untrained operators are also likely to reduce the working life of the kiln.

Experience so far has shown that these kilns can be expected to survive continual operation for a period of three years. After this time the bottom cylinder will usually need replacing or substantial repairs. The top section and cover are not subjected to the same amount of heat stress as the bottom section. Provided care has been taken in transport and assembly a longer period of use should be expected. In West Africa, a TPI designed kiln in continuous operation after a period of two years showed only minor signs of distortion in the lower part of the bottom section. The top section and cover remained in first-class condition.

The components of the kiln which suffer most in use are the 8 inlet/outlet channels. The high temperatures experienced at the inner ends of the channel distort the metal in these localised areas. Channels have to be regularly reshaped and will almost certainly have to be replaced after a period of three years continuous use.


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