5.1. The pit method
5.2. Technical and cost data for pit charcoal production
Using earth as a shield against oxygen and to insulate the carbonising wood against excessive loss of heat is the oldest system of carbonization and surely goes back to the dawn of history. Even today it is perhaps used to make more charcoal than any other method. It is, therefore, worthy of careful study to find out its advantages and disadvantages. Obviously it keeps its place because of its low cost. Wherever trees grow, earth must be available and it is natural that mankind turned to this cheap available non-combustible material, as a sealing material for enclosing the carbonising wood.
There are two distinct ways to use an earth barrier in charcoal making: one is to excavate a pit, put in the charge of wood and cover the pit with excavated earth to seal up the chamber. The other is to cover a mound or pile of wood on the ground with earth (5, 12, 16, 19, 20, 21, 28). The earth forms the necessary gas-tight insulating barrier behind which carbonization can take place without leakage of air, which would allow the charcoal to burn away to ashes. Both methods, if skilfully carried out, can produce good charcoal within their technological limitations.
5.1.1. Making charcoal in miniature pits
5.1.2. Making charcoal in large pits
A stratum of deep soil is needed for this method. Suitable deposits of soft soil will usually be found along the banks of a creek. Pits can be made very large and a cycle may take up to three months to complete (13, 31, 32). Capital investment is minimal; nothing more than a shovel, an axe and a box of matches is required. But the method is wasteful in resource. It is very difficult to control the circulation of the gases in the pit. Much of the wood is burned to ashes because it gets too much air. Another portion remains only partly carbonised, because it was never properly heated and dried out during the burn. Apart from the gross variations in quality, there is variation in volatile matter, i.e. degree of carbonization within the acceptable charcoal. This is because in the pit carbonization is started at one end and progresses towards the other. Hence, the charcoal at the start of the burn, being heated longer, is much lower in volatiles than the charcoal at the end. For domestic purposes, this is not a serious problem, though it does reduce the overall yield, since the "hard" or overburned charcoal at the firing end with its low volatiles, high final carbon content, implies a low yield (theoretically about 30%30%). Overburning at one end is unavoidable in order to burn the charge as a whole.
A further problem with pits is reabsorption of pyroligneous acid through rain falling on the pit. The pyroligneous acids tend to condense in the foliage and earth used to cover the pit. When heavy rain falls they are washed back down and are absorbed by the charcoal. They cause jute bags to rot and, on burning, the charcoal produces unpleasant smoke. Nevertheless, skilled operators using pits which are not too big can make excellent quality charcoal. (31). The low capital cost of the system commends its use where wood is abundant and labour costs are low.
Small pits or holes up to a cubic metre or so are useful for producing small amounts of charcoal from small, fairly dry wood. The method is employed at the village level, but is usually too low in productivity to supply large amounts commercially. To burn charcoal this way a fire is first started in the pit and dry small fuel is added to make a strong fire. More wood is added to fill the pit, the fire continuing to burn steadily. A layer of leaves about 20 cm thick is placed over the wood fuel and then earth about 20 cm thick shovelled on. The pit is left to complete carbonization and can be opened in two days or less. Water may be needed to prevent ignition when the pit is unloaded. Charcoal is not uniform in quality and, if small wood and bark is used, the proportion of fines is excessive. Sometimes pits axe covered by a sheet of old roof iron covered with earth, allowing a few small openings for escape of smoke and entry of air.
Typical pits for charcoal are large and burning takes place progressively from one end to the other. The larger pits producing 6 tons or more charcoal per burn are difficult to control, but are more efficient in use of labour. Somewhat smaller ones have better air flow and produce more uniform charcoal, but the output is lower and use of labour less efficient.
Figure 2 shows a large pit of about 30 m³ gross volume. It will hold a charge of about 26 m³. A sandy loam is preferred with adequate depth. About three man days are needed to dig the pit and a day to add the channels for lighting and for smoke exist.
Fig. 2. 30 m³ Charcoal Pit - Longitudinal section
Fig. 2. 30 m³ Charcoal Pit - Plan view - without earth cover
The pit is loaded with logs measuring 2.4 m or less, which will fit easily across the pit. Care must be taken, during loading, to fill as many gaps between logs as possible with branches and small wood to improve volumetric efficiency. The long length of the wood which can be loaded into pits means that crosscutting with axes is still a practical method for the small operator without capital. Nevertheless, chain saws are widely used. To ensure that the wood is properly heated for carbonization, the hot gas is allowed to pass along the floor of the pit by placing the charge on a crib of logs.
First, about five logs, cut to the width of the pit, are laid evenly spaced along the length; then four logs each equal to the length of the pit are evenly spaced on top of the first layer. This crib structure supports the charge and yet allows hot gases once the pit is lit at one end, to pass beneath the charge, heating it as they travel to the flue at the opposite end. These hot gases produced by partial burning of the wood charged slowly dry out the earth and heat up the rest of the wood to the carbonization point, about 280°C. Spontaneous decomposition of the wood, with evolution of heat, then occurs to form charcoal. Copious volumes of water vapour, acetic and other acids, methanol and tars, are produced at the same time. These also transfer their heat to the drying wood charge on their way to the outlet. Finally, the last of the wood is dried out, heated to carbonization point, and transforms itself into charcoal. The carbonization stage may take 20 to 30 days to complete and it is accompanied by a marked volume reduction of the wood charge to 50-70% of its initial volume. The earth covering the pit slowly sinks during the carbonization and any cracks or holes which form must be closed to prevent air leakage. There is danger of fatal burning to any person or animal falling or walking on the pit and care must be taken to avoid this.
When the covering of the pit has sunk from one end to the other, the burn is considered complete and openings are sealed and the pit allowed to cool, which can take 40 days approximately, depending on the weather. After cooling, the pit is opened and the charcoal unloaded, taking care to separate it from earth and sand and partially carbonised wood. Forks and rakes are useful for this.
The nature of carbonization in a pit makes it difficult to achieve a uniformly carbonised charge. The charcoal at the firing end is normally low in volatiles and the last formed charcoal near the smoke flue is high in volatiles, since it was subjected to carbonising temperatures for only a short time. Further, because air flow may not be uniform, there can be a considerable volume of brands in a charge. Although brands can be recovered and recycled, they represent an inefficiency in operations.
Smaller pits than the one shown in Fig. 2 are used. A typical small pit may measure 3 m long by 1.2 m wide and 1.2 m deep. The length of wood charged measures about one metre and, as in the large kiln, the spaces between large logs must be blocked carefully with small pieces of wood to increase charge efficiency and prevent uneven channelling of the gas from one end to the other, leading to production of "brands".
The following technical and cost data is given for the production of charcoal by the large pit system in Guyana where the process has been operated on a commercial level for many years. Annual production has varied widely. Peak output was around 6,000 tons per year in the 1950's.
From data collected in the field and from discussions with people who have been working with this method for many years, the following evaluation for one man or a crew of five men working with one pit are given. The values are in days/per man/per pit.
Size of the pit:
Length 6 m x 2.70 width x lighting point depth 1.20 m x opposite extreme depth 2.40 m:
Nominal volume = 29 m³
Real utilized volume = 26 m³
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Time employed: |
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- to dig it in soft sand |
3.0 man days |
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- to prepare the inlet and outlet air circulation channels |
1.0 man days |
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- felling-bucking with axe, transporting firewood and packing (length of firewood 2.40 m (8')) |
14.0 man days |
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- to cut bush and cover firewood (thickness of bush 3 cm) |
2.0 man days |
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- to cover with sand 30 cm |
1.0 man days |
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- to prepare sand and stakes around pit |
0.5 man days |
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- to discharge charcoal |
1.0 man days |
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22.5 man days |
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Process time |
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- carbonisation |
20.0 man days |
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- cooling (depends on weather, 30 days when rainy, 50 days when dry) average |
40.0 man days |
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60.0 man days |
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In practice, this pit is prepared by a crew of five men. This crew can produce 1.5 pits per week (7 days), working with axe and shovel, which is equal to 23 days per man per pit. To this value must be added another 8 man days. This is for taking care of the carbonization process during the 60 days it takes. Thus, the total man/days/pit is about 31. The average production of the pit is 6.0t. The nominal volume of the kiln is 29 m³. The useful capacity 90%, so the real volume utilized is 26 m³. By charging the pit with heavy woods 1,000-1,100 kg/m³ the total possible charge will be 27-28t. With a ratio of fuelwood: charcoal of 4.5 to 1, it is possible to obtain 6t per burn with an 82 day cycle. Three burns per year are possible, giving an annual production of 18 tons. This production requires 31 man days per pit x 3 = 93 man days per year.
If, say, 10,000 tons of charcoal are to be produced per year then 10,000/6= 1 666 pits must be burned per year. Each pit burned requires 31 man days, making the total man days 1 666 x 31 = 51 646.
Cost of producing 10,000 tons per year
The exact cost depends on the way labour can be organised around the cycle time of a pit which is 82 days. If 5 men work steadily digging, filling, lighting and unloading, then the number of pits which can be made and unloaded in a year will be as follows:
Number of days per year available = 365 - 82 = 283, say 280 days since pits started after day 280 approximately will not be unloaded in that year and their production will not count. Therefore, a 280 working day year is assumed, with labour employed the whole year. No pits are started after day 280.5 men can produce at 1.5 pits each 7 days in 280 days 280/7 x 1.5 = 60 pits each pit produces 6 tons charcoal: yearly production per team = 360 tons
Burners labour is 8 man days per pit. 60 pits need 60 x 8 = 480 man/days, i.e. approximately 2 men working through the year.
A nominal net wage without overheads of US$ 10.00 per day is assumed.
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Pit builders 5 men x 360 days x $10 = |
$18,000 |
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Burners 2 men x 360 x $10 = |
7,200 |
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$25,200 |
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Cost per ton = 25,200/360 = |
US$70,00 |
To produce approximately 10 000 tons per year will need 28 teams each of 5 builders and 2 burners.
Total = 196 men and 1,680 pits must be made and burned each year.
These calculations assume perfect organisation and do not allow for labour overheads or profit. The indicated cost is for charcoal piled for transport at the side of the kiln.
