by Keith Openshaw1
1 Keith Openshaw has worked on surveys of wood fuels in Gambia, Kenya, Tanzania and Thailand. At the time of writing this note he was Senior Lecturer in Forest Economics at the University of Dar es Salaam, Tanzania.
When measuring the consumption of fuelwood and charcoal, conversion factors have to be used in order to convert field observations into standard units. These factors will have to be compiled for the particular country in question but certain procedures are common to all countries.
Much fuelwood is collected by the headload. A number of headloads could be measured either by volume or weight and an average for a particular district or country could be established. The main collectors of fuelwood in each district should also be recorded, for if children are the principal collectors in one area, the headload size may differ considerably from another district where women are the collectors. Therefore it is advisable to record the incidence of collectors by sex and age. Again the bicycle or handcart may be used to collect the wood in some areas and naturally the size and weight of the bundle will differ from the average headload size, so the method of conveying the wood should also be recorded.
The two methods of measuring, by volume or by weight, each has drawbacks. If volume is used, then the conversion factor from the bundle to solid measure can vary enormously, depending on whether the headload consists of one large log or many small branches, though the average headload conversion factor may be between 0.35 and 0.400 For example, in the Machakos district of Kenya the average headload volume was found to be 0.087 m3 and the average conversion factor measured by the water displacement method was 0.38, so the solid volume of the average headload was 0.33 m3.
In some countries the stere or stacked cubic metre is the standard measure, but in using this measure the surveyor does not know the correct conversion factor to apply. If the stere is made up from bundles, then the conversion factor will be much lower than if it is made up from stacked stem wood. Again the stacked measure is not an exact measure and can be up to 20 percent more than a true stere (but it is rarely less than a true stere). This also applies to other stacked measures in use such as a cord (4 feet × 4 feet × 8 feet = 128 stacked cubic feet or the metric cord 1 m × 1 m × 3). One advantage the volume measure has over the weight measure is that the volume of 'wet' wood does not differ greatly from air dry wood (may be up to 5 percent); if a standard conversion factor to convert weight into volume is used, without accounting for the moisture content of the wood, then there can be 100 percent difference in volume estimation, depending on whether the wood is green or oven dry.
(i) Convenience
Weight may be a more convenient measure to use to ascertain solid volume, for the weight of a bundle of wood (or crop residue) is easier and quicker to determine (using a spring balance) than trying to determine the gross volume of an irregularly shaped headload of fuelwood. If the solid volume is to be measured, then every piece of wood will have to be measured separately, or the water displacement method used. This entails the submerging of a fuelwood bundle in a tank of water and determining the volume of displaced water. Of course, if the fuelwood is dry then some water will be absorbed unless the displaced water is measured immediately.
(ii) Moisture content
From surveys in Gambia and Tanzania, the average headload weighed about 26 kg whereas in Kenya it weighs 25 kg and in Sri Lanka 20 kg. However, if weight is used two drawbacks must be noted. First, the weight of wood depends upon the moisture content (m.c.). If wood is cut green and then collected, the moisture content could be 100 percent or more (dry basis measure), i.e. its weight is double the oven dry (bone dry) weight.1 If the wood is first allowed to dry out and become 'air' dry then the moisture content may be around 12 to 15 percent, depending on the relative humidity of the atmosphere, so the same piece of wood may weigh up to 75 percent more if it is freshly felled compared to air dry. Therefore, it is important to know the moisture content of the wood if weight is the measure for assessing the solid round wood volume. In areas where wood is reasonably available then dead (air dry) wood is collected. However, in areas of scarcity, live branches are lopped. The collecting of green wood means that the collector is carrying 'unwanted moisture'; second, if green wood is burnt, energy is required to evaporate this water and less energy will be available for cooking and heating.
1 Moisture content can be measured in two ways:
(a) on a dry basis, that is:
(b) on a wet basis, that is:
In this section all moisture contents have been given on a dry basis but in some countries fuelwood moisture content is given on a wet basis. The formula for changing from dry to wet basis is as follows:
where D = moisture content as a percent of the dry weight basis
and W = moisture content as a percent of the wet weight basis
If the moisture content is 100 percent on the dry basis then the wet basis moisture content = 50 percent.
Similarly a 15 percent dry basis moisture content = 13 percent (wet basis).
The reverse formula for changing from wet basis moisture content to dry basis moisture content is as follows:
The following table gives an idea of the volume of wood at different moisture contents for an average tropical fuelwood species per tonne weight, using a standard conversion factor of 1 t - 1.39 m3 at 15% m.c.
|
Moisture content % | |||||||||
|
(dry basis) |
100 |
80 |
60 |
40 |
20 |
15 |
12 |
10 |
0 |
|
Volume (m3) | |||||||||
|
Solid |
0.80 |
0.89 |
1.00 |
1.14 |
1.33 |
1.39 |
1.43 |
1.45 |
1.60 |
Therefore, if the wood was assumed to be air dry (15 percent m.c.) when in fact it was green (100 percent m.c.) then the estimated solid volume of 1 tonne would be 1.39 m3 and not 0.80 m3, an error of 74 percent.
The above table could be reworked using 1 m3 as the standard and the weights for different moisture contents would be as follows:
|
Moisture content % | |||||||||
|
(dry basis) |
100 |
80 |
60 |
40 |
20 |
15 |
12 |
10 |
0 |
|
Weight (tonnes) per m3 | |||||||||
|
Solid |
1.25 |
1.12 |
1.00 |
0.88 |
0.75 |
0.72 |
0.70 |
0.69 |
0.625 |
Therefore, if weight is going to be the unit of measure then the approximate moisture content should be ascertained. There are moisture content meters which give immediate readings, but they may be difficult to obtain. Another method is to collect samples of the wood in airtight containers and measure the moisture content in a laboratory.
(iii) Density
The second problem with using weight as a measure to determine solid volume is that the weight depends on density, and the density within and between wood species is not uniform. Juvenile wood in young trees is less dense than mature wood in old trees or the same species, and sapwood is less dense than heartwood. Similarly, non-conifers are usually denser than conifers. However, what is important to note is that if the moisture content is the same, the energy given off from a piece of wood is more or less the same on a weight basis irrespective of species, and this is an important factor when planning the needs of a household, community or industry. One kilogram of wood at 6 15 percent moisture content when burnt will give off about 16.0 megajoules (16 × 106 J) (MJ/kg) that is 3,820 kilocalories (kcals) or 15,170 British thermal units (Btus).1 Wood with 40 percent m.c. has a heat value of 12.7 MJ/kg and that of a 100 percent m.c. 8. 2 MJ/kg.2
1 This is the low heat value and is equivalent to the heat available when cooking food etc. It can be calculated as follows:
where x = (high) heat value of wood » 20.0 MJ/kg.
and m = the moisture content. The factor of 2.4 MJ per kg of water is the amount of heat required to drive off 1 kg of water from the wood.
2 The low heat values of III tropical woods from Africa, Asia and South America were found to be on average 18.3 MJ/kg (oven dry state) with a coefficient of variation of less than 8%. The lowest value was 16.7 MJ/kg and the highest 20.3 MJ/kg. The corresponding values for temperate zone non-resinous hardwoods vary from 18.0 to 18.8 MJ/kg and between 19.2 and 20.1 MJ/kg for resinous softwoods (Bialy 1979 To obtain high heat values add 1.3 MJ/kg to the above values.
One survey could conclude that the average per 3 capita consumption of fuelwood in an area is 1.4 m3 whereas in another area it is 2.4 m3. If the principal species in the first case is Acacia mearnsii and in the second case Pinus patula then the two different volumes would yield approximately the same amount of heat, therefore it is important to record the species of firewood used in order to work out an approximate energy yield value. When planning fuelwood plantations the potential energy yield per unit area rather than volume is the important measures
Some preferred fuelwood (and charcoal) species are very dense, and even for air dry logs 1 tonne may give a volume less than 1 m3, whereas the average is about 1.4 m3 per tonne (air dry) and for conifers it can be 2.0 m3 per tonne.
2.1 Volume and weight
2.2 Fines
2.3 Converting to roundwood equivalent
Charcoal is usually sold by volume - per standard bag or basket, per tin or per pile but sometimes directly by weight. If it is sold by weight and is dry (1-15 percent m.c. but on average about 5 percent) then the energy value of the charcoal will be about 33.0 MJ per kilogram (7,890 kcals/kg or 31,300 Btus/kg), or twice that of wood per kg.
Most frequently charcoal is sold by the standard bag, which can vary from area to area and country to country. A 50 kg 'sugar bag' in many areas is considered standard and this has a volume of about 0.1 De The weight of charcoal, like fuelwood, depends on the moisture content and the density of the parent wood. Unless the charcoal has been deliberately wet or stood out in the rain the moisture content will be about 5 percent, with little significant variation. Therefore the weight of charcoal will only depend on the density of the parent wood assuming it has been completely or near completely carbonised. Normal tropical hardwoods with a volume of approximately 1.4 m3/tonne (15 percent m.c.) will weigh about 33 kg per bag whereas preferred charcoal species will give an average weight of 36 kg per bag. This is equivalent to a wood volume of about 1.3 m3/tonne (15 percent m.c.).
If the charcoal is made from softwood then each bag will weigh on average about 23 kg equivalent to a wood volume of about 2.0 m3/tonne (15 percent m.c.). On the other hand mangrove (Rhizophora) charcoal will give a bag weight of 56 kg (wood volume equivalent 0.8 m3/tonne of 15 percent m.c.). It can be seen that there is over 100 percent difference in weight between the two extremes quoted here and therefore it is important to know the species from which the charcoal is made, and as will be discussed later the method by which it is made. When undertaking a survey, a number of bags in each district/region should be weighed and if there are several sizes of bags (baskets) then they should be categorized (large, medium and small) and the average weight of each class of bag obtained by district.
Charcoal is also sold by the tin and the pile. The tin can vary in size, but if it is a 20-liter paraffin tin then it will contain about 7 kg of charcoal (tropical hardwoods) as mentioned above; if the sides have been forced in, the weight may be reduced to 4-5 kg. The tin and the bag are sold at prices which fluctuate according to season, inflation and statutory controls. On the other hand, the pile is usually sold at a fixed price and therefore the quantity in the pile varies from season to season and over time. Therefore, if the pile is being used as a measure then it should be weighed periodically to see if there are variations. At the same time as recording weight of charcoal, the price per bag, tin, pile should also be recorded.
When charcoal is manufactured there is always a certain amount of powdered charcoal or 'fines'. This may be as much as 30 percent of the volume. Some of these fines, maybe up to 5 percent of the volume, are always included in a bag of charcoal and the quantity of fines will increase more or less in proportion to the distance the charcoal is transported due to the vibration of the lorry, etc. If the charcoal is sold by the pile then it must be remembered that the pile does not contain fines and when converting back to bags from piles (and to a lesser extent tins) then only the solid volume in each bag should be considered. This may be 80-90 percent of the weight but it can be determined by actual weighing.
If briquetting of charcoal is undertaken then the output by weight of charcoal may be increased by as much as 50-100 percent because powdered charcoal weighs approximately three times that of unpowdered charcoal on a volume for volume basis. Therefore, the conversion factor will have to be adjusted accordingly.
Once the weight of a charcoal bag or tin has been determined a consumption figure by individual end uses could be obtained in a similar fashion to that described for fuelwood. However, conversion factors are required to convert the weight of charcoal back to roundwood equivalent and three basic problems arise; wood density, moisture content of the wood raw material and the method of conversion.
The density of the wood governs the yield of charcoal and therefore as explained above a given volume of charcoal will give different weights of charcoal. For example, 1 m3 of air dry (15 percent m.c.) wood will give the following weight of charcoal including fines for various species.
|
Species: |
Pines |
Average Tropical hardwoods |
Preferred tropical hardwoods charcoal species |
Rhizophora |
|
Weight of charcoal per m3 (kg) |
115 |
170 |
180 |
285 |
Moisture content also has an effect on the yield of charcoal; the drier the wood the greater the yield of charcoal. As a first approximation the yield of charcoal from silar wood at various moisture contents is as follows:
|
Wood moisture content % |
12 |
15 |
20 |
40 |
60 |
80 |
100 |
|
Yield as a percentage |
(a)100 |
93 |
76 |
59 |
44 |
38 |
35 |
|
|
(b) 108 |
100 |
82 |
63 |
47 |
41 |
38 |
Source: (Adapted from Earl, 1973)
Air dry wood at 12 percent moisture content will give approximately three times more charcoal than green wood (100 percent m.c.). Therefore, charcoal production could be increased by using suitably dried wood.
Lastly the method of production can affect the yield of charcoal considerably and the range for average tropical hardwoods at 15 percent moisture content can be from about 4,5 m3 per tonne produced in a metal retort where most of the fines are briquetted to 27 m3 per tonne at a 100 percent moisture content in a poorly designed earth kiln with no sale of fines. It is therefore important to know the method of production, the species and their moisture content in order to arrive at a meaningful roundwood conversion factor. Most of the charcoal produced in developing countries is produced in earth kilns and the conversion factor can vary from about 10 m3 per tonne of charcoal up to 27 m3 per tonne depending on the moisture content, species and skill of the operator. Therefore, in order to determine the roundwood equivalent, the production method should be known - earth, portable steel kiln, brick kiln, a retort, etc. Then local conversion factors may be worked out from actual observation, knowing the species and their moisture content. However, the following conversion factors are given as a guide:
Conversion factors per tonne of charcoal sold. 1 (Average volume 1.4 m3/t at 15% m.c.) 2
1 It is assumed that the fines are briquetted in the retort.
2 With softwoods about 60 percent, more volume is required per tonne of charcoal and with dense hardwoods such as mangrove about 30 percent less volume is required.
|
|
Units m3 n e | |||||
|
|
Moisture Content | |||||
|
Kiln type |
15% |
20% |
40% |
60% |
80% |
100% |
|
Earth kiln |
10 |
13 |
16 |
21 |
24 |
27 |
|
Portable steel kiln |
6 |
7 |
9 |
13 |
15 |
16 |
|
Brick kiln |
6 |
6 |
7 |
10 |
11 |
12 |
|
Retort |
4.5 |
4.5 |
5 |
7 |
8 |
9 |
