Energy sources and commodities may be measured by their mass or weight or even volume, but the essential factor is the energy content related to these sources and commodities. That energy worth must be evaluated in terms of energy parameters, always using standard units. This standardization in the recording and presentation of original units is a primary task of energy and forestry statisticians before quantities can be analyzed or compared. It is recommended that for international reporting, and as far as possible in national accounting procedures, energy and forestry statistics should use the International System of Units, officially abbreviated to SI. Two basic relationships for bioenergy evaluation are introduced as follows, bearing in mind that both the heating value and density depend mainly on the moisture of the woodfuel.
(1)
(2)
The following paragraphs cover particular parameters of interest.
· Mass: some woodfuels, such as charcoal and black liquor, are measured in units of mass. The principal units of mass used to measure energy commodities include the kilogram and the metric ton. The metric ton (1 000 kg) is the most widely adopted.
· Volume: units of volume are typical units for round wood and fuelwood measurement. The basic SI units of volume are the litre and the kilolitre, which is equivalent to the cubic metre. The stere or stacked volume, usually considered as equal to 0.65 solid cubic metre, has been widely used in the past when measuring woodfuel volume. Nowadays, FAO, as well as foresters and other related experts, prefer to measure timber and fuelwood using solid volume units, usually in cubic metres (CUM).
· Density: the density of wood, i.e., the weight per unit of volume, varies widely between different wood species and types. The usual species used for fuelwood are around 650 and 750 kg/CUM. It is important to observe the influence of the moisture content on the wood density. The more water per unit weight, the less fuelwood. Therefore, it is imperative that the moisture content be accurately specified when fuelwood is measured by weight.
· Moisture: the amount of water in biofuel affects, in a decisive manner, the available energy from each biofuel. Two methods (dry and wet basis) are commonly used to specify the moisture content, depending on the adopted basis used to account for the water mass. It is important to distinguish between them, especially when moisture content is high.
(3)
(4)
In the above expressions, wet weight refers to the burned condition and dry weight refers to wood after a standardized drying process. It is important to state on which basis the moisture content is measured. Most biofuel moisture is measured on the dry basis, but some is measured on the wet basis.
· Ash content: another important factor of biofuel energy content is ash content, always measured on the dry basis, which refers to the solid residue remaining after complete combustion. While the ash content of fuelwood is generally around 1%, some species of agrofuels can register a very high ash content. This affects the energy value of the biofuels since the substances that form the ashes generally have no energy value. Thus dry woodfuels with a 4% ash content will have 3% less energy than biomass with a 1% ash content .
· Heating value (or calorific value): biofuel is essentially a material for burning as fire or as a thermal source of energy. The amount of thermal energy stored can be measured through the heating value or calorific value of fuels. The higher heating value (HHV), or gross calorific value (GCV), measures the total amount of heat that will be produced by combustion. However, part of this heat will be locked up in the latent heat of the evaporation of any water existing in the fuel during combustion. The lower heating value (LHV), or net calorific value (NCV), excludes this latent heat. Thus, the lower heating value is that amount of heat which is actually available from the combustion process for capture and use. The higher the moisture content of a fuel, the greater the difference between GCV and NCV and the less total energy will be available, as shown in Figure 2. These parameters are usually expressed in megajoules per kg (MJ/kg) or kilojoules per kg (kJ/kg).
Figure 2. Effect of moisture (wet basis) on heating value
· Charcoal productivity - When statistically recording the conversion from fuelwood (or woodfuels) to charcoal, three principal aspects must be dealt with: wood density, moisture content of the wood, and the means of charcoal production. The yield of charcoal from fuelwood, using different types of kiln, is presented in Table 5 (FAO, Woodfuel Survey, 1983). It is usually accepted that 165 kg of charcoal is produced from one cubic meter of fuelwood.
Table 5. Fuelwood requirement for charcoal production (CUM/ton of charcoal)
Kiln type |
Fuelwood moisture (%, dry basis) | |||||
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 |
· Agrofuels - The energy values of agricultural by-products are determined by moisture content and ash content. Data for these energy sources are rarely collected directly but are derived from crop/waste or end-product/waste ratios. Given the importance of the use of bagasse (the fibrous cane residues from the production of sugar from sugar cane), possible estimation procedures will be outlined for this case. Bagasse is used as a fuel mostly for the sugar industry's own energy needs, but surpluses are sold to the public grid in many sugar-producing countries. Table 6 presents data for typical agricultural by-products.
Table 6: Energy data for selected agricultural by-products
Product |
Moisture |
Approx. Ash content |
LHV |
(%, dry basis) |
(%) |
(MJ/kg) | |
Bagasse |
40-50 |
10-12 |
8.4-10.5 |
Groundnut shells |
3-10 |
4-14 |
16.7 |
Coffee husks |
13 |
8-10 |
16.7 |
Cotton husks |
5-10 |
3 |
16.7 |
Coconut husks |
5-10 |
6 |
16.7 |
Rice hulls |
9-11 |
15-20 |
13.8-15.1 |
Olives (pressed) |
15-18 |
3 |
16.7 |
Oil-palm fibres |
55 |
10 |
7.5-8.4 |
Oil-palm husks |
55 |
5 |
7.5-8.4 |
Corncobs |
15 |
1-2 |
19.3 |
Rice straw and husk |
15 |
15-20 |
13.4 |
Wheat straw and husk |
15 |
8-9 |
19.1 |