5. Parameters, Units and Conversion factors

To identify the biofuel quality certain properties must be described adequately. 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 theses sources and commodities. That must be evaluated in terms of energy parameters, using standard units. This standardisation in the recording and presentation of original units is a primary task of the 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).⁵ Additionally, when statistically recording charcoal consumption, charcoal productivity has to be taken into consideration.

5.1 Properties of biofuels

The most relevant biofuel properties in practice are moisture, energy content, mass, volume and density as well as shape and particle size and total ash. Data for the main biofuels is given in Table 9.

The importance of other biofuel properties (i.e. content of different elements, ash fusibility) depends on the type of solid biofuel, the specific conditions at the combustion plant, the emission control etc. For most of the currently used woodfuels those properties have no significant relevance for the use of biofuels and should only be taken into consideration in particular circumstances.

5.1.1 Moisture

The moisture of solid biofuels varies widely, since the moisture of woodfuels depends on the time of harvesting, the location, type and duration of the storage and the fuel preparation. It varies from less than 10% (wood processing industry by-products) up to 50% (forest wood chips). The moisture is relevant not only for the calorific value but for the storage conditions, the combustion temperature and also the amount of exhaust gas.

Two methods (dry basis and wet basis) are commonly used to specify the total moisture. It is important to distinguish between them, especially when total moisture is high.

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 the basis on which total moisture is measured. Most biofuel moisture is measured on a dry basis.

Table 10: Energy content and concentrations of some elements in untreated biomass compared with coal

Source: Smith, K.R.; Kaltschmitt, M.; Thrän, D. 2001 [19].

5.1.2 Energy content

In most practical applications the energy content of biofuel is best described by the net calorific value. This is greatly influenced by the total moisture of the biomass as well as total hydrogen of the fuel. The actual net calorific value of biomass containing a known percentage of water can be calculated from the net calorific value of the absolute dry biomass, which is available from the literature. In equation 3 H_u (w) describes the net calorific value (in MJ/kg) of the biomass at a specific total moisture, H_u (wf) the net calorific value of the fully dry biomass, and w the total moisture (in %). The constant '2.44' results from the evaporation energy of water.

(3)

Figure 7 shows that the net calorific value of wood decreases from approximately 18.5 MJ/kg with increasing total moisture. The net calorific value is zero at total moisture of approximately 88%. Normally the total moisture of air-dried wood is between 12 and 20% yielding a calorific value of 13 to 16 MJ/kg. Freshly harvested wood is characterized by a total moisture of about 50% or more. A low net calorific value is the result.

Source: Smith, K.R.; Kaltschmitt, M.; Thrän, D. 2001 [19].

Figure 7:. Net calorific value of wood depending on the total moisture.

5.1.3 Mass, Volume and Density

Two basic relationships for bioenergy evaluation are introduced as follows, bearing in mind that both the calorific value and density depend mainly on the moisture of the woodfuel.

The main parameters of interest are:

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 are 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. Although the stere or stacked volume, usually considered as equal to 0.65 solid cubic metres, has been widely used in the past when measuring woodfuel volume, the main units currently used are solid volume units, usually cubic metres (CUM). Wood chips and pellets are generally measured in bulk volume units, usually in cubic metres (CUM). Table 11 contains typical conversion factors of mass, solid volume and bulk volume for fuelwood.

Table 11: Conversion factors for fuelwood accounting

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 total moisture on the wood density. The more water per unit weight, the less fuelwood. Therefore, it is imperative that the total moisture be accurately specified when fuelwood is measured by weight. There are two different types of density relevant for the use of solid biofuels:

particle density describes the density of the material itself and is relevant for the combustion process (i.e. evaporation rate, energy density etc.), some feeding aspects (i.e. for pneumatic equipment), and storage. The particle density of the usual species used for fuelwood are around 650 and 750 kg/CUM. The particle density can only be varied by producing compressed biofuels and is used to describe the quality of those products (i.e. high particle density is an indicator for a high pellet quality).

bulk density is defined as ratio of dry material to bulk volume and is relevant for the volume needed for transportation and storage. It is very important for trading and supply. The bulk density varies widely. Typical bulk densities of biofuels are given in Table 12.

Table 12: Typical bulk density of biofuels

5.1.4 Total ash and ash melting temperature

The total ash of solid biofuels depends on the type of biomass and the impurities. It is relevant for the calorific value and determines if the biofuel is fit for use in particular combustion plants. The total ash is always measured on a dry basis, which refers to the solid residue remaining after complete combustion. While the total ash of fuelwood is generally around 1%, some species of agrofuels can register a much higher total ash. 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% total ash will have 3% less energy than biomass with a 1% total ash.

For certain biofuels the ash melting temperature is a relevant driving force for combustion processes, since a high process temperature initiates ash melting and slag expansion, resulting in plant breakdown and high maintenance. Low ash melting temperatures are characteristic for most of the herbaceous biomass and energy grain, while woody biomass is normally not affected by ash melting problems.

5.1.5 Shape and particle size

Particle shape, particle size and particle size distribution of solid biofuels are relevant for transportation, and handling at the conversion plant. In practice the shape and size varies widely i.e. between milled biofuels (i.e. wood flour), compressed biofuels (i.e. straw pellets), cut biofuels (i.e. chips) and baled biofuels (i.e. straw bales). Those different forms need specific equipment for production, transportation, storage, feeding and combustion. A wide particle size range can lead to trouble in fully-automated feeding systems by bridging, obstruction or adhesion processes.

5.2 Charcoal productivity

When statistically recording the conversion from fuelwood (or woodfuels) to charcoal, three principal aspects must be dealt with: wood density, moisture of the wood, and the means of charcoal production. The yield of charcoal from fuelwood using different types of kilns is given in Table 13 (FAO, Woodfuel Survey, 1983). FAO, in its statistics (FAOSTAT), uses a conversion factor of 165 kg of charcoal produced from one cubic metre of fuelwood.

Table 13: Fuelwood requirement for charcoal production (CUM/ton of charcoal)

5.3 Conversion factors

The main factors to be used for bioenergy accounting, covering the various types of biofuel and considering the usual information available from primary data sources, are shown in Table 14. It presents the values for density (necessary when only the biofuel volume is given) and the calorific value for typical moisture. It should be noted that the objective is to obtain the energy worth of a mass or volume flow of some biofuel, so expressions (1) and (2), already presented above, must be used. However, taking into account the substantial variations in calorific value and volume with moisture, it is advisable to express the values of biofuels in a dry and ash-free basis, especially for accounting in energy balances.

Table 14: Basic parameters in accounting biofuels

* for black liquor accounting as woodfuel, it can be assumed that from one ton of chemical cellulosic pulp production, an amount of liquor equal to 2.27 CUM of woodfuel, in energy terms, results.
Source: FAO, 1997 [10].

⁵ See the International Bureau of Measures at http://www.bipm.fr/en/home/