The classification system requires a detailed terminology of the biofuel sources, which considers not only the kind of biomass (woody biomass, herbaceous biomass etc.) but also its origin (i. e. logging residues, energy plantation wood, whole trees). This information mentions important conclusions on the biofuel properties (i. e. the ash content of forest wood mainly depends on the amount of bark).

However, besides the terminology, some biofuel properties have to be further classified. To identify the biofuel quality, the shape and size, density, moisture content and ash content of the parameters are reckoned as the most relevant (Rosch and Kaltschmitt, 2001; Rijpkema, 2001). These are further explained here. The importance of other biofuel properties (i. e. content of different elements, ash melting behavior) depends on the type of solid biofuel, the specific conditions at the combustion plant, the emissions control etc. For most of the currently used woodfuels, these properties have no significant relevance and thus should be taken into consideration only under particular circumstances. Approaches to quality assurance are discussed in the next section and exemplified through the case of straw.

Shape and size. The mechanical properties of solid biofuels are relevant for trans­portation and reaction at the conversion plant. In practice the shape and size vary 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 types of fuels need specific equipment for production, transportation, storage, feeding and combustion. For example, the trouble-free handling of chips is limited by a certain amount of over-sized particles as well as a certain amount of very fine particles (dust). A wide range of particle sizes can cause trouble in fully automated feeding systems due to bridging, obstruction or adhesion.

Density. There are two different types of density which are relevant for solid biofuels. The 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 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). The bulk density is defined as the ratio of dry material to bulk volume and is relevant for the volume needed for transportation and storage.

Moisture content. The moisture content of solid biofuels varies within a wide range For example, the moisture content of woodfuels depends on the time of harvesting the location, type and duration of the storage and the fuel preparation. It varies frorr less than 10 per cent (residues from wood processing industry) up to 50 per cent (forest wood chips). The moisture content is relevant not only for the heating value but also for the storage conditions, the combustion temperature and the amount ol exhaust gas.

Ash content. The ash content of solid biofuels depends on the type of biomass anc the impurities. It is relevant for the heating value and to decide whether the biofuel i: appropriate for use in particular combustion plants.