Biogenous fuels

Woodchips

The term woodchips refers to mechanically processed wood particles, ranging in size from 1 to 100 mm. As far as quality standards are concerned, ÖNORM M7 133 ("Woodchips for energy generation: quality and testing requirements") applies.

The main quality criteria for woodchips are:

• chip size: only the "fine" (smaller than 30 mm) and "medium" grades (below 50 mm) are suitable for small-scale installations;
• water content: this determines the energy content of the fuel on the one hand and its storability on the other;
• bulk density: this indicates the weight per cubic metre (bulk volume) and depends on wood type, particle shape, degree of compaction and water content.

The price of woodchips largely depends on their water content, as a high water content reduces the energy content. Moisture is therefore just as important as chip size in determining the price of woodchips.

In the following table, this is reflected by the percentages in the column headed "relative value". Quality class W35 (limited storability), which designates woodchips that have been stored over one summer, was taken as a basis.

Pellets

Pellets are wood wastes from the woodworking industry (wood shavings, saw dust, sanding dust) formed into cylindrical shape under high pressure with no bonding agent added. Typically, pellets are 6-8mm in diameter and 5-30 mm long (see figure 5). The maximum water content is 8%. The relevant quality standards are laid down in ÖNORM M 7135.

Fig. 5: Pellets

Pellets are therefore the first pumpable wood-based fuel: they can be supplied by tanker, just like heating oil. They are also just as convenient to use, with the added advantage that pellets are a domestically produced, and therefore dependable, energy carrier.

As a result of the pressing process, pellets have a very high energy content (4.3 to 5.0 kWh/kg at a density of 1.2 ± 0.1 t/m³). The energy content of pellets is therefore about 3 times that of woodchips, reducing the required storage space accordingly. Although the energy content of pellets is about 1/3 that of heating oil, they require practically the same amount of storage room as they do not constitute an environmental hazard and therefore need not be stored in a safe tank. However, it is essential to keep them dry. Appropriate storage is only possible if a dry cellar room is available; wooden floor grids may be used to avoid direct exposure to moist cellar floors. Pellets are exclusively produced by the domestic woodworking industry and sold by fuel dealers. This helps to allay many customers' fears regarding security of supply. Furthermore, homogeneous pellet structure can be ensured thanks to industrial production.

Given the precise fuel specifications, boilers can be optimally adjusted, which makes the investment more cost-effective and, most importantly, makes small-scale installations safer to use.

Pellets can currently be purchased in bulk (i.e. delivered by tanker) at a price of ATS 2 per kg. In provinces where pellets have been in use for longer periods of time, this price has dropped to as little as ATS 1 per kg.

For more detailed information on pellets, please contact the Austrian Pellets Association (office@pelletsverband.at)

Other biogenous fuels

In addition to wood-based fuels (logwood, woodchips and pellets), the following other plant-based energy carriers are subsumed under the umbrella term "biomass":

• straw;
• biogas und sewage gas;
• residues from fruit processing (e.g. stones, husks, …);
• wood wastes from the woodworking industry (saw dust, sanding dust);
• biogenous residues from the woodworking industry (e.g. from the pulp industry);
• biogenous fuels (biodiesel from oil seeds such as rape, methanol from cereals).

These particular energy carriers are currently mainly used industrially.

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