Wood pellets

  Wood pellets are a type of wood fuel, generally made from compacted sawdust. They are usually produced as a byproduct of sawmilling and other wood transformation activities. The pellets are extremely dense and can be produced with a low humidity content (below 10%) that allows them to be burned with a very high combustion efficiency. Further, their regular geometry and small size allow automatic feeding with very fine calibration. They can be fed to a burner by auger feeding or by pneumatic conveying.

Their high density also permits compact storage and rational transport over long distance. They can be conveniently blown from a tanker to a storage bunker or silo on a customer's premises. As the price of heating with fossil fuels increases, more capacity for pellet heating has been installed. A large number of models of pellet stoves, central heating furnaces and other heating appliances has been developed and marketed since about 1999. With the surge in the price of fossil fuels in 2005, the demand has increased all over Europe and a sizable industry is emerging.

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Production

Pellets are produced by compressing the wood material which has first passed through a hammer mill to provide a uniform dough-like mass. This mass is fed to a press where it is squeezed through a die having holes of the size required (normally 6 mm diameter, sometimes 8 mm or larger). The high pressure of the press causes the temperature of the wood to increase greatly, and the lignin plasifies slightly forming a natural 'glue' that holds the pellet together as it cools.

Pellets conforming to the norms commonly used (DIN 51731 or Ö-Norm M-7135) have less than 10% water content, are uniform in density (density in excess of 1 ton / cubic meter, so they do not float if placed in water), have good structural strength, and low dust and ash content. Because the wood fibres are broken down by the hammer mill, there is virtually no difference in the finished pellets between different wood types. Pellets can be made from nearly any wood variety, provided the pellet press is equipped with good instrumentation, the differences in feed material can be compenstated for in the press regulation.

Pellets conforming to the above norms cannot contain any recycled wood or outside contaminants. Recycled materials such particle board, treated or painted wood, melamine resin-coated panels and the like are particularly unsuitable for use in pellets, since the may produce noxious emissions and / or uncontrolled variations in the burning characteristics of the pellets.

Energy output

  Pellet heating systems provide a low-net-CO₂ solution, because the quantity of CO₂ emitted during combustion is equal to the CO₂ absorbed by the tree during its growth. With the high efficiency burners developed in recent years, other emissions such as NO_x and volatile organic compounds are very low, making this one of the most non-polluting heating options available. One remaining problem is emission of fine dust in urban areas due to a high concentration of pellet heating systems. Electrostatic particle filters for pellet heaters have however been developed and considerably reduce the problem when installed as standard.

The energy content of wood pellets is approximately 4.8 MWh/ton (or about 17 MBTU/ton).

Potential Issues

• Due to the rapid increase in popularity since 2005, Pellet availability and cost may be an issue. This is an important consideration when buying a pellet stove or furnace.

Use in Europe

In Austria, the leading market for pellet central heating furnaces (relative to its population), it is estimated that 2/3 of all new domestic heating furnaces are pellet burners. In Italy, a large market for automatically-fed pellet stoves has developed.

See also

• Solid fuel
• Renewable heat

References

1. ^ Bioenergie: Großes Potenzial für Pellets-Märkte in Europa und weltweit 9.7.2007 Solarserver.de
2. ^ Pellets for small-scale domestic heating systems 5/2007, European Biomass Association Aebiom (Resmac project)