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What are wood pellets?

Wood pellets are a type of biomass 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 8-9%) that allows them to be burned with a very high combustion efficiency.
The heating value of 2 pounds of wood pellets coresponds to the heating value of 0.2 gallons of heating oil.

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. The wood pellet stoves and wood pellet boilers on the market today can reach efficiencies up to 95%. Okofen, an austrian brand, even has a condensing boiler with an efficiency grade up to 103%.

Wood pellet fuel standards

According to the Pellet Fuels Institute (PFI) there are four standards for wood pellets:

Utility
Standard
Premium
Super Premium

Strangly enough heating value is not required in these standards. It is however required for pellet manufactures that produce in compliance with the standards of the Pellet Fuels Institute to print the mean higher heating value in BTU per pound ass well as the ash content on the bag label using a scale bar respresenting the mean value +/- 2 standard deviations. The label on the bag must indicate the PFI standard and the type of materials and additives used.

Energy content of wood pellets

The energy content of wood pellets is approximately 4.7-5.0 MWh/tonne (~7450 BTU/lb).

High-efficiency wood pellet stoves and boilers have been developed in recent years, offering combustion efficiencies of over 90%. Wood pellet boilers - having less control over the rate and presence of combustion compared to liquid or gaseous-fired systems - are particularly suited for hydronic systems due to the hydronic system's greater ability to store heat.

Production of wood pellets

Pellets are produced by compressing the wood material (sawdust and/or wood shavings) 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, 10 mm or larger). The high pressure of the press causes the temperature of the wood to increase greatly, and the lignin plastifies slightly forming a natural 'glue' that holds the pellet together as it cools.

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 compensated for in the press regulation. Recycled materials such particle board, treated or painted wood, melamine resin-coated panels and the like are particularly unsuitable for use in pellets, since they may produce noxious emissions and / or uncontrolled variations in the burning characteristics of the pellets.

The carbon footprint of wood pellets

Wood: short carbon cycle

Wood pellets have a small carbon footprint. Plants convert sunlight and carbondioxide into sugars. Oxygen is produced as a secondary product. Wood is made of these sugars and so it is a form of solar energy. Biomass is in fact portable, solid, solar energy. By burning wood in a pellet stove or boiler this energy is released in the form of heat. Carbon dioxide is produced and oxygen consumed.

Wood absorbs as much carbon when it is growing as is released when it is burnt. If the wood is harvested sustainably, e.g. Forest Stewardship Council (FSC) labeled wood, approximate the same amount is grown back as is cut. Therefore wood is assumed to be a carbon-neutral energy source.

The Gray energy of wood pellets

Pelletinfo Wood pellets are made by drying and compressing the raw material: saw dust. Unless these processes use carbon neutral green energy, wood pellets consume fossil energy or nuclear energy. Not only production but also transport produce fossil-carbon releases. The amounts of fossil fuels used for this purpose differ from one pellet factory to another, and according to the distance and manner of transport of the pellets. We call them 'gray energy'. Unless the factory is very energy inefficient and the pellets are transported over a long distance by truck, the gray energy of wood pellets is only a small fraction of the total embodied energy of the pellets.