Wood pellets are a form of wood energy that is clean-burning, convenient and cost-effective. They are made by drying and compressing sawdust and wood shavings.
The sawdust and shavings used usually comes from residue at wood processing sites (i.e. sawmills) but residue from forest harvesting sites (i.e. log landings and in-forest cutover sites) may also be used.
The wood pellet industry is well established in Europe and North America and over 1.2 million pellet fires are in use throughout North America. However, the wood pellet industry is relatively new to New Zealand, but it is fast gaining popularity. The first pellet mill was established in Canterbury, about a decade ago, and there are now mills in Huntly, Rotorua, Taupo, Nelson, Christchurch, Tiumaru, Dunedin and Invercargill.
How are wood pellets made?
Wood pellets are produced using waste material such as untreated sawdust and shavings from local sawmills. This woody material is dried to and compressed into 6mm diameter pellets under high pressures and temperatures. This process solidifies the pellet, by using the natural lignin found in wood, no artificial additives or binders are required. Pellet moisture content varies between <15% (for industrial grade pellets which are not produced in New Zealand) and <8% (for pellets rated as ‘Category A1 Premium Pellets’).
Energy from wood pellets
The ratio of energy used for processing compared to the energy value of the produced pellets depends on whether the material used is wet or dry. The energy yield ratio is 20:1 for dry material and 7:1 for wet material.
- Example 1: To produce 1 tonne of wood pellets from dry shavings requires approx 250kWh of electrical energy. One tonne of wood pellets contains 5000kWh of energy. For every 1 unit of energy put in there are 20 units out. This gives an energy yield ratio of 20:1, or 5%.
- Example 2: To produce 1 tonne of wood pellets from wet sawdust (50% m.c), requires 300kWh of electrical energy and 400kWh of heat energy. One tonne of wood pellets contains 5000kWh of energy. For every 1 unit of energy in there are 7 units out. This gives an energy yield ratio of 7:1 or 14%.
Pros and cons of using wood pellets
Wood pellets are a highly standardised and compact fuel, this allows for cost-efficient transportation and storage. The design of pellet stoves ensures almost complete combustion, resulting in the cleanest burn and lowest ash of any solid fuel. Low emissions mean there is a very low impact on air quality.
However, wood pellets can sometimes be more expensive than other forms of wood energy.
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- Wood pellets interest group website