Pelletising

There are other reconstituted wood fuels such as briquettes which are larger and suitable for either open fires or log boilers, but less commonly used.

Quality and standards

Wood pellets have a number of characteristics which make them suitable for woodfuel:

• Consistent density and heat content
• Low moisture content
• Consistent size
• A clean fuel which should be free from dust, bark and slivers

These characteristics make pellets easier to handle, cheaper to transport and more suited to automated handling systems than other types of woodfuel.

Quality criteria are set out in European standards (CEN 355). Within these standards are specifications for wood pellets.

Production process and models

Wood pellets cost more to produce than other wood-based fuels: the process is more energy intensive and requires a high capital investment in plant and machinery.

Pellets are usually formed from sawdust, or other suitable sawmill by-products. This is less production intensive and more energy efficient than using freshly felled timber.

Dry sawdust of a consistent moisture content and a uniform particle size is generally required. This makes the process less energy-intensive: it takes approximately 10% of the pellet's own energy content for fresh wet sawdust, and 2% if the sawdust is dry.

Wood pellets are produced by forcing dried sawdust (or other sawmill and forestry by-products) through holes in a rotating dye, to form tightly sealed pellets. Friction generated during manufacture creates heat that releases the natural lignins within the wood which act as a glue to bind the pellet together.

The pellets are cut to length as they emerge from the die and then allowed to cool before being stored in bulk or bagged for delivery.

This can be a small or large scale process.

Large scale wood pellet production

For large scale production, reliable supplies of local, good quality sawdust or other suitable raw material are essential. Consequently large scale pellet production plants are only appropriate in certain locations. There are currently four wood pellet production plants in Scotland with another in the development phase.

Most pellet production is by large scale industrial pellet manufacture, integrated with a large sawmill and/or heat user. This is to make the project economics viable.

An investment of £300,000 upwards is required.

Small scale pellet machines

Small-scale pellet machines are available with an output of around 200-300 kg per hour. These cost anywhere from £50,000 to £150,000, depending on the level of quality required and the level of automation.

At this scale, it is easier to source raw material and supply pellets locally, keeping both production and transport costs down, and ensuring the least environmental impact. They are most economically viable when you have to pay to get rid of waste wood.

It is possible to convert an existing animal grass-feed mill to make wood pellets, as the equipment needed is similar and many feed factories have facilities for drying the feedstock as well. This could be a profitable diversification for feed mills, particularly during the winter when grass is not growing. The capital costs of conversion are much less than for a dedicated plant.

Pellets produced from smaller scale machines are not likely to be of high enough quality for commercial distribution.

Storage and delivery

Pellets should be stored in dry conditions. If exposed to a damp environment, pellets may absorb water, subsequently crumbling and breaking down to sawdust.

The durability of wood pellets is important, especially if the delivery process to the end user involves several handling stages. Pellets which break up into sawdust during handling or storage can cause difficulties with both wood fuel boilers and their fuel feed systems.

For blown pellet delivery, ensure the pressure is not too high as this can lead to pellet damage, and potentially a high proportion of dust which can damage boiler equipment.