The main variation in technology for wood fuelled boilers revolves around the choice of fuel – wood pellets or wood chips. This section sets out the difference in these two fuel types.

1. Wood Pellets

Wood Pellets are made up solely of natural, renewable raw material for which no forests have to be felled. Practically no sulphur dioxide, one of the main causes of acid rain and thus of forests dying, is emitted when pellets are burned. The ash is an excellent natural fertilizer for fields and gardens.

Wood pellets can be used to fuel pellet burning boilers especially designed for the purpose. Integral fuel hoppers store enough pellets for 1 to 3 days operation allowing the boiler to operate continuously and automatically, making them ideal for domestic usage. In addition, the ash pan only needs emptying between once a month and once a year. Wood pellet boilers are fully automatic and almost as convenient as using gas or oil. They are well suited to meet variable load demands and can be operated on a timer. Pellet boilers operate at high efficiencies of around 90%. Being a very dense fuel pellets require less storage space than logs or chips.

Wood pellets are made from the compression of sawdust, a significant waste product for many industrial processes. Normally, no other ingredients are required. Pellets are generally between 5 and 8mm in diameter and typically 25mm in length. They are 3 times as dense as wood chip and it is therefore more reasonable to transport them further - although minimising transport is of course ideal.

This product is consistent and therefore is suited to small boilers as the boiler mechanics can be reduced to suit the lighter physical requirements to pull the fuel into the combustion chamber.

Pellets have the following advantages over other types of wood fuel:-

• Less volume to transport and store (due to higher energy density)
• Fewer deliveries
• Consistent size and moisture content
• Less ash
• Lower emissions CO2 neutral
• Exempt from CO2 tax
• Pellets are dry and can be stored without degrading
• Flow like a liquid and can be used in automatic machinery
• Easier to handle
• Easier to ignite

Being a source of renewable energy, wood pellets are exempt from the Climate Change Levy.

2 . Wood Chip

As with wood pellets modern wood chip boilers can provide a high level of automation and convenience for wood fuelled space heating. Wood chip systems generally have an output of greater than 20 kW (suitable for a large farmhouse or larger) and are not cost effective or appropriate for typical domestic scale applications. Extensive fuel handling systems and fuel storage facilities are required for automated operation. It is important to be able to source a steady supply of woodchip with a consistent size and moisture content suitable for burning in a boiler, as not all chips are suitable for burning.

Wood chips are generally taken from forestry thinnings and crops within a small radius (around 10 miles) of the boiler. The wood is then left to dry for a period before being chipped.