Chips are small pieces of wood that are 5-50 mm long (measured in the direction of the fibre). There may also be some longer twigs and finer material among them. The quality of the chips depends on the raw material and the chipping process (sharp chipper blades).
Two main sources for chips are available:
- Chips from the sawmill industry: should have a maximum water content of 30% and be of uniform quality and size.
- Forest chips: Given their water content of between 40% to 60%, they can only be used in large boilers unless they are dried.
Large pieces of wood or high humidity can cause problems with boiler operation. For this reason ensuring the quality of woodchips is an essential precondition for their successful use as fuel.
Good quality wood chips with a moisture content of 30% will have a calorific value of approximately 3,700 kWh/tonne.
Fresh cut trees and their residues will have moisture contents in the region of 55%. For smaller boilers it is advisable to have moisture contents of 30% or lower. An option with fresh cut trees is to leave them dry naturally over a season after felling. The wood can be artificially dried but this has energy implications for the drying process and can result in higher per tonne costs.
Pellets and chips have various advantages and disadvantages that have to be weighed up. Which fuel is used will depend very much on local conditions. Preferably systems should be installed, that can use both fuels and can therefore respond flexibly to the future market situation. Some of the main advantages and disadvantages are outlined below.
Logging residues can be chipped using specialised chipping equipment. Fresh forest chips would typically have moisture content of 55%. Allowing it to dry by storing it either in the forest or at a different location, will bring the moisture level down Technology now allows for whole tree chipping and stump harvesting for energy.
To see video files of forest chip harvesting, visit www.woodenergy.ie.
Wood chip supply infrastructures in Ireland are developing rapidly. In an ideal situation, wood chips should be sourced as locally as possible. This has benefits for local jobs and the economy. A list of known wood fuel suppliers, including wood chips, is provided in the publications list below.