Wood Fuels

Keywords -Bioenergy, Wood Fuel, Biomass, Wood Chips, Wood Pellets, Energy Crops

Wood fuels are available in many different forms. Some of the more common types are reviewed in this section.

Common forms of wood fuels include:

• Wood chips
• Wood pellets
• Firewood logs
• Sawdust
• Bark
• Raw wood wastes

Wood energy can be generated from industrial wood wastes, forest residues and energy crops.

Wood Wastes - Wood wastes or by-products from wood processing industries e.g. chips, bark and sawdust. These residues are used in boardmills as feedstocks for production and within sawmills and boardmills to provide heat for drying or space heating and to produce steam for the manufacturing process. Additional residues are available from forestry and industrial processes for the generation of high quality wood fuels. High quality wood fuels such as wood chips and wood pellets can be used for domestic buildings, commercial sized buildings and to drive process heat in industry.

Forest residues - these consist of the tree tops and branches remaining after timber is harvested. Some forest residues need to be left on the forest floor to decompose and return nutrients to the soil and also to act as brash mats, which allow machinery to travel across soft ground. However, much of this material could be harvested with suitable machinery and used as a renewable fuel for energy production.

Short Rotation Forestry (SRF) / Energy Crops are grown specifically for energy purposes. Typically includes the production of wood fuel from trees with high juvenile growth, which are planted at close spacing and harvested on short rotations (typically every three years). Species such as Willow are ideal for SRF, as they are easy to establish, fast growing and suitable for a variety of sites and resistant to pests and disease. Land for short rotation forestry is likely to come from: non-rotational arable set aside land and land that is not in arable use - i.e. beef or sheep production. Changes in the agricultural sector in Ireland due to CAP reform mean that more land will become available for forestry and energy crop cultivation.

Download Short Rotation Coppice Willow Best Practice Guidelines 2010 - Teagasc and Agri-Food and Biosciences Institute of Northern Ireland

Wood fuels (pellets/chips)

Wood pellets and chips are the two most suitable fuels for automatically fired heating systems in large buildings. The bio-fuels to be used in this programme should comply with I.S. CEN / TS 14961:2005 or equivalent. Post consumer wood waste is not acceptable as a fuel for this programme.

Selection

Pellets are a standardised fuel that are made by pressing dry shavings or saw dust. The production process does not use chemical additives - only high pressure and steam. To improve the mechanical stability of pellets often 1-3% of organic additives, such as potato starch, corn flour or waste liquor from the paper and pulp industry are added. Depending on the moisture, the energy content of pellets lies between 4.7-4.9 kWh/kg - 2 kilos of pellets therefore have a slightly lower calorific value than a litre of extra light fuel oil (10 kWh).

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 sources for chips are available:

• Chips from the sawmill industry: should have a maximum moisture content of 30% and be of uniform quality and size. They are suitable for boilers in large buildings.
• Forest chips: Given their water content of between 40% to 60%, they can only be used in large boilers. 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.

It is important that the moisture content of the wood chip is compatible with the boiler in which it is being used.

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. Such boilers have an electronic control system that adjusts the combustion parameters to the selected fuel. It is important that the feed system is suitable for handling both fuels. As chips (unlike pellets) are not generally blown in, the store should be designed to enable the fuel to be delivered by tipper truck if chips are expected to be used. The advantage of above ground silos for pellets is their lower cost. The following table highlights the advantages of the two fuels and compares them.

Supply

Wood fuels are generally delivered by truck or tractor trailer that tips the fuel into the opening in the store. Pellets are usually delivered in tankers. As 1 cubic metre of pellets has four times the calorific value of 1 cubic metre of dry chips, the frequency of deliveries is much lower than for chips. As a result, pellet heating systems may be a better solution in urban areas where the traffic plays an important role. There should be enough room for the delivery vehicle to turn.

Storage

The size of the fuel store depends on many factors: anticipated fuel requirements, fuel type, reliability of deliveries, space available, delivery vehicle capacity etc. In existing buildings adjusting the fuel delivery intervals to the available storage space is cheaper in most cases than putting in a new store outside the building.