Wood Chips For Heating

It is important to have an understanding of the practicalities of wood fuel supply for both domestic biomass boilers and commercial heating systems, and where interactions between supply and utilisation occur. The entire fuel supply chain will ultimately depend upon the requirements of the market. 

Production processes do not necessarily produce fuel of a specification needed by the end user, either for particle size or moisture content. As a result, further well controlled processing at some kind of wood-fuel depot may be required.

Where wood chip is being supplied this will often mean that the wood is best air dried before processing as drying chipped material can present its own difficulties.

Choice of harvesting equipment will depend on terrain and availability. Farm tractor based equipment will be appropriate on less steep sites. More specialist forestry equipment will be required on difficult sites with bumpy terrain. The main elements of any wood fuel supply chain are as follows, but not necessarily in this order:

  • Fell trees; if not already done.
  • Primary process (e.g. cross cut and branch removal).
  • Extract to processing point.
  • Store and dry.
  • Process to logs or chips.
  • Transport to end user.

These elements need to be tied to the resource and product specification.

Chipping Wood Residues

Chippers are used to reduce the size of wood residues for ease of handling and to fit boiler feed systems. There are 3 main types of chippers:

The disc chipper consists of a heavy rotating disc with usually 2, 3 or 4 blades mounted on the face of the disc. Material to be chipped is fed in, towards the blades. The rotating knives cut woody material into chips as they pass an anvil or fixed knife. Blower paddles on the back of the disc accelerate the chips up a spout where they are discharged.

The quality of chips varies between different makes of chippers and it is important to select a machine that will make good chips from the material at hand.

The drum chipper has a rotating drum with 2-4 knives inserted on its circumference. Wood to be chipped is fed in and cut against an anvil, chip size can be adjusted. Separate blowing provision is required.

The screw cone chipper consists of a conical screw with a sharp edge. When rotated, the screw pulls the tree into the chipper and cuts it into large chips or chunks. This type of chipper is unsuitable for twiggy material. To chip for fuel use it is essential to have a consistent particle size. Chippers are available with a special grading device to restrict chips to a suitable size at a small extra cost.

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Fuel Handling

Wood chips do not flow and therefore cannot be pumped. Most systems propel the chips horizontally by means of a conveyor or auger. Wood chips can be delivered to customers using a number of different systems depending on location, vehicle access and size of load.

Most wood chips are delivered using a tipper truck or tipping farm trailer, particularly for larger loads. ‘Big bags’ similar to those used in the building and agricultural trades would suit some systems with appropriate hoppers. Large bins delivered by ‘hook’ lorries (called Ro-Ro Bins) can act as a delivery container and fuel feed hopper if fitted with a scraper or moving floor out-feeder system.

The storage system for chipped wood fuel being fed into the boiler for smaller systems is known as a hopper or silo. For a larger system a fuel store room separated from the boiler room by a fire-proof partition is used. A screw auger conveyor system moves the fuel to the boiler.

The size of hopper depends on the availability of space adjacent to the boiler and the level of wood fuel being consumed. A hopper containing 5m3 of wood chips will provide 5-15 days of running for a farmhouse sized unit if the chips are fairly dry (25% moisture content). A careful estimate must be made of the heating load of a particular site and the hopper sized to reflect the life-style requirements of the site owners or the building services available at larger commercial sites.

Production

The efficiency of wood fuel is closely linked to the moisture content of the wood when it is burnt in a boiler or burner. Therefore the key question regarding using wood as a fuel is “how is the moisture content of the raw wood to be reduced?”

There are three main approaches:

  • Traditionally logs are ‘seasoned’ by being left out or undercover for a summer and dry naturally. Moisture contents of about 25% are typically achieved, less if the logs are well stacked and covered.
  • An alternative approach is simply to burn the ‘green’ wood (i.e. freshly cut) directly. In this case the moisture is reduced in the boiler or fire. However, this greatly reduces the calorific value of the fuel, makes good combustion difficult unless the boiler is specifically designed for this fuel, such as the Binder Step-Grate machines (USRF), needed to avoid greater accumulation of tar deposits in the flue/chimney.
  • Reduce the moisture content close to where the wood is produced by forced drying. This happens in a pelleting plant where the raw material is finely divided, force-dried and compressed. The end product is a dense, low moisture content fuel (typically less than 10%), which is convenient to transport and handle.
  • The downside of this fuel processing is the cost involved both in terms of money and energy input. Despite this, pellet fuel is a fast growing sector in Europe and North America and wood pellets are now being made in the UK such as at Balcas, Fermanagh, Northern the UK. A new pellet mill opened at Brookridge Timber in Devon, in April 2006.

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Calculation of Moisture Content (MC)

There are two different ways of specifying the moisture content of wood; on a ‘wet’ or ‘dry’ basis. The moisture content of a single piece of wood will be different using the two methods and so care must be taken to be clear which method is being used.

The moisture content of a piece of wood is given by the mass of water contained in the wood divided by the total mass of the piece of wood as found.

Example: A sample of wood chip has a mass of 10kg. It is dried to an oven-dried condition, and then it has a mass of 8kg. The mass of ‘wet’ wood (10kg) minus the mass of oven-dried wood (8kg) = mass of water (2kg).

2kg water / 10kg wet wood x 100 = 20% moisture content.

Transportation

The transport cost of low bulk density materials is a significant factor, which will affect the cost of fuel delivered into a fuel store. A 20m3 agricultural trailer will contain about 3.5-5 tonnes of wood chips at 25% moisture depending on the species of wood.

Wood chips bought commercially are likely to cost about a third the current price of heating oil for the same energy. To make this economic, the supply of wood chips should be as local as possible – ideally within a 10-mile radius. This is also desirable from an environmental perspective, reducing the amount of diesel used to deliver the wood fuel.

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