Chipping

Standards

It is essential that wood chips meet the criteria specified for the wood fuel system. If certain physical requirements aren't met, the chips will reduce the boiler's operational efficiency, cause blockages and even damage the system itself. Many chippers are designed for volume reduction and to assist handling and transport, where chip quality is unimportant, and so won't be appropriate for wood fuel systems.

The main criteria are: origin, size of chip, moisture content and ash content. These and other criteria are set out in European standards (CEN 355). Within these standards are specifications for wood chip. If a chipper is to be used for wood fuel systems, it should be able to produce chips to the required specifications.

Possibly most important is the absence of slivers within wood fuel. These long pieces can cause bridging within fuel feed systems, potentially causing a blockage.

Typically larger wood fuel systems will handle chips of a far wider size and quality range than smaller systems. Suppliers need to establish what market they are chipping for when purchasing equipment and processing wood for fuel.

Chippers

Chippers vary in size from the small hand-fed chipper for dealing with arboricultural waste to machines which can chip large diameter roundwood with outputs of over 100 tonnes an hour. Principally, chippers are used to reduce the size of wood residues to fit boiler feed systems.

Each type of chipper will have its own strengths and weaknesses. 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.

Your choice of chipper must take into consideration the many different types of feed mechanisms and different boiler requirements.

There are three main types of chipper:

• Disc chipper: A heavy rotating disc with two to four 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. Disc chippers will produce a lower quality of chip than a drum chipper and are often used by tree surgeons; the chip produced may not be of a suitable quality for smaller wood fuel boilers.

• Drum chipper: A rotating drum with two to four blades inserted on its circumference. Wood to be chipped is fed in and cut against an anvil. Chip size can be adjusted. Drum chippers require a separate blowing provision, produced either by the airflow of the blades themselves or a separate fan and can be fed either via gravity or a conveyor belt.

• Screw cone chipper: The screw cone chipper consists of a conical screw with a sharp edge that cuts against the inside of the housing. When rotated, the screw pulls the tree into the chipper and cuts it into large chips or chunks; there is, therefore, no separate mechanical feed mechanism required. The base of the blade generally includes a disk to generate the airflow to propel the chips outwards and may also incorporate a sliver breaker. Chip size is determined by the pitch of the screw and can be varied by replacing the blade with one of a different pitch. This type of chipper is unsuitable for twiggy material.

Note on blades

Chippers are very sensitive to metal and stones. Blades will have a relatively fast wear and will require changing at regular intervals to avoid the slowing of production and increased use of fuel for operation.

Contract Chipping

If you do not want to purchase your own chipping equipment then there are a number of companies in Scotland who provide contract chipping services, to find out about your local options submit an enquiry form here.

Training

Anyone using a wood chipper must undergo appropriate training. Free leaflets on using chippers, and information about training and certification, are published by the Health and Safety Executive's Agricultural and Forestry Advisory Group. You can also speak to members of your local Woodfuel Forum to see what type of training is available in your area.

Transport

Wood chips may be more easily handled, transported and stored than branches and brash, so chipping on the roadside makes sense. Small roundwood and logs have a higher bulk density, if stacked properly, and so should be transported before chipping.

Drying and storage

Drying the chips increases their calorific value and also reduces their bulk density which can, in turn, reduce transport costs.

There are two main approaches to drying:

• Roundwood and logging residue is left in the forest for a year or 18 months to dry before chipping into a shed
• Freshly harvested material can be chipped and dried later.

As wet wood chips can begin to degrade through the composting actions of micro organisms, the second approach has health hazards associated with fungal growth and requires a drying shed. So, drying wood in the round prior to chipping is the most common method.

Wood chips should be stored under cover to prevent wetting, however good airflow is necessary to disperse water vapour and minimise the chance of composting and mould formation. The stack height should be kept below 10 metres to prevent heat build up from composting and spontaneous combustion.

To produce the highest grade of wood chips, some kind of forced drying facility is needed to lower the moisture content to 20-25%. Wood chip driers can be designed using similar principles to a grain-drying floor, or mobile wood chip-drying trailers can be used. A wood fuel boiler can be used to produce the heat required for drying. Force drying will add to your production costs so consider carefully whether you need to produce this quality of chip.