The rules for using wood as a fuel are fairly simple in concept although not always so easy to follow in practice. The moisture content, the fuel size and type must be suitable for the equipment in which it being burned.
The presence of tramp material, stones, metal etc is to be avoided to prevent damage to the equipment. Wood that contains paint or preservative will produce atmospheric pollution an must not be burned in boilers.
If dried to the correct level wood fuel will keep indefinitely without degrading.

Wood chips
The major benefit of wood chip fuel is the ability to use it in automated stoker boilers. Fuel chips are contained in a store equipped with a feed auger and an agitator. This is usually achieved by a slow moving rotary arm which breaks any tendency for the chip to bridge across the auger infeed this is achieved by storing the chips adjacent to the boiler.
The density of wood chip is relatively low with one tonne of dry wood chip occupying five cubic meters. The fuel store needs to be larger than that required for pellets for the same heat output.
Their are a number of national standards which define the size and the moisture content of fuelwood chips. These are in the process of being consolidated under the European Standard CEN TS 335. For the UK this has been adopted as the British Standard.
The fuel is produced by specialised chippers which are able to create chips of a consistent particle size in accordance with national standards. If fuel is produced by conventional arboricultural chippers there is a danger that the wide range of chip sizes will cause feed problems with the sensors on the boilers. In addition production of long slivers can cause jamming in the feed augers.
Leaves and needles must be excluded and the proportion of bark controlled if excessive ash production is to be avoided.

Wood pellets
Wood pellets are produced by compressing small particles of wood such as sawdust. The pressure in the mill raises the temperature of the wood such that the pellets are held together by their natural resins. Some smaller pellet mills utilise additives such as plant oil to increase production. Pellets sizes cover a wide range with 6 or 8 mm diameter being the most common. Pellet length is random as a result of breakage as they come out of what is essentially a large mincing machine.
Pellets are also covered by the British Standard TS 14961 which determines the tolerances on length and diameter together with their hardness, ash content etc. The Austrian ONORM and the German DIN Standards are also used in this country to define pellet quality.
It is important that this fuel is kept dry as only a small amount of moisture can cause disintegration of the pellets with consequent problems in the feeding mechanism and reduced heat production.
Pellets are a free flowing fuel with a density of approx 600 kg per cubic meter loose. This together with their smooth shape means pellets can be stored in a conventional silo with transfer to the boiler firebox by either an auger or a pneumatic system.
Hand loading of the integrated store is the method of fuelling smaller boilers.

Briquettes
These are produced by the same method as pellets but are considerably larger. They can be burned in boilers or conventional fires taking the place of logs.

Logs
Logs must of a suitable size to fit the fire box. The amount of space they occupy in storage varies with the neatness of the stack. Logs should be stored where air can freely circulate and covered to prevent re-wetting. At least one full summers drying is required to give the best energy yield.