Energy content
Wood contains moisture, freshly felled conifer can contain up to 55% of its total weight in moisture. Before the wood can be burned the moisture must be removed, most usually by air drying. It is apparently possible to burn wet fuel in some specialised boilers. In fact the chips dried earlier in the process are being burned providing the energy to remove moisture from subsequent fuel as it feeds along the grate to the point of ignition. This process reduces the amount of useable heat energy available from the boiler. This system is used on larger boilers and when there is an advantage to be gained by utilising freshly felled wood.
If there is time available it is more usual to air dry the wood before it is burned.
Wood can be air dried to 35% moisture content or less if it is stored in a suitable location and covered against re-wetting. The drier the fuel the more available energy it contains.
In the case of wood pellets the sawdust feedstock must be dried to less than 15% moisture content if good quality pellets are to be produced. This is usually by means of an artificial heat source. The pellets will have a moisture content of 8-10% after processing. If pellets are allowed into contact with moisture they will absorb it and disintegrate. This will cause problems with the boiler fuel delivery system and it will not burn effectively.

Ash
The weight of ash produced from wood is between 0.5% and 1.0% of the weight of the original fuel burned. 1 tonne of wood fuel will produce between 5 – 10 Kilograms of ash. For example a house will produce between 40 and 90kg of ash per year. It contains short term fertiliser and can therefore be incorporated into the land.

Fuel Delivery
Delivery costs are based on deliveries to sites with good access and easy unloading, usually by tipping the bulk fuel into a storage hopper sited at a lower level than the access road. If there are difficulties in accessing the store and fuel has to be transferred by blowing equipment, conveyor belts or intermediate handling equipment then additional cost may be incurred.
Pellet fuel delivered in bags will attract a price premium due to the cost of bagging and the manual handling involved.
It is vitally important that the access to and location of the fuel store be properly considered at the planning stage in order to minimise delivery costs in the long term.