How to Procure Wood Fuel

Wood fuels have a greater range of energy content than fossil fuels like oil and gas, mostly due to variations in its moisture content.  The higher the moisture content, the less energy there is per tonne. The relationship between moisture and energy content is shown here on a downloadable spreadsheet if you follow the link.

It is essential to account for moisture content, and therefore energy content, when purchasing wood fuel and there are various ways to measure and pay for this:

• £ Per tonne
• £ per Oven Dried Tonne (ODT)
• By energy content
• As delivered heat

It is recommended that fuel should always be purchased based upon its energy content.

£ Per Tonne

If wood fuel is purchased by the tonne there must be a way of determining both the weight and moisture content of each delivered load. Wood fuel suppliers should be asked to submit evidence of both with each invoice. This can be in the form of weighbridge readings for the tonnage. In terms of moisture content this requires a clear and pre-agreed system of taking moisture content readings using a calibrated bucket moisture meter, taking samples in a prescribed manner from either the delivery wagon or the fuel silo. In addition random samples of fuel should be sent for independent laboratory testing.

£ per Oven Dried Tonne (ODT)

ODT is a theoretical measurement, that assumes the wood chips are bone dry. Payment can be made as if the fuel was ‘bone dry’ and so contains 5,300kWh per tonne.

By Energy Content

If wood fuel is purchased by its energy content the end user will pay for it measured in £/kWh or £/MWh. As for buying by tonne or per ODT there still must be a way of determining both the weight and moisture content of each delivered load in order to calculate energy content

As Delivered Heat

Delivered heat costs are measured by the heat meter and account for the efficiency losses experienced through the combustion of the fuel in the biomass installation. Further details on efficiency can be found here. Losses should be in the range 70% to 89% depending upon the efficiency of the installation. If they are selling metered heat wood fuel suppliers are obliged to determine the biomass installation losses and include these in their cost calculations. This transfers efficiency risk to the supplier and away from the customer.