The calorific value of a fuel expresses the amount of energy released during the complete combustion of a mass unit of a fuel.
The moisture content of wood changes the calorific value of the latter by lowering it. Part of the energy released during the combustion process is spent in water evaporation and is consequently not available for any wished-for thermal use.
Water evaporation involves the “consumption” of 2.44 MJ per kilo of water. It is thereby possible to distinguish between the following:
Net calorific value (NVC):
The water released is treated as a vapour, i.e. the thermal energy required to vaporize the water (latent heat of vaporization of water at 25°C) has been subtracted.
Gross calorific value (GVC):
The water in the combustion products is treated as liquid. When not specified, “calorific value” is to be intended as net calorific value.
The oven-dry calorific value (NCV0) of wood of different wood species varies within a very narrow interval, from 18.5 to 19 MJ/Kg. In conifers it is 2% higher than in broad-leaved. This difference is due especially to the higher lignin content and partly also to the higher resin, wax and oil content present in conifers. Compared to cellulose (17.2-17.5 MJ/kg) and hemicellulose (16 MJ/kg), lignin has a higher NCV0 (26-27 MJ/kg). Some variability in the anhydrous calorific value is also due to the slight variability in hydrogen (H) content and to the comparatively much wider variability in ash contents.
However, when taking into account agricultural biofuels as well, the oven-dry calorific value varies within a 16.5 to 19 MJ/Kg interval. The NCV0 of wood fuels is on average 9% higher than that of herbaceous plants.