Therefore the calorific value of dry wood is approximately proportional to its density. The density of solid oven-dry wood is referred to as its basic density. Basic density of wood within and between trees varies a lot, so the figures quoted are normally averages. Native hardwoods generally have average basic densities above 500 kg per cubic metre, although poplar is an exception at 380 kg/m³. Most conifers (softwoods) have average basic density below 400 kg/m³.
Freshly-harvested wood from living trees (‘green’ wood) normally contains a lot of water. Typically, approximately 50% of the weight of a piece of green wood is water. This is why green wood does not burn (there are exceptions in cases in which green wood has low moisture content), and why fuelwood normally needs to be dried before use. Fully air-dried wood (wood that has been stored outdoors under cover in well-ventilated conditions until it no longer loses weight due to evaporation of water) contains around 20% moisture in its total weight. Logs of such air-dried wood are suitable for domestic stoves and open fires.
Wood consists of sapwood and heartwood. Sapwood conducts water up the tree and hence water leaves sapwood relatively easily when logs are dried. Sapwood therefore dries quickly.Heartwood is not involved in water conduction in living trees and is much less permeable, so it dries more slowly than sapwood.
Wood of high density tends to be preferred when wood is used as fuel. On a domestic scale, logs of high density have a higher fuel content than equivalent logs of lower density, and so last longer in a fire. This is why native hardwoods such as hornbeam and oak (typical average basic densities of 640 and 580 kg per cubic metre of solid wood, respectively) are premium fuelwood species. Logs of such species are mainly heartwood, and typically require two years of air-drying before they are suitable for use.
Logs of most conifers (softwoods) and of chestnut are not recommended for use on open grates indoors because the logs emit sparks that are potentially hazardous.
In industry, and for heating in commercial premises, the use of woodchips and wood pellets is preferred. In these cases, the boilers are designed for fuels of specific moisture content. The composition of the wood fuel may still be important, because the ash content and composition can affect performance of some types of boilers. Wood density is generally not so important in larger boilers, because the fuel feed is automated. However, woodfuel of higher density is cheaper to transport, especially if it is also dry. This is one reason for the interest in wood pellets for use as fuel in industrial and commercial premises: the pelletising process both densifies and dries the wood. An important consequence is the scope to reduce delivered fuel costs
Wood pellets are already the preferred fuel for large-scale industrial use, due to their ease of handling and their uniform, predictable properties. There are several practical and economic incentives to improve the properties of pellets, including pellet density, still further. This is resulting in further innovations. A good example is the launch of ‘black pellets’ by Zilka, whose technology turns biomass into pellets whose physical properties are similar enough to coal for them to be transported, stored and used in the manner of coal.