Species
Generally speaking, given their density, hardwood species have a
higher calorific value and increased burn time over softwood
species by volume, although by weight the two are comparable.
Some of the very dense hardwoods such as oak and elm can be very
difficult to burn, so it is usually best to burn them with another
type of wood as well. Softwoods tend to be easy to light and will
burn quickly, proving ideal fuel for kindling
Most existing log burning equipment can operate effectively on
either hardwood or softwood logs, or a mixture. The design of log
burning equipment will determine whether it can accommodate
hardwood or softwood.
Size
Firewood lengths range from 15-50 cm depending on individual
system requirements. Optimum firewood width is between 6-8 cm, if
larger than 10-15 cm in diameter logs should be split.
Consistency of log size is important for uniform combustion and
to achieve an even burn rate from loading the fire.
Moisture content
The optimum moisture content for logs is 20%, although some
modern wood fuel boilers may be able to process wood of a higher
moisture content.
Burning logs above the specified moisture content for any wood
fuel boiler may result in corrosion of the boiler and chimney,
reduced efficiency and generally a lowering of economic return on
wood fuel investment.
Contamination/bark content
Wood fuel contamination and/or a high bark content will reduce
the burn rate of logs and increase emissions and ash from woodfuel
systems.
Sampling and testing
The CEN/TC 355 standards include detailed specifications for
sampling and testing biomass fuel.