Burning wood to produce heat is probably the oldest form of energy production known to man. Equally, domestic wood fires represent one of the most common forms of heating in New Zealand.
While competition from other fuel-types has seen firewood replaced in many homes, domestic log fires can still be a highly economical form of heating. For many people, wood fires provide a uniquely pleasing form of comfort.
Anti-pollution legislation prohibits open fires in the home and today only approved woodburners can be installed. Improved technology has seen wood fires become much more efficient. Of the total energy available from burning wood, the open fire produces only 10-20%. In a simple woodburner, this rises to 40-50%, and sophisticated designs can raise efficiencies as high as 85% or more.
Any wood burns, especially when it is dry. However, some species are better than others for firewood. Traditional firewood sources in New Zealand have included native forest and scrub, exotic plantations, shelterbelts and woodlots.
Although a number of native species make excellent firewood, it is increasingly expensive and undesirable to burn such valued wood. Exotic hardwood species such as eucalypts and acacia (wattle) provide the best alternative, since they grow fast, dry quickly, split easily and burn with a strong heat.
The potential heat available from firewood is greatly affected by moisture content. During combustion the moisture in the wood is evaporated which reduces the heat available. To ensure complete combustion and minimum air pollution, the use of wood fuels in domestic fires should contain less than 25% moisture.
Bark contains a high level of extractives, so its calorific value is normally higher than that of wood. For this reason, it is preferable to use firewood that still has bark on it.
There are three distinct phases that take place in the burning of wood: moisture evaporation, distillation and combustion of volatile matter, and the reaction of fixed carbon with oxygen. For combustion to occur, evaporation of moisture must take place; this requires heat. In the second stage, volatile hydrocarbon gases are generated and mixed with oxygen, resulting in a release of heat. Heat is also released during the final stage of combustion when the fixed carbon combines with oxygen at high temperatures to produce carbon dioxide.
More information on firewood