Wood fuel

From Academic Kids

Wood burning is the largest current use of biomass derived energy. Wood can be used as a solid fuel for cooking or heating, or occasionally for steam engines.

The use of wood as a fuel source for home heat is as old as civilization itself. Historically, it was limited in use only by the distribution of technology required to make a spark. Wood heat is still common throughout much of the world, although it has been mainly replaced with coal, oil or natural gas heating.

Early examples include the use of wood heat in tents. Fires were constructed on the ground, and a smoke hole in the top of the tent allowed the smoke to escape by convection.

In permanent structures, hearths were constructed -- surfaces of stone or another noncombustible material upon which a fire could be built. Smoke escaped through a smoke hole in the roof.

The development of the chimney and the fireplace allowed for more effective exhaustion of the smoke.

The stove was a technological development concurrent with the industrial revolution. Stoves were manufactured or constructed pieces of equipment that contained the fire on all sides and provided a means for controlling the draft - the amount of air allowed to reach the fire. Stoves have been made of a variety of materials. Cast iron is among the more common. Soapstone (talc), tile, and steel have all been used. Metal stoves are often lined with refractory materials such as firebrick, since the hottest part of a woodburning fire will burn away steel over the course of several years' use.

The Franklin stove was developed in the United States by Benjamin Franklin. More a manufactured fireplace than a stove, it had an open front and a heat exchanger in the back that was designed to draw air from the cellar and heat it before releasing it out the sides. The heat exchanger was never a popular feature and was omitted in later versions. So-called "Franklin" stoves today are made in a great variety of styles, though none resembles the original design.

The airtight stove, originally made of steel, allowed greater control of combustion, being more tightly fitted than other stoves of the day. Airtight stoves became common in the 19th century.


Energy Content

The "caloric content" (energy content available from combustion) of firewood depends mainly on how dry it is. "Green" wood is about 10 MJ/kg (megajoule per kilogram), air-seasoned wood about 16 MJ/kg, while kiln dried wood is about 19 to 20 MJ/kg. The "caloric content" of firewood on a volume basis also depends upon the species of tree from which the wood is cut. In the United States, firewood is typically broadly classified into two categories: "hardwood" (any broadleaf tree) and "softwood" (any species of conifer). These labels are often misleading, as some species of conifer have harder wood than some species of broad-leaf tree. Generally, the harder the wood (which results from slower growth), the denser it is and the greater the amount of biomass per unit volume. Such woods, when well-seasoned, produce hot, long-burning fires with practically no particulate emissions. In the United States, varieties of wood such as Oak, Hard Maple, Hickory, and most of the fruit woods (apple, cherry, etc.) have the hardest, most dense wood, and are most desirable for firewood. Broad-leafed varieties such as willow, aspen, or poplar have less-dense wood and require a greater volume of wood to produce the same amount of heat. In areas where broad-leafed trees do not grow, varieties such as slash pine, Western Larch, and Yew are desirable varieties of firewood.

Combustion by-products

A by-product of wood burning is wood ash, which in moderate amounts is a fertilizer, but is strongly alkaline. Wood ash can also be used to manufacture soap.

Environmental Impact

Wood is also a greenhouse gas neutral form of heating, since the combustion of the fuel releases the same amount of carbon dioxide as is bound up by growing the next generation of trees. High levels of wood consumption in urban areas can cause air pollution, particularly particulates. "Slow combustion stoves" increase efficiency of wood heaters, but also increase particulate production. Low pollution slow combustion stoves are a current area of research. An alternative approach is to use pyrolysis to produce several useful biochemical byproducts, and clean burning charcoal.


Some firewood is harvested in purpose grown "wood lots", but in heavily wooded areas it is more usually harvested from natural forests. Deadfall that has not started to rot is preferred, since it is already partly seasoned. Standing dead timber is considered better still, as it is both seasoned, and has less rot. Harvesting this form of timber reduces the speed and intensity of bushfires. Harvesting timber for firewood is normally carried out by hand with chainsaws. Thus, longer pieces - requiring less manual labour, and less chainsaw fuel - are less expensive (but the user must ensure that the lengths will fit in the firebox!) Prices also vary considerably with the distance from wood lots, and quality of the wood.

Firewood - usually relates to timber or trees unsuitable for building or construction, Firewood is a renewable resource provided the consumption rate is controlled to sustainable levels. The shortage of suitable firewood in some places has seen local populations damaging huge tracts of bush thus leading to further desertification.

Measurement of firewood

In the United States, firewood is sold by the cord (a unit of volume equal to 4 by 4 by 8 feet, or 128 cubic feet), or by the "face cord", which is actually a unit of area, equal to 32 square feet (this unit of area is used exclusively for describing stacks of firewood). A pile of wood four feet high, eight feet long, and cut to, say, sixteen inch lengths will often be sold as a "face cord" of 16 inch logs, even though the actual volume of the wood pile is only one-third of a cord. Hence, only a "face cord" of 48 inch logs would actually equal a "cord" of firewood. In Australia, it is normally sold by the tonne. In Sweden it's sold by volume.

European use of wood fuel

Some countries produce a significant fraction of their electricity needs from wood or wood wastes. Sweden, for example. produces 1490 megawatts of electricity this way and Austria produces 747 megawatts [1] (http://www.renewable-energy-policy.info/relec/index.html).

United States use of wood heat

Use of wood heat declined in popularity with the growing availability of other, less labor-intensive fuels. Wood heat was gradually replaced by coal and later by fuel oil, natural gas and propane heating except in rural areas with available forests.

1973 energy crisis

A brief resurgence in popularity occurred during and after the 1973 energy crisis, when some believed that fossil fuels would become so expensive as to preclude their use. A period of innovation followed, with many small manufacturers producing stoves based on designs old and new. Notable innovations from that era include the Ashley heater, a thermostatically-controlled stove with an optional perforated steel enclosure that prevented accidental contact with hot surfaces.

A number of dual-fuel furnaces and boilers were made, which utilized ductwork and piping to deliver heat throughout a house or other building.

The growth in popularity of wood heat also led to the development and marketing of a greater variety of equipment for cutting and splitting wood. New products included the stickler log splitter, and hydraulic log splitters previously developed found greater popularity.

The magazine "Wood Burning Quarterly" was published for several years before changing its name to "Home Energy Digest" and, subsequently, disappearing.


Wood heat continues to be used in areas where firewood is abundant. For serious attempts at heating, rather than mere ambiance, stoves and furnaces are most commonly used today. In rural, forested parts of the U.S., freestanding boilers are increasingly common. They are installed outdoors, some distance from the house, and connected to a heat exchanger in the house using underground piping. The mess of wood, bark, smoke, and ashes is kept outside and the risk of fire is reduced. The boilers are large enough to hold a fire all night, and can burn larger pieces of wood, so that less cutting and splitting is required. There is no need to retrofit a chimney in the house.

See also


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