Wood fuel

2007 Schools Wikipedia Selection. Related subjects: Environment

Firewood, stacked to dry
Firewood, stacked to dry

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. Wood heating has been singled out as a serious health hazard in many regions of the world.

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. Masonry heaters or stoves went a step further by capturing much of the heat of the fire and exhaust in a large thermal mass, becoming much more efficient than a fireplace alone.

The metal 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The airtight stove, originally made of steel, allowed greater control of combustion, being more tightly fitted than other stoves of the day.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 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 potential heat content per kilogram of wood is roughly equal for all wood varieties.

However, the heating potential of firewood per cubic meter or per log varies widely, depending upon the species of tree from which the wood is cut and thus the density of the log. 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 relatively little particulate emissions. 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.

For those burning firewood in a fireplace or old fashioned wood stove, the following list may be helpful. For heat produced and best burning trees when air dried consider the following which is listed in order of best to least--Rock Elm, Shagbark Hickory, White Oak (dried at least 24 months), Bitternut Hickory, Sugar Maple (also called Hard Maple), Beech, Red Oak (dried at least 24 months), Yellow Birch, Red Elm, White Ash, Red Maple, Tamarack, Black Cherry, White Birch, Black Ash, Green Ash, Silver Maple. All the above are excellent firewoods with over 20,000 BTUs of heat produced when dry.

Techniques to increase wood density (such as pelletizing or compression of wood pulp into molded 'logs' can increase the heat content (per cubic meter) dramatically. At the same time, if the moisture content of this type of fabricated wood fuel is maintained at a consistently low level (typically 8 - 10%), the heating value is maximized and very high combustion efficiencies are possible. This in turn provides very clean burning:low emissions, little ash production and a minimum of soot and deposits in the combustion chamber and chimney.

Combustion by-products

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

Environmental Impact

Depending on population density, topography, climatic conditions and combustion equipment used, wood heating may cause serious air pollution problems, particularly particulates. Wood combustion is also known to release various quantities of toxic and carcinogenic substances. The conditions in which wood is burnt will greatly influence the content of the emission, but in general, health officials in most countries have maintained that wood heating is not a healthy solution for residential heating.

" Slow combustion stoves" increase efficiency of wood heaters burning logs, 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, or to burn fuel extremely quickly inside a large thermal mass, such as a masonry heater. This has the effect of allowing the fuel to burn completely without producing particulates while maintaining the efficiency of the system.

Wood is said to be a greenhouse gas lean form of heating since the combustion of a tree releases the same amount of carbon dioxide as is bound up by a growing tree. Therefore, for this to be true, the resource must be managed accordingly.

The technique of compressing wood pulp into pellets or artficial logs provides an excellent means of reducing emissions : not only is the combustion very clean (this of course depends on a well-designed combustion chamber and feeding system), but because of the increased wood density and reduced water content, the transport bulk is reduced by 30 to 70%. Thus the fossil energy consumed in transport is reduced (and in fact represents a tiny fraction on the fossil fuel consumed in producing and distributing heating oil or gas).


Stacking firewood in a shed
Stacking firewood in a shed

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 metric system, firewood is normally sold by the stere (1 m³ = ~0.276 cords).

In the United States, firewood is usually sold by the cord, 128 ft³ (3.62 m³), corresponding to a woodpile 8 ft wide × 4 ft high of 4 ft-long logs. The cord is legally defined by statute in most states. It is also common to see wood sold by the "face cord", which is usually not legally defined, and varies from one area to another. For example, in one state a pile of wood 8 feet wide × 4 feet high of 16"-long logs will often be sold as a "face cord", though its volume is only one-third of a cord. In another state, or even another area of the same state, the volume of a face cord may be considerably different. Hence, it is risky to buy wood sold in this manner, as the transaction is not based on a legally enforceable unit of measure.

In Australia, it is normally sold by the tonne.

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 . In Finland, there is a growing interest in using wood waste as fuel for home and industrial heating, in the form of compacted pellets.

In Scandinavian countries the costs of manual labour to process firewood is very high. Therefore it is common to import firewood from countries with cheap labour and natural resources. The main exporters to Scandinavia are the Baltic countries (Estonia, Lithuania, Latvia).

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. Consumer grade hydraulic log splitters were developed to be powered by electricity, gasoline, or PTO of farm tractors.

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. However, outdoor wood boilers emit more wood smoke and associated pollutants than other wood burning appliances. This is due to design characteristics such as the water-filled jacket surrounding the firebox, which acts to cool the fire and leads to incomplete combustion. Outdoor wood boilers also typically have short stack heights in comparison to other wood burning appliances, contributing to ambient levels of particulates at ground level.

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