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Wood fuel

2007 Schools Wikipedia Selection. Related subjects: Environment

   Firewood, stacked to dry
   Enlarge
   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).

Firewood

   Stacking firewood in a shed
   Enlarge
   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.

Today

   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.

   Retrieved from " http://en.wikipedia.org/wiki/Wood_fuel"
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   with only minor checks and changes (see www.wikipedia.org for details
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