Wood fuel

Wood fuel

Wood fuel is wood used as fuel. The burning of wood is currently the largest use of energy derived from a solid fuel biomass. Wood fuel can be used for cooking and heating, and occasionally for fueling steam engines and steam turbines that generate electricity. Wood fuel may be available as firewood (e.g. logs, bolts, blocks), charcoal, chips, sheets, pellets and sawdust. The particular form used depends upon factors such as source, quantity, quality and application. Sawmill waste and construction industry by-products also include various forms of lumber tailings. Some consider wood fuel bad for the environment, however this is not the case if proper techniques are used.[1] One might increase carbon emissions using gas powered saws and splitters in the production of firewood, but when wood heat replaces carbon-producing fuels such as propane, heating oil or electricity from a coal-burning plant, then wood burning has a positive impact on the carbon footprint.

Wood may be used indoors in a furnace, stove, or fireplace. Wood also may be burned outdoors in a campfire, or bonfire. Wood is the most easily available form of fuel, requiring no tools in the case of picking up dead wood, or little tools, although as in any industry, specialized tools, such as skidders and hydraulic wood splitters, have evolved to mechanize production.

The discovery of how to make fire for the purpose of burning wood is regarded as one of humanity's most important advances.


Historical development

The use of wood as a fuel source for heating is much older than civilization and was used by neanderthals. 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.

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.

Campfires have been used for ages: fires are integral to humanity.

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

Wood has been used as fuel for millennia. The Greeks, Romans, Celts, Britons, and Gauls all had access to forests suitable for using as fuel. Over the centuries there was a partial deforestation of climax forests and the evolution of the remainder to coppice with standards woodland as the primary source of wood fuel. These woodlands involved a continuous cycle of new stems harvested from old stumps, on rotations between seven and thirty years. One of the earliest printed books in English was John Evelyn "Sylva, or a discourse on forest trees" (1664) advising landowners on the proper management of forest estates. H.L.Edlin, in "Woodland Crafts in Britain", 1949 outlines the extraordinary techniques employed, and range of wood products that have been produced from these managed forests since pre-roman times. And throughout this time the preferred form of wood fuel was the branches of cut coppice stems bundled into faggots. Larger, bent or deformed stems that were of no other use to the woodland craftsmen were converted to charcoal.

As with most of Europe, these managed woodlands continued to supply their markets right up to the end of World War two. Since then much of these woodlands have been converted to broadscale agriculture. Total demand for fuel increased considerably with the industrial revolution but most of this increased demand was met by the new fuel source, Coal, which was more compact and more suited to the larger scale of the new industries.

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.

Fireplace and chimney; fire in the wrong place--Witch Fire, California

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 1800s became the high point of the cast iron stove. Each local foundry would make their own design, and stoves were built for myriads of purposes—parlour stoves, box stoves, camp stoves, railroad stoves, portable stoves, cooking stoves and so on. Elaborate nickel and chrome edged models took designs to the edge, with cast ornaments, feet and doors. Wood or coal could be burnt in the stoves and thus they were popular for over one hundred years. The action of the fire, combined with the causticity of the ash, ensured that the stove would eventually disintegrate or crack over time. Thus a steady supply of stoves was needed. The maintenance of stoves, needing to be blacked, their smokiness, and the need to split wood meant that oil or electric heat found favour.

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.

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.

After the 1967 Oil Embargo, many in the United States used wood for the first time. The EPA provided information on clean stoves, which burned much more efficiently.[2]


Fuels for heating

Heating oil
Wood pellet
Natural gas

Stapled birch wood

Some firewood is harvested in "woodlots" managed for that purpose, but in heavily wooded areas it is more usually harvested as a byproduct of 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 and only limited by the size of their 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.

Potbelly stove at the Museum of Appalachia

Energy content

A common hardwood, red oak, has an energy content of 14.89 megajoules per kilogram (6,388 BTU per pound), and 10.423 megajoules recoverable if burned at 70% efficiency.[3]

The Sustainable Energy Development Office (SEDO), part of the Government of Western Australia states that the energy content of wood is 16.2 megajoules per kilogram (4.5 kWh/kg).[4]

According to The Bioenergy Knowledge Centre, the energy content of wood is much more dependent on the moisture content than the species. The energy content (number of joules of heat produced) improves towards the total number of joules stored in the wood as it dries.[5]

Heat Output / Moisture Content

There is some variation between species of wood as to the heat output, but it is small. Species also have different green moisture contents.

Measurement of firewood

In the metric system, firewood is normally sold by the cubic metre or 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. A "thrown cord" is firewood that has not been stacked and is defined as 4 ft wide x 4 ft tall x 10 ft long. The additional volume is to make it equivalent to a standard stacked cord, where there is less void space. 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.

Combustion by-products

As with any fire, burning wood fuel creates numerous by-products, some of which may be useful (heat and steam), and others that are undesirable, irritating or dangerous.

One by-product of wood burning is wood ash, which in moderate amounts is a fertilizer (mainly potash), contributing minerals, but is strongly alkaline as it contains potassium hydroxide[6] (lye). Wood ash can also be used to manufacture soap.

Smoke, containing water vapor, carbon dioxide and other chemicals and aerosol particulates, can be an irritating (and potentially dangerous) by-product of partially burnt wood fuel. A major component of wood smoke is fine particles that may account for a large portion of particulate air pollution in some regions. During cooler months, wood heating accounts for as much as 60% of fine particles in Melbourne, Australia.[7]

Wood-burning fireplace with burning log.

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.[citation needed] 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.[citation needed]

In some of the most efficient burners, the temperature of the smoke is raised to a much higher temperature where the smoke will itself burn (e.g. 609°C[8] for igniting carbon monoxide gas). This may result in significant reduction of smoke hazards while also providing additional heat from the process. By using a catalytic converter, the temperature for obtaining cleaner smoke can be reduced. Some U.S. jurisdictions prohibit sale or installation of stoves that do not incorporate catalytic converters.[citation needed]

Combustion by-product effects on human health

Depending on population density, topography, climatic conditions and combustion equipment used, wood heating may substantially contribute to air pollution, particularly particulates. The conditions in which wood is burnt will greatly influence the content of the emission.[citation needed] Particulate air pollution can contribute to human health problems and increased hospital admissions for asthma & heart diseases.[7]

The technique of compressing wood pulp into pellets or artificial logs can reduce emissions. The combustion is cleaner, and the increased wood density and reduced water content can eliminate 3 to 7% of the transport bulk.[citation needed] Thus the fossil energy consumed in transport is reduced (and in fact represents a tiny fraction of the fossil fuel consumed in producing and distributing heating oil or gas).

Wood combustion products can include toxic and carcinogenic substances. Generally, the heartwood of a tree contains the highest amounts of toxic substances, but precautions should be taken if one is burning wood of an unknown nature, since some trees' woodsmoke can be highly toxic. [9]

Environmental impact

Harvesting operations

Much wood fuel comes from native forests around the world. Plantation wood is rarely used for firewood, as it is more valuable as timber or wood pulp, however, some wood fuel is gathered from trees planted amongst crops, also known as agroforestry[10]. The collection or harvesting of this wood can have serious environmental implications for the collection area. The concerns are often specific to the particular area, but can include all the problems that regular logging create. The heavy removal of wood from forests can cause habitat destruction and soil erosion. However, in many countries, for example in Europe and Canada, the forest residues are being collected and turned into useful wood fuels with minimal impact on the environment. Consideration is given to soil nutrition as well as erosion. The environmental impact of using wood as a fuel depends on how it is burnt, but even if a fire gives off lots of smoke and particulates at least it is using a sustainable fuel, compared with fossil fuels. When wood that is sourced from a sustainable plantation, it can be regarded as being carbon-neutral. That is, a tree absorbs as much carbon (or carbon dioxide) as it releases when burnt.

Greenhouse gases

Wood burning does not release any more carbon dioxide than the eventual biodegradation of the wood if it was not burned. However, the carbon dioxide released through incineration occurs at a much faster rate than decomposition because burning wood takes a few seconds and decomposition takes years. Therefore, by burning wood one is releasing carbon dioxide into the atmosphere at a more concentrated rate than if one was to allow the wood fuel to decompose in soil. Wood harvesting and transport operations do produce varying degrees of greenhouse gas pollution. Inefficient and incomplete combustion of wood can result in elevated levels of greenhouse gases other than CO2, which may result in positive emissions where the byproducts have greater Carbon dioxide equivalent values.[11]

The intentional and controlled charring of wood and its incorporation into the soil is an effective method for carbon sequestration as well as an important technique to improve soil conditions for agriculture, particularly in heavily forested regions. It forms the basis of the rich soils known as Terra preta.

Wood fuels around the world

Ceramic stoves are traditional in Northern Europe: an 18th-century faience stove at Łańcut Castle, Poland


Some countries produce a significant fraction of their electricity needs from wood or wood wastes. 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, and Latvia).

Historic Japanese use

Wood, during the Edo period, was used for many purposes, and the consumption of wood led Japan to develop a forest management policy during that era.[12] Demand for timber resources was on the rise not only for fuel, but also for construction of ships and buildings, and consequently deforestation was widespread. As a result, forest fires occurred, along with floods and soil erosion. Around 1666, the shogun made it a policy to reduce logging and increase the planting of trees. This policy decreed that only the shogun, and/or a daimyo, could authorize the use of wood. By the 18th century, Japan had developed detailed scientific knowledge about silviculture and plantation forestry.

A pile of firewood logged from the Barmah Forest in Victoria.


About 1.5 million households in Australia use firewood as the main form of domestic heating.[13] As of 1995, approximately 1.85 million cubic metres of firewood (1m³ equals approximately one car trailer load) was used in Victoria annually, with half being consumed in Melbourne.[14] This amount is comparable to the wood consumed by all of Victoria’s sawlog and pulplog forestry operations (1.9 million m³).[citation needed]

Chimneypiece and overmantel, about 1750 V&A Museum no. 738:1 to 3-1897

Species used as sources of firewood include:

Environmental concerns

The environmental impact of burning wood fossil fuel is debatable. Several cities have moved towards setting standards of use and/or bans of wood burning fireplaces. For example, the city of Montréal, Québec passed a resolution to ban wood fireplace installation in new construction.

The environmental impact is debatable, however, as many wood burning advocates claim that properly harvested wood in carbon-neutral, therefore off-setting the negative impact of by-product particles given off during the burning process.

Efficiency and sustainability

With appropriately certified and operated modern wood heaters, the use of good quality wood fuel is one of the most efficient and cheapest forms of heating in Australia.[citation needed] The replacement of existing national domestic heating needs supplied by wood with gas and electricity would result in a significant net increase in carbon dioxide emissions,[16] while the application and enforcement of national standards for wood heaters and wood fuel would substantially reduce particulate emissions.[17] The peak industry body, the Australian Home Heating Association Inc is a major financial supporter of Landcare Australia, sponsoring the planting of over 40,000 trees per year. Landcare groups have planted millions of trees in revegetation programs to replace the estimated 20 billion trees removed since European settlement, laid thousands of kilometres of protective fencing, introduced sustainable farming techniques, removed hundreds of thousands of tonnes of weeds, and volunteered countless hours to the land care ethic.[13]

Firewood plantations also provide alternative financial opportunities for farmers and local government, with fuel being one of the multi-uses of tree plantations.

Sawmills create and burn sawdust: it can be pelletized and used at home

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.


A wood pellet stove.

A pellet stove is an appliance that burns compressed wood or biomass pellets.

Wood heat continues to be used in areas where firewood is abundant. For serious attempts at heating, rather than mere ambience (open fireplaces), stoves, fireplace inserts, 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. An alternative that is increasing in popularity are wood gasification boilers, which burn wood at very high efficiencies (85-91%) and can be placed indoors or in an outbuilding.

Wood is still used today for cooking in many places, either in a stove or an open fire. It is also used as a fuel in many industrial processes, including smoking meat and making maple syrup.

As a sustainable energy source, wood fuel also remains viable for generating electricity in areas with easy access to forest products and by-products.

Retail cost

United States

In 2008, wood for fuel cost $15.15 per 1 million BTUs.[18]

Potential use in renewable energy technologies

  • Efficient stove for developing nations
  • Pellet stove
  • Sawdust can be pelletized
  • Wood pellets

See also


  1. ^ "Wood-Burning Power Plants--Carbon-Neutral or High Carbon Emitters?: Scientific American". http://www.scientificamerican.com/article.cfm?id=wood-burning-power-plants-carbon-neutral-high-emitter. 
  2. ^ "Clean Burning Wood Stoves and Fireplaces". epa.gov. http://www.epa.gov/woodstoves/index.html. 
  3. ^ "Wood heat value (BTU)". daviddarling.info. http://www.daviddarling.info/encyclopedia/W/AE_wood_heat_value_BTU.html. 
  4. ^ "?". http://www.sedo.energy.wa.gov.au/pages/heat_run.asp. [dead link]
  5. ^ "Bioenergy Knowledge Centre's Calculators". bkc.co.nz. http://www.bkc.co.nz/BioenergyResources/Bioenergycalculationtools/tabid/86/Default.aspx.  (includes a range of calculators, including one for calculating the energy content of wood, taking into account the moisture content)
  6. ^ "Wood ash composition as a function of furnace temperature". Pergamon Press. 1993. http://www.fpl.fs.fed.us/documnts/pdf1993/misra93a.pdf. Retrieved 26 November 2010. 
  7. ^ a b Environment Protection Authority (2002) Wood heaters, open fires and air quality. Publication 851 EPA Victoria.
  8. ^ "Fuel Ignition Temperatures". Engineering Toolbox. http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html. 
  9. ^ "Toxic woods list". Degutopia. 2011. http://www.degutopia.co.uk/degutoxic.htm. Retrieved 17 October 2011. 
  10. ^ Ray, James, "Wood Usage in Rural Tanzania: An Investigation into the Sources and Accessibility of Fuelwood and Polewood for the Residents of Kizanda Village, West Usambara Mountains" (2011). ISP Collection. Paper 984. http://digitalcollections.sit.edu/isp_collection/984
  11. ^ "Greenhouse gases from biomass and fossil fuel stoves in developing countries: A Manila pilot study". sciencedirect.com. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V74-4876JPP-32&_user=559483&_coverDate=02%2F28%2F1993&_fmt=abstract&_orig=search&_cdi=5832&view=c&_acct=C000028178&_version=1&_urlVersion=0&_userid=559483&md5=ef69cfc726051ccbfa8a46d89c4d5d13&ref=full. 
  12. ^ Diamond, Jared. 2005 Collapse: How Societies Choose to Fail or Succeed. Penguin Books. New York. 294-304 pp. ISBN 0-14-303655-6
  13. ^ a b Matthew (26 December 2009). "The Truth about the Australian Home Heating Association". Clean Air Society of Kapiti Coast. http://cleanairkapiti.wordpress.com/2009/12/26/the-truth-about-the-australian-home-heating-association/. Retrieved 26 November 2010. 
  14. ^ "Firewood". birdsaustralia.com. http://www.birdsaustralia.com.au/rtbced/firewood.htm. 
  15. ^ NRE 2002 Forest Management Plan for the Mid-Murray Forest Management Area
  16. ^ "Pollution Issues". cleanairkapiti.wordpress.com. http://cleanairkapiti.wordpress.com/2009/12/26/the-truth-about-the-australian-home-heating-association/. 
  17. ^ "Benefits". cleanairkapiti.wordpress.com. http://cleanairkapiti.wordpress.com/2009/12/26/the-truth-about-the-australian-home-heating-association/. 
  18. ^ Ryan, Matt (June 20, 2008). Homeowners seek cheaper winter heat. Burlington Free Press. 

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