Wood fuel describes wood used as fuel. Wood fuel may be
available as firewood, charcoal, chips, sheets, pellets, and sawdust. The
particular form used depends upon factors such as source, quantity, quality and
application. In many areas, wood is the most easily available form of fuel,
requiring no tools in the case of picking up dead wood, or few tools, although
as in any industry, specialized tools, such as skidders and hydraulic wood splitters,
have been developed to mechanize production. Sawmill waste and construction
industry by-products also include various forms of lumber tailings.
The discovery of how to make fire for the purpose of burning
wood is regarded as one of humanity's most important advances. The use of wood
as a fuel source for heating is much older than civilization and is assumed to
have been used by Neanderthals. Today, burning of wood is 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 may be used indoors in a furnace, stove, or
fireplace, or outdoors in a furnace, campfire, or bonfire.
Historical development
Further information: Firewood
Campfires have been used for ages: fires are integral to
humanity.
Charcoal, a derivative of wood, was traditionally an
important fuel in ironmaking and other processes
Wood has been used as fuel for millennia. Historically, it
was limited in use only by the distribution of technology required to make a
spark. Heat derived from wood is still common throughout much of the world.
Early examples included a fire constructed inside a tent. 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 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.
In contrast to civilizations in relatively arid regions
(such as Mesopotamia and Egypt), 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 on woodland management, in English, was John Evelyn's
"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.
During the Edo period of Japan, wood was used for many
purposes, and the consumption of wood led Japan to develop a forest management
policy during that era. 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.
Fireplaces and stoves
Ceramic stoves are traditional in Northern Europe: an
18th-century faience stove at Łańcut Castle, Poland
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.
Potbelly stove at the Museum of Appalachia
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 people in the United States
used wood as fuel for the first time. The EPA provided information on clean
stoves, which burned much more efficiently.
1970s
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. The decade also saw a number of dual-fuel
furnaces and boilers made, which utilized ductwork and piping to deliver heat
throughout a house or other building.
1980s
The growth in popularity of wood heat also led to the
development and marketing of a greater variety of equipment for cutting,
splitting and processing firewood. Consumer grade hydraulic log splitters were
developed to be powered by electricity, gasoline, or PTO of farm tractors. In
1987 the US Department of Agriculture published a method for producing kiln
dried firewood, on the basis that better heat output and increased combustion
efficiency can be achieved with logs containing lower moisture content.
The magazine "Wood Burning Quarterly" was
published for several years before changing its name to "Home Energy
Digest" and, subsequently, disappearing.
Today
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.
Measurement of firewood
Stapled birch wood
In the metric system, firewood is normally sold by the cubic
metre or stere (1 m³ = ~0.276 cords).
In the United States and Canada, 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 U.S. 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.
Energy content
A common hardwood, red oak, has an energy content (Heat
value) of 14.9 megajoules per kilogram (6,388 BTU per pound), and 10.4
megajoules recoverable if burned at 70% efficiency.
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).[5]
According to The Bioenergy Knowledge Centre, the energy
content of wood is more closely related to its moisture content than its
species. The energy content improves as moisture content decreases.
In 2008, wood for fuel cost $15.15 per 1 million BTUs (0.041
EUR per KWh).
Environmental impacts
Fireplace and chimney after a wildfire, Witch Fire,
California
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 (lye). Wood ash can also
be used to manufacture soap.
Smoke, containing water vapor, carbon dioxide and other
chemicals and aerosol particulates, including caustic alkali fly ash, which 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.
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 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
Wood-burning fireplace with burning log.
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.
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 some of the
transport bulk. The fossil energy consumed in transport is reduced and
represents a small 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.not in citation
given]
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. 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.
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 labor, 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 possibly leading to further
desertification.
Greenhouse gases
Wood burning creates more atmospheric CO2 than
biodegradation of wood in a forest (in a given period of time) because by the
time the bark of a dead tree has rotted, the log has already been occupied by
other plants and micro-organisms which continue to sequester the CO2 by
integrating the hydrocarbons of the wood into their own life cycle. Wood
harvesting and transport operations 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.
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.
Regulation and Legislation
The environmental impact of burning wood 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. However many
wood burning advocates claim[weasel words] that properly harvested wood is
carbon-neutral, therefore off-setting the negative impact of by-product particles
given off during the burning process. In the context of forest wildfires, wood
removed from the forest setting for use as wood fuel can reduce overall
emissions by decreasing the quantity of open burned wood and the severity of
the burn while combusting the remaining material under regulated conditions.
Potential use in renewable energy technologies
Sawmills create and burn sawdust: it can be pelletized and
used at home
Efficient stove
for developing nations
Pellet stove
Sawdust can be
pelletized
Wood pellets
Usage
Some European countries produce a significant fraction of
their electricity needs from wood or wood wastes. In Scandinavian countries the
costs of manual labor to process firewood is very high. Therefore it is common
to import firewood from countries with cheap labor and natural
resources.[citation needed] The main exporters to Scandinavia are the Baltic
countries (Estonia, Lithuania, and Latvia). 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 the United States, wood fuel is the second-leading form
of renewable energy (behind hydro-electric).
Australia
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. 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. This amount is
comparable to the wood consumed by all of Victoria’s sawlog and pulplog
forestry operations (1.9 million m³).[citation needed]
Species used as sources of firewood include:
Red Gum, from
forests along the Murray River (the Mid-Murray Forest Management Area, including
the Barmah and Gunbower forests, provides about 80% of Victoria’s red gum
timber).
Box and Messmate
Stringybark, in southern Australia.
Sugar gum, a wood
with high thermal efficiency that usually comes from small
plantations.[citation needed]
Jarrah, in the
Southwest of Western Australia. It generates a greater heat than most other
available woods and is usually sold by the tonne.
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