Wood pellet

A small huddle of wood pellets

A handful of wood pellets.

Wood pellets are a type of wood fuel, generally made from compacted sawdust. They are usually produced as a byproduct of sawmilling and other wood transformation activities. The pellets are extremely dense and can be produced with a low humidity content (below 10%) that allows them to be burned with a very high combustion efficiency. Further, their regular geometry and small size allow automatic feeding with very fine calibration. They can be fed to a burner by auger feeding or by pneumatic conveying.

Their high density also permits compact storage and rational transport over long distance. They can be conveniently blown from a tanker to a storage bunker or silo on a customer's premises. As the price of heating with fossil fuels increases, more capacity for pellet heating has been installed. A large number of models of pellet stoves, central heating furnaces and other heating appliances have been developed and marketed since about 1999. With the surge in the price of fossil fuels in 2005, the demand has increased all over Europe and a sizable industry is emerging. The reason being is that customers on oil or LPG could be saving from 35-50% on their heating bill in the United Kingdom.

Production

Pellets are produced by compressing the wood material which has first passed through a hammer mill to provide a uniform dough-like mass. This mass is fed to a press where it is squeezed through a die having holes of the size required (normally 6 mm diameter, sometimes 8 mm or larger). The high pressure of the press causes the temperature of the wood to increase greatly, and the lignin plastifies slightly forming a natural 'glue' that holds the pellet together as it cools.

Pellets conforming to the norms commonly used in Europe (DIN 51731 or Ö-Norm M-7135) have less than 10% water content, are uniform in density (density in excess of 1 ton / cubic meter, so they do not float if placed in water), have good structural strength, and low dust and ash content. Because the wood fibres are broken down by the hammer mill, there is virtually no difference in the finished pellets between different wood types. Pellets can be made from nearly any wood variety, provided the pellet press is equipped with good instrumentation, the differences in feed material can be compensated for in the press regulation.

Pellets conforming to the European standards norms cannot contain any recycled wood or outside contaminants. Recycled materials such particle board, treated or painted wood, melamine resin-coated panels and the like are particularly unsuitable for use in pellets, since they may produce noxious emissions and / or uncontrolled variations in the burning characteristics of the pellets.

New pellet mills are being opened in the United Kingdom and other nations of Europe on a rapid basis, decreasing the price of a tonne of pellets to new lows. The scarcity and unreliable supply that used to come with the dependence on wood pellets has vanished.

Standards used in the United States are different, developed by the Pellet Fuel Institute, are not mandatory, and are generally less strict than those of Europe; for example, it is accepted that pellets exposed to large volumes of water in the US may significantly degrade (turning into "mush"). Still, many manufacturers comply, as warranties of US-manufactured or imported combustion equipment may not cover damage by pellets non-conformant with regulations. Prices for US pellets surged during the fossil fuel price inflation of 2007 - 2008, but subsequently have decreased significantly in late 2008 and early 2009, and are generally lower on a per-BTU basis than most fossil fuels, excluding coal, which is not an option highly favored for heating by many residential and commercial consumers, due to frequent maintenance/tending requirements for end users, high carbon emissions, air pollution (often leading to nuisance complaints from neighbors and/or investigation by boards of health and environmental agencies), the mess generated by coal dust and/or improper storage, as well as potentially dangerous fly ash.

Energy output and efficiency

Wood-pellet heater

The energy content of wood pellets is approximately 4.7-4.9 MWh/tonne^[1] (~7450 BTU/lb).

High-efficiency wood pellet stoves and boilers have been developed in recent years, offering combustion efficiencies of over 90%. Wood pellet boilers - having limited control over the rate and presence of combustion compared to liquid or gaseous-fired systems - are particularly suited for hydronic systems due to the hydronic system's greater ability to store heat. In particular, pellet burners capable of being retrofitted to oil-burning boilers are predicted to be available on the market within the next several years.

Emissions of regular air pollution

Emissions such as NO_x, SO_x and volatile organic compounds from pellet burning equipment, are, in general, very low in comparison to other forms of combustion heating, making this one of the less-polluting heating options available.^{[citation needed]} One remaining problem is emission of fine dust (particulate matter), especially in urban areas, when there is a high concentration of pellet heating systems, coal, or oil heating systems in close proximity. The PM_2.5 emissions of older pellet stoves and boilers can be problematic in close quarters, especially in comparison to natural gas (or renewable biogas), though electrostatic precipitators and baghouse particle filters for pellet heaters have been developed and considerably reduce the problem when installed as standard.

Wood pellet stoves have a number of advantages over normal wood stoves. Because the stoves are so efficient, there is almost no smoke or creosote produced, in fact the exhaust is barely even hot so the stove does not need a masonry chimney and can be installed anywhere a tin metal liner can be put in, either directly into the roof, or sideways out a wall. They can be stand-alone stoves on legs in the corner of a room, or chimney inserts using an existing chimney. Unlike wood stoves, pellet stoves work well in urban environments Because of little exhaust and no need for a chimney and can be installed in any room.

A fully automated stove requires filling up with the pellets and turning on, the stove does the rest: it automatically lights, automatically feeds the pellets into the flame with an auger, automatically adjusts the rate to keep the room at a pre-set temperature with an electric thermostat.

Global warming potential, sustainablility, and lifecycle emissions concerns

The climate impact of wood pellets is disputed^{[by whom?]}, though biomass fuels, including wood pellets (and other wood fuels, such as regular cordwood) produced using best practices from sustainably-managed forests, fuel crops, or other forms of biomass waste are generally recognized as having far lower net lifecycle carbon dioxide emissions than fossil fuel equivalents, to the order of 98% fewer emissions.^[2] However, it must be emphasized that if best practices and sustainable biomass management is not instituted, carbon emissions can exceed those of natural gas combustion.^[2]

Others counter that the source sawdust would not otherwise have contributed to greenhouse gases^{[dubious – discuss]}, and that burning fuel pellets releases a large amount of CO₂ into the air. The release of CO2 from burning sawdust, when derrived from forest biomass in Boreal forest ecosystems, is carbon-neutral. Boreal Forests are disturbance driven ecosystems. Boreal carbon regularly cycles between standing, stored terrestrial carbon and free atmospheric carbon in the transition from regrowth to maturity, decline, decomposition/destruction and regrowth. Secondary emissions related to pellet manufacture also have a carbon impact, with the release of carbon involved in the harvesting, manufacturing, and process of transport to the end-user for these pellets and other biomass fuels (known as grey energy) - though with appropriate practices, such as the use of biodiesel for transport fuel and to power harvesting equipment, along with solar thermal energy in wood drying facilities, this can be reduced to a minimum. Indeed, it is undoubtedly the case that pellet combustion releases carbon dioxide, but the use of pellet fuels is argued to have a low net lifecycle impact because the carbon dioxide released is ultimately consumed in the regrowth of the trees or biomass products used to produce the pellets; this process takes approximately 15 years (for trees) and as little as 1 season (for crop-based biomass) to sequester the carbon released by their combustion. Thus, although there is a short term radiative forcing effect from biomass combustion, there is a generally very low long term radiative forcing effect from biomass combustion if best practices for sustainability are followed.^[2]

This is because wood/plant biomass is made up of mostly carbon (and water); carbon came from the carbon dioxide absorbed from the atmosphere by the tree or grass while it grew through the process of photosynthesis, and the carbon returns to the atmosphere when the wood is either burned or left to decompose. It is true that in combustion, most of the carbon joins with oxygen and returns to the atmosphere as carbon dioxide, with a GWP₁₀₀ of 1. But - if the wood is instead left to decompose through natural processes, the carbon still partially returns to the air - but in the form of the gas methane, which has a GWP₁₀₀ of 21 - thus causing 21 times more damage to the climate over 100 years than the same quantity of CO₂ does over 100 years. Only part of the decaying wood is released as methane; other parts stay within the soil and often improve it substantially over time. This criticism of the climate aspect of wood decay does not apply if the wood is used in a process that fixates the carbon, such as in building materials, or, in the case of sawdust, in particle board, as these media sequester carbon unless or until they are allowed to decay or are consumed by fire.

Some^[who?] argue that the speed of return matters; the release of methane is, for example, over 5 years, the release of CO₂ is over five minutes if burned. Residence time also matters - and the methane of wood decay is not absorbed by the oceans or by growing biomass, as is carbon dioxide. However, one undoubted source of potential radiative forcing from wood pellets is if inefficient combustion technologies are used to burn them, and this results in black carbon emissions.

Organizations like the Sierra Club cautiously support use of biomass, though they are opposed to what might be termed wanton or reckless use of biomass, expressing concern about biomass that is grown without regard to techniques for sustainable forestry or agriculture, such as the use of unsustainable fertilizers, the wholesale removal of tree roots, which fixate carbon, or unsustainable harvesting is used, such as clearcutting, leading to erosion. They warn that if unsustainable land use practices are instituted in the production of biomass fuels, this may aggravate problems with global warming.^[3]

The United States forest products industry, with apparent justification, fears that if massive use of wood as a fuel is instituted, forestry will no longer be able to be sustainably managed, and may result in ecological devastation. This is especially the case if wood is extensively used to generate electricity, though this would probably not be in the form of wood pellets.^[4]

Still, the combustion of sustainably managed and harvested wood or biomass is generally considered to represent one of the best practical and available means for sustainable use of low net-carbon combustion for economic sectors that require the use of combustion. The medium of wood or biomass pellets as a form of energy transfer provide a fungible, mass-producible, commerce-ready product capable of putting sustainable biomass combustion technology to work for the modern-day consumer - and, in many cases, wood/biomass pellets are less expensive and nearly as convenient as other fuel sources.

Potential issues

• Due to the rapid increase in popularity since 2005, Pellet availability and cost may be an issue. This is an important consideration when buying a pellet stove, furnace, pellet baskets or other devices known in the industry as Bradley Burners. However, current pellet production is increasing and there are plans to bring several new pellet mills online in 2008-2009, in the US alone.
• Cost of the pellets has begun to rise as less wood by-products are being created due to the slowing of the housing boom.

Usage by region

Europe

Pellets are most widely used in Sweden - mainly as an alternative to oil-fired central heating. In Austria, the leading market for pellet central heating furnaces (relative to its population), it is estimated that 2/3 of all new domestic heating furnaces are pellet burners. In Italy, a large market for automatically-fed pellet stoves has developed.

New Zealand

The total sales of wood pellets in New Zealand was 3-5,000 tonnes in 2003. Recent construction of new wood pallet plants has given a huge increase in production capacity.^[7]

United States

Currently, there are several companies investing in pellet burning technology. TARM USA has been importing European-made boilers for five years, and Maine Energy Systems [1] hopes to convert 10% of Maine households to wood pellet heat in the next 5 years. Maine Energy Systems is using an Austrian system made by Okofen.

Retail cost

United States

In 2008, the cost for heating with pellets was $19.59 per million BTU.^[8] This corresponds to a price of $6.66 per 40 pound bag.

Europe

In 2009, the cost of woodpellets was 4 eurocent per kWh or 16 $ per million BTU^[9]. Retail prices depend on the size and specifications of the order (bulk, big bag, 15kg sacks,...).

See also

• Solid fuel
• Renewable heat
• Pellet mill
• Pellet stove
• Woodchips
• Wood briquettes
• Firelog

References

External links

• Pellet Fuels Institute - Non-profit association that serves the pellet industry
• [2] - Biomass Energy centre, compare costs in GBP

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