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
The energy content of wood pellets is approximately 4.7-4.9 MWh/tonne (~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. 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, 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. 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.
Others counter that the source sawdust would not otherwise have contributed to greenhouse gases, and that burning fuel pellets releases a large amount of CO 2 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.
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 o
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