Wood pellet

Wood pellets.

Fuels for heating

Heating oil Wood pellet Kerosene Propane Natural gas Wood Coal

Wood pellets are a type of wood fuel, generally made from compacted sawdust or other wastes from sawmilling and other wood products manufacture, but also sometimes from sources such as whole-tree removal or tree tops and branches leftover after logging and which otherwise help replenish soil nutrients.^{[citation needed]} Pellets are manufactured in several types and grades as fuels for electric power plants, homes, and other applications in between. Pellets are extremely dense and can be produced with a low moisture 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.

A broad range of pellet stoves, central heating furnaces, and other heating appliances have been developed and marketed since 1999.^{[citation needed]} With the surge in the price of fossil fuels since 2005, the demand for pellet heating has increased in Europe and North America, and a sizable industry is emerging. In a 2012 report, the Biomass Energy Resource Center says that it expects wood pellet production in North America to double in the next five years.

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. A report to the Massachusetts Department of Energy Resources estimates the energy required to manufacture and transport pellets is less than 2% of the energy content of the pellets.

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 (higher than 1 ton per cubic meter, thus it sinks in water)(bulk density about 0.6-0.7 ton per cubic meter), 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.^{[citation needed]} 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.^{[citation needed]}

Pellets conforming to the European standards norms which contain recycled wood or outside contaminants are considered Class B pellets. 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 uncontrolled variations in the burning characteristics of the pellets.^{[citation needed]}

Standards used in the United States are different, developed by the Pellet Fuels Institute and, as in Europe, are not mandatory.^{[citation needed]} Still, many manufacturers comply, as warranties of US-manufactured or imported combustion equipment may not cover damage by pellets non-conformant with regulations.^{[citation needed]} Prices for US pellets surged during the fossil fuel price inflation of 2007–2008, but subsequently have decreased significantly, and are generally lower on a per-BTU basis than most fossil fuels, excluding coal.

Regulatory agencies in Europe and North America are in the process of tightening the emissions standards for all forms of wood heat, including wood pellets and pellet stoves. These standards will become mandatory, with independently certified testing to ensure compliance. In the United States, the new rules are expected to be issued in July 2012. The American Lumber Standard Committee will be the independent certification agency for the new pellet standards.

Pellet stove operation

Main article: Pellet stove

A fully automated pellet 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.

Energy output and efficiency

Wood-pellet heater

The energy content of wood pellets is approximately 4.7 – 5.2 MWh/tonne (~7450 BTU/lb).

High-efficiency wood pellet stoves and boilers have been developed in recent years, typically offering combustion efficiencies of over 85%. Wood pellet boilers, having limited control over the rate and presence of combustion compared to liquid or gaseous-fired systems; however, for this reason they are better suited for hydronic heating systems due to the hydronic system's greater ability to store heat.^{[citation needed]} Pellet burners capable of being retrofitted to oil-burning boilers are also available.

Air pollution emissions

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. An additional consideration, though, is such air pollutant emissions caused in producing the energy used to manufacture pellets. A recognized problem is the emission of fine (particulate matter) to the air, especially in urban areas that have a high concentration of pellet heating systems or coal or oil heating systems in close proximity. This 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 on large installations electrostatic precipitators, cyclonic separators, or baghouse particle filters can control particulates when properly maintained and operated.

Global warming

There is uncertainty to what degree making heat or electricity by burning wood pellets contributes to global climate change, as well as how the impact on climate compares to the impact of using competing sources of heat. Factors in the uncertainty include the wood source, carbon dioxide emissions from production and transport as well as from final combustion, and what time scale is appropriate for the consideration.

A report by the Manomet Center for Conservation Sciences, "Biomass Sustainability and Carbon Policy Study" issued in June 2010 for the Massachusetts Department of Energy Resources, concludes that burning biomass such as wood pellets or wood chips releases a large amount of CO2 into the air, creating a "carbon debt" that is not retired for 20-25 years and after which there is a net benefit. In June 2011 the department was preparing to file its final regulation, expecting to significantly tighten controls on the use of biomass for energy, including wood pellets. Biomass energy proponents have disputed the Manomet report's conclusions, and scientists have pointed out oversights in the report, suggesting that climate impacts are worse than reported.

Until ca. 2008 it was commonly assumed, even in scientific papers, that biomass energy (including from wood pellets) is carbon neutral, largely because regrowth of vegetation was believed to recapture and store the carbon that is emitted to the air. Then, scientific papers began to appear which took at the climate implications of biomass and refuted the simplistic assumption of its carbon neutrality. According to the Biomass Energy Resource Center, the assumption of carbon neutrality "has shifted to a recognition that the carbon implications of biomass depend on how the fuel is harvested, from what forest types, what kinds of forest management are applied, and how biomass is used over time and across the landscape.”

In 2011 twelve prominent U.S. environmental organizations adopted policy setting a high bar for government incentives of biomass energy, including wood pellets. It states in part that, "[b]iomass sources and facilities qualifying for (government) incentives must result in lower life-cycle, cumulative and net GHG and ocean acidifying emissions, within 20 years and also over the longer term, than the energy sources they replace or compete with."

Sustainability

The wood products industry is concerned that if large-scale use of wood energy is instituted, the supply of raw materials for construction and manufacturing will be significantly curtailed. forests will no longer be able to be Ecological sustainability of large-scale use of wood energy has also been a topic of concern from divergent points of view.

Cost

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 the US in 2008–2009.

The cost of the pellets can be effected by the building cycle leading to fluctuations in the supply of sawdust and offcuts.

Usage by region

Europe

Pellet Use (ton)
Country	2006
Sweden	1 400 000
Italy	550 000
Germany	450 000
Austria	400 000
Denmark*	n. 400 000
Finland*	n. 50 000
*Households 2005.

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.^{[citation needed]}

New Zealand

The total sales of wood pellets in New Zealand was 3–5,000 tonnes in 2003. Recent construction of new wood pellet plants has given a huge increase in production capacity.

United States

Some companies import European-made boilers. As of 2009, about 800,000 Americans were using wood pellets for heat.

Retail cost

United States

In 2009, the cost for heating with pellets was $15.15 per million BTU.

Europe

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

Other uses

Horse bedding

When small amounts of water is added to wood pellets they expand and revert back to sawdust. This makes them suitable to use as a horse bedding. The easy of storage and transportation being additional benefits over traditional bedding.

See also

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

External links

• Pellet Fuels Institute – A non-profit trade association made up of pellet manufacturers and associated industry suppliers
• Mass. Dept. of Energy Resources (DOER)
• Manomet Center for Conservation Science