Pellet fuel

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 plasticizes slightly, forming a natural "glue" that holds the pellet together as it cools. In 2012, the Department of Agriculture of Nova Scotia announced as a demonstration project conversion of an oil-fired boiler to grass pellets at a research facility. Rice-husk fuel-pellets are made by compacting rice-husk obtained as by-product of rice-growing from the fields. It also has similar characteristics to the wood-pellets and more environment-friendly, as the raw material is a waste-product. The energy content is about 4-4.2 kcal/kg and moisture content is typically less than 10%. The size of pellets is generally kept to be about 6 mm diameter and 25 mm length in the form of a cylinder; though larger cylinder or briquette forms are not uncommon. It is much cheaper than similar energy-pellets and can be compacted/manufactured from the husk at the farm itself, using cheap machinery. They generally are more environment-friendly as compared to wood-pellets. In the regions of the world where wheat is the predominant food-crop, wheat husk can also be compacted to produce energy-pellets, with characteristics similar to rice-husk pellets. A report by CORRIM (Consortium On Research on Renewable Industrial Material) for the Life-Cycle Inventory of Wood Pellet Manufacturing and Utilization estimates the energy required to dry, pelletize and transport pellets is less than 11% of the energy content of the pellets if using pre-dried industrial wood waste. If the pellets are made directly from forest material, it takes up to 18% of the energy to dry the wood and additional 8% for transportation and manufacturing energy. An environmental impact assessment of exported wood pellets by the Department of Chemical and Mineral Engineering, University of Bologna, Italy and the Clean Energy Research Centre, at the University of British Columbia, published in 2009, concluded that the energy consumed to ship Canadian wood pellets from Vancouver to Stockholm (15,500 km via the Panama Canal), is about 14% of the total energy content of the wood pellets. Pellet standards , NB Pellets conforming to the norms commonly used in Europe (DIN 51731 or Ö-Norm M-7135) have less than 10% water content, and are uniform in density, higher than 1 ton per cubic meter, thus they sink in water. (In contrast, the bulk density of a mass of pellets, including trapped air, is only about 0.6-0.7 ton per cubic meter). The pellets have good structural strength, and low dust and ash content. Pellets conforming to the European standards norms which contain recycled wood or outside contaminants are considered Class B pellets. In the United States, the new rules initiated in 2009 have completed the EPA regulatory review process, with final new rules issued for comment on June 24, 2014. The American Lumber Standard Committee will be the independent certification agency for the new pellet standards. ==Hazards==

Wood pellets can emit large quantities of poisonous carbon monoxide during storage by autoxidation. Fatal accidents have taken place in private storerooms and onboard marine vessels. When handled, wood pellets give off fine dust which can cause serious dust explosions. Wood pellets are typically stored in bulk in large silos. Pellets may self-heat, ignite and give rise to a deep-seated smoldering fire that is very difficult to extinguish. The smoldering fire produces toxic carbon monoxide and flammable pyrolysis gases that can lead to silo explosions. ==Pellet stove operation==

There are three general types of pellet heating appliances: free standing pellet stoves, pellet stove inserts and pellet boilers. Pellet stoves work like modern furnaces, where fuel, wood, or other biomass pellets, is stored in a storage bin called a hopper. The hopper can be located on the top of the appliance, the side of it or remotely. A mechanical auger automatically feeds pellets into a burn pot. From there, they burn at high temperatures with minimal emissions. Heat-exchange tubes send air heated by fire into room. Convection fans circulate air through heat-exchange tubes and into room. Pellet stoves have circuit boards inside that act like a thermostat and to regulate temperature. A pellet stove insert is a stove that is inserted into an existing masonry or prefabricated wood fireplace, similar to a fireplace insert. Pellet boilers are standalone central heating and hot water systems designed to replace traditional fossil fuel systems in residential, commercial and institutional applications. Automatic or auto-pellet boilers include silos for bulk storage of pellets, a fuel delivery system that moves the fuel from the silo to the hopper, a logic controller to regulate temperature across multiple heating zones and an automated ash removal system for long-term automated operations. Pellet baskets allow one to heat one's home using pellets in existing stoves or fireplaces. == Energy output and efficiency ==

The energy content of wood pellets is approximately 4.7 – 5.2 MWh/tonne (~7450 BTU/lb), 14.4-20.3 MJ/kg. High-efficiency wood pellet stoves and boilers have been developed in recent years, typically offering combustion efficiencies of over 85%. The newest generation of wood pellet boilers can work in condensing mode and therefore achieve 12% higher efficiency values. Wood pellet boilers have limited control over the rate and presence of combustion compared to liquid or gaseous-fired systems. For this reason, they are better suited for hydronic heating systems due to the hydronic system's greater ability to store heat. Pellet burners capable of being retrofitted to oil-burning boilers are also available. ==Air pollution emissions==

Emissions such as NOx, SOx and volatile organic compounds from pellet burning equipment are in general very low in comparison to other forms of combustion heating. 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 PM2.5 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. ==Climate change aspects==

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 and scientists have pointed out oversights in the report, suggesting that climate impacts are worse than reported. Until around 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 studying the climate implications of biomass began to appear which 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." ==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 (lumber) will be significantly curtailed. == 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 affected by the building cycle leading to fluctuations in the supply of sawdust and offcuts. Per the New Hampshire Office of Energy and Planning release on Fuel Prices updated on 5 Oct 2015, the cost of #2 fuel oil delivered can be compared to the cost of Bulk Delivered Wood Fuel Pellets using their BTU equivalent: 1 ton pellets = 118.97 gallon of #2 Fuel Oil. This assumes that one ton of pellets produces 16,500,000 BTU and one gallon of #2 Fuel Oil produces 138,690 BTU. Thus if #2 Fuel Oil delivered costs $1.90/Gal, the breakeven price for pellets is $238.00/Ton delivered. == Usage by region ==

Europe Usage across Europe varies due to government regulations. In the Netherlands, Belgium, and the UK, pellets are used mainly in large-scale power plants. The UK's largest power plant, the Drax power station, converted some of its units to pellet burners starting in 2012; by 2015 Drax had made the UK the largest recipient of exports of wood pellets from the US. In Denmark and Sweden, pellets are used in large-scale power plants, medium-scale district heating systems, and small-scale residential heat. In Germany, Austria, Italy, and France, pellets are used mostly for small-scale residential and industrial heat. The UK has initiated a grant scheme called the Renewable Heat Incentive (RHI) allowing non-domestic and domestic wood pellet boiler installations to receive payments over a period of between 7 and 20 years. It is the first such scheme in the world and aims to increase the amount of renewable energy generated in the UK, in line with EU commitments. Scotland and Northern Ireland have separate but similar schemes. From Spring 2015, any biomass owners—whether domestic or commercial—must buy their fuels from BSL (Biomass Suppliers List) approved suppliers in order to receive RHI payments. The Renewable Heat Incentive scandal also referred to as the "cash for ash scandal", was a political scandal in Northern Ireland that centred on a failed renewable energy (wood pellet burning) incentive scheme. Pellets are widely used in Sweden, the main pellet producer in Europe, In 2014 in Germany, the overall wood pellet consumption per year comprised 2,2 million tonnes. These pellets are consumed predominantly by residential small-scale heating sector. The co-firing plants which use pellet sector for energy production are not widespread in the country. The largest amount of wood pellets is certified with DINplus, and these are the pellets of the highest quality. As a rule, the pellets of lower quality are exported. As early as 1997 a fully automatic wood pellet boilers with similar comfort level as oil and gas boilers became available in Austria. India In 2019, India started co-firing biomass pellets in coal-fired power stations around its capital city Delhi to reduce the air pollution caused by the stubble/biomass burning in open fields to clear the fields for sowing next crop. Plans are made to use biomass pellets for power generation throughout the country to utilize nearly 145 million tonnes of agricultural residue to replace equal quantity of imported coal in power generation. New Zealand The total sales of wood pellets in New Zealand was 300,000–500,000 tonnes in 2013. Recent construction of new wood pellet plants has given a huge increase in production capacity. Nature's Flame wood pellet processing plant, in Taupo, is due in late 2019 to double its annual production capacity to 85,000 tonnes. Azwood Energy operates a wood pellet processing plant in Nelson, utilising more than 1.2 million cubic metres of forestry residue each year to provide carbon neutral fuel for domestic use, hospitals, schools and industrial processes, including milk-processing. In 2025, the thermal electricity plant at Huntly is looking for a supply of black pellets. United States Some companies import European-made boilers. , about 800,000 Americans were using wood pellets for heat. The US wood pellet export to Europe grew from 1.24 million tons in 2006 to 7 million tons in 2012, but forests grew even more. ==Other uses==

Horse bedding When small amounts of water are added to wood pellets, they expand and revert to sawdust. This makes them suitable to use as a horse bedding. The ease of storage and transportation are additional benefits over traditional bedding. However, some species of wood, including walnut, can be toxic to horses and should never be used for bedding. In Thailand, rice husk pellets are being produced for animal bedding. They have a high absorption rate which makes them ideal for the purpose. Cattle fodder The biomass pellets made from edible matter can also be used as cattle fodder by importing from far away fodder surplus places to overcome the fodder shortage. Absorbents Wood pellets are also used to absorb contaminated water when drilling oil or gas wells. == See also ==