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Pellets are a solid biomass fuel, mainly produced from wood residues but also from agricultural by-products such as straw. They have a cylindrical form with a diameter of 6 – 12 mm. Specific advantages of pellets as compared to un- processed biomass include: standardized properties, high energy content, high density and therefore reduced costs for transport, storage and handling. Pellets are used for residential heating in pellet stoves and pellet boilers, for the gene- ration of heat, steam and electricity in the service industry, manufacturing and power generation.
Bioenergy is the most important renewable energy . When compared to other types of bioenergy, the pellet sector is one of the fastest growing.
Pelletizing technology was invented in the USA in the 1930’s for the production of feed pellets. Bark was pelleted in the United States in the 1950‘s as a way to reduce waste volumes in the sawmill industry. Production of wood pellets for the heating market took off in USA in the 1970s by the time of the first oil crisis followed by Swedish start-ups ten years later.
Sawmills in northern Sweden introduced pellets into the market around 30 to 40 years ago in order to reduce the cost of transport from Northern Sweden to the main consumption regions in the South. Since then, the use of pellets grew continuously for residential heating, power generation, and for heat generation in the manufacturing and energy sectors.
Pellet is a term used for a small particle of cylindrical form produced by compressing an original material. The term is used in a variety of contexts. In the context of energy, we distinguish between wood pellets and agro pellets based on the feed- stock used. At present, pellets are mainly produced from wood residues, though the volume of pellets produced from agricultural by-products such as straw, husks of sunflower seeds and stalks and leaves of corn etc. is increasing.
A key advantages of pellets compared to unprocessed biomass is the high density and high energy content per unit volume . This leads to significantly reduced costs for transport, storage, handling and use.
Another advantage is that they can be produced with standardized properties. This enhances their usability because boilers, stoves or pellet burners can be designed and constructed taking into ac- count the standard fuel properties.
As a rule of thumb it can be said: 3m³ pellets ~ 2 ton pellets ~ 1 000 l heating oil. Standardized quality is also a pre- condition for trading. ISO standards for pellets have recently been adopted that specify different quality classes for industrial use and for use in devices for residential heating (ISO 17225-2). In addition, a quality certification scheme has been in- troduced by the European Pellet Council,ENplus,that guarantees consistent quality of pellets for residential heating purposes by introducing standardized quality management procedures for production and trade. This so-called ENplus certification is now widely used both in Europe and North America.
Compared to other technologies of up- grading biomass, pelletisation is a fairly efficient, simple and low cost process. The four key steps within this process are:
• pre-milling of raw material
• drying of raw material
• milling of raw material
• densification of the product
These steps enable the production of a homogeneous fuel with low humidity and high energy density. In case dry raw materials are available, only milling and densification is necessary.
Currently about 80 % of globally produced pellets are made from woody bio- mass. In most cases, by-products from saw mills such as saw- dust and shavings are used. Some large pellet mills also use low value wood as raw material. An increasing volume of traded pellets are being made from such materials as empty fruit bunch (from oil palm), bagasse, and rice husk.
The world’s largest pellet plant in terms of pellet output is the Georgia Biomass Plant (USA) constructed by Andritz. This plant uses fast growing wood logs produced in pine plantations. The logs are debarked, chipped, dried and milled before densification in pellet mills. The Georgia Biomass Plant capacity is about 750 000 tonnes of pellets a year . The wood demand of this plant is similar to that of an average paper mill.
Small-scale technology for pellet production is typically based on sawdust shavings and off-cuts from sawmills or wood processing industries (Producers of floors, doors and furniture etc.) which adds value to their by-products by converting into pellets. Dry raw material is milled, and if needed, adjusted to precisely the right amount of humidity and the optimum temperature by pre-conditioning with steam prior to entering the pellet mill where it is densified. A cooler after the pellet mill reduces the temperature of the hot pellets after which the pellets are sifted be- fore being bagged, or conveyed to finished product storage.
A pelleting plant needs electric energy mainly for the milling and the densification of raw material. The amount of electricity needed can vary between 100 to 200 kWh per ton of produced pellets, depending on the raw material used and the efficiency of the plant. If the prod- uct needs to be dried, additional thermal energy is necessary. Most plants use bark as a renewable fuel for generating the necessary heat. The heat demand depends on the humidity of the raw material and can amount up to 1 000 kWh per ton if wet saw dust is used .
The energy content of one ton of pellets is approximately 4 700 - 4 900 kWh. Consequently, the energy needed to produce the pellets can vary between 2 % and 25 % of their energy content.
Energy is also needed to transport the fuel from the producer to the user. The amount of energy required for transportation depends on the type of transport technology used. For a given distance, the lowest energy demand is related to ship transport, the highest energy demand is for truck transport.
Transporting pellets in a truck over 200 km needs an amount of energy equivalent to 1 % of the energy contained in the pellets. Transporting pellets in a large ocean vessel over 5 000 km needs an amount of energy equivalent to 1 – 2 % of the energy content.
As the major part of energy demand (which is used for drying) is usually provided by renewable energy (bark), the CO 2 balance of using pellets is very positive. CO 2 reduction by conversion of a heating system or a power plant from using fossil fuels to fuelling by pellets is typically be- tween 80 and 90 % . In case of short trans- port distances from local pellet producers to residential consumers, CO 2 reduction compared to use of heating oil can reach 95 %.
Europe is currently the largest market for pellets. About half of the pellets are being used for power generation plants that have been converted from coal to pellet use or to partially replace the coal as fuel (co-firing). The other half of pellet consumption is used mainly for the generation of heat in households – either by pellet stoves or pellet boilers, for heating residential blocks, public or commercial buildings and for industrial steam demand. The possibility to use pellets at almost any scale of demand, from small domestic appliances with few kW of output all the way up to huge power plants with hundreds of MW of power is one of the big advantages of pellets.
Pellet stoves are highly efficient clean- burning devices that offer automatic operation and heat at very competitive prices. They can be purchased for 1 000 to 2 000 Euro and lead to annual savings of heating costs of 400 – 900 Euro , depending on the country and the replaced fuel. If pellets replace electric heating or heating oil, a payback of 1- 2 years can be achieved. Pellet stoves were invented in the USA during the first oil crisis and are currently extensively used in the USA, Italy, France, Spain, Greece and a number of other countries.
In developing countries like India, pellets are being used for cooking. They are usually sourced from agricultural residues and they help replace the burning of firewood in rural communities. This has many advantages including: cheap fuel, improving health and economic conditions of rural communities.
Residential pellet boilers offer even more convenience than pellet stoves . They use large bulk storage and offer fully automated heat supply. They are frequently used in countries with cold climates, such as Austria, Germany, Denmark or Sweden.
The advantage of using pellets in coal-fired power plants is that only minor modifications of the power plant need to be realized to use pellets as a fuel. This enables a plant to produce large amounts of renewable electricity with comparatively low investment costs. Usually plants being converted are adapted for co-firing which use pulverised coal as fuel. Pellets are milled and turned into wood powder, which is either burned together with coal dust or burned alone.
Pellets have great possibilities to re- place fossil fuel in large industries, for example in the pulp industry. Pellets are normally pulverized (ground) into wood powder, which can be burned as such or together with oil, gas or ground black coal.
Most pellets are currently being produced in Europe. This is due to the fact that European policies aimed at increasing the use of renewable energy have led to a significant demand for pellets. More recently, growth of European pellet production has slowed down while North American pellet production has accelerated.
As demand for fibre wood in the U.S. has declined by almost 100 million tonnes over the last 15 years – due to the decreased demand for paper production – ample raw material is available at low prices . This creates a very attractive situation for pellet production. A major part of U.S. pellet production is exported to Europe, mainly for use in power plants. Russia could – in the future – also become a major supplier of pellets, in view of the vast forest resources of this country. Recently China has emerged as a major pellet producer. In China, mainly straw and stalks from corn and other agricultural crops is used as raw material for pellet production.
Pellets are more expensive than coal but significantly cheaper than heating oil. Consequently, pellets replacing coal for use in power plants needs to be subsidised while its use in the heat market is economic on its own, without subsidy in most European Union countries.
Pellets cost only half as much as heating oil. This makes the conversion to pellet heating attractive, particularly for buildings with large heat demand. Technically, pellet boilers are more complex than boilers fuelled by heating oil and gas. Consequently investment costs are significantly higher and this may form a barrier for market development. For this reason, investment subsidies have been an important tool to kick off market development for pellet boilers. Pellet stove markets have been able to develop with or without financial incentives due to the low upfront investment cost.
In the power sector, the use of biomass pellets simplifies and reduces the costs of the conversion of coal fired power stations to use of renewable energy . Further the boiler capacity is better maintained compared to fuelling by fossil fuels due to the higher energy content and low moisture of the pellets vs. raw biomass.
It is noteworthy that in many countries, market development has hardly started. This is due to the fact that market kick-off usually needs policy initiatives, which have not yet been adopted by many countries. There is an imbalance between the support for renewable electricity – which has been promoted throughout Europe and in many countries worldwide – and the promotion of renewable heat, which has been widely neglected even though the economics of using renewable heat are better than the economics of using renewable electricity.