Biomass briquettes and pellets
With increasing pressure on the earth's resources, turning different types of organic waste into clean-burning fuel helps save forests and cuts greenhouse gas emissions by replacing wood, charcoal and fossil fuels for cooking and industrial processes.Aside from being cleaner and easier to handle, biomass briquettes are also less polluting.
- High pressure briquetting plant can produce over 200 tonnes of briquettes per day.
- About five million tonnes of biomass pellets used in Europe in 2010.
- Briquettes and pellets sell for between about US$80 and US$300 per tonne.
How briquetting works
There are two approaches to briquetting, both of which require the loose biomass to be ground into a coarse powder like sawdust.
High pressure briquetting
High pressure briquetting uses a power-driven press to raise the pressure of dry, powdered biomass to about 1,500 bar (150 MPa). This compression heats the biomass to a temperature of about 120°C, which melts the lignin in the woody material. The press forces the hot material through a die at a controlled rate. As the pressure decreases, the lignin cools and re-solidifies, binding the biomass powder into uniform, solid briquettes.
The three main types of high-pressure briquetting machine are (see diagrams):The piston press, which uses an oscillating piston to compress the biomass, and produces cylindrical briquettes, 50 to 100 mm in diameter.
The screw press, which uses a tapered screw, and produces longer, hollow briquettes.
The pellet mill, which compresses the biomass between rollers and makes smaller cylindrical pellets (similar to animal feed pellets) 6 to 12 mm in diameter.
The dies and moving components in the machines need to be made from hardened steel, because they are worn down by the biomass at the high pressures used. Even so, they need to be replaced quite often. Lower pressures can be used if the die is heated, but this requires additional energy.
High pressure briquetting machines are produced in a wide range of sizes. For example, one supplier provides a range capable of processing 30 kg/hour to 1,300 kg/hour. The energy required to run the plant is only about 5% of the heat produced by the briquettes.
Low pressure briquetting
Low pressure briquetting can be used for materials with a low amount of lignin, such as paper, and can also make use of charred residues and charcoal dust. In this process, the powdered material is mixed into a paste with a binder such as starch or clay, and water. A briquetting press is used to push the paste into a mould or through an extruder, or it can simply be shaped by hand. The briquettes produced in this way are dried so that the binder sets and holds the biomass powder together. Drying can be done outdoors, particularly if it is sunny, but can also make use of waste heat from other processes. Low-pressure briquetting machines can be electrically powered, but are often hand operated, using a lever that drives a piston to compress the paste.
How briquetting systems and briquettes are used
Ashden Award winner Kampala Jellitone Suppliers (KJS) runs a biomass briquetting business in Uganda, supplying customers such as schools and bakeries which previously used wood as a fuel. Briquettes are made using a high-pressure briquetting press, fed with sawdust from local sawmills and furniture workshops, coffee husk and other agricultural residues. High-pressure piston presses produce briquettes, which are delivered in sacks to customers.
India has many local briquette producers, so Ashden Award winner Nishant Bioenergy focuses on developing stoves to burn the briquettes, to give users such as schools and food stalls an alternative to cooking on increasingly expensive LPG.
Pellet production has started in India. Ashden winner Abellon CleanEnergy produces over 70,000 tonnes of pellets each year from agricultural residues and sawdust. Most are used to replace lignite and coal in factory boilers. The market for biomass pellets is also increasing in both the USA and Europe, because pellets can be used in domestic and institutional boilers with automated fuel handling systems. Ashden UK Award winner Nottinghamshire County Council ran a programme to convert coal boilers in schools to run on pellets, and also install new pellet-fuelled boilers.
Ashden winner Sustainable Green Fuel Enterprise (SGFE) uses low pressure briquetting to make char-briquettes, which are a replacement for wood-charcoal. SGFE buys waste char from wood-fired power plants, and also produces char by gasifying coconut shells. The char is mixed with tapioca flour and water and extruded in an electrically powered briquetting machine. The waste heat from the coconut shell gasifier is used to dry the briquettes, producing a strong, uniform product.
What are the benefits of briquetting?
Briquetting makes use of biomass residues that would otherwise go to waste, this can bring a useful income stream to the farms and sawmills that produce residues. By replacing the use of wood and charcoal (often produced unsustainably) and fossil fuels, briquettes cut greenhouse gas emissions and deforestation.
Briquettes are easier to store and use for cooking than wood or charcoal, because they are uniform in size and composition. They are cleaner to handle than charcoal or coal, and produce less local air pollution.
There are some concerns about using field waste for briquettes, because they are also valuable as a soil improver. However, residues like sawdust and rice husk have limited agricultural use and can be a fire hazard, as can pine needles.
High-pressure briquetting plants are costly because the equipment needs to withstand the pressures involved. For example, a typical piston press from India with a 65 kW motor costs about U$17,000, and the whole plant (including driers, grinders and handling equipment as well as the press) costs about US$50,000. Such a plant produces about 700 kg/hour of briquettes, or about 1,500 tonnes per year. In India, production costs about US$60 per tonne, although this varies depending on the cost of the agricultural residues.
The cost of making briquettes depends on the cost and availability of raw materials, and the price that briquettes can fetch depends on the price of the fuels that they replace: this makes the viability of briquetting as a business very site specific. In India the price is typically US$80 per tonne delivered, to replace coal or LPG. In Uganda, wood costs about US$140 per tonne and briquettes can fetch US$160 per tonne because they are more convenient to use and easy to store.
Per tonne, charcoal is more expensive than wood because of its higher energy content, and the same applies to char-briquettes. SFGE’s cheapest char-briquettes retail for about the same price as wood-charcoal in Cambodia, about US$0.35 per kg (US$350 per tonne). However, many restaurants are prepared to pay twice as much for their highest-quality briquettes, because these burn more slowly and evenly.
Briquettes have been used as fuel for many years in Europe and the USA, mainly on remote farms. Since 2000, there has been a rapid increase in the production and use of wood pellets, particularly in Sweden, Germany and Austria, because they can be used in automated boilers for space heating. An estimated five million tonnes of biomass pellets were used in Europe in 2010.
Briquetting plants have been manufactured in both India and China since the 1990s. A 2007 estimate suggested about 250 operating plants in India, producing approximately 750,000 tonnes of briquettes per year, with strong demand from industry. There is a huge resource of biomass residues that could be made into briquettes in China, and it is actively promoted by the government, although the availability of cheap coal means that they are often not cost competitive.
In many countries there are significant resources of biomass residues, which are not needed for agriculture. Thus as the cost of conventional fuels increases, the use of biomass briquettes and pellets is likely to grow.
- View our biomass briquettes and pellets photo collection on flickr
- FAO (1996) report on briquetting
- FAO (1990) report on biomass briquetting (including pictures)
Lead authors: Dr David Fulford and Dr Anne Wheldon