Briquetting is a way to convert loose biomass residues, such as sawdust, straw or rice husk, into high density solid blocks that can be used as a fuel. Biomass briquettes (including pellets, which are very small briquettes) replace fossil fuels or wood for cooking and industrial processes. They are cleaner and easier to handle, and cut greenhouse gas emissions.
- 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.
Read here to find out more about briquetting, and follow the links to watch short films and read detailed case studies of Ashden Award winners who make and use biomass briquettes.
How briquetting works
There are two approaches to briquetting. Both require the loose biomass to be ground to 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 1500 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.
There are three main types of briquetting machine (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 abraded by the biomass at the high pressures used. Even so, they wear out and need to be replaced. Lower pressures can be used if the die is heated, but this requires additional energy for heating.
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 1300 kg/hour.
Low pressure briquetting
Low pressure briquetting can be used for materials with a low amount of lignin, such as paper and charcoal dust. In this process, the powdered biomass 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 thus produced are left to dry, so that the binder sets and holds the biomass powder together. Low pressure briquetting machines are often hand operated, using a lever that drives a piston to compress the paste. In East Africa, people have traditionally made briquettes by hand from the dust that is left over from charcoal production. The charcoal dust is mixed with clay and rolled into balls which are left to dry. Ashden Award winner ARTI developed a portable kiln to produce char from sugar-cane leaves that are usually burned in the fields, and a hand press to make the loose char into briquettes. Similar systems have been developed for use with coconut husks and nut shells.
How briquetting systems and briquettes are used
Ashden Award winner Kampala Jellitone Suppliers (KJS) runs a biomass briquetting business in Uganda. The biomass used includes sawdust from local sawmills and furniture workshops, coffee husk, and other agricultural residues including rice husk and straw. Each material is ground, dried and stored separately, and appropriate proportions of each material are then mixed and fed to the briquetting machine. KJS runs two high-pressure piston machines to produce briquettes about 50 mm in diameter and 400 mm long. The briquettes are stored in sacks for delivery to customers, most of whom previously burned wood in stoves and furnaces. Customers are public institutions like schools and university halls that provide hot meals for students, and businesses like bakers, brewers and coffee roasters.
In India there are many local briquette producers. Ashden Award winner Nishant Bioenergy therefore focuses on developing stoves to burn the briquettes, to give users an alternative to cooking on increasingly expensive LPG. Their first product was a large briquette stove for schools, designed so that three cooks could work at the same time and cook meals for up to 650 students. A smaller version has now been developed for restaurants and roadside food stalls.
Pellet production has started in India. Ashden 2011 winner Abellon CleanEnergy produces pellets using both 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 pellet-fuelled boilers when replacements were needed.
What are the benefits of briquetting?
Briquetting is a way to make use of biomass residues that would otherwise go to waste, and replace the use of wood and charcoal (often produced unsustainably) and fossil fuels, thus cutting greenhouse gas emissions. Briquettes are easier to store and use for cooking than wood, because they are uniform in size and composition. They are much cleaner to handle than charcoal or coal, and produce less local air pollution.
There are some concerns about using field waste for briquettes, because it is sometimes 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 requires electricity or mechanical power. The energy input depends on the biomass used and the quality of the briquette produced, but is typically between 40 and 60 kWh/tonne, or only 3 to 9% of the heat produced by the briquettes. Extra heat may also be needed to dry the biomass, but this can usually be provided by burning below-specification briquettes.
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 US$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 price that briquettes can fetch depends on the price of the fuels that they replace. In India the price is typically US$80 per tonne delivered, to replace coal or LPG. Briquette use is more attractive to restaurants than schools because schools can buy subsidised 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. The viability of briquetting as a business is thus very site specific.
The cost of making wood pellets in Europe and USA is about U$130 per tonne, but they can be sold for US$290 per tonne to replace natural gas for heating. A small pellet machine capable of 450 kg/hour costs about US$11,000, but a complete automated plant with driers, grinders and handling equipment is much more expensive.
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, some which had to be imported from North America. Even in the UK, the demand and supply of wood pellets is increasing.
Since the 1990s, briquetting plants have been manufactured in both India and China. A 2007 estimate suggested about 250 operating plants in India, producing approximately 750,000 tonnes of briquettes per year. The demand for briquettes from industry is increasing, especially in South India. In 2002, there were about 600 briquetting plants in China and the number was increasing. There is a huge resource of biomass residues that could be made into briquettes, but the availability of cheap coal means that they are not cost competitive. However, the Chinese government has a programme to increase the production and use of biomass briquettes, with a target of one million tonnes in 2010.
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