Indoor air pollution is caused by cooking with solid fuels and using old-fashioned cookstoves and fires. This makes homes dirty, causes eye and breathing problems, and contributes to deforestation. Many waste materials, from sawdust to rice husks, can be turned into clean-burning, easy-to-handle fuels that cut waste and carbon emissions. These can alleviate poverty and illness in off-grid homes.
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.
Ashden Award winner Kampala Jellitone Suppliers runs a biomass briquetting business in Uganda, supplying customers such as schools and bakeries which previously used firewood 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.
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. 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 winners Nottinghamshire County Council and Suffolk County Council have converted schools to run on biomass pellets, and Forest Fuels is a large-scale supplier of biomass fuel.
Ashden winner Sustainable Green Fuel Enterprise (SGFE) uses low pressure briquetting to make char-briquettes, which are a replacement for charcoal made from wood. SGFE buys waste char from wood-fired power plants, and 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 uniform product that is resistant to crumbling.
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 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 it. 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 or pellets.
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 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. 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.
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, 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.
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.
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.
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.