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Biomass boilers

Information

Biomass boilers burn biomass fuel (most often wood) to create heat for domestic and commercial properties.

There are various different types of boilers, which use differing forms of processed fuel including wood logs, wood pellets and wood chips, miscanthus (an energy crop grass), straw, or other biomass (waste wood, willow, linseed, bean, rape, straw, etc.). However, the general design of the boilers, despite their fuel type, is fairly similar. This is a relatively new technology, resulting in safety standards being high. Emergency stops and controls will be accessible in close proximity to the apparatus.

Biomass fuel is usually stored on site and automatically fed into a boiler system through a hopper or feed system. The fuel is burnt in a combustion chamber in the boiler. Commonly, the combustion chamber has an automatic ignition system. Sensors closely monitor the temperature in the chamber, which can be adjusted through the supply of fuel and fan speeds, as well as shut off completely. The heat produced is transferred to the heat exchanger in the boiler, which heats up water to be pumped into the central heating system.

Some forms of biomass installations may include a buffer tank (also called a thermal store), which is located between the boiler and the rest of the heating system. These are highly insulated water tanks that act as heat stores and give instant hot water if demand is high. Some modern pellet boilers may pump straight into underfloor heating or radiators, but log and chip boilers need buffer tanks. Other forms of heat sources, such as solar thermal and heat pumps, may also be connected to the buffer tank.

A secondary product of the combustion process is ash, which is collected in the bottom of the biomass boiler system. Ash primarily consists of the non-combustible mineral constituents of the fuel as oxides or salts. In some instances, ash is used as a fertiliser.

Biomass boilers in buildings will vary in size and will be located according to building requirements. They range from domestic units to industrial-sized boilers supplying buildings such as schools or office blocks.

Large-scale biomass boiler systems may be connected to a district heating system to provide heat for a number of buildings. A pipeline to distribute the heat to neighbouring properties will connect these buildings.

Biomass stoves are purpose-built closed metal containers in which biomass can be combusted to create hot air to heat a room. The outer surface of these stoves gets very hot during operation. A chimney extracts emissions from the combustion process. Fuel is commonly stored on site for such stoves.

Both log and pellet stoves are available. Pellet stoves can operate automatically, with pellets loaded into a storage space in the stove; from there, they are fed automatically and ignited electrically.

Hazards (for further information refer to National Operational Guidance: Utilities and fuel)

  • Hot outer surfaces
  • Electricity supplies present
  • Hypoxic atmospheres (for example, wood storage facilities)

References and further reading

Information provided by the Renewable Energy Association (www.r-e-a.net/)