Examples of how biomass is used
Here we explain biomass fuel types, applications and the four main technologies that use biomass.
Biomass generally refers to technology that is fuelled by organic material to generate electricity and or heat. The technology used is dependent on two factors:
1. Fuel type
Biomass fuels can be categorised as either dry or wet.
The energy conversion of dry biomass (energy crops, woody material from the timber industry and forestry as well as some agricultural residues) generally involves using heat.
The conversion of wet biomass (waste streams and agricultural residues) generally involves fermentation or digestion.
Biomass technologies can be used to generate electricity, heat, or both in a combined heat and power (CHP) plant.
The following applications are possible for biomass:
Electricity: plants designed primarily for the production of electricity are generally larger schemes, in the range 10 to 40MW. Excess heat generated as part of the process is usually not productively utilised.
Heat: plants designed for the production of heat cover a wide range of applications and sizes can range from a few kilowatts (domestic boilers) to above 5MWth (district heating schemes).
Combined Heat and Power (CHP): the primary product of CHP plants (typically sized from 5 to 30MWth) is the generation of electricity, but the excess heat is used productively. This could be through providing heat for industrial processes or in a district heating scheme.
There are four main biomass technologies. This section explains them and which applications are best suited for the various technologies.
Direct combustion is a mature and reliable technology. It is used to heat space or water (domestic boilers or wood stoves) or to raise steam to drive a steam engine or turbine to generate electricity (mainly co-firing in coal power plants). The fuel is dry biomass including wood pellets and wood chips.
The equipment ranges from very small wood stoves used for domestic heating to multi-megawatt plants for electricity production. State-of-the-art systems achieve efficiencies greater than 90 per cent.
- Domestic or single large buildings, such as schools and leisure centres
- District heating
- Large scale power or CHP plants.
Anaerobic digestion (AD) is a readily available and proven technology. It involves the decomposition of organic materials. These can be solid or fluid biomass or waste stream, such as agricultural, household and industrial residues and sewage sludge.
The decomposition occurs in the absence of air to produce biogas (which is high in methane content). This is a known as a wet process as the feedstock is liquefied prior to digestion.
The biogas is then burnt in a gas turbine, internal combustion engine or domestic boilers (through the gas grid) to produce heat and or electricity. Only minimal pre-treatment of the gas is required before combustion.
- community district heating
- biogas cleaned and fed into the existing gas grid.
Pyrolysis is an emerging and relatively expensive technology. It involves heat treatment of the feedstock (solid biomass or waste) at very high temperatures, in the absence of oxygen. This is similar to traditional charcoal production. Pyrolosis produces a combustible gas or liquid (oil).
The fuel produced is used to generate heat and or electricity in an internal combustion engine or gas turbine. Total electrical efficiencies are approximately 20 per cent.
The only application is commercial at a large scale (>2MWe).
Similarly to pyrolysis, gasification is an emerging technology although it is more commercially viable at large scales. Solid fuel (solid biomass or waste) undergoes incomplete combustion in a limited air supply to produce a combustible gas - syngas.
Syngas can be burned in a boiler, or used as fuel for an engine or gas turbine. The fuel conversion efficiency of the gasification process is in the range of 60 to 70 percent.
- industrial and commercial buildings (scale =100kWe)
- smaller plants of under 5 MW are becoming more common.
1 May 2012