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Terms and abbreviations used within this website and the biomass sector

Accumulator Tank

A large, well insulated hot water tank. Accumulators are plumbed in to take heat directly from the boiler, they then have a secondary heating coil to take heat for heat and hot water to the rest of the system. This has the effect of decoupling heat demand from heat production, allowing the boiler to operate at best efficiency and to accept interruptions to the heat supply. It also allows some leeway in the specification of the boiler, as the accumulator is able to smooth some of the peaks and troughs in demand


The non-combustible, mineral content of biomass. During combustion bottom ash is the ash that is left behind in, or under the grate or combustion region, or at the bottom of a gasifier. Fly ash consists of very small particles of ash that are carried out of the system along with the flue gases. Both types of ash can potentially be spread on the land as a soil conditioner, to help return the mineral content to the soil, however owing to the potentially very high concentration of minerals it is important that this is done with caution. In particular, if there is the potential presence of heavy metals or arsenic in ash as a result of accumulation from soil or air, or from wood treatment, it must be checked before the ash is used in this way. Alternatively, ash may be used in the manufacture of construction blocks and cement. Ash that melts or fuses is known as slag or clinker.


Solid, liquid or gaseous fuel that is derived from biomass. Biomass that has been processed or converted in some way into a more convenient form, principally to increase energy density. This may involve physical pre-processing simply to cut it into more manageable pieces or reduce the moisture content, or may involve thermal or chemical processing to convert it into a sold, liquid or gas.


A mixture of methane (CH4) and carbon dioxide (CO2) produced by anaerobic digestion, with small amounts of other gases. The methane is a flammable gas, chemically identical to the main constituent of natural gas, and can be used as a fuel for heat and/or electricity generation. Biogas is effectively the same as landfill gas, which is produced by the anaerobic decomposition of organic material in landfill sites.


Material that is derived from living, or recently living biological organisms. In the energy context it is often used to refer to plant material, however by-products and waste from livestock farming, food processing and preparation and domestic organic waste, can all form sources of biomass. With such a wide range of material potentially described as biomass, the range of methods to process it must be equally broad.


Low density forestry material consisting of tops of trees and small branches. Also referred to as ‘lop and top’.


Common Agricultural Policy. The system of support for agriculture in the EU. Proposed in 1960, expenditure under CAP represents about 44% of the EU budget, however it is currently under major reform and is being replaced by single farm payments. CAP included import tariffs, minimum prices, and subsidies to farmers.

Carbon cycle

The cycle by which carbon in various forms moves between the various components of the Earth’s biosphere, between the atmosphere, hydrosphere (seas and oceans), lithosphere (rocks, soils and mineral deposits, including fossil fuels) and biological material including plants and animals. Carbon is constantly moving between some of these forms, maintaining a state of dynamic equilibrium. Other forms, most notably fossil fuels, can potentially store carbon indefinitely, however if they are burned the carbon is released and makes a net addition to the carbon cycle and raising the total free carbon. If biomass is used without replacement, as for instance can happen in the case of forest clearance such as in the Amazon rain forest, this too can make a net addition to the carbon cycle. Sustainable use of biomass, however, makes no such direct net addition as growing replacement plant material absorbs the carbon released by the harvested biomass.


Chromated Copper Arsenate (CCA). A water-borne, combined fungicide and insecticide that includes arsenic for the treatment of wood. It was developed in 1933 and has been used widely in the UK and around the world. The arsenic is carried off in flue gases during combustion if treated wood is burned, and can be volatilized, making it difficult to trap with conventional filters so CCA treated timber must not be burned except in suitable equipment. Modern, less toxic alternatives are now widely available.

Combined heat and power (CHP)

A system in which the heat associated with electricity generation is also used for space heating or process heat. In this way the overall efficiency of the process in terms of the proportion of the energy in the biomass fuel that is made use of is increased considerably. Also known as co-generation.


Chopping up material, such as biomass, into small fragments.

Embedded energy

The total amount of energy used in the production of a fuel or product. For woodfuel this may include contributions from land preparation, planting, fertilizer and pesticide inputs, thinning, harvesting, comminution, processing, and transport.

Energy density

The amount of energy stored per unit volume (volume energy density) or mass (mass energy density) of a fuel. A high energy density generally make storage and transport of a fuel more convenient. Fossil fuels typically have higher energy density than solid or wet biomass fuels, though converted liquid biofuels and biogas are similar to those of their fossil counterparts.


Energy supply company. Sells heat to the customer instead of a boiler and/or fuel. May install, own and maintain the boiler, or may sub-contract some or all of that. Heat used by the customer is metered, usually as hot water flow rate and temperature difference between outflow and return. Fully responsible for ensuring continuous operation and suitable quality fuel supply. Particularly well suited to district, site or large or multiple building heating systems.

Greenhouse effect

The effect of trapping heat by the transmission of visible solar energy, and the absorption (or reflection) of infra red heat energy. This can be done by a sheet of glass or a layer of some gases, known as greenhouse gases. The greenhouse effect is beneficial to mankind as well as harmful as, without it, the surface of the earth would be around 33ºC cooler than it is. The majority of this is natural, changes very slowly and consequently the earth’s ecosystems have evolved to function best with the greenhouse effect at this natural level. Since the industrial revolution, however, and the rapidly increasing use of fossil fuels, atmospheric levels of greenhouse gases, particularly carbon dioxide (CO2) have increased extremely rapidly to the highest level they have been at for 650,000 years. This is because carbon that was removed from the atmosphere many millions of years ago, and resulted in a reduction of atmospheric CO2 levels then, is now being extracted as coal, oil and natural gas and returned to the atmosphere.

Greenhouse gas (GHG)

Any one of a number of gases that trap heat in the earth’s atmosphere in a similar way to which the glass traps heat in a greenhouse. They work because they transmit light at the visible wavelengths that form the majority of the sun’s energy incident on the surface of the earth. When the surface of the earth absorbs this and warms up it re-radiates heat energy at much longer wavelengths, in the infra red, and this is absorbed by GHG molecules such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), ozone (O3) and water vapour (H2O). Although carbon dioxide is the most widely known about, as it is responsible for the majority of anthropogenic climate change, other GHGs have a much greater greenhouse gas potential per molecule.

Iodine value

A measure of the number of unsaturated carbon-carbon double bonds in a vegetable oil molecule. By titrating the oil with iodine, which reacts at the double bond sites, the number of such sites can be measured. Double bonds in vegetable oils make the molecules less flexible, and mean that they solidify at a lower temperature. In liquid biofuel applications this gives a lower cold filter plugging point (CFPP) or cloud point. While this makes it good for use in cooler temperatures, double bonds can allow polymerization, leading to the formation of lacquers and possibly blockage and damage to engine or fuel train components.

Landfill gas

The mixture of gases produced from the anaerobic breakdown of organic material in landfill. Typically consists of 50-60% methane (CH4), 30-40% carbon dioxide (CO2), 10% nitrogen (N2), small amounts of other volatile organic compounds (VOCs). Methane is a very potent greenhouse gas, 21 times more so than carbon dioxide.

Moisture content

The proportion of water in a sample of biomass, defined as the weight of water as a percentage of the weight of biomass. This can be defined on either a wet basis, as a percentage of the total (wet) weight of the sample, or a dry basis, as a percentage of the oven dry weight of biomass. Wet basis is usually used for fuel purposes.

Producer gas

The mixture of gases produced by the gasification (BEC Gasification) of organic material such as biomass at relatively low temperatures (700 to 1000ºC). Producer gas is composed of carbon monoxide (CO) and hydrogen (H2) plus carbon dioxide (CO2) and typically a range of hydrocarbons such as methane (CH4). Producer gas can be burned as a fuel gas such as in a boiler for heat or in an internal combustion gas engine for electricity generation or CHP. The composition of the gas can be modified by choice of gasification parameters to be optimized as a fuel gas (producer gas) or synthesis gas which contains almost exclusively CO and H2 and is suitable for synthesis of liquid biofuels.


Power take-off. Drive shaft from tractor, or other vehicle engine used to drive ancillary machinery. Can be used to power some wood chippers.

Pyrolysis oil

Bio-oil produced by fast pyrolysis of biomass. A dark brown, mobile liquid containing much of the energy content of the original biomass, with a heating value about half that of conventional fuel oil. Can be burned directly, either alone or co-fired with other fuels, gasified or otherwise upgraded.

Conversion of raw biomass to pyrolysis oil represents a considerable increase in energy density and it can thus represent a more efficient form in which to transport it.

Small round wood (SRW)

Small trunk or branch wood of diameter 7-14 cm.

Synthesis gas

A mixture of carbon monoxide (CO) and hydrogen (H2) which is the product of high temperature gasification of organic material such as biomass. Following clean-up to remove any impurities such as tars, synthesis gas (syngas) can be used to synthesize organic molecules such as synthetic natural gas (SNG- methane (CH4)) or liquid biofuels such as synthetic diesel (via Fischer-Tropsch synthesis).


Tonnes of oil equivalent. A measure of energy used to relate different fuels to the equivalent oil requirement based on an energy value for oil of 42 MJ/kg.

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