Forestry Commission Scotland logo



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    Accumulator tank

    A large, well insulated hot water tank, plumbed in to take heat directly from a boiler. They are often used to help smooth some of the peaks and troughs in demand.

    Arboricultural arisings

    Woody material produced during arboriculture (see below).


    The management of individual trees or groups of trees primarily for their amenity value (e.g. timber).

    Archimedes screw (auger)

    Rotating spiral of metal inside a tube used in boiler feed mechanisms for wood chips or pellets, and in automated ash extraction mechanisms.


    The non-combustible, mineral content of biomass. During burning ash is left behind in, or under, the grate or combustion region, or at the bottom of a gasifier. Fly ash is the very small particles of ash that are carried out of the system along with the flue gases.


    See archimedes screw.

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    Diesel made from biomass, such as oilseed rape.


    Energy produced from biomass. Sometimes the potential energy within biomass.


    Ethanol made from biomass.


    Liquid fuel made from biomass, such as biodiesel or bioethanol.


    Biological material such as wood, miscanthus, dung or straw.


    Device for extracting heat energy from fuel to provide hot water or steam.


    The branch-wood and leaf material that is generally too small in diameter to be considered part of the timber product from a harvesting site.


    Large cylindrical section of compressed dried sawdust. Used as fuel in a similar way to logs.

    Bulk density

    The weight per unit volume for a solid materials.

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    Calorific value (CV)

    The amount of heat produced by the complete combustion of a unit weight of fuel, often expressed in megawatt hours per cubic metre (MWh/m3).

    Carbon cycle

    The cycle by which carbon in various forms moves between the Earth's atmosphere, hydrosphere (seas and oceans), lithosphere (rocks, soils and mineral deposits, including fossil fuels) and biological material including plants and animals.

    Carbon emissions

    The level of carbon, usually in the form of carbon dioxide, exhausted into the atmosphere by a process.

    Carbon neutral (low carbon)

    Processes that have a zero or very low contribution to atmospheric carbon.


    Machine, mobile or static, used for the production of woodchips.


    The process of making small fragments of timber (usually to a specified size) by passing larger logs or pieces of timber through a chipper.


    See wood chips.

    Clear felling

    A harvesting operation that removes all the trees from a given area.


    Genetically identical trees propagated commercially because of desirable properties. Often used in, for example, short rotation coppice.

    Combined heat and power (CHP)

    Systems that derive and utilise both thermal and electricity energy from a given fuel source.


    The process of converting fuel into heat energy.


    The practice of harvesting the above ground part of a tree for timber and allowing the new shoots to re-grow in a cycle normally extending over a number of years.


    In forestry, collectively describes the trees in a productive woodland.

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    District heating scheme

    System by which heat is distributed via underground pipe-work from a single centralised boiler to a number of domestic or commercial units.

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    Embedded energy

    The total amount of energy used in the production of a fuel or product (e.g. planting, harvesting, processing, transport).

    Energy crop

    Typically a crop of trees, woody plants or grasses grown for the purpose of providing biomass for energy.

    Energy density

    The amount of energy stored per fuel unit, by volume or mass.


    Energy services company. Sells heat to the customer rather than a boiler and/or fuel.

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    Cutting down standing trees.


    A chimney-like tube for the venting of gases and vapour from the boiler after combustion.


    The process of extracting harvested timber from a woodland whilst keeping the timber off the ground (involving a tractor and trailer unit known as a forwarder).

    Fossil fuels

    Fossilised fuel (e.g. coal, mineral oil and natural gas) that, when burnt, releases carbon into the atmosphere. Burning fossil fuel can contribute to an increase in atmospheric greenhouse gases.

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    Gas turbine

    Device for turning thermal energy from gasified biomass into electrical energy.


    Conversion of solid material such as woodchip into a gas for use as a fuel.

    Global warming

    The rise in the earth's atmospheric temperature. Consider to be partly due to man-made atmospheric pollution contributed to by the use of fossil fuels.

    Greenhouse gas

    Gas capable of absorbing infra-red radiation before it escapes the earth's atmosphere (e.g. carbon dioxide, methane).

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    The branch wood and leaf material that is generally too small in diameter to be considered part of the timber product from a harvesting site.


    In forestry, the practice of felling and removing trees from a woodland.


    The movement of material (e.g. timber) from site to site, usually by lorry.

    Heat exchanger

    Device, within a boiler or water tank, to transfer heat from one system to another.


    Fuel storage container usually adjacent to a boiler house.

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    A unit of energy equal to 1000 watts.

    Kilowatt hour (kwh)

    Energy unit equal to one kilowatt of power used for one hour. A single bar electrical fire would typically use about 1 kwh of electricity if operated for 1 hour.

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    The energy generated by a boiler.


    Sections of a tree or branch cut to a length. When cut for use in a stove or boiler, the length is specified.

    Lop & top

    Alternative term for Brash.

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    Megawatt (Mw)

    Unit of energy equal to one million watts, or one thousand kilowatts. Sometimes MWe is used.

    Megawatt hour (Mwh)

    Measure of energy production or consumption equal to one million watts produced or consumed for one hour.


    A grass often planted as an energy crop.


    The small scale production of heat and/or electricity, usually from a low carbon source. The Energy Act 2004 gives maximum size limits of 50 kW electricity and 45 kW thermal.

    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.

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    Oven dry tonne (odt)

    One tonne of material (woodfuel) at 0% moisture content.

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    See wood pellets


    The chemical decomposition of wood chips by the action of heat to produce a liquid fuel.

    Pyrolysis oil

    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.

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    Renewable energy

    Energy that has been obtained from a potentially infinite resource, such as wood, sunshine or the wind.

    Renewable fuel

    Fuel that is considered to be an infinite resource if its source is sustainably managed. Examples include woodfuel, miscanthus or the wind.


    The period of growth of a forest crop before it is considered mature.


    See small round wood.

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    Short rotation coppice

    The practice of growing and harvesting a crop (usually willow) just above ground and then allowing new shoots to re-grow from the harvested stumps (also known as stools) in a short cycle (3 to 5 years).

    Short rotation forestry

    The practice of growing a specific crop of trees, usually poplar, for biomass over a relatively short rotation (usually no more than 15 years).


    Fuel storage container usually adjacent to a boiler house; can be below ground.


    The care and cultivation of forests.


    The process of extracting timber from a woodland site by winch, usually dragging part of the timber on the ground.

    Small round wood

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

    Solid fuel

    Fuel in solid form, typically wood or coal, often used in stoves, open fires and some boilers.


    A group of plants (e.g. trees).

    Steam turbine

    Device for converting thermal energy from steam into electrical energy.


    Device for use indoors to provide room heat, usually from solid fuel.


    Meeting the needs of the present without compromising the ability of future generations to meet their needs.

    Sustainable forest management

    The process of managing forests to provide goods and services without degradation of the site quality, and without a decline in the yield of goods and services over time.

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    Thermal power station

    A power plant in which water is heated, turns into steam which spins a turbine and drives a generator.


    The process of removing selected trees from an immature woodland to allow other trees to increase in diameter.

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    A unit of energy defined as one joule transferred or dissipated in one second.

    Wood chips

    Small fragments of timber sliced from a larger log or piece of timber by a chipper, usually to a specified size.

    Wood pellets

    Small diameter cylindrical section of compressed sawdust formed by passing dry sawdust under high pressure through a dye. Usually between 6 to 8mm in diameter and 10 to 30mm in length.

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    The timber volume increment of a specified area of woodland over a specified time. Usually expressed in cubic meters per hectare per year.

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