Biomass Glossary

This section contains a glossary of biomass terms that you might find useful when researching biomass information. This list will be added to on a regular basis, however, if you feel that there should be a term here that is currently not listed, please email a request to add it to:

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Accumulator Tank: similar to a battery storing electricity, a well insulated hot water tank stores hot water produced by a boiler for later use, acting as a “buffer” which allows the boiler to operate at maximum efficiency without the need to switch on and off constantly during the peaks and troughs in demand. Some accumulator tanks have a secondary heating coil which allow them to store heat from multiple sources such as a solar thermal panel. Accumulator tanks can also feature built-in domestic hot water tanks which can supply hot water as well as space heat.

Ash: The incombustible constituents of wood fuel, mainly potassium (K), sodium (Na), phosphorus (P), calcium (Ca) and silicon (Si). Typically, wood contains between 0.5% and 2% ash. However this level of deposit can only be achieved at very high combustion temperatures and efficiencies. Wood ash is traditionally used by gardeners as a good source of potash. See our wood fuel information page for more information.

Auger: A rotating, screw-type narrow cylinder device (like a cork screw) that allows the automatic transfer of wood pellets from the fuel pellet hopper to the pellet boiler. Standard Varmebaronen augers come in 1.5 meter and 2.5 meter lengths.


Bark: A non-technical term for the outer protective layer of a tree, consisting of inner bark and outer bark. The inner bark is a layer of living bark that separates the outer bark from the cambium and in a living tree is generally soft and moist. The outer bark is a layer of dead bark that forms the exterior surface of the tree stem. The outer bark is frequently dry and corky.

Back Flow Prevention: a device used to protect potable water supplies from contamination or pollution due to backflow. In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower, or other fixture.

Back Flow Protection: See back flow prevention.

Biomass: Biological material derived from living, or recently living organisms – the most common biomass fuel in the UK is wood fuel in the form of logs, pellets or wood chips. Biomass available for energy on a sustainable basis includes herbaceous and woody energy crops, agricultural food and feed crops, agricultural crop wastes and residues, wood wastes and residues, aquatic plants, and other waste materials including some municipal wastes.

Buffer Tank: also known as an accumulator tank, or a thermal store, a buffer tank is an insulated tank that stores hot water produced by a wood burning biomass log boiler, pellet boiler or wood chip boiler, allowing the biomass boiler to run for longer periods without cycling on and off, thus increasing the efficiency and lifespan of the biomass heating system. Buffer tank designs range from basic hot water stores through to stratified, flow restricted tanks with extra heat coils that support multiple heat sources.


Carbon Cycle: The cycle by which carbon 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. Fossil fuels store carbon indefinitely and release it when it is burned – adding to the carbon in the atmosphere. Sustainable use of biomass such as wood fuel, however, makes no direct addition to the carbon levels because the carbon released when the wood is burned is the same amount as the tree will absorb during its growth cycle.

Carbon Dioxide (CO2): An odorless, transparent gas produced by respiration and combustion of carbon-containing fuels. Plants and trees use CO2 as a food during the process of photosynthesis. Wood fuel is carbon-neutral because the amount of CO2 released when it is burned is the same amount as the tree absorbed during its growing phase.

Carbon Monoxide (CO): A poinsonous, odorless, tasteless and transparent gas produced by the incomplete combustion of wood and fossil fuels. Carbon Monoxide is slightly less dense than air and circulates easily with air in a room with, therefore Carbon Monoxide detector alarms can be placed at a variety of heights in a room, but level with human breathing is a good rule of thumb.

Catalyst: A substance that increases the rate of a chemical reaction without being consumed or produced by the reaction.

Chips: Wood chips are small fragments of wood that have been mechanically chopped or broken from logs or waste wood such as off-cuts from saw mills etc. Wood chips requre a large amount of storage space and are therefore only suitable for commercial applications where storage is not an issue.

Combustion: A chemical reaction between a fuel and oxygen that produces heat (and light).

Combustion Air: The air fed to a fire to provide oxygen for the combustion process of the wood fuel.


DHW: Domestic hot water

Drying: Moisture removal from biomass fuel to improve the combustion process and energy efficiency of the biomass boiler. Also referred to as Seasoning.


ENplus: Quality certification that oversees the production of wood pellets throughout UK and Europe. Wood pellets must conform to the new standard EN 17225-2. The ENplus certification scheme defines three pellet quality classes: ENplus A1, ENplus A2 & ENplus B. See our wood fuel information page for more information.


Flue: A duct, pipe, or opening in a chimney for conveying exhaust gases from a fireplace, boiler, furnace or generator to the outdoors. Historically the term flue meant the chimney itself. The construction and application of chimneys and flues is covered by the UK Building Regulations – section J

Forestry Residues: Includes tops, limbs, and other woody material not removed in forest harvesting operations, as well as woody material resulting from forest management operations such as pre-commercial thinnings and removal of dead or dying trees.

Fossil Fuel: A carbon or hydrocarbon fuel formed in the ground over millions of years from the remains of dead plants and animals. Fossil fuels include oil, natural gas, LPG and coal. When fossil fuels are burned, they release all the carbon they have stored back into the atmosphere with any means to recapture the carbon.


Gasification: A process that converts organic or fossil fuel based carbonaceous materials into carbon monoxide, hydrogen and carbon dioxide by reacting the material at high temperatures (>700 °C), without combustion, with a controlled amount of oxygen and/or steam. The resulting gas mixture is called syngas (from synthesis gas or synthetic gas) or producer gas. The power derived from gasification and combustion of the resultant gas is considered to be a source of renewable energy if the gasified compounds were obtained from biomass.

Gasifier: A device that converts solid fuel such as wood logs to a gas by reacting the material at temperatures in excess of 700 °C.

Global Warming: A term used to describe the increase in average global temperatures due to the greenhouse effect caused by natural processes and manmade carbon emissions.


Hardwood: One of the botanical groups of dicotyledonous trees that have broad leaves in contrast to the conifers or softwoods. The term has no reference to the actual hardness of the wood. The botanical name for hardwoods is angiosperms. Short-rotation, fast growing hardwood trees are being developed as future energy crops such as Hybrid poplars (Populus sp.), Hybrid willows (Salix sp.), Silver maple (Acer saccharinum), and Black locust (Robinia pseudoacacia).

Hopper: a container for loose bulk material such as pellets or grain, typically tapering downward and able to discharge its contents at the bottom. Wood pellet hoppers are used as fuel store to hold wood pellets which are typically transported from the hopper to the pellet boiler via an auger such as a screw auger.




Kilowatt (kW): a measure of electrical power equal to 1,000 watts. 1 kW = 3412 Btu/hr = 1.341 horsepower. See also watt.


Lignin: The major non-carbohydrate, polypenolic structural constituent of wood and other native plant material that encrusts the cell walls and cements the cells together. Lingin is the natural binding agent that liquifies, cools and sets, holding wood pellets together after they have been compressed.


Moisture: The amount of water and other components present in the biomass sample that are volatilized at 105 °C.


Nitrogen Oxides (NOx): A product of photochemical reactions of nitric oxide in ambient air, and the major component of photochemical smog.



Particulates: A fine liquid or solid particle such as dust, smoke, mist, fumes, or smog, found in air or emissions.

Pellet Hopper: a pellet hopper holds biomass pellet fuel which feeds a biomass pellet boiler via an auger. Pellet hoppers range in size from small, 200 Litre hand filled internal pellet hoppers (4-7 days) through to 10+ tonne, galvanised outdoor pellet hoppers that are filled annually by special delivery lorry with blower unit.

Pellets: Wood pellets come in either 6mm or 8mm sizes and there regular shape helps facilitate automatic and regulated fuel deliver to the pellet boiler. Wood pellets are formed by compacting sawdust through a die under pressure which forces the lignin in the wood to liquefy. When the pellet cools, this lignin sets binding the dense saw-dust together, making wood pellets extremely dense, with a low moisture content (below 10%) that allows them to be burned with a very high combustion efficiency. Wood pellets sold in the UK are regulated by the ENplus standard EN 17225-2.

Photosynthesis: The process used by plants to build carbohydrates from carbon dioxide and water, using energy derived from light. Photosynthesis is the key initial step in the growth of biomass and is depicted by the equation: CO2 + H2O + light + chlorophyll = (CH2O) + O2.

Process heat: Energy, usually in the form of hot air, hot water or steam, used in the manufacturing operations of a commercial application such as an industrial plant or a district heating system.

Pyrolysis: The process by which complex molecules break apart by heating in the absence of oxygen; producing solid, liquid, and gaseous fuels.



Renewable Heat Incentive: Also referred to as the RHI, the Renewable Heat Incentive is a financial incentive in Britain set up by the government to encourage households and businesses to replace their old carbon polluting gas and oil boilers with a carbon-neutral MCS accredited heating technology such as a Varmebaronen biomass boiler.

RHI: The Renewable Heat Incentive is a UK government financial incentive to encourage the uptake of renewable sources of heating for domestic and commercial properties and applications.

ROI: Return on investment.


Seasoning: The process of lowering the moisture content of wood to make it more suitable for use as timber or wood fuel. Seasoned wood burns much more efficiently than unseasoned wood, producing less ash and minimising the chance of future problems with your biomass boiler. Wood logs are seasoned by splitting and stacking under cover to allow airflow to pass through while keeping rain off – it can take 1 – 2 years for the moisture content to drop to around 20-25%, which is a good moisture target for your firewood. Logs can also be seasoned in a commercial oven, but this increases energy consumption and therefore cost.

Slow Pyrolysis: Thermal conversion of biomass to fuel by slow heating to less that 450 °C in the absence of oxygen.

Softwood: Generally, one of the botanical groups of trees that in most cases have needle-like or scale-like leaves; the conifers; also the wood produced by such trees. The term has no reference to the actual hardness of the wood. The botanical name for softwoods is gymnosperms.

Stratification: The process by which water masses with different properties, such as temperature, form layers that act as barriers to water mixing. The Aqualux Teknik accumulator tank features stratification plates to stop the water from different renewable heat sources (pellet boiler, solar thermal panel etc) from mixing and cooling down all the water in the tank.

Water stratification also creates barriers to nutrient mixing between layers. This can affect the primary production in an area by limiting photosynthetic processes. When nutrients from the benthos cannot travel up into the photic zone, phytoplankton may be limited by nutrient availability. Lower primary production also leads to lower net productivity in waters.[2]


Tar: A liquid product of thermal processing of carbonaceous materials.

Thermal Store: Also known as a buffer tank or accumulator tank, a thermal store is a well-insulated water tank capable of storing renewable heat produced by a biomass boiler or solar thermal panel as hot water for later use. Traditionally, hot water is normally stored in a medium to large cylinder often called a buffer or accumulator tank. A thermal store can contain one or more heat exchangers, usually in the form of internal coiled pipes or external flat-plate heat exchangers. Thermal stores can also include an electrical heating element such as an immersion heater.




Watt: common base unit of power in the metric system. One watt equals one joule per second, or the power developed in a circuit by a current of one ampere flowing through a potential difference of one volt. One Watt = 3.412 Btu/hr. See also kilowatt.

Wood: A solid lignocellulosic material naturally produced in trees and made of up to 40%-50% cellulose, 20%-30% hemicellulose and 20%-30% lignin.