Glossary of Terms


Appliance efficiency

The ratio of output energy to input energy when the combustion system is running under design conditions. Also called steady state efficiency.

Arboricultural residues

The prunings of trees (such as apple, olive, and pear trees) and grapevines in vineyards. Backup system: An alternate fuel combustion system used to provide heat when the primary system is out of service or unable to meet the full heat load.



A gas produced from biomass. Can be used as a combustion fuel.


Any organic matter that can be burned for energy. Here used as synonymous with wood in its various forms.

Biomass energy crops

Defined as crops specifically bred and cultivated for energy production by direct conversion to heat and electricity.


A heat exchanger used to extract heat from hot combustion gases and transfer the heat to water. The boiler output can be either hot water or, if the water is allowed to boil, steam.

Bottom ash

Ash that collects under the grates of a combustion furnace.


Calorific value

The energy content of a fuel, expressed in units such as joules per kilogram.

Carbon burn-out

The end of the combustion process in which all uncombined gaseous and solid carbon is oxidized to carbon dioxide.


Carbon-rich combustible solids that result from pyrolysis of wood in the early stages of combustion. Char can be converted to combustible gases under certain conditions or burned directly on the grates.


A large device that reduces logs, whole trees, slab wood, or lumber to chips of more or less uniform-size. Stationary chippers are used in sawmills, while trailer-mounted whole-tree chippers are used in the woods. Cogeneration: Combined heat and power (CHP). A term used in industrial settings, now being displaced by the more descriptive term CHP.

Combined heat and power (CHP)

The simultaneous production of heat and electrical power from a single fuel. Combustion efficiency: The efficiency of converting available chemical energy in the fuel to heat, typically in excess of 99% in biomass burners. Efficiencies of conversion to usable heat are much lower.

Cyclone separator

A flue gas particulate removal device, which creates a vortex that separates solid particles from the hot gas stream.


Day bin

An intermediary solid fuel storage bin that holds enough fuel to last approximately one day. Could be designed with the capacity to feed the combustion system for a weekend.

Design specifications

For mechanical systems, specifications (and drawings) produced by the owner’s mechanical or design engineer. Design specifications become part of the contract for the installation. The designer bears ultimate responsibility for the design and function of the system.


Domestic hot water.

Direct-burn system

A biomass combustion system in which the primary combustion chamber is located under and directly connected to the combustion chamber of the boiler itself.

District heating

The use of a single boiler plant to provide hot water or steam for heating a number of buildings in a locality.


Energy services company (ESCO)

A company that provides a broad range of energy services to a building owner, typically including the financing and installation of energy improvements under a contract that allows some of the dollar savings to accrue to the company.

Excess air

The amount of combustion air supplied to the fire that exceeds the theoretical air requirement to give complete combustion. Expressed as a percentage.