Glossary of Bioenergy Terms
The following is a short glossary of useful bioenergy terms. Many of the definitions were derived from ones listed in the U.S. Department of Energy Biomass Power Glossary. Another glossary of energy terms is available here (clicking on this link will open a new window). You might also find the Bioenergy Conversion Factors page useful.
A
Anaerobic digestion: Decomposition of biological wastes by micro-organisms, usually under wet conditions, in the absence of air (oxygen), to produce a gas comprising mostly methane and carbon dioxide.
B
Barrel of oil equivalent: (boe) The amount of energy contained in a barrel of crude oil, i.e. approximately 6.1 GJ (5.8 million Btu), equivalent to 1,700 kWh. A "petroleum barrel" is a liquid measure equal to 42 U.S. gallons (35 Imperial gallons or 159 liters); about 7.2 barrels are equivalent to one tonne of oil (metric).
Biochemical conversion: The use of fermentation or anaerobic digestion to produce fuels and chemicals from organic sources.
Bioenergy: Useful, renewable energy produced from organic matter. The conversion of the complex carbohydrates in organic matter to energy. Organic matter may either be used directly as a fuel or processed into liquids and gases.
Biofuel: See biomass fuel.
Biogas: A combustible gas derived from decomposing biological waste under anaerobic conditions. Biogas normally consists of 50 to 60 percent methane. See also landfill gas.
Biomass: Organic matter available on a renewable basis. Biomass includes forest and mill residues, agricultural crops and wastes, wood and wood wastes, animal wastes, livestock operation residues, aquatic plants, fast-growing trees and plants, and municipal and industrial wastes.
Biomass fuel: Liquid, solid, or gaseous fuel produced by conversion of biomass. Examples include bioethanol from sugar cane or corn, charcoal or woodchips, and biogas from anaerobic decomposition of wastes.
Biomass energy: See Bioenergy.
Bone dry: Having zero percent moisture content. Wood heated in an oven at a constant temperature of 100°C (212°F) or above until its weight stabilizes is considered bone dry or oven dry.
Bottoming cycle: A cogeneration system in which steam is used first for process heat and then for electric power production.
British thermal unit: (Btu) A non-metric unit of heat, still widely used by engineers. One Btu is the heat energy needed to raise the temperature of one pound of water from 60°F to 61°F at one atmosphere pressure. 1 Btu = 1055 joules (1.055 kJ).
C
Capacity: The maximum power that a machine or system can produce or carry safely. The maximum instantaneous output of a resource under specified conditions. The capacity of generating equipment is generally expressed in kilowatts or megawatts.
Capital cost: The total investment needed to complete a project and bring it to a commercially operable status. The cost of construction of a new plant. The expenditures for the purchase or acquisition of existing facilities.
cfm: Cubic feet per minute (1000 cfm = 0.472 cubic meters per second, m3/s)
Cellulose: The principal chemical constituent of cell walls of plants: a long chain of simple sugar molecules.
Char: The remains of solid biomass that has been incompletely combusted, such as charcoal if wood is incompletely burned.
Chipper: A machine that produces wood chips by knife action.
Chips: Woody material cut into short, thin wafers. Chips are used as a raw material for pulping and fiberboard or as biomass fuel.
Conifer: Tree, usually evergreen, with cones and needle-shaped or scalelike leaves, producing wood known commercially as softwood.
Cogeneration: The sequential production of electricity and useful thermal energy from a common fuel source. Reject heat from industrial processes can be used to power an electric generator (bottoming cycle). Conversely, surplus heat from an electric generating plant can be used for industrial processes, or space and water heating purposes (topping cycle).
Combined cycle: Two or more generation processes in series or in parallel, configured to optimize the energy output of the system.
Combined-cycle power plant: The combination of a gas turbine and a steam turbine in an electric generation plant. The waste heat from the gas turbine provides the heat energy for the steam turbine.
Combined heat and power: (CHP) See Cogeneration.
Combustion: Burning. The transformation of biomass fuel into heat, chemicals, and gases through chemical combination of hydrogen and carbon in the fuel with oxygen in the air.
Combustion air: The air fed to a fire to provide oxygen for combustion of fuel. It may be preheated before injection into a furnace.
Combustion efficiency: (actual heat produced by combustion) divided by (total heat potential of the fuel consumed)
Commercial forest land: Forested land which is capable of producing new growth at a minimum rate of 20 cubic feet per acre/per year, excluding lands withdrawn from timber production by statute or administrative regulation.
Coppice regeneration: The ability of certain hardwood species to regenerate by producing multiple new shoots from a stump left after harvest.
Cord: A stack of wood consisting of 128 cubic feet (3.62 cubic meters). A cord has standard dimensions of 4 x 4 x 8 feet, including air space and bark. One cord contains about 1.2 U.S. tons (oven-dry), i.e. 2400 pounds or 1089 kg.
D
Diameter at breast height: (DBH) The diameter of a tree measured 4 feet 6 inches above the ground.
Digester: An airtight vessel or enclosure in which bacteria decomposes biomass in water to produce biogas.
Discount rate: A rate used to convert future costs or benefits to their present value.
Downdraft gasifier: A gasifier in which the product gases pass through a combustion zone at the bottom of the gasifier.
Dutch oven furnace: One of the earliest types of furnaces, having a large, rectangular box lined with firebrick (refractory) on the sides and top. Commonly used for burning wood. Heat is stored in the refractory and radiated to a conical fuel pile in the center of the furnace.
E
Effluent: The liquid or gas discharged from a process or chemical reactor, usually containing residues from that process.
Emissions: Waste substances released into the air or water. See also Effluent.
Energy crops: Crops grown specifically for their fuel value. These include food crops such as corn and sugarcane, and nonfood crops such as poplar trees and switchgrass. Currently, two energy crops are under development: short-rotation woody crops, which are fast-growing hardwood trees harvested in 5 to 8 years, and herbaceous energy crops, such as perennial grasses, which are harvested annually after taking 2 to 3 years to reach full productivity.
Externality: A cost or benefit not accounted for in the price of goods or services. Often "externality" refers to the cost of pollution and other environmental impacts.
F
Feedstock: Any material which is converted to another form or product.
Feller-buncher: A self-propelled machine that cuts trees with giant shears near ground level and then stacks the trees into piles to await skidding.
Fermentation: Conversion of carbon-containing compounds by micro-organisms for production of fuels and checmicals such as alcohols, acids or energy-rich gases.
Firm power: (firm energy) Power which is guaranteed by the supplier to be available at all times during a period covered by a commitment. That portion of a customer's energy load for which service is assured by the utility provider.
Fluidized-bed boiler: A large, refractory-lined vessel with an air distribution member or plate in the bottom, a hot gas outlet in or near the top, and some provisions for introducing fuel. The fluidized bed is formed by blowing air up through a layer of inert particles (such as sand or limestone) at a rate that causes the particles to go into suspension and continuous motion. The super-hot bed material increased combustion efficiency by its direct contact with the fuel.
Fly ash: Small ash particles carried in suspension in combustion products.
Forest residues: Material not harvested or removed from logging sites in commercial hardwood and softwood stands as well as material resulting from forest management operations such as precommercial thinnings and removal of dead and dying trees.
Forest health: A condition of ecosystem sustainability and attainment of management objectives for a given forest area. Usually considered to include green trees, snags, resilient stands growing at a moderate rate, and endemic levels of insects and disease. Natural processes still function or are duplicated through management intervention.
Fossil fuel: Solid, liquid, or gaseous fuels formed in the ground after millions of years by chemical and physical changes in plant and animal residues under high temperature and pressure. Oil, natural gas, and coal are fossil fuels.
Fuel cell: A device that converts the energy of a fuel directly to electricity and heat, without combustion.
Fuel cycle: The series of steps required to produce electricity. The fuel cycle includes mining or otherwise acquiring the raw fuel source, processing and cleaning the fuel, transport, electricity generation, waste management and plant decommissioning.
Fuel handling system: A system for unloading wood fuel from vans or trucks, transporting the fuel to a storage pile or bin, and conveying the fuel from storage to the boiler or other energy conversion equipment.
Furnace: An enclosed chamber or container used to burn biomass in a controlled manner to produce heat for space or process heating.
G
Gas turbine: (combustion turbine) A turbine that converts the energy of hot compressed gases (produced by burning fuel in compressed air) into mechanical power. Often fired by natural gas or fuel oil.
Gasification: A chemical or heat process to convert a solid fuel to a gaseous form.
Gasifier: A device for converting solid fuel into gaseous fuel. In biomass systems, the process is referred to as pyrolitic distillation. See Pyrolysis.
Genetic selection: Application of science to systematic improvement of a population, e.g. through selective breeding.
Gigawatt: (GW) A measure of electrical power equal to one billion watts (1,000,000 kW). A large coal or nuclear power station typically has a capacity of about 1 GW.
Greenhouse effect: The effect of certain gases in the Earth's atmosphere in trapping heat from the sun.
Greenhouse gases: Gases that trap the heat of the sun in the Earth's atmosphere, producing the greenhouse effect. The two major greenhouse gases are water vapor and carbon dioxide. Other greenhouse gases include methane, ozone, chlorofluorocarbons, and nitrous oxide.
Grid: An electric utility company's system for distributing power.
H
Habitat: The area where a plant or animal lives and grows under natural conditions. Habitat includes living and non-living attributes and provides all requirements for food and shelter.
Hardwoods: Usually broad-leaved and deciduous trees.
Heat Rate: The amount of fuel energy required by a power plant to produce one kilowatt-hour of electrical output. A measure of generating station thermal efficiency, generally expressed in Btu per net kWh. It is computed by dividing the total Btu content of fuel burned for electric generation by the resulting net kWh generation.
Heat transfer efficiency: useful heat output released / actual heat produced in the firebox
Heating value: The maximum amount of energy that is available from burning a substance.
Hectare: Common metric unit of area, equal to 2.47 acres. 100 hectares = 1 square kilometer.
Herbaceous: Non-woody type of vegetation, usually lacking permanent strong stems, such as grasses, cereals and canola (rape).
Higher heating value: (HHV) The maximum potential energy in dry fuel. For wood, the range is from 7,600 to 9,600 Btu/lb (17.7 to 22.3 GJ/t).
Horsepower: (electrical horsepower; hp) A unit for measuring the rate of mechanical energy output, usually used to describe the maximum output of engines or electric motors. 1 hp = 550 foot-pounds per second = 2,545 Btu per hour = 745.7 watts = 0.746 kW
Hydrocarbon: Any chemical compound containing hydrogen, oxygen, and carbon.
I
Incinerator: Any device used to burn solid or liquid residues or wastes as a method of disposal. In some incinerators, provisions are made for recovering the heat produced.
Inclined grate: A type of furnace in which fuel enters at the top part of a grate in a continuous ribbon, passes over the upper drying section where moisture is removed, and descends into the lower burning section. Ash is removed at the lower part of the grate.
Incremental energy costs: The cost of producing and transporting the next available unit of electrical energy. Short run incremental costs (SRIC) include only incremental operating costs. Long run incremental costs (LRIC) include the capital cost of new resources or capital equipment.
Independent power producer: A power production facility that is not part of a regulated utility.
Indirect liquefaction: Conversion of biomass to a liquid fuel through a synthesis gas intermediate step.
J
Joule: Metric unit of energy, equivalent to the work done by a force of one Newton applied over a distance of one meter (= 1 kg m2/s2). One joule (J) = 0.239 calories (1 calorie = 4.187 J).
K
Kilowatt: (kW) A measure of electrical power equal to 1,000 watts. 1 kW = 3,413 Btu/hr = 1.341 horsepower. See also watt..
Kilowatt hour: (kWh) A measure of energy equivalent to the expenditure of one kilowatt for one hour. For example, 1 kWh will light a 100-watt light bulb for 10 hours. 1 kWh = 3,413 Btu.
L
Landfill gas: A type of biogas that is generated by decomposition of organic material at landfill disposal sites. Landfill gas is approximately 50 percent methane. See also biogas.
Levelized life-cycle cost: The present value of the cost of a resource, including capital, financing and operating costs, expressed as a stream of equal annual payments. This stream of payments can be converted to a unit cost of energy by dividing the annual payment amount by the annual kilowatt-hours produced or saved. By levelizing costs, resources with different lifetimes and generating capabilities can be compared.
Lignin: Structural constituent of wood and (to a lesser extent) other plant tissues, which encrusts the cell walls and cements the cells together.
M
Megawatt: (MW) A measure of electrical power equal to one million watts (1,000 kW). See also watt.
Mill/kWh: A common method of pricing electricity in the U.S. Tenths of a U.S. cent per kilowatt hour.
Mill residue: Wood and bark residues produced in processing logs into lumber, plywood, and paper.
MMBtu: One million British thermal units.
Moisture content: (MC) The weight of the water contained in wood, usually expressed as a percentage of weight, either oven-dry or as received.
Moisture content, dry basis: Moisture content expressed as a percentage
of the weight of oven-dry wood, i.e.:
[(weight of wet sample - weight of dry sample) / weight of dry sample ] x 100
Moisture content, wet basis: Moisture content expressed as a percentage
of the weight of wood as-received, i.e.:
[(weight of wet sample - weight of dry sample) / weight of wet sample ] x 100
Monoculture: The cultivation of a single species crop.
N
Net present value: The sum of the costs and benefits of a project or activity. Future benefits and costs are discounted to account for interest costs.
Nitrogen fixation: The transformation of atmospheric nitrogen into nitrogen compounds that can be used by growing plants.
Noncondensing, controlled extraction turbine: A turbine that bleeds part of the main steam flow at one (single extraction) or two (double extraction) points.
O
Old growth: Timber stands with the following characteristics: large mature and over-mature trees in the overstory, snags, dead and decaying logs on the ground, and a multi-layered canopy with trees of several age classes.
Organic compounds: Chemical compounds based on carbon chains or rings and also containing hydrogen, with or without oxygen, nitrogen, and other elements.
P
Particulate: A small, discrete mass of solid or liquid matter that remains individually dispersed in gas or liquid emissions. Particulates take the form of aerosol, dust, fume, mist, smoke, or spray. Each of these forms has different properties.
Photosynthesis: Process by which chlorophyll-containing cells in green plants concert incident light to chemical energy, capturing carbon dioxide in the form of carbohydrates.
Pilot scale: The size of a system between the small laboratory model size (bench scale) and a full-size system.
Present value: The worth of future receipts or costs expressed in current value. To obtain present value, an interest rate is used to discount future receipts or costs.
Process heat: Heat used in an industrial process rather than for space heating or other housekeeping purposes.
Producer gas: Fuel gas high in carbon monoxide (CO) and hydrogen (H2), produced by burning a solid fuel with insufficient air or by passing a mixture of air and steam through a burning bed of solid fuel.
Public utility commissions: State agencies that regulate investor-owned utilities operating in the state.
Public Utility Regulatory Policies Act: (PURPA) A federal law requiring a utility to buy the power produced by a qualifying facility at a price equal to that which the utility would otherwise pay if it were to build its own power plant or buy power from another source.
Pyrolysis: The thermal decomposition of biomass at high temperatures (greater than 400° F, or 200° C) in the absence of air. The end product of pyrolysis is a mixture of solids (char), liquids (oxygenated oils), and gases (methane, carbon monoxide, and carbon dioxide) with proportions determined by operating temperature, pressure, oxygen content, and other conditions.
Q
Quad: One quadrillion Btu (10^15 Btu) = 1.055 exajoules (EJ), or approximately 172 million barrels of oil equivalent.
R
Recovery boiler: A pulp mill boiler in which lignin and spent cooking liquor (black liquor) is burned to generate steam.
Refractory Lining: A lining, usually of ceramic, capable of resisting and maintaining high temperatures.
Refuse-derived fuel: (RDF) Fuel prepared from municipal solid waste. Noncombustible materials such as rocks, glass, and metals are removed, and the remaining combustible portion of the solid waste is chopped or shredded. RDF facilities process typically between 100 and 3,000 tons of MSW per day.
Reserve Margin: The amount by which the utility's total electric power capacity exceeds maximum electric demand.
Return on investment: (ROI) The interest rate at which the net present value of a project is zero. Multiple values are possible.
Rotation: Period of years between establishment of a stand of timber and the time when it is considered ready for final harvest and regeneration.
S
Saturated steam: Steam at boiling temperature for a given pressure.
Shaft horsepower: A measure of the actual mechanical energy per unit time delivered to a turning shaft. See also horsepower.
Silviculture: Theory and practice of controlling the establishment, composition, structure and growth of forests and woodlands.
SRIC: Short rotation intensive culture - the growing of tree crops for bioenergy or fiber, characterized by detailed site preparation, usually less than 10 years between harvests, usually fast-growing hybrid trees and intensive management (some fertilization, weed and pest control, and possibly irrigation).
Stand: (of trees) A tree community that possesses sufficient uniformity in composition, constitution, age, spatial arrangement, or condition to be distinguishable from adjacent communities.
Steam turbine: A device for converting energy of high-pressure steam (produced in a boiler) into mechanical power which can then be used to generate electricity.
Superheated steam: Steam which is hotter than boiling temperature for a given pressure.
Surplus electricity: Electricity produced by cogeneration equipment in excess of the needs of an associated factory or business.
Sustainable: An ecosystem condition in which biodiversity, renewability, and resource productivity are maintained over time.
T
Therm: A unit of energy equal to 100,000 Btus (= 105.5 MJ); used primarily for natural gas.
Thermochemical conversion: Use of heat to chemically change substances from one state to another, e.g. to make useful energy products.
Tipping fee: A fee for disposal of waste.
Ton, tonne: One U.S. ton (short ton) = 2,000 pounds. One Imperial ton (long ton or shipping ton) = 2,240 pounds. One metric tonne (tonne) = 1,000 kilograms (2,205 pounds). One oven-dry ton or tonne (ODT, sometimes termed bone-dry ton/tonne) is the amount of wood that weighs one ton/tonne at 0% moisture content. One green ton/tonne refers to the weight of undried (fresh) biomass material - moisture content must be specified if green weight is used as a fuel measure.
Topping cycle: A cogeneration system in which electric power is produced first. The reject heat from power production is then used to produce useful process heat.
Topping and back pressure turbines: Turbines which operate at exhaust pressure considerably higher than atmospheric (noncondensing turbines). These turbines are often multistage types with relatively high efficiency.
Transmission: The process of long-distance transport of electrical energy, generally accomplished by raising the electric current to high voltages.
Traveling grate: A type of furnace in which assembled links of grates are joined together in a perpetual belt arrangement. Fuel is fed in at one end and ash is discharged at the other.
Turbine: A machine for converting the heat energy in steam or high temperature gas into mechanical energy. In a turbine, a high velocity flow of steam or gas passes through successive rows of radial blades fastened to a central shaft.
Turn down ratio: The lowest load at which a boiler will operate efficiently as compared to the boiler's maximum design load.
U
V
W
Waste streams: Unused solid or liquid by-products of a process.
Water-cooled vibrating grate: A boiler grate made up of a tuyere grate surface mounted on a grid of water tubes interconnected with the boiler circulation system for positive cooling. The structure is supported by flexing plates allowing the grid and grate to move in a vibrating action. Ashes are automatically discharged.
Watershed: The drainage basin contributing water, organic matter, dissolved nutrients, and sediments to a stream or lake.
Watt: The 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 = 3.413 Btu/hr. See also kilowatt.
Wheeling: The process of transferring electrical energy between buyer and seller by way of an intermediate utility or utilities.
Whole-tree harvesting: A harvesting method in which the whole tree (above the stump) is removed.
X
Y
Yarding: The initial movement of logs from the point of felling to a central loading area or landing.