Animal Unit (AU) - A measurement of herd size used in the dairy industry. One animal unit is equal to 1000 pounds of animals. For example, a 1,200-pound cow equals 1.2 AU.

Attainment area - A geographic region where the concentration of a specific air pollutant does not exceed federal standards.

Average megawatt (MWa or aMW) - One megawatt of capacity produced continuously over a period of one year. 1 aMW = 1 MW x 8760 hours/year = 8,760 MWh = 8,760,000 kWh.

Avoided costs - An investment guideline describing the value of a conservation or generation resource investment by the cost of more expensive resources that a utility would otherwise have to acquire.

Bag filter - A device containing one or more cloth bags for recovering particles from dust laden gas or air which is blown through it.

Barrel of oil equivalent - A unit of energy equal to the amount of energy contained in a barrel of crude oil. Approximately 5.78 million Btu or 1,700 kWh. A barrel is a liquid measure equal to 42 gallons.

Base load - The term applied to that portion of a station or boiler load that is practically constant for long periods.

Baseload capacity - The power output that generating equipment can continuously produce.

Baseload demand - The minimum demand experienced by an electric utility, usually 30-40% of the utility´s peak demand.

Best available control technology (BACT) - That combination of production processes, methods, systems, and techniques that will result in the lowest achievable level of emissions of air pollutants from a given facility. BACT is an emission limitation that the permitting authority determines on a case-by-case basis, taking into account energy, environmental, economic and other costs of control. BACT may include fuel cleaning or treatment or innovative fuel combustion techniques.

Biodegradable - Capable of decomposing rapidly under natural conditions.

Bioenergy - 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 or gases.

Biomass- In the energy production industry, refers to living and recently dead biological material which can be used as fuel or for industrial production. Most commonly, biomass refers to plant matter grown for use as biofuel, but it also includes plant or animal matter used for production of fibres, chemicals or heat. Biomass also includes biodegradable wastes that can be burnt as fuel. It excludes organic material which has been transformed by geological processes into substances such as coal or petroleum. It is usually measured by dry weight.

Blowback - The number of pounds per square inch of pressure drop in a boiler from the point where the safety valve pops to the point where the safety valve reseats.

Blowdown - The drain connection including the pipe and the valve at the lowest practical part of a boiler, or at the normal water level in the case of a surface blowdown. The amount of water blown down.

Blowdown valve - A valve generally used to continuously regulate concentration of solids in the boiler, not the drain valve.

Blower - A fan used to force air under pressure.

Boiler - A closed vessel in which water is heated, steam is generated, steam is superheated, or any combination thereof, under pressure or vacuum by the application of heat from combustible fuels, electricity, or nuclear energy. The term dose not include such facilities of an integral part of a continuous processing unit but does include fired units of heating or vaporizing liquids other than water where these units are separate from processing systems and are complete within themselves.

Boiler, high-pressure, steam or vapor - A boiler in which steam or vapor is generated at a pressure exceeding 15 psig.

Boiler, hot-water-heating - A boiler in which no steam is generated and from which hot water is circulated for heating purposes and then returned to the boiler.

Boiler, hot-water-supply - A boiler functioning as a water heater.

Boiler, low-pressure-steam or vapor - A boiler in which steam or vapor is generated at a pressure not exceeding 15 psig.

Boiler horsepower - The evaporation of 34 1⁄2 lbs. of water per hour from a temperature of 212oF into dry saturated steam at the same temperature. Equivalent to 33,475 Btu.

Boiler water - A term construed to mean a representative sample of the circulating boiler water, after generated steam has been separated and before the incoming feed water or added chemical becomes mixed with it so that its composition is affected. (ASTM - D860)

Bottom ash - Noncombustible ash that is left after solid fuel has been burned.

Btu (British Thermal Unit) - A standard measure of energy in the British unit system. 1 Btu is the amount of heat required to raise the temperature of a liquid by 1 degree.

Burner assembly - A burner that is factory-built as a single assembly or as two or more assemblies which include all parts necessary for its normal function when installed as intended.

Burner capacity - Amount of heat release a burner can deliver (i.e., amount of fuel which can be completely burned through a burner) at a given set of operating conditions.

C unit (CCF) - One hundred cubic feet of solid wood. Used as a log measure or as a measure of solid wood content. 1 CCF contains typically 1.4 BDT.

Calorie - The amount of energy required to raise the temperature of 1 gram of water by 1 degree C. The kilocalorie (kcal) is a typical unit of measure in the process industry, 1 kcal = 1000 calories.

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.

Carbon Dioxide (CO2) - A product of combustion. The most common greenhouse gas.

Carbon Monoxide (CO) - A colorless, odorless gas produced by incomplete combustion. Carbon monoxide is poisonous if inhaled.

Carbon Sequestration - The absorption and storage of carbon dioxide from the atmosphere. Naturally-occurring in plants.

Carbonization - the term for the conversion of an organic substance into carbon or a carbon-containing residue through pyrolysis.

Cellulose - The main carbohydrate in living plants. Cellulose forms the skeletal structure of the plant cell wall.

cfm - Cubic feet per minute.

Closed feedwater heater - An indirect -contact feedwater heater; that is, one in which the steam and water are separated by tubes or coils.

Cogeneration - The sequential production of electricity and useful thermal energy from a common fuel source. Rejected 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 heat and power (CHP) - See cogeneration. The term is more commonly used in Europe and other foreign countries.

Combustion - The rapid reaction of fuel and oxidant (usually oxygen in air) to produce light, heat and noise. Major products of combustion for hydrocarbon fuels (e.g., natural gas, refinery gas, fuel oils) are carbon dioxide and water vapor. Trace products include carbon monoxide and nitrogen oxides, which are pollutants.

Combustion efficiency - The fraction of carbon in the fuel that is converted into CO2 in the flue gas, customarily expressed as a percent.

Combustion rate - The quantity of fuel fired per unit of time, as pounds of coal; per hour or cubic feet of gas per hour.

Combustion (flame) safeguard - A system for sensing the pressure or absence of flame and indicating, alarming or initiating control action.

Condensate - Condensed water resulting from the removal of latent heat from steam.

Condensing, controlled extraction turbines - A controlled turbine that bleeds off (condenses) part of the main stream flow at one (single extraction) or two (double extraction) points. Used when process steam is required at pressures below the inlet pressure and above the exhaust pressure.

Condensing turbine - A turbine used for electrical power generation from a minimum amount of steam. To increase plant efficiency, these units can have multiple uncontrolled extraction openings for feed-water heating.

Conduction - The transfer of heat by molecular collision. This process is more efficient in metals and other thermal conductors and poorer in fluids and insulators such as refractory.

Conductivity - The amount of heat (Btu) transmitted in 1 hour through 1 sq. ft. of a homogenous material 1 in. thick for a difference in temperature of 1o F. between two surfaces of the material.

Conservation - Efficiency of energy use, production, transmission, or distribution that results in a decrease of energy consumption while providing the same level of service.

Continuous blowdown - The uninterrupted removal of concentrated boiler water from a boiler to control total solids concentration in the remaining water.

Control - A device designed to regulate the fuel, air, water, steam, or electrical supply to the controlled equipment. It may be automatic, semi-automatic or manual.

Control, limit - An automatic safety control responsive to changes in liquid level, pressure, or temperature; normally set beyond the operating range for limiting the operation of controlled equipment.

Control, operating - A control, other than s safety control or interlock, to start or regulate input according to demand and to stop or regulate input on satisfaction of demand. Operating controls may also actuate auxiliary equipment.

Conversion efficiency - A comparison of the useful energy output to the potential energy contained in the fuel. The efficiency calculation relates to the form of energy produced. A direct comparison of the efficiency of different conversion processes can be made only when the processes produce the same form of energy output.

Conveyor - A mechanical apparatus for carrying bulk material from place to place; for example, an endless moving belt or a chain of receptacles.

Corn stover - Residue materials from harvesting corn consisting of the cob, leaves and stalk.

Critical pressure and critical temperature - That point at which the difference between the liquid and vapor states for water completely disappears.

Cyclone Filter - A filtration mechanism that uses centrifugal force to separate particles (dust) from an air stream.

Deaerator - A type of feedwater heater operating with water and steam in direct contact. It is designed to heat the water and drive off oxygen.

Dioxin -A family of compounds, some of which are hazardous, that result from combustion of carbon materials. The most toxic of these compounds is 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin.

District heating or cooling - A system that involves the central production of hot water, steam, or chilled water and the distribution of these transfer media to heat or cool buildings.

Draft - The difference between atmospheric pressure and some lower pressure existing in the furnace or gas passages of the steam-generating unit.

Draft environmental impact statement (DEIS) - A draft statement of environmental effects. Section 102 of the National Environmental Policy Act requires a DEIS for all major federal actions. The DEIS is released to the public and other agencies for comment and review.

Dry Ton - 2,000 pounds of material dried to a constant weight.

Duct - A passage for air or gas flow.

Economizer - A series of tubes located in the path of flue gases. Feedwater is pumped through these tubes on its way to the boiler in order to absorb waste heat from the flue gas.

Efficiency - Of boiler operation: Output in heat units divided by input in heat units. The number of Btus contained in all steam evaporated is useful output. The number of Btus contained in all fuel supplied to the boiler is input.

Emissions - Waste substances released into the air or water.

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 in the United States: 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.

Environmental assessment (EA) - A public document that analyzes a proposed federal action for the possibility of significant environmental impacts. The analysis is required by the National Environmental Policy Act. If the environmental impacts will be significant, the federal agency must then prepare an environmental impact statement.

Environmental impact statement (EIS) - A statement of the environmental effects of a proposed action and of alternative actions. Section 102 of the National Environmental Policy Act requires an EIS for all major federal actions.

Feedstock - Any material that can be converted to another form of fuel or energy product.

Fell - To cut down a tree. Cutting down trees and sawing them to manageable lengths is referred to as "felling and bucking" or "falling and bucking."

Firebox - The equivalent of a furnace. A term usually used for furnaces of locomotive boilers and similar types of boilers.

Firing rate control - A pressure temperature or flow controller which controls the firing rate of a burner according to the deviation from pressure or temperature set point. The system may be arranged to operate the burner on-off, high-low or in proportion to load demand.

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.

Flue - A passage for products of combustion.

Flue gas - The gaseous products of combustion in the flue to the stack.

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.

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.

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.

Fouling - Deposits from materials that have vaporized, then condensed in cooler regions of the furnace.

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.

Fuel NOx - NOx that is formed from nitrogen that is organically bound to the fuel molecule. Fuel NOx is most often a problem with liquid fuel or coal burning. Once the nitrogen has been cracked from the fuel molecule, the mechanism follows basically the same path as the prompt NOx mechanism.

Furnace - An enclosed space provided for the combustion of fuel.

Gasification - The process of converting solid or liquid fuel into a gaseous fuel

Gasifier - A device for converting solid fuel into gaseous fuel. In biomass systems, the process is also referred to as pyrolitic distillation.

Generator - A machine used for converting rotating mechanical energy to electrical energy.

Granulometry - The determination of the different grain size in a granular material. 2) The proportion by weight of particles of different sizes in granular material.

Green ton - 2,000 pounds of undried biomass material. Moisture content must be specified if green tons are used as a measure of fuel energy.

Greenhouse effect - A warming of the Earth and its atmosphere caused by greenhouse gases and water vapor 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´s system for distributing power.

Grid connection - Joining a plant that generates electric power to a utility system so that electricity can flow in either direction between the utility system and the plant.

Gross heating value (GHV) - The maximum potential energy in the fuel as received, considering moisture content (MC). It reflects the heat used to evaporate moisture. Compare higher heating value (HHV).
Expressed as: GHV = HHV ((1 - MC) / 100)

Header - A distribution pipe supplying a number of smaller lines tapped off of it. A main receiving pipe supplying one or more main pipe lines and receiving a number of supply lines tapped into it.

High fire - The input rate of a burner at or near maximum.

Higher heating value (HHV) - The maximum potential energy in dry fuel. For wood, the range is 7,600 to 9,600 Btu/lb.

Igniter - A burner smaller than the main burner, which is ignited by a spark or other independent and stable ignition source and which provides proven ignition energy required to immediately light off the main burner.

Interconnection - A connection or link between power systems that enables them to draw on one another´s reserve in time of need.

Intermittent firing - A method of firing by which fuel and air are introduced and burned in a furnace for a short period after which flow is stopped, this succession occurring in a sequence of frequent cycles.

Investment tax credit - A specified percentage of the dollar amount of certain new investments that a company can deduct as a credit against its income tax bill.

Kilowatt (kW) - A measure of electrical power equal to 1,000 Watts. 1 kW = 3,413 Btu/hr = 1.341 horsepower.

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.

Klinker- A klinker is a solid residue formed by melting of minerals or through sintering.

Load factor - Load factor is the ratio of average demand to maximum demand or to capacity.

Logging residues - The unused portion of wood and bark left on the ground after harvesting merchantable wood. The material may include tops, broken pieces, and unmerchantable species.

Lower heating value (LHV) - The potential energy in a fuel if the water vapor from combustion of hydrogen is not condensed.

Lowest achievable emissions rate (LAER) - Used to describe air emissions control technology. A rate of emissions defined by the permitting agency. LEAR sets emission limits for non-attainment areas.
Long-term productivity: The capacity of a site to support forest ecosystems over generations of humans and trees as measured against some defined reference.

Makeup water - The amount of raw water necessary to compensate for the amount of condensate that is not returned in the feedwater supply to the boiler.

Manifold - A pipe or header for collecting a fluid from, or the distributing of a fluid to a number of pipes or tubes.

Megawatt (MW) - The electrical unit of power that equals one million Watts (1,000 kW).

Micron - One millionth of a meter, or 0.000039 in. or 1/25400 in The diameter of dust particles is often expressed in microns.

MMBtu - One million British thermal units.

Moisture content, wet basis - Moisture content expressed as a percentage of the weight of wood as-received. Expressed as:

weight of wet sample - weight of dry sample
-------------------------------------------------- x 100
weight of wet sample

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.

weight of wet sample - weight of dry sample
----------------------------------------------------- x 100
weight of dry sample

Municipal solid waste (MSW) - Garbage. Refuse offering the potential for energy recovery; includes residential, commercial, and institutional wastes.

Net Metering - Net metering is a electricity policy for consumers who own, generally small, renewable energy facilities. "Net", in this context, is used in the sense of meaning "what remains after deductions" -- in this case, the deduction of any energy outflows from metered energy inflows. Under net metering, a system owner receives retail credit for at least a portion of the electricity they generate. The ideal has your existing electricity meter spinning backwards, effectively banking excess electricity production for future credit. In reality, the rules vary significantly by country and possibly state/province; if net metering is available, if and how long you can keep your banked credits, how much the credits are worth (retail/wholesale), etc.

NOx - Any combination of nitrogen and oxygen in a compound form. The most common in terms of environmental considerations is NO, which constitutes 90% of combustion NOx emissions, and NO2. All NO is eventually converted to NO2 in the atmosphere. Hence, most regulations are written to assume that the NOx which is emitted is in the form of NO2. Nox emissions are influenced by many factors, including furnace temperature, flame temperature, burner design, combustion air temperature, nitrogen content of liquid fuels, ammonia content of gas fuels, and other factors.

Oil burner - A burner that atomizes fuel oil and blows it into the combustion chamber in the form of a fine mist or vapor. Steam or mechanical motion plus air may be used as the operating medium.

Opportunity Fuel - (See waste fuel) Any biomass by-product that is waste from a commercial or industrial operation.

Organic compounds - Chemical compounds based on carbon chains or rings and also containing hydrogen, with or without oxygen, nitrogen, and other elements.

Oxidation - Chemical combination with oxygen.

Particulates - The residue left over from coal and fuel oil combustion. Particulates, alternatively referred to as particulate matter (PM), aerosols or fine particles, are tiny particles of solid or liquid suspended in a gas. They range in size from less than 10 nanometres to more than 100 micrometres in diameter.

Pascals - A unit of pressure. One Pascal (Pa) is equal to a force of one Newton per square meter.

Perfect or Stoichiometric combustion - The complete oxidation of all the combustible constitutes of a fuel, utilizing all the oxygen supplied.

pH - The hydrogen ion concentration of a water to denote acidity or alkalinity. A pH of 7 is neutral. A pH above 7 denotes alkalinity while one below 7 denotes acidity.

Pound of steam- One pound mass of water converted to steam.

Pressure - As applied to boilers, the force exerted by a liquid or gas on a unit area. Three pressures may be involved: gauge pressure, the unit pressure above atmospheric pressure; absolute pressure, gauge pressure plus the atmospheric pressure; vacuum pressure; the pressure below atmospheric pressure usually expressed in inches of Hg.

Primary air - Air introduced with fuel at the burners.

Pilot - A small burner which is used to light the main burner.

Prevention of significant deterioration (PSD) - Under the Clean Air Act, a planning and management process for air quality when a new source of air pollution is proposed in an area where ambient air quality is better than applicable standards (areas of special importance).

Process steam - Steam used for industrial purposes other than for producing power.

Prompt NOx - NOx formed at the initial stages of combustion that cannot be explained by either the thermal mechanism or the fuel NOx mechanism. The prompt NOx mechanism requires the CH radical as an intermediate, so the fuel must have carbon present to create prompt NOx.

Proximate and Ultimate Analyses (PU) - Process that characterizes Biomass fuel. The "proximate" analysis gauges moisture content, volatile content (when heated to 950 C), the free carbon remaining at that point, the ash (mineral) in the sample and the high heating value (HHV) based on the complete combustion of the sample to carbon dioxide and liquid water. (The low heating value, LHV, gives the heat released when the hydrogen is burned to gaseous water, corresponding to most heating applications and can be calculated from the HHV and H2 fraction.) The "ultimate" analysis" gives the composition of the biomass in wt% of carbon, hydrogen and oxygen (the major components) as well as sulfur and nitrogen (if any).

Pyrolisis- Chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except possibly steam.

Raw water - Untreated feedwater.

Renewable energy source - An energy source that is replenished continuously in nature or that is replaced after use through natural means. Sustainable energy source. Renewable energy sources include the sun, the winds, flowing water, biomass and geothermal energy.

Refractory - A heat-insulating material, such as firebrick or plastic fire clay, used for purposes as lining combustion chambers.

Safety shut down - The action of shutting off all fuel and ignition energy to the burner by means of safety control or controls such that restart cannot be accomplished without operator action.

Safety valve - A valve that automatically opens when pressure attains the valve setting which is adjustable; used to prevent excessive pressure from building up in a boiler.

Saturated steam - Steam at the temperature that corresponds to its boiling temperature at the same pressure.

Scrubber - An apparatus for the removal of solids from gases by entrainment in water.

Secondary combustion - Combustion which occurs as a result of ignition at a point beyond the furnace.

Secondary treatment - Treatment of boiler feedwater or internal treatment of boiler water after primary treatment.

Shaft horsepower - A measure of the actual mechanical energy per unit time delivered to a turning shaft. 1 shaft horsepower = 1 electric horsepower = 550 ft-lb/second.

Silica - A scale-forming element found in some boiler feedwaters.

Slagging - Deposits in a molten or highly viscous state found in the flame section of a furnace.

Sludge - The mixture of organic and inorganic substances separated from sewage.

Soot blower - A tube from which jets of steam or compressed air are blown for cleaning the fireside of tubes or other parts of the boiler.

SOx - Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula SO2. This important gas is the main product from the combustion of sulfur compounds and is of significant environmental concern.

Spontaneous combustion - Ignition of combustible material following slow oxidation without the application of high temperature from an external source.

Stack - A vertical conduit, which due to the difference in density between internal and external gases creates a draft at its base.

Stack loss - The fraction of total heat which exits with the flue gas through the stack. The quantity is customarily expressed as a percent of the total heat input. The stack loss is directly proportional to the stack exit temperature; the higher the temperature, the greater the stack loss.

Staged air - NOx reduction technique predominantly used for fuel oil firing. The fuel is injected into a fuel-rich primary zone. This stoichiometry helps to control the fuel Nox mechanism. When firing gas, staged air produces higher NOx emissions than staged fuel.

Staged fuel - NOx reduction technique whereby a small portion of the fuel is injected in a lean primary combustion zone. The flue products from this region flow to the secondary combustion zone where the remainder of the fuel is burned out. The lengthening of the flame creates cooler flame temperatures, thus lowering thermal NOx.

Steam Accumulator - The purpose of a steam accumulator is to release steam when the demand is greater than the boiler's ability to supply at that time, and to accept steam when demand is low.

Steam scrubber - A series of screens, wires, or plates through which steam is passed to remove entrained moisture.

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.

Superheat - To raise the temperature of steam above its saturation temperature. The temperature in excess of its saturation temperature.

Superheated steam - Steam at higher temperature than its saturation temperature. Superheated steam can be utilized by turbines or steam powered engines for electrical generation

Surface blowoff - Removal of water, foam, etc. from the surface at the water level in a boiler. The equipment for such removal.

Suspended solids - Waste particles suspended in water. Suspended solids can harbor harmful microorganisms and toxic chemicals. Suspended solids cloud the water and make disinfection more difficult and costly.

Sustainable - An ecosystem condition in which biodiversity, renewability, and resource productivity are maintained over time.

Swinging load - A load that changes at relatively short intervals.

Syngas - A syntheses gas produced through gasification of biomass. Syngas is similar to natural gas and can be cleaned and conditioned to form a feedstock for production of methanol.

Tertiary air - Air for combustion supplied to the furnace to supplement the primary and secondary air.

Tipping fee - A fee for disposal of waste.

Therm - A unit of heat applied especially to gas. One therm = 100,000 Btu.

Thermal conductivity - The ability of a material to conduct heat, expressed as thermal power conducted per unit temperature and thickness. Metals and other thermal "conductors" have a large thermal conductivity. Refractories and other thermal "insulators" have a low thermal conductivity.

Thermal NOx - NOx formed via the Zeldovich mechanism. The rate-limiting step in this mechanism is the formation of the O radical. This occurs only at high temperatures (above about 2400o F.). Hence the term thermal NOx, since it is NOX produced in the highest temperature regions of the flame.

Total solids concentration - The weight of dissolved and suspended impurities in a unit weight of boiler water, usually expressed as ppm.

Total suspended particulates - The quantity of solid particles in a gas or exhaust stream. Any finely divided material (solid or liquid) that is airborne with a diameter smaller than a few hundred micrometers.

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.

Treated water - Water which has been chemically treated to make suitable for boiler feed.

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.

UHC - Any unburned hydrocarbon that is emitted in a combustion process. Also termed VOC (volatile organic compound).

Unaccounted for loss - That portion of a boiler heat balance, which represents the difference between 100 per cent and the sum of the heat absorbed by the unit and all the classified losses expressed as per cent.

Vaporization - The change from liquid or solid phase to the vapor phase.

Vertical firing - An arrangement of a burner such that air and fuel are discharged into the furnace, in practically a vertical direction.

Volume of air - The number of cubic feet of air per minute expressed at fan outlet conditions.

Waste fuel - (See opportunity fuel) Any biomass by-product that is waste from a commercial or industrial operation.

Waste heat - Sensible heat in non-combustible gases.

Watt - The common base unit of power in the metric system. One watt equals one joule per second. It is 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.

Wet steam - Steam containing moisture.