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  • Glossary
| Last Updated:22/09/2014


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Acid Gas

Any gas or gaseous mixture that forms an acidic compound when mixed with water, such as hydrogen chloride (HCl), hydrogen fluoride (HF) and sulphur dioxide (SO2).

acid mine drainage (AMD)

water exhibiting a pH of less than 6.0 and in which total acidity exceeds total alkalinity, discharged from an active, inactive or abandoned mine and reclamation operation or from an area affected by surface mining and reclamation operations.

acid mine water

water exhibiting a pH of less than 6.0 and in which total acidity exceeds total alkalinity, discharged from an active, inactive or abandoned mine and reclamation operation or from an area affected by surface mining and reclamation operations.


a material that causes a catalyst to begin a function which, in the case of a coal combustion product based flowable fill or controlled low-strength material (CLSM), causes cementitious reactions to occur.


a material other than water, aggregates, hydraulic cement and fibre reinforcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing. Fly ash is used as an admixture in concrete.

advanced sulfur control products (ASC)

products generated from advanced coal conversion technologies including fluidised bed combustion (FBC) and products from advanced environmental emission clean-up technologies such as duct injection and lime injection multiphase burners (LIMB). The type of product is technology-dependent and could be a bed ash and/or high lime fly ash derived from FBC technology.


the process of exposing a substance or area to air circulation; the process of mixing air with a pulverised fuel or a powdered material such as fly ash in a transport pipe or storage bin. Example: the aeration of the fly ash in a silo to facilitate flow.


granular material such as sand, gravel, crushed stone, crushed hydraulic-cement concrete, iron blast furnace slag, or furnace bottom ash and boiler slag used as a component in concrete or mortar with a hydraulic cementing medium to produce either concrete or mortar.

lightweight aggregate (LWA)

Aggregate of low density used to produce lightweight concrete or concrete products. Examples of LWA include furnace bottom ash, pumice, scoria, volcanic cinders, tuff and diatomite; expanded or sintered clay, shale, slate, diatomaceous shale, perlite, vermiculite or slag; and bonded or sintered coal combustion products (CCPs) used to produce lightweight concrete or component products.

Air entraining

The capability of a material or process to develop a system of microscopic bubbles of air in cement paste, mortar or concrete during mixing.

air-entraining agent (AEA)

An addition for hydraulic cement; also an admixture for concrete or mortar which causes air to be entrained in the concrete or mortar during mixing, usually to increase workability and frost resistance. The quantity of unburned carbon in fly ash can affect the dosage of the AEA in the concrete and the quality of the concrete.

Air entrainment

The incorporation of air in the form of microscopic bubbles (generally smaller than 1 mm) during the mixing of concrete or mortar.

air separator

An apparatus that separates various size fractions of ground material pneumatically; fine particles are discharged as product; oversize is returned to the mill as tailings.


Salts of alkali metals, principally sodium and potassium; a hydroxide or carbonate of an alkali metal.


The capacity of water to neutralise acids, a property imparted by the water’s content of carbonates, bicarbonates, hydroxides and occasionally borates, silicates and phosphates. It is often expressed in milligrams per litre of equivalent calcium carbonate.

alkali-silicate reaction (ASR)

the reaction between the alkalis (sodium and potassium) in Portland cement and certain siliceous rocks and minerals, such as opaline chert, strained quartz, and acidic volcanic glass, present in some aggregates; the products of the reaction may cause abnormal expansion and cracking of concrete in service. Class F fly ash is used in concrete to reduce the occurrence of ASR.

alkali-silicate reactivity (ASR)

another name for alkali-silicate reaction.

ammonia slip

the unreacted ammonia that occurs in the flue gas downstream of the selective catalytic reduction (SCR) reactor and from the selective non–catalytic reduction (SNCR) nitrogen oxides control technologies. An ammonia slip results in the adsorption of the ammonia on to the surface of the fly ash particles in the ESP. An ammonia slip of 2 ppm yields 100 ppm adsorbed on to the fly ash, based on the European experience with SCR. This 100 ppm level of ammonia in fly ash has allowed for the unrestricted use of this ammoniated fly ash in concrete in Europe.

ammoniated ash

ash that contains ammonia and/or ammonium salts as a result of the addition of ammonia or ammonium salts to the flue gas at the power plant for purposes that include removing nitrogen oxides (NOx) from the combustion flue gases, conditioning the flue gas in order to improve the performance of electrostatic precipitators (ESP) or to reduce the opacity of the emissions from the stack. Ammonia levels occur primarily in the fly ash due to the adsorption of the ammonia on the surface of the fly ash particles in the ESP, although there could be some minor carryover of the ammonia to the scrubber residue when scrubbers are installed downstream of the ESP. Ammonia levels in fly ash have been reported to exceed 800 ppm for gas conditioning applications and to be less than 100 ppm for the nitrogen oxides removal applications. The latter applies to the selective catalytic reduction (SCR) process only. Fly ash with ammonia levels of less than 100 ppm has been reported to be used in concrete that is placed in a closed environment (building enclosure) without causing health and safety concerns (this is based on the European experience). Also, fly ash with ammonia levels of as much as 300 ppm has been reported to be used in concrete without affecting the structural performance of the concrete.

angle of repose

the maximum angle from horizontal at which a given material (such as fly ash, bottom ash or fixated FGD material) will rest on a particular stationary surface without sliding or rolling.


a geologic formation, group of formations, or part of a formation that is saturated with water and capable of providing a significant quantity of water; a geologic unit that contains and can transmit water at rates fast enough to yield useable quantities of water.


the incombustible inorganic matter in fuels such as coal; the non-combustible residue of a coal’s mineral matter.

ash-free basis

the method of reporting fuel analysis whereby ash is deducted and other constituents are recalculated to 100%.

ash fuel

the use of high-carbon ash to produce energy.

ash pond [dam]

an impoundment or surface impoundment used to store or dispose of ash primarily from the combustion of coal; a type of waste management facility consisting of an excavated, dammed or diked reservoir in which coal ashes are stored for future removal or disposed of as a slurry or sludge. The coal ash solids settle out and leave relatively clear water at the surface that is discharged through a designed and managed outlet structure to a nearby stream, surface water or plant process water system. Ash pond designs reflect local site conditions, relevant regulations and whether fly ash, bottom ash, boiler slag or a combination of coal ashes are disposed in the ash pond. While some power-generating facilities combine the ashes during storage or disposal, others use separate ash ponds for fly ash, bottom ash and boiler slag. The ash pond is referred to as a bottom ash pond, fly ash pond or boiler slag pond when it receives one type of ash. A large ash pond is referred to as an ash impoundment, ash reservoir or surface impoundment. (See surface impoundment and ponded ash)

ash pond water outlet works

an engineering construction that consists of either a stop-logged vertical riser or sloping shaft within the pond, a pipe or conduit that runs from the base of the riser inside the pond to a receiving stream or other plant process water system. It is used to decant the ash transport water and normal precipitation. The term may also refer to the skimming device at the vertical riser or sloping shaft within the pond, which prevents the floating fraction of ash or other material to enter the discharge from the pond.

Ash Processing Facility

a facility that uses technology to enhance the physical characteristics of boiler slag, furnace bottom ash or fly ash in order to meet specifications for the particular market to which it is targeted.