Glossary

This glossary contains terms and phrases that are commonly used by those who are involved with the management and use of coal ash and combustion products (CCPs). While stakeholders in the ‘CCP industry’ have used many of these terms for many years, other terms have evolved over time, with some still relatively new. Even the acronym, CCPs, was first used in the USA in 1998 and then globally agreed to through the World Wide Coal Combustion Products Network in 2005. ‘CCPs’ is now the preferred term throughout the world for coal ash that is used.

The terms and phrases in this glossary are associated with the production, handling, storage and use of CCPs and coal ash emplacement. Additionally, some environmental and regulatory terms associated with the use and disposal of these materials are included. There is a continuing global effort to reflect common usage in the industry. The glossary is drawn from compilations by the American Coal Ash Association and the World Wide Coal Combustion Products Network, with updates for this edition of the Coal Combustion Products Handbook. Some of the technology listed, such as that associated with flue gas desulfurisation systems, is not used in Australia at the present time. If you would like to purchase your copy of the Coal Combustion Products: Second Edition, please visit the ADAA Store.

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 – (See AMD) 

activator – 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. 

admixture – 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.

aeration – 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. 

aggregate – 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. 

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

alkalinity – 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. 

aquifer – 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. 

ash – 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.

bag filter – a device or equipment containing one or more cloth bags for recovering particles (fly ash) from the dust-laden gas or air which is blown through it. Bag filters are used in the fly ash transport system in series with mechanical equipment (dust collectors, referred to as primary and secondary dust collectors) to remove fine particulate fly ash from the conveying air. The bag filter dust collector is usually referred to as a tertiary dust collector in such cases.

baghouse – a facility constructed at some coal-fired power plants to remove particulate matter (fly ash) from the flue gas by the use of fabric filter bags that mechanically trap particulates (fly ash) carried in the flue gases; a facility that removes fly ash from the flue gas by the use of fabric filter bags. 

batch – quantity of either concrete, mortar, ash grout or flowable fill mixed at one time. 

bed ash – the spent bed material that is produced by fluidised bed combustion generating plants. The bed ash is usually collected separately and can be considered as being equivalent to bottom ash in a dry-bottom furnace or a wet-bottom wall-fired furnace. If limestone is used to absorb sulfur, the bed ash is mainly composed of calcium oxide, calcium sulfate and coal ash, with minor proportions of calcium carbonate and unburnt carbon. Also, because of the free lime content, heat is evolved when water is added. The collected bed ash is conveyed to a silo (which may store only the bed ash or a combination of bed ash and fly ash) from where it is loaded into trucks or other vehicles and transported to ground storage for reuse or to a disposal site. 

beneficial use of a CCP– the use of or substitution of the coal combustion product (CCP) for another product based on performance criteria. For purposes of this definition, beneficial use includes, but is not restricted to: raw feed for cement clinker, concrete, grout, flowable fill, controlled low-strength material; structural fill; road base/sub-base, soil modification, mineral filler, blasting grit and abrasives, roofing granules, mining applications, wallboard, waste stabilisation/solidification, soil amendment and agriculture.

beneficiation – improvement of the chemical or physical properties of a raw material or intermediate product by the removal or modification of undesirable components or impurities. The removal of unburned carbon in fly ash is an example of beneficiation of the raw fly ash. 

biomass – a synonym for biological solids. In the CCP context the term refers to material that originated from the growth of agricultural crops (including food crops), trees, grasses, aquatic plants and their residues or waste materials. It includes residues from crop harvesting, forest maintenance, road or land clearing, and utility line maintenance, as well as post-consumer materials such as used wood, food processing, paper mill sludge, industrial by-products and other waste materials that may result from the processing of these goods. Biomass also typically refers to agricultural and forest products or residues derived from living plants, as well as a range of other biologically-generated organic materials. 

biomass coal co-firing – the firing of two dissimilar fuels at the same time in the same boiler, namely coal and biomass. 

blended fly ash – fly ash resulting from the combustion of a mixture of coals having different classifications, such as bituminous and sub-bituminous coals; also the mixing of ash from different sources. (See fly ash sub-bituminous/bituminous coal blends) 

boiler slag – a molten ash collected at the base of slag tap and cyclone furnaces that is quenched with water and shatters into black, angular particles having a smooth, glassy appearance. 

boiler slag fines – the small boiler slag particles that are produced as a result of dry or wet boiler slag screening processes whose end product is directed to several markets that include use as a blasting grit. Beneficial use of the fines includes their use in the glass industry. 

bottom ash – agglomerated ash particles formed in pulverised coal furnaces that are too large to be carried in the flue gases and impinge on the furnace walls or fall through open grates to an ash hopper at the bottom of the furnace. Bottom ash is typically grey to black in color, is quite angular, and has a porous surface structure. Bottom ash is used as an aggregate, as feed stock for cement manufacturing or in construction applications in lieu of other constituents (such as sand and gravel). 

bottom ash dewatering bin/tank – an elevated sedimentation tank that is designed with baffles and other devices to receive the bottom ash slurry, settle out and store hours or days of bottom ash production, and discharge the stored and dewatered bottom ash into trucks. This tank is usually in series with a separate tank or tanks that settle out the very small bottom ash particles. Bottom ash dewatering bins are used where there are space limitations at a coal-fired power plant and where other process or site requirements do not allow for the use of ponds. 

bottom ash fines – the small bottom ash particles that are produced as a result of dry or wet bottom ash screening processes whose end product is an aggregate. 

bottom ash pond – an impoundment or surface impoundment used to store or dispose of bottom ash primarily from the combustion of coal. (See ash pond) 

bound – coal combustion products blended with cementititous binders, lime and other activators for use in applications, where the materials must be hydraulically or chemically bound together

bulk density – the mass of a material per unit volume, including voids. Bulk density is usually reported on a dry basis. 

bulk density of aggregate – the mass of a unit volume of bulk aggregate material (the unit volume includes the volume of the individual particles and the volume of the voids between particles). 

by-product – a material that is not one of the primary products of a production process and that is not solely or separately produced by the production process.  

by-product utilisation – the recycling or use of coal combustion products. 

cake – the solids discharged from dewatering equipment such as rotating drum vacuum filters, where the material is then referred to as filter cake. 

calcium carbonate equivalent (CCE) – the content of carbonate in a liming material or calcareous soil calculated as if all the carbonate is in the form of CaCO3. 

calcium sulfate dihydrate (CaSO4·2H2O) – gypsum; the primary by-product of a forced oxidation wet flue gas desulfurisation system in which additional air is introduced and lime or limestone is used as the reagent. (See FGD gypsum and gypsum) 

calcium sulfite (CaSO3) – the primary product (or by-product) of a wet flue gas desulfurisation system where there is no forced oxidation and lime or limestone is used as the reagent. 

cap – a layer of clay or other impermeable material installed over the top of a closed landfill to prevent entry of rainwater and minimise leachate. 

captive facilities – facilities that are located upon lands owned by the generator of coal combustion products or coal combustion/flue gas cleaning wastes and which are operated to provide for the treatment or disposal solely of the generator’s by-products or wastes. (Example: captive landfill) 

carbon – a chemical element; the principal combustible constituent of all fossil fuels. 

carbon dioxide (CO2) – a colorless, odorless, incombustible gas formed during combustion in fossil-fuel electric generation plants. 

carbon in ash (unburned carbon in ash, LOI) – the unburned carbons in fly ash include both carbon carried over as uncombusted coal particles (typically inertinite) and chars or cokes resulting from the incomplete combustion of thermoplastic, largely vitrinite-derived phases. The latter includes ‘isotropic coke’ and ‘anisotropic coke’.

carbon reduction process – a process to reduce the concentration of carbon (typically measured by the loss on ignition or LOI) in high-carbon fly ash. The reduction in the overall carbon content of a fly ash can be achieved through one or more of several different methods including thermal reduction, tribo-electric separation, flotation, classification, etc. In general, the goal in a carbon reduction processes is to remove carbon without significantly altering the hydraulic/pozzolanic properties of the fly ash, thereby making the material suitable for use in concrete applications. These processes are often proprietary and there are several firms that specialise in carbon reduction. 

cell – a portion of a landfill which is isolated, usually by means of soil or an impermeable barrier, from its surroundings. 

cementitious ash – fly ash that hardens irreversibly when mixed with water; also referred to as self-cementing ash. 

cementitious material (hydraulic) – an inorganic material or a mixture of inorganic materials that sets and develops strength by chemical reaction with water through formation of hydrates and is capable of doing so under water. 

cementitious mixture – A combination of more than any one of the following materials to make a cement paste: hydraulic cement; Portland cement; coal fly ash; FBC ash; lime; ground granulated blast furnace slag; lime kiln dust; cement kiln dust. It may be used by itself for grout, or used to bind aggregates or fine materials to make concrete or controlled low-strength materials (CLSM), or used for soil stabilisation and solidification. 

cenospheres – a portion of fly ash once referred to as the ‘floating fraction’ of fly ash because of its occurrence on the surface of fly ash ponds and from where it was and still is harvested for beneficial use. Cenospheres are lightweight, inert, hollow, essentially thin-walled glass spheres (10–350 microns) composed largely of silica and alumina and filled with air and/or gases. Cenospheres are formed from the ash when it is in a molten state. The thickness of cenosphere walls may be very small and the resultant bulk densities are less than 1 gm/cm³. Cenospheres that are harvested from the surfaces of ash ponds are processed (by drying, classifying, etc.) and marketed as a high-value product in applications for performance enhancement of products such as paints, coatings, adhesives etc. Cenospheres are also extracted from dry fly ash by proprietary processes and marketed under registered trade names. The proportion of 1 gm/cm³ cenospheres that exists in fly ash has not been quantified definitively, but the literature indicates an overall maximum of about 2%. (See microspheres) 

char – the solid carbonaceous residue that results from incomplete combustion of organic material which includes coal. It can be burned for heat or, if pure, processed for production of activated carbon for use as a filtering medium. In the electricity generation industry the overall proportion is represented by the fixed carbon of the coal, and current usage in coal combustion will be ‘the time to burn out the char’. It can also apply to the unburned portion of the coal and would have the same usage as unburned carbon in ash. 

char/fly ash – another term for the carbon in the fly ash which is now commonly referred to as unburned carbon (UBC) in ash.

cinders – an old term in ash marketing for the ash from utility stoker boilers. Cinders were used as an aggregate, especially in concrete block manufacturing, and hence the term cinder blocks. 

class C fly ash – fly ash which meets criteria defined in e.g. ASTM C 618, EN450 for use in concrete. 

class F fly ash – fly ash which meets criteria defined in e.g. AS 3582.1, ASTM C 618, EN 450  for use in concrete. 

classifier – device or processing machine used to separate fly ash particles according to size and density.

clean coal combustion – the burning of coal using technology designed to minimise the level of emissions such as carbon dioxide, sulfur oxides, nitrogen oxides or particulate materials in relation to the energy or other output of the system; may include fluidized-bed combustion, oxy-fuel combustion or integrated coal gasification combined cycle plants. 

clean coal technology combustion products – products generated from any technology,  including technologies applied at the pre-combustion, combustion, or post-combustion stage, at a new or existing facility, which will achieve significant reductions in air emissions of carbon dioxide, sulfur dioxide or oxides of nitrogen associated with the utilisation of coal.

clinker – a hard compact congealed mass of fused furnace refuse, usually slag. Bottom ash and boiler slag usually contain clinker. 

cement clinker – the fused particles or pellets produced from the sintering or burning zone of a rotary kiln in the cement manufacturing process. Raw materials (limestone, shale, iron ore and sand) are proportioned and ground to a powder and blended before being processed through the rotary kiln. 

closure plan – a written plan that describes the steps the owner or operator of the disposal facility will take to close the facility in accordance with regulatory or other requirements. 

coal – a brown to black combustible sedimentary rock (in the geologic sense) composed principally of consolidated and chemically altered plant remains; solid carbonaceous fuel formed by ancient decomposition of woody substances followed by changes due to burial under conditions of heat and pressure. Depending on the extent of the post-burial changes (the coal rank), coals may be classified as anthracite, bituminous, sub-bituminous or lignite. 

coal ash – a collective term referring to any solid materials or residues (such as fly ash, bottom ash or boiler slag) produced primarily from the combustion of coal. The term refers to any materials or residues produced directly from the combustion of coal and especially from coal-fired power plants. It resembles volcanic ash and typically consists of quartz, a range of aluminosilicate minerals and glasses, iron oxides and, in some cases, calcium-bearing phases and other inorganic components. In addition it usually contains trace quantities (in the parts per million range) of other naturally-occurring elements. These same elements exist in soil, rock and coal. The coal may be bituminous, sub-bituminous, lignite or a mixture of such coals. Residues from small quantities of other fuels such as petroleum coke, fuel oil, biomass etc. mixed with coal may also be part of the coal ash. Current usage of coal ash as a collective term is synonymous with the terms coal combustion products (CCPs) and coal combustion residue (CCR). Coal ash is a component of the term coal combustion by-product (CCB), covering only the materials or residues associated with the combustion of coal and not the residues from flue gas cleaning. (See coal combustion ash, CCPs, CCR and CCBs) 

coal ash landfills – a landfill that receives only coal ash. It is usually regulated by a sovereign  governmental agency and subjected to the waste management practices and alternative disposal practices of the particular state. (See mono-fill) 

coal combustion ash – collective term referring to any materials or residues produced from the combustion of coal. (See coal ash, CCPs) 

coal combustion by-products (CCBs) – collective term referring to fly ash, bottom ash, boiler slag, fluidised bed combustion ash or FGD material resulting from the combustion of coal and the cleaning of the stack gases. CCBs is also a collective term referring to any large-volume material or residue produced from the combustion of coal or the cleaning of the stack gases regardless of ultimate commercial application or disposal. The term coal combustion products (CCPs) has replaced the term coal combustion by-products, and this usage is intended to clearly identify the products from the combustion of coal or the cleaning of the stack gases that are manufactured or processed to meet standards, guidelines etc. and used commercially. (See coal combustion products)

coal combustion products (CCPs) – fly ash, bottom ash, boiler slag, FBC ash or FGD material produced primarily from the combustion of coal or the cleaning of the stack gases. It is also a collective term referring to materials produced in this way and manufactured either as part of coal-fired power plant operating processes or otherwise to meet standards, guidelines etc. and used commercially. (Examples include but are not limited to the production of FGD gypsum; the production of specification fly ash either as part of the power plant’s normal basis of operation or through the use of beneficiation processes; the production of commercial materials through screening, drying, classifying of bottom ash, boiler slag or fly ash.) The term coal combustion products (CCPs) has replaced the term coal combustion by-products (CCBs).

coal combustion residue (CCR) – collective term referring to any materials or residues produced from the combustion of coal. (See coal ash and coal combustion products) 

coal combustion wastes (CCWs) – a collective term for materials or residues produced from the combustion of coal or the cleaning of stack gases that are disposed of as a solid waste. 

coal fly ash – a product derived from burning finely ground coal in a boiler to produce electricity. It is removed from the plant exhaust gases, primarily by electrostatic precipitators or baghouses and secondarily by wet scrubbers. Physically, fly ash is a very fine, powdery material, composed mostly of silica and alumina, and nearly all particles are spherical in shape. Coal fly ash is a pozzolan. (See fly ash) 

fly ash–bituminous coal – fly ash resulting from the combustion of a bituminous coal in a boiler for the production of electricity; it is generally low in lime (less than 10% CaO), and its chemistry would usually classify it as a Class F fly ash; this fly ash does have pozzolanic characteristics. 

fly ash–sub-bituminous/bituminous coal blends – fly ash resulting from the combustion of sub-bituminous/bituminous coal blends in a boiler for the production of electricity; its lime (CaO) content can exceed 10%; its chemistry would classify it as a Class C (ASTM C618) or as a Class F fly ash having a lime content that exceeds the level normally associated with Class F fly ashes. 

coal mine – an area of land and all structures, facilities, machinery, tools, equipment, shafts, slopes, tunnels, excavations and other property, real or personal, placed upon, under or above the surface of such land by any person, used in extracting coal from its natural deposits in the earth by any means or method, and the work of preparing the coal so extracted, including coal preparation facilities. The British term is ‘colliery’. Coal combustion products are sometimes used in construction projects at coal mines and coal combustion products are sometimes emplaced in coal mines as part of remediation works. 

coal mine waste – the waste material produced from coal mining. May include overburden rock removed from open-cut mines, non-coal rock removed from underground mine development, and reject materials from coal preparation and processing plants. 

coal preparation waste – the non-coal rock and associated material separated as a reject or waste fraction from mined coal during cleaning, concentrating or other preparation processes. 

co-firing – the term applied to the firing of two dissimilar fuels, such as biomass and coal, at the same time in the same boiler. 

coke – a carbonaceous solid produced from coal, petroleum or other materials by thermal decomposition with passage through a plastic state; fuel consisting largely of the fixed carbon and ash obtained by the destructive distillation of bituminous coal. 

compaction – the densification of a soil or coal combustion product by means of mechanical manipulation; reduction in bulk volume of solid waste by rolling and tamping. Compaction also refers to the reduction in bulk volume or thickness of a body of fine-grained sediments in response to increasing weight of overlying material.

compliance coal – a coal or a blend of coals that meets sulfur dioxide emission standards for air quality without the need for flue gas desulfurisation. 

compost – relatively stable decomposed organic material, often associated with agriculture or gardening soil enrichment. Sometimes ash is blended with materials such as leaves, wood chips, peanut shell, poultry waste, etc. to form compost. 

composting – an aerobic process involving the biological stabilisation of sludge and other wastes by micro-organisms. Generally the process comprises spreading or windrowing the material; sometimes the sludge or other waste is mixed with a bulking agent such as coal ash to maximise air contact. 

conditioned ash – ash that has been moistened with water during the load-out process at a temporary storage silo at a power plant, to allow for its handling, transport and placement without causing fugitive dust emission. The water added can vary from 5% to 30% by weight of the dry ash, which may be fly ash from an electrostatic precipitator (ESP), fluidised bed material that could be a combination of fly ash and bed ash, and bag-house material from a dry scrubber that could be a combination of fly ash, unreacted lime (as calcium hydroxide), calcium sulfate and calcium sulfite. The water is added in a pugmill or pugmill-type equipment as the ash is fed from the silo and loaded into open-body trucks or other hauling equipment. The conditioned ash is usually designated for placing in a landfill, although it can be used in beneficial applications. (See pugmill) 

conditioned fly ash – dry fly ash that has been moistened with water during the load-out process at a temporary storage silo at a power plant, to allow for its handling, transport and placement without causing fugitive dust emission. The water added can vary from 5% to 30% by weight of the dry fly ash. Water content of 5% to 10% is sometimes added for high-lime fly ashes because of the quick setting that occurs and to allow a designated time so that the hauling equipment can easily discharge the conditioned fly ash without it sticking in the truck bed. Water content for low-lime fly ash is generally from 10% to 30%. The water is added in a pugmill or pugmill-type of equipment (dustless unloader) as the dry fly ash is fed from the silo and loaded into open-body trucks or other hauling equipment for placing in a landfill or for beneficial use. (See dustless unloader and pugmill) 

consolidation – the reduction in volume of a fill caused by movement of water out of the fill mass. Consolidation generally occurs due to an increase in the vertical stress on a fill. It is the movement of water rather than the compression of air-filled voids that distinguishes consolidation from compaction. 

controlled low-strength material (CLSM) – a flowable fill that is stabilised with the addition of a binder. 

cyclone – a cone-shaped air-cleaning apparatus that operates by centrifugal separation and is used in particle collecting and fine-grinding operations. Cyclone particle collection equipment is widely used in CCP handling and storage systems. 

cyclone boiler – a type of coal-fired boiler. The coarsely pulverised coal undergoes slagging combustion in a cylindrical (cyclone) burner. Some wet-bottom boilers are not cyclone-fired. The primary by-product is a glassy slag referred to as boiler slag, which is in great demand for beneficial use, but the supplies are declining because of the retirement from service of cyclone boiler electric power generating plants. 

cyclone dust collector – a type of particle collection equipment that is used in particle transport systems and is usually in series with other types of dust collection equipment such as a baghouse dust collector. It is used in CCP handling and storage systems.

deep mine injection – placement of materials such as ash and flue gas cleaning material into underground depleted mine cavities, through boreholes, either pneumatically or hydraulically. Proper filling may help control acid mine drainage by reducing oxidation of pyrite, addition of alkalinity or reducing groundwater flow through the mine. This is a beneficial use of coal ash when performed for mine subsidence control. 

density – the mass per unit volume; weight per unit volume, expressed as grams per cubic centimetre for solids and liquids and usually as grams per litre for gases. 

dewatering – a physical process that removes sufficient water from a sludge, FGD material or ponded ash and FGD solids so that its physical form is changed from essentially that of a fluid to that of a damp solid. Some major types of equipment and facilities used for dewatering are: rotary drum vacuum filters, centrifuges, horizontal belt filters, lagoons and ponds. 

dike – an embankment or ridge of either natural or synthetic materials used to contain or hold a liquid, slurry, sludge or other material in ponds. 

discharge – the release of any solid, liquid or gas waste stream or any constituent thereof to the environment. (Example: a coal combustion product surface impoundment will have a water discharge that has a permit issued by the relevant government agency) 

disposal – the discharge, deposit, injection, dumping, spilling, leaking or placing of any solid waste into or on any land or water so that such solid waste or any constituent thereof may not enter the environment or be emitted into the air or be discharged into any waters including groundwater. 

disposal facility – a facility or part of a facility at which waste is intentionally placed into or on any land or water, and at which waste will remain after closure; the necessary equipment and associated land area that serve to receive and manage waste. The facility may make use of one, many or all of a large number of disposal methods. A coal combustion products disposal facility is usually either a landfill or surface impoundment and may be located on site or off site from the power plant. 

drainage blanket – a uniform layer of permeable material such as sand, crushed stone or bottom ash/boiler slag installed with properly designed filter media at the base of a structural fill to maintain the fill in a drained condition. 

dredging – an excavation practice employed by the electric power generation industry to remove coal combustion products from a temporary storage to a long-term disposal facility. 

dry ash removal – the method of accumulating and removing dry ash from a dry bottom pulverised fuel-fired furnace. 

dry bottom furnace – a pulverised fuel-fired furnace in which the ash particles (bottom ash) are deposited on the furnace bottom in a dry, non-adherent condition. 

dry fly ash disposal system – the overall fly ash transport and storage system that involves the dry fly ash removal system at a coal-fired power plant, the truck (vehicular) transport of the conditioned fly ash and the placing of the conditioned fly ash in a landfill. 

dry fly ash removal system – the equipment system used to remove and convey fly ash from the hoppers of particulate collection equipment, ESP or other to a silo (including the silo aeration and unloading equipment) or to a wetting water eductor. 

dust – particles of gas-borne solid matter larger than one micron in diameter. 

economiser – a heat-recovery device designed to transfer heat from the products of combustion to a fluid, usually feedwater. 

economiser ash – ash which is collected in ash hoppers that are located below the economiser section of a boiler. The majority of the economiser ash is plus 200 mesh and it is referred to as having a popcorn consistency. Generally economiser ash is removed from its hopper(s) as part of the fly ash removal system and is conveyed along with the fly ash to a silo (for dry removal systems) or to a pond (for wet removal systems). However the size of the economiser ash particles when combined with the dry fly ash in a silo can cause the fly ash to be out of specification with standards for use in concrete. As a result, economiser ash is conveyed either to the bottom ash transport system and combined with the bottom ash or to a dedicated silo. 

effluent – the final discharge from any process such as from an ash impoundment. 

electrostatic precipitator (ESP) – a facility that removes fly ash from flue gas by producing an electric charge on the fly ash and collecting the ash electrostatically.

encapsulation – complete coating or enclosure of particles by an additive so as to sequester particles from any environmental receptors that may otherwise have been negatively impacted by particles; the complete enclosure of a waste in another material in such a way as to isolate it from external effects such as those of water or air. 

ettringite – a high-calcium sulfoaluminate mineral [Ca6.Al2(SO4)3(OH)12.26H2O] that is expansive because of its crystal structure; a mineral composed of hydrous basic calcium and aluminum sulfate that expands when wet upon forming its crystalline structure.  

ettringite formation – the phenomenon that leads to the formation of ettringite and can occur in coal ash/lime/sulfur mixtures. Ettringite is formed by the combination of aluminium from the coal ash with lime and sulfates and with water. These four substances are required for ettringite to form. Swelling problems due to ettringite formation have occurred with coal ash that contains scrubber or FBC residue. Swelling problems rarely occur with coal ash that does not contain scrubber or FBC residue. 

exothermic – a process or chemical reaction which is accompanied by the evolution of heat, for example, combustion reactions. 

facility – all contiguous land and structures, other appurtenances and improvements on the land used for processing, treating, storing or disposing of CCPs. This can include all buildings, equipment, structures and other stationary items which are located on a single site or contiguous or adjacent sites and which are owned or operated by the same person (or any persons under the direction or control of such person). 

filler – a substance added to a system or product to increase bulk, weight, viscosity, opacity or strength and often used to reduce cost. CCPs are used as fillers in many applications, for example, in solidification/stabilisation of wastes, or in concrete, flowable fills/controlled low strength materials. Fly ash in particular is used as a mineral filler. 

final closure – the measures that are specified by the permitting agency of a waste management facility and implemented by the owner of the facility to render a part of or the entire facility environmentally innocuous when it is no longer used to accept waste for treatment, storage or disposal. 

final cover – cover material that is applied upon closure of a landfill or surface impoundment. 

financial assurance – demonstration by an owner of a waste management facility to the permitting governmental agency of financial assets to guarantee closure and post-closure care. 

fineness – the percentage by weight of a standard sample of a pulverised material that passes through a standard screen of specific mesh size when subjected to prescribed sampling and screening procedures. It is an important quality factor for the use of fly ash in concrete. The fineness of a particular fly ash is related to the operating condition of the coal crushers and the grindability of the coal itself. It is a measure of the percentage retained on a defined sieve. A coarser gradation can result in a less reactive fly ash. 
 
fixed carbon – the part of the carbonaceous matter in coal that remains behind when the coal is heated in a closed vessel until all of the volatile matter is driven off. 
 
flowable fill – a material that flows like a liquid, is self-levelling, requires no compaction or vibration to achieve maximum density, hardens to a predetermined strength and is sometimes a controlled low-strength material (CLSM). CCPs are used in manufacturing flowable fills. The proportion of the CCPs in the flowable fill mixture can be 100% for an all-ash flowable fill that consists of a combination of fly ash and water. It can be a major portion of a mixture that consists primarily of fly ash or fly ash and bottom ash and a small amount of cement or cement and lime.  

flue gas conditioning – the process of adding chemicals such as sulfur trioxide or ammonia to flue gas in order to improve the performance of the electrostatic precipitator (ESP) or reduce the opacity of the emissions from the stack. 
 
flue gas desulfurisation (FGD) – removal of gaseous sulfur dioxide from boiler exhaust gas. Primary types of FGD processes are wet scrubbers, dry scrubbers and sorbent injection. Sorbents include lime, limestone, sodium-based compounds and high-calcium coal fly ash. 

dry FGD – an FGD system in which calcium or sodium-based sorbents, usually hydrated lime, are introduced to the flue gas. Dry FGD systems use less water than wet systems, usually remove fly ash and sulfur dioxide simultaneously, and generate a dry by-product. Spray dryer systems are the most common design. In a spray dryer, slaked lime slurry is sprayed into the flue gas and the resulting by-product, dried by the heat of the flue gas, is collected in a particulate control device with the fly ash. Other dry systems inject dry sodium sorbent directly into the boiler exhaust duct. The by-product of a dry FGD system is referred to by various names that include dry FGD ash, dry FGD material and dry scrubber material. 
 
dry sodium injection – (See dry FGD) 
 
ex situ oxidation (wet FGD) – forced oxidation that occurs outside of the scrubber and is used to produce FGD gypsum. 
 
forced oxidation – a process employed to supply additional air in wet FGD systems, resulting in a predominantly calcium sulfate dihydrate (gypsum) by-product with improved storage characteristics as well as greater commercial potential. 

in situ oxidation (wet FGD) – a process in which both SO2 absorption and oxidation are carried out within the scrubber. 

spray dryer – a type of dry FGD system. (See dry FGD) 
 
wet FGD – an FGD system that uses a wet scrubber to introduce an aqueous solution of either slaked lime (calcium hydroxide) or limestone (principally calcium carbonate) into the flue gas in a spray tower. The sorbent reacts with or oxidises the sulfur dioxide in the flue gas and coverts it to a by-product that is referred to as scrubber sludge, scrubber material or wet FGD material. 

scrubber – any of several forms of chemical or physical devices that remove sulfur compounds formed during coal combustion, especially from some coal-fired power plants. (See wet FGD and wet scrubbers)

FGD material – a product of an FGD process typically using a high-calcium sorbent such as lime or limestone. Sodium-based sorbent and high-calcium coal fly ashes are also used in some systems. The physical nature of these materials varies from a wet thixotropic sludge to a dry powdered material depending on the process. The wet thixotropic sludge is usually from a lime-based reagent wet scrubbing process and is predominantly calcium sulfite. It is the end product of dewatering equipment such as vacuum filters or centrifuges, although it can be the end product of a sedimentation pond. This dewatered end product is usually stabilised by mixing with lime and fly ash or other materials for disposal in landfills. (There are systems where the end product is not dewatered but is highly concentrated in solids as the underflow from a thickener – it is then mixed with fly ash and another material and pumped to a surface impoundment for disposal). The wet product from limestone-based reagent wet scrubbing processes is predominantly hydrated calcium sulfate (gypsum). This material readily dewaters and there are systems in use where the slurry is transported to a pond and construction equipment is used to excavate and stockpile the gypsum. The production of commercial-grade FGD gypsum used for wallboard manufacturing usually requires forced oxidation in the scrubbers or external to the scrubbers and dewatering by filtration equipment such as vacuum filters or centrifuges and sedimentation ponds. The dry material from dry scrubbers that is captured in a baghouse along with fly ash consists of a mixture of sulfites and sulfates in addition to fly ash. This powdered material is referred to as dry FGD ash, dry FGD Material, lime spray dryer ash, lime spray dryer or lime spray dryer residue. 
 
dry FGD ash – (See dry FGD material) 
 
dry FGD material – the product that is produced from dry FGD systems and consists primarily of calcium sulfite, fly ash, portlandite [Ca(OH)2], and/or calcite. The main constituents of the dry FGD material from lime-based sorbent systems are calcium sulfite and dry fly ash, along with minor quantities of calcium sulfate. The main constituents of sodium-based sorbent systems are sodium sulfite and dry fly ash, along with minor quantities of sodium sulfate. Dry FGD material is used in construction, engineering and agricultural applications; however, most of the material is stored in landfills. 
 
FGD by-products – a term for the by-products from wet and dry FGD systems. (See wet and dry FGD by-products) 
 
FGD gypsum – gypsum formed from an oxidising and calcium-based flue gas desulfurisation process; also a precipitated gypsum formed through the neutralisation of sulfuric acid in flue gas desulfurisation processes at coal-fired power plants. This gypsum can vary in purity, which is defined as the percentage of CaSO4·2H2O and generally is over 94% for use in wallboard manufacturing. The less pure gypsum can be stockpiled (gypsum stacking), placed in ponds or captive landfills, or utilised in agriculture or construction. The nearly pure or pure FGD gypsum is utilised beneficially. The pure FGD gypsum is produced to meet the specifications of wallboard manufacturing companies and is used for wallboard manufacturing, for cement production and as plasters. Large quantities of FGD gypsum are produced and utilised. (See gypsum and synthetic gypsum) 

FGD material dry scrubbers – the dry powdered material from dry scrubbers that is collected in a baghouse along with fly ash and consists of a mixture of sulfites, sulfates and fly ash. (See dry FGD ash) 

FGD products – another term for the by-products from wet and dry FGD systems. 

FGD sludge – another name for scrubber sludge, wet FGD material or filter cake. (See wet FGD material) 
 
filter cake – the material produced by filtering equipment such as vacuum filters for dewatering wet FGD material. (See wet FGD material) 

fixated FGD material – a designed mixture of dewatered FGD sludge that is primarily calcium sulfite with either a high-lime (Class C) fly ash, or a low-lime fly ash (Class F), combined with a cementitious material (such as cement kiln dust, lime kiln dust or FBC ash). FGD sludge is also known as scrubber sludge, scrubber material, FGD solids, filter cake or centrifuge cake. The designed mixture is produced in a mixing facility that is sometimes referred to as a sludge treatment plant (STP), then transported by belt conveyor to an area where it is stockpiled for a number of hours or days to undergo an initial chemical set. The stockpiled material is then excavated, loaded on to trucks or other earthmoving equipment for transport and placement as a fill in beneficial use applications, or for placement in a landfill for storage or disposal where it undergoes a further chemical set. After placement, the fixated material forms a stable, monolithic mass of low permeability. 
 
fixated scrubber sludge – another name for fixated FGD material. 
 
lime spray dryer ash – the residue from a spray dryer FGD system. The resulting by-product is dried by the heat of the flue gas and is collected in a particulate control device with the fly ash. (See dry FGD material) 
 
lime spray dryer residue – another name for lime spray dryer ash. 
 
scrubber sludge – another name for wet FGD material. (See wet FGD material) 
 
stabilised FGD material – another name for fixated FGD material. (See fixated FGD material) 
 
wet FGD material – the by-product of wet FGD processes or systems. It is composed primarily of water, calcium sulfite/sulfate solids and small quantities of fly ash. It has the consistency of a sludge when allowed to settle in a pond or when the water is removed by filtering equipment such as vacuum filters. It is commonly referred to as scrubber sludge. Depending on the composition of the injected lime or limestone, some by-products will also contain magnesium sulfite and/or sulfate, and possibly traces of barium sulfite or boron in addition to some trace metals.
 
fixated FGD material pad area – the engineered area that receives the fixated FGD material from the processing facility (sludge treatment plant) where the filter cake is mixed with fly ash and lime or other material, via a belt conveyor and radial stacker. The material is stockpiled on this pad and allowed to cure (hours or days). The cured material is then excavated, loaded on to trucks or other transportation equipment for beneficial use or disposal. 
 
fluidised-bed combustion (FBC) ash – the fly ash and bed ash produced by an FBC boiler. FBC fly ash is collected in the flue of an FBC boiler using a baghouse filter or electrostatic precipitator. FBC bed ash is the residue that is removed from the bottom of the FBC boiler. Some FBC fly ashes exhibit self-hardening properties in the presence of moisture. 
 
fluidised-bed combustion (FBC) bed ash – the spent bed material that is produced by an FBC boiler. The bed ash is usually collected separately and can be considered as being equivalent to bottom ash in dry-bottom or wet-bottom wall-fired furnaces. 
 
fluidised-bed combustion (FBC) boiler – a type of coal boiler in which the coal particles are lifted in an up-current of air and burn in a fluidised bed. In many cases the coal is mixed with a sorbent such as limestone, or other bed material, to capture sulfur oxides and reduce gaseous sulfur emissions. The fuel and bed material mixture is fluidised during the combustion process to allow complete combustion and removal of sulfur gases. Atmospheric FBC (AFBC) systems may be bubbling (BFBC) or circulating (CFBC). Pressurised FBC (PFBC) is an emerging coal combustion technology. 
 
fluidised-bed combustion (FBC) materials – the unburned coal, ash and spent bed material produced by an FBC boiler. The bed ash is usually collected separately and can be considered as being equivalent to bottom ash in dry-bottom or wet-bottom wall-fired furnaces. 
 
fluidised-bed combustion (FBC) products – the unburned coal, ash, spent bed material and unreacted sorbent produced by an FBC boiler. 

fluidising – the causing of a mass of finely divided solid particles to assume some of the properties of a fluid, as aeration. (Example: the fly ash in a silo is usually fluidised to facilitate its flow and allow for the load-out of the ash from the silo.) 
 
fly ash – coal ash that exits a combustion chamber in the flue gas and is captured by air pollution control equipment such as electrostatic precipitators, baghouses, and wet scrubbers. Fly ash is typically a pozzolan. Some fly ashes also exhibit self-hardening properties in the presence of moisture. (See coal fly ash) 
 
fly ash–high lime – fly ash typically resulting from the combustion of sub-bituminous and some lignite coal that contains a significantly higher percentage of calcium compounds than the fly ash resulting from the combustion of bituminous coal; its chemistry would make it fall under, but may not conform with, the ASTM C 618 classification of a Class C fly ash; it may contain in excess of 20% CaO. 
 
fly ash–low lime – fly ash typically resulting from the combustion of anthracite or bituminous coal; it is relatively low in lime (less than 2%); its chemistry would make it fall under, but may not conform with, the ASTM C 618 classification of a Class F fly ash. This fly ash does have pozzolanic characteristics. 

fly ash ‘graded’ capability – the production or processing capability within a facility infrastructure to process fly ash to specified properties, for example, physical or chemical properties. Should there be no capacity within a facility to collect, capture and load vehicles with CCPs then NO capability exists to produce graded fly ash. 

fly ash ‘ungraded’ capability – the production or processing capability within a facility infrastructure to collect, capture and load vehicles with CCP ungraded fly ash; for example, dedicated infrastructure or loading access point to load vehicles with fly ash from the facility without further processing. This does not include ash handling systems to transport or transfer fly ash to ash dams. 
 
fly ash–lime content – the total calcium content of fly ash, including reactive and non-reactive calcium species, expressed as calcium oxide (CaO). 
 
forced oxidation – a process employed to supply additional air in wet FGD systems, resulting in the production of gypsum. 
 
fossil fuel combustion wastes (FFCWs) – a collective term used by the US-EPA for materials or residues produced from the combustion of coal or the cleaning of stack gases. (See coal combustion wastes, coal combustion by-products) 
 
free lime – reactive lime and hydroxide species available to react with a pozzolan to form a cementitious product, usually expressed as a percentage by total weight of the product. 
 
friable – easily crumbled or pulverised. (Example: some coal bottom ashes are reported to be friable.) 
 
fuel switching – a pre-combustion process whereby a low-sulfur coal is used in place of a higher-sulfur coal in a power plant to reduce sulfur dioxide emissions. 
 
fugitive emissions – emissions other than those from stacks or vents. (Example: dust emissions from unpaved roads, the surfaces of landfills, etc.) 
 
gasification – the conversion of coal to a combustible gas, volatiles, char, and ash/slag; any of various processes by which coal is turned into low, medium, or high energy gases. By-products from gasification systems vary widely. Gasification is generally regarded as a clean coal technology. 
 
grab sample – a single sample of material or liquid taken at neither set time nor rate.  

graded – CCPs that have been processed to a particle size distribution that meets particular requirements and/or specifications, for example, a given specified fineness as nominated in AS3582.1 (Grade 1 – Fly ash and Grade 2 – Fly ash).

ground granulated blast furnace slag (GGBFS) – the granular material formed when molten iron blast furnace slag is rapidly chilled (quenched) by immersion in water. It is a granular product with very limited crystal formation, is highly cementitious in nature and, ground to cement fineness, hydrates like Portland cement. Use of GGBFS as a mineral admixture in concrete and water is covered by AS 5382.2. 

ground water – that part of the subsurface water that is in the saturated zone; the upper level of the saturated zone is referred to as the water table. 
 
grout – a mixture of cementitious material and water, with or without aggregate, sometimes incorporating CCPs, proportioned to produce a pourable consistency without segregation of the constituents. It is used for filling voids and spaces.
 
gypsum – name for calcium sulfate dihydrate (CaSO4·2H2O); the common name for the mineral consisting primarily of fully hydrated calcium sulfate or calcium sulfate dihydrate. Gypsum occurs naturally in many areas, and is produced by some wet flue gas desulfurisation (FGD) processes. (See FGD gypsum and synthetic gypsum) 
 
gypsum stacking – the method used in the phosphate fertiliser industry and applied to the power industry for stacking the wet FGD by-product (material) that is predominantly calcium sulfate (gypsum). It involves placement of the FGD by-product slurry in an impoundment and stacking of the reclaimed settled solid in two operations. The primary operation accepts the FGD by-product slurry directly from the scrubber in a diked or bermed ponding area (settling ponds). These settling ponds provide for primary settling of the FGD solids. The effluent from the ponds is decanted and either recycled back to the scrubber operation or sent to treatment and discharge. The solids that are settled in the primary/ponding operation are periodically excavated and placed into piles or stacks, typically adjoining the ponds to minimise the distance for transporting the dewatered material. Draining/excavating and stacking/drying operations alternate between diked areas to enable continuous storage, and excavated material is used to raise dikes and to increase the site capacity. 
 
heat of hydration – heat evolved by chemical reactions with water such as that evolved during the setting and hardening of Portland cement, fly ash, dry FGD material or quicklime, with or without pozzolans. 
 
high volume waste – a regulatory term for fly ash, bottom ash, boiler slag and flue gas desulfurisation wastes. 
 
highwall – the unexcavated face of exposed overburden and coal in a surface or open-cut mine. 
 
hygroscopic – the term describing a compound that can absorb water vapour from the atmosphere; for example, some high-lime fly ashes, when stored in buildings will absorb moisture in the air. 
 
impoundment – the restraint of a flowable material such as a slurry or sludge behind a structural barrier, such as a dam, dike etc. (See ash pond) 
 
in situ – a Latin term meaning in place. 
 
landfill – a disposal facility where waste is placed in or on land; a facility where ‘dry’ (actually moistened) coal combustion or flue gas cleaning products are placed for disposal in or on land. CCPs are transported to this facility directly from the coal-fired plant after they are produced or after they are dredged from storage impoundments that are used as interim facilities. The disposed CCPs remain in the landfill after closure. These CCPs have the consistency of soil and, as a result, dikes are not required to provide stability. Most large landfills are divided into sections or cells and the CCPs are placed in layers (referred to as lifts) that can vary in thickness. Typically, captive CCP landfills are designed and permitted to receive only CCPs, and are thus classified as mono-fills. 
 
leachate – the liquid, including any suspended components in the liquid, that has percolated through or drained from a pile or cell of solid materials; the liquid stream that issues from a pile (stockpile of ash, coal, etc.) or cell of solid materials (an ash landfill) and which contains water, dissolved solids and decomposition products of the solids. Leachate may enter the groundwater system and contaminate water supplies. 
 
leaching – the operation, natural or designed, of producing leachate. 
 
lift – the construction/earth moving industry term for placing or spreading soil and other materials in layers in a fill; the depth of soil or other materials placed in an embankment or fill that can be compacted to the specified density with the available equipment. 
 
lightweight aggregate – (See aggregate) 
 
lime – calcium oxide (CaO); also, loosely, a general term for the various chemical and physical forms of quicklime, hydrated lime and hydraulic hydrated lime. 
 
liner – a structure of natural and/or manufactured products that serves as a barrier to minimise the amount of leachate reaching or mixing with the adjacent groundwater. 
 
loss on ignition (LOI) – the weight change of a material when it is heated under prescribed conditions. Often used as a measure of the unburned carbon remaining in fly ash, LOI is perhaps the single most critical characteristic of fly ash when used in concrete. Higher carbon contents can result in significant air-entrainment problems and can adversely affect the performance of concrete incorporating the ash.  
 
low NOx burners (LNB) – a combustion technology for reducing the emissions of nitrogen oxides (NO and NO2, collectively referred to as NOx) from coal-fired power plants. The principle of LNB involves decreasing the amount of air introduced into the primary combustion zone, thereby creating a fuel-rich, reducing environment and lowering the temperature, both of which suppress NOx formation. The remaining air required for complete burnout of combustibles is added after the primary combustion zone, where the temperature is sufficiently low so that additional NOx formation is minimised. 
 
manufactured aggregates from CCPs – a commercial product made by the intentional size-enlargement and hardening of fine-particulate CCPs for use as a substitute for crushed stone, sand and gravel, and lightweight aggregate in the construction materials industry. The commercial products that use CCPs all have trade names. 
 
microspheres – micrometre-sized fly ash particles that are formed from fly ash when it is in a molten state. In this state, a spherical shape is formed because it minimises surface tension. These ash particles have diameters near or less than about 5µm. Although they may contain gas bubbles, in which case they would be considered as cenospheres, it is usually the case that microspheres are solid in form. Their bulk densities approach or are equal to that of the pure compounds. Microspheres are recovered from fly ash by air classification and other techniques. Microspheres have a high intrinsic value and have many known applications. (See cenospheres) 
 
mill rejects – the waste product from a pulveriser mill, which consists of coal, non-coal rock and, in some cases, pyrite. The mill rejects are sometimes combined with the bottom ash in ash handling systems. (See pyrite) 
 
milligrams per litre (mg/l) – essentially the same as parts per million when applied to water solutions, where the relative density is 1. 
 
mine subsidence – the downward displacement of the natural land surface in response to the removal of underlying supporting material by mining. 
 
mono-fill – a landfill that is composed of a single fill material. (Example: a fixated FGD material landfill or an ash landfill) 
 
naturally occurring radioactive material (NORM) – the trace quantities of naturally occurring radionuclides 238U, 232Th and 40K, as well as their associated decay chain products, that are emitted from coal or its ash. Coal ash is considered to be a diffuse naturally occurring radioactive material – the most benign classification. The Ash Development Association of Australia has published reports that show ‘CCPs are not significantly enriched in radioactive elements or in associated radioactivity, compared with common soils or rocks’. 
 
non-point sources – diffuse pollution sources (i.e., without a single point of origin or not introduced into a receiving stream from a specific outlet). The pollutants are generally carried off the land by storm water. Common non-point sources are agriculture, forestry, urban, mining, construction, dams, channels, land disposal, saltwater intrusion and city streets. 
 
off-gases – vapors and gases (including air) given off during a process. (Example: ammonia gas that is given off when ammoniated fly ash is mixed with cement and water) 

organic – chemical compounds that contain carbon and hydrogen; chemicals associated with living entities. 

outfall – the point at which a sewer, ash impoundment or drainage channel discharges to a river or other body of work. An outfall is also the narrow part of a stream, lake or other body of water where it drops away into a larger body. 

particle size – refers in this context to the composition of the solid particles of products from coal combustion or flue gas cleaning. The smaller the particle, the greater will be the exposed surface area of a given volume. 
 
particulate matter – the solid and liquid matter of organic or inorganic composition that is suspended as the result of stack or fugitive emissions. The matter may consist of individual elements and/or compounds, and may or may not be emitted along with gaseous contaminants. 
 
parts per billion (PPB) – 1 x 10-9;  a proportion by weight measurement equivalent to one unit weight of analyte per billion unit weights of matrix; in water treatment terminology, one microgram per litre of water. 
 
parts per million (PPM) – 1 x 10-6; a proportion by weight measurement equivalent to one unit weight of analyte per million unit weights of matrix; in water treatment terminology, one milligram per litre of water. 
 
permits – the official approval of and permission to proceed with an activity controlled by the permitting authority. Permits may be required from several government agencies for landfills and surface impoundments at coal-fired power plants. 
 
petrographic analysis – the determination of the structural, mineralogical and chemical character of coal, ash or slag by examination under an optical microscope. 
 
petroleum coke (pet-coke) – the solid carbonaceous residues remaining in oil refining stills after the distillation process and used in combination with coal at some coal-fired power plants. Fly ashes from pet-coke/sub-bituminous coal blends where the amount of pet-coke in the blend was less than 3% by weight have been marketed for use in fly ash concrete. The carbon in the fly ash from the blend consists primarily of unburned pet-coke. 
 
petrology – the branch of geology that deals with the study of rocks, in particular their formation and chemical and physical structure. 
 
pH – the logarithm of the reciprocal of the hydrogen ion activity in aqueous solutions; a measure of the strength or intensity of a water’s acidity or alkalinity. Water with a pH of 7.0 is neutral. A pH less than 7.0 indicates an acidic solution, while a pH greater than 7.0 indicates an alkaline solution. 
 
pneumatic conveying – the transportation of a powdered material, which would include ash, through a conduit by air. 
 
point source – a stationary location or fixed facility from which pollutants are discharged; any single identifiable source of pollution. (Example: a pipe from an ash pond) 
 
ponded ash – ash that is either in an ash pond or has been excavated from an ash pond. The ash pond is usually prepared to facilitate excavation of the ash by removing the surface water and lowering the water table within the pond. The ash, if it is predominantly fly ash, still retains moisture in excess of 30% unless construction practices of removing the ash in layers, and stockpiling the ash, are followed. The ponded ash tends to be segregated by particle size in the pond, with the coarser ash particles being located in the environs of the discharge of the ash transport pipeline(s) and the finer ash particles being located in the environs of the outfall from the pond. The ponded ash may also contain other materials that are transported to the ash pond as a part of the wastewater sedimentation process for the particular coal-fired power plant. These other materials could include coal fines from the coal pile run-off control system, or solids from the cooling tower blow-down and wastewater collection systems. 
 
ponded bottom ash – bottom ash that has been excavated from an ash pond. 
 
ponded fly ash – fly ash that has been excavated from an ash pond. 
 
pozzolan – primarily siliceous or siliceous-and-aluminous materials that will, in finely divided form and in the presence of moisture, chemically react with calcium oxide at ordinary temperatures to form compounds possessing cementitious properties. 
 
pozzolanic activity – the phenomenon of strength development that occurs when lime and certain aluminosilicates react at ambient temperatures in the presence of water. 
 
pozzolanic-activity index – an index that measures pozzolanic activity based on the strength of cementitious mixtures containing hydraulic cement with and without the pozzolan, or containing the pozzolan with lime. 
 
processed ash – ash that has been put through a cleaning, sieving or other commercial manufacturing process to make the ash meet specifications for a particular beneficial use or to make a proprietary product that is marketed for niche or other applications. 

product – any object possessing intrinsic value, capable of delivery either as an assembled whole or as a component part or parts and produced for introduction into trade or commerce. 
 
pugmill – a mixer having a stationary cylindrical mixing compartment, with the axis of the cylinder horizontal, and one or more rotating horizontal shafts to which mixing blades or paddles are attached. Pugmill-type equipment is used at coal-fired power plants for mixing fly ash and water, FGD cake/lime/fly ash or other materials. This is done to facilitate the handling of the CCPs without creating fugitive dust and/or providing for stabilising of FGD material in particular. 
 
pulverised coal (PC) combustion – refers to any combustion process that uses very finely ground (pulverised) coal in the process. Pulverised coal combustion processes usually result in the production of bottom and fly ashes. 
 
pulverised fuel ash (PFA) – another name for fly ash, used primarily in the United Kingdom. 
 
pulveriser – a machine that reduces a solid fuel such as coal to fine particles suitable for burning in suspension. 
 
pyrite – a common mineral that consists of iron disulfide (FeS2) and has a pale brass–yellow colour and metallic lustre; if present in the coal, coarse accumulations of pyrite may be rejected by the coal pulverisers at a coal-fired power plant and discarded. The waste product from a pulveriser mill, which consists of poor-quality coal, non-coal rock particles and (in some cases) pyrite, is commonly referred to as the ‘mill rejects’. 
 
ready-mixed concrete – concrete manufactured for delivery to a purchaser in a plastic and unhardened state. The use of fly ash in ready-mixed concrete is one of the largest markets for fly ash. 
 
reagent – a substance used because of its chemical activity; in the CCP context reagents are typically used to reduce emissions or improve opacity from coal-fired power plants. Examples of such reagents include lime and limestone used for wet scrubbing of the combustion flue gas to remove sulfur dioxide. The term is also applied to the substances used in solidification or stabilisation of wastes; these materials may include liquids or solids such as sodium silicate, cement, fly ash, etc. 
 
reclamation – actions taken to restore mined land to a post-mining land use approved by the regulatory authority. 
 
reuse – the utilisation of a coal combustion product as is or slightly refurbished by an organisation different from the generator of the CCPs. 
 
run-off – water which, having fallen on a landfill (or other) surface, flows across the surface, picking up materials and will, if not collected, continue into a watercourse; also any rainwater, leachate or other liquid that drains over land from any part of a facility. 
 
run-of-station [ash] – a term used to describe ungraded CCPs. Run-of-station (ROS) materials are typically collected from fly ash collection equipment. The term refers to ungraded fly ash with a particle size range typically between 1 and 300 microns. ROS also refers to ungraded furnace bottom ash. (See ungraded)

sample, composite – a sample that is constructed by combining equal portions of grab or regular samples. 
 
sample, grab – a single sample of a material or liquid (coal combustion product or coal combustion waste) taken at neither set time nor rate. (Example: a grab sample is taken in a single operation from a conveyor delivering fly ash to bulk storage.) 
 
sample, regular – a sample that is constructed by combining equal portions of grab samples that were taken at predetermined times or locations from any single lot. 
 
scrubber – (See flue gas desulfurisation) 
 
scrubber cake – another name for scrubber sludge. (See flue gas desulfurisation) 
 
scrubber material – another name for scrubber sludge. (See flue gas desulfurisation) 
 
scrubber sludge – another name for FGD material. (See flue gas desulfurisation) 
 
sedimentation – gravitational settling of solid particles in a liquid system. This is a widely used method in wet ash or flue gas cleaning material handling and disposal.
 
selective catalytic reduction (SCR) – a post-combustion technology for control of nitrogen oxides (NOx) emissions from coal-fired boilers, gas-fired industrial and utility boilers and combustion turbines. The SCR process consists of injecting ammonia (NH3) into boiler flue gas and passing the flue gas through a catalyst bed where the NOx and NH3 react to form nitrogen and water vapor. Unreacted ammonia will pass through the SCR reactor with the flue gases, with most of it being deposited on the fly ash in the electrostatic precipitators. The levels of ammonia in the fly ash have an effect on ash quality, especially its use as a pozzolan. 
 
selective non-catalytic reduction (SNCR) – a post-combustion technology for control of nitrogen oxides (NOx) emissions from coal-fired boilers, gas-fired industrial and utility boilers. The process consists of the injection of ammonia (NH3) or urea in an optimal temperature window (850°C to 1100°C) to produce a non-catalytic reaction between NH2 radicals and NOx. Ammonium bisulfate precipitation on the fly ash occurs with this process, and this can have an effect on disposal and beneficial use of the fly ash. 
 
self-cementing coal fly ash – fly ash resulting from the combustion of lignite or sub-bituminous coal in a boiler for the production of electricity or steam. Such fly ash, in addition to having pozzolanic characteristics, hardens and gains strength over time following contact with water. Self-cementing fly ash as described here does not include fly ash from fluidised-bed combustion boilers, nor fly ash from boilers that inject lime or other sorbents (either wet or dry), nor does it include fly ash collected with a flue gas desulfurisation material. (See cementitious ash) 
 
silo – a storage vessel, generally tall relative to its cross-section, for dry solids such as fly ash, FBC ash, etc. The dry solids are fed into the top of the silo and withdrawn from the bottom through a controlled mechanism. Silos are extensively used in dry fly ash removal and storage systems. They may be flat-bottomed or cone-bottomed and may be made of concrete or steel. 
 
silo aeration and unloading equipment – the collective term for the equipment used, especially in flat-bottomed fly ash silos, for fluidising the fly ash to facilitate flow and for removing the fly ash from the silo in either a conditioned or dry state. 
 
slag – the nonmetallic product resulting from the interaction of flux and impurities in the smelting and refining of metals; also molten or fused ash in the furnace of a coal-fired power plant. (See boiler slag) 
 
sludge – any solid or semisolid or liquid waste generated from a municipal, commercial or industrial wastewater treatment plant, water supply treatment plant or air pollution control facility (wet scrubbers), or any other such waste having similar characteristics and effect. 
 
slurry – a mixture of water and any finely divided insoluble material (fly ash, slaked lime, etc.) in suspension. 
 
soil cover – the clean earth fill, or combination of earth and other materials, capable of supporting vegetation that is used to cover the surface of a completed landfill, surface impoundment or other waste management facility. 

soil modification – a change to the physical or chemical characteristics of soils; any change to in situ soils that results in immediate effects that can expedite construction operations. These changes can be measured in terms of moisture reduction, improved California bearing ratio (CBR) and/or decrease in plasticity. CCPs are used to modify soils in applications such as surface mine land reclamation, soil stabilisation and base stabilisation. 
 
solidification – the conversion of liquids, slurries or sludges into a material that can be more easily handled or compacted for disposal or use; a process for converting a liquid waste to a solidified material. Solidification also refers to a process in which materials are added to the waste to produce a solid. In the solidification/stabilisation industry this process is usually monitored for completion by applying the ‘paint filter test’ and engineering tests such as unconfined compressive strength; fly ash is often used as a reagent or filler. 
 
sorbent – the term applied in some combustion systems, to the chemical compounds that are added to the gas side of the steam generator to reduce (sorb) emissions. (Example: Limestone is used in fluidised-bed steam generators to reduce sulfur dioxide emissions.) 
 
spoil – material overlying a coal seam that is removed during a surface coal mining operation. 
 
stabilisation – a process for treating a waste to minimise an undesirable attribute of that waste. In the stabilisation/solidification industry, the stabilisation process is typically monitored for completion by applying leachate testing. ‘Stabilisation’ of biological wastes may infer the elimination of pathogens (or their minimisation); fly ash is used as a reagent or filler. In the power generation industry, the terminology is typically applied to the treating of solids from wet scrubbing or other air pollution control processes. 
 
soil stabilisation – a permanent change to in-situ soils which improves their physical characteristics. Soil stabilisation allows the soil layer to be assigned a structural support value as an integral part of a pavement structure. CCPs are used as reagents in soil stabilisation. 
 
stabilised CCPs – CCPs that are blended with a cementitious binder to induce or enhance a pozzolanic reaction. (Example: wet FGD material mixed with fly ash and lime) (See fixated CCPs) 
 
stoker boiler – a type of coal-fired boiler in which the combustion of coal takes place on a grate, which may be stationary or moving. 
 
structural fill – an engineered fill with a projected beneficial end use that is typically constructed in layers of uniform thickness and compacted to a desired unit weight in a manner to control the compressibility, strength and hydraulic conductivity. 
 
subsidence – the downward displacement of the overburden (rock or soil or both) lying above an underground excavation or adjoining a surface excavation. Subsidence can also refer to the sinking of the earth’s crust or the lowering of the natural land surface in response to: earth movements; lowering of fluid pressure; removal of underlying supporting material by mining; or added load on the land surface. CCPs are used in a grout or flowable fill to reduce subsidence. 
 
sulfate attack – either a chemical or physical reaction, or both, between sulfates, usually in soil or ground water, and concrete and mortar; the chemical reaction is primarily with calcium aluminate hydrates in the cement–paste matrix, often causing a deterioration. 
 
sulfate resistance – ability of concrete or mortar to withstand sulfate attack. Fly ash concrete helps to reduce sulfate attack. (See alkali–silicate reaction) 
 
sulfur – one of the elements present in varying quantities in coal that may contribute to environmental degradation when coal is burned.  When coal is sampled, sulfur content is measured as a percent by weight of coal on an ‘as received’, ‘air-dried’ or ‘dry’ (moisture-free) basis. Sulfur occurs in coal in three forms: (1) iron sulfides (pyrite and marcasite), (2) secondary sulfates (e.g. gypsum and hydrous iron sulfates) and (3) organic sulfur (chemically bonded to the coal-forming organic matter). 
 
surface impoundment – a facility or part of a facility that is a natural topographic depression, human-made excavation, or diked area formed primarily of earthen materials (although it may be lined with human-made materials), which is designed to hold an accumulation of liquid wastes or materials containing free liquids and which is not an injection well; a type of waste management facility consisting of an excavated, a dammed or diked reservoir in which coal combustion and flue gas cleaning wastes are disposed of as a slurry or sludge. (See ash pond) 
 
surface mine – a coal mine in which soil and rock above or around the coal (overburden) is removed to expose the coal bed, which is then mined with surface excavation equipment such as draglines, power shovels, bulldozers, loaders or augers. Such mines are also referred to as open-cut mines.
 
surfactant – a material added to a liquid (usually water) to reduce its surface tension, enabling it to wet a solid surface effectively rather than running off the surface in droplets; a substance that affects markedly the interfacial or surface tension of solutions even when present in very low concentrations. Surfactants are added at some installations to the water in the fly ash load-out process from a silo, to wet the conditioned fly ash more effectively. 
 
supernatant – the liquid remaining above a layer of solids after the solids settle and collect at the bottom of a pond or vessel. (Example: the clear water at the outlet area/water discharge structure of an ash pond) 
 
suspended solids – solids that either float on the surface of or are in suspension in water, wastewater or other liquids and which are largely removed by laboratory filtering. The term also refers to the quantity of material removed from wastewater in a laboratory test as prescribed in Standard Methods for the Examination of Water and Wastewater and referred to as nonfilterable residue. 
 
suspended solids, total (TSS) – the sum of all insoluble particles suspended in a liquid. 
 
synthetic gypsum – a precipitated gypsum formed through the neutralisation of sulfuric acid in an industrial process. (Examples are phosphorus (phospho) gypsum from phosphoric acid production, titano gypsum from titanium oxide production, citro gypsum for citric acid production and flue gas desulfurisation (FGD) gypsum from flue gas cleaning of utility boilers.) In North America there are large quantities of synthetic gypsum such as phospho-gypsum that are being produced and stockpiled but not being used. The exception is FGD gypsum where large volumes are being generated and utilised in wallboard manufacturing, cement production and plasters. (See gypsum and FGD gypsum) 
 
thickener – a vessel or apparatus for concentrating the solid particles and reducing the proportion of water in a slurry. Thickeners are used in wet FGD systems. 
 
thickener underflow – the settled solids that are extracted from the bottom of a thickener as a slurry. The thickener underflow in a wet FGD process is conveyed either for dewatering to equipment such as vacuum filters or to a pond. 
 
thixotropic – the property of a material that enables it to stiffen (increase its viscosity) in a relatively short time on standing, but to change upon agitation or manipulation to a very soft consistency or to a fluid of lower viscosity, the process being completely reversible. Some CCPs are thixotropic. 
  
toxic – term describing a harmful effect by a substance as the result of physical contact, ingestion, or inhalation over a given exposure period. 
 
toxicity characteristic leaching procedure (TCLP) – a laboratory procedure that is designed to simulate leaching under a specific set of low-pH disposal conditions.  
 
trace element – an element present in extremely small quantities. Metals are the predominant, naturally occurring trace elements in coal; they are also in the coal ash. As well as the total concentration of individual trace elements in the coal or ash, the concentrations leached under particular conditions may also be evaluated.
 
treatment – any method, technique, or process designed to change the physical or chemical or biological character of a waste to neutralise the waste, render safer to transport or manage, or reduce its volume. 
 
triboelectric separation process – an electrostatic technology with patented processes to remove carbon from high loss on ignition (LOI) fly ash and to produce a concrete grade fly ash. 

Unbound -- coal combustion products that is not blended with cementititous binders, lime and other activators for applications, where the material is not hydraulically  or chemically  bound together.

unburned carbon (UBC) in fly ash – the unburned carbon in fly ash includes both carbon carried over as uncombusted, mainly inertinite components and chars or cokes resulting from the incomplete combustion of thermoplastic, largely vitrinite-derived phases. The latter include ‘isotropic coke’ and ‘anisotropic coke’, a measure of the actual amount of carbon found in coal fly ash or other coal combustion products determined by a mineral analysis of the material. UBC levels can affect the beneficial use of fly ash. (See loss on ignition) 

ungraded – CCPs that have not been processed to a specific particle size distribution meeting requirements and/or specifications, but which may be suitable for various applications without further processing.

waste – material that has no identifiable future use for which suitable disposal must be found. Wastes would include: inorganic solutions/solids/sludges, organic wastes, sewage sludge, animal wastes, waste contaminated soils, wastewater treatment sludge, complex mixtures, sludges from air pollution control facilities. CCPs are used in the stabilisation or solidification of wastes. 

wet ash disposal system – an overall coal ash transport and storage system that involves conveying the ash in a fluid (slurry) state by pipeline to a surface impoundment or holding pond for future excavation. 

wet ash removal systems – the overall system of mixing the ash and water, and transporting the ash via a slurry tank(s), pump(s) and pipeline(s) to a pond, surface impoundment or dewatering bin/tank (for bottom ash only). 

wet bottom furnace – a pulverised fuel-fired furnace in which the ash particles are deposited and retained on the floor and molten ash is removed by tapping either continuously or intermittently. 

wet fly ash disposal system – an overall fly ash transport and storage system that involves conveying the fly ash in a fluid (slurry) state by pipeline to a surface impoundment. 

wet scrubbers – equipment that is used to remove ash from the combustion flue gas of coal-fired power plants, where fuels are burned in suspension, by collecting it with a suitable liquid. The term also refers to equipment used to remove sulfur oxides from the combustion flue gas of fossil-fuelled power plants in a gas-liquid contactor using lime or limestone. The use of wet scrubbers for particulate removal results in the collected fly ash being in a slurry form that requires, as a general practice, wet disposal of the fly ash and very limited opportunity for beneficial use. (See FGD) 

wetland – those areas that are inundated or saturated by surface water or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs and similar areas. 

XRD – X-ray diffraction; the scattering of X-rays by crystal atoms, producing a diffraction pattern that yields information about the structure of the crystal. Used to identify the minerals and other phases (including glass) in coal and coal ash, and, with appropriate techniques, the relative proportions of each.