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Glossary of Terms - FGT

The Carmeuse Lime & Stone glossary of terms is a great resource for our customers and visitors. Here you can find definitions for many industry terms and acronyms regarding the multiple uses of lime. You'll find this section to be a useful supplement to the research papers and case studies available through our website. Locate a definition in the immediate display of general terms or choose an industry from the list to find terms specific to their application.

 

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

 

Advanced Sulfur Control Products (ASC)

Byproducts generated from advanced coal conversion technologies including Fluidized Bed Combustion (FBC) and gasification and byproducts from advanced environmental emissions cleanup technologies such as duct injection and lime injection multiphase burners (LIMB). The type of byproduct is technology dependent and could be a bed ash and high lime fly ash for an FBC technology.

Alkalinity

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

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 onto the surface of the fly ash particles in the ESP. An ammonia slip of 2 ppm yields 100 ppm adsorbed onto 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 as a result of the use of processes at the power plant for removing Nitrogen Oxide (NOx) from the combustion flue gases, and for flue gas conditioning in order to improve the performance of Electrostatic Precipitators (ESP) or to reduce the opacity of the emissions from the stack. Ammonia levels occurs 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 can exceed 800 ppm for gas conditioning applications and 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 can 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 reportedly been used in concrete without affecting the structural performance of the concrete.

Ash

The incombustible inorganic matter in fuels such as coal.

 

Bed Ash

The spent bed material that is produced by Fluidized 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. The bed ash is composed of calcium oxide (35+% by weight), calcium sulfate (30+%), coal ash (26+%), calcium carbonate (5+%) and carbon (4+%). 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 only store the bed ash or may store 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

Projects promoting public health and environmental protection, offering equivalent success relative to other alternatives, and preserving natural resources. Example: the use of fly ash in concrete and FGD gypsum in wallboard manufacturing.

Bottom Ash

Agglomerated ash particles formed in pulverized 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 texture. Bottom ash is used mainly as an aggregate and competes with natural aggregates.

Byproduct Utilization

The recycling or use of coal combustion wastes.

 

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) x 2H2O), gypsum; the primary byproduct of a forced oxidation wet flue gas desulfurization system where bowers are used to supply additional air to the process and where lime or limestone is used as the reagent. See FGD gypsum and gypsum.

Calcium Sulfite

(CaSO3, the primary byproduct of a natural oxidation wet flue gas desulfurization system where only the oxygen available in the flue gas is used and where lime or limestone is used as the reagent.

Carbon Dioxide

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

Cementitious Ash

Fly ash, which is pozzolanic and hardens irreversibly when mixed with water. Also referred to as self-cementing ash. Cementitious fly ash is produced from the combustion of sub-bituminous and some lignite coalas. See Class C fly ash.

Class C Fly Ash

Fly ash, which meets criteria, defined in ASTM C618, including the requirement that the ash be produced solely from the combustion of coal. Class C fly ash is produced from the combustion of sub-bituminous and some lignite coals. It is "self reactive" or "cementitious" in the presence of water, in addition to being pozzolanic. It usually has a calcium oxide content that is greater than 10 percent. See cementitious ash.

Class F Fly Ash

Fly ash, which meets criteria, defined in ASTM C618, including the requirement that the ash be produced solely from the combustion of coal. Class F fly ash is produced from the combustion of bituminous, anthracite and some lignite coals. It usually has a low calcium oxide content.

Clean Coal Combustion

The burning of coal, coal culm, or coal fines in a furnace designed to operate to minimize emissions (that is a fluidized bed or aerated fluidized bed, etc.) or coal burned in the presence of alkaline materials, which combine to reduce emissions.

Clean Coal Technology

A government and industry co-funded effort to demonstrate a new generation of innovative coal utilization processes in "showcase" projects conducted across the country.

Clean Coal Technology Combustion Byproducts

Byproducts generated from clean coal conversion technologies including fluidized bed combustion (FBC), coal gasification, and byproducts from advanced environmental emissions cleanup technologies such as duct injection and lime injection multi-phase burners (LIMB).

Coal Ash

Collective term referring to any materials or residues produced directly from the combustion of coal and especially from coal-fired power plants. It is much like volcanic ash. It consists of limestone, iron, aluminum, silica sand and clay. In addition it contains trace quantities (in the parts per million range) of the oxidized forms of other naturally occurring elements. These same elements exist in soil, rock and coal. The coal can be bituminous, sub-bituminous, lignite or a mixture of these coals. The residues of mixtures of small quantities of other fuels such as petroleum coke, fuel oil, etc. with coal are also referred to as coal ash. Current usage of the coal ash collective term is synonymous with the term coal combustion ash and coal combustion residue (CCR). Also coal ash is a component of the term coal combustion byproduct (CCBs) covering only the materials or residues associated with the combustion of coal and not the residues from flue gas cleaning. See coal combustion byproducts.

Coal Combustion Byproducts (CCBs)

Collective term referring to fly ash, bottom ash, boiler slag, fluidized bed combustion ash or flue gas desulfurization (FGD) material resulting from the combustion of coal and the clenaing of the stack gases. 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. Coal combustion products (CCPs) have replaced the term coal combustion byproducts 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. However, many government agencies (Federal and State) and other organizations continue to use the term CCBs. In addition federal regulations also use the term coal combustion wastes (CCWs) and fossil fuel combustion wastes (FFCWs) in the same context as the term CCBs. As a result of the interchangeable use of these terms (CCBs, CCPs, CCWs and FFCWs) there is an industry movement to provide clarity based on use of the products. This clarification based on current usage by the Department of Energy National Energy Technology Laboratory is that the term "Products" applies when the material is used and "Wastes" applies when the material is discarded.

Coal Fly Ash

A product of 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 nearly all particles are spherical in shape. Coal fly ash is a pozzolan. See fly ash.

Coal Refuse

Waste products of coal mining, cleaning, and coal preparation operation (for example culm, gob, etc.) containing coal, matrix material, clay and other organic and inorganic 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 desulfurization.

Conditioned Ash

Ash that has been moistened with water during the load out process at the temporary storage silo at the power plant to allow for its handling, transport, and placement without causing fugitive dusting. The water that is added can vary from 5 to 30 percent by weight of the dry ash which can be fly ash from an ESP, fluidized bed material which could be a combination of fly ash and bed ash, and baghouse 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 handling equipment. The conditioned ash is usually designated for placing in a landfill, although it can be used in beneficial applications. See pugmill.

Continuous Emission Monitoring (CEM)

The measurement on a continuous basis of pollutants emitted into the atmosphere in the exhaust of gases for combustion processes or as the byproduct of industrial processes.

Culm

Anthracite tailings, especially prevalent in eastern Pennsylvania, that are a source of energy which could be used for example in fluidized bed boilers.

Cyclone Boiler

A type of coal-fired boiler. The coarsely pulverized coal undergoes slaging combustion in a cylindrical (cyclone) burner. Some wet-bottom boilers are not cyclone-fired. The primary byproduct 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.

 

Deep Mine Injection

Disposal 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.

Dewatering

A physical process which 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 slurry or damp solid. Some major types of equipment and facilities are: Rotary Drum Vacuum Fitlers, Centrifuges, Horizontal Belt Filters, Lagoons, Ponds, etc.

Dry Bottom Furnace

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

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 byproduct. Spray dryer systems are the most common design. In a spray dryer, slaked lime slurry is sprayed into the flue gas and the resulting byproduct, 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 byproduct of a dry FGD system is referred to by various names that include dry FGD ash, dry FGD material, dry scrubber material.

Dry FGD Ash

The byproduct from dry FGD systems and consists primarily of calcium sulfite, fly ash, portlandite (Ca(OH)2), and/or calcite. Lime based sorbent systems dry FGD material main constituents are calcium sulfite and dry fly ash, along with minor quantities of calcium sulfate. Sodium based sorbent systems main constituents are sodium sulfite and dry fly ash along with minor quantities of sodium sulfate. Dry FGD material is beng used in construction, engineering and agricultural applications; however, most of the material is stored in landfills.

Dry FGD Material

The byproduct from dry FGD systems and consists primarily of calcium sulfite, fly ash, portlandite (Ca(OH)2), and/or calcite. Lime based sorbent systems dry FGD material main constituents are calcium sulfite and dry fly ash, along with minor quantities of calcium sulfate. Sodium based sorbent systems main constituents are sodium sulfite and dry fly ash along with minor quantities of sodium sulfate. Dry FGD material is beng used in construction, engineering and agricultural applications; however, most of the material is stored in landfills.

Dry Fly Ash

Fly ash that has been collected by the particulate removal equipment such as Electrostatic Precipitators (ESP), Baghouses, Mechanical Collectors or Fabric Filters at coal-fired power plants. The collected fly ash is in a dry state, less than 3 percent moisture, and it is transported via an ash removal system to either a silo for temporary storage or to a wetting water eductor for sluicing to an ash pond. The fly ash in the silo is loaded out through specially designed equipment either in its present dry state into pneumatic or bulk carriers (truck or rail) for beneficial reuse or moistened with water for disposal and/or beneficial reuse. Also high lime fly ashes, especially Class C fly ashes, that are not beneficially used are sometimes transported by pneumatic bulk trucks to a facility where the ash is mixed with water to form a slurry and then discharged into a pond -- the pond is then dewatered and the hardened material is excavated and placed in a landfill or beneficially used. See fly ash.

Dry Sodium Injection

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 byproduct. Spray dryer systems are the most common design. In a spray dryer, slaked lime slurry is sprayed into the flue gas and the resulting byproduct, 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 byproduct of a dry FGD system is referred to by various names that include dry FGD ash, dry FGD material, dry scrubber material.

 

Electrostatic Precipitator (ESP)

A facility constructed at some coal-fired power plants to remove particulate matter (fly ash) from the flue gas by producing an electric charge on the particles to be collected and then propelling the charged particles by electrostatic forces to collecting curtains.

Ettringite

A high-calcium sulfoaluminate mineral (3CaO x Al2O3 x 3CaSO4 x 30-32H2O) that is expansive because of its crystal structure; a mineral composed of hydrous basic calcium and aluminum sulfate that expands when wet. The Electric Power Research Institute (EPRI) in its "Environmental Focus report for Flue Gas Desulfurization (FGD) Byproducts" has indicated that 'FGD byproducts containing a hgh percentage of this material are not appropriate for most construction applications'. Ettringite can be formed in the FGD byproduct by the combination of aluminum from the coal ash, lime and sulfates from the scrubber process and water.

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 aluminum from the coal ash, lime and sulfates from the scrubber process and 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.

Ex situ

A Latin term meaning off-location. Example: ex situ oxidation (Wet FGD Scrubbers).

Ex situ Oxidation (wet FGD)

Forced oxidation that occurs outside of the scrubber and used to produce FGD gypsum.

 

FGD Byproducts

The term for the byproducts from wet and dry FGD systems. See wet and dry FGD material.

FGD Gypsum

A precipitated gypsum formed through the neutralization of sulfuric acid in flue gas desulfurization processes at coal-fired power plants. This gypsum can vary in purity, which is defined as the percenrage of CaSO4 x 2H2O and generally is over 94% for use in wallboard manufcturing. The less pure gypsum can be stockpiled (gypsum stacking), placed in ponds or captive landfills or utilized in agriculture or construction. The nearly pure or pure FGD gypsum is utilized beneficially. The pure FGD gypsum is manufactured 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 utilized. See gypsum and synthetic gypsum.

FGD Material

A byproduct of the processes employed for the removal of gaseous sulfur dioxide from boiler exhaust gas at coal-fired electricity-generating plants. In the United States, it is primarily a byproduct of the Wet FGD process or the Dry FGD process. (See Dry FGD and Wet FGD). The physical nature of these byproducts varies from a wet thixotropic sludge to a dry powdered material depending on the process. The wet thixotropic slduge 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 stabilized 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 calcium sulfate which is 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. (See byproducts, FGD gypsum, etc.)

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 byproducts from wet and dry FGD systems.

FGD Sludge

Another name for scrubber sludge wet FGD material or filter cake. See wet FGD material.

Filtercake

The material produced by filtering equipment such as vacuum filters for dewatering wet FGD material. See wet FGD material.

Fixated CCPs

CCPs that are blended with a cementitious binder to induce or enhance a pozzolanic reaction.

Fixated FGD Material

A designed mixture of dewatered FGD sludge (also known as scrubber sludge or filtercake) which is primarily calcium sulfite with either a low lime (Class F) fly ash and lime, with a high lime fly ash (Class C), or with a low lime fly ash and a cementitious material such as cement kiln dust, lime kiln dust or FBC ash. The designed mixture is produced in a mixing facility that is sometimes referred to as a Sludge Treatment Plant (STP), transported by a 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 onto trucks or other earthmoving equipment for 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 FGD Material Pad Area

The engineered area that receives the fixated FGD material from the processing facility (Sludge Treatment Plant) where the filtercake 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 onto trucks or other transportation equipment for beneficial use or disposal.

Fixated Scrubber Sludge

A designed mixture of dewatered FGD sludge (also known as scrubber sludge or filtercake) which is primarily calcium sulfite with either a low lime (Class F) fly ash and lime, with a high lime fly ash (Class C), or with a low lime fly ash and a cementitious material such as cement kiln dust, lime kiln dust or FBC ash. The designed mixture is produced in a mixing facility that is sometimes referred to as a Sludge Treatment Plant (STP), transported by a 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 onto trucks or other earthmoving equipment for 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.

Fixation

A physical immobilizing of particulates achieved by the development of chemical cementation bonds. This term is used by the US EPA to mean either solidification or stabilization.

Flue Gas Conditioning

The process of adding chemicals such as sulfur trioxide or ammonia to the flue gas in order to improve the performance of the electrostatic precipitator or reduce the opacity of the emissions from the stack.

Flue Gas Desulfurization (FGD)

The processes employed for the removal of gaseous sulfur dioxide from boiler exhaust gas at coal-fired electricity generating plants. These processes transform the gaseous sulfur dioxide (SO2) to primarily solid sulfur compounds that are collected for safe disposal or beneficial use. Although similar in concept these processes are characterized as wet or dry and they differ as to the sorbents used and the byproducts produced. The two most prevalent process types currently used in the United States are wet FGD using lime or limestone as the reagent and dry FGD using calcium or sodium based sorbents.

Fluidized Bed Combustion (FBC) Ash

Fly ash, bottom ash or boiler slag generated by the combustion of coal in a bed of fluidized sorbents such as limestone. FBC fly ash is collected in the flue of an FBC boiler using a baghouse filter or electrostatic precipitator. FBC bottom 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.

Fluidized-Bed Combustion (FBC) Boiler

A type of coal boiler which accomplishes coal combustion by mixing the coal with a sorbent such as limestone or other bed material. The fuel and bed material mixture is fluidized during the combustion process to allow complete combustion and removal of sulfur gases. Atmospheric FBC (AFBC) systems may be bubbling (BFBC) or circulating (CFBC). Pressurized FBC (PFBC) is an emerging coal combustion technology.

Fly Ash

Coal ash that exits a combustion chamber in the flue gas and is captured by equipment that includes Electrostatic Precipitators, bag filters, 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-Bituminous Coal

Fly ash resulting from the combustion of a bituminous coal in a boiler for the production of electricity is generally low in lime (less than 2 percent); its chemistry would make it fall under but may not conform with the ASTM C618 classification of a Class F fly ash; this fly ash does have pozzolanic characteristics.

Fly Ash-High Lime

Fly ash 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 C618 classification of a Class C fly ash; it may contain in excess of 20% CaO.

Fly Ash-Lime Content

The calcium content of fly ash expressed as calcium oxide (CaO). Class F fly ash generally has less than 10 percent CaO, whereas Class C fly ash may contain in excess of 20 percent CaO. Free lime is typically not available in Class F fly ash and typically only 1 to 3 percent or less free lime is available in Class C fly ash.

Fly Ash-Low Lime

Fly ash resulting from the combustion of anthracite or bituminous coal is relatively low in lime (less than 2 percent); its chemistry would make it fall under, but may not conform with, the ASTM C618 classification of a Class F fly ash. This fly ash does have pozzolanic characteristics.

Forced Oxidation

A process employed to supply additional air in wet FGD systems, resulting in a predominantly calcium sulfate dihydrate (gypsum) byproduct with improved storage characteristics as well as greater commercial potential.

Fossil Fuel Combustion Wastes (FFCWs)

A collective term utilized by the US EPA for materials or residues produced from the combustion of coal or the cleaning of stack gases. See coal combustion wastes (CCWs), coal combustion byproducts (CCBs).

Friable

Easily crumbled or pulverized. 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.

 

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 Btu gases. Byproducts from gasification systems vary widely. Gasification is a clean coal technology.

Gypsum

The common name for the mineral consisting primarily of fully hydrated calcium sulfate, CaSO4 x 2H2O or calcium sulfate dihydrate. Gypsum occurs naturally in many areas, and is produced by some wet Flue Gas Desulfurization (FGD) processes. See FGD gypsum and Synthetic gypsum.

Gypsum Stacking

The method used in the phosphate fertilizer industry and applied to the power industry for stacking the wet FGD byproduct (material) that is predominantly calcium sulfate (gypsum). It involves placement of the FGD byproduct slurry in an impoundment and stacking of the reclaimed settled solid in two operations. The primary operation accepts the FGD byproduct 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 are decanted from the pond 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 minimize 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.

 

In situ Oxidation (wet FGD)

A process in which both SO2 absorption and oxidation are carried out within the scrubber.

 

Landfill

A disposal facility where waste is placed in or on land; a facility where "dry" (actually moistened) coal combustion or flue gas cleaning byproducts (CCBs) are placed for disposal in or on land. CCBs 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 CCBs remain in the landfill after closure. Also these CCBs are dry (moistened) and have the consistency of soil. As a result dikes are not required to provide stability. Most large landfills are divided into sections or cells and the CCBs are placed in layers which are referred to as lifts that can vary in thickness. Typically captive CCBs landfills are designed and permitted to receive only CCBs and are classified as mono-fills.

Leachate

The fluid stream which issues from a pile (stockpile of ash, coal, etc.) or cell of solid materials (an ash landfill) and which contains water, dissolved solid and decomposition products of the solids. Leachate may enter the groundwater and contaminate drinking water supplies.

Leaching

The operation, natural or designed, of producing leachate.

Lime Spray Dryer Ash

The residue from a spray dryer FGD system. The resulting byproduct is dried by the heat of the flue gas and it is collected in a particulate control device with the fly ash. See dry FGD material.

Lime Spray Dryer Residue

The residue from a spray dryer FGD system. The resulting byproduct is dried by the heat of the flue gas and it is collected in a particulate control device with the fly ash. See dry FGD material.

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 minimized.

 

Manufactured Aggregates from CCPs

A commercial product made by the intentional size-enlargement and hardening of fine-particulate coal combustion products (CCP) 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.

 

New Source Performance Standards (NSPS)

Uniform national US EPA air emission and water effluent standards which limit the amount of pollution allowed from new sources or from modified existing sources.

Non Self-Cementing Fly Ash (NSPS)

A coal combustion product resulting from the combustion of anthracite or bituminous coal and some lignite coal in a boiler for the production of electricity or steam. This fly ash does have pozzolanic characteristics; it is usually relatively low in lime (less than 2 percent); its chemistry would make it fall under the ASTM C618 classification of a Class F fly ash; generally it will not harden or gain strength over time following contact with water.

 

Particle Size

This term refers in this context to the composition of the solid particles of the 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 liqulid matter of organic or inorganic composition that is suspended as the result of stack or fugitive emissions. The matter may be 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 pound per one billion pounds 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 pound per one million pounds of water or one milligram per liter of water.

Petroleum Coke (pet-coke)

The solid carbonaceous residues remaining in oil refining stills after the distillation process and sometimes used in combination with coal at some coal-fired power plants. The fly ash from pet-coke/sub-bituminous coal blends where the percentage of pet-coke in the blend was less than 3 percent by weight have been marketed for use in fly ash concrete. The carbon in the fly ash from the blend consisted primarily of unburned pet-coke.

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 water, while a pH greater than 7.0 indicates an alkaline water.

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 has been excavated from an ash pond or ash reservoir for beneficial use or for placement in a landfill type disposal facility. 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 percent 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. Also the ponded ash may contain other materials which are transported to the ash pond as a part of the wastewater sedimentation process for the particular coal-fired power plants. The other materials could include coal fines from the coal pile runoff system, solids from the cooling tower blowdown and wastewater collection systems.

Pozzolanic Activity

The conventional term given to 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.

Pozzolans

Siliceous or siliceous and aluminous materials that in themselves possess little or no cementitious value but 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.

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 orther to facilitate the handling of the CCBs without creating fugitive dusting and/or providing for stabilizing of FGD material in particular.

 

Reagent

A substance used, because of its chemical activity, typically to reduce emissions or improve opacity from coal-fired power plants. Examples of reagents include lime and limestone used for wet scrubbing of the combustion flue gas to remove sulfur dioxide. Also it is a term applied to the substances used in solidification or stabilization of wastes. These materials may include liquids or solids such as sodium silicate, cement, fly ash, etc.

 

Scrubber

Any of several forms of chemical/physical devices that remove sulfur compounds formed during coal combustion and especially from coal-fired power plants. See wet FGD and wet scrubbers.

Scrubber Cake

Another name for scrubber sludge. See flue gas desulfurization.

Scrubber Material

Another name for scrubber sludge. See flue gas desulfurization.

Scrubber Sludge

Another name for FGD material. See flue gas desulfurization.

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 described here does not include fly ash from fluidized 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 desulfurization material. See cementitious ash.

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.

Solidification

A process for converting a liquid waste to a solidified product. Also a process in which materials are added to the waste to produce a solid. In the Solidification/Stabilization Industry, this process is usually monitored for completion by applying the "paint filter test" and engineering tests such as unconfined compressive strengths; fly ash is 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. For example, limestone is used in fluidized-bed steam generators to reduce sulfur dioxide emissions.

Spray Dryer

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 byproduct. Spray dryer systems are the most common design. In a spray dryer, slaked lime slurry is sprayed into the flue gas and the resulting byproduct, 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 byproduct of a dry FGD system is referred to by various names that include dry FGD ash, dry FGD material, dry scrubber material.

Stabilization

A process for treating a waste to mnimize an undesirable attriburte of that waste. In the Stabilization/Solidification Industry, typically the stabilization process is monitored for completion by applying leachate testing; "Stabilization" of biological wastes may infer the elimination of pathogens (or their minimization); fly ash is used as a reagent or filler. In the power generation industry, typically the terminology is applied to the treating of solids from wet scrubbing or other air pollution control processes.

Stabilized FGD Material

A designed mixture of dewatered FGD sludge (also known as scrubber sludge or filtercake) which is primarily calcium sulfite with either a low lime (Class F) fly ash and lime, with a high lime fly ash (Class C), or with a low lime fly ash and a cementitious material such as cement kiln dust, lime kiln dust or FBC ash. The designed mixture is produced in a mixing facility that is sometimes referred to as a Sludge Treatment Plant (STP), transported by a 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 onto trucks or other earthmoving equipment for 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.

Sulfur

One of the elements present in varying quantities in coal that contributes to environmental degradation when coal is burned. EIA classifies coal, in terms of pounds of sulfur per million Btu as low (less than or equal to 0.60 pounds of sulfur), medium (between 0.61 ad 1.67 pounds of sulfur), and high (greater than or equal to 1.68 pounds of sulfur). When coal is sampled, sulfur content is measured as a percent by weight of coal on an "as received" or "dry" (moisture-free) basis. Sulfur occurs in coal in three forms: (1) iron sulfides (pyrite and marcasite), (2) secondary sulfates (gypsum and hydrous ferrous sulfate), and (3) organic sulfur chemically bonded to the coal forming plant material.

Supernatant

The liquid remaining above a layer of settleable solids after the solids collect at the bottom of a pond or vessel. Example: the clear water at the outlet area/water discharge structure of an ash pond.

Synthetic Gypsum

A precipitated gypsum formed through the neutralization 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, flue gas desulfurization (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 and not being used. The exception is FGD gypsum where large volumes are being generated and utilized in wallboard manufacturing, cement production and plasters. See gypsum and FGD gypsum.

 

Thickener

A vessel or apparatus for 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 and measured in percentage solids by weight. The thickener underflow in a wet FGD process is either conveyed for dewatering to equipment such as vacuum filters or to a pond.

Thixotropic

The property of a material that enables it to stiffen in a relatively short time on standing, but upon agitation or manipulation to change to a very soft consistency or to a fluid of high viscosity, the process being completely reversible. Some CCPs are thixotropic.

Trace Element

An element present in extremely small quantities. Metals are the predominant, naturally occurring trace elements in coal; they are also in its ash. Most of the trace elements in FGD sludge/cake originate from the small amounts of coal ash that elude the particulate collection device.

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.

 

Wet FGD

An FGD system which 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 oxidizes the sulfur dioxide in the flue gas and converts it to a byproduct that is referred to as scrubber sludge, scrubber material, wet FGD material.

Wet FGD Material

The byproduct 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 byproducts will also contain magnesium sulfite and/or sulfate and possibly traces of barium sulfite, barium sulfate or boron in addition to some trace metals.

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. Also equipment used to remove sulfur oxides from the combustion flue gas of fossil-fueled power plants in a gas-liquid contractor 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.

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