Coal Ash

Coal Ash

Coal ash is the mineral matter present in the coal. It is a waste which is left after coal is combusted (burned). It is the particulate material which remains after coal is burned. It includes fly ash (fine powdery particles which are carried up the smoke stack and captured by pollution control devices) as well as coarser materials which fall to the bottom of the furnace. It has different physical and chemical properties depending on the geochemical properties of the coal being used and how that coal is burned.

Coal ash is also referred to as coal combustion residuals. It has very little organic fraction. Chemical constituents of coal ash may include nitrogen (N2), sulphur (S), unburned carbon (C), heavy metals, radioactive elements, and polycyclic aromatic hydrocarbons (PAHs). Coal ash also contains coarse particles and fine particles which can be inhaled and may contribute to public health and environmental problems. Coal ash contains many toxic contaminants. When coal ash spills, leaks or leaches into nearby ground water or waterways, the toxins contained within pose serious health risks to nearby communities.

Depending on where the coal was mined, coal ash typically contains heavy metals including arsenic, lead, mercury, cadmium, chromium and selenium, as well as aluminum, antimony, barium, beryllium, boron, chlorine, cobalt, manganese, molybdenum, nickel, thallium, vanadium, and zinc. If eaten, drunk or inhaled, these toxicants can cause cancer and nervous system impacts such as cognitive deficits, developmental delays and behavioral problems. They can also cause heart damage, lung disease, respiratory distress, kidney disease, reproductive problems, gastrointestinal illness, birth defects, and impaired bone growth in children.

A large amount of coal ash is disposed in dry landfills, frequently at the power plant where the coal was burned. Coal can also be mixed with water and disposed in so-called ‘ash ponds’, some are more like small lakes, behind earthen walls. These wet ‘surface impoundments’ account for substantial quantity of coal ash disposal. A fair amount coal ash is ‘recycled’ in agricultural and engineering applications rather than being disposed, and some amount is dumped in abandoned mines.

Coal ash is a complex mixture that varies with the type of coal, the pulverization and combustion processes used, and precipitation techniques. The variable nature of coal ash makes it difficult to regulate effectively because some coal ash may contain significantly more contaminants than average. Further, the additives used, including oil additives for flame stabilization and corrosion control additives affects the coal ash. Also the minerals present in the coal dictates the elemental composition of the ash. But the mineralogy and crystallinity of the ash is dictated by the furnace design and operation.

Coal ash is of several different kinds. Broadly coal ash consists of (i) fly ash, (ii) bottom ash, (iii) boiler slag, and (iv) flue gas desulphurization waste  (Fig 1). The ash collected from pulverized-coal-fired furnaces is fly ash and bottom ash. For such furnaces, fly ash constitutes a major component (80 % to 90 %) and the bottom ash component is in the range of 10 % to 20 %. Boiler slag is formed when a wet-bottom furnace is used. Flue gas desulfurization (FGD) waste is the byproduct of air pollution control systems used to reduce the sulphur dioxide emissions from coal fired furnaces.

Fig 1 Types of coal ash

Fly ash

Fly ash consists of inorganic matter present in the coal which has been fused during coal combustion. This material is solidified while suspended in the exhaust gases and is collected from the exhaust gases by electrostatic precipitators (ESP. In ESP the flue gas is passed between electrically charged plates where the fly ash particles are then attracted to the plates. Bag house can also be used to collect ash with bags which filter the fly ash out of the flue gas stream. The fly ash particles are periodically knocked off the plates or bags and fall into the hoppers located at the bottom of the ESP or bag house. The fly ash is then pneumatically transported to storage silos. The storage silos are equipped with dry unloaders for loading dry bulk semi tankers or rail cars, and wet unloaders for conditioned ash or disposal applications.

Fly ash consists of fine, powdery particles which are predominantly spherical in shape, either solid or hollow, and mostly glassy (amorphous) in nature. The carbonaceous material in FA is composed of angular particles. The specific gravity of fly ash usually ranges from 2.1 to 3.0, while its specific surface area may range from 170 sq m/kg to 1000 sq m/kg. The colour of fly ash can vary from tan to gray to black, depending on the amount of unburned carbon in the ash. The lower the carbon (C) content, the lighter the fly ash colour. Lignite or sub-bituminous fly ashes are usually light tan to buff in colour, indicating relatively low amounts of C as well as the presence of some lime (CaO) or calcium (Ca). Bituminous fly ashes are usually shade of gray, with the lighter shades of gray generally indicating a higher quality of ash.

The chemical properties of fly ash are determined by the type of the coal burned and the techniques used for handling and storage. There are basically four types of coal, each of which varies in terms of its heating value, its chemical composition, ash content, and geological origin. They are anthracite, bituminous, sub bituminous, and lignite. Fly ash is also sometimes classified according to the type of coal from which the ash was derived. The principal components of bituminous coal fly ash are silica (SiO2), alumina (AL2O3), iron (Fe) oxide, and Ca, with varying amounts of C. Lignite and sub bituminous coal fly ashes are characterized by higher concentrations of Ca and magnesium oxide (MgO) and reduced percentages of SiO2 and Fe oxide, as well as lower C content, compared with bituminous coal fly ash.

Fly ash is a fine powder. Its particles are very fine, mostly spherical and vary in diameter. Under a microscope they look like tiny solidified bubbles or spheres of various sizes. The average particle size is about 10 micro meters (microns) but can vary from less than 1 micron to over 150 micron. The properties of fly ash vary with the mineral make-up of coal used, grinding equipment, the furnace and the combustion process itself. ASTM C618 classifies fly ash into two categories namely (i) Class F fly ash, and (ii) Class C fly ash. Combustion of bituminous or anthracite coal normally produces Class F (low calcium) fly ash and combustion of lignite or sub-bituminous coal normally produces Class C (high calcium) fly ash. Tab 1 shows the typical range of the chemical composition for fly ash produced from different types of coals.

Tab 1 Typical range of fly ash chemical compositions produced from different types of coals
CompoundsUnitBituminous coalSub-bituminous coalLignite

ASTM does not differentiate fly ash by CaO content, but class C fly ash normally contains more than 15 % of CaO, and class F fly ash usually contains less than 5 % of CaO. In addition to class F and class C fly ash, ASTM C618 also defines a third class of mineral admixture named Class N. Class N mineral admixtures are raw or natural pozzolans such as diatomaceous earths, opaline cherts and shales, volcanic ashes or pumicites, calcined or uncalcined, and various other materials which need calcination to induce pozzolanic or cementitious properties, such as some shales and clays. Tab 2 gives the typical composition of Class F fly ash, Class C fly ash and Portland cement.

Tab 2 Typical composition of fly ash and Portland cement
CompoundsUnitClass F fly ashClass C fly ashPortland cement
  ASTM C618TypicalASTM C618TypicalASTM C618Typical
SiO2+Al2O3+Fe2O3%70 min87.950 min64.3
MgO%6 max2.24
SO3%5 max0.45 max2.53 max
LOI%6 max3.26 max0.43 max0.55
Moisture%3 max0.13 max0.1
Insoluble residue%0.75 max
Available alkalis as equivalent  Na2O%1.5 max0.81.5 max1.40.2

The loss on ignition (LOI) is a very important property for determining the quality of fly ash for use in concrete. The LOI value primarily represents residual carbonaceous material which can negatively impact fly ash use in air entrained concrete. A low and consistent LOI value is desirable in minimizing the quantity of chemical admixtures used and producing consistent durable concrete.

Activated carbon (C) powder is sometimes now being used in power plant air quality control systems to remove mercury (Hg) from combustion gases. Ordinary activated C which is commingled with fly ash can present two issues when used as a cementitious material in concrete. First, conventional activated C has a high affinity for air entraining admixtures, making predictable air content in concrete very difficult. This phenomenon may also be true for other chemical admixtures as well. Secondly, C particles can present aesthetic issues for architectural concrete in terms of a darker colour or black surface speckles.

Another important fly ash parameter with respect to affecting concrete quality is fineness, which is a measure of the percent of material retained on the 45 micron size sieve. The condition and the type of coal crusher can affect the particle size of the coal itself. A coarser ground coal can leave a higher percentage of unburned residues. Also, a coarser resulting fly ash gradation means there is less particle surface area of contact, which leads to a less reactive ash.

Uniformity of fly ash is important in most applications. The characteristics of the fly ash can change when a new coal source is introduced. Fly ash generated from each type of coal is different and it is important to determine its chemical and physical properties before it is used in commercial applications.

Fly ash particles are primarily in the silt size range with the low end falling in the clay category and top end in the sand range based on the Unified Soil Classifications System. Fly ash is sometimes classified as a sandy silt or silty sand, having a group symbol of ML or SM for geotechnical applications.

The specific gravity of fly ash is generally lower than that of Portland cement (SG = 3.15) and is typically range from a specific gravity of 2.05 to 2.68. Tab 3 gives some typical geotechnical engineering properties of fly ash. These properties are useful when fly ash is designed for use in applications such as backfilling for retaining walls or constructing embankments.


Tab 3 Typical geotechnical properties of fly ash
Testing parameterUnitValue
Internal friction angleDegree26-42
Initial stress-strain modules (triaxial test)Mpa30
Stress-strain module (plate load tests)Mpa100
Modules of subplates reactions (300 mm diameter plates (Ks)Kpa/mm130
California Bearing Ratio, Unsoaked (low lime fly ash)10.8-15.4
California Bearing Ratio, Soaked (low lime fly ash)6.8-13.5
Permeabilitycm/sec0.0001- 0.000001
Maximum dry densitykg/cum960-1760
* C=0 is recommended for Class F fly ash. Class C fly ash self-hardens when hydrated and gain strength over time

Class C fly ash has been widely used for soil stabilization. It can be incorporated into the soil by disking or mixing. Fly ash can increase the subgrade support capacity for pavements and increase the shear strength of soils in embankment sections when proportioned, disked and compacted properly.

One of the ways that fly ash stabilizes soil is by acting as a drying agent. Soil with high moisture content can be difficult to compact. Adding fly ash to the soil and mixing will quickly reduce the moisture content of the soil to levels suitable for compaction. Fly ash has been widely used to reduce the shrink-swell potential of clay soils. Due to the cementitious products formed by the hydration of fly ash bond with the clay particles, the swell potential is substantially reduced to levels comparable to lime treatment.

When fly ash is used to stabilize subgrades for pavements, or to stabilize backfill to reduce lateral earth pressure or to stabilize embankments to improve slope stability, better control of moisture content and compaction is needed.

Class C and F fly ashes are pozzolanic and Class C fly ash is also cementitious. It reacts with calcium hydroxide [Ca(OH)2] produced by the hydration of cement in the presence of water to form additional cementitious compounds. This property of fly ash gives it wide acceptance in the concrete industry.

Class C fly ash has been successfully used in reconstructing and/or upgrading existing pavements. In this process, commonly known as cold-in-place recycling (CIR) or full depth reclamation (FDR), existing asphalt pavement is pulverized with its base, and the pulverized mixture is stabilized by the addition of fly ash and water. The cementitious and pozzolanic properties of fly ash enhance the stability of the section. Fly ash recycled pavement sections have structural capacities substantially higher than crushed stone aggregate base. A new asphaltic concrete or other wearing surface is then installed above the stabilized section.

Fly ash is a by-product pozzolan. The active silica (SiO2) and alumina (Al2O3) in the ash combined with the lime (CaO) and is used to produce pozzolanic cement. Extensive studies have been carried out in utilizing fly ash in concrete, masonry products, precast concrete, controlled low strength materials, asphalt and other applications.

Bottom ash

Bottom ash is formed when ash particles soften or melt and adhere to the furnace walls and boiler tubes. These larger particles agglomerate and fall to hoppers located at the base of the furnace where they are collected and normally ground to a predominantly sand size gradation. Some bottom ash is transported to storage dry, but most is transported wet from the furnace bottom to dewatering bins where water is removed prior to unloading and its transport to construction sites or storage stockpiles. Bottom ash is coarser and more granular than fly ash and can have the consistency of sand or gravel.

Bottom ash particles are much coarser than fly ash. The grain size typically ranges from fine sand to gravel in size. The chemical composition of bottom ash is similar to that of fly ash but typically contains greater quantities of C. Bottom ash tends to be relatively more inert because the particles are larger and more fused than fly ash. Since these particles are highly fused, they tend to show less pozzolanic activity and are less suited as a binder constituent in cement or concrete products. However, bottom ash can be used as a concrete aggregate or for several other civil engineering applications where sand, gravel and crushed stone are used. Tab 4 shows the typical chemical composition of bottom ash obtained by burning bituminous coal and subbituminous coal.

Tab 4 Typical chemical composition of bottom ash
CompoundUnitBottom ash from bituminous coalBottom ash from sub- bituminous coal

Tab 5 shows typical gradation of bottom ash from two power plants. The gradation of bottom ash can vary widely based on the coal pulverization and burning processes in the power plant, the variety of coal burned, and the bottom ash handling equipment.

Tab 5 Typical gradation of bottom ash
% passing sizePercentage
20 mm100
10 mm95 – 97
5 mm85 – 90
2.5 mm75 – 80
1.2 mm65 – 70
0.6 mm55 – 60
0.3 mm40 – 45
 0.15 mm25 – 30
0.075 mm15 – 20

Typical geotechnical properties of bottom ash produced from the combustion of bituminous coal are given in Tab 6.

Tab 6 Typical Geotechnical properties of bottom ash
PropertyUnitBituminous coalSub-bituminous coal
Specific gravity2.332.28
Minimum dry densitygrams/cc1.040.77
Maximum dry densitygrams/cc1.351.07
Optimum moisture content,      Air dry%28.732.3
Los Angeles Abrasion%49.250.4
Linear shrinkage1.090.59
California Bearing Ratio @ 95 %%2622

Boiler slag  

Boiler slag is formed when a wet-bottom furnace is used. The non-combustible minerals are kept in a molten state and tapped off as a liquid. The ash hopper furnace contains quenching water. When the molten slag contacts quenching water, it fractures, crystallizes, and forms pellets, resulting in the coarse, black, angular, and glassy boiler slag. The boiler slag constitutes the major component of cyclone boiler by-products (70 % to 85 %). The remaining combustion products exit along with the flue gases.

Boiler slags are predominantly single-sized and within a range of 5 mm to 0.5 mm. Boiler slag particles normally have a smooth texture, but if gases are trapped in the slag as it is tapped from the furnace, the quenched slag will become somewhat vesicular or porous. Boiler slag from the burning of lignite or subbituminous coal tends to be more porous than that of the bituminous coals. Table 7 gives the gradation of typical boiler slag. Compared to natural granular materials, the maximum dry density values of boiler slag are from 10 % to 25 % lower and the optimum moisture content values are higher.

Tab 7 Typical gradation of boiler slag
% passing sizePercentage
20 mm100
10 mm99
5 mm97
2.5 mm85
1.2 mm46
0.6 mm23
0.3 mm12
0.15 mm6
0.075 mm4

Tab 8 shows the chemical composition of boiler slag. The chemical composition of boiler slag is similar to that of bottom ash, though the production process of boiler slag and bottom ash is relatively different.

Tab 8 Typical chemical composition of boiler slag
CompoundUnitBottom ash from bituminous coalBottom ash from lignite coal

Tab 9 gives the typical geotechnical properties of the boiler slag. The friction angle of boiler slag is within the same range as those for sand and other conventional fine aggregates. Boiler slag shows high CBR value, comparable to those of high-quality base materials. Compared to bottom ash, boiler slag shows less abrasion and soundness loss because of its glassy surface texture and lower porosity.

Tab 9 Typical Geotechnical properties of boiler slag
PropertyUnitBoiler slag
Specific gravity2.3 – 2.9
Minimum dry densitygrams/cc0.96 – 1.44
Maximum dry densitygrams/cc1.31 – 1.63
Optimum moisture content%8-20
Los Angeles Abrasion%24-48
Friction angleDegree36 – 46
California Bearing Ratio @ 95 %%40-70

Boiler slag has been frequently used in hot mix asphalt because of its hard durable particles and resistance to surface wear. It can also be used in asphalt wearing surface mixtures because of its affinity for asphalt and its dust-free surface, thus increasing the asphalt adhesion and anti-stripping characteristics. Since boiler slag has a uniform particle size, it is usually mixed with other size aggregates to achieve the target gradation used in hot mix asphalt. Boiler slag has also been used very successfully as a seal coat aggregate for bituminous surface treatments to enhance skid resistance.

Flue gas desulfurization waste

Flue gas desulfurization (FGD) waste is the solid material resulting from the removal of sulphur dioxide (SO2) gas from the furnace stack gases in the FGD process. The material is produced in the flue gas scrubbers by reacting slurried limestone or lime with the gaseous SO2 to produce calcium sulphite. Usually, the calcium sulphite (CaSO3) is further oxidized to calcium sulphate (CaSO4) known as synthetic gypsum which has the same chemical composition as natural gypsum. The dewatering system removes water from the CaSO4 leaving the FGD absorber modules into hydrocyclone centrifuges and onto belt filter presses. Vacuum pumps beneath the belt siphon the water out of the material, leaving it with around 10 % moisture content. A belt conveyor system transports the dewatered materials from the dewatering building to an adjacent storage shed.

In the FGD process, a small fraction of the CaSO4 slurry is regularly removed to a water treatment system for dewatering to remove chlorides and fines from the process. The solids from the water treatment system are captured and removed in a filter press. This material is typically referred to as waste water system filter cake (a second by-product) and consists of fine gypsum particles, unreacted limestone fines, CaSO3 particles and a minor amount of fly ash. It is a brown clay-like chunky material with high water content. Due to the high content of water, chlorides, sulphites and trace metals, filter cake cannot be used in pavements or other applications without stabilization.

FGD scrubber material is initially generated as CaSO3. But when using wet FGD systems which utilize calcium-based sorbents and forced oxidation which converts CaSO3 to CaSO4. Since this process is carried out in the aqueous phase, FGD gypsum is produced. CaSO3 FGD scrubber material can be expansive and needs to be fixated or stabilized prior to most construction uses. FGD gypsum is often used for wallboard, in agriculture, and as a cement additive. FGD gypsum (CaSO4) is typically coarser than CaSO3. The purity of FGD gypsum typically ranges from 96 %-99 %, depending on the sorbent used for desulphurization.

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