What is Fly Ash? Physical and Chemical Properties of Fly Ash

What is Fly Ash?

Fly ash, a byproduct from coal-based thermal power plants, is abundantly available in India. Estimates show that more than 100 million tons of fly ash are produced annually from around 83 major coal-based power stations. A small fraction of this is fruitfully utilized.

Fly ash is a solid fine-grained material resulting from the combustion of pulverized coal in power stations. It is one of the residues produced by flaming& comprises fine particles that rise with the flue gases. Ash that does not rise is termed as bottom ash.

In the industrial context, fly ash normally refers to ash produced during the combustion of coal. It is a waste product of a power plant. It is generally detained by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys of coal-fired power plants. The ash is collected in mechanical & electrostatic separators/bag filters.

Formation of Fly Ash

Fly ash is formed during the coal burning process in the boiler. Fly ash material solidifies while in suspension in the exhaust gases and is collected by electrostatic precipitators or filter bags. Since the particles are generally spherical in shape and range in size from 0.5 µm to 100 µm. They contain most of the silicon dioxide (Sio2), which is available in two forms: amorphous, which is rounded and smooth, and crystalline, which is sharp, pointed, and hazardous; aluminum oxide (Al2O3) and iron oxide (Fe2O3). The formation of fly ash is mostly highly heterogeneous, consisting of a mixture of glassy particles with different identifiable crystalline phases like quartz, mullite & various iron oxides (Fe2O3). In dry bottom furnaces, the retention time of the coal in the boiler is between 3-10 seconds. During this time coal is burnt, non-coal particles remain partially in their original shape or form spherical particles. Bloating of molten glass phases results in hollow spherical particles. Bloating of molten glass phases results in hollow spherical particles so-called cenospheres.

Due to the different temperatures and oxygen regimes in the boiler, the ash formation process is very complex and difficult to predict. Nevertheless, from the long-term monitoring of coal properties and corresponding fly ash properties, one can predict fly ash properties possible in a specific unit.

Types of Fly Ash

Two classes of fly ash are differentiated by ASTM C618:

  • Class F fly ash
  • Class C fly ash.

The main difference between class F and class C fly ash is in the amount of lime, silica, alumina, and iron content in the ash. In Class F ash, total lime typically ranges from 1 to 12 percent, mostly in the form of calcium hydroxide, calcium sulfate, and glassy components in combination with silica and alumina. Class C fly ash may have reported calcium oxide contents as high as 30% to 40%. Another difference between class F and class C is that the number of alkalis (combined sodium and potassium) and sulfate (SO4) are generally higher in Class C fly ash than in Class F fly ash. The chemical properties of the fly ash are greatly affected by the chemical content of the coal burnt (i.e., anthracite, bituminous, and lignite).

Class F Fly Ash

The flaming of harder, older anthracite & bituminous coal typically produces Class F fly ash. This fly ash is pozzolanic contains less than 10% lime (CaO) and possesses pozzolanic properties. The glassy silica & alumina of Class F fly ash require a cementing agent, such as Portland cement, quicklime, or hydrated lime.

Class C Fly Ash

Fly ash manufactured from the flaming of lignite or sub-bituminous coal, in addition to having pozzolanic properties, also has some self-cementing properties. In the presence of water, Class C fly ash will harden & gain strength over time. Class C fly ash generally contains more than 10% lime (Cao). Unlike Class F, self-cementing Class C fly ash doesn’t require an activator. Alkali and sulfate (SO2) content are generally higher in Class C fly ash.

Indian fly ash is mainly of class F type. Lignite-based fly ash is of Class C fly ash. Indian Standard IS 3812 refers to Class F and Class C fly ash as “Siliceous fly ash” and “Calcareous fly ash” respectively.

Based on the method of collection, fly ash is classified as Pulverized fuel ash, Bottom ash, Pond ash, and Mound ash.

Pulverized fuel ash is brought out from fuel gases by a necessary process such as an electrostatic precipitator or cyclone separator.

Pond Ash: Pulverized fuel ash or bottom ash or both mixed in any proportion and conveyed in the form of water slurry and deposited in pond or lagoon.

Mound Ash: Pulverized fuel ash or bottom ash both mixed in any ratio and conveyed in dry form & deposited in dry form.

What Fly Ash is Suitable?

Of all the fly ash generated, only the volume that is collected in dry form is more suitable for addition in cement and concrete. Though a large quantity of fly ash is available, all the ash is not suitable for use in cement and concrete. Carbon content, fineness, gradation, lime reactivity, etc. of fly ash are important properties that have a bearing on the properties of concrete, both in its fresh and hardened states. Processed fly ash is being made available in recent times.

Properties of Fly Ash

Physical Properties

Fly ash includes fine& powdery particles that are predominantly spherical, either solid or hollow, and mostly glassy (amorphous) in nature. The carbonaceous material in fly ash is consists of angular particles. The chart of the particle size distribution of every bituminous coal fly ash is usually similar to that of silt, which is less than 0.075 mm or No. 200 sieve. Although sub-bituminous coal fly ash is also silt-sized, they are generally slightly coarser than bituminous coal fly ash.

The specific gravity of fly ash generally varies between 2.1 to 3.0, while the specific surface area of fly ash (measured by the Blaine air permeability method) may range from 170 m2/kg to 1000 m2/kg.

The color of fly ash can vary from tan to gray to black, depending on the amount of unburnt carbon in the ash. Lighter the color, lower the carbon content. Lignite or sub-bituminous fly ash is commonly light tan to buff in color, indicating a relatively low amount of carbon as well as the presence of some lime or calcium. Bituminous fly ash has generally some shade of gray. Lighter shades of gray generally indicate higher quality ash.

Chemical Properties

The chemical properties of fly ash are influenced to a great extent by those of the coal burnt and the process used for handling and storage. There are four types, or ranks, of coal, each of which varies in terms of its heating value, its chemical composition, ash content, and geological origin. The four types, or ranks, of coal are anthracite, bituminous, sub-bituminous, and lignite (See below table).

ComponentBituminousSub bituminousLignite
The initial range of chemical composition of fly ash produced from different coal types (expressed as percent by weight).

The main ingredients of bituminous coal fly ash are silica, alumina, iron oxide & calcium, with varying amounts of carbon, as measured by the loss on ignition (LOI). Lignite & sub-bituminous coal fly ash are characterized by higher concentrations of calcium and magnesium oxide & reduced percentages of silica and iron oxide, as well as lower carbon content, compared with bituminous coal fly ash. Some anthracite coal is burned in utility boilers, so there are only small amounts of anthracite coal fly ash.

The above table differentiates the normal range of the chemical compositions of bituminous coal fly ash from those of lignite coal fly ash & sub-bituminous coal fly ash. From the table, it is evident that lignite and sub-bituminous coal fly ash have a higher calcium oxide content and lower loss on ignition than fly ash from bituminous coal. Lignite & sub-bituminous coal fly ash may have a higher concentration of sulfate compounds than bituminous coal fly ash.

Different Particle Types

Fly ash is composed of a range of different particle types with different origins and reactivity.

Generally, 5 types of particles can be differentiated:

Type 1 – Crystalline particles not affected by thermal impact morphology: original shape, round to sharp-edged: Quartz SiO2, (non-reactive / inert).

Type 2 – Sintered particles, sheeted structure, high inner surface area: dewatered clays (reactive), partially molten SiO2, (low reactivity).

Type 3 – Glass phases, spherical shape, hollow, solid, or filed spheres: completely molten mineral phases Silica Alumina based, Ca, Fe-based (carrier of pozzolanic reaction).

Type 4 – New crystalline phases: Mullite, Fe-Oxides (inert).

Type 5 – Unburnt carbon or coke, original shaped coal, and highly porous coke (sponge-like).

Production and Present Status of Fly Ash in India

According to the Central Electricity Authority of India, there are around 83 major coal-fired thermal power plants and more than 30 plants are in the pipeline in India. More than 110 million tons of ash is generated every year and is expected to reach 225 million tons by 2017. As per the Ministry of Environment and Forest, presently about 30% of ash is being used in fillings, embankments, construction, cement, block, tiles, etc. More than 65,000 acres of land are occupied by fly ash ponds. Thus, unutilized fly ash is causing enormous environmental problems and needs attention on a war footing.

Codes on Fly Ash

In India, two standards on fly ash are available:

  1. IS 3812 – (Part 1) 2013 – Pulverized Fuel Ash – Specification for use as Pozzolana in cement, cement mortar, and concrete.
  2. IS 3812 – (Part II) 2013 – Pulverized Fuel Ash – Specification for use as an admixture in cement mortar and concrete.

Factors Impacting Fly Ash Quality and Quantity

The quality and quantity of fly ash depend upon coal type, process technology, method of collection, and storage. The influencing factors and their effect on quality and quantity are summarized in the below table.

Boiler TypeDry bottom furnaceWet bottom furnaceFluidized Bed (without DeSOx)
AnthraciteHigh LOI >5%Fine spherical fly ashHigh LOI No glass
Hard coalLOI <7% High FinenessFine spherical fly ashNo glass
LigniteLow LOI <3% Low FinenessNot ExistingNo glass
Impact of boiler type on fly ash quality

Filter Types

  • Electrostatic precipitator

– Most common filter types

– Discharge by electrostatic charging and attraction by electrodes

– Multiple stage filter (3-7 stages)

– Possibility to select fractions of fly ash in different stages

  • Baghouse

– Separation through textile filters

– High separation rate

– High fineness

– No fractioning of fly ash by filter stages

  • Cyclones

– Limited number

– Separation by gravimetry

– Coarse fly ash

Impact of Dust Filter on Fly Ash Quality

  • Problems

– Use of separation enhancers (SO3, NH3)

– No proper discharge of hoppers, shot through of coarse fly ash in later stages

  • Improvement

– Increase clocking rates of filter plates at ESPs

Evaluation of Fly Ash

For evaluation of fly ash, it is important to know that each fly ash has its specific properties and the impact factors on quality are manifold. Each fly ash source has to be seen individually with its specific characteristics.

In general, it has to be noted that a prediction of fly ash quality is very complex. But the knowledge about the type of coal, boiler, filter, loading factor is giving a fair idea of the quantity and quality to be expected. Long term correlation between coal properties and fly ash properties of one source make a reasonably good prediction of fly ash quality.

It is important to know that types of fly ash differ in their properties depending on the coal burning, the boiler type, environmental measures, and filter systems. No information about the suitability of the fly ash can be given before studying the complete background of the ash. Each of the factors influencing the fly ash quality can be excluding factor for utilization.

Application of Fly Ash

Fly ash from the thermal industry is no longer considered a waste product. It has numerous applications. They are:

In Civil Engineering Work

– In Cement Production – PPC

– In concrete as mineral admixture / as a partial replacement of cement.

– In cement Mortar

– Like a fine aggregate

– As a raw material in brick/block making

– As an embankment material

– In roller compacted concrete

– In soil stabilization/sub-base formation, etc.

– In roads/pavements.

Agricultural use of Fly ash

Research on agricultural uses of fly ash has been going on in different universities and research institutes across the country for several years.

The same fly ash that causes destruction when it settles on leaves can prove beneficial when applied scientifically to agricultural fields. It can be a soil modifier & increase the moisture-retaining capacity & fertility of crops. It enhances the plant’s water & nutrient uptake, helps in the development of roots and soil–binding, stores carbohydrates & oils for use when needed, protects the plants from soil-borne diseases, and detoxifies contaminated soils.

Fly ash as fill material

Large scale utilization of ash as a fill material can be applied where fly ash replaces another material & is therefore in direct competition with that material. Fly ash itself is utilized by the power generating company producing the fly ash to improve the economics of the overall disposal of surplus fly ash.

As an adsorbent material of heavy metals

Studies indicate that heavy metals in fly ash are not leached out as fly ash-based concrete binds heavy metals effectively in structure.

Benefits of Fly ash integration

Product differentiation – Fly ash offers a wide range of specific properties to be exploited in new specific composite cement. Fly ash simply as an extender undervalues the real benefits. Products using specific properties of fly ash.

– Low heat cement

– Low shrink cement

– Sulphate/Chloride resistant cement

– Self Compacting Concrete (SCC)

– Road binders

– Masonry cement

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