Fly Ash

Context:

Seeking potential overseas off-takers for the fly ash produced by its thermal power plants, NTPC has invited Expression of Interest (EoI) for sale of the residual product to the West Asian and other regions.

• It will supply the ash from power plants to ports and the total quantum earmarked for export is 14.5 million tonne (MT) per year.

What is Fly Ash:

Fly ash or flue ash, coal ash, and also known as pulverized fuel ash in the, or coal combustion residuals (CCRs), is a coal combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired boilers together with the flue gases. Ash that falls to the bottom of the boiler’s combustion chamber (commonly called a firebox) is called bottom ash. In modern coal-fired power plants, fly ash is generally captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys. Together with bottom ash removed from the bottom of the boiler, it is known as coal ash.

Composition of fly ash:

Composed of the particulates that are driven out of coal-fired boilers together with the flue gases.

• Depending upon the source and composition of the coal being burned, the components of fly ash vary considerably, but all fly ash includes substantial amounts of silicon dioxide (SiO2), aluminium oxide (Al2O3) and calcium oxide (CaO), the main mineral compounds in coal-bearing rock strata.
• Minor constituents include: arsenic, beryllium, boron, cadmium, chromium, hexavalent chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium, along with very small concentrations of dioxins and PAH compounds. It also has unburnt carbon.

Properties of Fly Ash:

• Resemble Portland cement but is chemically different.
  • Portland cement is a binding material in the form of a finely ground powder that is manufactured by burning and grinding a mixture of limestone and clay.
  • Its chemical composition includes calcium silicates, calcium aluminate and calcium aluminoferrite.
  • Exhibit cementitious properties.
    • A cementitious material is one that hardens when mixed with water.

Harmful Effects:

• Fly ash particles are toxic air pollutants. They can trigger heart disease, cancer, respiratory diseases and stroke.
  • When combined with water they cause leaching of heavy metals in ground water.
  • It also pollutes the soil, and affects the root development system of trees.

Fly Ash uses:

fly ash can be used in the following ways:

1. Concrete production, as a substitute material for Portland cement, sand.
2. Fly-ash pellets which can replace normal aggregate in concrete mixture.
3. Embankments and other structural fills.
4. Cement clinker production – (as a substitute material for clay).
5. Stabilization of soft soils.
6. Road subbase construction.
7. As aggregate substitute material (e.g. for brick production).
8. Agricultural uses: soil amendment, fertilizer, cattle feeders, soil stabilization in stock feed yards, and agricultural stakes.
9. Loose application on rivers to melt ice.
10. Loose application on roads and parking lots for ice control.

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