Fly Ash

  • Recently, two people died and four were missing after a dyke of a fly ash pond of Reliance’s Sasan Ultra Mega Power Project in Singrauli breached , causing spillage on several acres of agricultural fields.
  • The project, owned by Anil Ambani's Reliance Power, is 780 kilometres northeast of the state capital of Bhopal.
  • Singrauli houses various power projects, including NTPC, Coal India, Reliance Power and Essar Power.

About Fly Ash

  • Also known as flue ash and pulverised fuel ash, fly ash is a fine powder that is a byproduct of burning pulverized coal in electric generation power plants.

Production

  • It is produced by coal-fired electric and steam generating plants. Typically, coal is pulverized and blown with air into the boiler's combustion chamber where it immediately ignites, generating heat and producing a molten mineral residue.
  • Boiler tubes extract heat from the boiler, cooling the flue gas and causing the molten mineral residue to harden and form ash.
  • Coarse ash particles, referred to as bottom ash or slag, fall to the bottom of the combustion chamber, while the lighter fine ash particles, termed fly ash, remain suspended in the flue gas.

Properties

Physical Properties

  • It depend on the nature of coal; mineral matter chemistry and mineralogy, furnace  design,  furnace  operation  and  method  of  particulate  control;  such  as Sulphur Oxide (SOx) and Nitrogen Oxide (NOx) control technologies.
  • Fly ash particles are generally spherical in shape and ranging from 0.5 µm to 100 µm.

Chemical Properties

  • Major Constituents: Fly ash is a pozzolan, a substance containing aluminous and siliceous material that forms cement in the presence of water. In addition, it contains ferric oxide and calcium oxide.
  • Minor Constituents: 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.

Classification

  • Basically, there are two classes of fly ash: Class F and Class C fly ash.
  • The chief difference between these classes is the amount of calcium, silica, alumina, and iron content in the ash.

Class F

  • The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash.
  • This fly ash is pozzolanic in nature, and contains less than 7% lime (CaO).

Class C

  • Fly ash produced from the burning of younger lignite or sub-bituminous coal, in addition to having pozzolanic properties, also has some self-cementing properties.
  • Class C fly ash generally contains more than 20% lime (CaO).
  • Unlike Class F, self-cementing Class C fly ash does not require an activator.

Applications of Fly Ash

  • Fly-ash pellets which can replace normal aggregate in concrete mixture
  • Embankments and other structural fills (usually for road construction)
  • Grout and Flowable fill production
  • Waste stabilization and solidification
  • Cement clinker production - (as a substitute material for clay)
  • Mine reclamation
  • Stabilization of soft soils
  • Road subbase construction
  • As aggregate substitute material (e.g. for brick production)
  • Mineral filler in asphaltic concrete
  • Agricultural uses: soil amendment, fertilizer, cattle feeders, soil stabilization in stock feed yards, and agricultural stakes
  • Loose application on rivers to melt ice
  • Loose application on roads and parking lots for ice control

Benefits

  • Fly ash can be a cost-effective substitute for Portland cement in many markets.
  • It is also recognized as an environmentally friendly material because it is a byproduct and has low embodied energy, the measure of how much energy is consumed in producing and shipping a building material.
  • Fly ash utilization, especially in concrete, has significant environmental benefits including:
  • increasing the life of concrete roads and structures by improving concrete durability,
  • net reduction in energy use and greenhouse gas and other adverse air emissions when fly ash is used to replace or displace manufactured cement
  • reduction in amount of coal combustion products that must be disposed in landfills
  • conservation of other natural resources and materials.

Environmental Hazard

  • When ash is disposed in dry landfills or wet ponds, there are associated environmental effects. Wet surface impoundments account for a fifth of coal ash disposal.
  • These wet impoundments can be an issue if they do not have proper liners for the landfill or pond to prevent leaking and leaching.
  • Both leaking and leaching lead to groundwater contamination.
  • This groundwater contamination can be harmful to human heath if the groundwater is a source of drinking water.
  • In addition to leaching, fly ash toxics are able to travel through the environment as a result of erosion, runoff, or through the air as fine dust.
  • Application of flyash, particularly unweathered ones, shows a tendency of accumulating elements like B, Mo, Se and Al. The accumulations of these elements to toxic levels are responsible for reductions in the crop yields and consequently influence animal and human health.

Health Hazard

  • Fly ash contains crystalline silica which is known to cause lung disease, in particular silicosis, if inhaled.
  • It impacts on the nervous system, causing cognitive defects, developmental delays, and behavioral problems while also increasing a person's chance of developing kidney disease, and gastrointestinal illness.

Source : Civil Services Chronicle Online, April, 2020