FLY ASH

With the boom population and industrial growth, the need for power has increased manifold. Nearly73% of India’s total installed power generation capacity is thermal, of which 90% is coal-based generation, with diesel, wind, gas and stream making up the rest. Thermal power generation through coal combustion produces minute particles of ash that causes serious problem. Commonly known as fly ash. These ash particles consists of silica, alumina, oxides of iron, calcium, magnesium and toxic heavy metals like lead, arsenic, cobalt, and copper.

          The 80-odd utility thermal power stations in India use bituminous coal and produce large quantities of fly ash. According to one estimate up to 150 million tones of fly ash will be produced in India during the year 2000. It is expected that the quantity of fly ash available will touch the figure of 1000 million KN per annum. The disposal of such a large quantities of fly ash is certainly a gigantic problem and a matter of national concern. This poses problem in form of land use, health hazards, and environmental dangers.

          The prevalent particle is to dump fly ash on wastelands, and this has lain to waste thousands of hectares all over the country. To prevent the fly ash from getting airborne, the dumping sites have to be constantly kept wet by sprinkling water over area. And in India, the sites are not lines and leads to seepage, contaminating groundwater and soil. It lowers the soil fertility and contaminates surface and ground water as it can leach into subsoil. When fly ash gets into the natural draining system, it results in siltation and clogs the system. It also reduces the pH balance and portability of water. Fly ash interferes with the process of photosynthesis of aquatic plants and thus disturbs the food chain.

Properties

          Fly ash is a residue resulting from combustion of pulverized coal or lignite in thermal power plants. About 80% of total ash is in finely divided form, which is carried away with flue gases and is collected by electrostatic precipitator or other suitable technology. This ash is called as dry ash or chimney or hopper ash. The balance 20% of ash gets collected at the bottom of the boiler and is referred as bottom ash. Fly ash is very fine comparable to cement, however some particles have size less than 1 micron in equivalent diameter

Chemical composition of Fly Ash.

Components	Percentage (%)
Silica as Sio2	35-59
Iron as Fe2O3	0.5-2
Alumina as Al2O3	20-33
Calcium as CaO	5-16
Magnesium as MgO	1-5.5
Sulphate as So3	0.5-1.5
Loss on ignition	1-2

Above the Table gives the chemical component of Fly ash. Fly ash posses pozzolana characteristics. Pozzolana are defined as silicious and aluminous materials, which in themselves posses little or no cementitious value but, will in finally divided from and in the presence of moisture chemically react with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties. So this pozzolanic material can be used for the manufacture of cement, concrete, and lime/cement based bricks/blocks.