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| Last Updated:: 29/03/2016

Introduction

    : Fly ash is the by-product of coal combustion collected by the mechanical or electrostatic precipitator (ESP) before the flue gases reach the chimneys of thermal power stations in very large volumes.


    All types of fly ash contain significant amounts of silicon dioxide (SiO2), aluminium oxide (Al2O3), iron oxide (Fe2O3), calcium oxide (CaO), and magnesium oxide (MgO) however, the  actual composition varies from plant to plant depending on the coal burned and the type of burner employed. Fly ash also contains trace elements such as mercury, arsenic, antimony, chromium, selenium, lead, cadmium, nickel and zinc. The texture, physicochemical and geotechnical properties of flyash generated from coal including trace/ heavy metal content and radioactivity are similar to those of common soils. Due to these properties of flyash, it can be safely used in agriculture, mines, road/ embankment, as building material etc. The pozzolanic property of flyash/lime reactivity enables it to be used in manufacture of cement and in concrete to replace a part of cement. The use of flyash for various applications is beneficial from health and environment point of view also. 

     

     


    Classification of flyash

    Flyash is generally produced at coal based thermal power stations. It is available mainly in four forms namely dry flyash,  bottom ash, pond ash and mound ash.

     

      • Hopper Ash: About 80% of the total ash is finely divided and comes out of boiler along with flue gases. It is collected by cyclone separators or electrostatic precipitators (ESP) or bag filters or other suitable means. Dry fly ash, collected from different fields of ESP, are of different fineness. The fly ash collected in the ESP field, nearer to boiler, are coarser than the fly ash collected in the ESP fields away from the boiler (i.e. nearer to the chimney).
      • Bottom Ash: The balance about twenty per cent of ash gets collected at the bottom of the boiler furnace and is referred to as bottom ash. It is coarser then dry fly ash/hopper ash.
      • Pond Ash: When dry fly ash alone or along with bottom ash is carried, in the form of water slurry, to storage or deposition lagoon or pond and deposited, is termed as pond ash.