Environmental concerns associated with ash generation in thermal power plants (TPPs) have resulted in utilities adopting various measures such as reducing the water requirement for wet ash disposal, dry disposal of fly ash and utilisation of ash in various applications. One of the major challenges in the management of ash disposal is minimising the amount of water required for ash handling. To this end, it is necessary to have a well-planned ash disposal system in place.
The measures for reducing water consumption in ash management include decreasing the water-to-ash ratio for slurry disposal, recirculating ash pond water and using a high concentration slurry disposal (HCSD) system for fly ash.
Slurry, which is a mixture of ash and water, is transportable at different levels of concentration. The current practices for fly ash handling and management include lean concentration (10 per cent) slurry conveying, medium concentration (25-30 per cent) slurry conveying and high concentration (50-55 per cent) slurry conveying. For bottom ash, in addition to lean concentration slurry conveying, the dry collection method is also being adopted. About 75 per cent of TPPs adopt the lean slurry practice. However, in recently commissioned plants, wet disposal of ash has been adopted with a slurry concentration of 30 per cent for fly ash and 25 per cent for bottom ash.
The conventional lean slurry disposal system and ash water recovery system suffer from several disadvantages such as a large amount of water wastage, groundwater contamination, potential ash pond collapse, vast land requirement for ash dykes, higher costs for ash pond construction and greater power consumption. These limitations have led to the adoption of new environment-friendly ash disposal techniques like HCSD, which help minimise water consumption due to the high concentration in HCSD (60-70 per cent) as compared to lean concentration slurry disposal (LCSD) (15-25 per cent). In addition, the specific energy consumption is also lower and dust is minimal as compared to LCSD. HCSD systems include intermediate surge hoppers, stream surge hoppers, screw conveyors, ash mixers, agitator retention tanks, charge pumps and ABEL pumps.
Ash water recirculation system
The installation of an ash water recirculation system is another measure being taken for reducing the water requirement in ash handling. In plants using ash water recirculation, typically, 70 per cent of the ash pond water can be recovered and reused in the ash handling plant (AHP). In this condition, the available cooling tower (CT) blowdown is sufficient to meet the make-up water requirement of the AHP and no additional water is required to be drawn from raw water sources. As such, additional water from raw water sources will be required only till the ash water recirculation system becomes operational. Therefore, the net water to be supplied for ash disposal is reduced to about 30 per cent of the total requirement of the AHP.
Changes in the internal design of the centrifugal pumps can also lower the water requirement. Centrifugal pumps with improved design can take the slurry up to 50-55 per cent concentration. However, distance is a constraint for these pumps, as they can carry slurry up to only 2-3 km. For longer distances, positive displacement pumps are ideal as they can carry slurry concentration up to 65-70 per cent for a distance of 8-10 km. High rating positive displacement pumps can go up to a longer distance. Typically, the pumps capable of transporting slurry of medium and high concentration are often made to operate on low concentration slurry, which increases the water requirement for ash management at a plant.
The way forward
Reducing the water requirement for wet ash disposal is possible through 100 per cent ash utilisation and storage of dry ash in silos. Going forward, in the medium term, lean concentration slurry can be converted to medium concentration slurry and ash pond water can be recycled. In the long term, redesigned centrifugal pumps can be used for transporting fly ash slurry.