Ash constitutes 26-40 per cent of the coal used for power generation. A coal-based unit ranging from 250 MW to 800 MW of capacity typically produces 100-320 million tonnes of ash per hour. Of the total ash, only about 20 per cent takes the form of bottom ash and the remaining is fly ash. Coal ash contains several distinct materials, including a toxic stew of chemicals like lead, arsenic and mercury. Coal ash can be used as a raw material in concrete and cement or as a filling material in stabilisation projects and road beds. However, by utilising coal ash instead of disposing it of in landfills, significant environmental degradation and energy costs can be avoided. In addition to this, for every tonne of fly ash used in place of cement, about a tonne of carbon dioxide is prevented from entering the atmosphere. These opportunities, along with the regulations regarding the management of ash generated by coal-based power plants, emphasise the need for plants to adopt more efficient and effective technologies for ash handling.
Ash handling systems mainly collect and transport the fly ash and bottom ash from the boiler to the respective storage points. The bottom ash is generally collected and crushed to produce fine powdered ash. The powdered ash is then mixed with water to produce slurry, which is disposed of in an open pond called the ash dyke. Ash handling systems for bottom ash include submerged scrapper conveyors (SSCs) and high pressure water pumps.
Fly ash handling systems include fly ash slurry transportation with bottom ash and fly ash dry pneumatic transportation. Fly ash is collected using electrostatic precipitators (ESPs) that operate by charging the particulate matter entrained in the flue gas stream through high voltage and then electrostatically influencing those particles to oppositely charged electrodes. ESPs are installed near the flue gas chimney. The fly ash particles in the flue gas stick to the screen of the ESP and the remaining flue gas escapes through the chimney. The stuck ash is then scraped from the screen using an automated scraper mechanism and collected in a hopper. The collected fly ash is then transported either hydraulically or pneumatically. In the pneumatic method, fly ash is transported to silos using pressurised air in order to deliver it to potential buyers and not disposed of like bottom ash.
Earlier, almost all the ash from coal-based power plants was conveyed as slurry and collected in ash ponds. Then gradually, dry fly ash collection systems became more popular, especially with new coal-based power plants. In recent years, existing plants have also phased out pond-based systems for fly ash disposal. The ever-growing drive to conserve water facilitates the shift to dry ash handling.
Some of the techniques being used for ash handling are given below.
High concentration slurry disposal (HCSD)
In this method, fly ash from silos is fed to the mixing tank/agitator retention tank through the weighting unit, rotary feeder and ash conditioner. The conditioned fly ash is wetted further by adding water in the mixing tank, following which the entire ash is blended using a mixer to form a uniform slurry. The quantities of ash and water are controlled in the mixing tank to achieve the desired characteristics. The mixer tank is equipped with an agitator to create smooth and homogeneous slurry that is lump-free. The slurry so prepared is then regulated through a control loop, water control system and material feed rate control system. The control loop continuously monitors the density of the slurry and automatically instructs for the addition of water, if required. Thereafter, the slurry is transferred to a positive displacement type high concentration ash slurry disposal pump. The HCSD pump discharges the concentrated fly ash slurry up to the ash dyke through seamless pipelines.
In HCSD, the concentration of ash (by weight) is 60 per cent or above. Therefore, special pumps like reciprocating or diaphragm pumps are required to transport the slurry. Since the ash concentration is high in this method, water consumption is reduced significantly and minimal water is released in the disposal area. The ash slurry is generally disposed of on a slope where it spreads over a substantial area and solidifies, leaving hardly any ash to fly.
Dry fly ash disposal system
Power plants are increasingly adopting dry fly ash disposal systems since the ash collected using these systems can be used as a raw material for cement manufacturing, thereby providing economic benefits to the plant owner. In this method, fly ash from the ESP hoppers is collected in ash vessels. To ensure free flow of ash into the ash vessels from the hopper, the lower portions of the hoppers are provided with electric heaters. The heaters maintain the temperature well above the ash fusion temperature to prevent the formation of clusters of ash and ensure smooth functioning of the conveying system. The ash is then transported to fly ash silos with the help of compressed air. The moisture from the compressed air is removed using an adsorbent air dryer or refrigeration air dryer. From the ash silos, the ash is finally transported to sealed vessel trucks. The bottom of each fly ash silo is also equipped with an additional discharging channel, which is connected with a wet mixer as a backup for the dry ash disposal system.
Dry bottom ash system
Dry bottom ash handling techniques offer increased thermal efficiency, reduced unburned carbon and improved ash quality. The dry bottom system comprises a submerged scraper conveyor for cooling and transmitting the hot bottom ash to the crusher. The conveyor is enclosed in a tight housing to preclude uncontrolled air infiltration. The fine ash is collected by a spill chain located at the bottom of the housing. The ash so collected is then cooled by the controlled flow of ambient air.
One of the biggest advantages of the dry bottom ash system is that it is environment friendly. There is no requirement for water for bottom ash cooling and conveying. As a result, the need for wastewater treatment does not arise. In addition, the costs associated with pumps, piping, dewatering bins and corrosion damage are also avoided. The operations and maintenance (O&M) costs of the dry bottom ash system are also considerably lower than those of the wet system. Further, being a zero-discharge system, it can be easily integrated with the environmental management system.
O&M issues
In dry ash disposal, one of the major issues is the blockage of lines due to low transportation capacity. This generally happens when the ash handling plant is designed on the basis of assumptions regarding particle size distribution and not as per actual particle size distribution. Hence, it becomes important to analyse the ash particles prior to the design of the ash handling plant. Other issues include the need for appropriate sizing of the air compressor and possibilities of choking of the wetting head and air washer nozzles and corrosion of the water pump.
In cases where the concentration of ash in the slurry is low (10-15 per cent), the transportation of ash becomes highly uneconomical. In addition, water consumption is very high and the method leads to excessive wear and tear of pipelines carrying the slurry due to higher transportation velocity. Further, due to excessive water usage, the hazard of polluting both ground and surface water sources is quite high.
The way forward
With regulatory changes and technology developments, ash management has evolved significantly over the years and continues to improve. Technologies like HCSD and dry ash disposal systems benefit operators by improving the overall efficiency of plants as well as providing economically valuable waste material. Most importantly, these technologies help operators in conforming to emission regulations and standards. In addition, these technologies reduce the adverse effects of coal power production on the environment by making the otherwise toxic material useful. In the long run, these technologies are seen to be highly cost-effective and environment friendly. In view of this, it is not just the new plants that are installing these systems, but also many old coal power plants, which have been retrofitting them.