The combustion of coal in thermal power plants (TPPs) generates large volumes of coal combustion residuals (CCRs), such as fly ash, bottom ash, boiler slag and flue gas desulphurisation materials. These residuals are collectively referred to as coal ash. For decades, coal ash has been managed using river or lake water to sluice it into a large surface water impoundment (coal ash pond) for its final settlement. Coal ash contains substances such as arsenic, boron, selenium and heavy metals (cadmium, copper, chromium, lead and mercury, among others). As a result, coal ash pond water, if released into the environment through embankment breaches and seepage of water through the pond bottom as leachate, may contaminate soil, rivers, lakes and groundwater. Besides, given the fact that many regions in the country are struggling to support essential services such as drinking water and irrigation, water is set to become a limiting factor in the growth and productivity of TPPs.
Moreover, more than 50 per cent of the TPPs continue to use traditional semi-wet or wet ash handling systems. Due to low reutilisation of ash generated in several plants, about 35 per cent of the ash on an average has remained unutilised every year for the past 10 years. The unutilised ash ends up in ash ponds, polluting the water and soil around them.
Thus, for improving water use efficiency in TPPs, developers are investing in technologies that optimise the use of water in ash handling systems. Dry bottom ash systems, high concentration slurry disposal (HCSD), ash water recirculation systems, restricting the water-to-ash ratio for slurry disposal, among others, are some of the measures that the industry has adopted over the years to tackle this problem.
Methods to reduce water consumption
In plants using ash water recirculation, typically 70 per cent of ash pond water can be recovered and reused in the ash handling plant. Accordingly, the available cooling tower’s (CT) blowdown will reduce the water requirement of the ash handling plant because it will blow air to eliminate the ash. This will, thereby, serve to reduce the water requirement of the ash handling plant and no additional water will be required from a raw water source. Thus, net water to be supplied for ash disposal gets reduced to about 30 per cent of the requirement of the ash handling plant. The consumptive water requirement for old TPPs with CTs is as high as 8-9 cubic metres (cum) per MWh without ash water recirculation and 5 cum per MWh with ash water recirculation. Recently, TPPs have been designed with consumptive water requirement in the range of 3.5-4 cum per hour per MW.
In addition, the HCSD system for fly ash involves pumping of high solid concentration slurry with more than 60 per cent solids by weight, employing positive displacement pumps, as compared to lean slurry transportation at 25-30 per cent concentration. Overall, if HCSD is used instead of the wet slurry system for fly ash disposal as well as in ash water recirculation systems, the available CT blowdown shall suffice to meet the water requirement and the withdrawal of water from raw water sources will be minimised.
With regard to the ash-to-water ratio, optimising the ratio for slurry disposal helps reduce the water consumption. In most plants, the ash-to-water ratio is found to be 1:20. Restricting this to 1:5 for fly ash and 1:8 for bottom ash can significantly help in conserving water. For every percentage reduction in the ash-to-water ratio, there is a saving potential of 60 cum per hour of water. After initiating water conservation measures, many TPPs have brought down their ash-water ratios to a reasonable range of 1:10 to 1:12.
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 water requirement 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 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.
Notably, NTPC has put in place an ash water recirculation system to reuse the decanted ash slurry water and toe drain water from the ash pond for meeting the requirements of ash handling and service water. Further, it is developing an eco-park on the ash disposal area at the Badarpur TPP, to have an environmentally sustainable neighbourhood and the area is being developed as a green island. NTPC has also undertaken various initiatives such as installation of a dry bottom ash handling system, dewatering bins, rail loading system for transportation of dry fly ash, as well as a feasibility study for large capacity silo, etc.
Conclusion
Given the impending water crisis, it is critical to restrict water consumption. Ash management has evolved significantly over the years as a result of regulatory changes and technological advancements and it continues to improve. Technologies such as HCSD and dry ash disposal systems help operators by increasing plant efficiency and providing economically valuable waste material. Above all, these technologies help operators in adhering to emission norms and standards. These technologies are expected to be very cost-effective and environmentally friendly in the future.
To summarise, safe ash disposal is necessary for power plants to use water efficiently and optimally, since it is advantageous not only to the environment but also to TPPs.