In coal-based thermal power plants (TPPs), nearly 40 per cent of the coal consumed is converted into ash. It is a major by-product of the generation process, which is difficult to dispose of, and has numerous uses that require it to be gainfully utilised. Most plants thus engage in ash management.
However, ash handling requires huge amounts of water, second only to cooling systems in TPPs. With the present water norms and the ongoing shift to highly water-intensive flue gas desulphurisation systems, optimising water consumption in the ash handling process is necessary.
Strategies for water efficiency
In most plants, the ash-to-water ratio has been 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 of the ash-to-water ratio that is reduced, 60 cubic metres per hour of water can potentially be saved.
There are two types of ash generated in coal-based TPPs: Bottom ash and fly ash. Bottom ash is generated under the furnace of the TPP, and accounts for nearly 20 per cent of the total amount of ash generated. It is coarse in nature and needs to be crushed for further handling.
Fly ash, contrast, comprises about 80 per cent of the total amount of ash generated in TPPs. It consists of very fine particles that are collected via an economiser hopper, an air preheater hopper or an electrostatic precipitator (ESP), and need to be disposed of properly.
Ash handling systems
Bottom ash from boiler requires water during handling and disposal due to the presence of clinkers, while fly ash from electrostatic precipitators can be disposed in dry form completely. But wet disposal (slurry of 10-20% ash by weight with water) is more common due to economic reasons. High Concentration Slurry Disposal (HCSD) system uses lesser water than wet system (concentration of ash 50-60% by weight).
Dry handling of fly ash can be an effective method of reducing water consumption. The material can be collected from the ESP through a pneumatic system and transferred to an intermediate storage silo that is adequately aerated. It can then be sent to the end users. One of the major issues with this method is the blockage of lines due to low transportation capacity. Hence, it is important to analyse ash particles prior to designing an ash handling plant (AHP). Other issues include the need for appropriate sizing of the air compressor, and the possibilities of choking of the wetting head and air washer nozzles, and corrosion of the water pump.
Mechanical removal of the bottom ash is another effective method, which involves the use of mechanised bottom ash conveyor systems as an alternative to conventional bottom ash sluice water systems.
Another alternative is the high concentration slurry disposal system, which uses intermediate surge hoppers, stream surge hoppers, screw conveyor, ash mixer, agitator retention tank and charge pump. The fly ash is collected in dry form in each intermediate surge hopper, and is discharged to agitated mixing tanks via screw conveyors and ash mixers. Water flow to the ash mixer is controlled via a motor. This process reduces the dilution of the slurry, saving both energy and water. These systems need less space and have lower capital and operating costs. The process uses 8.34 per cent of the water required in other ash handling systems, has minimal to zero risk of contamination through water leakage. It requires negligible run-off water and water recycling system capacity. It also reduces the need for dusting, as the slurry hardens out, allowing rehabilitation.
Other measures such as reusing water for other plant functions and rainwater harvesting for ash ponds can reduce water requirements. In plants that use ash water recirculation, typically, 70 per cent of the ash pond water can be recovered and reused in the AHP.
Industry initiatives
NTPC leads by example and adopts the latest technology for thermal power generation. NTPC’s Sipat Super Thermal Power Station has taken some innovative water conservation measures that can be considered by other utilities as a roadmap for water-efficient ash handling. NTPC Sipat consumes some of the least water among the company’s stations. The station is zero liquid discharge-compliant as it uses and reuses ash water, drain water, effluent-treated water, sewage water, etc., through various recycling systems such as effluent treatment plants, sewage treatment plants and ash water recirculation systems.
The station evacuates fly ash through dry evacuation technology. This ash has been used in low land filling, ash brick manufacturing and as input materials for cement plants. These efforts have reduced the use of water for ash disposal significantly. With its real-time water system monitoring dashboard, NTPC monitors the water system right from the drawl point to the end-consumption, greatly improving water utilisation. The station engages in practices such as rainwater harvesting and reservoir lining to reduce water consumption and ensure sustainability. The station also operates on a higher cycle of concentration, reducing the requirement of fresh water for the water cooling system, thereby saving water on all ends. This has ultimately helped reduce the specific water consumption of the station on a year-on-year basis to 2.66 litres/kwh in 2021-22, which is far below the prescribed norms.
Many utilities have the potential to alter their water consumption requirements. Internet of things- and artificial intelligence-based water management systems, coupled with the above-mentioned measures, can make TPPs cleaner and more sustainable. Gencos can deploy online ash analysers to measure the ash content and calorific value in coal. Such analysers measure the total ash content in coal on a belt conveyor and report the results to the plant operator in real time, assisting informed decision-making and process control. Apart from these measures, focusing on operations and maintenance/repair and maintenance practices will ensure that the system stays robust enough to make space for cleaner technologies.
Given that the power sector is a major water consumer, these measures can save a vital resource while also safeguarding our environment.