Water is one of the key inputs for thermal power generation as it is required for multiple purposes including cooling tower, ash handling, coal dust suppression, removal of plant heat from plant auxiliaries and cycle makeup. As a matter of fact, the thermal power segment accounts for the highest share of freshwater use in the industrial sector. A major portion of the water requirement in a thermal power plant (TPP) goes towards makeup water for cooling towers, followed by ash handling.
The water requirement of TPPs is governed by a number of factors such as the quality of raw water, type of condenser cooling system, quality of coal, ash utilisation, type of ash disposal system and efficient wastewater management. It is estimated that most of India’s TPPs today are located in water-scarce regions and water shortages have led to electricity generation disruptions. This underlines the importance of lowering the water consumption by TPPs.
Water consumption norms
The Ministry of Environment, Forest and Climate Change (MoEFCC), through its notification issued in December 2015, has revised standards on water consumption by TPPs. As per the norms, all freshwater-based once-through cooling (OTC) plants are required to install cooling towers and achieve a maximum specific water consumption (SWC) of 3.5 cubic metres (cum) per MWh. Further, plants installed after January 1, 2017, are required to meet an SWC limit of 3 cum per MWh, as well as achieve zero wastewater discharge. Meanwhile, all seawater-based plants have been exempted from meeting these norms.
In addition to this, the government, in January 2016, notified the revised tariff policy, wherein it has mandated that all TPPs located within a 50 km radius of a sewage treatment plant use treated wastewater for their water requirement. The cost of treated water is allowed as pass-through tariff without affecting the merit order despatch. Further, to set up TPPs in water-scarce regions, state and central agencies that are responsible for water allocation from rivers/reservoirs need to ensure water availability before granting environmental clearance to a power plant. Further, in June-July 2019, the Central Pollution Control Board issued directions to freshwater-based OTC plants (about 17 GW capacity), requiring them to install cooling towers and comply with standards by June 30, 2022. Further, the Ministry of Power, vide letter dated March 4, 2020, has issued an advisory to stakeholders for taking appropriate measures for the use of treated sewage water.
Various water management strategies
Cooling towers: A cooling water system cools the hot water ejected from different heat generation operations in a TPP and sends it back into the system. It helps in reducing water consumption in general. To reduce water consumption in cooling towers, utilities can opt for recycle options such as reusing water used once through a cooling system, using pretreated effluent from other processes, or using high quality municipal wastewater effluent or recycled water. Further, cycles of concentration (CoC) of cooling water can be increased, thereby reducing the blowdown and makeup water quantity. Increasing the number of cycles from three to six can reduce cooling tower makeup water by 20 per cent and cooling tower blowdown by 50 per cent. Some of the retrofit options to meet stringent environmental specifications include installing a side-stream filtration system, installing side-stream softening system for high CoC, installing covers on open distribution decks to significantly reduce biological growth, installing automated chemical feed systems on large cooling tower systems and water treatment options such as ozonation or ionisation.
Ash handling systems: Ash handling technologies help in collecting, conveying, storing and loading different types of residual ash generated by coal-based power plants. Some of the emerging technologies for optimising the use of water in ash handling systems include 100 per cent dry ash handling systems up to silos; transportation of silo ash by bulkers/trucks or to dykes through conveyor belts after 20 per cent water mixing by hydro mix dust conditioners; high concentration slurry disposal ash transportation up to dykes; installation of dry ash handling systems; and 100 per cent recycling of bottom ash handling water through dewatering bins. Conservation of water in wet ash handling plants is a key area for gencos deploying conventional conveying technologies. One key measure to reduce specific consumption is to use recycled water or condenser outlet water, instead of fresh raw water for ash handling. Further, in such systems, optimising the ash-to-water ratio can help in reducing 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.
FGD systems: Water is required in flue gas desulphurisation (FGD) systems for various processes including absorber systems, mist eliminator wash systems, limestone grinding and slurry preparation systems and gypsum dewatering systems. Process water is pumped from storage tanks to cater to the water requirement of the entire FGD system. Also, demineralised water is required for cooling the FGD plant equipment. According to ICRA estimates, in a 500 MW TPP, FGD plants are expected to use 110-130 km per hour of fresh water for the desulphurisation process. This translates into about 0.3 km per MWh of water consumption per MW hour of generation. Further, an FGD plant is expected to discharge 20-25 km per hour as wastewater, which has to be treated to achieve zero liquid discharge (ZLD). FGD systems can also make use of process water, such as from cooling tower blowdown, for processes such as grinding limestone.
ZLD systems: ZLD is a wastewater treatment technology used to purify and recycle all the wastewater produced in industries, including TPPs. The implementation of ZLD ensures that the discharged water is recycled back to the plant. A ZLD system includes a range of technologies for recovery, recycling and reuse of treated wastewater. It is a system wherein all wastewater is either retained on site or reduced to solids by a method of concentration and thermal evaporation. A ZLD system typically includes one or more advanced treatment technologies such as lime soda ash softening, reverse osmosis (RO), electrodialysis and evaporation. It typically comprises three components – pretreatment (for chemical and biological processes), RO (for the membrane process) and evaporator and crystalliser (for the thermal process).
Other solutions
Other activities to improve SWC include identification of leakages and arresting of water leakages; changing the mode of operation of inclined surface settler hopper blowdown from continuous to intermittent; diversion of activated carbon filter backwash effluent to a holding pond for recycling; recycling ultra-filtration reject in the raw water treatment plant; a dry fog system at wagon tippler, crusher house and bunker floor; automation of cooling tower water blowdown by sensing conductivity of water, rainwater harvesting at the circulating water pump house and demineralisation plant roofs (use as cooling tower makeup). A drastic reduction in the use of water can also be achieved through better coal pile management using thermography, segregation and compaction.
Various digital tools are being explored by power developers to optimise water consumption. Cooling tower performance monitoring and diagnosis is one such tool. It monitors the key performance indicators of a cooling tower in real time and evaluates its capabilities with respect to ambient conditions. It predicts the optimum cooling water temperature and gives an early warning alert for performance deviation and root-cause analysis. It is useful in reducing auxiliary power and water consumption, while enhancing cooling tower performance. Another digital solution for water management at TPPs is the steam water chemistry monitoring solution, which is a digital tool that monitors and optimises the power plant chemistry performance. It provides real-time monitoring of steam or water chemistry parameters and gives accurate, optimised chemical dosing. The solution improves the equipment’s life cycle and the availability of the dosing system, while increasing the turbine efficiency.
Issues and the way ahead
In January 2022, the World Economic Forum (WEF) released its latest Global Risk Report, listing natural water crisis among the top 10 risks to humanity. Hence, it is essential to examine water use across the value chain. The government has given an impetus to an improve water use efficiency by limiting it for TPPs. However, even after six years of the introduction of water consumption norms for TPPs, there has been no progress on compliance. According to a study conducted by the Centre for Science and Environment in 2021, around 41 per cent of TPPs in India were non-compliant with the MoEFCC’s notification (for cooling tower and OTC systems).
Additionally, the majority of the plants have still not installed cooling towers and continue to flout the norms. Further, many TPPs in India are financially stressed and are unwilling to invest in reducing water consumption of their units. Moreover, there is no uniform format for reporting SWC. The lack of on-ground monitoring and inspection by regulatory authorities to check compliance with zero discharge norms is also a major concern. Although stringent water consumption norms have been put in place, their effective implementation and enforcement will be crucial for meeting the targets, going ahead.