Water is used in thermal power plants (TPPs) mainly for cooling purposes, ash handling, coal dust suppression, removal of plant heat from plant auxiliaries and cycle makeup. Notably, the thermal power segment accounts for the highest share of freshwater use in the industrial sector. 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 wastewater management. Further, in photovoltaic (PV) power plants, water is primarily needed for cleaning the modules. The estimated water requirement is 3,000 litres per MW for one cleaning cycle. The cleaning frequency is generally two times in a month, thus, requiring 6,000 litres of water per month for a 1 MW plant. Water usage varies based on factors such as soiling conditions, PV module size, frequency of cleaning of PV modules and cleaning method.
Considering environmental factors, such high water consumption is a major concern. Therefore, there is a need for adopting innovative technologies to reduce water consumption, particularly in areas facing water shortage.
Water consumption norms
The Ministry of Environment, Forest and Climate Change, through its notification issued in December 2015, defined the specific water consumption (SWC) standards for TPPs. As per the norms, all freshwater-based once-through cooling plants are required to install cooling towers and achieve a maximum 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 water discharge. These norms are, however, not applicable to TPPs that use seawater.
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.
New and emerging technologies
Air-cooled condenser (ACC): An ACC is a direct dry cooling system where steam is condensed inside air-cooled finned tubes. Power plants are equipped with ACCs, do not require a large volume of cooling water and can easily be built in water-scarce regions. Around 63 per cent of the water is saved in an ACC as compared to a conventional water-cooled condenser.
In March 2023, NTPC commissioned India’s first ACC at the 660 MW unit of the North Karanpura super TPP in Jharkhand. This project would result in water saving of around 30.5 mcm annually. After this, NTPC will focus on completing its second ACC installation at the Patratu super TPP, which is under construction in Jharkhand. The uptake of ACCs has been limited so far as most of the integral ACC components would typically need to be imported, which will hinder cost competitiveness.
Dry bottom ash handling systems: The traditional method of bottom ash disposal is to make a slurry of ash by mixing the water and pumping the same to distant ash ponds through long distance pipes. The process requires a considerable amount of water and is known as a conventional lean concentration slurry disposal (LCSD) system. One of the key technologies for optimising water usage in ash handling is dry bottom ash handling systems. In these systems, the bottom ash is air-cooled as it is being removed from the boiler and transported using a pneumatic vacuum system, eliminating the need for water, resulting in significant water saving. These systems are currently not manufactured in the country. NTPC has adopted a dry ash extraction system to make dry ash available to other end-users. Dry bottom ash handling systems are being installed in units at Patratu and North Karanpura.
High concentration slurry disposal (HCSD): One of the emerging technologies for optimising the use of water in ash handling is HCSD for ash transportation up to dykes. In HCSD, ash slurry is produced at a concentration of 60-75 per cent of ash by weight and pumped through piston diaphragm slurry pumps to the disposal area. In HCSD systems, the water requirement is one-tenth as compared to LCSD systems. HCSD systems require less land for the disposal of fly ash, and have a lower specific energy consumption. However, HCSD systems have not been fully indigenised with major components like slurry pumps being imported. Notably, NTPC is implementing HCSD systems at the Patratu super TPP.
FGD waste water management: 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, gypsum dewatering systems, and 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. Controlling coal quality and checking for impurities and heavy metals is a very important step to ensure that fewer impurities enter the wastewater. Various technologies are being deployed to treat effluent generated from FGD processes, including forward osmosis (FO) and metal ion precipitation. FO treats fouling waste streams with high salinity and suspended solids by separating fluids and solids via osmotic separation, while the metal ion precipitation process removes heavy metals and other pollutants from the water.
Digital tools: Various digital tools are being explored by power developers to optimise water consumption. Cooling tower performance monitoring and diagnosis is one such tool. It predicts the optimum cooling water temperature and gives early warning alerts for performance deviation and root-cause analysis. In November 2022, Vedanta Aluminium deployed internet of things (IoT)-based technology for cooling water and overall reduction in water consumption at its TPPs at Jharsuguda, Odisha. IoT technology helps analyse real-time operational data and recommends optimum water quality parameters to improve water conservation. Another digital solution for water management at TPPs is the steam water chemistry monitoring solution. It provides real-time monitoring of steam or water chemistry parameters and ensures accurate, optimised chemical dosing. The solution improves the equipment’s life cycle and the availability of the dosing system, while increasing turbine efficiency.
Other solutions: Other activities to improve SWC include identifying and arresting water leakages; changing the mode of operation of inclined surface settler hopper blowdown from continuous to intermittent; diverting activated carbon filter backwash effluent to a holding pond for recycling; installing a dry fog system at the wagon tippler, crusher house and bunker floor; and implementing rainwater harvesting at the circulating water pump house. A significant reduction in water usage can also be achieved through better coal pile management using thermography, segregation and compaction.
Notably, NTPC’s research unit, NETRA, is working on the development of innovative water technologies such as non-thermal FO, electrodialysis reversal, and desalination for processing of sewage and seawater.
In view of the significant increase in solar PV capacity, it is important to promote innovative methods such as dry cleaning and robotic cleaning of panels and modules to reduce water usage. For instance, the Dadri solar PV plant of NTPC utilises 64 robots for dry cleaning of 4 MWp solar panels, thereby saving 4,000 kilolitres of water annually. Floating solar PV is an emerging renewable energy technology that involves installing solar PV panels on top of a floating platform. While the primary benefit of these systems is energy generation without using land areas, they can also play a major role in water conservation by reducing the natural evaporation rate. NTPC has installed floating solar plants in the reservoirs of NTPC power stations at Kayamkulam, Simhadri and Ramagundam.
Issues and the way ahead
One of the key issues in water management is the lack of progress on compliance, even six years after the introduction of water consumption norms for TPPs. Many TPPs in India are financially stressed and are unwilling to invest in reducing the water consumption of their units. Furthermore, the majority of plants have yet to install cooling towers and continue to disregard the norms. The lack of a uniform format for reporting SWC, and the lack of on-ground monitoring and inspection by regulatory authorities to check compliance with zero discharge norms is another major concern.
Water is becoming an increasingly scarce resource, and NTPC has implemented various water conservation measures to reduce its SWC to 3 litres per kWh. It has also developed a roadmap to achieve an SWC of 2.5 litres per kWh by 2032. Similar efforts to adopt low-water-use technologies by other generation companies, and the effective implementation and enforcement of water consumption norms, will be crucial going forward.