Among the industrial consumers of water, thermal power plants (TPPs) are the largest ones. It is estimated that TPPs account for nearly 88 per cent of industrial water consumption in the country. Moreover, 48 per cent of coal-fired power plants are in water-stressed areas. The majority of industrial water is consumed in TPPs. The consumptive water requirement for old TPPs with cooling towers is as high as 8-9 cubic metres (cum) per MWh without ash water recirculation, and 5 cum per MWh with it. Recently, TPPs have been designed with consumptive water requirement in the range of 3.5-4 cum per MWh, while the plants commissioned after January 2017 are likely to have a maximum water consumption of 2.5 cum per MWh. In TPPs, the majority of water is typically used for cooling tower make-up (47.35 per cent) followed by firefighting (9.97 per cent), ash handling (8.41 per cent), drinking water (6.54 per cent), demineralising water make-up (4.67 per cent) and other activities (23.05 per cent).
Water consumption norms and compliance so far
The Ministry of Environment, Forest and Climate Change (MoEFCC), through notifications issues in 2015 and thereafter, has introduced new emission norms for reducing water consumption. As per the norms, all TPPs with once-through cooling (OTC) are required to install cooling towers. Existing plants with cooling towers are required to achieve a maximum specific water consumption (SWC) of 3.5 cum per MWh. Also, 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. All seawater-based plants have been exempted from meeting these norms. Only freshwater-based plants with OTC have to convert to cooling towers and follow the limit of 3.5 cum per MWh.
In addition to this, as an alternative to fresh water, the government has mandated that all coal-based power plants located within a 50 km radius of a sewage treatment plant must use treated wastewater in their operations. To set up TPPs in water-scarce regions, the 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. TPPs are required to meet the water consumption norms prescribed by the MoEFCC.
Notably, Maharashtra Power Generation Company Limited (Mahagenco) was the first in India to implement a reuse project. Mahagenco has initiated three projects so far – the 130 minimal liquid discharge (MLD) reuse project for the Koradi TPP (EPC model), the 190 MLD reuse project in the Koradi and Khaperkheda TPPs (PPP model), and the 50 MLD reuse project for the Chandrapur TPP (PPP model).
Water management in cooling towers
Innovative technologies are being adopted for wastewater recycling, water flow and quality monitoring. To ensure water conservation in the cooling system, the cycles of concentration (CoC) of cooling water need to be increased, thus reducing the blowdown and make-up water quantity. Many systems operate at two to four CoC, but six cycles or more may be possible. Increasing the number of cycles from three to six reduces cooling tower make-up water by 20 per cent and cooling tower blowdown by 50 per cent.
Wastewater produced by cooling towers contains salts such as sodium sulphate, sodium chloride, calcium, magnesium and bicarbonates. A significant quantity of blowdown water is generated when cooling towers are operated at different cycles of concentration. A cooling tower blowdown treatment plant reduces corrosion in the blowdown system and discharges cleaner water into the environment. New technologies such as microfiltration are also increasingly being employed in cooling tower blowdown treatment plants to meet the stringent environmental specifications. The treated blowdown water (permeate or filtrate) is then reused as boiler feed or process water.
Zero liquid discharge (ZLD) is an advanced wastewater treatment technology used to purify and recycle all the wastewater produced in industries, including TPPs. A ZLD system includes a range of technologies for the recovery, recycling and reuse of treated wastewater. Implementation of ZLD ensures that the discharged water is recycled back to the plant. 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). Some examples of water efficient ZLD plants are CSEC’s Budge Budge power plant, located in West Bengal, and JSW Energy’s Torangallu Bellary power plant, situated in Karnataka.
Ash handling systems
Ash handling technologies help in collecting, conveying, storing and loading different types of residual ash generated by coal-based power plants. New and 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 (HCSD) ash transportation up to dykes, installation of dewatering bins, and augmentation of dry ash handling systems in old projects.
Conservation of water in wet ash handling plants is a key area for gencos deploying conventional conveying technologies. 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. After initiating water conservation measures, many TPPs have brought down their ash-water ratios to a reasonable range of 1:10 to 1:12. Another alternative is the HCSD method, which involves pumping high concentration slurry with over 60 per cent solids by weight through positive displacement pumps, as compared to lean slurry transportation at 25-30 per cent concentration.
Water requirement for FGD
Water is required in flue gas desulphurisation (FGD) plants 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 coal-based power project, the FGD plants are expected to use 110-130 kilolitres per hour of fresh water for the desulphurisation process. This translates to about 0.3 kilolitre per MWh. Further, an FGD plant is expected to discharge 20-25 kilolitres per hour as wastewater, which has to be treated to achieve ZLD. Apart from the available sources of water, FGD systems can also make use of process water, such as from cooling tower blowdown, for processes such as grinding limestone.
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 a cooling tower’s performance on a real-time basis 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 the 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 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 turbine efficiency and reducing the start-up time. It also controls corrosion and deposits inside boilers and turbines.
Challenges and the way forward
Coal power plants were supposed to meet the water norms by December 2017. However, even after six years of the introduction of water consumption norms for TPPs, there has been no progress on compliance. Freshwater-based OTC plants withdraw enormous amounts of water, and many of these plants continue to operate, with no plans to upgrade or install cooling towers. Further, there is no uniform format for reporting specific water consumption. The lack of on-ground monitoring and inspection by regulatory authorities to check compliance with zero discharge norms is also a major concern. The use of wastewater for cooling is also limited, with just 5-8 per cent of all coal-based TPPs in India having access to treated wastewater, as per the International Energy Agency. Overall, efficient water management practices at TPPs are gaining traction, and the tightened water consumption norms are promoting judicious water consumption at TPPs.