Coal-based thermal power plants (TPPs) in India consume a significant amount of water, accounting for approximately 70 per cent of the overall water used for industrial purposes. Moreover, a recent study has suggested that about 30 per cent of India’s coal-based power plants are located in regions with negative trends of freshwater availability. The heavy water consumption by TPPs can lead to significant water stress, especially when multiple power plants are clustered together. As the electricity demand continues to rise, the extensive water withdrawal for generating power will not only impact the watersheds all over India but also lead to disruptions in the power plant’s operations during low-demand periods. Water plays a crucial role in various plant processes such as cooling towers and ash handling systems, prompting power plant operators to recognise the importance of reducing water consumption through improved water efficiency practices and technological advancements. Apart from this, due to the notification issued by the Ministry of Environment and Forest (2014), the TPPs, far from coal mines, must use coal with ash content below 34 per cent. This creates the need to wash the coal before transport, further increasing water consumption.
In December 2015, the Ministry of Environment, Forest and Climate Change introduced a notification that established limits on the amount of water a TPP can use per unit of electricity generated, known as “specific water consumption”. According to the 2015 norms (revised again in 2018), plants installed before January 1, 2017 were required to meet a specific water consumption limit of 3.5 cubic metres per MWh, whereas plants installed after January 1, 2017 had to meet the norm of 3 cubic metres of water per MWh, besides adopting zero liquid discharge. Additionally, all freshwater-based once-through cooling plants still operating in the country were required to install cooling towers, and subsequently achieve a norm of 3.5 cubic metres of water per MWh. All seawater-based plants were exempted from meeting the norms. All power plants were required to comply with these norms within two years of the notification, that is, by December 2017. The deadline to meet the water use norms has passed. There is, thus, a scope and an urgent need for action to reduce the sector’s water demand by ensuring implementation of the standards. Moreover, water management and optimising resource consumption have emerged as key focal points in the thermal power generation segment.
Water management technologies and solutions
TPPs have various water requirements, including coal handling, ash handling, steam production, condensation, maintenance of green areas, firefighting and domestic purposes. The most significant water demand arises from cooling processes, which typically account for 80-90 per cent of the total water requirement, particularly in coal-based TPPs. Consequently, enhancing water use efficiency in power plants heavily relies on improving cooling technologies.
According to industry estimates, in India, older coal-fired power plants utilising open loop cooling systems consume around 80-160 cubic metres of water per MWh of electricity generated. Meanwhile, closed loop cooling plants use 2.8-3.4 cubic metres of water per MWh, which is double the global average of 1.2-1.5 cubic metres of water per MWh. The implementation of dry cooling technology could potentially reduce water use intensity significantly to 0.45-0.65 cubic metres of water per MWh, but it comes with a higher cost. While the OTC system is cost-effective and has low water consumption, it involves high withdrawal rates and causes thermal pollution in the receiving waterbody. In order to optimise water use in a power plant, the use of dry cooling systems is gaining prominence. Dry cooling systems can be either direct, where the steam is condensed in an air-cooled condenser, or indirect, where cooling water is used to condense the turbine steam in a conventional surface condenser or a contact condenser. However, dry cooling technology reduces power production by approximately 7-8 per cent, and also entails higher costs.
One of the key emerging technologies is the air-cooled condenser (ACC), which is a dry cooling system that condenses steam inside air-cooled finned tubes. Power plants equipped with ACCs do not require a large volume of cooling water, making them suitable for water-scarce regions. While ACCs are expensive, they reduce the water intensity to 0.45-0.65 cubic metres of water per MWh. Notably, in March 2023, NTPC Limited successfully commissioned India’s first ACC at the 660 MW unit of the North Karanpura super TPP in Jharkhand. The North Karanpura super TPP is equipped with a direct ACC, A-frame configuration with single-row, flat-finned tubes and mechanical ventilation. This project is expected to save around 30.5 million cubic metres of water annually. Following this, NTPC plans to focus on the installation of a second ACC at its under-construction Patratu super TPP. One of the challenges in the adoption of ACCs is the use of many imported components, which hampers cost competitiveness.
One key aspect of water conservation in cooling systems involves increasing the cycles of concentration (CoCs) of cooling water. By elevating the CoCs, the quantity of blowdown and make-up water required can be reduced. While many systems currently operate at two to four CoCs, it is possible to achieve six cycles or more. Transitioning from three to six cycles, for instance, can lead to a 20 per cent decrease in cooling tower make-up water, and a 50 per cent reduction in cooling tower blowdown. The blowdown water generated by cooling towers contains various salts such as sodium sulphate, sodium chloride, calcium, magnesium and bicarbonates. To mitigate this, a cooling tower blowdown treatment plant is employed, which helps minimise corrosion within the blowdown system, and releases cleaner water into the environment.
The TPPs are increasingly becoming zero liquid discharge (ZLD), wherein an advanced wastewater treatment technology is being deployed to purify and recycle all the wastewater generated from TPPs in order to make it suitable for reuse. A ZLD system encompasses a variety of technologies aimed at recovering, recycling and reusing treated wastewater. By implementing ZLD, the discharged water is effectively recycled back into the plant. This system ensures that all wastewater is either retained on-site or reduced to solids through concentration and thermal evaporation methods. 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.
The way forward
Promoting the efficient and optimal utilisation of water resources by power plants is not only environmentally beneficial, but also advantageous for the sustainable operation of TPPs. That said, by prioritising water conservation practices, TPPs can address the pressing water challenges faced by communities, while ensuring their own long-term viability.