The water situation in India is progressively becoming alarming with the demand-supply gap increasing and the quality of water degrading. According to the Central Water Commission, the total water demand will overshoot supply by 2050. The World Resource Institute estimated that 54 per cent of the areas in India face extreme water stress.
Power generation is a water-intensive industry. Thermal power plants (TPPs) are reported to account for over 87 per cent of the total industrial water consumption in the country. The industry has been making efforts to reduce its water consumption, minimise freshwater usage, and improve the water efficiency of processes. Advanced technology solutions such as air-cooled condensers and desalination plants are also being deployed to conserve water.
A look at some of the best O&M practices and technologies being adopted to reduce the water footprint…
Cooling water blowdown
A cooling tower is an important component of a TPP cycle. It brings down the temperature of condenser cooling water so that it can be used in a closed cycle. When water passes through a cooling tower, it is lost in evaporation, which increases the salt concentration. To prevent this gradual escalation of salt concentration, some water is taken out continuously and fresh water is introduced in the form of make-up. Typically, the blowdown water is taken out from the cold side of the cooling water system to maintain the desired cycle of concentration. If the water is taken from the hot side of the water cycle such as the condenser outlet, the evaporation loss and the make-up water consumption can be reduced. Equivalent fresh water can be reserved using this practice.
Adoption of ZLD concept
In most subcritical power plants, the excess domestic/industrial wastewater is discharged into drains after treatment in a sewage treatment plant or an effluent treatment plant. It eventually gets discharged into rivers/waterbodies. The adoption of zero liquid discharge (ZLD) is a practice wherein the entire industrial and domestic wastewater can be reused after treatment/recycling without discharging a drop of water outside the project boundary in waterbodies.
Changes in the original design of systems, such as the installation of cooling towers in the cooling water system, modifications in wastewater disposal systems and use of an independent storm water drain, can help achieve ZLD and comply with the new regulations of water use by the Ministry of Environment, Forest and Climate Change (MoEFCC). Further, the TPP operators are targeting the segregation of plant effluents, process waste, oil-sleek, coal-dust-laden water from catchment rainwater, and their treatment and recycling for the graded use of various locations of power plants. The wastewater from flue gas desulphurisation (FGD) systems, which contains high total dissolved solids, high chlorides and heavy metals, is planned to be used in the ash water system to conserve water and achieve ZLD.
Also, by making use of sewage water from the municipality, a large amount of fresh water can be saved. NTPC Limited, the country’s largest power generator, is currently making attempts in this endeavour for its plants at Dadri, Solapur, Mouda, Meja, etc.
Dry bottom ash handling systems and HCSD
Ash is a major by-product of coal-based TPPs and must be safely disposed of from plant premises. According to an assessment by NITI Aayog, power plants use around 40 per cent of fresh water in ash handling systems.
The traditional method of ash disposal is to make a slurry by mixing ash with water and pumping the same to ash ponds through long distance pipes. This method requires a considerable amount of water. One method to reduce water consumption is to adopt dry ash handling systems, in which ash is extracted and disposed of in dry form using pneumatic vacuum systems. Dry bottom ash handling systems are also being used. At the Patratu and Dadri power stations of NTPC, such a system has brought down the requirement of bottom ash handling water by up to 700 cubic metres per hour and 350 cubic metres per hour on an average, respectively.
Another ash disposal technology that is being adopted is high concentration slurry disposal (HCSD), which is a modern method of disposing of fly ash from TPPs into ash ponds. In this method, ash slurry is produced at a concentration of 60-75 per cent ash by weight, and pumped through slurry pumps into the disposal area. The water requirement in HCSD is about one-tenth of low concentration slurry disposal.
To ensure water conservation in the cooling system, the cycle of concentration (CoC) of cooling water needs to be increased. CoC is the ratio of dissolved solids in circulating water and make-up water. The increase in CoC decreases the requirement for make-up water. Many systems operate at two to four cycles of concentration, while 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. The actual number of CoCs the cooling tower system can handle depends on the make-up water quality and cooling tower water treatment regimen. Typical treatment programmes include corrosion and scaling inhibitors along with biological fouling inhibitors.
The desalination of seawater and brackish water is another feasible and attractive option for water conservation. This technology is particularly used in water-stressed regions like Gujarat and Rajasthan. Further, the use of an online water purification system fitted in the steam cycle can ensure quick start-up and unit operation during condenser leakages. However, these units are required to be regenerated using chemicals, such as acids and alkali, when exhausted. These exhaustion and regeneration cycles happen after treating the condensate, which is called output between regeneration. By increasing the output before regeneration, the number of regenerations also reduces, thereby saving water. Also, power plants can conduct water audits, which help them in understanding the specific water consumption and degree of water use efficiency in their operations. Another new emerging technology for conserving water is floating solar PV plants. Although the primary importance of such a system is energy generation without using land areas, it can serve as a major means of water conservation. The extent of saving in water evaporation is primarily dependent on the surface area of the floating platform.
According to a Council on Energy, Environment and Water study, the consumptive water requirement for old TPPs with cooling towers is as high as around 8-9 cubic metres per hour per MW without ash water recirculation and 5 cubic metres per hour per MW with ash water recirculation. Recently, TPPs have been designed with consumptive water requirement in the range of 3.5-4 cubic metres per hour per MW.
As per the MoEFCC norms issued in 2015, power plants installed after January 1, 2017 will have to achieve a specific water consumption of 2.5 cubic metres per MWh, while existing TPPs are required to limit their specific water consumption to 3.5 cubic metres per hour per MW by December 2017. Once implemented, these standards have the potential to save water equivalent to a per capita water requirement of about 8 per cent of the country’s population as per research estimates.
To this end, there is a need for immediate action to conserve water by maximising the water use efficiency of power plants and widely adopting technology measures like dry cooling, closed loop cooling and ZLD.