Water is indispensable for the functioning of thermal power plants (TPPs) and is a key input for power generation. However, in recent times, the twin challenges of limited availability and conflicting demands have created immense pressure on water resources. The consequent water shortage will have repercussions in the thermal power segment, which is the largest consumer of water, accounting for 80 per cent usage among all industrial sectors.
Recently, NTPC Limited, India’s largest power producer, had to shut down its Farakka TPP in West Bengal owing to a shortage of water. A water crisis is also taking a toll on power plants in Maharashtra. While the 1,130 MW Parli and 620 MW Koradi TPPs have been shut down, the 2,340 MW Chandrapur and 1,340 MW Khaperkheda TPPs are operating at half their capacities. The crisis has also severely affected hydropower generation in the state, owing to the low water levels in dams.
This highlights the need to rationalise water consumption for TPPs. New technologies can be deployed to minimise water consumption, enable water and wastewater recycling, and make use of municipal treated sewage water. As per estimates, wastewater treatment systems alone could enable power plants to recycle 60-80 per cent of the total wastewater generated during operations. In addition, there should be a focus on driving improvements in the cooling towers (CTs) used in power plants as they account for the highest water losses due to evaporation.
In December 2015, the Ministry of Environment, Forest and Climate Change (MoEFCC) took a step in this direction and notified new water consumption limits for TPPs. As per the modified norms, all existing TPPs with once-through cooling systems need to install CTs and achieve specific water consumption of up to 4 cubic metres per MWh (m3 per MWh). Meanwhile, the existing CT-based plants will have to reduce their specific water consumption to 3.5 m3 per MWh.
The existing plants will have to comply with the water consumption limit within two years from the date of notification (December 7, 2015). However, new units, commissioned from January 1, 2017 onwards, will have to restrict their specific water consumption to 2.5 m3per MWh. In addition, these plants will be required to achieve zero liquid discharge (ZLD).
The recent tariff policy amendments also mandate TPPs within 50 km of sewage treatment facilities to use treated sewage water for their functioning.
Wastewater treatment
Recycling and reuse of wastewater can effectively reduce the water footprint of TPPs as well as the overall costs involved in water processing. The cess levied on freshwater intake is one of the factors driving power plants towards the adoption of wastewater treatment solutions.
However, the use of conventional technologies for treatment has reduced over the years. This is because these technologies require more time, have a larger water footprint, involve costs of conveyance and power consumption, and may not produce treated wastewater of the desired quality for reuse. As a result, the deployment of advanced technologies has gained momentum as they reduce the total dissolved solid (TDS) content, and remove sodium, nitrate, sulphates, heavy metals, etc. These technologies recycle all types of wastewater including brackish seawater, and enable high levels of water recovery. Further, large plant sizes can help in achieving economies of scale.
At present, one of the most widely deployed wastewater treatment technologies is membrane technology. This technology makes use of a number of filtration processes – microfiltration, ultrafiltration, nanofiltration and reverse osmosis. In this technology, membranes are used as filters or barriers separating two fluids, allowing certain substances to be transported across. This technology is more efficient and improves the effluent grade as compared to conventional wastewater treatment techniques. However, new TPPs commissioned post January 2017 will have to use ZLD solutions to meet the new MoEFCC guidelines. In the ZLD process, wastewater is purified and recycled, leaving zero discharge at the end of the treatment, thus fully optimising water resources. It is an advanced treatment method, which involves ultrafiltration, reverse osmosis, evaporation/crystallisation, and fractional electrodeionisation.
Cooling towers
As per estimates, more than 80 per cent of input water is required for make-up in the CT. This is mainly due to water evaporation in the CT while cooling the circulating water (CW). With the new norms mandating all TPPs to deploy CTs, there is a need to opt for solutions in this segment that can help optimise water intake.
CTs are essentially heat exchangers, which remove heat from the CW and transfer it to atmospheric air. During the process, some of the water evaporates and sustains the cooling process (evaporative cooling). However, evaporation during the cooling process increases the concentration of TDS. The ratio of dissolved solids in CW to make-up water (water required to compensate for evaporation due to cooling) is defined as the cycle of concentration (CoC). In order to maintain a desirable CoC, it is necessary to remove a certain amount of CW from the system on a continuous basis, which is known as blowdown. The drained water is compensated for by make-up water.
In recent times, improvements in raw water quality through the use of various water treatment technologies have helped increase the CoC of the CW system from one to five, thereby reducing the blowdown water requirements. In addition, CoCs can be significantly increased through the use of stabilising chemicals and disinfectants.
The deployment of different types of CTs – dry CTs, or a combination of wet and dry CTs – can also help in minimising water requirements.
At sites where an adequate quantity of water is not available, the use of dry cooling systems in place of wet cooling condenser systems is recommended. As per estimates, the use of dry CT technology can reduce a plant’s water requirement by up to 80 per cent. Dry cooling systems do not require any make-up water as rejection of waste heat from the condenser into the atmosphere takes place in finned tubes through the ambient air and thus involves no evaporative cooling.
There is also the option of deploying a combination of wet and dry cooling technologies. Such CT passes the working fluid through a tube bundle on which clean water is sprayed and a fan-induced draught is applied. The resulting heat transfer performance is much closer to that of a wet CT, with the dry-cooler advantage protecting the working fluid from environmental exposure.
Ash disposal
Another area for minimising water requirements is the ash handling system. Fly ash and bottom ash generated in a thermal plant are disposed of into ash ponds in the form of wet slurry. Some of the methods for reducing water requirements during ash disposal are decreasing the water-ash ratio for slurry disposal, recirculating pond water, and using high concentration slurry disposal systems for fly ash.
The way ahead
To comply with the stringent ZLD condition, upcoming TPPs will have to combine and deploy various technologies so as to optimise the existing processes as well as water consumption. In such a scenario, recycling and reuse of water will be the focal point. The deployment of ZLD systems will result in cost savings and help in raw water treatment. Moreover, as there will be no effluent discharge from the new plants, the environmental costs arising from the power generation activity will be minimised to a considerable extent.