Water is one of the key input requirements for thermal power generation. It is required for various uses like process cooling in the condenser, ash disposal, removal of heat generated in plant auxiliaries and various other plant uses like service water, potable water, etc. It is generally sourced from freshwater bodies such as rivers, lakes, canals, reservoirs and barrages for inland power stations. Coastal plants draw from the sea or creeks. Water shortages have been affecting thermal power plants (TPPs) on a regular basis.
Research estimates indicate that 14, out of the 20 largest thermal utilities in India, experienced at least one shutdown due to water shortage during 2013-16. Besides chronic availability concerns, rising water charges by states, water quality concerns and considering the future pressure from competing uses, have had direct financial consequences for the power generation industry.
A look at the water consumption requirements for power plants, recent policy initiatives for reducing consumption as well as the way forward…
Water use for cooling
A Cooling water system requires huge quantities of water in TPPs. The choice of cooling system has the greatest impact on water requirements for a given type of thermal power generation.
Two broad categories of cooling systems are available: once-through and recirculating, which is further divided into wet, dry and hybrid systems. Each involves trade-offs in terms of water use, impacts on water quality, plant efficiency and cost. In a once-through system, the consumption of water is essentially very less as compared to the cooling tower system. Once-through (or open-loop) systems withdraw fresh water (or non-fresh water), pass it through a steam condenser and return it at higher temperature to a nearby waterbody. The capital costs of once-through systems are lowest, compared with other cooling systems, but they require considerably higher water withdrawals and the large intake and subsequent discharge downstream at higher temperatures can be detrimental to aquatic life and ecosystems. As a result, such systems have been phased out and changed to closed-cycle systems since 1998.
Closed-cycle systems withdraw fresh water and pass it through a steam condenser, but instead of being discharged downstream, the heated water is cooled in wet tower or pond. Water not consumed by evaporation is returned to the steam condenser for reuse. Water withdrawals are much lower than in once-through systems, reducing exposure to risks posed by constrained water resources as well as environmental impacts. Trade-offs relative to once-through systems include higher water consumption (as opposed to withdrawal) and greater land area requirements.
Dry cooling is also where the water requirement is less compared to the wet closed-loop system. Dry cooling systems use air flow through a cooling tower to condense steam. Their water requirements are minimal, compared with other systems, and they, therefore, are better suited to dry climates. Their cost is about three to four times higher than that of wet tower or pond systems.
Management of cooling tower blowdown is one of the key components of cooling tower operation and requires significant attention due to its important economic and environmental implications. Due to fairly low water quality, blowdown is typically subjected to some treatment in order to meet the local or state discharge requirements that are governed by the final disposal options. Management or disposal options available for cooling tower blowdown depend mainly on its quality, local discharge regulations and capabilities of treatment processes under consideration.
Discharge to surface waters is one of the main options for once-through cooling systems that is not feasible for recirculating cooling systems because of the low blowdown quality. Meanwhile, discharge to waste water treatment plants (WWTPs) is probably the most cost-effective management alternative, but may not be feasible for many plants since the WWTP may not accept the blowdown without any pretreatment due to extremely high solids and the presence of other chemicals in the blowdown that were added to control corrosion, scaling and biofouling.
After the 1988 gazette notification, once-through systems were to be changed and cooling tower systems for new plants were to be adopted, and more stringent norms were introduced in 2012 with the water consumption limited to 30 cusecs for 1,000 MW capacity by the Central Electricity Authority. Subsequently, in 2015, a new gazette notification further restricted the water consumption to 25 cusecs for 1,000 MW capacity in the case of new plants and 35 cusecs for older plants.
One of the key recent policy developments for promoting water management at TPPs in the country is the tightening of the water consumption standards by the Ministry of Environment, Forest and Climate Change in December 2015. The revised norms required all existing plants to achieve specific water consumption of 3.5 m3 per MWh by December 2017. Further, plants with once-through cooling were required to install cooling towers.
Meanwhile, all plants set up after January 2017 are required to operate at a water consumption level of 2.5 m3 per MWh and achieve zero discharge. Power plant owners, therefore, need to recycle waste water and look at installing zero liquid discharge (ZLD) systems. A ZLD system involves a range of advanced waste water treatment technologies to recycle, recover and reuse the treated waste water, and thereby, ensure that there is no discharge of waste water into the environment. A typical ZLD system comprises the following components: pretreatment, reverse osmosis, and the evaporator and crystalliser. While ZLD systems have higher operating costs, investments are justified by high recovery of water (>90-95 per cent) and recovering of several by-products from the salt.
Another gazette notification in January 2016 mandated the use of treated sewage water from sewage treatment plants (STPs) if it is available within a 50 km radius of a TPP, including the existing plants. Recently, India’s largest power generator, NTPC signed an MoU with the Noida Authority in June 2018 at Noida, for the supply of 80 million litres per day (mld) treated sewage water to NTPC’s Dadri plant. Earlier, in January 2018, the Maharashtra state power genco, Mahagenco, had signed a pact with the Nagpur Municipal Corporation for the use of 150 mld of treated sewage water for power generation.
The way forward
Conservation of input water can be achieved by economising system consumption and cutting losses. For instance, to reduce wastage, leakage in plants and reuse of the waste water after treatment must be done. Further, the use of good quality water should only be restricted to stringent process requirement and low-grade water such as process reject water should be used for low-grade purposes. For instance, the cooling water system’s blowdown water can be used in ash handling or high concentration ash slurry. Further, using methods that totally eliminate the use of water, such as air-cooled condensers, dry cooling towers and dry ash handling must also be encouraged.
Also, there is a need to address the concerns related to STPs. For instance, while the policy requires all TPPs to use treated sewage water, coastal plants, where seawater is used, should be exempted from using treated sewage water. Also, the minimum capacity of an STP should be at least 30 per cent of the power station make-up water requirement in order to avoid multiple pipe connections from individual STPs to achieve economy. Further, in some cases, raw sewage water is contaminated with industrial waste and will require costly tertiary treatment involving reverse osmosis. Industrial waste needs to be segregated to reduce the cost of tertiary treatment. Lastly, routing of cross-country piping through congested municipal areas is difficult. State governments need to ensure availability of right of use /right of way.
To sum up, management of water resources is one of the biggest challenges for the industry. To the extent possible it is required that the TPPs adopt measures that are taken after economic consideration. The plants should reduce, reuse and recycle waste water and also increase the use of low-grade water. More research and development efforts and technological innovation in this area would also be fruitful.