Water is one of the key inputs for thermal power generation. It is required for process cooling in the condenser, ash disposal, the removal of heat generated in plant auxiliaries, and various other plant consumptive uses. For power plants located on mainland, raw water is generally drawn from fresh water sources such as river, lake, canal, reservoir, and barrage. Treated sewage water may also be used as a source of raw water for power plants located adjacent to cities. For power plants located in coastal areas, water for cooling the condenser and auxiliaries is drawn from the sea or a creek, which also provides water for the wet ash handling system. The requirement of water for other plant consumptive uses is met from an alternative source or by installing a desalination plant. The thermal power generation industry is one of the highest users of water. Therefore, a complete water management plan with a comprehensive site-wise strategy is required for maximising efficiency, minimising waste water discharge, encouraging sustainability, water conservation, and spreading environmental awareness. In order to reduce water consumption, utilities such as NTPC Limited and the Damodar Valley Corporation (DVC) have taken a number of steps at their thermal power plants (TPPs).
Powerline looks at the measures taken by the utilities to minimise their water footprint at TPPs…
NTPC’s strategy
Thermal power major NTPC is committed to reducing its water footprint. The company is minimising its fresh water usage and making its processes water efficient through the implementation of advanced technologies and process reengineering. Efforts are on to transform all its power stations into zero liquid discharge (ZLD) plants and emphasis is on recycling waste water. All its future stations are envisaged to have air cooled condensers, which have a potential to save 75 per cent of water when compared to conventional cooling systems. Further, cycles of concentration (CoC) are being increased at all stations of NTPC for reducing fresh water intake.
During 2018-19, specific water consumption was reduced by 1.3 per cent (3.02 l/kWh) when compared to the financial year 2018. Implementation of the ZLD scheme has been completed at ten stations. For the first time, NTPC has adopted an air cooled condenser system at the North Karanpura STPP and the Patratu STPP. This will bring a significant reduction in make-up water requirement for the projects.
Further, the company is taking initiatives to use the treated sewage water from municipal sewage treatment plants (STP) nearby to fulfil bulk water requirement in its power plants. Recently, for the Dadri STPP, the company signed an MoU with the NOIDA authority for utilising 80 MLD treated sewage water from Noida STPs. The contract for installing secondary and tertiary treatment plants for sewage water by the Solapur Municipal Corporation is underway to provide 52 MLD of treated sewage to the Solapur Thermal Power Station. Agreements with the Nagpur Municipal Corporation for the MoudaThermal Power Station and the Ramagundam Municipal Corporation for the Ramagundam Power Station are under discussion.
Another key initiative taken by the company is to minimise water consumption by adopting dry bottom ash handling instead of conventional wet bottom ash handling for upcoming coal-based thermal power plants at Patratu, Singrauli III and Lara II. Dry bottom ash handling facilitates the extraction of bottom ash in dry form and practically eliminates water requirement. Only a meagre quantity of water will be required for conditioning and dust suppression. The system not only reduces water consumption but also results in the reduction in power consumption for bottom ash disposal and facilitates the separation of bottom ash and fly ash. This will result in better utilisation of fly ash.
NTPC is implementing advanced waste water treatment facilities such as STP, liquid waste treatment plants (LWTP), coal slurry settlement pit (CSSP), ash water recycling systems (AWRS) and closed cycle condenser cooling water systems with higher CoC across all its stations. Other measures include rain water harvesting, the reuse of treated effluent in ash slurry disposal, and the reuse of treated sewage effluent for horticulture purposes. For effectively monitoring water use, flow meters with integrators are installed at all designated locations in all stations.
The company is also planning to install floating solar panels at its reservoirs; it is a step towards saving land, and conserving water by reducing surface evaporation. The company is carrying out water audit at stations.
Initiatives by DVC
The specific water consumption at DVC was 3.63 cubic metres/hour per MW during 2018-19, as compared to 3.32 during 2017-18 and 3.45 during 2016-17.
DVC has deployed a number of water conservation measures such as the concepts of three Rs – reduce, reuse and recycle. SCADA-based water monitoring systems are being used to monitor real-time water consumption. Further, dry fly ash utilisation is being increased to reduce water consumption. Dry fly ash utilisation increased from 2.22 mt during 2016-17 to 3.31 mt during 2018-19, resulting in a reduction in ash water make-up. Also, cooling tower blowdown is being used as service water/ash water, and cooling water (CW) treatment is being done by online chemical treatment.
The utility is focusing on optimising the CW system make-up water. For every kWh of energy generated, around 1.5 kWh of waste energy is released into the environment. Cooling water is required for condensing of steam in a surface condenser and for secondary cooling in heat exchangers of the cooling system for plant auxiliaries. For a typical 500 MW coal-fired unit, the amount of cooling water required for the condenser and auxiliary cooling is of the order of 58,000 cubic metres/hour with temperature rise across the condenser being about 10.3 degree celsius. In a closed cycle system, make-up water is added in the CW system to compensate for the loss of water on account of evaporation, drift and blowdown to maintain a desired level of dissolved solids in the circulating water.
Another key initiative has been to optimise power cycle make-up. This is required to compensate for the loss of water due to boiler blowdown and other losses from the system. Currently, all 500 MW plants, including the 210 MW and 250 MW units, are running with less than 1 per cent make-up. With the use of condensate polishing units, better metallurgy of feed cycle and efficient boiler components in 500 MW units, make-up requirements have further reduced.
Ultra filtration systems in demineralisation plants have been installed in 500 MW units. As a result, the problem of ingress of colloidal silica in the boiler water has been reduced and this has minimised the boiler blowdown to a great extent. Also, the CHP dust suppression system is working in almost a closed cycle mode with lesser make-up water. Drift eliminator blades have been installed in the natural-draft cooling tower of the Bokaro TPS to reduce drift losses.
In addition, DVC has reduced the potable water and service water consumption by 3-5 per cent by periodic checking and the rectification of float valves in different overhead tanks. The utility has also conserved 575 cubic metres/hour of water by utilising surface drainage water in its ash handling systems.
Conclusion
In the water-stressed scenario and going by the new norms for specific water consumption, water conservation enables utilities to ensure compliance.