Sustainable Practices: Reducing the water footprint of TPPs

Reducing the water footprint of TPPs

Water plays a crucial role in ensuring smooth operations of a thermal power plant (TPP). It is typically used at a plant for cooling tower make-up, ash disposal, demineralising (DM) water make-up, etc. In the recent past, the availability of water has been a key concern for power developers. Competing uses, limited supplies as well as shortages in various parts of the country have resulted in a shortage of water for power generation. The shut-down of TPPs owing to water shortage has been increasing year on year. Around 21 thermal power units have had to shut down during 2016-17 as against 15 in 2015-16. Taking cognisance of the scenario, it has become imperative to undertake water management measures to minimise consumption through judicious use and reuse of wastewater wherever possible.

Promoting water management

One of the key policy measures for promoting water management at TPPs was the tightening of water consumption standards by the Ministry of Environment, Forest and Climate Change in December 2015. As per the revised norms, the existing plants are required to achieve a specific water consumption of 3.5 cubic metres per MWh by December 2017. Further, plants deploying the once-through cooling mechanism are required to install a cooling tower. Meanwhile, all plants being set up after January 2017 need to operate at a water consumption level of 2.5 cubic metres per MWh and achieve zero discharge.

Another policy push for water management at TPPs was the mandate under the amended tariff policy of 2016 to use treated sewage water at plants located within a radius of 50 km from a sewage treatment plant (STP). Significant progress has been made on this front. The government has completed the mapping of power stations that are within a 50 km radius of STPs across the country. Besides, thermal power major NTPC Limited has identified five coal-based plants at Solapur, Mouda, Meja, Dadri and the upcoming Patratu TPP for using treated sewage water to meet the make-up water requirement. The company is in talks with the respective municipalities for the same. Apart from this, Maharashtra State Power Generation Company Limited has entered into a long-term contract with the Nagpur Municipal Corporation for the supply of treated sewage water at the Koradi TPP.

Key drivers for water management

One unit of electricity generated from a coal-based plant requires around three litres of water. At the current level of power generation of around 950 BUs, nearly 2,800 million cubic metres per annum (around 7,700 million litres per day [mld]) of water is required. With the country’s power demand growing at a rate of 6 per cent per annum, the coal-based installed capacity is estimated to reach 277 GW by end-2027. This is expected to increase the water requirement of thermal plants to about 12,000 mld. Unfortunately, with the existing TPPs facing operational hurdles owing to water shortage, water availability is likely to be one of the biggest challenges for new TPPs.

Another growth driver for undertaking water management at TPPs is the increase in water charges by the state governments. In order to meet the growing water demand, the state governments have resorted to increasing water charges. In the past six to seven years, the increase in water charges across states varied from 94 per cent to 718 per cent. States such as Odisha, Chhattisgarh and Madhya Pradesh have increased water charges significantly.

Cooling system

The most important use of water in a TPP is as a coolant in a cooling system. Around 85 per cent heat is discharged through cooling water in a condenser.

Typically, three types of cooling systems are used in TPPs. These are the once-through cooling, evaporative cooling and dry cooling systems. Each of these systems has its own merits and demerits. The once-through cooling system involves the direct method of cooling, wherein water is withdrawn from the source (river or ocean) and returned to the same source after heat rejection. It provides high plant efficiency, but requires large quantities of water and has an adverse impact on the environment. The evaporative cooling system is based on the indirect closed loop method of cooling and results in the second best plant efficiency. However, it also entails higher water consumption to make up for the loss due to evaporation, drift and blowdown. Besides, the performance of evaporative cooling is affected by ambient temperature and humidity. In the dry cooling system, power cycle waste heat is released from the condenser into the atmosphere by air cooling. Low water requirement is the key benefit of this system. The water requirement in dry cooling towers is one-tenth of that in wet cooling towers. However, it increases the cost of power generation, resulting in a tariff increase of 8-9 per cent. This type of cooling system is mostly deployed in water-scarce areas.

An important step for ensuring water conservation in the cooling system is increasing the cycle of concentration (CoC) of cooling water. The CoC is the ratio of dissolved solids in the circulating water to that in the make-up water. Some amount of water is taken out of the cooling system as blowdown to reduce the salt concentration and fresh make-up water is added to take care of evaporation, drift and blowdown water loss. The higher the CoC, the lower is the make-up water and blowdown water requirement.

Wastewater treatment

Wastewater treatment is an important component of water management in a power plant. In order to operate a zero liquid discharge power plant, it is essential to monitor effluent streams, conduct regular water audits, undertake process change to minimise effluent generation at the existing treatment plants, and reuse wastewater wherever required.

An effluent treatment plant is crucial for wastewater management in a TPP. It is designed for treating the cooling tower blowdown and the neutralised regeneration wastes from denim and condensate polishing units, making it reusable. The effluent treatment process broadly entails precipitating hardness and silica in reactor clarifiers with chemicals such as lime, soda ash or dolomite. It also involves the deployment of a high recovery plant using ultrafiltration and reverse osmosis (RO) membranes to achieve over 90 per cent reusable water for cooling tower make-up and evaporator or crystalliser followed by centrifuge for separating solids in the RO brine. Apart from setting up an effluent treatment plant, it is important to undertake process changes at the TPP to reuse waste streams with minimal or no treatment. This not only optimises the effluent treatment plant capacity but also  the capital and operating costs.

Ash handling system

Ash handling is another water-intensive process in TPPs. Transporting ash to an ash pond for storage for the handling of unutilised ash requires large quantities of water. Slurry, which is a mixture of ash and water, is transportable at varying levels of concentration. Broadly, lean concentration (around 10 per cent), medium concentration (25-30 per cent) and high concentration (50-55 per cent) slurry conveying are considered feasible. However, it has been found that pumps that are capable of transporting slurry of medium and high concentration are often made to operate on low concentration slurry. This in turn increases the water requirement in ash management at the plant.

In order to minimise the water requirement for ash handling, TPP developers are re-evaluating wet ash handling practices. Dry bottom ash handling techniques, optimisation of ash-water ratios and use of treated water for ash handling are some of the solutions being considered. To minimise the water requirement in ash management, in the medium term, all lean concentration slurry should be converted to medium concentration and recycling of ash pond water must be undertaken. This is expected to reduce water consumption in ash handling to 750 million cubic metres per annum. Meanwhile, in the long run, redesigned centrifugal pumps should be used for transporting 45 per cent concentration fly ash slurry for distances up to about 3 km, while positive displacement pumps should be used for transporting 50-55 per cent concentration fly ash slurry for distances between 3 km and 8 km. These measures are likely to reduce water consumption to 350 million cubic metres per year. Besides, waterless management of ash is possible through 100 per cent ash utilitisation and by storing dry ash in silos.

In sum, given the stringent water consumption norms and growing water shortages, developing more sustainable practices and adopting new water conservation technologies at TPPs has become critical. Although there are several water conservation technologies available, it is important to select the one best suited for the plant, considering parameters such as water source and the age of the plant. Undertaking proper water management at TPPs is essential to ensure the vitality of thermal power generation in the coming years, which continues to be the chief source of power generation in the country.