Water is a key requirement in thermal power generation for condenser cooling, removal of heat from other plant auxiliaries, cycle make-up and other consumptive uses. With increasing pressure on natural resources like water, optimisation of water consumption for thermal power plants (TPPs) is inevitable. Further, the new environment ministry regulations requiring TPPs to reduce their specific water consumption have necessitated greater attention to this area.
A look at the major water requirements for TPPs as well as the key steps that need to be adopted by operators to minimise their water footprint…
Water sources and consumption areas
For power plants located on main land, raw water is generally drawn from rivers, lakes, canals, reservoirs and barrages, whereas plants located adjacent to cities make use of treated sewage water. For power plants located in coastal areas, water for cooling of condensers and auxiliaries is drawn from the sea or creek, which meets the water requirement of wet ash handling systems also. The requirement of water for other plant uses is met from an alternative source or by installing a desalination plant.
The water requirement of TPPs is governed by a number of factors such as quality of raw water, type of condenser cooling system, quality of coal, ash utilisation, type of ash disposal system and waste water management aspects. The major water consumption areas in TPPs include cooling water (CW) make-up (in the case of wet cooling systems), ash handling system make-up (in the case of wet ash systems), power cycle make-up, coal handling plant (CHP) dust suppression, potable water use, service water use and fire water. Efforts are made during the design and operation of a plant to minimise the make-up water requirement for plants.
Water conservation measures
CW make-up accounts for over 80 per cent of water consumption in a TPP with wet cooling systems. The major component (75 per cent) in CW make-up is evaporation loss, which depends on heat energy being rejected in cooling towers. Wet cooling water systems may be of the “once-through type” or “closed-cycle type”. In a once-through system, the additional evaporation from the surface of the waterbody to dissipate the imposed heat load by the power station amounts to about 1 per cent of the circulating water flow rate. In a closed-cycle system, the use of a cooling tower results in the evaporation of 1.5-1.7 per cent of CW flow for heat removal from the circulating water.
There is not much scope for reduction in evaporation loss, except for changing the cooling system from wet to dry. This is because in direct dry cooling systems, low pressure (LP) exhaust steam is directly cooled using finned tubes and ambient air, whereas in indirect dry cooling systems, the LP exhaust steam is cooled by water, and the water, in turn, is cooled by the ambient air using finned tubes. However, a wet cooling system finds more usage because of low capital costs and high cycle efficiency. Dry cooling systems are costly technologies and are not comparable to wet cooling systems on techno-economic considerations. However, for sites where an adequate quantity of water is not available, dry cooling systems offer a possible solution for power plant installation, with much reduced water requirement.
Various measures are adopted to reduce the water consumption of ash handling systems. Using an optimum ash-water ratio (25-30 per cent ash by weight in slurry), a dry fly ash handling system and a high concentration slurry disposal system for fly ash (typically more than 60 per cent solid by weight) are a few of them.
Power cycle make-up is required to compensate the losses due to boiler blowdown among other reasons. With the use of condensate polishing units, better metallurgy of feed cycle and boiler components, this requirement can be reduced to less than 2 per cent. Some other measures for optimisation/minimisation of plant consumptive water are judicious utilisation of water in different applications, adoption of reduced margins in various consumptive uses, adequate treatment for deteriorating quality of raw water, use of plant wastewater in various low-grade applications and recycling of plant wastewater to the maximum extent.
As water resources continue to become scarce, drinking and irrigation uses will receive higher priority over industrial uses. In addition, with the issue of new norms for TPPs by the Ministry of Environment, Forest and Climate Change, water usage would need to be regulated. All these facts make it imperative for TPP owners to deploy various options for reducing plant water consumption. In this regard, the development of highly efficient and low-cost dry cooling systems could play an important role in the optimisation of water consumption. n
Based on inputs from a presentation by C.P. Tiwari, Head, BTG & Renewables, Core Technology, Tata Power Company Limited