HOW COOLING TOWER EVAPORATION DRIVES WATER INTENSITY IN COAL-FIRED POWER GENERATION

Power generation is generally acknowledged as a water-intensive activity. Eskom, South Africa’s power utility, reports a specific water consumption of some 1.3 litre/kWh sent out to the grid, with a standard consumption target of 1.4 litre/kWh sent out for the organisation as a whole. The actual amount of water consumed at individual power stations depends strongly on the cooling technology employed. To understand the crucial role of cooling as regards water use efficiency in the power generation process, refer to the simplified flow diagram below.

In simple terms, coal-fired power stations burn coal in order to heat up demineralised water and produce superheated steam. This steam is then passed through turbines which convert kinetic energy in the steam into electrical energy. The steam emerges from the turbines at a lower temperature and pressure, and is then condensed before being pumped back to the boilers to produce yet more superheated steam in what is essentially a closed cycle.

The condensing of the steam is achieved in massive heat exchangers (condensers) using water as a coolant. The cooling water removes energy from the steam as the steam is de-superheated and condensed, and increases in temperature as a result. In some countries this warm water is discharged (once-through cooling) and may cause modification of the receiving environment due to thermal pollution (this depends on the capacity of the sink used). In South Africa, the warmed cooling water is instead cooled and reused.

The cooling device employed may be “wet” or “dry”, with “dry” cooling being the most efficient technology from a water use perspective. Dry cooling technology cools the warm cooling water through a process similar to that of the radiator in your motor car. The water is passed through a large heat exchanger, and air is passed over the heat exchange surface, either through natural draught or using fans.  The air is heated and the water is cooled. Eskom estimates that dry-cooled stations use 15 times less water than wet-cooled stations. It must however be noted that this excludes the use of flue gas desulphurisation technology, which could significantly increase water use at dry-cooled stations.

At power stations employing wet cooling systems, warm cooling water is passed over packing in a cooling tower. Air is passed up the tower in counter-current flow to the water. The surface area of the packing facilitates heat and mass transfer, and a portion of the water is evaporated, humidifying the air. Since this evaporation requires energy, the main body of water is cooled in the process. Fresh water has to be constantly added to the cooling tower to compensate for evaporation and blow-down losses. Blow-down water is typically recycled to other processes e.g. ash handling.

Evaporation is the the primary driver of water consumption at wet-cooled power stations, typically accounting for more than 80% of the water used. The conversion of existing wet-cooled power stations to a dry cooling system is technically possible, but would incur significant capital costs and introduce the potential for serious operational disruptions. It is therfore not really feasible. Dry cooling is however a real option for new power stations and is in fact already in place at a few South African power stations, such as Kendal, Matimba and Majuba Power Stations. Eskom has committed to the use of dry cooling for all new coal-fired power stations.

There is an interesting relationship between water use and energy efficiency at wet-cooled power stations. The energy contained in coal is expressed as its calorific value, which is in effect the amount of heat energy produced when a given mass of coal is completely burnt with oxygen. The efficiency with which this energy is converted into electrical energy depends on two primary factors:

• the efficiency with which the boilers convert the energy in the coal into energy in steam - given that in power generation coal is typically pulverised, and boiler controls are very sophisticated, efficiencies in excess of 90% are achievable;
• the efficiency with which the energy in this steam is converted into rotational energy at the turbines, and then to electrical energy  - this is limited by the thermodynamics involved and depends in part on the steam temperature and pressure. Values in the neighbourhood of 40% are typical, with higher efficiencies possible with supercritical boilers. 

Both boiler and turbine efficiency levels are important in terms of GHG emissions and power station operating costs. It is however turbine efficiency which is the ultimate driver of evaporative losses at wet-cooled power stations. The more efficiently the turbines operate, the less heat has to be removed by the condensers, and hence, for wet-cooled power stations, the lower the level of evaporation at the cooling towers.

Copyright 2011-2013, Craig van Wyk, all rights reserved

All information regarding Eskom from www.eskom.co.za