Engineering background

Overview of operational water use at coal power plants

As shown by the schematic of a coal-fired power plant in Figure 2.1, high-temperature and high-pressure steam is produced by burning coal in a furnace and converting the chemical energy stored in the coal into thermal energy that produces steam through the boiler. The high-temperature and high-pressure steam then drives the turbine and converts the energy into mechanical shaft motive energy, which drives the electric generator and eventually produces electricity. After exiting the turbine, the steam is routed to a condenser, and the condensed water is then pumped back into the boiler, repeating the cycle. The condensation process usually uses a cooling medium (i.e. water and air) to dissipate the residual heat carried in the exiting steam.

As can be seen from Figure 2.1, various processes at a coal-fired power plant require water as an input, for example, to clean and then transfer the coal, to remove the lime-ash and to desulfurize the flue gas, among which the largest amount of water is used to cool down the steam exiting the turbines. The water-using processes in a typical coal-fired power plant can be categorized as follows:

1 Industrial water use: Raw water withdrawn from the natural environment (e.g. river and ocean) first undergoes clarification and then can be used for different industrial purposes, such as ash removal and wet desulfurization, as well as cooling of the steam that drives the turbine and other auxiliary engines (including induced air fan, forced draft fan, primary air fan and so forth). Flue gas desulfurization systems use water combined with limestone or other agents to treat the flue gas to remove sulfur and lower SO, emissions. Furthermore, coal transportation also often uses water to create slurry that is a mixture of crushed coal suspended in liquid, usually water, as a means of transporting coal.

Figure 2.1 Water and energy flows in a coal-fired power plant Source: Stone et al., 1982.

• 2 Domestic water use: Water is required to meet the living needs of power plant workers for drinking, cooking and sanitation sendees.
• 3 Boiler water use: Boiler needs to be regularly blown down to reduce the accumulating density of salt and other dissolved and suspended solids, which may cause foaming, priming and corrosion in the boiler system if not properly removed.

Two concepts, water withdrawal and water consumption, should be differentiated before proceeding further. The quantity of water withdrawn from the natural environment (e.g. rivers, lakes and oceans) is known as ’water withdrawal’. That water may be discharged back into the natural environment. Water that is withdrawn but not discharged back (because, for example, it is emitted into the atmosphere as steam) is defined as ’water consumption’ (AQUASTAT, 1998). The term ’water use' is used generally to refer to water consumption or water withdrawal. Water intensity is defined as water use (m³) per unit of electricity produced (KWh).

To understand cooling water uses, the three types of cooling systems currently being used should first be explained: open-loop and closed-loop wet cooling, and air-cooling:

Open-loop, or ‘once-through’ cooling (Figure 2.2): If the power plant is located close to a large body of water, either to the sea or a big river, openloop cooling systems can be used to run a large amount of cooling water through the condenser in a single pass and discharge the water withdrawn back into the environment a few degrees warmer. A very small amount of water will be consumed. Open-loop cooling systems withdraw the largest amount of water, but more than 99% is returned to the water source.

Figure 2.2 Water flows in open-loop cooling

Closed-loop, or recirculating cooling and cooling tower (Figure 2.3)

If the coal power plant is not built in water-abundant areas, a closed-loop cooling system can be employed that recirculates the cooling water in a closed loop where upward flowing air through water droplets cools the water. The cooling water is then condensed and can be reused. A large proportion of water withdrawn is eventually lost due to evaporation. As shown in Table 2.1. consumptive water use in a power plant with an open-loop cooling system is 70 to 80% lower than one with a closed-loop cooling system. However, open-loop cooling systems withdraw 30 to 60 times more water than closed-loop cooling systems (World Resource Institute, 2015).

Air-cooling systems, also called dry cooling (Figure 2.4)

In very arid places, air-cooling systems can be used to cool steam exiting turbines in a coal power plant. Without relying on evaporation, air-cooling

Figure 2.3 Water flows in closed-loop cooling

systems use cooling towers with a closed circuit, or high-forced draft air flow through a finned assembly. Power plants with air-cooling systems have the lowest water consumption and withdrawal intensities as they lose negligible quantities of water during the cooling process. However, water is still required for other processes, such as coal washing, desulfurization and domestic uses.

The designed water consumption intensities of different processes in a typical coal power plant with either an open-loop cooling system or a closed-loop cooling system in southern China are detailed in Table 2.1. It can be seen that cooling water makes up the largest proportion (80%) of water consumption in power plants with closed-loop cooling systems, which is followed by wet desulfurization, ash removal, boiler makeup, coal transport and domestic use, in descending order.

The choice of cooling technology also affects water use efficiencies at coal power plants by affecting their energy conversion efficiency. The conversion efficiency of current coal power plants from chemical energy stored in coal to electricity is about 30 to 40%. Open-loop cooling systems use running water and therefore have the highest cooling efficiency, while air-cooling systems have the lowest. High cooling efficiency reduces the backward pressure exiting steam exacts on the turbine and therefore improves energy conversion efficiency of the coal power plant. Therefore, power plants with open-loop cooling systems consume less water in other

Table 2.1 Designed water consumption factor of power plants with different cooling systems and capacity

Source: East China Electric Power Design Institute (2012).

Figure 2.4 Schematic of air-cooling systems

processes due to its higher efficiency. Several hundred coal power plants in China were required to report their water consumption factors to China Electricity Council (CEC, 2012, 2013). A summary of their reported values is presented in Table 2.2.

It can be seen that, for small-scale units, closed-loop cooling systems consume the most water, while air-cooling systems consume the least. Elowever, when the capacity exceeds 600 MW, air-cooling systems lose their advantage to open-loop systems in terms of water consumption. The reason is presumably because air-cooling systems are less efficient and, although they do not need water for cooling purposes, they require a larger amount of coal inputs and hence a larger amount of water used for other purposes, such as coal washing and transporting, wet desulfurization, dust control and so forth.

Reducing water use by deploying air-cooling systems comes at a price. First of all, air-cooling systems require a larger land area, and therefore the capital cost of power plants with dry cooling systems can be at around 2.5 times of that of plants with wet cooling systems (Zhai and Rubin, 2010).

Table 2.2 Average water consumption factors of units of different capacities with different cooling systems in China

Secondly, for the reasons discussed earlier, power plants with air-cooling systems face 5-10% of thermal efficiency losses (Electric Power Research Institute, 2004). Not only does this add to the cost of coal inputs, but it also implies greater greenhouse gas emissions. Consequently, the deployment of air-cooled power plants in China contributed to an additional 24.3-31.9 million tons of CO, emissions in 2012 (Zhang et al., 2014). In summary, the choice of cooling technologies should be made based on a comprehensive evaluation considering multiple factors, including local water availabilities, coal prices, power plant construction costs, climate change mitigation and so forth.

Table 2.2 also shows that water use intensities differ by boiler size. The reason is because larger units tend to adopt more advanced technologies, such as supercritical systems that have higher energy conversion efficiencies and therefore lower water intensities (Liao et al., 2017).

Other generating technologies can also affect coal power plants' water use intensity. Macknick et al. (2012) reviewed a large amount of existing literature and technical documents on water use in American power plants with different fuels and generating technologies and summarized their average operational water use intensity as shown in Figure 2.5.

It can be seen that compared to generic, subcritical and supercritical coal power plants. Integrated Gasification Combined Cycle (IGCC) technology lowers the water use intensity, while carbon capture and storage (CCS) increases water use. IGCC uses a high-pressure gasifier to mm coal into pressurized gas - synthesis gas and then use the synthesis gas as the primary energy to generate electricity. Water consumption is dramatically reduced in an IGCC power plant since the syngas is combusted in a gas turbine, and steam is not used as the primary way to convert the energy from the primary energy source (i.e. coal) to electricity. Therefore, a much lower amount of

Figure 2.5 Operational water withdrawals for fuel-based electricity-generating technologies. IGCC: Integrated Gasification Combined Cycle. CCS: carbon capture and storage

Source: Adapted from Macknick et al., 2012.

Engineering background 19 water is needed to condense the steam exiting the turbines as in a normal pulverized coal power plant.

When carbon capture and storage (CCS) is installed at a power plant, an amine-based solvent is usually used to absorb CO, from the flue gas at a direct contact cooler (Rochelle, 2009). The flue gas then needs to be washed by water to remove any residual ammonia. The CO,-rich solvent is then pumped to a stripper where the CO, is separated by heat. It should be noted that the heat is often provided by low-quality steam extracted from the steam turbine, which also incurs efficiency loss in the steam cycle. The concentrated CO, gas stream is then compressed and transported to a storage site. Water is used for cooling the direct contact cooler, washing flue gas and supporting the CO, absorber and stripper. Moreover, as discussed previously, additional water is required for the steam cycle due to the efficiency loss incurred (Zhai. Rubin and Versteeg, 2011). As a result, water intensity almost doubles when CCS is installed to capture emissions from coal power plants (Webster, Donohoo and Palmintier, 2013; Byers et al., 2016).