Saving water in coal fired plants

Areport from the IEA Clean Coal Centre, by Anne Carpenter, Water conservation in coal-fired power plants, CCC/275, discusses where water can be conserved or recovered within pulverised coal-fired power plants.

Water conservation in coal-fired power plants is becoming an important issue as fresh water becomes scarcer and regulations on water use more stringent. Regions where water is scarce face obvious risks, but even regions with ample resources can experience constraints due to droughts, heat waves, seasonal variations and other factors. For instance, a number of power plants in India, such as the 2600 MW Tiroda plant in Maharashtra, had to shut down temporarily in 2016 due to a lack of water. The number of power plants affected is likely to increase in the future, with serious economic consequences.

There are a large array of opportunities and technologies available to conserve water within a power plant, the report notes. The cooling system (when water is the coolant) uses the most water. More efficient designs can lower the amount of water withdrawn in an open-loop (once-through) system. In these systems, water is withdrawn from the water source, used once to condense the steam, before it is returned to the source at a slightly higher temperature. Water can be saved from a closed-loop cooling tower by reducing the amount of water lost through evaporation, drift and blowdown. Most water is lost through evaporation. This can be reduced by cooling the inlet air or the water from the steam condenser before it enters the tower. Installing state-of-the-art drift eliminators can reduce drift losses to 0.0005–0.002% of the circulating water flow.

Treating blowdown for reuse can be expensive, but it is already carried out at a number of plants. The introduction of zero liquid discharge regulations will force more power plants to treat blowdown, the report says.

Adoption of dry cooling systems, using air as the cooling medium, could save substantial amounts of water. However, air is a less efficient cooling agent and consequently, efficiency is lower compared to a similar capacity power plant with a wet cooling tower – about 2 to 7 percentage points (more on hot days), the report estimates.

Despite the drawbacks, some national governments and regions are promoting dry cooling as a way of relieving pressure on water resources. For example, the local government in some water-stressed regions in China has set compulsory requirements for new coal-fired power plants to install these systems. A considerable amount of research is being conducted to mitigate the drawbacks of dry cooling and on the development of alternative dry cooling technologies.

The second largest consumer of water in a wet-cooled coal-fired power plant is typically the wet FGD system. Cooling the flue gas before it enters the wet scrubber (for example,withregenerativeheatexchangers) can lower water consumption by some 40–50%. Although semi-dry scrubbers (spray dry scrubbers and circulating dry scrubbers) consume some 60% less water than limestone wet scrubbers, and produce no waste water, they capture less SO2. Nevertheless, semi-dry scrubbers can capture more SO3 (over 99%) and oxidised mercury than wet scrubbers. These pollutants are now starting to be regulated. On the other hand, wet scrubbers usealessexpensivesorbent(limestone)than semi-dry scrubbers, and can more easily adjust to varying boiler loads.

There are smart systems available that can minimise the steam consumption associated with sootblowing. Alternatively, compressed air or acoustic sootblowers could be installed.

Water recovered from coal or flue gas can be used for boiler feedwater make- up to reduce fresh water consumption. Recovering water vapour from the mill or pre-dryer exhausts, prior to coal combustion, or from the flue gas, after combustion, can save a significant amount of water. In some cases, enough water could possibly be recovered from the flue gas to enable a dry- cooled power plant to become a co-producer of electricity and water.

However, none of these systems (condensing heat exchangers, membranes and desiccants) have been demonstrated at full-scale on a coal-fired power plant.

Another way to save water, the report notes, is to replace the wet bottom ash handling system (such as ash sluicing) with a semi-dry (where the ash is dewatered and the collected water recycled) or completely dry system (which uses air to cool the ash).