Global energy demand is rising and water is becoming scarcer in many parts of the world. The power generation industry is often the largest industrial user of fresh water. Many countries with a growing demand for energy are also water stressed. Coal-fired power plants need a reliable supply of water, of a specified quality, that will be available over the lifetime of the plant (often more than 40 years). This means alternative water sources will become increasingly important.
In her latest report for the IEA Clean Coal Centre, Anne Carpenter examines non-fresh water sources for coal-fired power plants in China, India, South Africa and the USA. These are the top four thermal coal consuming countries and all have water-stressed regions. The potential water sources are:
- Waste water from municipal water treatment plants.
- Brackish and sea water.
- Mine water.
- Water produced from oil and gas wells (including coalbed methane wells).
- Water from deep saline aquifers.
Alternative water sources
Treated municipal waste water (MWW) (or reclaimed water) is abundant and widespread and some power plants already use it for cooling. More could use it, especially in China and India. The Chinese government now requires new thermal power plants to use nearby MWW sources. In India it is mandatory for power plants to use treated MWW if it is available within a 100 km radius. MWW is the most used alternative water supply at US thermal power plants, with around 5% of existing cooling systems using it – and more could.
Corrosion, scaling and biofouling of pipes and cooling systems associated with the use of MWW can be controlled with adequate water treatment. Both power plant operators and municipalities can benefit financially and environmentally from the reuse of MWW. The problem is a lack of data on its availability, quantity and quality, and increasing competition for it in some areas.
Sea water can provide an unlimited supply for coastal power plants and brackish ground water is also an also important resource for inland power plants. Both can be used directly (with minimal treatment) for cooling purposes, if the plant is designed for its use. However, desalination is required to supply fresh water needs. The governments of China, India and South Africa all recognise that desalination is likely to be important. China, for example, requires all new power plants in coastal regions to use sea water desalination to supply their fresh water requirements.
Integrating the power plant and desalination units has economic and environmental benefits. Most of the energy needs of a desalination plant can be met by using the low-grade heat from the power plant, which reduces energy costs. This improves the efficiency of the desalination plant and less cooling water is required in the power plant. If the desalination plant has excess capacity, the power plant can become a co-producer of power and water, instead of a water consumer. The main disadvantage is that the integrated system is harder to operate due to seasonal variability in electricity demand.
Mine water from abandoned and active mines could be an important source for nearby power plants in some regions. Its use could turn a water pollution liability into a water resource. A number of power plants currently use it for cooling purposes. Legal and fiscal incentives would encourage more use of mine water. But, there are no comprehensive inventories of mine pools and drainage available. China has set targets for the reuse of mine water and new power plants in North China have been given priority access to mine drainage and recycled water. South Africa has recognised the importance of recovering water from acid mine drainage and the reuse of mine water.
Water produced from oil and gas wells is limited. It can be difficult to collect from each well within a field, transport it, and manage the variability in flow and quality over time. However, the combination of heat, pressure and salinity in the produced water may provide opportunities for energy recovery, and help lower the cost of its treatment. Only a few power plants, mainly in Australia, are currently exploiting this source. They are firing coalbed methane (CBM) and utilising the produced water from the coalfield for cooling purposes.
The amount of produced water may increase in the future as countries develop their unconventional oil and gas resources. India and China are both supporting unconventional gas development. In the US, produced water from oil and gas wells and from CBM activities, could potentially become a significant source of water. Data on the quantity and quality of produced water is lacking.
Water from deep saline aquifers could be used as part of carbon capture and storage (CCS), depending on site-specific conditions. Substantial quantities of water may need to be extracted to facilitate storage. The volume may be enough to replace, or even exceed, the increased water requirements of carbon capture and, in some cases, may even enable a power plant to become a net producer of both water and electricity. Also, using the heat, pressure and salinity in the extracted water, where possible, could help lower water treatment costs. This water source is not yet being used by a power plant, although a few projects are planned.
The use of economically treated non-fresh water by coal power plants can reduce the burden on fresh water supplies, whilst enabling the plants to continue to deliver the energy that is needed. In some cases, with a suitable on-site water treatment plant, a coal power plant could even become a supplier of both energy and fresh water, instead of a water consumer.
The economic feasibility of using alternative water sources depends on local conditions, such as the distance to the power plant, the amount of water available, its price, and treatment costs. Treatment, such as desalination, is required to avoid problems in the power plant. But it can be energy-intensive and expensive. New treatment technologies and new materials that are tolerant of lower water quality are needed. This could encourage more use of the alternative water sources.
Edited by Jonathan Rowland.
Read the article online at: https://www.worldcoal.com/power/06072016/alternative-water-supplies-for-coal-power-plants-2016-1078/