Today, 90 percent of electricity generated in the United States comes from coal, nuclear or natural gas plants. Thermoelectric power plants boil water by burning a fossil fuel or through a nuclear reaction to create steam that spins turbines. Once steam passes through a turbine, it has to be cooled back into water before it can be reused to produce more electricity. Colder and cleaner water cools steam more effectively and allows more efficient electricity generation.
Fifty years ago, inefficient once-through cooling systems were used to take water from rivers, lakes or oceans and circulate it through pipes to absorb heat from the steam in condensers. Once used, the water was discharged back to its local source. Today, there are still a few legacy plants in the U.S. that use this process, but almost all of them are in line for renovation as part of repowering efforts.
Most power plants built in the U.S. after 1980 rely on closed-loop recirculating cooling processes. These systems use cooling towers to expose water to ambient air. Water that evaporates is replaced, but most of the water is sent back to the plant’s condenser to be used again multiple times.
A third, even more efficient cooling approach, called dry-cooling, uses air instead of water to cool steam in turbines. Most plants that use this system run on natural gas. Despite the name, dry-cooling systems still require water for system maintenance, cleaning and boiler blowdown.
How Much Water Do Power Plants Need?
Coal-fired plants, relying on once-through cooling processes, need about 30,000 gallons of water for every megawatt of electricity produced. But plants with recirculating systems can make the same megawatt of electricity with just 5 percent as much water (about 1,200 gallons per megawatt). Nuclear plants, regardless of their cooling systems, require twice as much water as coal-fired plants.
This unquenchable appetite for water, combined with the goal of reducing harmful carbon dioxide (CO₂) emissions, is driving momentum toward natural gas-powered plants that leverage dry-cooling processes. This is why almost all of the new power plants being built in China, India, Africa and other parts of the developing world are standardizing on natural gas combined-cycle (NGCC) techniques.
NGCC systems can increase a plant’s efficiency by up to 40 percent over coal-fired plants. Natural gas feed stocks are abundant, less expensive and easier to transport via pipelines. In Europe, where more than 40 percent of electricity is generated by coal, and China, which has an even higher percentage, large coal-fired plants struggle to ratchet down production, causing wasteful surpluses that often need to be exported to neighboring markets at lower prices. But natural gas-fired plants give operators greater control over feedstock inputs, which provides flexibility to meet demand.
How Natural Gas Power Plants Operate
A NGCC power plant uses a gas combustion turbine to generate electricity, and it also uses waste heat to make steam, which then generates additional electricity in a steam turbine. Because the first stage (the gas combustion turbine) has no steam to condense, it does not require cooling. As a result, the combined process requires 25 percent as much water to generate the same electricity as a coal-fired plant.
Even though natural gas plants use less water, they still need to treat water, because there is a direct correlation between the quality of the water and the efficiency of the plant. The quality and the turbidity of a plant’s incoming water can vary depending on storms or man-made interactions. A number of pretreatment activities must be performed, such as removing sludge and sedimentation, dissolving suspended organic material, adjusting pH levels and disinfecting water by killing disease-causing microorganisms.
The following applications are administered by injecting chemicals with metering pumps:
Disinfection is accomplished by dosing specific quantities of high-concentration sodium hypochlorite (bleach). When sodium hypochlorite comes in contact with bacteria, it oxidizes molecules in the cells of the germs and kills them. Even though this simple but harsh chemical has been used for more than a hundred years, it is prone to causing problems by “off-gassing.” So the pumps used to deliver it must be able to pass the gas bubbles through without locking or clogging the pump.
pH adjustment. Power plants operate best when the pH of the water is as close to neutral (7) as possible. Specific volumes of acids are administered to alkaline feedwater (pH higher than 7) to adjust the pH, while similar volumes of caustics are dosed to acidic feedwater (pH lower than 7) to raise its alkalinity.
Boiler feedwater must be treated to avoid scale and corrosion that could damage or impede the boiler’s performance. Boilers and other plant equipment are protected by metering pumps that dose precise volumes of corrosion inhibitors.