by Steve Rosenberg, The Dow Chemical Co.
While industrialization and population growth create a greater need for electricity, power-generating technologies increase fuel efficiency and flexibility and reduce air and water emissions. Water must be optimized in power generation to enhance operation efficiencies and minimize environmental impact.
This water vs. energy nexus is a major driver of innovation in the power industry. Recent advances include plant design improvements, upgrades to water management systems and innovations to supporting purification technology.
China and the U.S. represent the two extremes of power plant designs. China is the largest producer and consumer of coal in the world and accounts for nearly half of the world’s coal consumption, according to the Energy Information Administration (EIA). It is reasonable for China to invest predominantly in coal-fired plants. Electricity demand growth has caused China to also invest in nuclear power. The U.S., on the other hand, with more moderate electricity demand growth is investing primarily in natural gas to reduce its coal dependence while still reticent to adopt a notable nuclear power plant investment strategy.
Coal has been the largest source of electricity generation for more than 60 years, but its annual share of generation declined from 49 percent in 2007 to 42 percent in 2011. Some power producers switched to lower-priced and more environmentally friendly natural gas that emits about half as much carbon dioxide as coal, according to EIA data.
The Environmental Protection Agency (EPA) in 2012 proposed the first Carbon Pollution Standard for New Power Plants. It released these proposed standards on Sept, 20, 2013, setting national limits on the amount of carbon pollution new fosil-fueled electricity generating plants can emit. The proposed standard limits new coal-fired power plants’ CO2 emissions to 1,100 pounds for each megawatt hour of power produced. This standard which can’t be met without carbon-capture technology, is for only new plants. The EPA will likely announce standards for existing coal plants in about 18 months. The standards are designed to create a cleaner, safer and more modern power sector, according to the EPA.
There are four generally recognized ways to reduce carbon emission via the choice of fuel source:
1) More efficient boiler designs that use less coal per unit of electricity;
2) Use of natural gas, which has half the CO2 emissions of coal;
3) Use of biomass fuels, which are treated as having zero emissions because they are renewable; and
4) Use of alternative technologies with low to zero emissions, such as hydroelectric, geothermal, solar, wind and nuclear power.
Industry Design Trends for Fossil Plants
With two-thirds of the planet’s electricity coming from burning fossil fuels, fossil power plants are experiencing some of the greatest design changes. Fossil plants typically have multiple water treatment systems: a demineralization system to provide water for steam generation; a condensate polishing system to repurify condensed steam for reuse in the steam system; a cooling tower to cool steam and a flue gas desulphurization system to prevent toxic volatile emissions. The need for higher operating efficiency and lower environmental emissions has led to design changes in each subsystem. The greatest changes have been in steam generation and cooling methods.
During the past 40 years, improved fuel efficiency has driven fossil designs with ultrasupercritical steam and increased steam pressures and temperatures. Fuel flexibility has facilitated the development of combined-cycle designs that lower water usage. This new development has been particularly helpful in regions with water scarcity. These regions were the first to adopt recirculation water cooling towers and use air for cooling instead of water.
A major design trend that affects steam generation for fossil fuels is the use of supercritical steam generators. By operating at higher pressures and temperatures, fossil plants become more efficient and require less fuel consumption, creating cost and environmental emissions benefits. To achieve such designs, the required water quality has increased dramatically. In older nonsupercritical designs, water is boiled, the steam separates and is then sent to spin steam turbines. Minerals dissolved in water will not boil, and the water reservoir (drum system) can be bled periodically to remove unwanted contaminants.
Supercritical designs send 100 percent of the feed water to the turbines without an intermediate water reservoir. As a result, all minerals dissolved in the water will end up in the steam and can form scale on the turbines, hurt system operation and damage expensive equipment. The result is a high dependency in supercritical designs on very high-purity water. Ion exchange resins are the predominant condensate polishing technology and there has been continuous innovation in these systems to meet the higher water-quality demand.
A second design trend that affects steam generation is the adoption of combined-cycle gas turbine (CCGT) plants where natural gas is the fuel. The combustion products are gases that directly turn the gas-powered turbines while they provide heat to create steam for a steam turbine, hence the name combined cycle. CCGTs reduce water demand for power generation because steam is not required for all the turbines. CCGTs and renewable energy facilities are being built in the U.S. to replace retired coal fired plants wherever possible.
Cooling system design has undergone major changes in areas with severe water shortages. Cooling the steam back to condensate is essential to drive the pressure gradient that spins the turbine. Typically, water is cooled to less than 55 C. Where practical, such as near large rivers or the ocean, water can be sent through a cooling tower in a single pass. This consumes large quantities of water and is practical only if the power plant does not disturb the natural environment or compete for water usage with the needs of human consumption.
In many parts of China and other select regions of the world with poor water availability, air cooling is being adopted in place of water cooling. Condensate temperatures often exceed 60 C on air-cooled systems. This places severe demands on traditional ion exchange resins (which typically are unstable at these elevated temperatures) used to purify the condensed steam before cycling back to the steam generation system. Manufacturers of ion exchange resins continue to innovate to meet these challenges.
An alternative to air cooling for water-starved regions is the recycling of cooling water. Recycling results in salinity increases in the cooling tower owing to evaporation. It is possible to occasionally blow down or bleed off mineral-enriched water, but that water must be treated further. This has created the need for brine concentration systems to reduce disposal and recover as much water as practical. Reverse osmosis has been the technology of choice for the primary concentration stage.
Ahead–Water Stress, CO2 Emissions Will Drive Plant Design
Because fossil power generates one-third of the world’s CO2 emissions, more countries are implementing strict environmental standards. Where coal will be used, supercritical designs are the standard; however, where possible, natural gas and renewable energy sources are being adopted.
Water shortages worldwide will continue to drive innovation in fossil plant design and accelerate the adoption of alternative power generation technologies. Using the U.S. as an example, industrialization has driven a change in water usage. During the 1950s, agricultural consumption used the most water, but now electricity generation uses the most water. As more countries industrialize, they will undergo similar trends as industry competes with humans for water. This already is occurring in China where the trend is moving from water-cooled to air-cooled towers.
Steven Rosenberg is a Research Fellow for Dow Water and Process Solutions with more than 28 years of industrial experience in the development of advanced materials. His main focus is on developing breakthrough innovations for water treatment technologies. He has a doctorate in chemistry from The Pennsylvania State University.