There are, according to estimates by the U.S. Geological Survey and other organizations, some 980 billion metric tons of coal buried worldwide. Taken at face value, that coal could supply the energy needs of the world for many years. But while coal is plentiful, burning it to generate power places a tremendous burden on the global water supply. At present, although coal plants are subject to a variety of regulations to reduce emissions of pollutants including mercury, sulfur dioxide, carbon dioxide, and particulate matter, there is no technology in place to recover water, one of the world's most valuable resources.
Now, with the impetus of a $930,000 contract from the Department of Energy National Energy Technology Laboratory (NETL) and a $470,000 buy-in from industry giant Siemens Westinghouse, the University of North: Dakota Energy & Environmental Research Center (EERC) has begun a two-year pilot program to study the use of commercial desiccant technology as a way to remove, treat, and use water from Coal power plant flue gas--water that originates within the coal itself. Using de desiccant technology may prove to be one way that coal-fired power plants can reduce their draw on local fresh water supplies.
Coal, the fruit of the earth, can actually have a significant percentage of water in its makeup, says EERC director Gerald H. Groenewold. North Dakota lignite, the type that the EERC researchers are currently working with, can be 30% watch he says, and he and his colleagues have even worked with some coal from Australia that is 67% water. "At that point, it's hard to recognize it as a solid resource," says Groenewold, "but it is." Ideally, some of that water, which generally goes out the stack, would be captured and reused in the plant or returned to the local water supply.
Desiccant technology is currently used to dry the air in facilities such as ice rinks, where there is a large volume of water but the surrounding air must be kept relatively dry. Besides reducing water use, the technology has other potential advantages for power plants, says EERC associate research director Thomas A. Erickson. "For one, if you dry the stack gas, it actually becomes easier to capture the suite of contaminants involved in coal firing.... And although research is ongoing, it appears possible that capture of excess water might have a positive impact on the formation of certain pollutants, such as sulfur dioxide, downwind from the plant."
Water for Power
According to Groenewold, power generation overall is second only to agriculture as the largest domestic user of water. Thermoelectric generating facilities make electricity by converting water into high-pressure steam to drive turbines. Once through this cycle, the steam is cooled and condensed back into water (some current technologies also use water for this step, which increases the plant's need for water still further). In coal plants, water is also used to clean and process the fuel itself.
The U.S. Geological Survey estimates that as of 2000, thermoelectric plants (including both nuclear and fossil fuel) withdrew 195 billion gallons of water per day, of which 136 billion gallons was fresh water. About 97% of the water that these plants withdraw is returned to the source water, says Thomas Feeley, technology manager for environmental and water resources programs for the NETL, and about 3% goes up the stack as evaporative loss.
"Loss through evaporation becomes an issue [for power plants] because water is so very limited in many parts of the country, such as the western United States," says Feeley. In many of these same areas, a great deal of agriculture and some of the most rapid population growth is taking place, placing further demands on water resources.
Feeley adds, "Heavy water usage can have a long-term impact on aquifers in the region, because once depleted, they can take hundreds of years to recharge. …