TECHNOLOGY UPDATE
The future of coal power technology looks hotter A new technology for producing electricity from burning coal has the potential to dramatically increase the efficiency of a traditional means of producing power, while decreasing the amount of greenhouse gas and pollutant emissions associated with it. Officials from the Department of Energy (DOE) are proclaiming it a "major advance in the state of the art." Known in DOE lingo as a highperformance power system because it employs advanced technologies that boost its overall efficiency beyond 47%, die new technology was designed by researchers at the United Technologies Research Center of East Hartford, Conn. Its components are currently being tested at a pilot facility in the University of North Dakota's Energy and Environmental Research Center (EERC) in Grand Forks, N.Dak. Late last year, researchers used the technology to heat air for driving a turbine to 2000 °F, achieving what they believe to be a world record temperature for a coal-fired power system The fact that the new technology uses a heat exchanger to warm up air sets it apart from conventional coal-fired power systems. By using hot air to push an off-the-shelf gas turbine, of the sort used in natural gas-based electricity generation systems, the researchers succeeded in making a "huge leap" in the temperatures that can be reached, explained John Hurley, senior research manager for die EERC. This makes the process more efficient, he continued. Today's power plants use the heat from burning coal to boil water, then they compress the resulting steam to a high pressure and use it to spin a turbine. The steam must be condensed back to water to restart the cycle. In the process a great deal of energy is released from the cooling tower, rather
The walls on the left side of this high-temperature, advanced furnace being tested at the Energy and Environmental Research Center are made of a high-temperature refractory similar to those used in the glass-making industry. (Courtesy United Technologies Research Center)
than being used to turn the turbine, Hurley said. "With the high-performance power system, there's no condensation, so you don't lose that heat of vaporization," he added. Also, because the power generation process is more efficient, it generates a third less carbon dioxide, he said. United Technologies' high-performance power system does include a steam turbine, but it is a secondary process, which is known as a bottoming cycle in the electricity world. The waste heat from the air-driven turbine is tunneled into a boiler system, which produces steam that drives a second turbine. Overall, the process operates at over 50% efficiency; conventional coal-fired electricity production is 35% efficient. United Technology's power system is an outgrowth of a project started in 1992 funded by DOE's Combustion 2000 program. It is one of two high-performance power systems being developed (the second is a project contracted out to Foster
Wheeler of Clinton, N.J. In addition to coal, the United Technologies design can use "virtually any solid fuel," according to Dan Seery, senior program manager for United Technologies Research Center. He stressed that die technology's main innovations lie in its heat exchangers and the materials from which diey are constructed. "Today's [commonly used] steels can only go to around 1000 °F before they start to sag a bit and start to corrode," Hurley said. The design can only feature an off-the-shelf gas turbine because the air that spins that turbine is never exposed to the combustion gases, Seery added. The refractory wall of the furnace in which the coal is combusted would dissolve if they were made from commercial materials, Hurley said. The parts of the heat exchanger that are exposed to those corrosive furgases cire constructed from high-temperature refractories similar to those used in the glass-making
MARCH 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 1 2 9 A
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industry, which allow them to withstand the 2600 °F temperature. The heat exchanger's internal tubes are composed of oxide-dispersionstrengthened alloys. The emissions reduction technologies in United Technology's power system were developed through DOE's low-emission boiler program, said Seery. By incorporating selective noncatalytic reduction to remove NO,,., and high-efficiency scrubbers to remove S0 2 and particulates, Seery said that United Technologies expects to reduce emissions for these pollutants by 90%. Though mercury abatement was not an issue when the technology WclS designed, Hurley said that it will generate a third less mercury because of the coal being combusted more efficiently DOE anticipates that high-performance power systems, such as the ones being developed by United Technologies and Foster Wheeler, will be commercially available by the year 2005. Though Seery acknowledged that there is currently not much market demand for new coalfired power plants, he stressed that United Technologies' power system technology can also be used as part of a retrofit to improve the efficiency of an existing power plant, bumping it up from 35-45% efficiency. There is a large and emerging market for such "repowering" designs for existing coal power plants, Seery said. Additionally both high-performance power system technologies are candidates for the Vision 21 program that DOE expects to inaugurate in 2000 said Victor Derr director of power systems for the agencv's Office of Fossil Enerev DOE hoDes to help dpsien the enerev generation tpchnolnpips that will he ?010 artr\
program In the meantime, United Technologies and EERC have conducted more than 1000 hours of tests on the power system using three different kinds of coal. Because of their continued success with running the system in 100-hour increments, they were poised to begin testing in 200and 500-hour blocks at the beginning of the year. The researchers are also altering the design to allow it to incorporate solid oxide fuel cells, an improvement they believe can ratchet up the system's fuel efficiency to over 60%. —KELLYN S. BETTS
