Clean Coal Power Technologies

Securing our future energy supply begins at home. The challenge is to develop the technologies needed to help coal remain a viable energy option for the United States in the face of increasing international pressure to reduce carbon dioxide emissions. The events of September 11 further increase the need to ensure that our nation’s plentiful supply of coal is not interrupted, especially if imported oil and gas supplies are.

The United States has a 1500-year supply of coal, and 54% of the electricity produced in the country comes from coal combustion, averting the need to use large amounts of imported oil and gas for generating electricity. Although oil was used to supply 20% of the nation’s electricity in the 1970s, it is now the source of only 1% of our power, as other fuels have become more available.

Although our nation has many coal-fueled power plants, very few new ones have been built in the past 15 to 20 years. While this may seem strange in a time of ever-increasing energy demands, the reason is primarily economics. Numerous new natural-gas-combined-cycle (NGCC) plants—plants that use natural-gas- fueled turbines as the primary electricity-generating technology and steam-driven turbines as a secondary generating technology in the same system—have been built. Capital costs of these NGCC plants are much lower than for conventional coal-fired steam plants.

Other compelling reasons for the trend toward building more NGCC plants are that natural-gas-combined-cycle plants are more efficient and have much lower emissions of sulfur and nitrogen oxides and carbon dioxide (CO₂) than do coal-fired plants. The NGCC plants can also be brought on line in much less time than can coal-fired plants.

TVA’s Bull Run coal-fired power plant in East Tennessee. (Photo by Tom Cerniglio)

The problem is explained this way from the local perspective, according to the Southern Alliance for Clean Energy for the Tennessee Clean Air Task Force: Assume that you purchase electricity from the Tennessee Valley Authority (TVA) and your monthly bill is $100. Without significant investment in pollution-control equipment, meeting your electrical needs each year would result in the combustion of 9 tons of coal and the release to the atmosphere of 335 pounds of sulfur dioxide (SO₂), 177 pounds of nitrogen oxides (NO_x), and 42,197 pounds of CO₂. Both SO₂ and NO_x (through the production of ground-level ozone) contribute to lung damage in humans. Additionally, rising atmospheric concentrations of CO₂ may alter the climate in undesirable ways. Coal-fired power plants also emit hazardous mercury, which eventually is deposited from the air to waterways, where it can be taken up by fish that humans eat (possibly causing neurological damage).

“Because of dramatic advances in pollution-control technology, these emissions from coal plants, except for CO₂, can actually be controlled by commercially available equipment to about the same level as natural-gas electricity generation,” says Rod Judkins, director of ORNL’s Fossil Energy Program. “These pollution control measures also impact the economics of coal power generation.”

TVA, which produces 64% of its power from fossil fuel plants, is the largest single-utility buyer of coal in the United States. The agency has made great strides in reducing its SO₂ and NO_x emissions by purchasing low-sulfur coal and using the best available emissions control technologies. By installing scrubbers and electrostatic precipitators, TVA has cut its SO₂ emissions by 65% since 1977 and plans to reduce it by 75 to 85% by 2005. Through installation of low NO_x burners and selective catalytic reduction systems, TVA plans to increase its reduction of NO_x emissions from 45% (since 1995) to 75% by 2005.

In February 2002 President Bush delivered a speech calling for a reduction of greenhouse gas emissions (including CO₂) by 18% in 10 years. He also asked for an “unprecedented” reduction in power plant emissions of sulfur dioxide, nitrogen oxide, and mercury, which the administration calls the worst air pollutants. Almost a year earlier, President Bush urged the U.S. government to commit $2 billion over the next 10 years to the development of advanced clean–coal power technologies In response, Secretary of Energy Spencer Abraham said, “This country will have a much stronger, more reliable electrical industry if we keep coal in the power mix. New technology is the way to do that.”

One possible approach to keeping coal in the nation’s energy portfolio is to develop economical methods for capturing CO₂ from the stack gases released from coal-fired power plants. Some of the captured carbon could be used to make marketable products. The rest of the recovered carbon gas could be collected in a vessel for transport to a carbon sequestration site to isolate it from the environment. It could be then injected into an underground coal bed, depleted oil reservoir, or the ocean, although ocean sequestration may be becoming a less preferred approach.

MOLECULAR SIEVES AND DESIGNER SOLVENTS

ORNL and University of Kentucky researchers recently developed a carbon-fiber composite molecular sieve (CFCMS) adsorbent filter that has a great affinity for CO₂ and that is quite effective in separating CO₂ from mixtures of gas containing it. Because the CFCMS adsorbent is electrically conductive, the CO₂ can be removed from the saturated sieve by running an electrical current through it at low voltage.

This CO₂-adsorption technology, developed largely by Tim Burchell of ORNL’s Metals and Ceramics Division, has several advantages over conventional granular activated–carbon beds. “It adsorbs more carbon and takes it up 5 to 10 times faster,” says Judkins. “Also, 2 to 10 times less energy is required to recover the adsorbed CO₂ and regenerate the filter so it can be used again.”

In addition to addressing the burning issue of CO₂, another group of ORNL researchers is trying to tackle the problem of hazardous mercury emissions from coal-fired power plants. Using internal funding from ORNL’s Laboratory Directed Research and Development Program, they are searching for and trying to synthesize “ionic liquid” compounds that most effectively take up and separate CO₂ and mercury from power-plant stack gases.

Members of the group are David DePaoli of the Nuclear Science and Technology Division; Moonis Ally of the Engineering Science and Technology Division; and Mike Simonson, Sheng Dai, and Doug Duckworth, all of the Chemical Sciences Division. Working with them is Ruth Baltus, visiting from Clarkson University in New York, and former ORNL staff scientist Jerry Braunstein.

David DePaoli (left) and Moonis Ally (right) examine a vial of ionic liquid shown by Sheng Dai, who synthesized it. This ionic liquid will be tested to determine its effectiveness in removing carbon dioxide from a mixture of gases. (Photo by Curtis Boles; enhanced by Gail Sweeden)

Dai has synthesized organic “designer solvents” that absorb CO₂. Simonson, Baltus, and DePaoli, using special experimental facilities to reproduce stack-gas conditions, measure their thermodynamic and kinetic properties, including their ability to take up and transport CO₂. In a parallel effort, Duckworth uses mass spectrometry to measure each solvent’s affinity for CO₂. Guided by the results of Ally’s computer modeling of the properties of CO₂-ionic liquid mixtures, these experimental results are contributing to the design of new solvents for enhanced performance under field conditions.

Dai has synthesized a promising compound that can be impregnated in a glass-fiber or porous polymer membrane. The goal is to work with the coal power industry to create a “reactive membrane” through which CO₂ will flow so that it can be captured for industrial use or carbon sequestration.

“Ionic liquids are ideal for this application because they do not evaporate easily, they are stable in a wide temperature range, and they have adjustable properties,” says DePaoli. “We have shown in preliminary experiments that we can design one liquid for which CO₂ has a high affinity and another for which mercury has a high affinity. We hope to find the best compounds that can be used together to extract both CO₂ and mercury from stack gases.”

COAL GASIFICATION

Ruth Baltus, a chemical engineering faculty member from Clarkson University on sabbatical at ORNL, assembles a quartz crystal microbalance, which is used to screen different ionic liquids for their ability to absorb CO2 from a gas mixture (such as stack gases from a coal-fired power plant). The ionic liquid is coated on a piezoelectric quartz crystal that is electronically excited into resonance. As the ionic-liquid-coated quartz is exposed to varying concentrations of CO2, the resonance frequency changes because of the uptake and release of dissolved CO2. These frequency changes are interpreted in terms of the effectiveness of the liquid in removing CO2. (Photo by Curtis Boles; enhanced by Gail Sweeden)

Since 1973 when the Organization of Petroleum Exporting Countries (OPEC) embargoed sales of oil to the United States, the U.S. government has explored the use of direct and indirect coal liquefaction to make liquid fuels and coal gasification as a means of producing gaseous fuels such as synthesis gas and substitute natural gas. Most current government-sponsored activities are directed toward coal gasification for power generation in integrated (coal) gasification, combined-cycle systems.

Reforming of natural gas (breaking up gas molecules) and gasification of coal are considered excellent approaches to the production of hydrogen. Because the United States has enough coal deposits to last for 1500 years, coal represents a tremendous asset for the production of fuels, including hydrogen, that could be used for transportation and power production.

“Unfortunately,” Judkins says, “the economics for these types of plants are poor. We could build an integrated coal-gasification, combined-cycle plant that is 42% efficient. It would produce electricity, partly by using the gas produced from the coal to run a gas turbine for generating power. Its capital cost would be $1000 to $1500 per kilowatt of electricity generation capacity. However, the commercial natural-gas combined-cycle plant commonly being used today costs only about $300 to $500 per kilowatt of installed electricity-generating capacity and has a conversion efficiency of 50 to 58%.”

The cost of gasifying coal may become attractive if the U.S. supply of natural gas declines or other factors cause the cost of natural gas to increase dramatically. “We probably have 65 to 120 years of natural gas left, based on known reserves, current consumption rates, and anticipated growth in gas-usage scenarios,” Judkins states. “But, of course, if we can get natural gas from methane hydrates, we will have a much larger supply that might last for centuries.”

Today, efforts are being made to burn coal at a higher temperature to produce electricity more efficiently, thereby reducing emissions of CO₂ and other pollutants. New technologies, such as selective catalytic reduction, low-NO_x burners, and scrubbers, will help decrease these emissions.

Advanced coal-fired power plants, such as this 2544-ton-per-day coal gasification demonstration pilot plant, will have energy conversion efficiencies 20 to 35% higher than those of conventional pulverized-coal steam power plants.

ORNL is a partner with Foster Wheeler, Babcock and Wilcox, and Alstom Power (based in Brussels, Belgium) in developing an Ultra Supercritical Coal Plant. The plant will produce steam as hot as 1400°F. It would have a higher efficiency than the Eddystone supercritical coal plant in Pennsylvania, which operates at 1100°F but was designed to operate (and did) at 1200°F in the 1950s, still a record for commercial operation. This plant was de-rated and required to operate at less severe conditions as a result of materials issues that arose at the higher temperatures.

ORNL researchers are also involved in a $5-million program to identify materials that can withstand the 1400°F temperature of this ad-vanced plant’s steam system. Materials scientists will focus on modifications of conventional steels and nickel-based alloys for use in a steam system, so that it can stand up to these harsh conditions.

ORNL, which is located near the Cumberland Mountains of East Tennessee where coal is abundant, is doing its part to help demonstrate that coal really can be made clean and can, thus, remain in the nation’s energy mix.

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Related Web sites

• ORNL's Fossil Energy Program
• ORNL's Metals and Ceramics Division
• ORNL's Nuclear Science and Technology Division
• ORNL's Engineering Science and Technology Division
• ORNL's Chemical and Analytical Sciences Division
• Tennessee Valley Authority

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