SCIENCE & TECHNOLOGY
CLEANER FUTURE FOR FOSSIL FUELS DOE initiatives focus on near-zero-emissions electricity generation and ultraclean gasoline STEPHEN K. RITTER, C&EN WASHINGTON
T
HE U.S. IS THE LARGEST CON-
sumer of energy in the world, in terms of both electricity and transportation fuels. The federal government's greatest challenge in meeting the ongoing demand for energy in the coming decades will be to keep a steady balance between the interests of oil companies and the deregulated electric power industry on the one hand and a prudent course of environmental protection on the other. The Department of Energy has stepped forward with several R&D initiatives that it hopes will make useful contributions to this task. One of the initiatives, called Vision 21, has the goal of designing a new generation of near-zero-emissions fossil fuel energy plants. These plants will inte-
Ruth, senior adviser at DOE's National Energy Technology Laboratory (NETL), in Pittsburgh. "It's different from the traditional fossil energy R&D program, which addresses different areas of power technology separately" In a keynote address at the recent American Chemical Society national meeting in Boston, Ruth provided an overview of Vision 21 and discussed the key role that chemistry and chemical engineering will play in future energy production. Ruth's presentation and related talks by several of his NETL colleagues and other scientists were made in symposia sponsored by the Division of Fuel Chemistry Topics included ultraclean transportation fuels, advances in fuel cells, monitoringair toxics and particulates, and hydrogen production and storage.
ENERGY CONSUMPTION Fossil fuels dominate U.S. electricity generation and total energy needs Natural
r—
9 as \ . ^ l · ^ ^ ^
11%
^
M
^
Oil
^
3%
Nuclear
8%
r~
\^~~
t \
Hydro & other renewables
?X
7%
Hydro & / ^ B ^ ^ ^ ^ Coal ^ ^ ^ L other 7 ^ ^ ^ ^ ^ B _ 54% 22% — ^ ^ ^ B renewables ^ 12% ^ ^ ^ ^ ^ ^ ^ V L ^
Nuclear 20%
"^^^^^^^^^7 ^ ^ ^ ^ ^
2000 electricity generation = 39.8 quadrillion Btu
Natural gas J^^^^^^L· 23% ^ ^ ^ ^
/
7
0il 40%
/ ^
2000 energy c:onsumption = 98.5 quad rillion Btu
SOURCE: D e p a r t m e n t of Energy
grate technologies such as coal gasification and fuel cells to produce electricity and, where feasible, auxiliary products such as transportation fuels and chemical feedstocks. "Vision 21 is agovernment-industry-academia cost-shared partnership that stresses technology innovation and a diverse mix of energy resources," noted Lawrence A.
Although DOE stresses that the country will need to continue to utilize all of its energy resources—fossil fuels, nuclear, and renewables—the Bush Administration has made a bet on coal to be the principal energy resource for the future. The U.S. has more than 250 billion tons of coal reserves, the most of any country and the energy equivalent of the world's known oil reserves,
Ruth noted. That's enough coal to last about 250 years at current use rates, he said. The U.S. generates just over 50% of its electricity from coal, Ruth said. That percentage is expected to drop slightly by 2020, he added, but the amount of electricity produced annually by coal in the U.S. is expected to grow 25% over the same period, from 1.97 trillion k W h to 2.47 trillion kWh. "We need to rely on our abundant and secure coal reserves to meet the growing demand for electricity, maintain competitive energy prices, and sustain economic growth," Ruth stated. "However, coal must meet increasingly stringent environmental standards, a requirement that can be achieved through better technology" THE ENVIRONMENTAL concerns that Vision 21 will address include the traditional pollutants sulfur and nitrogen oxides, mercury, particulate matter, and liquid and solid wastes, Ruth said. A special emphasis for Vision 21 is cost-effective C 0 2 management, he pointed out. "One of the big environmental issues today is C 0 2 and its association with global climate change," Ruth noted. "To manage C 0 2 emissions, you first have to capture it and then decide what to do with it, such as sequestering it in a way that is environmentally acceptable." Controlling C 0 2 emissions at current coal-fired power plants would be extremely expensive and energy-intensive because of the low concentration of C 0 2 in large volumes of flue gas, Ruth said. Under Vision 21, gasification technologies and some types of combustion technologies would generate a concentrated stream of C 0 2 that could be easily managed, he noted. A suite of technology modules is being developed under Vision 21 to become the building blocks of future power plants, Ruth explained. In most cases, electricity would be the primary or only product of the Vision 21 energy plant. "But when it makes sense economically, other products such as chemicals or clean liquid transportation fuels could be coproduced," he added. A typical integrated-design plant would include a gasification unit that produces synthesis gas—a mixture of CO, H 2 , C 0 2 , and H 2 0—from coal or other fuel using oxygen produced with low-cost air-separation membranes, he said. The syngas would be cleaned to remove sulfur and ni-
"We need to rely on our abundant and secure coal reserves to meet the growing demand for electricity/' 30
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Coal and other fuels
POWER
High-efficiency turbines powered by H2 fuel cell or syngas
Excess process heat/steam
Off-peak diversion
02 Syngas/H^/C02 separation
Air-separation membrane
Syngas production
Gas stream cleanup to remove S and N compounds
C02 sequestration
SOURCE: National Energy Technology Laboratory
Fuels/chemicals by Fischer-Tropsch reactions
Electricity
LOOKING AHEAD DOE's Vision 21 initiative has the goal of developing designs for near-zero-emissions fossil fuel energy plants. These plants would integrate coal gasification, fuel cells, and other new technologies to produce electricity and, where feasible, auxiliary products such as transportation fuels and chemical feedstocks. trogen oxides, and a second membrane would be used to separate H 2 . Unused CO and steam in the syngas could be catalytically shifted to H 2 and C 0 2 as needed, and the total C 0 2 generated in a plant could be sequestered. Syngas, H 2 , or another fuel would power gas turbines and/or fuel cells to produce electricity, he added. A portion of the syngas or other fuels could be diverted at offpeak times, such as at night, to produce clean gasoline or commodity chemicals by Fischer-Tropsch chemistry Some 60 companies and universities currently are working on various Vision 21 projects supported by DOE funding. The completed modules will be interconnected in different configurations to utilize fossil fuels and opportunity fuels, such as biomass and municipal waste, Ruth said. The selection of fuels, products, plant configurations and sizes, and environmental controls will be site-specific and determined by prevailing market conditions. To be successful, costs for Vision 21 plant products will need to be on par with competing technologies, he said. Ruth
cause Vision 21 is being carried out as a partnership, Ruth added, the technologies that make it to demonstration models will require larger cost-sharing by the industry partner, increasing the likelihood that the technology will be implemented. Vision 21 is operating in parallel with the Bush Administration's Clear Skies initiative and in conjunction with DOE's $2 billion Clean Coal Power Initiative. Clear Skies sets targets for reducing emissions of S 0 2 , NO x , and mercury by existing power plants over the next 15 years, while the clean-coal initiative has the goal
THE TIMELINE for Vision 21 has spin-off technology developments operating by 2005, completed module designs prepared by 2012, and commercial plant designs ready by 2015, Ruth noted. Actual construction and use ofplants will be left up to private industry, he said, which in theory will do so depending on prevailing economic conditions and regulatory requirements. But be-
of accelerating commercial deployment of advanced technologies over the next 10 years. The clean-coal initiative could be a route to demonstrate some of the early Vision 21 technologies, Ruth said. One Vision 21 project that is nearly ready to be put into practice is the first large-scale fuel cell to be powered by a fuel produced from coal. The 2-MW fuel
HTTP://PUBS.ACS.ORG/CEN
Cuqini
cell, developed by FuelCell Energy, Danbury, Conn., is expected to be running by late next year. The unit will be installed at Wabash River Energy's power plant in WestTerre Haute, Ind. The 2 6 0 - M W Wabash facility is currently one of only two commercial-scale coal gasification-combined cycle plants operating in the U.S. The plant converts coal into syngas, and after sulfur and other contaminants are removed, the gas powers a turbine to produce electricity The turbine's hot exhaust gas is captured and used to generate steam to operate a conventional A turbine. DOE is funding half of the S $32.3 million fuel-cell project. ^ An important part of the future ° energy equation will be the supply of ultraclean fuels. In Boston, Anthony V. Cugini, NETL focus area leader for Advanced Fuels Systems and Computational Energy Sciences, provided an overview of his groups' efforts to support the development of Vision 21 technologies to meet new federal environmental standards in the production of affordable fuels. 'A key aspect of our support is to develop computational models of advanced processes and systems," Cugini explained. "This includes molecular modeling of hydrogen interactions with surfaces as applied to hydrogen production, storage, and separation; Fischer-Tropsch chemistry; investigations of electron transfer and oxygen mobility in fuel cells; and desulfurization processes." One element of the research is verifying C&EN
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SCIENCE & TECHNOLOGY computational results with experimental tests. An example Cugini described is the chemisorption of hydrogen on substrates such as carbon nanotubes as a means to store hydrogen for fuel-cell applications (C&EN,Jan. 14, page 25). "We believe that a coordinated computational modeling and validation program will help us to more rapidly develop advanced technologies to benefit the Vision 21 program," Cugini said. An effort within the Advanced Fuels Systems group is to develop and test some poison-tolerant materials such as palladium-based composites for hydrogen separation applications. Membrane reactors made with these materials could enhance hydrogen production from coal gasification plants via the water-gas shift reaction, he noted. In addition to developing their own membranes, N E T L researchers have tested membranes from several collaborators in academia, industry, and at other national labs. Some advanced analytical techniques also are of interest for monitoring trace quantities of sulfur and other impurities in fuels, Cugini pointed out. For example, accurate measurements will be needed to determine if removing sulfur or other impurities of environmental concern will affect fuel performance properties. N E T L researchers have been working to enhance the analytical capabilities of element-specific detectors, such as gas chromatography with atomic emission detection, he said. One method already being tested can measure sulfur compounds at the parts-per-billion level, Cugini noted. REDUCTION OR elimination of sulfur in diesel and gasoline is currently one of the toughest challenges in clean fuels research. New U.S. regulations to reduce the amount of allowable sulfur in gasoline from 350 ppm to 30 ppm and in diesel from 500 ppm to 15 ppm are set to take effect in 2006. These regulations are in addition to required reductions of aromatics in fuels. "The sulfur problem is becoming more serious, particularly for on-road diesel fuel and gasoline, as the regulated sulfur content is being reduced by an order of magnitude," noted associate professor of fuel science Chunshan Song of Pennsylvania State University, University Park. Song, who heads the university's Clean Fuels & Catalysis Program, discussed current and 32
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developing desulfurization technologies and catalyst design during a keynote address in Boston. Most refineries have a fluid catalytic cracking (FCC) unit that converts heavy oil fractions to naphtha that is used to make gasoline and light cycle oil that is used to make diesel, Song explained. These primary fuel blending stocks contain the most sulfur. A variety of desulfurization processes are used to remove sulfur either before FCC, during FCC, or after FCC. These processes require a catalyst and hydrogen that react under high temperature and pressure, Song noted. The thiophene ring is hydrogenated before the sulfur can be stripped out as H 2 S, leaving butene or other hydrocarbons that can be added back to the fuel stream. Typical catalysts may be molybdenum disulfide clusters that contain small amounts of "promoter" metals such as cobalt or nickel, he noted. There are a number of complicating factors to meeting the new regulations, Song pointed out. Removing sulfur and maintaining the desired high octane number of the gasoline is tricky This is primarily related to undesirable hydrogenation of olefins in the naphtha fraction.
tions to clean fuel technology is a process called selective adsorption for removing sulfur (SARS). This process uses a transition-metal-based adsorbent loaded on a porous solid support, such as MCM-41, an aluminosilicate molecular sieve. Rather than catalytically removing sulfur from thiophenes, Song said, sulfur is selectively bound to the metal atoms at low temperature under ambient pressure, while desirable aromatics that don't contain sulfur, such as alkylbenzenes and naphthalene, pass through. SARS does not use hydrogen, so olefins and other aromatics are not hydrogenated. The hydrogen can thus be used for other purposes, such as powering fuel cells, he noted. The adsorbent can clean 10 times its volume of fuel before becoming saturated, Song said. Once the adsorbent is saturated, the sulfur compounds can be stripped off by washing it with a polar solvent, and the adsorbent can be reused. In lab tests on jet fuel, the SARS method has reduced 500-ppm sulfur fuel to less than 1-ppm sulfur. The method is in the process of being patented, Song noted, and Penn State is looking for potential industrial partners to move up to production scale. Song said SARS is an alternative to existing desulfurization processes. Another alternative is Phillips Petroleum's new S-Zorb process, which also uses an adsorbent to trap sulfur compounds. "Our SARS process for sulfur adsorption is carried out
"Coal must meet increasingly stringent environmental standards, a requirement that can be achieved through better technology." Another problem, he added, is that as allowable sulfur levels in finished gasoline and diesel are being lowered, the average sulfur content of available crude oil has been on the rise. Requirements also will be more stringent for fuels to power fuel cells, should they be broadly used, he said. Liquid hydrocarbon fuels will need to have less than 1-ppm sulfur to avoid poisoning both fuel-processing catalysts and fuel-cell electrode catalysts. Current technology being developed allows much of the sulfur to be removed, he said, but the remaining thiophene compounds, particularly 4,6-dime thyldibenzothiophene, are refractory Finding a catalyst or economical process that is efficient against those compounds should allow enough sulfur to be removed to meet the upcoming regulations, especially for diesel. One of Song and coworkers' contribu-
at ambient temperature and pressure without using hydrogen, whereas S-Zorb uses elevated temperature and low hydrogen pressure, which converts some sulfur to sulfide with hydrogen loss." Most of the major oil companies likely will try to find a desulfurization method that meets the new federal regulations with a minimal amount of additional investment, Song observed. These approaches could include coming up with new catalyst formulations, tailoring reaction and process conditions, designing new reactors, or developing completely new processes based on adsorption, oxidation, membrane separation, biotechnology, or ionic liquids. One or more of these approaches may end up being used by a refinery to produce ultraclean fuels, he concluded, and they could also end up as Vision 21 plant technologies. • HTTP://PUBS.ACS.ORG/CEN
