How Clean Can Coal Get?
April / May 2009
Volume 7 Number 2
We're sitting on the world's largest supply of coal reserves. Finding a way to control greenhouse gas emissions is vital if those reserves are to do more than just sit there. The National Energy Technology Laboratory is working toward a solution. This is its definitive report on progress to date.
We rely on fossil fuels such as coal, oil and natural gas to produce electricity, operate automobiles and heat homes and industries. In the U.S., pollution from fossil fuels has been reduced significantly by new technologies, but fossil fuels produce large amounts of greenhouse gases such as carbon dioxide (CO2), that have been implicated in climate change. While there are no U. S. regulations on greenhouse gas emissions, there is a growing worldwide movement to reduce them. Scientists and engineers at the Department of Energy's National Energy Technology Laboratory and its partners in industry and academia are developing advanced technologies to reduce pollutant emissions from fossil fuels to very low levels and to capture and sequester carbon emissions. By carefully engineering the process of combustion, improving the technology of gasification and developing the new technology of carbon sequestration, researchers are creating new technologies to capture and store greenhouse gases in stable forms for long times.
Can fossil fuels really be scrubbed clean?
Coal has historically met over half of U.S. electricity demand. This electricity comes from plants that burn coal to heat water and create steam, which drives large turbine generators. Since the passage of the Clean Air Act in 1970, emissions of the "criteria pollutants" (particles, sulfur dioxide, and nitrogen oxides) from coal-based power plants have been greatly reduced. In 2004, the EPA announced that U. S. air pollutant emissions had been cut by more than half since 1970. These reductions continue as older plants are replaced or retrofitted with advanced controls.
In power plants, particles are removed by electrostatic precipitators, sulfur dioxide is removed by scrubbers, and nitrogen oxide is reduced by selective catalytic reduction (SCR) systems. For a new coal-fired plant equipped with these systems, emissions of sulfur dioxide and nitrogen oxides can be reduced more than 90 per cent from uncontrolled levels. Much of the environmental control technology now available derives from research and development by NETL and its partners in ongoing demonstration programs that began in the 1980s. New technologies being developed are bringing us closer to near-zero level emissions of criteria pollutants and toxic emissions such as mercury.
Current research focuses on mercury controls, fine particulates, recycling of solid coal utilization byproducts and technologies to better manage how power plants use water. Removing mercury is difficult because it is present in coal in minute amounts and is difficult to measure. However, NETL has sponsored industry demonstrations that have reduced mercury emissions from coal-fired plants by 90 per cent or more. One new process for mercury capture using palladium sorbents developed by NETL researchers received an R&D 100 Award in 2008. The process was licensed to an industry partner who is commercializing it.
The most advanced low-emission, coal-based electric power plants are based on coal gasification, a process that converts coal to a mixture of hydrogen (H2) and carbon monoxide (CO) called "syngas," removes pollutants and then burns the clean syngas in a gas turbine, similar to a large jet engine. This technology is called integrated gasification combined cycle (IGCC) because both gas and steam turbines are used to convert the chemical energy in coal into electric power. The pollutants in the syngas can be reduced to low levels before burning the gas. Two IGCC demonstration plants have been sponsored by NETL under DOE's demonstration programs.
The Wabash River Coal Gasification Repowering Project in Indiana is the first full-size commercial IGCC plant built in the United States. The Polk Power Station near Mulberry, Florida, is the nation's first "greenfield" (built as a brand new plant) commercial IGCC power plant. Its gas cleaning technology removes more than 98 per cent of the sulfur in coal, converting it to a commercial product. Nitrogen oxide emissions are reduced by more than 90 per cent. These plants have shown conclusively that electric power can be generated from coal with high efficiency and low emissions. Since CO2 emissions are not regulated in the U.S., and removal increases the cost of power, there are no commercial electric power plants now that remove and sequester CO2. However, gasification technology makes the capture of CO2 emissions easier and can be the basis for electric power generation with carbon capture.
Can we really capture carbon and make use of it?
The primary objectives of the carbon capture and sequestration (CCS) research and development program, managed by NETL, are to reduce the cost and energy penalty associated with CO2 capture from large point sources, such as power plants, and improve the understanding of factors affecting CO2 storage permanence, capacity and safety in geologic formations and terrestrial ecosystems. Carbon capture involves separating CO2 from other components in a power plant. The CO2 can then be injected for long-term storage in deep geological formations of porous rock. The ease of capturing CO2, and therefore the cost, depends on the type of technology used. Commercial technologies exist to capture CO2 but better processes are needed to reduce the cost and increase efficiency. The permanence of geological storage also needs to be proven.
Coal produces the highest amount of CO2 of any fossil fuel—€”fuels derived from ancient biomass. The biggest challenge of capturing this CO2 from coal-based energy plants is the sheer volume produced. This challenge is better grasped if we understand that coal is mostly carbon, so the potential emissions of carbon are roughly equal to the amount of coal burned. NETL's research, development and demonstration initiatives are designed to make future coal systems dramatically more efficient and cleaner than today's plants, while keeping electricity affordable. The Clean Coal Power Initiative is a 10-year, multi-billion program with the government providing up to 50 percent of the cost of demonstrating promising technologies. Coal-based power technologies may produce heat, fuels, chemicals, hydrogen or other useful by-products in combination with electricity.
NETL analyzes the attributes of energy technologies and examines research, development and demonstration areas in terms of potential costs, benefits, risks, uncertainties, and timeframes. NETL engineers, with industry and university partners, have developed two R&D 100 award-winning technologies to simulate advanced energy systems. These technologies "build the plant in software" before companies commit billions of dollars to actual construction.
The Advanced Process Engineering Co-Simulator (APECS) allows engineers to simulate entire plant designs and individual processes simultaneously, a new capability that is being commercialized through ANSYS and other companies.
Post-combustion capture of CO2 is being studied extensively by NETL because it could be a retrofit solution for the pulverized coal plants that represent 99 percent of the country's coal-fired power plants. In this process, CO2 is removed from the flue gas, which contains CO2 together with atmospheric nitrogen. NETL is researching materials that absorb large amounts of CO2, including aqueous ammonia and solids such as amine-grafted substrates. Another retrofit option is to modify boilers to burn coal in an oxygen atmosphere, so-called "oxy-fuel combustion." Here coal is burned in oxygen diluted with recycled CO2 or H2O. The CO2 can then be captured by condensing the water in the exhaust stream. NETL researchers are developing advanced materials that will allow these processes to operate efficiently at high temperatures.
The next-generation power plants will remove CO2 prior to combustion, where it is more concentrated and at high pressure, making separation and capture easier. This "pre-combustion" carbon capture technology will most likely involve integrated gasification combined cycle (IGCC) power plants. As noted, there are no gasification-based power plants in the U.S. that capture and sequester CO2.
However, there is a large coal gasification plant in Beulah, N.D., that uses coal gasification to produce 54 billion standard cubic feet per year of substitute natural gas that is sold into the national natural gas grid, while capturing CO2. The Dakota Gasification Plant sells the CO2 to Canada for use in improving oil recovery. It was built in the early 80s under sponsorship of the U.S. Synthetic Fuels Program; NETL participates in the environmental monitoring program.
Current commercial designs for CO2 separation and capture in gasification would raise the cost of electricity by about 30 per cent. Not surprisingly, less costly methods of separating CO2 from the valuable H2 produced in gasification are being investigated. NETL researchers are developing sorbents and membrane materials; conducting research to better understand combustion and gasification processes; and devising equipment that maximizes efficiency while capturing CO2.
NETL is also developing membranes that will pass through pure hydrogen while rejecting CO2 and other syngas contaminants. The H2-rich syngas can be used as a fuel in a combustion turbine to produce electrical or thermal power, or to power fuel cells. NETL engineers are studying issues in gas turbine combustion and heat transfer associated with hydrogen-rich fuels, and conducting research on advanced fuel cells that will use gasified coal more efficiently than combustion-based processes.
First Capture It. Then Deliver It
Once captured, CO2 must be delivered to the sequestration site through high pressure pipelines. NETL has recently completed an atlas of U.S. sequestration resources. The most likely geologic sequestration formations are older oil producing reservoirs, deep saline formations, and unmineable coal seams.
Long-term, deep saline formations have the most capacity. However, the best near-term targets for sequestration may be oil fields where oil left behind in earlier production is being recovered. These fields, with their porous underground rock structures and their impervious cap rock, could prove to be ideal carbon sequestration sites where CO2 can be trapped while enhancing oil recovery. Similarly, pumping CO2 into unmineable coal seams could release some of the trapped methane, a technique called enhanced coal-bed methane (ECBM) recovery.
NETL is managing large-volume development projects that are storing CO2 in a variety of geological settings. These projects are under way in cooperation with seven Regional Carbon Sequestration Partnerships across the United States. The partnerships are collaborations among government, industry, universities and international organizations funded by DOE to determine the most suitable technologies, regulations and infrastructure needs for carbon capture and sequestration. Large-volume sequestration tests are scheduled to demonstrate the potential to store hundreds of years of CO2 emissions.
Technologies are being validated at test sites in the United States and Canada, and ongoing data collection is confirming geologic and terrestrial sequestration capacity and effectiveness. Monitoring, verification and accounting are necessary to ensure that the CO2 remains within the targeted formation. In fact, before the CO2 is ever pumped underground, a detailed site characterization and survey of the surrounding area is conducted to make sure all previously drilled and abandoned well openings are sealed. In 2004, in response to a request from a petroleum company, NETL researchers applied their helicopter-based SEQURE—„ Well Finding Technologies to a 40-square-mile oil field in Wyoming.
In a "proof of concept" test flight over a subsection of the oil field, SEQURE located 93 percent of the wells within the test area, including 100 percent of the pre-1930 wells, whose locations were more likely to be unknown because of insufficient records. SEQURE was recognized with an R&D 100 Award in 2007. It promises to be a valuable tool in the monitoring, verification and accounting end of the carbon sequestration chain.
Thus, NETL is involved in all aspects of advanced energy systems, from modeling and testing of novel combustion and gasification systems to large scale demonstrations of carbon sequestration, including monitoring and verification. These efforts are developing technologies to convert domestic resources into a variety of clean fuels with low emissions, capture carbon dioxide, and demonstrate value-added carbon sequestration as an option for the nation.
Diane Newlon is a technology transfer officer at NETL; Larry Headley and Timothy Palucka are senior associates at the lab's technology and management services.
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