Winner: Restoring Coal’s Sheen

The industrial age, wrote historian Barbara Freese, “emerged literally in a haze of coal smoke, and in that smoke we can read much of the history of the modern world." In boom economies like India's and China's, where coal meets about three-quarters of the electrical demand, that haze still hangs heavily. Globally, according to a recent influential study done at MIT and data from the International Energy Agency, in Paris, coal accounts for a quarter of energy consumed and more than two-fifths of ­the electricity generated. That makes it the second leading fuel after oil and the world's main source of ­greenhouse-gas emissions.

You can add up all the electricity produced in the world from renewable sources plus nuclear reactors, and it doesn't amount to what coal generates just in the United States and China. It's impossible to imagine our getting along without coal anytime soon. And yet, with concerns rising sharply about climate change, the general expectation is that governments will increasingly be penalizing carbon emissions by taxing them, regulating them, or forcing companies to trade in them. So burning coal could become radically more expensive unless efficient means are found to capture and permanently store carbon dioxide, which right now is pumped into the atmosphere in astonishing quantities.

In the United States alone, according to MIT, coal-­burning power plants produce about 1.5 ­billion metric tons of CO₂ a year—roughly a quarter of the world's total—which is about three times the weight of the total amount of natural gas the country uses each year and nearly twice the volume of oil it consumes annually.

Just capturing the carbon, not to mention finding sound ways of sequestering it, is a job of staggering dimensions and one that the world has just barely begun to address, as the MIT report emphasized. There's been a lot of talk about it, but hardly anybody is doing anything about it. “We need large-scale demonstration projects," a summary of the MIT report said, bluntly.

One company that is doing that kind of demonstration right now is Vattenfall, Sweden's national energy company, in Stockholm. It's building a novel clean-coal plant in southeastern Germany, in a town called Schwarze Pumpe. The approach Vattenfall will test and evaluate at the 30-megawatt ­facility—a technology called oxyfuel, or sometimes oxyfiring—is not the one most favored by students of carbon capture. But it appealed to Vattenfall partly because of its disarming simplicity.

In the oxyfuel process, instead of burning coal in air, the nitrogen is first extracted from the air using standard industrial equipment, so that the coal can be combusted in an atmosphere of oxygen and recycled flue gases. The result is a flue-gas stream containing almost none of the nitrogen that otherwise complicates the separation of carbon dioxide. Once the sulfur has been scrubbed using standard procedures, the flue gases consist essentially of just water vapor and carbon dioxide. The water is separated by condensation, and presto, the carbon dioxide is ready to be compressed and liquefied for transport to a final storage site. In this particular case, Vattenfall will have the CO₂ trucked to a region called Altmark, where it will be injected into a natural gas reservoir, initially to enhance gas recovery, and ultimately for final disposal.

Why did Vattenfall settle on this somewhat eccentric approach to carbon capture? Back home, as Sweden's state-owned national utility, it traditionally has produced the country's electricity in hydropower ­stations and nuclear reactors, which for all practical purposes emit no carbon dioxide. But with the opening of Europe's ­electricity system to competition in the 1990s, Vattenfall began to expand outside Sweden and is more or less Europe's fourth largest electricity producer in terms of revenues.

At the end of the 1990s, Vat­tenfall acquired much of what had been East Germany's electricity system from West German energy companies, which had to sell them to meet competition rules. Those West German companies had already begun to improve and clean up the East German power system—which is based almost entirely on lignite—building several giant coal-burning plants, including a 1600-MW pulverized coal plant at Schwarze Pumpe.

The acquisition of the ­lignite plants in eastern Germany, together with the establishment of a European carbon trading system that will make emitting coal increasingly expensive, got Vattenfall's executives thinking about how to secure a future for its coal holdings and help meet commitments under the Kyoto Protocol. “The position we take is that there is a threat to the society and to the whole globe, actually. And so we need to do something," says Lennart Billfalk, an advisor to Vattenfall's CEO and the former manager of its R&D program.

Vattenfall is building the oxyfuel pilot plant at Schwarze Pumpe in close cooperation with the French firm Alstom Power, which is supplying almost all the major components except for the oxygen-­nitrogen separator, the desulfurization system, and the condenser that will remove the water, ­leaving CO₂ .

Best known for its supersleek and very fast TGV trains, Alstom, based in Levallois-Perret, is the world's No. 2 transportation company and No. 3 in power generation, behind GE and Siemens. The company sees oxyfuel as a growth opportunity and the Schwarze Pumpe project as a learning experience, says John Marion, vice president for global technology at Alstom's U.S. power subsidiary in Windsor, Conn. Marion says that Alstom has been looking closely at oxyfuel and that the Schwarze Pumpe project is the "most significant and advanced step globally" in the field of coal power with carbon capture. He adds that the company has been looking closely at oxyfuel prospects since 1997, because of Kyoto.

A quirky but important aspect of the Schwarze Pumpe plant [see diagram, "Just Take Out the Nitrogen"] is that flue gas is recirculated back into the combustion chamber in order to keep burning temperatures close to their levels in a regular coal-fired plant, near 1000 °C. Research engineers originally devised this procedure when oxyfuel combustion—which, by the way, is common in other industries such as steel, aluminum, and glass—was first visualized mainly as a retrofit technology for existing coal plants. If coal were burned in pure oxygen without recirculation, temperatures would get high enough to melt boiler walls. Recirculating the flue gases simulates, in effect, atmospheric burning conditions, with carbon ­dioxide substituting for nitrogen.

When a plant like the one at Schwarze Pumpe is custom designed, recirculation is theoretically not necessary; the boiler could be designed to withstand higher operating temperatures, and higher-temperature combustion could produce efficiencies. But the Vattenfall and Alstom designers wanted the boiler to be as similar as possible to standard boilers so that they could make close comparisons and scale up with greater confidence, says Marion. Also, coal typically contains between 5 and 30 percent ash, and if the ash melts in excessively high temperatures, it gets sticky, glasslike, and hard to handle.

Alstom would like to be able to sell utility-scale oxyfuel plants—not just major components—on a turnkey basis with the usual full guarantees by the middle of the next decade. And Vattenfall, too, would like to move aggressively with oxyfuel and have a precommercial plant in the ­­­250- to 300-MW range running by 2014 or 2015. Right now Vattenfall is evaluating seven larger carbon-capture projects in Denmark, Germany, and Poland and expects soon to select two, one of which is likely to be an oxyfuel plant. The company's economic target is to develop plants that will pay for themselves if carbon prices in the European cap-and-trade system stabilize at 20 per metric ton or higher.

The oxyfuel concept for coal-fired power generation origi­nated in the late 1970s at Argonne National Laboratory, near Chicago, according to Alan Wolsky, the leader of the team that pioneered the idea there. Wolsky, now a visiting fellow at the University of Cambridge, in England, recalls that the U.S. Department of Energy supported the team's research mainly on the grounds that more CO₂ was needed to inject into oil wells for enhanced recovery. Members of the group and their government sponsors were well aware, even then, that climate change was going to be a growing issue, says Wolsky, but neither they nor the Energy Department promoted the research on that basis.

The Argonne-led group did a series of small-scale demonstrations, controlling for factors such as the coal and gas mixture, temperature, and turbulence, and did computer simulations and ­analysis. The work attracted attention worldwide, and other experiments followed in Canada, Japan, the Netherlands, and the United Kingdom. It was a time when most work done at U.S. national ­laboratories was considered public property, and there was not much incentive to secure intellectual property. Wolsky remembers giving oxyfuel talks in Canada, only to be told a year later that Shell Oil had patented the content of his speech.

The initial oxyfuel demonstrations confirmed the technology's promise but also demonstrated the importance of implementing it carefully. For example, when a stoker-fed furnace was used in one demonstration, it was hard to keep air from leaking into the recirculation system; CO₂ concentrations in the flue gas were correspondingly low. Handling pure oxygen is always a dicey business, of course, and so there were concerns about safety. Nevertheless, nothing suggested that oxyfuel firing couldn't work or wouldn't work in a pulverized coal system.

Although Vattenfall itself believes that custom oxyfuel design is the way to go, the retrofit option continues to be assessed by a number of companies, including notably Babcock & Wilcox in Barberton, Ohio. B&W owns a relevant patent portfolio, and its executives have testified to the U.S. Congress on the promise of oxyfiring.

B&W was participating in a plan by SaskPower in Regina, Sask., Canada, to build a 300-MW lignite-burning oxyfuel plant, but that project was put on hold earlier this year and will be reassessed in 2009. Meanwhile, however, B&W has converted a test reactor in Alliance, Ohio, to do oxyfuel combustion. The program of oxyfiring tests began last October and will cost B&W US $14 million to $16 million. It concluded a run with bituminous coal in November and early this year will burn Saskatchewan lignite. B&W is partnering in this demonstration with the French company Air Liquide, a leading provider of liquid oxygen.

The Alliance test reactor, like Schwarze Pumpe, produces 30 MW of thermal energy. But it does not have an oxygen- nitrogen separation facility, and carbon dioxide is not being captured in the tests. B&W is ­planning a commercial-scale demonstration soon, with both custom-designed new units and retrofit in mind, and it considers itself, with Vattenfall and Alstom, a world leader in oxyfuel.

In terms of retrofit, the most important oxyfuel project on the books is in Australia, where the technology got a government go-ahead in November 2006. (Though Australia, until a new government was elected last fall, had declined to ratify the Kyoto Protocol, it authorized spending 400 ­million Australian dollars on the development of greenhouse gas– reduction technologies.) CS Energy, of Brisbane, Australia, working with partners in Australia's coal industry and Japanese manu­facturers, wants to backfit a decommissioned 30-MW boiler, Callide A, in Queensland. To that end, CS Energy is doing front-end design work and specifying costs for a project that would involve installing a nitrogen separation plant, flue-gas recycling equipment, a facility to compress and liquefy the carbon dioxide, and the means to transport the CO₂ to a storage site. There are at least a half dozen possible sequestration sites within several hundred kilometers of the plant, both depleted gas f­ields and saline aquifers, according to Chris Spero, who is in charge of oxyfuel research at CS Energy.

The retrofitted Callide A plant will burn bituminous coal, not lignite. Spero notes that Australia's soft coals are especially advantageous for oxyfuel retrofit because they are low in sulfur: the flue-gas recirculation system tends to concentrate the sulfur, making its removal more of a problem.

If oxyfuel retrofit could be made to work at low enough costs, the implications would be enormous. In principle, all the existing coal plants in the world could be refitted to run carbon free. But Vattenfall is quite skeptical about that scenario. Particularly because so much energy has to be used to separate oxygen from nitrogen at the front end, the whole process will probably be made economically attractive only when plants are scaled up and customized specially for oxyfiring, says Lars Strömberg, until recently chief engineer and project manager at Schwarze Pumpe and now Vattenfall's head of R&D.

Right now the standard oxygen-nitrogen separation equipment runs on electricity, which has to be obtained from the plant itself, reducing the plant's efficiency of energy conversion by several percentage points. With the development of membrane separation ­systems, however, the electrical cost of oxygen might come down. And if heat or steam were recovered from an oxyfuel plant to drive air separation, says Strömberg, and the whole plant were customized for oxyfuel at whatever scale turns out to be optimal, then the plant might register an efficiency gain of several points rather than a loss.

Oxyfuel is but one of three basic approaches to carbon capture and storage. In general terms, carbon can be separated from postcombustion flue gases by chemical means, as sulfur and nitrogen oxides are scrubbed, or the bigger part of the job can be done precombustion, either by gasifying the coal or by oxyfiring. In the United States, discussion of carbon sequestration has been dominated by the coal gasification ­scenario, which generally goes by the acronym IGCC, for integrated gasification, ­combined cycle.

IGCC involves converting coal into a synthetic gas that can be burned to drive steam turbines, just as if it were natural gas; the waste stream consists mainly of hydrogen, carbon dioxide, and water vapor. Four commercial-scale demonstration plants have been built and are operating, two in the United States and two in Europe. Studies comparing IGCC with oxyfuel and postcombustion carbon capture generally find costs in the same ballpark: the total cost of doing carbon capture and storage using any of the three approaches is likely to be between 25 and 75 percent higher, by comparison with standard pulverized coal. IGCC is generally considered slightly cheaper than oxyfuel, but with large uncertainties.

“There's a perception that IGCC is the only game in town, but our calculations indicate it's not the optimal choice, either for hard coal or lignite," says Alstom's John Marion.

IGCC plants are complicated structures that resemble small refineries. They tended to have problems in their early years of operation and by nature require a great deal of maintenance. Their relative economic attractiveness won't really be known until all three ­carbon-­capture approaches have been tested at much larger scales.

And although there are several IGCC plants that are considered adaptable to capture carbon, none have actually done so. So if carbon is captured at Schwarze Pumpe and disposed of permanently in a geologic repository, it will be a first—not just for oxyfuel, but for coal. Although carbon sequestration is not seen as an essential aspect of the project, Vattenfall wants to do a fully integrated demonstration to win public confidence. Stabilizing liquefied carbon dioxide at depths of a kilometer or more has been demonstrated in the North Sea, Canada, and northern Africa.

Vattenfall's Schwarze Pumpe plant builds on a well-developed approach that seems sure to be a part of the solution to the coal-carbon problem. Even if other approaches turn out to be superior for some types of coal, oxyfuel is uniquely suited to lignite, a low-grade and dirty coal found in superabundance in eastern Germany and in some other parts of the world, including Poland and regions of the United States and China. It's likely to be suitable as well for low-sulfur bituminous coals and anthracite.

But even if—contrary to expert expectations—oxyfuel proves to be a technical or economic failure, Vattenfall will still have achieved a moral victory of sorts. This is because Vattenfall will have been the first to initiate and complete a project of significant scale to demonstrate carbon capture and storage with a coal plant.