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.

What The Experts Say

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.