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The Search for Clean Coal

March 5, 2008


Coal is still the fuel of choice to generate electrical power for many nations; including some of its greatest polluters like the U.S., China and India. One of the reasons is that coal reserves remain abundant — the U.S., Russia, China, India, and Australia have the world’s largest coal reserves. Environmentalists decry the continued use of coal-fired power plants, but alternatives are expensive and, in the case of nuclear power, controversial. One of the questions being asked during the current race for president in the U.S. is “who is the greenest candidate?” One of things that environmentalists look at to make such a judgment is the candidate’s stand on building new coal-fired power plants. Hillary Clinton’s position is that utility companies must show that current and future demands cannot be met by creating more system efficiencies before she would be willing to support building new coal plants. She supports investing in liquid coal if it reduces carbon pollution by 20%. Her opponent, Barak Obama, is a little less green when it comes to supporting liquid coal — he supports investing in liquid coal if it reduces carbon pollution by 10% — but he is considered “greener” on the issue of coal-fired power plants because he is willing consider standards that ban new conventional coal plants. The Republican nominee, John McCain, has articulated no position on these issues. Whichever of these candidates is eventually elected president, he or she will face two daunting challenges: keeping the U.S. economy growing (which means increasing the supply of electricity) and addressing global climate change (which means reducing emissions). Add to that the growing call for energy security and it is easy to understand why finding a way to burn coal cleanly is the Holy Grail. Into this picture steps a Swedish energy company, Vattenfall, that is trying a novel approach to capturing carbon emissions at coal-fired power plants [“Restoring Coal’s Sheen,” by William Sweet, IEEE Spectrum, January 2008]. Sweet begins his article by stating the challenge.

“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.”

That’s where Vattenfall, Sweden’s national energy company enters the picture. Surprisingly, the process they are testing is being operated at a plant in Germany not in Sweden. The reason, as explained later in the article, is that Vattenfall produces almost all of its energy in Sweden using hydropower or nuclear power. When competing for business in a coal-rich country like Germany, however, Vattenfall realized it needed to master coal-fired processes.

“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.”

When you read “simplicity,” you normally interpret that to mean cost effective and reliable; characteristics valued by corporations. The problem right now is that oxyfuel plants are experimental and expensive. Nevertheless, here is how the process works.

“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.”

Vattenfall became interested in coal-fired plants at the end of the last century when Europe opened up electrical generation and distribution for competition.

“At the end of the 1990s, Vatttenfall 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.”

For those unfamiliar with variations in coal, here’s a quick primer. All coal is believed to be of plant origin. As plants die and accumulate, they are compressed and changed over time to become peat, lignite, bituminous coal, or anthracite coal (with even more variations in between). The futher up that chain the harder the coal. Lignite, the fuel being burned in the Schwarze Pumpe plant, is sometimes called brown coal. It doesn’t burn as hot as true coal. Vattenfall’s strategic partner in this venture is the French firm Alstom Power, which Sweet reports, is the world’s third largest producer of power generation equipment, behind GE and Siemens.

“Alstom Power … is supplying almost all the major components [for the Vattenfall plant] except for the oxygen-­nitrogen separator, the desulfurization system, and the condenser that will remove the water, ­leaving CO2. … 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.”

As anyone who has any experience in a lab or a hospital, or who has parents who need oxygen tanks in order to breathe, fire and oxygen is dangerous mixture — it burns hot and fast. That could be a problem in an oxyfuel system. Sweet explains how that challenge is handled.

“A quirky but important aspect of the Schwarze Pumpe plant 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 clo
se 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.”

Sweet reports that Alstom hopes to market turn-key oxfuel power plants around the world while Vattenfall is looking to build and operate oxfuel plants elsewhere in Europe. The fact that the technology is being considered in Europe, which has a much better environmental reputation than the United States, indicates that such plants could play a vital future role in helping control greenhouse emissions. Ironically, the oxyfuel concept originated in America.

“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 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 new, custom-designed oxyfuel plants are probably preferable to retrofitting older coal-fired plants, the fact is that retrofitting is likely to provide a significant business opportunity.

“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. … 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.”

Getting environmentalists to buy off on any kind of coal-fired plant would be a major coup. If Vattenfall can prove the plants can operate effectively, efficiently, and competitively, coal may yet have a future in even in the greenest of countries. For countries like the U.S., China, and India, that want to use their coal resources, finding a clean coal-burning plant should be high on their list of technologies to watch.

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