COALCELL The gasifier will supply the fuel, and an electrochernical reaction will generate the power.

By Jeffrey Winters, Associate Editor Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/125/12/42/6355526/me-2003-dec5.pdf by guest on 25 September 2021

IT'S A CLlCH E in disc ussions about energy gen- preparing to receive a two-megawatt fuel cell system. de­ eration that is the dirtiest signed to convert gasified coal into electricity. fuel around. And, sure, there's a lot of evidence around The cells were fabricated by FuelCell Energy of Dan­ to support that view. Coal mining is incredibly disrup­ bury, Conn., at a 65,000-square-foot facility in nearby tive, and coal is heavy and bulky, involving rumNing Torrington. They are scheduled to be installed at the freight trains to transport it. And most of all, to make Wabash River plant by the end of the month and run for ""OLOG~ electricity, coal is burned in boilers, one year. If the concept lives up to its promise, it might ...",L ~V~ "oop with the exhaust vented through smoke- well revolutionize the way we think about coal as a fuel. -;;'))i\~ stacks and the ash carted to landfills. Coal isn't generally thought of as being compatible "'v ~.... "'rA I NABI\.\ The idea that fuel cells are every bit as with fuel cells. But the coal at the Wabash River plant is clean as coal is dirty is just as widespread. Fuel cells, after first turned into a hydrogen-rich gas. all, take hydrogen and oxygen, and combine those ele­ Coal is hardly a new technology. The first ments to make electricity and water. Because it is a com­ instances of gasifYing coal date to the late 1700s, when it bustionless reaction, there are no byproducts such as ni­ was part of a method for making tar. By 1812, commer­ trogen oxides, and the whole process promises to be far cial companies were supplying for heating and more efficient than burning fuel. lighting London. (The famous "gaslights" were powered So, what would yo u get if you could combine coal and by coal gas .) By the start of the 20th century, most major fuel cells? Which attribute would dominate-dirtiness or cities in the developed world had gasworks, which often cleanliness? dominated skylines with their towering tanks. That's the question about to be answered in a power To get coal gas (also known at the time as "town gas," plant in Indiana as part of a pilot program, sponsored by and today as ""), coal or another hydrocarbon was

the U.S. Department of Energy. The Wabash River Coal A FuelCell Energy power plant like this one will soon convert coal gas into Gasification Repowering Project in West Terre Haute is electricity. Such future combinations could provide zero-emission energy.

42 D ECEMBE I1. 2003 MEC HA N I CAL ENG I NEE RI NG pulverized and placed in a sealed container. The fuel was River Plant. " It's cheaper than coal, but it's 5.5 percent sul­ heated to over 1,500°F until compounds, such as carbon fur. A conventional utility boiler would never have enough monoxide, , and hydrogen, separated from gas cleanup to run on a fuel with that much SUlfi.lr." heavier tars and solid . In many ways, gasification At the Wabash plant, is transformed from a prob­ can be thought of as a type of fuel-rich, low-oxygen lem to an asset. "We make about a railroad tank car a day combustion. of sulfur," Amick said. "We sell that to a broker, and it The process was not without its problems. Coal is a ends up in fertilizer." complex fuel, with sulfur, metals and other impurities, The system, which included a heat-recovery steam and the residue from the gasification process was often generator, started operations in 1995 and proved to be 40 toxic. Former gasworks sites are often contaminated with percent efficient over a four-year demonstration period. hazardous wastes. Another critical concern was efficien­ What's more, the plant (now rated at more than 260 cy: Early gasification methods delivered only 25 percent MW) captured sulfur with more than 99 percent effi­ Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/125/12/42/6355526/me-2003-dec5.pdf by guest on 25 September 2021 of the coal's original energy content into gas form. Of­ ciency and generated undetectable amounts of particu­ ten, gasification was viewed primarily as a means to pro­ late emissions. duce coke rather than gas. Another DOE demonstration project, the Polk Power Gasification technology improved in the 1920s and Station near Tampa, Fla., rated at 250 MW, was started 1930s, yielding more gas and less coke. But by then, nat­ from scratch. The Polk Power Station uses a slightly dif­ ural gas was being piped into industrial areas. Cheaper ferent gasification technology, but turned out to be every and cleaner than coal gas, began to dominate bit as clean; in fact, both plants lay claim to being the the market wherever they competed head to head. cleanest coal-fired generators in the world. In places such as Germany, where coal is plentiful and The success of these gasifiers in electrical generating fa­ natural gas is rare, gasification remained an important cilities has spurred the DOE to push for more uses of the technology. In recent years, coal gas has been produced technology. Gasifiers are seen as potential sources for in the United States for niche applications, such as raw mass-producing hydrogen for transportation uses. And in material for chemical plants. February, Secretary of Energy Spencer Abraham un­ The energy crisis of the 1970s brought an upsurge in veiled plans for building a billion-dollar prototype fossil interest about portable fuels from coal and other miner­ fuel power plant that would combine electricity and hy­ als. But the spike in oil prices was too short-lived, and drogen production with the virtual total elimination of the push for alternative fuels died out. The technological harmful emissions, including greenhouse gases. development continued, however, and by the 1990s, the Department of Energy began funding Trainloads of coal enter research as a possible pathway to reduce carbon dioxide and electricity leaves. It sounds enussions from power plants. less like industlY than like nlagic.

TRANSFORMING WABASH RIVER Such a plant, called FutureGen, would have a gasifier at The program, called the Clean Coal Technology Pro­ its heart. gram, was, in part, an effort to promote commercial­ "Gasification is the cleanest way to make power from scale integrated gasification combined-cycle, or IGCC, coal," Amick said. "And since you've converted all the coal power plants in the United States. Two such plants energy to a gas and pu t it in a pipe at press ure, you can came online in the ITlid-1990s. The first was a 1950s­ even remove the CO2. It's still expensive, but it's easier vintage pulverized coal-burning boiler in western Ind.i­ with a gasifier than with anything else." ana, the Wabash River plant, operated by Global Energy Coal gas trad.itionally is made of a nllxture-mostly hy­ of Houston. drogen and carbon monoxide. H ydrogen can be used in a The plant was rated at 90 MW and ran at 33 percent effi­ number of applications, from gas turbines to fuel cells. ciency. As part of the demonstration project, the plant was Carbon monoxide is a bit trickier to deal with. It burns gutted and connected to an advanced gasification system. just fine, though it doesn't have as high an energy density The coal is first slurried, then combined with pure oxy­ as other fuels. But carbon monoxide can foul many fuel gen and subjected to high temperatures and pressure. This cells. It gloms onto the catalysts and destroys their ability not only partially oxid.izes the slurry, but also melts the coal to catalyze chemical reactions. For cells such as proton ex­ ash, enabling it to flow out of the gasification chamber. change membranes or phosphoric acid fuel cells, CO The remaining gas moves into a second stage gasifier must be scrubbed from the fuel stream at all costs. that enhances its heating value. After the gas is cooled, Removing carbon monoxide is not a large problem. It re­ it's scrubbed of chlorides and sulfides, leaving a mostly quires making it react with high-temperature steam to make pure fuel stream to be piped into a gas turbine. hydrogen and carbon dioxide. But it comes at a price in "We've been running on petroleum coke since 2000," terms of energy consumption and complexity. Researchers said Phil Amick, director of gasification technology at at Ohio University in Athens are working to develop a fuel ConocoPhillips and former project manager of the Wabash cell system tl1at can run on normal coal gas.

M ECH AN I CA L ENG I NEERI N G D ECEMB ER 2003 43 Professo r D avid Bayless is experi­ menting wi th small stacks of pl anar solid oxide fu el cell s, or SO FCs . " With the planar SO FCs, ca rbon monoxide is not a po ison," Bayless said. "It's not as good a fu el as hydro­ gen-it doesn't have the energy con­ tent- but planar SOFCs can use it." The question, Bayless said, is how many impurities-such as and metals such as mercury­ the SOFCs ca n handle. The greater Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/125/12/42/6355526/me-2003-dec5.pdf by guest on 25 September 2021 tolerance they have for impure fu el streams, the cheaper and easier it will be to connect them to coal gasifiers. " If this is going to be viable in the long term," Bayless said, "the cost of th e fu el cell has to be competitive with other energy conversion sys­ telns." The Wabash River plant in Indiana will start sending coal gas into a fuel cell early next year. I n fa ct, Bayless envisions gasifi ers and fu el cells as pieces of a revamped rural energy system. ket." And FuelCell Energy believes that its technology is Coal gas can be separated into two streams: pure hydro­ well suited to coal gas since it can run directly on methane gen, which can be used for transportati on purposes, and and carbon monoxide as well as just pure hydrogen. (A car­ ca rbon monoxide mixed with tra ce impurities . If this bonate fuel cell needs a carbon fu el to provide the carbon second stream can be converted directl y into energy, it dioxide needed on the ca thode side of the cell. ) could make rural coal not just a cheap fu el, but a clea n "The fu el is m ore dilute than pure m ethane," Maru and effi cient one as well. said, "so we have to design the fu el cell system for this "The effi ciencies of coal plants right now are about 37 dilute stream." percent," Bayless sa id. "With fuel cells, you're talking One way to increase the energy density of the stream is about a theoretical effi ciency of 70 percent. So right by methanation. The two major components of coal gas, there, you're almost doubling the energy conversion rate. carbon monoxide and hydrogen, are reacted catalytical­ And if you are using SOFCs, it's hot enough to make ly to make methane and steam. On the one hand, this steam or to convert into another industrial process, so you adds a processing step and thus unwa nted complexity. have the potential for much higher energy utiliza ti on. But the reaction is exothermic-and so is self-sustain­ "This is good for coal, long-term. If you are using it ing. And the extra heat ca n help drive other processes, more effi ciently, it makes it a more valuable fu el. And less including ones designed to remove trace levels of sulfur input for the sa me usable output just has to be good for from the fu el stream . th e envirol1.n1.ent." Steinfeld said the project is aiming for effi ciencies in the 48 percent range, a 20 percent improve ment ove r the effi cien cy of the current rGCC plant at Wabas h CARBONATE DESIGN River. But that increase will be scarcely noticeable in The experiment in Indiana starting next year won't in­ the overall operation of the plant: The eight fu el cell volve a solid oxide fu el cell , though. Instead, FuelCell stac ks will be receivin g less than 1 percent of the coal Energy will be installing a molten ca rbonate fuel cell gas stream, with the rest being sent on to the turbines . stack power plant. And the cell s w ill operate at times from natural gas One adva ntage of the carbonate design is in scale. Pla­ ra ther than coal gas. nar SO FCs are still so mewhat e)..'}J erimental and availabl e Still, if the demonstra ti on proves successful , it might in sta cks on the order of a few thousa nd watts. Molten well lea d to new plants along this m odel. (All parties carbonate fu el cell stacks routinely weigh in at 250 kilo­ agree that established plants are unlikely to convert to wa tts. For the Wabash River demonstrati on, eight stacks this technology.) This is es pecially true of new plants in will be combined for a total of2 MW George Steil1.feld, built-up areas, where gas turbines have prove n fa r more director of systems development at FuelCell Energy, said popular of late than coal-fired boilers. "The typical im­ it will be the largest carbonate fuel cell power plant oper­ age of coal goes away when you combine it with a fuel ating on coal in the world. cell," Steinfeld said. FuelCell Energy has been planning for this sort of project Indeed, such a power plant probably wouldn 't need a for more than 20 years. " Fifty percent of U.S. power is traditional smokestack. Trainloads of coal enter; electric­ generated from coal," says Hansraj C. Maru, FuelCell En­ ity, ca rb o n di oxide, sul fur and vario us trace m etals ergy's chief technology offi cer. "It's a large part of the mar- leave-it so unds less like industry than like magic. _

44 D ECEMBER 2003 M EC H A I CA L E G INEERI NG