publications and resources
Interstate Water Report
 |
| Download PDF of current issue |
Current Issue
Firsthand Look
How One New England Power Plant Plans to Get the Mercury Out
by Stephen Hochbrunn, NEIWPCC
As rule, people don’t visit coal-fired power plants. We know they’re out there, we occasionally see them lurking heavily in the distance, but go to one? The grim, fortress-like structures seem to be telling us—stay away.
Despite their central role in producing the nation’s power, we rarely even think of them. They cross our minds when there’s a problem—as on November 6, when three workers died in an explosion at the power plant in Salem, Mass. We may think of them when we hear of a mine collapse, of men dying in search of the raw material to electrify our homes, businesses, and iPods. But usually, they are far from our thoughts. And nowhere in our travels.
That is, unless your goal is to find out exactly how a coal-fired power plant removes mercury from its emissions. The Northeast Regional Mercury TMDL urges EPA to enact a rule requiring all such plants to control mercury emissions by 90 percent. But it doesn’t specify how this is to be achieved, other than to say cost-effective and available technologies should be used. What are these technologies? It seemed there was no better way to find out than to do the unusual—visit a coal plant. And preferably one well down the road to employing mercury-reducing systems.
We found one right in the neighborhood. An hour’s drive northwest of NEIWPCC’s Lowell, Mass., headquarters, along the west bank of the Merrimack River in Bow, N.H., sits a facility that burns through 4,000 tons of coal a day as it generates enough power to satisfy the needs of a third of New Hampshire’s population. Public Service of New Hampshire (PSNH), the state’s largest utility, owns and operates the plant, known as Merrimack Station. Since it burns coal, which contains a trace amount of mercury, the plant emits mercury into the atmosphere—not a lot compared with many other plants around the country, but it doesn’t take much to threaten the environment and public health.
Reducing mercury emissions is now a priority at Merrimack Station, especially after the passing of strict legislation in New Hampshire in 2006. Of course, -making something a priority doesn’t guarantee success. But interesting, innovative mercury strategies are being pursued at the plant. It’s just going to take time.
Close Encounter
Located several miles to the east of the heart of Bow, Merrimack Station stands alone, its great mass towering over low, barren surroundings. The sheer hulk of the plant and its worn, drab skin make it appear primeval, as though forged in a bygone era. In truth, the plant is less than 50 years old. But on August 28, the day of our visit, it looked from the parking lot as if it had always been there—a fixed feature of the landscape, immovable, relentless and unceasing in the creation of its elemental product.
It had been months since the initial request for a visit was posed to Martin Murray, who does media relations for PSNH. Given the bashing Big Coal has taken lately in the press and in Washington, the delay was understandable; clearly PSNH was in no particular hurry to entertain visitors at the plant, especially ones from an organization they’d never heard of. But our request must eventually have been deemed straightforward enough. Murray greeted us amiably, and introduced Harold Keyes, the plant manager.
Keyes was prepared. He said he’d read the Mercury TMDL the night before, and posed pointed questions to NEIWPCC’s Susy King, an author of the TMDL who had come along on the visit. Her thoughtful responses diffused skepticism about our intentions, which were simple—to observe and to learn. Keyes warmed. He began speaking proudly of his plant, which stood silently, stolidly behind him, betraying none of the extreme ferocity of the process underway within.
Merrimack Station generates 478 megawatts—enough electricity for roughly 200,000 residential, commercial, and industrial customers—through a process that at its core features two boilers, where coal is burned around the clock at 3,500 degrees Fahrenheit. They’re called boilers because they contain tubes filled with water that, amid the intense heat, rapidly becomes steam, which is both exceedingly hot (1,000° F) and highly pressurized (2,400 lbs. per square inch). The searing steam screams through tubes to a turbine, where the blast of force is powerful enough to turn the turbine’s blades at 3,600 rotations a minute. It’s that rotation that creates electricity. Connected to the plant are six high-voltage transmission lines that carry the power to substations throughout the state.
It’s a remarkable process, awesome in its brute ability to produce mass amounts of electricity. But you can’t ignore the downside: a coal-fired power plant without emissions controls emits dangerous levels of pollutants, including sulfur dioxide and nitrogen oxides, which cause acid rain and smog.
At Merrimack Station, they’ve installed systems to reduce pollution, including two selective catalytic reduction systems, known as SCRs, that dramatically lower emissions of nitrogen oxides by inducing chemical reactions that turn the NOx into harmless water vapor and nitrogen gas. The plant also has two electrostatic precipitators, or ESPs, that impart a charge to the coal ash (or fly ash) in the plant’s exhaust so the potentially polluting ash sticks to metal plates instead of soaring out the smokestack.
For several minutes, Keyes boasted about the progress the plant’s made on cleaning up emissions, and you couldn’t blame him. The plant after all has received two Environmental Merit Awards from EPA as well as a Governor’s Award for Pollution Prevention.
“Historically at Merrimack Station, they’ve been fairly proactive in responding to environmental concerns,” said Craig Wright, a permitting bureau administrator with the New Hampshire Department of Environmental Services’ Air Resources Division, in an interview before our visit to Bow. “They went way above and beyond [environmental regulations] with their SCR systems.”
But Keyes knows all too well that there’s a new issue in town.
“Just when we thought we were turning the corner environmentally,” he said, “mercury pops up.”
Evolution of a Solution
Actually, Keyes should have seen it coming, and no doubt did. For years, environmental groups pushed for more aggressive government action on mercury emissions by power plants. The power industry fought back, emphasizing that U.S. electric utilities contribute just one percent of total global mercury emissions. But there is no getting around disturbing data: according to the latest EPA figures on mercury emitted by U.S. power plants, the top 50 mercury polluters sent almost 21 tons of mercury into the environment in 2005. The worst offender, the massive Martin Lake Steam Electric Station in Texas, alone emitted more than 1,700 pounds of mercury.
By comparison, Merrimack Station’s reported contribution of 130 pounds seems tiny—but it’s hardly insignificant. Mercury is highly bioaccumulative, meaning its concentration increases as it moves up the food chain; that’s why levels in fish tissue are far higher than in the surrounding water. So while the amount of mercury in coal is minute, and the amount being emitted at any given moment by a power plant is exceedingly small, it adds up in the environment—and to alarming levels.
In March 2005, the Conservation Law Foundation, an environmental advocacy group, went on the offensive against Merrimack Station, saying its mercury emissions were enough to poison 60 million pounds of fish and threatening to sue the plant to force it to slash its mercury output. CLF and other likeminded groups weren’t particularly appeased five days later when EPA issued its Clean Air Mercury Rule, which aims to achieve a 70 percent reduction in total power plant mercury emissions by 2018 through a cap-and-trade approach: companies that reduce emissions can sell credits to those that don’t reduce them.
Environmental groups pushed for tougher restrictions, and in early 2006, state lawmakers in New Hampshire upped EPA’s ante and passed legislation specifically targeting Merrimack Station. The law prohibits PSNH from buying credits to meet mercury emissions restrictions, and mandates an 80 percent drop in the plant’s mercury discharge by 2013. The law specifies how it’s to be done—through the installation of a wet flue gas desulphurization system. As the name implies, such systems were designed to reduce sulfur dioxide, which they do very well. But they can also capture mercury, and at very high levels.
Keyes quickly dashed our hopes of seeing signs of this system under development during our visit. “We’re far from construction,” he said. “This is a very complicated project, and the amount of engineering required to make it happen is immense.”
Keyes did come armed with a diagram (similar to the one on this page) that explained how the system, commonly called a wet scrubber, will work at Merrimack Station. A boiler’s exhaust, or flue gas, will first pass through an ESP before being directed into a large vessel (the scrubber). There, the gas will be sprayed with a mix, or slurry, created at the plant by combining limestone and water. A chemical reaction ensues, in which the slurry will absorb virtually all the sulfur dioxide and mercury in the flue gas, removing them almost entirely from the emissions stream.
Actually, mercury in its simple, elemental state isn’t captured in a wet scrubber. But if the mercury has been oxidized, it’s captured very efficiently. How to accomplish the oxidation? Here, Merrimack Station gets off easy. It so happens that the mercury in its flue gas is already being oxidized as it passes through the plant’s SCRs, the units installed to reduce nitrogen oxides.
PSNH expects to spend about $250 million on designing and building the wet scrubber and various projects related to it—a new chimney must be installed, and systems for handling the limestone, mixing the slurry, and removing the waste products must be developed. A scrubber of course doesn’t eliminate mercury; it just scrubs it, so to speak, out of a plant’s emissions. Just how PSNH will deal with its scrubber waste, and the very small amount of mercury within it, is still being determined. One option takes into account that the waste produced by a limestone-based scrubber is -actually a synthetic gypsum, the very material used in the making of drywall, plaster, and other products.
Although the mercury in the gypsum is a concern, and has been the focus of research, it’s hard to find any definitive conclusions about potential hazards. Other plants with scrubbers successfully sell the gypsum for industrial use, and PSNH is open to that idea. But Keyes emphasized it wouldn’t be a moneymaker.
“We might be able to break even,” he said, “and cover the cost of transporting [the gypsum]. Selling it wouldn’t save us money, but it might prevent us from spending more.”
A Cheaper, Challenging Fix
Any mercury reduction from a wet scrubber is a long way off—but evidence of another mercury project, smaller in scale but already underway, couldn’t be missed. A gleaming new silver silo stood out in sharp contrast to the largely colorless structures elsewhere. Keyes made it the first stop as we embarked on a tour of the plant.
The silo, he explained, is an integral part of PSNH’s experimentation with activated carbon injection. In ACI, as it’s called, powdered carbon is injected into a power plant’s flue gas, and if it’s working right, the mercury in the flue gas is adsorbed by the carbon sorbent—that is, it collects on the surface of the carbon particles. The mercury-laden particles are then carried off with the rest of the flue gas into whatever system a plant is using to remove particulates, which in Merrimack Station’s case, is an ESP. There, the carbon, along with its adsorbed mercury, is trapped like any other particle.
ACI is a fairly well established technology; it’s been employed at plants across the country and more often than not achieves high mercury removal rates. But it can be tricky. There are different types of carbon sorbents, and no guarantees about their effectiveness.
“A sorbent that works on one plant,” Keyes said, “may not work on another. They’re site specific.” That’s because not all coal-fired power plants are alike. The boilers at Merrimack Station, for example, are cyclone boilers, which are hardly commonplace. In a cyclone boiler, circulating air spins the coal as fast as 100 miles per hour to maximize combustion. The plant’s SCR units also affect the chemical makeup of its flue gas—in a good way for scrubbers in that they oxidize mercury, but in a bad way for ACIs. The SCR process creates sulfur trioxide, which interferes with the adsorption of mercury by the carbon particles.
Merrimack Station’s two ESPs are also unusual—most plants have just one—but for an ACI system, that comes in handy. Before embarking on its first ACI test, PSNH decided to inject the carbon between the two ESPs. To understand why, consider how the ESPs work together: the first one knocks out a lot of the fly ash in the flue gas, the second one collects what’s left. Put your ACI in between, and you end up with a lot less ash containing mercury and increased carbon levels than if you’d injected before the entire ESP process. That’s important since post-ACI ash has to be disposed of properly, and can’t be sold for use in concrete as can regular fly ash.
The first test of an ACI system at Merrimack Station proved to be disappointing, with mercury emissions reduced by just 15 percent. So PSNH applied to the U.S. Department of Energy for a grant to try again—and got the money. In February 2006, DOE announced a $2.5 million award for full-scale ACI field testing at the plant. PSNH kicked in $1.5 million of its own, and ADA Environmental Solutions, the Colorado-based contractor selected for the project, put up $300,000. A new round of tests began.
Again, disappointment. “We were nowhere near the 70 to 90 percent [mercury] reduction we’d hoped for,” Keyes said.
The problem was the sulfur trioxide from the SCRs. Keyes and his cohorts decided on a new approach: to inject not just carbon into the flue gas, but also a sorbent specifically for the SO3. It was that sorbent that was being stored in the new silver silo that we admired as Keyes told his tale—a tale which, at this point, got vague, intentionally.
“We did get better results [in the second DOE test],” he said. “But if I told you what they were, I’d have to kill you.” Hyperbole of course, but not without some truth. The DOE grant stipulates that anything learned from the testing must be shared freely with all interested parties, but that only DOE can disseminate the information.
In the spring of 2007, PSNH began a long-term six-month test of the ACI system, but quickly suspended the test as the plant underwent annual maintenance procedures. The test was still on hold during our visit, but Keyes said he hoped to restart it in October. If the long-term test reveals acceptable mercury removal rates, PSNH plans to keep the system running—at least until the wet scrubber comes on line.
Learning Excursion
After our stop at the ACI silo, our tour continued on to the rest of the plant—though from a mercury standpoint, there was little more to see. Keyes did take us into the cramped area that houses the base units of the plant’s two mercury continuous emissions monitors, made by Waltham, Mass.-based Thermo Fisher Scientific. He called their performance a work in progress, and said he wasn’t sure when they’d be ready for “prime time”—that is, January 1, 2009, when EPA’s strict new requirements for mercury monitors go into effect.
As the tour went on, the focus on mercury waned amid the astonishing nature of what we were seeing and feeling: the blanketing heat as we walked in the building’s bowels, near the base of a boiler; the small, thick window through which molten residue from the coal-burning could be seen falling in fiery vertical streams; the cleanliness in this place of epic combustion (“We work hard to keep it that way,” Keyes said); the control room where operators monitored all that occurred in the vast complex, eyes fixed on screens to catch the slightest deviation from the norm.
Outside the plant, we strode aside railroad tracks, upon which twice a week rumble trains 90 cars long, each car holding 100 tons of coal from mines in Pennsylvania, West Virginia, and two sources in South America. We looked out at a yard where mountains of coal lay on standby. Like any coal-fired power plant, Merrimack Station is vulnerable to strikes in the railroad and mining industries; regulations require that it keep 300,000 tons of coal always on hand, just in case.
Emerging from our immersion in the plant’s operations, the talk returned quickly to mercury. Will it really be 2013 before the wet scrubber is up and running?
“If we were the only ones building a scrubber,” Keyes said, “we could get it done a lot quicker. We’re trying to build in the biggest scrubber boom this country’s ever had.” The wait for scrubber parts, he said, can last as long as two years.
Still, the New Hampshire law contains incentives for early compliance with the mercury reduction targets.
“If we can do it sooner, we will,” Murray said.
Not long after our visit came evidence that the process of building a scrubber was indeed moving along. On October 3, Washington Group International, a Boise, Idaho-based firm, announced it had entered into a contract with PSNH to manage the scrubber project. In an email in early November, Murray wrote that Washington Group had already started engineering and design work for the job.
As for the ACI system, Murray wrote that the long-term test had yet to resume, but should soon. He blamed the delay on the need to buy new equipment to dispose of the ash collected by the second ESP, ash that will contain the injected carbon and mercury. The new equipment, Murray wrote, will be used to moisten the ash so it can be used as a landfill cover.
Promising Progress
The mercury technologies being implemented at Merrimack Station are not the only options available to power plants; there are others with exotic names—corona discharge, circulating fluid bed, electro-catalytic oxidation. But they’ve yet to catch on to the extent of ACI and wet scrubber systems.
For now, PSNH’s actions on mercury at Merrimack Station have placated its critics. The Conservation Law Foundation never has filed its threatened suit against the plant, and in a November interview, Melissa Hoffer, the director of CLF’s New Hampshire Advocacy Center,
said it appeared that PSNH was moving in the right direction.
“We’re aware they’ve been making some significant investments at Merrimack,” Hoffer said, “and we remain hopeful that the ACI and other actions they’re taking will reduce mercury emissions.”
With the ACI system on hold, and the wet scrubber many years from completion, hopeful is about all anyone can be. As we stood in the parking lot again, our tour complete, it was clear that despite the impressive projects outlined by Keyes, no mercury reduction was being done at that moment. As much was being emitted by the plant’s ominous smokestack as ever had been. That didn’t seem to bother Keyes. He’s got his plans. And his pride in his company’s environmental record.
“We’ve done everything we’ve been told to do,” he said, “and often gone beyond that. Our job is to provide electricity, and to provide it in a clean way.”
Given it’s coal we’re talking about, not solar or wind power, Keyes might have worded that differently: can burning fossil fuels for power ever really be clean? The best we can hope for is to make the inherently dirty process as clean as possible, to keep emissions of its hazardous by-products such as mercury to a bare minimum.
Keyes knows this of course, and it’s hard to fault him for indulging in a little PR-speak toward the end of a long interview. The line about clean power sounded like one he’d uttered hundreds of times, in contrast to the unrehearsed feel that had permeated our conversation. He deserves credit for that open exchange, just as PSNH deserves credit for the work being done: at Merrimack Station, they are moving ahead with strategies proven to reduce mercury. Visiting the plant made that perfectly clear.
True, the progress may be happening a little later than some would have liked. But it’s happening. That was good to see.