Environmental showcase

Building a coal-fired power plant in Wisconsin isn’t easy given the state’s tough environmental regulations. Visions of smokestacks and acid rain are incompatible with its draw as an outdoor playground of woods, lakes, fishing, and hunting, so it’s no surprise that Wisconsin has historically been a net importer of electric power.

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Building a coal-fired power plant in Wisconsin isn’t easy given the state’s tough environmental regulations. Visions of smokestacks and acid rain are incompatible with its draw as an outdoor playground of woods, lakes, fishing, and hunting, so it’s no surprise that Wisconsin has historically been a net importer of electric power.

That could change as We Energies works to construct its new Elm Road Generating Station, part of its existing Oak Creek Power Plant complex south of Milwaukee. But, Elm Road is no ordinary project. It will be an environmental showcase bringing clean coal technology into operation using an unprecedented combination of technologies to boost efficiency and reduce emissions. The two-unit, 1,230 MW total plant, combines supercritical boiler technology with what is likely the most comprehensive U.S. pollution control system. None of the technology used is considered experimental, but it is being applied in ways to ensure exceptionally high removal levels of key pollutants. Construction of the first unit is scheduled to be complete in 2009 with the second unit due one year later.

Supercritical boiler

Elm Road will join a growing number of U.S. power plants using supercritical boiler designs. These boilers run with steam pressures around 3,500 psi, as opposed to more common subcritical boilers operating at 2,400 psi. While generating steam at higher pressures demands more robust equipment, it pays off in higher efficiency. “Units with subcritical boilers operate at 30-38% efficiency, depending on their age and design, while units featuring supercritical boilers increase efficiency to 42-48%,” says Joyce Dasch, power marketing manager for the power & water solutions division of Emerson Process Management. This gain can more than offset the additional costs for equipment and maintenance, plus reducing fuel consumption per kWh creates less pollution of all kinds.

While supercritical boiler technology has been in use for decades, most installations are in Europe and Asia with fewer plants, proportionally, in the U.S. This is changing however, with a growing number of gas- and coal-fired sites in North America. In addition to their efficiency, supercritical plants can change output more quickly than subcritical facilities and have larger operating output range than combined cycle facilities, allowing utilities to more easily modulate output to match demand.

Air pollution control

There are five major air pollutants that coal-fired plants need to control to stay within environmental regulations. Elm Road targets each using equipment that is exceptionally comprehensive:

• SO _x is neutralized in a wet flue gas desulfurization (FGD) scrubber using limestone slurry to react with sulfur compounds before they can create acid rain in the atmosphere;

• NO _x is neutralized using selective catalytic reduction (SCR) with ammonia injection and a catalyst bed to break down harmful compounds into nitrogen and water vapor;

• Ash and other particulates are captured using a fabric filter (baghouse) and a wet electrostatic precipitator (ESP). Normally, only one or the other of these is used, but at Elm Road they have both;

• Mercury is captured primarily in the scrubber, resulting in a 90% reduction, and;

• CO ₂ cannot be removed by normal technologies, however the output is mitigated by reducing overall fuel consumption.

“The really unique thing about the plant we are constructing is that it will be (to the best of our knowledge) the first plant in the country that contains an SCR, baghouse, wet FGD, and wet ESP on the same unit. The really unique part from a new plant point of view is the wet ESP,” says Barry McNulty, manager of media relations for We Energies.

Controlling the operation

As a new plant with no existing legacy equipment, We Energies, working with Bechtel Corp., the project’s engineering, procurement, and construction (EPC) contractor, was able to start with a clean slate and choose one platform for Elm Road’s control systems. Emerson Process Management’s PlantWeb digital plant architecture with the Ovation expert control system is expected to keep the operation on an even keel.

Supercritical installations can be tricky to operate and depend on having real-time temperature information from many measuring points. “Things happen very quickly on supercritical boilers,” says Dasch. “You have to monitor your field temperature, pressure, and flow devices very closely and know the state of your equipment in real-time. When that is combined with intelligent alarms and controls, you can utilize all the advantages of the boiler technology.”

When operating with as complex a pollution control system as Elm Road, the challenge is to keep the operations and maintenance budgets to a minimum by making the control system do most of the work. “PlantWeb makes it cost-effective and convenient to put the operations consoles for all the pollution control devices in one room rather than distributed around the plant,” says Tom Snowdon, performance improvement specialist for Emerson.

Looking ahead

Once the units are running, Elm Road will be a base-loaded facility, meaning it will operate around the clock for at least 7,400 hours per year, or 85% or greater availability. There will be two planned shutdowns and a small cushion for unexpected equipment outages. This should continue for the next 40 to 45 years.

“Our advanced control technology will help manage critical power generation processes that are central to Elm Road’s role as a clean, efficient, and affordable source of energy for the area,” says Bob Yeager, president of Emerson’s power and water solutions division. “We’re gratified to be involved in a project that will be integral to meeting the region’s electricity needs for years to come.”