The fine print on fine particles — facts about PM2.5 and how we’re addressing it.

If you’ve heard or read anything about air quality, you may have come across the term “PM2.5” — a topic being addressed by the U.S. Environmental Protection Agency (EPA) and companies such as Mirant. We’ve put together this information sheet to help you understand the particulars of PM2.5 and the progress we’re making to reduce its impact on the community.

What is this stuff?

PM — which stands for particulate matter — is a mixture of solid particles and liquid droplets containing varying combinations of elements, including nitrogen oxides (NOx), sulfur dioxide (SO2), organic chemicals, and ordinary soil and dust. These compounds react in the atmosphere, creating particles too small to be seen by the naked eye. Particles measuring 10 micrometers in diameter or less are referred to as “PM10”. The fraction of particles that are 2.5 micrometers in diameter or less are considered fine particles, or PM2.5. (To put the size in perspective, consider this: a single human hair is about 70 micrometers in diameter.)

Adapted from graphic courtesy of Metropolitan Washington Council of Governments

Because these particles are small enough to be inhaled, there is concern about the effect of PM on human health. PM levels are monitored by the EPA as part of the National Ambient Air Quality Standards (NAAQS) that companies like Mirant are required to meet. In 2006, the EPA tightened the standards for PM2.5 cutting in half the amount of PM2.5 emissions allowed in a 24-hour period. At the same time, the EPA revoked the standards for annual PM10 emissions, citing that available evidence does not suggest a link between long-term exposure to PM10 and health problems.

Where does PM2.5 come from?

PM2.5 originates from a variety of sources, near and far:

• Cars and trucks, construction sites, unpaved roads, open fields, fires, manufacturing plants and power plants — they’re all contributors to PM2.5.
• Fine particles also result from the reaction of gases or droplets in the atmosphere. These chemical reactions can occur miles from the original source of the emissions. Because fine particles can be carried long distances from their source, events such as wildfires or volcanic eruptions can raise fine particle concentrations hundreds of miles from the event — making it hard to pinpoint and control the source.
• PM2.5 is also produced by common indoor activities such as smoking tobacco, cooking (e.g., frying, sautéing, and broiling), burning candles or oil lamps, and operating fireplaces and fuel-burning space heaters (such as kerosene heaters).

What is Mirant doing to control PM2.5 at Potomac River Generating Station?

The majority of PM2.5 particles that can be traced to power plants are not dust or ash particles. They are sulfates and nitrates, fine particles that transform naturally in the atmosphere from plant SO2 and NOx emissions. Therefore, the best way for power plants to reduce PM2.5 emissions is to lower NOx and SO2 emissions — and we’re doing this in several ways.

For NOx control, we use Separated Over-Fire Air (SOFA) technology in the boiler to reduce NOx by “staging” combustion. We’ve cut NOx in half using this technology, compared to 2002 emissions.

For SO2 control, we use trona injection — a patent-pending process — to lower SO2 emissions by 50 percent. Trona is a mineral that looks like ordinary baking soda, and is just as safe to handle. When injected into the exhaust gas stream, it bonds with SO2. The compounded material is then removed from the exhaust gas by existing emissions control equipment.

We employ a number of other methods to control NOx and SO2 emissions, including:

• Low NOx burners;
• Modern computerized control systems to optimize combustion and minimize NOx formation;
• Combustion tuning; and
• The use of low-sulfur coal.

If you’ve visited our plant, you may also have seen two very large pieces of equipment called “hot” and “cold” electrostatic precipitators to control dust emissions. They work by capturing and containing particles through static created by an electric charge, allowing them to be removed so they don’t escape through the exhaust. These units are capable of removing 99.7 percent of fly-ash from combustion gases. The hot precipitator has a particulate collecting area equivalent to six acres — that’s approximately three times larger than typical units of this type.

Is that enough?

No way. We’re proud of our success so far in curbing plant emissions, but there’s more work to be done.

By 2011, we expect to reduce NOx emissions from our four Mid-Atlantic-area plants — including Potomac River — by 65 percent, through investments in equipment and technology. At our Morgantown station in Maryland, selective catalytic reduction (SCR) technology is being installed on both units, which will benefit the Washington D.C. metropolitan area. In addition, we plan to make a significant investment in our Maryland facilities to lower SO2 emissions to comply with Maryland’s Healthy Air Act.

Who’s holding Mirant accountable?

Our operations are continually monitored by a number of groups. Real-time data from our Continuous Emissions Monitoring System is collected and reported to the EPA and the Virginia Department of Environmental Quality (DEQ) on a quarterly basis.

In addition, the DEQ and the City of Alexandria Department of Transportation and Environmental Services conduct semi-annual inspections of the plant’s air quality equipment and record keeping.

To check our progress specific to fine particles, a PM2.5 monitor has been installed on nearby Marina Towers.