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How to clean coal

By Nancy S. Grant from February 2013 Issue

Coal is and will remain the workhorse of electricity generation worldwide for decades to come. Around the globe, coal use as a share of electricity generation is climbing above 40 percent. Coal can produce huge quantities of electricity all day and all night. But coal isn't perfect. Burning coal to release its energy also releases many different kinds of gases and particles into the environment.

The battle to better control unwanted emissions at coal power plants in the United States kicked into high gear more than 40 years ago. The original Clean Air Act of 1970 set ambitious goals and timetables to reduce certain power plant emissions known as "criteria air pollutants." The list includes nitrogen oxides and sulfur dioxide (often shortened to "NOx" and "SOx") and certain sizes of particulate matter (fancy words for soot and dust).

$100 billion on cleaner coal
Each time that law's been modified (in 1977 and again in 1990), the requirements have become even stricter. Meeting these standards is an engineering success story.

Paul Bailey, senior vice president for Federal Affairs and Policy at the American Coalition for Clean Coal Electricity (ACCCE), says utilities have three basic choices. "You can improve the combustion process so it's more efficient," Bailey says, "or you can add controls during a retrofit, or you can use a cleaner fuel. There are two kinds of cleaning, physical coal cleaning and chemical coal cleaning, that can help remove some of the pollutants before the fuels burn."

Throughout the United States, electric utilities have taken an "all of the above" approach, picking and choosing one, two, or all three options. Sometimes their options are limited by the amount of physical space within and surrounding an existing power plant. Some clean coal technology is relatively simple to add. In other situations the first technology available may not fit, but a second or third version might be just right.

Bailey says, "As the control technologies have become more sophisticated, more power plants have been installing them over time."

One useful device is a "scrubber." This form of clean coal technology uses pulverized limestone to help remove sulfur dioxide from flue gases as the coal is burned.

Bailey says that putting a scrubber on a medium sized coal unit would cost more than $100 million.

Another utility might choose to install a "baghouse" that can trap various sizes of soot and dust particles to keep them from entering the environment outside the power plant. Or an electric utility might choose a "selective catalytic reduction" device to reduce NOx emissions. Each coal power plant presents a unique situation, with the cost to add technology also changing from plant to plant.

ACCCE recently examined data assembled by the U.S. Environmental Protection Agency and the U.S. Energy Information Administration about the changes in equipment and processes at coal-based power plants during the last 40 years. ACCCE calculates that utilities have already invested more than $100 billion in cleaner coal.

All that money has accomplished a lot. Examining EPA and EIA data in more detail, ACCCE notes that the "emissions of sulfur dioxide, nitrogen oxides, and particulate matter per kilowatt-hour from coal-fueled electricity generation have been reduced by almost 90 percent over the period 1970-2011."

Reducing greenhouse gas emissions
Modifying existing coal power plants to use coal more cleanly is only part of the story. During the last decade, engineers have been busy designing new coal power plants from the ground up. In a new circulating fluidized bed (CFB) power plant, crushed coal is mixed with some other material to help reduce SOx emissions during combustion.

In 2005 and 2009, East Kentucky Power Cooperative, which generates and transmits power to 16 local co-ops for their members in 87 counties, completed construction of two new CFB units. These coal power plants include built-in baghouses to reduce particulate matter, built-in SCRs to reduce NOx emissions, and the innovative CFB combustion section to reduce SOx emissions. Building these two clean coal power plants cost about 1 billion dollars.

Most emissions control investments made during the last 40 years have been focused on the environmental concerns of the 20th century, which were mainly how to have cleaner air. But new concerns in the 21st century about greenhouse gases (such as carbon dioxide) are inspiring a second wave of coal power plant innovations.

In one design, known as the integrated gasification combined cycle (IGCC) process, solid coal is changed into a gas. This synthesis gas (syngas) can be used not only to heat water to make steam, but also to turn the turbines directly, an added feature that greatly improves efficiency. The design of an IGCC coal power plant also includes processes that can capture much of the carbon dioxide emitted during operations and prevent it from entering the atmosphere.

An early version of IGCC technology can be found at the Wabash River Coal Gasification demonstration project in Indiana. James F. Wood, deputy assistant secretary of the Office of Clean Coal at the U.S. Department of Energy, recently described Round 3 of the department's Clean Coal Power Initiative, which includes several IGCC power plants, as the next step toward reducing coal power plant emissions below the levels in current and proposed EPA rules.