Above: The Boundary Dam Power Station, where SaskPower is installing a carbon capture and sequestration system (Wtshymanski)

Coal remains one of the world’s most popular fuels. Up to 41% of all electricity is produced by coal-fired power plants. Coal is cheap to mine, easy to use, and readily available in many countries.

Unfortunately, coal is also very dirty, causing more pollution per unit of energy produced than any other fuel (as measured by carbon intensity).

Did you know? The first sulfur-reducing scrubber was installed at a power plant in London, England, in 1931.

Coal-fired power plants release a variety of greenhouse gases, including sulfur dioxide (which contributes to acid rain), carbon dioxide (which contributes to global warming), nitric oxide, and nitrogen dioxide. Dangerous heavy metals such as vanadium and lead are also present in coal exhaust.

Thankfully, there are ways of reducing the amount of pollution caused by burning coal—often called clean coal technology—even if coal remains a relatively dirty source of energy. In particular, there are methods for significantly reducing the amount of sulfur and carbon dioxide released by coal-fired power plants.

Sulfur

Efforts to develop clean coal technology have been ongoing for several decades. Initially, the focus was on reducing the effects of acid rain, mainly caused by sulfur dioxide. Coal-fired power plants were equipped with devices called scrubbers, which chemically neutralize sulfur and lock it away instead of releasing it into the air. Modern scrubber technology can reduce sulfur emissions by up to 98%.

Did you know? In the case of an average-sized coal plant, a carbon capture and sequestration system (CCS) can prevent the release of about 1.4 million tonnes of CO2 per year.

It would take 62 million trees 10 years to remove that much carbon from the atmosphere. There are three main types of scrubbers: wet, semi-dry, and dry. Wet scrubbers spray the coal exhaust, trapping sulfur in the liquid. The sulfur liquid is collected and stored for disposal. Semi-dry scrubbers also spray the exhaust, but they use chemicals that bind to the sulfur and dry in the air. The exhaust is then passed through a very fine filter, which separates out the sulfur.

Dry scrubbers use a completely different approach. A dry powder is added to the furnace as the coal is burned. When sulfur is released, it interacts with the powder and forms a new compound. The exhaust then passed by an electrostatic precipitator (ESP), which creates an electric field that attracts the sulfur and prevents it from being released into the air.

Sulfur scrubbing produces a sulfur sludge that must be disposed of properly. Traditionally, the sludge has been transported to local landfills certified to accept toxic waste. More recently, new technologies have allowed some of the sludge to be recycled into building materials such as gypsum, which is used in making drywall.

Carbon dioxide

Burning coal also releases large amounts of carbon dioxide (CO2) into the air. CO2 is the most abundant greenhouse gas and a major contributor to climate change.

Coal-fired power plants can reduce the amount of CO2 they release by 80-90% using carbon capture and sequestration (CCS) systems. This technology captures CO2 from coal exhaust, storing it away before it can enter the atmosphere. CCS is an effective way of reducing emissions, but it does not remove any of the CO2 already in the atmosphere.

Did you know? SaskPower in Saskatchewan will become one of the first electric utilities in the world to use a commercial-scale carbon capture system.

It will be installed at the Boundary Dam Power Station, near Estevan. What can be done with all the CO2 that gets stored? There are currently two options. One is called geologic sequestration. An extremely deep hole (often about 2 km deep) is drilled and the CO2 is pumped into the rock. At these depths, the layers of rock above form an impermeable seal, meaning the CO2 cannot escape. Instead, the CO2 chemically bonds with the rock.

Captured CO2 can also be used in oil extraction. The CO2 is pumped into underground chambers to help force oil to the surface. The well site is then sealed, trapping the CO2 underground.

* * *

Despite the popularity of the term “clean coal”, this technology can only reduce the environmental impact of burning coal, not eliminate it altogether. Fossil fuels will continue to be a relatively dirty source of energy, even as new technologies help balance our growing appetite for energy with our need for a healthy environment.

Update (October 1, 2014): Canada switches on world's first carbon capture power plant (The Guardian UK)

References

General information

Carbon Dioxide Capture and Sequestration (US Environmental Protection Agency) Key World Energy Statistics 2012 (International Energy Agency)

Industry publications

Coal Ash: Characteristics, Management and Environmental Issues (Electrical Power Research Institute) SaskPower, Hitachi to develop carbon capture test facility (SaskPower press release)

Technical publications

Air pollution Control Technology Fact Sheet: Flue Gas Desulfurization (US Environmental Protection Agency) Separation of flue-gas scrubber sludge into marketable products. Second year, third quarterly technical progress report, March 1, 1995 – May 31, 1995 (S.K. Kawatra and T.C. Eisele, Michigan Technological University and US Department of Energy)

Mike MacPherson

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