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Flue gas desulfurization

Partial flue gas desulfurization (FGD) can achieve about 50-70 % removal of sulfur dioxide by the injection of dry limestone just downstream of the the air preheater. The resultant solids are recovered in the ESPs along with the fly ash.

In power plants burning pulverized coal, wet flue gas desulfurization (FGD) that contacts the flue gases with lime slurries (in what are called wet lime scrubbers) can achieve 95% sulfur dioxide removal without additives and 99+% removal with additives. Wet FGD has the greatest share of the FGD usage in the United States and it is commercially proven, well established technology.

The typical older FGD units in power plants burning pulverized coal within the United States achieve average sulfur dioxide emission levels of about 0.340 kg/MWh (0.22 lb SO₂ /10⁶ Btu), which meets the level to which those units were permitted.

The lowest demonstrated sulfur dioxide emission level (in 2005) for power plants burning pulverized high-sulfur coal within the United States was 1.08 kg/MWh (0.07 lb SO₂ /10⁶ Btu) and 0.046 kg/MWh (0.03 lb SO₂ /10⁶ Btu) for plants burning low-sulfur pulverized coal.

In 2006, power plants in the United States that burned fossil fuels (coal, fuel oil or natural gas) generated 327 GW of electric power, of which about 70% was generated in plants burning coal. Only about 30% of that 327 GW of generated power was derived from plants equipped with flue gas desulfurization. The other 70% of the generated power was derived from power plants that met their permitted levels by burning low sulfur coals, fuel oil or natural gas.

In March 2005, the U.S. Environmental Protection Agency promulgated the Clean Air Interstate Rule (CAIR) which sets emission caps for particulate matter, sulfur dioxide and nitrogen oxides is expected to result in more efficient FGD units being installed and more coal-fired power plants using FGD units or switching to burning oil or natural gas. The CAIR rules are still in litigation as of November 2008.<ref>[http://www.epa.gov/cair/ Clean Air Interstate Rule}

Coal-fired power plants also emit large quantities of carbon dioxide (CO₂) which is not a pollutant in the traditional sense. In fact, it is essential for all plant life on Earth through photosynthesis. However, it is a greenhouse gas considered to have a major role in so-called global warming. 50% of the electricity generated in the U.S. is from coal.2 � Th ere are the equivalent of more than fi ve hundred, 500 megawatt, coal-fi red power plants in the United States with an average age of 35 years.2 � China is currently constructing the equivalent of two, 500 megawatt, coal-fi red power plants per week and a capacity comparable to the entire UK power grid each year.3 � One 500 megawatt coal-fi red power plant produces approximately 3 million tons/year of carbon dioxide (CO2).3 � Th e United States produces about 1.5 billion tons per year of CO2 from coal-burning power plants. � If all of this CO2 is transported for sequestration, the quantity is equivalent to three times the weight and, under typical operating conditions, one-third of the annual volume of natural gas transported by the U.S. gas pipeline system. � If 60% of the CO2 produced from U.S. coal-based power generation were to be captured and compressed to a liquid for geologic sequestration, its volume would about equal the total U.S. oil consumption of 20 million barrels per day.