Coal-Fired Power Plants (CFPPs)
Causes of greenhouse gases, Including NOx
What causes SO2 emissions in coal-fired power plants?
To generate the energy we need every day, coal-fired power plants burn coal to produce electricity. Virtually all coal contains some amount of sulfur and when coal is burned that sulfur reacts with oxygen in the atmosphere to form sulfur dioxide (SO2) and lesser amounts of sulfur trioxide (SO3). Collectively, these are sulfur oxides and can contribute to air pollution. The amount of sulfur oxide produced by a particular coal-fired power plant depends on the following:
- Sulfur content of the coal itself (western coal differs from eastern coal),
- Coal treatment and handling methods,
- Combustion efficiency of the power plant, and
- Efficiency of the power plant’s pollution control system.
Sulfur content of coal varies geographically. Western coal in the U.S. contains about 0.3% and eastern coal about 2.5%. This means controlling emissions from a plant burning eastern coal can be more challenging that controlling the same emissions from a plant burning western coal. Utilities often seek to reduce SO2 emissions by switching to lower-sulfur coals. However, the greatest advances in reducing emissions come from technology advances in power production and the efficiency of the plant itself.
Allowable Emissions
Emission limits for SO2 have steadily decreased since regulations were first enacted in 1971. At that time, we set a limit of 1.2 pounds of SO2 per million British Thermal Units (Btu) heat content. The heat content of coal varies – from 12,000 Btu per pound for eastern coal to around 9,000 Btu per pound for western coal. In round numbers, this emission limit translated to one pound of SO2 per 100 pounds of coal burned.
In 1979, revised standards required that SO2 emissions for boilers constructed after 1978 be controlled by at least 90%, or limited to 0.6 pounds per million Btus. Additional reductions mandated decreasing overall caps on nationwide SO2 emissions from power generation. The result was, and is, continued reductions in SO2 emissions since 1970, with corresponding improvements in air quality. The chart below shows this steady decrease.
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Pollution Emissions From Coal-Fired Power Generation
Pollution shown: Particulate Mater (PM10) (less than 10 microns), Sulfur Dioxide (SO2),
and Nitrogen Oxide (NOx).
Shown in thousands of tons per billion kilowatt-hours of electricity production. |
| Note: A kilowatt-hour is a unit of electric power, equivalent to 1000 watts of electricity for a one-hour period.
Household light bulbs are commonly rated at 100 watts; powering 10 such
bulbs for a one-hour period uses one kilowatt-hour of electric power. |
What are the estimated SO2 discharge levels for 2018 and beyond?
The Bush Administration’s Clear Skies Initiative of 2005 mandates that total nationwide SO2 emissions from major power generation facilities be reduced to 3 million tons per year by 2018, compared to 11.2 million in 2000. This can be accomplished by establishing steadily decreasing overall emission caps. Because approximately two-thirds of SO2 emissions in the U.S. are associated with power generation, these caps will greatly reduce overall SO2 emissions. The chart below shows projected reduction in emissions. Although the projections show emissions at 3.6 million tons per year in 2030 rather then the mandated 3 million, this chart includes emissions from small utilities, small power plants, and minor industries so the numbers are larger. The important point here is the steady, continual decline.
U.S. Sulfur Dioxide Emissions From All* Power Generation
(Millions of tons per year)
Source: Annual Energy Outlook 2008 with Projections to 2030
*Values shown above include facilities not classified as major under the
Clear Skies Initiative such as minor industry or small utilities.
Additional Studies - Global Emissions
One study by Pacific Northwest National Laboratory (PNNL) looked at manmade (or anthropogenic) sulfur emissions from the last century and a half. The graph below shows PNNL's results on the dark red line. Other lines on the chart show results from other studies. Specifically, the chart shows a consensus that most researchers agree upon: that sulfur emissions went from near zero in the early 1800s, peaked in the 1980s, and have been steadily decreasing since then.
Note: Total emissions are given in teragrams (TgS).
1 teragram = 1,000,000 metric tons, and 1 metric ton is about 1.1 US ton.
http://www.pnl.gov/main/publications/external/technical_reports/PNNL-14537.pdf
This next graph shows the different sources of SO2 emissions. Coal is the greatest contributor and oil next. It is important to note that there is a decline in the emissions of major contributors.
Global Sulfur Dioxide Emissions by Source
Finally, this last graph shows we are not alone in SO2 emissions. Many countries contribute to and deal with SO2 emissions. However, the chart shows that the largest contributors, Europe and North America, have the greatest reduction of emissions. This is quite an accomplishment for these countries.
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