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See also Utility Air Pollution Control Technology

The U.S. Environmental Protection Agency (EPA) is implementing a number of regulations that are currently targeting or soon are set to target SO2 emissions, including the Clean Air Transport Rule (CATR) and the SO2 National Ambient Air Quality Standards (NAAQS). In addition, Hazardous air pollutants (HAPs), are poised for regulation under the Air Toxics Rule for utilities via the proposed Maximum Achievable Control Technologies (MACT) standard.

According to the EPA, power plants are responsible for 66 percent of worldwide SO2 emissions, with the majority—more than 98 percent—coming from coal-fired power plants.  According to the Edison Electric InstituteWet FGD, semi-dry FGD, dry sorbent injection (DSI) and other technologies have been responsible for cutting SO2 emissions by 57 percent between 1980 and 2008.

McGill Air Clean’s spray dry scrubber and fabric filter
 operating on a Bubbling Fluidized Bed boiler.
 Photo courtesy McGill Air Clean.
The Clean Air Transport Rule (CATR)—proposed by the EPA in July 2010—was created to reduce the interstate transport of emissions from power plants in the eastern U.S. as required by the Clean Air Act. EPA is expected to finalize the CATR by the end of 2011. The Transport Rule replaces the Clean Air Interstate Rule (CAIR) that EPA issued in March 2005. “Good neighbor” provisions of the Act require states to prohibit emissions that contribute significantly to a downwind state’s air quality problems. One example is West Virginia, where the EPA found that power plants significantly affect air quality statuses of counties in Ohio, Indiana, Kentucky, Pennsylvania and Michigan, preventing these states from achieving federal air quality standards.

The Transport Rule will go into effect under two phases: the Phase 1 compliance date of 2012 and the Phase 2 compliance date of 2014. The Transport Rule also establishes two independent trading programs for SO2: Group 1 states and Group 2 states (See figures 1 and 2). SO2 emissions from Group 1 states would be capped at 3.1 million tons per year beginning in 2012 and 1.7 million tons per year beginning in 2014. The 2012 cap represents a 13 percent reduction below 2009 emissions levels. SO2 emissions from Group 2 states would be capped at 0.8 million tons beginning in 2012. The 2012 cap for Group 2 states represents a 29 percent reduction below 2009 emissions levels. The rule will affect fossil fuel-fired power plants 25 MW and larger in 31 states and Washington D.C., encompassing both SO2 and NOx reductions.

The Air Toxics Rule is EPA’s replacement for the Clean Air Mercury Rule (CAMR), an interstate cap-and-trade program issued in 2005. On March 8, 2008, a federal court ruled that EPA violated the Clean Air Act when it sought to regulate mercury-emitting power plants through CAMR. EPA estimates the proposed Air Toxics Rule will reduce mercury emissions from covered power plants by 91 percent, acid gas emissions by 91 percent and SO2 by 55 percent. This is the first time federal limits have been established for hazardous air pollutants (HAPs) and mercury control. A consent degree with public health and environmental groups requires EPA to finalize the standards by Nov. 16, 2011. Compliance to the MACT ruling is expected by around 2015.

The SO2 NAAQS are another concern for utilities. The Clean Air Act requires EPA to set national air quality standards for SO2 and five other emissions types. On June 2, 2010, the primary SO2 NAAQS was revised to set the SO2 standard at 75 parts per billion (ppb), which is attained when the three-year average of the 99th percentile of one-hour daily maximum concentrations does not exceed 75 parts per billion (ppb). SO2 emissions must now be recorded at a one-hour measurement, which raises a new set of challenges for utilities. The Clean Air Act directs states to submit their SO2 designation recommendation by June 3, 2011.

On March 16, 2011, EPA proposed new source performance standards and Maximum Achievable Control Technologies (MACT) standards for fossil fuel-fired units under the Air Toxics Rule. This ushers in the first-ever national standards for mercury, arsenic and other hazardous air pollutants (HAPs). Since SO2 is a surrogate for HAPs, the Utility MACT standard could also introduce or reinforce a push toward SO2 control installation.

Best Available Retrofit Technology (BART) guidelines under the Clean Air Visibility Rule will also take a toll on SO2 emissions (as well as NOx, ammonia and certain volatile organic compounds), affecting facilities built or reconstructed between Aug. 7, 1962, and Aug. 7, 1977, that have the potential to emit more than 250 tons a year of emissions and fall into one of 26 different categories. These include utility and industrial boilers and large industrial plants such as pulp mills, refineries and smelters. The EPA’s presumptive BART emission limit for SO2 is 0.15 lb/mmBtu, or 95 percent removal.

Instead of undergoing extensive retrofits, a percentage of coal-fired generation—an estimated 50 to 60 GW by 2020—will likely be retired, according to a 2010 Credit Suisse report. (Power Engineering, 6/1/2011)