Operators of about 16 percent of U.S. coal-generating capacity need to make decisions about the April 2015 deadline for compliance with EPA’s mercury air toxic standard (MATS), reports the U.S. Energy Information Administration (EIA). Basically, there are two options.  One, operators can install a technology to meet the maximum achievable control technology (MACT) requirement to control emissions of acid gases and metals, including mercury.  Or, two, operators can retire the plant. 

The EIA recently reviewed the benefits and drawbacks of each of two technological choices—flue gas desulfurization (FGD) and dry sorbent injection (DSI).  The MATS includes a provision that allows state environmental permitting agencies to grant 1-year compliance exemptions.  Many states are likely to use this flexibility, says the EIA.

According to the EIA, at the end of 2012, 70 percent of the U.S. coal-generating capacity already had the appropriate environmental control equipment to comply with the MATS and allow their operation past 2016.  Another 6 percent plan to add control equipment, while      8 percent have announced plans to retire.  Owners of the remaining 16 percent are faced with the decision of upgrading or retiring their plants.

Control technologies
As the EIA describes it, the three key pollutants targeted by the MATS have MACT compliance options that sometimes overlap.

• Acid gas.  In most cases, an FGD or DSI system will qualify as the MACT for acid gases. Both of these systems can effectively remove sulfur dioxide (SO2) emissions as well.  While not directly addressed by the MATS, SO2 is regulated under Title IV of the Clean Air Act and the Clean Air Interstate Rule (CAIR).  FGD systems are already installed on 69 percent of the U.S. coal fleet, while 1 percent has DSI systems.
• Toxic metals.  FGD scrubbers with a fabric filter or an advanced electrostatic precipitator will likely qualify as the MACT for toxic metals.  If a DSI system is used to control acid gases, a fabric filter must be included to remove the appropriate level of toxic metals to complement the performance of the DSI system.  The available equipment options are determined by the characteristics of each plant.
• Mercury.  The control equipment needed to reduce mercury is driven by the type of coal burned and the plant configuration. In some cases, an existing FGD scrubber and selective catalytic reduction system (SCR) can lower mercury emissions to the point where no additional controls are needed.  In other cases, activated carbon injection (ACI) systems may be necessary to bring plants into compliance.

Costs

FGD scrubbers have higher capital costs but lower operating costs than DSI systems, along with a higher removal rate for SO2 and other acid gases (90 percent of SO2 emissions for FGD, compared to 70 percent for DSI).  DSI systems have much lower capital costs than FGDs, but significantly higher operating costs because of the high cost of the reagent needed to remove emissions.  This characteristic makes DSI systems more attractive for plants burning lower sulfur coal or for plants that operate infrequently. The low capital investment required for a DSI system makes it easier to recover the investment in the controls if the plant is not expected to operate frequently. Installation of an FGD scrubber would require the plant to operate more often in order to earn enough revenue to pay for the significant capital investment.

The EIA adds that implementation of the MATS will result in significant reductions in SO2 emissions from electric power plants.  However, compliance with the MATS will have no effect on NOx emissions.  NOx emissions have declined primarily through implementation of CAIR and the decline in coal-fired electricity.