Progress Report - Emission Controls and Monitoring

Emission Controls and Monitoring

Electricity generating units (EGUs) and other industrial units that are regulated under Acid Rain Program (ARP), the Cross-State Air Pollution Rule (CSAPR) programs, and the Mercury and Air Toxics Standards (MATS) must monitor and report emissions to EPA to ensure accountability in meeting emission reduction targets.

The EGUs have several effective options to control SO₂, NO_X, particulate matter, and hazardous air pollutants (HAPS), including mercury. Coal-fired power plants are the primary source of SO₂ in the power sector. To control SO₂ emissions, EGUs can employ different types of flue gas desulfurization (FGD, or scrubber) technologies that introduce a material to scrub out SO₂. To control NO_X emissions, EGUs can use advanced post-combustion controls—controls that remove NO_X after it is formed in the combustion process—such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) technologies. These technologies use ammonia or other chemical to react, with or without a metal catalyst, with gaseous NO_X. The byproduct is nitrogen and water. In addition to advanced post-combustion controls, EGUs can employ combustion controls that reduce the formation of NO_X during the combustion process. These technologies include low-NO_X burners. Some of the same technologies employed for SO₂ and NO_X control also reduce mercury emissions. For example, an SCR to control NO_X also oxidizes mercury which is water soluble and can be captured in a wet FGD designed to control SO₂. EGUs can optimize their existing controls to also capture mercury and install mercury-specific controls such as activated carbon injection (ACI) technology. ACI works by injecting processed carbon (e.g., activated charcoal) with very small pores to capture mercury emissions. The carbon is then captured in a particulate matter control device, such as a fabric filter. 

Sources have a number of options to control emissions of SO₂, NO_X, and hazardous air pollutants (HAPS) including mercury. Under the ARP or the CSAPR programs, sources may reduce SO₂ emissions by switching to low sulfur coal or natural gas, employing various types of flue gas desulfurization (FGDs, or scrubbers), or, in the case of fluidized bed boilers, injecting limestone into the furnace. FGDs on coal-fired electricity generating units are the principal means of controlling SO₂ emissions. To control NO_X emissions under these programs, sources can employ advanced post-combustion controls such as selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR), as well as combustion controls, such as low NO_X burners. Sources in MATS can improve operation of existing controls, add pollution controls, and switch fuels (including coal blending). Specific pollution control devices that reduce mercury include wet FGDs, activated carbon injection (ACI), dry sorbent injection (DSI), and fabric filters.

EPA has developed detailed procedures codified in federal regulations (40 CFR Part 75) to ensure that sources monitor and report emissions with a high degree of precision, reliability, accuracy, and timeliness. Most EGUs must continuously monitor their emissions of SO₂, NO_X, and CO₂. For EGUs that burn coal or other solid fuel, this can be done with continuous emission monitoring systems (CEMS) to measure SO₂, NO_X CO₂, and mercury or sorbent trap monitoring systems for mercury. Most EGUs that burn natural gas or oil have the option of continuously measuring fuel quantity and evaluating fuel quality to determine SO₂ and CO₂ emissions. Some qualifying units with low emissions can conduct periodic stack tests in lieu of continuous monitoring.

Sources must also conduct stringent quality assurance tests of their monitoring systems to ensure the accuracy of emissions data and to provide assurance that a quantity of emissions measured at one facility is equivalent to the same quantity measured at a different facility. EPA conducts comprehensive electronic and desk data audits to validate the reported data. The hourly CEMS data consists of greater than 99% quality assured measurements and less than 1% calculated values used to provide a complete record of all emissions. For more information on substitute data refer to  Substitute data in EPA CAPD's Power Sector Emissions Data (pdf) (144.22 KB, February 16, 2021)

ARP and CSAPR SO2 Program Controls and Monitoring

CSAPR NO_X Annual Program Controls and Monitoring

CSAPR NO_X Ozone Season Program Controls and Monitoring

MATS Controls and Monitoring

Mercury Compliance and Monitoring Methods used by Units Reporting Hourly Data under MATS, 2022

Reporting Hourly Data		Compliance Method (# of Units)		Monitoring Method
Number of reporting units	Number of reporting facilities	Electrical Output	Heat Input	Sorbent Trap	CEMS	CEMS and Sorbent Trap
387	177	112	275	173	179	35