Emission Controls and Monitoring
Emission Controls and Monitoring Figures
Source: EPA, 2022
Last updated: 07/2022
ARP and CSAPR SO₂ Program Controls and Monitoring
- Units with advanced flue gas desulfurization (FGD) controls (also known as scrubbers) accounted for 71 percent of coal-fired units and 81 percent of coal-fired electricity generation, measured in megawatt hours, or MWh, in 2021.
- In 2021, 20 percent of the CSAPR units (including 100 percent of coal-fired units) monitored SO₂ emissions using CEMS. Ninety-nine percent of SO₂ emissions were measured by CEMS.
CSAPR NOₓ Annual Program Controls and Monitoring
- Eighty-one percent of fossil fuel-fired generation was produced by units with advanced add-on controls (either selective catalytic reduction [SCR] or selective non-catalytic reduction [SNCR]).
- In 2021, the 236 coal-fired units with advanced add-on controls (either SCRs or SNCRs) generated 78 percent of coal-fired electricity. At oil and natural gas-fired units, SCR- and SNCR- controlled units produced 84 percent of electricity generation.
- In 2021, 67 percent of the CSAPR units (including 100 percent of coal-fired units) monitored NOₓ emissions using CEMS. Ninety-seven percent of NOₓ emissions were measured by CEMS.
CSAPR NOₓ Ozone Season Program Controls and Monitoring
- Seventy-three percent of all the fossil fuel-fired generation was produced by units with advanced add-on controls (either SCRs or SNCRs).
- In 2021, 213 units with advanced add-on controls (either SCR or SNCR) accounted for 71 percent of coal-fired electricity generation. At oil- and natural gas-fired units, SCR- and SNCR-controlled units produced 75 percent of electricity generation.
- In 2020, 73 percent of the CSAPR units (including 100 percent of coal-fired units) monitored ozone season NOₓ emissions using CEMS. Ninety-seven percent of ozone season NOₓ emissions were measured by CEMS.
MATS Controls and Monitoring
- In 2021, forty-six percent of the MATS units reporting mercury emissions and 52 percent of the electricity generation at the MATS reporting units used activated carbon injection (ACI), a mercury-specific pollution control method to reduce mercury emissions and SO₂.
- About 81 percent of units that reported continuous mercury emissions data (or 82 percent of the total electricity generation from units that reported data) reported the use of advanced controls, such as wet scrubbers, dry scrubbers, or ACI, to reduce hazardous air pollutant emissions in 2021. These controls also reduce other pollutants, including SO₂. Some oil-fired units can meet the MATS emission limits through the use of particulate matter (PM) controls such as electrostatic precipitators (ESPs) or fabric filters (FFs).
Continuous Emission Monitoring Systems (CEMS)
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. Sources are required to use CEMS or other approved methods to record and report pollutant emissions data. Sources conduct stringent quality assurance tests of their monitoring systems to ensure the accuracy of emissions data and to provide assurance to market participants 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. While some units with low levels of SO₂ or NOₓ emissions are allowed to use other approved monitoring methods, the vast majority of SO₂ and NOₓ emissions are measured by CEMS.
Affected units have a variety of monitoring options, but most use either CEMS or sorbent traps for mercury (Hg). Some qualifying units with low emissions can conduct periodic stack tests in lieu of continuous monitoring.
SO₂ Emission Controls
Sources in the ARP or the CSAPR SO₂ programs have a number of SO₂ emission control options available. These include switching to low sulfur coal or natural gas, employing various types of FGDs, 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 and tend to be present on the highest generating coal-fired units.
NOₓ Emission Controls
Sources in the ARP or the CSAPR NOₓ annual and ozone season programs have a variety of options by which to reduce NOₓ emissions, including advanced add-on controls such as SCR or SNCR, and combustion controls, such as low NOₓ burners.
Hazardous Air Pollutant Controls
Sources in MATS have a number of options available to reduce hazardous air pollutants (HAPs), including mercury, PM (a surrogate for toxic non-mercury metals), HCl, HF, and other acid gases. Sources can improve operation of existing controls, add pollution controls, and switch fuels (including coal blending). Specific pollution control devices that reduce mercury and HCl include wet FGDs, activated carbon injection (ACI), dry sorbent injection (DSI), and fabric filters.