Introduction

Stationary source emissions monitoring is composed of four elements. These include (1) indicator(s) of performance, (2) measurement technique, (3) monitoring frequency, and (4) averaging time. A continuous emission monitoring system (CEMS) is only one type of measurement technique that monitoring devices may use. Continuous opacity monitoring systems (COMS) and continuous parametric monitoring systems (CPMS) are two other common techniques. Regardless of the technique, there is a lot involved in continuous emission monitoring (CEM). This Fact File gives a detailed look at the history of CEM. It also explains basic requirements under various programs and standards for CEM. 

Background

A continuous emission monitoring system is the total equipment essential for the determination of a gas or particulate matter concentration or emission rate using pollutant analyzer measurements and a conversion equation, graph, or computer program to yield results in units of the appropriate emission limitation or standard. Federal continuous emission monitoring requirements were first recognized in the U.S. for tracking the performance of air pollution control equipment under the Environmental Protection Agency’s (EPA’s) New Source Performance Standards (NSPS). Data from this program are reported when emission standards are surpassed. Then, these “excess emission reports” are used to decide if control equipment performance warrants an inspection or a reference method test to identify whether the source follows emission standards.

Positive CEM experiences helped set it up as the bedrock for many other subsequent programs including the Acid Rain Program of the 1990 Clean Air Act Amendments (CAAA). In this Program, CEMS supplies the means to determine who has how many allowances, how many are used, and how many open for trading.

CEM Requirements for NSPS

NSPS regulations are intended to require the installation and correct operation of “best demonstrated control technology” to lessen emissions of pollutants. Test methods are found in Appendix A and Performance Specifications for continuous emission monitors are included in Appendix B of 40 CFR 60. Continuous monitoring of opacity of emissions is required for some sources. NSPS regulations have set “percent removal” requirements in addition to emission limitations for certain sources. Sometimes, continuous emissions monitoring is required to demonstrate compliance with percent removal requirements.

CEM Requirements for Acid Rain Program

Acid Rain monitoring regulations at 40 CFR 75 require continuous monitoring of sulfur dioxide (SO₂) and stack gas flow rate to determine the mass emission rate of SO₂. It is also required to monitor nitrogen oxides (NO_x) and diluent concentrations to govern emissions in units of pounds NO_x per million British thermal units (Btu) of heat input. The largest 263 SO₂ emission sources were identified within the 1990 CAAA as Phase 1 sources. All other affected utility sources (about 2,000 units) are designated as Phase 2 units. Phase 1 units had to have a CEM system installed and certified before November 1994. Phase 2 units were required to have a CEM system installed and certified by the end of December 1995. Certain gas-fired units and peaking units were the exception.

CEM Requirements for NESHAP 

Several of the national emission standards for hazardous air pollutants (NESHAP) regulations include CEM Requirements:

• Opacity monitors are required for sources affected by Subparts N, O, and P regulating inorganic arsenic emissions from glass manufacturing plants, copper smelters, and arsenic trioxide and metal arsenic production plants, respectively.
• Subpart F requires installation of vinyl chloride CEM systems at certain emission points in ethylene dichloride, vinyl chloride, and polyvinyl chloride plants.

CEM Requirements for Sources Burning Hazardous Waste and Sewage Sludge Incinerators

CEM requirements for carbon monoxide (CO) and total hydrocarbons (THC), as well as incinerator and control device operation are included in hazardous waste incinerator operating permits. Concentrations of CO and hydrocarbons must be continuously monitored and corrected to seven percent oxygen on a dry basis. CEM systems for CO and hydrocarbons must complete a minimum of one cycle of sampling and analysis every 15 seconds. It must record one data point each successive minute.

EPA promulgated CEM requirements for sewage sludge incinerators on February 19, 1993. It falls under 40 CFR 503 under authority of the Clean Water Act. Subpart E requires the installation of a THC monitor and an oxygen monitor on each incinerator.

Applicable Laws & Regulations

40 CFR 60 – Standards of Performance for New Stationary Sources

40 CFR 61 – National Emission Standards for Hazardous Air Pollutants

40 CFR 63 – National Emission Standards for Hazardous Air Pollutants for Source Categories

40 CFR 75 – Continuous Emission Monitoring

40 CFR 270 – EPA Administered Permit Programs: the Hazardous Waste Permit Program

40 CFR 503 – Standards for the Use or Disposal of Sewage Sludge

Related Definitions

“Opacity” means the degree to which emissions reduce the transmission of light and obscure the view of an object in the background.

“Monitoring” means ongoing collection and use of measurement data or other information for assessing performance against a standard or status with respect to a specific requirement. 

“Stationary source” means any building, structure, facility, or installation which emits or may emit any air pollutant.

Key to Remember

Both federal and state air quality regulations may require continuous emissions monitoring systems. Make sure to consult your state and applicable State Implementation Plans.

Real World Example

EPA can assess large civil administrative penalties under the Clean Air Act that go as high as $37,500 per day, per violation against federal agencies for noncompliance. In the fourth quarter of 2021, a chemical manufacturer in Louisiana was fined $275,000 for NSPS violations under the CAA. They failed to maintain and operate the facility and related air pollution control equipment in a way consistent with proper air pollution control practice for lowering emissions. Make sure you follow any CEM requirements to avoid these high fines.