What is a continuous emissions monitoring system?
A continuous emissions monitoring system (CEMS) is the combination of sampling hardware, analyzers and data software that measures pollutants leaving a stack in real time and produces the records regulators require. Typical CEMS measure SO2, NOx, CO, CO2 or O2, and depending on the source, also mercury, HCl, particulate matter and volumetric flow.
A CEMS does more than read a gas value. It must prove, day after day, that its data is valid. That means automated calibration checks, quality-assured data substitution when a monitor is down, and emission calculations that match the averaging periods written into the facility’s permit.
How CEMS technology works
There are three basic architectures:
- Dry extractive: A heated probe and sample line carry flue gas to a conditioning system that filters and dries it before the analyzers. This setup is well proven for SO2, NOx, CO, CO2 and O2.
- Hot-wet extractive and close-coupled: The sample stays above its dew point, or is conditioned right at the stack, so water-soluble gases such as HCl and NH3 survive to the analyzer.
- In-situ: The analyzer or its optics mount directly on the stack or duct and measure the gas without extracting it.
Inside the cabinet, each gas is matched to a measurement principle:
| Measurement | Common principle | Typical CEMS use |
|---|---|---|
| SO2, NO, CO, CO2 | NDIR gas filter correlation | Part 60 and Part 75 compliance |
| NOx | Chemiluminescence | Low-NOx combustion sources |
| HCl, NH3, multi-component | FTIR | MACT sources, ammonia slip |
| Total hydrocarbons | Heated FID | VOC and organic HAP limits |
| O2 diluent | Paramagnetic or zirconia | Correcting concentrations |
| Mercury | Thermal conversion with atomic absorption | Utility MATS, cement NESHAP |
| Particulate | Light scatter, electrodynamic, beta | PM CEMS, bag leak detection |
| Flow | Ultrasonic time-of-flight | Mass emission rates |
Mercury monitoring deserves special mention. Mercury appears in flue gas in both elemental and oxidized forms. An Hg CEMS converts it all to elemental mercury, typically with high-temperature thermal conversion, then measures it by atomic absorption at trace µg/m³ levels. Zeeman background correction, the specialty of Ohio Lumex, separates true mercury absorption from interference caused by SO2 and other gases.
Common applications
- Power generation: coal- and gas-fired units across the Southeast report SO2, NOx, CO2 and flow under Part 75, and coal units add mercury under MATS.
- Pulp and paper mills: recovery boilers, lime kilns and power boilers in Alabama, Georgia, Mississippi and Louisiana.
- Gulf Coast refining and chemical plants: heaters, boilers, flares and thermal oxidizers along the Mississippi River corridor.
- Cement kilns and waste combustion: mercury, HCl and particulate under NESHAP rules.
Choosing the right CEMS
A defensible CEMS starts with the permit and the stack. Gas composition, moisture, temperature and dust loading determine whether dry extractive, hot-wet or in-situ sampling will produce stable data. Required ranges and certified low scales determine the analyzer. After installation, the real cost shows up in quality assurance: calibration gases, audits, RATAs and technician hours. Compare lifetime maintenance, not just purchase price, and consider how quickly spare parts and factory service reach your site.
Manufacturers we represent for emissions monitoring
ENVEA, formerly Altech Environment USA, supplies analyzers and complete integrated systems from its Geneva, Illinois operation. Its range includes the MIR 9000 and MIR IS multi-gas analyzers listed for 40 CFR 60 and 75, FTIR and heated FID analyzers, SM-5 mercury CEMS, particulate monitors, STACKFLOW ultrasonic flow monitors and reporting software. Many ENVEA products carry both US EPA compliance claims and TÜV QAL1 or MCERTS certification.
Ohio Lumex of Solon, Ohio, has specialized in trace mercury measurement since 1999. It offers the 915J continuous mercury monitor, Method 30B and PS-12B sorbent traps and samplers, an ISO/IEC 17025-accredited laboratory, and field engineers who support mercury RATAs.
Get local application support
Our Baton Rouge-based area managers specialize in analyzers, CEMS and gas detection for refineries, chemical plants and power stations. We can review your permit requirements, visit the stack and coordinate with the factory on system design. Visit engineering support, request a quote, or find your area manager on the territories page.
