Continuous Emissions Monitoring Systems (CEMS) Information

Continuous Emissions Monitoring (CEM) System via Thermo Fisher ScientificContinuous emissions monitoring systems (CEMS) measure and record concentrations of pollutants emitted from industrial activities. CEMS allow operators to comply with federal or local environmental regulations gaseous waste products to fall within a closely monitored range. In addition to regulatory compliance, CEMS can be useful in process control and optimization applications.


A common type of CEMS is an extractive monitoring system. In this type of CEMS, the first step in the measurement process involves extracting a sample of gas emissions with a probe. The sample is filtered, conditioned, and transferred to one or more gas analyzers. Gas concentration measurements are carried out by the analyzers and transmitted to a data acquisition system for collection and recording of emission data.

Filtration and conditioning of the gas sample before analysis is often necessary to remove moisture or particulate matter that could damage instrumentation, clog conduits, or hinder accurate sensing of the target pollutants.

The central function of a CEMS data acquisition system is to record measurements, but it can also create alarm signals to indicate that processes are operating outside of normal tolerances. Data acquisition systems can display data in real time or generate reports to evaluate system parameters and assess compliance.

To ensure reliable and accurate emissions data, CEMS must undergo regular calibration and quality checks. Calibration gases of certified concentrations can be measured by the CEMS analysis instrument to verify system accuracy.


Continuous emissions monitoring systems measure a wide range of pollutant concentrations and other process variables, such as:

  • Sulfur Dioxide (SO2)
  • Nitrogen Monoxide (NO)
  • Nitrogen Dioxide (NO2)
  • Carbon Monoxide (CO)Continuous Emissions Monitoring (CEM) System via Emerson Process Inc.
  • Carbon Dioxide (CO2)
  • Oxygen (O2)
  • Water (H2O)
  • Total Hydrocarbon (THC)
  • Total Organic Carbon (TOC)
  • Hydrogen Sulfide (H2S)
  • Hydrogen Chloride (HCl)
  • Ammonia (NH3)
  • Total Reduced Sulfur (TRS)
  • Mercury
  • Volatile Organic Compounds (VOCs)
  • Formaldehyde (CH2O)
  • Hydrogen Fluoride (HF)
  • Methane (CH4)
  • Ethane (C2H6)
  • Propane (C3H8)
  • Ethylene (C2H4)
  • Airborne Particulate Matter
  • Moisture
  • Opacity
  • Air Flow
  • Stack Temperature

CEMS are deployed in many different applications, including:

  • Power plants
  • Waste incineration plants
  • Wastewater treatment plants
  • Cement kilns
  • Nitric acid plants
  • Aluminum and steel smelters
  • Brick and glass manufacturers
  • Pulp and paper facilities
  • Gas turbines
  • Commercial and industrial boilers
  • Cogeneration facilities
  • Refineries
  • Pharmaceutical facilities
  • Chemical manufacturers
  • Selective catalytic reduction (SCR) systems

In the United States, regulations of the Environmental Protection Agency (EPA) require power plants to monitor and limit emissions such as sulfur dioxide and nitrogen dioxide to reduce acid rain, ozone, and fine particle pollution.


The gas analyzer in a continuous emissions monitoring system may use one of a variety of detection technologies, such as:

Fourier Transform Infrared (FTIR) spectroscopy measures a relatively wide range of the infrared spectrum of source emissions. It is useful for analysis of a variety of gases. Non-Dispersive Infrared (NDIR) spectroscopy does not alter the wavelength of the infrared beam entering the sampling chamber.

Chemiluminescence is often used to measure nitrogen oxides and detects the intensity of light produced by a nitric oxide and ozone reaction.

Pulsed fluorescence is frequently used to measure SO2. It involves exposing a gas to ultraviolet radiation and measuring the resulting fluorescence emitted as the gas absorbs the UV light. Atomic fluorescence spectroscopy is also used to measure mercury in emissions.

Flame ionization detection (FID) involves analyzing the ions produced during combustion of emissions in a hydrogen flame.

Image credits:

Thermo Fisher Scientific Inc. | Emerson Process Management


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