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CEMS continuous emission monitoring system: definition, types and main features

As environmental regulations become more restrictive, adequate environmental monitoring systems are essential for both new projects and existing industrial plants.

Continuous Emission Monitoring Systems (CEMS) are used to measure gases and particulate matter from stationary sources in a wide variety of industrial facilities.

The main industries known to be sources of pollutant emissions are:

  • Power and thermal power plants
  • Iron and steel industry
  • Non-ferrous metallurgy
  • Cement factories
  • Oil refineries
  • Petrochemical plants
  • Waste incineration plants
  • Pulp and paper producers
  • And many others…

What is a CEMS system?

The acronym CEMS stands for Continuous Emission Monitoring System and is an Automatic Measurement System (AMS) applied to emissions of air pollutants produced in industrial plants and processes.

One of the main functionalities of the CEMS is to assess compliance with the Emission Limit Values (ELVs) set according to the Industrial Emissions Directive. 

What are the gas emissions?

The parameters usually taken into account for continuous emission monitoring are:

Ammonia (NH3)Methane (CH4)
Benzene C6H6Carbon monoxide (CO)
Total Organic Carbon (TOC)Oxigen (O2)
Hydrogen cyanide (HCN)Nitrous oxide (N2O)
Chlorine and inorganic compounds (such as HCl)Sulfur Oxides (SOx/SO2)
Total reduced sulphur compounds (TRS)Nitrogen oxides (NOx/NO2)
Non-methane volatile organic compounds (NMVOCs)Particulate matter (PM10)
Carbon dioxide (CO2)Total suspended particulate matter (PST)
Fluorine and inorganic compounds (such as HF)Air Flow Rate
Mercury and compounds (such as Hg)


elegir un CEMS

Steps for choosing a CEMS

The first thing to do before choosing a Continuous Emission Monitoring System is to know the applicable regulations, which will depend on the type of industry, where the emission source to be monitored is located, and which parameters and concentrations have to be monitored continuously.

At national level, the competent authorities are based on the Integrated Environmental Authorization (IEA), an administrative intervention figure that includes all the environmental aspects, and any other related aspects considered by the competent environmental authorities, in accordance with the basic state legislation and the corresponding autonomous regional legislation.

  • European Standard (EN): Establishes a series of assurance levels and monitoring tests to ensure that systems comply with the maximum measurement uncertainty established in the standard and that control is maintained throughout the measurement period. Generally speaking, they are: NGC1 (QAL1), NGC2 (QAL2) and NGC3 (QAL3).
  • EPA Emissions Standard: Regulated by the United States Environmental Protection Agency and its Title 40 Code of Federal Regulations (40 CFR).

There are a multitude of analysis devices and instrumentation that vary according to the parameters to be measured, the types of systems, the auxiliary equipment required for their design and even the measurement techniques to be used.

Therefore, the design and configuration of a CEMS will vary according to the nature of each measurement project.

The most effective way to choose the most suitable Continuous Emissions Monitoring System is to have an Environmental Engineer study in detail the client’s needs, identify the requirements according to the Permit and current activity of the plant and be able to design a CEMS system adapted to the production process.


cems system


cems extractivo

What types of CEMS exist?

We already know that it is not the same to design and implement a CEMS solution for gas measurement in a desulphurisation plant, a denitrification plant or in a stack emission source.

But what we can do is to classify Continuous Emission Monitoring Systems into 2 main groups:

  • In-situ or non-extractive CEMS (measurement on wet basis)
  • Extractive CEMS (dry or wet basis)

In-situ CEMS: In this architecture, the analysers and sensors are mounted in the stack and in direct contact with the gas, without the need to extract and transport it.

The main advantages are that the emissions are determined under the same conditions as the gas in the stack, no probes or heated lines are required for sample collection and transport, and no conditioning system is required.

The main disadvantage is that all the instrumentation is located at height and therefore maintenance must be carried out at height and in the open.

Extractive CEMS: In contrast to the in-situ architecture, in extractive CEMS it is necessary to extract the gas from the stack using vacuum pumps and transport it without altering its composition to the location of the analysers.

Within the extractive configuration, there are two possible methods: dry basis and wet basis. Both systems can be with or without dilution.

The main advantages are undoubtedly the ease of access to equipment for diagnostics and maintenance, as well as a wider choice of equipment.

On the other hand, it requires more maintenance resources associated with the extraction methods.


Software adquisición de datos

CEMS data acquisition and logging: emissions monitored in real-time

The functionality of the CEMS does not end with continuous monitoring, but by regulation all acquired data must be controlled and reported to the competent authorities.

It is therefore essential to have data acquisition and control software integrated with the plant’s control systems that also allows the validation of environmental data, their classification and the generation of reports both for the company’s own use and for the authorities.

At Envira we are experts in continuous environmental measurement

As we have seen throughout this article, the correct choice of a Continuous Emissions Monitoring System (CEMS) is a task that requires in-depth technical knowledge of the subject and of the sector so that the implementation of the system ensures the greatest profitability for the company and provides security in relation to the regulations in force.

With more than 37 years of experience in emissions, Envira specialises in the control of gases and instrumentation and has maintenance teams distributed throughout the peninsula to be able to provide assistance in the event of any eventuality.

We also have an ENAC accredited laboratory and our own data acquisition and validation software developed and updated by our team of programmers.

For all these reasons, we can say that Envira is the most complete partner for the integral management of any continuous monitoring project of environmental emissions.


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