• What is Continuous Emission Monitoring?

Air Monitoring

What is Continuous Emission Monitoring?

Oct 01 2021

Continuous emission monitoring – or CEM for short – is a method of monitoring the emissions from the smokestacks of industrial sites such as power stations, manufacturing plants and other facilities where combustion of fuel sources is in occurrence. It is a legal requirement in many countries around the world, including the UK, the EU and the USA.

Site owners are required by law to ensure that the appropriate mechanisms are in place for continuous emission monitoring. Not only this, but they must also regularly check the equipment is functioning properly, as well as store and analyse the data collected by the system. This is to verify that the site in question is being operated in a safe and responsible manner, as well as complying with national and international regulations regarding emissions, pollution and climate change.

Why is continuous emissions monitoring important?

CEM is crucial for all plants, power stations and other industrial sites where the combustion of fuels is a regular occurrence for a number of different reasons. Firstly, it can provide vital information about the levels of carbon monoxide (CO), carbon dioxide (CO2) and oxygen (O) in any given site, thus ensuring they remain safe for human occupation and do not pose a fire risk. This can be especially pertinent at facilities where flames are a necessary part of daily operation.

Besides this basic function of enhancing site safety, CEM is also the optimum method of ensuring that the facility in question complies with its legal obligations. There are a number of different standards and methods for environmental monitoring in the UK, with all combustion plants obliged to monitor and analyse their gas flue effluents to ensure they comply with the relevant maximum thresholds for certain contaminants.

The reasons that these limits are in place are twofold. On the one hand, a high concentration of contaminants like CO2 and PM can negatively impact upon air quality levels in the vicinity of the site. This can be problematic for a wide variety of reasons, but it is especially concerning if there is a sizable human habitation nearby.

On the other hand, certain contaminants like methane and CO2 are particularly damaging for the planet. Collectively known as greenhouse gases (GHGs), these contaminants help to trap heat in the environment and contribute to global warming, which can in turn exacerbate climate change. This manifests itself in a number of undesirable phenomena, such as more frequent and more intense extreme weather events, acidification of the seas and oceans, rising sea levels and many more catastrophic outcomes.

Due to a growing awareness of the cumulative negative effects of anthropogenic industrial activity, emissions from major sources of pollution (such as power stations, manufacturing plants and industrial facilities) must now be closely monitored to address the situation. CEM is the most effective way of doing so and curbing the emissions produced by industrial sources as much as possible.

What are continuous emission monitoring systems (CEMS)?

Continuous emission monitoring systems (CEMS) are one among several methods of monitoring the emissions from a stationary source of pollution. Alongside continuous opacity monitoring systems (COMS) and continuous parametric monitoring systems (CPMS), CEMS are deployed by site owners to determine the concentrations of various contaminants in the effluent stream of their facility’s smokestacks as a means of meeting their legal obligations.

However, unlike COMS and CPMS, CEMS extract a sample of gas from the smokestack effluent and analyse it specifically to quantify the concentration of one or more pollutant. CEMS can either be used to directly measure the target contaminant, or else to ascertain the levels of a surrogate contaminant which can reveal more information about the target contaminant.

For example, a nitrogen oxides (NOx) CEMS is used to directly measure NOx concentrations in a site’s sample. Meanwhile, carbon monoxide (CO) CEMS are often used to determine CO levels as a surrogate for concentrations of volatile organic compounds (VOCs), since there are strict limits on VOCs in place throughout the EU and beyond.

What does a typical CEMS consist of?

There are a variety of different operating methods and protocols when it comes to handling CEM at industrial sites, with owners and national authorities implementing distinct processes for sampling, conditioning and assessing the gases removed from the flue effluent. Although the exact components of a system can vary from plant to plant and purpose to purpose, a typical CEMS consists of the following different elements:

  • A sample probe, which is the piece of equipment responsible for removing the sample from the flue gas stream.
  • A pump or other pneumatic plumbing equipment, which contains valves that regulate the extraction and distribution of the sample along the sample line. This regulation is normally conducted remotely by a programmable logic controller (PLC).
  • A sample line, along which the sample is transmitted to direct it to the analysing hardware.
  • An analyser (or analysers), which are capable of assessing the concentration of certain pollutants within the gas sample.
  • Filters to remove harmful contaminants like particulate matter (PM), which can damage the analyser equipment.
  • Conditioning equipment to remove water or other elements which could impact upon the capabilities of the analyser equipment and skew the results it produces.
  • A calibration system, which is responsible for the injection of certain gases into the sample line in order to correctly calibrate the sample.
  • A Data Acquisition and Handling System (DAHS), which is a centralised computer system that can collect, store and analyse the data, including the ability to perform calculations aimed at revealing the total mass emissions of a site over time.

While not all of these components may be present and in operation at every site, several of them (such as the sample probe, analysers and filtration equipment) are mandatory for all facilities and for all pollutants. Additionally, DAHS are also a legal requirement in many sites and can incorporate flow rate data in order to produce the concentration measures in a mass per hour format.

What types of pollutants are measured using CEMS?

CEMS are capable of monitoring the concentrations of a wide variety of different pollutants, depending on the site in question, the purpose for which it is used and the specific type of contamination which must be measured. Not all CEMS are capable of measuring all types of pollution, with targeted monitoring systems in place at sites across the UK and beyond based upon the targeted contaminants in question.

Generally speaking, however, the main pollutants measured by CEMS include carbon monoxide (CO), carbon dioxide (CO2), sulphur dioxide (SO2), nitrogen oxides (NOx), hydrogen chloride (HCI), particulate matter (PM), volatile organic compounds (VOCs), ozone (O3) and heavy metals such as mercury (Hg). It should be noted that a monitoring system which is specifically designed to quantify levels of particulate matter requires extra filtration and conditioning steps to ensure the contaminant does not damage the analysing equipment. This type of system is known as a PEMS.

Meanwhile, CEMS can also be set up to monitor other parameters of a site’s flue gas effluent in order to reveal insights into its day-to-day functioning. These include such variables as the air flow in the sample (which can help to produce readings of contamination concentration on a mass per hour basis), the moisture levels of the gas and the opacity of the flue gases. Systems specifically designed to calculate pollutant levels by measuring the transmittance and opacity of the sample are called COMS.

What are the maximum emissions limits in the UK?

Even though the UK has now left the EU, the maximum emissions limits employed in the country still mirror those of the bloc. These are defined by the Industrial Emissions Directive and are updated on a regular basis. The maximum thresholds differ depending on the size of the facility in question and the specific pollutants being targeted. Here’s a breakdown of the maximum emissions limits for both medium combustion plants (MCPs) and large combustion plants (LCPs).

Existing MCPs

Pollutant

Biomass

Other solids

Gas oil

Other liquid

Natural gas

Other gases

SO2

200

400

-

350

-

35

NOx

650

650

200

650

200

250

Dust

30

30

-

30

-

-

New MCPs

Pollutant

Biomass

Other solids

Gas oil

Other liquid

Natural gas

Other gases

SO2

200

400

-

350

-

35

NOx

300

300

200

300

100

200

Dust

20

20

-

20

-

-

LCPs

Pollutant

Existing plants

New plants

 

Solids

Liquids

Gases

Solids

Liquids

Gases

SO2

200

200

35

150

150

35

NOx

200

150

100

150

100

100

Dust

20

20

5

10

10

5

*All values are mg/m3

For reference, an MCP is defined as a plant which involves a thermal input of between 1MW and 50MW. By contrast, LCPs are defined as all sites whose thermal input exceeds 50MW. For more information on the specific legislation surrounding maximum emissions limits in this country, please consult the informative guide What Are the Normal Limits for Emissions Monitoring and the investigate article Should We Measure Emissions at Lower Limits?, both of which examine the issue in greater detail.


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