Considerations for Procuring a Continuous Emissions Monitoring System

by | Jan 13, 2021 | CEMs, Emissions Monitoring

Before construction of an Energy from Waste (EfW) plant, a power plant or biomass gasification plant can be signed over to the site operator, it’s the responsibility of the EPC (Engineering, Procurement, Construction) to procure and install Continuous Emissions Monitoring Systems (CEMS). We explore the most important considerations for procuring a CEMS.

Why are Continuous Emissions Monitoring Systems Required?

In the UK, large combustion plants and waste incineration plants must operate with a permit within the strict conditions of the Industrial Emissions Directive (IED 2010/85/EU). The IED requires sites to control and reduce the impact of their emissions on the environment.

To monitor pollutants emitted to atmosphere and to ensure measurement parameters are within the permit, this is achieved using CEMS. A CEM system continuously collects, records and reports gas and dust emissions data to the authorities.

A complete CEM system could comprise of:

  • > A sample extraction and conditioning probe (with integrated temperature, pressure, and flow measurement)
  • > Sample lines that transport the gas and dust sample to a conditioning system
  • > Sample conditioning system which may filter, cool, and remove moisture from the sample
  • > Housing
  • > Analysers
  • > Maintenance & Calibration
  • > Data acquisition software where gas concentrations are measured, recorded, and stored as data. The data is used to generate reports, alarms or control a process.


Which Sites Require CEMS?

The requirement for CEMS depends on various factors; the industry regulations, the size of the plant, which components are being measured and what is required by the environmental permit.

Generally, sites with a combined rated thermal input of greater than 50MW and waste incineration plants require Continuous Emissions Monitoring Systems.

The Environment Agency operates the MCERTS scheme which sets standards obliging sites to use the Best Available Techniques (BAT). BAT highlights the types of CEM needed (in-situ or extractive) and monitoring technique (sensor technology) depending on the gases you need to measure.


Selecting a Suitable CEM System

To ensure that your CEMS provides a good return on investment, it pays to understand which factors maximise system efficiency. CEMS is the result of careful planning. By understanding and applying the relative legislation & permit requirements to your process conditions, you will be able to choose the correct CEMS analytical techniques, system positioning, ongoing maintenance, and cost control.

Working with a supplier that has the expertise and experience to help you procure the right system first time and manage ongoing costs is key to this outcome.


Legislation, Approvals & Permit Requirements

In the UK, where continuous monitoring is required, the EA require the Continuous Emissions Monitoring Systems and data acquisition software to be MCERTS accredited for the parameters specified in the permit, and it must comply with EN14181.

In general, the lower the certified range, the better the performance of the CEM is likely to be. This is because the majority of performance standards are expressed as a percentage of the certified range.

With talk of the BREF regulations tightening the emissions monitoring limits, it pays to understand what you need to do and how you need to be doing it. The Environment Agency’s approach to selecting suitable CEMS is to apply range multipliers. The lowest certified range is not to be more than 1.5x the daily average (DA) ELV for waste incineration processes as specified under Chapter IV of the IED and 2.5x the DA-ELV for large combustion-plant (as specified under Chapter III of the IED) and other types of process.

As there is a linear relationship between certified ranges and uncertainties, these multipliers provide assurance that CEMS with appropriate ranges will meet the uncertainty requirements specified in the incineration and large combustion plant Directives.


CEMS Techniques

Generally, there are two techniques for monitoring emissions from stack gases: in-situ or extractive analysis.


In-situ monitors are mounted on the stack and measure gas in the stack at process conditions (temperature, humidity, moisture)


  • > No sample conditioning or sample transport (heated lines) required so in-situ offers a lower manufacturing cost
  • > Low installation costs as there’s no need for fitting heated lines
  • > No filters or sample conditioning components to maintain, therefore, low service costs
  • > In-situ gives a true measurement as you don’t need to remove the moisture from the sample being measured,, negating the risk of water soluble compounds, such SOx, NH3, HCl, etc, from being removed


  • > Calibration by certified span or zero gas is very costly or impossible due to very high volume of gas. (QAL3 weekly zero and span injection, annual linearity check etc). Failure to meet requirements of EN 14181
  • > Angled sample planes can not be validated (EN 15259)
  • > On small stacks, there’s not always enough space for multiple flanges


However, there are also some important questions to consider before committing to an in-situ stack gas analyser.


  • How easy will it be to remove the analyser from the stack?


Ambient temperatures and weather conditions

  • Will the analyser be stable and accurate to face extreme temperatures or conditions?
  • How accessible are the flanges on the stack and could weather conditions pose a risk to a service engineer?


A scramble for positions

  • Several instruments for a complete CEMS means several flanges are required: gases, HF, flow probe, temp probe, O2, Dust, VOC. Are there other probes competing for space on the stack?

Extractive Methods

Extractive systems are, in general, more complex and more costly than in-situ analysers because they require the installation of a sample line however, it’s safer and more convenient to perform maintenance routines away from potentially poor weather conditions or the dangers associated of working at height.

There are 4 main types: Cold extraction, Dilution, Hot/Wet, or Drying System (Permeation or Chiller)


Cold Extraction 

Cold extraction is the simplest and cheapest method but it can only be used with non-soluble gases and on ambient temperature processes/stacks making it for ambient monitoring solutions.


Extractive Dilution

During extractive dilution analysis, sample gas is extracted from the stack at a known flow rate, mixed with a known flow rate of dry air and then transported to the analyser for measurement.

  • > Can be used to reduce moisture content so no heated sampling components are required to bring the sample gas from the stack to the analyser system.
  • > No power required at the probe so it can be used in hazardous areas.
  • > Low extraction rate
  • > Quenches most reactions
  • > No corrosive gases transported

  • > Flow rates need to be controlled to avoid varying dilution ratios.
  • > Sample gas needs to be a high concentration to avoid analyser sensitivity issues (direct calibration)
  • > Wet gas basis only
  • > Requires purified air
  • > Requires separate O2 and moisture measurements
  • > Slow response time
  • > Single dilution ratio
  • > Requires a tracer gas at the probe to verify dilution ratio
  • > Does not operate at high temperature
  • > Lack of MCERTS approval


Extractive Hot/Wet

Extractive hot/wet analysis extracts the sample from the stack and transports it to the analyser using heated line & sampling components. The temperature of all components in contact with the sample gas (including the analyser measurement cell) is typically at 180oC to avoid condensation and loss of soluble gases.

  • > Can be used for the majority of gases
  • > Dry and wet analysis
  • > Ease and accuracy of calibration
  • > Ease of maintenance
  • > Integral O2 and moisture measurements
  • > Analysers run under controlled conditions: high stability
  • > Optimal control of sample conditioning
  • > Provides accurate analysis

  • > Optical parts could fail if any of the temperature controlled parts fail
  • > Systems could have long lengths of heated lines, so tendency is not to span at the probe
  • > Heat lines & components are costly
  • > Heated lines have high power consumption


Drying System

Chiller Dryer

When sample gas is passed through a chiller to take the sample gas down to a low temperature, it removes water so the gas sample is almost dry in preparation for the analyser.

  • > Can use analysers suitable for operating at low/ambient temperatures
  • > Verification of efficiency is possible through temperature measurement
  • > Very robust, reliable, and a well proven method
  • > Quick water removal minimises solubility problems
  • > No dirty condensate problems
  • > Maximum removal of acids and hydrocarbons


  • > Cannot be used on very soluble/corrosive gases
  • > Still requires heated sampling components/heated line up to chiller which adds to costs


Permeation Dryer

The permeation dryer is the most efficient method of cooling a gas sample on the stack. Permeation is performed at a low temperature therefore resulting in a low power consumption.

  • > No heated line needed therefore lower cost compared to other chiller methods
  • > Low temperature means low power consumption
  • > Sample is dry and clean so no H2O measurement required or is there H2O interference
  • > Removes water while in gaseous phase; no problems with solubility of SO2, NO2 etc.
  • > Removes some hydrocarbons and NH3
  • > Desiccant membrane has the tendency to be clogged up by HCs and other sticky condensates
  • > Difficult to check efficiency of membrane


Understanding the advantages and disadvantages to both in-situ and extractive gas measurement can help you determine the correct approach for your application.


Analytical Techniques

The choice of analytical technique depends on the gas species required by the permit.

The main gases are: Carbon Monoxide (CO),  Nitric Oxide (NO), Nitrogen Dioxide (NO2) Nitrogen Oxides (NO+NO2) (NOx), Sulphur Dioxide (SO2), Total Organic Compounds (TOC), Hydrogen Chloride (HCl), Oxygen (O2), Carbon Dioxide (CO2).

The secondary gases are: Hydrogen Fluoride (HF), Ammonia (NH3), Nitrous Oxide (N2O).

For a full lust of available techniques, take a look at the Government guidance


System Redundancy

Under the IED, the maximum period allowed for any one episode of abatement or equipment failure must not exceed four hours.

The a1-cbiss duplex philosophy has therefore been specifically developed to assist you in complying with IED by reducing downtime, maximising availability and ensuring you achieve a sound return on your investment.


Maintenance & Calibration

The procurement team should also pay close attention to the potential service costs. Continuous Emissions Monitoring Systems require ongoing maintenance on a regular basis because the process conditions that they’re subject to. Problems are less likely to arise is the CEMS have a regular service plan. All customers of a1-cbiss are supported by the UK’s largest service network which offers around the clock reliability and performance.

a1-cbiss assemble CEM Systems with top-quality components from reputable brands with the best reliability and lowest maintenance cost.

Supplier Support

a1-cbiss work with you on your journey to compliance and process efficiency. Our strengths enable us to consult with you to design and integrate a monitoring system that not only achieves regulatory compliance but allows you to operate the process efficiently to reduce wastage, control costs and reduce emissions.

Our scope of supply is not simply offering CEMS at lowest price but, rather, a partnership to assist the site operator throughout the system’s lifecycle.



The CEMS (Continuous Emissions Monitoring Systems) supplier is a critical element. Maintenance and service are a natural requirement of any CEMS. Be sure to choose a company that not only supports the immediate need of supplying a reliable CEMS, but also provides ongoing support to ensure years of reliable service. Also, by choosing a vendor that can supply all types of measurement techniques, you can ensure you will get the best solutions for your application.

Remember that it’s no good spending money on CEMS if they are not specified or installed correctly.


Contact a1-cbiss for support with procuring your CEM System