There has never been such a wide range of analysers and systems available for continuous emissions monitoring from industrial processes as there is today. In the UK there are many manufacturers and distributors offering CEMS (continuous emissions monitoring systems) using a variety of technologies. These include Ultraviolet (UV) and Infrared (IR) absorption, Fourier Transform Infrared (FTIR), Differential Optical Absorption Spectroscopy (DOAS), chemiluminescence and photoacoustic spectroscopy (PAS) for gaseous species as well as various techniques for particulate monitors.

A large number of CEMS are now approved under the Environment Agency’s Monitoring certification (MCERTS) scheme which aims to give operators and regulators confidence that the CEMS are fit for purpose on a particular process (e.g. waste incineration). Although manufacturers may offer instrumentation with a wide range of analysis techniques, the sampling of gaseous pollutants comes down to two main techniques – extractive and in-situ monitoring. In-stack measurement techniques are generally employed for particulate species, although there are some special cases where extractive systems have been designed.

MCERTS schemes – background

The Environment Agency Monitoring Certification scheme (MCERTS) was first introduced in 1997 for CEMS. It has been expanded subsequently and now encompasses the following:

Monitoring emissions to air:

  • Continuous monitoring of industrial chimneys, stacks and flues
  • Emissions monitoring from chimney stacks – using accredited laboratories and certified staff
  • Monitoring ambient air quality
  • Portable equipment for emissions monitoring
  • Monitoring with isokinetic samplers

Monitoring emissions to water:

  • Equipment for monitoring discharges to rivers, smaller watercourses and the sea
  • Self monitoring of effluent flow
  • Direct toxicity assessment of effluents

Continuous emission monitoring systems

The initial focus of MCERTS was on continuous emissions monitoring systems (CEMS) for monitoring emissions from chimney stacks.

The scheme covers:

  • Extractive stack-emission monitoring instruments
  • Cross-stack or in-situ emissions monitoring instruments

Performance standards were developed which covered:

  • Large combustion plant
  • Incineration of municipal and hazardous wastes
  • Solvent-using processes

The most recent versions of the performance standards include additional categories for other process industries.

The atmospheric pollutants covered by the scheme were selected so that there was maximum overlap with, and benefits to, a wide range of industrial processes. These now include greenhouse gases and pollutants such as hydrogen fluoride and ammonia.

The measurement ranges covered for each of the atmospheric pollutants depends on the specific, process application for the CEM. The instrument manufacturer and the certification body (Sira Certification Service) agree this scope at the time of the application for certification.

Benefits of the MCERTS scheme for CEMS

  • It delivers a certification scheme that is both accepted and formally recognised within the UK and internationally
  • It provides assurance to regulatory authorities that equipment and services which are approved to MCERTS standards are suitable, and capable of producing results of the required quality and reliability
  • It gives users of monitoring equipment confidence that equipment approved by MCERTS is robust and conforms to performance standards related to current international standards
  • It supports the delivery of accurate and reliable data to regulators and the public
  • It provides the framework whereby further monitoring instrumentation and other aspects of compliance monitoring can be formally certified
  • It meets the growing requirements of EU Directives, which increasingly specify that monitoring systems must meet minimum performance requirements

Techniques for gas emission monitoring CEMS for measurement of gaseous species

The two general sampling techniques can be split further into several categories:

Extractive Techniques Heated extraction . This technique involves extracting the sample gas from the stack using a sample probe, heated line, gas conditioning equipment and a heated sample pump. The gas sample is not diluted, so higher range analysers are used (i.e. 0-5000 ppm). Generally the gas is transported via a heated sample line to the analyser(s). Before being introduced into the analyser, the sample is usually “conditioned” to remove any condensates and to reduce the temperature in order to protect the analysers. It is during this transportation and conditioning stage that problems may occur.

The biggest challenge is to ensure that the gas that was introduced into the sampling system from the stack at the point of measurement arrives at the inlet of the analyser in exactly the same state as it was in the stack. Many gases that are measured commonly (SO 2 NO 2 HCl etc.) are, to varying degrees, water soluble and situations can arise in parts of a sampling system for analytes to ‘drop out’ due to dissolution in condensation (eg if ‘cold spots’ develop). Considerable care must therefore be taken to ensure that the sampling system is designed in such a way as to eliminate as far as possible any sample loss or degradation. A wide range of single component and multi-gas analysers are available for use with extractive sampling systems.

Dilution extraction . As above, this technique involves the use of a probe and sample line to transport the sample gas to the analyser(s). The main difference between the two techniques is that with dilution extraction, the sample is diluted with clean, dry air to a predetermined factor (eg 100:1). With dilution extraction, there is less of a requirement for the sample line to be heated except in special circumstances. Because the sample gas is diluted, lower range “ambient” gas analysers (i.e. 0-10,000 ppb) can be used.

Generally speaking dilution extraction requires less operator maintenance than heated extraction as there is often less gas conditioning equipment to maintain.

In-situ techniques In-situ “probe” analysers . Several manufacturers use the in-situ probe analyser design. This can be described as a “close coupled” analyser and probe, where the analyser is directly connected to the probe installed in-situ at the point of measurement. This reduces any problems that may occur with extractive monitoring, especially heated extraction.

Most in-situ probe-based systems use an infrared measurement principle that can often measure more than one component (e.g. SO 2 , NO, CO). Non-dispersive IR, FTIR and Gas Filter Correlation are all examples of the infrared techniques used. Although they can be described as “in-situ” these techniques still rely on a probe being inserted into the stack, therefore care has to be taken to ensure that the gas is homogenous, uniform, well mixed and therefore representative at the point of measurement.

Cross-duct analysers . Cross-duct continuous emissions monitoring systems have many advantages over the other techniques listed here and are becoming very popular. Basically, an energy source (IR or UV) is projected across the stack from one side to the other. Specified target gases absorb some of this energy at specific wavelengths resulting in an altered energy state of the gas molecule.

Various analytical techniques including FTIR and spectroscopy are employed in cross-duct monitoring systems. Most cross-duct systems have the ability to measure multiple gases over a range of wavelengths (mainly IR) although some manufacturers are combining UV and IR absorption systems to further extend the range of gases. As there is no sample system used with these techniques, and the analysers do not come into contact with the target gases, far less maintenance and operator involvement is required.

Calibration of cross-duct systems can be more complicated and, unlike extraction systems, it is impossible to calibrate the “whole system”. However, the benefits of cross-duct monitoring often outweigh such difficulties, especially if an automatic calibration system can be supplied to demonstrate accurate and reliable calibration checking.

Technique Type Gases Measured Comments
Chemiluminescence Extractive NO NO x NO 2 NO 2 calculated (NO x – NO)
UV Fluorescence Extractive SO 2 (H 2 S, Total Reduced Sulphur) Gases in brackets can also be measured but not simultaneously
IR Gas Filter Correlation (GFC) Extractive CO CO 2 NO x SO 2 N 2 O Multiple gases can be monitored, generally no more than 2-5
Fourier Transform Infra-red (FTIR) Extractive / Cross duct CO CO 2 SO 2 NO x HCl HF etc Multiple gases can be monitored, typically 5+
Differential optical absorption spectroscopy (DOAS) Cross-duct CO CO 2 SO 2 HCL HF NO NO 2 NH 3 VOCs H 2 O Multiple gases can be monitored, typically 5+ NO 2 measured directly. Additional gases can be added at relatively low cost
Flame Ionisation Detector (FID) Extractive Total Hydrocarbons Requires hydrogen carrier gas
Tuneable Laser Diode (TLD) Cross-duct HCl HF NH 3 CH 4 CO CO 2 H 2 O Cost effective for single component applications
Zirconia oxide cell In-situ O 2 Widely used, max temp generally 500ºC
Paramagnetic Extractive O 2 None
Photoacoustic spectroscopy (PAS) Extractive CO CO 2 SO 2 HCL HF NO NO 2 NH 3 VOCs H 2 O Can measure virtually any gas that absorbs IR. Detailed analysis of other compounds that may be present other than target gases required
Pb 0 – 5200 ppm 8 ppm 0.0021

Types of CEMS analysers

Extractive gas analysers are designed to be used with heated or dilution sampling systems and therefore draw a gas sample into the analyser for analysis. These systems are therefore of pneumatic/ electronic design and many techniques are available depending on the type and number of gases to be monitored. Some techniques only measure a single component, others can measure multiple gases.

Cross duct analysers do not involve pneumatics (e.g. pumps and sample lines) and generally work on optical gas absorption techniques such as FTIR or DOAS. Both of these techniques involve a transmitter and receiver system (or a transceiver) and rely on gas absorption laws such as the Beer-Lambert law. These analysers measure gases passing through the stack due to specific energy-absorbing spectral characteristics of the various gases.

CEM particulate monitors

Instruments for measuring particulate emissions to atmosphere are some of the most common to be fitted to industrial processes. There are a number of techniques used for quantifying particulate emissions which include;

Optical transmission These instruments are capable of measuring smoke density in terms of transmission, opacity, Ringelmann units or optical density (extinction co-efficient). To derive an output in mg/m 3 requires the generation of a “transfer function” which is applied to the extinction co-efficient. The transfer function is attained by performing a number of isokinetic extractive samples. The extinction coefficient is used because of the requirement in BS EN 14181 that the relationship between mass concentration and CEM output be linear.

Light scatter devices There are several types of light scatter devices, side scatter, back scatter and forward scatter. A light scatter instrument measures the amount of light scattered in a particular direction (i.e. forward, side or back) and outputs a signal proportional to the amount of scattering material (e.g. particulate matter) in the sample stream. As with transmissometers, mass concentration can only be derived by applying a transfer function, derived from a series of isokinetic tests. Because light scatter instruments achieve higher sensitivities than transmissometers, they are normally used for measuring low particle mass concentrations in smaller ducts (typically less than 2 metre diameter).

Dynamic transmission This is a form of opacity measurement where the light transmission value is averaged over a long period (up to several hours) and measures dynamic variations in light intensity. The method works by measuring small variations in opacity caused by changes in mass loading, incomplete mixing of particles, turbulence induced inhomogeniety in particle distribution and pressure/temperature variations in the flowing gas. These monitors are able to achieve much better sensitivities than transmissometers over short path lengths, typically 1 mg/m 3 at 1 metre, but are not as sensitive as light scattering devices.

The averaged transmission level is used to perform continuous zero offsets to compensate for material on the lenses or misalignment of the heads. This procedure makes the instruments relatively immune to the effects of window and lens contamination.

Probe electrification devices These devices detect charge imparted to an intrusive steel probe by three mechanisms, charge transfer, frictional electrification and charge induction. These devices are inherently reliable as they have no moving parts and use electronic systems to check their performance. Their sensitivity is a function of the gain employed to amplify very small signals and the length of the probe. Tribo-electric CEMs can achieve similar sensitivities to scatter devices but are affected to some extent by gas velocity variation. Tribo-electric monitors measure total mass flow directly and accurately at very low cost without the need for separate velocity measurements.

Inertial mass measurement The tapered element oscillating microbalance (TEOM) generates stack particulate mass concentration information in real-time using a direct mass measurement approach. The instrument collects particulate matter from the flue using a dynamic isokinetic sampling system that continually adjusts to reflect changing stack velocity conditions.

The instrument contains a unique system for measuring the stack gas moisture content in real time, along with the flue gas velocity. With the addition of flue gas O 2 and CO 2 data, the instrument computes the molecular weight of the stack gas. Stack particulate mass concentration values are generated on a dry, standard volume, basis.

Beta ray absorption devices Beta (β) gauge samplers are the only systems that continuously measure the mass concentration of particulate by extraction. The particles are collected isokinetically on a filter tape and the change in transmission of a stream of β-particles from a radioactive source is monitored. The particulate laden gas is extracted via a small nozzle from the duct, the extraction rate being controlled by a duct flow sensing system. The captured material is placed on a moveable sticky tape and then presented to a β gauge to measure the mass of particles. These devices are not continuous monitors and there are periods when they are not monitoring. The output they give is not a real-time determination of mass concentration, rather a series of discrete time weighted averages.

Because the cross sectional area of the sampling nozzle is very small in comparison to the total area of the duct, there is a strong possibility that the sample is unrepresentative of the average mass concentration within the duct. In consequence, these systems are used more for ambient air applications than stack emissions.

With tougher regulatory demands with regard to data availability and equipment downtime along with the often harsh environmental conditions that CEMS often have to work in, accuracy, reliability and fitness-for-purpose should be considered along with price.

Which system is right for you?

There is no such thing as a “one size fits all” CEMS system. The type of CEMS for your application depends on many factors including:

  • Stack conditions (temperature, dust levels, condensing water, gases present)
  • Stack dynamics and dimensions (velocity, flow, diameter, location)
  • External environmental conditions (weather, temperature, corrosive environment)
  • Single or multiple gases
  • Single or multiple monitoring points
  • Type of gases to be measured
  • Concentration of gases to be measured
  • Location of monitoring points
  • Access to monitoring points
  • Compliance with legislation and availability of data
  • Reason for monitoring (compliance monitoring or process control)
  • Resources for operation and maintenance of system
  • Budget for purchasing equipment
  • Budget for running equipment

This is a non-exhaustive list of factors that will influence the choice and suitability of monitoring equipment.

Although often cited as the main driver, equipment price should not be the main consideration when choosing a CEM system.

With tougher regulatory demands with regard to data availability and equipment downtime along with the often harsh environmental conditions that CEMS often have to work in, accuracy, reliability and fitness-for-purpose should be considered along with price.

As stated above, many CEMS are now MCERTS certified. Although many manufacturers have MCERTS approval, the difference in scope and approved measurement range differs considerably from system to system. For example, a system that is to be used for waste incineration (to meet the requirements of the EU Directive 2000/76/EC) needs to be able to measure down to the very low emissions limits laid out in the directive. It is important to understand the scope of the certification for the instrument. An instrument with a lower range than is required can be used for emission measurement at a higher range, the reverse situation is not acceptable.

Finally, overall system quality and life-time maintenance costs should also be considered.

Published: 10th Sep 2006 in AWE International