The topic of monitoring stationary sources emissions is very broad and many books have been published on the subject. This article is aimed at giving an overview of some of the techniques employed.

Continuous Emission Monitoring Systems (CEMS) or Automated Measuring Systems (AMS) or CEM systems can be categorised as being either extractive (source-level, dilution or close coupled) remote, parameter-based or in-situ systems. In-situ systems can be further separated into point in-situ, sometimes referred to as in-stack monitors, and path monitors, which are also known as cross-stack monitors. A summary of the different techniques available is presented in Figure 1.

When considering installation of continuous monitors the CEN Standard EN 14181 Stationary source emissions – quality assurance of automated measuring systems must be taken into account. Under the Monitoring and Certification Scheme MCERTS, the Environment Agency has produced a Method Implementation Documents (MID) for EN 14181 along with a technical guidance note M20, which set out the requirements for regulators, end users and manufacturers alike, and will specify how the standard will be adopted in England and Wales. EN 14181 and the accompanying MID will specifies the quality assurance requirements for operators of continuous monitoring equipment in terms of instrument specification, calibration and validation. The introduction of EN 14181 has had a major impact on AMS best practice methods.

AMS or CEM systems can be categorised as being either extractive (source-level, dilution or close coupled) remote, parameter-based or in-situ systems. In-situ systems can be further separated into point in-situ, sometimes referred to as in-stack monitors, and path monitors, which are also known as cross-stack monitors. A summary of the different techniques available is presented in Figure 1. The most common pollutants that are monitored continuously are:

  • Total particulate matter (TPM)
  • Sulphur dioxide (SO 2 )
  • Oxides of nitrogen (NO X )
  • Carbon monoxide (CO)
  • Oxygen (for correction to standard reference conditions) (O 2 )
  • Speciated and total volatile organic compounds (VOC or TOC)
  • Hydrogen chloride (HCl)
  • Hydrogen fluoride (HF)
  • Flue gas flow rate, water vapour content, temperature and pressure

CEM (AMS) systems

The following provides a short description of the main types of sampling systems offered by manufacturers and system builders, together with an overview of currently available technologies for measuring the major pollutant releases to atmosphere.

Extractive systems

Source-level extractive systems

Source-level extractive systems are those in which a sample of flue gas is continuously extracted and conveyed to the analyser using a sampling line. Particulate matter may be removed from the gas, and it may be cooled and dried, but in all other respects the sample is not altered by the sampling process. Three types of source-level extractive systems are marketed commercially:

  • Cool/dry systems with conditioning at the analyser enclosure or with conditioning at the probe
  • Hot/wet systems – analysers are heated
  • Close coupled systems

In some cases a combination of these systems may be used, for example when there is a requirement to measure both highly reactive and less reactive determinands concurrently. A source-level extractive system consists of a number of basic components: probe, sample line, filters, moisture-removal system and pump. In some source-level systems the analyser, eg O 2 zirconia sensor or TOC flame ionisation detector (FID), is mounted on the stack or duct, and the sample line is very short. In this case analyser response times are very fast and, apart from particulate filtering, sample conditioning is not required. These systems are known as close coupled.

Dilution extractive systems

The main issue associated with source-level extractive systems is the need to transport the sample hot, and to filter and dry relatively large volumes of flue gas. This can be largely avoided by using dilution systems, where gas is drawn into the probe at much lower flow rates than in a source-level system. Dilution systems are used in conjunction with ambient air level gas analysers. Oxygen must be measured separately for correction purposes (the diluted sample is ‘swamped’ by dilution air). There are two types of commercially available dilution systems: dilution probes, where dilution of the sample gas takes place in the stack, and out-stack dilution systems, where dilution is carried out external to the stack. The latter are less sensitive to changes in stack gas temperature, pressure and density.

In-situ systems

Path monitors

Path monitors usually measure over the entire stack or duct diameter. They are based on a beam of a certain wavelength that crosses the duct and is attenuated proportionately to the concentration of the target compound. In some systems a pipe may be used in the stack for support or calibration purposes, or to reduce optical path lengths in very large stacks or ducts. There are two basic types of path systems: single pass and double pass. These systems are much simpler than extractive systems.

Point in-situ

Point in-situ systems perform measurements at a single point in the stack, as do extractive system probes. The point, however, may extend over a few centimetres to a metre or more. In point in-situ systems the sensing optics are contained in a tube fitted with holes or filters to allow flow-through of stack gases. The sampling path will be relatively short compared to the stack or duct diameter, so the sampling location must be carefully chosen to ensure that the sample is representative of the flue gas.

Parameter monitoring

Parameter surrogate

In parameter surrogate approaches to emission monitoring direct analysis of pollutant species is not undertaken. Instead, data are gathered from process sensors such as thermocouples, flow meters and pressure transducers. The information is then either used directly as a surrogate to substitute for emission data, or it can be incorporated into a computer model in order to predict pollutant emission levels.

Predictive

Predictive monitoring can be theory-based or empirical. In theory-based monitoring, pollutant emission levels are calculated from basic principles. Alternatively, the process characteristics may be modelled using historical emissions monitoring data. Theory-based techniques are widely used in the European Union for the estimation of pollutant mass flow releases from large combustion plant.

Currently available technologies

Analytical techniques used for the continuous measurement of pollutant species can be divided into optical methods and non-optical methods. Optical methods include techniques employing infra-red, ultra violet and visible radiation. Non-optical methods include paramagnetic analysers, electrochemical cells, zirconia sensors, flame ionisation detectors, mass spectrometers etc.

Table 2 summarises currently available techniques for the continuous and semi-continuous measurement of gaseous and phase-partitioned species commonly monitored in releases to atmosphere from coal and waste-fired combustion and gasification plant. It should be noted that some techniques, e.g. FTIR, mass spectroscopy and ion-mobility spectrometry, are capable of measuring more determinands than are identified in the table. The following is a brief descriptions of a selection of the technologies identified in the above table.

Optical methods for particulate monitoring

Opacity monitors or transmissometers

Opacity monitors or transmissometers are based on the measurement of the optical transmission of a light beam as it passes through the absorbing flue gas stream, which contains particulate matter together with a mixture of gases. As a result of absorption and scattering, the transmitted light beam suffers a reduction in intensity. As with in-situ path systems for monitoring gaseous pollutants, transmissometers may be single-pass or double-pass design. Double-pass types use a reflector on the opposite side of the stack or duct so that the light is transmitted twice through the flue gas. Some modern single-pass designs use two identical senders and receivers on each side of the stack to transmit and receive alternatively in order to increase sensitivity and reduce the effects of fouling of the optical surfaces. The light sources used include filament bulbs, light emitting diodes and lasers.

Scintillation or received light modulation

In recent years, a variation of the opacity monitor or transmissometer has been developed, which is based on the flicker of the light beam as dust particles pass through it, rather that the overall attenuation. Dust particles passing through the light beam cause the receiver to detect a modulating signal. The ratio of the fluctuations in the received light (scintillation) to the average light intensity at the detector is used to produce a signal proportional to changes in particulate concentration. The use of the ratio of fluctuating light intensity to average light intensity means that fouling of the optics, lamp degradation and detector drift biases cancel out.

Light scattering

When light is directed toward a particle, the particle may both absorb and scatter the light, deflecting it from its incident path. An opacity monitor or transmissometer measures the intensity of light that is not scattered. Other instruments have been developed to measure the scattered light. The intensity of the scattered light depends on the angle of observation, the size of the particle, its refractive index and shape, and the wavelength of the incident light. Both in-situ and extractive analysers of this type have been developed. A light beam is passed through the PM laden flue gas. Absorption and scatter attenuate the light. Light scatter analysers measure the intensity of the scattered light at a predetermined angle to the beam direction. The amount of light scattered in any direction is dependent on the size distribution and shape of the dust particles. Variations in the intensity of the light source and in the sensitivity of the detector are compensated for by the use of a reference beam, in the opposite phase to the measuring beam, onto the photoelectric detector.

Probe electrification instruments

Probe Electrification devices measure the current produced at a grounded sensor rod installed across a duct or stack by particles interacting with the rod. There are a number of types of probe electrification instruments available, each processing and interpreting the current in different ways.

Triboelectric instruments measure the DC component of the current and are therefore predominantly sensitive to the collision signal. Electrodynamic and AC tribolectric instruments measure the alternating component of the signal and hence are sensitive to the signal induced by particles passing by the sensor rod. All three techniques are highly sensitive and are responsive at concentrations below 0.1mg/m 3 . Triboelectric instruments are widely used in bagfilter applications in the metals, mineral and process industries. In the main, their use has been confined to process, qualitative or broken bag monitoring due to perceived performance limitations and in the US due to lack of regulatory requirements and approvals.

Triboelectric instruments have received regulatory approval for measurement in specific applications in Germany. Electrodynamic and AC Triboelectric instruments are used to satisfy measurement requirements on Bagfilters in the, power, incineration, metals, mineral and chemical industries. Regulatory approvals exist for QAL1 electrodynamic instruments in the UK and Germany

Optical methods for the analysis of gases

Simple Non Dispersive Infrared (NDIR)

Many gaseous pollutants absorb light energy in one or more regions of the spectrum. Sulphur dioxide and a wide range of other gases absorb infrared radiation and ultraviolet radiation. Each type of pollutant molecule will absorb light at a characteristic wavelength, and therefore it can be distinguished from other pollutant species. Non-dispersive photometry analysers using infrared (NDIR) and ultraviolet (NDUV) have been developed for a wide range of gases.

Luft detector NDIR

The Luft type detector or pneumatic detector consists of a reference cell, a sample cell and, in the case of single component analysers, two gas-filled absorption cells arranged in series. The cells are connected in such a way that any pressure difference between the cells can be detected by either a flow sensor, or in some designs by a type of strain gauge known as a microflow sensor. The cells in the detector are filled with the gas to be measured. Appropriately filtered infrared light is made to pass through both cells. More light is absorbed in the first cell than in the second cell due to the distribution of energy in the light (higher energy at centre of band, lower at band ‘wings’). The light energy is absorbed by the gas in the cells, causing the gas to heat up. Differential heating caused by the difference in incident IR light causes a differential pressure between the two cells in the detector, which is detected by either distortion of the diaphragm (measured by strain gauges) or by flow between the cells.

Gas Filter Correlation (GFC) NDIR

A type of NDIR technique, which is widely used in in-situ monitors, is also applied to extractive system analysers. The gas filter correlation (GFC) technique uses a reference cell that contains a 100% concentration of the pollutant, instead of the 0% concentration in the techniques discussed previously. Radiation from an infrared source passes through a filter wheel, which contains a neutral gas, such as N 2 , in one cell and the gas of interest in the other cell. The light is then passed through a modulator that creates an alternating signal. When the instrument is operating the filter wheel is continuously rotating. When light passes through the gas filter it will be attenuated. The gas filter contains enough of the target gas to remove most of the light at the wavelengths where the target gas absorbs. The wavelengths not absorbed are not removed and are passed on to the detector.

The net result is a reduction of light energy reaching the detector. When the light passes through the neutral cell its intensity is not reduced. If a sample of gas containing the target pollutant is introduced into the sample cell the molecules will absorb light energy at the absorption wavelength of the target gas. Because the gas filter was chosen to absorb energy at the same wavelengths, the absorption is already complete in the gas filter cell beam, and the detector will see the same signal as it did when the sample cell contained zero gas.

The beam passing through the N 2 side however will carry less energy because light is absorbed by the target gas in the sample cell. The difference between the two beams is monitored, and can be related to the concentration of the gas of interest in the sample. Other gases having spectral patterns in the same regions as the target gas will not affect the measurement, as they do not correlate. This technique is popular in the UK, with most manufacturers offering analysers based on the GFC principle of operation. Analysers are robust, relatively low cost and less sensitive to vibration effects.

Differential Optical Absorption Spectroscopy (DOAS)

Another non-dispersive method measures light absorption at different wavelengths, those at which the molecule absorbs energy and those that do not. In this system a reference wavelength is used instead of a reference cell as in techniques described previously. The underlying principle behind the method once again is derived from the Beer-Lambert law. The technique, known as differential absorption spectroscopy or differential optical absorption spectroscopy (DOAS), is applicable to both extractive system analysers and in-situ systems. In a typical system a light source is used to emit light at many different wavelengths and transmit it through a cell containing the sample gas, or across the stack. The detector signal at the light wavelength where no energy is absorbed is used as a reference measurement for the signal obtained at the wavelength where energy is absorbed.

Fourier Transform Infrared Spectroscopy (FTIR)

The technique of Fourier transform infrared spectroscopy is growing in popularity for the multi component analysis of stack gases. Analysers based on this principle of operation are capable of measuring up to 50 determinands concurrently with very fast response times and fewer cross-interferences than NDIR analysis methods. A further attraction of the FTIR technique is that analysers do not require frequent calibration against reference materials. Once the instrument has been calibrated the calibration data are stored as a spectral library, which is stored as software. Essentially the FTIR technique provides a ‘picture’ of the total absorption spectrum of the sample gas over a broad spectral range.

Non Dispersive Ultraviolet (NDUV)

The characteristics of light in the ultraviolet (UV) region of the spectrum lead to molecular electronic transitions when the light is absorbed. Absorption of ultraviolet photons excites the electrons of the atoms within the molecule to a higher energy state. The excited electrons quickly loose the energy by returning to the ground state by one of four methods; disassociation, where absorption of high-energy photons can cause the electron to leave the molecule completely, causing it to fragment; re-emission, where an identical photon is re-emitted as the electron decays back to its ground state; fluorescence, where a photon is emitted at a lower frequency than the original absorption as the electron decays back to its ground state, causing the gas to appear to glow; and phosphorescence, which is a similar process to fluorescence, but takes place over a longer time period.

Ultraviolet fluorescence

Ultraviolet fluorescence analysers for SO 2 are based on the absorption of UV light at one specific wavelength by the SO 2 molecules, and its re-emission at a different wavelength. Commercially available instruments contain either a continuous or pulsed source of UV radiation. Filters are used to produce a narrow waveband around 210nm. The light emitted from the exited molecules is first passed through a filter and then to a detector photomultiplier tube. The amount of light received at the specific wavelength is directly proportional to the number of SO 2 molecules and is a measure of concentration in the measurement cell, provided the sample flow rate is tightly controlled.

Chemiluminescence analysers

Chemiluminescence is the emission of light energy that results from a chemical reaction. It was found in the late 1960s that the reaction of nitric oxide (NO) and ozone (O 3 ) produced infrared radiation from about 500 to 3000nm.

Nitrogen dioxide (NO 2 ) does not undergo this reaction and must be reduced to NO before it can be measured by this method. Most commercial analysers contain a converter that catalytically reduces NO 2 to NO. The NO (converted from NO 2 ) plus the original NO in the sample is then reacted with O 3 as described above to give a total NO + NO 2 (NO x ) reading.

Published: 10th Mar 2009 in AWE International