THE SOURCE TESTING ASSOCIATION ANNUAL GUIDE 2011
Light attenuation (Transmissometry): In which the amount of light absorbed by particles crossing a light beam is measured and correlated to dust concentration. In Opacity/Extinction instru- ments the amount of light reduction in measured directly whereas in Ratiometric Opacity the ratio of the amount of light variation (flicker) to the transmitted light is measured.
Light scattering: in which the amount of light scattered (reflected) by the particles in a specific direction is measured. Forward, side and back scatter are a function of the angle of scattered light that is measured by the detector. Light scattering techniques (especially forward scatter) are capable of measuring dust concentrations several magnitudes smaller than that measured by light attenuation techniques.
Using control charts for filter leak monitoring
A Continuous Emission Monitor (CEM) which has been indepen- dently certified to meet the European Standard EN -15267-3 (which is adopted in the MCERTS class 1 performance standard) may carry the certificate as QAL1 approved. This instrument may be used to continuously measure mg/m3 according to the uncertainties expressed in the WID and LCPD directive as well as used in other applications as specified by regulators on a case by case basis.
2 Quantitative monitor less certainty). 3 Filter leak monitor
A filter leakage monitor monitors for changes in the operation of the dust arrestment plant (typically bagfilter).The plant operator is warned when there is any leakage and can fix the leak and hence bring the process in control. The trend of emissions and arrestment plant condition are obtained from an associated report. Importantly, there is confidence in data because there are self-checks on the instrument.
4 Gross Filter failure detector
A simple instrument provides an alarm when there is a very significant change in emissions associated with filter rupture. These instruments tend to be used only on smaller filters since they provide no information to improve the plant performance and have no trend output or Quality Assurance (QA) features to provide confidence they are working correctly.
Emissions data from quantative monitor
A particulate CEM which may be used to monitor mg/m3 continuously is referred to as a quantitative monitor. In the UK it is common that this instrument is also certified. The specific approval relevant to this type of instrument is MCERTS class 2 which allow slightly higher uncertainty compared to a class 1 instrument and allows different Quality Assurance (QA) procedures (automatic internal zero and reference) built into the instrument. A class 1 instrument may also be used to satisfy these requirements.
Note: both QAL 1 approved CEMS and Quantitative monitors may be used in Qualitative mode when a limited number of Isokinetic samples are performed or there is reason to believe there is increased uncertainty in the results of the isokinetic sample (e.g. at dust levels below 1 mg/m3 where isokinetic sampling provides confidence that dust levels are low but with
10 Source Testing Association Annual Guide 2011
Technology and design of instruments Particulate CEMS measure a parameter (eg scattered light) which can be correlated to dust concentration by comparison to a gravi- metric sample taken under iso-kinetic conditions (eg EN-13284-1) rather than the mass concentration directly. The performance and suitability of any particulate monitor is therefore application dependent. A number of techniques are used in practice which between them provides a practical and robust solution for most industrial applications. The core techniques used for continuous for monitoring particulate are:
Light scattering
Probe electrification: in which the electrical current produced by particles interacting with a grounded rod protruding across the stack/duct is measured and correlated to dust concentration. Charge induction (AC Tribolectric and ElectroDynamicTM) and dc Triboelectric instruments are types of probe electrification devices in which differ- ent signal and current analysis are performed.
Probe electrification
One of the fundamental issues in obtaining good results from particulate instruments is to ensure that the instrument is fit for purpose for the intended application. This means that the instrument: 1 has a stable, reliable response which can be directly correlated to dust concentration with limited cross interference from likely changes in process or flue gas conditions. MCERTS certificates for certified products provide guidance on the application suitability of different instruments. Manufacturers should be contacted for more detailed guidance on the application suitability of a specific type of instrument.
Light attenuation
2 can operate long term in the application without the need for maintenance or cleaning. The Maintenance Interval as stated in the MCERTS certificate can provide guidance on servicing issues and longer duration tests and experience with an instrument also are very relevant
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