18 Environmental Analysis - Focus on Mercury
Some manufacturers also offer dilution probes for gas sampling. Sample gas dilution however requires even more sensitive measuring methods, for
example atomic fluorescence. On the downside, fluorescence methods are also known to be sensitive to quenching effects, which leads to wrong mercury detection signals.
High temperature conversion
Other mercury systems use single or twin beam UV photometers. An amalgamation step is carried out before measurement (e.g. in some measuring systems to prevent cross- interference from other gas constituents). Due to the separation of the sample and the flue gas, cross interference effects are minimised. A major disadvantage of this amalgamation process is that mercury values can only be output "semi-continuously" (approximately 180 - 400 seconds per measurement).
In order to prevent from the influence
of cross interferences together with the advantage of a continuous and direct measurement of mercury, a new device using the Zeeman AAS principle has been introduced. Due to the Zeeman effect all interferences are directly compensated for during measurement without the need of any amalgamation step or any separation of mercury from the flue gas. With this approach a fast, reliable and cost effective mercury monitoring was established.
Validating the measuring device
As previously described, various points must be taken into account due to the complexity involved with checking analysers. Some devices use a mercury permeation chamber to create a mercury test gas. With this approach it is only possible to check the analyser with elemental mercury test gas. There are also some gas suppliers
on the market, who can deliver such metallic mercury test gases.
However, complete and sustainable conversion of oxidised mercury to metallic mercury is becoming the focal point for measuring systems. Test gases with oxidised mercury are not commercially available. Gaseous mercury compounds can only be
created through vaporisation of a HgCl2 solution. Such vaporisers are already available on the market and normally have suitable mass flow controllers for accurate dilution with instrument air to create different mercury calibration gas concentrations. The conversion of the mercury compounds to metallic mercury can now be checked in measuring systems by feeding those mercury test gases to the sampling probe. Some mercury monitoring devices have as an option, an integrated test gas generator, which is the only reliable check of the entire CEMS.
A new measuring method for continuous mercury measurement MERCEM300Z
Measuring gas transportation, secure and reliable conversion to metallic mercury and cross sensitivity compensations are difficult in mercury monitoring. But how should an ideal mercury measuring system be designed?
Existing physical and chemical conditions are such that extractive gas sampling is always necessary for an ideal mercury measuring system. It should allow reliable conversion with the lowest possible maintenance effort and should not have any cooling stages or converters when possible. The measuring method should be free from cross-sensitivity or at least with reliable correction of cross- sensitivity and be able to measure the total mercury in the flue gas with shortest T-90 reaction time. An automatic check of measuring precision to ensure the reliability of the measuring system should also be available.
The newly developed mercury measuring system MERCEM300Z fulfills these criteria. The so-called Zeeman AAS method is used here for continuous mercury measurement for the first time, coupled with a high temperature gas cell at 1,000°C. All mercury compounds in the sample gas are reduced to metallic mercury at this temperature. The fact that no gas cooling is done, means that recombination of Hg° to oxidised mercury compounds (oxide or chloride) cannot occur. The TÜV type approval tests have also shown a faster running-in behavior (T-90 time) than ever seen
before as well as excellent linearity. Typical cross-sensitivities such
as those known for SO2 and other gas components do not occur when using the Zeeman AAS method. Comparative measurements on plants with existing mercury CEMS show excellent conformity, also compared to reference measurements of a certified Test Institute according to German stack testing institute “Messstelle § 26”.
For quality control of measurements, known HgCl2 concentrations can be fed to the measuring system automatically via a vaporiser. A
swiveled-in integrated calibration filter is also available for simple drift checks. Due to this major advantages, the MERCEM300Z is the first device which is certified according to EN15267 with a lowest measuring range of 0 – 10 µg/m³. A unique maintenance interval of 6 months has been reached during 12 months field test for the standard range of 0 – 45 µg/m³.
Features and Benefits
The decisive advantages over mercury CEMS available on the market up to now are: • Smallest certified measuring range 0 -10 µg/m³ • Maintenance interval of 6 months
• Measuring system suitability tested with a heating hose length of 35 m
• Direct conversion with simultaneous measurement in a sample gas cell at 1,000°C
• Certification according to EN 15267
• Gas sampling with ejector instead of heated diaphragm vacuum pump (quasi maintenance free)
• No consumables such as conversion substances or solutions
• No additional amalgamation step, continuous real-time measurement instead
• Drift check of analysers without test gas with internal filter control
• No cross-sensitivities determined for typical accompanying components in exhaust gases
• Optional automated driven HgCl2 test gas evaporation unit inside the analyser cabinet
MERCURY IS
OUR BUSINESS - THE MERCURY EXPERTS
Analysers for Mercury in
Solids, Liquids, Gases
Seefelder Messtechnik GmbH Oedenpullach 2 D-82041 Oedenpullach / Germany Tel: +49 89 63866980 Fax: +49 89 62819839
www.seefelder-messtechnik.com Reader Reply Card No.
See you in 2013 at ICMGP, Edinburgh 57
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