Focus on Mercury - Environmental Analysis
Monitor Mercury – But How?
Due to its high toxicity mercury has to be closely monitored in flue gases from coal fired power plants, waste incinerators and cement plants all around the world. Limit values must be maintained
in the lower µg-range. For this reason, measuring devices need to be extremely sensitive, highly accurate as well as being able to withstand the harsh industrial environment. Meanwhile different types of mercury monitoring systems are available. But how should the ideal system perform in terms of performance and acceptable low maintenance for the plant operator?
In order to ensure accurate measurements, the detection of the mercury should directly take place within the conversion process.
Sample gas conditioning, reduction and existing measurement technology
Existing measurement technology is based on extractive gas sampling, conversion, possibly amalgamation and UV measurement. Sample gas conditioning plays a special role in mercury monitoring. All mercury analysers have one thing in common: They can only detect metallic mercury (Hg°). The flue gas does however not only contain metallic mercury but also oxidised mercury compounds, mainly mercury
chloride (HgCl, HgCl2). These compounds cannot be detected directly and must be reduced to metallic mercury by appropriate measures in sample gas conditioning.
Various methods are used for reduction of oxidised mercury. One method uses the classic wet chemical reduction, for example, with a tin chloride solution, as also used in laboratories for reference measurements. Another method which is more often used over the past years is the so-called dry reduction. Dry reduction uses converters in gas sampling which convert the oxidised mercury compounds to metallic mercury either at low temperatures (approximately 250°C) or high temperatures (approximately 450 – 700°C). Both methods have advantages and disadvantages, and are rated by operators very differently regarding operating, efficiency and reliability. One major disadvantage here is the risk of contamination as a result of sulphur and chlorine compounds, which may be present in the flue gas. This leads to
increased operating costs as a result of the shortened operational lifetime of the converter.
A new approach dispenses of any consumables – dry converters and chemical solutions. At very high temperatures (above 900°C), mercury only exists in its elemental form. This allows for so- called thermal conversion. The big advantage of this approach is the increase of the total availability of the measurement without being poisoned by any other flue gas component.
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In order to ensure accurate measurements, the detection of the mercury should directly take place within the conversion process. This prevents any losses of mercury detection by recombination effects during the transportation of the mercury sample to the analyser.
Mercury measurement with MERCEM300Z
The heated sample gas line, heated at least to 180°C, should be kept as short as possible, to prevent any absorption during transportation.
Author Details: Florian Greiter
co-author Dr. Michael Boness SICK MAIHAK GmbH
Dr.-Zimmermann-Str. 18 D-88709 Meersburg GERMANY
Tel: +49 (0) 7532 801-111
Email:
Florian.Greiter@
sick.de Web:
www.sick.com
Best possible interference compensation due to the Zeeman atomic absorption spectroscopy
www.envirotech-online.com IET November / December 2012
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