RENEWABLE ENERGY
a normalised gaseous mercury concentration in the flue gas, would ideally fall below 5 µg/m3
and in all cases must be kept below 20 µg/m3 .
The mercury concentration must be monitored continuously unless the waste can be proven to contain a low and stable level of mercury content. Such proof can be hard to obtain, notably in cases of mixed-waste or hazardous-waste incineration. What then remains is to make
sure that the flue gas cleaning equipment is installed and working, and to monitor the remaining mercury emissions continuously. When it comes to the monitoring part, the word ‘total’ in total mercury must be considered. The mercury BAT-AEL applies not only to elementary (metallic) mercury in the flue gas but to all gaseous mercury including compounds such as the relatively commonly occurring mercury chloride (HgCl2) and methylmercury (CH3Hg+). Accordingly, a monitor of total mercury is needed, preferably with a proven reliability and a low total cost of ownership. The Opsis System 400HG is
an example of such a monitoring system. It is based on a technology called differential optical absorption spectroscopy, DOAS. A beam of light is shone through the gas mixture to be monitored. Each type of molecule in the gas gives rise to a specific pattern of light absorption. The more molecules of a given type, the more absorption. By analysing the spectrum of the remaining light, specific gas concentrations can be calculated. The DOAS method works excellently for elementary mercury (Hg0
), where
extremely low levels can be detected. This is extended to apply also to total mercury (THg) by introducing a durable catalytic converter. In the complete system, a small amount of flue gas is extracted from the duct and led through the converter. In the converter, all mercury compounds are split up while what already is Hg0
mercury only existing as Hg0
remains unaffected, resulting in after
the converter. The gas sample is then led to a monitoring cell where a gas analyser easily detects the Hg0 concentration, and the concentration of total mercury in the flue gas. The Opsis System 400HG has
been thoroughly tested and verified
Coal-fired power plants and waste incineration facilities may emit mercury
by the company in harsh industrial environments, and is now being rolled out to customers both within the European Union and elsewhere in the world. The system is QAL1 certified for the measurement ranges 0-10 µg/m3
as well as 0-45 µg/m3 . Operators requiring a complete
multi-gas monitoring system can take full advantage of the Opsis monitoring system. A single system can be extended and configured to measure concentrations of many gaseous substances simultaneously, not only
Bengt Löfstedt is with Opsis AB, Sweden For more information visit:
www.opsis.se
www.engineerlive.com 37 .
The system has a a supplementary range of 0-100 µg/m3
Hg0 and THg but also for example
CO, CO2, HCl, HF, NH3, NO, NO2, and SO2. This is applicable to continuous emissions monitoring in many different types of industries as well as to process control needs, for example to control and protect scrubbers and catalytic converters used for flue gas treatment.
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