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OPERATION & MAINTENANCE OF POWER PLANTS


MERCURY T


MONITORING


Svante Wallin discusses raw gas measurement of mercury


he fuel for a waste incinerator is unknown. Most of the time the fuel is standard household waste with small amounts of mercury.


Te mercury is there because of disposal of batteries and low-energy light bulbs. But it could happen that some industrial waste is being burned with high levels of mercury. Te mercury could come from thermometers or old types of power switches. Te amount of mercury burned in a few minutes could easily be 0.1kg. Most waste incinerators have injection


of active coal to trap the gaseous mercury and thereby reduce the amount of mercury emitted to the air. Te injection of active coal runs at a fixed speed with a few kilos per hour. Tis will take care of the normal operation but not an event where 0.1kg mercury is burned in a few minutes. To handle the event, raw gas measurement of mercury is needed. Te level of mercury is measured in the raw gas directly after the furnace with fast speed. Te rising levels of mercury are detected and the signal is used to increase the amount of active coal that is injected.


MEASUREMENT CHALLENGE To measure gaseous mercury in the raw gas is a challenge. High levels of dust, high temperature and corrosive gases make the monitoring difficult. To use an extractive system for this application will slow down the speed thus giving less time to inject additional active coal. An extractive system will also need a lot of maintenance as filters will have to be cleaned. To measure total gaseous mercury could include a converter that will suffer from the corrosive gas composition. It is possible to measure mercury atoms using an optical path across the stack. Light is emitted from one side of the stack


THE


Mercury measurement is challenging but optical light technology is proving to be a useful solution


and detected on the other side of the stack. Te amount of mercury atoms is detected using a spectrometer that can separate wavelengths and detect how much light the mercury atoms have absorbed. No pumps, filters, converters or other parts that need maintenance are used. Te speed of the measurement is fast, thus allowing fast regulation of the active coal injection. To simplify the method, an optical fibre is used to guide the light from the monitoring path to the spectrometer that is placed in an air- conditioned cabinet. Te new BAT regulation asks for


lower emission of mercury to the air. Raw gas monitoring of mercury atoms and fast regulation of the active coal injection will be important tools to lower the mercury emissions.


Overview of the Opsis solution 16 www.engineerlive.com


Svante Wallin is with Opsis www.opsis.se


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