The fuel for a waste incinerator is unknown. Most of the time the fuel is standard household waste with small amounts of mercury. The 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. The mercury could come from thermometers or old types of power switches. The 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. The injection of active coal runs at a fixed speed with a few kilos per hour.
This 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. The level of mercury is measured in the raw gas directly after the furnace with fast speed. The rising levels of mercury are detected and the signal is used to increase the amount of active coal that is injected.
How To Measure Gaseous Mercury In Raw Gas
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 and detected on the other side of the stack. The 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. The 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.
The 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.
Svante Wallin is with Opsis