Emissions of oxidised mercury may be efficiently controlled by some FGD systems. Some activated carbons have the potential to control the oxidised mercury. Any oxidised mercury escaping from the stack is deposited on a local or regional scale. On the other hand, elemental mercury is extremely volatile and insoluble and is not captured by FGD systems. Elemental mercury may be removed by some chemically treated activated carbons or selective sorbents but these are only currently being tested at pilot scale on coal-fired power stations.
Elemental mercury travels hundreds of kilometres and contributes to the increasing atmospheric load.
Emissions of mercury from coal combustion have not been associated directly with any detrimental environmental effects. More research is required on the development of sampling and analysis systems which can measure the individual mercury species at the concentrations encountered in coal combustion systems and in the environment. A complete evaluation of mercury emissions from coal combustion requires more high quality data on speciation. Current research into sampling and analysis is now starting to produce such data.