|Abstract Title:||Technical challenges of continuous mercury measurements|
|Session Choice:||Innovative Measurement Technology|
|Presenter Name:||Ms Emilia Jyrkiäinen|
Abstract Information :
Many pollutants such as CO2, NOx and SO2 have been monitored continuously already for decades. In the last years the awareness for the need to measure mercury has also grown, and continuous mercury monitoring is becoming more and more relevant. Furthermore, the mercury emission regulations are expected to become stricter in the future.
Mercury is naturally occurring element which can be found for example in rock and coal. Mercury causes health problems when emitted to air as it is toxic to humans. Therefore, many industries burning coal, oil or waste, share the responsibility to monitor their mercury emissions. As mercury is emitted, it is transported through air to soil and water, where some of the mercury is consumed by fish, and eventually humans.
There are three main measurement techniques used for continuous mercury measurements: Cold vapor atomic absorption (CVAAS), cold vapor atomic fluorescence (CVAFS) and differential optical absorption spectroscopy (DOAS). All of these measuring techniques need to be able to tackle the challenges mercury monitoring has.
First of all, mercury is occurring in various forms, including elemental mercury and mercury compounds, as well as mercury that is bound to dust particles. Flue gas includes elemental mercury Hg0 and oxidized mercury Hg2+ in the form of HgCl2. Most continuous mercury measurement methods are based on measuring elemental mercury. This means that the oxidized mercury compounds must be converted to elemental mercury before the measurement. It has to be made sure that none of the mercury compounds are lost before the conversion, and also that there is no recombination of mercury compounds afterwards.
Secondly, the mercury concentrations are really low compared to other gas compounds. The concentrations are generally around 0.1-5 ?g/m3 in waste incineration and power plants. These extremely low concentrations naturally require that the measurement technique has low detection limits and therefore, low measurement range as well. With this in mind, it should be noted that depending on the application and plant, there can also be relatively high concentration peaks from time to time. Because of that the measuring system has to be able to withstand and detect those peaks as reliably as the lower concentrations.
Other features expected from mercury monitoring systems are cross-sensitivity of the system and stable calibration check routines. Cross-interference effects need to be minimized especially as the mercury concentrations are very low compared to other gases in the stack gas. The quality of the measurements can be tested with frequent calibrations and for example drift checks. Check routines are preferred to be automatic and adjustable by the user if needed.
Especially with CVAF, the high sensitivity and selectivity of the fluorescence technique ensures that extremely low mercury levels can be measured accurately. Low detection limit of CVAF also enables diluting the stack gas. With dilution, cross-interference effects from other gases like SO2 and O2 can be eliminated.