2018-11-15
Continuous Emissions Monitoring of Total Mercury
Over the past decades, mercury (Hg, quicksilver) has gone from being a versatile commodity used in everything from thermometers to herbicides, to a partially banned substance, which name alone signals environmental issues and health concerns. Today, much use of mercury has been phased out in favour of more environmental- and health-friendly substances, and the remaining circulation of mercury is often governed by strict regulations and control measures. This includes emissions to air from for example energy production, in particular coal-fired power plants, and incineration facilities for toxic waste. In these areas, there are often facility-specific limits to the amount of mercury allowed to be released, and requirements to monitor such releases by means of flue gas analysers.
Atomic mercury as well as mercury compounds are very toxic. There are risks of both chronic and acute poisoning, and exposure to living organisms must be minimized. Yet, releases of mercury cannot always be avoided. A major natural source is erupting volcanos. However, what can be avoided or at least minimized are emissions from man-made sources. Here, coal-fired power plants constitute the notably largest source of potential mercury emissions to air, due to the mercury compounds residing in the fuel. Also for example gold production industries, metal smelters, and toxic waste incinerator are notable potential sources of mercury releases.
Thankfully, the toxicity of mercury is, nowadays, well acknowledged, and there are both international treaties and national legislations banning or limiting its use. As a result, for example mercury-based batteries and thermometers are no longer produced or in active use, although discarded units still can pose a problem as waste. A notable surviving area of use of mercury is fluorescent light bulbs, produced in high quantities, which sooner or later still have to be disposed of and then will pose a potential threat of mercury contamination.
There are flue gas cleaning methods to capture and aggregate mercury under controlled forms. The fraction not captured often has to be kept below certain release limits, as governed by for example national or local legislation. In relation hereto, it is often a requirement to monitor the concentrations of mercury in the flue gases from the combustion processes.
Atomic mercury (Hg0) has very favourable optical properties making it possible to design monitoring equipment able to measure Hg0 concentrations down to very low levels. However, notable amounts of mercury are often bound in a variety of organic or inorganic compounds, also after a combustion process. Examples of such compounds are mercury chloride, mercury sulfide, the methylmercury cation, and the very toxic dimethyl-mercury. The mercury in such compounds is not detected by regular Hg0 monitors.
However, OPSIS has developed a method for fast and precise measurements of total mercury (THg) for continuous emissions monitoring purposes. The flue gas from the combustion process is extracted and sent to a converter where any compound-bound mercury is released to form atomic mercury, Hg0. The concentration of Hg0, and thereby effectively the concentration of THg in the flue gas, can then easily be determined using the proven OPSIS DOAS gas analysers. The same monitoring system can also be configured to measure concentrations of a number of other gases, such as NO, NO2, SO2, CO, CO2, NH3, H2O, HF, and HCl, thereby constituting a very cost-effective monitoring solution operating with a minimum of maintenance.
