Bengt Löfstedt explains why monitoring of total mercury emissions has become a focus area within the waste incineration industry, and how to meet the monitoring needs
Historically, mercury (Hg) has found its use in many products such as thermometers, batteries and fluorescent lamps. However, mercury is extremely toxic and, therefore, it has in modern days been banned from use in most applications. By example, the Minamata Convention on Mercury from 2013 has been designed to prevent releases of mercury and mercury compounds to protect human health and the environment. It is signed and ratified by well above 100 countries around the world.
Nevertheless, mercury continues to be released into the environment. Major anthropogenic sources are coal-fired power plants due to the natural occurrence of mercury in coal, and waste incineration facilities. In the latter case, the mercury emissions are due to old mercury-containing products ending up in the waste to be burnt.
Reducing mercury emissions
Emissions of mercury from combustion facilities can be reduced by appropriate flue gas cleaning equipment. To drive the installation and use of such equipment, legislation can dictate emission limits. A recent example of this is found within the European Union, where the Industrial Emissions Directive (the IED) was supplemented by a new set of so-called Associated Emission Levels for waste incineration in December 2019. These are referred to as the BAT-AELs of the WI-BATC, where BAT stands for “best available techniques” and BATC for “BAT conclusions”.
One of the new BAT-AELs states that the total mercury emissions, expressed as a daily average of a normalised gaseous mercury concentration in the flue gas, are possible to keep below 5 µg/m3 and in all cases must be kept below 20 µg/m3. The mercury concentration must be monitored continuously unless the waste can be proven to contain a low and stable level of mercury content. Such proof can be hard or impossible to obtain, in particular in cases of mixed-waste or hazardous-waste incineration.
What then remains is to make sure that the flue gas cleaning equipment is installed and working, and then monitor the remaining mercury emissions continuously. This must all be up and running not later than by the end of 2023, when the new BAT-AELs come into effect.
Monitoring total mercury emissions
When it comes to the monitoring part, the word “total” in total mercury must be considered. The mercury BAT-AEL applies not just to atomic (metallic) mercury in the flue gas but to all gaseous mercury, including compounds such as the relatively commonly occurring mercury chloride (HgCl2) and methylmercury (CH3Hg+). Accordingly, a monitor of total mercury is needed, preferably with a proven reliability and a low total cost of ownership. Here is where Opsis offers an ideal solution: the Opsis System 400Hg.
The Opsis System 400Hg is based on a technology called differential optical absorption spectroscopy (DOAS). A beam of light is led through the gas mixture to be monitored. Each type of molecule in the gas gives rise to a specific pattern of light absorption. The more molecules of a given type, the more absorption. By analysing the spectrum of the remaining light, specific gas concentrations can be calculated.
The DOAS method works excellently for atomic mercury (Hg0), where extremely low levels can be detected. This is extended to apply to total mercury (THg) by introducing a durable catalytic converter. In the complete system, a small amount of flue gas is extracted from the duct and led through the converter. In the converter, all mercury compounds are split up while what already is Hg0 remains unaffected, resulting in mercury only existing as Hg0 after the converter. The gas sample is then led to a monitoring cell where a gas analyser easily detects the Hg0 concentration, and thereby effectively the concentration of total mercury in the flue gas.
The Opsis System 400Hg has been thoroughly tested and verified in a variety of harsh industrial environments over the past years, with excellent results. It is currently being rolled out to customers both within the EU and elsewhere in the world.
Operators looking for a complete multi-gas monitoring system can take advantage of the fact that a single Opsis system can be extended and configured to measure concentrations of many gaseous substances simultaneously, not only Hg0 and THg but also for example CO, CO2, HCl, HF, NH3, NO, NO2, and SO2. This is applicable to continuous emissions monitoring in many different industry types, as well as to process control needs, for example to control and protect scrubbers and catalytic converters used in the flue gas cleaning.
Bengt Löfstedt is with Opsis