Although Mercury is a naturally occurring element that is found in air, water and soil, the World Health Organization has reported that exposure to mercury – even small amounts – may cause serious health problems, including having toxic effects on the nervous, digestive and immune systems, and on lungs, kidneys, skin and eyes, as well as is a threat to the development of the child in utero and early in life. It is one of the top ten chemicals or groups of chemicals of major public health concern.
Due to this global level of risk, governments are looking for ways to limit Mercury emissions into the environment. The European Union has passed stringent new requirements on Mercury (Hg) emitters bringing levels closer to those regulated by the US EPA. In 2011, the U.S. EPA promulgated the MATS, Mercury Air Toxin Standards for reduction of mercury in air emissions. This solidified the need for low level mercury standards, pollution controls, and calibration standards. To meet the new regulations, the U.S. market overwhelmingly gravitated toward three technologies in mercury monitoring – Cold Vapor Atomic Florescence (CVAF), Elemental calibration standards with an oxidized sample, and Speciation of Hg analytes.
Cold Vapor Atomic Florescence is a form of measurement technology employed in a Mercury Continuous Emissions Monitoring System (CEMS). It operates on the principle that Hg atoms absorb ultraviolet (UV) light at 253.5nm, which excites the molecules in the sample. The excited Hg atoms then decay back to their equilibrium, emitting a UV fluorescence (photons) at the same wavelength when doing so. The measurement of the emitted photons is proportional to the Hg concentration in the sample. The equation is as follows,
Hg +hν (253.5nm)→Hg *→ Hg +hν (~253.5 nm)
The result is a highly sensitive measurement method with a lower detectable limit (LDL) of 1ng/m3 by the analyzer alone. With the extremely low detection limit of CVAF a dilution ratio of typically between 20:1 and 40:1 can be used depending on the application. The dilution of the sample with clean, dry, nitrogen reduces the moisture and temperature effects of sampling flue gas. At a 30:1 dilution ratio, the lower detection limit becomes 30 ng/m3, low enough for measuring a typical flue gas stream from a coal-fired power plant under the new EU directives.
To calibrate an analyzer at such a low level, a stable and reliable method is required to check the analyzer and system integrity. By employing elemental calibrations on a daily schedule, the system is challenged with a very precise, very stable concentration of Hg0. A Peltier cooler and mass flow controller are used to create a Hg0 vapor that is able to be NIST calibrate to the lowest EN 15267 certified range of 0-5 µg/m3. The result is a stable reliable standard to check the measurement of a mercury analyzer, but does not check the system integrity of total mercury. There are a number of ways to accomplish this – an oxidizer and chlorine gas to create HgCL2, the use of Sorbent traps, known as an EPA Method 30B, or a liquid standard, which has fallen out of favor in the U.S. for aforementioned methods, but is the only way to calibrate according to the QAL3 in EN 14181.
Part of the reason a system integrity check with an HgCL2 is so important, is that this species of mercury is often present in flue gas streams. Because the CVAF and most other mercury analysis methods only measure Hg0, a high temperature (760oC), catalytic converter is used to convert all mercury species into the more stable Hg0 analyte for analysis. This stream of converted mercury is known as Total Mercury, or HgT, and is the primary flue gas emittance value that is regulated. However, by analyzing a stream of flue gas sample that bypasses the high temp converter, the result is a known value of the Hg0 of the flue gas, and important distinctions can be made. By calculating the oxidized value of Hg2+, HgT minus Hg0, a plant can make better process decisions on how to remove and control Hg emissions below the allowable limits.
The latest European regulations may seem to be unachievable with the ability of currently qualified measurement and calibration techniques of the past. These are just some of the techniques used to measure such low concentrations in the U.S., and there are many parallels to the recent European directives for Large Combustion Plants and Waste Incinerators. The need for lower level performance and qualification, as has been seen by the regulations and implementation in the U.S. and E.U. alike, is on the increase as the global community looks to decrease Hg emissions to their lowest level into the future.
Want to know more about mercury monitoring?
Download our free book “What You Need to Know About Mercury Emissions, Regulations, and Monitoring”
Editor’s Note: You can also hear more about this topic at this upcoming event:
POWER PLANT TECHNICAL COLLOQUIUM
International Congress Center Dresden, Germany
October 22-23, 2019
Follow on: Twitter: @tudresden_de and Facebook: @TUDnews