Frequently Asked Questions about Continuous Mercury Monitoring

In this blog post, we answer the most frequently asked questions about our fully optimized solutions for continuous monitoring of mercury emissions, CMM and CMM AutoQAL, our new continuous mercury monitoring system that has an EN 15267 certified, integrated and automatic QAL3 Validation Tool.

1. What is the detection limit of the CMM AutoQAL & CMM?

Taking the dilution ratio into account, CMM and CMM AutoQAL complete system detection limit is 0.02 μg/m³.

2. What are the lowest and highest certified measuring ranges?

CMM and CMM AutoQAL systems allow measuring with multiple ranges. The lowest EN 15267 certified range is 0 – 5 μg/m³. This is the lowest certified range for Hg in the world and there is no other system or technology with this low certified range. However, according to EN 15267 certificate, measuring ranges up to 1000 μg/m³ can be used in both systems.

CMM and CMM AutoQAL is suitable for performing measurements, not only in plants where mercury concentration is low (meaning lower ranges are needed) but also in plants that have higher mercury concentrations (meaning higher ranges are needed).

Some of the systems available certified ranges are the following:

0 to 5 μg/Nm³
0 to 10 μg/Nm³
0 to 45 μg/Nm³
0 to 100 μg/Nm³
0 to 1000 μg/Nm³

3. How is the total gaseous mercury measured, including the ionic species?

CVAF instrument only detects atomic mercury vapor (Hg⁰) whereas stack gas contains also oxidized mercury compounds such as HgCl₂. In the CMM and CMM AutoQAL systems, the sample gas passes through an optimized 700 °C thermal converter, located just before the sample cell.

Mercury compounds are converted to elemental mercury, enabling measurement of total gaseous mercury emissions and leaving no Hg-Derivatives undetected. As the measurement is made immediately after conversion, no recombination of mercury compounds takes place. Emissions monitoring always looks at the total amount of mercury. Consequently, different forms of mercury don’t need to be measured separately.

4. Why and how is the dilution of sample gas performed?

Dilution of the sample gas has many advantages when measuring total gaseous mercury. First of all, the dilution gives better response times, as the amount of the sticky mercury compounds in the diluted gas is then lower, and reactions of mercury compounds with sampling surfaces is reduced. The diluted sample gas also eliminates quenching effects (see question 5).

The system dilutes the sample gas with nitrogen in a 1:50 ratio. Thanks to the excellent sensitivity of CVAF, a fairly high dilution ratio can be used. The nitrogen gas used for the dilution is generated inside the CMM system from compressed air via a nitrogen generator.

Dilution is achieved by using an eductor pump with an orifice that limits the flow of undiluted sample gas into the pump. Dilution gas (nitrogen) enters the eductor and provides the flow which pulls sample gas from the probe into the analyzer.

5. What is quenching and how it is minimized?

Quenching is an effect where collisions between mercury atoms and molecules such as O₂ take place in the tiny timeframe between absorption of UV light from the lamp and emission of fluorescent light from the mercury atoms. A collision at this time may remove the energy stored in the mercury atom and transfer it to the quenching molecule. Quenching lowers the amount of light coming to a detector and makes the instrument read low unless measures are taken to prevent it.

Quenching gases include O₂, CO₂, and H₂O. Nitrogen (N₂) is not a strong quencher and other gases in a stack sample matrix have typically sub-% concentrations and can be ignored.

The system dilutes the sample gas in a 1:50 ratio, which lowers the concentrations of quenching gases considerably. Thanks to the excellent sensitivity of CVAF, a fairly high dilution ratio can be used. If the diluent would be air, the O₂ content of the air would cause quenching of the fluorescence signal, and for this reason, the system uses nitrogen for dilution. The nitrogen gas for dilution is generated inside the system from compressed air via a nitrogen generator, as explained in question 4.

6. How are zero and span calibrations and checks performed?

All calibrations are performed automatically at user-defined intervals. The factory setting for calibration interval is one day. For zero calibration, the CMM AutoQAL and CMM both use synthetic nitrogen gas generated in the cabinet. The nitrogen is generated from compressed air via a nitrogen generator.

For span calibrations, systems are using Hg0 vapor span gas generated in the test gas generator, which is an important part of the CMM AutoQAL and CMM systems. The system can also perform automatic zero and span drift tests, and optional converter efficiency and system integrity tests using HgCl2 test gas generated in the calibrator. CMM AutoQAL has a certified test gas generator with possibility to do both Hg⁰ and HgCl₂ checks, which means that there is no longer a need for an external gas generator for QAL3 operations.

7. What are the key features of the CMM AutoQAL and CMM software?

Mercury Analyzer User Interface (MAUI) is the software for operating and controlling the CMM and CMM AutoQAL system. The key features of MAUI are:

Enables fully automatic operation of the system
Analyzer & test gas generator settings are easily accessible when needed
User-friendly interface with a touch panel access
User-defined automatic calibration intervals and probe blowback routines
Status and warning signals are sorted into four categories (system alarm, service request, maintenance, result valid)

8. How is sample contamination and memory effect avoided?

Sample contamination is taken into account in various ways in the system designs. First of all, the metal parts of the probe and filters have special coatings to ensure that mercury does not stick or react with the surfaces. Furthermore, the flexible tubing of the sample lines is made from perfluoropolymer, enabling an excellent chemical resistance.

In addition to the material choices explained above, the following precautions ensure minimal analyte loss or memory effects:

Trace heating of the entire sampling, beginning at the probe tube
Reduced size of the filter to block dust from entering the system
Two-stage blowback to prevent dust from accumulating on the filter
Sample lines are only in contact with the diluted sample gas
Sample cell is maintained at low pressure

Memory effects and sample contamination are minimized with optimized material selection and technology design.

9. What is the advantage of having shorter sampling lines?

Short sampling lines ensure more reliable measurement results and short response times. Mercury is a sticky compound that is easily attached to the surface of sampling lines. A shorter sampling line means a shorter response time and thus a faster response to changes in Hg concentrations.

10. What are the typical applications for the systems?

CMM AutoQAL and CMM are designed for continuous measurement of mercury from hot, wet and corrosive gas streams. The system is used in a wide range of industrial processes requiring mercury emissions monitoring. Continuous mercury monitoring is generally a regulatory requirement for waste incineration plants, power plants and cement kilns.

Thanks to the high sensitivity and selectivity of the CVAF measurement technique, both systems are well suited for many different emission monitoring applications even with extremely low mercury concentrations. The CMM AutoQAL and CMM have been granted the lowest EN 15267-3 certified range in the world (0-5 μg/m³).

11. What is a memory effect and how is it avoided?

Memory effects, due to sample gas reactions with selected sampling materials, can cause delays in system response time for real sample gas concentration level changes. Selecting proper materials and design for both elementary and ionic mercury is essential. Gasmet CMM and CMM AutoQAL have no memory effects since systems are reacting quickly to real concentration changes in the process. Thanks to Gasmet CMM and CMM AutoQAL system’s unique design and careful selection of materials, possible memory effects are minimized.

Mercury Emission Monitoring – CMM & CMM AutoQAL

Continuous Mercury Monitoring systems CMM AutoQAL and CMM are the perfect solutions for monitoring mercury continuously from hot, wet and corrosive gas streams. These future-proof systems have the lowest certified range in the world (0-5 µg/m³) and CMM AutoQAL has an automatic and integrated QAL3 validation tool.