- What does CVAF mean? How does it work?
- What is UV fluorescence?
- What are the advantages of Gasmet CMM compared to similar and other techniques measuring mercury?
- Is there any interference from SO2?
- What is quenching and how it is minimized?
- What is the detection limit of Gasmet CMM?
- How do you take all of the mercury compounds into account?
- What are the typical applications for Gasmet CMM?
- What calibrations are required?
- How sensitive Gasmet CMM is for reactive gases?
- What are the special features of MAUI™software?
- What is the material used for the parts in touch with sample gas?
- How do you avoid mercury sticking to surfaces?
- Why should we dilute the sample and how can that be done?
CVAF stands for Cold Vapour Atomic Fluorescence. It is an extremely sensitive and selective measurement principle and the current state of the art method of measurement of trace Mercury levels. In a CVAF Mercury analyzer the main parts are:
- Mercury vapor UV lamp emitting UV light at the precise wavelength specific for Mercury
- Sample flow through cell where the gas stream crosses UV beam coming from the lamp
- Photomultiplier tube (PMT) UV detector capable of detecting single photons, mounted at right angle compared to the UV lamp
- Light traps, polarizer plates, and other devices used to eliminate stray light
The UV detector does not see the light from the lamp (the 90° measurement geometry and light traps take care of this) and the only signal picked up by the detector is the fluorescent light coming from the mercury atoms in the sample gas.
Fluorescence is an effect seen with many substances that absorb ultraviolet or visible light. In the case of a Mercury atom (Hg) the sequence of interactions with light can be summarized below:
Hg (ground state) + hν → Hg (excited state) (1)
Hg (excited state) → Hg (ground state) + hν (2)
The fluorescence photons (hν) produced in step 2 have the same wavelength as the photons from UV lamp in step 1. However, they can be distinguished from each other by the fact that fluorescence photons (2) are emitted in all directions from the mercury atoms, whereas the UV light from the lamp in step 1 is in a parallel beam directed at a light trap at the end of the sample cell. By the use of light traps and 90° measurement geometry the photons produced in step 2 can be detected without interference from the photons in step 1.
3. What are the advantages of Gasmet CMM compared to similar and other techniques measuring mercury?
Compared to atomic absorption measurement technique, atomic fluorescence used in the Gasmet CMM offers superior sensitivity and reduced cross-interference effects thanks to the very specific nature of the fluorescence effect. Especially in applications where the concentration of Mercury is low and the concentration of other UV absorbing gases such as SO2 is high, CVAF is the best available technology. This includes especially coal-fired power plants and cement kilns.
Compared to other atomic fluorescence analyzers, the Gasmet CMM is a compact and cost-effective solution with several accuracy and reliability increasing features:
- converter for measuring total mercury is directly coupled with sample cell
- no recombination of mercury compounds after converter
- no pre-concentration of mercury into gold traps or scrubbing of acid gases is required
- sample probe has a unique two-stage blowback mechanism for removing dust from filter surfaces. Clean filters prevent analyte loss in the probe and minimize memory effects
Other UV absorbing gases such as SO2 do not interfere with this measurement as the light source is selective to Mercury and only the fluorescent light from the sample is detected. The CMM has been used to measure Mercury in a Sulphuric acid production process with 5–10 vol-% SO2 present, and even in this extreme gas matrix the SO2 interference is not an issue.
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 (O2). Quenching lowers the amount of light falling on the UV detector and makes the instrument read low unless measures are taken to prevent it.
Quenching gases include O2, CO2, and H2O. N2 is not a strong quencher and other gases in stack sample matrix have typically sub-% concentrations and can be ignored. Diluting sample gas in a 1 : 50 ratio lowers the concentrations of quenching gases considerably and the excellent sensitivity of CVAF means that a fairly high dilution ratio can be used. If the diluent is air, the O2 content of the air still causes considerable quenching of the fluorescence signal and for this reason, the CMM system uses nitrogen for dilution. The nitrogen diluent gas is made in a nitrogen generator inside the CMM system from compressed air. By replacing air with N2 as the diluent the sensitivity of the system goes up by a factor of 10.
The detection limit is 0.5 nanograms per cubic meter in sample gas diluted with synthetic nitrogen. Taking the dilution ratio into account the detection limit is 25 ng/m3 (0.025 µg/m3) in an undiluted stack gas.
CVAF instruments only detect atomic mercury vapor (Hg0) whereas stack gas contains also oxidized mercury compounds such as HgCl2. In the CMM system, the gas passes through a thermal converter just before the fluorescence measurement cell. Mercury compounds break and atomic mercury vapor is released in the converter, enabling a measurement of total gaseous mercury emissions. As the measurement is done immediately after conversion, no recombination reactions take place between conversion and measurement.
Continuous Mercury Monitoring is required in various legislations for waste incineration plants, coal-fired power plants, and cement kilns. Thanks to the high sensitivity (low measuring ranges) and reduced cross-sensitivity to eg. SO2 the Gasmet CMM is well suited to all of the above emission monitoring applications.
The CMM also has a process control application in Sulphuric Acid manufacturing. The raw Sulphur used in this process frequently has mercury impurities from mineral sources and the CMM is used to monitor trace levels of mercury in a process stream containing 5 to 10 vol-% SO2 to ensure that the end product has acceptably low levels of mercury.
The CMM is calibrated for zero and span using synthetic nitrogen zero gas generated in the cabinet and Hg0 vapor span gas generated in the mercury calibrator which is an important part of the CMM system. The recommended calibration interval is one day, and the calibration is done automatically at user defined intervals. The calibrator can also measure automatic zero and span drift tests and optionally also converter efficiency and system integrity tests using HgCl2 test gas generated in the calibrator.
The CMM withstands high concentrations of e.g. SO2 and the cross-interference effects are virtually eliminated by the use of Atomic Fluorescence measurement principle.
MAUI (Mercury Analyzer User Interface) is easy to use, touch panel oriented software for controlling the analyzer, calibrator and sampling probe blowback mechanism. MAUI visualizes Mercury measurement values in trend (60 min / 24 hrs) and latest measured concentration displays. The software shows status/warning messages sorted into four categories (system alarm, service request, maintenance, result valid) and allows the user to define calibrations, calibration checks, and probe blowback routines as required.
The metal parts of CMM probe and filters have special proprietary coatings to ensure that Mercury does not react with the metals, and the flexible tubing parts are made from PFA, a perfluoropolymer with 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 train beginning at the probe tube
- Reduced size filter element with frequent, two-stage blowback to prevent dust from accumulating in large amounts on the filter element
- Sample lines are only in contact with diluted sample gas
- Sample cell is maintained at low pressure (below 100 mbar)
A sample gas is diluted with nitrogen to eliminate quenching (see above) and to reduce the reactions between Mercury in the sample gas and sample line surfaces. Dilution is achieved by using an eductor pump with an orifice which limits the flow of undiluted sample gas into the eductor pump. Dilution gas (nitrogen) enters the eductor and provides the flow which pulls sample gas from the probe into the analyzer.