This\u00a0\u00a0 system \u00a0\u00a0involves \u00a0\u00a0continuous and \u00a0\u00a0repetitive \u00a0\u00a0stack sampling, using paired sorbent traps in subsequent analysis as time integrated samples.\u00a0 This system can be a primary system for compliance monitoring and as back up during CEMS maintenance & verification of performance.<\/p>\n
ONLINE ANALYSIS <\/b><\/p>\n
Online\u00a0\u00a0 CEMS is \u00a0\u00a0\u00a0\u00a0used for real\u00a0 \u00a0time monitoring and analysis.<\/p>\n
The popular advanced techniques \u00a0\u00a0used are<\/p>\n
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- Cold vapor atomic absorption spectrometry (CVAAS)<\/li>\n
- Cold vapor atomic fluorescence spectroscopy (CVAFS)<\/li>\n<\/ul>\n
The working principle of above online mercury monitoring is similar to other flue gas monitoring emission systems. Objects to be analyzed are taken by sampler from flue gas .The\u00a0 \u00a0interferential components available in sample gas such as HCI, SO2<\/sub>, SO3,<\/sub> andother acidic gases are removed before going to detector<\/p>\n
Cold Vapor Atomic Absorption Spectrometer (CVAAS<\/b>)<\/p>\n
It is widely accepted\u00a0 \u00a0methods \u00a0\u00a0for determining mercury i.e total mercury content. Mercury is reduced to elemental form\u00a0 for by using \u00a0Tin Chloride or Sodium borohydroide \u00a0\u00a0From solution, vapor mercury is purged by air, nitrogen or argon carrier gas.\u00a0 After \u00a0\u00a0it passes through gas liquid separately, it is led into optical path of atomic absorption spectrometer.<\/p>\n
These analyzers use a electrode less\u00a0\u00a0 discharge lamp EDL as light source which is driven by high frequency. Extremely narrow band width emission\u00a0\u00a0 lines are generated by lamp which is congruent with mercury atom \u00a0\u00a0absorption lines .<\/p>\n
CVAAS is primary technique for analyzing mercury. These \u00a0\u00a0instruments have peristaltic pump \u00a0\u00a0that transports \u00a0\u00a0sample & stannous chloride separately\u00a0 into gas liquid , where pure dry gas (argon) stream\u00a0 is introduced into \u00a0liquid mixture to release mercury vapour\u00a0 . Mercury shall be carried by dry argon gas\u00a0 \u00a0in vapour phase through dryer and into \u00a0atomic absorption optical cell. In absorption cell, the elemental mercury will absorb the light at 253.7 nm in logarithm which is proportion to its actual concentration of the sample.<\/p>\n
Cold Vapor Atomic Fluorescence Spectroscopy (CVAFS) <\/b><\/p>\n
These mercury analysis include sub ppt detection limits and much wider dynamic range can be achieved. CVAFS analyzer \u00a0\u00a0are available with 2 configurations \u2013 one which employs single atomic fluorescence and other which employs gold amalgamation system to pre concentrated mercury. The detection limit for only fluorescence approach \u00a0\u00a0is 0.2 ppt whereas method using pre-concentration approach with fluorescence\u00a0\u00a0 detection is low as 0.02 ppt.<\/p>\n
The sample is introduced by peristaltic pump and stannous chloride into gas liquid separator where \u00a0\u00a0pure dry argon \u00a0\u00a0gas stream is \u00a0bubbled through a mixture to release mercury vapor.\u00a0\u00a0 This technique uses unique characteristic of mercury that allows measurement at room temperature.<\/p>\n
The mercury is transported in carrier gas through a dryer\u00a0\u00a0 and then to a valve which selects\u00a0\u00a0 between fluorescence & pre concentration approach. In the detector mercury vapor absorbs \u00a0\u00a0254.7 nm and fluorescence at the same wavelength. Fluorescence signal is the measurement at 90 deg to incident beam to minimize scatter from excitation source. The intensity of the fluorescence light is directly proportional to the concentration of mercury.<\/p>\n
Typical flow diagram of CVAAS monitor<\/p>\n
Courtesy- Sick instruments<\/p>\n
![\"1asd\"](\"https:\/\/www.tce.co.in\/blogs\/wp-content\/uploads\/2018\/03\/1asd.png\")
<\/a><\/p>\nCourtesy- Sick instruments<\/p>\n
The Gas flow path consists of strong bypass stream from which for a sample preparation a \u00a0small partial stream is extracted\u00a0 by a built in gas transfer pump. The 2nd<\/sup>\u00a0 \u00a0pump extracts the partial stream and sends it to reduction step & amalgamation unit. All the components in contact with gas are heated electrically \u00a0\u00a0to high temperature.<\/p>\n
Reduction<\/p>\n
Mercury chloride compounds are reduced to elemental mercury by wet chemical reduction process with SnCl2 <\/sub>solution within reduction vessel. Excess condensate is removed by peristaltic pumps & feed in reductant \u00a0solution. This sampling gas\u00a0\u00a0 is conducted into the gold trap after \u00a0\u00a0processing.<\/p>\n
Amalgamation<\/p>\n
During this\u00a0 process \u00a0a defined sample gas volume\u00a0 is conducted through gold trap, so \u00a0metallic mercury forms an amalgam with gold. Gold trap is heated electrically and in collection phase they are released and later transported through photometer cell by inert carrier stream of gas. In this amalgamation technique interferences of other flue gas components are avoided \u00a0since analyzer is in contact with mercury & carrier gas<\/p>\n
Photometric measurement<\/p>\n
Atomic absorption spectrometry is used to measure mercury.\u00a0 Single beam photometer with Hg discharge lamp, photodiode detector, quartz cell & assembly is thermo-stated.\u00a0 So \u00a0high stability measurement is ensured due to baseline correction automatically.<\/p>\n