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Dilution-Extractive CEMS vs Extractive CEMS

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Schematic of Dilution-Extractive Continuous Emission Monitoring System
Schematic of Extractive Continuous Emission Monitoring System (Cool - Dry)
Schematic of Extractive Continuous Emission Monitoring System (Hot - Wet)
Schematic of Portable Emission Monitoring


Dilution-Extractive Continuous Emission
Monitoring System


Extractive Continuous or Portable Emission
Monitoring System


Primary Benefits of Dilution-Extractive Systems:

  • Ability to measure flue gas concentrations on a wet basis. All flue gas flow rate measuring techniques are on a wet basis, consequently, e.g. wet basis SO2 emission data can be used more conveniently to calculate SO2 mass emission rates
  • The CFR, vol. 40, Part 75 requirement to measure CO2 makes the use of a dilution-extractive system even more convenient because CO2 is measured as the diluent gas (instead of O2)
  • Dilution-extractive systems are extractive systems that dilute the sample gas with dry contamination-free dilution air to a level below the dew point of the diluted flue gas to eliminate condensation problems in the CEM system (instead of using a moisture condenser)
  • Ideal for conducting gaseous pollutant sampling from sources where moisture content or pollutant concentrations are too high for accurate measurements
  • No need of heated sample transport lines. Heat traced umbilical bundles or freeze protection is occasionally required only in very cold ambient conditions or when dilution ratio less than 25 to 1 is used in climate with temperatures below zero Co
  • An extractive dilution system only requires 100-300 cc/min of sample gas as opposed to the 2-5 litres required by extractive systems. Therefore, only 200-400 cc/min of calibration gas is required for a full calibration. Typically with daily calibrations a "D" size cylinder will last for up to two years
  • A diluted sample can be readily measured by existing off-the-shelf ambient monitoring instrumentation

Primary Benefits of Extractive Systems:

  • Probe materials and designs have been improved; some probe materials and coatings currently available are more resistant to corrosion, while other materials can withstand temperatures exceeding 3000 oF.
  • The availability of easily spliced heated sample lines
  • The use of secondary conditioners ensures that particulate matter and moisture removal are adequate for those monitors that measure gas concentrations on a dry basis
  • Wet-basis gas analysis without dilution and without sample cooling is accurate method of analyzing a wet-basis gas sample. Such systems can be used to measure very wide variences in concentrations total hydrocarbon, oxides of nitrogen, oxygen and moisture. For the measurement of very low levels of nitrogen oxides (without loss of NO2), this is the only appropriate technique currently available

Primary Limitations of Dilution-Extractive Systems:

  • Requires Dilution Air-Cleanup System according to CFR, vol. 40, Part 75
  • Dilution-extractive probe systems require a constant source of contamination free dilution air. The air supply should be dry (-20 to -40 F) and delivered at 90 +/-10 psig. Additionally, the dilution air should be free of oils, particulates, CO2, NOx, and SO2.
  • The dilution ratio must be carefully selected. Two criteria are used to determine the desired dilution ratio: (1) the analyzer span range must correspond to the diluted sample gas concentration, and (2) the ratio must be selected to ensure that no condensation occurs in the sample line at the lowest possible ambient temperature
  • Dilution extractive techniques have dominated for monitoring acid rain, due to the desirability of a wet basis measurement. These types of systems have been successful to this point, but have severe limitations as pollutant concentrations reach continually lower levels. Variables such as stack temperature, stack pressure, and molecular weight have an unfavourable impact on the dilution technique
  • A dilution sample system cannot be used unless an analyser with adequate sensitivity is available

Primary Limitations of Extractive Systems:

  • Sample Moisture Removal System is needed for cold/dry extractive systems i.e. systems where the moisture is removed from the flue gas sample prior to entering the analyzers
  • The sample pump must be designed so no lubricating oil can contact and contaminate the sample gas, it has to be corrosion resistant and no air in-leakage occurs
  • The straight extractive method requires that a water removal system is used prior to the sample reaching the analysers. This process of removing water also removes part of the very water soluble sample gases, thus introducing an error into the measurement of SO2, NO2 and making it impossible to measure very soluble gases like NH3, HCl and HF
  • Heated sample transport lines are needed for cold/dry extractive systems
  • The removal of water from the sample gas (extractive systems) alters the ratio of gases in the sample and this requires that sample temperature and a % moisture curve for saturated conditions is required in order to determine moisture content of the stack gas and correct the reading for this loss of water
  • For hot/wet systems i.e. systems where the moisture is not removed from the flue gas sample prior to entering the analyzers, there are not so many instruments working at such conditions.

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