Methods for Continuous Emission Gas Monitoring

1. Methods for Continuous Emission Gas Monitoring

2. Factors to consider when choosing a CEM System • Compliance with Legislation • Suitable Analytical Method • Appropriate Analytical Techniques • Correct System Design • Reliability & Availability • Overall operating Cost

3. Compliance with Legislation • For any given process, the Legislation imposes limits on how much can be emitted to the atmosphere for particular species • These limits can be expressed as: • Maximum gas concentration • ppm • mg/m3 • Maximum mass emission

4. Gas Concentration • When the limits imposed specify gas concentration, it must often be expressed at the following conditions: • Dry basis • Temperature: 0 deg C (273 K) • Pressure: 1 atm. (101.3 KPa) • Corrected for the O2 level specified for the given process

5. To comply with most Legislation,ideally,all gases should be measured on a dry basis, i.e after removal of water.

6. However, certain gases must be measured in a hot wet state because: • They are soluble in water • They would otherwise condense and lose their gaseous state

7. Factors to consider when choosing a CEM System • Compliance with Legislation • Suitable Analytical Method • Appropriate Analytical Technique • Correct System Design • Reliability & Availability • Overall operating Cost

8. Analytical Methods available • In Situ • Dilution Probe • Extractive

9. In Situ - Optical Cross Stack U.V or I.R flue gas

10. In Situ - Optical Cross Stack Advantages: • Low installation cost • No sample system required

11. In Situ - Optical Cross Stack Disadvantages: • True traceable calibration not possible • Wet gas basis only • Difficult access for maintenance • Significant interferences • Limited sensitivity • Temperature limitation • Not suitable for applications with high dust loading • Requires separate O2 and moisture measurements • Does not normally measure NO2

12. Dilution Probe dilution probe pump & calibration flue gas analysers clean air

13. Dilution Probe Advantages: • In-situ conditioning • Low extraction rate • Quenches most reactions • No heated lines • No corrosive gases transported

14. Dilution Probe Disadvantages: • Wet gas basis only • Requires purified air • Requires separate O2 and moisture measurements • Problems with ambient air analysers • Slow response time • Single dilution ratio • Needs tracer gas at probe to verify dilution ratio • Does not operate at high temperature

15. Extractive Method sample transport line probe sample conditioning flue gas analysers calibration

16. Extractive Method Advantages: • Dry and wet analysis • Ease and accuracy of calibration • Ease of maintenance • Integral O2 and moisture measurements • Analysers run under controlled conditions: high stability • Optimal control of sample conditioning • Much more accurate analysis

17. Extractive Method Disadvantages: • Needs heated lines (when hot analysis required) • Needs sample conditioning

18. Factors to consider when choosing a CEM System • Compliance with Legislation • Suitable Analytical Method • Appropriate Analytical Technique • Correct System Design • Reliability & Availability • Overall operating Cost

19. The Choice of Analytical Technique depends on the Gas Species. The main Gases are: • CO : Carbon Monoxide • CO2: Carbon Dioxide • NO : Nitric Oxide • NO2: Nitrogen Dioxide • NOx: Nitrogen Oxides ( NO+NO2) • SO2: Sulfur Dioxide • HCs: Hydrocarbons (Volatile Organic Compounds: VOCs) • O2 : Oxygen • Others – HCl, HF

20. Carbon Oxides: • CO and CO2 normally measured by Infra Red • With Infra Red, water vapour interference • Therefore, dry analysis (after removal of water)

21. Sulphur Dioxide: • SO2: Slightly soluble. • Prolonged contact with water generates corroding acid. • Can be analysed in dry condition if transported in hot, wet condition and water removed quickly by chilling sample. • IR: dry analysis • UV: wet or dry analysis, only if no HCs in flue gas

22. Dispersive and Non-Dispersive Optical Techniques Light is re-emitted or scattered in all directions

23. Beer’s Law Absorption of light follows Beer’s Law which states that the degree of absorption of light varies exponentially with the thickness of the layer of absorbing medium, it’s molar concentration and it’s absorption constant. Mathematically this is: I = Io exp(-axc) I = Intensity of light after absorption Io = Intensity of light before absorption a = Absorption constant (different for each gas) x = Absorption path length c = Concentration

24. If path length is chosen to suit a particular concentration range, then the response of an analyser to varying concentrations will be of this nature:

25. Single Beam NDIR chopper blade motor optical filter sample cell I.R. source detector sample in sample out

26. Gas Filter Correlation NDIR Gas filter wheel motor detector sample cell I.R. source optical filter sample out sample in

27. Dual Beam NDIR chopper blade reference cell synchronous motor flow detector I.R. source sample cell sample in sample out

28. Other optical techniques • Fourier Transform Infrared – very useful for investigative work, but requires very skilled operatives and all components have to be referenced. It is also expensive. • FT-IR is good for N2O and can analyse multiple components simultaneously • Laser based systems – precise wavelength, usually use IR or visible frequencies, in-situ location.

29. Nitrogen Oxides: • 3 techniques: Infra Red, Ultra Violet and Chemiluminescence. • Infra Red requires dry analysis. • With Ultra Violet, Hydrocarbons interference. • Chemiluminescence, very sensitive technique, not affected by water or Hydrocarbons.

30. Nitrogen Oxides • NO2 is very soluble in water. Must be maintained at temperature above water dew point. • NO: dry analysis if IR ;dry or wet analysis if UV (only when no HCs in flue gas) or Chemiluminescence. • NO/NO2/NOx: wet analysis only, UV (when no HCs in flue gas) or Chemiluminescnce

31. Chemiluminescence clean dry air reaction chamber O3 generator photomultiplier tube (PMT) --------signal to vent pump NO + O3 ---> NO2* + O2 NO2* ---> NO2 + hv sample NO2 / NO converter

32. Hydrocarbons: • Must be analysed in a hot, wet condition as would otherwise condense. • Normally analysed by Flame Ionisation Detection (FID)

33. Flame Ionisation Detector (FID) gas outlet anode ionisation current air cathode sample H2 + He

34. Oxygen: • Two techniques: Paramagnetic and Zirconia Paramagnetic: • Dry analysis only • Susceptible to corrosion and damage by water • NO2 interference Zirconia: • Rugged and reliable • Wet or dry basis

35. Oxygen • Zirconia probe – electrochemical sensor, using zirconia at 800 centigrade Outer electrode Inner electrode Reference air Sample stream At 800 degrees the Zirconia becomes a solid electrolyte and O2 will migrate from the high to low concentration to maintain an equilibrium. Usually from the reference to the sample.

36. Factors to consider when choosing a CEM System • Compliance with Legislation • Suitable Analytical Method • Appropriate Analytical Technique • Correct System Design • Reliability & Availability • Overall operating Cost

37. Correct System Design : Aspects to consider. • Sample extraction & transportation • Sample Conditioning • Analysis • Calibration • Data collection • Housing

38. Sample Extraction : Typical Probe. Cal gas and air for blow back sinter To heated line Heated compartment flue gas

39. ON-STACK PROBES

40. Sample Extraction : Factors to consider. • Stack temperature • Dust loading • Corrosiveness

41. Sample Transport : Typical heated Line. Teflon/PFA core Steel braid heating element electrical insulation thermal insulation scuff resistant jacket

42. Sample Conditioning : • Wet analysis (keeping sample in hot and wet condition) • Dry analysis (removing water from sample) : • Mixed analysis (splitting sample into wet and dry streams)

43. Sample Conditioning for Wet Analysis : Moisture Measurement. • To comply with the legislation, the moisture must be measured so that the correction for water can be made. • Main moisture measurement techniques: • Chilled mirror • RH sensor • Dual O2 measurement

44. Moisture MeasurementChilled Mirror & RH sensors • Chilled mirror systems- Accurate but very difficult to use and very expensive. • RH sensors – cost effective but not robust in hot and dirty atmospheres

45. Moisture Measurement : Dual O2 Measurement Zirconia Zirconia O2 wet O2 dry ( O2 Wet ) { } % (H2O) = 100 1- microprocessor ( O2 Dry ) %moisture

46. Moisture Measurement : Dual O2 Measurement Advantages: • Continuous measurement • No extra stack insertion (if using extractive) • Not affected by stack conditions • Not temperature dependent • Rugged Disadvantages: • Restricted accuracy at low O2 level

47. Sample Conditioning for Dry Analysis : Removal of Water. • Two factors can affect the performance of an analyser after removal of water: residual acids and residual hydrocarbons. • When choosing the water removal technique,it is therefore vital to consider the levels of acids and hydrocarbons generated by the process. • Main methods: • Front end permeation dryer • Chiller

48. Removal of Water : Front End Permeation Dryer wet purge gas sample dry - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - sample wet - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - desiccant membrane dry purge gas

49. Removal of Water : Front End Permeation Dryer Advantages: • Low cost • No heated line needed • Removes water while in gaseous phase: no problems with solubility of SO2, NO2 etc..

50. Removal of Water : Front End Permeation Dryer Disadvantages: • Desiccant membrane easily clogged up by hydrocarbons and other sticky condensates • Does not remove acids and some hydrocarbons. • Removes some HCs and NH3 • Results affected by ambient temperature variations • Unpredictable stabilisation time • No efficiency checks possible