emission 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
flue
gas
U.V or I.R

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

11. 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
In Situ - Optical Cross Stack

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

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
flue
gas
probe
sample
conditioning
analysers
calibration
sample transport line

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
I.R.
source
motor
chopper
blade
detector
sample cell
sample
in
sample
out
optical filter

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

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

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

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)
ionisation
current
air
sample H2 + He
gas outlet
cathode
anode

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
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.
Inner electrode
Reference air
Sample stream

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.
flue
gas
sinter To
heated
line
Cal gas and
air for
blow back
Heated compartment

39. ON-STACK PROBES

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

41. Sample Transport : Typical
heated Line.
heating element
Teflon/PFA core
Steel braid
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 Measurement
Chilled 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
O2 wet O2 dry
microprocessor
%moisture
% (H2O) = 100 { }
1-
( O2 Wet )
( O2 Dry )
Zirconia Zirconia

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
- - - - - - - - - - -- - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
desiccant membrane
sample wet sample dry
dry purge
gas
wet 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

51. Removal of Water : Chiller
to analysers
1
2
3
4
6
1= Sample probe
2= Heated line
3= Dual path chiller
4= Sample pump
5= Condensate sensor
6= Filter coalescer
.. .. .. .. .. .. .. .. .. .. .. .. .. .
: :
: :
: :
:. .. .. .. .. .. .. .. .. .. .. .. .. ..:
flue
gas
5

52. Removal of Water : Chiller
Advantages:
• Verification of efficiency possible through
temperature measurement
• Very robust, reliable, well proven method
• Quick water removal minimises solubility
problems
• No dirty condensate problems
• Maximum removal of acids and hydrocarbons
Disadvantages:
• Higher cost
• Needs heated line

53. Extractive Sample Conditioning
for Mixed Analysis (wet and dry)
heated
module
with flow
control
chiller with
flow control
system
control
moisture
O2
O2
(wet)
(dry)
SO2
HCs
NOx
CO
CO2
probe
heated
line

54. Calibration.
• Normally made using traceable gases from
bottles certified for the right concentration
(within the range chosen)
• Calibration gases can be inserted, either
directly at the analysers, or, via the probe,
through the line and the sample conditioning
system so as to verify the system integrity.
• New guidance requires more calibration
points, use of dilutors and dividers

55. Having now carefully considered
the following Aspects :
• Compliance with Legislation
• Suitable Analytical Method
• Appropriate Analytical Technique
• Correct System Design

56. Check your supplier and the
equipment for:
Reliability & Availability
Value for money
Service and support