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
Maximum gas concentration is normally expressed in
ppm: parts per million
mg/m3: milligrams per cubic meter
Maximum mass emission is normally expressed in units of:
time: g/h (grams per hour)
fuel burnt: g/kg ( grams per kilograms)
energy: g/kW (grams per kw of energy produced)

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
Dry basis: This is because water vapour can represent a fair proportion of the flue gas composition.As some of the water will condense immediately on leaving the stack, the concentration of the other components of the flue gas actually reaching the atmosphere would be lower than the reality if measured in the stack on a wet basis. Furthermore, dry analysis offers a stable basis for comparison between similar processes.
0 deg C.; 1 atm. : These are the STP conditions (Standard Temperature Pressure). Note that in ambient air measurement, the requirement is often for correction to NPT (Normal Temperature Pressure) conditions which are at 20 deg C.
Oxygen correction: It is used to prevent polluters diluting the emission with excess air. The reference level of O2 to correct to is normally fixed by Legislation and correspond to an acceptable excess oxygen for a given process. Some typical O2 reference used in the EC: oil- and gas-fired combustion: 3%; coal-fired: 6%; incinerators : 11%; gas turbines: 15%. In Japan and Taiwan, however , O2 reference level for incinerators is 10%.

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
Gases soluble in water: This means that by removing the water, you also remove the dissolved component, hence showing results lower than reality. Certain gases, when dissolved, form acids that will corrode the system.
Component condensing: Most hydrocarbons have a high dew point. Condensing the water will also condense the hydrocarbons, hence making it impossible for them to be analysed. Furthermore, in a liquid state, they tend to be very sticky and would foul the analysers.

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
Source and detector collimated by lenses or mirrors.
Stack acts as optical chamber.
Uses gas or interference filters to select the appropriate wave band. The absorption of light in the selected band is proportional to the concentration of the corresponding gas.

10. In Situ - Optical Cross Stack
Advantages:
Low installation cost
No sample system required
Low installation cost: simple flange mount and insertion into opposite sides of the stack.
Difficulty: alignment of source and detector.

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
True traceable calibration:impossible to have accurate zero:
if zero cal. achieved during shut down, process conditions are different. One cannot wait for shut down to calibrate.
if zero cal. achieved by blanking off source and replacing by similar source near detector: (a) sources are never identical, (b) it ignores the effect of wall radiations, particules and remaining hot gases still present in the stack.
Access for maintenance: needs regular maintenance to clean windows and to make the necessary adjustments; it requires a trip up the stack.
Interferences: water for IR and HCs for UV
Limited sensitivity: intensity of light decreases with square of path length. High noise level created by temperature induced rogue IR emissions.
Temperature limitation: ca. 300 deg C.
Dust loading : causes windows fouling + obscuration of light

12. Dilution Probe dilution probe flue gas analysers clean air pump &
calibration
Compressed, highly purified air is brought to the probe and used to dilute the sample.
This method was invented to allow manufacturers of ambient air analysers to measure stack emissions.
analysers
clean air

13. Dilution Probe Advantages: In-situ conditioning Low extraction rate
Quenches most reactions
No heated lines
No corrosive gases transported
In-situ conditioning: by diluting, it minimises the concentration of water and acids, hence bringing their respective dew point below the ambient temperature
Low extraction rate: typical ratio of dilution :100:1, therefore only one volume of sample for 100 volumes of air.
Quenches most reactions: by increasing the distance between molecules

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
Requires purified air: the air must be very highly purified as any small contamination will be detected by the ambient air analysers normally measuring low concentrations. This means either a high capital expenditure to purchase a high quality purifier, or a high maintenance cost for lower spec. equipment.
Separate O2 and moisture measurement: O2 to discriminate stack oxygen from dilution air; moisture because, although water vapour has been diluted below the interference level, the concentration % is still the same.
Ambient air analysers generally cost up to 50% more .
Slow response time: with ambient air analysers, the response time tends to be around 2-3 min; for emission analysers, it is between 10 and 30 seconds.
Single dilution ratio: it often means a compromise . The ideal dilution ratio to reach the best range for one given analyser is often not ideal for another analyser.

15. Extractive Method sample transport line probe sample flue gas
conditioning
flue
gas
analysers
The pump in the analyser system extracts the appropriate amount of sample from the stack via the probe and the transport line and brings it to the analysers after sample conditioning.
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
This method has suffered in the past serious problems due to poor conditioning of sample. Bad reputation is hard to change. However, technological advance in materials and the knowledge acquired through experience have virtually eliminated the problems encountered in the past.

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 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 I = Io exp(-axc)
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 in
sample
out

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
It is considered that if the NO2 concentration is negligible (less than 10% of total NOX), dry analysis can be used.
In the early days of gas turbines, the fuel was directly injected into the combustion air feed, and this resulted in quite high levels of NOX with only approximately 10% of NO2. This argument is used by the manufacturers of IR analysers to justify their technique: suitable for high levels NOX (does not require very high accuracy) and a relatively low level of NO2 (proportion of NO2 lost in water relatively unimportant). The solution was often to measure NO only and use a simple correction factor to cater for the little NO2 not measured.
With the new dry low NOX turbines, they now premix the fuel and air and this has 2 effects:on one hand, it reduces the overall NOX content, hence requiring a more sensitive measurement technique such as chemiluminescence; on the other hand, it increases the ratio of NO2 to NO (around 50%) hence making it far more important to measure the NOX “hot” so as to eliminate the loss of NO2 .

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
Paramagnetic O2 analysers: O2 molecules exhibit a magnetic susceptibility. When flowing through a strong magnetic field, they are deflected. This deflection can be used to either affect the flow of an inert gas or move a dumbbell by upsetting a magnetic equilibrium. The flow change or the the dumbbell deflection is proportional to the O2 concentration.
Zirconia O2 analysers: The 360 Zirconia O2 analyser uses a high temperature (800 deg C) electrochemical cell. It consists of a closed tube of yttrium stabilised zirconia with 2 electrodes, one on the outer surface exposed to the sample gas , one on the inner side exposed to reference air. The tube is located inside a heater. At 800 deg C, the zirconia becomes a solid electrolyte in which O2 ions can move. When there is a difference in O2 concentration across the cell, O2 ions migrate from the higher to the lower concentration (usually from reference to sample) to re-establish equilibrium. This generates a small voltage logarithmically related to the concentration difference, This is then amplified.

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 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
Zirconia
Zirconia
O2 wet
O2 dry {
( O2 Wet ) }
% (H2O) = 100 1-
microprocessor
( O2 Dry )
Example:
If the content of O2 is, say, 10% in the wet gas and 20% when it has been dried, the formula gives the following result:
% H2O= 100-(1- 10/20)= 50
This means that for the same volume of O2, the concentration in the wet gas compared with the concentration in the dry gas depends on the dilution of the gas by water vapour. In the above example, the dilution ratio was 1:1 and therefore, the concentration of O2 in the wet gas was halved.
%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
The measurement of moisture is made by differential oxygen. The accuracy of of the moisture measurement is therefore the sum of the errors for each oxygen measurement, typically + or - 2% of the measured moisture. However, at low oxygen concentration (less than 1% in stack), a small leak in the system allowing air to enter the sample could distort the reading as there is 20.9% oxygen in air.

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 wet
sample dry
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

51. Removal of Water : Chiller
1= Sample probe
2= Heated line
3= Dual path chiller
4= Sample pump
5= Condensate sensor
6= Filter coalescer 1 2
to analysers 4 5 6
flue
gas
: :
: : 3
The chiller used is a dual path one. The chiller temperature is set at 3 deg C. The pump pressurises the sample to 1.33 atm. absolute pressure. The condensation at 1.33 atm. and 3 deg. C is equivalent to the condensation at 1 atm. and 0 deg. C. This chiller therefore removes all the water without the danger of “icing”.

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)
probe
heated
line
HCs
NOx
heated
module
with flow
control O2
(wet)
moisture O2
(dry)
chiller with
flow control
SO2
system
control CO
CO2

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