Changing Best Practices in Flue Gas Analysis

Copyright © Yokogawa Corporation of America
1
Flue Gas Analysis
Best Practices
Jesse Underwood

2
The Early Days

3
Modern Day Fired Equipment

4
Modern RPs & Codes for Flue Gas Analysis
 NFPA 85 – Boiler and Combustion Systems Hazard
Code
 API 556 – RP Instrumentation, Control and Protective
Systems for Gas Fired Heaters
 API 538 – RP for Industrial Fired Boilers for General
Refining and Petrochemical Service
 API 561 – RP for Steam Methane Reformers

5
3,000+ refinery heaters
Approximately 10,000,000,000,000,000 BTUs consumed for fired heater
6,500+ industrial boilers
Approximately 8,000,000,000,000,000 BTUs consumed for steam generation
Energy Consumption of Fired Equipment

6
What is Flue Gas?

7
N2
N2
N2
N2
N2
N2
O2
O2
CH4
N2
Complete Stoichiometric Combustion
N2N2N2
N2N2N2N2
N2H2OCO2H2O

8
N2
N2
N2
N2
N2
N2
O2
O2
CH4
N2
N2
N2
N2
N2
N2
O2
Practical Combustion
 Why would we run with
slight excess oxygen?
System design
Burner flame shaping
Balancing safety while
minimizing heat losses
N2N2N2
N2N2N2N2
N2H2OCO2
H2ON2N2
N2N2O2

9
N2
N2
N2
N2
N2
N2
O2
O2
CH4
N2
N2
N2
N2
N2
N2
N2
N2 N2
N2
O2
O2
Excess Air (Fuel Lean)
 Signs of fuel lean conditions:
 NOX formation
 Increased fuel usage
 Consequences:
 Fuel and thermal
efficiency loss
 Elevated NOX emissions
N2N2N2
N2N2N2N2
N2H2OCO2
H2ON2
N2N2
N2N2
N2N2NOXNOXO2

10
N2
N2
N2
N2
N2
N2
O2
O2
CH4
N2
CH4
CH4
Incomplete Combustion (Fuel Rich)
 Signs of incomplete
combustion:
 CO breakthrough
 High fuel usage and
breakthrough in extreme
cases
 Consequences:
 Unsafe condition
 Wasted fuel
 Efficiency loss
N2N2N2
N2N2N2N2
N2H2OCO2CH4CO

11
The State of Most Fired Systems – Fuel
Fuel –
•Volumetric flow used to determine the value of fuel
going into the fired system
•Pressure is used to control the amount of fuel
delivered to the burners
•BTU analysis is performed at the mixing tank
•No compensation for a density change
Fuel
 Volumetric flow used to determine the value of fuel going into
the fired system
 Pressure is used to control the amount of fuel delivered to
the burners
 BTU analysis is performed at the mixing tank
 No compensation for a density change

12
The Sate of Most Fired Systems - Air
 Air –
The value of air entering the burner is not measured
Modulated manually via dampers, registers or louvers in
front of fans
If automated dampers are used, no automation is used on
the registers
No flow measurement to determine availability for
changing fuel density

13
Impact of Fuel Composition Changes
Air –
•The value of air entering the burner is not
measured
•Modulated manually via dampers, registers or
louvers in front of fans
•If automated dampers are used, no automation
is used on the registers
•No flow measurement to determine availability
for changing fuel density
Air demand changes with fuel composition
 Methane as Fuel
CH4+2O2 = CO2+2H2O
1 Mol CH4(volume) requires 2/0.21=9.52 Mols Air
 Propane as Fuel
C3H8+5O2=3CO2+4H2O
1 Mol C3H8(volume) requires 5/0.21=23.81 Mols Air

14
Excess Oxygen Targets by Fuel
 Typical design excess Oxygen concentrations by fuel
Natural gas: 1 – 3%
Fuel oil: 1 – 4%
Coal: 1.5 – 10%

15
Emissivity as a Function of Excess Oxygen
 Emissivity is in part determined by the partial pressure
of H2O and CO2
0
5
10
15
20
25
30
1.00% 2.00% 3.00% 4.00% 5.00% 6.00% 7.00% 8.00% 9.00% 10.00%
PercentH2OandCO2(%)
Excess Oxygen (%)
% H2O
% CO2
0.43
0.44
0.45
0.46
0.47
0.48
0.49
0.5
0.51
0.52
0.53
0.54
0
5
10
15
20
25
30
1.00% 2.00% 3.00% 4.00% 5.00% 6.00% 7.00% 8.00% 9.00% 10.00%
GasEmissivity
PercentH2OandCO2(%)
Excess Oxygen (%)
% H2O
% CO2
Gas Emissivity

16
Radiation Efficiency vs Emissivity
 Increased concentration of these gases will increase
radiative heat transfer
0.43
0.44
0.45
0.46
0.47
0.48
0.49
0.5
0.51
0.52
0.53
0.54
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
GasEmissivity
RadiationEfficiency(%)
Excess Oxygen (%)
Radiation Efficiency vs Emissivity
Radiation Efficiency
Gas Emissivity

17
Radiation Efficiency and Flue Gas Heat
 This is important because low radiative heat transfer
increases the amount of heat needed to achieve the
same coil outlet temperature
0
50
100
150
200
250
300
350
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
FlueGasHeat(mmBTU/h)
RadiationEfficiency(%)
Excess Oxygen (%)
Radiation Efficiency and Flue Gas Heat
Radiation Efficiency
Total Heat Liberation
Flue Gas Heat

18
API-556 Excerpt on Oxygen Set Point
 3.4.4.9.2
A) Combustibles breakthrough testing is recommended
to establish Oxygen concentration when combustibles
breakthrough occurs. Combustibles breakthrough
typically occurs between 0.5% and 2.5% Oxygen
depending on tramp air flow rate, condition of the burners,
fuel gas composition and bridgewall temperatures.
B) The operating margin between the %O₂ setpoint and
breakthrough must be sufficient to allow a process step
change to be detected within the overall response time of
the control loop (see 3.4.4.1.3).
F) in a properly designed system with fast response
infrared or lased based O₂/CO measurements, Oxygen
control at less than 1% may be acceptable (see 3.2.4.4).

19
Excess O2 Change and CO Spike
19
0
500
1000
1500
2000
2500
3000
3500
4000
4500
1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253 265 277 289
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
TDL CO ppm
TDL O2 %
0
500
1000
1500
2000
2500
3000
3500
4000
4500
1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253 265 277 289
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
TDL CO ppm
TDL O2 %
Operator Test. Adjust O2 downward to cause CO breakthroughs.
Its Reproducible
First breakthrough. Operator increases
O2 and CO goes down.
Second breakthrough. Operator increases
O2 and CO goes down.

20
The Search For an Adequate Analyzer

21
Typical Measurements Today
 Oxygen –
Common Technologies are:
 Paramagnetic
 Zirconium Oxide
 Tunable Diode Laser Spectrometers
 Uses:
Dry Oxygen measurement for emissions
Mostly used as a monitor with an alarm for an operator
response
Sometimes used for close loop control on boilers, but
seldom for process fluid heaters or furnaces

22
 Combustibles –
 Catalytic Bead
 Thick/Thin Film
 Uses:
Indicator for upset
response
Sometimes tied to safety instrument systems to trip fuel
gas

23
 CO Specific –
 Infrared Photometers
 Laser Spectrometers
 Uses:
Indicator for upset
response
Sometimes tied to BPCS as an override to fuel pressure
Sometimes used as part of a cross limiting control system
for auto-tuning

24
 Methane Specific –
 Catalytic Bead
 Thick/Thin Film
 Laser Spectrometers
 Uses:
Diagnostic for loss of flame
Startup permissive after purge cycle

25
Zirconium Oxide Analyzer
The most common technology for
Oxygen analysis
Low cost, solid state, reliable
Analyzers generally divide into three
types
Close coupled extractive (CCE).
Sensor is removed from the process to
allow higher gas temperatures
In-situ with heater. Sensor is in the
process, limited to ~700°C
In-situ w/o heater. Allows higher gas
temperature, no measurement at lower
gas temperatures
25
Diffusion
sensors
sensors
Low Flow
Extractive
High Flow
Extractive
sensors

26
API-556 Comments on Zirconium Oxide
 3.2.4.2
During combustibles breakthrough…at low oxygen levels, it is
possible for a high concentrations of Hydrogen and CO to mask
(malfunction low) the true oxygen concentration at the sensor
Upon complete loss of flame…in a fuel rich environment, it is
possible for a high concentration of methane to mask
(malfunction low) the true oxygen concentration at the sensor
Nitrogen backup to the instrument air system has the potential
to create an oxygen analyzer malfunction high
An oxygen analyzer with heated ZrO₂ sensor is a potential
ignition source during purge cycle. Mitigation options include a
purge interlock to disconnect sensor power, reverse flow of
close-coupled extractive systems or flame arrestors.

27
“Before” “After”
Pictures from Dow’s Publication on the hazards of Zr Oxide as
a potential ignition source

28
NFPA 85 Unsure about Flame Arrestors
 Analyzers could contain heated elements that exceed the auto-ignition
temperature of many fuels during pre-purge, startup, or fuel trips
 1076˚F for Natural Gas
 850˚F for Bituminous Coal
 494˚F for Number 2 Fuel Oil
 Many manufacturers have begun to include flame arrestors with their
probes, but these can be corroded or may not work below certain
temperatures
 How do you test a corrosion limit?
 What does this do to your speed of response?
 Consideration should be given to powering down analyzers during
boiler or fuel trip situations if they can exceed the auto-ignition
temperature of the fuel being fired
 How can you effectively do this?
 How do you compensate for running blind or having a warmup time?
 The metal, even on non-heated probes, can exceed the auto-ignition
temperature of some fuels

29
Potential Additional Options
 Isolate the probe
from the process
 Purge the area
around the probe

30
A word about COe
COe is a combination of H2 and CO plus response to
other hydrocarbons
COe does not include methane as methane cracks at a
higher temperature
API 556:
“The term COe is used in this manual to describe the sensor output.
This term indicates that the sensor is calibrated in terms of CO, and
that the sensor output is equivalent to CO but not specific to CO”

31
API-556 Comments on CO(E) Analyzers
 3.2.4.3
 Since catalytic bead or hot wire technology requires the presence of
oxygen for combustibles detection, some sensors may report lower
than actual combustible values at low oxygen concentrations
 Since the Methane molecule cracks at a high temperature, detecting
Methane typically requires a separate sensor
 A combustibles analyzer with a heated catalytic sensor is a potential
ignition source during the purge cycle
 3.4.4.1.5
 Measurement delay due to sensor response – Typical published T90
specification from catalytic bead and film sensors can range from <20
seconds to 30 seconds. These manufacturer specifications often do
not define the concentration step change at which the T90 applies
which may have a significant impact on T90 response. Analytical
sensor test data may indicate the true T90 response time to a
concentration step change of 0 ppm to 1000ppm and 1000ppm to
5000ppm CO may be > 2 minutes

32
Cross Stack NIR
CO Specific Measurement
Commercially available 1980’s
Gas filter correlation
Chopper motor, gas cell filtration
sample cell with detector
reference cell with detector
Sample temp 0 – 300C
Optical Path Length max ~30 feet

33
TDLS Platform
33
O2 C
O
H2O

34
API-556 Comments on Laser Based Analysis
3.2.4.2
Laser based technology…is not an ignition source to flue gas
and does not require reference air
It has a response time of < 5 seconds and can measure across
a radiant section up to 100ft
3.2.4.4.1
When controlling a fired heater’s air/fuel ratio near the CO
breakthrough point, an IR or laser based CO specific
measurement is recommended
3.4.4.9.2.F
In a properly designed system with fast response infrared or
laser based O₂/CO measurements, Oxygen control at less than
1% may be acceptable (see 3.2.4.4).

35
Point Measurement Considerations
Placement
 Oxygen and CO concentrations can have varied
distribution in large systems (vertical and horizontal)
 Vertical distribution is due to tramp air (air leaks) for
oxygen and “afterburning” for CO
 Horizontal distribution is due to burner variations and
flow effects

36
Path Type Measurement
 Measurement Approach with TDL:
 Measurement in Radiant Section
 Fast Detection of CO Breakthrough
 Methane Detection
 LOP for Startup Safety
 Averaging Oxygen Across Radiant
Section
 Solid State Device
 Not a potential ignition source
Convection
Section
Oxygen
CO + CH4
Radiant Section

37
API-556 Comments on Analyzer Placement
3.2.4.2
Oxygen measurement should be taken as near as
possible to the point where combustion is completed,
normally at the exit of the radiant section
To minimize the impact of air ingress, stack measurement
for oxygen concentrations should be avoided where
possible
Combustibles should not be measured in the stack due to
the potential for afterburning in the convection section
For large combustion zones one analyzer for every 30ft of
firebox length is recommended due to non-uniformities in
the firebox flue gas circulation and to facilitate balancing
the burners

38
Typical Fired System Control Scheme
 Air flow is set but not measured
 Fuel flow is measured by volume, not mass
 COT determines fuel pressure to burners
 No Combustible feedback
Murphy’s Law:
1. Fuel density increases
2. Fuel rich flame is produced and cooled
3. Flue gas is cooled
4. COT drops
5. Controller demands more fuel pressure
6. More fuel is delivered to burners
7. Flue gas is cooled more
8. COT continues to drop
9. Controller demands more fuel
10.More fuel is delivered to burners

39
Nova Chemical’s Recommendation
Reduce the fuel input into the fired system at a rate of 1%
every 𝑡 seconds
𝑡 = 𝑡 𝑟𝑒𝑠(3550𝑝𝑝𝑚)/𝐶 𝑎𝑐𝑐𝑒𝑝𝑡𝑎𝑏𝑙𝑒
3550ppm combustibles = 1% change in fuel pressure
71sec=8(3500ppm)/400ppm
20sec=8(1000ppm)/400ppm

40
Internal Test at Third Party Facility

41
Thank you!

Changing Best Practices in Flue Gas Analysis

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Changing Best Practices in Flue Gas Analysis (20)

More from Yokogawa1 (20)

Recently uploaded (20)

Changing Best Practices in Flue Gas Analysis

Editor's Notes