Changing Best Practices in Flue Gas Analysis

Copyright © Yokogawa Corporation of America

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Flue Gas Analysis Best Practices

Jesse Underwood


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The Early Days


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Modern Day Fired Equipment


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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


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3,000+ refinery heaters Approximately 10,000,000,000,000,000 BTUs consumed for fired heater

6,500+ industrial boilersApproximately 8,000,000,000,000,000 BTUs consumed for steam generation

Energy Consumption of Fired Equipment


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What is Flue Gas?


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N2

N2

N2

N2

N2

N2

O2

O2

CH4

N2

Complete Stoichiometric Combustion

N2N2N2N2N2N2N2N2H2OCO2H2O


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N2

N2

N2

N2

N2

N2

O2

O2

CH4

N2

N2

N2

N2

N2

N2O2

Practical Combustion

Why would we run with slight excess oxygen?System designBurner flame shapingBalancing safety while

minimizing heat losses

N2N2N2N2N2N2N2N2H2OCO2

H2ON2N2N2N2O2


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N2

N2

N2

N2

N2

N2

O2

O2

CH4

N2

N2

N2N2

N2

N2

N2

N2 N2

N2O2

O2

Excess Air (Fuel Lean) Signs of fuel lean conditions:

NOX formation Increased fuel usage

Consequences: Fuel and thermal

efficiency loss Elevated NOX emissions

N2N2N2N2N2N2N2N2H2OCO2

H2ON2N2N2N2N2N2N2NOXNOXO2


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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

N2N2N2N2N2N2N2N2H2OCO2CH4CO


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The State of Most Fired Systems – FuelFuel –•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


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The Sate of Most Fired Systems - Air

Air –The value of air entering the burner is not measuredModulated manually via dampers, registers or louvers in

front of fansIf automated dampers are used, no automation is used on

the registersNo flow measurement to determine availability for

changing fuel density


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Impact of Fuel Composition ChangesAir –•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 FuelCH4+2O2 = CO2+2H2O 1 Mol CH4(volume) requires 2/0.21=9.52 Mols Air

Propane as FuelC3H8+5O2=3CO2+4H2O1 Mol C3H8(volume) requires 5/0.21=23.81 Mols Air


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Excess Oxygen Targets by Fuel

Typical design excess Oxygen concentrations by fuelNatural gas: 1 – 3%Fuel oil: 1 – 4%Coal: 1.5 – 10%


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Emissivity as a Function of Excess Oxygen

Emissivity is in part determined by the partial pressure of H2O and CO2

1.00% 2.00% 3.00% 4.00% 5.00% 6.00% 7.00% 8.00% 9.00% 10.00%0

5

10

15

20

25

30Emissivity as a Function of Excess Oxygen

% H2O% CO2

Excess Oxygen (%)

Perc

ent H

2O a

nd C

O2

(%)

1.00% 2.00% 3.00% 4.00% 5.00% 6.00% 7.00% 8.00% 9.00% 10.00%0

5

10

15

20

25

30

0.42

0.44

0.46

0.48

0.5

0.52

0.54Emissivity as a Function of Excess Oxygen

Gas Emissivity% H2O% CO2

Excess Oxygen (%)

Perc

ent H

2O a

nd C

O2

(%)

Gas E

miss

ivity


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Radiation Efficiency vs Emissivity

Increased concentration of these gases will increase radiative heat transfer

1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.42

0.44

0.46

0.48

0.5

0.52

0.54

Radiation Efficiency vs Emissivity

Gas EmissivityRadiation Efficiency

Excess Oxygen (%)

Radi

ation

Effi

cienc

y (%

)

Gas

Emiss

ivity


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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

1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

50

100

150

200

250

300

350

Radiation Efficiency and Flue Gas Heat

Total Heat LiberationFlue Gas HeatRadiation Efficiency

Excess Oxygen (%)

Radi

ation

Effi

cienc

y (%

)

Flue

Gas

Hea

t (m

mBT

U/h)


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API-556 Excerpt on Oxygen Set Point

3.4.4.9.2A) 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).


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Excess O2 Change and CO Spike

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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 2890

0.5

1

1.5

2

2.5

3

3.5

4

4.5

TDL CO ppmTDL 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 2890

0.5

1

1.5

2

2.5

3

3.5

4

4.5

TDL CO ppmTDL O2 %

Operator Test. Adjust O2 downward to cause CO breakthroughs.

Its ReproducibleFirst breakthrough. Operator increasesO2 and CO goes down.

Second breakthrough. Operator increases

O2 and CO goes down.


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The Search For an Adequate Analyzer


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Typical Measurements Today

Oxygen –Common Technologies are:

Paramagnetic Zirconium Oxide Tunable Diode Laser Spectrometers

Uses:Dry Oxygen measurement for emissionsMostly used as a monitor with an alarm for an operator

responseSometimes used for close loop control on boilers, but

seldom for process fluid heaters or furnaces


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Combustibles –Common Technologies are:

Catalytic Bead Thick/Thin Film

Uses:Indicator for upset Mostly used as a monitor with an alarm for an operator

responseSometimes tied to safety instrument systems to trip fuel

gas


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CO Specific –Common Technologies are:

Infrared Photometers Laser Spectrometers

Uses:Indicator for upset Mostly used as a monitor with an alarm for an operator

responseSometimes tied to BPCS as an override to fuel pressureSometimes used as part of a cross limiting control system

for auto-tuning


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Methane Specific –Common Technologies are:

Catalytic Bead Thick/Thin Film Laser Spectrometers

Uses:Diagnostic for loss of flame Startup permissive after purge cycle


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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

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Diffusionsensors

sensorsLow Flow Extractive

High Flow Extractive

sensors


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API-556 Comments on Zirconium Oxide

3.2.4.2During 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.


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“Before” “After”

Pictures from Dow’s Publication on the hazards of Zr Oxide as a potential ignition source


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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


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Potential Additional Options

Isolate the probe from the process

Purge the area around the probe


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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”


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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


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Cross Stack NIR

CO Specific MeasurementCommercially available 1980’sGas filter correlation

Chopper motor, gas cell filtrationsample cell with detectorreference cell with detector

Sample temp 0 – 300COptical Path Length max ~30 feet


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TDLS Platform

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O2 CO

H2O


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API-556 Comments on Laser Based Analysis

3.2.4.2Laser 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

3.2.4.4.1When 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).


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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


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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


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API-556 Comments on Analyzer Placement

3.2.4.2Oxygen 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


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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 increases2. Fuel rich flame is produced and cooled3. Flue gas is cooled4. COT drops5. Controller demands more fuel pressure6. More fuel is delivered to burners7. Flue gas is cooled more8. COT continues to drop9. Controller demands more fuel10.More fuel is delivered to burners


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Nova Chemical’s Recommendation

Reduce the fuel input into the fired system at a rate of 1% every seconds

3550ppm combustibles = 1% change in fuel pressure

71sec=8(3500ppm)/400ppm

20sec=8(1000ppm)/400ppm


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Internal Test at Third Party Facility


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Thank you!

Technology

Changing Best Practices in Flue Gas Analysis