Welcome to Fired Heater Training!

1
Welcome to Fired Heater Training!

• The course is designed to give you some
  background information needed to operate a fired
  heater

2
Agenda

1. Introduction
2. Air/Fuel Ratios
3. Fundamentals of Burners
4. Fundamentals of Furnaces
5. Furnace Tuning and Use of Analyzer
6. NOx and Advanced Burner Design
7. Field Tuning of Heaters
8. Q A and Wrap-up

3
Heater and Burner Operation
4
Course Objective

• To ensure that everyone fully understands how
  burners and heaters work.

5
Course Topics

• Combustion Essentials
• Basic Burner Designs
• Furnace Types
• Draught
• Heater Tuning
• Low NOx Burner Designs

6
Combustion Essentials
7
What is Combustion?

• A chemical reaction between fuel and oxygen
  producing heat.
• Air is usually the source of oxygen.
• The chemical reaction produces flue gases

8
What Is Required For Combustion?

• Three Elements
• Fuel
• Air
• Source of Ignition

9
Fuel Components

• Gas, Oil and Coal are all basically a mix of
  Hydrocarbons.
• During combustion these break down progressively
  as some parts burn more easily.
• The most important components are Carbon and
  Hydrogen compounds.

10
Other Components

• In addition to the Carbon and Hydrogen many fuels
  contain Sulphur.
• Sulphur also burns but produces hazardous
  products.
• Liquid and solid fuels can contain other
  non-combustibles which form ash.
• Nitrogen may be present as a gas or in compound
  form in liquid/solid fuels.

11
Chemical Formulas

• In formulas we will use the following basic
  components
• Carbon C
• Hydrogen H2
• Oxygen O2
• Nitrogen N2
• Water H2O
• Carbon Dioxide CO2
• Methane CH4

12
Note on Calculations

• Each component in a formula is a Molecule (of
  gas)
• A Molecule of any gas occupies the same Volume
• The number of Molecules is therefore the same as
  the number of Volumes
• All calculations are therefore Volumetric,
  including measured Gas Analyses
• e.g. 2 CO 2 volumes of CO

13
Examples Of Combustion For Typical Fuel
Componentswith Oxygen

• C O2 CO2
• 2H2 O2 2H2O
• S O2 SO2

14
Heat

• Where does the heat come from?
• Heat
• C O2 CO2

15
But we dont have Pure Oxygen available
Oxygen in Air (by volume)
Air 21 O2 79 N2
Ratio 1 O2 3.75 N2
The other main component in air is Water Vapour.
In humid conditions this can be 5 or more and
affects efficiency
16
Examples Of Combustion For Basic Fuel Components
with Air

• C O2 3.75N2 CO2 3.75N2
• 2H2 O2 3.75N2 2H2O 3.75N2
• S O2 3.75N2 SO2 3.75N2

17
Example - Combustion Of Methane

• CH4 2O2 7.5N2 CO2 2H2O 7.5N2
• Heat

18
Stoichiometry

• The technical term used to define the theoretical
  amount of air or oxygen required for complete
  combustion of a fuel is the Stoichiometric ratio.
• e.g. - for a typical Natural Gas the
  Stoichiometric Ratio is approximately 10 volumes
  of Air to one of Gas.

19
Excess Air

• Because of many factors, including imperfect
  mixing, extra air is always needed to ensure
  complete combustion.
• The extra air above the Stoichiometric amount
  required is known as the excess air.

20
Stoichiometric Air Example

• CH4 2O2 7.5N2
• CO2 2H2O 7.5N2 Heat
• Note no Excess Oxygen in Flue Gas

21
Excess Air Example

• CH4 (2 0.4)O2 (7.5 1.5)N2 ? CO2 2H2O
  9N2 0.4O2 Heat
• 0.4/2.0 0.2 or 20 excess air
• 0.4/(1290.4)0.032 or 3.2O2 in flue gases
  (wet)
• 0.4/(190.4)0.038 or 3.8O2 (dry)

22
Fuel Rich Examples(Sub-stoichiometric)

• 3C O2 ? 2 CO C heat
• 4H2 O2 ? 2 H2O 2H2 heat
• Products include Combustible Gases and free
  Carbon (soot)

23
Some Dangers of operatingbelow Stoichiometric

• Flue gases contain combustibles.
• When these gases find a supply of air they will
  burn.
• If this happens in the convection tubes it can
  damage the tubes.
• Pockets of gas can build up in ducting and cause
  explosions.
• Flames eventually back out of burners.

24
Heater Control Problems with Sub-Stoichiometric
Combustion

• Increasing fuel flow will reduce heat to the
  process as more combustibles are generated.
• This can lead to total loss of control and very
  high levels of unburned gases in the heater.

25
How do you get out of this situation?

• Do not open up air suddenly, as this will cause
  unburned gas to burn rapidly and possibly
  explosively.
• Reduce the gas flow slowly until temperature
  starts to recover. This allows unburned gases to
  disperse safely.

26
Flue Gas Analysis

• We control the excess air by measuring the excess
  Oxygen in the Flue Gas
• The amount of excess air we need to know is what
  goes through the burners.
• The ideal sample point is at the exit of the
  firebox, as there should be little or no air
  leaks in this box.

27
Sample Points
28
On-Line Analysis

• The oxygen analyser is located in the stack.
• This analyser measures in the gas stream, so it
  indicates what we call a WET analysis since
  water vapour is present.
• Air leaks between the firebox and stack affect
  the readings.

29
Portable (off-line) Analysis

• Portable analysers can be used to check gases
  wherever a test point is available.
• They draw a sample through a cold line so water
  condenses out. The analysis is therefore known as
  DRY. This gives higher O2 readings but standard
  compensations can be made.
• Analysers can also measure CO and NOx for
  combustion efficiency and emissions checks.

30
Flue Gas Losses

• The gases passing out of the stack are above the
  ambient temperature, so they carry unused heat
  into the atmosphere.
• Increasing Flue gas temperature increases these
  losses.
• Increasing Excess air increases the amount of
  flue gases, giving even more loss.

31
Units Of Heat Flow

• British Thermal Unit BTU/hr
• Kilocalorie 1 KCal/hr3.938 BTU/hr
• KiloJoule 1 KJ/hr 0.9478 Btu/hr
• Kilowatt 1 KW 3,413 BTU/hr (1W 1J/s)

32
Gross and Net Heating Value

• Higher (Gross) Heating Value (HHV)
• The total heat theoretically available from
  combustion of a fuel.
• Lower (Net) Heating Value (LHV) the HHV less the
  latent heat used to convert the produced water to
  vapour.

33
Heating Values (Btu/Ft3)

• LHV HHV
• --------------------------------------------------
  ------
• Methane (CH4) 911 1012
• Ethane (C2H6) 1622 1773
• Propane (C3H8) 2322 2524
• Butane (C4H10) 3018 3271
• Hydrogen (H2) 275 325
• Carbon Monoxide (CO) 321 321

34
Wobbe Index

• This is a factor used in the design of Premix
  Burners only.
• It is based on Calorific Value and Density.
• If 2 gases have the same Wobbe index they should
  work equally well in the same premix burner.

35
Products Of Combustion

• Water Vapour - H2O
• Carbon Dioxide - CO2
• Sulphur Dioxide - SO2, SO3
• Carbon Monoxide - CO
• Unburned Hydrocarbons - UBC
• Nitrogen Oxides - NO, NO2

36
Flame Speed

• Another important factor in Combustion is the
  Flame Speed
• Each gas burns in air at a particular speed under
  reference conditions
• A stable flame is produced when the Flame Speed
  and gas/air mixture velocity correspond

37
Typical Flame Speeds (ft/sec)
Methane 1.48
Ethane 2.30
Propane 2.78
Butane 2.85
Hydrogen 9.30
Carbon Monoxide 1.70
38
Other Gas Characteristics

• All fuel gases will burn within a mixture range
  both below Stoichiometric and above
  Stoichiometric.
• The flammability range varies between gases,
  and is another indicator of how easily a gas will
  burn.
• Gas density affects burner design as heavier
  gases have higher pressure drops though gas jets.

39
So why have burners?
40
Basic Objects of a Burner

• The burner must mix the fuel and the air
  effectively to ensure complete combustion.
• The flame must be stabilised in a fixed position
  so that its heat can be absorbed effectively.
• The flame shape must be controlled to suit its
  working environment.

41
Process Heater Burners
42
Basic Burner TypesNatural Draught

• Premix
• Raw Gas (Nozzle Mix)
• Combination Oil Gas

43
Natural Draught

• Air is pulled through the burner by draft created
  by the heat in the furnace and stack (explained
  in a later section).
• Since air velocity is low we need to use the
  energy in the gas (typically at 1 barg) to
  improve the gas/air mixing.
• We have 2 basic ways we do this.

44
Premix Burners

• Fuel pressure drop occurs in the gas jet.
• Gas velocity in venturi induces part of the air
  so air flow adjusts with gas flow.
• Fuel and primary air mix before the nozzle.
• Secondary air mixes in burner throat.
• All domestic gas burners are premix, including
  cooking appliances.

45
Basic Burner TypesPre-Mix Heater Burner
GAS NOZZLE
46
Pre-Mix Burner Advantages

• Large fuel gas discharge orifice.
• Large ports in firing nozzle.
• Small flame volume.
• Automatic variation of air flow with varying fuel
  rates.

47
Premix Burner Disadvantages

• Can only accept small variations in gas quality
  without adjustment (n.b. unless Wobbe Index is
  maintained)
• Limited turndown.
• Difficult to adapt for combination gas/oil firing
  (but not impossible)
• Maintenance more difficult.
• Hard to reduce NOx.

48
Raw Gas Burners(Nozzle Mix)

• Gas and air are kept separate until discharged
  into the combustion zone.
• Fuel pressure drop occurs at the combustion zone.
• The energy in the gas helps mix fuel and air.

49
Basic Burner TypesNozzle Mixing Gas Burner
GAS NOZZLE
BURNER THROAT
FLAME HOLDER
50
Basic Burner Types

• Raw Gas

51
Zeeco Burner for United
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Test Burner Flame
54
Nozzle Mixing Gas Burner Advantages

• A high turndown ratio
• No possibility of flashback
• The ability to burn a wide variety of fuels with
  differing heating values
• Flame shape can be controlled as required by gas
  tip and tile design.
• Can be adapted many ways to reduce NOx

55
Nozzle Mixing Gas Burner Disadvantages

• Small fuel discharge ports
• "Large" flame volume
• Fuel/air ratio is dependent on operators

56
Raw Gas Combination

• Designed to burn gas and fuel oil either
  separately or together.
• Inner tile stabilizes oil flame with controlled
  primary air.
• Gas burners stabilize in secondary tile throat.
• Oil guns remove easily for cleaning while gas
  burners are in service.
• Gas burners can also be maintained while oil
  burners are in service.

57
Combination Natural Draught Gas and Oil Burner
GAS TIPS
PRIMARY TILE
58
Combination Burner Limitations

• Oil guns need frequent maintenance.
• Oil firing problems can cause fouling of gas
  tips.
• Total capacity of burner is set by air flow
  available, so firing gas and oil at the same time
  requires both fuels to be limited to give correct
  total Heat Flow.

59
Forced Draught Burners

• Basically similar to Natural Draught Raw Gas
  Burners (including Combination Oil/Gas Burners).
• Higher air velocities give better mixing and
  smaller flames.
• Air can be preheated, using various types of heat
  exchanger.
• Flames are hotter, giving higher rates of heat
  transfer.

60
Gas pilots

• Most process burners use a pilot to provide the
  basic source of ignition.
• Pilot is usually fully premixed.
• Pilot can be ignited manually or have a built-in
  spark ignition.
• Some pilots have flame rods to check flame is
  alight.

61
Pilot Burner
62
Burners are only part of the system
63
Furnaces

• A furnace is basically an insulated box lined
  with tubes containing the process fluid.
• We fire burners inside the box to heat the tubes
  by a mixture of radiation and convection heat
  transfer.
• There are many different furnace designs
  depending on the process application and the
  companies involved.
• The next 2 slides show some basic types.

64
Heater Types
65
Heater Types
66
Heater Parts
67
Burner Locations

• Depending on the heater layout burners may be
  installed up-fired, side-fired, end-fired and
  down-fired.
• Most heaters are up-fired, except for special
  types such as Ethylene Crackers and Reformers.

68
Heat Transfer(a) - Radiation

• In the firebox we get heat transferred initially
  by direct radiation from the flames to the tubes.
• Additional heat is radiated to the back of the
  tubes from the hot furnace walls.
• Radiant efficiency depends on the emissivity of
  the flame and of the tube surfaces, plus the
  temperatures of both.

69
Heat Transfer(b) - Convection

• Hot gases passing over tube surfaces heat the
  tubes mainly by Convection.
• Away from the Flames most heat is transferred by
  Convection.
• A Convection Bank is a section of the Heater
  where Radiation is insignificant, normally just
  below the Stack.

70
Process Flow

• In most heaters the coolest fluid is exposed to
  the coolest heat source.
• Fluid passes first through the Convection Tubes,
  where available.
• Fluid exits near the burners.

71
Furnace Draught

• Natural Draught burners depend on the air flow
  being created by the difference in air pressure
  between the inside of the heater and outside.
• The reason the pressure is different is that the
  air inside the heater is hotter than the air
  outside.
• Since hot air is lighter it rises and reduces the
  pressure inside the heater.

72
Furnace Draught

• Typically the temperature in a firebox is 500 -
  800C.
• At this temperature the draft increases by about
  2.5 mm water for every 3 metres of firebox
  height.
• If we have a convection section we need more
  draught above it to overcome the pressure drop
  through the tube bank.

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Where Draught comes from
10ft column of cold air 0.15w.g.
10ft column of air at 1000degF 0.05w.g.
DRAUGHT 0.1 /2.5 m.m.
74
Furnace Draught

• The temperature in the stack is lower, so we need
  more stack height to give us the required
  draught.
• The next chart shows what happens in our heater
  with a convection bank and a stack damper

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More on Draught

• We need just enough air to burn our fuel
  properly.
• We do not want any air to get in except through
  the burners.
• Any air which does not pass through the burners
  just absorbs some of the heat available and
  throws it away up the stack.

77
Even more on Draught

• We need to keep draught negative all the way
  through the heater.
• If we get a positive draught then hot gases will
  find small holes and make them bigger.
• The critical point is usually at the top of the
  firebox look at the chart again.
• Many heaters have alarms for positive pressure.

78
Smallest Draught
79
Heater Tuning
80
Before Tuning

• Before tuning make a full check of the burner
  conditions.
• Ensure air doors are open equally and gas valves
  open completely.
• Check flame appearance / stability. Close all
  peep doors.
• Keep in Radio touch with panel operators.

81
Heater TuningDraught Calculation / Setting

• For a typical heater as in the sketch we should
  have about 2 mm draught at the arch.
• If the heater is 10 metres high we can expect an
  additional 8-9 mm at the floor
• This gives us 12 mm total.
• Burners should have been designed for slightly
  less than this theoretical draught, so we close
  the air doors to control the excess air through
  the burners.
• After we close the air doors we may need to
  adjust the stack damper to maintain 2 mm at the
  arch.
• We check O2 and draught and repeat adjustments
  until we get both figures correct.

82
HEATER ADJUSTMENT FLOW CHART
TARGET DRAFT 1 to 3 mm water
TARGET OXYGEN 2 3
START

CHECK DRAFT
LOW
HIGH
CHECK O2
CHECK O2
TARGET
LOW
HIGH
HIGH
LOW
CLOSE STACK DAMPER
OPEN AIR REGISTERS
CLOSE AIR REGISTERS
OPEN STACK DAMPER
RETURN TO START
RETURN TO START
CHECK O2
HIGH
LOW
ON TARGET
CLOSE AIR REGISTERS
OPEN AIR REGISTERS
RETURN TO START
RETURN TO START
GOOD OPERATION
83
HEATER ADJUSTMENT FLOW CHART
TARGET DRAFT 1 to 3 mm water
TARGET OXYGEN 2 3
START

CHECK DRAFT
TARGET
CHECK O2
HIGH
LOW
ON TARGET
CLOSE AIR REGISTERS
OPEN AIR REGISTERS
RETURN TO START
RETURN TO START
GOOD OPERATION
84
Heater TuningDraught Control General

• There are differences in approach depending on
  the type of burner, if the heater has a
  convection bank, and if there is a stack damper.
• If the burners are in a plenum and have their own
  air doors then we have an extra adjustment point.
  In such cases the individual burner air doors
  should be fixed open unless a burner is stopped,
  when they should be shut.
• Sinclair has almost every combination possible,
  so we have to look at all the possibilities.

85
Heater TuningDraught Control Raw Gas Burners

• Basically the Flowchart given applies to this
  type of burner.
• If there is no stack damper we need to monitor
  the arch Oxygen assuming that the furnace leaks
  have been fixed.
• We must still check that Draught is negative as
  putting too much air through burners can cause
  draft to go positive at the arch.

86
Heater Start-up

• During start-up draught is low as temperatures
  are low.
• Pilots self-inspirate so should work normally.
• High excess air is used to control furnace
  temperature rise.
• Individual Burner light-off should be done with
  air doors nearly closed, so gas lights more
  smoothly.
• Increase air opening slowly so burner heats up
  quickly and flame can stabilize properly.

87
Heater TuningFuel Gas Valves

• Valves fitted upstream of each burner are for
  isolation only.
• The only time a valve should not be opened fully
  is during light-off.
• If any valves are not completely open then the
  burners are not all firing at the same rate.
• Gas pressure trip settings are established on the
  basis that valves are fully open.
• If a trip setting interferes during normal
  operation it should be checked and may be
  changed, provided that the burner stability is
  checked at the revised setting.
• If an individual burner gives a problem with the
  valve open then the problem should be
  investigated. On many burners there are small gas
  jets which can plug easily and will affect flame
  stability.

88
What can go Wrong?

1. O2 falls too low Temperature control is lost as
   fuel does not burn flames search for air and
   blow back through registers Puffing CUT
   BACK ON FUEL FIRST
2. Draught goes positive gas leaks out of any gaps
   and causes damage, but O2 still looks OK. Heaters
   should have an alarm for high pressure.

89
Heater TuningFlue Gas Analysis

• In general a good target for excess Oxygen is 3
• We need this level in the firebox that should
  mean we are getting the right amount of air
  through the burners.
• Gas samples taken above convection banks include
  any air which leaks in around the tubes.
• These leaks should always be minimised as they
  affect the convection bank efficiency.
• In serious cases the leaks can exceed our 3
  target, so we could actually be firing below
  stoichiometric.

90
Heater TuningFlue Gas Analysis

• One way to check what is really happening is to
  also measure CO levels.
• Typically it is safe to run with a maximum of 50
  ppm of CO in flue gases.
• Older burners will start producing CO at around
  2 excess Oxygen, so we have a good indication of
  the actual excess air through the burners.
• On-line CO analysers allow burners to be run
  safely right down to their minimum achievable
  levels of excess air.

91
Heater TuningSummary

• We are aiming to have 3 excess oxygen in the
  firebox.
• We need all the burners in each heater to be
  operating with the same amount of fuel and air.
• This means air doors set equally, gas valves full
  open, and clean gas tips.
• If there is a stack damper, it should normally be
  set to give a draft of 0.1 maximum at the heater
  arch.
• Some heaters may still need more draft to get
  enough air through the burners.

92
Nitrogen Oxides (NOx) Formation
93
What is the Problem?

• All combustion processes produce some Nitrogen
  Oxides
• In the atmosphere these oxides can form Nitric
  acid and fall as acid rain
• They react with other gases and sunlight,
  producing ozone and smog

94
NOx Formation in Combustion

• In ambient conditions Nitrogen is an inert gas

95
NOx Formation in Combustion

• In hot flames we get
• Thermal NOx
• Fuel NOx

96
Thermal NOx
Created from atmospheric Nitrogen Formation
controlled by the breaking of N2 molecules to
reactive nitrogen atoms by the supply of heat.
The N atoms then react with available Oxygen to
form NO. Thermal NOx formation rate is
dependent on peak flame temperature and oxygen
availability.
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Controlling ReactionsThermal NOx
98
NOx definitions

• The primary component formed in a flame is NO.
• In the atmosphere this NO converts to NO2, which
  is the harmful form.
• We define limits as NOx, where all measured
  levels are treated as having converted to NO2.
• Fired Heater limits are always expressed as the
  equivalent levels of NOx at 3 excess Oxygen.
• EPA bases limits on lbs/million Btu rather than
  on percentages.

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Fuel NOx

• Some fuels contain fixed Nitrogen as compounds.
  Liquids and Solids contain more of these than
  most gases.
• These compounds break down in the combustion
  process and release the Nitrogen in a form which
  reacts easily to form NOx.
• Nitrogen as a gas component is not significant.
• NOx levels increase in direct proportion to the
  fixed Nitrogen in the fuel.
• NOx reduction techniques are also effective in
  reducing Fuel NOx.

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How can we reduce NOx?

• Reduce the Flame Temperature
• Reduce the Oxygen available
• Flue Gas Treatment

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Reducing Flame Temperature

• Slow down fuel / air mixing
• Inject cooler inert gases into the flame (steam
  or recycled flue gas)
• Increase the excess air
• Reduce air below stoichiometric
• Unfortunately all of these things conflict with
  our requirement to get maximum heat from the
  flames to the process

103
Reducing Available Oxygen

• Reduce the excess air
• Inject Inert gases into the flame to reduce the
  oxygen concentration available (recycled flue gas
  again)

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Low NOx Burners

• Staged Air
• Staged Fuel Low NOx
• Internal Flue Gas Recirculation
• Combination of Features

106
Staged Air Burner Features

• Sub-Stoichiometric Primary Combustion
• Presence of CO and H2
• Flame Cooling in Second Stage
• Works with Gas or Oil

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Staged Air Burner
108
Staged Air Burners Disadvantages

• Long Flames
• Complicated Air Adjustment
• Fuel Composition affects Performance
• Higher Excess Air Required
• Limited NOx Reduction

109
Staged Fuel Low NOx Burners

• Features / Advantages
• Disadvantages

110
Staged Fuel Burner Features

1. Two Stage Fuel Injection
2. Good Heat Transfer from Secondary Flame
3. Combustion Product Injection
4. "Compact Flame
5. Tolerates gas variations

111
1. Two stage fuel injection

• Primary gas burns with high excess air, cooling
  the flame
• Secondary gas mixes into flame above the burner,
  where oxygen level is low, so burns at a lower
  temperature

112
2. Heat Transfer from Secondary Flame

• Secondary Flame burns slowly above the burner
• Maintains uniform radiant Heat transfer further
  up the furnace

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3. Combustion Product Injection

• Secondary gas pokers are above the burner tile
• They induce furnace gases into the Secondary
  flame
• Oxygen is reduced but temperature increases,
  maintaining flame dimensions well

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4. Compact Flame

• High excess air primary flame gives strong core
  to flame
• Controlled secondary mixing and recirculation
  keeps flame relatively compact

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5. Tolerates Gas variations

• Balance of primary to secondary gas is fixed
  (typically 30-40 primary)
• Stoichiometry is not affected by fuel properties

116
Staged Fuel Burner
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Staged Fuel Burner Disadvantages

• Turndown is limited
• Stability sometimes a problem
• Small Gas Port Size
• Effectiveness of NOx reduction depends on fuel
  properties

120
Low Emission Burners

• Combination of Staged Fuel and Internal Flue Gas
  Recirculation

121
Low Emission Burner

• Based on Staged Fuel Burner
• Primary Gas induces furnace gases into Primary
  Flame
• Zoning of air in burner throat gives high
  stability
• Self compensates for gas changes

122
Internal Flue Gas Recirculation
Recycle Gas
Flue Gas
Burner
Recycle Gas
Furnace
123
Flue Gas Recirculation

• Hot flue gases rise fast up the centre of the
  furnace
• Cooler gases travel down wall around tubes to the
  floor
• Gases have only Excess Oxygen and relatively low
  temperature
• Lighter fuel gases run at higher pressure /
  velocity, maintaining recirculation levels

124
Flame Retention

• Primary gas induces inert gas into the burner
  throat.
• Flame holder mixes limited air with fuel and
  recirculated gases to give a fuel-rich zone
  around the outside of the flame holder for high
  stability
• Balance of air passes through centre of flame
  holder to mix into the primary flame

125
Staged Fuel

• Staged fuel induces more inert gases into flame
• Mixing is delayed by the fuel-rich zone on the
  outside of the primary flame

126
Internal Flue Gas Recirculation Burner
127
Relative Process Heater Burner NOx Levels for
Conventional and Low NOx Burners

• Conventional - 0.12 NOx/MMBtu, 100 ppmv
• Staged Fuel - 0.06 NOx/MMBtu, 50 ppmv
• Low Emission - 0.03 NOx/MMBtu, 25 ppmv

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Boustead International Heaters.
END