Transcript No Slide Title

Welcome to
Fired Heater Training!
The course is designed to give you
some background information
needed to operate a fired heater
Agenda
1.
2.
3.
4.
5.
6.
7.
8.
Introduction
Air/Fuel Ratios
Fundamentals of Burners
Fundamentals of Furnaces
Furnace Tuning and Use of Analyzer
NOx and Advanced Burner Design
Field Tuning of Heaters
Q & A and Wrap-up
Heater and Burner Operation
Course Objective
• To ensure that everyone fully understands
how burners and heaters work.
Course Topics
• Combustion Essentials
• Basic Burner Designs
• Furnace Types
• Draught
• Heater Tuning
• Low NOx Burner Designs
Combustion Essentials
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”
What Is Required For
Combustion?
Three Elements:
Fuel
Air
Source of Ignition
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.
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.
Chemical Formulas
• In formulas we will use the following basic
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•
•
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•
components
Carbon = C
Hydrogen = H2
Oxygen = O2
Nitrogen = N2
Water = H2O
Carbon Dioxide = CO2
Methane = CH4
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
Examples Of Combustion For
Typical Fuel Components
with Oxygen
C + O2  CO2
2H2 + O2  2H2O
S + O2  SO2
Heat
• Where does the heat come from?
Heat
+
C + O2
CO2
But we don’t 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
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
Example - Combustion Of
Methane
CH4 + 2O2 + 7.5N2
CO2 + 2H2O + 7.5N2
+ Heat
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.
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.
Stoichiometric Air Example
CH4 + 2O2 + 7.5N2
CO2 + 2H2O + 7.5N2 + Heat
Note – no Excess Oxygen in Flue Gas
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/(1+2+9+0.4)=0.032 or 3.2%O2 in flue
gases (wet)
0.4/(1+9+0.4)=0.038 or 3.8%O2 (dry)
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)
Some Dangers of operating
below 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.
Heater Control Problems with SubStoichiometric 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.
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.
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.
Sample Points
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.
Portable (off-line) Analysis
• Portable analysers can be used to check gases
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•
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.
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.
Units Of Heat Flow
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British Thermal Unit BTU/hr
Kilocalorie
1 KCal/hr=3.938 BTU/hr
KiloJoule
1 KJ/hr = 0.9478 Btu/hr
Kilowatt
1 KW = 3,413 BTU/hr
(1W = 1J/s)
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.
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
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.
Products Of Combustion
• Water Vapour - H2O
• Carbon Dioxide - CO2
• Sulphur Dioxide - SO2, SO3
• Carbon Monoxide - CO
• Unburned Hydrocarbons - UBC
• Nitrogen Oxides - NO, NO2
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
Typical Flame Speeds (ft/sec)
Methane
1.48
Ethane
2.30
Propane
2.78
Butane
2.85
Hydrogen
9.30
Carbon Monoxide
1.70
Other Gas Characteristics
• All fuel gases will burn within a mixture range
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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.
So
why
have
burners?
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.
Process Heater Burners
Basic Burner Types
Natural Draught
• Premix
• Raw Gas (Nozzle Mix)
• Combination Oil & Gas
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.
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.
Basic Burner Types
Pre-Mix Heater Burner
GAS NOZZLE
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.
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.
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.
Basic Burner Types
Nozzle Mixing Gas Burner
BURNER
THROAT
GAS NOZZLE
FLAME
HOLDER
Basic Burner Types
Raw Gas
Zeeco Burner for United
Test Burner Flame
Nozzle Mixing Gas Burner Advantages
• A high turndown ratio
• No possibility of flashback
• The ability to burn a wide variety of fuels with
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differing heating values
Flame shape can be controlled as required by
gas tip and tile design.
Can be adapted many ways to reduce NOx
Nozzle Mixing Gas Burner
Disadvantages
• Small fuel discharge ports
• "Large" flame volume
• Fuel/air ratio is dependent on operators
Raw Gas Combination
• Designed to burn gas and fuel oil either
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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.
Combination Natural Draught
Gas and Oil Burner
PRIMARY
TILE
GAS TIPS
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.
Forced Draught Burners
• Basically similar to Natural Draught Raw Gas
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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.
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.
Pilot Burner
Burners are only part of the
system
Furnaces
• A furnace is basically an insulated box lined with
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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.
Heater Types
Heater Types
Heater Parts
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.
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.
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.
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.
Furnace Draught
• Natural Draught burners depend on the air flow
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•
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.
Furnace Draught
• Typically the temperature in a firebox is
500 - 800°C.
• 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.
Where Draught comes from
10ft
column of
air at
1000degF
=
0.05”w.g.
10ft
column of
cold air
=
0.15“w.g.
DRAUGHT = 0.1” /2.5 m.m.
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
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.
Even more on Draught
• We need to keep draught negative all the way
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•
•
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.
Smallest
Draught
Heater Tuning
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.
Heater Tuning
Draught Calculation / Setting
• For a typical heater as in the sketch we should have
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•
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.
HEATER ADJUSTMENT FLOW CHART
TARGET DRAFT
1 to 3 mm
water
TARGET
OXYGEN
2–3%
START
CHECK DRAFT
HIGH
LOW
CHECK O2
CHECK O2
TARGET
HIGH
LOW
HIGH
LOW
CLOSE STACK
DAMPER
OPEN AIR
REGISTERS
CLOSE AIR
REGISTERS
OPEN STACK
DAMPER
RETURN TO START
RETURN TO START
CHECK O2
HIGH
CLOSE AIR
REGISTERS
LOW
ON TARGET
RETURN TO START
OPEN AIR
REGISTERS
RETURN TO START
GOOD OPERATION
HEATER ADJUSTMENT FLOW CHART
TARGET DRAFT
1 to 3 mm
water
START
TARGET
OXYGEN
2–3%
CHECK DRAFT
TARGET
CHECK O2
HIGH
CLOSE AIR
REGISTERS
LOW
ON TARGET
RETURN TO START
OPEN AIR
REGISTERS
RETURN TO START
GOOD OPERATION
Heater Tuning
Draught Control – General
• There are differences in approach depending on the type
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•
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.
Heater Tuning
Draught Control – Raw Gas
Burners
• Basically the Flowchart given applies to this
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•
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.
Heater Start-up
• During start-up draught is low as temperatures
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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.
Heater Tuning
Fuel Gas Valves
• Valves fitted upstream of each burner are for isolation only.
• The only time a valve should not be opened fully is during
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•
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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.
What can go Wrong?
1. O2 falls too low – Temperature control is lost
2.
as fuel does not burn – flames search for air
and blow back through registers – “Puffing” –
CUT BACK ON FUEL FIRST
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.
Heater Tuning
Flue 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.
Heater Tuning
Flue Gas Analysis
• One way to check what is really happening is to
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•
•
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.
Heater Tuning
Summary
• 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.
Nitrogen Oxides (NOx)
Formation
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
NOx Formation in Combustion
In ambient conditions
Nitrogen is an inert gas
NOx Formation in Combustion
In hot flames we get
Thermal NOx
Fuel NOx
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.
Controlling Reactions
Thermal NOx
h
N 2  N  N
N  O2  NO  O
NOx definitions
• The primary component formed in a flame is NO.
• In the atmosphere this NO converts to NO2, which is the
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•
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.
Fuel NOx
• Some fuels contain ‘fixed’ Nitrogen as compounds.
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•
•
•
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.
How can we reduce NOx?
• Reduce the Flame Temperature
• Reduce the Oxygen available
• Flue Gas Treatment
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
Reducing Available Oxygen
• Reduce the excess air
• Inject Inert gases into the flame to reduce
the oxygen concentration available
(recycled flue gas again)
Low NOx Burners
• Staged Air
• Staged Fuel Low NOx
• Internal Flue Gas Recirculation
• Combination of Features
Staged Air Burner Features
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•
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•
Sub-Stoichiometric Primary Combustion
Presence of CO and H2
Flame Cooling in Second Stage
Works with Gas or Oil
Staged Air Burner
Staged Air Burners Disadvantages
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•
Long Flames
Complicated Air Adjustment
Fuel Composition affects Performance
Higher Excess Air Required
Limited NOx Reduction
Staged Fuel Low NOx Burners
• Features / Advantages
• Disadvantages
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
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
2. Heat Transfer from Secondary Flame
• Secondary Flame burns slowly above the
burner
• Maintains uniform radiant Heat transfer
further up the furnace
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
4. Compact Flame
• High excess air primary flame gives strong
core to flame
• Controlled secondary mixing and
recirculation keeps flame relatively
compact
5. Tolerates Gas variations
• Balance of primary to secondary gas is
fixed (typically 30-40% primary)
• Stoichiometry is not affected by fuel
properties
Staged Fuel Burner
Staged Fuel Burner Disadvantages
•
•
•
•
Turndown is limited
Stability sometimes a problem
Small Gas Port Size
Effectiveness of NOx reduction depends
on fuel properties
Low Emission Burners
Combination of Staged Fuel and
Internal Flue Gas Recirculation
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
Internal Flue Gas Recirculation
Recycle Gas
Flue Gas
Burner
Recycle Gas
Furnace
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
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
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
Internal Flue Gas Recirculation Burner
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
Boustead International Heaters.
END