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7/21/2019 Combustion Fundamentals
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Combustion Fundamentals
Dr. Mike Klassen, P.E.
Combustion Science &
Engineering, [email protected]
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What is NOx?
• NOx stands for Nitrogen Oxides
– NO, N2O, NO2, etc.
• Some NOx will always be formed when fuel is
burned in air • There are several ways that NOx is formed
– Most important path is the reaction of the N2 and
O2 to form NO – the Zeldovich reaction
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NOx formation pathways
• Zeldovich reaction (thermal NOx)
• N2O reaction
• Prompt NOx
• Fuel NOx
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Zeldovich Reaction
• Reaction 1: O + N2 => NO + N
• Reaction 2: N + O2 => NO + O
• Net reaction: N2 + O2 => 2NO
• Reaction rate increases exponentially
with flame temperature
• Often called “thermal” NOx
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Impact of GT Conditions on
Thermal NOx formation
• The Zeldovich reaction also increaseswith the square root of pressure, so gasturbine designers are faced with a truedilemma
• Higher pressure ratios and higher firingtemperatures yield higher efficienciesbut also produce more thermal NOx
• Which would you choose?
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Impact of GT Compressor Discharge
Conditions on NO Formation
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N2O Pathway
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Prompt NOx
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Impact of Fuel Nitrogen
Content
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NO2 Formation
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How is CO formed?
• In a flame the carbon atoms in a fuel will
react with O2 in the air to form CO2
• But this occurs in a “two step” process
– Step 1: one oxygen atom reacts with acarbon atom to form CO
– Step 2: another oxygen atom reacts with
CO to form CO2
• Without “step 2” you get CO emissions
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“Quenching” causes CO
• Step 2 will not happen if the combustionproducts are “quenched” or cooled
prematurely
– Typically this happens in regions where cooling air
is mixed into the flow
• Step will also not happen if there is a
shortage of O atoms – fuel-rich combustion
– Typically not encountered in gas turbines
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Regions of High CO in a GT
combustor
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NO & CO
• In general, hotter firing temperatures
produce more NOx and less CO
• Conversely, lower firing temperatures
produce more CO and less NOx• Also, longer residence time in the flame
zone gives more time for NOx form and
for CO to be consumed
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Optimal Combustor Design
• Now we will examine how to design a
combustor to minimize both NOx and
CO
• But first, we need to go over somecombustion fundamentals
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Important Concepts
• Air/Fuel Ratio (A/F)
• Stoichiometric A/F Ratio (A/F)ST
• Equivalence Ratio (ø)
• Adiabatic Flame Temperature
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Air/Fuel Ratio
• Ratio of air flow to fuel flow in a flame
• Can be a volume (or mole) ratio or a
mass ratio (lbs-air/lbs-fuel)
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Stoichiometric A/F
• The air/fuel ratio that results in all of the
fuel and oxygen being consumed
• In air, for every mole (or molecule or
cubic foot) of O2 there are 3.77 moles
(or molecules or cubic feet) of N2
• So after all the O2 has been consumed
you still have a lot of N2 in the exhaust
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Stoichiometric Combustion of
Methane• CH4 + 2x(O2 + 3.77N2) => CO2 + 2H2O +
7.54N2
• (A/F)ST = 2x(1+3.77)/1 =
9.54 ft
3
-air/ft
3
-CH4
• (A/F)ST = (2x32+2x3.77x28)/16 =
17.2 lbs-air/lb-CH4
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Equivalence Ratio (Ø)
• Ø = (A/F)ST /(A/F)actual
• Ø = 1, stoichiometric combustion
• Ø < 1, fuel-lean combustion (excess air)
• Ø > 1, fuel-rich combustion (excess
fuel)
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Adiabatic Flame Temperature
• The temperature the products ofcombustion will reach if there is no heatloss from the flame zone
• Function of (A/F), fuel type, and thetemperature of the reactants
• For CH4, with Ø = 1 & 59°F reactants, AFT = 3565°F (note: titanium melts at3036°F!!!)
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Combustion Properties of
some Gaseous Fuels
38322.5CarbonMonoxide, CO
382434.2Hydrogen, H2
359015.6Propane, C3H8
357916.1Ethane, C2H6
356517.2Methane, CH4
Adia. Flame
Temp., °F
(A/F)ST
(lb/lb)
Fuel
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How will Adiabatic Flame T
change as Ø changes?
AFT
Ø10 2
(no fuel) (stoich.) (excess fuel)
CDT
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Adiabatic Flame T is
maximized around Ø = 1
Thermal NOx formation is also
maximized around Ø = 1
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Ø vs NOx and AFT
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Optimal Conditions
• To minimize NOx, must operate in fuel-
lean conditions (Ø < 1)
• Cannot be too lean or CO emissions will
become too high
• Cannot operate fuel-rich because
unburned hydrocarbons and CO will be
too high
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AIRFLOW
60%
FUEL
2900°F
1870 K
40%AIRFLOW
30%4100°F
2530 K
70%
FUEL
Conventional
Lean-Premixed
Same
Turbine
Inlet
Temp
Diffusion vs Pre-Mixed
©Solar Turbines Incorporated
©Solar Turbines Incorporated
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Diffusion vs Pre-Mixed Flames
• Fuel & air are separatebefore flame zone
• Fuel burns over rangeof fuel/air mixtures
• Burn rate depends on
rate of fuel & air supply
and degree of mixing
• “Over-ventilated” flame
temps near max• No flammability limits
• Fuel and air are well-mixed before flamezone
• Fuel burns at specificair/fuel ratio, Ø
• Flame temperaturevaries as function of Øand fuel type
• Flammability limits are a
function of Ø and fueltype
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Special Considerations for
Pre-mixed Flames
• Good Mixing
• Flammability limits
• Blow-off & Flashback
• Dynamic Instabilities or “Humming”
• Auto-Ignition
• Part-Load Operation
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Ø = 1.68
Ø = 1.0
Ø = 0.50
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Flammability Limits &
Materials Limits• The maximum adiabatic flame temperature a
turbine blade can withstand corresponds to Ø
< 0.5 (typically 0.4 for modern designs)
• The lower flammability limit of CH4 is at Ø =0.5 (and CO emissions would be too high at
that condition also)
• Conclusion: some air must by-pass the flame
zone even in a pre-mixed combustor
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Blow-off and Flashback
• If the “flame speed” does not match theflow speed of the reactants, the flamefront will move
• If flame speed is too high, you can getflashback (flame moving upstream intofuel nozzle)
• If flame speed is too low, you can getblow-off (flame pushed downstream)
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What determines flame
speed?
• Flame speed is determined by the
combustion reaction rates and those
rates depend on:
– Equivalence Ratio, Ø (there is it again!) – Fuel type
– Flow regime (laminar or turbulent)
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Flame speed vs fuel type
42.9Carbon Monoxide, CO
291.2Hydrogen, H2
42.7Propane, C3H8
44.2Ethane, C2H6
37.3Methane, CH4
Max. flame speed
(cm/s)
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Pre-mixed Combustor with Diffusion Pilot Flame
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Dynamic Instabilities or
“Humming”
• Combustion instabilities occur when a
forcing mechanism drives pressure
fluctuations at the resonant frequencies
of the the combustion chamber.
• Combustion instabilities can lead to:
– Excessive wear and eventually component
failure – Increased emissions of NOx and UHC
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What Can Cause Humming?
• Combustion instabilities can be caused bynumerous mechanisms, but are generallyrelated to the coupling of heat release withpressure (or acoustic) waves. Amplitudes
and frequencies of the instabilities candepend on: – Inlet air and fuel temperatures
– Fuel Type
– Fuel Injector geometry
– Combustor geometry
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Auto-Ignition
• When fuel and air are pre-mixed, one
always has to worry about the mixture
igniting before it reaches a spark (or
flame)• The temperature above which a fuel-air
mixture can spontaneously ignite is
called the auto-ignition temperature.
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Part-Load Operation
• As gas turbines reduce load, the turbine
rotor inlet temperature decreases, which
means that the overall fuel-air ratio must
decrease• This poses problems for pre-mixed flames
because of the lower flammability limit
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Part-Load Strategies
• Some manufacturers only operate in pre-
mixed mode over a limited load range
– At low loads the combustion switches to a
diffusion flame (with higher emissions)
• Some manufacturers extend the load range
of pre-mixed operation by air-staging or
fuel-staging
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Fuel-Staging Example
Source: GE Report 3568E
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Annular Combustor
Fuel Staging
Source: GE Report 3568E
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Gas
Turbines
with Air-
Staging
(combustor by-pass
valve)