Combustion Fundamentals

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7/21/2019 Combustion Fundamentals

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Dr. Mike Klassen, P.E.

Combustion Science &

Engineering, Inc.MKlassen@csefire.com

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

NOx formation pathways

• Zeldovich reaction (thermal NOx)

• N2O reaction

• Prompt NOx

• Fuel NOx

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

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?

Impact of GT Compressor Discharge

Conditions on NO Formation

N2O Pathway

Prompt NOx

Impact of Fuel Nitrogen

Content

NO2 Formation

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

“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

Regions of High CO in a GT

combustor

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

Optimal Combustor Design

• Now we will examine how to design a

combustor to minimize both NOx and

• But first, we need to go over somecombustion fundamentals

Important Concepts

• Air/Fuel Ratio (A/F)

• Stoichiometric A/F Ratio (A/F)ST

• Equivalence Ratio (ø)

• Adiabatic Flame Temperature

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)

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

Stoichiometric Combustion of

Methane• CH4 + 2x(O2 + 3.77N2) => CO2 + 2H2O +

7.54N2

• (A/F)ST = 2x(1+3.77)/1 =

9.54 ft

-air/ft

• (A/F)ST = (2x32+2x3.77x28)/16 =

17.2 lbs-air/lb-CH4

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

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!!!)

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)

How will Adiabatic Flame T

change as Ø changes?

Ø10 2

(no fuel) (stoich.) (excess fuel)

Adiabatic Flame T is

maximized around Ø = 1

Thermal NOx formation is also

maximized around Ø = 1

Ø vs NOx and AFT

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

AIRFLOW

2900°F

1870 K

40%AIRFLOW

30%4100°F

2530 K

Conventional

Lean-Premixed

Turbine

Diffusion vs Pre-Mixed

©Solar Turbines Incorporated

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

Special Considerations for

Pre-mixed Flames

• Good Mixing

• Flammability limits

• Blow-off & Flashback

• Dynamic Instabilities or “Humming”

• Auto-Ignition

• Part-Load Operation

Ø = 1.68

Ø = 1.0

Ø = 0.50

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

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)

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)

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)

Pre-mixed Combustor with Diffusion Pilot Flame

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

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

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.

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

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

Fuel-Staging Example

Source: GE Report 3568E

Annular Combustor

Fuel Staging

Source: GE Report 3568E

Turbines

with Air-

Staging

(combustor by-pass

valve)

Combustion Fundamentals

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