ME529 Combustion and Air Pollution
Topic 16. Gas Turbines
Gas turbines are characterized by high power output from an internal combustion engine
that is lighter and smaller than those of reciprocating engines of comparable power.
Hence, the original application of the gas turbine was in aircraft. The high power output
makes gas turbines useful for electricity generation. They are the preferred engine to use
in applications requiring more than 10MW of power. Large units operate in the 100 to
200MW range.
The Brayton (or Joule) Cycle describes the combustion in turbines thermodynamically.
For engineering applications, air standard calculations are used regardless of the actual
fuel because of the large air-fuel ratio needed to keep temperature below metallurgical
limits.
Fuels: aviation kerosene; in stationary systems, a wide range including gasified coal
(synfuel), heavy oils, natural gas, propane
Combustion air enters via a centrifugal compressor. Part of the air is used in the primary
combustion zone, into which fuel is sprayed and burns intensely. Gas volume increases
with combustion. As gases pass at high velocity through the turbine, work is generated
beyond that needed to operate the compressor. The excess work can turn a shaft (to drive
an electrical power generator) or released at high velocity to provide thrust for an aircraft.
1
500
450
2
Brayton Gas
3
Turbine Cycle
P[i] [Pa]
400
350
300
250
200
150
50
0
4
1
100
10
20
30
40
50
60
70
v[i] [m^3/kmol]
1400
Brayton Gas Turbine Cycle
3
T[i] [K]
1200
1000
4
800
600
2
400
1
200
150
155
160
165
170
175
180
185
s[i] [kJ/kmol-K]
1 – 2: isentropic compression
2 – 3: constant pressure heat addition
3 – 4: isentropic expansion
4 – 1: constant pressure cooling
2
190
195
200
Gas turbine characteristics:
1. Operates in steady flow, as opposed to reciprocating
2. Compressor raises pressure from 1 to 5 atm, as high as 30 atm
3. Short residence time (milliseconds) for droplet evaporation and combustion
4. Flame stability accomplished by swirl or obstructions
5. Inlet temperature is limited to 1500K because of thermal expansion of the turbine
blade and the blade material’s temperature limits.
6. To keep the temperature of the ‘combustion can’ cool, additional air is injected
through the walls
7. Large single chambers are appropriate for heavy fuels, which require long
residence times to burn completely. Small, multi-chamber designs are better
suited for gaseous fuels and light distillates.
8. Staged combustion can be used to control NO formation. Steam or water
injection can also be used. Post-combustion control is achieved via SCR. Steam
can also be injected for a power boost, and for cooling.
The combustion conditions vary widely inside the turbine. At low loads, φprimary < 0.5.
Once ignited, the combustion is rapidly quenched. NOx emissions are low, but CO and
unburned hydrocarbon emissions are high. Dilution air can lower φ to 0.15 at the turbine
inlet and temperature to 700 K.
At high loads, φprimary can range from 0.80 to 1.0. Combustion is maintained for a longer
time. Hence, there is more time for NOx formation but there is also better oxidation of
CO and HCs - and better soot formation because of imperfect mixing. Once a soot
particle forms, it is difficult to burn out. Dilution air can lower φ to 0.3 at the turbine
inlet; temperature must be below ~1500 K.
3
Gas turbine engine emissions – variation with engine power
(Flagan and Seinfeld, 1988).
The very short residence time in the combustor requires droplet evaporation, mixing and
reaction to occur very quickly. The residence time affects pollutant formation. At low
load, NOx emissions are low because of dilution and rapid quenching – and these same
factors cause significant CO and HC emissions. At high load, gases remain hot longer
and dilution is lower – more NO is formed (and more soot due to poorer mixing), but
there is better oxidation of CO and HCs.
NO2 levels are comparable to NO levels in gas turbines. This characteristic contrasts
with most other steady-flow combustors.
4
NO and NO2 emissions from a gas turbine power generating unit
(Johnson and Smith, 1978).
To control emissions from gas turbine engines:
1. Improve atomization and mixing of liquid fuels.
2. Lower peak combustion temperature at high load by lowering the equivalence
ratio in the primary zone – in combination with improved mixing.
3. Staged dilution – first dilute products to an equivalence ratio where CO and HC
oxidation is still rapid and allow time for the oxidation to finish before further
dilution. Staging dilution is limited by the combustor volume and the need to cool
the can walls.
4. Optimize performance at high and low loads – use a number (100’s) of small
burners that operate at near optimum level at all time. Vary the load by varying
the number of burners operating.
5. Pre-mix fuel and air – be wary of flame stabilization problems.
6. Staged combustion with additional fuel injection in the secondary zone.
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7. Divert some of the fuel/air for catalytic ignition and inject remaining fuel into the
combustion products.
8. Other ideas?
Appendix A.
Air standard Brayton cycle expressions:
State 1.
v
p1  p 4  p 2  1
 v2
v
T1  T4  1
 v4
T
v1  v 4  1
 T4
k
k

 T  k 1
  p 2  1 

 T2 

v
  T2  2

 v1



k 1
p 
 T2  1 
 p2 
1
k

p 
T
  v 2  2   v 2  2

 p1 
 T1
v 
 T  k 1
p 2  p3  p1  1   p1  2 
 v2 
 T1 
v 
T2  T1  1 
 v2 
k 1
v
 T3  2
 v3
1
 p k
T
v 2  v1  1   v1  1
 p2 
 T2

p 
  T1  2 
 p1 

1
v 
T 
p3  p 2  p 4  4   p 4  3 
 T4 
 v3 
v 
v 
T3  T2  3   T4  4 
 v2 
 v3 
T
v3  v 2  3
 T2
p

  v 4  4

 T3
k 1



k
k 1
p 
 T4  3 
 p4 
1
k
T
  v 4  4

 T3
k
k
State 4.
k 1
k
T
 k 1
  v3  2

 T3
k
State 3.
1
k 1
k
k
State 2.



k 1
k
 T  k 1
v 
p 4  p1  p3  3   p3  4 
 v4 
 T3 
6
k 1
k
1
 k 1


v 
T4  T3  3 
 v4 
k 1
p 
 T3  4 
 p3 
1
 p k
T
v 4  v3  3   v3  3
 p4 
 T4
k 1
k
v 
 T1  4 
 p1 
1
 k 1
T
  v1  4

 T1



Work and heat transfer expressions:
qin  C P T3  T2   h3  h2
qout  C P T1  T4   h1  h4
Wturbine  C P T4  T3   h4  h3
Wcompressor  C P T2  T1   h2  h1
th 
Qin  Qout Wturbine  Wcompressor h3  h2   h4  h1 


Qin
Qin
h3  h2
If the isentropic efficiency is less than 100% for either or both the compressor and the
turbine, the actual enthalpies must be used to calculate work, heat and thermal efficiency.
h  h1

h2  h1  2
 s ,compressor

h4  h3   s ,turbineh3  h4 
References
Flagan, R. C., and Seinfeld, J. H., Fundamentals of Air Pollution Engineering, Prentice
Hall, NY,1988.
Johnson, G. M., and Smith, M. Y., “Emissions of Nitrogen Dioxide from a Large GasTurbine Power Station,” Combustion Science and Technology, 19, 131-160, 1978.
Lefebvre, A. H., Gas Turbine Combustion, Taylor and Francis, PA, 1983.
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