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Microsoft PowerPoint - Gas cycles by lindayy

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Microsoft PowerPoint - Gas cycles

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									          ENGG1050
Gas turbines and combined cycles
               C&B 9-8, 10-9




One pass gas engines
 Characteristics
 – Burn a fuel
 – Compress air
 – Expand combustion
   gases
 Examples
 – Internal combustion
   engine
 – Diesel engine
 – Gas turbine
 C&B Chapter 9 (5th Edn)




                                   1
One pass gas engines
 “Ideal” model for the
 engine
 – Closed cycle
 – Ideal gas working fluid
   similar to air


 Examples of ideal
 cycles:
 – I.C.E. = Otto cycle
 – Diesel = Diesel cycle
 – Gas turbine = Brayton
                  cycle




Gas turbine cycle
 Working fluid = ideal gas
 4 steps:
 – Adiabatic, reversible
   compression A-B
 – Constant pressure heat
   transfer (combustion) B-C
 – Adiabatic, reversible
   expansion C-D
 – Constant pressure heat
   transfer (exhaust gas) D-A
                ( k −1) / k                  (k −1) / k
   TB ⎛ p B ⎞                  ⎛p ⎞                           TC
     =⎜     ⎟                 =⎜ C ⎟                      =
   TA ⎜ p A ⎟
      ⎝     ⎠
                               ⎜p ⎟
                               ⎝ D⎠                           TD
                              (1− k ) / k
            TA ⎛ p B ⎞                                    (1− k ) / k
   η = 1−     =⎜     ⎟                      = 1 − rp
            TB ⎜ p A ⎟
               ⎝     ⎠




                                                                        2
Brayton Cycle




Gas turbine efficiency




                         3
Gas turbine efficiency


• Best rp from T limits
• Tmax determined by
 metallurgy of turbine
• Large gas turbines
efficiencies ~0.4




Gas turbine cycle
   Natural gas as a fuel
                                                      TA      T
   Typical values:                           η = 1−      = 1 − D = 0.45
    – pb=8 atm                                        TB      TC
    – TC=1500 °C
   It follows that:
                                               Issues
            ( k −1) / k                         – Large fraction of
TC ⎛ pC ⎞                  0.4
                                                  output work to run
   =⎜ ⎟                   =8  1.4
                                    = 1.81
TD ⎜ pD ⎟
     ⎝ ⎠                                          compressor
TD                                              – High fuel cost
   = 0.55                                       – Easy to start up
TC
                                                – Good greenhouse
TD = 0.55TC = 0.55(273+ 1500) = 970K              credentials
                                                – Low pollution




                                                                          4
A new approach to power generation?

    Standard Rankine:            Standard gas
                                 turbine




   Can you come up with an improved approach to give a
   cycle with much higher efficiency?




  CURRENT TECHNOLOGY
  No. 2 boiler in Pleystowe Sugar Mill




                                                         5
Fluid bed combustor




Combined Cycles

 Use gas turbine cycle to deliver high
 temperature energy to a Rankine
 cycle
 High temperature exhaust gas to
 generate steam for power plant
 turbine
 Higher real efficiencies ~0.6,
 compared with separate cycles (~0.4)




                                         6
Combined cycle




Gas-steam combined power plant

     IGCC Plants




                                 7

								
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