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# Microsoft PowerPoint - Gas cycles by lindayy

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Microsoft PowerPoint - Gas cycles

• pg 1
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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:
compression A-B
– Constant pressure heat
transfer (combustion) B-C
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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