The air standard cycle for a gas turbine power plant  is the Brayton cycle which, like Rankine cycle, also  consists of two reversible adiabatics and two reversible

isobars,  but  unlike  Rankine  cycle  the  working  fluid  does  not  undergo  phase change.

A gas turbine plant can be either open or closed. Simple, open gas turbine plant is shown in Fig. 3.6.

Since the product of combustion is the working  fluid which produces power by doing  work on the blades of the gas turbine,  it  is an  internal  combustion plant. However, unlike the reciprocating internal combustion engine, the gas turbine is a steady  flow  device  and  the  blades  are  always  subjected  to  the  highest  gas temperature.

To  limit  the  maximum  gas  temperature to  about  1200  K  at  inlet  to  the  turbine consistent with the materialized, a high air-fuel ratio is used.

The  disadvantages  of  a  gas  turbine  power  plant  in  a  utility  system  are  the

1.  Large  compressor  work  input,  since  the  power  required  to  drive  the  compressor  is considerably  higher  than  that  required  by  a  pump  for  the  same  pressure  rise.  The compressor thus consumes a large part of the work produced by the turbine.

2. Large exhaust loss, since the exhaust gas temperature is quite high and also the mass
flow rate of gas is large due to high air —fuel ratio used.

3. Machine  inefficiencies, since with the decrease  in compressor  efficiency (i the work
input to the compressor increases and with the decrease in turbine efficiency ( the work output  from  the  turbine  decreases.  At  certain  values  of  i  and  17  a  situation  may  arise when  the  compressor  consumes  more  power  than  what  the  turbine  develops.  So,  the machine efficiencies of the compressor and the turbine have to be high enough to yield justifiable net work output.

4.  Low  cycle  efficiency,  due  to  the  large  exhaust  loss,  large  compressor  work  and
machine inefficiencies.

5. Costly fuel, since the cost of kerosene and other fuels used is much higher than that of
coal. Its availability is also not always guaranteed.

Due to the above factors, the cost of power generated by a stationary gas turbine plant for
a utility system is high. However, a gas turbine plant offers certain advantages also, as given below:

1. Less installation cost

2. Less installation time

3. Quick starting and stopping

4. Fast response to load changes

So, a gas turbine plant is often used as a peaking unit for certain hours of the day when the  energy  demand  is  high.  A  large  steam  plant  designed  to  meet  peak  loads  would operate at an uneconomical load factor during most of the year.

Thermodynamics of Brayton-Rankine Combined Cycle Plant

Let  us  consider  two  cyclic  power  plants  coupled  in  series,  the  topping  plant operating  on  Brayton  cycle  and  the  bottoming  one  operating  on  Rankine  cycle
(Fig. 3.7).

Helium  gas  may  be  the  working  fluid  in  the  topping  plant  and  water  in  the bottoming   plant.   As  shown   in   Section  3.4.1,   the  overall  efficiency  of  the combined plant is given by Eq. (3.6)

neglecting the pump work.As  inlet  temperatures  to  gas  turbine  keep  increasing  (due  to  the  use  of  better material  and  blade  cooling),  the  importance  of  supplementary  firing  diminishes further. However, supplementary firing may provide increased operating and fuel flexibilities in CC plants, which may fall into the following two categories.

1. Combined cycle plants with limited supplementary firing Supplementary  firing  raises  the  temperature  of  the  exhaust  gas  to  800  to  900  ° C.

Relatively  high  flue  gas  temperature  raises  the  condition  of  steam  (84  bar,  525  ° C), thereby  improving  the  efficiency  of  the  steam  cycle.

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