Power Plant Engineering

296 POWER PLANT ENGINEERING

Fig. 9.29 shows a combination of an open
cycle gas turbine and steam turbine. The exhaust
of gas turbine which has high oxygen content is
used as the inlet gas to the steam generator where
the combustion of additional fuel takes place. This
combination allows nearer equality between the
power outputs of the two units than is obtained with
the simple recuperative heat exchanger. For a given
total power output the energy input is reduced (i.e.,
saving in fuel) and the installed cost of gas turbine
per unit of power output is about one-fourth of that
of steam turbine. In other words, the combination cycles exhibit higher efficiency. The greater disadvan-
tages include the complexity of the plant, different fuel requirements and possible loss of flexibility and
reliability. The most recent technology in the field of co-generation developed in USA utilizes the gas-
eous fuel in the combustion chambers produced by the gasification of low quality of coal. The system is
efficient and the cost of power production per kW is less.

9.12 Applications of Gas Turbine

1. Gas turbine plants are used as standby plants for the hydro-electric power plants.


2. Gas turbine power plants may be used as peak loads plant and standby plants for smaller
   power units.


3. Gas turbines are used in jet aircrafts and ships. Pulverised fuel fired plants are used in loco-
   motive.

9.13 Advantages of Gas Turbine Power Plant

The economics of power generation by gas turbines is proving to be more attractive, due to low
capital cost, and high reliability and flexibility in operation. Quick starting and capability of using wide
variety of fuels from natural gas to residual oil or powdered coal are other outstanding features of gas
turbine power plants. Major progress has been made in three directions namely increase in unit capaci-
ties of gas turbine units (50—100 mW), increase in their efficiency and drop in capital cost, (about
Rs. 700 per kW installed). Primary application of gas turbine plant is to supply peak load. However gas
turbine plants now-a-days are universally used as peak load, base lead as well as standby plants.

1. It is smaller in size and weight as compared to an equivalent steam power plant. For smaller
   capacities the size of the gas turbine power plant is appreciably greater than a high speed
   diesel engine plant but for larger capacities it is smaller in size than a comparable diesel
   engine plant. If size and weight are the main consideration such as in ships, aircraft engines
   and locomotives, gas turbines are more suitable.


2. The initial cost and operating cost of the plant is lower than an equivalent steam power plant.
   A thermal plant of 250 mW capacity cost about Rs. 250 crores. Presently whereas a gas
   turbine plant of that same-size cost nearly 70 crores.


3. The plant requires less water as compared to a condensing steam power plant.


4. The plant can be started quickly, and can be put on load in a very short time.

Fig. 9.29. Combined Cycle (Co-generation).

Fuel

CC

Gas Turbine

Fuel Steam Turbine

Pump

Boiler

Steam

T

Chimney

CT