PRACTICAL
-6
AIM:- TO STUDY ABOUT USE OF COMBINED CYCLE POWER PLANT AND USE OF CO-GENERATION PLANT
·
USE OF COMBINED
CIRCLE POWER PLANT:
Power
supply industry is highly capital intensive. Therefore, it is desirable to
utilize it in optimum manner. There are two aims of the national economy.
(I)To
create the maximum amount of generating capacity with the available funds.
(ii)To
generate power at the cheaper rate.
So, before investing huge amount in this
industry, the most economic generating scheme should be selected to supply
power at the lowest cost.
The maximum steam temperature in a
power plant not exceeds 6000C. However, the flame temperature in a
boiler burning fossile fuels is about 16000C.as heat transferred
from combustion gases to thermal irreversibility. So the use of gas turbine
alone to drive the generator, would be uneconomical, however, the high exhaust
temperature of the gas turbine can drive a steam turbine in the second stage.
This is possible through combined cycle.
The production of electricity using two
or more heat engines one by one as prime movers to operate electric generators
called combined cycle power generation. First heat engine works at higher
temperature. Part of heat from first discharge at lower temperature serves as
the source for the next engine. The net result is a greater overall operating
temperature range.
The combined cycle concept combines
two thermodynamic cycles.
(i)
Brayton
cycle
(ii)
Rankine
cycle
The Brayton cycle has the capability to
operate at very high combustion temperature (say 12000C) and gives high
efficiency at high temperature. The Rankine cycle will operate at low
combustion temperature (say 5400C) and gives high efficiency at low temperature.
If steam turbine or gas turbine operates independently, system efficiency will
be lower. If both turbines operate in combined cycle, system efficiency will be
of the order of 42-47%. The first installation of the combined cycle was
commissioned in Germany in 1973.the system has a net output of approximately
35MW. In India, gas turbine can be used for power generation to exploit the
vast reserves of natural gas.
First
of all, air is compressed from atmosphere and given to combustion chamber. Fuel
is burned by keeping air-fuel ratio constant. So inlet temperature ay Gas
turbine is approximately 1200-13000 C. mechanical output is obtained
by decompressing it in the gas turbine. This mechanical output is given to the
electric generator-1 and electricity is produced by it. Outlet temperature of
gas turbine is approximately 6000C. Gas from outlet is given to the
HRSG (Heat Recovery Steam Generator). Steam is generated from HRSG with the
help of super heater, evaporator, economizer etc and its temperature is about
5500C. Gas is exhausted to atmosphere from HRSG. Steam at lower
temperature (approximately 5500C). Is given to steam turbine.
Mechanical output is given to electrical generator-2 and electricity is
produced by it. Steam from outlet of steam turbine is at approximately 400C
to 800C. This steam is condensed in condenser and hot water is
pumped to HRSG by feed water pump.
Combined
cycle plants may of the following types.
(i)
Gas
turbine-steam turbine plant
(ii)
MHD-steam
plant
(iii)
Thermionic-system
plant
(iv)
Thermoelectric-steam
plan
v ADVANTAGES OF COMBINED CYCLE POWER GENERATION:
The following are the advantages of
combined cycle power generation.
(1)
Higher
efficiency
(2)
Low
investment costs
(3)
Small
amount
(4)
Great
operating flexibility
(5)
Simplicity
of operation
(6)
Low
environmental impact
(7)
Small
amount of water required
v USE OF CO-GENERATION PLANTS:
Co-generation is a procedure for
generating electric power and useful heat in a single installation. The useful
heat may be in the form of steam, hot water or hot air. Thus, cogeneration is
the simultaneous generation of electricity and steam (or heat) in a single
power plant. In cogeneration system, mechanical work is converted into
electrical energy in an electric generator and the discharge heat is utilized
in an industrial process or in other ways. So the net result is an overall
increase in the efficiency of fuel utilization.
v Many industries
like chemicals, fertilizers, papers, textiles, sugar, food processing etc.
require steam for heating and electric power for drives. Earlier, due to
reliable electric power from government, these industries started generating
only steam for process requirements. Now, electrical form state electricity
boards are more expensive and lead to shortages, so cogeneration is being used
extensively. It saves approximately 20% fuel and reduces the burden on
electrically boards. Government is also encouraging.
(1)TYPES OF COGENERATION:
There
are two board categories of co-generation.
v The bottoming
cycle
v The topping
cycle
v The bottoming cycle: Primary heat is
used at high temperature directly for process requirements. Waste heat from
process is used for electrical power generation. It is difficult to extract
power economically in the bottoming cycle plants. It has low efficiency and
therefore it is not widely used.
v
The topping
cycle:
First electricity is generated by steam turbine or gas turbine or diesel
engine, and then exhaust steam or gas is used for process equipments. Process
steam at low pressure and temperature is exhausted by two ways depending on
process requirements.
(A)Extracted
from turbine at an intermediate stage
(B)
Taken at the turbine exhaust.
Therefore, topping cycle can provide
true savings in primary energy. Also process applications require low pressure,
low temperature. (Sometimes moderate temperature) steam. Such steam is easily
produced in a topping cycle.
v Advantages of Co-generation:
The
advantages of Co-generation applications for industrial use as
(I)Lower capital costs: The capital
investment for co-generation is much lower than that of two separate systems
for steam and electric power. Industries needing steam for their processes have
to install boilers. To a kW of power from this steam is about ½ costly than the
cost of installing and supplying a kW by a grid.
(ii)Energy efficiency: The energy
efficiency can reach to 80% or even higher. This depends on the co-generation
plant configuration and the utilization of heat. Energy is also saved in transportation
of fuel.
(iii)Lower Gestation period: Co-generation
plants can be commissioned in 1.5 years to 3 years. A central station may take
more than 5 to 6 years. The shorter gestation period of cogeneration curtails
the scope of time over-runs and reduces the cost of interest during constriction
(iv)Quality of supply: Since cogeneration
uses simpler equipment which operates on lower temperatures and pressures,
reliability of cogeneration is high. They reduce dependence of industries on
the unreliable grid supply. Voltage of supply can also be much better
controlled. Here, transmission and distribution losses are negligible.
(v)Reduction in Pollution levels:
Cogeneration systems require 20% to 25% lesser fuel than for two separate
systems for electricity and steam generation. Due to less usage of fuel,
pollution levels will be lower.
v Constraints of Co-generation Plants:
(i)
Limitations faced by Industries: Due to lack of
awareness, financial constrains, technology inadequacy, problems of coal etc.,
cogeneration capacity is not much accelerated of 8 to 10%. High T&D losses
in the power system have been the subject of criticism particularly in concern
with power shortages. The power shortages may lead to put up additional highly
cost intensive generating capacity. Since power is backbone of development of
any country. So the percentages of losses in T&S system are alarming.
LIST
OF FIGURE.
Fig.1-
combined cycle without reheat of exhaust gases.
Fig.2-
Bottoming cycle.
Fig.3-
Topping cycle.
Fig.4-
Gas turbine plant with exhaust heat recovery system.
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