This
section provides an understanding, at an overview level, of the steam power
cycle. References were selected for the next level of study if required. There
are noteworthy omissions in the section: site selection, fuel handling, civil
engineering-related activities (like foundations), controls, and nuclear power.
Thermal
power cycles take many forms, but the majority is fossil steam, nuclear, simple
cycle gas turbine, and combined cycle. Of those listed, conventional coal-fired
steam power is predominant. This is especially true in developing third-world
countries that either have indigenous coal or can import coal inexpensively.
These countries make up the largest new product market. A typical unit is shown
in Figure 1.
The
Rankin cycle is overwhelmingly the preferred cycle in the case of steam power
and is discussed first.
Topping
and bottoming cycles, with one exception, are rare and mentioned only for
completeness.
The
exception is the combined cycle, where the steam turbine cycle is a bottoming
cycle. In the developed countries, there has been a move to the combined cycle
because of cheap natural gas or oil. Combined cycles still use a reasonably
standard steam power cycle except for the boiler. The complexity of a combined
cycle is justified by the high thermal efficiency, which will soon approach
60%.
The
core components of a steam power plant are boiler, turbine, condenser and feed
water pump, and generator. These are covered in successive subsections.
The
final subsection is an example of the layout/and contents of a modern steam
power plant.
As
a frame of reference for the reader, the following efficiencies/effectiveness’s
are typical of modern fossil fuel steam power plants. The specific example
chosen had steam conditions of 2400 psia, 1000°F main steam temperature, 1000°F reheat steam temperature: boiler thermal 92; turbine/generator
thermal 44; turbine isentropic 89; generator 98.5; boiler feed water pump and
turbine combined isentropic 82; condenser 85; plant overall 34 (Carnot 64).
Nuclear
power stations are so singular that they are worthy of a few closing comments.
Modern stations are all large, varying from 600 to 1500 MW. The steam is both low
temperature and low pressure (~600 °F and ~1000 psia), compared with fossil applications, and hovers
around saturation conditions or is slightly superheated.
Therefore, the boiler(s), superheated equivalent (actually a combined moisture separator and reheated), and turbines are unique to
this cycle. The turbine generator thermal efficiency is
around 36%.
Rankin Cycle Analysis
Modern
steam power plants are based on the Rankine cycle. The basic, ideal Rankine
cycle is shown in Figure 2.
The
ideal cycle comprises the processes from state
1-Saturated
liquid from the condenser at state l is pumped isentropically (i.e.,S1=S2)
to state
2
-and into the boiler.
3-
Liquid is heated at constant pressure in the boiler to state 3 (saturated
steam).
4-Steam
expands isentropic ally (i.e.,S3=S4)
through the turbine to state 4 where it enters the condenser as a wet vapor.
Constant-pressure
transfer of heat in the condenser to return the steam back to state 1
(saturated liquid).
If
changes in kinetic and potential energy are neglected, the total heat added to
the rankine cycle can be represented by the shaded area on the T-S diagram in Figure 8.1.2, while the work done by this
cycle can be represented by the crosshatching within the shaded area. The
thermal efficiency of the cycle (h) is defined as the work (WNET) divided by the heat input to
the cycle (QH).
The
Rankine cycle is preferred over the Carnot cycle for the following reasons:
The
heat transfer process in the boiler has to be at constant temperature for the
Carnot cycle, whereas in the Rankine cycle it is superheated at constant
pressure. Superheating the steam can be achieved in the Carnot cycle during
heat addition, but the pressure has to drop to maintain constant temperature.
This
means the steam is expanding in the boiler while heat added which is not a
practical method.
The
Carnot cycle requires that the working fluid be compressed at constant entropy
to boiler pressure.
This
would require taking wet steam from point 1¢ in Figure 2 and compressing it to saturated liquid condition at 2
.A
pump required to compress a mixture of liquid and vapor isentropically is difficult
to design and operate. In comparison, the Rankine cycle takes the saturated
liquid and compresses it to boiler pressure. This is more practical and
requires much less work.
The
efficiency of the Rankine cycle can be increased by utilizing a number of
variations to the basic cycle. One such variation is superheating the steam in
the boiler. The additional work done by the cycle is shown in the crosshatched
area in Figure 3.
The
efficiency of the Rankine cycle can also be increased by increasing the
pressure in the boiler.
However,
increasing the steam generator pressure at a constant temperature will result
in the excess moisture content of the steam exiting the turbine. In order to
take advantage of higher steam generator pressures and keep turbine exhaust
moistures at safe values, the steam is expanded to some intermediate pressure
in the turbine and then reheated in the boiler. Following reheat, the steam is
expanded to the cycle exhaust pressure. The reheat cycle is shown in Figure 4.
Another
variation of the Rankine cycle is the regenerative cycle, which involves the
use of feed water heaters. The regenerative cycle regains some of the
irreversible heat lost when condensed liquid is pumped directly into the boiler
by extracting steam from various points in the turbine and heating the condensed
liquid with this steam in feed water heaters. Figure
5 shows the Rankine cycle with regeneration.
The actual Rankine cycle is far from ideal as there are losses associated with
the cycle. They include the piping losses due to friction and heat transfer,
turbine losses associated with steam flow, pump losses due to friction, and
condenser losses when condensate is sub cooled. The losses in the compression
(Pump) and
expansion process (turbine) result in an increase in entropy. Also, there is
lost energy in heat addition (boiler) and rejection (condenser) processes as
they occur over a finite temperature difference.
Most modern
power plants employ some variation of the basic Rankine cycle in order to
improve thermal efficiency. For larger power plants, economies of scale will
dictate the use of one or all of the variations listed above to improve thermal
efficiency. Power plants in excess of 200,000 kW will in most cases have 300 °
F superheated steam leaving the boiler reheat, and seven to eight stages of feed
water heating.
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