By:Prayag nao
Thermal Power Plants
Thermal power plants use water as working fluid. Nuclear and coal
based power plants fall under this category. The way energy from fuel
gets transformed into electricity forms the working of a power plant. In
a thermal power plant a steam turbine is rotated with help of high
pressure and high temperature steam and this rotation is transferred to a
generator to produce electricity.
Energy absorption from steam
When turbine blades get rotated by high pressure high temperature
steam, the steam loses its energy. This in turn will result in a low
pressure and low temperature steam at the outlet of the turbine. Here
steam is expanded till saturation point is reached. Since there is no
heat addition or removal from the steam, ideally entropy of the steam
remains same. This change is depicted in the following p-v and T-s
diagrams. If we can bring this low pressure, low temperature steam back
to its original state, then we can produce electricity continuously.
Use of Condenser
Compressing a fluid which is in gaseous state requires a huge amount
of energy,so before compressing the fluid it should be converted into
liquid state. A condenser is used for this purpose, which rejects heat
to the surrounding and converts steam into liquid. Ideally there will
not be any pressure change during this heat rejection process, since the
fluid is free to expand in a condenser. Changes in fluid are shown in
the p-v and T-s diagram below.
Compressor
At exit of the condenser fluid is in liquid state, so it is easy for a
compressor to raise its pressure.During this process the volume and
temperature (2-3 deg.C rise)of fluid hardly changes, since it is in
liquid state. Now the fluid has regained its original pressure.
Heat Addition in Boiler & Rankine Cycle
Here external heat is added to the fluid in order to bring fluid back
to its original temperature. This heat is added through a heat
exchanger called a boiler. Here the pressure of the fluid remains the
same, since it is free to expand in heat exchanger tubes. Temperature
rises and liquid gets transformed to vapor and regains its original
temperature. This completes the thermodynamic cycle of a thermal power
plant, called Rankine Cycle. This cycle can be repeated and continuous
power production is possible.
Condenser Heat Rejection - Cooling Tower
In order to reject heat from the condenser a colder liquid should
make contact with it. In a thermal power plant continuous supply of cold
liquid is produced with the help of a cooling tower. Cold fluid from
the cooling tower absorbs heat from a condenser and gets heated, this
heat is rejected to the atmosphere via natural convection with the help
of a cooling tower.
Boiler furnace for Heat Addition
Heat is added to the boiler with help of a boiler furnace. Here fuel
reacts with air and produces heat. In a thermal power plant, the fuel
can be either coal or nuclear. When coal is used as a fuel it produces a
lot of pollutants which have to be removed before ejecting to the
surroundings. This is done using a series of steps, the most important
of them is an electro static precipitator (ESP) which removes ash
particles from the exhaust. Now much cleaner exhaust is ejected into the
atmosphere via a stack.
Optimizing a Thermal plant performance
There are various flow parameters which have to be fine-tuned in
order to get optimum performance from a thermal power plant.Lowering the
condenser temperature or raising the average boiler temperature will
result in a high efficiency power plant cycle according to the 2nd law
of thermodynamics (Carnot efficiency),most of the performance improving
technologies are working on this idea. Some latest trends are listed
below.
Expanding Turbine After Saturation
Expanding the steam in the turbine even after reaching the saturation
point may be a dangerous affair. As the steam goes below saturation,
wetness of the steam increases. These condensed water droplets collide
with the turbine blades rotating at a high speed, thus it can cause
extreme tip erosion to the blades. Turbine blade tip erosion is shown in
figure below. But as you expand more you will be able to absorb more
energy from the steam, thus increasing power plant efficiency. Up to 15%
wetness level is considered to be safe for steam turbine operation. So
most of the steam turbine will expand up to this point in order to
extract maximum energy from the fluid. This is shown in figure below.
Raising average boiler temperature
If you can increase the average heat addition temperature of the
boiler, that will result in a power plant with higher efficiency. One
way to do this is to increase the compressor pressure. This will shift
the saturation point of the fluid to a higher level, thus providing
higher average temperature of heat addition. This is shown in the figure
below. The blue line represents change in the cycle after raising the
compressor pressure.
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