How does a Thermal Power Plant Work ?

Thermal Power Plants

They use water as working fluid. Nuclear and coal based power plants fall under this category. The way energy from fuel is getting transformed into electricity forms 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.

Fig.1 Power is produced in thermal power plants by rotating steam turbine

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 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 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.

Fig.2 Pressure and temperature drop of steam when turbine absorbs energy from it

Use of Condenser

Compressing a fluid which is in gaseous state requires 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 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 condenser. Changes in fluid are shown in p-v and T-s diagram below.

Fig.3 Use of condenser in order to transform vapor into liquid state

Compressor

At exit of condenser fluid is in liquid state, so it is easy for a compressor to raise its pressure.During this process 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.

Fig.4 Compressor pumps the fluid to 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 boiler. Here pressure of the fluid remains same, since it is free to expand in heat exchanger tubes. Temperature rises and liquid gets transformed to vapour and regains its original temperature. This completes the thermodynamic cycle of thermal power plant called Rankine Cycle. This cycle can be repeated and continuous power production is possible.

Fig.5 Heat addition at boiler brings the fluid to its original temperature

Condenser Heat Rejection - Cooling Tower

In order to reject heat from condenser a colder liquid should made contact with it. In a thermal power plant continuous supply of cold liquid is produced with help of a cooling tower. Cold fluid from cooling tower absorbs heat from condenser and gets heated, this heat is rejected to atmosphere via natural convection with help of a cooling tower.

Boiler furnace for Heat Addition

Heat is added to the boiler with help of boiler furnace. Here fuel get react with air produces heat. In a thermal power plant fuel can be either coal or nuclear. When coal is used as a fuel it produces lot of pollutants which has to be removed before ejecting to the surrounding. This is done using a series of steps, most important of them is electro static precipitator (ESP) which removes ash particles from exhaust. Now much cleaner exhaust is rejected to atmosphere via a stack.

Fig.6 Main accessories of Rankine cycle - Cooling tower, Boiler furnace, ESP & Chimney

Optimizing a Thermal plant performance

There are various flow parameters which has 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 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. This condensed water droplets collide with turbine blade rotating at high speed, thus can cause extreme tip erosion to the turbine 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.

Fig.7 Expanding turbine below saturation point in order to gain maximum power from steam

Raising average boiler temperature

If you can increase average heat addition temperature of boiler that will result in power plant with higher efficiency. One way to do this is to increase the compressor pressure. This will allow shifting saturation point of boiler heat addition will to higher level, thus providing higher average temperature of heat addition. This is shown inf figure below. The blue line represents change in cycle after raising the compressor pressure.

Fig.8 Raising compressor pressure in order achieve higher average boiler temperature