About 70 % of energy used by India is produced in Coal fired thermal power plants. Not just India, people all over the world heavily rely on thermal power stations. This is because of the abundant availability of coal, reliable cheap power and early advent of ‘steam engine’ technology. Though there is a lot of hue and cry over the CO2 emissions and diminishing coal reserves, coal power continues to dominate the energy sector.
Rankine cycle is the working principle of the plants all over the world. Water is boiled into steam which is super heated. This is the phase where the energy of the coal is give to the steam/water. The high pressure and high temperature steam is allowed to expand in turbines coupled with generators. Here , a part of energy is given back by the steam. Most of remaining heat is dissipated to atmosphere. More about this will be discussed in the efficiency discussion.
The TS graph for Basic Rankine cycle
For an ideal plant, there are good number of specifications to be satisfied. The power plant must be located to a coal mine as close as possible. If the plant is dependent on the imported coal it should be closely located to the sea port. In either cases, dedicated transporation system must exist for transmission if coal reserves. Another important aspect is the ash disposable facility. Indian Coal has a higgm amount of ash content which turns out to be around 30 -40 %. This if not disposed properly, results in health hazards in and around the plant leading to numerous other problems. Presently the ash is used for various industries and also used for domestic purposes. In most cases it is stored in propoer places.
Huge quantities of water must be required for condenser, disposal of ash and feed water circuit etc. It is therefore desirable to locate plant on side of river. For example, VTPS in Vijayawada is located on the banks of river Krishna.
The basic entities in the power plant are Boiler, Turbine , Condenser , pump. Here, the principle is explained with the help of Temperature and Entropy (TS) curve.
Starting at 1, the water at room temperature is boiled at constant temperature in the boiler. This process has a T constant for it is the boiling of water which takes at a constant temperature. Here we are increasing the entropy of by phase change and the energy is sorted in the form of latent heat also. As you might assume, the temperature is not 1000c because this boiler is at higher pressure and the boiling point is also high. As shown in the figure the temperature is constant and the line is parallel to S axis. All this process happens in a boiler. The end product is steam.
Once you have steam, you cannot immediately pump it to the turbine to extract energy because once it enters turbine and starts condensing forming water droplets without giving much energy. The point here is, just a phase change won’t work out for energy transfer from coal in boiler to turbine. The steam must be heated to higher temperatures. This phase is called super heater and this phase is mainly executed in ‘super heater’. This super heater will be located in between boiler and the turbine. The source of heat for the super heater is the hot flue gases obtained in the boiler after burning coal. This explains the phase 2-3 in the T-S curve. Note that , in this process, neither the temperature nor the entropy remains constant.
Now, in the next process, the super heated steam is allowed to expand in the turbine. As, the high pressure steam is allowed through a small nozzle ,steam acquires kinetic energy. This kinetic energy of the steam will exert required force on the turbine blades. The turbine is designed well to receive maximum force from the steam. This process is a constant entropy process. The steam is allowed inside the turbine until water droplets begin to form. In practice, formation of water drops is strictly prohibited for the water drops will impinge the turbine blades and cause corrosion. Now the ouput of the turbine is low pressure and low temperature steam. This accounts for the phase 3-4 of the cycle.
In this phase of condenser, the heat of steam is exchanged with a heat exchanger, essentially water. The steam now turns into water and this is processed again and sent into boiler for the next cycle. The heat exchanger gets heated up and this needs to be cooled for further use. Hence the heat of exchanger is dissipated in atmosphere through large cooling towers. A lot of energy from the entire system remains unused in this 4-5 . The analysis can be obtained in the efficiency discussion at the end of the tutorial.
As you have seen throughout the process, water needs to be flown from one part to other. The necessary draught is created by the pump. The step 5-6 is a pump which is used to circulate water. During this process, a little temperature change can be observed. Finally, the cooled water cannot be directly sent into boiler. Because the boiler is at higher temperature, it causes irregularly expansion which results in collapse of the boiler. Hence, the water should be heated to higher temperature. This is done in economizer which uses heat from flue gases. Thus this accounts for the 6-1 phase of the Rankine cycle. The efficiency of the Rankine cycle is given by 1-T2T1. Where T2 is the temperature of super heated steam and the T1 is the temperature of the water entering inside.
Basic Flow Chart of Power Plant
As cited above, the primary parts of the plant are the boiler, turbine, condenser are explained below. Other numerous parts including pulveriser, water treatment plant, cooling towers etc are also discussed in detail.
Boiler : It is used to convert the water into steam where coal is burnt. It is a relatively huge structure with a typical boiler of a 500 MW plant would be equivalent to 5 storied building. The boiler material will mostly made of cast iron to with stand high temperature and pressure. The construction of boiler varies depending upon the heat transfer method used. In a traditional boiler, the boiler has holes on the lower bottom for the coil powder to enter. The coal enters in such a way that , it creates a vortex inside the boiler. This is to ensure that coal spends maximum time before settling down and gets burnt completely. The outer surface of the boiler has thousands of pipes in which water runs through. This is the process in which heat is exchanged. The flue gases rising out of burning coal pass through the super heater as shown in the figure.
Steam turbines convert the energy acquired by the steam in to the mechanical energy. These turbines are couple with the alternators which produce the electrical energy from the mechanical energy. Two types of turbines are widely prevalent : impulse turbines and reaction turbines.
In the impulse turbines, steam expands at the nozzles and achieved kinetic energy is used to rotate the blades of the turbine. The blades change the direction of steam but not the pressure. Thus change in momentum can be accounted for rotation of the rotor. In the reaction turbines, steam is partially expanded on the nozzles and remaining expansion takes place during the flow over moving blades.
Generally there are two or three sets of turbines at one go. All the enrgy stored in the steam cannot be obtained at one go from a single turbine. So, there are two or three sets of turbines located which are connected by a shaft. Now the high pressure steam enters into the first turbine, lets call it HP turbine. Once the expansion takes place, the pressure falls. Hence we need to use a turbine designed for lower pressure appropriate to the out coming steam. So the second turbine will be a medium pressure turbine (MP turbine).
Further , in some cases a third turbine is also added to make more energy out of steam and this is called a low pressure turbine. The specifications of turbines are calclcuated during the plant design and later during operations, same ratings of steam pressure and temperature need to be maintained for optimum operation. Given below is a figure illustrating the construction of the three stage turbine.
Three turbine model for a plant
To generate the massive amount of heat which is required instantaneously , a lot of coal is to be burnt. If chunks of coal is used, very less surface area of coal is exposed and it takes a lot of space to burn enough coal chunks for required power. As a result, to overcome this problem, coal is pulverized into powder which is as smooth as talc. Now this powder is blown into the boiler. Thus, as powder has a very higher surface area compared to chunks of coal, very large amount of coal can be burnt instantaneously in less volume efficiently. This is the underlying interesting fact for using of a pulveriser.
The steam is super heated in order to make it hold more energy and transfer it to the turbine. This job is accomplished by the super heater. Super heater is showed in the boiler schematic. The flue gases coming out of the boiler are used to super heat the steam.
The water entering into the boiler must have a temperature compatible with the boiler temperature. So, the heat left with the flue gases after super heater is used to heat the water in the economizer. The economizer has convoluted tubes in which water flows and the flue gases flow over these tubes in a closed structure.
The air used for combustion of the coal is also pre heated by the flue gases so as to take maximum heat from the gases before they diffuse in to the atmosphere. It is also to ensure that the un heated air should not interfere with proper combustion inside the boiler.Re heater : To improve the efficiency of the plant, there is something interesting done. The area under curve is the output or work done. If we could improve the area, we can improve the efficiency of the system. The steam which comes out of high pressure turbine is taken out and heated using flue gases and this reheated steam is sent into IP turbine. As a result the new TS graph looks like below.
As discussed earlier, the job of condenser is to turn the steam from the turbine into water and thereby reducing the amount of water required for each cycle. There are many types of condensers. The familiar ones are Jet type and Surface type. In the jet type , the cooling water and the steam are mixed and the resultant steam water mixture is drawn outside. Surface type uses a different circuit for both and the steam is converted into water and cooling water turns hot. The surface type are the widely prevalent ones.
The generators also called alternators are coupled with turbines which generate electrical energy. The output of the generator at 11KV is stepped up to higher voltage of 220KV and transmitted through the transmission lines. Here , the interesting area of study is to control the output power of the generator. As the load on the system continuously vary and as the energy cannot be stored, the output of generator has to be varied according to load. This aspect will be covered in Power Systems Operations and Control tutorial.
Ash handling plant, Ash precipitators, pumps for draught, turbine governing system etc.
The efficiency of the thermal powerplant calculated from the Rankine cycle will be around 45% . But practically efficiencies of 35%-38% only have been achieved so far. This is due to various losses present in the entire system when put into practice. Out of the losses, energy lost as heat takes a major chunk. This energy is lost at mainly two points: flue gases entering into atmosphere and cooling of the condensate. The cooling of the condensate is a part and parcel of the cycle and nothing can be done there to increase the efficiency. Now we can consider decreasing the temperature of flue gases as much as possible up to room temperatures. But it is also disadvantageous , for, the cooled gases do not flow outside the boiler circuit on their own. They need forced draught to go out if the temperature is equal to room temperature. So there is a lower limit for temperature of flue gases.
Also note that, high pressure cannot be maintained inside the boiler. It is recommended to maintain slightly low pressure in the boiler otherwise, the fire will come out from every possible gap in the boiler.
Thermal energy is the most unclean energy. It causes thermal pollution and air pollution apart from leaving off a lot of ash. The ash can be used for other purposes or should be disposed properly otherwise during dry season , it mixes with air and makes the surrounding places uncomfortable to live. The plant also produces thermal pollution ie by adding more and more heat to the atmosphere. But as Nature is a huge sink of heat this doesn’t add much trouble. Other pollution from the plant is due to production of soot, SO x , COx gases and consequent problems. Nowadays, latest technologies are being implemented to minimize the emission of these gases by designing the boiler in a special way and adding other compounds so as to neutralize these gases.
Problem : Find out the theoretical efficiency of a power plant whose steam is heated up to a temperature of 4000 Celsius and water temperature at the initial stages is 75 0 Celsius.
Efficiency : 1-(75+273)/(400+273) = 1- 0.51 = 0.49 = 49 % efficiency.