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International Journal of Mechanical And Production , ISSN: 2320-2092, - 3, Issue-11, Nov.-2015 ANALYSIS OF AND ITS UTILITY IN THERMAL PLANT - A THEORETICAL APPROACH

PROSUN ROY

Bachelor of Technology (4th Year Student), Department Of Mechanical Engineering, Maulana Abul Kalam Azad University of Technology (Formerly West Bengal University of Technology) Techno India College of Technology, E-mail: [email protected]

Abstract - Most of the thermal power plants according to Rankin cycle with superheat, reheat and regeneration. Thermodynamic analysis of Rankin cycle has been undertaken to enhance the efficiency and reliability of power plant. However, electricity is being produced by some other sources like hydro power, , , etc. but absolute majority of electricity production throughout the world is from steam power plant. Based on a project activity and local power plant experience some key observation has made and is presented in this paper. A comparative study between Rankin cycle and efficiency has been analyzed. The study on -fired power plant analysis gives a better insight into the cycle operation with various system components and components required for operation. Factors affecting efficiency of Rankin cycle have been identified and analyzed for improved working of thermal power plant. Some recent developments to enhance the efficiency of Rankin cycle have been analyzed.

Keywords - Bleed Steam, Feed- Heater (Fwh), Resuperheat, Regeneration, Rate. water along with steam and so it requires more power INTRODUCTION which finally results in poor plant efficiency. These deficiencies all are eliminated in Rankin cycle. The A producing net amount of work Rankin cycle uses complete of steam and as output is called power cycle. The power cycle only liquid water is pumped back to . Actually, is a cycle with a working substance which Rankin cycle requires very less pumping work (back alternatively vaporizes and condenses. Rankin cycle is work), therefore, it has practically higher efficiency the standard thermodynamic cycle in general use for than that of Carnot vapor power cycle. Thermal generation and this cycle is a heat efficiency of Rankin cycle can be improved by with a vapor power cycle. The common increasing the mean of heat addition and is water. Thermodynamically, it is true the mean temperature of heat addition can be that no can be more efficient than a increasing by using reheat, superheat and reversible heat engine working between two fixed regeneration. In a word, a suitable model for vapor temperature limits. The most efficient cycle operating power cycles is the Rankin cycle. between a source temperature H and the sink RANKINE CYCLE & ITS FEATURES temperature  is the Carnot vapor power cycle and L Rankin cycle has less practical difficulties and it is its is given by TL CARNOT 1 TH found to be more economical. For the steam boiler, where  and  are the temperature range over this would be a reversible constant heating H L process of water to form steam, for the the which the working fluid operates. Now increased ideal process would be a reversible adiabatic cause more difficulty with the materials expansion of steam, for the condenser it would be a of the plant components than increased pressure. reversible constant pressure heat rejection as the steam Moreover, irrevsibility of Carnot cycle cannot be condenses till it becomes saturated liquid, and for the approximated in actual practice. Due to this, Rankin , the ideal process would be the reversible cycle, which operates between two constant adiabatic compression of this liquid ending at the rather than two constant temperatures, is universally initial pressure. When all these four processes are adopted for steam power plants. ideal, the cycle is an ideal cycle, called a Rankin cycle. No doubt Carnot cycle is considered to be the most efficient cycle, but for steam power plants it has Components associated with Rankin cycle several impracticalities associated with this cycle such The four basic components of a vapor power plant are as steam is not fully condensed because condensation shown in Fig 1.Each component in the cycle is stops in between, as a result compressor has to handle

Analysis Of Rankine Cycle And Its Utility In Thermal Power Plant - A Theoretical Approach

53 International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 3, Issue-11, Nov.-2015 regarded as a , operating at steady state. Turbine: The vapor leaving the boiler enters the turbine, where it expands isentropic ally to the condenser pressure at the state 2 (shown in Fig1&2). The work produced by the turbine is rotary (shaft) work and is used to drive an . Condenser: The condenser is attached at the exit of the turbine. The vapor leaving the turbine is wet vapor and it is condensed completely in the condenser to the state 3 (shown in Fig 1&2), by giving its to Cycle analysis some other cooling fluid like water. Pump: The liquid condensate leaving the condenser at For purposes of analysis the Rankin cycle is assumed the state 3 (shown in Fig1&2) is pumped to the to be carried out in a steady flow operation. Applying operating pressure of the boiler. The pump operation the steady flow equation to each of the is considered isentropic. processes on the basis of unit mass of fluid, and Boiler: The heat is supplied to the working fluid (feed neglecting changes in kinetic and potential energy, the water) in the boiler and thus vapor is generated. The work and heat quantities can be vapor leaving the boiler is either saturated at the state 1 or superheated at the state1’ (shown in Fig1&2), depending upon the amount of heat supplied in the boiler.

Fig 3: 1kg of H2O executing a heat engine cycle evaluated in terms of the properties of the fluid.

For 1kg fluid The S.F.E.E. for the boiler control volume) gives

h4 + Q1 = h1

So, Q1 = h1  h4 The S.F.E.E. for the turbine as the control volume gives

h1 = WT + h2

so, WT = h1  h2 Cycle operation The S.F.E.E. for the condenser is

h = Q + h The Figure 2 shows Rankine cycle on T-s coordinates 2 2 3 . The liquid, vapour and wet vapour regions are also So, Q = h  h indicated with the help of saturation curve. 2 2 3 In the Figure 2, the cycle 1-2-3-4-1 represents an ideal The S.F.E.E. for the gives Rankine cycle using saturated steam and the cycle h3 + WP = h4 1’-2’-3-4-1’ represents an ideal Rankine cycle with So, W = h  h superheated steam at the turbine entry. The Rankine P 4 3 cycle consists of the following four internally A measure of the effectiveness of an energy reversible processes :- conversion device is its thermal efficiency. By using Process 1-2 Isentropic expansion of the working the above equation the Rankin cycle thermal fluid in the turbine from boiler pressure to condenser efficiency in terms of cycle is given as pressure. W  = net = Process 2-3 Heat rejection from the working fluid rankine Q at constant pressure in the condenser till the fluid 1 reaches the saturated liquid state 3 (shown in Fig2) WT  WP (h  h )  (h  h ) = 1 2 4 3 Process 3-4 isentropic compression of the working Q1 h1  h4 fluid in the pump to the boiler pressure at the state 4 The pump handles liquid water which is (shown in Fig2) in the compressed liquid region. incompressible, i.e. its density or specific volume Process 4-1 heat addition of the working fluid at undergoes little change with an increase in pressure. constant pressure in the boiler from state 4 to 1 (shown in Fig2)

Analysis Of Rankine Cycle And Its Utility In Thermal Power Plant - A Theoretical Approach

54 International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 3, Issue-11, Nov.-2015 For reversible adiabatic compression, by the use if the the pressure losses in various other components. general property relation, Again, the pump handles liquid, which has a very Tds = dh  vdp ; ds=0 small specific volume, and the turbine handles vapor, and dh= vdp whose specific volume is many times larger. since change in specific volume is negligible Therefore, the work output of the turbine is much h = v p larger than the work input to the pump. This is one of the reasons for the overwhelming popularity of steam Or, h  h  v ( p  p ) 4 3 3 1 2 power plants in electric power generation. Usually, the pump work is quiet small compared to the Another practical aspect affecting the efficiency is turbine work and is sometimes neglected. the heat rate. It is the amount of heat supplied in Btu to Then , and the cycle efficiency approximately h4  h3 generate 1Kwh of electricity. It is a convention still becomes practice in USA to express the conversion efficiency of a power plant. The relation between the heat rate and h1  h2  rankine  thermal efficiency can be expressed as h  h 1 4 3142 Btu kWh One of the significant advantages of Rankin cycle is  th  that the pump power is usually quite small compared Heat rate Btu kWh with turbine power. This is indicated by work-ratio, The low value of heat rate represents high thermal efficiency and is of course desirable. WT which is large compared with one of the WP Rankin cycle. As a result, the pumping power is THERMAL POWER PLANT BASED ON neglected in approximating the Rankin cycle net RANKINE CYCLE power output. It is assumed that the liquid at a pump In a simple Rankin cycle, steam is used as the working entrance is saturated liquid because the condensate fluid, generated from saturated liquid water (feed temperature never reaches below exit cooling water water). This saturated steam flows through the temperature. This is usually the case for power plant turbine, where its is converted into feed-eater pumps. mechanical work to run an electricity generating Sub-cooling would increase the heat input in the steam system. All the energy from steam cannot be utilized generator, and on the other hand, the introduction of for running the generating system because of losses steam into the pump would cause poor performance. due to , viscosity, bend-on-blade etc. Most of The properties of the pump inlet or condenser exit the heat energy is rejected in the steam condenser. The (state 3 Fig2) therefore, may be obtained directly feed-water brings the condensed water back to the from the saturated liquid curve at the (usually) known boiler. condenser pressure. The properties for an isentropic The heat rejected during condensation of steam in the pump discharge at state 4 could be obtained from a condenser is given away by a sink. As a result of the sub-cooled water property table at the known inlet conversion of much of its thermal energy into and the throttle pressure. However, such or work, steam leaves the turbine tables are not widely available. The of a at a pressure and temperature well below the turbine sub-cooled state is commonly approximately by the entrance valves. Basically the low pressure steam enthalpy of the saturated-liquid evaluated at the leaving the turbine at the state 2 is first condensed to a temperature of the sub-cooled liquid. This usually liquid at state 3 and then pressurized in a pump to state quite accurate because the enthalpy of a liquid is 4, and this high pressure liquid water is then ready for almost independent of pressure as the Rankin cycle its next pass through the steam generator to state 1 and based thermal power plant works at atmospheric is reused around the Rankin cycle again as shown in pressure. Fig 1 and 2. In accordance with the second law of thermodynamic, The steam generator and the condenser both function the Rankin cycle efficiency must be less than Carnot as heat exchangers. In a well-designed heat engine operating between the same temperature exchanger, both hot and cold fluids flow with little extremes. As with the Carnot-cycle efficiency, Rankin pressure loss. Therefore, ideally it can be considered cycle efficiency improves when the average that steam generators and condensers have negligible heat-addition temperature increases and the pressure loss, where fluids are operating without any heat-rejection temperature decreases. The cycle change in pressure. Hence, the Rankin cycle is efficiency may be improved by increasing turbine inlet operating between two fixed pressure levels, i.e. the temperature and decreasing condenser pressure (and pressure in the steam generator and pressure in the thus condenser temperature). condenser. A pump provides the pressure increase, In steam power plants, the pressure rise in the pump is and a turbine provides the controlled pressure drop equal to the pressure drop in the turbine if we neglect

Analysis Of Rankine Cycle And Its Utility In Thermal Power Plant - A Theoretical Approach

55 International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 3, Issue-11, Nov.-2015 between these levels. The Rankin cycle as a system Effect of superheating (Fig 1) converts thermal energy of the steam into Superheating of steam increases the mean temperature mechanical energy by means of a turbine rotation, of heat addition. The effect of superheated steam on which runs the generator to produce . the performance of the Rankin cycle is shown in Fig 5. The increase in superheat is shown by the line 1-1’. EFFECT OF OPERATING VARIABLES ON The hatched area 1-1’-2’-2-1 represents an increase in RANKINE CYCLE net work done during the cycle. The area under curve 1-1’ represents increase in the heat input. Thus, both Steam power plants are responsible for the production the net work done and increase as a result of most electric power in the world, and even small of superheating the steam to higher temperature. increase in thermal efficiency can mean large savings Therefore, superheating begets higher cycle from the requirements. Therefore, every effort is efficiency. made to improve the efficiency of the cycle on which It is observed that the specific steam consumption steam power plant operate. The basic idea behind all decreases as steam is superheated. Superheating of he modifications to increase the thermal efficiency of steam to higher temperature is desirable, because the the power cycle is the same: the average fluid moisture content of steam leaving the turbine temperature should be as high as possible during heat decreases as indicated by the state 2’. However, the addition and as low as possible during heat rejection. metallurgical considerations restrict the superheating Next we discuss three ways of accomplishing this for of steam to a very high temperature. the simple ideal Rankine cycle. Effect of reducing condenser pressure The steam enters the condenser as a saturated mixture of vapor and moisture at the turbine back pressure p2 . If this pressure is lowered, the saturated temperature of exhaust steam decreases, and thus, the amount of heat rejection in the condenser also decreases. The efficiency of the Rankin cycle increases by lowering condenser pressure.  As turbine back pressure p2 decreases to p2 , the heat rejection decreases by an area 2-3-3’-2’-2. The Effect of increase in boiler pressure heat transfer to steam is also increased by an area a’-4’-4-a-a’. Thus, the net work done and efficiency of By increasing the boiler pressure, the mean the cycle increases. (Shown in Fig4) temperature of the heat addition increases, and thus However, lowering condenser pressure is not without raises the thermal efficiency of the cycle. By keeping side effects such as:  Lowering the back pressure causes an increase in the maximum temperature Tmax and condenser moisture content of the steam leaving the turbine. It is pressure p2 constant it boiler pressure increases, the an unfavorable factor, because, if the moisture content heat rejection decreases by an area b’-2’-2-b-b’ (Fig of steam in low pressure stages of the turbine exceeds 6). The net work done by the cycle remains almost 10%, there is a decrease in turbine efficiency and same, thus, the Rankine cycle efficiency increases, erosion of turbine blade may be a very serious with an increase in maximum pressure. Actually, the problem. operating conditions of the condenser remain  To maintain the high vacuum, the air extraction unchanged and there is no drastic gain in the work pump will run continuously and its work input will output. Specific steam consumption decreases first increase , thus reducing the useful work. and then increases after reaching a minimum level at

160 bar.

Fig 4: Effect of lowering condenser pressure on the Rankin cycle

Analysis Of Rankine Cycle And Its Utility In Thermal Power Plant - A Theoretical Approach

56 International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 3, Issue-11, Nov.-2015 Table 1: Effect of operating variables on work done and thermal T-s diagram of reheat Rankin cycle (Fig 8) shows efficiency of Rankin cycle some increase in the area which is attained because of the introduction of the reheating steam. This results in low pressure turbine expansion work. Thus, reheat increases work output because of low pressure turbine expansion work. Indeed, the net work of the reheat cycle is the algebraic sum of the work of the two and the pump REHEAT RANKINE CYCLE work and the total heat addition is the sum of the total heat added in the feed-water and reheat passes through Actually, the Rankin cycle has been modified to boiler. Thus, the thermal efficiency of reheat cycle is: produce more work output by introducing two stage , using intermediate heating. Reheating (h  h )  (h  h )  (h  h ) also lowers the moisture load of the steam at the 1 2 3 4 5 6  reheat  turbine exit. (h1  h6 )  (h3  h2 ) If the steam expands completely in a single stage then In actual, when steam expands through the turbine, a steam coming out the turbine is very wet. The wet considerable friction is always involved when steam steam carries suspended moisture particles, which are glides over the blades. This friction resists the flow of heavier than the vapour particle, thus deposited on the steam. The isentropic enthalpy drop is not fully blades and causing its erosion. In order to increase the converted into kinetic energy but some of its part is life of turbine blades, it is necessary to keep the steam utilized to overcome the frictional resistances. Thus, dry during its expansion. It is done by allowing the the kinetic energy produced is less than that steam to expand to an intermediate pressure in a corresponding to theoretical isentropic enthalpy drop. high-pressure turbine, and then taking it out and Further, this friction is converted into heat, sending back to the boiler, where it is reheated consequently, the steam becomes dry and saturated, (resuperheated) at constant pressure, until it reaches even superheated. The frictional heating causes an the inlet temperature of the first stage as shown in Fig increase in entropy and hence actual entropy drop is 7. This process is called reheating during which heat always less than the isentropic enthalpy drop. is added to the steam. The reheated steam then further The ratio of cumulative isentropic enthalpy drop to expands in the next stage of the turbine. Due to isentropic enthalpy drop from initial pressure to final reheating, the work output of the turbine increases, pressure is called reheat factor. thus improving the thermal efficiency. The reheat cycle is designed to take advantage of higher boiler pressure by eliminating the problem of excessive moisture content in the exhaust steam. In a reheat Rankin cycle, the steam is expanded in a number of stages. After each stage of expansion, the steam is reheated in the boiler. Then, it expands in the next stage of turbine and is finally exhausted to the condenser. Also, a low reheat pressure may bring down mean temperature of heat addition and hence, cycle efficiency. Again, a high reheat pressure increases the moisture content at turbine exhaust. So, optimization Since, the superheated steam is charged to the turbine, of reheat pressure is necessary. The optimum reheat reheating resuperheats the extraction steam. And this pressure for most of the modern power plants is about is nowadays universe rally practiced when HP (high 0.2 to 0.25 of the initial steam pressure. pressure) and HT (high temperature) steams are employed for throttling process. For power plants harnessing a still higher pressures and temperatures, a double reheat scheme may be incorporated.

REGENERATION

Another way of increasing the thermal efficiency of Rankin cycle is ‘regeneration’. In a simple Rankin cycle the condensate accumulated in the condenser is pumped to the boiler by BFW (boiler feed water) pump where it is heated to produce saturated steam and then

Analysis Of Rankine Cycle And Its Utility In Thermal Power Plant - A Theoretical Approach

57 International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 3, Issue-11, Nov.-2015 to superheated steam. Now the feed-water at the pump supplied to the boiler is known as ‘bleeding’. The outlet is heated in a feed-water pre-heater at a number of feed-water heaters to be inducted into the relatively low temperature (much lower than source circuit is dictated by the turbine throttle conditions temperature). This lowers the average heat addition which fix the most advantageous condensate heating temperature and thus, lowers the overall cycle temperature. efficiency than that of Carnot vapor cycle. To redress Assuming, 1 kg of steam be leaving the boiler and this problem, the process of regeneration is resorted. entering the turbine. m1 kg of steam per kg, is The feed-water from the pump exit is pre-heated by extracted at the state2 from the turbine at intermediate the bleed steam (extraction steam) from the turbine at pressure and kg of steam enters in open various points. The device where the feed-water is p2 (1  m1 ) heated by regeneration is called ‘regenerator’ or a feed water heater and mixed with m1 kg of steam ‘feed-water heater’. blown from the turbine at state 2. After mixing, the Ideal Regeneration mass of saturated liquid becomes 1kg at the state 6 and The mean temperature of heat addition in the Rankin it is pumped to the boiler pressure at state 7. (Shown in cycle can be improved by increasing the heat supplied Fig 11) at high temperature such as increasing superheat, If there were a large number of extraction stages of using reheat and increasing boiler pressure. The mean steam for feed-water heating, then the resulting cycle temperature of heat addition can also be increased by would approach to be a Carnot cycle. decreasing the amount of heat supplied at lower temperatures. In a simple Rankin cycle (shown in Fig  Comparative study between REGENERATIVE 9), a considerable part of the total heat supplied is in CYCLE and simple RANKINE CYCLE on the the liquid when heating up water from 4 to basis of Mean Temperature of Heat Addition:- 4’(Fig 2), at a temperature lower than source In the Regenerative cycle (in Fig 11) , the feed water temperature, the maximum temperature of the cycle. enters the boiler at the temperature T and its mean For maximum efficiency, all heat should be supplied 7 at maximum temperature of the cycle, and feed-water temperature of heat addition is should enter the boiler at state 4’. This may be q in h 1  h 7 T m , Re g   accomplished in what is known as an ‘ideal s 1  s 7 s 1  s 7 regenerative Rankin cycle’. The mean temperature of heat addition without heat The unique feature of the ideal regenerative cycle is generation for simple Rankin cycle operating between that the condensate, after leaving the pump circulates same pressure p1 and p3 would be around the turbine casing; counter flow to the q h  h direction of vapor flow in the turbine. Thus, it is in 1 7 T m, Rankine   possible to transfer heat from the vapor as it flows s1  s 5 s1  s 5 through the turbine to the liquid flowing around the Since, T  T turbine. m , Re g m , Rankine Thus, the amount of heat supplied is decreased and However, this solution is not practical because: the efficiency of Regenerative cycle will be higher than ‘inbuilt within the turbine’ is the most that of Rankin cycle. But at the same time, the turbine impractical part of the system and the moisture work and heat rejection are also reduced due to content of the steam in the turbine will be high for that extraction of steam at an intermediate pressure. reason. Regeneration rewards a higher thermal efficiency to

the Rankin cycle. Greater the amount of heat transferred to the BFW before it is fed to the boiler, higher is the thermal efficiency of the cycle. The thermal efficiency of the cycle is also boosted as the number of feed-water heaters is increased. However, the optimum number of feed-water heaters is dictated by economy. Many large thermal power plants make use of as many as eight to nine feed-water heaters. Practical Regeneration In actual practice (Fig 10), advantage of regenerative heating principle is used by extracting a part of the steam from turbine at a certain stage of the expansion and it is used for heating of feed-water in separate feed-water heaters. The process of draining steam from the turbine at certain point during its expansion and using this steam for heating the feed-water

Analysis Of Rankine Cycle And Its Utility In Thermal Power Plant - A Theoretical Approach

58 International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 3, Issue-11, Nov.-2015 FEEDWATER HEATERS CONCLUSION

Feed-water heaters are two types, viz. open feed The technological investigations and analysis of water heater and close feed water heater. The open various important characters affecting the thermal feed water heater is essentially a direct contact heater power plant’s overall efficiency and other related which is nothing but a mixing chamber where the factors responsible for deviation from ideal working of bleed steam is blown through the BFW exiting the Rankin cycle have been discussed. The efficiency of feed-water pump. The closed feed water heater simple Rankin cycle is improved by using (CFWH) is basically a shell-and-tube heat exchanger intermediate reheat cycle, enabling improved thermal that up the BFW by the bleed steam flowing conditions of the working fluid. However, it cannot counter currently in the tube side. The two streams reach the thermal conditions as in the case of the (extracted steam and feed water) now can be at Carnot cycle where heat addition and heat rejection different pressures, since they do not mix. occurs at a specified temperature range. The Additionally, each open feed water heater requires a regeneration is vital to improve the efficiency as it uses pump to handle the feed-water but CFWH do not the of exhaust steam for the preheating of require a separate feed pump for each heater. feed water. Inclusion of FWH also introduces an Actually, the main function of feed-water heater is to additional pressure level into the Rankin cycle as seen raise the temperature of the feed-water by means of in the T-s diagram. Hence, the extraction pressure bled steam before the feed water is supplied to the level is another parameter under the control of boiler. designer. The control of steam condenser pressure i.e. The temperature of feed-water leaving a particular condenser vacuum and supply of condenser tube heater is always less than the saturation temperature at cooling water is another parameter which affects the the steam extraction pressure, the difference being steam thermal power plant efficiency. known as ‘terminal temperature difference’ (TTD). TTD=saturation temp. of extracted steam  outlet REFERENCES feed water temp. TTD should be as low as possible because if TTD is [1] Arora, C.P., (1998), , Tata McGraw Hill, New Delhi. reduced, more work could be got from the bled-steam [2] Ballaney, P.L. (1999), Thermal Engineering, Khanna, because it could be expanded further down the turbine Delhi. and thus, getting maximum thermal efficiency as well. [3] Cengel, Y.A. , and Boles , M.A. , (2003), Thermodynamics an Engineering Approach , Tata McGraw Hill, New Delhi. [4] Nag, P.K. , (2005), Engineering Thermodynamics , Tata McGraw Hill, New Delhi , 3rd Edition. [5] Rajput , R.K. , (2001), Thermal Engineering , Laxmi , New Delhi. [6] Rathore, M.M.,(2010), Thermal Engineering, Tata McGraw Hill, New Delhi . [7] Sharma, P.C., (2008), Power Plant Engineering, Katson , New Delhi, 9th Edition. [8] Kapooria , R.K. , Kumar, S. , and Kasana , K.S. , ‘ An analysis of thermal power plant working on a Rankine cycle’ published in ‘Journal of Energy in Southern Africa’ , vol 19 (1) , pp. 77-83 , 2008 . [9] Understanding Rankine cycle and its associate components in CESC Ltd. (Budge Budge Generating Station)

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Analysis Of Rankine Cycle And Its Utility In Thermal Power Plant - A Theoretical Approach

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