Gas Turbine Power Plants

Transcription

1 14 Gas Turbine Power Plants Introduction The gas turbine in its simplest form is a heat engine operating by means of a series of processes consisting of compression of air drawn from the atmosphere, increase of air temperature by the combustion of fuel in the air, expansion of hot gases to atmosphere, the who"le being a continuous flow process. It is thus similar to gasoline and diesel engines in its working medium and internal combustion, but is akin to the steam turbine in its aspect of the steady flow of the working medium. The compression and expansion processes are carried out in turbomachines, that is by means of rotating elements in which the energy transfer between fluid and rotor is effected by means of kinetic action, rather than by positive displacement as in reciprocating machinery. Thus in its simplest form a gas turbine consists of a compressor, a combustion chamber, and a turbine unit. Air which acts as a working fluid is compressed in the compressor and energy is added to it in the combustion chamber. The high energy fluid is then expands in the turbine and thus mechanical energy is produced. Part of this energy is used up in driving the compressor, which is usually mounted on the same shaft as that of turbine, and rest of the energy may be utilised for various purposes. Since the compressor is coupled with the turbine shaft, the come pressor absorbs some of power produced, by the turbine and hence lowers the efficiency. The network is therefore the difference between the turbine work and work required by the compressor to drive it. The gas turbine prime mover was first used in 1939 for large central station service. Since then several stations have been built with gas turbines to drive electric generators. Gas turbine generators have been built and electrical outputs upto 100 MW. In some situations gas turbines are the cheapest type of plants available. These situations are when they are used as intermittent or peak load plants in combination with the base load plants. These are particularly useful and economical when the amount of energy required is a small part of the total energy to be supplied by the whole system and the lcpad factor of the plant is less

2 GAS TURBINE POWER PLANTS 589 than 15%. In a large system the size of the gas turbine plants normally employed varies from 10 to 30 MW. These plants are cheaper in capital cost compared with steam stations of the same size. Also the fixed charges of these plants are comparatively lower than those for steam plants. The thermal efficiency of gas turbine plants is however lower compared to that of condensing steam 'plants (20 to 25% compared to 25 to 30%). No doubt a lower thermal efficiency results in increased fuel costs at low load factors, but this is compensated by lower fixed charges as well as lower operating and maintenance charges. A gas turbine plant has the advantage of high reliability, flexibility, low start up time and less space requirements. They are ideally suitable as peak load plants. At some places they are also used as base load plants. In India the 70 MW gas turbine plant at Namrup in Assam works as base load plant with natural gas as fuel. Uran-Gas turbine power plant in Maharastra is the second power plant established in the country Application of gas Turbine Plants Gas turbine plants have the following applications: 1. To drive generators and supply peak loads to steam, diesel or hydro plants. 2. To work as combination plants. 3. To supply mechanical drive for auxiliaries. These plants are suited for peak load purposes as already mentioned because their fuel costs are some what higher while their initial costs are low when these plants are used with conventional boilers they may be used for (a) supercharging or (b) heat recovery from exhaust gases. In supercharging system air is supplied to the boilers under pressure by a compressor mounted on the common shaft with turbine and gases formed as a result of combustion after coming out of the boiler; work in the gas turbine before passing through the economiser and exhausting through the chimney. The turbine drives the compressor and also generators, producing some additional power for the station.

3 590 POWER PLANT TECHNOLOGY In the exhaust heat recovery cycle the gas turbine plant is fitted with usual combustor and gets the gas supply from the combustor. The gases after expanding in the turbine enter the boiler and transfer part of the heat, to the boiler tubes. In the supercharging system heat transfer in the boiler increases by about 7 to 8%, while in the exhaust heat recovery cycle the heat rates are improved by about 4 to 5%. Also in the later case no mechanical draught is required because due to pressure of exhaust gases the furnace is under positive pressure. The gas turbine is widely used in air craft. There are many installations in ships as propulsion unit. Attempts are also being made to develop it as an engine for automobile use. There is a wide range of industrial applications ranging from petro-chemical, thermal process industries to generate utility industries Types of Gas Turbine Plants On the basis of combustion process the gas turbine may be classified as follows: 1. Continuous combustion of constant pressure type, the cycle working on this principle is called Joule or Brayton cycle. \ 2. The explosion or constant volume cycle; the cycle working on this principle is called Atkinson cycle. Another classification based on the path of the working ~1Jbstance, it is classified as : (i) Open cycle gas turbine in which working fluid enters from atmosphere and exhausts to atmosphere. The working substance air first is compressed in the compressor, and after compression, its temperature is raised by burning fuel in it. The products of combustion along with the excess air are passed through the turbine, developing power and then exhausted into the atmosphere. For next cycle, fresh air is taken in the compressor. (ii) Closed cycle gas turbine, in which working fluid is confined within the plant. The air is heated in an air heater (refer Fig (b) by burning fuel externally. The working air does not come in contact with the products of combustion.'the hot air expands in the turbine and then cooled in a precooler and supplied back to the compressor. The same working fluid circulates over and again in the system.

4 GAS TURBINE POWER PLANTS 591 Combustion chamber Compressor Air inta~e (a) Open cycle. -'- FUE L Gases. out c.c. Compressor Gas turbine Heat exchanger Fig Schematic diagram of open cycle and closed cycle gas turbines. Gas turbine power plants can be anyone of the following type. (a) Simple cycle Gas turbine power plant. (b) Combined cycle Gas turbine power plant. (c) Co-generation Gas turbine power plant. (a) Simple cycle Gas turbine power plant. It is based on Brayton cycle as stated above in which air is compressed to a higher pressure with the help of compressor and temperature of air firing fuel in the combustion chamber before expanding in the turbine. The difference between work output in expansion process and tl:1e work input in compression process is the net oqtput of Gas turbine.which will be converted into electricity.

5 592 POWER PLANT TECHNOLOGY (b) Combined cycle Gas turbine Power plant. This type of power plant is combination of simple Brayton cycle gas turbine and Rankine steam cycle as bottoming cycle. Exhaust gases from Gas turbine whose temperature is of the order of 550 C are led the heat recovery steam generator to generate steam which in turn drives steam turbine producing additional power. This cycle derives the advantage of higher temperature achieved in Brayton cycle and lower heat rejection (sink) temperature of Rankine cycle. Gross efficiency of the order of 47% can be achieved in such combined cycle power plant which is higher that super critical pressure conventional power plant. (c) Co-generation Gas turbine power plant: These power plants are similar to combinedcyclepower plants; the basic difference being that the steam generated in the heat recovery steam generator by the gas turbine exhaust gases is used for process application either fully or partially instead of generating electricity only Open and Closed Cycle Gas Turbine (1) Open Cycle Gas Turbine. The arrangement that has proved most successful in the continuous combustion or constant pressure gas turbine which is described as follows: A simple open cycle gas turbine plant consists of the turbine itself, a compressor mounted on the shaft or coupled to the turbine, the combustor, and auxiliaries, such as starting device, auxiliary lubrication system, fuel system, the dust system etc. A modified plant may have in addition to the above, an intercooler, a regenerator and a reheater. The arrangement of a simple gas turbine plant is shown in Fig Combustion Fuel "/chamber 3 c Power output Air from atmosphere Exhaust gas to atmosphere (a) Schematic diagram of gas turbine cycle.

6 (;AS TURBINE POWER PLANTS 593 ip 2 \ I 4 Constant pressure heat rejection r- --- "\r (b) P.V. diagram. 3 Turbine \ work \ \ 4 \ -----'il,-=- 'ill., f,t ~ \~';;;;,o~ cp --- (e) T-4J diagram. }<'ig Open cycle gas turbine. It will be noted that the essential components are three in number, namely, air compressor, a combustion chamber (combustor) and a turbine. The method of operation is as follows: Air enters the air compressor in which it is compressed, through a pressure compression ratio of some 4 or 6 : 1. There are some installations in which the pressure compression ratio is as high as 10 : 1 or even 18 : 1, although these llre not common. The quantity of the working fluid and speed required are more, so generally, a centrifugal,or an axial compressor is employed. Centrifugal compressors are often used in small gas turbines. An axial-flow compressor consists of sets of moving and fixed blades, resembling a turbine in reverse. In traversing the passages between the blades, the kinetic (motion) energy of the gas imparted by the rotation is changed into pressure (internal) energy (i.e. the pressure of the gas is increased). In the centrifugal compressor, air taken in

7 594 POWER PLANT TECHNOLOGY near the shaft of a rotating impeller blade is accelerated outward by centrifugal force. At the periphery, the high-speed air enters a diffuser, that is, a nozzle designed to convert kinetic energy into pressure energy. The compressed air is passed from the air compressor into the combustion chamber through a duct. If there are several combustion chambers then the take off volute from the air compressor will have ducts feeding the combustion chamber equispaced around it. In the combustion chamber fuel, which is usually a fuel oil, such as gas oil or kerosene is sprayed inform a burner and is burnt continuously. Thus the air passing through the combustion chamber has its temperature and volume increased while its pressure remains constant. Due to combustion, heat is added to the working fluid from T2 to T3. The product of combustion from the combustion chamber are expanded in the turbine from P3 to atmosphere. It will be noted upon inspecting Fig (a) that the turbine is coupled back to the air compressor by a coupling shaft. On the other side of tl1e turbine there is a coupling by means of which the turbine can be coupled to drive some external equipment. From this it will be observed that, in this case part of the turbine output is used to drive the air compressor and it is the net output which appears for driving external equipment. Due to continuous combustion which occurs in the combustion chamber, steps are taken to ensure that temperatures do not become too high. This is usually dealt with by supplying considerable excess air above the required for complete combustion. A special shroud is usually built round the burner in order to meter the air to the combustion space. This ensures that there is good burning of the fuel and that further air is fixed with the very hot combustion products further down to the combustion chamber. This brings the final combustion product temperature down to something workable before entry to the turbine. The mass (or weight) of air supplied to the compressor is three to four times the amount required theoretically for complete combustion (about 50 to 60 parts by weight of air to one part of fuel). The excess air mixes with the very hot combustion products and moderates the temperature of the gas somewhat, thus protecting both the combustor and the turbine blades from damage. The turbine illustrated in Fig (a) is arranged to develop shaft power. This being the case, the turbine would be designed to extract as much energy from the combustion products as possible before they are passed to exhaust. On the other hand, the gas turbine has a very wide use as ajet

8 GAS TURBINE POWER PLANTS 595 propulsion unit for air craft. The basic element of the jet propulsion unit are the same. There is however, no power out shaft and the turbine itself its built just large enough to drive the air compressor and auxiliaries, such as fuel-pump and oil pump are necessary. In these circumstances the combustion products will leave the turbine still with a high energy content. They are then passed rearward of into a nozzle from which they issue with a high velocity and thus they provide the necessary thrust for propulsion of the air craft. Gas turbines are not self starting machine as the reciprocating internal combustion engine it is necessary only to turn the engine over one compression, the engine will fire and then it will pick up speed on its own. The gas turbine will not start simply turning the burner on. It must first be motored up to some minimum speed, called the 'coming in speed' before the fuel is turned on.when this speed has been reached, the fuel is turned on ignited and the turbine will then pick up speed of its own. The turbine rotor is usually motored upto 'coming in speed' by a starter motor. This can either be electrical or some times it is a small turbine. Gas turbine C,C.II Combustion chamber AC Genera"tor Exhaust gases Fig Simple cycle glib turbine plant. The speed of the gas turbines varies considerably. It can be as low as 3000 rev/min. and as high as 35,000 rev/mm. Reduction gear boxes are fitted to high-speed turbines for coupling to external equipment in order to reduce this speed. Turbine of output as high as 20,000 kw have now been built, and air consumption as much as 130 kg/s is recorded. 2. Closed Cycle Gas Turbine. In an open cycle gas turbine plant, the fuel is mixed with air in the combustor and combustion gases are expanded in the gas turbine; the hot gases cause erosion and corrosion of turbine blades. To minimize these superior quality

9 596 POWER PLANT TECHNOLOGY of filel has to be used in the combustion chamber. The trouble or drawback of an open cycle plant is overcome in a closed cycle plant, where the fuel does not mix with the working medium air or gas. Since the close cycle continuously circulates the same working fluid, air or gases of a higlier density than air, the heat added must be supplied through a heat exchanger from an external source and the heat rejected from the system must be through a heat exchanger and a cooling medium. A schematic sketch of a simple closed cycle gas turbine is shown in Fig Combustion of the fuel takes place in the air heater and is external to the working medium of the system. The working fluid leaving the turbine is cooled down by the cooling water in the precooler and is recirculated to the compressor. Gases of combustion Air heater Fuel' Accumulators -r~jp",:':, Cooling medium The advantages Jo'ig Simple closed cycle glis turbine. of this system over that of the open cycle are: L R.educed size. The density of the working fluid is increased in the closed cycle by placing the system under an initial overall high pressure. Also since the working- medium is not required to support combustion, it is not mandatory that it should be air. It is possible to use a gas of heavier density and higher specific heat than air, such as the monoatomic gases; krypton, argon, Xenon and mercury vapour. This increase in the density, red4ces the physical size of all components and ducts of the system for the same power output and permits the use of higher temperatures for a given stress limit. Other working mediums may be helium, hydrogen or neon. The heat conductivity of hydrogen is about 6-8 times that of air and therefore requires smaller heat exchangers.

10 <;AS TURBINE POWER PLANTS Fuel. The closed cycle utilizing external heating can use an inexpensive solid fuel, such as coal. 3. No contamination. Since the working medium does not contain the gases of combustion the turbine and the generator are not subjected to carbon deposits and should remain relatively clean. There is absence of risk of corrosion and abrasion of the interior of the turbines. The compressor should remain free of dust and other foreign deposits since the working medium can be cleaned before being put into the systems. This means that \the periodic cleaning of the component is not necessary and the component efficienciesshould not change appreeiably with continued operation. Thus continued operation should not reduce the thermal efficiently. 4. Improved part load efficiency. The control of a closed cycle system is different from the open cycle. The power output of a closed cycle gas turbine can be controlled by changing the mass flow. The system pressure is proportional to the gas mass flow. By changing the pressure and mass flow, output changes but the temperature drop remains the same. Constant temperatures lead to constant heat drop and constant velocities in the turbine blading and hence the velocity triangles and consequently the turbine and compressor efficiencies remain constant for every power output. In case of an open cycle gas turbine the power control is affected by controlling temperature which affects the efficiency of the turbine at part load. 5. Fluid friction loss is reduced due to the higher Reyhold's number. 6. Improvement in the rate of heat transmission. 7. The regulation of the closed cycle gas turbine is simpler. The power output at constant speed can be varied by adding or subtracting the working fluid and thus altering the charge weight. Disadvantages of the closed cycle as compared to open cycle gas turbine engine are: 1. The use of high pressure requires a strong heat exchanger. 2. The complexity and cost of the system particularly in the load control, is increased. Since the system is under an initial high pressure with a working medium other than air, it is necessary that the system be gas tight. This add~to the cost and increases the engineering problems.

11 ~~._... _. JL J..IJ.l.l i, ". 598 POWER PLANT TECHNOLOGY 3. It is a dependent system Cooling water must be provided for the precooler. This eliminates the use of the system as an 'aeronautical' engine. The provision of cooling water is not a problem in marine propulsion and many land based applications but this is a disadvantages as compared to an open cycle plant. 4. A heavy, large air heater is required. The air heater is relatively inefficient compared to the internal combustion chamber of the open cycle gas turbine engine. Poor combustion efficiency results due to losses as radiation and other, since heat transfer is indirect. Inspite of the disadvantages and complexities of the closed cycle, it is higher in efficiency, smaller in weight and space, and is easier to adopt to marine propulsion than the open cycle gas turbine. It has a comparable or better efficiency than steam plants of the same power output with a great saving in weight and space Work Output and Thermal Efficiency of Constant Pressure Gas Turbine Plant The ideal gas turbine cycle using isentropic compression and isentropic expansion is called constant pressure cycle or Brayton or Joule cycle. Such a cycle is never possible in practice due to irreversibilities introduced in the operation on account of leakage, turbulence and internal friction. The actual processes of compression and expansion are not isentropic, and temperature of air (or gas) at the end of compression and at the end of expansion are higher than those in the case of an ideal cycle. The representation on pressure volume and temperature entropy planes is shown in Fig The actual cycle is represented by points 1, 2', 3, 4' and the ideal by points 1, 2,3, 4. where Since the compressor is coupled to the turbine then. Net work output-turbine output-compressor work But compressor work = rha CPa (Ti - T1) rha = mass of air flow/second CPa = specific heat of air at constant pressure T~= final compression absolute temperature~ Tl = intake absolute temperature....(14 5 1)...(14 5 2)

12 GAS TURBINE POWER PLANTS 59f / --~-_. \ \ \ l., /'.\ IsenlroplC\, 3 IT where Volume - cp,entropy Fig Ideal and actual gas turbine cycles on P-V and T-f diagrams. But the final compression temperature is above the normal adiabatic compression temperature du,e to turbulence as stated above. The frictionless adiabatic temperature is calculated using gas laws and is obtained from the equation, r. -1 T2 = Tl (~)----r;;-...(14.5.3) Ya = adiabatic index for air. The connection between the frictionless adiabatic compression temperature T2' and the final compression temperature Ti, is by means of the adiabatic or isentropic efficiency equation,. T2-T1 () lsentrop1c1]comp= 'r.' T Knowing the isentropic efficiency of the compressor, T2 can be calculated. Now to consider the turbine, by a similar analogy to that used for the air compressor, the turbine output is obtained from the equation. _ where Turbine output = mt Cpt (T3 - T';)..(14 5 5) mt = mass of the combustion products through turbine per second / CPt = specific heat of combustion products ~t constant pressure T3= inlet absolute t~perature of the turbine T'; = exhaust absolute temperature of the turbine;

13 600 POWER PLANT TECHNOLOGY Now the final exhaust temperature from the turbine will be above the frictionless adiabatic exhaust temperature as a result of turbulence, etc., which occurs in the turbine. In a similar way to that adopted in the case of the air compressor, so these two temperatures are connected by the equation where where. T3 - T4 IsentropIc TJturb - m T. 3-4 Isentropic TJturb = Isentropic efficiency at the turbine T4 = Frictionless adiabatic absolute exhaust temperature. T4 may be calculated from the gas law equation r, - 1 T4 = T3 (~;)r;-...(14 5 7) It = adiabatic index for the combustion products through the turbine. Net Workoutput = mt CPt (T3 - T';) - ma Cpa (Ti - T1)... (14 5 8) Now the mass of fuel used is usually small compared with the mass of air, and hence the mass of the fuel is often neglected. If this is the case, then and (T2 - m; = ma = m', say If the fuel is neglected the it can be considered that Then from Equation (14 5 8). Net turbine output CPt = CPa = Cp say. = mcp (T3 - T4) - mcp (Ti - T1) = mcp {(T3 - T4) - (Ti - TJ)}...(14 5 9) If m: = mass of air in kg/s, then from equation (14 5 9) Net power output of the turbine = mcp {(T3 - T4) - (Ti - T1)} Substituting for (T3 - T4) and (Ti - T1) in terms of (T3 - T4) TI)

14 GAS TURBINE POWER PLANTS p-- =mcptjt1'a j1- r r~li-tk1'1 1 C [r- r r 1-1 ]...( ) where r is the compression ratio, and r is the ratio of the two specific heats at constant pressure and at constant volume. This expression can be differentiated w.r.t. compression ratio r, keeping 1'a and 1'1 as constants, and equated to zero to find the value of r for maximum net work. The energy received in the gas turbine is in the combustion chamber at constant pressure. In the combustion chamber the temperature is raised from 1'';' to 1\ If the fuel mass is neglected as before, then energy received at constant pressure in combustion chamber Now thermal efficiency Which from equations = ritcp (T:l - 1'2) = change in enthalpy _ work output - energy input (14 5 9) and ( ) becomes...( ) Thermal 11= mcp {(I:1-1't) - (1'2 - I'd} --' mcp(t:1-1'2) = (T:l - 1';) - (1'2-1'1) (T:l - 1'2)...( ) With TIc = 11t = 100%, the following relationship is obtained for ideal conditions. rr r = (~~)2(r- I) )

15 ---~=~ IIIIIIIIlnRII 602 POWER PLANT TECHNOLOGY i.e., the maximum network from the plant, theoretically, the temperature of t)1e gas at the end of compression is the same as the temperature at the end of expansion. Heat supplied to plant = Cp (Ta - Ti) = mc p (Ta - T2) approximately = mc p Ta - Tlr) 7 [ r- 1] = mcpt1 ~: - [ (r)-rr- 1]... ( ) Thermal efficiency T/th = net work ';C T1 [ r-r-- r- 1 1 ]...( ) From equation ( ) it is evident that thermal efficiency of gas turbine plant depends upon the ratio of compression, the efficiencies of compression and expansion, the turbine inlet temperature and compressor inlet temperature. The expression ( ) can be differentiated with respect to r and equated to zero to find the compression ratio for maximum thermal efficiency. This results in the following expression for r.

16 GAS TURBINE POWER PLANTS ( ) If we assume nc = 100%, i.e. under ideal conditions y Then ry-1 = ~:...( ) The equation gives the optimum compression ratio for maximum thermal efficiency. Example A continuous combustion constant pressure gas turbine takes in air at 0 95 kg Icm2 (93 kn I m2) with a temperature of 20 C. A rotary air compressor compresses the air to a pressure of 5 70 kglcm2 (558 kn 1m2), with an isentropic efficiency of 83%. The compressed air is passed to a combustion chamber in which its temperature is raised to 867 C. From the combustion chamber the high temperature air passes into a gas turbine in which it is expanded to 0 95 kg I cm2 (93 kn I m2) with an isentropic efficiency of 80%. For an air flow of 10 kg Is and neglecting the fuel mass as small, determine : (a) the net power output of the plant the compressor; (b) the thermal efficiency of the plant. Take r= 1 4, Cp = 0 24 keall kg K) (= 1 00 kj / kg ok) Solution. For the compressor (refer Fig ) r-1 T2 = T1 (~~)-r- Isentropic = :4 = 293 x = 464 K efficiency for the compressor T2 - T1 1/c = mj.12 - T1 T2 - T1 _ = 206 K Ti. - T1 = 1/c if the turbine is coupled to

17 .;. 604 POWER PLANT TECHNOLOGY Ti = = 500 K. t2 = = 227 C For the turbine r- 1 T4 = T3 (~:)-r-= 510~2~~:: 7200K Isentropic efficiency of turbine ~1 - T, 7J - -'-- t- ~1-T T'; :: (T3 - T4)rlt = ( ) 0 80 = 336 K T, = T3-336 = = 804 K t" = 531 C Net power out put (MK8) Net power output (81 system) = mcp {(T3 - T';) - (Ti - T1)} = 10 x 0 24 ( ) = 312 kcal/s = 312 x 427 :: 1776HP. Ans. 75 = 10 x 1 x ( ) :: 1300 kw Ans. Thermal Work output 7J:: Energy input rhcp {(T3 - T4) - (Ti - T1)} = rhcp (~1- T2) (~1- T, )- (Ti - T1) _ = :: I 3 - "'~ = 20 31% Aus Methods to Improve Thermal Efficiency of Gas Turbine Plant. The efficiency and the specific work output of the simple gas turbine cycle is quite low inspite of increased component efficiencies.

18 GAS TURBINE POWER PLANl'S 605 Some modifications improve the thermal efficiency of a simple open cycle gas burbine, they are: (1) Regeneration (4) Gas Temperature (2) Intercooling and (5) Pressure ratio (3) Reheating (6) Combined cycle and Co-generation (1) Regeneration. One of the main reasons for the low efficiency of a simple gas turbine plant is the large amount of heat which is rejected in the turbine exhaust. Due to limitations of maximum turbine inlet temperature ann the pressure ratio which may be used with it, the turbine exhaust temperature is always greater than the temperature at the outlet of the compressor. So, if this temperature difference is used to increase the temperature of the compressed air before entering the combustion chamber and, thereby, reducing the heat which must be supplied in the combustion chamber for a given turbine inlet temperature, an improvement in efficiency can be attained. This utilization of heat in turbine exhaust can be affected in a heat exchanger called re-generator. In the regenerator the heat energy from the exhaust gases is transferred to the compressed air, before it enters the combustion chamber. Therefore, by this process there will be a saving in fuel used in the combustion chamber, if the same final temperature of the combustion gases is to be attained and also there will be a reduction of waste heat, thus there will be improvement in the cycle thermal efficiency. Fig shows a schematic diagram of such an arrangement. The exhaust gases from the turbine pass through the regenerator and give their heat to the compressed air, before it enters the combustion chamber, thereby reducing the amount of heat which must be supplied in combustion chamber to get a given turbine inlet temperature T:l' Thus regeneration improves fuel economy. The power output will be slightly reduced because of the pressure losses in regenerator and its associated pipework Heat exchanger 2 Air intake Gas turbine Generator Fig Temperature entropy diagram for regenerative cycle. )

19 606 POWER PLANT TECHNOLOGY The energy recovered from the exhaust in actual gas turbines varies from 50 to 90%. They operate most commonly between 70% and 80%, recovery. They percentage recovery of the heat exchanger is called its effectiveness. The thermal efficiency of gas turbines without heat exchanger is usually in the range of 15% to 20%. With a heat exchanger fitted, the thermal efficiency is pushed upto the range 20% to 30%. The temperature entropy diagram for the turbine arrangement with heat exchanger is illustrated in Fig It will be noted t T t Compressed air temp. lncrease in combustion JChamber Exhaust temp. 4' t T drop l in HE Max. temp. drop Jin H.E. ct>- Fig Temperpture entropy diagram for regenerative cycle. that the maximum exhaust temperature drop available in the exchanger = (T4 - Ti), since Ti is the lowest temperature in the heat exchanger. The effectiveness of the regeneration is defined as: e = Effectiveness = Rise in air temperature Max. possible rise Or it can be written _ T., - Ti - T4 - Ti as EmCp (T4 - Ti) = mcp (T4 - n) = mcp (T., - Ti) assuming Tit and Cp constant throughout then E (T4 - Ti) = (T4 - T6) = (T5 - Ti)...(14 6 1)....(14 6 2)

20 GAS TURBINE POWER PLANTS 607 Equation (14 6 2) will -enable the exhaust temperature from the exchanger, T6 to be determined. The nett turbine output will be as before....(14 6 3) However the energy required from the fuel is that required to increase the temperature from Ts to T3....(14 6 4) This is evidently less than that which would be required if no heat exchanger was fitted in which case the temperature increase required from the fuel would be from Ti. to Ts. Thus with a heat exchanger the thermal efficiency of the plant is increased. Thermal 11 = mcp {(7:1 - TI)- (Ti. - TI)}. r'<p (7:1-7:,;...(14 6 5) and again assuming m and Cp constant throughout, (T3- T';)- (Ti. - TJ) Thermal 1] = (T3_ T5)...(14 6 6) Example In a gas turbine cycle, the compressor compresses air from 100 kpa and 22 C to 600 kpa. The turbine inlet temperature is 800 C. It is lmown that a regenerator with 80% effu:iency is available, the isentropic efficiencies of the compressor and the turbine are 0 90 and 0 85 respectively. Determine the improvement in the efficiency resulting from the installation of the regenerator. Assume y == ] 4 and Cp = ] 03 kj / kg K. Solution. PI = Data given 100 kpa, P2 = 600 kpa, TI = = 295 Ie e = regenerator effectiveness = 0 8 1]is comp = 0 90, 1]is turbine = 0 85 T3 = = 1073 K First we will determine the thermal efficiency without regenerator and then with regenerator and calculate the improvement in the efficiency due to the regenerator. Referring to figure

21 608 POWER PLANT TECHNOLOGY t ~ W 0: ::J ~ <t 0: W (L ~ W 1-' 30 Ok ENTROPY ~ Fig K T.S. diagram for cxmplc (14 6 2). For the isentropic process 1-2 and T2- TI _ (~)(7- PI I)/r = 295 (0)(14-1)/1.4 = 492 K Similarly T4 = T:I P2 ( PI. )(7- I)/r X 6.4/14 = 643 K Equation for the isentropic compressor efficiency is T2 -TI 11i", =- ~--1' on rearrangement I 'T" _ T T2 - TI Jis C = : = 514 K 0 90 The expression for the isentropic turbine efficiency is i.e. 1;1 - T4 T/is turbine =,." T 1;1-4 T4 = T3 - T/isl (T3-1',.,) = ( ) = K.

22 GAS TURBINE POWER PLANTS 609 Also E =.0.80 = T.., - Ti. "" rn' 4-12 For thermal T5 = 0 80 ( ) = 669 K efficiency without regeneration Cp (T3 - T4) - Cp (Ti. - T1) 71th = Cp (T3 - T2) _ ( ) - ( ) = 0 26 or 26% Thermal efficiency with regeneration 71th = Cp (T.1- T4) - Cp (Ti. - T1) Cp (T.1 - T5) _ ( ) - ( ) = 0 36 or 36% The improvement in the thermal efficiency of the plant, due to regenerator instajjation is = = 38m (2) Intercooling. A regenerator, as discussed above does not change the work output of a gas turbine cycle. Two possible methods for increasing the work output are: (i) by reducing the work of compression, and (ii) by increasing the work done by the turbine. IntercooJing is used for decreasing the workdone on the compressor. One of the ways to achieve this is to cool the air after it has been partiajjy compressed, and this is accomplished by employing multi-stage compression and intercoojing between stages. Usually 2 to 3 stages of compression are used. IntercooJing improves the thermal efficiency, air rate and work ratio. Thus, if intercooling is used the size of the turbine and compressor can be reduced for the same output or alternatively greater work can be obtained from the plant of the same size. Fig shows the schematic diagram of a two stage intercooled gas turbine and Fig shows the indicator diagram for a two

23 610 POWER PLANT TECHNOLOGY Compressor Combustion chamber 2 3 Exhaust Intercoole r Fig Schematic diagram of a two-stage intercooled gas turbine. S it Entropy, _ Fig T- diagram for t\\o stage intcrcoolcd gas turbine cycle. stage compressor with intercooler. By employing multistage compression with intercooling between stages, the compression process in the compressor can be made to approach isothermal compression which requires less powr~r than the adiabatic compression. In the ideal state of intercooling the fluid should be cooled to its ambient temperature, i.e. the temperature of the fluid before compression, in each'stage ; and there should be no loss of pressure in the system. Also the maximum advantage of intercooling is obtained when the pressure ratio for each stage is the same. Maximum advantage of intercooling occurs when pressure ratio is high, compression efficiency is low and regeneration is employed.

24 GAS TURBINE POWER PLANTS 611 (3) Reheating. It is arfother method of increasing the specific work output of the cycle. An intercooling improves compressor performance, reheating improves the output from the turbine due to multiple heating. The gain in work output is obtained be cause of divergence of constant pressure lines on T > diagram, with an increase in temperature. Thus for the same expansion ratio if the exhaust from one stage is reheated in a separate combustion chamber and expanded, more output will be obtained than that obtained by expansion in a single stage. Fig shows the schematic diagram of a reheat gas turbine plant and Fig the corresponding T- > diagram. Reheater 3 Compressor HP Turbine lp Turbine Generator Fig Schematic diagram of a reheat gas turbine plant. 6" T t -4> Fig T-~ diagram of reheat cycle. Reheating involves extra equipment of combustors and high temperature resistance material for construction which adds to cost. Also, the complication of spliting the turbine and of producing suitlable controls may offsets much of the gain by use of reheater in m~ny cases.

25 612 POWER PLANT TECHNOLOGY In order to increase the thermal efficiency of a Brayton cycle, we can inaease the pressure ratio. However, as the pressure ratio is increased, the t<!mperature at the outlet of the compressor increases, causing problems with seals and metal fatigue. Also, the physical size of a compressor incr~ases with the increase in the pressure ratio. To minimize such problems, compression is accomplished in two or more stages. Ideally, in intercooling the compressed air at the exit of one stage is cooled to the inlet temperature of that stage and then compressed in the next stage. The compressor work required may be reduced by dividing the compression into two or more stages, and to cool the air or working fluid between them. Most of the heat of compression may then be removed by inter-cooling. The effect of i.nter cooling, when carried to the theoretical limits, is to have the work of compression approach an isothermal process (i.e., compression at a constant temperature). Ideally, intercooling the compressed air at the exit of one stage is cooled to the inlet temperature of that stage and then compressed in the next stage. 'i'he total work of compression in the cycle is the sum of the work for each compression stage. From a practical stand point, however, the effect of inter-cooling is to reduce the work of compression required to achieve a given pressure. The reduction of compressor work achieved in this manner results in an increase in the overall gas turbine output and usually improves the overall plant efficiency. In a reverse manner, the output of the turbine may be increased by dividing the expansion of the working media into a number of steps and the gas reheated between them. The reheating of the gas or working media back to the limiting turbine-inlet temperature allows a greater portion of the expansion to take place at higher temperatures. The theoretical limit of reheating would of course, be an isothermal expansion at the turbine inlet temperature. Again, from a practical stand point, the result of reheating is an increase in the output of the turbine through the same expansion pressure range although it has a negligible effect upon the overall efficiency. However, when reheating is properly utilised in conjunction with regeneration, the increase in overall efficiency is appreciable. The mechanical components and T-S diagram for the Brayton cycle with intercooling and reheat is shown in l<'ig.(14 6 7). As can be seen from the T-S d'iagram in the figure, both intercooling and reheat increase the network available from the cycle. For the cycle shown in Fig. (b). The thermal efficiency of the cycle with intercooling and reheat can then be calculated in the usual manner. If the compressor and the turbines have isentropic efficiencies of less than 100 percent and if there is also regeneration, the analysis becomes a bit more complex but presents no extraordinary difficulty.

26 GAS TURBINE POWER PLANTS 613 (a) t-- 11 :::! w t-- ENTROPY S ~ (b) Fig Brayton cycle with inter cooling and reheater. (a) Mechanical components, (b) T-S. diagram. We = (h4 - h3) + (hz - hi) WT = (hn - N;) + (h7 - hs) qin = (hn - h4) + (h7 - N;)

27 614 POWER PLANT TECHNOLOGY The open cycle gas turbine with regenerator, intercooler, and reheater is shown in Fig. (14 6 8) with T. S. diagram. REGENERATOR 9 T GENERATOR 8 TURBINE INTERCOOLER REHEATER CaOUNG' MEDIAM (a) ENTROPY.. S (b) Fig S. Brayton cycle with regenerator, intercooler, and rcheater. (a) Mechanical components (b) T-B diagram.

28 GAS TURBINE POWER PLANTS 615 We = (112- hi) + (h4 - h3) Wr = (~ - h7) + (hs - 119) qin = (~ - ~) + (hs - h7) The thermal efficiency of the cycle is expressed as _ [(~ - h7) + (hs - h9)j-=.[(h2 - hi) + (h4 - h3)) 17th - (~ _ ~) + (hs - h7)... (14 6 7) Gas Temperature. The thermal efficiency of a gas turbine, as defined earlier, depends in the first place on the intake gas temperature, which should be as high as possible. In practice, this temperature is limited by the potential for blade damage. Gas temperatures are commonly in the range from 800 to 900 C. By the use of special alloys and protective refractory coatings for the blades, the temperature can be increased to about 1250 C or so. For still higher temperature, it would probably be necessary to use special means of cooling the blades. However, the increase in thermal efficiency resulting from an increase in gas temperature must be balanced against the greater cost of the turbine. (5) Pressure ratio. The thermal efficiency of a gas turbine is related to the pressure ratio (i.e. the pressure in the combustor relative to the ex.haust gas pressure). Upto a point, an increase in the pressure ratio, to about 10 at moderate gas intake temperatures or to 20 at high temperatures, is accompanied by an increase in efficiency once again, however, the increased cost of the equipment must be taken into account. Combined cycle and Cogeneration. Another approach to increasing the efficiency of fuel utilization would be in a combined cycle or cogeneration system. The still hot exhaust gas from the turbine provides the heat for generating steam in a waste heat boiler. The steam is then used to operate a steam turbine i.e. combinedcycle generation. Alternatively, the hot gas might be used to produce process heat i.e. cogeneration. Example In a two stage gas turbine cycle with ideal inter cooling and reheat, the pressure ratio in each stage is 3 5. The inlet conditions are 300 K and 100 k pa and the temperature at the inlet to the turbines is 1300 K. A regenerator with an efficiency of 70% is used to improve the efficiency. Determine the compressor work, the turbine work, and the thermal efficiency of the cycle; Take r = 1 4, and Cp = 1 03 kj / kg K.

29 616 POWER PLANT TECHNOLOGY Solution. T.8. diagram for Two-stage regenerative gas turbine cycle with ideal intercooling and reheat is shown in Fig. (14 6 7). For isentropic process 1 2 We havet2 = TI ('7PI / )(r- I)/r = 300 X (3 5) 4/1.4 = K = T4 also Ideal intercooling and reheating is to be considered. (r- I) Likewise T7 = Tg = T6 (3'5f -r- The compressor input is The turbine output is For the regenerator = 1300 (3 5)-04/14 = K. We = Cp [(T4 - T3) + (T2 - T1)] = 2 Cp (T2- T1) = 2 x 1 03 ( ) = kj/kg Ans. WI = Cp [(T6- T7) + (T8 - Tg)] = 2 Cp (T6 - T7) = 2 x 1 03 ( ) = kj/kg Ans. 0.7 = (1:<; - T4) (Tg - T4) or T5 = 0 7 ( ) = K. The heat supplied is given by qif/t'=(h6- h5) + (h8- h7) = Cp [(T6 - T5) + (T8 - T7)] = 1 03 [( ) + ( )] = 954kJ/kg

30 GAS TURBINE POWER PLANTS 617 Now thermal efficiency can be calculated 11th = Turbine work - compressor work Heat input = WT - We = or 56 5% Ans Main Components of A Gas Turbine Plant The basic gas turbine components are: (1) Compressor, (2) Combustion chamber, (3) Turbine and (4) Heat exchangers. 1. Compressor. A gas turbine compressor should be able to handle a relatively large volume of air or working media and delivering it at 4 to 6 atmospheric pressure with the highest possible efficiencies, moreover, the compressor should be such as can be coupled to the turbine shaft which runs at very high speed ranging from about 600 rpm to 40,000 rpm. On the above basic requirements, only a centrifugal or axial compressors can be employed. Reciprocating compressors can not be used, because it sutters from a number of disadvantages, such as, inertia of moving parts, sliding friction of the piston inside the cylinder, limitations in speed, etc. and are not considered suitable for use in gas turbine plants. However, a version of this compressor in the 'free piston' design, which eliminates use of crank shaft and connecting rods and is at present being developed for use in these plants. The centrifugal compressor consists of a rotor called impeller provided with vanes and moving in a casing or scroll. The inlet section at the hub of the impeller on one side, called the inducer, is curved to minimize entry losses and is provided with vanes to direct the air to the eye. A schematic diagram of a radial bladed centrifugal compressor is shown in Fig Air is given a whirling motion at high velocity by the impeller and is thrown out of it by centrifugal force. The static pressure of air increases to the tip. A stationary I passage surrounding the impeller diffuser helps to convert most o~ the velocity head into pressure head as the air has a high velocity when it leaves the impeller. The impeller converts the mechanical

31 618 POWER PLANT TECHNOLOGY Diffuser throat (a) Collector Depth of vaned diffuser Vaneless diffuser (b) (e) Fig Schematic of a centrifugal compressor. energy imparted to air by the rotation of the impeller into pressure and kinetic energy. The pressure rise in the impeller is due to diffusion action (i.e. the relative velocity decreases from inlet to outlet due to diverging channel area) and the centrifugal action (i.e. the air enters at lower diameter and comes out at higher diameter). The rest of the kinetic energy available at the tip of the impeller is converted into pressure energy in the vaneless and vaned diffuser. The vane less diffuser converts some part of the kinetic energy into

32 GAS TURBINE POWER PLANTS 619 pressure energy and stabilizes the flow so that it enters the bleded diffuser without shock. From the vaned diffuser the air is collected in the volute casing and comes out from the outlet pipe. For the gas turbine (instead of putting the volute casing), a 90 bend is provided to take air to the combustion chambers. The present day practice is to design the centrifugal compressor such that about half the pressure rise occurs in the impeller and half in the diffuser. The impeller blades are made in two types, the radial blades and the backward curved blades. Radial bladed impeller is suitable where low weight and dimension are required, whereas the backward turned blade is suitable where higher efficiency is preferred. In the gas turbine radial bladed impeller is used due to lighter construction and less stressed impeller. A pressure ratio of 4 5 : 1 may be obtained in a single stage centrifugal compressor. In a multistage centrifugal two or more impellers operating in series on a single shaft are provided in a single casing. The effect of multi-staging is to increase the delivery pressure of air, as air compressed in one stage of machine is fed into the next stage for further compression and pressure is multiplied in each stage. The overall efficiency of a multistage compressor is lower than the efficiency of individual stages. Labyrinth packings provide sealing effect on the air and prevent leakage between the impellers of various stages and from inside the compressor to outside through shaft end connections. The compressor discharge can be controlled by varying the speed. The centrifugal compressor is superior to the axial flow compressor in that a high pressure ratio can be obtained in a short rugged single stage machine. It is relatively insensitive to surface deposits, has a wider stability range and less expensive. However the efficiency is lower, the diameter greater and it is not as readily adoptable to multistaging. For higher pressure ratios multistage centrifugal compressor does not prove to be as useful as an equivalent axial flow compressor. Therefore, when high pressure ratios are needed, axial compressor is advantageous and it always used for industrial gas turbine installations. Although, the axial compressor is heavier than the centrifugal compressor but it has higher efficiency than the centrifugal compressor. The axil fzow compressor consist of a rotor and a stator as shown in Fig The rotor (i.e. moving element) consists of rows of moving blades and the stator (i.e. stationary component) consists energy imparted of a rows to air of stationary by the rotorblades. is converted Someinto partpressure of the kinetic energy ( in the rotor due to diffusion action and the rest is converted in the

33 620 POWER PLANT TECHNOLOGY Rotating blades Guide blades Stat ionary blades ROTOR Casing Fig Arrangement of rotor and stator in axiall10w compressor. stator. The stat,or blades also redirects the air into an angle suitable for entry to the succeeding rows of moving blades. The rotor as well as the stator blade channels are of diverging type. A row of moving blade with a succeeding row of stationery blades is called a stage of axial compressor. Blades are usually made of air foil section. The important characteristics of the axial flow compressor are its high peak efficiencies, adoptability to multi staging to obtain higher overall pressure ratio, high flow rate capabilities, and relatively small diameter. However, the axial flow compressor is sensitive to changes in air flow and rpm, which results in a rapid drop off in efficiency, i.e. the stability range of speeds for good efficiencies is smal1. These latter characteristics limit the part load capabilities; of this type of compressor and are considered undesirable in some installations. (3) Combustion Chamber (Combustor) : Generally the air fuel ratio in open gas turbine varies from 50 : 1 to 250 : 1, to keep the turbine inlet temperature down to permissible limits. The combustion process taking place inside the combustion chamber is quite important because it is in this process that energy, which is later converted into work by the turbine is supplied. Therefore, the combustion chamber should provide thorough mixing of fuel and air as well as combustion products and air so that complete combustion and uniform temperature distribution in the combustion gases may be achieved. Combustion slwuld take place at high efficiency because losses incurred in the combustion process have a direct effect on the thermal efficiency of the gas turbine cycle. Further the pressure losses in the combustion chamber should be low and the combustion chamber should provide sufficient volume and length for complete

34 GAS TURBINE POWER PLANTS 621 combustion of the fuel. Hence requirements are: of a comhustion chamber (a) lower pressure loss; (b) high combustion efficiency; (c) good flame stability; (d) low carbon deposit in the combustion chamber, turbine and regenerators; (e) low weight and frontal area; (n reliability and serviceability with reasonal1ife, and (g) through mixing of cold air with the hot products of combustion. The types of construction of the combustion chambers in use are in general: (1) tubular or 'can' counter flow. (2) tubular or can straight-through flow, and (3) annular parallel flow. Although theoretically the annular chamber possesses advantages over the 'can type', this has not been realized in practice. Also, for test and for replacement of burned out chambers, the 'can' type is cheaper and more practical. For these two reasons, the 'can' type predominates in current practice. A typical combustion chamber design employs an outer cylindrical shell with a conical inner sleeve which is provided with ports or slots along its length. At the cone apex is fitted a nozzle through which fuel is sprayed in a conical pattern into the sleeve, with an igniting device or sparking nearly. (Refer Fig ) A few air ports provided close to the location of the nozzle, supply the combustion air directly to the fuel and are fitted with vanes to produce a whirling motion of oil and thereby to create turbulence. The rest of the. air admitted ahead of the combustion zone serves to cool the combustion chamber and the outlet gases. I Gases /T- ". '--'--,,---- Nozzle Outer shell \ \" --I --~UL~-~~~'~~ ~ conic~~::eve Fig Arrangement ;~~:~:gt ~~:I of a combustor.

35 622 POWER PLANT TECHNOLOGY The combustion chamber in the open cycle gas turbine engine is the most efficient component of the gas turbine. Efficiencies of between 95 and 98 percent are obtained over a fairly large operating range. The combustion chamber of a closed cycle gas turbine engine is actually a heat exchanger. The heat added to the working fluid air or gases of higher density, must be supplied through a heat exchanger from an external source. The working media is thus not contaminated with the products of combustion. The problem has been to design an efficient heat exchanger of a practical size, capable of supplying the heat addition required. However cheaper fuels such as soft coal may be used in the heat exchanger of a closed cycle gas turbine. (3) The gas turbine. The construction and shape of the gas turbine blades are very similar to that of steam turbines. It differs only in the blading material, the means for cooling the bearings and highly stressed parts, the thermal distortion due to higher temperatures, and high ratio of blade length to wheel diameter to accommodate large gas flows. The main requirements for the gas turbines are light weight, high efficiency, ability to operate at high temperatures for long periods, reliability and serviceability. Special cooling arrangements for the blades may some times be used in gas turbines. These include supply of cooling air near the rim or use of different materials for rim and hub sections. The blade speed is selected on the strength consideration of the wheel. The arrangement of the rotor and stator blades in the gas turbine is similar to that of steam turbine. As in the case of steam turbines, gas turbines may be irripulse or reaction. If the entire pressure drop of the turbine occurs across the fixed blades, the design is impulse type, while if this drop takes place in the moving blades, the fixed blades serving only as deflectors, the design is called reaction type. Generally the blades are made of Nimic 80 alloy (heat resisting). (4) Heat Exchangers. Regenerator and the intercooler are the heat exchangers used in gas turbine plants. In the heat exchangers, heat transfer takes place between exhaust gases and cool incoming air, while in the int.ercooler the heat transfer occurs between the hot air under compression and cooling water. Since water has a much better heat transfer coefficient than do air and gases, the surface required for the same amount of heat transfer is much less in the case of the intercooler than for the regenerator. The regenerator is generally shell and tube construction, with gas flowing inside the tubes and air flowing outside, in opposite direction.

36 GAS TURBINE POWER PLANTS Auxiliaries and Controls Gas turbine engines need additional equipment to serve the main comp9nents ; these include; starting motor or engine, auxiliary lubricating oil pump, fuel control system, oil coolers and filters, inlet and exhaust mumers (silencers), air and gas ducts and plant control panel. In addition there are also automatic devices for alarm and shut down. The starting motor or engine drives the gas turbine ana compressor through a clutch and step up gear. The starting gear is mounted on the shaft at one end. The clutch is often made to work under air pressure. The rotation of the turbine-compressor shaft, for about 5 minutes at speed of 500 to 1000 rpm results inelimination of un burnt fuel from the air-gas flow system. Speed is then increased to 4000 to 5000 rpm and fuel is allowed to enter the compressor where it is made to ignite and gases produced are passed on to the turbine. The turbine then slowly starts under influence of the gases, and at about 6000 rpm, the starting motor is shutdown and clutch disconnected automatically. Feeding in additional fuel brings the turbine upto rated speed. Often the drive for the lubricating oil pump is taken from the starting-up gear. High pressure oil from the pump is supplied to the hydraulic control system and low-pressure lubricating oil for the gas turbine, gears and driven apparatus. A separate motor-driver pump usually acts as stand-by if main pump fails. A failure of the lubricating pump system results in stopping of the unit automatically. The fuel feed is made responsive to the speed governor, for generator drive. A reduction in speed (when load increases) opens the fuel valves to restore normal speed. A rise in speed (when load decreases) closes the fuel valves to lower the rate of fuel feed and restore speed. The duct system includes the main connection between the compressor and combustor, and between the combustor and the turbine in the simple cycle plant, and an addition of other such connection when additional heat exchangers are employed. Compressor inlet air usually enters t.he gas turbine unit from outdoors through a filter and duct. The filter proves necessary because a slight buildup of solids (fouling) on compressor blading can seriously reduce its efficiency. Both filter and ducts must be sized to minimize air-pressure drop. Any undue loss in pressure directly reduces the capacity of the unit. The exhaust duct and stack must also be size to minimize pressure drop because this loss raises the turbine-exhaust pressure and reduces turbine capacity and efficiency. Exhaust gas from the

37 624 POWER PLANT TECHNOLOGY stack must not be allowed to recirculate to the compressor intake; this can be minimized by increasing stack height, which also raises pressure loss. Another important point regarding ducts in to support them suitably so that vibrations are reduced to minimum. They should be stift enough to resists vibration caused by the air and gas-cows. Furthermore, the ducts should be capable of taking up the expansion at joints due to changes in temperature, and so adequate expansion joints should be incorporated wherever necessary. The exhaust duct and connections between combustor and turbine, in particular, should be capable of standing high temperatures (about 500 C for simple open cycle turbines and 350 C for regenerative cycle). Filters of various types are used on air compressor inlets. They may be oil bath type or dry type. The viscous types are use a filtering material dipped in oil that catches air-brone particles as they pass through. They dry type filters use glass fibres as the trapping agent. The air capacity through the filter should not increase about 2 mi see, which gives a pressure drop of about 13 to 19 mm of water gauge. Silencers may be used at the inlet and exhaust of air and gas respectively. The air velocity through the inlet muller may be about 60 mlsec Fuels for Gas Turbine Plants Gas turbines can use a wide variety of fuels, solid, liquid and gases. The ideal fuel is ofcourse natural gas but this is not always available. Natural gas which is mainly methane has a very high calorific value and is generally used for auxiliary power generation within the oil fields. Blast furnace and producer gas can also be used for these plants. Liquid fuels of petroleum origin such as distillate oils or residual fuels (including fuel oils, furnace oils, boiler fuel oils) are most commonly used for such plants. These fuels are generally costly. When using such fuels one has to be very careful that the fuel used possess proper volatility, viscosity and calorific value. Moisture and suspended impurities should not be there, as they may clog the small passage ofthe.nozzles and damage valves and plungers of the fuel pumps. Residual oil usually contain sodium, vanadium, and calcium as part of the ash constituent. They corrode hot metals and build up hard deposits that choke gas passage in the blading. Residual oil may be treated by heating it, then mixing it, then mixing it with 5% of water. Two centrifuges in series receive the mixture to remove the water that takes with it most of the sodi urn originally in oil.

38 GAS TURBINE POWER PLANTS 625 The increased use of heavy oils has been limited by the effect of vanadium corrosion and deposits build up on blades. Distillate fuels burns more easy than doresiduals fuels. Therefore when starting the unit for cold initially distillate fuels feed into the combustor after which residual fuels may be fed. In cold climate it may be necessary to preheat the residual fuels. Solid fuels (for example pulverised coal) may be used but they create coal handling and ash handling problems. The efficiency of coal fired gas turbine plant is lower than that of oil fired plant. Present day gas turbine plants use mainly natural gas liquid petroleum fuels Plant Layout In the case of a gas turbine plant the main building is the turbine house in which major portion of the plant as well as auxiliaries are installed. In many respects it is similar to the steam plant turbine house. The fuel oil storage tanks are arranged outside but adjoining the turbine house. In some installations even heat exchangers are placed out doors. The rotating parts of the plant form a very small part of the total volume of the plant since it is the intercoolers, combustion chambers, heat exchangers, waste heat boilers and interconnecting ducts work which have to be arranged and accommodated. It is these components which occupy the major portion of the total space. Air filter Intercooler. LP Turbine Heat exchanger,. I, \ " \ ~-,:\ 1 Alternator g~~ motor /" --' /" --[,../ I I H.P --//1 I Turbine --- -'''0- // CombusflO chamber Fig Layout of a gas turbine power plant.. A typical layout of gas turbine plant is shown in Fig

39 626 POWER PLANT TECHNOLOG'Y The purpose of the air filter is to clean air. From this air filter air flows to the L.P. compressor. From there the compressed air enters H.P. compressor via intercooler. The air leaving the H.P. compressor enters heat exchanger, the- hot air from there flows to the combustion chamber. Products of combustion are first expanded in H.P. turbine and than in L.P. turbine. The layout of a gas turbine plant has a very important effect on the overall performance of the plant. Since there may be a loss of as much as 20% of power developed in the interconnecting ducts with a large number of sharp bends. Great care has therefore to be exercised in the design and layout of the air as well as gas circuits Comparison of Gas Turbine Plants with Other Plants (A) Comparison with Steam Power Plants 1. Space requirement for a gas turbine plant is smaller compared to a condensing steam plant of equal size. 2. A gas turbine plant can be started quickly and has a short starting time in comparison to steam plant. 3. The capital cost of a gas turbine plant is lower than that of a comparable steam power plant. 4. The circulating water consumption is less in comparsion to that of a steam turbine plant of the same size. This makes site selection easier. In water scarcity areas they have great application. 5. These plants can be readily located in cities and industrial centres very near to the areas of heavy power demand. 6. The gas turbine plant uses fewer auxiliaries compared with steam plant. Therefore smaller size of the gas turbine components enables complete work tested units to be transported to the site. 7. Storage of fuel is much smaller and its handling is easy. 8. The fuel consumption during starting and shutting-down periods is low. 9. Foundations and buildings are less costly. 10. Time for installation required is less 11. Number of personnel required for operation is hardly one-third compared with that for a steam plant of same size.

40 GAS TURBINE POWER PLANTS Problems of coal and ash handling as encounter in case, of steam plants are eliminated in open cycle gas turbine plants using gas or liquid fuel. 13. The components and circuits of a gas turbine plant may be arranged to give the most economic results in any given situation. This is not possible in case of steam power plant. 14. A gas turbine plant becomes more economical for operating below a given load factor as saving on the capital charges outweighs the additional cost of fuel. 15. The heat rate of gas turbine is gerierally higher than the heat rate of steam turbine. 16. Specific weight of steam turbine is generally more than twice of the specific weight of gas turbine. 17. The operation of turbine is simpler and its capital and maintenance costs are lower than those of steam turbine plant. (B) Comparison with Diesel Power Plants 1. As compared to diesel power plants, gas turbine have higher mechanical efficiency due to fewer sliding parts in construction. While the adiabatic expansion of gases in the cylinder of diesel engine is incomplete, the gas turbine allows for a more or less complete expansion of gases which increases power output. 2. Gas turbine plants have easier maintenance and reduced attendance charges. 3. Gas turbine plants have lower cost of buildings and smaller site area. 4. Gas turbine being rotating machine is well balanced at all speeds, so less vibrations. 5. There is greater flexibility in design of a gas turbine plant as the processes of compression, combustion and expansion occur independent units unlike diesel plant in which operations occurs in the cylinder of the engine. 6. The gas turbine is a compact powerful machine and specific weights are low, as 15 kglh.p. (20 kgikw), as compared to 85 kglh.p. (112 kglkw) for diesel engine. 7. The gas turbine, is able to operate with lower graaer of fuel oils than is possible with diesel engines. Also low grade waste gases may be utiliz~ as fuel.

41 628 POWER PLANT TECHNOLOGY 8. Heat rate of a gas turbine is generally better than the heat rate of a diesel engine. 9. Water requirements are much less in gas turbine plant in contrast to a diesel plant. However, the gas turbine plant has lower thermal efficiency as compared with a diesel plant, as a great deal of its power output is used to run the compressor. The diesel plant is somewhat easier to start and needs less elaborate cooling arrangements Combination Gas Turbine Cycle Gas turbines after several advantages for Jifferent type of service peak load, emergency standby, base load, hydrostation stand-by etc. In some of th.ese services the quick starting ability makes the gas turbine plant desirable. The combination gas turbine-steam turbme cycles aims at utilizing the heat of exhaust gases from the gas turbine and thus, to improve the ov~rall plant efficiency. The heat content of gas turbine exhaust is quite substantial. Gas turbine exhaust has a temperature of around 500 C. The oxygen content in this exhaust is around 16% compared with 21% in atmospheric air. A simple cycle gas turbine plant wastes this energy to atmosphere, while a regenerative gas turbine plant recovers much of this heat to raise overall thermal efficiency. But instead we can use the gas turbine exhaust as a heat source for a steam plant cycle. The combined steam and gas turbine cycle provides the highest efficiency turbine system available at the present time. The efficiency of the combined cycle is higher than efficiency of a standard regenerative cycle gas turbine. There are three popular designs of the combination cycles: 1. Gas turbine exhaust gases used for feed water heating, 2. Employing the exhaust gases as combustion air. in the steam boiler, and 3. Employing the gases from a suppercharged boiler to expand in the gas turbine. Fig shows a combined cycle in which the gas turbine exhaust passes through a heat exchanger to feed water for the boiler of the system plant. When this arrangement is used, bleeding of steam from the steam turbine (for the purpose of fed water heating) is not necessary. The full steam supply to the steam turbine is available for expansion and producing mechanical power.

42 GAS TURBINE POWER PLANTS 62fl Fuel to combustor Feed water heater To stacks Air in Turbine exhaust Fig Use of exhaust gases to heat feed water of steam cycle. If bleeding is also used, the requirement of bled steam is much less than what would be required when no feedwater heating with exhaust gases is employed. Arrangement is shown in figure, using both exhaust gases and bled steam for the feedwater heating. Further the gas inlet temperature to turbine can be increased and this results in an overall increase in efficiency of the plant. Fig shows a combined cycle in which the gas turbine exhaust is used as preheated air for the boiler of the steam plant. To stock Baiter G.T. rg Generator Air in Gas turbine 12xhoust Fig Combined gas and steam plant (Heat reccvery boiler). The gas turbine exhaust has around 16% oxygen which is enough to support combustion in the boiler. Supplementary fuel and air can be fed to the boiler, which would be larger than the conventional boiler. About 5% improvement in plant heat rate can be obtained by the use of combined cycle. Fig shows a flow diagram for the supercharged boiler. Here the combustors of the gas turbine unit are replaced by a steam generator having a supercharged fumace, and the gas-turbine exhaust, heats the feedwater before it enters the boiler. The heat transfer rate in the boiler are increased due to the high density of air. So, the boiler weight get reduced by as much as about 50%. Heat rate also gets improved by about 7 to 8 per cent. The station capacity is also increased and there is only a slight increase in the cooling water arrangement.

43 630 POWER PLANT TECHNOLOGY Feed water To stack Flue gases Feed water Exhaust gases Alternator Star(lng motor tu~brne ~~F=() Fig Flow diagram of supercharged boiler cycle Advantages and Disadvantages of the Gas Turbine Power plants. Advantages (1) Low Installation cost. Presently, the installation cost / MW capacity for a conventional fossil fuel power plant of unit size 200/500 MW is nearly Rs 10 millions, whereas, it is only Rs. 6 millions in case of combined cycle power plant of capacity 300 MW which comprises of two nos. Gas turbine of 100 MW each and a steam turbine of 100 MW. In case of simple cycle, Gasturbine power plant installation costjmw capacity is only Rs 35 millions for a unit of size 100 MW. (2) Higher Efficiency. Combined cycle plant efficiency is of the order of 42-47% which are nearly 10-20% more efficient than fossil fuel conventional power plants. (3) High Reliability/Availability. Combined cycle power plant is highly reliable to the extent of 85% to 90%. Some combined cycle power plants achieved even 95% reliability for years long. As per the North American Electric Reliability Council (NERC) which collects and analyses data of such electricity generating plants, simple cycle Gas turbine power plants achieved a reliability of 95 7%. These figures are very high when compared to reliability figures of the order of 65% generally achieved for conventional power plants. Meantime between failure CMTBF) which is mean operating duration between two forced outages, for Gas turbine power plant is above 1000 hours whereas it is nearly 500 hrs for conventional power plants.

44 (:AS TURBINE POWER PLANTS 631 (4) Low Gestation Time. The installation time for a simple cycle gas turbine power plant capacity can be installed in months and the rest of the capacity which is steam cycle plant can be added in months more. Some manufacturers keep gas turbine units as off-shelf items, since gas turbines are of standard equipment. These durations are very much on favourable side when compared to installation time of 48 and 60 months for conventional 200 MW and 500 MW plants respectively. This advantage contributes to less interest charges during construction and escalation of power plant cost due to inflation, faster returns on the investment and indirectly helps in improving the national economy in promoting the unrestricted growth. (5) Fast Starting Characteristics. Gas turbine power plants can achieve full load within minutes from cold start condition. Even in combined power plants, two-third (2/3) of full load can be realised within minutes by operating gas turbines in simple cycle with the help of by-pass stack. Entire combined cycle plant can be brought to full load within, 1/2-2 hrs (one and half-two hrs). This fast starting characteristics make them favourable to run as peaking power plants or two shifts in a day mode unlike in conventional thermal power plants. (6) Less Water Requirements. Simple cycle gas turbine power plants need neglibrible amount of cooling water for its auxiliaries only. However, this water consumption may also'be eliminated by resorting to air cooling methods. Water consumption is nearly 40% of the requirement of conventional thermal power plants. In combined cycle power plants since steam cycle generates only one third (1/3) of the total power output. If availability of water for condenser cooling purpose in difficult even for this quantity, air cooled type condenser can be envisaged. Efficiency deterioration due to higher condenser back pressure is not as much as in conventional power plants due to only one-third (1/3) of power output contributed by steam cycle in combined cycle power plants. (7) Less Pollution Problems. Gas turbine power plants do not have dust pollution problem unlike in coal fired power plants. Thermal pollution is also comparatively less due to higher efficiency in case of combined cycle plants. There is no ground water/water pollution around ash disposal area due to ash dumping in coal fired power plant. Pollution due to blow down from cooling water system is also less when compared to conventional thermal power plants. Though gas turbine power plants emit more oxides of nitrogen (Nox) when compared to conventional thermal power plants. These emissions can be controlled easily to/the acceptable levels by stearil!

45 632 POWER PLANT TECHNOLOGY water injection into gas turbine combustion chamber unlike in conventional thermal power plants. (8) Flexibility in locating Power plants. Gas turbine power plants can be located in areas where minimal infrastructural facilities are available. There is no absolute requirement of site to be connected by rail road unlike in coal fired power plant to transport vast quantities of coal to the power plant. One of the main factors to decide the location of a thermal power station is availability of sufficient quantity of water for its consumption in the near vicinity. Since combined cycle power plants need only 40% of water requirement of conventional thermal power plant, its flexibility with respect to water availability is more. Combined cycle power plants can be located even in deserts by envisaging air cooled condenser with marginal loss in its thermal efficiency unlike conventional thermal power plants. The requirement ofland for combined cycle power plant is only 10 to 12% of coal fired power plant of equal capacity. There is no requirement of land for disposing ash and storing coal. Land requirement for water storage is also less unless on perennial water supply facility is existing. Less land requirement and minima] pollution problems favour these p]ants to be located near load centres like cities. (9) Less man/mw Ratio. Gas turbine power plants need less man power to erect, operate, and maintain than conventional thermal power plants. So human management problems are less comparatively. (10) Clean operating Conditions. These power plants can be kept in absolutely dean and tidy condition unlike coal fired power plants which adds to the morale of work force. Disadvantages (1) Need of Good Quality Fuels. Gas turbine power plants can be operated only with gaseous fuels mainly natura] gas or liquid fuels such as HSD, naptha, natura] Gasoline Liquid (NGL), heavy residual oils etc., which are scarce in our country and their inevitable (or unavoidable) requirement in other industries such as petrochemical industry, fertilizer industry and transport industry. Leser Plant Life. Gas turbine power plants have operating life of around years when compared to 25 years for conventional power plants. Gas turbine power plants life depends mainly on the type of fuel used and the actual combustion temperatures subjected over design combustion temperature. With heavy residua] fuels such

46 GAS TURBINE POWER PLANTS 633 as LSHS, HPS etc., gas turbine life deteriorates faster than with clear fuels such as natural gas, HSD etc. Uneconomical Partial Load Operation. Gas turbine power plants efficiency is considerably low when they are operated at partial loads. However, with inlet guide vane system, efficiency deterioration can be checked upto 80% ofrated load Prospects of Gas Turbine Power Plants in India The application of gas turbine power plants can be foreseen in the following fields in India: (a) (b) (c) base load gas turbine power plants peak load gas turbine power plants Captive power combined cycle plants (d) Retrofitting of old combined cycle and uneconomical power plants. (e) Co-generation gas turbine plants. Base Load Gas Turbine Power Plants. Power supply situation in our country is becoming worse year by year due to under utilization of existing capacity and faster pace of demand growth. Power shortage can be mitigated on crash programmes by installing combined cycle utility power plants due to their less gestation periods and low installation cost in this age of diminishing capital availability. Though, India has vast coal deposits but these deposits are mainly concentrated in central and eastern parts. Our coals have less calorific value and high ash content which cost excessively in transporting to the power plants located in extreme south, south western regions western regions and north western region. At present 40% of the rail freight is mainly due to coal transportation in India. Fortunately some of the above regions have fairly good other hydro-carbon deposits such as oil and natural gas. India's gas reserves have increased five folds during the last ten years and these are expected to increase further. Present gas reserves is of the order of billion cubic metres by the end of the seventh plan. To exploit the natural gas reserves, pipe line networks interconnecting the production and consumption points to meet the requirements of various industries are being contemplated. Phase - 1 of this pipe network is based on the proven reserves while phase II & III are base on the additional proven reserves through conversion of prognosticated resources. Phase 1 is HBJ pipe line from Hazira (Gujarat) to Babrala (D.P.) which will supply

47 634 POWER PLANT TECHNOLOGY N. G. (Natural Gas) to three combined cycle power plants of total capacity 1600 MW being under execution. There is further scope of two more combined cycle power plants of total capacity nearly 1000 MW, since plants to install some gas based fertiliser plants along this pipe line are not materialising. Phase 1A is an extension of HBJ pipe line from Auraiya to Kapurthala, while phase 1 B is a pipe line connecting Bombay south terminal to Banglore which will cater natural gas to combined cycle power plants of capacity 2875 MW. Phase II envisages a pipe line network extension in southern sector upto 'T'rivandrum and its connection to Northern pipe line network, which will cater to 13 combined cycle power plants of total capacity 3875 MW. In phase III, the North East and Eastern part of the gas fields are proposed to be connected to the gas grid, creating scope for some more combined cycle power plants. So, it can be visualised that there is a scope of atleast 10,000 MW capacity gas based combined cycle power plants in India by 2000 AD. There is scope for simple cycle gas turbine base load power plants to utilize the associated gas from crude oil wells which will be flared otherwise in Bombay High region and north eastern region, mainly due to their low gestation periods and least installation cost. One example of this type of power plant is Uran Gas Turbine Power Station (MSEB), with installed capacity of 672 MW. In nearly 2718 million cubic metres of Natural gas was flared which is colossal wastage of natural resource. Combined cycle power plants need not depend on the availability of liquid or gaseous fuels in entire future. Coal can be gasified to produce lower caloric value gas which can be utilised in pit head combined cycle power plants most economically for power generation. Coal gasification technology is on the threshold of commercial utilization. Uneconomical coal deposits by present technology which are deep in earth and of less seam thickness, can be exploited economically only by underground coal gasification technology, which will enhance scope for pit head combined cycle power plants further. Peak Load Gas Turbine Power Plants. These power plants are mainly simple cycle gas turbine power plants because of their shorter gestation period, low cost of installation and fast starting characteristics though their thermal efficiency is relatively unfavourable. All large load centres in India, need this type of power plants to stabilize the grid when frequency is falling either due to overdrawing of power or less feeding to grid due to failure of few operating power plants. These power plants can also rectify the

48 GAS TURBINE POWER PLANTS 635 complete grid failures very quickly since they can achieve their full load within 20 minutes restoring partial. Supply to the grid catering to essential load requirements and to restart the tripped power plants. Gas turbine units operating on simple cycle will be the ideal solution to act as spinning reserve to cater to peak demand and demand-fluctuations of the grid. The primary fuel for these plants will be gas or liquid fuel. The generator of these gas turbine power plants can also be utili sed as synchrounous condenser to improve power factor of the grid, when gas turbine power plants are not generating power. Captive Power Combined Cycle Power Plants. When unit capacities are below 100 MW, combined cycle power plants are best suitable to generate electricity at lesser cost and than coal fired conventional thermal power plants even at existing liquid/gaseous fuel prices in India due to their higher efficiency of the order of 40 45%, low cost of installationlkw and high reliability. At present, unit capacities of the order of 100 MW and below are being mainly installed as captive power plants since most of the regional grids in India can accommodate larger size single unit of 200 MW and more. Retrofitting of old and uneconomical power plants. Some of the power plants which can not generate electricity at economical cost due to their less design thermal efficiencycan be converted into combined cycle power plants. This modification can be done by replacing the existing steam generation by HRSG (Heat recovery steam generator) and Gas Turbine, such that existing steam cycle facilities can be utilised as bottoming cycle to the gas turbine. Sometimes, it is also possible to use Gas turbine exhaust gases as a practical source of heat energy in already existing coal fired steam generator by doing moderate alternations in the steam generator. Co-Generation Gas Turbine Power Plants. Efficiencies of the order of 80-85% can be achieved in these power plants. These power plants find its application in process industries like petrochemical, fertilizers, paper industries etc, where large quantities of steam and auxiliary power are required. In some applications instead of producing steam by the gas turbine exhaust gases, exhaust gases can be used directly for heating\ requirement such as in centralised adsorption, refl;geration/air-~onditioning! plants, food processing, plastic industries etc. The cost of power generated by these plants is less than the larger size utility coal fired power projects. Co-generation systems are not only decentralised but also integrated systems of energy based on the total energy concept.