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Applied Thermal Engineering 56 (2013) 83e90

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Possibilities for reconstruction of existing steam boilers for thepurpose of using exhaust gases from 14 MW or 17 MW gas turbine

Dragan Tucakovic a,*, Goran Stupar a, Titoslav Zivanovic a, Milan Petrovic a,Srdjan Belosevic b

a Faculty of Mechanical Engineering, University of Belgrade, Kraljice Marije 16, 11120 Belgrade 35, SerbiabVin�ca Institute of Nuclear Sciences, University of Belgrade, Lab. for Thermal Engn. and Energy, P.O. Box 522, 11001 Belgrade, Serbia

h i g h l i g h t s

� Utilization of gas turbine exhaust gas heat.� Reconstruction of the existing steam boiler in purpose of utilizing exhaust gases from a 14 MW gas turbine.� Reconstruction of the existing steam boiler in purpose of utilizing exhaust gases from a 17 MW gas turbine.� Selection of the most favorable solution.

a r t i c l e i n f o

Article history:Received 1 June 2012Accepted 14 March 2013Available online 28 March 2013

Keywords:Gas turbineExhaust gasSteam boiler

* Corresponding author. Tel.: þ381 11 3370 373; faxE-mail address: dtucakovic@mas.bg.ac.rs (D. Tucak

1359-4311/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2013.03.02

a b s t r a c t

Within the energy system in Methanolevinegar complex (MVC) in Kikinda, beside process boiler andauxiliary equipment, there are three equal steam boilers made by “Minel Kotlogradnja”, provided forcombustion of natural gas, fuel oil and process gases. Aiming to increase the MVC Kikinda energy plantcapacity, one gas turbine of 14 MW or 17 MW is going to be installed. In regard to relatively high gastemperature and a large amount of the unused oxygen from the air in the exhaust gas, it is specified tosplit exhaust gas into the two equal streams and import them into the two existing steam boilers, eachhaving production of 16.67 kg/s (60 t/h). In order to use the exhaust gas heat, as well as oxygen containedwithin, it is necessary to replace the existing burners and to reconstruct the heat exchangers in the steamboiler vertical convective pass. Besides, it is necessary to verify if the existing flue gases fan can complywith the new operating regime, during which a half of the turbine exhaust gas is imported into the steamboiler.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Within the energy system of Methanolevinegar complex (MVC)in Kikinda, beside the process boiler and auxiliary equipment, thereare three equal steam boilers, having steam production of 16.67 kg/s (60 t/h) each, with turbine (superheated steam temperaturets¼ 455 �C and pressure ps¼ 77 bar) made by “Minel Kotlogradnja”,designed for combustion of natural gas, fuel oil and process gases.The steam is used for process in the plant and to run two steamturbines. The first steam turbine drives an air compressor and thesecond one drives a generator with electrical output of 11.5 MW.Aiming to increase energy efficiency in the MVC Kikinda plant, aninstallation of one gas turbine of 14 MW or 17 MW is considered.

: þ381 11 3370 364.ovic).

All rights reserved.8

The gas turbine should replace the existing steam turbine satisfyingthe electrical consumption in the factory.

For the purpose of using the heat of hot exhaust gases comingfrom the gas turbine (temperature around 500 �C) it is necessary toeither install a new waste-heat boiler, or carry out a reconstructionof two existing boilers. The exhaust gases, containing a largeamount of air (excess air is 3.3), are intended to be distributed intwo equal streams, and to be inducted into these two reconstructedsteam boilers (Fig. 1). To obtain nominal production of steam boiler,with guaranteed temperature of superheated steam, it is necessaryto enable additional natural gas combustion. By using the heat ofexhaust gases in cogeneration with steam cycle, it is possible toachieve higher plant efficiency compared to individual cycles. Ef-ficiency model of combined cycle gas turbine (CCGT) power plantsis shown in Ref. [1]. Authors of paper [2] considered thermody-namic efficiency of CCGT power plants by taking into account realvalues of cycle parameters. Paper [3] regards the energy and exergy

Fig. 1. Exhaust gases duct route from gas turbine to new biler burners 1 and 2.

Fig. 2. Connection of gas turbine and steam boiler.

D. Tucakovic et al. / Applied Thermal Engineering 56 (2013) 83e9084

analysis of combined cycle during the combustion of municipalwaste, while paper [4] analyses the advantages of combined cyclewith closed-loop steam cooling. Bandyopadhyay, in Refs. [5], em-phasizes pitch point as a dominant influence on the efficient ofcombined cycle, while Reddy, in Refs. [6], in addition defines thetemperature of flue gases at the HRSG outlet, pressure drop in theunit, and the environmental temperature as influential parameters.Authors in Ref. [7] considered the correlation between efficiencyand cost in the design of a power plant, while [8] represents aperennial experience of combined cycle usage. Manassaldi in Ref.[9] presented a model for optimal design of HRSG, taking intoconsideration the geometry of heating surfaces.

Usage of combined cycles also reduces the emission of pollutinggases, which are harmful to the environment [10]. These cycles areused worldwide because of their advantages [11].

This study highlights the significant energy and economicalbenefits which could be achieved through implementation of thecombined cycle.

In this paper, heating recovery steam generator is representedwith twoexisting steamboilers, after the suggested reconstructions.Reconstruction of the steam boiler, presented here, considers thereplacement of existing burners with the newones that are suitablefor newworking conditions. Scheme of connection between the gasturbine and the steam boiler by means of the burners is shown inFig. 2. Fresh air fan (Fig. 2) is used if greater steam boiler productionis required, when the amount of oxygen in exhaust gases isinsufficient.

Besides, it is necessary to complete the reconstruction of heatingsurfaces in the vertical convective pass of the steam boiler. Becauseof the maximum temperature reduction in the furnace, caused bymixing of turbine exhaust gases with gaseous products of addi-tional, natural gas combustion in the burners, heat exchanged inthe steam boiler furnace is insufficient for complete vaporization inradiant evaporator. Therefore it is necessary to install an additionalconvective evaporating heating surface in the form of inclinedevaporator. It is also required to disassemble the existing air heater,and to install an additional economizer section in its place.

Bearing in mind the specified reconstructions, research withinthis project included optimization of individual heating surfacesfor the purpose of gaining the higher boiler efficiency andretaining the existing steam production and parameters. Based on

D. Tucakovic et al. / Applied Thermal Engineering 56 (2013) 83e90 85

the optimization done in the conditions of lower investmentpossibilities, the selection of an appropriate gas turbine wasperformed.

Considering the required reconstructions of the steam boilerand the new operating regime, in which one half of the turbineexhaust gases are inducted into one boiler, it is necessary to check ifthe existing flue gas fan can meet the new conditions.

Normative method presented in Ref. [12] was used for thermalcalculations of both the unreconstructed and the reconstructedboiler, giving the temperatures of the heat transmitter and thereceiver similar to the measured ones.

2. Technical description of the steam boiler

One-drum vertical steam boiler, with maximum steam flow of16.67 kg/s (D ¼ 16.67 kg/s) is shown in Fig. 3. Two burners (1) forcombined combustion of natural gas, heavy fuel oil and processgases are located on the front wall of the furnace, one above theother. Burning fuel in the steam boiler furnace (2) releases heat,which is being exchanged via heating surfaces, placed in the steamboiler gas tract, for the purpose of gaining the corresponding pro-duction of superheated steam.

The first gas channel represents the furnace (2), with screenedwalls, ceiling and bottom, the tubes being very close to each other.Every third tube on the furnace rear wall (6), after the ridge, makesthe first tube screen (8) which consists of one row of tubes, whereastwo remaining tubes are located on the floor of horizontal gas

Fig. 3. Disposition of steam boiler. 1. Combined burner; 2. Furnace; 3. Furnace front wall; 4gases; 8. First tube screen; 9. Horizontal gas channel; 10. Second tube screen; 11. Furnace latfor furnace lateral walls; 14. Corner tubes; 15. Steam drum; 16. Surface steam cooler; 17. Boile21. Guide chamber; 22. Vertical convection flue gases channel. 23. Third economizer sectioneconomizer section; 27. Flue gases duct; 28. Main steam header; 29. Light bricking; 30. Pla

channel, inwhich the superheaters are placed, and form the secondtube screen (10) with two rows of staggered tubes.

A transversal steam drum (15), which is included in naturalcirculation circuit, is placed on the top of steam boiler. There is asurface steam cooler (16) in his water part.

Between the two tube screens, in horizontal (second) gaschannel (9), secondary (18) and primary (19) superheaters areplaced. Surface steam cooler, which regulates superheated steamtemperature at the boiler outlet (16), is located between the su-perheaters. After the final superheating in the secondary super-heater, steam streams to the main steam header (28) and fromthere, to consumption.

After the second tube screen, flue gases are flowing down intothe vertical convective pass (22), across the guide chamber (21).The vertical convective pass represents the third gas channel, withthe gases streaming downwards. Economizer (23 and 24), built inthree sections, and three-pass tubular air heater (25) are located inthis gas channel. Heated air, suppressed by air fan, reaches theburners (1), in which it is mixed with fuel, for the purpose ofachieving the complete combustion.

Flue gases are being transported by an induced fan, and aredischarged to the environment.

3. Technical description of the reconstruction

Steam boiler reconstructed for the purpose of using hot exhaustgases from 14 MW or 17 MW gas turbine is shown in Fig. 4.

. Furnace ceiling; 5. Furnace floor; 6. Furnace rear wall; 7. Ridge for the stream of flueeral walls; 12. Downcomer tubes for furnace front and rear wall; 13. Downcomer tubesr drum fittings; 18. Secondary superheater; 19. Primary superheater; 20. Boiler fittings;; 24. First and second economizer section; 25. Air heater; 26. Connecting tubes of thirdte casing; 31. Vertical convection channel bricking; 32. Galleries; 33. Staircase.

D. Tucakovic et al. / Applied Thermal Engineering 56 (2013) 83e9086

On the furnace front wall (2), which should be adjusted to thenew burners, two burners (34) are located. The burners are used forcombustion of natural gas, in the presence of gas turbine exhaustgases. Oxygen from these gases is used for combustion of naturalgas in steam boiler furnace and it reduces themass of gases, flowingthrough the steam boiler, which would be discharged into the at-mosphere through the chimney, using the existing fan.

The first gas channel, i.e. the furnace (2), the horizontal gaschannel (9) and the guide chamber (21), don’t need reconstructions.After the second tube screen, the mixture of flue and exhaust gasesflow across the guide chamber and enters the vertical convectivepass (22). Inclined evaporator (36), which should be installed in thebeginning of this channel, should be connected to the steam drum(15) by means of the falling and the connecting tubes. Boiling waterfromsteamdrumgets to the lowerheader (35) of inclinedevaporator(36), through the falling tubes, and it evaporates. Through the largerdiameter tubes (37) on the rearwall of the guide chamber, steam andwatermixture gets to theupperheader (38), fromwhere it is broughtto the steam drum by connection tubes, for phase separation.

Economizer of the existing boiler is being reconstructed, whilethe air heater is being disassembled and removed.

Reconstructed steam boiler needs to be fitted with more econ-omizer sections (40) in the vertical convective pass (22). In thatpurpose, steam boiler steel structure needs to be reconstructed inthe section where the air heater was placed before, and the lastthree sections of the new economizer are to be installed in its place.All vertical convection flue gas channel (22) manholes remainfunctional.

Fig. 4. Disposition of the reconstructed steam boiler. 2. Furnace; 9. Horizontal gas channel; 1gas burner; 35. Convective evaporator lower header; 36. Inclined evaporator; 37. Tubes of inc40. Economizer; 41. Economizer upper header; 42. Boiler bricking; 43. Additional economi

Air channel also needs to be reconstructed, from the boiler roomentrance, all the way to the burner, because the location of the airheater is now taken by tubes of the economizer. If the new boiler isworkingwithout exhaust gases, approximately sameamountof freshair is needed, so that the air channel has the same cross section asbefore reconstruction. When gas turbine is working, hot exhaustedgases enriched with oxygen and cold air are used for combustion ofnatural gas. Therefore the existing air fan has to provide extraamount of cold air, which would be lead to the new burner.

The existing boilers are predicted to work with negative pres-sure in the gas tract, and this has to be take into account during theboiler reconstruction. This negative pressure should be providedwith the existing flue gas fan.

4. The steam boiler heat balance and thermal calculation

The steam boiler heat balance is being made for the purpose ofdefining its efficiency, namely the fuel consumption. For theexisting boiler, the inlet heat quantity is defined first. In the givencase, it consists of lower heating value of natural gas and itsphysical heat. Besides, the mass air balance for the boiler has to bedefined, meaning that the coefficient of excess air at the furnaceoutlet, increase of excess air in the furnace and along the boiler, aswell as the relation of the amount of air that leaks in heater and thetheoretical air quantity, have to be assumed.

For the reconstructed boiler, while working with hot exhaustgases and air, the inlet heat quantity consists of natural gas lowerheating value, its physical heat and the exhaust gases heat quantity.

5. Steam drum; 21. Guide chamber; 22. Vertical convection flue gases channel; 34. Newlined convective evaporator; 38. Convective evaporator upper header; 39. Feed header;zer in the flue gas duct.

D. Tucakovic et al. / Applied Thermal Engineering 56 (2013) 83e90 87

For combustion of natural gas in these conditions, ratio of real andtheoretical air quantity is assumed to be by 0.1 higher than thestandard one. Excess air at the end of the furnace is defined as aratio of unused oxygen residue from exhaust gases and theoreticalair quantity. Quantity and coefficient of excess air and temperatureof exhaust gases for normal working regime of 14 MW and 17 MWgas turbine are shown in Table 1.

Heat balance of steam boiler (Table 2) is made for maximumload (100%) for both the existing and the reconstructed steamboiler. Heat balance is shown for the reconstructed steam boiler formaximum load of 100%when it is workingwithout usage of turbineexhaust gases, as well as heat balances for 100% and 50% loadswhen the boiler is working with one half of exhaust gases being ledinto the burner of one of the boilers. The consumption of naturalgas in the burners of steam boiler (BF) is given at the end of heatbalance (Table 2), as well as half of consumption of natural gas ingas turbine (BT) and overall consumption of natural gas by onesteam boiler (B) in m3/s.

Based on geometrical characteristics of the existing and thereconstructed steam boiler and heat balance shown in Table 2,suitable thermal calculations have been made [12] for differentsteam productions of both the existing and the reconstructedboiler. Table 3 shows the temperature of flue gases behind indi-vidual heating surfaces, and water temperature at the outlet ofeconomizer, which are given based on thermal calculation. Namely,while determining the needed convective evaporator and econo-mizer heating surfaces, it was important that the water tempera-ture at economizer outlet was not higher than the appropriateboiling temperature for the boiler working pressure. The differencebetween these two temperatures in the existing boiler (58 �C) waslowest during maximum boiler load. For the reconstructed boiler,this difference was lowest during 50% of the boiler load and whenusing half of the turbine exhaust gases, but for all of the reviewedcases the boiling temperature was not reached.

5. Results and discussion

5.1. Using the exhaust gases from 14 MW gas turbine

Based on calculation of the reconstructed steam boiler, duringsupply of half of hot exhaust gases from 14 MWgas turbine, and for100% load, the calculated temperature of flue gases at the steamboiler outlet is 173 �C, the excess air at the boiler outlet is 1.36, andthe boiler efficiency is 89.12%. For 50% load, temperature of fluegases at the steam boiler outlet is 181 �C, the excess air at the boileroutlet is 2.13 and efficiency is 81.37%. During lower loads, excess airthrough boiler increases, considering that not all of the oxygenfrom turbine exhaust gases is being used, which substantially re-duces the boiler efficiency. During the highest boiler loads, all of theoxygen from turbine exhaust gases is used, and thus it is necessaryto import fresh air, using an appropriate fan. Only 5% of theoreticalair quantity has to be imported. Based on thermal calculation for all

Table 1Parameters of nominal gas turbine working regime.

Name Unit Gas turbine, MW

14 17

Mass air flow rate at the compressor inlet kg s�1 49.76 59.59Mass fuel flow rate kg s�1 0.854 1.054Mass exhaust gas flow rate kg s�1 50.614 60.644Volume air flow rate at the compressor inlet m3 s�1 38.101 45.750Air quantity at the compressor inlet m3 m�3 31.672 31.748Theoretical air quantity m3 m�3 9.534 9.534Excess air e 3.32 3.33Temperature �C 485 467

considered cases, the capacity of the surface cooler of superheatedsteam is lower than its maximum capacity, which is 4561 kW.

For the purpose of checking whether the existing flue gases fansatisfies new steam boiler working conditions, the gas tract aero-dynamic calculation [13] of considered steam boiler was done formost unfavorable conditions, namely during maximum steamboiler load, and with supply of one half of turbine exhaust gases. Itwas found that calculated flow rate of flue gases was 46.56m3/s andthe corrected calculated stress was 3145 Pa. These values have beendrawn in the fan aerodynamic characteristic curve (Fig. 5). There-fore, based on the position of working point, it can be concludedthat the existing fan will satisfy in considered conditions, providedthat the angle of inlet guide vane control is 4 z20�. The fan willwork with somewhat reduced efficiency (around 78% in relation tothe projected one, which is 79.5%).

Considering the lower boiler efficiency, after its reconstruction,the possibility of installing an additional economizer of 130 m2 inthe flue gas duct (position 27, Fig. 3) was also considered. In thatcase, during supply of one half of the 14 MW turbine hot exhaustgases, and 100% boiler load, the temperature of outlet flue gaseswould be 155 �C, and the boiler efficiency 90.29%, and during 50%boiler load, gas temperature would be 167 �C, and the boiler effi-ciency 82.64%. In this way, the consumption of natural gas in theboiler would be reduced by 0.020 m3/s (from 1.158 to 1.138 m3/s)during 100% boiler load, and by 0.013m3/s (from 0.458 to 0.445m3/s) during 50% boiler load. In case that reconstructed steam boiler,with an additional economizer, works without turbine exhaustgases, during 100% boiler load, the temperature of outlet gaseswould be 146 �C, and the boiler efficiency would be 91.98%, which isclose to the designed values. It should also be highlighted that evenin these cases, the maximum capacity of superheated steam cooleris satisfactory.

Economizer in flue gas duct causes an increase in resistance ingas tract, which is why the flue gas fan capacity was checked again,and it was determined that the existing fan could not overcome theadditional resistances.

5.2. Using the exhaust gases from 17 MW gas turbine

Based on calculation of the reconstructed steam boiler, duringsupply of half of hot exhaust gases from 17 MWgas turbine, and for100% load, the calculated temperature of flue gases in steam boileroutlet is 187 �C, the excess air at the boiler outlet is 1.36, the boilerefficiency is 86.84%, and consumption of natural gas for boilercombustion is 1.128 m3/s. For 50% load, temperature of flue gases atthe steam boiler outlet is 201 �C, efficiency is 76.68%, and con-sumption of natural gas is 0.439 m3/s. In the considered cases it isnot necessary to import fresh air for combustion, but the air fan hasto exist, in case that gas turbine is out of order.

Even though the consumption of natural gas for boiler com-bustion was lower than with the use of 14 MW turbine exhaustgases, the boiler efficiency was substantially reduced, so thatinstalling of an additional, 130 m2 economizer in the flue gas ductwas also considered in this case. In this way, for 100% boiler load,the temperature of outlet flue gases would be 168 �C, boiler effi-ciency 88.13%, and consumption of natural gas used in the boilerwould be reduced by 0.023 m3/s (from 1.128 to 1.105 m3/s). During50% of the boiler load, the temperature of outlet flue gases would be183 �C, boiler efficiency 78.55%, and consumption of natural gaswould be reduced by 0.021 m3/s (from 0.439 to 0.418 m3/s).

As already mentioned, maximum capacity of superheated steamcooler is 4561 kW. In the case of the boiler reconstruction, during100% load, required power of the cooler would be 4697.9 kW, but ifan additional economizer is installed in the flue gas duct, theneeded cooler power would be reduced to 4478.7 kW.

Table 2Heat balance of steam boiler.

No. Name Mark Unit Exhaust gases of gas turbine

Without Without With 14 MW With 14 MW With 17 MW With 17 MW

Existing boiler Reconstructed boiler

Steam boiler load [%]

100 100 100 50 100 50

1. Natural gas lower heating value Hlow kJ m�3F 34,065.0 34,065.0 34,065.0 34,065.0 34,065.0 34,065.0

2. Exhaust gases heat quantity Qtur kJ m�3T e e 22,153.7 22,153.7 21,355.1 21,355.1

3. Excess air in exhaust gases atur e e e 3.32 3.32 3.33 3.334. Ratio of real and theoretical air quantity bF e 1.15 1.15 1.25 1.25 1.25 1.255. Furnace excess air lf e 1.15 1.15 1.16 1.88 1.32 2.096. Fuel temperature tF �C 20 20 20 20 20 207. Physical natural gas heat QF kJ m�3

F 31.91 31.91 31.91 31.91 31.91 31.918. Disposed heat quantity QS kJ m�3

S 34,096.9 34,096.9 30,017.1 27,320.4 29,066.5 26,115.49. Excess air at the boiler outlet aout e 1.35 1.35 1.36 2.13 1.52 2.3410. Temperature of outlet gases tout �C 145 162 173 181 187 20111. Outlet gases enthalpy Iout kJ/m3

S 2794.7 3133.2 3376.4 5296.8 4007.8 6400.112. Loss due to incomplete mechanical combustion q4 % 0 0 0 0 0 013. Waste gas loss q2 % 6.70 7.69 9.53 16.43 11.81 21.1214. Loss due to unburned gases q3 % 0.50 0.50 0.50 0.50 0.50 0.5015. Radiation loss q5 % 0.85 0.85 0.85 1.70 0.85 1.7016. Boiler efficiency hb % 91.95 90.96 89.12 81.37 86.84 76.6817. Superheated steam production D kg s�1 16.667 16.667 16.667 8.333 16.667 8.33318. Superheated steam production D t/h 60 60 60 30 60 3019. Superheated steam temperature ts �C 455 455 455 455 455 45520. Superheated steam pressure ps bar 77 77 77 77 77 7721. Superheated steam enthalpy hs kJ kg�1 3291.3 3291.3 3291.3 3291.3 3291.3 3291.322. Feed water temperature tfw �C 110 110 110 110 110 11023. Feed water pressure pfw bar 92 92 97 82.8 97 82.824. Feed water enthalpy hfw kJ kg�1 467.9 467.9 468.9 467.3 468.9 467.325. Heat quantity used in steam boiler Qsb kW 47,057.1 47,057.1 47,040.9 23,532.4 47,040.9 23,532.426. Natural gas consumption BF m3

F s�1 1.501 1.517 1.158 0.458 1.128 0.43927. Turbine fuel consumption by half of

the exhaust gases brought into the boilerBT m3

T s�1 e e 0.601 0.601 0.736 0.736

28. Total fuel consumption BS m3S s�1 1.501 1.517 1.759 1.059 1.864 1.175

D. Tucakovic et al. / Applied Thermal Engineering 56 (2013) 83e9088

In case that the reconstructed boiler, during maximum boilerload, works with the supply of half of the hot exhaust gases from17 MW turbine, the flow of flue gases through the boiler wouldincrease by 8.384 m3/s, that is by 20.5%, compared to work with14 MW turbine. In that case, the existing flue gas fan could notovercome the resistances, and would have to be replaced.

Table 3Temperature of flue gases.

Name Mark Unit Exhaust gas

Without

Existing boi

Steam boile

100

Flue gas temperature Furnace outlet tfout �C 1147First tube screen outlet tts1 �C 1114Secondary superheater outlet tss �C 902Primary superheater outlet tps �C 630Second tube screen tts2 �C 618Guide chamber outlet tgc �C 614Convective evaporator outlet tce �C e

Economizer outlet teco �C 249Air heater outlet tah �C 145Steam boiler outlet tout �C 145

Boiler drum pressure pbd bar 81.40Saturated temperature tsat �C 296.19Water temperature at the economizer outlet tWeco

�C 238COOLER BALANCE (cooler maximum capacity is 4561 kW)Cooler heat quantity QH kW 4314.9

6. Techno-economic analysis

The gas turbine with the reconstructed steam boilers shouldreplace the existing steam turbine and boilers in regard of energysupply in the plant. Since the gas turbine with the utilization ofwaste heat gas better efficiency comparing to a simple steam

es of gas turbine

Without With 14 MW With 14 MW With 17 MW With 17 MW

ler Reconstructed boiler

r load [%]

100 100 50 100 50

1122 1082 790 1036 7331091 1056 776 1015 722889 879 689 865 656625 636 550 647 541613 624 542 636 534609 620 540 633 533418 430 396 445 401162 173 181 187 201e e e e e

162 173 181 187 20181.40 81.40 78.10 81.40 78.10296.19 296.19 293.31 296.19 293.31209 225 275 244 292

3898.7 4451.5 1779.2 4697.9 1515.0

Fig. 5. Aerodynamic characteristic of the existing flue gas fan.

D. Tucakovic et al. / Applied Thermal Engineering 56 (2013) 83e90 89

turbine cycle, it was expected that a considerable reduction of fuelcost could be achieved. The techno-economic analysis should showif the project feasible, i.e. if the investment cost could be covered byfuel saving.

Table 4Techno-economic analysts of the 14 MW gas turbine project.

Input dataLow heat value of natural gas kJ/m3 34,065Price of the fuel EUR/m3 0.225Price of electrical energy EUR/kWh 0.055Operation hours per year h/y 7920Current situation with the steam turbineAverage electrical output of the steam

turbine netMW 10.5

Overall production of steam for theturbine and for process

t/h 122(113 in summer)

Efficiency of the boilers % 89.1Annual natural gas consumption m3/y 86,405,000Annual fuel cost EUR/y 19,441,000Maintenance cost for steam turbine EUR/y 455,500Overall annual cost EUR/y 19,896,500Overall cost in the 14 MW gas turbine plantGas turbine efficiency % 34.6Own consumption of el. energy MJ/s 0.5Fuel consumption in gas turbine m3/y 35,429,200Addition fuel consumption in the

reconstructed boilerm3/y 31,570,600

Overall annual fuel consumption m3/y 66,999,800Overall annual fuel cost EUR/y 15,074,955Maintenance of the gas turbine EUR/y 635,000Electrical energy surplus MJ/s 3Income due to delivered electrical energy EUR/y 1,188,000Overall annual cost e income due el. energy EUR/y 14,521,955Overall annual saving in case of gas

turbine applicationEUR/y 5,493,345

Investment cost in gas turbine EUR 7,605,000Investment cost in boilers reconstruction EUR 1,215,000Economic parameters of the projectSimple pay back period (SPB) year 1.64Internal rate of return (IRR) % 191.3Net present value (NPV) EUR 24,580,000

The most important economic results are shown in Table 4.Applying the gas turbine with the reconstructed boilers the factorywill be able to save about 19.4 M m3 of natural gas i.e.5,374,545 EUR/y.

Since the estimated investment cost is 8.2 M EUR, the economicanalysis shows very good parameters: the simple pay back periodshorter then two years, the internal rate of return 192% and netpresent value of more then 24 M EUR.

7. Conclusion

Based on the presented results, and in order to be able to use theexhaust gases of a 14MW turbine in the case-study boilers, it wouldbe necessary to make these changes:

- to replace existing burners with special burners;- to install an inclined evaporator of required surface at thevertical convective pass inlet, and put it into the steam boilernatural circulation cycle;

- to disassemble the existing air heater and economizer, and puta new economizer of significantly larger heating surface intheir place;

- to build an exhaust gas duct between the gas turbine and theboilers number 1 and 2 and connect it with the new burners, sothat one half of these gases is led into the boiler number 1 andthe other half into the boiler number 2 and

- to reconstruct the fresh air ducts from the fan to the newburner in the cases when the steam boiler is working withoutexhaust gases.

With this reconstruction, at 100% boiler load, the efficiency is89.12%. In case of placing an additional economizer in the flue gasduct, the boiler efficiency would increase by 1.2%, but the existingflue gas fan would have to be replaced with a new one.

Using the exhaust gases of a 17 MW gas turbine would increasethe flow of flue gases through the boiler from 46.56 m3/s to54.94 m3/s, which would additionally increase the pressure drop inthe gas tract, so it would be necessary to replace the existing fan.Besides, it is necessary to install an additional economizer in theflue gas duct, considering that it is the only case, during the 100%boiler load, in which the maximum capacity of the cooler is satis-factory. After these reconstructions, during 100% boiler load, theefficiency would be 88.13%.

It should also be mentioned that stress check of the feed waterpump was performed, and that it was concluded that it couldprovide the additional increase of water pressure at the boiler inlet,considering the increase of the economizer surface in the boiler.However, if an additional economizer would be installed into theflue gas duct, so that feed water is led into it first, the requiredwater pressure would not be satisfied by the existing feed waterpump.

Comparing the necessary boiler reconstructions by using theexhaust gases from gas turbines of 14 MW and 17 MW, it is clearthat using the 17 MW turbine additionally requires the replace-ment of flue gas fan, feed water pump and installation of an addi-tional economizer in the flue gas duct. Besides, when using theexhaust gases from the 14 MW gas turbine, the boiler efficiency isby 2.28 percentage points greater than for the 17 MW turbine.

Taking into account also the investment possibilities of MVCKikinda, the best solution would be to install the 14 MW gas tur-bine, considering that it satisfies the plant need for electricity. Forthe purpose of using the heat of exhaust gases from the 14 MWturbine, it is necessary to perform the reconstruction of the twosteam boilers, as described, but without installation of an addi-tional economizer in the flue gas duct.

D. Tucakovic et al. / Applied Thermal Engineering 56 (2013) 83e9090

The installation of the 14 MW gas turbine with the recon-structed boilers will enable considerable improvement in energyefficiency in the factory. The company will save 19.4 M m3 of nat-ural gas per year. Since the investment cost of the project is rela-tively low, the techno-economic analysis shows very positiveresults of the project.

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