A Study on Energy Optimization of Heat Exchangers in a Gasification System

Transcription

1 International Journal of Meanial Engineering and Aliations 016; 4(3: tt://wwwsieneublisinggrouom/j/ijmea doi: /jijmea ISSN: X (Print; ISSN: (Online A Study on Energy Otimization of Heat Exangers in a Gasifiation System Le Min Nut 1, Young-Sub Moon, Youn Ceol Park 3, * 1 Deartment of Termal Engineering, Faulty of Veile and Energy Engineering, Ho Ci Min City University of Tenology and Eduation, Ho Ci Min City, Vietnam Resear and Develoment Center, POSCO E&C, Ineon City, Reubli of Korea 3 Deartment of Termal Engineering, Faulty Meanial Engineering, Jeju National University, Jeju, Reubli of Korea address: nutlm@muteeduvn (Le M N, ysmoon@osoenom (Young-Sub M, yark@jejunuakr (Youn C P * Corresonding autor To ite tis artile: Le Min Nut, Young-Sub Moon, Youn Ceol Park A Study on Energy Otimization of Heat Exangers in a Gasifiation System International Journal of Meanial Engineering and Aliations Vol 4, No 3, 016, doi: /jijmea Reeived: May 16, 016; Aeted: May 7, 016; Publised: June 13, 016 Abstrat: Te objetive of tis resear is te otimization of energy arameters su as te temerature and flow rate of te fluid in eat exangers in te gasifiation system in order to inrease te reovery rate of energy in te system A matematial model of tese eat exangers is develoed to redit teir oerating erformane under te seified gasifiation system Te otimal flow rate and temerature of te fluid in te eat exanger based on te effetiveness - number of transfer units(ntu metod is investigated Te result of te simulation sows tat te otimal mass flow rate and temerature of te ig ressure (HP boiler feed water are determined at 175,907 kg/ and 110 C, resetively, wile te otimal mass flow rate and temerature of ig ressure saturated steam of boiler are determined at 38,430 kg/, 905 C, resetively At tese values, te total eat amount obtained at tese eat exangers is igest wit 169 MW Besides, te total eat amount obtained at eat exangers ould be inreased by 461% (78 MW wen sixty erent of te eat release amount from air ooler (178 MW is used Keywords: Otimization, Heat Exanger, Gasifiation System, Effetiveness-NTU 1 Introdution Te demand for natural gas due to te inreased oulation and eonomi growt of te world s oulation as aused te deletion of natural gas resoures, as well as rie inrements, in reent years It is neessary to disover alternative ways to develo a substitute for natural gas resoures Natural gas obtained from te oal gasifiation roess is ounted as a substitute resoure to satisfy te growing demands of ower generation and ome utilization in te near future Syntesized natural gas (SNG as many advantages It an be rodued from inexensive arbonaeous feedstoks and te removal of ontaminants su as sulur from oal makes oal gasifiation a more environmentally friendly means of energy onversion omared to te normal ombustion of solid oal Also, it as a ig onversion effiieny, is easy, and it is ost effetive to remove arbon dioxide by te searation of igly onentrated CO stream, as inerent to all SNGroesses [1-3] Te gasifiation roess of a ressurized, oxygen-blown, entrained-flow E-Gas like gasifier troug numerial modeling is investigated by solving te 3-D, steady-state Navier Stokes equations wit te Eulerian Lagrangian metod [4] Te study indiates tat te inreasing O /Coal ratio results in a derease of CO, but an inrease of CO and exit temerature Jaojaruek [5] resented a study to redit te temerature rofile, feedstok onsumtion rate (FCR and reation equivalene ratio (RER A matematial model for te entire lengt of a downdraft gasifier is roosed using termoemial riniles to derive energy and mass onversion equations Te analysis results sow tat model-redited temerature fitted well wit exerimental data eseially on te yrolysis zone Combustion and gasifiation zones ad maximum temerature error of 5 C or 78% In tis study, a simulation rogram is develoed, based on te matematial model of te waste eat reovery eat

2 14 Le Min Nut et al: A Study on Energy Otimization of Heat Exangers in a Gasifiation System exangers to redit teir oerating erformane and to obtain otimal information of te temerature and flow rate of te fluid in te eat exangers of te gasifiation roess Matematial Model of Heat Exanger Gasifiation is a roven manufaturing roess tat onverts ydroarbons su as oal, etroleum oke, and biomass to a syntesis gas (syngas In te oal gasifiation roess of onverting oal to SNG, oal is gasified wit steam and oxygen Te gasifiation roess rodues arbon monoxide (CO, ydrogen (H, arbon dioxide (CO, metane (CH 4, and iger ydroarbons su as etane and roane [5] Metanation reation is te main roess of onverting oal to gas In te metanation reators CO, CO, and H are onverted into CH 4 aording to te following reations: CO 3 H CH + H O + eat (1 + 4 CO 4 H CH + H O + eat ( + 4 Parallel wit tese metanation reations, te water gas sift reation will arouse to make equilibrium in all metanators: CO + H O CO + H + eat (3 Metanation reations in equations (1 and ( are igly exotermi In order to reover te waste eat from tese reations, eat exangers need to be installed between ea metanator to imrove system erformane and in order to utilize te igly ressurized suereated steam for ower generation wit a steam turbine Te eat exanger used in tis gasifiation system for te waste eat reovery is te ounterflow eat exanger, wile te effetiveness - number of transfer units (NTU metod is used for te analysis Te exanger eat transfer effetiveness is determined by Sadik Kaka [0]: Q ε = (4 Q max Tis is te ratio of te atual eat transfer rate Q in a eat exanger to te termodynamially limited maximum ossible eat transfer rate Q max if an infinite eat transfer area were available in a ounterflow eat exanger Te value of ε ranges between 0 and 1 and te atual eat transfer is alulated by: were ( m 1 1 ( T T = ( m ( T T Q = (5 ( m and ( m are te aaity rate of te ot fluid and te old fluid, resetively T 1 and T are te inlet temerature and te outlet temerature of te ot fluid and T 1 and T are te inlet temerature and te outlet temerature of te old fluid, resetively Te maximum ossible eat transfer is exressed as: if and if Q max = ( m 1 1 ( T T C = ( m < C = ( m (6 Q max = ( m 1 1 ( T T C = ( m < C = ( m (7 Te eat transfer area number is given by Sadik Kaka [9]: AU NTU = (8 C min were U and A are te overall eat transfer oeffiient and te eat transfer surfae area between te ot fluid and te old fluid, resetively, and C min is te smaller of C and C Eq (4 may be re-written using Eqs (5, 6, (7, (8, and (9 as follows: [ NTU(1 Cmin / Cmax ] ex[ NTU(1 C / C ] 1 ex ε = (9 1 ( C / C min max In tis study, te total inlet energy of te eat exangers of te gasifiation system was alulated as follows: i ihpbfw ihpstb min max H = H + H (10 were H ihpbfw, H ihpstb are te inlet energy of te ig ressure boiler feed water (HPBFW and te inlet energy of te ig ressure saturated steam of te boiler (HPSTB, resetively Te total outlet energy of te eat exangers of te gasifiation system was given by Eq (11: were H H + H + H o = (11 ohpsst H ohpsst, ofpre H and ofpre olpcd H olpcd are te outlet energy of te ig ressure suereated steam to turbine (HPSST and te outlet energy of te feed reeater (FPRE, and te outlet energy of te low ressure ondensate drum (LPCD, resetively Te total reovery energy of te eat exangers was given as Eq (1 δ (1 H = H o H i

3 International Journal of Meanial Engineering and Aliations 016; 4(3: From te matematial model of eat exanger based on te effetiveness-ntu metod, an EES (Engineering Equation Solver rogram is written to simulate te otimal mass flow rate and temerature of te fluid in te eat exangers of te gasifiation system 3 System Desrition Te semati diagram of te steam rodution by waste eat reovery from metanation reations of te gasifiation system is resented in Fig 1 Normally, te main roess in te metanation unit inlude gas leaning, bulk metanation, trim metanation, atalyti oxidation, and drying roesses However, tis investigation is foused only on te main eat exangers and metanators beause te gasifiation system as too many devies wit omlexities Te designed metanation roess as rodution aaity wit 500,000 metri tons er year based on feedstok from a oal gasifiation unit During te metanation roess, te lant also generates igly ressurized suereated steam at 70 bar, 490 C from te waste eat reovery of metanation reations for steam turbine uses Fig 1 Semati diagram of te steam rodution by waste eat reovery of te gasifiation systemin te metanator, CO, CO, and H are onverted into CH 4 tat is igly exotermi In order to otimize te onversion of CO, CO, and H to CH 4 and to utilize te amount of eat of te reation, te roess is divided into a series of adiabati metanators wit interstage ooling, as illustrated in Fig 1 After te 1st metanator, te roess gas is ooled down at te downstream waste eat reovery setion by te 1st waste eat boiler and te nd waste eat boiler Te nd waste eat boiler is followed by two suer eaters, te 1st HP steam suereater and nd HP steam suereater, and te temerature of te roess gas is redued from 675 C to 30 C (b Effet of te total reovery energy δ H Fig Effet of te inlet mass flow rate of te ig ressure boiler feed water on te total inlet energy, te total outlet energy and te total reovery energy (a Effet of te total inlet energy (Hi Te total outlet energy (H o; Te inlet energy of te HPBFW (H ihpbfw and te outlet energy of te HPSST (H ohpsst

4 16 Le Min Nut et al: A Study on Energy Otimization of Heat Exangers in a Gasifiation System Te major art of te onversion of CO, CO, is omleted in te two subsequent metanators: te nd metanator and te 3 rd metanator Te nd metanator oerates at a ig temerature wile te 3 rd metanator oerates at a lower temerature After te nd metanator, te roess gas is ooled by te 3 rd metanator s waste eat boiler and te 3 rd metanator s boiler feed water (BFW re-eater Wile after 3 rd metanator te roess gas is ooled by 4 t te waste eat boiler tat is followed by a series of eat exangers, were te eat is utilized to reeat boiler feed water and demineralised water in te 4 t BFW Preeater, and DMW Preeater, resetively Te roess gas is finally ooled in an Air Cooler After ooling te roess gas, te roess ondensate is searated in te 1 st Proess Condensate Searator and enters into te 4 t metanator Te effluent gas from te 4 t metanator is ooled in te nd BFW Pre-eater and te roess gas ontinually enters into te 5 t metanator 4 Results and Disussion Te otimization of energy arameters su as te temerature and flow rate of te fluid in te eat exanger in te gasifiation system based on several reasons as follows Te temerature and ressure of te ig ressure suereated steam suly to te steam turbine are 490 C and 70 bar, resetively Te eat transfer roess at te eat exanger sould be ensured tat te temerature of te roess gas t after te 5 Metanator is 80 C Fig (a sows te effet of te inlet mass flow rate of te ig ressure boiler feed water on te inlet energy of its H ihpbfw, and te outlet energy of te ig ressure suereated steam to turbine H ohpsst, and te total inlet energy H i and total outlet energy H o of te eat exangers of te gasifiation system Wen te mass flow rate of te ig ressure boiler feed water in te range of 13,134 kg/ to 175,907 kg/, te total outlet energy of te eat exangers H o and te total outlet energy of te ig ressure suereated steam to turbine H ohpsst inreased quikly and ten inreased slowly Tis is exlained briefly as follows In te range of mass flow rate from 13,134 kg/ to 175,907 kg/, te state of te work liquid in te steam drum is suereated vaor state and redues gradually to saturated vaor state Terefore, its entaly is also orresonding redution However, te inreasing of te mass flow rate of te ig ressure boiler feed water rater tan te reduing of te entaly lead to te outlet energy H ohpsst and H o are inrease (see Fig (a After te value of 175,907 kg/, its state is redued from saturated vaor state to saturated water-vaor mixture state but is lose to saturated state Terefore, its entaly is strongly dereased from te saturated vaor entaly to near te saturated water entaly For tis reason, te outlet energy H ohpsst and H o are slowly augmented Fig 3 Effet of te inlet mass flow rate of te ig ressure boiler feed water on te outlet temerature of te ig ressure suereated steam to turbine Te effet of inlet mass flow rate of te ig ressure boiler feed water on te total reovery energy of te eat exangers δ H is sown as Fig (b As seen in Fig (b, it demonstrates tat inreasing of te total reovery energy of te eat exangers δh is igest at 193,4977 kg/ Also, wen te inlet mass flow rate of te ig ressure boiler feed water is inreased, it will lead to te outlet temerature of te ig ressure suereated steam to turbine is redued (see Fig 3 As indiated in Fig 4(a, wen te inlet temerature of te ig ressure boiler feed water ranges from 77 C to 110 C, te state of te work liquid in te steam drum is te saturated mixture state At te value 110 C, its state is saturated vaor state and ten beomes suereated vaor state Te state ange of te work liquid in te steam drum is te ause of te ange of total outlet energy of te eat exangers H o and total outlet energy of te ig ressure suereated steam to turbine H ohpsst is sown in Fig 4(a Fig 4(b sows tat te total reovery energy of te eat exangers δh is igest at 110 C As seen in Fig 5, wen te inlet temerature of te ig ressure boiler feed water ranges from 77 C to 143 C, te outlet temerature of te ig ressure suereated steam to turbine inreased from 476 C to 518 C (a Effet of te total inlet energy (Hi

5 International Journal of Meanial Engineering and Aliations 016; 4(3: (b Effet of te total reovery energy δ H Fig 4 Effet of inlet temerature of te ig ressure boiler feed water on te total inlet energy, te total outlet energy and te total reovery energy ket onstant at value of 175,907 kg/, 110 C and 100 bars, resetively As seen in Fig 6(a, wen te inlet mass flow rate of te ig ressure saturated steam of te boiler rises, te inlet energy of its H ihpstb and te outlet energy of te ig ressure suereated steam to turbine HoHPSST, and te total inlet energy Hi and total outlet energy Ho of te eat exangers of te gasifiation system are inreased Tis inreasing is due to te main inreasing of inlet mass flow rate of te ig ressure saturated steam of te boiler beause of its saturated vaor entaly does not ange state For tis reasons, te amount of te total reovery energy of te eat exangers is almost unanged (see Fig 6(a However, te outlet temerature of te ig ressure suereated steam to turbine is redued wen te inlet mass flow rate of te ig ressure saturated steam of te boiler is inreased (see Fig 7 At te value of 38,430 kg/, te outlet temerature of te ig ressure suereated steam to turbine is 4998 C Te total outlet energy (H o; Te inlet energy of te HPBFW (H ihpbfw and te outlet energy of te HPSST (H ohpsst (a Effet of te total inlet energy (Hi Fig 5 Effet of inlet temerature of te ig ressure boiler feed water on outlet temerature of te ig ressure suereated steam to turbine Altoug te igest value of total reovery energy of te eat exangers δh obtained at 193,4977 kg/ (see Fig (b However, at te value 193,4977 kg/, te outlet temerature of te ig ressure suereated steam to turbine is 458 C, tis temerature does not meet te above requirement temerature Furtermore, in te range of inlet mass flow rate of te ig ressure boiler feed water from 175,907 kg/ to 193,4977 kg/, te inreasing of total reovery energy of te eat exangers δh is sligtly From te above analyses, te otimal mass flow rate and temerature of ig ressure boiler feed water are determined at 175,907 kg/ and 110 C, resetively At tese values, te outlet temerature of ig ressure suereated steam to turbine is 4998 C In addition, tese values also satisfy te above requirements To alulate for ase of te ig ressure saturated steam of te boiler, te arameters of te ig ressure boiler feed water su as mass flow rate, temerature and ressure are (b Effet of te total reovery energy δ H Fig 6 Effet of inlet mass flow rate of te ig ressure saturated steam of te boiler on te total inlet energy, te total outlet energy and te total reovery energy Te total outlet energy (Ho; Te inlet energy of te HPBFW (HiHPBFW and te outlet energy of te HPSST (HoHPSST

6 18 Le Min Nut et al: A Study on Energy Otimization of Heat Exangers in a Gasifiation System Fig 7 Effet of inlet mass flow rate of te ig ressure saturated steam of te boiler on outlet temerature of te ig ressure suereated steam to turbine (a Effet of te total inlet energy (Hi Fig 8 (a sows te variations of te inlet energy of te ig ressure saturated steam of te boiler H ihpstb, te outlet energy of te ig ressure suereated steam to turbine H ohpsst, and te total inlet energy H i and total outlet energy H o of te eat exangers wit reset to te inlet temerature of te ig ressure saturated steam of te boiler By inreasing te inlet temerature of te ig ressure saturated steam of te boiler until te value of 6145 C, te inlet energy of te ig ressure saturated steam of te boiler H ihpstb, te outlet energy of te ig ressure suereated steam to turbine H ohpsst, and te total inlet energy H i and total outlet energy H o of te eat exangers inreased slowly; owever, from 6145 C to 905 C, tey inrease quikly and ten inreased slowly Tis is beause, in te range of temeratures from 0335 C to 6145 C, te state of te ig ressure saturated steam of te boiler is saturated liquid-vaor mixture state but lose to saturated liquid state wile at 905 C it is saturated vaor state and ten beomes suereated vaor state Terefore, entaly anges tat orresond to te state of te ig ressure saturated steam of te boiler lead to te igest total reovery energy of te eat exangers δ H at 905 C (see Fig 8 (b Fig 9 sows te effet of inlet temerature of te ig ressure saturated steam of te boiler on outlet temerature of te ig ressure suereated steam to turbine In te range of 0335 C to 6145 C, te outlet temerature of te ig ressure suereated steam to turbine is onstant at 89 C; owever, from 6145 C to 905 C, it inrease quikly and ten inreased slowly As te above analysis sows, te otimal mass flow rate of ig ressure saturated steam of te boiler is determined at 38, 430kg/ wile otimal temerature is 905 C and ressure is 74 bars In tis study, te waste eat amount from air ooler is also investigated Te simulation results sow tat, wen sixty erent of te eat release amount of te air ooler (178MW is used, te total reovery energy of te eat exangers δ H ould be inreased by 461% (78MW (b Effet of te total reovery energy δ H Fig 8 Effet of inlet temerature of te ig ressure saturated steam of te boiler on te total inlet energy, te total outlet energy and te total reovery energy Te total outlet energy (H o; Te inlet energy of te HPBFW (H ihpbfw and te outlet energy of te HPSST (H ohpsst Fig 9 Effet of inlet temerature of te ig ressure saturated steam of te boiler on outlet temerature of te ig ressure suereated steam to turbine

7 International Journal of Meanial Engineering and Aliations 016; 4(3: Conlusions In tis study, a matematial model of a eat exanger based on te effetiveness-ntu metod is develoed to redit its oerating erformane for te urose of otimal temerature and mass flow rate of te fluid in te eat exangers of te gasifiation system Comutational simulations ave roved tat te otimal mass flow rate, temerature and ressure of te ig ressure boiler feed water are determined at 175,907kg/, 110 C and 100 bars, resetively, wile te otimal mass flow rate, temerature and ressure of te ig ressure saturated steam of te boiler are determined at 38,430 kg/, 905 C and 74 bars, resetively Moreover, te sixty erent of te eat release amount of te air ooler (178MW is also roosed to use beause te total reovery energy of te eat exangers δh ould be inreased by 461% (78MW Aknowledgements Tis work was suorted by te Tenology Innovation Program (No 011T , Tenology develoment for te demo-sale SNG syntesis funded by te Ministry of Knowledge Eonomy (MKE, Reubli of Korea Referenes [1] Koysinski J, Sildauer T J, Boillaz S M A, Prodution of synteti natural gas (SNG from oal and dry biomass A tenology review from 1950 to 009, Fuel, Vol 89, , 010 [] Cronomarkovi N, Rei B, Neskovi O, Veljkovi M, Mladenovi R, Exerimental investigation of role of steam in entrained flow oal gasifiation, Fuel, Vol 86, 194-0, 007 [3] Bell D A, Towler B F, Fan M H, Coal Gasifiation and its Aliations, Oxford OX5 1GB, UK, 011 [4] Luan Y T, Cyou Y P, Wang T, Numerial analysis of gasifiation erformane via finite-rate model in a ross-tye two-stage gasifier, International Journal of Heat and Mass Transfer, Vol 57, , 013 [5] Jaojaruek K, Matematial model to redit temerature rofile and air fuel equivalene ratio of a downdraft gasifiation roess, Energy Conversion and Management, Vol 83, 3-31, 014 [6] Materazzi & et al, Termodynami modelling and evaluation of a two-stage termal roess for waste gasifiation, Fuel, Vol 108, , 013 [7] Coi Y C, Li X Y, Park T J, Kim J H, Lee J G, Numerial study on te oal gasifiation arateristis in an entrained flow oal gasifier, Fuel, Vol 80, , 001 [8] Morris M, Waldeim L, Energy reovery from solid waste fuels using advaned gasifiation tenology, Waste management, Vol 18, , 1998 [9] Stoeker W F, Design of termal system, 3t ed MGraw-Hill Book, 8-93, 1989 [10] Inroera F P, Dewitt D P, Bergman T L, Lavine A S, Fundamentals of Heat and Mass Transfer, 6t ed Jon Wiley & Sons In, 006, , 006 [11] Hodge B K, Analysis and Design of Energy system Prentie-Hall, In, 1985 [1] Zedtwitz P V, Steinfeld A, Te solar termal gasifiation of oal-energy onversion effiieny and CO mitigation otential, Energy, Vol 8, , 003 [13] Ye D P, Agnew J B, Zang D K, Gasifiation of a Sout Australian low-rank oal wit arbon dioxide and steam: kinetis and reativity studies, Fuel, Vol 77, , 1998 [14] Cavan P D, Sarma T, Mall B K, Rajurkar B D, Sarma B K, Kulkarni B D, Develoment of data-driven models for fluidized-bed oal gasifiation roess, Fuel, Vol 93, 44-51, 01 [15] Abani N, Goniem A F, Large eddy simulation of oal gasifiation in an entrained flow gasifier, Fuel, Vol 104, , 013 [16] Andrew J M, Coal gasifiation for advaned ower generation, Fuel, Vol 84, - 35, 005 [17] Amed I, Guta A K, Evolution of syngas from ardboard gasifiation, Alied Energy, Vol 86, , 009 [18] Kruse A, Hydrotermal biomass gasifiation, Journal of Suerritial Fluids, Vol 47, , 009 [19] Erla B, Harder B, Tsatsaronis G, Combined ydrotermal arbonization and gasifiation of biomass wit arbon ature, Vol 45, , 01 [0] Kaka S, Boilers, Evaorators, and Condensers, New York, Jon Wiley & Sons In, 11-67, 1991 Biogray Le Min Nut reeived is BS in Heat and Refrigeration Tenology from Danang University of Tenology in 003 and MS degrees (in Termal Engineering from Vietnam National University, Ho Ci Min City University of Tenology in 006, resetively He ten reeived is PD degree from Jeju National University, Reubli of Korea He is urrently a leturer at te Ho Ci Min City University of Tenology and Eduation, Vietnam His fields of interest inlude solar energy and aliation, refrigeration and air onditioning, and renewable energy Youn Ceol Park reeived is BE in Meanial Engineering, MS, and PD degrees from Korea University in 1990, 199 and 1997 resetively He is now a Professor of te faulty of Meanial Engineering, Jeju National University, Reubli of Korea His urrent resear interests inlude Refrigeration, and Air onditioning