Selected Topics in Energy, Environment, Sustainable Development and Landscaping

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1 Modeling and optiization of a natural gas pressure reduction station to produce electricity using genetic algorith SEPEHR SANAYE AMIR MOHAMMADI NASAB Energy Systes Iproveent Laboratory (ESIL Departent of Mechanical Engineering Iran University of Science and Technology (IUST Energy Systes Iproveent Laboratory, Mechanical Engineering Departent, Iran University of Science and Technology, Narak, Tehran, IRAN Sepehr@iustacir, Airns2004@gailco Abstract: For longer distances, natural gas is transported through transit pipeline at high pressures In a place of consuption or at passing into a lower pressure pipeline, the pressure of the gas ust be reduced In Iran, the coon procedure of reducing pressure in natural gas station is using expansion valves, which causes the waste of large aount of energy Cobined heat and power (CHP systes have any econoic and environental benefits In this paper a CHP syste which its prie coponents are boiler, expander, pup, heat exchanger and gas engine, is used in a pressure reduction station to produce electricity A new and relatively quick ethod for selecting the nuber of each type of prie coponent, and deterining their noinal power and their efficiency is also presented In this ethod an objective function naed Actual Annual Benefit in ters of dollar has been defined as the su of the outcoes fro selling electricity and natural gas inus capital cost, operating cost and aintenance cost of prie coponents of the syste Subsequently different parts of the objective function have been expressed in ters of 7 decision variables Finally the optiu values of decision variables have been obtained by axiizing the objective function using genetic algorith Results of this research show that abient condition, pressure and ass flow rate of inlet natural gas, efficiency of prie coponents and prices of electricity and natural gas have the key role in selecting the appropriate strategy for the syste Keywords: "pressure reduction", "natural gas", "genetic algorith", "expander" Noenclature 1 Introduction Cobined heat and power (CHP syste ay be defined as a syste which generates electricity and heat siultaneously using a single source of fuel CHP systes play a significant role in efficient usage of energy in industrial and doestic applications Furtherore, these systes have less harful effects on the environent Properly designed CHP systes ay provide a theral efficiency ore than 80% [1] In these systes, efficiency is defined as the su of useful heat and power generated per unit of the input energy The theral efficiency of a CHP syste (soeties called energy utilization factor (EUF defined by Eq (1, is the ost iportant criterion to evaluate the theral efficiency of a CHP syste [2]: η CHP (1 E CHP + H Q f CHP ISSN: / ISSN: ISBN:

2 This efficiency is the su of the power output and recovered heat by CHP syste, divided by the energy input Selection of CHP systes have been investigated in soe references [1 5] In this paper, a new and relatively quick ethod for selecting the nuber of each type of prie coponent (boiler and gas engine, and deterining their noinal power and therodynaics paraeters is presented At first, a therodynaic odeling (siulation of a cobined heat and power (CHP syste shown in (Fig1 has been perfored Then by introducing an objective function and a set of design variables (including thero-physical constraints of the CHP syste, the optial design variables have been obtained The selected decision (design variables for this syste were: expander efficiency ( η EX, nuber of boilers (, power of boilers ( H bl, nuber of engines (n, noinal power of gas engines ( E no,en, outlet water teperature of heat exchanger ( T 7, ass flow rate of water crossing heat exchanger ( ɺ w In the next step, the influences of changes in abient condition, ass flow rate and pressure of inlet natural gas and prices of selling electricity and natural gas on the design paraeters and Actual Annual Benefit have also been studied below, have been coputed to use in the next pace [6, 7, 8, 9, 10, 11] Heat exchanger Q (2 (3 A req T o (4 1 cpng( T2 T1 w cpw( T8 T9 ( T o 8 Qreq U T Expander (5 T 2 T2 ( T9 T1 ( T8 T2 ln ( T T 1 η EX 9 T 3 1 P3 1 P2 k 1 k 2 Theral odeling Having known the values of decision variables ( η EX, n, E no,en, j, H bl, T 7, ɺ w with the aid of genetic algorith, for specific given values of ass flow rate, pressure and teperature of the inlet natural gas and pressure and teperature of the outlet natural gas, all the required inforation of the syste, using equation (6 P 2 P 1 P HeatExchan ger 2 ng 2 T3 (7 W cp ( T EX Reciprocating gas engine ISSN: / ISSN: ISBN:

3 Energy re covered fro jacket water LHV f, PL f 1749 exp( ( PL exp( ( PL (8 η η (9 th,pl th,no f,pl f,no exp( ( PL exp( ( PL (10 H ext, PL f, PL ( PL 01423( PL (For gas exhaust enthalpy (11 E en f pl (12 Boiler H bl f bl (13 Pup W pu (14 LHV LHV f f η η PL bl wν ( p7 p 9 w ν ( phe + pen& bl Natural gas has been assued an ideal gas with below coposition and characteristic The specific heat capacity of the cobustion gases is a coposition of the equations of each coponent in its ass percentage [12] Thus: T ( PL ( PL cpng ( T T T (15 One of iportant paraeters for estiating the available exhaust heat is the stack teperature In cobustion process the sulfur contents of fuel change to SO 2 and SO 3 In reaction with produced water, SO 3 creates sulfuric acid which is very corrosive This is the reason for keeping the stack teperature above the dew point teperature The dew point teperature varies with variations of atospheric conditions usually in the range of 516 o C ( 125 o F to 655 o C ( 150 o F [8] Therefore with ore sulfur content in fuel, the stack teperature should be higher for not having water vapor condensed Natural gas has little aount of sulfur content, however, carbonic acid is one of unwanted products in this case Heat transfer to the atosphere also exposes soe liitations on stack teperature Therefore it is recoended to consider stack teperature not lower than 1488 o C ( 300 o F for the boiler and the gas engine [8, 13] Furtherore for a naturally aspirated engine for about each 303 ( 1000 ft increase in elevation, efficiency decreases about 3%, this value is 2% for a typical turbocharged engine Power output also decreases 1% for each 55 o C ( 10 o F increase in abient teperature [1, 8] ISSN: / ISSN: ISBN:

4 3 Econoic considerations and estiating the Actual Annual Benefit (AAB In selection of a CHP syste, both theral and econoic analysis have to be applied Econoic analysis of the syste includes both capital as well as operational and aintenance costs Since the capital cost should be considered during engine lifetie, the annualized capital cost (R is obtained using the following relations [2, 11, 13, 14]: R βc (16 β (17 k i(1+ i (1+ i 1 k Where, β is capital recovery factor, i is the interest rate, k is the equipent lifetie and C is the initial capital cost Equivalent unifor annual cost (EUAC is also obtained by the following relation [14]: EUAC R A (18 Where: A (19 $ i Salvage value ( kw (1+ i ( k 1 In order to evaluate the investent and aintenance costs, the following relations were used [9, 11, 15, 16, 17, 18]: Heat exchanger (20 C A o,he ($ (21 M 06 EUAC ($ Expander C W 0 HE 0354 EX (22 $ M ( kw (23 Gas engine $ ( kw C ln( E, 17271($ / kw (24 M E 02 no (25 Boiler C 205 H ( bl no en + E, en ($/ kw E no, en ($ / kw ISSN: / ISSN: ISBN:

5 M 00027($ / kw (27 Pup C (28 w ($ M 006 EUACPu ($ (29 Then the actual annual benefit (AAB was defined as: Een φe + ( W EX W Pu φe J AAB + ( 1 f LHV f φf X j φe, j j 1 τ ( M + EUAC E ( M + EUAC H ( M + EUAC ( M + EUAC P CHP bl bl HE pu (30 4 Deterining design paraeters of syste Fig 1 illustrates the scheatic diagra of the analyzed CHP syste Prie coponents which are studied in this paper are gas engine, boiler, expander, pup and heat exchanger The data provided by the sellers of prie coponents ay be directly used in this step instead of relations entioned in Section 2 and 3 According to the flowchart shown in (Fig 2 a coputer progra using genetic algorith was developed to perfor the econoic and therodynaic analysis of this syste This progra will be repeated with the aid of genetic algorith to reach the optiu aount of AAB 5 Case study Case study of this research is designing an optiu CHP syste to produce electricity fro pressure of natural gas in an Iranian city In this pressure reduction station, natural gas enters with ass flow rate of 238 (kg/s, pressure of 40 (bar and teperature of 15 (C and desired outlet properties are pressure of 8 (bar and teperature of 8 (C The electricity and fuel cost (for natural gas with LHV about kj/kg are listed in Table 2 [11] The penalty cost of eissions in this case study was zero τ is assued 8760(hr (the nuber of hours in a year The values of equipent lifetie (k and interest rate (i were considered to be 15 years and 10%, respectively The salvage value was 20% of initial capital cost 6 Discussion and Results Considering natural gas condition (entioned in section 5 and proble constrains (entioned in section 4, results have been presented in table 3 It should be noted that with the optiu values of decision variables, properties of other prie coponents of the syste are: a heat exchanger with heat transfer surface of ( 2, a water pup with power of 1834 (W and an expander with power of 3247 (MW Due to the results of optiu nuerical values of decision variables and price of electricity and natural gas, using boilers is ore econoic than using gas engines However, it is ore econoical to use gas engines in case of a 50% increase in electricity prices or a 50% decrease in natural gas prices There are two contradictory factors which have the key role in deterining the quantity of T 2 At first, with the rise of T 2, the outlet electricity of the station increases which is a factor that raises AAB Secondly, as a result of T 2 rise, Q req increases; therefore, fuel consuption and cost of equipents applied ISSN: / ISSN: ISBN:

6 for heat exchange in the station becoes ore, thus reducing the AAB According to Eq (5, with the rise of station inlet gas ass flow rate, T 2 increases and with the reduction of ηex the quantity of T 2 becoes less A study of the results shows that the ore the pressure of station inlet gas, the less the efficiency of expander to oderate T 2 aount and consequently increase AAB However, when the pressure of station inlet gas reaches 429 bars, with theη EX at its possible iniu aount, the actual annual benefit is insignificantly less than it correspondent in 42 bars (Fig 3 With a teperature rise in inlet gas, Q req and power of the water pup decrease, leading to a rise in AAB (Fig 4 With inlet gas ass flow rate increasing, and at the sae tie input pressure kept constant, the quantity of station output gas and electricity increase, leading to a rise in actual annual benefit (Fig 5 There are two key factors deterining the aount of T 7 in the syste Firstly, the higher the T 7, the higher also the T Thus, based on Eq (4, A O decreases and the cost of purchase, operating and aintenance of heat exchanger becoes less Secondly, the less T 7, the less the water needed to pass through the heat exchanger, eaning that a less powerful pup can be used These two factors are contradictory however, and since in the conditions of the station, the first factor plays a ore iportant role, T 7 selects References [1] A Beyene, Cobined heat and power sizing ethodology, in: Proceeding of ASME Turbo Expo 2002, June 3-6, Asterda, The Netherlands [2] JH Horlock, Cogeneration Cobined Heat and Power (CHP, Pergaon Press, Oxford, 1987 [3] LR Hinojosa, AR Day, GG Maident, C Dunha, P Kirk, A coparison of cobined its axiu quantity fro that range However, with gradual increase of inlet gas ass flow rate, the rate of water passing through the heat exchanger increases When the inlet gas ass flow rate reaches 30 kg/s, the ass flow rate of the water passing through the heat exchanger eets it axiu aount With further increase of inlet gas ass flow rate, T 7 gradually increases so the syste can supply ore Q req (Figs 6 and 7 7 Conclusions A new and relatively quick ethod for selecting the nuber of each type of prie coponent, and deterining their noinal power and their efficiency based on both the econoic and the therodynaic analysis and base on desired inlet and outlet condition is presented The results obtained fro using this ethod to deterining the design variables of the syste were presented in table (3 A thero-econoic objective function naed Actual Annual Benefit was used in this paper and the condition of axiu AAB was considered as desired condition of the syste Maxiu aount of AAB for satisfying desired outlet condition were et when five boilers with H bl (w, a heat exchanger with A O ( 2, a water pup with W pu (w and an expander with W EX 3,247 (Mw was used heat and power feasibility odels, Applied Theral Engineering , 2007 [4] R Mikalsen, YD Wang, AP Roskilly, A coparison of Miller and Otto cycle natural gas engines for sall scale CHP applications, Applied Energy , 2009 [5] José Alexandre Matelli, Edson Bazzo, Jonny Carlos da Silva, An expert syste prototype for designing natural gas cogeneration plants, Expert Systes with Applications , 2009 ISSN: / ISSN: ISBN:

7 [6] S Sanaye, Shahabeddin Shokrollahi, selection and sizing of prie overs in cobined heat and power systes, in: Proceeding of ASME Turbo Expo, June 14 17, Vienna, Austria, 2004 [7] S Sanaye, et al, Nuber and noinal power of prie overs in cobined heat and power systes, in: Proceeding of ASME Turbo Expo, May 8-11, Barcelona, Spain, 2006 [8] ASHRAE HANDBOOK, 1999, Chapter S7, Cogeneration Systes and Engine and Turbine Drivers [9] M S Peters, M S Peters, K Tierhaus, R West, Plant Design and Econoics for Cheical Engineers, New York: McGraw-Hill, 2003 [10] A Kostyuk, V Frolov, Stea and Gas Turbines, Mir Publishers, Moscow, 1988 [11] S Sanaye, Mehdi Aghaei Meybodi, Shahabeddin Shokrollahi, Selecting the prie overs and noinal powers in cobined heat and power systes, Applied Theral Engineering, , 2007 [12] JL Silveira, CE Tuna, Theroeconoic analysis ethod for optiization of cobined heat and power systes Part I, Pergaon Press, Progress in Energy and Cobustion Science , 2003 [13] wwwcogenorg, The European Educational Tool on Cogeneration, Second ed, Deceber 2001 [14] KK Hupherys, S Katell, Basic Cost Engineering, Marcel Dekker, New York, 1981 [15] MP Boyce, Handbook for Cogeneration and Cobined Cycle Power Plants, ASME PRESS, 2002 [16] J Kalina, J Skorek, CHP Plants for Distributed Generation Equipent Sizing and Syste Perforance Evaluation, in: Proceeding of ECOS, 2002, Berlin, Gerany [17] J Kalina, J Skorek, Siulation of sallscale CHP syste perforance for given heat and electricity load profile, in: Proceeding of ECOS, 2000, Enschede, Netherlands [18] L Hongwei, R Nali, PA Haldi, Theral-econoic optiization of a distributed ulti-generation energy syste-a case study of Beijing, Appl Theral Eng 26 (2006, Fig (1: The scheatic diagra of studied pressure reduction station Fig (2: Flowchart of coputer progra ISSN: / ISSN: ISBN:

8 Actual annual benefit ($ Thousands Pressure of inlet gas (bar M238 M2379 M2381 Mass flow rate of water P40 (bar P20 (bar P30 (bar Fig (3: The effects of inlet gas pressure on aount of AAB Actual annual benefit($ Millions Tepreture of inlet gas(c M238 M2379 M2381 Fig (4: The effects of inlet gas teperature on aount of AAB Mass flow rate of inlet gas (kg/s Fig (7: The effects of inlet gas ass flow rate on ass flow rate of water Table (1: Natural gas coposition [12] Coponen t Volu e (% Mass (% Molecula r Mass (% LHV (kj/kg CH ,000 C 2 H ,525 C 3 H ,390 Actual annual benefit Thousands P10 (bar P20(bar P40 (bar C 4 H ,775 C 5 H ,400 CO N Mass flow rate of inlet gas Fig (5: The effects of inlet gas ass flow rate on aount of AAB 1000 Total ,966 0 Table (2: The price list [11] T7(C P40 P30 p20 Ites Selling electricity Selling natural gas Price ( $/kwh Mass flow rate of inlet gas(kg/s Fig (6: The effects of inlet gas ass flow rate on T 7 ISSN: / ISSN: ISBN:

9 Table (3: Optiu values of decision variables Design paraeters η EX n Eno,en j H bl ɺ w T 7 AAB Optiu design values Table (4: The list of constrains Constrains 065 ηex 09 kg 0 w 20( s 0 n 5 05 Eno,en 6(Mw 0 j 5 05 Hbl 6(Mw 60 T7 95( C T8 140( C Texi 1488( C Reason Coercial availability Coercial availability Space liitation Coercial availability Space liitation Coercial availability To provide condition for engines to work better To avoid evaporation of water in heat exchanger To avoid production of corrosive acids ISSN: / ISSN: ISBN:

Table 1- An overview of the different options of repowering. %-point Boiler Boiler

Table 1- An overview of the different options of repowering. %-point Boiler Boiler Proceedings of the International Gas Turbine Congress 23 Tokyo Noveber 2-7, 23 DRAFT IGTC23Tokyo-TS-11 Repowering of Lowshan Power Plant Mohaad R. Shahnazari,Dariush Foroughi,Haid Fakharian Niroo Research