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October 2011, Volume 2, No.5International Journal of Chemical and Environmental Engineering

A Feasibility Study of Employing an InternalCombustion Engine and a Turbo-expander in a

CGSHasan Eftekhari a,b , Kourosh Akhlaghi b , Mahmood Farzaneh-Gord c, Mohsen Khatib c b National Iranian Gas Company, South Khorasan Gas Company, Iranc The Faculty of Mechanical Engineering, Shahrood University of Technology, Shahrood, IranaCorresponding Author

Email: [email protected]

Abstract Most of the natural gas reaches its end users through transmission and distribution pipelines. Transmission pipelines operate at high

pressures. In a place of consumption or at passing into a lower pressure pipeline the pressure of the gas must be reduced. This pressurereduction takes place in places which called City Gate Stations (CGS). In CGSs the pressure must be reduced from 5-7 MPa to 1.5-2.0MPa (usually to 1.7 MPa) into high pressure intrastate pipeline, then to approximately 0.3 MPa into medium pressure intrastate

pipeline. In this study, based on a comprehensive program, Inlet and outlet properties of natural gas flow and daily flow rate through atypical CGS were measured and recorded for a whole year. Based on this data recording, the amount of electricity which can be

produced from natural gas pressure has been calculated when utilizing a turbo-expander. The amount of obtainable work could beincreased significantly through the use of a CHP system. The idea is to utilize an additional prime mover as a heat source whilesimultaneously generating additional work along a turbo expander. The additional prime mover was chosen to be an internalcombustion engine driving an additional generator. Some Internal combustion engines have been added to preheat the natural gas aswell as generating electricity. Preheating the gas rise the amount of electricity generation as well as preventing hydrate forming innatural gas stream. The amount of electricity generation for both systems have been calculated and compared. The economics of sucha system are very complex, and there are many variables that must be considered. Some of the major considerations are total installedcost, the load factor or capacity factor, the value of the electricity, and, where preheat is required, the cost differential between theelectricity produced and the fuel used. Any design must make assumptions about these variables and then the final design must be aseries of compromises, which will yield the optimum combination. The result shows that a considerable amount of energy can be

produced in a CGS. The result also showed that if a gas turbine utilized along to the turbo-expander, the amount of electrical energycan be doubled comparing when only a turbo-expander is installed.

Keywords: Gas Pressure Station, Turbo Expander, Internal Combustion Engine, Cogeneration

1. IntroductionThe increasing scarcity of energy resources, global

warming and blackouts resulting from weather conditionshave stimulated the search for more efficient methods ofenergy conservation, reducing greenhouse gas emissionsand ensuring power supplies. Meanwhile, natural gas

pressure is one of the best sources ofobtainable exergy potentially, which can be used to provide heat and poweras electrical or the other shapes of energy such as heat.

Natural gas is considered as an environmentallyfriendly clean fuel, offering important environmental

benefits when compared to other fossil fuels. The superiorenvironmental qualities over coal or crude oil are thatemissions of sulfur dioxide are negligible or that thelevels of nitrous oxide and carbon dioxide emissions arelower. This helps to reduce problems of acid rain, ozonelayer, or greenhouse gases. Natural gas is also a very safesource of energy when transported, stored, and used.

Natural gas flow through the high pressure pipelinescontains a valuable pressure exergy (available energy).

Transmission pipelines operate at high pressures (5 7MPa). In CGSs the pressure must be reduced from 5-7MPa to 1.5-2.0 MPa (usually to 1.7 MPa) into high

pressure intrastate pipeline, then to approximately 0.3MPa into medium pressure intrastate pipeline.

Figure 1 shows the schematic diagram of a typical pressure reduction stations. The CGSs essentially performs a safety function: the limitation of the pressurein the downstream system to a safe value. But thisconfiguration is not useful to employ the potential exergyof the high pressure flowing natural gas. So there is no

chance to produce heat and power in form of electricitynamely.

Figure 1: Process flow sheet diagram of a usual CGS

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Much research has been carried out to study the useof natural gas pressure exergy, focusing on the pressuredrop stations. One example is the work of Bisio [1].Heexamined systems to use this exergy, including amechanical system to compress air.In a limited number ofcases pressurereduction is achieved using a turbo-expander. This method has the added bonus of

powergeneration. The first turbo expander application fornatural gas processing wasaccomplished using Rotoflowtechnology in Texas in the early 1960s.It dramaticallydemonstrated how efficiently the expansion turbinecouldcondense heavier components of the gas stream, while atthesame time providing power to re-compress the leanergas. Turbo-expanders are mainly used in industry to createvery low temperature natural gas streams in ethaneextraction plants, or for LNG production. Because work isextracted from the expanding high pressure gas, theexpansion process closely follows an isentropic process.

Greeffet al. [2] has studied the integration of turbo-expanders into various high-pressure exothermicchemical-synthesis processes. They demonstratedsuccessful integration of a turbo-expander withmeaningful energy savings.

Hinderinket al. [3] proposed a method for calculatingthe absolute exergy of multi-component liquid, vapour ortwo-phase flows. This method enabled the clear divisionof the total exergy of a material stream into three terms,thus the exergy change of mixing was calculatedseparately from the chemical and the physical exergy.Exergies were calculated as extensive stream propertiesthrough the use of some external subroutines.

Poivill [4] has studied the use of turbo -expanders innatural gas pressure-drop stations with commercialsoftware, AspenTechs HYSYS process simulator. He

investigated the effects of turbo-expander isentropicefficiency on temperature reduction and powergeneration.

Farzaneh and Magrebi [5] studied exergy destructionin Irans natural gas fields. They concluded that one couldgenerate 4,200 MW of electricity from this pressureexergy. Farzanehet al. [6] studied methods of using the

pressure exergy of natural gas in the Bandar Abbas (Iran)refinery pressure-drop station. They investigated theeffect of gas pre-heating on the amount of electricitygeneration. They have also proposed somethermodynamics systems to produce and use refrigeration.Farzaneh et al. [7] investigated the amount of energy

destruction in Iran's pipeline network. Based on acomprehensive program, they concluded that one cangenerate 109.68 kjkg-1 of natural gas flow through thenetwork.

C.R.Howard [8], has been investigated the performance of a hybrid turbo expander and fuel cell(HTEFC) system for power recovery at natural gas

pressure reduction stations. Simulations were created to predict the performance of various system configurations.

In this study, a comprehensive program has beencarried out to record and measure natural gas properties

and flow rates at Mashhad west CGS for a year. Based onthis data, the amount of energy which could be generated

by utilizing a combined heat and power system wascalculated. The system consists of a preheat section, aturbo expander and additional prime movers. Theadditional prime moverwas selected to be an internalcombustion engine (ICE). Some Internal combustionengines have been added to preheat the natural gas as wellas generating electricity. Preheating the gas rise theamount of electricity generation as well as preventinghydrate forming in natural gas stream. The amount ofelectricity generation for both systems have beencalculated and compared.

The economics of such a system are very complex,and there are many variables that must be considered.Some of the major considerations are total installed cost,the load factor or capacity factor, the value of theelectricity, and, where preheat is required, the costdifferential between the electricity produced and the fuelused. Any design must make assumptions about thesevariables and then the final design must be a series ofcompromises, which will yield the optimum combination.

2. Power generation in a CGS with a CombinedHeat and Power System

In theory, it is possible to utilize a system to generateelectricity and refrigeration through the natural pressuredrop process. In this study, a combined heat power systemhas been proposed to capture this energy. The systemconsists of a turbo expander as a main and an internalcombustion engine (ICE) as additional prime mover.

Fig. 2 shows a schematic diagram of a typical CGSwith a CHP system. The amount of obtainable work could

be increased significantly through the use of a CombinedHeat and Power (CHP) system. The idea is to utilize anadditional prime mover as a heat source whilesimultaneously generating additional work along a turboexpander. The additional prime mover was chosen to be anatural gas-powered ICE driving an additional generator.The ICEs are available in different size and easily could

be combined to address seasonal change in the CGSs. Inthe CHP system, the heat lost from the ICE is utilized to

preheat the natural gas stream. This combination is moreeconomical than just burning gas to preheat the naturalgas.

In case of installing a turbo expander in a CGS, theinlet natural gas is needed to be heated up more than asimple throttling valve system. The temperature after theheater is calculated based on gas temperature after theexpander or throttle valve, by means of try and see

process. Certainly the outlet gas temperature is assumedto be greater than 5C to ensure that no hydration ishappened.

The average natural gas ICE is between 30% to 40%efficient in converting the energy content of the naturalgas into electricity. The majority of the other 70% to 60%is waste heat, and the majority of this waste heat can be

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used to preheat the natural gas. This is a very effectiveCHP, where nearly all the heat is additionally convertedinto electricity via turbo expander power generation. TheICE is widely available in different sizes.

The turbo expander is unable to provide powergeneration and pressure reduction in all possible flowsituations. It is therefore necessary for the system to haveconventional expansion valves installed in parallel withthe turbo expander to ensure consistent and safe pressurereduction.Flow is controlled between the turbo expanderand valves in order to maximize power production andensure accurate pressure control.

The turbo expander was simulated as an adiabaticexpansion of gas with work extracted from thecontrolvolume. Irreversibilities in the process were included asan isentropic efficiency. This is the ratio of the actualwork done by the system to the amount of work that istheoreticallypossible without irreversibilities. Enthalpiesfor known states were found as well as entropyandenthalpy for the ideal isentropic expansion. Thesewere combined with the isentropic efficiency inorder to

determine the actual outlet enthalpy. The power output ofthe turbo expander was thencalculated using the massflow and enthalpy change and the outlet temperature wasfound usingthe outlet enthalpy and pressure. Thefollowing expressions were used in EES in ordertocalculate these values.

Where h1 and h2 are the enthalpies at the inlet andoutlet of the turbo expander, is the mechanical power ofthe turbo expander in kW, is the mass flow ratethrough the turbo expander The isentropic efficiency ofthe turbo expander was approximated based on the gasflow rate.Efficiency curves for this simulation were basedon the efficiency relationship of a constant speedturboexpander with respect to flow given in [9].

A second order approximation for efficiency curvesas

flow rate varies from design flow was used to determinethe efficiency for each flowcondition. This relationship isshown in Figure 3.

For the purposes of this model, design efficiency,design flow, maximum flow and minimumflow werevaried in order to represent different turbo expanderconfigurations. Maximum andminimum flows for theturbo expander were used to allocate gas flow to the turboexpander andexpansion valves. Excess flow from theturbo expander at maximum flow was divertedtoexpansion valve components in the model. Flow rates

below minimum for the turbo expanderwere completelydiverted to the expansion valves.

The mechanical power output from the turbo expandercomponent of thesimulation was multiplied by thegearbox and generator efficiencies to calculate theelectricalpower output of the turbo expander. Totalelectrical power output from the entire system wasfoundusing the following expression.

Where is the total electric power of the systemin kW, is the mechanical power of the turbo expanderin kW, and are the mechanical

efficiencies of the gearbox and generator respectively,is the electric power supplied by the internal

combustion engine and finally is the number of enginesthat must be in the circuit.

3. Results and Discussion3.1- Measured value of natural gas properties flowing throughthe CGSs

Figure 4 shows the daily Inlet and outlet gas pressuresof the station. Thisstation exhibited seasonal inlet

pressure drops, due to pipeline network design and highnatural gas demand during the winter. But the outlet

pressure remains nearly constant in all the days.

Fi ure 2: A schematic dia ram of a CGS with a CHP

Figure 3: Turbo expander efficiency curve [9].

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Figure 5, shows the daily gas inlet temperature to thegas station in a year. It can be seen that this parameterexhibited severe seasonal changes in a year.

Figure 6, shows daily mass flow rates. As it can beseen, the largest flow occurred during December at theWest (Mashhad) CGS. This is due to cold weather andhigh demand for space heating. It can also be seen thatwinter natural gas demand is about three times larger thansummer demand in all CGSs.

3.2- Calculation of output workThe calculation procedure starts by computing

enthalpies of natural gas stream at inlet, preheated andoutlets positions. This is done by knowing inlet,

preheated, outlet pressures and temperatures and help ofexergetic efficiency.

Figure 7 shows the heat requirement of the system forvarious days of the year. It is obvious that the heatdemand in December and January is greater than the heat

demand in the other months. So the power production will be greater in these months compared with the other times.

Fig.8 shows that, nearly in all days, the majority of theheat requirement is obtained with the internal combustionengine and the heater is off all days approximately.

Figure 9 shows the power production of the singleturbo expander and ICE system, where the number ofgenerator sets will be changed in different days due tovariations of conditions and requirements.

Total power production ranges from 3000 to 14000kW when the single turbo expander and some internalcombustion engines are operational.Variations in flowrates have a direct effect on power production. As flowrates vary seasonally, the power production from theturbo expander will also vary. The output electrical powerof the expander is nearly a constant magnitude comparedwith the power production of the ICEs.

Figure 4: Gas inlet and outlet pressures of the Mashhadwest CGS.

Figure 5: Gas Inlet temperature of the Mashhad west CGS.

Figure 6: Daily mass flow rate of the Mashhad west CGS.

Figure 7: Heat requirement of the system

Figure 8: Daily Fuel Consumption Rate of SingleTurbo ex ander and Heater S stem

Figure 9: Daily Power Output of Single Turbo Expander andsome ICEs System

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Electrical efficiency of the internal combustionengine is dependent to the load percent where engine isworking on it. Figure 10 shows this variation with respectto the load percent of genset based on information of [10].

Also the output heat that can be obtained using thegenerator sets is dependent to the input fuel flow rate andthe load percentage of the genset. These variations areused in the modeling on the basis of Ref.10.

3.3- Number of Internal combustion enginesIt should be noted that the number of generator sets

are different for various days of year, where themaximum magnitude is 3 for winter.

The engines work at different load percent due tochanges of conditions and input parameters. So the load

percentage of the generator sets is calculated for various

days. Figure 11 shows the total number of internalcombustion engines that are in the circuit in a day. Alsothe number of full load is obvious in this figure.

The part load working engine has different load percent per day. Figure.12 shows this clearly.

4. Feasibility StudyThe cost analysis is carried out for the combined inlet

air coolingsystem and turbo-expander installation.Thecost analysis of the combined system is based on thecostbenefit analysis. In this method the additionalrevenues are calculatedas a result of additional electricity

production in MWh. Thenet electricity production isaround 22,602 MWh for turbo-expanderand additional

production is around 30,707 MWh for the internalcombustion engines in a year. The net cash flow of thiselectricity production is calculatedto be2,385,000USDbased on the current electricity price inIranwhich is 6 Cents/kW h. Table 1shows the detailed costanalysisfor the combined system. A payback periodanalysis wasalso carried out on these simulation results.There are two definitions for payback ratio, as given

below:Simple payback ratio (SPB)Discounted payback ratio (DPB)Where are calculated as follow:

Where F t is the net flow cash in period t . in

Figure 10: variation of electrical efficiency of theICE versus load percent

Figure 11: number of working generator sets, a) total numberof working engines and b) number of full load engines.

Figure 12: The load percentage of the part load working engine

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preliminary assessments, F t is often considered constant(i.e. it does not change with t): F t=F. Then Eq.4 leads to:

Which, of course, gives a positive value of SPB, if F >0; otherwise the investment results in a loss.

Also by assuming F t=F, the DPB is calculated asfollow:

There are investments which have a reasonable simple payback period, but their discounted payback periodshows that the investment cost will never be recovered.

Table 1: The cost analysis of the combined system

Total capital cost for turbo-expander

purchasing (Qty=1) 2,100,000 USDTotal capital cost for generator sets

purchasing (Qty=3) 1,437,416 USD

Annual O&M costs per year 125,682 USDDirect investments (i.e. installation, piping ,instrument, )

5,341,000 USD

Indirect investments(i.e. engineering andsupervision, ) 942,617 USD

Annual fuel consumption costs 262,472 USDAnnual net cash flow 2,385,000 USDCost of electricity per kWh 0.06 Cents/kWhrSimple payback ratio(SPB) 2.927 yearsDiscounted payback ratio(DPB) 3.243 years

Based on these values, the discounted paybackperiod

(DPB) has been calculated to be around 3.2 years. Thisunveiledthe cost effectiveness of the proposed system.

5. ConclusionThis work resulted in valuable insights into the design

of hybrid turbo expander and internal combustion enginefor power recovery and natural gas pressure reduction. Italso led to the identification ofmany areas for future workon the topic.A comprehensive program has been carriedout to record and measure natural gas properties and flowrates at Mashhad west CGS for a year. A comprehensive

program has been carried out to record and measurenatural gas properties and flow rates at Mashhad west

CGS for a year. Based on this data, the amount of energywhich could be generated by utilizing a combined heatand power system was calculated. The system consists ofa preheat section, a turbo expander and additional primemovers. The additional prime moverwas selected to be aninternal combustion engine (ICE). Some Internalcombustion engines have been added to preheat thenatural gas as well as generating electricity. Preheatingthe gas rise the amount of electricity generation as well as

preventing hydrate forming in natural gas stream. The

amount of electricity generation for both systems have been calculated and compared.

The isentropic efficiency of the turbo expander wasapproximated based on the gas flow rate.Efficiencycurves for this simulation were based on the efficiencyrelationship of a constant speedturbo expander withrespect to flow given in [9].

The results show that this configuration is capable to provide nearly 13MW electrical power in the coolest dayof the year. The expander is working in all the yearapproximately and so we can obtain minimum 1.8MW ina hot day of the year. The numbers of generator sets aredifferent for various days of year, where the maximummagnitude is 3 for winter. The engines work at differentload percent due to changes of conditions and input

parameters. So the load percentage of the generator sets iscalculated for various days.

A feasibility analysis has been performed to calculatethe payback ratio of this proposed configuration. Theresults show that the discounted payback ratio is nearly3.2 years. So this unveiled the cost effectiveness of the

proposed system.

6. List of symbolsDPB Discounted payback ratio (year)Ft, F Net cash flow per year (USD)F0 Investment costs (USD)h1 Inlet enthalpies of turbo-expander (kJ/kg)h2 Outlet enthalpies of turbo-expander (kJ/kg)ICE Internal combustion engine

Mass flow rate through turbo expandern Number of engines

P electrical Total produced electrical power (kW) P ICE electrical power of internal combustion engineSPB Simple payback ratio (year)

Mechanical power of turbo expander (kW)

Greek symbols

Turbo expander efficiency

Gearbox efficiency

Generator efficiency

R EFERENCES

[1] "Thermodynamic Analysis of the Use of Pressure Exergy of Natural Gas". Bisio.G. 2, 1995, Energy, Vol. 20, pp. 161-167.

[2] "Using turbine expanders to recover exothermic reaction heat-flowsheet development for typical chemical processes". Greeff, I.L, etal., et al. 2004, Energy, Vol. 229, pp. 2045-2060.

[3] " Exergy analysis with a flowsheeting simulator-I. Theory;calculating exergies of material streams". Hinderink, A.P, et al., etal. 20, Chemical Engineering science : s.n., Oct 1996, Vol. 51, pp.4693-4700.

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[4] "Use of Expansion Turbines in Natural Gas Pressure ReductionStations. Pozivill.J. 2004, Vol. 3, pp. 258-260.

[5] "Exergy of Natural Gas Flow in Iran's Natural Gas Fields".Farzaneh-Gord, M and Maghrebi, J. Forthcoming in internationalJournal of Exergy.

[6] "Using Pressure Exergy of Natural Gas In Bandar-Abbas Refinery

Gas Pressure Drop Station". Farzaneh-Gord, M, et al., et al. AbuDhabi,UAE : s.n., 24-27 2007. The second internationalconference on modeling, simulation and applied optimization.

[7] "Energy destruction in Iran's Natural Gas Pipe Line Network".Farzaneh-Gord, M, Hashemi, S and Sadi, M. 6, 2007, EnergyExploration and Exploitation, Vol. 25.

[8] "Hybrid Turbo expander And Fuel Cell System For PowerRecovery At Natural Gas Pressure Reduction Stations". Howard,C.R and engineering, A thesis submitted to the Department ofMechanical. s.l. : Queen's University , 2009.

[9] Bloch, Heinz,Claire Soares." Turbo expanders and ProcessApplications ". s.l. : Gulf Professional Publishing, 2001.

[10] under the co-ordination Ramsay, Bruce." The EuropeanEducational Tool on Cogeneration". 2. s.l. : University of Dundee,December 2001.

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