Waste heat utilization in natural gas pipeline compression stations by an organic rankine cycle

ENERGY EXPLORATION & EXPLOITATION · Volume 32 · Number 2 · 2014 pp. 317–328 317

Waste heat utilization in natural gas pipelinecompression stations by an organic rankine cycle

M. Zeki Yilmazoglu1,*, Ehsan Amirabedin1 and Babak Shotorban2

1Gazi University, Faculty of Engineering, Department of Mechanical Engineering,Maltepe, Ankara, TURKEY

2Department of Mechanical and Aerospace Eng. The University of Alabama Huntsville,AL 35899, USA

*Author for corresponding. Email: [email protected]

(Received 11 March 2013; accepted 16 December 2013)

Abstract Utilizing compressors, natural gas compression stations (NGCS) supply therequired pressure to transport the natural gas in pipelines. This study proposesto utilize the waste heat of gas turbines (GT), employed in these stations, toproduce shaft power for compressors. A NGCS coupled to an Organic RankineCycle (ORC) is simulated by THERMOFLEX in this study. Ten differentworking fluids are compared against each other in the ORC to select a workingfluid with the highest possible efficiency and obtainable power, and lowestpossible environmental impacts. According to the results, n-Pentane is selectedto be the working fluid, as compared to others, which better meets these criteria.The effects of the variation of the ambient temperature and the condenserpressure on the net power of the cycle working with n-Pentane are investigatedin details. As a general result, the utilization of waste heat in NGCS by an ORCcan improve the energy efficiency and generates electricity for near rural areas.

Keywords: Natural gas transportation, Natural gas compression station,Organic Rankine Cycle, Waste heat recovery, Pipeline

1. INTRODUCTIONWhile transporting the Natural Gas (NG) in pipelines, the pressure of the gas drops dueto friction so the gas must be constantly compressed in compression stations to providethe necessary pressure for the gas transportation. The compression processes requirelarge amounts of energy that are typically supplied by compressors with shaft worksproduced by gas turbines (GT). This production is made using a very small proportionof the transported NG as fuel. However, a simple analysis of the energy balance forthe compression station indicates that almost 60% of the total supplied energy to a gasturbine is emitted as heat to the atmosphere mainly by hot combustion gases leadingto so-called heat pollution. Although such heat emission is inevitable because of thesecond law, its minimization essentially reduces undesired environmental impactwhile saving some energy (Wei et al., 2007).

The waste heat from GTs can be somewhat useful for purposes such as electricitygeneration. However, since the temperature and the mass flow rate of the exhaust gasof a GT in a NG compression station is quite low, it cannot be utilized as the heatsource for a conventional Rankine cycle that operates with steam (Schuster et al.,2009). On the other hand, an Organic Rankine Cycle (ORC) has shown to be capableof working with such a low flow rate. An ORC involves components similar to thoseused in a conventional steam power plant. However, in an ORC, the working fluid isan organic material characterized by a lower boiling point than water, which, in turn,allows lower evaporating and superheating temperatures. The use of an ORC in anatural gas compression station (NGCS) provides the possibility for the developmentof a process to improve the energy efficiency of its compression process, so thecompression station can be self-sufficient in electricity consumption.

Various aspects of ORCs were investigated in recent years. Wei et al. (2007)analyzed the thermodynamic performance of an ORC system under disturbances andshowed that the degree of sub-cooling at the condenser outlet should be small (0.5–0.6K). Schuster et al., examined the state-of-the-art applications of ORCs addressing theirenergy and economy issues. Dai et al., conducted a study for ORCs comparingdifferent working fluids in order to find optimum performance of cycles. Hung alsocompared the influences of various working fluids including benzene, toluene, p-xylene, R113 and R123 on the recovered waste heat by an ORC (Hung, 2001). P-xylene showed the highest efficiency and the lowest irreversibility.

In the current study, a parametric analysis is conducted to determine an appropriateworking fluid for an ORC coupled with compression station. Moreover, the parametricoptimization of the selected working fluid is fulfilled in order to have an ORC withboth best thermodynamic performance and less environment impact.

2. DESCRIPTION OF THE SYSTEMTwo thermodynamic cycles coupled to each other are considered in this work, whichare a compression station used to provide the required pressure for the gas flow in aNG pipeline and an ORC. These two systems were described in detail.

2.1. Compression station and modeling A schematic diagram of the compression station is shown in Figure 1. Siemens SGT-200-1S model gas turbine with a total of 6250 kW mechanical shaft power isconsidered for the simulations that are performed with THERMOFLEX software(Thermoflex, 2010). The compression station inputs were taken from a real NGCS inTurkey. Ambient air enters the compression station cycle with a temperature of 15∞C,and then it is compressed with a pressure ratio of 12.11 in the compressor. Compressedair then enters to the combustion chamber where the combustion takes place using asmall proportion of the natural gas as the fuel source with a mass flow rate of 0.382kg/s. The mass flow rate of the natural gas entering the combustion chamber can bevaried in different weather conditions. General properties and the composition of thenatural gas are given in Table 1. Combustion gas leaves the gas turbine and enters theheat exchanger. The gas compressor is connected via a drive shaft to other threecompressors that supply the required pressure of the natural gas for pipeline

318 Waste heat utilization in natural gas pipeline compression stationsby an organic rankine cycle

transportation. There are two intercoolers between the compressors in order to reducethe power consumption. In brief, the compression station system consists of a gasturbine and three serial compressors on the same drive shaft and two intercoolers.

ENERGY EXPLORATION & EXPLOITATION · Volume 32 · Number 2 · 2014 319

Figure 1. Schematic diagram of the compression station and Organic Rankine Cycle

Table 1. The composition and general properties of the natural gas

Element Volume percentage [%] Hydrogen H2 0.36 Oxygen O2 0.07 Nitrogen N2 3.65 Carbon Monoxide CO 0.09 Carbon Dioxide CO2 0.34 Methane CH4 87.00 Ethane C2H6 8.46 Ethylene C2H4 0.03 Property Value Average temperature in pipeline [¡C] 7.7 Inlet pressure [bar] 48 Outlet pressure [bar] 73 Mass flow rate [kg/s] 90.72 Lower Heating Value [kJ/kg] 46280.2

2.2. Organic Rankine CycleFigure 1 displays an ORC power plant consisting of an evaporator, a single-stageorganic turbine, an air-cooled condenser and a pump. The pump supplies the workingfluid to the evaporator where the working fluid is evaporated by the exhaust heat ofthe compression station. After the working fluid evaporates, high-pressure vapor flowsinto the turbine and expands to produce power. The low-pressure vapor is led to theair-cooled condenser after which the condensed working fluid is pumped back to theevaporator, and a new cycle begins.

Depending on the shape of the saturation vapor curve of the working fluid the typeis called dry or wet in an ORC. In the dry type, the working fluid after expanding inthe turbine lies in the superheated region whereas in the wet type, the working fluidlies in the two-phase region. The organic working fluids of dry type are moreapplicable to ORCs since their superheated state eliminates the concerns about theimpingement of liquid droplets on the turbine blades (Liu et al., 2004). However, inthe dry type ORC, the temperature of the working fluid remains higher than itscondensing temperature after expanding in the turbine leading to extra heat that mustbe extracted in the condenser. Therefore, the heat efficiency and power output of thedry type ORC could be remarkably lower. To minimize this loss of efficiency andpower, another heat exchanger can be added to preheat the cold working fluid beforeentering the evaporator. Figure 2 displays an NGCS-ORC system with regenerator.


Figure 2. Schematic diagram of the compression station and Organic Rankine Cyclewith a regenerator

2.3. Working fluid selectionThe selection of the working fluid for an ORC is critical. The choice of proper workingfluids has been discussed in the literature (Karellas and Schuster, 2008; Saleh et al.,2007; Borsukiewcz-Gozdur and Nowak, 2007; Quoilin and Lemort, 2007; Somayaji,2008). According to the results the working fluid selection can be summarized asfollows.

• Thermodynamic performance: the efficiency and/or output power should beas high as possible for the given heat source and heat sink temperatures. Thisgenerally involves low pump consumption and high critical point (Quoilinand Lemort, 2007)

• High heat of evaporation (latent heat) and density: a fluid with high latentheat and density will absorb more energy from the source in the evaporatorand thus reduce the required flow rate, the size of the facility, and the pumpconsumption (Quoilin and Lemort, 2007)

• High specific heat: the liquid specific heat should be high meaning that lesspreheating is required (Somayaji, 2008)

• Low freezing point: the freezing point of the fluid must be lower than thelowest temperature of the ORC

• Minimal environmental impact • Safety: the working fluid must account for toxicity, danger of explosion and

flammability• Low cost.It is emphasized that two major parameters that should be taken into account with

respect to the environmental impact of the working fluids of ORCs are the OzoneDepleting Potential (ODP) and Global Warming Potential (GWP). The ODP is therelative amount of degradation to the ozone layer that a fluid can cause. Datumreference of all the organic working fluids is R11 with an ODP of unit. The lower thevalue of ODP is, the less harmful the organic working fluid is to the ozone layer. TheGWP is the potential of an organic working fluid in affecting global warming inrelation to Carbon Dioxide, where CO2 has a GWP of 1. The lower the value of GWPis, the less harmful the organic working fluid is to the environment. Table 2 lists thecritical thermodynamic properties and environmental data of different types of ORCworking fluids investigated in this study.

Other properties which have to be taken into account for the operation of an ORCare the temperature control of cycle, decomposition control and auto ignitiontemperature control of the working fluid. The temperature of the combustion gasstream could be adjusted by fresh air dilution or a heat exchanger. In the simulations,total heat flux of the combustion gas to ORC is assumed constant. Thermaldecomposition of n-pentane was investigated by Morgan and Munday (1935).According to their results, decomposition percentage was found approximately 11% at600°C. Therefore, an ORC could be positioned after a gas turbine which total heat fluxis not enough to operate a steam cycle such as in NGCS. The highest temperature ofthe working fluid is at the outlet of the evaporator which is also lower than autoignition temperature of n-pentane, 309°C (Internet, 2011). Hung et al. (1997) analyzedsome working fluids in the case of efficiency and power generation below 370°C heat


source temperature. Yilmazoglu and Amirabedin (2010a; 2010b) analyzed the effectsof environmental and natural gas temperature conditions and economic analysis ofoffered system. According to the results low natural gas temperature and surroundingtemperature increase the system performance. For the economic analysis of the systemthe payback time is found to be 3.8 years.


Table 2. Thermodynamic properties and environmental data of different ORCworking fluids

Substance Molecular formula

Molecular weight

Critical point

[¡C], [bar]

Boiling temperature

[¡C]

ODP ASHARE level of safety

GWP over 100 year

Type

Butane C4H10 58 152, 38 -0.5 0 A3 20 Dry

Isobutene C4H10 58 134.7, 36.4 -11.7 0 A3 20 Dry

Propane C3H8 44 96.7, 42.5 -42.2 0 A3 20 Wet

n-Pentane C5H12 72 196.3, 33.6 35.5 0 A3 20 Dry

R11 CCl3F 137 198, 44.1 23.7 1 A1 4600 Isentropic

R113 C2Cl3F3 187 241.1, 33.9 47.6 0.9 A1 6000 Dry

R123 C2Cl2F4 171 183.8, 36.6 27.8 0.01 B1 120 Dry, isentropic

R236EA C3H2F6 152 139.3, 35.0 -1.4 0 A1 1200 Dry

R245FA C3H3F5 134 157.6, 36.4 15.1 0 A1 820 Dry

Ammonia NH3 17 132, 113.4 -33.3 0 B2 <1 Wet

3. RESULTS In this study, a comparison is made for ten different organic working fluids. Emphasisis put on the net power and efficiency of the ORC for every considered fluid as well asits environmental impact. Table 3 displays important calculated parameters for variousORC working fluids simulated in the present work. These parameters include turbinegross output power and other auxiliary power with the various turbine and flue gasoutlet temperatures and pressures, and also mass flow rate of the working fluid for theirrespective optimal pressures. In Table 3, T5 and P5 are, respectively, the outlettemperature and pressure of the turbine. T10 and P10 are, respectively, the outlettemperature and pressure of the flue gas. m denotes mass flow rate of the workingfluid. Wtrbn, Wpmp, Wfn are the time rate of the work of the turbine, pump and fanrespectively. Qeva and Qcnds are, respectively, the time rate of the transferred heat inthe evaporator and condenser.

Figure 3 displays the net power output and the net efficiency of the NGS-ORC foreach working fluid. As seen in this figure, R113, R123 and n-Pentane are top threefluids with maximum power outputs, which are 2081, 1966 and 1824 kW, respectively.These ORC working fluids also have the highest efficiency among all fluids, with 21.3,20.36 and 21.07%, respectively, as can be seen in this figure. However, as explainedin the previous section, having the largest power output and energy efficiency does notnecessarily render a working fluid the best as an environmental impact of the fluidmust be taken into account as well. As could be seen in Figure 3, R113, for instance,

has the maximum value of the net power in comparison to other fluids; however, itsGWP and ODP values are 6000 and 0.9, respectively, which are notably higher thanthose of other working fluids making it not a good choice with respect toenvironmental impacts.


Table 3. Various calculated parameters for the NGCS-ORC operating with 10different types of ORC working fluids (with regenerator unit).

Substance T ambient T5 T10

P5 P10

m Wtrbn Wpmp Wfn Qeva Qcnds

Unit [¡C] [¡C] [¡C] [bar] [bar] [kg/s] [kW] [kW] [kW] [kW] [kW]

Butane 15 200 126 3 1 16 1910 143 109 9121 7201

Isobutene 15 210 119 4 1 18 1799 162 113 9287 7490

Propane 15 228 97 11 1 23 1560 184 125 9842 8271

n-Pentane 15 189 145 1 1 17 2225 149 98 8658 6506

R11 15 150 82 2 1 37 2082 138 123 10200 8128

R113 15 151 100 1 1 42 2410 151 111 9772 7354

R123 15 165 104 1 1 37 2282 140 111 9657 7358

R236EA 15 169 110 2 1 41 2008 152 125 9512 7581

R245FA 15 93 82 2 1 39 1903 155 127 10200 8409

Ammonia 15 188 82 11 1 7 1136 45 137 10200 9075

Figure 3. Net power output and net efficiency of the NGCS-ORC for 10 differenttypes of working fluids

Considering all mentioned criteria, n-Pentane seems to be a good choice for theorganic working fluid as according to Table 3, it has the third highest net output poweramong all fluids while having minimum environmental impacts according to Table 2.n-Pentane was selected as the working fluid in the system for which the simulationsperformed in this study. With this fluid, the calculated nominal power andthermodynamic efficiency of the considered system are 1.8 MW and 21.07%,respectively. Other thermodynamic properties of the system are tabulated in Table 4.


Table 4. Thermodynamic properties of each stream of the NGCS and ORCoperating with n-Pentane (with regenerator unit)

Point T[ßC] P [kPa] m [kg/s] 1 41.17 120 17.1 2 45 3005 17.1 3 142.5 2946 17.1 4 260 2600 17.1 5 189 124 17.1 6 65 120 17.1 7 15 0.87 641.7 8 25 0.87 641.7 9 484 89 23.61 10 145 88 23.61

Figure 4 shows the effect of the regenerator unit in the system. In the case aregenerator unit is employed. All input parameters are the same in both cases. Thesimulation results are shown in Fig. 5 that displays the variation of net power against

Figure 4. Net power output x 100 [kW] with and without pre-heater

the ambient temperature. It is observed in this figure that net power is significantlyhigher for the case with a regenerator. It could be also seen in this figure that in bothcases, with the increase of the ambient temperature from -15 °C to 30 °C, the net powerslightly decreases, roughly with a constant slope.


Figure 5. Effect of ambient temperature on net power [kW]

In addition to the ambient temperature, the variation of condenser pressure on thenet power and is also examined. The variation of parameters such as the exhaust massflow rate and temperature of the combustion gas or heat source temperature are notinvestigated in this study since the combustion gas properties of the compressionstation gas turbine are approximately constant. Figure 6 shows the effect of thecondenser pressure on the net power of the system. It is obvious from the figure thatwith the increase of the condenser pressure in the air-cooled condenser, the pressure ofthe turbine exhaust sharply increases. This increase means that a less amount ofworking fluid energy is utilized in the turbine, which consequently leads to a reductionin the general performance of the system.

4. CONCLUSIONS Organic Rankine Cycles can be integrated into various mid-temperature industrialwaste heat applications to partially recover the waste heat. One of the benefits ofORCs is their low temperature requirement with respect to conventional steam cyclesto generate electricity. Compression stations of natural gas pipelines are one of theindustrial sites generally located in distant areas. These stations release waste heat totheir environment; therefore, they could be good candidates for coupling with ORCs.In this manner, the waste heat is utilized and electricity is generated for generalrequirements of the site.

In this study an ORC is designed and coupled to an NGCS and perform analysis ofits performance. The important conclusions drawn from this study can be summarizedas follow:

• Using combustion exhaust gases originated from compression stations, whichare normally released to the atmosphere at high temperature, it is possible toproduce electricity through utilizing an ORC with limited temperatures. (Thetotal heat flux of combustion gas does not enough to operate a steam cycledue to gas turbine selection.)

• Amongst various ORC working fluids considered in this study, n-Pentanewas determined to be the best working fluid to achieve good efficiency withless environmental drawbacks. (Moreover, decomposition temperature andauto ignition temperature is less than operation temperatures.)

• The regenerative ORC was shown to have higher net output power andenergy efficiency than basic ORC.


Figure 6. Effect of air-cooled condenser pressure net power [kW]

• Under a high ambient temperature, the net output power of the NGCS-ORCwas indicated to deviate from the nominal power and the thermodynamicefficiency was shown to deteriorate remarkably.

• The increase of the condenser pressure led to a reduction of net output powerof the NGCS-ORC power plant.

REFERENCESAmirabedin E., Yilmazoglu M.Z. and Durmaz A., 2010. Utilization of the exhaust gas

of a gas pipeline compression station to generate electricity, 7th Int. Conf. onNatural Sciences and Tech. for Waste and Wastewater Treatment, Remediation,Emissions Related to Climate and Economic Effects, ECO-TECH’10, Kalmar,Sweden, 22-24 November 2010.

Borsukiewcz-Gozdur A. and Nowak W., 2007. Comparative analysis of natural andsynthetic refrigerants in application to low temperature Clausius–Rankine cycle,Energy 32, 344–352.

Dai Y., Wang J. and Gao L., 2009. Parametric optimization and comparative study oforganic Rankine cycle (ORC) for low grade waste heat recovery. EnergyConversion and Management 50, 576–582.

Hung T.C., Shai T.Y., Wang S.K., 1997. A review of Organic Rankine Cycles for therecovery of low-grade waste heat. Energy 22, 7, 661-667.

Hung T.C., 2001. Waste heat recovery of organic Rankine cycle using dry fluids.Energy Conversion and Management 42, 539–53.

Internet, <http://www.inchem.org/documents/icsc/icsc/eics0534.htm>, Access date2011.

Karellas S. and Schuster A., 2008. Supercritical fluid parameters in Organic RankineCycle applications. Int. J. of Thermodynamics 11, 101-108.

Liu B.T., Chien K.H. and Wang C.C., 2004. Effect of Working fluids on OrganicRankine Cycle for waste heat recovery. Energy 29, 1207–17.

Morgan J.J. and Munday J.C. 1935. Thermal decomposition of n-pentane. Ind. Eng.Chem. 27, 9, 1082-86.

Quoilin S. and Lemort V., 2007. Technological and Economical Survey of OrganicRankine Cycle Systems, 5th European Conference Economics and Managementof Energy Industry, Portugal, 2007.

Saleh B., Koglbauer G., Wendland M. and Fischer J., 2007. Working fluids for low-temperature Organic Rankine Cycles. Energy 32, 1210–21.

Schuster A., Karellas S., Kakaras E. and Spliethoff H., 2009. Energetic and economicinvestigation of Organic Rankine Cycle applications. Applied ThermalEngineering 29, 1809–17.

Somayaji C., 2008. First and second law analysis of organic Rankine cycle, DoctoralThesis, Mississippi State University, Mississippi.

Thermoflex Software, 2010. Thermoflow. Inc. 29 Hudson Road Sudbury. MA 01776· USA


Wei D., Lu X., Lu Z. and Gu J., 2007. Performance analysis and optimization oforganic Rankine cycle (ORC) for waste heat recovery. Energy Conversion andManagement 48, 1113–19.

Yilmazoglu M.Z., Amirabedin E., 2010. Sensitivity analysis of the effect ofenvironment and fuel conditions on a natural gas compression station operatedwith gas turbine. TUBAV Journal of Science 3(4), 298-305.


Waste heat utilization in natural gas pipeline compression stations by an organic rankine cycle

Documents

Transcript of Waste heat utilization in natural gas pipeline compression stations by an organic rankine cycle