A novel modification on preheating process of natural gas in pressure reduction stations to improve energy consumption, exergy destruction and CO2 emission: Preheating based on real demand

Accepted Manuscript A novel modification on preheating process of natural gas in pressure reduction stations to improve energy consumption, exergy destruction and CO2 emission: Preheating based on real demand

Mohammad Olfati, Mehdi Bahiraei, Farzad Veysi PII:

S0360-5442(19)30285-3

DOI:

10.1016/j.energy.2019.02.090

Reference:

EGY 14732

To appear in:

Energy

Received Date:

17 October 2018

Accepted Date:

12 February 2019

Please cite this article as: Mohammad Olfati, Mehdi Bahiraei, Farzad Veysi, A novel modification on preheating process of natural gas in pressure reduction stations to improve energy consumption, exergy destruction and CO2 emission: Preheating based on real demand, Energy (2019), doi: 10.1016/j.energy.2019.02.090

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ACCEPTED MANUSCRIPT A novel modification on preheating process of natural gas in pressure reduction stations to improve energy consumption, exergy destruction and CO2 emission: Preheating based on real demand Mohammad Olfati1, 2, Mehdi Bahiraei3, Farzad Veysi1,* 1 Mechanical 2 Head 3

Engineering Department, Faculty of Engineering, Razi University, Kermanshah, Iran

of Technical and Engineering Department, National Iranian Gas Company (NIGC), Kermanshah, Iran

Department of Mechanical Engineering, Kermanshah University of Technology, Kermanshah, Iran

* Corresponding Author: Farzad Veysi Email: [email protected]

Abstract One of the tools for optimizing energy systems is the design of the system output based on real (desired) demand. Thermodynamic performance of natural gas pressure reduction stations are functions of inlet conditions. In order to investigate the impacts of changes in inlet pressure and temperature on performance of a natural gas pressure reduction station, energy consumption and exergy destruction of a natural gas pressure reduction station of 10,000 SCMH are evaluated for different inlet conditions. In order to improve the station performance, a novel modification is proposed in the present research based on the real demand of preheating, wherein thermodynamic operation of the regulator is modeled and minimum pre-heating temperature of natural gas is calculated based on desirable temperature at the regulator outlet (natural gas hydrate formation temperature). Indeed, once the temperature at the heater outlet reaches the calculated minimum temperature, the heater is turned off. Compared to conventional stations, the modified station exhibits at least 33% and 15% reductions in energy consumption and exergy destruction, respectively. The results of investigating the performance of two sample stations also show that by implementing the proposed modification, CO2 emission can be reduced by up to 80% or even higher. Keywords: Pressure reduction station modification, Energy performance improvement, Real demand, Heater CO2 emission, Exergy destruction, Natural gas

ACCEPTED MANUSCRIPT 1. Introduction With the development of industrial communities and ever-increasing demand for energy, the consumption of fossil fuels has increased significantly, which intensifies the concerns associated with limited nature of the sources of such energy [1]. With the increase in the demand for energy and limited nature of the sources of fossil fuels and also concerns about climate changes, upgrading and improving the energy-consuming facilities based on their real demand are inevitable [2]. Natural gas (NG) is one of the cleanest fossil fuels, making it particularly preferred across industrial communities [3]. NG, in addition to supplying residential, commercial and industrial needs, has a major role in supplying electricity in power plants [4], therefore, this fuel plays an important role in the future development of the countries, and its continuous and safe supply are of particular importance [5]. According to credible international standards, NG is a mixture of a maximum of 21 gaseous components, with methane being its major partial component [6]. Having one of the largest reserves of NG in the world, Iran plays a significant role in supplying this fossil fuel [7]. Once exploited from well, sour gas is subjected to sweetening and dehydration to come with sweetened gas which is then introduced into the consumption cycle. NG can be transmitted from the point of production to the points of consumption in a number of forms, including LNG (liquefied natural gas), CNG (compressed natural gas), and pipeline-transmitted gas [8]. In Iran, almost entire deal of produced NG is distributed from refineries to consumption points via a network of pipelines at three classes of operating pressure: 6.89 MPa (1,000 psi), 1.72 MPa (250 psi), and 0.41 MPa (60 psi). Main NG transmission pipelines operate at a pressure of 6.89 MPa (1,000 psi) and are constructed beyond the territories of cities and villages. At the entrance of urban territories, safety and operating requirements imply that the operating pressure of NG should be reduced to 1.72 MPa (250 psi). The pressure reduction of NG is performed via a rapid expansion at regulator of a pressure reduction station (PRS) [9]. Given that Joule-Thomson coefficient of NG is a positive value, the pressure reduction of NG is accompanied with a drop of temperature. At lower temperatures, the pressure and composition of NG may favor the formation of NG hydrates in the regulator of the PRS. NG hydrate is a combination of light gases such as methane, ethane, or carbon dioxide that are bound to water molecules under particular conditions in terms of temperature and pressure to form an ice-like material [10]. The formation of NG hydrate may interrupt the operation of regulator and hence the pressure reduction operation at the PRS. In addition, the NG hydrate may end up damaging the equipment sets at downstream of the PRS. In order to avoid the formation of NG hydrate, prior to the pressure reduction, NG is pre-heated indirectly in water bath heaters. Given the large number of PRSs installed on the NG pipelines all over the world and the fact that heaters at such PRSs consume large amounts of energy, a wide spectrum of research works have been performed to model the operation of and thermodynamically analyze PRSs. Rashid-Mardani et 2

ACCEPTED MANUSCRIPT al. [11] calculated thermal efficiency of a heater at a PRS in Mahshahr city based on experimental data (53%). They predicted that, upon designing an auxiliary solar system, one can save some 39,000 m3 of NG per year by cutting the fuel consumed at the heater. Ashoori et al. [12] used the data from a PRS of 20,000 SCMH in capacity and calculated the Joule-Thomson coefficient with the help of AGA-8 equation of state to formulate a methodology where minimum required temperature at regulator inlet for preventing NG hydrate at the regulator outlet was calculated for different values of inlet pressure to the PRS. They considered a constant inlet NG temperature to the PRS of 15℃, and showed a 43% reduction in fuel consumption with pre-heating the NG to the calculated temperature at each pressure. In a PRS, Borelli et al. [13] reduced the NG pressure in two stages using a pair of turbo-expanders. They numerically simulated dynamic performance of the PRS and showed that, compared to the conventional single-stage pressure reduction procedure, the two-stage process consumes lower energy for pre-heating the NG. In recent years, numerous research works have also been performed on exergy analysis of PRS. Olfati et al. [14] examined a PRS of 20,000 SCMH in capacity in terms of both energy and exergy. They showed that seasonal changes over a year contribute to thermodynamic performance of such a PRS, and highest rate of exergy loss (15.33 kW) and exergy destruction (153.85 kW) occurred at the heater exhaust and combustion chamber of the heater, respectively. They further showed that, the exergy losses via heat transfer from the heater and filter cases and also the exergy destruction induced by friction along pipelines of the PRS were extremely negligible compared to those of other equipment sets. Farzanehgard et al. [15] referred to the advantages of using a solar auxiliary system at PRS in terms of reduced fuel consumption and exergy destruction. Their investigations were indicative of the fact that, overall cost of the plan (including 380 flat solar collectors along with a storage tank of 38 m3 in capacity) reached 144,000 USD. Accordingly, annual savings achievable via modification plan reached as high as 27,011 USD, so that payback period based on simple payback period method and net present value method were estimated at 5.5 and 8 years, respectively. Thermoeconomic investigation and exergy analysis of the plan indicated superiority of the plan in terms of exergy efficiency. Neseli et al. [16] simulated the operation of a PRS in Izmir, Turkey. At the studied PRS, the pressure reduction was practiced in a turbo-expander, with the NG pressure-induced power used for generating electricity. They evaluated energy and exergy efficiencies of the entire system and its components with respect to changes in NG pressure and volumetric flow rate, thereby showing that, at the studied PRS and based on average capacity, each year, 4,113,026 kWh of power could be generated at an efficiency of 69.24%. Xiong et al. [17] showed that in a PRS if a single screw expander to be used instead of the throttling valve, some portion of NG energy can be harvested during pressure reduction. They also showed that if the daily

3

ACCEPTED MANUSCRIPT exergy efficiency and daily power output reach 37.02% and 60.9 kWh, respectively, the daily round-trip efficiency of the system would increase to more than 25 %. In energy conversion systems, some portion of input power (fuel) is spent on generating output (product) and the remained is lost or destroyed. During the operation of the system, due to the change in loading conditions, the desired product may change in time and intensity [18]. Often, in the design of energy systems, the minimum amount of input power is considered to be capable of generating the maximum desired output (Design based on the worst case). In such a system, when the system output exceeds the desired output, the surplus output is not used and in the same time, the losses and destruction of the system would increase. So in the energy system optimization, prediction of the real desired demand and regulation of input power based on it, are of particular importance [19, 20]. The increase in fuel consumption not only wastes the energy resources, but also adds to environmental pollution resulted from the combustion products. Carbon dioxide is among the combustion products which significantly contributes to environmental pollution [21]. The heaters at PRSs are among the main fuel-consuming facilities, so that a reduction in fuel consumption of the heaters can reduce the deal of disposed environmental pollutants [22]. Based on the technical specification published by National Iranian Gas Company (NIGC) for heater manufacturing [23], NG can be heated to a maximum of 38℃. On this basis, all heater systems at Iranian PRSs are equipped with a controlling system which shuts down the heater once the NG temperature at outlet of the heater reaches 38℃. The NG temperature drop in the regulator depends on its pressure drop (Joule-Thomson effect). According to the fact that inlet NG to the PRS possesses various pressures during a year [14], one may expect different values of pressure drop and hence temperature drop at regulator, so that pre-heating NG to 38℃ is not necessarily required in all operating conditions. In order to calculate the minimum required pre-heating temperature (real preheating demand) for NG, one can determine a minimum NG temperature at regulator inlet for each inlet pressure into the PRS in such a way to maintain the NG temperature at regulator outlet within an engineering safety margin above the NG hydrate formation temperature. This can lower the energy consumed by heater significantly. On this basis, in the present research, firstly, a study is performed on the effects of different parameters at PRS inlet (temperature and pressure of the inlet NG) on energy consumption of heater and total exergy destruction of the PRS for a given composition of NG and during the operating period of a PRS with a capacity of 10,000 SCMH. Then, based on the inlet pressure of the PRS, an algorithm is proposed to calculate the minimum required temperature for pre-heating the NG. In the next step, based on the temperature of the inlet NG into the PRS, minimum pre-heating of the NG in the heater is determined. For this purpose, along with developing thermodynamic model of the PRS, thermodynamic properties of the NG are 4

ACCEPTED MANUSCRIPT calculated using conventional thermodynamic relationships as well as the AGA-8 equation of state. Accurate calculation of thermodynamic properties of NG and identification and examination of all possible inlet conditions (pressure and temperature of inlet NG) are outstanding highlights of the present study. The optimization is performed for both temperature and pressure assuming a given composition of NG. The values of energy consumption and exergy destruction are calculated for all possible cases at high accuracy, and the improvement obtained with the proposed modification under each set of inlet conditions during an operating period is evaluated. Finally, results of the analysis on two PRSs at two locations of different climatic conditions are examined, and the improvement obtained at each PRS upon implementing the proposed modification is computed in terms of energy consumption, exergy destruction, and annual carbon dioxide emission. To the best of our knowledge, no improvement has yet been conducted on PRSs based on real demand of preheating.

2. Definition of the problem Main NG transmission pipelines are extended from the NG supply premises (NG refineries and compressor stations) until the last consumption sites. Friction-induced pressure drop along NG transmission pipelines is governed by the following general relationship [24] (Equation 1): (1)

𝑃12 ― 𝑃22 = 𝐾𝑄𝑏𝑛

where P1 and P2 are upstream and downstream pressures along the pipeline, respectively, Qb is the volumetric flow rate of NG, n is exponent of NG flow rate (ranging between 1.74 and 2), and K = R × L is overall resistance of the pipeline (where R is the resistance per unit length of the pipe and L is the length of the pipeline). As can be observed, pressure of NG is a function of its flow rate, so that the pressure drops nonlinearly as one gets farther from the source of NG. Overall flow rate of main NG transmission lines depends on the removal amount of NG from the line at different points along the line. Given that a major portion of NG removal from the pipes is utilized for heating applications, the removal volume at each PRS may differ depending on local climatic conditions at final consumption sites supplied by the line [14, 25]. As such, it can be concluded that, inlet NG pressure into each PRS is a function of upstream climatic conditions. Temperature of inlet NG to each PRS is equal to the ground temperature at burial depth of the pipeline, with the ground temperature being a function of ambient temperature [26]. As such, it can be inferred that, temperature of the inlet NG at each PRS is determined by the ambient temperature of the PRS installation site. Considering the inlet NG pressure and temperature variations at each PRS throughout a year, it is unreasonable to consider a steady performance for the PRS throughout a year, and such an assumption may be true only during shorter periods of time wherein merely 5

ACCEPTED MANUSCRIPT negligible changes in inlet NG pressure and temperature occur. Consider, for example, a PRS installed within a tropical area such as the one under study, while the pipeline supplying this PRS passes through a pretty cold area before delivering NG to the PRS. Because a significant removal of NG is carried out at PRSs installed within the cold upstream region, the primary NG flow rate passing through the main pipeline must be greater and hence, the pressure decreases with higher intensity towards downstream locations. As a result, pressure at the inlet of downstream stations will be lower. On the other hand, ambient temperature at the PRS under study increases the ground temperature which can rise the NG temperature. So, it can be stipulated that, in the majority of days in a year, the inlet NG to the PRS under study is expected to exhibit relatively low pressure and high temperature values. Table 1 shows the values of inlet NG pressure and temperature for a PRS based on climatic conditions in the vicinity of the PRS as well as those of upstream PRSs. Given that ambient temperature of a meteorological region varies between minimum and maximum values throughout a year, the intermediate pressure and temperature values should also been considered when addressing the problem. Accordingly, in an attempt to account for all possible combinations of inlet pressure and temperature at different PRSs installed along a NG transmission pipeline, the problem is solved for 8 values of local ambient temperature at the studied PRS (-5℃ to 30℃ at step size of 5℃) and 8 values of inlet NG pressure to the PRS 6.89 MPa (1,000 psi) to 1.72 MPa (250 psi) at step size of 0.69 MPa (100 psi). Table 1. The effect of climate conditions on pressure and temperature of the NG entering the PRS. Climate

Inlet values of NG for PRS under study

Upstream PRSs

Under study PRS

Pressure

Temperature

Cold

Tropical

Low

High

Tropical

Tropical

High

High

Cold

Cold

Low

Low

Tropical

Cold

High

Low

3. Mathematical formulation Schematic of a PRS is shown in Figure 1. In this process, after filtering the NG, it is pre-heated in a heater and finally depressurized at regulator.

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ACCEPTED MANUSCRIPT

Fig. 1. Schematic diagram of the PRS.

The heater is the only item of equipment at a PRS that consumes energy. Pressure drops in the filter and the pipelines, combustion and heat exchange in the heater, and rapid expansion in the regulator tend to destruct exergy at a PRS. According to the literature, the exergy destruction resulted from the pressure drop through the filter and friction along the pipelines and also energy and exergy losses due to heat exchange through the heater wall are negligible compared to those associated with other parts of the PRS [14]. On this basis, the model demonstrated in Figure 2 is considered for the present analysis.

Fig. 2. Selected model of PRS.

The following assumptions are considered to solve the problem: -

Problem conditions are steady. 7

ACCEPTED MANUSCRIPT -

Air is introduced into the heater with environmental conditions in terms of pressure and temperature.

-

The fuel is introduced into the heater with environmental conditions in terms of pressure and temperature.

-

The flue gas emitted through the exhaust is already reached the environmental conditions in terms of pressure, temperature and concentration.

-

The studied PRS supplies NG for an industrial complex. This implies that the NG mass flown within the PRS remains rather invariant throughout a year.

-

The pressure at the PRS installation site is 1 atm.

Conventionally deployed heaters at PRSs heat the NG to 38℃ prior to actual pressure reduction process. The reduction in NG temperature is a function of the drop in its pressure. Based on the pressure of the inlet NG into the PRS, one can calculate the minimum required pre-heating temperature for the NG. Then, installing a control system, the PRS can be programmed in such a way to turn off the heater once the temperature of the inlet NG reaches the calculated minimum requirement. In this way, thermodynamic performance of the PRS is improved. Continuing with the article, we begin with investigating thermodynamic performance of conventional PRSs followed by studying the effect of the proposed modification on improved energy consumption and reduced irreversibility of the PRSs. 3.1.

Thermodynamic model of conventional PRS

In this section, thermodynamic model of a conventional PRS is presented. Temperature of the inlet NG into the PRS is equal to the ground temperature at burial depth of the pipe. According to Equation (2), the ground temperature at depth of 1 m (burial depth of the NG pipeline) can be obtained from the ambient temperature [26]: 𝑇1 = 0.0084 × 𝑇𝑎𝑚𝑏2 +0.3182 × 𝑇𝑎𝑚𝑏 +11. 403

(2)

where Tamb is ambient temperature in ℃. According to the technical specification published by NIGC, NG should be pre-heated to 38℃ before actual pressure reduction [23]. As such, in any case, output temperature of the heater (T2) is considered as 38℃. In the regulator, NG pressure is decreased from an initial value to 1.72 MPa (250 psi). In order to predict NG temperature after pressure reduction stage, Joule-Thomson coefficient of the natural is calculated from the following relationship [12]: 𝑅𝑇2 ∂𝑍 ∂𝑇 𝑃

𝜇𝐽𝑇 = 𝑃𝐶𝑚,𝑝

( )

(3)

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ACCEPTED MANUSCRIPT where T and P are temperature and pressure of the NG, respectively, Cm,p is constant pressure molar heat capacity of the NG, and Z is compressibility factor of the NG. In order to calculate the value of Z, the AGA-8 equation of state is utilized. The AGA-8 equation of state [27] is the most common equation of state for predicting NG properties in industry, by which one can determine compressibility factor and density of NG based on pressure, temperature and molar composition of the NG. Molar composition of the NG used as both working fluid and fuel for the heater is detailed in Table 2. This molar composition is for the NG flowing through pipelines in Kermanshah Province, Iran. Table 2. The composition of NG in Kermanshah province pipelines [14]. Component

Mole Fraction (%)

Co2

1

N2

3.77

CH4

88.54

C2H6

4.55

C3H8

1.16

ISO-C4H10

0.34

N-C4H10

0.34

ISO-C5H12

0.15

N-C5H12

0.15

The regulator installed at the studied PRS is an axial-type regulator, with NG temperature at its output calculated via the following relationship [12]: (4)

𝑇3 = 𝑇2 ―0.734 × ∆𝑃 × 𝜇𝐽𝑇(𝑥𝑖,𝑇2,𝑃3) (5)

∆𝑃 = 𝑃2 ― 𝑃3 3.2.

Thermodynamic model of modified PRS

In this section, based on the conditions of inlet NG to the PRS and considering the target preheating temperature, the processes performed in the PRS are planned according to a modified thermodynamic model. The purpose of this modification is to reduce fuel consumption at preheating stage and hence prevent the waste of fossil reserves while cutting the resultant environmental pollution. The pre-heating stage is performed to prevent the formation of NG hydrates at outlet of the regulator. Since output temperature of the regulator is a function of its inlet temperature and pressure drop, one can predict minimum required NG temperature at inlet of the regulator using thermodynamic properties of the NG in such a way that the temperature at the 9

ACCEPTED MANUSCRIPT regulator outlet (𝑇3) remains above the NG hydrate formation temperature. It is worth mentioning that in Iran, in PRSs called City Gate Station (CGS), the inlet pressure is reduced to 250 psi (i.e. 1.72 MPa). So, in this study, the PRS output pressure (𝑃3) is considered constant and equal to 250 psi. Considering a safety margin of 5℃ at outlet of the regulator, minimum required temperature at inlet of the regulator can be obtained from Equation (7). 𝑇 = 𝑇ℎ𝑦𝑑(𝑃3) +5

(6)

𝑇2,𝑚𝑖𝑛 = 𝑇 +0.734 × ∆𝑃 × 𝜇𝐽𝑇(𝑥𝑖, 𝑇,𝑃2)

(7)

where Thyd (P3) is the NG hydrate formation temperature at output pressure of the regulator and can be obtained from the following equation [28]: 𝑇ℎ𝑦𝑑 = 13.47𝑙𝑛 (𝑃3) +34.27𝑙𝑛 (𝐺) ―1.675𝑙𝑛 (𝐺)𝑙𝑛(𝑃3) ―20.35

(8)

where Thyd (°F) is the NG hydrate formation temperature at pressure P3 (psi), and G is the relative density of NG (relative to air). In this way, for each inlet pressure into the PRS, the minimum required pre-heating temperature is calculated. Subsequently, given the temperature of inlet NG into the PRS, the requirement of a preheating stage is investigated and the heater is turned on/shut down accordingly. Operation of the modified PRS is demonstrated in the flowchart of Figure 3.

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Fig. 3. Performance flowchart of modified PRS.

3.3.

Energy and exergy analyses

Mass flow rate of the consumed fuel by the heater is calculated as follows: 𝑄𝑁𝐺

(9)

𝑚𝑓 = 𝜂𝐼 × 𝐿𝐻𝑉

where 𝑄𝑁𝐺 is the absorbed energy by NG in the heater, LHV is lower heating value of NG, and η1 is the first-law efficiency of the heater. LHV of the fuel used in the heater of the PRS (Table 2) is calculated to be 45,466 kJ/kg. Moreover, investigations showed that, first-law efficiency of the heater of the PRS is about 0.5 [11, 14]. In the heater, NG is heated via a constant-pressure process. The energy received by the NG can be calculated from the following relationship: (10)

𝑄𝑁𝐺 = 𝑚𝑁𝐺(ℎ2 ― ℎ1)

where 𝑚𝑁𝐺 and h are mass flow rate and enthalpy of the NG, respectively. Mass flow rate of NG utilized in the analyses is obtained from the following relationship: 𝑚𝑁𝐺 =

𝑀𝑁𝐺 × 𝑃𝑠𝑡 × 𝑉𝑠𝑡

(11)

𝑅 × 𝑇𝑠𝑡

11

ACCEPTED MANUSCRIPT where 𝑀𝑁𝐺 is the molecular weight of NG mixture, 𝑃𝑠𝑡 and 𝑇𝑠𝑡 are the pressure and temperature of the standard condition respectively (1 atm and 25℃), 𝑉𝑠𝑡 is the volumetric flow rate of NG at standard condition and 𝑅 denotes the gas universal constant. Molar enthalpy of the NG is obtained from the following relationship [29]: 𝜌

ℎ𝑚 = ℎ𝑚,𝐼 ― 𝑅𝑇2∫0𝑚

(∂𝑇∂𝑍)𝜌

𝑑𝜌𝑚 𝑚

𝜌𝑚

𝑧

(12)

+𝑅𝑇∫1𝑑𝑍

where hm,I is the molar enthalpy of ideal NG, ρm is molar density of NG, and R is the universal gas constant. In this equation, the parameter Z is calculated from the AGA-8 equation of state. Exergy shows the ability of a mass or energy to perform work. In a spontaneous process, energy of higher quality changes to energy of lower quality. Equation (13) provides the exergy destruction equation for steady state [30]: 𝑇0

(13)

𝐸𝑑 = (1 ― 𝑇𝑗 )𝑄𝑗 ― 𝑊𝑐.𝑣. + ∑𝑗𝑚𝑖𝑒𝑖 ― ∑𝑗𝑚𝑒𝑒𝑒

where 𝑚𝑖𝑒𝑖 and 𝑚𝑒𝑒𝑒 are rates of exergy transfer at inlet (i) and outlet (e), respectively. Moreover, 𝑄𝑗 is the heat transfer rate at somewhere along the boundary of control volume where instantaneous temperature is Tj. 𝑊𝑐.𝑣. is the energy transfer rate via non-flow work and 𝐸𝑑 is rate of exergy destruction induced by irreversibilities within the control volume. According to the equation (𝐸𝑑 = 𝑇0 𝑆𝑔𝑒𝑛) rate of exergy destruction is proportional to the entropy generated during the process (𝑆𝑔𝑒𝑛 ). The subscript 0 refers to reference conditions. In the present research, atmospheric air with temperature and pressure of 298.2 K and 0.101325 MPa, respectively is considered as the reference environment. Flow exergy is written as per Equation (14): 𝑒 = 𝑒𝑃𝐻 + 𝑒𝐾𝑁 + 𝑒𝑃𝑇 + 𝑒𝐶𝐻

(14)

where ePH is physical exergy, eKN is kinetic exergy, ePT is potential exergy, and eCH is chemical exergy. For the purpose of this research, the potential and kinetic exergies are neglected. Physical exergy of a mass flow can be obtained from the following relationship: 𝑒𝑃𝐻 = (ℎ ― ℎ0) ― 𝑇0(𝑠 ― 𝑠0)

(15)

where h and s are enthalpy and entropy of NG, respectively. For a given mixture, chemical exergy is evaluated via Equation (16) [31]: 𝐶𝐻 𝑒𝐶𝐻 𝑚𝑖𝑥 = ∑𝑖𝑥𝑖𝑒𝑖 + 𝑅𝑇0∑𝑖𝑥𝑖𝑙𝑛 (𝑥𝑖)

(16)

where xi is molar fraction of the ith component, and 𝑒𝐶𝐻 𝑖 is chemical exergy of each component of the mixture. Entropy of NG can be obtained from the following relationship [29]: 𝜌

[

𝑠𝑚 = 𝑠𝑚,𝐼 ―𝑅∫0𝑚 𝑍 + 𝑇

(∂𝑇∂𝑍)𝜌 ] 𝜌

𝑑𝜌𝑚

𝑚

(17)

𝑚

12

ACCEPTED MANUSCRIPT where sm,I is molar entropy of ideal gas. Destructed exergy at a PRS includes the exergy destruction at the heater and the one at the regulator of the PRS: (18)

𝐸𝑑,𝑆 = 𝐸𝑑,𝐻 + 𝐸𝑑,𝑅

The destructed exergy at heater of the PRS (𝐸𝑑,𝐻) includes the destructed exergy due to combustion process in the burner of the heater, the destructed exergy due to heat transfer caused by temperature difference at heat exchanger of the heater, and the destructed exergy due to effluent of hot combustion products through the exhaust into the environment. Additionally, the exergy destruction in the regulator (𝐸𝑑,𝑅) is due to rapid expansion of NG. Exergy destruction at the heater can be obtained from the following relationship: 𝐸𝑑,𝐻 = 𝑚𝑓𝑒𝑓 + 𝑚𝑁𝐺(𝑒1 ― 𝑒2)

(19)

The fuel is fed into the heater at environment temperature and pressure and therefore, it has not any physical exergy while possesses chemical exergy. The following equation gives chemical exergy of NG with good accuracy [14, 31]. (20)

𝑒𝑓 = 1.04 × 𝐿𝐻𝑉 Exergy destruction at the regulator can be calculated from the following equation: 𝐸𝑑,𝑅 = 𝑚𝑁𝐺(𝑒2 ― 𝑒3)

(21)

Exergy destruction of each member of a system can be further decomposed into avoidable and unavoidable parts [32]. The unavoidable part refers to the part of destructed exergy which cannot be avoided due to technical and/or economic limitations. In order to eliminate the avoidable exergy destruction, processes shall be modified in such a way to minimize the generated entropy in each process. For this purpose, two corrective measures can be carried out: 1. Adopting alternative processes where lower entropy is generated for the same product, thereby ending up with lower exergy destruction. An example is the use of turboexpander rather than regulator at PRSs [33]. 2. Implementing process modification practices including (A) improving the process performance by taking some measures such as enhancing the heat transfer area to reduce the temperature difference, or for the burners, lowering temperature variations along the combustion chamber by pre-heating the air and step-wise injection of fuel-air mixture while controlling the flow velocity through the fuel nozzles [34], and (B) process optimization based on the requirements and constraints (real demand); in the present study, the option (B) is utilized. In this research, operation of the heater of the PRS is

13

ACCEPTED MANUSCRIPT controlled based on the predicted performance of the regulator for minimizing the fuel consumption by the heater while avoiding hydrate formation at the regulator outlet. 3.4. CO2 emission In order to calculate CO2 emission value from a PRS, the fuel combustion equation at the burner of the heater is written as follows [35]:

where a, b, c, and d are mole numbers of incoming air, CO2, water, and oxygen involved in the combustion reaction, and 𝛽 is defined as: 𝛽=

𝑀𝑎𝑖𝑟 𝑀𝐻2𝑂

(23)

×𝜔

where 𝑀𝑎𝑖𝑟 and 𝑀𝐻2𝑂 are molecular masses of dry air and water, respectively and ω is the humidity ratio of the incoming air which is evaluated as follows: ∅ × 𝑃𝑔

(24)

𝜔 = 0.622 × 𝑃𝑎𝑡𝑚 ― 𝑃𝑣

where ∅ is relative humidity of the incoming air, Pv is partial pressure of the air moisture content, Pg is saturation pressure of the air moisture content, and Patm is atmospheric pressure. Mass flow rate of the CO2 content of the combustion products is obtained from the following equation: 𝑚𝐶𝑂2 =

𝑀𝐶𝑂2 𝑀𝑓

(25)

× 𝑏 × 𝑚𝑓

where MCO2 and Mf are molecular masses of CO2 and the fuel, respectively. Given the type of burner usually used in a PRS, excess combustion air is considered to reach 20%. 4. Results and discussion 4.1. Conventional PRS In the conventional PRS, the temperature at inlet of the regulator (T2) is 38℃ while the one at the regulator outlet (T3) is obtained from Equation (4). Regulator output temperatures are calculated for different inlet pressures, with the results shown in Table 3. As predicted, with increasing the inlet pressure, the temperature at the regulator outlet experiences a more significant reduction. Table 3. Calculated temperatures at the regulator output (conventional PRS). P2 T3

Mpa

2.07

2.76

3.45

4.14

4.83

5.52

6.21

6.89

psi

300

400

500

600

700

800

900

1000

˚C

36.7

34.5

32.2

30

27.7

25.8

23.6

21.4

14

ACCEPTED MANUSCRIPT Since NG expansion at regulator occurs via a throttling process, from an energy analysis point of view, the only unit consuming energy in a PRS is the heater. The results of energy analysis on the heater of the PRS under different sets of inlet conditions are demonstrated in Figure 4. As can be seen in this figure, since final pre-heating temperature of NG is the constant value of 38℃, a decrease in the temperature of inlet NG (ambient temperature) increases the required energy for preheating the NG. On the other hand, at a constant inlet temperature, with increasing the NG pressure, the absorbed energy for pre-heating increases. At higher NG pressures, the effect of increased temperature on the absorbed energy by NG is further pronounced. The reason is that, within the operating range of the PRS, an increment in pressure increases heat capacity of the NG, so that more energy is required to heat the NG to the same temperature [36]. It is worth mentioning that in the conventional PRS, as stated, the outlet temperature of the heater (T2) is invariant and therefore, only the heat capacity variation can change the absorbed heat for a constant inlet temperature. Maximum rate of energy consumption (151.9 kW) by the heater at the PRS is observed at the lowest and highest NG temperature and pressure at inlet of the PRS, respectively. 160

Energy of NG Preheating (kW)

140 120 100 80 60 40 20

-5 (˚C)

0 (˚C)

5 (˚C)

10 (˚C)

15 (˚C)

20 (˚C)

25 (˚C)

30 (˚C)

0 300

400

500

600

700

800

900

1000

PRS Inlet Pressure (psi) Fig. 4. Energy consumption of the heater for different inlet pressures at various ambient temperatures (conventional PRS).

In the heater, exergy is destructed due to the combustion and heat transfer while in the regulator, it is destructed due to the expansion. Figure 5 provides the overall exergy destruction at the 15

ACCEPTED MANUSCRIPT conventional PRS for different values of inlet pressure and ambient temperature. As can be observed on the figure, highest rate of exergy destruction (679.1 kW) is perceived at the lowest ambient temperature and maximum inlet pressure to the PRS. This set of conditions can be observed at PRSs located in cold regions and close to NG refineries or compressor stations. On the other hand, the lowest rate of exergy destruction (138.1 kW) is found to correspond to the lowest inlet pressure along with the highest ambient temperature: i.e., PRSs at the end of the NG transmission line, far from NG refineries or installed compressor stations.

Exergy Destruction in PRS (kW)

700 600 500 400 300 200 100

-5 (˚C)

0 (˚C)

5 (˚C)

10 (˚C)

15 (˚C)

20 (˚C)

25 (˚C)

30 (˚C)

0 300

400

500

600

700

PRS Inlet Pressure (psi)

800

900

1000

Fig. 5. Exergy destruction of PRS for different inlet pressures at various ambient temperatures (conventional PRS).

At lower levels of pressure and temperature, the contribution of exergy destruction at the heater is larger than that at regulator, so that the heater destructs more than five times as large exergy as that destructed in the regulator at an inlet pressure of 2.07 MPa (300 psi) and ambient temperature of 5℃. With increasing the inlet pressure and temperature into the PRS, relative amount of exergy destruction at the heater decreases.

4.2. Modified PRS Inlet NG temperature (T1) is a function of ambient temperature and can be obtained from Equation (2). In the modified system, the heater output temperature (T2) is determined based on the inlet pressure into the PRS. At each particular inlet pressure, minimum required pre-heating temperature 16

ACCEPTED MANUSCRIPT (T2, min) is calculated based on Equation (7), and the NG introduced into the heater is heated to the desired temperature. As long as the inlet NG temperature is either equal or higher than T2, min, preheating is not required and the burner of the heater remains shutdown. Temperature profile of the PRS with the modified system (the flowchart in Figure 3) is shown in Figure 6. As can be seen on this figure, at lower inlet pressures, since the regulator experiences lower pressure drop and hence lower decrease in temperature, the minimum required pre-heating temperature (T2, min) is relatively low, and there is even no need for any pre-heating in many months of year as the inlet NG temperature is adequately high, in which case the burner of the heater remains shutdown (T2 = T1). In the case that the pre-heating stage is necessary, the burner remains on as long as the output NG temperature is below the T2,

min.

Regulator output temperature (T3) can be calculated via either

Equation (4) or Equation (6) if the burner is on or off, respectively.

17

ACCEPTED MANUSCRIPT

Fig. 6. Temperature profiles of modified PRS at different inlet conditions.

In order to clarify the influence of the modified system on energy consumption and exergy destruction of the PRS, performance of the modified system is investigated in terms of the first and second laws of thermodynamics. Figure 7 presents energy consumption of the modified PRS for 18

ACCEPTED MANUSCRIPT different sets of inlet conditions. As can be seen, at lower pressures and many ambient temperatures, there is no need for pre-heating the NG, so that the burner remains off. Maximum rate of energy consumption (102.3 kW) by the heater of the modified PRS happens at lowest inlet temperature and highest inlet pressure into the PRS.

120

Energy of NG Preheating (kW)

100

-5 ˚C

0 ˚C

5 ˚C

10 ˚C

15 ˚C

20 ˚C

25 ˚C

30 ˚C

80

60

40

20

0 300

400

500

600

700

800

900

1000

PRS Inlet Pressure (psi) Fig. 7. Energy consumption of the heater for different inlet pressures at various ambient temperatures (modified PRS).

Figure 8 refers to the overall exergy destruction of the modified system for all combinations of the inlet conditions. As is evident, in the modified PRS, with increasing the inlet temperature and decreasing the inlet pressure, the level of exergy destruction decreases. When the heater is shutdown, the exergy destruction occurs in the regulator only and remains almost the same at any given pressure and different inlet temperatures. As an example, at inlet pressure of 300 psi, the heater is shutdown at temperatures ranging from 5℃ to 30℃, while the heater remains shutdown at temperatures in the range 25-30℃ only when the inlet pressure is elevated to 800 psi. Evidently, the exergy destruction in these two temperature ranges depends on the inlet pressure only.

19

Exergy Destruction in Modified PRS (kW)

ACCEPTED MANUSCRIPT

700

1000 Psi

900 Psi

800 Psi

700 Psi

600 Psi

500 Psi

400 Psi

300 Psi

600 500 400 300 200 100 0 -5

0

5 10 15 20 Ambient Temperature (˚C)

25

30

Fig. 8. Exergy destruction of overall PRS for different ambient temperatures at various inlet pressures (modified PRS).

In contrary to the conventional PRS, it is observed considerably lower contribution of the heater into the overall exergy destruction, so that the largest contribution from the heater is observed at the highest pressure and lowest temperature into the PRS in which the ratio of exergy destruction in the heater to the regulator is 0.6.

4.3.

Comparison

In this section, performance of the conventional and modified PRSs is compared under different inlet conditions. For this purpose, the improvement in each item is calculated from the following equation for each combination of inlet parameters.

𝐼𝑚𝑝𝑟𝑜𝑣𝑒𝑚𝑒𝑛𝑡 =

𝐶𝑜𝑛𝑣𝑒𝑛𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 ― 𝑀𝑜𝑑𝑖𝑓𝑖𝑒𝑑 𝑎𝑚𝑜𝑢𝑛𝑡 𝐶𝑜𝑛𝑣𝑒𝑛𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡

× 100

(26)

Figure 9 demonstrates the improvement in the energy consumption of the PRS under different combinations of inlet conditions. As can be seen, at lower pressures and higher temperatures, since the heater of the PRS remains off, 100% improvement is achieved. At a particular temperature, with decreasing the pressure, the improvement increases. The lowest improvement (33%) is obtained at inlet pressure and temperature of 1000 psi and -5℃, respectively. Moreover, when the inlet NG temperature is 30℃, 100% improvement is achieved at all inlet pressures.

. 20

Improvement in Energy Consumption (%)

ACCEPTED MANUSCRIPT

100

80

60

40

20

300 Psi 600 Psi 900 Psi

0 -5

0

5

400 Psi 700 Psi 1000 Psi 10

15

500 Psi 800 Psi 20

Ambient Temperature (˚C)

25

30

Fig. 9. Improvement in performance of PRS regarding energy consumption.

Figure 10 demonstrates the improvement of the PRS in terms of reduced exergy destruction at the heater. As can be seen, at lower pressures and higher temperatures, since the heater burner is frequently off, a 100% improvement in the performance of the PRS in terms of reduced exergy destruction is obtained. At a particular pressure, with increasing the temperature, the improvement increases. The improvement follows an even faster rate at higher temperatures.

21

Improvement in Heater Exergy Destruction (%)

ACCEPTED MANUSCRIPT

100

80

60

40

20

1000 Psi

900 Psi

800 Psi

700 Psi

600 Psi

500 Psi

400 Psi

300 Psi

0 -5

0

5

10

15

Ambient Temperature (˚C)

20

25

30

Fig. 10. Improvement in performance of heater regarding exergy destruction.

Figure 11 demonstrates the improvement of the PRS in terms of exergy destruction at the regulator. In the modified PRS, performance of the heater is seen to improve, while the improvement in the performance of the regulator of the PRS is negligible. This is why, the exergy destruction at the regulator is sourced from rapid NG expansion and depends on the inlet pressure to the regulator. As can be seen from the figure, as long as the burner of the modified PRS is shutdown, the improvement exhibits an ascending trend versus the pressure while the opposite is true for the cases that the burner is on.

Improvement in Regulator Exergy Destruction (%)

1.2 1 0.8 0.6 0.4 0.2

-5 ˚C

0 ˚C

5 ˚C

10 ˚C

15 ˚C

20 ˚C

25 ˚C

30 ˚C

0 300

400

500

600

700

PRS Inlet Pressure (psi)

22

800

900

1000

ACCEPTED MANUSCRIPT Fig. 11. Improvement in performance of regulator regarding exergy destruction.

Figure 12 shows the improvement in overall performance of the modified PRS in terms of reduced exergy destruction under different combinations of inlet conditions. As can be observed, the largest improvement in overall performance occurs at the lowest inlet pressure and ambient temperature of 5℃. As the inlet pressure increases, the corresponding temperature to peak improvement increases gradually. That is, upon modifying the system, the PRSs located at the end of the NG transmission pipeline and those with dominantly moderate climatic conditions experience the largest second-law efficiency improvements. Furthermore, the PRSs located in hot regions and close to the source of NG supply are likely to experience larger improvements in the second-law efficiency as compared to the PRSs located in cold regions close to the source of the NG supply. It is worth mentioning that the most improvement (curve peak) for any inlet pressure occurs at the ambient temperature at which the burner is turned off. It means that at a constant inlet pressure, when the ambient temperature is increasing, at first, improvement in exergy destruction of the PRS has an increasing trend. However, as the ambient temperature reaches a level, in which the burner is shut off, the trend reverses and "improvement" begins to decrease. This change in the trend of "improvement" has two reasons: first, as the burner is turned off, with the further increase of ambient temperature, the reduction of exergy destruction in the heater of the modified PRS remains constant and equal to zero, while the exergy destruction of conventional PRS is decreasing due to decrease in the preheating duty. The second reason is that when the burner of modified PRS is shut down, with increasing ambient temperature, the exergy destruction in the regulator of modified PRS increases somewhat because of the temperature increment.

23

PRS Improvement in Exergy Destruction (%)

ACCEPTED MANUSCRIPT

90

1000 Psi

900 Psi

800 Psi

700 Psi

600 Psi

500 Psi

400 Psi

300 Psi

80 70 60 50 40 30 20 10 0 -5

0

5

10

15

20

Ambient Temperature (˚C)

25

30

Fig. 12. Improvement in overall performance of PRS regarding exergy destruction.

4.4.

Case study

Kermanshah Province in Iran hosts a variety of different climatic conditions. In addition, the NG transmission pipelines entering the province come from different sources, so that the PRSs located across the province have different inlet parameters in terms of NG pressure and temperature. In this section, the data recorded in 2017 (pipeline pressure, ambient temperature, and relative humidity) at two different PRSs across Kermanshah Province is used to evaluate the values of energy consumption, exergy destruction, and CO2 emission at these two PRSs for a one-year period with both the conventional and modified systems and then, the results are compared to one another. As can be observed in Tables 4 and 5, given the changes in the inlet parameters, performances of PRS 1 and 2 during the studied year are divided into three and five periods of time, respectively.

Table 4. Inlet conditions of PRS 1.

24

ACCEPTED MANUSCRIPT Period No.

1

2

3

Inlet Pressure (psi)

500

600

700

Ambient Temperature (˚C)

5

10

20

Relative Humidity (%)

65

50

40

Duration (Days)

90

91

184

Table 5. Inlet conditions of PRS 2.

Period No.

1

2

3

4

5

Inlet Pressure (psi)

400

700

600

600

700

Ambient Temperature (˚C)

0

5

10

15

20

Relative Humidity (%)

70

60

50

45

40

Duration (Days)

89

90

62

62

62

Figures 13 and 14 provide the results of energy analysis on PRS 1 and 2, respectively. At PRS 1, the annual energy consumption is seen to reduce by 86% (3,113.6 GJ with the conventional system down to 432.95 GJ with the modified system). At this PRS, although the time Interval 3 is the longest interval and represents the maximal energy consumption in the conventional system, the lowest deal of energy is consumed in this interval with implementing the modified system. This is because the inlet pressure is maximum during this period and hence, a more significant improvement occurs. Accordingly, knowing that Joule–Thomson coefficient of NG is lower at higher inlet pressures, the regulator would experience a smaller temperature drop, so that less energy is required for pre-heating the NG. At PRS 2, comparing the time Intervals 1 and 2 with identical number of days, the modified PRS exhibits more reduction in energy consumption during the time Interval 1, because of lower inlet pressure to the PRS during this interval which results in lower T2,

min.

Accordingly, despite the lower inlet temperature into the PRS, lower energy is

required to pre-heat the NG. Comparing time Intervals 3, 4, and 5 with identical number of days, the modified PRS shows further reduction in energy consumption during the time Interval 5 because the Joule-Thomson coefficient of NG decreases at higher pressures [36], thereby lowering the impact of inlet pressure on T2, min. On the other hand, within the operating range of the PRS, specific heat capacity of NG increases with pressure [36], thereby intensifying the effect of inlet temperature 25

ACCEPTED MANUSCRIPT on the amount of energy absorbed by the NG. Accordingly, the modified PRS experiences the largest reduction in the energy consumption during the time Interval 5 because of higher inlet temperature, despite the relatively high inlet pressure during this period.

Energy Absorbed by NG (GJ)

1600 Conventional

1400

Modified

1200 1000 800 600 400 200 0 1

2

3

Period Number

Fig. 13. Energy consumption of PRS 1 during the annual period of operation for the conventional and modified models.

1200

Energy Absorbed by NG (GJ)

Conventional Modified

1000 800 600 400 200 0 1

2

3

Period Number

4

5

Fig. 14. Energy consumption of PRS 2 during the annual period of operation for the conventional and modified models.

Figures 15 and 16 demonstrate the results of exergy analysis at PRS 1 and 2. At PRS 1, annual exergy destruction decreases by 41% (from 14,085 GJ in the conventional PRS to 8,461.8 GJ in the

26

ACCEPTED MANUSCRIPT modified PRS). At PRS 2, annual exergy destruction decreases by 40% (from 14,271 GJ in the conventional PRS to 8,442.71 GJ in the modified PRS).

8000

PRS Exergy Destruction (GJ)

Conventional 7000

Modified

6000 5000 4000 3000 2000 1000 0 1

2

3

Period Number

Fig. 15. Exergy destruction of PRS 1 during the annual period of operation for the conventional and modified models.

PRS Exergy Destruction (GJ)

4500 Conventional

4000

Modified

3500 3000 2500 2000 1500 1000 500 0 1

2

3

Period Number

4

5

Fig. 16. Exergy destruction of PRS 2 during the annual period of operation for the conventional and modified models.

Figure 17 displays the annual CO2 emission at PRS 1 in kg/yr. According to the figure, after implementing the modification on the PRS, 86% less CO2 is emitted by PRS 1 (from 352,303 kg/yr

27

ACCEPTED MANUSCRIPT at the conventional PRS down to 48,994 kg/yr at the modified PRS). Note that in this figure, the CO2 emission values are indicated separately for each of the time intervals.

Fig. 17. Annual CO2 emission of PRS 1 for the conventional (a) and modified (b) models.

Figure 18 shows the annual CO2 emission at PRS 2 in kg/yr. According to the figure, after implementing the modification on the PRS, 80% less CO2 is emitted by PRS 2 (from 392,376 kg/yr at the conventional PRS down to 77,748 kg/yr at the modified PRS).

Fig. 18. Annual CO2 emission of PRS 2 for the conventional (a) and modified (b) models.

5. Conclusion

28

ACCEPTED MANUSCRIPT In this study, a new modification based on real demand was applied on preheating process of NG in a PRS. In fact, such modification can avoid losses and destructions of system significantly. The PRS of 10,000 SCMH in capacity was thermodynamically modeled. In conventional PRSs, NG is pre-heated to 38℃ to prevent the formation of NG hydrates at outlet of the regulator. In the conventional PRS under study, the heater imposes the largest contributions into overall exergy destruction of the PRS, so that at lower values of temperature and pressure, the exergy destruction incurred in the heater increases to more than five times as large as that at the regulator. Depending on the position of a PRS across the NG transmission network, inlet NG pressure and temperature into the PRS may vary throughout the year. In the proposed modified PRS, minimum required pre-heating temperature for NG was predicted based on the pressure drop occurred in the regulator, and the NG was pre-heated to the predicted value for different combinations of inlet conditions. Moreover, in the cases where the inlet NG temperature was equal or higher than the minimum required pre-heating temperature, there was no need to any pre-heating stage, and thus the burner of the heater remained shut down. This modification reduced the energy consumption and exergy destruction considerably, although the reduction in exergy destruction was negligible in the regulator. Finally, an investigation on the performance of two test PRSs showed that, upon implementation of the proposed modification, one could reduce CO2 emission at the PRSs by more than 80%. Considering the large number of PRSs, undoubtedly, such a modification will save a lot of fuel and also reduce concerns about air pollution and climate changes. References [1] Kazas G, Fabrizio E, Perino M. Energy demand profile generation with detailed time resolution at an urban district scale: A reference building approach and case study. Applied Energy. 2017;193:243-62. [2] Grubler A, Wilson C, Bento N, Boza-Kiss B, Krey V, McCollum DL, et al. A low energy demand scenario for meeting the 1.5 °C target and sustainable development goals without negative emission technologies. Nature Energy. 2018;3:515-27. [3] Zhang X, Myhrvold NP, Hausfather Z, Caldeira K. Climate benefits of natural gas as a bridge fuel and potential delay of near-zero energy systems. Applied Energy. 2016;167:317-22. [4] Crow DJG, Giarola S, Hawkes AD. A dynamic model of global natural gas supply. Applied Energy. 2018;218:452-69. [5] Su H, Zhang J, Zio E, Yang N, Li X, Zhang Z. An integrated systemic method for supply reliability assessment of natural gas pipeline networks. Applied Energy. 2018;209:489-501. [6] ISO-12213-2:. Natural gas Calculation of compression factor Part 2: Calculation using molarcomposition analysis. ISO Copyright Office Geneva; 2005. [7] Statistical Review of World Energy, Natural Gas, 2017. British Petroleum Report. June 2018. [8] Ríos-Mercado RZ, Borraz-Sánchez C. Optimization problems in natural gas transportation systems: A state-of-the-art review. Applied Energy. 2015;147:536-55. [9] Jannatabadi M, Farzaneh-Gord M, Rahbari HR, Nersi A. Energy and exergy analysis of reciprocating natural gas expansion engine based on valve configurations. Energy. 2018;158:9861000. 29

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31

ACCEPTED MANUSCRIPT 

Energy and exergy analyses of a pressure reduction station



Different conditions for inlet pressure and temperature to station



A novel modification on preheating natural gas based on real demand



Changes of exergy destruction in terms of ambient temperature and inlet pressure



Effect of modification in terms of energy, exergy and CO2 emission