Performance analysis of liquefied natural gas storage tanks in refueling stations

Accepted Manuscript Performance analysis of liquefied natural gas storage tanks in refueling stations Amir Sharafian, Omar E. Herrera, Walter Mérida PII:

S1875-5100(16)30800-9

DOI:

10.1016/j.jngse.2016.10.062

Reference:

JNGSE 1908

To appear in:

Journal of Natural Gas Science and Engineering

Received Date: 5 August 2016 Revised Date:

28 September 2016

Accepted Date: 26 October 2016

Please cite this article as: Sharafian, A., Herrera, O.E., Mérida, W., Performance analysis of liquefied natural gas storage tanks in refueling stations, Journal of Natural Gas Science & Engineering (2016), doi: 10.1016/j.jngse.2016.10.062. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Performance analysis of liquefied natural gas storage tanks in refueling stations

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Amir Sharafian, Omar E. Herrera, Walter Mérida*

Clean Energy Research Centre, The University of British Columbia, 2360 East Mall, Vancouver,

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BC, Canada V6T 1Z3

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Abstract

Liquefied natural gas (LNG) could replace diesel in the transportation sector. However, fugitive emissions including boil-off gas (BOG) across the LNG supply chain have revealed uncertainties on the overall environmental benefits of such replacement. In this study, time-dependent thermodynamic models were developed to study the LNG holding time of storage tanks in

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refueling stations before BOG releases to the atmosphere. Previously overlooked factors, such as the thermal mass of storage tanks and the actual operating conditions at refueling stations, were included explicitly in the models. The effect of the thermal mass of storage tanks on holding time

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is illustrated by an analysis of 57.20 m3 storage tanks filled with LNG at -150°C and -126.5°C.

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The tank with the lower temperature fills show 3.7-times longer holding time. Further investigations highlight the importance of the ratio of heat transfer surface area to the LNG volume as a key factor in proper sizing of storage tanks to maximize the holding time. Finally, the modeling of a 57.20 m3 storage tank with a heat transfer coefficient of 0.022 W/m2K shows that fuel delivery rates as low as 1.89 m3/day are sufficient to maintain the tank pressure within allowable limits. *

Corresponding author: Tel.: +1 (604) 822-4189; Fax: +1 (604) 822-2403. E-mail address: [email protected] (W. Mérida).

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Keywords: Liquefied natural gas, Boil-off gas, Storage tank, Holding time, Refueling station.

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1. Introduction Climate change is one of the main concerns of today’s world [1], but greenhouse gas (GHG) emissions from industrial and transportation processes have steadily increased [2–4]. For instance, the GHG emissions from the U.S. medium- and heavy-duty trucks increased by 76%

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between 1990 and 2014 and reached 407.4 Mt CO2,eq [5]. According to the announcements at the

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21st Conference of Parties in Paris, mitigation of climate change and reaching the 2°C scenario targets would require immediate and significant changes over the next three decades (as opposed to changes occurring over centuries) [6].

It has been claimed that replacing conventional petroleum fuels, e.g., diesel and gasoline, with low-carbon content fuels reduces GHG emissions and climate change [7]. Natural gas (NG) is

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considered a low-carbon content fuel [8] and several studies reported the benefits of NG on economic and market growth [8–19]. However, and despite this significant body of work, recent

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studies revealed uncertainty in the overall benefits associated with NG use [20–25]. According to the Global Warming Potential (GWP), methane emissions contribute up to 72 times more to

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climate change than CO2 in a 20-year horizon [26]. Therefore, the reduction in CO2 emissions from NG use must be compared to the impact of the corresponding methane emissions. Without reliable data on the actual deployment technologies, most of the models and analyses comparing widespread NG use to the existing energy options will remain incomplete. Liquefied natural gas (LNG) is the condensed form of natural gas with 60% volumetric energy density of diesel [27]. The combustion of LNG in comparison with ultra-low sulfur diesel can reduce CO2, NOx, and particulate matter emissions by up to 20%, 90%, and 100%, respectively 2

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[28]. These features make LNG a candidate fuel in the transportation sector to reduce GHG emissions, especially for large, mobile applications, such as heavy-duty trucks [29,30], trains

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[31–33], and ships [34,35]. Nevertheless, fugitive emissions from LNG are a major concern. LNG is a cryogenic liquid stored at temperatures as low as -162ºC. Due to its large temperature gradient with the environment, LNG is gradually heated and evaporates (boil-off gas (BOG)). Methane (the major component in LNG mixtures) is a potent GHG and has more impact on

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climate change than CO2 due to methane higher radiative forcing [26]. The BOG leads to

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undesirable pressurization across the LNG distribution chain [36]. In large LNG carriers, the BOG is re-liquefied or used as fuel to keep the LNG at atmospheric pressure and low temperature [37]. In large LNG regasification plants and storage facilities, the BOG can be used to generate electricity [38]. However, in small-scale applications, such as LNG refueling stations, the BOG management is more challenging. BOG generation and methane emissions from LNG

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facilities are originated from [39]: 1) heat transfer to LNG and pressurization of storage tanks, 2) ventilation of displaced BOG when filling a tank, 3) heat transfer to hoses, lines, and pumps, 4) precooling of equipment prior to LNG transfer, and 5) LNG transfer from a high pressure tank to

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a low pressure tank.

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Burnham et al. [40] classified the natural gas (NG) well-to-tank into four sectors to quantify fugitive emissions rate: 1) gas field, 2) processing, 3) transmission and storage, and 4) distribution. Their analysis showed that on average 1.12% to 1.15% of NG was emitted to the atmosphere across the value chain. Their investigations also indicated that transportation and storage sector accounted for 35% of methane emissions followed by the distribution sector, which includes refueling stations and fueling process, with 28%. Therefore, the transportation,

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storage, and distribution sectors have significant opportunities to improve by preventing the BOG release and fixing existing leaks. The main focus of this study is on the distribution sector.

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According to Burnham et al. [40], the average methane venting from LNG refueling stations was about 0.32% per delivery of LNG. Hailer’s measurements from two LNG refueling stations in the U.S. indicated that one of the operating stations had methane emissions of 0.1% to 1.5% of fuel dispensed to vehicles [41]. However, the second station had methane emissions of 0.9% to

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5.3%. Field surveys from 2,400 LNG refueling stations in China demonstrated that more than

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1,600 stations had daily methane emissions of greater than 5% and in some cases 10% due to improper insulation [42]. China has currently about 3,200 LNG refueling stations [43] which account for 94% of the total refueling stations around the world [44]. It is expected that the number of LNG refueling stations in the U.S. and China will reach 223 and 5,000 by 2020, respectively.

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Developing appropriate thermodynamic models to determine the weaknesses in the design of LNG infrastructures and quantify their BOG generation rates is the first step toward the design of zero or near zero fugitive emissions LNG facilities. A summary of relevant thermodynamic

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studies on LNG infrastructures are listed in Table 1.

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The LNG facilities investigated in Table 1 illustrate the recent work on LNG storage tanks [36,45,48,51,54,55],

transportation

[46,49,51],

unloading

(tank-to-tank

transfer)

[47],

regasification terminals [38,50,52,53], and LNG weathering [49,51]. There are limited studies available in the literature on LNG storage tanks with less than 113.5 m3 capacity for LNG refueling station applications, such as Refs. [36,45,51]. These tanks can hold LNG at pressures about 1,300 kPa which are higher than those of large storage tanks (100,000-300,000 m3) used in tanker ships with operating pressures close to atmospheric pressure. The difference between 4

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these operating pressures affects the thermo-physical properties of LNG, LNG holding time, and the BOG generation rate. LNG holding time refers to the time a storage tank can hold the LNG

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without venting [25]. In this study, we analyze LNG storage tanks of refueling stations by using practical parameters such as tank storage size, LNG initial temperature, tank thermal insulation, and fuel delivery rate. The main differences between this study and previous models in Refs. [36,45,48,51] are to

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consider the thermal mass of storage tank and use real operating temperatures and pressures for

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the LNG at refueling stations. Previous studies used LNG at -162°C (the dew point of NG at atmospheric pressure) for the modeling which is not the case in a LNG refueling station. This study highlights the importance of these parameters on the model predictions, such as heat transfer rate to tank, LNG holding time, and the BOG generation rate.

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2. LNG refueling stations and vehicles’ fuel supply system

LNG is dispensed to vehicles in two conditions: 1) Unsaturated (cold) LNG at a less than -143°C

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and 340 kPa, and 2) saturated (warm) LNG at -125 to -131°C, and 690 to 930 kPa [56]. This is due to variations in vehicles’ fuel supply systems [44,57,58]. In a simple vehicle’s fuel supply

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system, the tank pressure transfers the LNG to the vaporizer and engine at appropriate temperature, pressure, and flow rate [57,58]. It is customary to fill these LNG tanks with saturated LNG, which has been heated prior to filling to increase its saturation pressure, to supply the required fuel flow rate. Other, more sophisticated fuel supply systems have auxiliary equipment, such as a pump or compressor, for transferring the LNG from the onboard tank to the vaporizer and engine [44,57]. These fuel supply systems are capable of accepting unsaturated LNG. 5

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The process of heating the LNG to increase its saturation temperature and pressure is known as “LNG conditioning”. There are two methods of LNG conditioning at refueling stations: bulk and on-the-fly conditioning. Figure 1a shows a simplified schematic of a LNG refueling station with

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bulk conditioning. After filling the storage tank with unsaturated LNG, the pump pushes the LNG to the heater to rise its temperature and pressure. This process continues until the LNG pressure stored in the storage tank reaches the set point.

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In contrast, a LNG refueling station with on-the-fly conditioning increases the pressure and

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temperature of unsaturated LNG simultaneously with the fueling process, as shown in Figure 1b. This method helps the storage tank to store more LNG with higher density for a longer time. However, the heater needs to be precisely designed to heat the LNG on-the-fly within a short time without overheating it.

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3. LNG storage tank modeling and assumptions

LNG storage tanks in refueling stations with 22.7 to 113.5 m3 capacity have a maximum allowable working pressure (MAWP) of about 1,300 kPa [57]. Figure 2 shows a schematic of a

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double-wall LNG storage tank with a net capacity of 57.2 m3.

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A combination of Aspen Plus and Aspen Plus Dynamics software [60] was used to model a LNG storage tank. The LNG storage tank was initially designed in Aspen Plus and was solved for the steady-state condition. Then, the model was exported to Aspen Plus Dynamics where the inlet and outlet valves were closed to model a stationary tank with no fuel delivery. The thermodynamic model included time-dependent conservation of mass and energy, heat and mass transfer equations and equation of state. In this study, pure methane was considered as the LNG because more than 90% of NG is composed of methane. A constant ambient temperature 6

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boundary condition was applied on the walls of the storage tank. The liquid-vapor phases, i.e., LNG and BOG, were assumed to be in equilibrium. The Peng-Robinson (PR) equation of state

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was used to determine the density of vapor phase for a given temperature and pressure. According to technical recommendations [57], LNG storage tanks at refueling stations should be filled to a maximum of 80% of their volume with unsaturated LNG. In the case of saturated LNG, storage tanks should not be filled more than 90% of their volume. In this study, the

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rationale behind this practical suggestion is discussed. To have a fair comparison, the initial

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volumes of unsaturated and saturated LNG in Aspen Plus Dynamics were set at 80% of the tank net volume. The pressure and temperature of LNG in the tank increased until the tank pressure reached its MAWP due to the heat transfer from the environment. This time is defined as the “LNG holding time” of the tank with no methane emissions. During the modeling, there is no LNG flow in and out of the storage tank unless otherwise specified. Further details about the

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specifications of LNG storage tank, ambient temperature, initial LNG level in the tank, and temperatures and pressures of saturated and unsaturated LNG are listed in Table 2. The overall heat transfer coefficient, Uinsulation, of the storage tank was calculated based on LNG

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at -162°C and 101.325 kPa, and the storage tank average daily BOG rate of 0.3% [57]:

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Q& = BOG % × ρ LNG × Vtank × hfg =

 kJ  0.3%  kg  × 423.0  3  × 57.20 m 3 × 512.59   = 430 W 24 × 3600 m   kg 

(UA )tank =

Uinsulation =

( )

430 (W ) Q& = = 2.3 W/K Tambient − TLNG 25 − ( −162 )

(UA)tank Atank

=

2.3(W / K )

( ) 2

104.35 m

(1)

(2)

= 0.022 W/m2K

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(3)

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& is the heat transfer rate to the tank, where, in Eq. (1), Q

ρLNG

and hfg are the LNG density and

heat of evaporation at the given temperature and pressure, and Vtank is the storage tank net

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volume. In Eqs. (2) and (3), (UA)tank is the thermal conductance of the storage tank and Atank is the total heat transfer surface area of the inner tank, as shown in Figure 2.

The average daily BOG generation rate in the model is calculated by Eq. (5): Q& i ∆ti × 100 ρ LNG × VLNG × hfg

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BOGi % =

i =tholding time i =0

BOGi %

tholding time

where in Eq. (4), BOGi ,

Q& i

× 24 × 3600

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BOG ( % / day ) =

∑

(4)

(5)

, and ∆ ti are the BOG generation, the heat leak rate to the tank, and

the time interval at time step i, respectively.

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For solving the time-dependent governing equations, the mixed Newton method was selected in Aspen Plus Dynamics. The relative error differences for all variables, namely, density, temperature, pressure, and liquid and vapor mass fractions, were set at 10-4 at each time step. The

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mixed Newton method uses the Newton method for initialization and the fast Newton method for dynamic iterations. Therefore, it provides a fast iteration speed and high convergence rate for

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most of time-dependent simulations [61].

4. Results

4.1. Model validation Experimental data [62] of a horizontal onboard tank with net capacity of 0.257 m3 were used to validate the model. The holding time of the tank was measured according to the SAE J2343 8

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Recommended Practice standard [63]. The tank was filled to 75% of its net capacity and changes in its pressure was recorded over time until the tank pressure reached its MAWP. Further details

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about the tank and initial temperature and pressure are shown in Table 3. Figure 3 shows the comparison of experimental data and the results of the model. The LNG pressure stored in the tank increased gradually due to the heat leak rate to the tank and after about 5.7 days (137 hours), it reached the MAWP. As shown in Figure 3, the model predicted the

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experimental data with good accuracy. The maximum and average relative differences between

4.2. Baseline model analysis

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the experimental and numerical modeling were 6.2% and 3.1%, respectively.

Variations in LNG pressure, temperature and level in the tank, and heat leak rate to the tank over time are discussed in this section for the given values in Table 2. Figure 4a and Figure 4b show

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the pressure and temperature variations of unsaturated and saturated LNG in a 57.2 m3 tank over time. Unsaturated LNG with initial pressure of 230 kPa is gradually heated and after about 87 days, the tank pressure reaches its maximum allowable working pressure (MAWP). Whereas, the

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pressure of saturated LNG initially at 900 kPa exceeds the MAWP of the tank after 23 days. As shown in Figure 4b, temperatures of unsaturated and saturated LNG increase linearly from -

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150°C and -126.5°C, respectively, to -118.3°C. Figure 4c demonstrates the LNG level variations in the tank due to the heat transfer from the environment and changes in the LNG density. Unsaturated and saturated LNG levels are initially at 10.11 m (corresponding to 80% of the length of the inner tank, Ltank, as shown in Figure 2). The unsaturated LNG level increases by increasing the LNG temperature and reaches 11.77 m when the tank pressure reaches its MAWP. This shows a 1.66 m change between the initial and the end levels of LNG in the tank due to decrease in the LNG density by 16.5% from 405.7 to 9

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348.2 kg/m3. Saturated LNG level also increases by 0.46 m and reaches 10.57 m. The level of saturated LNG increases less because the density of saturated LNG only decreases by 4.5% from its initial density. These data support the practical recommendation on filling storage tanks up to

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80% and 90% of their net volume with unsaturated and saturated LNG, respectively.

The amount of heat leak rate to the LNG storage tank is shown in Figure 4d. The heat leak rate to unsaturated and saturated LNG decreases over time as the LNG temperature gradually increases.

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The heat leak rate to the tank filled with saturated LNG is less than that filled with unsaturated

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LNG. As shown in Figure 4d, the heat leak rate to the tank shows an exponential decay in the first few days and then linearly decreases. This is because of the thermal mass of the storage tank that stores a portion of heat transferred to the LNG. This will be discussed further in Figure 6. The BOG generation of the storage tank filled with unsaturated and saturated LNG are depicted in Figure 4e and Figure 4f, respectively. The BOG generation of the tank filled with unsaturated

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LNG is greater than that of the tank filled with saturated LNG. This is due to a larger temperature gradient between the unsaturated LNG and the environment. As shown in Figure 4e and Figure 4f, the BOG generation decreases over time to a minimum and then increases.

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Increase in the pressure of the LNG storage tank minimizes the BOG generation. However, as

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the tank temperature and pressure increase, the heat of evaporation of LNG steadily decreases. For example, the enthalpy of evaporation of LNG decreases by 24% from 488.9 to 394.8 kJ/kg by increasing the LNG temperature and pressure from -150°C and 230.65 kPa to -118°C and 1,300 kPa. Therefore, a competing trend between increase in the pressure and decrease in the enthalpy of evaporation controls the BOG generation. These results refute the assumption of constant BOG generation and BOG generation reduction by increase in the tank pressure, Refs. [46,48] for instance. 10

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Figure 5 shows the LNG holding time and average daily BOG generation rate of the 57.2 m3 storage tank at the baseline operating conditions. It can be seen from Figure 5 that storage of unsaturated LNG in comparison with saturated LNG increases the LNG holding time by almost

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3.74 times from 23.4 days to 87.4 days. As a result, the average daily BOG generation rate of the tank filled with unsaturated LNG is 12.5% less than that of the tank filled with saturated LNG. Effects of the thermal mass of storage tank on the modeling results are shown in Figure 6. The

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thermal mass of the tank stores a portion of the heat transferred from the environment to the

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LNG as shown in Figure 6a and Figure 6b, as an example. The tank with zero thermal mass filled with saturated LNG has faster pressure and temperature rise than the tank with thermal mass. Also, the LNG level increases in a shorter time for the tank with zero thermal mass compared to that with thermal mass as shown in Figure 6c. These results indicate that thermodynamic models with zero thermal mass for storage tanks underestimate the LNG holding time and overestimate

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the average daily BOG generation rate.

Figure 6d shows that the heat leak rate to LNG in the storage tank with zero thermal mass decreases linearly with time. Whereas, the heat leak rate to the LNG in the storage tank with

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thermal mass initially reduces because a portion of the heat is stored in the walls of the storage

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tank. The remainder of the heat is transferred to the LNG. When the thermal mass of storage tank cannot store more heat, the heat leak to the tank is transferred mostly to the LNG. At this point, the slope of the heat leak rate curve is parallel to that of the storage tank with zero thermal mass, as shown in Figure 6d. This analysis highlights the importance of thermal mass of storage tanks that has been overlooked in the literature [36,48,51,55].

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4.3. Performance analysis of LNG storage tanks without fuel delivery Proper usage of insulation materials in LNG infrastructures can minimize the heat transfer to LNG. In a storage tank, the thermal resistance of metallic walls can be neglected in comparison

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with those of insulation materials. The heat transfer resistances due to insulation materials on the

the thermal conductance of the storage tank, (UA)tank:

1 1 1 + + t t D insulation insulation ln  o  D i  kinsulation Aroof kinsulation Aceiling 2π kinsulation Ltank

(6)

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(UA)tank =

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side walls, roof, and ceiling of the storage tank are assumed to be in parallel. Equation (6) gives

The parameters in Eq. (6) are given in Table 2 and Table 4. By replacing (UA)tank in Eq. (3), the overall heat transfer coefficient, Uinsulation, of the storage tank can be calculated. The thermal conductivity, kinsulation, thermal conductance, (UA)tank, and the overall heat transfer coefficient of

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the tank, Uinsulation, for different insulation materials are summarized in Table 4. Effects of Uinsulation ranging from 0.01 to 0.25 W/m2K on the unsaturated and saturated LNG pressures and heat leak rates to the tank are shown in Figure 7. For Uinsulation of 0.01 W/m2K, the

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unsaturated and saturated LNG pressures take more than 80 and 20 days, respectively, to reach the MAWP of the storage tank as shown in Figure 7a and Figure 7b. While, the storage tank with

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Uinsulation of 0.25 W/m2K can only hold unsaturated and saturated LNG for about 10 and 3 days, respectively, before releasing the BOG to the atmosphere. As shown in Figure 7c and Figure 7d, the heat leak rate to the tank varies from 165 to 2,870 W for unsaturated LNG and 154 to 2,500 W for saturated LNG under Uinsulation of 0.01 to 0.25 W/m2K. The amount of heat leak rates shown in Figure 7c and Figure 7d can be used as benchmark values in the design or selection of proper liquefier in LNG refueling stations.

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Figure 8 shows the LNG holding time and average daily BOG generation rate of the storage tank filled with unsaturated and saturated LNG under different Uinsulation. It can be seen in Figure 8a that the tank with Uinsulation of 0.01 W/m2K (similar to that of LCI insulation material under

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vacuum pressure of 133.3 Pa listed in Table 4) can hold unsaturated and saturated LNG for 188 and 50 days, respectively. Using conventional insulation materials, such as polyurethane with Uinsulation of 0.10 W/m2K, and no vacuum results in unsaturated and saturated LNG holding times

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of 22 and 5.7 days, respectively; more than 8 times reduction in the LNG holding time compared

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to that of Uinsulation of 0.01 W/m2K.

Figure 8b shows that the average daily BOG generation rate increases from 0.15% to 2.59% for unsaturated LNG and 0.16 to 2.93% for saturated LNG when Uinsulation increases from 0.01 to 0.25 W/m2K. In LNG refueling stations with high fuel deliver rate, Barclay et al. [45] suggested to use a single-wall tank insulated with traditional insulation materials and install an on-site

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liquefier to liquefy the BOG. This idea is investigated in Section 4.4. LNG refueling stations are exposed to different ambient temperatures, Tambient, over the course of a year. Also, they are installed in different geographic locations where Tambient can deviate from

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the design point of LNG storage tanks. Figure 9 shows the effects of Tambient on the LNG holding

LNG.

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time and average daily BOG generation rate of the tank filled with unsaturated and saturated

Increase in Tambient from -10°C to 45°C decreases the unsaturated LNG holding time from 112 to 78 days. For the saturated LNG, the tank holding time decreases by 50% when Tambient increases from -10°C to 45°C. It can be seen in Figure 9b that Tambient of 45°C causes the average daily BOG generation rates of 0.36% and 0.40% in the tank filled with unsaturated and saturated LNG, respectively. 13

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The thermal mass of LNG stored in the tank also affects the LNG holding time and BOG generation rate [55]. In low fuel throughput LNG refueling stations, one idea is to only refill half of the storage tanks to increase their LNG holding time. Figure 10 shows the effects of LNG

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filling level in a 57.2 m3 storage tank on LNG holding time and average daily BOG generation rate. Initial LNG level in the tank represents the volume percentage of the tank occupied with LNG. As shown in Figure 10a, increasing the initial LNG level in the tank from 50% to 90%

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increases the holding times of unsaturated and saturated LNG from 63 and 17 days to 95 and 25 days, respectively. Lower thermal mass of LNG, e.g., 50%, in the tank causes higher BOG

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generation rate as shown in Figure 10b. These data disprove the idea of filling the tank partially in order to increase the LNG holding time in refueling stations.

Smaller LNG storage tanks can be used in low throughput LNG refueling stations. Table 5 shows the dimensions of LNG storage tanks with 21.84 to 107.32 m3 net capacity. The surface area to

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tank net capacity ratio, Atank/Vtank, affects the BOG generation rate of LNG storage tanks [48]; Larger tanks have smaller Atank/Vtank as indicated in Table 5. This means that small Atank and large thermal mass of LNG stored in storage tanks are desirable features to reduce the BOG generation

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rate and increase the LNG holding time.

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Figure 11 demonstrates the holding time and average daily BOG generation rate of LNG storage tanks ranging from 21.84 to 107.33 m3. It can be seen in Figure 11a that a 21.84 m3 storage tank filled with unsaturated LNG has a holing time 56% shorter than a 107.33 m3 storage tank. Accordingly, the average daily BOG generation rates of the 21.84 m3 storage tank filled with unsaturated and saturated LNG are equal to 0.43% and 0.48% which are higher than those of the 107.33 m3 storage tank. This analysis shows that storing LNG in a storage tank with the right

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size provides longer holding time than storing the same amount of LNG in two or three lowercapacity storage tanks. The 57.2 m3 storage tank filled to 60% of its net capacity, as shown in Figure 10, holds the same

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amount of LNG as the 41.49 m3 storage tank filled to 80% of its net capacity, as shown in Figure 11. However, the 41.49 m3 storage tank holds the LNG for a longer time with lower average daily BOG generation rate than the 57.2 m3 storage tank. This is because of the difference

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between the surface area to LNG volume ratios, Atank/VLNG, of these tanks. The Atank/VLNG of the 41.49 m3 storage tank is equal to 2.36 (= 78.19 m2 / (80% × 41.49 m3)) whereas that of the 57.2

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m3 storage tank is equal to 3.04 (= 104.35 m2 / (60% × 57.20 m3)). It can be concluded that when comparing storage tanks of different sizes, smaller Atank/VLNG is preferred in order to achieving longer LNG holding time.

4.4. Performance analysis of LNG storage tanks with fuel delivery

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In this section, the performance of a 57.20 m3 LNG storage tank with fuel delivery to vehicles is investigated similar to the study of Powars [57]. Three fleet sizes of 5, 10, and 20 vehicles with

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fuel requirements of 1.89, 3.79, and 7.56 m3/day, respectively, are modeled. In this model, it is assumed that each vehicle is fueled with a 0.38 m3 of LNG. The fueling process is started every

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day at 6 pm and takes 5 min per vehicle with a fuel delivery rate of 0.076 m3/min. There is a 5 min time interval between the fueling of two consecutive vehicles. For example, fueling process for a fleet size of 5 vehicles takes 45 min (= 25 min fueling + 20 min intervals). Figure 12 shows the unsaturated and saturated LNG pressures and levels in the tank for daily fuel delivery rates of 1.89, 3.79, and 7.56 m3/day. Figure 12a and Figure 12b show that unsaturated and saturated LNG pressures do not reach the MAWP of the tank and decreases gradually, especially for fuel delivery rates of 3.79 and 7.57 15

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m3/day. LNG removal from the tank causes a portion of LNG evaporate to displace the liquid volume. LNG gains heat from the LNG thermal mass to evaporate and the LNG temperature reduces. Accordingly, the LNG pressure reduces. Figure 12c and Figure 12d show that the

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unsaturated and saturated LNG levels in the tank steadily decrease and the tank is fully depleted without releasing the BOG to the atmosphere. It should be noted that these data is true for the case of regular fuel delivery with appropriate rate. In the case of low fuel delivery rates, the LNG

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tank pressure can reach the MAWP of the tank, such as ones reported by Chen, et al. [36].

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Figure 13 shows the idea of using a single-wall storage tank insulated with conventional insulation materials and no vacuum condition, such as polyurethane, and daily fuel delivery rates of 1.89, 3.79, and 7.56 m3/day to fleet sizes of 5, 10, and 20 vehicles, respectively. Uinsulation in this modeling is equal to 0.1 W/m2K corresponds to that of polyurethane listed in Table 4. For the storage tank filled with unsaturated LNG, Figure 13a indicates that fuel delivery

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rate of 1.89 m3/day fails to maintain the storage tank pressure below its MAWP. At this condition, 25% of unsaturated LNG is still available in the tank as shown in Figure 13c. However, higher fuel delivery rates of 3.79 and 7.56 m3/day would be sufficient to contain the

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BOG in the tank with no release to the atmosphere.

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Figure 13b shows that the single-wall tank with Uinsulation of 0.1 W/m2K and fuel delivery rates of 1.89 and 3.79 m3/day cannot hold saturated LNG for more than 6.5 and 8.6 days, respectively, below the MAWP of the tank. At these times, 62% and 27% of saturated LNG is still available in the tank, as shown in Figure 13d. These results highlight that a single-wall storage tank cannot be used for LNG applications if the on-site liquefier does not operate efficiently and the refueling station has not a high fuel throughput greater than 7.56 m3/day. In this model, the insulation thickness corresponding to Uinsulation of 0.1 W/m2K was equal to 0.248 m. In order to achieve 16

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Uinsulation of 0.02 W/m2K (similar to that of the baseline model), the insulation thickness made out of polyurethane should be 1.5 m thick which is not practical. As a result, using a single-wall storage tank with conventional insulation materials and on-site liquefier cannot be a practical

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solution to reduce the BOG emissions and upfront capital cost of equipment in LNG refueling stations.

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5. Conclusions

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In this study, the performance of LNG storage tanks in refueling stations with and without fuel delivery were studied. Our main findings are summarised in the following list: •

Storage tanks filled with unsaturated LNG provided more than 3 times longer LNG holding time than those filled with saturated LNG.

The thermal mass of storage tanks had direct impact on LNG holding time and the BOG generation rate.

•

TE D

•

High vacuum insulation materials, such as LCI at vacuum pressure of 133.3 Pa, assisted

•

EP

to increase the LNG holding time up to 188 days. Changes in the ambient temperatures (-10°C to 45°C) and geographic location of

•

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refueling stations reduced LNG holding time up to 50%. The ratio of heat transfer surface area to LNG volume, Atank/VLNG, was introduced as a

crucial factor in comparing the holding times of storage tanks with different sizes.

•

Storage tanks with proper vacuum insulation (Uinsulation of 0.022 W/m2K) could function with no BOG emissions under fuel delivery rates as low as 1.89 m3/day.

17

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•

The single-wall tank insulated with polyurethane (Uinsulation of 0.1 W/m2K) could not hold BOG for fuel delivery rates less than 3.79 m3/day and was not a reliable solution for LNG

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refueling stations.

Acknowledgment

The authors gratefully acknowledge the financial support of the Natural Sciences and

Nomenclature

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SC

Engineering Research Council of Canada (NSERC), Mitacs Elevate, and Westport Power Inc.

area (m2)

BOG

boil-off gas

D

diameter (m)

∆t

time interval (s)

GHG

greenhouse gas

hfg

heat of evaporation (J/kg)

k

thermal conductivity (W/mK)

LCI

EP

AC C

L

TE D

A

length (m)

Layered composite insulation

LNG

liquefied natural gas

MAWP

maximum allowable working pressure

18

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Multi-layer insulation

NG

natural gas

Q&

heat transfer rate (W)

ρ

density (kg/m3)

t

time (s)

T

temperature (°C)

U

heat transfer coefficient (W/m2K)

UA

thermal conductance (W/K)

V

volume (m3)

ambient

ceiling

tank ceiling

holding time

holding time

i

ith time step, inner

roof

AC C

EP

TE D

ambient

tank

storage tank

insulation

insulation material

o

outer

SC

M AN U

Subscripts

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MLI

tank roof

19

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List of tables: Table 1. Thermodynamic modeling of different LNG facilities across the value chain to determine their BOG generation rates. Table 2. Specifications of LNG storage tank, and the base-case initial and boundary conditions.

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Table 3. Specifications of LNG storage tank and initial operating conditions.

Table 4. Thermal properties of different insulation material with/without vacuum pressure.

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Table 5. Specifications of different LNG storage tanks.

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Figure captions: Figure 1. LNG conditioning at a refueling station by: (a) bulk conditioning method [58] and (b) on-the-fly conditioning method.

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Figure 2. Schematic of a double-wall LNG storage tank with the net capacity of 57.2 m3. The dimensions on the figure belong to the inner tank [59]. Figure 3. Comparison of changes in LNG pressure between experimental data [62] and the model developed in this study.

Figure 4. Variations in unsaturated and saturated LNG (a) pressure, (b) temperature, and (c) level in tank, (d) heat leak rate to tank, (e) BOG generation% of unsaturated LNG, and (f) BOG generation% of saturated LNG vs. time.

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Figure 5. Unsaturated and saturated LNG holding time and average daily BOG rate of a 57.2 m3 storage tank at the base operating conditions given in Table 2.

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Figure 6. Variations in saturated LNG (a) pressure, (b) temperature, and (c) level in a 57.2 m3 storage tank with and without thermal mass (Other parameters are as given in Table 2). Figure 7. Effects of Uinsulation on unsaturated and saturated LNG (a)-(b) pressure, and (c)-(d) heat leak rate to tank (Other parameters are as given in Table 2). Figure 8. Effects of Uinsulation on (a) LNG holding time and (b) average daily BOG generation rate of a 57.2 m3 storage tank filled with unsaturated and saturated LNG (Other parameters are as given in Table 2).

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Figure 9. Effects of Tambient on (a) LNG holding time and (b) average daily BOG generation rate of a 57.2 m3 storage tank filled with unsaturated and saturated LNG (Other parameters are as given in Table 2). Figure 10. Effects of initial LNG level in tank on (a) LNG holding time and (b) average daily BOG generation rate of a 57.2 m3 storage tank filled with unsaturated and saturated LNG (Other parameters are as given in Table 2).

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Figure 11. Effects of storage tank size on (a) LNG holding time and (b) average daily BOG generation rate for unsaturated and saturated LNG (Other parameters are as given in Table 2).

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Figure 12. Effects of fueling fleet sizes of 5, 10, and 20 vehicles on unsaturated and saturated LNG (a)-(b) pressure, and (c)-(d) level in tank (Uinsulation is equal to 0.022 W/m2K. Other parameters are as given in Table 2). Figure 13. Effects of fueling fleet sizes of 5, 10, and 20 vehicles on unsaturated and saturated LNG (a)-(b) pressure, and (c)-(d) level in tank (Uinsulation is equal to 0.1 W/m2K. Other parameters are as given in Table 2).

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Table 1. Thermodynamic modeling of different LNG facilities across the value chain to determine their BOG generation rates. Scope of modeling

Approach*

Results

Barclay et al. [45], 1998

Optimization of LNG storage tank capacity for a fleet-size refueling station

• Conducted a thermoeconomic analysis to size LNG storage tanks with respect to the fleet size • Investigated the effects of LNG storage capacity, insulation material, and tank pressure on the capital cost of LNG refueling stations

• Capacity and capital cost of LNG storage tanks showed a linear relationship • Use of onsite liquefier assisted to use less efficient and less expensive storage tanks • A single-wall storage tank with polyurethane insulation equipped with a liquefier had lower lifecycle cost than a double-wall LNG storage tank

Chen et al. [36], 2004

Thermodynamic modeling of LNG storage tank and refueling station

• Developed steady-state and dynamic models • Analyzed the effects of number of vehicles fueled every day on methane emissions • Modeled 22.7 to 113.6 m3 LNG storage tanks

Hasan et al. [46], 2009

BOG management in LNG transportation from liquefaction plant to regasification terminal

Thermodynamic modeling to determine important parameters on BOG generation during LNG unloading in LNG terminals

SC

• Thermal conductance of LNG storage tanks could be calculated indirectly by using dynamic thermodynamic modeling and measuring temperature and pressure over time when no fuel is delivered • Delivering more than 2.8 m3 LNG per day eliminated methane emissions from a 49.0 m3 storage tank with thermal conductance of 2 W/K • An electric generator was recommended to be used for the BOG management and power generation • A liquefier was recommended to be used for the BOG management

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Yan and Gu [47], 2010

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Ref.

• Used Aspen HYSYS for thermodynamic modeling • Used Soave-Redlich-Kwong (SRK) equation of state

• Important factors on the BOG generation during LNG transportation were LNG sloshing, LNG composition, ambient temperature, tank thermal conductance, and tank pressure • The BOG generation was increased by increase in voyage distance

• Used Aspen Plus software for thermodynamic modeling • Used Peng-Robinson (PR) equation of state • Analyzed the effects of LNG flow rate, and pipe elevation change, roughness, and diameter on the BOG generation rate

• The BOG generation rate was minimized at a specific LNG flow rate and pump head • Increasing the elevation difference of pipelines between two tanks increased the BOG generation rate • Larger pipe diameter reduced the BOG generation rate and pump head

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• Pipe roughness had negligible effect on the BOG generation rate Thermodynamic modeling to calculate the BOG generation rate of LNG storage tanks

• Used Lee-Kesler-Plocker (LKP) and the Starling modified Benedict-WebbRubin (BWRS) empirical models to simulate compressibility factor and equation of state • Assumed LNG evaporation only at the liquid-vapor interface • Assumed constant LNG temperature and density during the whole process

• Structure of the tank affected the BOG generation rate • Smaller tanks had larger BOG generation rate because of large surface area to volume ratio • The BOG generation rate decreased as the tank pressure increased • Constant temperature and density during the whole process were reduced the accuracy of modeling • The BOG generation rate did not necessarily mean methane emissions to the atmosphere

Querol et al. [38], 2010

Thermodynamic modeling to determine the daily BOG generation rate in LNG terminals

• Used Aspen Plus software for thermodynamic modeling • Analyzed the usage of recondenser, compressor, and reliquefaction plant on the BOG management in receiving terminals

• Recondenser was the best option to manage the BOG due to its low power consumption and maintenance • Integration of cogeneration plants with LNG terminals were proposed to manage the BOG and generate the electricity required for operating terminals • The BOG was a mixture of methane and nitrogen not heavy hydrocarbons

Miana et al. [49], 2010

Thermodynamic and intelligent neural network modeling to determine LNG weathering in tanker ships

• Used MOLAS software to predict LNG composition and thermo-physical properties • Used a lumped body model which is independent from the shape of LNG storage tanks

• The intelligent neural network model had better prediction in LNG composition at the destination than thermodynamic modeling because of using historical data • For new tank designs, thermodynamic modeling provided better results

Park et al. [50], 2012

Thermodynamic modeling to calculate the BOG generation rate in receiving terminals

• Used Aspen Plus combined with Sequential Quadratic Programming (SQP) optimization solver in Matlab software for thermodynamic modeling • Used Peng-Robinson (PR) equation of state

• Proper design of receiving terminals could reduce the operating cost of compressors for handling the BOG • Optimal design of receiving terminals provided 23% more energy savings and a payback period of less than 0.2 year • Direct compression of the BOG had higher costs than recondensation

Pellegrini et al. [51], 2014

Thermodynamic modeling to study LNG weathering in LNG storage tanks

• Developed an in-house code for thermodynamic modeling of LNG storage tanks • Used Soave-Redlich-Kwong

• LNG composition had to be considered for an accurate thermoeconomic analysis • Using average BOG value

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Adom et al. [48], 2010

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(SRK) equation of state • Assumed constant heat flux on walls of a 190 L storage tank

reported in previous literature was misleading in prediction of LNG composition and density change

Thermodynamic modeling and cost optimization of regasification plants

• Used Aspen HYSYS software to determine the minimum lower heating value range of natural gas by recovering heavier hydrocarbons and allowing maximum net gain • Used Penge-RobinsoneStryjeke-Vera (PRSV) equation of state

• Removing heavier hydrocarbons from LNG resulted in higher net gain • A regasification plant with appropriate heavy hydrocarbon removal provided $14M/year net gain

Kurle et al. [53], 2015

Thermodynamic modeling to minimize the BOG generation rate in exporting terminals

• Used Aspen Plus software for thermodynamic modeling • Used Soave-Redlich-Kwong (SRK) equation of state • Used C3-MR (mixed refrigerant) process in the LNG liquefier

• Usage of the BOG as fuel gas was the most effective way to manage the BOG • Less than 20% of the liquefied BOG energy was consumed for the BOG recovery

Liu et al. [54], 2015

Thermodynamic modeling to determine the energy required to liquefy the BOG of 100,000 to 300,000 m3 LNG storage tanks

• Not clarified modeling software and assumptions • Analyzed four re-liquefiers powered by a coal driven Claude cycle, a Brayton cycle run on the BOG, a Claude cycle run by a BOG-driven spark-ignition engine, and a liquid nitrogen condenser

• The liquefier powered by the Claude cycle driven by a sparkignition engine had the highest BOG energy recovery • The liquefied nitrogen condenser provided the lowest BOG energy recovery

Migliore et al. [55], 2015

Thermodynamic modeling to investigate LNG weathering in storage tanks of regasification terminals

• Developed an in-house code for thermodynamic modeling of LNG storage tanks • Used the revised KlosekeMcKinley method (an empirical correlation) for calculating the LNG density • Assumed variable heat flux to tank due to the temperature difference between the ambient and LNG

• Initial composition and thermal mass of LNG affected the BOG generation rate • Presence of 0.5% nitrogen in LNG composition resulted in a 7% decrease in the BOG generation rate • Increasing the ambient temperature as low as 1°C increased the BOG generation rate by 0.2% independent from initial composition of LNG and tank size

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Fahmy et al. [52], 2015

* Common assumption in all studies: 1) Liquid-vapor phases are in equilibrium, and 2) steady-state assumption unless otherwise noted.

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Table 2. Specifications of LNG storage tank, and the base-case initial and boundary conditions. Parameter

Value

Description 3

57.20 m

A vertical double-wall storage tank with a vacuum insulation between two walls [59]

Length of inner tank, Ltank

12.64 m

Calculated from the length of outer tank

Outer diameter, Do

2.896 m

Ref. [59]

Inner diameter, Di

2.4 m

Calculated from the net capacity of the tank

Insulation thickness, tinsulation

0.248 m

Calculated from the tank inner and outer diameters

Mass of storage tank (empty)

21,455 kg

Ref. [59]

Specific heat capacity of storage tank, cp,tank

477 J/kgK

Assumed from stainless steel 304

Surface area of inner tank, Atank

104.35 m2 0.022 W/m K

Thermal conductance of tank, (UA)tank

2.30 W/K

Ambient temperature, Tambient

25°C

Maximum allowable working pressure of storage tank (MAWP)

1,300 kPa

Unsaturated LNG pressure Saturated LNG temperature

-150°C

230.65 kPa

-126.5°C 900 kpa

AC C

EP

Saturated LNG pressure

TE D

Unsaturated LNG temperature

10.11 m

Calculated from BOG generation rate of 0.3%/day for methane at -162°C and 101.325 kPa [57]

M AN U

Overall heat transfer coefficient of tank, Uinsulation

Initial LNG level in tank

SC

Calculated from inner tank geometry 2

Initial conditions

RI PT

Tank net capacity, Vtank

[57]

Calculated based on 80% of the length of inner tank

ACCEPTED MANUSCRIPT

Table 3. Specifications of LNG storage tank and initial operating conditions. Parameter

Value

Description 3

0.257 m

A horizontal double-wall tank with a vacuum insulation between two walls [62]

Length of inner tank, Ltank

0.909 m

Calculated from the net capacity of the tank

Outer diameter, Do

0.66 m

Ref. [62]

Inner diameter, Di

0.6 m

Ref. [62]

Insulation thickness, tinsulation

0.03 m

Calculated from the tank inner and outer diameters

Mass of storage tank (empty)

175 kg

Ref. [62]

Specific heat capacity of storage tank, cp,tank

477 J/kgK

Assumed from stainless steel 304

Surface area of inner tank, Atank

2.279 m2 0.075 W/m K

Thermal conductance of tank, (UA)tank

0.17 W/K

Ambient temperature, Tambient

25°C

Maximum allowable working pressure of storage tank (MAWP)

1,585 kPa

-147.4°C

283 kpa

AC C

EP

LNG pressure

TE D

LNG temperature

0.4212 m

Calculated from average heat leak rate of 20.9 W given in Ref. [62]

M AN U

Overall heat transfer coefficient of tank, Uinsulation

Initial LNG level in tank

SC

Calculated from inner tank geometry 2

Initial conditions

RI PT

Tank net capacity, Vtank

Assumed

Ref. [62]

Calculated based on 75% of the volume of inner tank in the horizontal position.

ACCEPTED MANUSCRIPT

Table 4. Thermal properties of different insulation material with/without vacuum pressure. Insulation material [64]

kinsulation

(UA)tank

Uinsulation

[Pa]*

[W/m.K]

[W/K]

[W/m2K]

Layered composite insulation (LCI)

133.3

0.0016

0.735

0.007

Aerogel blanket

133.3

0.0034

1.561

0.015

Multi-layer insulation (MLI)

133.3

0.010

4.592

0.044

Fiberglass

133.3

0.014

6.429

0.062

Perlite powder

133.3

0.016

7.348

0.070

Polyurethane

No vacuum

0.021

9.644

0.092

Cellular glass foam

No vacuum

0.033

15.155

0.145

AC C

EP

TE D

SC

M AN U

* 133.3 Pa = 1 torr

RI PT

Vacuum pressure

ACCEPTED MANUSCRIPT

Table 5. Specifications of different LNG storage tanks. Parameter

Storage tank sizes and specifications 57.20 m3

107.32 m3

9.29 m

9.17 m

12.64 m

19.0 m

Outer diameter, Do

1.978 m

2.896 m

2.896 m

2.93 m

Inner diameter, Di

1.73 m

2.4 m

2.4 m

2.682 m

Insulation thickness, tinsulation

0.248 m

0.248 m

0.248 m

0.248 m

Mass of storage tank (empty)

9,525 kg

15,694 kg

21,455 kg

38.555 kg

Surface area of inner tank, Atank

55.19 m2

78.19 m2

104.35 m2

171.37 m2

Initial LNG level in storage tank

5.94 m

7.34 m

10.11 m

15.2 m

Surface area of inner tank to net capacity ratio, Atank/Vtank

2.53 1/m

1.88 1/m

1.82 1/m

1.60 1/m

SC

Length of inner tank, Ltank

AC C

EP

TE D

M AN U

21.84 m

RI PT

41.49 m3

Tank net capacity, Vtank [59]

3

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

(a)

(b)

Figure 1. LNG conditioning at a refueling station by: (a) bulk conditioning method [58] and

AC C

EP

TE D

(b) on-the-fly conditioning method.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Figure 2. Schematic of a double-wall LNG storage tank with the net capacity of 57.2 m3. The

AC C

EP

TE D

dimensions on the figure belong to the inner tank [59].

ACCEPTED MANUSCRIPT

MAWP

1500 1200 900 600

RI PT

LNG pressure [kPa]

1800

Experiement

300

Model

0 1

2

3 4 Time [day]

5

6

SC

0

Figure 3. Comparison of changes in LNG pressure between experimental data [62] and the

AC C

EP

TE D

M AN U

model developed in this study.

MAWP

1200 1000 800 600 400 Unsaturated LNG Saturated LNG

200 0 0

10

20

30

40 50 60 Time [day]

70

80

-115 -120 -125 -130 -135 -140 -145 -150 -155 -160

Unsaturated LNG Saturated LNG

0

90

10

20

(a)

10.4

Unsaturated LNG Saturated LNG

10.0 20

30

40 50 60 Time [day]

70

80

(c) 0.006

90

40 50 60 Time [day]

70

80

90

SC

350 330 310 290

Unsaturated LNG Saturated LNG

270 250

0

10

20

30

40 50 60 Time [day]

70

80

90

(d)

0.0017

0.0058

TE D

BOG generation%

Heat leak rate to tank [W]

10.8

370

M AN U

11.2

0.0056 0.0054 0.0052

BOG generation%

LNG level in tank [m]

11.6

10

30

(b)

12.0

0

RI PT

LNG pressure [kPa]

1400

LNG temperature [°C]

ACCEPTED MANUSCRIPT

0.00165 0.0016 0.00155 0.0015 0.00145

Saturated LNG

0.005 0

10

20

EP

Unsaturated LNG

30

40 50 60 Time [day]

70

80

90

0.0014 0

4

8

AC C

(e)

12 16 Time [day]

20

24

(f)

Figure 4. Variations in unsaturated and saturated LNG (a) pressure, (b) temperature, and (c) level in tank, (d) heat leak rate to tank, (e) BOG generation% of unsaturated LNG, and (f) BOG generation% of saturated LNG vs. time.

100

0.5

BOG rate

0.4

0.36

0.32

80

0.3 60 0.2

40 23.4

0.1

20 0

Average BOG rate [%/day]

LNG holding time 87.4

RI PT

LNG holding time [day]

ACCEPTED MANUSCRIPT

0.0

Saturated LNG

SC

Unsaturated LNG

Figure 5. Unsaturated and saturated LNG holding time and average daily BOG rate of a 57.2

AC C

EP

TE D

M AN U

m3 storage tank at the base operating conditions given in Table 2.

Saturated LNG

MAWP

1300 1200 1100 1000

Tank with zero thermal mass Tank with thermal mass

900 800 0

5

10 15 Time [day]

20

-117

Saturated LNG

-119 -121 -123 -125

Tank with zero thermal mass Tank with thermal mass

-127

25

0

5

(a) 350 Saturated LNG

10.2

Tank with zero thermal mass Tank with thermal mass 5

10 15 Time [day]

20

(c)

25

SC

Heat leak rate [W]

10.3

330 310 290

M AN U

LNG level [m]

10.4

0

20

Saturated LNG

10.5

10

10 15 Time [day]

(b)

10.6

10.1

RI PT

1400

LNG tank temperature [°C]

LNG tank pressure [kPa]

ACCEPTED MANUSCRIPT

25

270 250

0

5

Tank with zero thermal mass Tank with thermal mass 10 15 Time [day]

20

25

(d)

Figure 6. Variations in saturated LNG (a) pressure, (b) temperature, and (c) level in a 57.2 m3

AC C

EP

TE D

storage tank with and without thermal mass (Other parameters are as given in Table 2).

ACCEPTED MANUSCRIPT

1350

1200

0.25

1000 0.1

800

0.05

0.02

600 400

Uinsulation= 0.01 W/m2K

200

Unsaturated LNG

0 0

10

20

MAWP 0.25

1250 1150

0.1 0.05 0.02

1050

Uinsulation= 0.01 W/m2K

950

Saturated LNG

850

30 40 50 Time [day]

60

70

80

RI PT

MAWP

LNG pressure [kPa]

LNG pressure [kPa]

1400

0

4

8 12 Time [day]

3000 0.25

2000

0.15 0.1

1500 1000

Uinsulation= 0.01

W/m2K

0.05

500

0.02

0 0

10

20

30 40 50 Time [day]

60

(c)

70

2800 2400

SC

Unsaturated LNG

Heat leak rate to tank [W]

3500 2500

20

(b)

80

Saturated LNG

0.25

2000 1600

0.15

1200

Uinsulation= 0.01 W/m2K

0.1

M AN U

Heat leak rate to tank [W]

(a)

16

800 400

0.05 0.02

0

0

4

8 12 Time [day]

16

20

(d)

Figure 7. Effects of Uinsulation on unsaturated and saturated LNG (a)-(b) pressure, and (c)-(d)

AC C

EP

TE D

heat leak rate to tank (Other parameters are as given in Table 2).

LCI Aerogel blanket

Unsaturated LNG Saturated LNG Polyurethane Cellular glass foam

0.20

0.25

1.0 0.5

0.32 0.36

1.5

0.0 0.02

0.05 0.10 0.15 Uinsulation [W/m2K]

TE D

0.01

0.20

2.59 2.93

1.79 2.03

2.0

1.28 1.45

2.5

0.69 0.78

3.0

M AN U

Unsaturated LNG Saturated LNG

0.15 0.16

Average BOG rate [%/day]

(a) 3.5

RI PT 11 2.8

0.05 0.10 0.15 Uinsulation [W/m2K]

SC

13 3.3

22 5.7

40 10.6

23.4 0.02

16 4.1

0.01

87

MLI

2.22 2.51

188

240 210 180 150 120 90 60 30 0

50.4

LNG holding time [day]

ACCEPTED MANUSCRIPT

0.25

(b)

Figure 8. Effects of Uinsulation on (a) LNG holding time and (b) average daily BOG generation

AC C

EP

rate of a 57.2 m3 storage tank filled with unsaturated and saturated LNG (Other parameters are as given in Table 2).

ACCEPTED MANUSCRIPT

Unsaturated LNG Saturated LNG

78

82

20 0

0

0.1

10 25 Tambient [°C]

TE D

0.0

0.32 0.36

-10

0.2

45

35

45

M AN U

0.3

0.29 0.32

0.4

0.27 0.29

Unsaturated LNG Saturated LNG

0.5

0.25 0.27

Average BOG rate [%/day]

(a)

35

SC

10 25 Tambient [°C]

0.36 0.40

0

0.34 0.38

-10

RI PT

21.9

28.2

40

26.0

60

23.4

80

20.5

100

87

97

104

112

120

30.7

LNG holding time [day]

140

(b)

Figure 9. Effects of Tambient on (a) LNG holding time and (b) average daily BOG generation

AC C

EP

rate of a 57.2 m3 storage tank filled with unsaturated and saturated LNG (Other parameters are as given in Table 2).

80

72

63

80

87

Unsaturated LNG Saturated LNG

100

0.4 0.3 0.2 0.1 0.0

RI PT

60% 70% 80% Initial LNG level in tank/LTank

TE D

50%

0.36 0.40

0.5

Unsaturated LNG Saturated LNG

M AN U

0.6

0.41 0.46

0.48 0.53

Average BOG rate [%/day]

(a) 0.7

90%

SC

60% 70% 80% Initial LNG level in tank/LTank

0.32 0.36

50%

25

23

0

0.29 0.32

20

19

40

21

60

17

LNG holding time [day]

120

95

ACCEPTED MANUSCRIPT

90%

(b)

Figure 10. Effects of initial LNG level in tank on (a) LNG holding time and (b) average daily

EP

BOG generation rate of a 57.2 m3 storage tank filled with unsaturated and saturated LNG

AC C

(Other parameters are as given in Table 2).

84

100 80

100

Unsaturated LNG Saturated LNG

87

120

64

41.49 (10,960)

57.20 (15,110)

SC

Tank size [m3] or [gal]

0.2 0.1 0.0

41.49 (10,960)

TE D

21.84 (5,770)

0.36

0.32

M AN U

0.3

0.37

0.4

Unsaturated LNG Saturated LNG

0.33

0.5

0.48

0.43

Average BOG rate [%/day]

(a) 0.6

107.33 (28,350)

57.20 (15,110)

0.31

21.84 (5,770)

RI PT

0

0.28

20

26.6

22.6

40

23.4

60

17.0

LNG holding time [day]

ACCEPTED MANUSCRIPT

107.33 (28,350)

Tank size [m3] or [gal]

(b)

EP

Figure 11. Effects of storage tank size on (a) LNG holding time and (b) average daily BOG

AC C

generation rate for unsaturated and saturated LNG (Other parameters are as given in Table 2).

m3/day

300 200 100

MAWP = 1,300 kPa Unsaturated LNG

0 0

5

10

15 20 Time [day]

25

1000 800 600

3.79 m3/day (10 vehicles)

400 200

Saturated LNG

0

30

0

5

4 3.79 m3/day (10 vehicles)

0 0

5

10

15 20 Time [day]

(c)

25

10 9 8 7 6 5 4 3 2 1 0

7.57 m3/day (20 vehicles)

SC

Fuel delivery rate = 1.85 m3/day (5 vehicles)

LNG level in tank [m]

Unsaturated LNG

8

2

15 20 Time [day]

25

30

(b)

7.57 m3/day (20 vehicles)

6

10

M AN U

LNG level in tank [m]

(a) 10

MAWP Fuel delivery rate = 1.89m3/day (5 vehicles)

7.57 m3/day (20 vehicles)

1200

RI PT

Fuel delivery rate = 1.85 m3/day (5 vehicles) 7.57 (20 vehicles) 3.79 m3/day (10 vehicles)

400

LNG pressure [kPa]

LNG pressure [kPa]

ACCEPTED MANUSCRIPT

30

0

5

10

Saturated LNG

Fuel delivery rate = 1.89 m3/day (5 vehicles)

3.79 m3/day (10 vehicles) 15 20 Time [day]

25

30

(d)

Figure 12. Effects of fueling fleet sizes of 5, 10, and 20 vehicles on unsaturated and saturated

TE D

LNG (a)-(b) pressure, and (c)-(d) level in tank (Uinsulation is equal to 0.022 W/m2K. Other

AC C

EP

parameters are as given in Table 2).

ACCEPTED MANUSCRIPT

1400

1000 800

7.57 m3/day (20 vehicles)

600

m3/day

3.79 (10 vehicles)

400 200

Unsaturated LNG

0 0

5

10 Time [day]

15

MAWP

1200 3.79 m3/day (10 vehicles)

1000 800

Fuel delivery rate = 1.89 m3/day (5 vehicles)

600 400 200

Saturated LNG

0 0

20

1

2

3

4 5 6 Time [day]

7

8

9

Fuel delivery rate = 1.89 m3/day (5 vehicles)

3.79 m3/day (10 vehicles)

10 Time [day]

15

(c)

Fuel delivery rate = 1.89 m3/day (5 vehicles)

10 9 8 7 6 5 4 3 2 1 0

SC

Unsaturated LNG

LNG level in tank [m]

(b)

M AN U

LNG level in tank [m]

(a) 10 9 8 7 6 5 4 3 2 7.57 m3/day 1 (20 vehicles) 0 0 5

7.57 m3/day (20 vehicles)

RI PT

MAWP Fuel delivery rate = 1.89 m3/day (5 vehicles)

1200

LNG pressure [kPa]

LNG pressure [kPa]

1400

20

7.57 m3/day (20 vehicles)

Saturated LNG

0

1

2

3

3.79 m3/day (10 vehicles)

4 5 6 Time [day]

7

8

9

(d)

Figure 13. Effects of fueling fleet sizes of 5, 10, and 20 vehicles on unsaturated and saturated

TE D

LNG (a)-(b) pressure, and (c)-(d) level in tank (Uinsulation is equal to 0.1 W/m2K. Other

AC C

EP

parameters are as given in Table 2).

ACCEPTED MANUSCRIPT

Research highlights

Holding time of liquefied natural gas storage tanks in refueling stations is analyzed.

•

The thermal mass of storage tanks and actual operating conditions are considered.

•

The thermal mass of storage tanks increases the liquefied natural gas holding time.

•

Boil-off gas generation in storage tanks minimizes at a specific temperature and pressure.

•

Insulation material has the highest effect on the liquefied natural gas holding time.

AC C

EP

TE D

M AN U

SC

RI PT

•