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Journal of Natural Gas Science and Engineering 36 (2016) 496e509

Contents lists available at ScienceDirect

Journal of Natural Gas Science and Engineering

journal homepage: www.elsevier.com/locate/jngse

Performance analysis of liqueﬁed natural gas storage tanks in refueling stations

Amir Sharaﬁan, Omar E. Herrera, Walter Meacute;rida*

Clean Energy Research Centre, The University of British Columbia, 2360 East Mall, Vancouver, BC, V6T 1Z3, Canada

a r t i c l e i n f o

Article history:

Received 5 August 2016 Received in revised form 28 September 2016

Accepted 26 October 2016

Available online 28 October 2016

Keywords:

Liqueﬁed natural gas Boil-off gas

Storage tank Holding time Refueling station

a b s t r a c t

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 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 is illustrated by an analysis of 57.20 m3 storage tanks ﬁlled

with LNG at —150 ○C and —126.5 ○C. The tank with the lower temperature ﬁlls shows 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.

Introduction

Climate change is one of the main concerns of todays world (Richter, 2014), but greenhouse gas (GHG) emissions from indus- trial and transportation processes have steadily increased (Global greenhouse, 2016; Sources of greenhouse gas emissions, 2016; Liimatainen et al., 2014). For instance, the GHG emissions from the U.S. medium- and heavy-duty trucks increased by 76% between 1990 and 2014 and reached 407.4 Mt CO2,eq (Inventory of U.S., 2016). According to the announcements at the 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) (21st session of the Conference of the Parties et al., 2015). 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 (van Der Hoeven, 2015). Natural gas (NG) is considered a low-carbon content fuel (Van Den Broek et al., 2015) and several studies reported the beneﬁts of NG on economic and market growth (Van Den Broek et al., 2015; Imran

* Corresponding author.

E-mail address: walter.merida@ubc.ca (W. Meacute;rida).

et al., 2016; Hao et al., 2016; Wang and Li, 2016; Furuoka, 2016; Balitskiy et al., 2016; Wang et al., 2016; Scedil; evik, 2015; Wang and Lin, 2014; Kakaee et al., 2014; Wang et al., 2014; Khan et al., 2015). However, and despite this signiﬁcant body of work, recent studies revealed uncertainty in the overall beneﬁts associated with NG use (Alvarez et al., 2012; Howarth et al., 2011; Venkatesh et al., 2011; Davis and Shearer, 2014; McJeon et al., 2014; Delgado and Muncrief, 2015). According to the Global Warming Potential (GWP), methane emissions contribute up to 72 times more to climate change than CO2 in a 20-year horizon (Solomon, 2007). 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 (Study on natural gas, 2014). 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 (International Gas Union, 2015). These features make LNG a candidate fuel in the transportation sector to reduce GHG emissions, especially for large, mobile ap- plications, such as heavy-duty trucks (Bassi,

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加气站液化天然气储罐的性能分析

摘要

液化天然气可以代替运输部门的柴气，但包括LNG供应链中的蒸发气体（BOG）逸散排放表明这种替代在整体环境效益有一定的不确定性。在这项研究中，通过开发基于时间的热力学模型，来研究在BOG释放到大气之前加气站储罐的LNG储存时间。以前被忽视的因素，如储罐的热质量和加气站的实际运行情况，在本次研究中都明确列入了模型。本研究通过对57.20立方米储罐分析来说明一定储存时间的储罐热质量。

液化天然气在-150℃和-126.5℃时与有较低温度的容器具有3.7倍的保持时间。通过进一步的调查证明了传热表面积与LNG体积比的重要性，作为储罐适当尺寸化的一个关键因素，应要最大化保温时间。最后，通过对传热系数为0.022 W/m2K的57.20立方米储罐的建模表明，燃气输送速率低至1.89msup3;/d足以将气箱压力维持在允许的范围内。

关键词：液化天然气蒸发气储存罐保温时间加气站

1.介绍

气候变化是当今世界的主要问题之一，工业和运输过程的温室气体（GHG）排放量一直稳步增长。例如，1990年至2014年期间，美国中型和重型卡车的温室气体排放量增加了76％，达到了4.040亿吨二氧化碳当量。根据巴黎第二十一届缔约方会议的公告，人类需要在未来三十年内立即缓解气候变化并达成二氧化碳目标。据称，用低碳含量燃料代替常规石气燃料，例如柴气和汽气，可减少温室气体排放和气候变化。天然气（NG）被认为是一种低碳含量燃料，有几项研究证明了NG对经济和市场增长的好处。然而，尽管这些研究的效果很明显，但最近的研究显示与NG使用相关的总体收益存在一定的不确定性。根据全球变暖潜势（GWP），导致气候变化的二氧化碳排放量比二氧化碳在二十年期间贡献高达72倍。因此，NG二氧化碳排放量的减少必须与甲烷的排放进行比较。由于没有关于实际实施技术的可靠数据，将NG的使用与现有能源进行比较的大多数模型和分析仍将是不完整的。

液化天然气（LNG）是天然气的浓缩形式，具有60％的柴气容积能量密度。与超低硫柴气相比，LNG的燃烧可以将二氧化碳、氮氧化物和颗粒物排放量分别降低20％，90％和100％。这些特点使LNG成为运输行业的候选燃料，以减少温室气体排放，特别是对于大型移动应用，如重型卡车、火车、和船。然而，液化天然气的逃逸排放仍是一个主要问题。

LNG是在低至-162℃下储存的低温液体。由于其环境温度梯度较大，LNG逐渐加热蒸发（蒸发气体（BOG））。甲烷（LNG混合物的主要成分）是一种影响比较大的温室气体，由于甲烷较高的碳含量，所以对气候变化的影响要大于二氧化碳。在大型液化天然气运输船上，BOG可被重新使用或用作燃料，可用于保持LNG处于大气压力和低温。在大型LNG再生设备和储存设施中，BOG可用于发电。然而，在小型应用，如LNG加气站，BOG管理更具挑战性。LNG设备的BOG生成和甲烷排放主要来自：1）向LNG的热转移和储罐的加压，2）在填充罐时通风排出的BOG，3）将热量转移到软管，线路和泵，4）在LNG转运之前对设备进行预冷，5）LNG从高压罐转移到低压罐。

天然气（NG）井库分为四个部门，以量化排放排放量：1）气田，2）加工，3）输送和储存，4）分配。分析显示，在价值链上，大气平均排放了1.12％的NG。他们的调查还表明，运输和仓储部门占分配部门中甲烷排放量的35％，其中包括加气站和加气过程，占28％。因此，运输、仓储和配送行业通过防止BOG释放和固定现有泄漏，有明显的改良余地。本研究的主要重点是配送行业。

根据Burnham等人（2015年）调查研究，液化天然气加气站的平均甲烷排放量约为LNG运输量的0.32％。