『Original by HaiChuan Translation Team』Answers to Common Questions about Liquefied Natural Gas

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English title: Frequently Asked Questions (FAQ) About Liquefied Natural Gas (LNG)
Original link: Click here
Original author: ankur2061
Translator ID: @yunshih
Proofreader ID: @YuHuan

Currently, liquefied natural gas is a hot topic; many countries lacking in fossil fuels have regarded LNG as a crucial primary energy resource. Compared to other fossil fuel sources such as coal and crude oil and their derivatives, liquefied natural gas is one of the cleanest fuels among all fossil fuels. Today's blog post will focus on liquefied natural gas, aiming to explain what LNG is to engineers who are just starting their careers. The blog is published in the form of “FAQs on liquefied natural gas”. We will provide hyperlinks to some websites to guide readers on where they can find more details. Here are the frequently asked questions: Question 1: What is liquefied natural gas? Answer 1: Liquefied natural gas (LNG) is natural gas (NG) that has been cooled to its freezing point. Question 2: What are the conditions for converting natural gas into liquefied natural gas? Answer 2: The conditions for natural gas to condense into a liquid, namely liquefied natural gas, are a temperature of around -161°C (-256°F) and an atmospheric pressure of 101.325 kPa (absolute pressure). Question 3: Why is it necessary to convert natural gas into liquefied natural gas? Answer 3: In situations where it is not economically viable to transport natural gas via pipelines or where there are other constraints, liquefying natural gas reduces its volume by about 600 times, thereby making its storage and long-distance transportation more economical. Question 4: How is liquefied natural gas stored? Answer 4: Liquefied natural gas is stored in double-walled tanks at atmospheric pressure. The storage tank has inner and outer layers; the annular space between the two tank walls is filled with insulating material. Question 5: What material is used for LNG storage tanks? Answer 5: The inner liner in contact with LNG is made of a material suitable for low-temperature conditions and capable of withstanding structural loads. These materials include 9% nickel steel, aluminum, and prestressed concrete. The outer storage tank is generally made of carbon steel or prestressed concrete. Question 6: How is liquefied natural gas transported by ship? Answer 6: Liquefied natural gas carriers are specially designed double-hulled ships to prevent leaks or ruptures in the event of an accident. Liquefied natural gas is stored in a special sealed system within the ship’s hull, at atmospheric pressure and low temperature (-161°C). Question 7: What is the regasification of liquefied natural gas? Answer 7: The liquefied natural gas at reception and regasification stations needs to be converted back into natural gas in a controlled environment. LNG can be heated in pipelines through direct flame heating, seawater baths, or hot water baths. Question 8: How is liquefied natural gas quantified in the international market (sold/bought)? Answer 8: Generally, liquefied natural gas is traded in units of millions of tons. Question 9: Under standard conditions (1 atmosphere, 60°F), what volume of gaseous natural gas can approximately be obtained from a unit mass of liquefied natural gas? Answer 9: Under standard conditions {1 atmosphere, 60°F}, the conversion relationship between unit mass of liquefied natural gas and gaseous natural gas NG is as follows: 1 metric ton of LNG or 1,000 kg of LNG = 48,700 Scf (1,379 Sm3) of NG. Note: The exact conversion relationship depends on the molecular weight of natural gas. Question 10: Is liquefied natural gas flammable? Answer 10: At ambient temperatures, if liquefied natural gas is suddenly released from a container, it will flash and form a vapor-air mixture. This vapor-air mixture forms a visible vapor-air cloud. When the steam cloud comes into contact with the surrounding air, its temperature rises, it becomes lighter than the air, and rises upward. A combustible gas-gas mixture can form a combustible vapor-air mixture only when its natural gas vapor concentration is between 5% and 15% (v). When the volume concentration of natural gas in the steam-air mixture is less than 5% (v), the concentration of steam (NG) in the steam-air cloud is not sufficient to ignite or sustain the flame; in other words, such a steam-air mixture is too lean to initiate a flame or fire. Therefore, 5% (v) is known as the lower flammability limit (LFL) of natural gas. Similarly, when the natural gas concentration in the steam-air mixture is greater than 15% (v), the air (oxygen) concentration in the steam-air mixture is insufficient to ignite or sustain the flame; in other words, such a steam-air mixture is too rich to cause a fire. Therefore, 15% (v) is referred to as the upper flammability limit (UFL) of natural gas. Question 11: Is liquefied natural gas explosive? Answer 11: Liquefied natural gas is not explosive in its liquid state. When liquefied natural gas forms a vapor-air mixture in a confined space with no way to disperse it, exposure to an ignition source can cause the vapor cloud to explode. For the explosion of a vapor-air mixture, the vapor concentration of natural gas in the vapor-air mixture must be between 5% (v) and 15% (v). Therefore, the term LFL/UFL can be used interchangeably with Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL). Question 12: What are the main uses of liquefied natural gas? Answer 12: Liquefied natural gas after being gasified is mainly used for power generation, home heating, and cooking gas.