Science Popularization of Ship Knowledge -- Differences among LPG Ship, LNG Ship and CNG Ship

LPG carrier: Liquid petroleum gas carrier

LPG ships are used to transport liquefied petroleum gas or ammonia. Generally, such gas can be liquefied at relatively high temperature or by pressurization. Generally, LPG ships are transported in the form of refrigeration (about - 60 ℃) or pressure tanks, that is, pressure tanks are installed on the ship to transport liquefied gas.

LPG, liquefied petroleum gas (LPG for short) is often confused with LNG. In fact, they are obviously different. The main component of LPG is propane (more than 95%), with a small amount of butane. LPG is stored as liquid in storage tanks under appropriate pressure, and is often used as cooking fuel. LPG has been used as light vehicle fuel for many years abroad.

LNG carrier: Liquid natrual gas carrier

LNG ships are used to transport liquefied natural gas (CH4). The liquefaction temperature of natural gas is low, and it is not suitable for pressure liquefaction. LNG ships are used to transport liquefied natural gas at minus 163 ℃. Because the temperature is low, and the natural gas is volatile, if it leaks and catches fire, the consequences will be unimaginable. Therefore, the design requirements of LNG ships are much higher than LPG.

LNG, liquefied natural gas, is a clean and efficient energy. Natural gas is the combustible gas naturally extracted from the gas field, which is mainly composed of methane. LNG is cooled to - 162 ℃ by gaseous natural gas under normal pressure to condense it into liquid. Liquefaction of natural gas can greatly save storage and transportation space and cost, and has the characteristics of large calorific value and high performance.

CNG ship: Compressed natural gas

CNG carrier refers to the cargo ship transporting compressed natural gas. CNG is in gaseous state at a pressure of 80~313bar, and its volume can be reduced to 1/200 of the standard volume of natural gas. Since expensive facilities such as liquefaction unit and regasification terminal are not required, CNG investment cost is significantly lower than LNG. Therefore, it is increasingly favored. This new way is compressed natural gas transportation.

The principle of CNG transportation is actually very simple. It is to compress the natural gas at the output terminal and then fill it into a specially designed CNG vessel. When the CNG ship arrives at the receiving terminal, it can directly transport the natural gas to the natural gas pipe network, unlike the liquefied natural gas (LNG), which transforms the gas into liquid at the output terminal, and then transforms the liquid into gas at the receiving terminal.

CNG (Compressed natural gas) is the abbreviation of Compressed natural gas, which is mainly used to supply gas for short distance vehicles and suburban communities. Common on-board CNGs are stored in high-strength tanks, and the storage pressure of natural gas is 20MPa

LNG can be used to make CNG, which is called NGV (Natural Gas Vehicle). Compared with the traditional method of producing CNG, this set of process requires lower cost of precision equipment, which only requires about 15% of the operation and maintenance costs. At present, most of the "green leather" buses in Beijing use CNG LPG.

The main components of CNG and LNG are both methane, and the main components of LPG are propane and butane. The calorific value of LPG is higher than the other two, and the risk is also higher. Under normal temperature and pressure, a cubic meter of the three will be gasified. After LNG gasification, the volume will increase by 625-650 times, and LPG will increase by 250-350 times. CNG will depend on how many times it is compressed. LPG is liquefied petroleum gas, CNG is compressed natural gas, and LNG is liquefied natural gas.

Liquefied petroleum gas (LPG) is often mistaken for propane. In fact, LPG is a mixture of oil and natural gas under appropriate pressure and exists as a liquid at room temperature. In the United States and Canada, the mixture of these two substances is generally considered to be mainly composed of propane, while in many European countries, the propane content in LPG is only 50% or less.