30, 60, and 100 cubic meter LNG storage tanks, LNG gasification stations, and CNG filling station process and equipment selection

1. LNG: Liquefied Natural Gas 2. CNG: Compressed Natural Gas 3. LPG: Liquefied Petroleum Gas

CNG: Compressed Natural Gas (CNG) refers to natural gas compressed to 20MPa and stored in high-pressure cylinders. Vehicles that use this as fuel are known as CNG cars. High-pressure cylinder storage at normal temperature: 20MPa.

LNG: Liquefied Natural Gas (LNG) is natural gas cooled to -162℃ and converted into a liquid form. It is stored in cryogenic tanks, and used as fuel in vehicles known as LNG cars. Cylinders for low-pressure, low-temperature storage: 1.6 MPa.

LNG and CNG refueling, storage, and supply systems have certain differences, but natural gas ultimately enters the car engine in a gaseous state after being pressurized through a pressure regulator to 0.15 MPa for combustion and work.

LPG: Liquefied Petroleum Gas

Section 2: Uses of LNG Tanks: Gasification Stations & Vehicle Fueling Stations

LNG liquefaction plant

(1) Peak load and accident load regulation for urban pipeline gas supply

(2) Gas source for city municipal gas networks in areas where natural gas pipelines have not been installed.

(3) Gas sources for community supply: Boiler gas and domestic gas supply.

(4) Source for corporate gas supply; (Image)

2. Auto Gas Filling Station: Used for automotive fuel

Section 3: Types of CNG Fueling Stations

LNG Vehicle Fueling Station

1) Conventional Station: Located at a fixed site, LNG is unloaded using unloading equipment, filled into storage tanks, and refueled into vehicles using fueling machines. (Image)

2) Skid-mounted Station: Equips gas station-related equipment and devices on a truck or skid, highly integrated for easy transportation and relocation, suitable for smaller-scale gas stations. (Image)

2. CNG Vehicle Refueling Station

3. L-CNG Vehicle Fueling Station

30, 60, 100 cubic meter LNG storage tanks, LNG gasification station process flow

As shown in the figure, LNG is transported to the LNG satellite station via low-temperature tanker trucks. It is pressurized in the tanker truck tanks using horizontal dedicated unloading boosters at the unloading platform. The LNG is then sent to the satellite station's low-temperature LNG storage tanks via pressure difference. Under operating conditions, the storage tank booster increases the pressure of the LNG inside the tank to 0.6 MPa. The pressurized low-temperature LNG enters an air-cooled gasifier, where it is heated by air exchange and converted into gaseous natural gas, increasing its temperature. The outlet temperature is 10°C lower than the ambient temperature, with a pressure of 0.45-0.60 MPa. If the temperature of the natural gas at the outlet of the air-cooled gasifier does not reach above 5°C, it is heated using a water bath heater. Finally, after pressure regulation (regulator outlet pressure of 0.35 MPa), metering, and odorization, it enters the city's transmission and distribution network, supplied to various users.

1. Unloading Process

LNG is transported to the city's LNG regasification station via highway tanker trucks or tank container vehicles from the LNG liquefaction plant. The tank trucks are pressurized using the tank trucks' empty-temperature pressure booster vaporizers (or through an onboard booster vaporizer at the station for tank container vehicles), creating a pressure difference between the tanker and the LNG storage tank. This pressure difference is used to discharge the LNG from the tanker into the station's storage tank. At the end of unloading, the gaseous natural gas is recovered from the tanker through the vapor phase pipeline on the unloading platform.

During unloading, to prevent an increase in pressure within the LNG storage tank from affecting unloading speed, the upward liquid method is used when the LNG temperature in the tanker is lower than that in the storage tank. The low-temperature LNG in the tanker is sprayed into the storage tank through the upward liquid nozzle, cooling some gas to a liquid state and reducing the tank pressure for smooth unloading. If the LNG temperature in the tanker is higher than that in the storage tank, the downward liquid method is employed. The high-temperature LNG enters the tank from the downward liquid inlet, mixing with the low-temperature LNG inside to cool down, preventing the high-temperature LNG from evaporating through the upward liquid inlet, which could raise the tank pressure and make unloading difficult. In practice, due to the long distance from the LNG gas source to the consuming cities, the LNG temperature inside the tanker is typically higher than that in the storage tanks at the gasification stations upon arrival, necessitating the use of the downward liquid method. Therefore, except for the initial LNG filling, the downward liquid method is predominantly used during regular tanker unloading.

To prevent significant temperature differences and stress from causing damage to the pipeline or affecting unloading speed during unloading, the unloading pipeline should be pre-cooled with LNG from the storage tank before each unloading. Additionally, it should be avoided to rapidly open or close valves to prevent sudden changes in LNG flow rate that could lead to liquid impact and damage the pipeline.

2. Storage Tank Pressurization Process

Driven by pressure, LNG flows from the storage tank to the regasifier, where it is vaporized into natural gas and supplied to customers. As LNG exits the tank, the internal pressure decreases, and the outflow rate of LNG gradually slows down until it stops. Therefore, during normal gas supply operations, it is necessary to continuously replenish the tank with gas to maintain the pressure within a certain range, ensuring the continuous vaporization process. The pressure increase of the tank is achieved using an automatic pressure booster valve and an auto-pressurizing atmospheric temperature regasifier. When the tank pressure falls below the set opening value of the automatic pressure booster valve, the valve opens, and the LNG in the tank flows into the auto-pressurizing atmospheric temperature regasifier (the installation height of the auto-pressurizing atmospheric temperature regasifier should be below the lowest liquid level in the tank) due to the liquid level difference. In the auto-pressurizing atmospheric temperature regasifier, the LNG vaporizes into natural gas by heat exchange with air, and then the natural gas flows back into the tank, raising the tank pressure to the required operating pressure.

3. LNG液化工艺

LNG undergoes heat exchange with the atmosphere in a vacuum gasifier, transitioning from liquid to gas. By the time it reaches the outlet, it is 10℃ cooler than the ambient temperature. If it falls below 5℃, it is heated by a water bath vaporizer. The hot water for the water bath vaporizer comes from the water circulation of a hot water boiler.

4. Gas Safety Release Process

LNG is a liquid mixture primarily composed of methane, with a boiling point of -161.5°C at atmospheric pressure and a storage temperature of -162.3°C, with a density of approximately 430 kg/m³. When LNG vaporizes into gaseous natural gas, its critical buoyancy temperature is -107°C. Above -107°C, the gaseous natural gas is lighter than air and will rise and drift away from the leak source. Below -107°C, the gaseous natural gas is heavier than air, and the cold gaseous natural gas will accumulate downward, forming a flammable explosive mixture with air. To prevent the accumulation of cold gaseous natural gas, which could form an explosive mixture, a 1-unit air-temperature safety vent gas heater is installed. The vent gas is first heated by this heater to reduce its density below that of air before being released into the atmosphere.

For LNG vaporization stations in southern regions without EAG heating devices, to prevent cold burns to operators caused by the release of low-temperature LNG gas-liquid mixture after the safety valve trips, the single safety valve vent pipe and the storage tank vent pipe should be connected to the central vent header for venting.