I. Introduction to LNG

Under normal pressure, natural gas, when cooled to about -162 ° C, changes from a gaseous state to a liquid state, called Liquefied Natural Gas (LNG). Natural gas is a kind of mixture, and its composition varies with gas fields. The main components are methane, nitrogen and C2 to C5 saturated alkanes. In addition, it contains trace amounts of antimony, carbon dioxide and hydrogen sulfide. After liquefaction, LNG is used. It becomes a cryogenic liquid containing methane (96% or more) and ethane (4%) and a small amount of C3-C5 alkane. LNG is another form of energy that is transformed by natural gas.

1. Physicochemical properties of LNG

1) The main component of LNG is methane, the chemical name is CH4, and there are a small amount of other components such as ethane C2H6, propane C3H8 and nitrogen N2.

2) The critical temperature is -82.3 ° C, and the critical pressure is 45.8 kg / cm 3 (4.58 MPa).

3) The boiling point is -162.5 ° C, the melting point is -182 ° C, and the ignition point is 650 ° C.

4) The liquid density is 0.430 T/m 3 and the gaseous density is 0.688 kg/Nm 3 .

5) The gaseous calorific value is 9100 Kcal/m3, and the liquid calorific value is 12000 Kcal/kg.

6) Explosion range: the upper limit is 15% and the lower limit is 5%.

7) Huabai Index (W) 44.94 MJ/Nm3.

8) Combustion potential (CP) 45.18.

9) Octane ASTM: 130 (research method).

10) Colorless, odorless, non-toxic and non-corrosive.

11) The volume is about 1/625 of the volume of the same amount of gaseous natural gas.

2, the advantages of LNG

1) LNG volume is 625 times smaller than natural gas of the same quality, so it is convenient to transport LNG to places where there is no natural gas.

2) LNG has high storage efficiency and a small footprint. In the province of investment, 10m3LNG storage capacity can provide 10,000 households with one-day living gas.

3) As a high-quality vehicle fuel, LNG has high octane number, good antiknock performance and long engine life compared with gasoline. Fuel costs are low. Good environmental performance and other advantages. It can reduce HC in gasoline vehicle exhaust by 72%, NOx by 39%, CO by 90%, and SOx and Pb to zero.

4) The latent heat of LNG vaporization is high, and the cold amount in the liquefaction process can be recycled.

5) Since the density of LNG vaporization is very low, only about half of the air, a slight leak immediately spreads out without causing an explosion.

6) Because the LNG component is purer, burns completely, and produces carbon dioxide and water after combustion, it is a good clean fuel, which is conducive to protecting the environment and reducing urban pollution.

3. The main use of LNG

1) It is safe, convenient, fast and less polluting for use as a clean fuel for urban residents.

2) Use as a substitute for automotive fuel. LNG is used as fuel for automobile engines. The engine only needs to be properly changed. The operation is not only safe and reliable, but also has low noise and low pollution. Especially in today's increasingly strict emission regulations, the exhaust gas is obviously improved in vehicles with LNG as fuel. According to the data report, compared with compressed natural gas (CNG), LNG vehicle fuel cost is 20% lower and 2/3 lighter than medium and medium-duty vehicles under the same stroke and running time conditions. The cost of the combustion system unit is also at least 2/3 lower. It can be shown that liquefying natural gas and storing it in liquid form is the most cost-effective way to promote its use in transportation fuels.

3) As a cold source for the production of fast-cooling foods, as well as low-temperature pulverization of plastics and rubber, it can also be used for seawater desalination and cable cooling.

4) As an industrial gas fuel, used in glass bulb factories, process glass factories, etc.

4. Production, transportation and storage of LNG

The production process of LNG is generally divided into the following three types: one is a step-mixing refrigeration process, the other is a mixed-stage cooling process, and the third is a compression and expansion refrigeration process. The main equipment of the LNG plant includes compressors, expanders, heat exchangers, separators, cryogenic storage tanks, cryogenic pumps, and cryogenic tank trucks.

LNG is mainly transported by ship or train. Car tanker and other means. It is an ideal way to transport LNG by car tanker within the economic transportation radius of 500-800 km. The tank car body adopts double-wall vacuum powder and is not insulated. It is equipped with an operating valve safety system and an infusion hose. The manufacturing technology of domestic cryogenic liquid tank trucks is relatively mature, and the tank trucks are safe to use.

LNG products are stored in cryogenic liquid storage tanks. The liquid storage tanks are not insulated by double-walled vacuum powder. The daily evaporation rate of LNG can be controlled within 0.46% and the storage period is 4 days.

2. Overview of the process flow of the LNG gasification station

The LNG is transported by the tank truck to the gasification station. The LNG unloading supercharger is used to increase the pressure inside the tank truck, and the LNG in the tank truck is sent to the LNG cryogenic storage tank for storage. When discharging from the LNG storage tank, the tank pressure is first increased by the self-pressurizing system of the storage tank, and then the liquid phase outlet valve of the storage tank is opened, and the LNG in the storage tank is sent to the gasifier through the pressure difference. It is sent to the municipal gas pipeline network through processes such as pressure regulation, metering and odor removal. When the outdoor ambient temperature is low and the natural gas temperature at the outlet of the air-temperature gasifier is lower than 5 °C, a water bath heater is connected in series at the outlet of the air-temperature gasifier to heat the gasified natural gas.

1. Storage of LNG

1) Determination of tank volume

The total volume of the storage tank is usually determined by the average monthly daily gas consumption of 3 to 7 days. The number of long-term gas supply plants, maintenance time, transportation cycle and fluctuations of user gas consumption should also be considered. The industrial gas consumption should be based on The specific requirements of the gas equipment and production are determined. If there is only one gas source plant, the total volume of the tank should be considered to ensure normal gas supply during maintenance of the gas source plant.

2) How to store LNG

The storage tank is the main equipment of the LNG gasification station, which directly affects the normal production of the gasification station and also has a large proportion of cost. According to the structure, it can be divided into underground storage tanks, above-ground metal storage tanks and metal prestressed concrete storage tanks. For LNG storage tanks, existing vacuum powder insulated storage tanks, positive pressure accumulation insulated storage tanks and high vacuum layer insulated storage tanks, and medium and small gasification stations generally use vacuum powder insulated low temperature storage tanks. The tank is divided into two layers, inner and outer, and the interlayer is filled with pearl sand and vacuumed to reduce the external heat transfer, ensuring that the daily gasification rate of LNG in the tank is less than 0.3%.

2. Gasification of LNG

The gasification device is the main device for supplying gas to the outside by the gasification station. In the design, we usually use an air-temperature gasifier, and its gasification capacity should be I.3~1.5 times of the flow rate calculated by the peak hour of the gas town. In 2 sets, and there should be 1 spare. When the ambient temperature is low, when the natural gas temperature at the outlet of the air-temperature gasifier is lower than 5 °C, the exported natural gas should be reheated twice to ensure the normal operation of the entire gas supply. Generally, natural gas heaters use water bath heaters.

3. Treatment of BOG (Boil Off Gas) and EAG (Safe Discharge Gas: Escape air gas)

BOG is mainly derived from the return gas of LNG tank trucks and 0.3% natural gasification per day of storage tanks. Now the commonly used tank truck has a volume of 40m3. The time for recovering BOG is calculated according to 30min. The gas pressure in the tank truck after unloading LNG is about 0.55MPa. According to the pressure of the end natural gas, the pressure inside the tank truck after recycling BOG is different. Can be calculated according to 0.2MPa. The return flow of the recovery tanker requires a flow rate of 280 m3/h for the BOG heater. When the natural evaporation of the LNG storage tank is added, the flow rate of the BOG heater can be calculated. The storage temperature of LNG is -163 ° C, that is, the temperature of BOG is about -163 ° C. To ensure the safety of the equipment, BOG is heated to 15 ° C. The specifications of the BOG heater can be determined based on flow rate and temperature. After the recovered BOG is pressure-regulated, metered, and odorized, it can directly enter the pipe network. If the user gas is not continuous, the BOG storage tank needs to be set for storage.

EAG is mainly emitted when equipment or pipelines are overpressured. When LNG is gasified to natural gas, the critical temperature of natural gas when it is lighter than normal temperature is -110 °C. To prevent the EAG from accumulating during the release, the EAG should be heated to above -110 °C and then released. The 500 m3/h EAG heater is selected for the LNG storage tank with a volume of 100 m3. The maximum outlet temperature is not lower than -15 °C.

Third, the main process equipment of LNG gasification station

1, LNG storage tank

1) LNG storage tank structure

LNG storage tanks can be divided into underground storage tanks, above-ground metal storage tanks and metal/prestressed concrete storage tanks according to the structure. The above-ground LNG storage tanks are further divided into two types: metal mother storage tanks and metal single tanks. The metal mother tank is assembled by juxtaposition of more than three sub-tanks in a large female tank (ie, outer tank). The sub-tank is usually a vertical cylindrical shape, and the mother tank is a vertical flat-bottomed arch cylindrical shape. The mother tank is mostly used in natural gas liquefaction plants. The storage tank of the urban LNG gasification station usually adopts a vertical double-layer metal single tank, and its internal structure is similar to an upright thermos bottle. The inner tank is supported on the outer tank, and the inner and outer tanks are vacuum powder insulation layers. The tank has a volume of 50 m3. And 100 m3. , more than 100 m3 storage tanks.

 For 100 m3. Vertical tank with inner tank inner diameter of 3 000mm, outer tank inner diameter of 3 200 mm, tank body and support total height of 17 100 mm, storage tank geometry volume of 105.28 m3.

2) Determination of design pressure and calculated pressure

At present, most of the 100 m3. The maximum working pressure of the vertical LNG storage tank is O. 8 MPa. According to the provisions of GB 150 "Steel Pressure Vessel", when the maximum working pressure of the tank is 0.8 MPa, the design pressure is 0.84 MPa. The filling factor of the storage tank is 0.95, the net pressure of the liquid column after the inner tank is filled with LNG is 0.062 MPa, and the absolute pressure between the inner and outer tanks is 5 Pa, and the calculated pressure of the inner tank is 1.01 MPa.

 The main function of the outer tank is to fix the inner tank and the heat insulating material by hanging or supporting, and at the same time form a high vacuum heat insulating layer with the inner tank. The loads acting on the outer tank are mainly the gravity load of the inner tank and the medium and the vacuum negative pressure of the heat insulating layer. Therefore, the outer tank is an external pressure vessel and the design pressure is 0.1 MPa.

3) Selection of LNG storage tanks

 The operating temperature of the LNG storage tank during normal operation is 162.3 ° C. Before the first application, the storage tank is pre-cooled with a liquid nitrogen of 196 ° C. The design temperature of the storage tank is 196 ° C. The inner tank must withstand the working pressure of the medium and the low temperature of the LNG. The inner tank material must have good low-temperature comprehensive mechanical properties, especially good low-temperature toughness. Therefore, the inner tank material adopts 0Crl8Ni9, which is equivalent to ASME (USA). Mechanical Engineers Association) Standard 304.

 The calculated thickness and design thickness of the inner can are 11.1 mm and 12.1, respectively, based on the calculated pressure of the inner can and the material selected. O mm. As a normal temperature external pressure vessel, the outer tank material is made of low-alloy container steel 16MnR, and its design thickness is 10.0 mm.

4) LNG tank connection design

The nozzles opened in the tanks of the storage tank are: upper inlet port, lower inlet port, outlet port, gas phase port, measuring full port, upper level gauge port, lower level gauge port, and 8 nozzles for process manhole . The material of the joint on the inner tank is OCrl8Ni9.

 In order to facilitate periodic measurement of vacuum and vacuuming, a vacuum port is opened on the lower head of the outer tank (the nozzle is closed after vacuuming). In order to prevent the vacuum from failing and the inner tank medium leaking into the outer tank, an explosion-proof device is arranged on the outer tank head.

5) Design of LNG storage tank liquid level measuring device

In order to prevent the LNG in the tank from being overfilled or the LNG in the tank is too small to endanger the safety of the tank and the process system, two sets of independent liquid level measuring devices for measuring the full mouth and the differential pressure level gauge are respectively set on the tank, and the sensitivity thereof is adopted. And reliability is critical to the safety of LNG storage tanks.

When filling the tank with LNG, the static pressure reading displayed by the differential pressure level gauge can be used to visually and easily read the liquid level, volume and mass of the LNG in the tank from the static pressure and filling quality comparison table. . When the filling limit is reached, the LNG liquid will overflow from the metering port, alerting the operator to manually cut the feed. The tank automatic control system is also equipped with a high-limit alarm (filling capacity is 85% of the tank capacity), emergency cut-off (filling capacity is 95% of the tank capacity), and low-limit alarm (the remaining LNG is 10% of the tank capacity) .

6) LNG storage tank insulation layer design

The insulation layer of the LNG storage tank has the following three forms:

 1 high vacuum multi-layer wound insulation layer. Mostly used in LNG tank trucks and tank container trucks.

 2 Positive pressure buildup insulation layer. This method of insulation is to deposit the heat insulating material in the interlayer between the inner and outer tanks, and the interlayer is filled with nitrogen gas, and the heat insulating layer is usually thick. Widely used in large and medium-sized LNG storage tanks and storage tanks, such as vertical metal LNG mother storage tanks.

 3 vacuum powder insulation layer. The commonly used single tank nominal volume is 100 m3. And 50 m3. The cylindrical bimetallic LNG storage tank is usually insulated. The interlayer between the inner and outer tanks of the LNG storage tank was filled with powder (pearl sand), and then the interlayer was evacuated to a high vacuum. The evaporation rate is usually used to measure the insulation performance of the tank. At present, the daily static evaporation rate volume fraction of domestic LNG storage tanks is ≤O. 3%.

2, BOG buffer tank

 For the peak-adjusting type LNG gasification station, in order to recover the residual gas of the non-peaking period unloading tank truck and the BOG in the storage tank, or for the natural gas mixing station for uniform gas mixing, BOG buffer is often added at the outlet of the BOG heater. The capacity of the tank is set according to the amount of gas remaining in the tank truck.

3, tank pressurized gasifier

After filling a 90 m3 LNG tank with a 90 m3 LNG, it will be 10 m3 in 30 min. The pressure in the gas phase space is calculated from the pressure of 0.4 MPa in the unloading state to 0.6 MPa in the working state. According to the calculation results, each tank uses one air-temperature gasifier with a gasification capacity of 200 m3/h to supercharge the tank, and the temperature of the LNG inlet pressurized gasifier is 162.3 °C, gaseous natural gas. The temperature of the pressurized gasifier is 145 °C.

 The design uses one LNG storage tank with one pressurized gasifier. It is also possible to share one or a group of gasifiers with multiple tanks, and the valve can be switched to simplify the process, reduce equipment and reduce the cost.

4, unloading pressurized gasifier

Since the LNG container tanker is not equipped with a supercharger, the unloading pressurized gasifier with a gasification capacity of 300 m3/h is installed in the station, and the tanker pressure is increased to 0.6 MPa. The LNG inlet gas temperature is 162.3 ° C, and the gaseous natural gas outlet gas temperature is 145 ° C.

 5, BOG heater

Since the largest amount of BOG in the station is the gas phase natural gas after the tank truck is unloaded, the design capability of the BOG air-temperature heater is calculated according to this, and the time of recovering the gas phase natural gas after unloading the tank truck is 30 min. Taking a 40m3 tanker pressure from 0.6MPa to 0.3 MPa as an example, the capacity of the required BOG air-temperature gasifier is calculated to be 240 m3/h. Generally, the BOG air temperature heater is selected according to the number of tank cars that can be connected and unloaded at the same time. Usually the BOG heater has a heating capacity of 500 to 1 000 m3/h. When using a water bath type natural gas heater in winter, BOG is used as a fuel for a hot water boiler, and is fed in the rest of the season.