The key technologies of CNG filling stations can be divided into multiple systems such as compression system, natural gas pretreatment system, storage system and gas sales system, control system, etc.
Natural gas compression system
The natural gas compression system is mainly composed of compressor host, air intake buffer tank, cooling system, lubrication system, etc. The compressor host is the most important. It is the heart of the gas station. Its performance directly affects the reliability and economy of the gas station operation. CNG filling stations generally use reciprocating compressors with high exhaust pressure and small displacement. From the lubrication method, there are mainly two types: oil lubrication and oil-free lubrication. For oil lubrication, a de-oiling device should be installed after the final exhaust port.
The exhaust pressure of the compressor used in the gas station is generally 25MPa, some are slightly higher, reaching 28MPa, and a few manufacturers' compressors reach 32MPa, but analysis shows that the exhaust pressure of 25MPa is the most economical, reliable and safe. The inlet pressure range is 0.035～9MPa, and the compressor exhaust volume can be selected according to different scales, ranging from 16～2000m3/h, and the commonly used exhaust volume is 200～300m3/h.
Natural gas pretreatment system (including purification and pressure regulation control)
The natural gas entering the gas filling station must be purified and dried before or after compression, that is, it must go through the desulfurization, dehydrogenation and dehydration process. Desulfurization refers to the removal of acidic gases such as hydrogen sulfide in natural gas to prevent corrosion of equipment pipelines and "hydrogen embrittlement" of steel gas cylinders. Dehydrogenation refers to the removal of light hydrocarbons in natural gas to make the ethane and heavy hydrocarbon content less than 3% to prevent abnormal engine ignition and combustion. Dehydration refers to the removal of water from natural gas to prevent ice blockage in the gas supply system during the decompression, expansion and cooling process of CNG.
Natural gas dehydration is the most critical part of the pretreatment process. CNG gas filling stations usually use double-tower dryers for dehydration. According to the position of the dehydration device in the station, it can be divided into three types of low-pressure, medium-pressure and high-pressure dehydration. If the high-pressure dehydration method can solve the relevant technical and process problems in terms of the quality of accessories and valves, it is a better dehydration solution because of its compact structure and good dehydration effect.
After purification and drying, natural gas must meet the standards for natural gas for vehicles before it can be filled into gas cylinders at gas stations, sold directly, or filled into CNG vehicle cylinders. my country has formulated the national standard for "compressed natural gas for vehicles" (GB 18047-2000). This standard has very strict restrictions on the sulfur and water content of CNG for vehicles, requiring the hydrogen sulfide content to be less than 15mg/m3 and the water dew point to be lower than the minimum ambient temperature of 5℃ under the maximum operating pressure.
Control system
The function of the control system is to control the normal operation of the gas station equipment and monitor the operating parameters of the relevant equipment and automatically alarm or shut down when the equipment fails. The control systems of foreign gas filling equipment mostly use PLC (programmable logic controller), which has high reliability and can realize fully automatic operation of the equipment. It can also be remotely transmitted to the duty room for unattended operation, reducing the labor intensity of the operator.
Storage and gas sales system
In order to avoid frequent startup of the compressor and provide gas source when inflation is not required, CNG gas filling stations need to be equipped with gas storage devices. The typical design is that the gas storage system and the gas sales system achieve efficient inflation and rapid gas filling through the order of the priority control panel. Usually, gas filling stations adopt a hierarchical storage method, dividing the gas cylinder group into high pressure, medium pressure and high pressure. The priority control panel automatically controls the inflation and gas extraction process of the high-pressure and low-pressure cylinder groups. When inflating, inflate the high-pressure group first. When the pressure of the high-pressure group rises to a certain value, the medium-pressure group starts to inflate. When the pressure of the medium-pressure group rises to a certain value, the low-pressure group starts to inflate. Then the three groups of cylinders are inflated together. When the pressure reaches the maximum storage pressure, stop inflating. When extracting gas, extract gas from the low-pressure group first. When the pressure of the low-pressure group drops to a certain value, start extracting gas from the medium-pressure group. Wait until the pressure of the medium-pressure group drops to a certain value. When the value is reached, gas is taken from the high-pressure group, and then from the three groups of gas cylinders together, until the pressure in the three groups of gas cylinders drops to the same level as the highest gas storage pressure of the vehicle-mounted gas cylinders, and then gas is stopped. If there are still cars that need to be refueled, gas is directly taken from the compressor exhaust pipeline. After the car is refueled, the compressor completes the refueling of the three groups of gas cylinders in the order of refueling, and then shuts down. The advantage of this working mode is that it can ensure that the gas cylinder group is refueled to the maximum, improve its utilization rate, and also make the car refueling at the fastest speed.
