Natural gas entering a CNG refueling station must undergo purification and drying before or after compression, specifically desulfurization, dehydrogenation, and dehydration. Desulfurization removes acidic gases such as hydrogen sulfide from the natural gas to prevent corrosion of equipment and pipelines and hydrogen embrittlement of steel cylinders. Dehydrogenation removes light hydrocarbons from the natural gas, ensuring that the ethane and heavy alkane content is less than 3% to prevent abnormal engine ignition and combustion. Dehydration removes moisture from the natural gas to prevent ice blockage in the CNG supply system during depressurization, expansion, and cooling.
Natural gas dehydration is the most critical step in the pretreatment process. CNG refueling stations typically use dual-tower dryers for dehydration. Based on the location of the dehydration unit within the station, dehydration methods can be categorized into low-pressure, medium-pressure, and high-pressure methods. High-pressure dehydration, if the related technical issues regarding the quality of components and valves are resolved, is a superior dehydration solution due to its compact structure and excellent dehydration effect.
Natural gas, after purification and drying, must meet the standards for vehicle-grade natural gas before it can be filled into gas cylinders at gas stations, sold directly, or used to refuel CNG vehicles. my country has established a national standard for "Compressed Natural Gas for Vehicles" (GB 18047-2000). This standard imposes very strict limits on the sulfur and water content of CNG for vehicles, requiring hydrogen sulfide content to be less than 15 mg/m³ and a water dew point 5°C lower than the lowest ambient temperature under the maximum operating pressure.
