Compressed Natural Gas (CNG) is natural gas, primarily methane, stored at extremely high pressure. The gas is compressed to less than one percent of its original volume at standard atmospheric pressure, significantly increasing its energy density for practical use. This process transforms the low-density gas, typically sourced from pipelines, into a viable, compact fuel source, most commonly used in the transportation sector as an alternative to gasoline and diesel.
Sourcing and Purification of Natural Gas
The creation of CNG begins with sourcing raw natural gas, typically drawn from existing pipeline feeds, wellheads, or processing plants. This raw gas is a mixture of hydrocarbons that requires extensive conditioning and purification before it can be compressed into a high-quality fuel.
One significant step is the removal of water vapor, known as dehydration, which prevents corrosion and the formation of ice or hydrates within the compression machinery and storage vessels. Specialized processes like acid gas removal are then used to strip out corrosive impurities such as hydrogen sulfide (\(H_2S\)) and carbon dioxide (\(CO_2\)). Cleaning the gas ensures it is non-corrosive and maintains the integrity of the high-pressure components used later in the process.
The Mechanical Compression Stage
Once purified, the natural gas moves into the mechanical compression stage, the defining process for creating CNG. This stage uses specialized, multi-stage compressors, often of the reciprocating or centrifugal type, to incrementally increase the gas pressure. Compressing the gas in multiple stages is necessary to manage the extreme heat generated, as increasing pressure raises the gas temperature.
The process incorporates intercoolers between the compression stages to cool the gas back down, protecting the compressor components and maximizing efficiency. The goal is to achieve a pressure ranging from 200 to 250 bar (3,000 to 3,600 psi). Achieving this high pressure drastically reduces the gas volume, making it dense and volumetrically efficient enough to be stored in compact, specialized cylinders on vehicles as a motor fuel.
Delivery and Dispensing of CNG
The finished CNG product is stored in high-pressure vessels, such as large-capacity storage cascades or fixed cylindrical tanks. The highly pressurized gas is transported either directly through pipelines to a refueling station or via specialized tube trailers designed to handle the high internal pressure.
At the fueling station, the CNG is dispensed into the vehicle tank using a nozzle connected to a dispenser unit. In a fast-fill scenario, the gas is rapidly transferred from the station’s storage vessels into the vehicle’s onboard cylinders. The dispensing system often includes a cooling mechanism, as cooling the fuel helps ensure the maximum possible amount of gas is transferred despite the heat generated by the rapid transfer.