Natural gas, which is mostly methane, is difficult to store or transport efficiently in its natural gaseous state. Moving this energy source requires changing its physical state to significantly reduce its volume. This “bottling” process uses two distinct industrial methods: compression and liquefaction. Both methods make natural gas portable but differ vastly in technology, resulting physical state, and optimal applications.
Compressed Natural Gas (CNG)
Compressed Natural Gas (CNG) is created by pressurizing natural gas to extremely high levels, typically between 2,900 and 3,600 pounds per square inch (psi). This intense compression reduces the gas volume to less than one percent of its volume at normal atmospheric pressure. The gas remains in a gaseous state, allowing for practical storage in rigid, high-pressure cylinders or tanks.
CNG is highly suitable for use in natural gas vehicles (NGVs), such as city buses, taxis, and small delivery fleets. Since the infrastructure is less complex than liquefaction, CNG is also used for short-distance transport and local distribution where pipelines are absent. CNG vehicles are typically limited to shorter ranges due to the lower energy density compared to liquid fuels, requiring more frequent refueling.
Liquefied Natural Gas (LNG)
Liquefied Natural Gas (LNG) is produced by cooling natural gas to approximately -260° Fahrenheit (-162° Celsius). This cryogenic process converts the gas into a liquid state at near-atmospheric pressure. The cooling process is highly energy-intensive and requires complex, specialized equipment at the liquefaction plant.
The primary benefit of liquefaction is the dramatic volume reduction, shrinking the gas volume to about 1/600th of its original size. This massive compaction makes it economical to transport huge volumes of natural gas over long distances. LNG is stored and transported in specialized, super-insulated cryogenic tanks on massive ships designed for transoceanic voyages. Upon reaching its destination, the LNG is warmed back into its gaseous state, a process called regasification, before being distributed through local pipelines.
Comparing CNG and LNG Use Cases
The choice between compression and liquefaction depends largely on the volume of gas needing to be moved and the distance of the journey. CNG is the preferred method for smaller volumes and shorter transport distances, often within a range of about 2,500 miles. It requires a lower capital investment for the compression equipment and storage tanks compared to the complex facilities needed for liquefaction.
LNG is the clear solution for transcontinental or transoceanic transport where pipelines are not feasible. The extraordinary volume reduction achieved by liquefaction makes shipping massive quantities of energy economically viable over thousands of miles. LNG possesses a higher energy density than CNG, meaning a given volume of LNG contains roughly twice the energy of the same volume of CNG. This makes it ideal for heavy-duty, long-range transport applications like large trucks and marine vessels.
Safety and Environmental Considerations
Handling natural gas in its concentrated forms introduces specific safety challenges related to its physical state. CNG’s primary safety concern is the high pressure at which it is stored, which necessitates extremely robust storage cylinders. If CNG were to leak, the gas is lighter than air and quickly rises and disperses into the atmosphere, reducing the risk of fire or explosion at ground level.
LNG presents different hazards due to its extremely cold temperature, which can cause severe cryogenic burns or frostbite upon contact. In enclosed spaces, a large LNG spill can vaporize and create a cloud of cold gas that is initially heavier than air, posing a risk of asphyxiation before it warms and disperses.
While natural gas is generally considered a cleaner-burning fuel than coal or oil, a significant environmental concern for both CNG and LNG is methane leakage. Methane is a highly potent greenhouse gas, and any unintentional release during the production, processing, or transport stages diminishes the overall environmental benefit.