Compressed natural gas (CNG) is made by taking pipeline-quality natural gas, purifying it to at least 90% methane, and compressing it to pressures between 2,900 and 3,600 psi. The process transforms a gas that flows through ordinary pipelines into a dense, portable fuel suitable for vehicles and industrial use. Each step, from cleaning the raw gas to squeezing it into high-pressure cylinders, is designed to produce a fuel that burns cleanly and stores safely.
Starting Material: Natural Gas From the Ground
CNG begins as ordinary natural gas, which is mostly methane mixed with smaller amounts of ethane, propane, carbon dioxide, hydrogen sulfide, and water vapor. This raw gas comes from underground wells, and it’s far too impure to compress directly into fuel. The goal of the entire production chain is to strip away everything that isn’t methane, then pack what’s left into a much smaller volume.
Pipeline-quality natural gas typically contains 96 to 98% methane. That’s the standard CNG needs to meet before compression. Gas that falls short of this purity can corrode equipment, freeze at pressure valves, or burn unevenly in an engine.
Removing Water: The Dehydration Step
Water vapor is one of the first impurities to go. Even small amounts of moisture can form ice or hydrate crystals inside high-pressure equipment, blocking valves and damaging compressors. The most common dehydration method uses a chemical called glycol as an absorbent.
In an absorption column, wet natural gas enters from the bottom while a stream of dry glycol flows in from the top. As the two meet, water molecules dissolve into the glycol. Dry gas exits the top of the column. The now water-rich glycol flows out the bottom, gets depressurized in a flash tank to release any dissolved hydrocarbons, and then enters a heated still where the water boils off as steam at around 204°C (400°F). The regenerated glycol cools back down and cycles through the absorber again.
For applications that demand extremely dry gas, an additional pass through molecular sieves (tiny porous materials that trap water molecules by size) catches whatever the glycol absorber missed.
Sweetening: Removing Sulfur and CO₂
Raw natural gas often contains hydrogen sulfide and carbon dioxide, both of which are corrosive and, in the case of hydrogen sulfide, toxic. The process of removing these is called “sweetening.” Chemical solvents or solid adsorbents pull these acid gases out of the methane stream, producing a clean, non-corrosive gas that won’t eat through compressor parts or storage tanks over time.
Adding a Safety Odor
Pure methane is colorless and odorless, which makes leaks impossible to detect by smell alone. Before the gas enters the distribution system, a tiny amount of sulfur-based odorant is injected, typically at concentrations of just 1 to 10 parts per million. The most common odorants are tert-butyl mercaptan and dimethyl sulfide, both of which have a strong, distinctive “rotten egg” smell.
U.S. federal regulations require the odorant to be strong enough that a person with a normal sense of smell can detect a gas leak when methane reaches just one-fifth of its lower explosive limit, or roughly 0.88% methane by volume in the surrounding air. That’s a generous safety margin: natural gas won’t actually ignite unless it reaches about 5% concentration in air.
Compression: The Core of CNG Production
Once purified and odorized, the gas is ready for compression. This is the step that turns pipeline gas into CNG. Multi-stage compressors squeeze the gas in successive stages, cooling it between each one to manage the heat generated by compression. The final pressure typically lands between 2,900 and 3,200 psi (200 to 220 bar), though it can range up to 3,600 psi for leaner gas that’s almost entirely methane.
Compressing the gas this much shrinks its volume dramatically. At 3,000 psi, the same amount of natural gas that would fill a large room at atmospheric pressure fits inside a tank you can mount under a vehicle. Even so, CNG is less energy-dense than liquid fuels. About 5.66 pounds of CNG holds the same energy as one gallon of gasoline, meaning CNG tanks need to be larger (or more numerous) than a conventional gas tank to achieve similar driving range.
Storage Cylinders and Tank Types
Holding gas at 3,000+ psi requires seriously robust containers. CNG storage cylinders fall into four categories, each trading off weight against cost:
- Type 1: All-metal construction, usually steel or aluminum alloy. Heaviest and cheapest. Common in stationary storage and budget vehicle conversions.
- Type 2: Metal liner wrapped with composite fiber around the middle (hoop-wrapped). Lighter than Type 1, moderate cost.
- Type 3: Thin metal liner fully wrapped in composite fiber. Significantly lighter than Types 1 and 2.
- Type 4: Plastic liner fully wrapped in carbon or glass fiber composite. Up to 75% lighter than all-metal tanks, making them ideal for vehicles where weight matters. They’re also the most expensive.
Most modern CNG vehicles use Type 3 or Type 4 tanks to keep weight down while safely containing the extreme pressure.
How CNG Gets to Vehicles
CNG reaches vehicles through a network of fueling stations, which come in a few distinct configurations. Mother stations connect directly to a natural gas pipeline and run their own compressors on-site. They can fill vehicles directly and also load portable high-pressure containers (called mobile cascades) onto trucks for delivery elsewhere.
Daughter stations have no pipeline connection at all. They receive pre-compressed gas via those mobile cascades from a mother station and dispense it to vehicles. Online stations sit along a pipeline and compress gas on-site, similar to a mother station but without the capability to fill mobile cascades for transport. Daughter booster stations are a hybrid: they receive mobile cascades but also have a small compressor to squeeze out more of the stored gas, improving how much of each delivery actually reaches customers.
For home refueling, small compressor units can connect to a residential natural gas line and slowly fill a vehicle’s tank overnight, though the flow rate is much slower than a commercial station.
Why CNG Performs Differently Than Gasoline
The compression process doesn’t change the chemistry of the gas, but the resulting fuel has some notable performance characteristics. CNG has an octane rating above 120, well beyond the 84 to 93 range of standard gasoline. Higher octane means greater resistance to engine knock, which allows CNG engines to run at higher compression ratios and extract more work from each combustion cycle.
CNG also has a high autoignition temperature of about 1,200°F, roughly double that of gasoline at around 600°F. Combined with its narrow flammability range (it only burns between 5% and 15% concentration in air), this makes CNG less likely to ignite accidentally in a crash or spill. Unlike gasoline, which pools on the ground as a liquid, CNG disperses upward into the atmosphere if it escapes, reducing the risk of a ground-level fire.
The tradeoff is energy density. Pound for pound, CNG and gasoline carry comparable energy, but because CNG is a compressed gas rather than a liquid, it takes up more physical space. A CNG tank sized to match the range of a standard 15-gallon gas tank would be substantially larger and heavier, which is why CNG vehicles often sacrifice some trunk or bed space to accommodate their fuel storage.
Renewable CNG: Same Process, Different Source
CNG doesn’t have to start with fossil natural gas. Renewable natural gas, captured from landfills, wastewater treatment plants, or agricultural waste, can go through the same purification and compression process. Raw biogas from these sources typically contains only 45 to 65% methane, so it requires more aggressive upgrading to reach the 90%+ purity needed for compression. Once cleaned, it’s chemically identical to conventional CNG and works in the same vehicles and stations.