Compressed Natural Gas (CNG) is a fuel derived from conventional natural gas, predominantly composed of methane (CHâ‚„). The gas is physically altered for practical storage and use. The “compressed” aspect refers to reducing the gas volume by forcing it into high-pressure storage vessels. This conversion makes the gas dense enough to be stored onboard vehicles, where its primary application is as an alternative transportation fuel.
The Primary Source Material
The raw material for CNG, natural gas, originates from deep within the Earth’s crust as a fossil fuel. It forms over millions of years from the decomposition of ancient organic matter, such as marine plants, buried under layers of sediment. Intense heat and pressure transform this material into hydrocarbons, which become trapped in porous rock formations beneath impermeable rock layers.
Extraction begins with drilling wells into these underground reservoirs. Conventional gas deposits are found in permeable rock where the gas flows easily once a well is drilled. However, a growing portion of the supply comes from unconventional sources, where the gas is trapped tightly within low-permeability rock, such as shale formations.
Accessing these unconventional reserves requires specialized techniques like hydraulic fracturing, commonly known as fracking. This method involves injecting high-pressure mixtures of water, sand, and chemicals into the wellbore to create rock fractures. These pathways allow the trapped methane to escape and flow up to the surface for collection and processing.
Transforming Gas into CNG
Before compression, the raw natural gas stream must undergo a rigorous purification process. The extracted gas contains impurities like water vapor, heavier hydrocarbons, and acidic gases such as hydrogen sulfide and carbon dioxide. These contaminants must be removed to prevent corrosion and ensure the final fuel meets quality standards.
This industrial processing involves steps like dehydration to remove water and acid gas removal, or “sweetening,” to eliminate corrosive sulfur compounds. Once purified, the gas stream consists almost entirely of methane, making it suitable for high-pressure applications.
Powerful compressors reduce the gas volume to less than one percent of its original state. This compression raises the pressure to a range of 200 to 250 bar (about 2,900 to 3,600 pounds per square inch). This high-pressure state allows a sufficient quantity of fuel to be stored efficiently in vehicle tanks.
Renewable Sources of CNG
CNG can also be produced from renewable sources, resulting in Renewable Natural Gas (RNG) or biomethane. RNG starts as biogas, generated through the natural process of anaerobic digestion. This decomposition occurs when microorganisms break down organic materials in oxygen-free environments.
Primary sources for this organic waste include municipal landfills, wastewater treatment plants, and agricultural operations. The raw biogas produced is a mixture of roughly 60% methane and 40% carbon dioxide, along with trace contaminants. This composition is not suitable for direct use as a vehicle fuel.
To create pipeline-quality RNG, the biogas must be upgraded to remove carbon dioxide, water, and other impurities. This purification results in biomethane, a gas with a methane concentration of 90% or greater. This biomethane is then compressed to the same high pressures as conventional CNG, creating bio-CNG for use in the transport sector.