Compressed Natural Gas (CNG) has become a widely used alternative fuel due to its cost-effectiveness and environmentally friendly attributes. It is natural gas under high pressure, used in vehicles as a clean alternative to gasoline or diesel. This article will provide a detailed explanation of how CNG is made, step by step, starting from natural gas extraction to its compression and storage.
1. What is Compressed Natural Gas?
Compressed Natural Gas, or CNG, is natural gas that has been pressurized and stored in a compact form. The main component of CNG is methane (CH₄), which burns cleaner than gasoline and diesel, reducing greenhouse gas emissions.
Natural gas is typically extracted from underground reserves, then processed and compressed for transportation and use. CNG is stored in high-pressure tanks and is widely used in vehicles because it emits fewer pollutants than other fossil fuels.
2. Extracting Natural Gas
Natural gas is extracted from the earth through drilling operations. There are two primary sources for natural gas extraction:
Conventional Natural Gas Wells:
These are found in large underground reservoirs. The gas is often trapped under layers of rock and can be extracted using standard drilling techniques.
Shale Gas:
This is obtained through a process called hydraulic fracturing, or “fracking.” Shale rock formations contain natural gas that requires high-pressure fluid injection to release the gas from the rock layers.
Once extracted, the raw natural gas is composed of methane along with other gases, including ethane, propane, butane, and impurities such as water vapor, sulfur compounds, and carbon dioxide.
3. Processing Natural Gas
Before natural gas can be compressed, it must be processed to remove impurities and separate its different components. The steps involved in processing natural gas include:
Removal of Impurities
Raw natural gas extracted from wells often contains water vapor, sulfur compounds, and carbon dioxide, which need to be removed. The removal process typically involves:
Dehydration: This step removes water vapor using either glycol dehydration or solid desiccant drying.
Sulfur and CO₂ Removal: Acid gases such as hydrogen sulfide (H₂S) and carbon dioxide (CO₂) are removed through chemical processes using amine solutions or membranes. This step ensures that the natural gas meets pipeline quality standards and is free from harmful contaminants.
Separation of Hydrocarbons
Natural gas also contains other hydrocarbons such as ethane, propane, butane, and even small amounts of heavier hydrocarbons. These components are separated from the methane to create “pipeline-quality” natural gas. This process occurs through:
Cryogenic Distillation: This method cools the gas to extremely low temperatures to separate the various hydrocarbon components based on their boiling points.
Fractionation: In this process, hydrocarbons are further separated into individual components such as ethane, propane, and butane, which can be sold separately or used for other purposes.
After processing, the natural gas is predominantly methane and is ready for compression.
4. Compression of Natural Gas
Once natural gas is processed and purified, it is ready to be compressed to make CNG. The compression process involves:
Multi-Stage Compression
The natural gas is pressurized using a multi-stage compression system. This system gradually increases the gas pressure while minimizing the risk of overheating. Compression typically occurs in three to five stages, depending on the desired final pressure.
Stage 1: The gas enters the first compression chamber, where it is pressurized slightly above atmospheric pressure.
Stage 2: The gas is cooled, then enters the second chamber, where the pressure is increased further.
Stage 3 and Beyond: The compression process continues, with the gas being pressurized in subsequent chambers until it reaches the required high pressure of approximately 200 to 250 bar (3,000 to 3,600 psi).
Cooling and Intercooling
During the compression process, the gas heats up as its pressure increases. To maintain safety and efficiency, intercoolers are used to reduce the temperature of the gas between compression stages. This cooling prevents equipment from overheating and ensures efficient compression.
Final Compression Stage
Once the gas reaches its target pressure (200-250 bar), it becomes compressed natural gas. The compressed gas is now ready for storage in high-pressure cylinders, either for direct use in vehicles or for further distribution to CNG stations.
5. Storing Compressed Natural Gas
After compression, CNG is stored in specially designed storage containers. These containers are designed to withstand the high pressure of compressed gas and are made from materials such as:
Steel Cylinders: These heavy-duty containers are designed for high-pressure storage and are commonly used for bulk storage of CNG at filling stations.
Composite Cylinders: These are made from lightweight materials such as aluminum or carbon fiber, reducing the overall weight and making them ideal for vehicle use. Composite cylinders offer improved safety and higher strength compared to steel cylinders.
SEE ALSO: Why Is Natural Gas Compressed?
6. Transportation and Distribution of CNG
Once the natural gas is compressed, it needs to be transported to CNG filling stations or used directly in vehicles. There are several methods of transporting CNG:
Pipelines
Compressed natural gas can be transported via pipeline networks directly to refueling stations or industrial facilities that use CNG. Pipeline transport is the most efficient and cost-effective way to move large volumes of natural gas over long distances.
Trailer Trucks
For areas without pipeline access, CNG can be transported in high-pressure trailer trucks. These trucks carry multiple CNG cylinders and deliver the compressed gas to refueling stations or other facilities.
Mother-Daughter Systems
This system is used in locations where pipeline access is limited. The “mother” station compresses and stores CNG, which is then transported by truck to a “daughter” station, where the gas is transferred for refueling.
7. Uses of Compressed Natural Gas
CNG has many applications, particularly in transportation, where it is used to power vehicles that run on natural gas. CNG-powered vehicles are popular for the following reasons:
Lower Emissions: CNG burns cleaner than gasoline or diesel, reducing harmful emissions such as carbon monoxide, nitrogen oxides, and particulate matter.
Cost Savings: CNG is generally cheaper than gasoline or diesel, leading to significant cost savings for fleets and individual vehicle owners.
Energy Security: Using CNG reduces dependence on imported oil, contributing to greater energy independence for countries with natural gas reserves.
In addition to transportation, CNG is used in industrial applications, heating, and electricity generation. Its versatility and cleaner combustion make it a favorable choice for various sectors.
8. Environmental Benefits of Compressed Natural Gas
CNG offers numerous environmental benefits, making it an attractive alternative to conventional fuels:
Reduction of Greenhouse Gases:
Because methane burns more efficiently than gasoline or diesel, it produces fewer carbon dioxide emissions, helping to mitigate climate change.
Lower Toxic Emissions:
CNG vehicles produce fewer pollutants such as nitrogen oxides, sulfur dioxide, and particulate matter, improving air quality.
Decreased Dependency on Oil:
As CNG is derived from natural gas, it reduces reliance on oil-based fuels, contributing to more diversified energy sources.
Conclusion
Compressed Natural Gas (CNG) is made through a multi-step process that begins with the extraction and processing of natural gas, followed by its compression and storage in high-pressure tanks. CNG serves as a cleaner, more cost-effective alternative to conventional fuels, with a wide range of uses in transportation and industry.
The process of making CNG involves extracting natural gas, removing impurities, compressing it in multiple stages, and storing it under high pressure. The growing adoption of CNG contributes to reduced emissions and greater energy security. As awareness of environmental sustainability continues to grow, CNG remains a vital component of the global energy landscape.
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