Residue gas is the final, purified product resulting from the processing of raw natural gas extracted from underground reservoirs. Often called sales gas or pipeline-quality natural gas, it is the marketable fuel stream ready for commercial distribution. It is the material remaining after valuable hydrocarbon liquids and various impurities have been stripped away at a processing plant. This refined gas stream is primarily composed of methane, making it suitable for injection into long-distance transmission networks.
How Residue Gas is Separated from Raw Gas
The process begins with raw natural gas, a complex mixture containing water, contaminants, and heavier hydrocarbons. Initial processing separates free liquids like water and crude oil condensates using specialized vessels at the wellhead. The gas is then moved to a processing plant for deeper purification, ensuring safety and commercial viability.
A significant step involves removing acid gases, primarily hydrogen sulfide (H₂S) and carbon dioxide (CO₂), through gas sweetening, often using amine solutions. H₂S is highly corrosive and toxic, making its removal mandatory to protect pipeline infrastructure. Water vapor is also removed through dehydration, typically using glycol, to prevent the formation of hydrates that can cause pipeline blockages.
The final major stage is the extraction of Natural Gas Liquids (NGLs), including ethane, propane, and butanes. These heavier components are separated from the methane stream, often using cryogenic technology that rapidly cools the gas. NGL removal is necessary to meet pipeline specifications and to capture these compounds for sale as high-value petrochemical feedstocks. The clean, dry gas that remains after these processes is the residue gas.
Chemical Composition and Quality Standards
Residue gas consists of methane (CH₄) at concentrations that often exceed 90% by volume. This high methane content dictates the gas’s heating value, which measures its energy output. Pipeline operators require a specific heating value, commonly 950 to 1,150 British thermal units (BTU) per standard cubic foot, to ensure consistent performance for end-users.
Pipeline specifications govern the allowable concentrations of non-methane components remaining in the residue gas. The gas must be considered “dry,” meaning its water vapor content is limited, often to a maximum of six or seven pounds per million cubic feet (lbs/MMcf). This low moisture level prevents internal corrosion and the formation of hydrates within the transmission lines.
Allowable levels for inert gases like nitrogen and carbon dioxide are tightly controlled, limited to a few percent by volume. Nitrogen lowers the overall heating value of the fuel, while excessive carbon dioxide can lead to corrosion in the presence of water.
Distribution and Primary Energy Applications
Once the residue gas meets quality specifications, it is injected into high-pressure transmission pipelines for long-distance transport. These pipelines move the gas from processing facilities to market hubs, where it is delivered to local distribution companies and large industrial consumers. Before the gas enters the local distribution system, a chemical odorant, usually mercaptan, is added for safety purposes.
The purified residue gas serves as a foundational source of energy across several economic sectors. A significant use is in electricity generation, fueling natural gas power plants, particularly efficient combined-cycle units. It is also a primary energy source for industrial processes, providing the necessary heat for manufacturing goods and operating large-scale equipment.
In the residential and commercial sectors, residue gas is widely used for space heating, water heating, and cooking appliances. Its clean-burning characteristics and efficient delivery make it a popular choice for consumers.