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Lower Explosive Limit Upper Explosive Limit Toxicity Specifc Gravity Color Odor Melting Point Boiling Point Methane, CH 4 5% 15% Nontoxic Asphyxiant 0.56 None None -182.5° C -161.5° C Ethane, C 2 H 6 3% 12.4% Nontoxic Asphyxiant 1.038 None None -182.8° C -88.6° C Propane, C 3 H 8 2.1% 9.5% Nontoxic Asphyxiant 1.522 None None -188° C -42.04° C Butane, C 4 H 10 1.8% 8.4% Nontoxic Asphyxiant 2.006 None None -138° C -0.5° C Pentane, C 5 H 12 1.5% 7.8% Irritant Asphyxiant 2.5 None Gasoline- like -129.7° C 36.1° C The Basics Natural gas is a naturally occurring gas mixture, composed mostly of methane, with varying amounts of impurities consisting of ethane, nitrogen, pro- pane, carbon dioxide, butane, pen- tane, oxygen, sulfur compounds, and water. Natural gas found in gas wells and condensate wells is called nonas- sociated gas, is not formed in conjunc- tion with crude oil, and is in a gaseous or semi-liquid state. Natural gas in oil wells is called associated gas, formed in conjunction with crude oil, and can be separate or dissolved in the crude oil. Domestic Transportation Raw natural gas must be purifed into pipeline-quality, dry natural gas for transportation. The gas is processed through heaters and scrubbers to remove large particles, such as sand. Further processing removes water, sulfur, carbon dioxide, oil, and natural gas liquids (NGLs) consisting of pro- pane, ethane, butane, and others. Transmission and distribution pipelines then transport the clean gas to the end user. Interstate pipelines use 24- and 36-inch diameter pipe to transport nat- ural gas at pressures ranging between 200 to 1,500 pounds per square inch. According to the Association of Oil Pipe Lines and the American Petroleum In stitute, in the U.S. there are approxi- mately 300,000 miles of interstate and intrastate transmission pipelines and 2.1 million miles of distribution pipelines that carry gas directly to the consumer. Global Transport Natural gas must be cooled and con- densed into liquefed natural gas (LNG) to transport it globally by sea, as in its liquid state, LNG takes up 1/600 th of the space than does its gaseous form at ambient temperature and pressure. The gas is cooled to -256 degrees F, converting it into a cryogenic liquid. During liquefaction, the natural gas is further purifed, resulting in nearly pure methane. The Gases Methane: a colorless, odorless gas pro- duced biologically through anaerobic, bacterial decomposition. It can also be produced through technological and synthetic processes. It contains one carbon atom surrounded by four hydrogen atoms. It is nontoxic, but is an asphyxiant at high concentrations. It can cause frostbite and severe cryo- genic burns in its liquid form. The gas is combustible at concentrations between fve and 15 percent. Methane is also a greenhouse gas 20 times more potent than carbon dioxide at trapping heat in the atmosphere, accounting for approximately 16 percent of all green- house gases. However, it produces fewer greenhouse gases than oil or coal when burned, making it an attractive alternative fuel. Primar y methane sources are not renewable; however, there are some renewable secondary sources, such as manure processing and landflls. Ethane: a nonrenewable, odorless, and colorless gas composed of two carbon and six hydrogen atoms. It is a clean- burning fuel that is explosive at con- centrations between 3.0 and 12.4 per- cent. It is a byproduct of natural gas purifcation and crude oil refning. It is the most abundant of the NGLs and is often blended in LPG to increase energy output. The shale gas boom has increased ethane supplies, reduced the price, and stimulated an interest in exporting the gas. Starting in early 2016, the U.S. will begin exporting 240,000 barrels of ethane a day. Propane: a nontoxic, colorless, and vir- tually odorless gas used for heating, cooking, and plastic manufacturing. Propane is a nonrenewable energy source and is made up of three carbon atoms and eight hydrogen atoms. Propane is produced from crude oil refning, in addition to being a compo- nent of natural gas. It is easily liquefed and stored at about 150 pounds per square inch (psi) or at -45 degrees F, and is commonly known as liquefed petro- leum gas (LPG). It can cause severe frostbite in its liquid form. 12 Proceedings Fall 2015 www.uscg.mil/proceedings