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Natural Gas Definitions, Measurement and Energy Content

Natural gas is a fuel composed primarily of methane (CH4) but can also contain up to 20% of other light hydrocarbons such as ethane, propane and butane and other trace gases.  Although the differences in energy content and trace gases tend to be small, the composition of natural gas can vary depending on the source of natural gas and the refining process used by natural gas producers to meet pipeline and utility requirements.  More information about the composition of natural gas can be found from the natural gas industry here.


There is no single standard to define natural gas sold to consumers; therefore buying natural can be confusing because of the many different ways it is used, produced, measured and sold.  Terms such as Btus, decatherms, standard cubic feet and gallons of gasoline equivalent are used in different parts of the natural gas industry and the natural gas from one utility can vary slightly from another utility or over different seasons due to changes in the source of the natural gas supplies and the utility or pipeline company's own requirements.  The Energy Information Agency has a useful description and calculator for natural gas units here.  


The following terms are frequently used in association with various parts of the natural gas network in Wisconsin:


Btus or mmBtus:  Natural gas is sold in large commodity markets, such as the New York Mercantile Exchange (NYMEX), in energy units of mmBtus, or one million Btus ("m" is the Roman symbol for one thousand, and a thousand thousands equals one million).  One Btu is equivalent to the amount of energy needed to raise the temperature of one pound of water one degree Fahrenheit. A large household furnace in Wisconsin can burn around 100,000 Btu's in an hour when running at full output, so an mmBtu represents the amount of natural gas used to heat a large or leaky house for one very cold night.  The average residential customer in Wisconsin used approximately 81.5 mmBtu (or 815 therms) of natural gas in 2009, according to the Wisconsin Energy Statistics 2010 publication of the State Energy Office. The amount of residential natural gas used in a year will vary with weather, but the long term trend is one of decreasing use as houses and heating appliances have become more energy efficient.  Recent market prices for natural gas have been around $3 or less per mmBtu, but have risen to over $10 per mmBtu as recently as 2008.


Therms:  Traditionally, natural gas is sold by Wisconsin utilities in units called therms which represent 100,000 British Thermal Units (Btus).  A decatherm is ten therms and is equal to one million Btus. Your bill for residential gas service is typically measured in 100 cubic feet but a heat factor adjustment is applied to this volume for therm usage.


Standard Cubic Feet (SCF):  Natural gas production is often measured in standard cubic feet which represents the volume of a gas at 60 degrees Fahrenheit and one atmosphere of pressure (approx. 14 pounds per square inch).  A large natural gas well can produce as much as a billion cubic feet (BCF) or more of natural gas over its productive life.  "Raw" natural gas from a well will typically require processing to remove impurities such as water vapor, carbon dioxide, and sulfur before sale.  The entire U.S. economy uses approximately 20 trillion cubic feet (TCF) of natural gas.  The energy contained in a standard cubic foot of natural gas will vary somewhat depending on the exact concentration of methane as well as the lesser hydrocarbons or trace gases that can be present in natural gas.  In 2010, the average heat (or energy) content of natural gas sold in the U.S. was 1,024 Btus per cubic foot.  Thus, a therm (100,000 Btus) of natural gas in 2010 contained an average of approximately 97 cubic feet of natural gas.


Compressed Natural Gas (CNG):  The required unit of sale for motor vehicle CNG is the Gasoline Gallon Equivalent(GGE), which is defined as 5.66 lbs. of CNG. CNG provided for vehicles is typically dispensed at pressures of 3,000 pounds per square inch (PSI) or 3,600 PSI.  The high pressure of CNG allows more natural gas, and therefore energy, to be stored in a compact container. Despite the high pressures, CNG still has less energy density (amount of energy in a given unit of space or weight) than gasoline.  For example, a CNG tank with an actual volume of 20 gallons (also called the water volume) at a pressure of 3,600 PSI will provide the energy equivalent to approximately 6 gallons of gasoline.


Liquified Natural Gas (LNG):  LNG is the liquid form of natural gas that occurs when natural gas is cooled to below negative 260 degrees Fahrenheit.  LNG is not compressed, but must be kept very cold.  When it evaporates, LNG expands to 600 times its volume.  It takes more processing and energy to liquify natural gas than to compress it, but for a given amount of space LNG holds over twice the energy of CNG.  LNG, however, still has a lower energy density than diesel fuel, and requires a more complex cryogenic tank to keep the natural gas in liquid form.  


Gallons of Gasoline Equivalent (GGE):  CNG is sold in units of GGE that represent approximately the same energy content as gasoline.  CNG meters use mass flow sensors to calculate the mass of natural gas and then converting this mass into an equivalent energy in a gallon of gasoline.  One GGE of CNG is equal to 5.66 Lbs of natural gas.  The variation in the energy content of natural gas per unit of volume, as discussed under SCF above, will also result in a minor variation in the energy content per unit mass.  This is because the different minor constituents of natural gas, such as propane and butane, have slightly different energy contents per unit mass.  The energy content of gasoline, however, also varies slightly depending on the specific blend used in different regions for air quality or seasonal requirements.  The conversion factor of 5.66 pounds of natural gas to one gallon of gasoline was developed by the National Conference of Weights and Measure, and published by the National Institute of Standards and Technology (NIST) in NIST Handbook 44. For state road taxes purposes, a GGE is equal to 1.25 therms.


Gallons of Diesel Equivalent (GDE):  LNG for motor vehicles is currently required to be sold by weight. Work is commencing to develop a standard for a Diesel Gallon Equivalent, since LNG is used primarily in heavy trucks that would otherwise use diesel.  The same amount of natural gas will have a lower GDE number because diesel fuel has more energy than gasoline.  The calculation of GGE and GDE, however, is based on the same principle: equivalent amounts of energy.


Finally, all of the above definitions represent the common units used in the U.S.  The energy content, volume and mass can all be converted to equivalent metric units (e.g., megajoules, liters and kilograms) using many different online calculators.




Last Modified:   3/20/2013 10:43:41 AM   
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Wisconsin Office of Energy Innovation
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Madison, Wisconsin  53705
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