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Did You Smell That?



You see it in the news, “residents call in a natural gas odor.”

You open up your natural gas bill, if you smell natural gas, “leave the area immediately and then call ….” (If you are an Oklahoma Natural Gas customer, the number to call is 800-458-4251.)

If you are using propane, you will see similar notices and warnings, “if you smell gas…”

Most of us recognize the odor associated with propane and natural gas. And, you may have even noticed, that while similar the smell associated with natural gas is slightly different than that of propane.

If you have traveled outside of the United States, you may also have noticed that the odor associated with natural gas may be slightly different, yet similar as well. But, what is the odor and where does it come from.

Propane and natural gas are light hydrocarbon gases. Propane is hydrocarbon consisting of three carbon atoms and eight hydrogen atoms. It is considered an alkane because the carbons are linked by a single carbon-carbon bond. Natural gas is a mixture.

While the majority of the mixture is methane (a molecule that consists of one carbon and four hydrogen atoms) typically about 95 percent for residential natural gas, it can also contain ethane, propane, butanes, pentanes, and other molecules.

The properties of these molecules make them excellent fuel sources for heating and cooking, but they have a downside. They are flammable (which makes them a good fuel). In higher concentrations, they are considered asphyxiants. And, they are colorless and odorless.

But, we are familiar with a “natural gas odor.” The odor isn’t from the hydrocarbons in the fuel. The odor is from an odorant added to the fuel.

The odorant is added to help us detect a problem such as a leak or an extinguished pilot light before the concentrations of the gas in a confined space become hazardous or reach a source of ignition.

The United States began requiring the use of odorants in natural gas systems after a school explosion in New London Texas on March 18, 1937. This disaster destroyed the school and killed over 290 students and teachers. The cause of the accident was a leak of the odorless natural gas that went undetected. It was believed that had the gas contained an odorant, which some suppliers were using to help customers identify potential leaks, the disaster could have been prevented.

In addition to the natural gas that comes through the residential distribution system, propane and liquefied petroleum gas (LPG) also contain odorants. (It dependents upon the ultimate use of the fuel. You may have seen propane or LPG transports that are specifically labeled as non-odorized.)

The chemicals used for odorization are typically sulfur containing. The primary odorant for propane is ethyl mercaptan, and for natural gas, it is likely to be tert-butyl mercaptan (this has other common names as well tert-butanethiol or TBM or 2-Methyl Propane-2-Thiol).

But, depending upon the specific use, source, and distribution system other odorants such as dimethyl sulfide, isopropyl mercaptan, sec-butyl mercaptan, and methyl ethyl sulfide may also be used.

The advantage of these materials is that they stink, and humans can detect them at very low concentrations. The odor threshold for ethyl mercaptan is 0.001 part per million. Compare this odor threshold with that of ethanol, which is at 10.0 parts per million.

It is the sulfur in the compounds which gives it the stink. It is sulfur that gives hydrogen sulfide, its characteristic “rotten egg” odor.

And, it is the sulfur in the thiol, an organic compound linked with a sulfur and hydrogen an R-SH group, in the 2-butene-1-thiol, 3-methyl-1-butanethiol, and 2-quinolinemethanethiol which combined are the stinky elements in a skunk’s spray.

It is our sensitivity to these odors that allows the odorants to be used in very small quantities and helps to provide us with a built-in detection system.

Editor’s note: This is a series of science-related articles by author Frankie Wood-Black, Ph.D., REM, MBA, to appear in Mid-Week section of the Ponca City News. The author currently runs her own environmental consulting firm based in Ponca City, Sophic Pursuits, Inc., and also serves as a Physics Instructor and the Director for Process Technology at Northern Oklahoma College.

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