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Why does natural gas have to be “dried” before being compressed into CNG?

All natural gas, as it comes out of the ground, contains some amount of moisture.  In a home cooking fuel application, consumers can observe moisture when they first light their stove and see a small amount of condensation at the burner tip.  On a natural gas stove, where the gas is uncompressed, a small amount of moisture is does not cause any problems.  When gas is compressed, moisture in the natural gas is a different story.

To understand why condensation becomes more significant when natural gas is compressed, think of a beach ball, filled with air at atmospheric pressure.  There will be some amount of moisture mixed into the air contained in the beach ball, but at atmospheric pressure the moisture will take up a very small amount of the total volume inside the ball.   Now imagine taking all the molecules present inside the beach ball and putting them into a space the size of a golf ball, thereby compressing them.  Since gaseous molecules can be compressed and liquid molecules cannot, the same amount of liquid that was present in the beach ball will still be present in the golf ball, but it will take up a larger portion of the total volume of the golf ball.

The same phenomena occurs when natural gas is compressed – the moisture that is present when natural gas comes out of the ground at atmospheric pressure comprises a much greater percentage of the total volume of the gas when it’s compressed to 3600 psig to be used as a vehicular fuel.   Exactly how much volume depends on the amount of moisture present when the gas comes out of the ground.

Natural gas utility companies typically provide pipeline quality gas at 7 pounds of water per million cubic feet (7#MMSCF) of gas, which is “dry” enough to avoid hydrate formation and freeze-ups for normal pipeline operations.  In a CNG fueling application, 7#MMSCF translates to a +52 degree Fahrenheit dew point at 3600 psig.  Because the relative volume of moisture in the gas is greater at pressure, potential issues for condensation, hydrate formation, and freezing can occur during fueling operations.

To avoid problems at a CNG station, the natural gas supply should be “dried” (have a percentage of the liquid molecules removed) to a pressure dew point (PDP) well below the minimum ambient temperature that will occur a the highest storage pressure.  Guidelines for how dry natural gas for vehicle fueling applications can be found in the Society of Automotive Engineers (SAE) fuel specification standard, J1616, and depends on the local conditions.  J1616 specifies that natural gas for fueling operations be dried to a local dew point temperature of -10 degrees Fahrenheit below the 99% winter design dry-bulb temperature as found in the 1989 American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) handbook at the maximum operating tank pressure.  The ASHRAE handbook should be referenced to find the recommended PDP in your area and ensure safe CNG vehicle fueling operations.

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