Page 66 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

• Net calorific value or net heat of combustion: is the net heat
generated when the water vapor in the gas does not condense
(water forms during combustion) and can be 10% lower.

Wobbe index measures the heating effect that a burner is exposed to during
combustion. A higher value means a greater thermal load on the burner.
Different gases with the same Wobbe index will impose the same load on
the burner. An excessively high value is a safety hazard as it can lead to
burner overheating and to excess production of carbon monoxide during
combustion.

Calorific value and Wobbe index can be adjusted by blending gas from
different sources as well as by addition or removal of nitrogen (N 2)

Methane Number is a value similar to octane value for gasoline, and is
important when the gas is used for internal combustion engines (as CNG).

Hydrogen Sulfide and Overall Sulfur Content both hydrogen sulfide (H 2S)
and total sulfur must be reduced. H 2S is toxic as well as corrosive for the
pipeline as it forms sulfuric acid (H 2SO 4) and should be kept as low as
possible. Typical maximum values are 5 mg per scm. of H 2S and total sulfur
at 10 mg per scm.

Mercury should be kept below 0.001 ppb (parts-per-billion) which is its
detectable limit. The goal is to limit emissions and to prevent damage to
equipment and pipelines by mercury amalgamation which would make
aluminum and other metals brittle.

Dew point is a temperature below which some of the hydrocarbons in the
gas might condense at pipeline pressure, forming liquid slugs which could
damage the pipeline. The gas must also be clear of all water vapor to
prevent the formation of methane hydrates within the gas processing plant or
within the sales gas transmission pipeline.

Particles and other substances: must be free of particulate solids and all
liquids to prevent erosion, corrosion or other damage to the pipeline and
satisfy limits on carbon dioxide, nitrogen, mercaptans etc.

Additives: when the natural gas is intended for domestic use,
Tetrahydrothiophene (THT) is added so that the otherwise odorless natural
gas can be detected in the event of a gas leak. The sulfurous smelling
substance added is equal to a sulfur content of 4-7 mg per scm.

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