Pipeline Natural

Natural gas is a mixture of several gases, most of which are hydrocarbons, i.e., molecules consisting of carbon and hydrogen atoms. These hydrocarbons are also known as paraffins (or alkenes) belonging to the alkyl group. The chemical formula of paraffins has the format of CnH2n+2, which includes methane (CH4), ethane (C£Hg), propane (CjHg), etc. Each succeeding member of the series has one more carbon (C) atom with two more hydrogen (H) atoms. Hydrocarbons deficient in one hydrogen atom take the names methyl, ethyl, etc.

Although the composition of natural gas varies from gas field to gas field, it is always a mixture, predominantly of methane (CH4), ethane (^Hg), propane (C3Hg), and butane (C^Hjq), and usually small amounts of helium (He), carbon dioxide (CO2), and nitrogen (N2). In some cases, hydrogen sulfide (H^S), pentane (C5H12), or hexane (C6Hl4) may also be present.

In its original state, as produced at the wellhead, natural gas may be referred to as field gas or wet gas. The term refers to the presence of liquefied hydrocarbons, such as butane and pentane. Prior to distribution, compounds containing sulfur are removed, as are helium, water, and the bulk of the wet ends (gases). The remaining constituents are sometimes adjusted to produce what is commonly known as "pipeline quality" natural gas. Generally, it contains less than 7 lbm of water vapor per million cf (MMcf) and is lower still in the winter. For most applications, this amount is insignificant and the gas is considered to be dry.

The remaining mixture, which is transported through pipelines, is typically composed of methane, ethane, propane, nitrogen, and carbon dioxide. Whereas field gas may be composed of anywhere from 70 to 99% methane, pipeline gas will generally constitute between g0 and 95% methane.

Natural gas is non-toxic, colorless, and odorless. Because natural gas cannot be smelled, an odorant is added to allow for detection for safety. The odorant most commonly used, Mercaptan, is a hydrocarbon with a sulfur atom added. Roughly, 0.002% of sulfur is added. Although Mercaptan can be corrosive, this is too small an amount to cause corrosion in the pipelines or contribute to air pollution.

Natural gas composition variability in most applications is of minor concern, because such variations are usually insignificant. The Wobbe Index is a useful measure of energy delivered to a burner, since the energy varies directly with the heating value of the gas and inversely with the square root of the relative density (also known as specific gravity, or SG) as it flows through an orifice. Thus, two gases with different heating values and different relative densities, but with the same Wobbe Index, will deliver the same heat at the burner and, hence, yield the same performance. Since the Wobbe Index allows comparison of the volumetric energy content of different gas fuels at different temperatures, it is key to defining which fuels can be run in the same fuel system and when multiple manifolds or gas systems are required. It is generally accepted that a difference between two Wobbe Indices of up to 5% will allow full interchangeability of such fuel gases. But, note that the Wobbe Index may be based on higher or lower heating values, depending on application. The internationally used symbols for Wobbe Index, according to International Gas Union, are Ws based on HHV and W; based on LHV.

The composition of pipeline natural gas is determined by gas chromatography and summarized in a gas quality report. The data generally includes the mole percent of each component present on dry volume basis, real specific gravity and compressibility of the gas at reference pressure and temperature, and adjusted HHV in Btu/scf. The adjusted HHV value in Btu/scf is obtained by dividing the calculated ideal values in Btu/cf by the compressibility factor, which indicates the degree to which the real gas mixture departs from results, calculated using the ideal gas law. There are several computer programs capable of doing this, that are available through the American Gas Association (A.G.A.).

For the composition of the pipeline gas, the computer program based on A.G.A. Transmission Measurement Committee Report No. 8 (A.G.A. Cat. #8806) defines the key gas parameters as shown in Tables 5-2 and 5-3:

Table 5-2 Key Gas Parameters

Component

Mole%

Molar Mass

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