Page 48 - Synthetic Fuels Handbook

36 CHAPTER TWO

TABLE 2.1 Range of Composition of Natural Gas
Constituents Formula Amount present

Methane CH 4 70–90%
Ethane C H 6
2
Propane C H 8 0–20%
3
Butane C H 10
4
Pentane and higher hydrocarbons C 5 H 12 0–10%
Carbon dioxide CO 2 0–8%
Oxygen O 2 0–0.2%
Nitrogen N 2 0–5%
Hydrogen sulfide, carbonyl sulfide H S, COS 0–5%
2
Rare gases: argon, helium, neon, xenon A, He, Ne, Xe Trace

[BTX—benzene (C H ), toluene (C H CH ), and the xylene (CH C H CH )] can also be
6
6
3
3
5
6
4
6
3
present, raising safety issues due to their toxicity. The non–hydrocarbon gas portion of the
natural gas contains nitrogen (N ), carbon dioxide (CO ), helium (He), hydrogen sulfide
2
2
(H S), water vapor (H O), and other sulfur compounds such as carbonyl sulfide (COS) and
2
2
mercaptans (e.g., CH SH) and trace amounts of other gases. Carbon dioxide and hydrogen
3
sulfide are commonly referred to as acid gases since they form corrosive compounds in
the presence of water. Nitrogen, helium, and carbon dioxide are also referred to as diluents
since none of these burn, and thus they have no heating value. Mercury can also be pres-
ent either as a metal in vapor phase or as an organo-metallic compound in liquid fractions.
Concentration levels are generally very small, but even at very small concentration levels,
mercury can be detrimental due its toxicity and its corrosive properties (reaction with
aluminum alloys).
However, in its purest form, the natural gas that is delivered to the consumer is almost
pure methane and the remaining hydrocarbons and non-hydrocarbons have been removed
through refining. The non-hydrocarbon constituents include, but are not limited to, carbon
dioxide (CO ), hydrogen sulfide (H S), nitrogen (N ), and helium (He). Because natural
2
2
2
gas is colorless, shapeless, odorless, and tasteless in its pure form, it is not possible to see
or smell natural gas. Therefore, an odorant (a mercaptan also called a thiol that is a sulfur-
containing compound having the general formula R-SH) is added to natural gas for safety
reasons so that it can be smelled if there is a gas leak. A mercaptan is a chemical odorant
that smells a little like rotten eggs or skunk spray.
There are several general definitions that have been applied to natural gas that are based
on composition. For example, lean gas is gas in which methane is the major constituent.
On the other hand, wet gas contains considerable amounts of the higher molecular weight
hydrocarbons. To further define the terms dry and wet in quantitative measures, the term
3
dry natural gas indicates that there is less than 0.1 gal (1 gal, U.S. = 264.2 m ) of gasoline
3
3
3
vapor (higher molecular weight paraffins) per 1000 ft (1 ft = 0.028 m ). The term wet
natural gas indicates that there are such paraffins present in the gas, in fact more than
3
0.1 gal/1000 ft . Natural gas is considered dry when it is almost pure methane, having had
most of the other commonly associated hydrocarbons removed. When other hydrocarbons
are present, the natural gas is wet.
Sour gas contains hydrogen sulfide whereas sweet gas contains very little, if any, hydrogen
sulfide. Residue gas is natural gas from which the higher molecular weight hydrocarbons
have been extracted and casing head gas is derived from petroleum but is separated at the
separation facility at the wellhead.