Speight J.G. Natural Gas: A Basic Handbook
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7.7
Process Selection
189
7.7
1.
2.
3.
4.
5.
6.
7.
8.
Process Selection
Tail-gas cleanup requirements
Type and concentration
of
impurities
in
the sour gas
Specifications
for
the residue gas
Specifications for the acid gas
Temperature and pressure at which the sour gas is available
and at which the sweet gas must be delivered
Volume
of
gas
to
be processed
Hydrocarbon composition
of
the gas
Selectivity required for acid-gas removal
Decisions in selecting a gas-treating process can many times be sim-
plified by gas composition and operating conditions. High partial
pressures
(50
psi)
of
acid gases enhance the probability of using a
physical solvent. The presence
of
significant quantities
of
heavy
hydrocarbons in the feed discourages using physical solvents. Low
partial pressures
of
acid gases and low outlet specifications generally
require the use of amines for adequate treating.
In
general, the batch and amine processes are used for over 90%
of
all
onshore wellhead applications. Amine processes are preferred when
the lower operating cost, the chemical cost
for
this process is prohibi-
tive, and justifies the higher equipment cost. The key is the sulfur con-
tent
of
the feed gas, where
<
20-lb sulfur/day batch processes are more
economical, and
>
100-lb sulfur/day amine solutions are preferred
(Manning and Thompson, 1991).
7.8
References
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P.D.,
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Chapter
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Arnold,
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Sept.
25-27.
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E.E.
and Bhatia,
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1985,
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J.B.,
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and Lunsford,
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2000,
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79‘h
GPA Annual Convention, Atlanta, Georgia,
March
2000.
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I.,
1981,
Process
for
Scavenging Hydrogen Sulfide from Hydrocarbon
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Gall,
A.L.
and Gadelle,
D.,
2003,
“Technical and Commercial Evaluation of
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1945,
in
Chemistry
of
Coal Utilization,
Lowry,
H.H.
(ed.), John
Wiley
&
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Inc., New York
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pp.
947-1007.
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1998,
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Book,
1
lth
Edition, Gas Processors Suppliers
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P.
and Lokhandwala, K.A.,
2004,
“Advances in Membrane Materials
Provide New Gas Processing Solutions,” Proceedings, GPA Annual
Convention, New Orleans, Louisiana, March.
Hohlfeld, R.W.,
1979,
Oil
&
Gus
I.,
October
15,
pp.
129-130.
Hoochgesand, G.,
1970,
Industrial
and Engineering Chemistry,
62(7): 37-43.
Kattner,
J.E.,
Samuels,
A.,
and Wendt, R.P.,
1988,
Journal
of
Petroleum
Kohl,
A.L.
and Nielsen,
R.B.,
1997,
Gas
Purification,
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Technology,
40(9): 1237.
Company, Houston, Texas.
7.8
References
191
Kohl,
A.L.
and Riesenfeld, F.C.,
1985,
Gas Purification,
4‘h
Edition, Gulf
Publishing Company, Houston, Texas.
Lallemand,
F.
and Minkkinen,
A.,
2001,
“High Sour Gas Processing in an
Ever-Greener World,” Proceedings,
gth
GPA GCC Chapter Technical
Conference, Abu Dhabi. May
23.
Lokhandwala,
K.A.,
2000,
Fuel Gas Conditioning Process, United States
Lokhandwala,
K.A.
and
Jacobs,
M.L.,
2000,
“New Membrane Application
in
Gas Processing,” paper presented at the GPA Annual Convention,
Atlanta, GA (March,
2000).
Maddox, R.N.,
1974,
Gas and Liquid Sweetening,
2nd
Edition, Campbell
Patent No.
6,053,965.
Petroleum Series, Norman, Oklahoma.
Manning,
F.S.
and Thompson, R.E.,
1991,
Oil Field Processing ofPetroleurn.
Vol.
I:
Natural Gas,
Pennwell Publishing Company, Tulsa, Oklahoma.
Processing. Proceedings,”
Sh
GPA-GCC Chapter Technical Conference,
Bahrain. May
6.
Mokhatab,
S.,
Poe, W.A., and Speight, J.G.,
2006,
Handbook
of
Natural Gas
Transmission and Processing.
Elsevier, Amsterdam, The Netherlands.
Morgan, D.J.,
1994,
“Selection Criteria for Gas Sweetening,’’ Proceedings,
GPA Technical Meeting, GCC Chapter, Bahrain. November
30.
Newman,
S.A.
(ed.),
1985,
Acid and Sour Gas Treating Processes,
Gulf
Publishing Company, Houston, Texas.
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1994,
“Selecting Amines for Sweetening Units,”
Proceedings, GPA Regional Meeting, Tulsa, Oklahoma.
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1985,
“The Rectisol Wash: New Developments in
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Processes,
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1997,
“Methanol Simplifies Gas
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1988,
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1990,
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of
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1988,
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192
Chapter
7
Processes
Speight,
J.G.,
1993,
Gas Processing: Environmental Aspects
and
Methods,
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F.,
1963,
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61(33): 103-104.
Butterworth Heinemann, Oxford, England.
CHAPTER
8
Emissions Control and
Environmental Aspects
All the fossil fuels-coal, petroleum, and natural gas-release pollutants
into the atmosphere when burned to provide energy. However, natural
gas is the most environmentally friendly fossil fuel.
It
is cleaner
burning than coal or petroleum, because it contains less carbon than
its fossil fuel cousins. Natural gas also has less sulfur and nitrogen com-
pounds, and, when it is burned, it emits less ash particulates into the
air than coal or petroleum fuels.
Currently, natural gas represents about
24%
of
the energy consumed
in
the United States, and there are projections that this figure will
increase to about
28%
by
2020.
This increase is expected because
emissions
of
greenhouse gases are much lower with the consumption
of
natural gas relative to other fossil fuel consumption.
Thus, natural gas is an extremely important source
of
energy for
reducing pollution and maintaining a clean and healthy environ-
ment (Speight,
2003
and references cited therein; Mokhatab et al.,
2006
and references cited therein; Speight
2007
and references cited
therein). In addition to being an abundant and secure source
of
energy in the United States, the use
of
natural gas also offers several
environmental benefits over other sources
of
energy, particularly
other fossil fuels.
Of course, there are environmental concerns with the use of any fuel.
As with other fossil fuels, burning natural gas produces carbon
dioxide, which is the most important greenhouse gas. Many scientists
believe that increasing levels
of
carbon dioxide and other greenhouse
gases in the earth’s atmosphere are changing the global climate.
193
194
Chapter
8
Emissions Control and Environmental Aspects
As
with other fuels, natural gas also affects the environment when it
is produced, stored, and transported. Because natural gas is made up
mostly
of
methane (another greenhouse gas), small amounts of
methane can sometimes leak into the atmosphere from wells, storage
tanks, and pipelines. The natural gas industry is working to prevent
any methane from escaping. Exploring and drilling for natural gas
will always have some impact
on
land and marine habitats. But new
technologies have greatly reduced the number and size of areas dis-
turbed by drilling, sometimes called “footprints.” Satellites, global
positioning systems, remote sensing devices, and
3-D
and
4-D
seismic
technologies, make it possible
to
discover natural gas reserves while
drilling fewer wells. Plus, use
of
horizontal drilling and directional
drilling make it possible for a single well
to
produce gas from much
bigger areas.
Natural gas pipelines and storage facilities have a very good safety
record. This is very important, because when natural gas leaks it can
cause explosions. Because raw natural gas has
no
odor, natural gas
companies add an odiferous chemical (usually a mercaptan,
R-SH)
to
the gas
it
so
that a leak can easily be detected.
Once brought from underground (Chapter
4),
the natural gas is
refined
to
remove impurities, such as water, other gases, sand, and
other compounds (Chapter
6
and Chapter
7).
Some hydrocarbons are
removed and sold separately, including propane and butane
(Chapter
6).
Other impurities, such as hydrogen sulfide, are also
removed and used to produce sulfur, which is then also sold sepa-
rately. After refining, the clean natural gas (methane) is transmitted
through a network of pipelines, thousands of miles
of
which exist in
the United States alone. From these pipelines, natural gas is delivered
to its point
of
use.
On
a relative basis, natural gas is the cleanest
of
all the fossil fuels.
Composed primarily
of
methane, the main products of the combus-
tion of natural gas are carbon dioxide and water vapor, the same com-
pounds we exhale when we breathe. Coal and petroleum are
composed
of
much more complex molecules, with a higher carbon
ratio and higher nitrogen and sulfur contents. Thus, when combusted,
coal and oil release higher levels
of
harmful emissions, including a
higher ratio of carbon emissions, nitrogen oxides (NO,), and sulfur
dioxide
(SO,).
Coal and fuel oil also release ash particles into the envi-
ronment, substances that do not burn but instead are carried into the
atmosphere and contribute to pollution. The combustion
of
natural
195
Table
8-1
Petroleum, and Coal
Emissions from the Combustion of Natural Gas,
Carbon
11
7,000 164,000 208,000
Dioxide
Carbon
40
33
208
Monoxide
Nitrogen
92 448 45 7
Oxides
Sulfur
1
1,122 2,591
Dioxide
Particulates
7
84 2,744
Mercury
0.000
0.007
0.016
gas,
on
the other hand, releases very small amounts of sulfur dioxide
and nitrogen oxides, virtually
no
ash or particulate matter, and lower
levels of carbon dioxide, carbon monoxide, and other reactive
hydrocarbons (Table
8-1).
The use of fossil fuels for energy contributes to several environmental
problems. Natural gas, as the cleanest of the fossil fuels, can be used
in many ways to help reduce the emissions of pollutants into the
atmosphere. Burning natural gas in the place of other fossil fuels
emits fewer harmful pollutants into the atmosphere, and an increased
reliance on natural gas can potentially reduce the emission of many
of these most harmful pollutants
(EIA,
2006).
Pollutants emitted in the United States, particularly from the combus-
tion of fossil fuels, have led to the development of many pressing
environmental problems. Natural gas, emitting fewer harmful chemi-
cals into the atmosphere than other fossil fuels, can help to
mitigate
some of these environmental issues.
196
Chapter
8
Emissions Control and Environmental Aspects
On
a unit basis, natural gas emits lower quantities
of
greenhouse
gases and criteria pollutants than other fossil fuels. This occurs in part
because natural gas is more easily fully combusted, and
in
part
because natural gas contains fewer impurities than any other fossil
fuel. For example,
U.S.
coal contains
1.6%
sulfur (a consumption-
weighted national average) by weight. The oil burned at electric
utility power plants ranges from
0.5
to
1.4%
sulfur. Diesel fuel has less
than
0.05%,
while the current national average for motor gasoline is
0.034%
sulfur. Comparatively, natural gas at the burner tip has less
than
0.0005%
sulfur compounds.
The amount of carbon dioxide produced for an equivalent amount
of
heat production varies substantially among the fossil fuels, with natural
gas producing the least. For the major fossil fuels, the amounts of
carbon dioxide produced for each billion Btu
of
heat energy extracted
are:
208,000
lb for coal,
164,000
lb for petroleum products, and
117,000
lb for natural gas (Table
8-1).
As
long as many countries have fossil-fuel-based economies, fossil-
fuel combustion will lead to environmental problems.
In
addition,
the venting or leaking
of
natural gas into the atmosphere can have a
significant effect with respect to greenhouse gases because methane,
the principal component
of
natural gas, is much more effective in
trapping these gases than carbon dioxide. The exploration, produc-
tion, and transmission
of
natural gas, as well, can have adverse effects
on
the environment.
This chapter addresses the many environmental aspects related to the
use
of
natural gas, including the environmental impact
of
natural gas
relative to other fossil fuels and some of the potential applications for
increased use of natural gas. These issues include:
1.
Greenhouse gas emissions
2.
Smog, air quality and acid rain
3.
Industrial and electric generation emissions
4.
Pollution from the transportation sector-natural gas vehicles
8.1
Greenhouse Gas Emissions
Global warming, or
the
greenhouse
erect,
is
an environmental issue
that deals with the potential for global climate change due to
8.1
Greenhouse
Gas
Emissions
197
increased levels of atmospheric
greenhouse
gases.
These are the gases in
the atmosphere that serve to regulate the amount of heat that is kept
close to the Earth’s surface.
It
is speculated that an increase in these
greenhouse gases will translate into increased temperatures around the
globe, which would result in many disastrous environmental effects.
The principle greenhouse gases include water vapor, carbon dioxide,
methane, nitrogen oxides, and some manufactures chemicals such as
chlorofluorocarbons. While most
of
these gases occur in the atmo-
sphere naturally, some scientists believe that levels have been
increasing due to the widespread burning of fossil fuels by growing
human populations. The reduction of greenhouse gas emissions has
become a primary focus of environmental programs in many (but not
all) countries around the world.
One of the principle greenhouse gases is carbon dioxide, and although
carbon dioxide does not trap heat as effectively as other greenhouse
gases (making it a less potent greenhouse gas), the sheer volume of
carbon dioxide emissions into the atmosphere is very high, particularly
from the burning of fossil fuels. Most of greenhouse gas emissions
come from carbon dioxide directly attributable to the combustion of
fossil fuels. Therefore, reducing carbon dioxide emissions can play a
huge role in combating the greenhouse effect and global warming. The
combustion of natural gas emits almost
30%
less carbon dioxide than
oil, and just under
45%
less carbon dioxide than coal.
One issue that has arisen with respect to natural gas and the green-
house effect is the fact that methane, the principle component
of
nat-
ural gas, is itself a very potent greenhouse gas. In fact, methane has
an ability to trap heat almost
21
times more effectively than carbon
dioxide.
Sources of methane emissions include the waste management and
operations industry, the agricultural industry, as well as leaks and
emissions from the crude oil natural and gas industry itself. It is felt
that the reduction in carbon dioxide emissions from increased nat-
ural gas use would strongly outweigh the detrimental effects of
increased methane emissions. Thus, the increased use of natural gas
in
the place of other, dirtier fossil fuels can serve
to
lessen
the
emission
of
greenhouse gases.
198
Chapter
8
Emissions
Control
and Environmental Aspects
Before describing the polluting nature of natural gas, it is worth
reviewing the composition of the gas as a means of understanding the
nature of the pollutants.
Briefly, natural gas is obtained principally from conventional crude
oil and non-associated gas reservoirs, and secondarily from coal beds,
tight sandstones, and Devonian shale. Some is also produced from
minor sources such as landfills. In the not too distant future, natural
gas may also be obtained from natural gas hydrate deposits located
beneath the sea floor in deep water
on
the continental shelves or
associated with thick subsurface permafrost zones in the Arctic.
While the primary constituent of natural gas is methane (CH,), it may
contain smaller amounts of other hydrocarbons, such as ethane
(C,H,) and various isomers of propane (C,H,), butane (C4Hlo), and
pentane (C5Hl,), as well as trace amounts of higher boiling hydrocar-
bons up to octane (C8H18). Non-hydrocarbon gases, such as carbon
dioxide (CO,), helium (He), hydrogen sulfide (H,S), nitrogen
(N,),
and
water vapor (H,O), may also be present (Chapter
4).
At the pressure
and temperature conditions of the source reservoir, natural gas may
occur as free gas (bubbles) or be dissolved in either crude oil or brine.
Pipeline-quality natural gas contains at least
80%
methane and has a
minimum heat content of
870
Btu/scf (Chapter
4).
Most pipeline nat-
ural gas significantly exceeds both minimum specifications. Because
natural gas has by far the lowest energy density of the common
hydrocarbon fuels, by volume (not weight) much more of
it
must be
used to provide a given amount of energy. Purified natural gas (specif-
ically methane and
not
the higher boiling constituents) is also much
less physically dense, weighing about half as much
(55%)
as the same
volume of dry air at the same pressure. It is consequently buoyant in
air, in which it is also combustible at concentrations of
5%
to
15%
by
volume.
8.2
Air
Pollutants and Greenhouse Gases
The earth’s atmosphere is a mixture primarily of the gases nitrogen
and oxygen, totaling
99%;
nearly
1%
water; and very small amounts
of other gases and substances, some of which are chemically reactive.
With the exception of oxygen, nitrogen, water, and the inert gases, all
constituents of air may be a source of concern owing either to their
potential health effects on humans, animals, and
plants,
or
to
their
influence
on
the climate.