Energy Solutions to Air Pollution and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 313

The use of CCS may reduce CO 2 (g) emissions from
the smokestacks of coal-fired power plants by 85 to
90 percent or more. However, the CCS process has no
effect on reducing other pollutants from the smokestack,
nor does it reduce emissions of any pollutant during
the mining and transport of the coal. In fact, because
a coal-CCS plant requires 25 percent more energy to
operate than does a conventional coal plant (Inter-
governmental Panel on Climate Change (IPCC), 2005),
a coal-CCS system requires 25 percent more coal min-
ing, transport, processing, and non-CO 2 (g) air pollution
per unit of net energy delivered than does a conven-
tional coal system. Furthermore, because of the signifi-
cant CO 2 (g) emissions due to coal mining and transport
and the time lag between planning and operation of
a coal plant with CCS equipment relative to that of
awind farm, a coal-CCS plant still emits about fifty
times more CO 2 (g) than does a wind farm producing
the same quantity of electric power (Jacobson, 2009).
As such, the use of coal-CCS represents an opportu-
nity cost in terms of air pollution and climate-relevant
emissions

CH 4 (g), and N 2 O(g) emissions of about 500 g-CO 2 (g)-
eq/kWh of electricity generated (Spath and Mann,
2000). About 74 percent of the emissions is due to
operating the plant, 25 percent is due to mining and
distributing the natural gas, and the rest is due to plant
construction and ammonia production. Although more
than 99 percent of the mass of all global warming-
relevant gases emitted by natural gas combustion is
CO 2 (g) mass, CO 2 (g) contributes to only about 88 per-
cent of natural gas's atmospheric warming. Almost 12
percent of the warming over a 100-year time frame is
due to the small amount of methane leaked during the
mining, transport, and use of natural gas when 1.4 per-
cent of all natural gas mined is leaked (Spath and Mann,
2000). Such a leakage rate is associated with conven-
tional sources of natural gas, such as wells containing
both oil and natural gas. The leakage rate from such
wells generally ranges from 1 to 4 percent.
However, when natural gas is extracted from shale
rock formations instead of wells, a larger percent of nat-
ural gas leaks. Shale is sedimentary rock composed of a
muddy mix of clay mineral flakes and small fragments
of quartz and calcite. Large shale formations contain-
ing natural gas can be found in eastern North Amer-
ica, close to population centers. The fraction of natural
gasproduced from shale is significant and increasing,
particularly in the United States. Extraction of natural
gas from shale requires large volumes of water forced
under pressure to fracture and refracture the rock to
increase the flow of natural gas. This process is referred
to as hydraulic fracturing (or fracking ). As the water
returns to the surface over days to weeks, it is accom-
panied by methane that escapes to the atmosphere.
One study estimates that 3.6 to 7.9 percent of all
natural gas mined from shale formations leaks to the
atmosphere (Howarth et al., 2011). If correct, these
numbers suggest that the 100-year global warming-
relevant natural gas emissions from shale are 600 to
700 g-CO 2 (g)-eq/kWh. This compares with 308 to 570
g-CO 2 (g)-eq/kWh for coal-CCS (accounting for both
life cycle emissions and the opportunity cost emissions
from planning-to-operation delays of coal-CCS plants)
and with more than 1,000 g-CO 2 (g)-eq/kWh for coal
without CCS (mining, transport, and use; Jacobson,
2009). Because natural gas carbon-equivalent emissions
fall between those of coal-CCS and coal without CCS
and because natural gas combustion emits air pollution,
it is not recommended as a future source of electricity in
aclean sustainable world. Because CNG requires even
more energy to produce than does natural gas without
compression, it too is not recommended.

compared with clean renewable energy

options.

13.1.3. Why Not Natural Gas?
Natural gas is a colorless, flammable gas made of about
90 percent methane plus other hydrocarbon gases that is
often found near petroleum deposits (Sections 3.6.4 and
5.2.1.5). Natural gas is combusted directly to produce
electricity. It is also compressed to less than 1 percent
of its gas volume to form compressed natural gas
(CNG) ,which is burned in vehicles for transportation.
Natural gas can be compressed and cooled to

162 ◦ C
to produce liquefied natural gas (LNG) .LNGisused
primarily to transport natural gas from a natural gas
field to a market or pipeline, where it is regassified.
Compressing and/or liquefying natural gas require more
energy input; thus, both are less efficient than is just
burning raw natural gas.
During combustion, natural gas emits CO 2 (g),
CH 4 (g), other hydrocarbon gases, NO x (g), SO x (g),
N 2 O(g), CO(g), and particulate matter, although gen-
erally in lower quantities [aside from CH 4 (g)] than the
combustion of most other fossil fuels. Among these pol-
lutants, CO 2 (g), CH 4 (g), N 2 O(g), and absorbing partic-
ulate matter contribute to global warming. All pollutants
contribute to human health problems.
The use of natural gas to generate electricity (with-
out compressing or liquefying it) in a combined cycle
power plant normally results in the combined CO 2 (g),

−