Natural gas has the
lowest carbon content of any fossil fuel. For example, bituminous coal has a
carbon content of about 205 lbs.
CO2/MMBtu,
and subbituminous coal has a carbon content of about 213 lbs.
CO2/MMBtu.
In contrast, the carbon content of natural gas is only about 117 lbs.
CO2/MMBtu.
By increasing the use of natural gas
for the production of electricity in lieu of other fossil fuels,
the CO2 emissions per MMBtu are reduced.
On a Btu basis, burning one
million Btu of natural gas will release about 117 lbs. of CO2.
Oil has a higher CO2
emissions factor, about 161 lb. CO2/MMBtu
for distillate and about 174 lb. CO2/MMBtu
for residual fuel. For coal, the CO2
emissions factor is higher still and varies by rank of coal, ranging from
about 205 lb. CO2/MMBtu
for bituminous coal to about 213 lb. CO2/MMBtu
for subbituminous coal. Hence, when natural gas is used instead of coal, CO2 emissions per MMBtu are
reduced by over 40 percent. CO2 emissions
are also reduced when natural gas substitutes for oil, though by a lesser
percentage.
In addition, further
CO2
reductions can be achieved when the natural
gas is used in new high-efficiency applications. In the electric power
industry, natural gas combined cycle units have a substantially lower heat
rate (Btu consumed per kilowatt-hour of generation) than conventional
turbines and steam generators. This, coupled with the inherent lower carbon
content, can achieve
CO2
reductions of 60 percent or more on a kilowatt-hour
basis.
Increased electric utility use of natural
gas can take several forms:
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 |
Build new electric generating capacity
that uses natural gas |
| |
|
Repower existing electric generation
with natural gas |
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Employ natural gas co-firing systems.
Cofiring is a technology that uses gas burners inserted into the
boiler in a way that allows both coal and natural gas to be burned at
the same time |
| |
|
Seasonal use of natural gas at
high-carbon fuel stations |
| |
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Dispatch gas-fueled capacity ahead of
higher carbon fuel stations |
Adding gas burning capability to boilers designed for coal and oil allows
flexibility to displace those fuels and reduce GHG emissions as
opportunities may arise. Additionally, because natural gas has lower fuel
nitrogen than other fossil fuels and because of its combustion
characteristics, NOx emissions will also be reduced. SO2
and particulate emissions will also be less when burning natural gas.
Although most of the recent expansion of
natural gas use for electricity generation has involved large gas turbines
(often paired with steam turbines in combined-cycle configurations), future
growth will depend in part on the successful introduction of new generating
technologies, both large and small. Today's NGCC plants can have outputs of
over 500 MW and overall efficiencies surpassing 50% (calculated using the
lower heating value of natural gas), compared with 35-44% for today's
simple-cycle gas turbine plants — the
workhorse for peaking capacity. Further improvements are expected; by
2020, an NGCC plant should have an overall operating efficiency of over 55%.
Such a plant would produce about half the CO2 emissions of
today's coal plants without carbon capture.
Meanwhile, a variety of smaller, gas-based
generation technologies are starting to become more popular. Microturbines,
for example, were commercially introduced in 2000 and are now either
available or being developed in the 30-350 kW capacity range. They are
considered ideal for distributed generation applications because of their
flexibility in connection methods, their ability to be stacked in parallel
to serve larger loads, and their improved reliability. Typical applications
include supplying either stand-alone or backup power for customers ranging
from financial services and data processors
to hospitals and office buildings. Most micro-turbines feature an internal
heat exchanger, called a recuperator, which increases efficiency by
preheating inlet air. An additional heat exchanger can be added for combined
heat and power applications.
Fuel cells, which substitute an
electrochemical process for direct combustion of fuel, potentially offer
very high efficiency and low emissions in their use of natural gas. About
200 units of the first commercially available 200 kW phosphoric acid
fuel cell are now in service worldwide,
often providing on-site premium power for sensitive operations, such as
credit card processing. At the same time, research is under way to develop a
hybrid generation technology in the 1-20 MW range combining small gas
turbines and solid oxide fuel cells (which operate at higher temperatures
than phosphoric acid cells); this approach could potentially offer electric
service providers their highest-efficiency generating option.
In recent years, the
volatility and level of natural gas prices has affected the outlook of
natural gas-fired power as a CO2 reduction strategy. Throughout
much of the 1990s, U.S. supplies of natural gas were abundant and readily
available at prices generally below $3 per million Btu. Coupled with the low
capital cost, quick construction, and relatively straightforward permitting
of gas-fired powerplants, new gas-fired units came to dominate new capacity
additions by the late 1990s. However, in recent years natural gas prices
have been far higher and far more volatile, rendering much of this recent
new capacity uneconomic, and discouraging further additions of new gas-fired
capacity.
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