Greenhouse
Effect: Carbon Removal & Recovery
Carbon dioxide
(CO2) is estimated to contribute to about half the total global warming.
Therefore, most of the attention on greenhouse gases has been concentrated
on the analysis of technologies to reduce fossil-fuel CO2 emissions.
CO2 is an
unavoidable byproduct of the combustion of carbonaceous fuels. Emissions
can be reduced by energy conservation, i.e., by applying technologies that
produce the same level of energy service while using less energy. However,
energy conservation alone cannot halt the buildup of CO2, so scientists
are looking for other ways to reduce the level of gas in the atmosphere.
Because the level of emission depends upon the amount of carbon per unit
energy contained in the fuel, emissions can be reduced by fuel substitution,
e.g., substitute natural gas (low-CO2-emitting) for coal (high-CO2-emitting)
or better yet, use non-fossil fuels (non-CO2-emitting) technologies such
as hydroelectric power, nuclear power, solar power... Furthermore, non-fossil
energies can be used to recycle CO2 into low-CO2-emitting fuels by combining
CO2 with hydrogen to from hydrocarbon fuels such as methane or methanol.
Researchers are trying to boost the performance of catalysts that foster
such reactions. An alternate option is to store carbon in the form of carbon
black. In this scheme, a hybrid process converts "dirty" fuels
like coal into cleaner burning methanol, while it turns CO2 into a powder
that can be easily stored or used as a fuel. Because the process splits
a dirty fuel into two cleaner fuels (carbon black and methanol), it might
offer a new way to reduce CO2 emissions (such as using them in efficient
combined-cycle plants). An extended option would be to use only the methanol,
while storing the carbon black in abandoned mines, reducing CO2 by more
than 50%. This would likely increase the cost of electricity, but if global
warming proves negligible, the carbon could be removed and used as a fuel
at a later time.
Those above
methods will reduce the amount of CO2 released into the atmosphere but
do not remove the CO2 out of the atmosphere. CO2 removal can be done by
capturing CO2 directly from the atmosphere by plants or from the flue gases
being expelled by power plants and other sources that emit large quantities.
After capture takes place, the carbon dioxide must be disposed of or recycled
into other fuels.
Atmospheric
CO2 capture can be done by biological means. Reforestation is one possibility.
Tree plantations might help counter rising CO2 levels, but this would require
huge expanses of land. A seemingly better solution is farming the ocean.
Researchers are investigating the feasibility of developing huge, floating
farms made of kelp. On top the the kelp, a special type of algae will be
attached. The algae will take up CO2 and convert it to calcium carbonate.
These algae would grow heavier as they consumed CO2, and eventually the
entire farm would sink to the ocean floor, only to be replaced by a new
farm. In another scenario, the floating farms would be launched in an area
of the ocean with plentiful nutrients. As ocean current carried the farms
into low-nutrient areas, the farms would die and sink. The cost for developing
such unmanned ocean farms is expected to be relatively low. However, researchers
have experienced problems attaching the algae to the kelp. And, according
to early calculations, the farms would have to cover about one million
square miles of ocean surface in order to handle current CO2 emissions.
To remove
CO2 from the flue gas, chemical absorbent (either solid pellets or liquid)
is used. The CO2-laden absorbent is then collected and heated to free the
CO2 which then compressed in tanks. Studies by various groups of researchers
have produced similar results: The cost is high. Capturing and compressing
CO2 from a fossil fuel power plant for disposal would require a very large
fraction (up to 35%) of the electricity generated by a power plant.
Cost is not
the only problem. Another problem is disposing the collected CO2. One option
is to pump the CO2 into working or abandoned oil and gas wells. CO2 is
a suitable injection gas for wells, because it is highly soluble in oil
and thus reduces the oil's viscosity. The thinned oil releases thicker
deposits that might otherwise be trapped in rock pores. Researchers think
that with CO2 injection, there could be a net gain up to 50% in the oil
extracted from a given well. However, this technique is only feasible for
power plants located near oil and gas fields. An extended use of the technique
would be liquifying CO2 at a power plant far from the fields and transporting
the liquified CO2 to the wells for disposal.
Another option
is to store liquified CO2 at the bottom of deep ocean. In 1989, a Japanese
deep-sea research vessel found liquid CO2 in an apparently stable state
occurring naturally on the ocean floor. At a depth of about 3 km, liquid
CO2 is heavier than seawater and is likely to stay where it's put. Because
of the high pressure, CO2 and seawater form solid compounds in which CO2
molecules are contained within lattices formed from water molecules. It
is thought that CO2 could be safely stored at that depth.
None of the
scientists working in this field foresee a quick fix. Many of them think
technological breakthroughs are needed, but they also recognize that those
breakthroughs are not going to be easy to achieve. They just have to start
research and development as soon as possible. And hopefully, the fruits
will come soon.
References:
D.J. Wuebbles and J. Edmonds in "Primer on Greenhouse Gases"
(Lewis Publishers, 1991). "Science: Gets the CO2 Out", D. Normile,
Popular Science, pp 65-70 (Feb 1994).
