Environmental Engineering Reference
In-Depth Information
scrubber and cryogenic cooling, which are both relatively expensive to operate (“Carbon
sequestration,” n.d.). It is expected that in the future, the development of other technologies,
for instance membranes, that could selectively separate the carbon dioxide from other gases
would reduce the capture expenses significantly.
There are two possible other options to increase the concentration of carbon dioxide and
facilitate its capture. These are precombustion with carbon dioxide capture and oxygen com-
bustion with postcombustion capture.
Precombustion capture is the removal of the carbon dioxide prior to combustion. The pro-
cess consists on converting the fuel into hydrogen gas (H 2 ) and carbon dioxide and then sepa-
rating the carbon dioxide using conventional scrubbing technology. The advantage in this case
is that the carbon dioxide is more concentrated and easier to separate from the hydrogen,
which can be burned in a boiler and produce only water vapor at the flue.
Fossil fuels can be converted into hydrogen and carbon dioxide by using steam reforming,
for liquid and gases, or by gasification of coal or biomass to produce syngas:
CH
+
H O
→+
CO
3 H (Steam reforming)
[8.20]
4
2
2
C
+
H O
→+
CO
H (Gasification)
[8.21]
2
2
Additional hydrogen can be produced via a water gas shift reaction:
CO
+
H O
→+
CO
H (Gas
shift reaction)
[8.22]
2
2
2
In oxygen combustion systems, oxygen is separated from nitrogen and used to burn the fuel
in an oxygen atmosphere. The by-products of the combustion process are mainly carbon diox-
ide and water that are separated by condensation.
Carbon storage
Geological sequestration
Once carbon dioxide is captured, a second problem is its permanent storage. Several ideas
have been suggested that primarily involve geologic sequestration in oil and gas reservoirs
(both depleted and active), coal beds, and saline formations. Injection of carbon dioxide in
oil reservoirs has been practiced for a while to enhance the recovery of petroleum. However,
it still needs to be proved if the concept will work for permanent storage of carbon dioxide.
Coal beds are typically porous and contain important amounts of methane adsorbed on the
internal surfaces. The idea is to inject carbon dioxide in these beds to displace and capture
the methane that then can be used as a fuel and at the same time permanently store carbon
dioxide. The third option is the injection of carbon dioxide in deep saline underground
formations, which is a concept that needs to be proved in the long run (“Carbon sequestra-
tion,” n.d.).
Unfortunately, sources of emission where the carbon dioxide is captured are not in the
vicinity of the geological formations where it could be stored; therefore, transportation
will be required. The use of pipelines is the most likely transportation method for big
emitters such us power plants. However, for facilities with not so massive production of
carbon dioxide, such as food-processing plants, installation of pipelines may not be cost
effective.
Search WWH ::




Custom Search