215
far, the two main uses for captured CO 2 appear to be the manufacture of carbonated
beverages and the re-pressurization of depleted oil and gas reservoirs for enhanced
hydrocarbon recovery operations.
There are some relatively simple ways to improve the efficiency of CCS. When
coal is burned in air, the concentration of carbon dioxide in the flue gases is generally less than 15% (Songolzadeh et al. 2014). Chemical absorbents are inefficient at
capturing low concentrations, but the levels of CO 2 in combustion gases can be
greatly increased by burning the coal in pure oxygen instead of air. This so-called
“oxy-combustion” process can produce concentrations of CO 2 of more than 80% in
the flue gases that can be easily captured. Of course, obtaining pure oxygen adds
another cost compared to burning in plain old air, so although the capture efficiency
is improved, the economics are not.
Storage of the captured carbon dioxide typically uses deep geologic formations
that will keep it isolated from the atmosphere (USDOE 2012). These include
depleted conventional oil and gas fields, which are known to have the capability of
trapping gases underground over geologic time, coal seams that hold the CO 2 by
adsorption, deep saline aquifers that hold the CO 2 in solution under great pressure,
and basaltic lava rocks, which react chemically with the CO 2 and turn it into the
solid mineral calcium carbonate, or calcite. All of these have pros and cons, for
example, the use of depleted conventional oil and gas fields saves drilling costs by
using existing wells for CO 2 injection, but also runs the risk of encountering deteriorated casing or cracked cement in old wells that might allow the gas to leak back to
the surface (Watson and Bachu 2009). Storing the carbon dioxide underground as a
gas or a supercritical fluid always leads to worries about potential leakage and
upward migration back to the atmosphere.
Fracked gas shales are also being considered for CO 2 storage when they become
depleted (Levine et al. 2016). Because the carbon dioxide adsorbs onto organic matter in the shale more strongly than methane, it might be possible to use CO 2 to
“sweep” methane out of the shale pores. This could improve the efficiency of natural gas recovery from shales, while leaving the CO 2 behind in storage.
Other ideas for the storage of captured carbon dioxide away from the atmosphere
include using it to cure concrete, piping it into sealed greenhouses to enhance plant
growth, or feeding it to cultures of anaerobic bacteria that will consume it and give
off methane gas as a byproduct. The methane could then be used for combustion,
repeating the cycle.
One of the best methods for carbon storage is to inject the CO 2 into basalt. These
rocks are formed from oceanic lavas rich in metals, and one of the major mineral
components is a type of crystalline feldspar called plagioclase. The composition of
plagioclase ranges from a sodium-rich end member known as albite, Na(AlSi 3 O 8 ),
to a calcium-rich end member called anorthite, Ca(Al 2 Si 2 O 8 ). Most plagioclase is a
mixture of the two, combined in what is known as a solid solution series. Plagioclase
is unstable at the surface of the Earth, and exposure to water and air causes it to
weather into clay. The anorthite end member releases calcium into solution during
this process, which reacts with carbon dioxide and oxygen to form the solid mineral
calcium carbonate (CaCO 3 ), a primary component of limestone.
11.3 Energy and Climate Sustainability
far, the two main uses for captured CO 2 appear to be the manufacture of carbonated
beverages and the re-pressurization of depleted oil and gas reservoirs for enhanced
hydrocarbon recovery operations.
There are some relatively simple ways to improve the efficiency of CCS. When
coal is burned in air, the concentration of carbon dioxide in the flue gases is generally less than 15% (Songolzadeh et al. 2014). Chemical absorbents are inefficient at
capturing low concentrations, but the levels of CO 2 in combustion gases can be
greatly increased by burning the coal in pure oxygen instead of air. This so-called
“oxy-combustion” process can produce concentrations of CO 2 of more than 80% in
the flue gases that can be easily captured. Of course, obtaining pure oxygen adds
another cost compared to burning in plain old air, so although the capture efficiency
is improved, the economics are not.
Storage of the captured carbon dioxide typically uses deep geologic formations
that will keep it isolated from the atmosphere (USDOE 2012). These include
depleted conventional oil and gas fields, which are known to have the capability of
trapping gases underground over geologic time, coal seams that hold the CO 2 by
adsorption, deep saline aquifers that hold the CO 2 in solution under great pressure,
and basaltic lava rocks, which react chemically with the CO 2 and turn it into the
solid mineral calcium carbonate, or calcite. All of these have pros and cons, for
example, the use of depleted conventional oil and gas fields saves drilling costs by
using existing wells for CO 2 injection, but also runs the risk of encountering deteriorated casing or cracked cement in old wells that might allow the gas to leak back to
the surface (Watson and Bachu 2009). Storing the carbon dioxide underground as a
gas or a supercritical fluid always leads to worries about potential leakage and
upward migration back to the atmosphere.
Fracked gas shales are also being considered for CO 2 storage when they become
depleted (Levine et al. 2016). Because the carbon dioxide adsorbs onto organic matter in the shale more strongly than methane, it might be possible to use CO 2 to
“sweep” methane out of the shale pores. This could improve the efficiency of natural gas recovery from shales, while leaving the CO 2 behind in storage.
Other ideas for the storage of captured carbon dioxide away from the atmosphere
include using it to cure concrete, piping it into sealed greenhouses to enhance plant
growth, or feeding it to cultures of anaerobic bacteria that will consume it and give
off methane gas as a byproduct. The methane could then be used for combustion,
repeating the cycle.
One of the best methods for carbon storage is to inject the CO 2 into basalt. These
rocks are formed from oceanic lavas rich in metals, and one of the major mineral
components is a type of crystalline feldspar called plagioclase. The composition of
plagioclase ranges from a sodium-rich end member known as albite, Na(AlSi 3 O 8 ),
to a calcium-rich end member called anorthite, Ca(Al 2 Si 2 O 8 ). Most plagioclase is a
mixture of the two, combined in what is known as a solid solution series. Plagioclase
is unstable at the surface of the Earth, and exposure to water and air causes it to
weather into clay. The anorthite end member releases calcium into solution during
this process, which reacts with carbon dioxide and oxygen to form the solid mineral
calcium carbonate (CaCO 3 ), a primary component of limestone.
11.3 Energy and Climate Sustainability