533
afforestation/reforestation. Natural photosynthesis is a very slow process and afforestation/ reforestation requires conversion of large land mass to forests.
Absorption of CO 2 in alkaline-based solutions such as sodium hydroxide solutions has been proposed and investigated for direct air capture of CO 2 . The process
involves contacting of air with a sodium hydroxide solution in a tower. The CO 2 is
absorbed in the sodium hydroxide solution and is converted to sodium carbonate.
The sodium carbonate is regenerated in a closed loop process and converted again
into sodium hydroxide, via the causticization process using lime. The CO 2 evolved
in the regeneration process is the captured CO 2 from the atmosphere [12, 13].
While this is thermodynamically feasible, the low concentrations in the atmosphere make it impractical for large-scale use.
1.4.2 CO 2 Capture from Industrial Point Sources
Industrial process emissions, on the other hand, have high carbon dioxide concentrations (ethanol production, natural gas processing, ethylene oxide production), and it
can be utilized directly. Depending on the composition of CO 2 in the exhaust stream,
CO 2 capture may require the use of conventional separation processes such as absorption and adsorption for the capture of carbon dioxide. Most carbon dioxide emissions
from industrial sources are from power plants and therefore significant efforts have
been expended in carbon dioxide capture from power plants. The main processes for
CO 2 capture from power plants include pre-combustion capture processes, post-combustion capture processes and oxy-combustion capture processes [14].
Pre-combustion CO 2 capture involves the conversion of fuels (gasification of coal,
reforming of natural gas and liquid fuels) into synthesis gas and further conversion
of synthesis gas into hydrogen via the water gas shift reaction. The CO 2 separated
from the H 2 is the captured CO 2 , while the H 2 is combusted for provide power. Postcombustion capture processes involve the separation of CO 2 from the exhaust gases
in a conventional fossil fuel power plant. Oxy-combustion capture process involves
the combustion of fuels with high-purity oxygen instead of air to provide an exhaust
gas with high-purity carbon dioxide which can be easily separated.
Carbon dioxide capture results in the production of a highly concentrated CO 2
stream. In the next section, we explore various utilization options for captured CO 2 .
2 Carbon Dioxide Utilization
Captured carbon dioxide can be either sequestered or utilised. Carbon sequestration
involves the long-term storage of captured CO 2 in suitable underground rock formations. While appropriate choice of injection areas will ensure the safety of the
sequestration process, it is associated with inherent risks. Furthermore, it represents
the conventional method of treating by-products as waste, and no economic gains
result from such processes. Carbon dioxide utilization on the other hand describes
Synthesis of Novel Catalysts for Carbon Dioxide Conversion to Products of Value
afforestation/reforestation. Natural photosynthesis is a very slow process and afforestation/ reforestation requires conversion of large land mass to forests.
Absorption of CO 2 in alkaline-based solutions such as sodium hydroxide solutions has been proposed and investigated for direct air capture of CO 2 . The process
involves contacting of air with a sodium hydroxide solution in a tower. The CO 2 is
absorbed in the sodium hydroxide solution and is converted to sodium carbonate.
The sodium carbonate is regenerated in a closed loop process and converted again
into sodium hydroxide, via the causticization process using lime. The CO 2 evolved
in the regeneration process is the captured CO 2 from the atmosphere [12, 13].
While this is thermodynamically feasible, the low concentrations in the atmosphere make it impractical for large-scale use.
1.4.2 CO 2 Capture from Industrial Point Sources
Industrial process emissions, on the other hand, have high carbon dioxide concentrations (ethanol production, natural gas processing, ethylene oxide production), and it
can be utilized directly. Depending on the composition of CO 2 in the exhaust stream,
CO 2 capture may require the use of conventional separation processes such as absorption and adsorption for the capture of carbon dioxide. Most carbon dioxide emissions
from industrial sources are from power plants and therefore significant efforts have
been expended in carbon dioxide capture from power plants. The main processes for
CO 2 capture from power plants include pre-combustion capture processes, post-combustion capture processes and oxy-combustion capture processes [14].
Pre-combustion CO 2 capture involves the conversion of fuels (gasification of coal,
reforming of natural gas and liquid fuels) into synthesis gas and further conversion
of synthesis gas into hydrogen via the water gas shift reaction. The CO 2 separated
from the H 2 is the captured CO 2 , while the H 2 is combusted for provide power. Postcombustion capture processes involve the separation of CO 2 from the exhaust gases
in a conventional fossil fuel power plant. Oxy-combustion capture process involves
the combustion of fuels with high-purity oxygen instead of air to provide an exhaust
gas with high-purity carbon dioxide which can be easily separated.
Carbon dioxide capture results in the production of a highly concentrated CO 2
stream. In the next section, we explore various utilization options for captured CO 2 .
2 Carbon Dioxide Utilization
Captured carbon dioxide can be either sequestered or utilised. Carbon sequestration
involves the long-term storage of captured CO 2 in suitable underground rock formations. While appropriate choice of injection areas will ensure the safety of the
sequestration process, it is associated with inherent risks. Furthermore, it represents
the conventional method of treating by-products as waste, and no economic gains
result from such processes. Carbon dioxide utilization on the other hand describes
Synthesis of Novel Catalysts for Carbon Dioxide Conversion to Products of Value