536
The carbon dioxide utilization technologies that will have an impact by reducing
emissions must be done at large scale. A large-scale implementation of CO 2 utilization technologies is necessary because current anthropogenic emissions are at the
giga ton scale and it enables the leverage of economies of scale. Table 1 shows the
emission rate of CO 2 from various industrial sources in the United States.
Requirements for economical large-scale implementation of CO 2 utilization to mitigate carbon dioxide emissions include
• High reaction rates and CO 2 conversion efficiencies
• Net reduction in CO 2 emitted when compared to conventional processes
• Low cost of operation
• High demand for product
• Low-cost and reliable supply of carbon dioxide
High reaction rates and CO 2 conversion efficiencies are required for reasonable
sizes of an industrial-scale plant and to reduce energy requirement for separation
and recycle of unreacted reactants. To achieve a net reduction in CO 2 emissions, the
utilization process should emit less CO 2 per unit of product when compared to the
conventional process. Low cost of operation for CO 2 utilization processes is essential to ensure the cost of products is comparable to that of the conventional process.
For economic justification of high production rates, typically obtained in industrial
scales, there should be at least an equivalently high demand for the product. A lowcost and reliable supply of carbon dioxide is necessary for CO 2 utilization processes
to ensure high on-stream time for the process at industrial scales.
Photocatalytic conversion of CO 2 avoids the energy penalty associated with
high-temperature catalytic conversion processes, but studies have shown that it is
highly kinetically limited with very low photoconversion efficiencies [17, 20–25].
This severely limits the effectiveness of photo-catalytic processes as high reaction
rates are required for reasonable sizes for an industrial plant.
Although conversion rates for algae-based processes are marginally more than
photocatalytic processes, they are still very low and their cost of operation can be
very high. A lot of space and water resources are required to get optimal performance, and frequent tank cleaning will be required to ensure light penetration. Also,
algae systems are less robust requiring a narrow range of operating conditions for
optimal performance and usually produce a mixture of various products which can
add complexity to downstream separations processes [17].
A major route for the catalytic conversion is the hydrogenation of CO 2 to produce fuels and chemical products such as methanol, methane, formaldehyde, formic
acid and others [26, 27]. However, hydrogen production via electrolysis or reforming processes is highly energy intensive and can impact negatively on hydrogenation processes [28, 29]. For example, the CAMERE process has not been
commercialized despite its development in 1999 and its successful pilot-scale demonstration (100 kg methanol/day) in 2004 [30, 31]. In the CAMERE process, CO 2
is hydrogenated in two steps to produce methanol. In the first reactor, CO 2 and H 2
are converted to CO via the reverse water gas shift reaction using a zinc aluminatebased catalyst at about 60% conversion per pass. By-product water is separated and
O. Okonkwo and P. Biswas
The carbon dioxide utilization technologies that will have an impact by reducing
emissions must be done at large scale. A large-scale implementation of CO 2 utilization technologies is necessary because current anthropogenic emissions are at the
giga ton scale and it enables the leverage of economies of scale. Table 1 shows the
emission rate of CO 2 from various industrial sources in the United States.
Requirements for economical large-scale implementation of CO 2 utilization to mitigate carbon dioxide emissions include
• High reaction rates and CO 2 conversion efficiencies
• Net reduction in CO 2 emitted when compared to conventional processes
• Low cost of operation
• High demand for product
• Low-cost and reliable supply of carbon dioxide
High reaction rates and CO 2 conversion efficiencies are required for reasonable
sizes of an industrial-scale plant and to reduce energy requirement for separation
and recycle of unreacted reactants. To achieve a net reduction in CO 2 emissions, the
utilization process should emit less CO 2 per unit of product when compared to the
conventional process. Low cost of operation for CO 2 utilization processes is essential to ensure the cost of products is comparable to that of the conventional process.
For economic justification of high production rates, typically obtained in industrial
scales, there should be at least an equivalently high demand for the product. A lowcost and reliable supply of carbon dioxide is necessary for CO 2 utilization processes
to ensure high on-stream time for the process at industrial scales.
Photocatalytic conversion of CO 2 avoids the energy penalty associated with
high-temperature catalytic conversion processes, but studies have shown that it is
highly kinetically limited with very low photoconversion efficiencies [17, 20–25].
This severely limits the effectiveness of photo-catalytic processes as high reaction
rates are required for reasonable sizes for an industrial plant.
Although conversion rates for algae-based processes are marginally more than
photocatalytic processes, they are still very low and their cost of operation can be
very high. A lot of space and water resources are required to get optimal performance, and frequent tank cleaning will be required to ensure light penetration. Also,
algae systems are less robust requiring a narrow range of operating conditions for
optimal performance and usually produce a mixture of various products which can
add complexity to downstream separations processes [17].
A major route for the catalytic conversion is the hydrogenation of CO 2 to produce fuels and chemical products such as methanol, methane, formaldehyde, formic
acid and others [26, 27]. However, hydrogen production via electrolysis or reforming processes is highly energy intensive and can impact negatively on hydrogenation processes [28, 29]. For example, the CAMERE process has not been
commercialized despite its development in 1999 and its successful pilot-scale demonstration (100 kg methanol/day) in 2004 [30, 31]. In the CAMERE process, CO 2
is hydrogenated in two steps to produce methanol. In the first reactor, CO 2 and H 2
are converted to CO via the reverse water gas shift reaction using a zinc aluminatebased catalyst at about 60% conversion per pass. By-product water is separated and
O. Okonkwo and P. Biswas