selected sectors, such as avio-, navy-, heavy road-transport where electric motors
(directly or indirectly) powered by solar energy cannot be conveniently used.
Noteworthy, it would make sense to implement such reduction reactions even today
if excess hydrogen is used. As already mentioned, flared-H 2 amounts at ca. 8 Gm
3 /
y, as it represents an average 5.54% of the total flared gases (150–179 Gm
3 ) which
cause the emission of over 450 Mt/yCO 2 [53]. However, the production of
large-scale fuels from CO 2 requires non-fossil-H 2 , such as low-cost PV-H 2 or H 2
that would be flared. Only under the latter conditions the conversion of CO 2 into Cn
hydrocarbons or olefins may become an option economically and energetically
viable.
The considerations just made, push us pay attention to an interesting issue: the
opportunity of “clustering processes and operations” for an efficient use of materials and even waste gases, with much benefit for the environment and our society.
Here, some key elements will be discussed for clarifying the frame in which such
option can really contribute to recycling carbon and reducing CO 2 emission.
Most likely, the existing industrial organization will be revolutionized in future
for generating better options of a cascade utilization of goods and residues.
Industrial processes dispersed in separated sites rise the problem of transportation of
specialty residues. In general, effluents and solid residues of an industrial process
cannot be freely transported on road but can circulate within an industrial site. In a
circular economy frame, effluents and residues can become “secondary raw
materials” for another process. Clustering of processes and diverse activities will
play a key role in order to optimize the utilization of raw materials and minimize
waste production and CO 2 emission. We have always believed that CO 2 is
renewable carbon [56], it can be recovered and cycled incessantly, as Nature has
always done. Clustering of processes is a strategic approach to the efficient use of
resources and valorization of “waste” streams. Approaching the conversion of CO 2
via “value-chains,” more than single reactions, will give a new system perspective
to CCU. Scheme 9.2 is an example of integration of processes for the production of
chemicals and fuels. It shows how it is possible to connect processes for going from
a putative waste (CO 2 ) to a variety of chemicals, materials, and fuels. If such
processes are present on the same site a great advantage is produced in chain
development in terms of economy of transport and storage.
Another example is given in Scheme 9.3 which shows how hydrogen can be
managed in a process system: if processes are clustered, “residual” H 2 can be more
easily cycled into a new process for CO 2 reduction opening to a range of opportunities. However, Scheme 9.3 shows that CO 2 (combined or not with oxygen) can
be used as dehydrogenating agent (Scheme 9.3, left part, reactions 1–3) toward
aliphatic hydrocarbons (CO 2 is a mild oxidant), namely in the coupling of methane
or dehydrogenation of propane to propene or ethylbenzene to styrene, a process that
is finding industrial exploitation with some demo-plants in Korea and China. Such
process brings to the production of single C–C or double C=C bonds and hydrogen
that could be used for the reduction of CO 2 to useful products in situ.
166
9 Circular Economy and Carbon Dioxide Conversion
(directly or indirectly) powered by solar energy cannot be conveniently used.
Noteworthy, it would make sense to implement such reduction reactions even today
if excess hydrogen is used. As already mentioned, flared-H 2 amounts at ca. 8 Gm
3 /
y, as it represents an average 5.54% of the total flared gases (150–179 Gm
3 ) which
cause the emission of over 450 Mt/yCO 2 [53]. However, the production of
large-scale fuels from CO 2 requires non-fossil-H 2 , such as low-cost PV-H 2 or H 2
that would be flared. Only under the latter conditions the conversion of CO 2 into Cn
hydrocarbons or olefins may become an option economically and energetically
viable.
The considerations just made, push us pay attention to an interesting issue: the
opportunity of “clustering processes and operations” for an efficient use of materials and even waste gases, with much benefit for the environment and our society.
Here, some key elements will be discussed for clarifying the frame in which such
option can really contribute to recycling carbon and reducing CO 2 emission.
Most likely, the existing industrial organization will be revolutionized in future
for generating better options of a cascade utilization of goods and residues.
Industrial processes dispersed in separated sites rise the problem of transportation of
specialty residues. In general, effluents and solid residues of an industrial process
cannot be freely transported on road but can circulate within an industrial site. In a
circular economy frame, effluents and residues can become “secondary raw
materials” for another process. Clustering of processes and diverse activities will
play a key role in order to optimize the utilization of raw materials and minimize
waste production and CO 2 emission. We have always believed that CO 2 is
renewable carbon [56], it can be recovered and cycled incessantly, as Nature has
always done. Clustering of processes is a strategic approach to the efficient use of
resources and valorization of “waste” streams. Approaching the conversion of CO 2
via “value-chains,” more than single reactions, will give a new system perspective
to CCU. Scheme 9.2 is an example of integration of processes for the production of
chemicals and fuels. It shows how it is possible to connect processes for going from
a putative waste (CO 2 ) to a variety of chemicals, materials, and fuels. If such
processes are present on the same site a great advantage is produced in chain
development in terms of economy of transport and storage.
Another example is given in Scheme 9.3 which shows how hydrogen can be
managed in a process system: if processes are clustered, “residual” H 2 can be more
easily cycled into a new process for CO 2 reduction opening to a range of opportunities. However, Scheme 9.3 shows that CO 2 (combined or not with oxygen) can
be used as dehydrogenating agent (Scheme 9.3, left part, reactions 1–3) toward
aliphatic hydrocarbons (CO 2 is a mild oxidant), namely in the coupling of methane
or dehydrogenation of propane to propene or ethylbenzene to styrene, a process that
is finding industrial exploitation with some demo-plants in Korea and China. Such
process brings to the production of single C–C or double C=C bonds and hydrogen
that could be used for the reduction of CO 2 to useful products in situ.
166
9 Circular Economy and Carbon Dioxide Conversion