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110th Anniversary : carbon dioxide and chemical looping : current research trends

Lukas Buelens1, Hilde Poelman1, Guy B. Marin1, Vladimir Galvita1 
18 Jul 2019-Industrial & Engineering Chemistry Research (American Chemical Society (ACS))-Vol. 58, Iss: 36, pp 16235-16257

AbstractDriven by the need to develop technologies for converting CO2, an extraordinary array of chemical looping based process concepts has been proposed and researched over the past 15 years. This review aims at providing first a historical context of the molecule CO2, which sits at the center of these developments. Then, different types of chemical looping related to CO2 are addressed, with attention to process concepts, looping materials, and reactor configurations. Herein, focus lies on the direct conversion of carbon dioxide into carbon monoxide, a process deemed to have economic potential.

Summary (2 min read)

2. CARBON DIOXIDE AND THE EARTH’S CLIMATE

  • In the 1820s, Joseph Fourier made heat transfer calculations based on knowledge of that time−between the Sun and the Earth.9,10 9,10 Among other possible explanations, Fourier proposed that the Earth’s atmosphere could be partially responsible for keeping the surface warm.9 11.
  • In 1896, Svante Arrhenius was the first to quantitatively estimate the relation between atmospheric CO2 concentration and the Earth’s surface temperature based on physical chemistry.
  • The evolution in carbon dioxide research trends can be illustrated by the number of publications treating certain topics, shown in Figure 1.

3. CARBON DIOXIDE CAPTURE, STORAGE, AND REUSE

  • Natural processes of atmospheric carbon dioxide capture occur through equilibrium with the biosphere via photosynthesis, with the lithosphere through carbonation of minerals and with the hydrosphere through absorption.
  • This biomass can also be converted into fossil resources with a significant energy input through high pressure and temperature, albeit on geological time scales.
  • Different methods for CO2 capture exist, most of which can be classified into one of the following categories: (i) gas absorption, (ii) gas adsorption, (iii) cryogenic separation, (iv) membrane separation, and (v) mineral carbonation.
  • Absorption relies on the use of a solvent (e.g., amines,46,47 alkali solutions,48 or ionic liquids49,50) contacted with a CO2 containing gas stream in one vessel, resulting in a CO2 lean gas effluent and a CO2 laden solvent.
  • Solvents should be resistant to degradation, nontoxic, and noncorrosive and should show low viscosity and fast reaction kinetics with.

CO2.

  • 46,47 Besides these sometimes conflicting requirements, they should preferably also be cheap.
  • Important properties of adsorbent materials include the CO2 adsorption selectivity, adsorption capacity, adsorption−desorption kinetics and resistance to poisoning and degradation.
  • Today, biological CO2 conversion and the subsequent use of produced biomass is mainly applied through agriculture for the food industry and the production of biofuels (bioethanol and biodiesel), forestry for the furniture and paper industry, and anaerobic digestion of organic waste for the production of biogas.
  • During the following decades, biomass-based processes are expected to increasingly focus not only on food and energy production but also on the production of value-added chemicals.66−68 3.3.3.
  • 4. Pioneers in Industrial Carbon Dioxide Capture, Storage, and Reuse.

4. CHEMICAL LOOPING

  • Chemical looping processes can play an important role in CO2 conversion.
  • These reactions as well as the global reaction are presented in eqs 4−6.
  • Recently, Kim and co-workers160 integrated CO2 capture and conversion and performed dry re-forming in which CO2 is fixed using CaO as sorbent in the first step.
  • Even though the addition of above-mentioned metal oxides improves the physical properties, resistance, and is sometimes also found to mitigate poisoning of the oxygen storage material,177 it may give rise to deactivation through formation of undesired phases.
  • Along with an appropriate choice of chemical looping material, the reactor configuration choice is vital for the practical realization of chemical looping processes.

5. CONCLUSION AND PERSPECTIVE

  • During the past centuries, mankind has improved its quality of life by making use of a seemingly endless source of fossil resources for energy production.
  • In the past decade, the sharp decrease in energy cost associated with wind and solar energy production has triggered the start of an energy revolution toward renewables.
  • Carbon recycling by conversion of CO2 into useful chemicals or fuels may allow to close the carbon loop.
  • The last two decades have witnessed intensified research on chemical looping processes.

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110th Anniversary: Carbon Dioxide and Chemical Looping: Current
Research Trends
Lukas C. Buelens, Hilde Poelman, Guy B. Marin, and Vladimir V. Galvita*
Laboratory for Chemical Technology, Ghent University, Technologiepark 125, B-9052 Ghent, Belgium
*
S
Supporting Information
ABSTRACT: Driven by the need to develop technologies for
converting CO
2
, an extraordinary array of chemical looping
based process concepts has been proposed and researched
over the past 15 years. This review aims at providing rst a
historical context of the molecule CO
2
, which sits at the center
of these developments. Then, dierent types of chemical
looping related to CO
2
are addressed, with attention to
process concepts, looping materials, and reactor congu-
rations. Herein, focus lies on the direct conversion of carbon
dioxide into carbon monoxide, a process deemed to have
economic potential.
1. THE DISCOVERY OF CARBON DIOXIDE
Carbon dioxide was probably discovered around 1640 by Jan
Baptist van Helmont, who named it spiritus sylvestre.
1,2
When
burning a piece of wood, van Helmont noticed that the mass of
the ash was considerably less than that of the original wood.
1,2
He argued that a gas had volatilized from the wood.
1,2
Over a
hundred years later, around 1750, Joseph Black looked into
carbon dioxide released by magnesium carbonate upon heating
or treatment with an acid.
3,4
Black studied carbon dioxide,
which he called f ixed air, in detail and recognized its presence
in air, mineral carbonates, and alkaline solutions, as well as its
formation not only during wood burning but also during
fermentation and human respiration.
4
In the next decades
Joseph Priestley would make signicant progress in the study
of fixed air and its interaction with organisms and water.
5
Indeed, Priestley studied the formation of sparkling water and
suggested that fixed air may be considered a weak acid.
5
In
1779, Jan Ingenhousz published his work on the respiratory
system of plants, proposing that plants take up carbon dioxide
and release oxygen in the presence of sunlight and vice versa in
its absence.
6
In his 1789 treatise, Antoine Lavoisier proposed a new
nomenclature and suggested that the combustion of carbon
(charbon or carbone) with oxygen (oxyge
ne) would result in the
formation of gaseous carbonic acid (acide carbonique)or
carbon oxide (oxide de carbone), deliberately dismissing the
previously proposed nomenclature of f ixed air (air fixe).
7
Two
decades later, in 1808, John Dalton proposed that a single
carbonic acid gas molecule should be composed of one carbon
atom linked with two oxygen atoms (carbon dioxide), while a
carbon oxide molecule should be composed of one carbon and
one oxygen atom (carbon monoxide).
8
The widespread use of
the term carbon dioxide (CO
2
) rather than carbonic acid
(H
2
CO
3
) and carbon monoxide rather than carbon oxide to
designate CO
2
and CO probably occurred only after the
foundation of the International Union on Pure and Applied
Chemistry (IUPAC) in 1919.
2. CARBON DIOXIDE AND THE EARTHS CLIMATE
In the 1820s, Joseph Fourier made heat transfer calculations
based on knowledge of that timebetween the Sun and the
Earth.
9,10
On the basis of his calculations, however, Fourier was
unable to explain why the Earths surface temperature was so
high.
9,10
Among other possible explanations, Fourier proposed
that the Earths atmosphere could be partially responsible for
keeping the surface warm.
9
In the early 1860s, John Tyndall
11
rst proved the greenhouse eect, the heat insulating eect of
the atmosphere owing to the presence of absorbing and
radiating greenhouse gases such as water (H
2
O), carbon
dioxide (CO
2
)andmethane(CH
4
). Tyndall correctly
measured the relative infrared absorptive powers of dierent
gases present in the atmosphere (nitrogen, oxygen, water
vapor, carbon dioxide, ozone, methane, among others).
11
In 1896, Svante Arrhenius was the rst to quantitatively
estimate the relation between atmospheric CO
2
concentration
and the Earth s surface temperature based on physical
chemistry.
12,13
From his calculations, Arrhenius concluded
that human-caused CO
2
emissions through the combustion of
carbonaceous fuels could be large enough to signicantly aect
the Earths climate through the greenhouse eect.
12,13
In 1931,
Edward O. Hulburt conrmed Arrhenius theory and estimated
that a doubling and tripling of the CO
2
levels in the
atmosphere would result in an increased surface temperature
Received: May 7, 2019
Revised: July 11, 2019
Accepted: July 18, 2019
Published: July 18, 2019
Review
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of 4 and 7 K, while the opposite would hold for cooling due to
a decrease in CO
2
levels.
14
With his work, published in 1938,
Guy S. Callendar was probably the rst to report the evolution
in the Earths average surface temperature since 1880 based on
collected data from dierent measurement stations.
15
Call-
endar proposed that the observed increase in average
temperature could be owing to accumulation of anthropogenic
CO
2
emissions into the atmosphere.
15
The discussion with respect to the eect of anthropogenic
CO
2
upon the Earths climate was revived by Gilbert N. Plass
in 1956.
16
Plass estimated that global atmospheric CO
2
concentrations increased with about 30% per century and
resulted in an average temperature increase of 1.1 K per
century.
16
Moreover, he calculated that doubling the levels of
CO
2
would cause an average temperature increase of 3.6 K.
16
One year later, in 1957, Roger Revelle and Hans E. Suess
published their ndings with respect to the role of the oceans
as CO
2
sinks.
17
Contrary to what had previously been
assumed, Revelle and Suess found that the average residence
time of CO
2
in the atmosphere before being absorbed in the
ocean is of the order of 10 years rather than hundreds of
years.
17
They initially contested earlier propositions that linked
the rise in the Earths surface temperature with an (unproven)
increase in atmospheric CO
2
concentration.
17
Revelle and
Suess argued that this would be unlikely because most of the
anthropogenic CO
2
produced by combustion of fossil fuels
since the beginning of the industrial revolutionwould have
been absorbed by the ocean.
17
While detailed measurements of
the global temperature had started around 1850,
18
Charles D.
Keeling started performing measurements of the atmospheric
CO
2
concentration in 1958.
19
In the 1960s and 1970s, many new models and estimates of
climate sensitivity to CO
2
levels were proposed and showed a
strongly varying outcome, causing quite some climate
confusion.
20
The model used by Syukuro Manabe and Richard
T. Wetherald in their 1967 publication,
21
however, was
exceptional in that it included all the major occurring physical
phenomena (including convection and conduction) while
taking into account temperatures at the Earths surface and in
the atmosphere. Indeed, their model, which predicted an
average temperature rise of 1.5 to 2.4 K upon a doubling of the
atmospheric CO
2
concentration, could be considered the rst
true meteorological model.
From 1968,
2224
Keeling started publishing the evolution of
his atmospheric CO
2
concentration measurements, which had
steadily increased from 312 to 318 ppm since 1958. In order to
acquire temperature data prior to 1850 and CO
2
concentration
levels before 1958, researchers started studying tree ring
cores
2530
and ice cores,
3036
while developing techniques to
accurately obtain climate information by calibration with
reliable direct measurements. During the 1980s, eorts in
raising awareness about climate change in the US were
orchestrated by scientists and activists such as Gordon
MacDonald, Rafe Pomerance, and James Hansen.
37
Although
their eorts remained largely unanswered by political leaders,
the perseverance of Pomerance and Hansen in particular
certainly contributed to the establishment of the International
Panel on Climate Change (IPCC) in 1988.
37
Additional
historical context to the relationship between CO
2
and the
environment is provided in Supporting Information (Historical
Context 1).
Current trends in global climate monitoring include the
analysis of data recorded by satellites.
3840
At the same time,
climate modeling has tremendously evolved in the past decades
with Earth System Models (ESMs) as the current state of the
art.
41
Nevertheless, capturing and modeling the complexity of
the Earths climate remains a huge task as it essentially
encompasses comprehending the (physical) chemistry, geology
(e.g., eect of volcanic activity), and uid dynamics of the
Earth, as well as the occurrence and inuence of processes
occurring in the Sun.
After the rst IPCC report was published in the 1990s,
awareness within the scienticcommunityhasgrown
tremendously. Today, a large majority of the scientic
community, estimated at over 95%,
42
acknowledges that
anthropogenic greenhouse gas (such as CO
2
and CH
4
)
emissions have a signicant eect on the Earths climate.
43
Nevertheless, this scientic consensus is not necessarily picked
up outside the scientic community, especially in Anglo-Saxon
countries.
43,44
Apart from purely environmental concerns, the presence of
excessive amounts of CO
2
in the atmosphere is increasingly
approached from a geo-engineering perspective, i.e., by aiming
to obtain deliberate control over the environment. Accord-
ingly, trends in research and development focus on reducing
CO
2
emissions by its capture, (geological) storage, or recycling
and the active control of atmospheric CO
2
levels.
The evolution in carbon dioxide research trends can be
illustrated by the number of publications treating certain
topics, shown in Figure 1. It can be seen that a stagnating
increase in the number of carbon dioxide studies occurred
between 1960 and 1990, followed by a rst growth wave in the
1990s and a second growth wave in the 2010s. The former may
be ascribed to increasing interest in atmospheric CO
2
levels
and its eect on climate, while the latter corresponds with
increasing interest in carbon dioxide capture, (geological)
Figure 1. Overview of the evolution of number of scientic
publications for dierent topics. The evolution of the number of
scientic publications (Web of Science online database) per year
ltered by dierent keywords is presented. These topical keywords are
the following: (
) CO
2
;(
) CO
2
AND atm osphere
(atmosphere); (
) CO
2
capture (capture); (
) CO
2
storage
(storage); (
) CO
2
utilization OR CO
2
conversion OR CO
2
recycling (reuse). The time spans above the graph indicate the years
with more than 50 publications/year. The jump between 1990 and
1991 can be (partially) explained by a sudden increase in the number
of publications in Web of Science per year, estimated by performing a
search with neutral keyword of (inset).
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storage, and reuse (utilization, conversion, recycling). In what
follows, some of the principles behind carbon dioxide capture,
storage, and reuse will briey be discussed.
3. CARBON DIOXIDE CAPTURE, STORAGE, AND
REUSE
3.1. Carbon Dioxide capture. Natural processes of
atmospheric carbon dioxide capture occur through equilibrium
with the biosphere via photosynthesis, with the lithosphere
through carbonation of minerals and with the hydrosphere
through absorption.
45
The former is the odd one out because,
through the photosynthesis process, CO
2
capture is accom-
panied by biochemical reduction of carbon dioxide with
formation of biomass (e.g., trees, crops and algae), which can
be reused as a resource of chemicals or energy. This biomass
can also be converted into fossil resources with a signicant
energy input through high pressure and temperature, albeit on
geological time scales.
45
However, the carbonation of minerals
in the lithosphere and the absorption of CO
2
by the oceans are
kinetically limited chemical processes, which result in
chemicals with an even lower energy content than gaseous
CO
2
(Figure 2).
In order to mitigate CO
2
emissions or control its
atmospheric concentration directly, CO
2
capture techniques
are required. Dierent methods for CO
2
capture exist, most of
which can be classied into one of the following categories: (i)
gas absorption, (ii) gas adsorption, (iii) cryogenic separation,
(iv) membrane separation, and (v) mineral carbonation.
Absorption relies on the use of a solvent (e.g., amines,
46,47
alkali solutions,
48
or ionic liquids
49,50
) contacted with a CO
2
containing gas stream in one vessel, resulting in a CO
2
lean gas
euent and a CO
2
laden solvent. This solvent is regenerated in
a second vessel with the release of enriched CO
2
. Solvents
should be resistant to degradation, nontoxic, and noncorrosive
and should show low viscosity and fast reaction kinetics with
CO
2
.
46,47
Besides these sometimes conicting requirements,
they should preferably also be cheap. While amine- and alkali-
based CO
2
absorption processes for gas purication have been
industrially used for many decades, challenges associated with
solvent degradation, volatility, and corrosion are yet to be
addressed, especially for amine-based solvents.
48
While alkali
solutions are less corrosive by nature than amine solutions,
corrosion remains an issue as the alkali solution absorbs ue
gas contaminants and hence degrades.
48
Besides a loss in
absorption capacity due to precipitation of alkali salts (e.g.,
sulfates or nitrates), the kinetics of CO
2
absorption are much
slower for alkali solutions compared with amines.
48
Ionic
liquids, however, are favorable in terms of their low volatility,
low ammability and thermal stability, though challenges
remain due to the increased viscosity of ionic liquids with
increasing CO
2
absorption and their toxicity.
51
Adsorption relies on preferential physisorption of CO
2
over
other compounds on a high-surface area adsorbent (e.g.,
zeolites
5153
or metalorganic frameworks
5154
). The adsorb-
ent is regenerated by applying heat (temperature swing) and/
or decreasing pressure (pressure swing). Important properties
of adsorbent materials include the CO
2
adsorption selectivity,
adsorption capacity, adsorptiondesorption kinetics and
resistance to poisoning and degradation.
51
In this sense, the
highly tailorable nature of metalorganic frameworks provides
a major opportunity in terms of adsorbent properties
optimization.
53,54
Cryogenic separation relies on extensive cooling in order to
separate CO
2
from other compounds based on its boiling
point. Because of the high cost of cooling, such a separation
process should only be considered when the concentration of
CO
2
is already high (e.g., higher than 50%).
55
Additional
challenges lie in the buildup of solid CO
2
on the heat-
exchanger surface, which deteriorates the overall energy
eciency of the process.
55
Membrane separation relies on the dierence in permeation
of dierent gases through a membrane (e.g., polymer
membrane
56
or ceramic membrane
56
). The preferential
permeation of either CO
2
or the diluting gases allows us to
obtain an enriched CO
2
stream. Critical properties of the
membrane include selectivity, permeability, and stability.
56
Carbonationdecarbonation makes use of the chemical
carbonation of solid minerals (typically alkali or earth alkali
metal oxides). By decarbonation of the mineral carbonate at
Figure 2. Overview of stability of selected chemical species. Gibbs free energy of formation as a function of molecular mass of dierent organic and
inorganic species with indication of some homologous series (olens, alkanes, and alcohols).
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elevated temperature or reduced pressure, a CO
2
rich gas
stream can be obtained. Important challenges with respect to
such minerals are the mechanical and structural stability after
prolonged carbonationdecarbonation cycling. Additionally,
the eect of SO
x
and NO
x
should be borne in mind.
Besides a distinction between CO
2
capture based on the
separation technology, eorts in CO
2
capture can be classied
as direct or indirect CO
2
capture depending on whether CO
2
is
caught from the air directly or from (industrial) exhaust gases.
3.2. Carbon Dioxide Storage. An early suggestion for
CO
2
capture and storage through geo-engineering came from
Cesare Marchetti in 1977, who proposed to separate CO
2
from
ue gases with subsequent disposal either (i) in exhausted gas
or oil elds and other geological cavities or (ii) by injection in
sinking ocean currents in order to improve the kinetics of CO
2
absorption.
57
Ocean absorption, however, does not seem to be
a favorable idea due to the resulting ocean acidication, which
harms marine ecosystems.
58
While several test projects have
been started with respect to geological CO
2
storage,
uncertainty remains with respect to unfavorable long-term
eects (e.g., seismic activity or leakage).
51,58,59
The reader is
referred elsewhere for a detailed overview.
58,59
No doubt the most elegant carbon dioxide storage strategy
would be biological storage by promoting reforestation. The
development of ecient methods for urban agriculture could
create opportunities for nature to reclaim rural agricultural
areas. When the produced biomass is periodically harvested
and consumed, biological carbon dioxide storage becomes a
strategy for carbon dioxide reuse, the topic of next paragraph.
3.3. Carbon Dioxide Reuse. Carbon dioxide reuse can be
divided into three categories: (i) carbon dioxide utilization
comprises processes in which carbon dioxide is utilized but not
necessarily conve rted; (ii) biological conversion includes
biochemical carbon dioxide conversion into biomass through
photosynthesis; (iii) carbon dioxide conversion is used for the
deliberate industrial conversion of carbon dioxide into
chemicals or fuels.
3.3.1. Carbon Dioxide Utilization. A rst example of CO
2
utilization is its use in enhanced oil recovery, typically a one-
time use accompanied by CO
2
storage, whereby CO
2
is used to
enhance the yield of fossil resource recovery by pumping CO
2
into geological reservoirs which contain oil and/or gas. While
one could argue that enhanced oil recovery results in CO
2
storage, it also provides new fossil resources that are likely to
cause signicant CO
2
emissions during their life cycle. A
second example is the production of carbonated beverages, in
which CO
2
is stored as H
2
CO
3
. Carbon dioxide utilization in
beverages is temporary as CO
2
will again be released from the
beverage when consumed. A third example is the use of
supercritical CO
2
as an extraction or reaction solvent.
6063
Because CO
2
is nonammable, nontoxic, and relatively inert, it
is perceived as an interesting candidate solvent for green
chemistry.
60
Moreover, the supercritical regime of CO
2
is
readily accessible contrary to, e.g., that of water.
60
All of the above-mentioned methods for carbon dioxide
utilization have in common that they do not contribute to CO
2
emission m itigation. In the following paragraphs, CO
2
conversion methods are discussed that have the potential to
Figure 3. Selection of carbon dioxide production, storage, and conversion processes. In this scheme, the potential of carbon monoxide (CO) as an
intermediate between carbon dioxide (CO
2
) and a variety of chemicals is presented with an indication of annual global production volumes
51,7280
(in Mt year
1
or Gt year
1
). These production volumes are not necessarily realized by the indicated process. The scheme indicates that the
combustion of fossil resources (coal, oil, and natural gas) results in CO
2
emission, part of which is (i) very slowly captured in the lithosphere by
formation of mineral carbonates, (ii) slowly captured in the hydrosphere by ocean absorption (acidication), and (iii) captured in the biosphere by
means of photosynthesis. Double arrows toward the hydrosphere and biosphere indicate the possibility of CO
2
emission by desorption from the
oceans (in case the atmospheric CO
2
concentration would decrease) and CO
2
emission by respiration of organisms (via biomass combustion). In
principle, CO
2
can be converted into methane (CH
4
), which is equivalent to natural gas. This work focuses on the conversion of CO
2
through the
intermediate CO, a platform molecule from which ethanol (C
2
H
5
OH, by fermentation), polycarbonate (through phosgene, COCl
2
, or dimethyl
carbonate, (CH
3
O)
2
CO), and acids (such as acetic acid, CH
3
COOH) can be formed. Additionally, CO can be transformed into alkanes (Fischer
Tropsch synthesis), methanol (CH
3
OH), formaldehyde (CH
2
O), and aldehydes when a source of (renewable) hydrogen (H
2
) is available. Alkanes
(catalytic dehydrogenation, catalytic cracking, or steam cracking) or CH
3
OH (methanol-to-olens process) can further be transformed into olens
(C
=
). Alternatively, alkanes can be further rened in order to yield hydrocarbon fuels (or other petrochemical building blocks), thereby allowing us
to close the carbon loop.
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contribute to CO
2
emission mitigation and closure of the
carbon loop.
3.3.2. Biological Conversion. Biological conversion of CO
2
into biomass is typically realized by plants through the process
of photosynthesis. Microalgae are considered among the fastest
growing species of plants (e.g., compared with crops and
trees), a property that makes them a promising candidate for
biological CO
2
capture and conversion.
64
Nevertheless,
economic feasibility currently hinders the use of algal biofuels
and biochemicals.
65
Product diversication in an algal
biorenery concept could provide future opportunities for
the valorization of algae.
65
The current situation for other
biomass feedstocks is similar to that of algae, as high
production costs associated with biomass processing
66
impair
their competitiveness with existing processes. Today, biological
CO
2
conversion and the subsequent use of produced biomass
is mainly applied through agriculture for the food industry and
the production of biofuels (bioethanol and biodiesel), forestry
for the furniture and paper industry, and anaerobic digestion of
organic waste for the production of biogas. During the
following decades, biomass-based processes are expected to
increasingly focus not only on food and energy production but
also on the production of value-added chemicals.
6668
3.3.3. Carbon Dioxide Conversion. The direct conversion
of carbon dioxide into useful chemicals includes the
production of urea, salicylic acid, dimethyl carbonate, polyur-
ethane, polycarbonate, polyacrylates, and (in)organic carbo-
nates.
45,69,70
Additional possibilities lie in its use as a mild
oxidant, for example, in the oxidative dehydrogenation of
alkanes with formation of alkenes, wa ter, and carbon
monoxide.
69
Finally, carbon dioxide can also be hydrogenated
into chemicals or fuels such as methane (methanation),
formaldehyde, dimethyl ether, formic acid, methanol, and
other alcohols.
45,69,70
The ecient methanation of carbon
dioxide using renewable hydrogen (e.g., from electrolysis using
wind or solar electricity) and subsequent injection in the
natural gas grid could be particularly interesting as a method of
energy storage.
Alternatively, carbon dioxide can rst be converted into
carbon monoxide, a building block for a broad variety of
chemicals and fuels. Figure 3 gives an overview of some carbon
dioxide sources and sinks. The main carbon dioxide sources
are the combustion of fossil fuels (such as coal, oil, and natural
gas), the respiration of organisms through biomass combustion
and desorption from the oceans (when the partial pressure of
carbon dioxide in the atmosphere decreases). As previously
mentioned, carbon dioxide can also be captured and stored in
the lithosphere (mineral carbonates), hydrosphere (absorption
by oceans), and biosphere (photosynthesis).
The main focus of this review lies on the direct conversion
of carbon dioxide, for example, into carbon monoxide, a
process deemed to have economic potential.
71
Carbon
monoxide can be converted into chemicals such as ethanol
(e.g., through fermentation), polycarbonates (with phosgene
or dimethyl carbonate as intermediate), and acids. By adding
(renewable) hydrogen, the production of aldehydes, methanol,
and alkanes (e.g., through Fischer Tropsch synthesis) can be
realized. Moreover, olens can be produced from carbon
monoxide via methanol (methanol-to-olens process) or from
alkanes (catalytic dehydrogenation, catalytic cracking or steam
cracking) as intermediate. Figure 3 indicates the global annual
production volume, not necessarily through the proposed
process, in order to give an estimate of the capacity of
chemicals or fuels as carbon sink. The gures show that, while
the production of chemicals could provide a carbon sink for
hundreds of Mt year
1
, production of fuels would be necessary
in order to reach the Gt year
1
level and close the carbon loop.
The conversion of CO
2
to CO can be realized by a
(photo)electric current, by thermal reduction or by chemical
reduction. Chemical reduction, which is the focus of this
review, relies on the rearrangement of chemical bonds in a
reducing agent such as CH
4
or H
2
.
3.4. Pioneers in Industrial Carbon Dioxide Capture,
Storage, and Reuse. Industrial (direct) air capture and the
subsequent recycling of CO
2
into chemicals/fuels is currently
under development and already being demonstrated by
pioneering companies such as Climeworks, Carbon Engineering,
Global Thermostat, Carbon Recycling International, INERATEC
GmbH and Nordic Blue Crude. Indeed, in recent years the
number of start-up companies envisaging to close the carbon
loop using renewable energy and CO
2
as a feedstock has
surged. Carbicrete,however,aimstoreducethecarbon
footprint of the construction sector by combining mineral
waste and CO
2
carbonation in a novel way of curing concrete
without making use of cement.
8183
4. CHEMICAL LOOPING
4.1. Principle. Chemical looping processes can play an
important role in CO
2
conversion. They make use of stable
solid intermediates in order to realize a redox reaction or gas
separation. In what follows, three categories of chemical
looping processes will be distinguished: (i) chemical looping
redox reactions; (ii) thermochemical looping redox reactions;
(iii) chemical looping carbon dioxide separation.
Chemical looping redox reactions generally make use of a
metal oxide oxygen storage material (reduced form, M;
oxidized form, MO, which transfers oxygen from the oxidant
(e.g., O
2
,H
2
O, or CO
2
) to the reductant (e.g., C, CH
4
,H
2
).
Vice versa, electrons are transferred by the oxygen storage
material from the reductant to the oxidant. Equations 1 and 2
illustrate the general concept of such redox reactions. The
global reaction is an ordinary redox reaction (eq 3). Because
the global reaction is split into two sub reactions, however, the
reduced and oxidized product can be inherently separated by
performing the two reactions at two dierent points in time or
space.
+→ +oxidant M reduced product MO
(s) (s)
(1)
+→ +reductant MO oxidized product M
(s) (s)
(2)
+
→+
oxidant reductant
reduced product oxidized product
(3)
Thermochemical looping redox reactions are similar to
ordinary chemical looping redox reactions but dier in the
method of metal oxide reduction. Here, the reduction of the
metal oxide occurs thermally through oxygen evolution. The
oxidation of the reduced metal oxide is analogous to the one in
chemical looping redox reactions. These reactions as well as
the global reaction are presented in eqs 46.
+→ +oxidant M reduced product MO
(s) (s)
(4)
⎯→⎯⎯ +
M
O
1
2
OM
(s)
heat
2(g) (s)
(5)
Industrial & Engineering Chemistry Research Review
DOI: 10.1021/acs.iecr.9b02521
Ind. Eng. Chem. Res. XXXX, XXX, XXXXXX
E

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Journal ArticleDOI
Abstract: As a promising approach for carbon dioxide capture, chemical looping combustion has been extensively investigated for more than two decades. However, the chemical looping strategy can be and has been extended well beyond carbon capture. In fact, significant impacts on emission reduction, energy conservation, and value-creation can be anticipated from chemical looping beyond combustion (CLBC). This article aims to demonstrate the versatility and transformational benefits of CLBC. Specifically, we focus on the use of oxygen carriers or redox catalysts for chemical production – a $4 trillion industry that consumes 40.9 quadrillion BTU of energy. Compared to state-of-the-art chemical production technologies, we illustrate that chemical looping offers significant opportunities for process intensification and exergy loss minimization. In many cases, an order of magnitude reduction in energy consumption and CO2 emission can be realized without the needs for carbon dioxide capture. In addition to providing various CLBC examples, this article elaborates on generalized design principles for CLBC, potential benefits and pitfalls, as well as redox catalyst selection, design, optimization, and redox reaction mechanism.

118 citations


Journal ArticleDOI
Abstract: A separated gasification chemical looping combustion (SG-CLC) system has been operated auto-thermally, which has high combustion performances and is beneficial for the lifetime of oxygen carrier (OC). In this work, the performances of SG-CLC were compared with in-situ gasification CLC (iG-CLC) in a two-stage reactor system. The iG-CLC has more favorable solid conversion than that of SG-CLC, namely the conversion of iG-CLC reached 97.81% after 10 min while that of SG-CLC was 88.31% at 1050 °C. When the oxygen ratio RO of gasification agent was 3.7%, the auto-thermal operation of SG-CLC could be realized. Under the auto-thermal condition, when the gasification temperature increased from 850 °C to 1050 °C, the carbon conversion increased from 61.02% to 96.64% while the gaseous combustion efficiency fluctuated near 98.66%. When the reduction temperature varied in that range, the carbon conversion was approximately 87.12% and the combustion efficiencies ηC increased from 97.91% to 99.96%. Finally, multi-cycle redox experiments were conducted using both iG-CLC and SG-CLC to probe into the reactivity stability of the OC. Results demonstrated that SG-CLC had satisfying performances in fuel conversion and the stability of reactivity, proving the benefit to lengthen the lifetime of oxygen carrier.

13 citations


Journal ArticleDOI
Abstract: The application of a CO2 sorbent which releases CO2 at a lower temperature than calcium oxide is of interest in view of reducing the operating temperature or increasing the operating pressure of the super-dry reforming process. To this end, three different CO2 sorbents based on lithium orthosilicate (Li4SiO4) were prepared and characterized: (i) Li4SiO4 as such; (ii) zirconia coated Li4SiO4, denoted Li4SiO4@ZrO2; (iii) Li4SiO4, coated with lithium metazirconate and denoted as Li4SiO4@Li2ZrO3. While the carbonation properties of Li4SiO4 were found satisfactory, its decarbonation was incomplete. Li4SiO4@ZrO2 on the other hand, was found to contain Li2SiO3 and Li2ZrO3 rather than the anticipated Li4SiO4 and showed fast carbonation and decarbonation of Li2ZrO3. The best performing material in terms of CO2 sorption capacity, stability and rate of carbonation-decarbonation was Li4SiO4@Li2ZrO3. Its superior performance is ensured by the core-shell structure saturated with Li, allowing for easy restructuring during carbonation and fast regeneration upon decarbonation.

9 citations


Journal ArticleDOI
Abstract: Fe-modified MgAl2O4 makes a surprisingly active catalyst support, likely linked to a structural effect of the Fe incorporation. Two catalyst supports, MgAl2O4 and MgFeAlO4, have been studied in fre...

8 citations


Journal ArticleDOI
Li Ma1, Yu Qiu1, Min Li1, Dongxu Cui1, Shuai Zhang1, Dewang Zeng1, Rui Xiao1 
Abstract: Reverse water gas shift on the basis of chemical looping technology provides a viable method for efficiently converting CO2 to CO for hydrocarbons at moderate temperature (<750 °C). However, the co...

8 citations


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TL;DR: Recent developments in genetic engineering, enhanced extraction methods, and a deeper understanding of the structure of lignin are yielding promising opportunities for efficient conversion of this renewable resource to carbon fibers, polymers, commodity chemicals, and fuels.
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