110th Anniversary : carbon dioxide and chemical looping : current research trends
18 Jul 2019-Industrial & Engineering Chemistry Research (American Chemical Society (ACS))-Vol. 58, Iss: 36, pp 16235-16257
TL;DR: A review of chemical looping-based process concepts for converting CO2 into carbon monoxide can be found in this article, where the focus lies on the direct conversion of carbon dioxide into CO2, a process deemed to have economic potential.
Abstract: Driven 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)
Jump to: [2. CARBON DIOXIDE AND THE EARTH’S CLIMATE] – [3. CARBON DIOXIDE CAPTURE, STORAGE, AND REUSE] – [CO2.] – [4. CHEMICAL LOOPING] and [5. CONCLUSION AND PERSPECTIVE]
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.
- 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
Lukas C. Buelens, Hilde Poelman, Guy B. Marin, and Vladimir V. Galvita*
Laboratory for Chemical Technology, Ghent University, Technologiepark 125, B-9052 Ghent, Belgium
ABSTRACT: Driven by the need to develop technologies for
, 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 CO
, which sits at the center
of these developments. Then, diﬀerent types of chemical
looping related to CO
are addressed, with attention to
process concepts, looping materials, and reactor conﬁgu-
rations. Herein, focus lies on the direct conversion of carbon
dioxide into carbon monoxide, a process deemed to have
1. THE DISCOVERY OF CARBON DIOXIDE
Carbon dioxide was probably discovered around 1640 by Jan
Baptist van Helmont, who named it spiritus sylvestre.
burning a piece of wood, van Helmont noticed that the mass of
the ash was considerably less than that of the original wood.
He argued that a gas had volatilized from the wood.
hundred years later, around 1750, Joseph Black looked into
carbon dioxide released by magnesium carbonate upon heating
or treatment with an acid.
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.
In the next decades
Joseph Priestley would make signiﬁcant progress in the study
of fixed air and its interaction with organisms and water.
Indeed, Priestley studied the formation of sparkling water and
suggested that fixed air may be considered a weak acid.
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
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).
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).
The widespread use of
the term carbon dioxide (CO
) rather than carbonic acid
) and carbon monoxide rather than carbon oxide to
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 EARTH’S CLIMATE
In the 1820s, Joseph Fourier made heat transfer calculations
based on knowledge of that time−between the Sun and the
On the basis of his calculations, however, Fourier was
unable to explain why the Earth’s surface temperature was so
Among other possible explanations, Fourier proposed
that the Earth’s atmosphere could be partially responsible for
keeping the surface warm.
In the early 1860s, John Tyndall
ﬁrst proved the greenhouse eﬀect, the heat insulating eﬀect of
the atmosphere owing to the presence of absorbing and
radiating “greenhouse” gases such as water (H
). Tyndall correctly
measured the relative infrared absorptive powers of diﬀerent
gases present in the atmosphere (nitrogen, oxygen, water
vapor, carbon dioxide, ozone, methane, among others).
In 1896, Svante Arrhenius was the ﬁrst to quantitatively
estimate the relation between atmospheric CO
and the Earth’ s surface temperature based on physical
From his calculations, Arrhenius concluded
that human-caused CO
emissions through the combustion of
carbonaceous fuels could be large enough to signiﬁcantly aﬀect
the Earth’s climate through the greenhouse eﬀect.
Edward O. Hulburt conﬁrmed Arrhenius’ theory and estimated
that a doubling and tripling of the CO
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
Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
© XXXX American Chemical Society A DOI: 10.1021/acs.iecr.9b02521
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of 4 and 7 K, while the opposite would hold for cooling due to
a decrease in CO
With his work, published in 1938,
Guy S. Callendar was probably the ﬁrst to report the evolution
in the Earth’s average surface temperature since 1880 based on
collected data from diﬀerent measurement stations.
endar proposed that the observed increase in average
temperature could be owing to accumulation of anthropogenic
emissions into the atmosphere.
The discussion with respect to the eﬀect of anthropogenic
upon the Earth’s climate was revived by Gilbert N. Plass
Plass estimated that global atmospheric CO
concentrations increased with about 30% per century and
resulted in an average temperature increase of 1.1 K per
Moreover, he calculated that doubling the levels of
would cause an average temperature increase of 3.6 K.
One year later, in 1957, Roger Revelle and Hans E. Suess
published their ﬁndings with respect to the role of the oceans
Contrary to what had previously been
assumed, Revelle and Suess found that the average residence
time of CO
in the atmosphere before being absorbed in the
ocean is of the order of 10 years rather than hundreds of
They initially contested earlier propositions that linked
the rise in the Earth’s surface temperature with an (unproven)
increase in atmospheric CO
Suess argued that this would be unlikely because most of the
produced by combustion of fossil fuels
since the beginning of the industrial revolutionwould have
been absorbed by the ocean.
While detailed measurements of
the global temperature had started around 1850,
Keeling started performing measurements of the atmospheric
concentration in 1958.
In the 1960s and 1970s, many new models and estimates of
climate sensitivity to CO
levels were proposed and showed a
strongly varying outcome, causing quite some climate
The model used by Syukuro Manabe and Richard
T. Wetherald in their 1967 publication,
exceptional in that it included all the major occurring physical
phenomena (including convection and conduction) while
taking into account temperatures at the Earth’s 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
concentration, could be considered the ﬁrst
true meteorological model.
Keeling started publishing the evolution of
his atmospheric CO
concentration measurements, which had
steadily increased from 312 to 318 ppm since 1958. In order to
acquire temperature data prior to 1850 and CO
levels before 1958, researchers started studying tree ring
and ice cores,
while developing techniques to
accurately obtain climate information by calibration with
reliable direct measurements. During the 1980s, eﬀorts in
raising awareness about climate change in the US were
orchestrated by scientists and activists such as Gordon
MacDonald, Rafe Pomerance, and James Hansen.
their eﬀorts 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.
historical context to the relationship between CO
environment is provided in Supporting Information (Historical
Current trends in global climate monitoring include the
analysis of data recorded by satellites.
At the same time,
climate modeling has tremendously evolved in the past decades
with “Earth System Models” (ESMs) as the current state of the
Nevertheless, capturing and modeling the complexity of
the Earth’s climate remains a huge task as it essentially
encompasses comprehending the (physical) chemistry, geology
(e.g., eﬀect of volcanic activity), and ﬂ uid dynamics of the
Earth, as well as the occurrence and inﬂuence of processes
occurring in the Sun.
After the ﬁrst IPCC report was published in the 1990s,
awareness within the scientiﬁccommunityhasgrown
tremendously. Today, a large majority of the scientiﬁc
community, estimated at over 95%,
anthropogenic greenhouse gas (such as CO
emissions have a signiﬁcant eﬀect on the Earth’s climate.
Nevertheless, this scientiﬁc consensus is not necessarily picked
up outside the scientiﬁc community, especially in Anglo-Saxon
Apart from purely environmental concerns, the presence of
excessive amounts of CO
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
emissions by its capture, (geological) storage, or recycling
and the active control of atmospheric CO
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 ﬁrst growth wave in the
1990s and a second growth wave in the 2010s. The former may
be ascribed to increasing interest in atmospheric CO
and its eﬀect on climate, while the latter corresponds with
increasing interest in carbon dioxide capture, (geological)
Figure 1. Overview of the evolution of number of scientiﬁc
publications for diﬀerent topics. The evolution of the number of
scientiﬁc publications (Web of Science online database) per year
ﬁltered by diﬀerent keywords is presented. These topical keywords are
the following: (
) “ CO
” AND “atm osphere”
capture” (capture); (
utilization” OR “CO
conversion” OR “CO
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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Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
storage, and reuse (utilization, conversion, recycling). In what
follows, some of the principles behind carbon dioxide capture,
storage, and reuse will brieﬂy be discussed.
3. CARBON DIOXIDE CAPTURE, STORAGE, AND
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
The former is the odd one out because,
through the photosynthesis process, CO
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 signiﬁcant
energy input through high pressure and temperature, albeit on
geological time scales.
However, the carbonation of minerals
in the lithosphere and the absorption of CO
by the oceans are
kinetically limited chemical processes, which result in
chemicals with an even lower energy content than gaseous
In order to mitigate CO
emissions or control its
atmospheric concentration directly, CO
are required. Diﬀerent methods for CO
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,
or ionic liquids
) contacted with a CO
containing gas stream in one vessel, resulting in a CO
eﬄuent and a CO
laden solvent. This solvent is regenerated in
a second vessel with the release of enriched CO
should be resistant to degradation, nontoxic, and noncorrosive
and should show low viscosity and fast reaction kinetics with
Besides these sometimes conﬂicting requirements,
they should preferably also be cheap. While amine- and alkali-
absorption processes for gas puriﬁcation 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.
solutions are less corrosive by nature than amine solutions,
corrosion remains an issue as the alkali solution absorbs ﬂue
gas contaminants and hence degrades.
Besides a loss in
absorption capacity due to precipitation of alkali salts (e.g.,
sulfates or nitrates), the kinetics of CO
absorption are much
slower for alkali solutions compared with amines.
liquids, however, are favorable in terms of their low volatility,
low ﬂammability and thermal stability, though challenges
remain due to the increased viscosity of ionic liquids with
absorption and their toxicity.
Adsorption relies on preferential physisorption of CO
other compounds on a high-surface area adsorbent (e.g.,
or metal−organic frameworks
). The adsorb-
ent is regenerated by applying heat (temperature swing) and/
or decreasing pressure (pressure swing). Important properties
of adsorbent materials include the CO
adsorption capacity, adsorption−desorption kinetics and
resistance to poisoning and degradation.
In this sense, the
highly tailorable nature of metal−organic frameworks provides
a major opportunity in terms of adsorbent properties
Cryogenic separation relies on extensive cooling in order to
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
is already high (e.g., higher than 50%).
challenges lie in the buildup of solid CO
on the heat-
exchanger surface, which deteriorates the overall energy
eﬃciency of the process.
Membrane separation relies on the diﬀerence in permeation
of diﬀerent gases through a membrane (e.g., polymer
or ceramic membrane
). The preferential
permeation of either CO
or the diluting gases allows us to
obtain an enriched CO
stream. Critical properties of the
membrane include selectivity, permeability, and stability.
Carbonation−decarbonation 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 diﬀerent organic and
inorganic species with indication of some homologous series (oleﬁns, alkanes, and alcohols).
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Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
elevated temperature or reduced pressure, a CO
stream can be obtained. Important challenges with respect to
such minerals are the mechanical and structural stability after
prolonged carbonation−decarbonation cycling. Additionally,
the eﬀect of SO
should be borne in mind.
Besides a distinction between CO
capture based on the
separation technology, eﬀorts in CO
capture can be classiﬁed
as direct or indirect CO
capture depending on whether CO
caught from the air directly or from (industrial) exhaust gases.
3.2. Carbon Dioxide Storage. An early suggestion for
capture and storage through geo-engineering came from
Cesare Marchetti in 1977, who proposed to separate CO
ﬂue gases with subsequent disposal either (i) in exhausted gas
or oil ﬁelds and other geological cavities or (ii) by injection in
sinking ocean currents in order to improve the kinetics of CO
Ocean absorption, however, does not seem to be
a favorable idea due to the resulting ocean acidiﬁcation, which
harms marine ecosystems.
While several test projects have
been started with respect to geological CO
uncertainty remains with respect to unfavorable long-term
eﬀects (e.g., seismic activity or leakage).
The reader is
referred elsewhere for a detailed overview.
No doubt the most elegant carbon dioxide storage strategy
would be biological storage by promoting reforestation. The
development of eﬃcient 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 ﬁrst example of CO
utilization is its use in enhanced oil recovery, typically a one-
time use accompanied by CO
storage, whereby CO
is used to
enhance the yield of fossil resource recovery by pumping CO
into geological reservoirs which contain oil and/or gas. While
one could argue that enhanced oil recovery results in CO
storage, it also provides new fossil resources that are likely to
cause signiﬁcant CO
emissions during their life cycle. A
second example is the production of carbonated beverages, in
is stored as H
. Carbon dioxide utilization in
beverages is temporary as CO
will again be released from the
beverage when consumed. A third example is the use of
as an extraction or reaction solvent.
is nonﬂammable, nontoxic, and relatively inert, it
is perceived as an interesting candidate solvent for green
Moreover, the supercritical regime of CO
readily accessible contrary to, e.g., that of water.
All of the above-mentioned methods for carbon dioxide
utilization have in common that they do not contribute to CO
emission m itigation. In the following paragraphs, CO
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
) and a variety of chemicals is presented with an indication of annual global production volumes
(in Mt year
or Gt year
). 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
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 (acidiﬁcation), and (iii) captured in the biosphere by
means of photosynthesis. Double arrows toward the hydrosphere and biosphere indicate the possibility of CO
emission by desorption from the
oceans (in case the atmospheric CO
concentration would decrease) and CO
emission by respiration of organisms (via biomass combustion). In
can be converted into methane (CH
), which is equivalent to natural gas. This work focuses on the conversion of CO
intermediate CO, a platform molecule from which ethanol (C
OH, by fermentation), polycarbonate (through phosgene, COCl
, or dimethyl
CO), and acids (such as acetic acid, CH
COOH) can be formed. Additionally, CO can be transformed into alkanes (Fischer−
Tropsch synthesis), methanol (CH
OH), formaldehyde (CH
O), and aldehydes when a source of (renewable) hydrogen (H
) is available. Alkanes
(catalytic dehydrogenation, catalytic cracking, or steam cracking) or CH
OH (methanol-to-oleﬁns process) can further be transformed into oleﬁns
). Alternatively, alkanes can be further reﬁned in order to yield hydrocarbon fuels (or other petrochemical building blocks), thereby allowing us
to close the carbon loop.
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Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
contribute to CO
emission mitigation and closure of the
3.3.2. Biological Conversion. Biological conversion of CO
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
capture and conversion.
economic feasibility currently hinders the use of algal biofuels
Product diversiﬁcation in an algal
bioreﬁnery concept could provide future opportunities for
the valorization of algae.
The current situation for other
biomass feedstocks is similar to that of algae, as high
production costs associated with biomass processing
their competitiveness with existing processes. Today, biological
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.
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-
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
Finally, carbon dioxide can also be hydrogenated
into chemicals or fuels such as methane (methanation),
formaldehyde, dimethyl ether, formic acid, methanol, and
The eﬃcient 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
Alternatively, carbon dioxide can ﬁrst 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.
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, oleﬁns can be produced from carbon
monoxide via methanol (methanol-to-oleﬁns 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 ﬁgures show that, while
the production of chemicals could provide a carbon sink for
hundreds of Mt year
, production of fuels would be necessary
in order to reach the Gt year
level and close the carbon loop.
The conversion of CO
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
3.4. Pioneers in Industrial Carbon Dioxide Capture,
Storage, and Reuse. Industrial (direct) air capture and the
subsequent recycling of CO
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
as a feedstock has
footprint of the construction sector by combining mineral
waste and CO
carbonation in a novel way of curing concrete
without making use of cement.
4. CHEMICAL LOOPING
4.1. Principle. Chemical looping processes can play an
important role in CO
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
O, or CO
) to the reductant (e.g., C, CH
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 diﬀerent points in time or
+→ +oxidant M reduced product MO
+→ +reductant MO oxidized product M
reduced product oxidized product
Thermochemical looping redox reactions are similar to
ordinary chemical looping redox reactions but diﬀer 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 4−6.
+→ +oxidant M reduced product MO
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TL;DR: In this article, the use of oxygen carriers or redox catalysts for chemical production has been investigated and shown to offer significant opportunities for process intensification and exergy loss minimization.
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.
TL;DR: In this paper, the authors explore the fundamental aspects underpinning solid CO2 sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use.
Abstract: Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO2 emissions. Solid materials capable of reversibly absorbing CO2 have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO2 sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO2 absorption both at surfaces and within solid materials.
TL;DR: In this paper, the authors analyze the kinetics of a single-path reaction represented as a closed sequence of two steps and discuss some examples of chemical looping processes providing, in contrast to catalytic technology, the possibility to perform the steps of a closed-sequence at different conditions.
Abstract: Combining spectroscopic and transient kinetic techniques provides access to the identification and quantification of active sites and corresponding turnover frequencies of catalyzed reactions  . Ab initio calculated rate coefficients combined with network generation codes allow so-called microkinetic models accounting for all relevant elementary steps of complex reaction networks. Computational Fluid Dynamics (CFD) account for the scale-dependent transport of mass, energy and momentum and, hence, renders the design of an industrial process based on the intrinsic chemical kinetics possible. These theoretical and experimental achievements cannot be a substitute for the insights provided by the analysis of a catalytic cycle in terms of a limited number of kinetically significant steps  . We analyse the kinetics of a single-path reaction represented as a closed sequence of two steps  . We discuss some examples of chemical looping processes providing, in contrast to catalytic technology, the possibility to perform the steps of a closed sequence at different conditions.
TL;DR: In this article, 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.
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.
TL;DR: In this paper, a review of recent advances of oxygen carriers (OCs) for chemical looping reforming (CLR) of CH4 which aims to generate high-quality syngas, an important chemical intermediate for the synthesis of chemicals and fuels is presented.
Abstract: This review discussed recent advances of oxygen carriers (OCs) for chemical looping reforming (CLR) of CH4 which aims to generate high‐quality syngas, an important chemical intermediate for the synthesis of chemicals and fuels. We firstly introduced CLR and that combined with CO2 and H2O splitting as well as clarified their advantages over conventional reforming processes. Then we reviewed the latest advances of metal‐based OCs, composite OCs including perovskites and hexaaluminates, and catalytic OCs, namely oxygen‐containing materials with catalytically metal sites, during the past five years. Specific discussion focused on the influence of composition, surface, and bulk structure of these OCs on conversion, selectivity, and lattice oxygen reactivity to summarize design and optimization strategies of the tailored OCs. Finally, a brief summary and an outlook on some of the scientific challenges and suggestions for future investigations on the development of outstanding OCs for CLR of CH4 are given.
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.
Abstract: Background Lignin, nature’s dominant aromatic polymer, is found in most terrestrial plants in the approximate range of 15 to 40% dry weight and provides structural integrity. Traditionally, most large-scale industrial processes that use plant polysaccharides have burned lignin to generate the power needed to productively transform biomass. The advent of biorefineries that convert cellulosic biomass into liquid transportation fuels will generate substantially more lignin than necessary to power the operation, and therefore efforts are underway to transform it to value-added products. Production of biofuels from cellulosic biomass requires separation of large quantities of the aromatic polymer lignin. In planta 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. [Credit: Oak Ridge National Laboratory, U.S. Department of Energy] Advances Bioengineering to modify lignin structure and/or incorporate atypical components has shown promise toward facilitating recovery and chemical transformation of lignin under biorefinery conditions. The flexibility in lignin monomer composition has proven useful for enhancing extraction efficiency. Both the mining of genetic variants in native populations of bioenergy crops and direct genetic manipulation of biosynthesis pathways have produced lignin feedstocks with unique properties for coproduct development. Advances in analytical chemistry and computational modeling detail the structure of the modified lignin and direct bioengineering strategies for targeted properties. Refinement of biomass pretreatment technologies has further facilitated lignin recovery and enables catalytic modifications for desired chemical and physical properties. Outlook Potential high-value products from isolated lignin include low-cost carbon fiber, engineering plastics and thermoplastic elastomers, polymeric foams and membranes, and a variety of fuels and chemicals all currently sourced from petroleum. These lignin coproducts must be low cost and perform as well as petroleum-derived counterparts. Each product stream has its own distinct challenges. Development of renewable lignin-based polymers requires improved processing technologies coupled to tailored bioenergy crops incorporating lignin with the desired chemical and physical properties. For fuels and chemicals, multiple strategies have emerged for lignin depolymerization and upgrading, including thermochemical treatments and homogeneous and heterogeneous catalysis. The multifunctional nature of lignin has historically yielded multiple product streams, which require extensive separation and purification procedures, but engineering plant feedstocks for greater structural homogeneity and tailored functionality reduces this challenge.
TL;DR: In this article, the authors derived formulas for the correlation coefficient between the average of a finite number of time series and the population average, where the subsample signal strength (SSS) and expressed population signal (EPS) were derived.
Abstract: In a number of areas of applied climatology, time series are either averaged to enhance a common underlying signal or combined to produce area averages. How well, then, does the average of a finite number (N) of time series represent the population average, and how well will a subset of series represent the N-series average? We have answered these questions by deriving formulas for 1) the correlation coefficient between the average of N time series and the average of n such series (where n is an arbitrary subset of N) and 2) the correlation between the N-series average and the population. We refer to these mean correlations as the subsample signal strength (SSS) and the expressed population signal (EPS). They may be expressed in terms of the mean inter-series correlation coefficient r as SSS ≡ (Rn,N)2 ≈ n(1 + (N − 1)r)/ N(1 + (N − 1)r), EPS ≡ RN)2 ≈ Nr/1 + (N − 1)r.Similar formulas are given relating these mean correlations to the fractional common variance which arises as a parameter in a...
TL;DR: The core Vema 28-238 as discussed by the authors preserves an excellent oxygen isotope and magnetic stratigraphy and is shown to contain undisturbed sediments deposited continuously through the past 870,000 yr.
Abstract: Core Vema 28-238 preserves an excellent oxygen isotope and magnetic stratigraphy and is shown to contain undisturbed sediments deposited continuously through the past 870,000 yr Detailed correlation with sequences described by Emiliani in the Caribbean and Atlantic Ocean is demonstrated The boundaries of 22 stages representing alternating times of high and low Northern Hemisphere ice volume are recognized and dated The record is interpreted in terms of Northern Hemisphere ice accumulation, and is used to estimate the range of temperature variation in the Caribbean
TL;DR: Using three million Landsat satellite images, this globally consistent, validated data set shows that impacts of climate change and climate oscillations on surface water occurrence can be measured and that evidence can be gathered to show how surface water is altered by human activities.
Abstract: A freely available dataset produced from three million Landsat satellite images reveals substantial changes in the distribution of global surface water over the past 32 years and their causes, from climate change to human actions. The distribution of surface water has been mapped globally, and local-to-regional studies have tracked changes over time. But to date, there has been no global and methodologically consistent quantification of changes in surface water over time. Jean-Francois Pekel and colleagues have analysed more than three million Landsat images to quantify month-to-month changes in surface water at a resolution of 30 metres and over a 32-year period. They find that surface waters have declined by almost 90,000 square kilometres—largely in the Middle East and Central Asia—but that surface waters equivalent to about twice that area have been created elsewhere. Drought, reservoir creation and water extraction appear to have driven most of the changes in surface water over the past decades. The location and persistence of surface water (inland and coastal) is both affected by climate and human activity1 and affects climate2,3, biological diversity4 and human wellbeing5,6. Global data sets documenting surface water location and seasonality have been produced from inventories and national descriptions7, statistical extrapolation of regional data8 and satellite imagery9,10,11,12, but measuring long-term changes at high resolution remains a challenge. Here, using three million Landsat satellite images13, we quantify changes in global surface water over the past 32 years at 30-metre resolution. We record the months and years when water was present, where occurrence changed and what form changes took in terms of seasonality and persistence. Between 1984 and 2015 permanent surface water has disappeared from an area of almost 90,000 square kilometres, roughly equivalent to that of Lake Superior, though new permanent bodies of surface water covering 184,000 square kilometres have formed elsewhere. All continental regions show a net increase in permanent water, except Oceania, which has a fractional (one per cent) net loss. Much of the increase is from reservoir filling, although climate change14 is also implicated. Loss is more geographically concentrated than gain. Over 70 per cent of global net permanent water loss occurred in the Middle East and Central Asia, linked to drought and human actions including river diversion or damming and unregulated withdrawal15,16. Losses in Australia17 and the USA18 linked to long-term droughts are also evident. This globally consistent, validated data set shows that impacts of climate change and climate oscillations on surface water occurrence can be measured and that evidence can be gathered to show how surface water is altered by human activities. We anticipate that this freely available data will improve the modelling of surface forcing, provide evidence of state and change in wetland ecotones (the transition areas between biomes), and inform water-management decision-making.
TL;DR: HadCRUT3 as mentioned in this paper is a new version of this data set, benefiting from recent improvements to the sea surface temperature data set which forms its marine component, and from improving to the station records which provide the land data.
Abstract:  The historical surface temperature data set HadCRUT provides a record of surface temperature trends and variability since 1850. A new version of this data set, HadCRUT3, has been produced, benefiting from recent improvements to the sea surface temperature data set which forms its marine component, and from improvements to the station records which provide the land data. A comprehensive set of uncertainty estimates has been derived to accompany the data: Estimates of measurement and sampling error, temperature bias effects, and the effect of limited observational coverage on large-scale averages have all been made. Since the mid twentieth century the uncertainties in global and hemispheric mean temperatures are small, and the temperature increase greatly exceeds its uncertainty. In earlier periods the uncertainties are larger, but the temperature increase over the twentieth century is still significantly larger than its uncertainty.