sustainability

Review
A Comprehensive Review on Recent Advancements in
Absorption-Based Post Combustion Carbon Capture
Technologies to Obtain a Sustainable Energy Sector with
Clean Environment
Susmita Datta Peu 1, * , Arnob Das 2, * , Md. Sanowar Hossain 2 , Md. Abdul Mannan Akanda 3 ,
Md. Muzaffer Hosen Akanda 4 , Mahbubur Rahman 5 , Md. Naim Miah 6 , Barun K. Das 2 ,
Abu Reza Md. Towfiqul Islam 7 and Mostafa M. Salah 8, *

1 Department of Agriculture, Hajee Mohammad Danesh Science and Technology University,

Dinajpur 5200, Bangladesh
2 Department of Mechanical Engineering, Rajshahi University of Engineering and Technology,
Rajshahi 6204, Bangladesh
3 School of Engineering and Technology, Central Michigan University, Mount Pleasant, MI 48859, USA
4 Department of Manufacturing Engineering, Texas State University, Waco, TX 78666, USA
5 Ingram School of Engineering, Texas State University, San Marcos, TX 78666, USA
6 Design Engineer, Regal Rexnord Corporation, Beloit, WI 53511, USA
7 Department of disaster management, Begum Rokeya University, Rangpur 5400, Bangladesh
8 Electrical Engineering Department, Future University in Egypt, Cairo 11835, Egypt
* Correspondence: susmitapeu77@gmail.com (S.D.P.); arnobarjun@gmail.com (A.D.);
mostafa.abdulkhalek@fue.edu.eg (M.M.S.)

Citation: Peu, S.D.; Das, A.; Hossain, Abstract: CO2 capture, use, and storage have been identified as significant strategies for reducing
M.S.; Akanda, M.A.M.; Akanda, greenhouse gas emissions induced by the usage of fossil fuels. The current review focuses on the
M.M.H.; Rahman, M.; Miah, M.N.; concepts of post-combustion capture technologies based on absorption mechanisms. Among all other
Das, B.K.; Islam, A.R.M.T.; Salah, developed technologies, researchers have proposed absorption as the most mature carbon capture
M.M. A Comprehensive Review on technology for industrial-scale application. Absorption-based carbon capture can be classified into
Recent Advancements in chemical and physical absorption, and researchers have developed different solvents and absorbent
Absorption-Based Post Combustion materials to investigate their performance in CO2 capture. This paper comprehensively reviewed
Carbon Capture Technologies to
these established solvents and absorbents with their performance parameters in the CO2 absorption
Obtain a Sustainable Energy Sector
approach. Besides the improvement in widely applied absorbents such as amine-based absorbents,
with Clean Environment.
recently, researchers have been working to develop some advanced nanomaterials such as nanofluids
Sustainability 2023, 15, 5827.
https://doi.org/10.3390/su15075827
and nano-emulsions. This review focuses on the application of such absorption mechanisms that
can contribute to capturing CO2 in a compact, environment-friendly, and safe way. This paper also
Academic Editors: Rosaria Volpe,
provides future research direction for further development in absorption-based CO2 capture.
Alberto Fichera and
Samiran Samanta
Keywords: carbon capture; post-combustion; absorption; absorbents; physical absorption; chemi-
Received: 24 February 2023 cal absorption
Revised: 16 March 2023
Accepted: 20 March 2023
Published: 27 March 2023
1. Introduction
Global warming caused by greenhouse gas emissions, especially carbon dioxide, is a
Copyright: © 2023 by the authors.
major concern throughout the world. Attempts are being undertaken continually to prevent
Licensee MDPI, Basel, Switzerland. the extent of future environmental change caused by rising emissions of greenhouse gases.
This article is an open access article If not regulated, rising temperatures will eventually lead to rising sea levels, increasing
distributed under the terms and the probability of flooding and storms. The Intergovernmental Panel on Climate Change
conditions of the Creative Commons (IPCC) estimates that by the year 2100, the CO2 content in the atmosphere shall reach
Attribution (CC BY) license (https:// 570 ppmv, the sea level will increase by 3.8 m, and the global mean temperature will rise by
creativecommons.org/licenses/by/ 2 ◦ C with major consequences on the environment [1–4]. According to the ensemble-mean
4.0/). results of state-of-the-art Earth System Models (ESMs), climate warming throughout the

Sustainability 2023, 15, 5827. https://doi.org/10.3390/su15075827 https://www.mdpi.com/journal/sustainability

Sustainability 2023, 15, 5827 2 of 33

21st century is forecasted to be between 1.0 and 3.7 ◦ C, depending on future greenhouse gas
emissions [5,6]. The United Nations Framework Convention on Climate Change (UNFCCC)
held its 21st Conference of the Parties (COP21) in Paris, France, where the primary goal
of COP21 was to establish a lawful climate agreement among 195 countries of the United
Nations to keep the global temperature rise since 1800 even below 2.0 ◦ C (ideally 1.5 ◦ C) by
2100 [7].
Absorption technologies can be integrated with both pre-combustion and post-combustion
processes for carbon capture, and this process can be classified into chemical absorption
and physical absorption. A basic chemical absorption system is composed of three main
parts: solvent, absorber, and stripper. In the absorber, flue gases from various CO2 emitters,
such as coal power plants, come into contact with the lean solution in a counter-current
mechanism in the absorber. The solvents then absorb CO2 , resulting in lower carbon
dioxide levels in the exhaust gases. The stripper then regenerates the solvent-rich CO2 .
This regenerated light solution comes back to the absorber, and compressed CO2 is gathered
and transferred to the stripper’s top. Chemical absorption has been the most reliable
marketed method for many years; however, it has yet to be scaled up in CO2 capture in
power plants. Numerous studies have been conducted and published for the creation of
efficient gas–liquid contactor systems, solvent structures, and stripper configurations to
maximize CC with little energy penalty [8,9]. The central concept was to optimize surface
area and mass transfer for absorption and desorption procedures [10]. Packed bed (PB),
bubble column, spray column, rotating packed bed (RPB), and tray tower absorber layouts
were applied. Some modification methods, such as the addition of numerous columns,
vapor recompression and heat integration in the stripping phase, split flows, and matrix
stripping, have been observed to increase performance [11–14].
In the preceding introductory part, the urgency of carbon capture and the many tech-
nologically viable alternatives in obtaining a similar result to mitigate the serious threat of
global warming have been highlighted. In the following sections, the development patterns
in the key technologies of carbon capture are thoroughly explored, and future difficulties
and possibilities in each of them are reviewed. In each section, a table summarizing the
overall findings has been included.

Status of Global CO2 Emissions

CO2 is emitted from different sources, and the largest amount of CO2 has been emitted
from power plants and energy sectors. Therefore, with increasing energy production, the
amount of CO2 emission also increases. Figure 1 shows the top 10 CO2 -emitting countries,
and it is clear that in recent years, China has been emitting the largest amount of CO2
because it has been producing the largest amount of energy. From the year 1990 to 2005,
CO2 emissions increased by nearly 4000 million metric tons, while a sudden increase was
observed in 2015, when the amount of CO2 emissions passed 10,000 million metric tons
(MMT). Finally, in 2021, China’s CO2 emissions reached 12,000 MMT, and China became the
largest CO2 emitter in the world. According to CO2 emissions statistics, the United States
has been the second-largest emitter for the last two decades (2000–2021). From Figure 1, it
can be observed that from 1990 to 2021, the CO2 emissions in the United States were not
drastic. The amount of CO2 emissions was always in the range of 4000 to 6000 MMT.
Sustainability 2023,
Sustainability 2023, 15,
15, x5827
x FOR
FOR PEER
PEER REVIEW
REVIEW 3of
of 33
33
Sustainability 2023, 15, 33 of 33

Top 10
Top 10 CO
CO22 Emitter
Emitter countries
countries ,, 1990
1990 – 2021
2021
14,000
14,000
Tons)
(MetricTons)

12,000
12,000
CO22(Metric

10,000
10,000
8,000
8,000
ofCO

6,000
6,000
Emissionsof

4,000
4,000
Emissions

2,000
2,000
00
1990
1990 2000
2000 2005
2005 2015
2015 2019
2019 2020
2020 2021
2021
Year
Year
United States
United States China
China Russia
Russia Germany
Germany Ukraine
Ukraine
India
India United Kingdom
United Kingdom Canada
Canada South Africa
South Africa Mexico
Mexico

Figure 1.
Figure 1. Top 10
10 countries that
that emit the
the largest amount
amount of CO
CO2, 1990–2021. (Data
(Data from the
the EDGAR
Figure 1. Top
Top 10 countries
countries that emit
emit the largest
largest amount of
of CO22,, 1990–2021.
1990–2021. (Data from
from the EDGAR
EDGAR
website).
website).
website).

Major
Major CO
Major CO emitters
CO222emitters in
emittersin the
inthe U.S.
theU.S. are
U.S.are listed
arelisted in
listedin Figure
inFigure 2,
Figure2, including
2,including power
includingpower plants,
powerplants, chemical
plants,chemical
chemical
and
and metal processing industries, petroleum and natural gas systems, food processing,
and metal processing industries, petroleum and natural gas systems, food processing, coal
metal processing industries, petroleum and natural gas systems, food processing, coal
coal
mining,
mining, and
and other
other different
different major
major sources.
sources. These
These major
major emitters
emitters can
can be
be promising
promising
mining, and other different major sources. These major emitters can be promising for for
for cap-
cap-
turing
turing CO22CO
CO duedue
due to the large amount of exhaust
exhaust CO22CO
in the
the system.
capturing 2 to the
to large amount
the large of
amount CO
of exhaust in 2 in system.
the system.

Figure 2. Different sources of Carbon Dioxide emissions in the environment.

Figure 2.
Figure 2. Different
Different sources
sources of
of Carbon
Carbon Dioxide
Dioxide emissions
emissions in
in the
the environment.
environment.
Power plants are the largest source of emitted CO2 , which is clear from Figure 3.
Powerto
According
Power plants are the
the EPA
plants are the largest
dataset, source
insource
largest of emitted
emitted
the United
of CO
States,CO 22,, which
there which is clear
are 1369
is clear from in
facilities
from Figure 3.from
total,3.
Figure Ac-
Ac-
cording to the EPA dataset, in the United States, there are 1369 facilities in total,
cording to the EPA dataset, in the United States, there are 1369 facilities in total, from from
Sustainability 2023, 15, 5827 4 of 33
Sustainability 2023, 15, x FOR PEER REVIEW 4 of 33

which 1660 MMT of CO2 was emitted in the year 2019. As populations grow, more power
whichare
sectors 1660 MMT
built, of COleads
which 2 was to
emitted in theenergy
increased year 2019. As populations
consumption. Asgrow, more
a result, power
more power
plants are developed, which causes a higher amount of CO2 emissions. Thepower
sectors are built, which leads to increased energy consumption. As a result, more EPA has
plants areadeveloped,
developed statewide which
plan tocauses a higher
control CO2 emissions CO2 emissions.
amount offrom The EPA
power plants, has de-plans
and these
veloped a statewide plan to control CO 2 emissions from power plants, and these plans are
are listed in Section 111(d) of the Clean Air Act, where the EPA set different performance
listed in Section 111(d) of the Clean Air Act, where the EPA set different performance
standards for multiple sources of pollution, which includes power plants. According to
standards for multiple sources of pollution, which includes power plants. According to
the EPA’s clean power plan, states have the responsibility to cut off 30% of CO2 emissions
the EPA’s clean power plan, states have the responsibility to cut off 30% of CO2 emissions
from power plant sectors from 2005 to 2030. To implement these acts, states have started
from power plant sectors from 2005 to 2030. To implement these acts, states have started
implementing
implementingdifferent
differentpolicies to minimize
policies to minimizepower powersectors’
sectors’ climate
climate impact
impact [15].[15].
The The
EPA EPA
hashas
focused on four major opportunities to reduce CO 2 emission from electricity
focused on four major opportunities to reduce CO2 emission from electricity produc- production
sectors, and these
tion sectors, and opportunities include
these opportunities the increased
include efficiency
the increased of fossil-fired
efficiency power
of fossil-fired plants
power
and fuel switching, renewable energy, increased end-use energy efficiency,
plants and fuel switching, renewable energy, increased end-use energy efficiency, and car- and carbon
capture and sequestration
bon capture (CCS)
and sequestration [16,17].
(CCS) [16,17].

Figure 3. Amount of greenhouse gas emissions during the period of 1990–2020 and CO2 emissions
Figure 3. Amount of greenhouse gas emissions during the period of 1990–2020 and CO2 emissions
from different sectors in the USA. Comm
from different sectors in the USA.
2. Absorption-Based Carbon Capture
2. Absorption-Based Carbon Capture
2.1. Physical Absorption
2.1. Physical Absorption
Henry’s law is considered the base of the physical absorption process for CO2 capture.
Henry’scondition
The operating law is considered the base
should have highofpressure
the physical absorption
and low processinfor
temperature theCO
case2 cap-
of CO2
ture. The operating condition should have high pressure and low temperature in the case
absorption, whereas it is just the opposite for CO2 desorption. Several solvents are already
of CO2 absorption, whereas it is just the opposite for CO2 desorption. Several solvents are
being used commercially to produce synthesis gas and hydrogen [18,19]. In recent years,
already being used commercially to produce synthesis gas and hydrogen [18,19]. In recent
ionic liquid (IL) is achieving much attention for physical absorption due to its uncommon
years, ionic liquid (IL) is achieving much attention for physical absorption due to its un-
characteristics such as non-toxicity,
common characteristics lower vapor
such as non-toxicity, pressure,
lower higher thermal
vapor pressure, stability,stabil-
higher thermal and high
polarity. Table 1 summarizes the physical absorption processes with applicable
ity, and high polarity. Table 1 summarizes the physical absorption processes with appli- absorbent
materials, advantages,
cable absorbent and advantages,
materials, applications.
and applications.

Table 1. Several physical absorption processes with applicable absorbent materials, advantages, and
applications.

Absorption Process Used Absorbent Advantage Application

Low vapor pressure, low tox-
Dimethyl ether/propylene Remove both CO2 and H2S at
Selexol icity, and less corrosive sol-
glycol low temperature
vent
Sustainability 2023, 15, 5827 5 of 33

Table 1. Several physical absorption processes with applicable absorbent materials, advantages, and
applications.

Absorption Process Used Absorbent Advantage Application

Dimethyl Low vapor pressure, Remove both CO2
Selexol ether/propylene low toxicity, and less and H2 S at low
glycol corrosive solvent temperature
Less corrosive and CO2 removal from
Rectisol Methanol
more stable absorbent sulfur-containing gas
Low energy CO2 removal from
Purisol N-methyl pyrrolidone
consumption sulfur-containing gas
Operation cost is 30%
Selective removal of
Morphysorb Morpholine to 40% lower than
H2 S from CO2
that for Selexol
Suitable for gases
Solubility of CO2 in containing CO2
Fluor Propylene carbonate
the solvent is high partial pressure
higher than 60 psig

2.2. Chemical Absorption

The most researched and established approach, the chemical absorption process,
normally consists of two phases. In the chemical absorption process, the exhaust CO2
mixes with a chemical solvent through a chemical reaction and forms an intermediate
compound whose bond is weak enough to get back the original form of CO2 and solvent
by applying heat to the intermediate compound. In the first step, when CO2 -containing
gas is introduced from the bottom into a column of packed bed absorber, the CO2 reacts
with the absorbent. In the second step, the CO2 -rich absorbent enters a stripper where it is
regenerated before being pumped back to the absorber for cyclic applications. The process’
compression, transportation, and storage portions receive the net CO2 emissions from
the stripper. In regard to chemical absorption, CO2 capture technology, alkali absorbents,
inorganic and organic solvents with amine bases, and ILs are the most employed absorbents.
Amine aqueous solvents were initially applied for chemical absorption CO2 capture. After
further research on chemical absorption technologies, mono-ethanolamine (MEA) appeared
to be the most applicable amine solvent due to its high absorptivity of CO2 .

3. Developed Chemical Absorption Processes

3.1. Amine-Based Absorption
Amines are ammonia-derived compounds where at least one hydrogen molecule is
replaced by an organic compound. Different amine solvents such as MEA, DEA, and
MFEA have been applied for the last few decades in natural gas industries, where they
also exhibit the potential to capture CO2 , making them attractive candidates in carbon
capture technologies [20]. The CO2 capture process via amine-based absorption follows the
reactions (Equations (1) and (2)), where R is an alkanol group. The amine reacts as a weak
base to neutralize acidic CO2 and forms carbamate (R-NHCOO− ) through Reaction (1) and
finally forms bicarbonate in the presence of moisture that follows Reaction (2) [18].

2R − NH 2 + CO2 → R − NHCOO− + R − NH 3+ + heat (1)

R − NHCOO− + H2 O → R − NH 2 + HCO3− (2)

According to this mechanism, most of the CO2 absorbed will cause the liquid amine
capture system to produce bicarbonate. The binding between the absorbent and CO2 is
weakened by either raising the temperature or decreasing the pressure of the solution,
which removes the CO2 from the liquid amine solvent to a water stream and regenerates
the solvent for further usage [21]. Higher energy requirements for solvent regeneration,
slower reactivity rates, and lower absorption rates are the major barriers to conventional
Sustainability 2023, 15, 5827 6 of 33

amine-based solvents. To withstand such limitations, Bita Karami et al. have reported that
the nonporous hyper-cross-linked polymeric (HCP) networks can be employed as CO2
absorption rate promoters, and they can dramatically increase CO2 capture via absorption
in N-methyldiethanolamine (MDEA) sorbents [22]. They synthesized two HCPs, namely
polystyrene (HCP-S) and benzene (HCP-B), from cost-effective monomers and suspended
these polymerics in MDEA solutions to form a novel slurry solvent. They discovered that
by employing HCP-B and HCP-S in MDEA solution, the CO2 absorption rate was increased
by 130 and 253%, respectively.

3.2. KMALC
The Kerr-McGee/AGG Lummus Crest (KMALC) is an emerging amine-based adsorp-
tion technology to capture CO2 from flue gases [23,24]. This technology utilizes 15–20 wt%
MEA solution for CO2 absorption, and the low cost of MEA makes this this approach more
applicable for CO2 capture. Researchers reported that a maximum of 800 tons/day of CO2
can be absorbed via KMALC processes, whereas 8000 tons/day of CO2 was emitted from
the reported fossil fuel power plant [25].

3.3. Fluor EFG + Process

The Fluor Economic FG PlusSM is an experimentally proven technology that can be
employed to capture CO2 from flue gases. For capturing CO2 from coal-based power plants’
flue gases, Fluor has been improved to further reduce the energy consumption, operating
and capital costs, and environmental influences of CO2 capture plants in commercial ap-
plications. This technology has the license to be employed in 28 plants around the world.
In EFG+ technology, a chemical solvent that absorbs CO2 via an exothermic reaction has
been used. NRG Energy designed, constructed, and operated a carbon capture demonstra-
tion plant at WA Parish Electric Generating Station, and in this project, they demonstrated
the ability to capture 90% inlet CO2 from a 240 MW equivalent flue gas slipstream that was
exhausted from a coal-fired boiler [26]. Among recent Fluor FG+ -based projects, the Electric
Power Research Institute (EPRI), along with California Resources Corporation (CRC) and
Fluor Corporation, has investigated Fluor’s economic FG OlusTM -based carbon capture
technology on a natural gas-fired combined cycle power plant in California. This carbon
capture technology was integrated with the Elk Hills Power Plant (EHPP), which is located
near Kern County, California. Flue gas can be divided in two ways: in one way, 79% of flue
gas moves through Fluor’s EFG+ unit, and in another way, 21% of flue gas is ventilated via
the stack. The CO2 capture unit can capture 90% inlet CO2 from the flue gas with 97+% CO2
purity, and 4000 tons of CO2 /day can be captured through this carbon capture technology
in this plant [27].

3.4. KM-CDR Process

Mitsubishi Heavy Industries, Ltd. (MHI) and the Kansai Electric Power Co., Inc.
(KEPCO, Osaka, Japan) have developed a CO2 capture technology based on the KM-CDR
(Kansai Mitsubishi Carbon Dioxide Recovery) process and have integrated the carbon
capture plant with the Petra Nova Project, which is considered the largest CO2 capture
plant. In this project, a new solvent for CO2 capture was prepared, and this new solvent
showed better performance compared to KS-1TM in terms of solvent degradation, emissions,
and steam consumption rate. The new solvent possessed 50% less solvent degradation,
50% less solvent emission, and approximately 5–10% less steam requirement compared to
KS-1TM [28]. MHI and the research and development activities of the Nanko Pilot Plant
have started the CO2 capture operation at a 25 MW coal-based power plant at Southern
Company’s Plant Barry, which is operated and developed by Alabama Power. This plant is
considered the world’s first carbon capture technology integrated with a coal-based power
plant. It was estimated that approximately 150,000 tons (500 tons/day) of CO2 can be
captured annually with a CO2 capture rate of 90+% [29]. MHIENG’s (Mitsubishi Heavy
Industries Engineering, Ltd., Singapore) latest process, the KM-CDR process with KS-1TM ,
Sustainability 2023, 15, 5827 7 of 33

investigated the near-zero emission and CO2 ratio and found a 99.5% CO2 capture ratio.
With their default system, it was found that at a 99.5% CO2 capture ratio, the reboiler steam
and solvent rate increased by 15% and 25%, respectively, whereas by employing 50% more
absorption packing and maintaining the operating expenditure (OPEX) ($/tonne CO2 ) as
the base case, a 50% reduction of reboiler steam and solvent rate can be achievable [30].

3.5. Chilled Ammonia Process (CAP)

Eli Gal has patented his research on the chilled ammonia process (CAP) that can
absorb CO2 at much lower temperatures, and this technique also minimized the density of
moisture and volatile and acidic elements existing in the gas. CAP also minimizes NH3
slip at a lower amount and controls the NH3 loss to less than 6% of the solvent compared
with the conventional aqueous ammonia process [31]. Daniel Sutter et al. developed an
advanced ammonia-based CO2 capture technology that controlled the solid formation
chilled ammonia process (CSF-CAP). A comparison of CSF-CAP was carried out with con-
ventional CAP (L-CAP) technology since a solid handling section, and from the scrutinized
comparison it was found that in the CSF-CAP process reduction of steam, the requirement
was minimized by 30% for CO2 desorption and the SPECCA (Specific Primary Energy
Consumption for CO2 Avoided) by 17% [32]. There are some existing limitations of CAP
technologies, including low absorption rates, the requirement of multiple absorber vessels,
and the volatility of ammonia [33].

3.6. Aqueous Ammonia Scrubbing

Ammonia is considered a potential alternative to MEA, as it possesses higher availabil-
ity, higher corrosion resistance, lower solvent degradation, lower expense, and higher CO2
absorption capacity compared to MEA. Aqueous ammonia can capture CO2 with higher
purity, and it also captures other coexisting elements in flue gases such as SO2 , NOX , HCL,
HF, and different acidic gases. This process can minimize the cost of a plant by eliminating
the acidic gas clean-up system, and the SOX and NOX can be converted to fertilizer that
can make money and reduce the plant’s cost. The CO2 absorption by ammonia is carried
out following Equations (1)–(3), where in the first step, ammonia reacts with CO2 and pro-
duces ammonium carbamate in dry conditions (Reaction (3)). After adding some moisture,
ammonium carbonate is formed from ammonium carbamate (Reaction (4)), and then it is
converted into ammonium bicarbonate (Reaction (5)).

2NH 3( g) + CO2( g) ⇐⇒ NH 2 COONH 4(s) (3)

NH 2 COONH 4(s) + H2 O( g) ⇐⇒ ( NH 4 )2 CO3(s) (4)

NH 2 COONH 4(s) + H2 O(l ) ⇐⇒ NH 4 HCO3(s) + NH 3( g) (5)

Aqueous ammonia scrubbing is more advantageous than amine-based processes, as
ammonia-based solvents consume a lower amount of energy for solvent regeneration and
are less expensive than amine-based processes [34]. There are also some limitations of
aqueous ammonia solvents compared to amine-based solvents, including the requirement
for a larger absorber area, higher initial capital cost, and lower CO2 absorption capacity [35].
Moreover, a higher loss of ammonia causes higher uses of wash water, which results in a
higher recovery cost of ammonia. To solve the problem of ammonia loss, Hamed Rashidi
et al. developed an ammonia-glycerol hybrid solvent to capture CO2 glycerol, which is a
byproduct of biodiesel has hydroxyl groups which can bind ammonia molecules and hence
the vaporization of ammonia reduces. Their experimental output reveals that applying
glycerol with aqueous ammonia causes an increase in the mass transfer coefficient and a
reduction in the vapor pressure of ammonia that results in lower losses of ammonia in the
CO2 capture system [36].
Sustainability 2023, 15, 5827 8 of 33

3.7. Amino Acid Absorption

The usability of amino acids in CO2 capture systems makes them a potential alterna-
tive to amine-based absorption systems. There are some crucial limitations in conventional
amine-based solvents, such as the volatility of solvents [37,38], degradation of solvents [39],
lower corrosive resistance [38,39], higher energy requirement for regeneration [38,40], and
lower CO2 loading capacity [41]. On the other side, amino acid solvents possess some
attractive features, such as a higher corrosion resistance, higher degradation resistance,
lower volatility [37,38], and lower energy requirements compared to amine-based solvents.
Moreover, the utilization of amino acids in CO2 is also preferable due to their natural avail-
ability and biodegradability, and so, these solvents can minimize environmental hindrances.
Additionally, hydrophilic amino acids are capable of operating under a wide range of tem-
peratures and pressure conditions required for carbon capture [42]. Cheng et al. reported
an ion pair arginine-arginine carbamate derived from L-arginine (amino acid) to capture
CO2 from gases, and their study expanded the understanding behavior of L-arginine and
other amino acids for CO2 absorption [43]. The application of amino acid anions in CO2
absorption can enhance the CO2 uptake twofold for some amino groups, and possible
reactions were reported by Stefano et al. [44]. Maria Castro analyzed the use of sodium
(Na) salts of aqueous solutions of different amino acids such as glycinate and prolinate for
CO2 capture via chemical absorption, and they experimented with this solution in a bubble
column reactor under various testing conditions such as flow rates of flue gas and solvent
concentrations [45]. Carboxylic groups have caused the major differences in reactions
and absorption rates, and it has also been reported that glycinate possessed greater CO2
loading than mono-ethanolamine (MEA) due to higher destabilization of carbamate of
glycinate. To minimize the energy requirement for solvent regeneration, different crystal-
lization processes can be employed with aqueous amino acid solvents. The crystallization
of bis-iminoguanidines (Glyoxal-bis-iminoguanidine or GBIG) employed with aqueous
amino acids is being developed as a potential technology to reduce the energy penalty from
a CO2 capture plant by minimizing the energy consumption for solvent regeneration, and
the CO2 loading capacity was reported as 1.36 mol per mol of GBIG [46].

3.8. Dual Alkali Absorption (DAA)

Alkali metal carbonate solvents are a potential alternative to the conventional solvents
applied in CO2 capture technologies, and these alkali-based solvents can be employed
through dual alkali absorption (DAA) processes. There are some attractive advantages to
applying this CO2 process, including lower degradation, lower emissions, and lower costs,
whereas the limitations are a lower rate of CO2 mass transfer and a lower and slower rate
of reaction. To overcome these limitations, Yang Li et al. developed a dual alkali solvent
(DAS) that is different from the conventional solvents in terms of phases as DAS has two
aqueous phases. In the first phase, an organic alkali 1-(2-hydroxyethyl) piperazine (HEP)
was employed for CO2 absorption, while in the second phase, a mixture of K2 CO3 /KHCO3
aqueous solution and KHCO3 precipitate was employed for CO2 stripping. From their
experimental investigation, they revealed that by implementing DAS to capture CO2 , 55.7%
of energy can be saved without lowering the CO2 absorption efficiency from 90% [47].
This novel system has faster CO2 absorption kinetics and a low energy need for solvent
regeneration, which are advantages of both amine and alkali metal carbonate, respectively.

3.9. Alkaline Solvent Absorption

Alkaline solvents such as sodium hydroxide or potassium hydroxide are potential
solvents that are widely applied for CO2 from the air. When air meets such absorbents,
then CO2 is captured via a chemical reaction following Reaction (6), where CO2 reacting
with NaOH forms sodium carbonate. Normally, the regeneration of this type of solvent
(NaOH) is carried out by dosing another alkaline solvent (Ca(OH)2 ), and calcite forms due
to this Reaction (7). For further treatment, CaCO3 is heated at 700 ◦ C to form calcium oxide
Sustainability 2023, 15, 5827 9 of 33

(CaO) and CO2 through Reaction (8), and then, by rehydrating CaO, regenerated Ca(OH)2
can be obtained through Reaction (9) [48].

2NaOH + CO2 → Na2 CO3 + H2 O ∆H 0 = 109.4 KJmol−1 (6)

Na2 CO3 + Ca(OH )2 → CaCO3 + 2NaOH (7)

CaCO3 → CaO + CO2 (8)

CaO + H2 O → Ca(OH )2 (9)

Alkali absorption is advantageous in terms of solvent availability and the cost-effectiveness
of the solvent, but the treatment of CaCO3 is much more expensive, which hinders the
economic feasibility of these absorbents.

4. Different Types of Absorbents

4.1. Polymeric Solvent
In recent times, polymeric solvents have attracted much attention for further research
due to their applicability in CO2 capture technologies. There are some potential conve-
niences of these technologies, including lower viscosity, thermal stability, hydrophobicity,
lower capital and operation costs, simplified installation and maintenance, and lower
volatility. Among several polymeric solvents, a few that have emerged, such as poly-
dimethylsiloxane (PDMS), polyesters, and polyethers (Pes), are remarkable. Miller et al.
experimented with several solvents to compare their performances regarding the selectivity
of CO2 , and these solvents included polypropylene glycol dimethyl ether (PPGDME),
polyethylene glycol dimethyl ether (PEGDME), poly-dimethyl siloxane (PDMS), perflu-
orpoly ether (PFPE), polybutylene glycol diacetate (PBGDAc), and polypropylene glycol
diacetate (PPGDAc) [49]. Their experiment revealed that PDMS and PPGDME were the
most promising alternative for CO2 capture from a mixed stream. Xingguang et al. prepared
novel absorbent amine-infused hydrogels (AIHs) by adding hydrogels with organic amine
solutions, and they found where hydrogels ensure the increased interfacial area to capture
the higher amount of CO2 . Their result also showed that the CO2 uptake capacity of AIHs is
higher than that of aqueous amine solutions operating under the same conditions. Kim et al.
prepared AIH in a disparate way than before by just adding a monoethanolamine (MEA)
solution with dried hydrogel particles, and their results showed that AIHs showed higher
CO2 uptake capacity and selectivity than MEA solutions [50]. Dimethyl ether of polyethy-
lene glycol (DMEPEG) is a novel polymeric solvent that offers potential conveniences
such as being suitable for both carbon and sulfur contents, less corrosion, and a lower
power consumption rate. In recent times, researchers have developed several integrated
systems where they prepared carbon capture technologies by adding several polymers and
copolymers with membrane-based materials. Tao et al. prepared such technologies to ex-
amine the effect of several copolymers by adding them during membrane fabrication time;
they applied commercially available poly(2,6-dimethyl-1,4-phenylene oxide) (PPE) and
additives including polyethylene glycol (PEG) and a PEG–PDMS copolymer (commercially
known as IM22). After the experiment, they compared with pure PPE membrane, and
they found that CO2 permeability rose nearly 5 times after adding 50 wt% IM22 and rose
4 times after adding 40 wt% PEG. Table 2 summarizes the merits and demerits of different
polymeric solvents.
Sustainability 2023, 15, 5827 10 of 33

Table 2. Merits and demerits of different polymeric solvents.

Polymeric Solvent Merits Demerits Ref.

Hydrophobic
Requires higher
polymeric solvents Selective CO2 absorption. [49,51]
temperature.
(PPGDME & PDMS)
Solid sorbents,
Tendency to agglomerate
can be easily manufactured
with each other without
Amine-infused at a large scale,
mixing, which resulted in [50,52,53]
hydrogels (AIHs) fast kinetics,
decreased CO2
minimal performance
absorption capacity.
degradation after recycling.
Suitable for both CO2 and
H2 S.
Less corrosive than
Di-Methyl-Ether of Varieties in solvent
chemical solvents.
poly-Ethylene-Glycol processing ability within [54,55]
Consume a lower amount
(DMEPEG) a specific tower.
of power.
Lower heat duty for gas
desorption.
For post-combustion CO2
Low-viscous High capacity for CO2 .
capture, poly ethers are [56–58]
branched polymers Large selectiveness.
not suitable.
Well-suited for
The existence of flowing
PEG-PDMS precombustion carbon
gas may cause severe [59,60]
copolymer capture.
foaming,
Selective CO2 capture.
Selective uptake of CO2 A fall in CO2 solubility
from different gases (H2 S, may be caused due to
PDMS solvents H2 O). increased temperature. [61,62]
Higher solubility and High cost for operational
thermal stability. processes.

4.2. Poly-Ionic Liquid

Poly-ionic liquids (PILs) are a class of ionic liquids (ILs) that are effective as CO2
absorbents. These liquids are characterized by the presence of multiple ionic groups in
the molecule, which allows for a high level of CO2 binding. PILs have been studied
extensively in recent years due to their potential use in carbon capture and storage (CCS)
technologies. One of the main advantages of PILs as CO2 absorbents is their high CO2
binding capacity. This is due to the presence of multiple ionic groups in the molecule, which
can form multiple complexation sites for CO2 . Additionally, PILs are more effective at
binding CO2 than traditional amine-based absorbents, as they can form stronger complexes
with CO2 . Another advantage of PILs is their thermal stability. Unlike traditional amine-
based absorbents, which can degrade at high temperatures, PILs can maintain their CO2
binding capacity even at high temperatures. This makes them well-suited for use in CCS
technologies that involve high-temperature processes, such as post-combustion capture.
PILs have also been found to be highly selective for CO2 , which means that they have a
low affinity for other gases, such as N2 and O2 . This is important in CCS applications,
as it allows for a higher concentration of CO2 to be captured. Additionally, PILs are non-
corrosive, which is a significant advantage over traditional amine-based absorbents that can
be corrosive to certain metals. Chau et al. developed a novel cyclic -5-valve pressure swing
membrane absorption where they applied ionic liquid (1-butyl-3-methyl-imidazolium
dicyanamide) as an absorbent, aiming to separate CO2 from lower temperature syngas [63].
Their experimental results revealed the potentiality of their system, and 95.5% CO2 can
be yielded via that mechanism. Ionic liquid membranes are potential elements for CO2
absorption, and these can be classified into supported ionic liquid membranes (SILMs) and
 Sustainability 2023, 15, x FOR PEER REVIEW 11 o

Sustainability 2023, 15, 5827 11 of 33

option for CCS. Several different types of PILs have been studied for use as CO2 ab
bents. These include quaternary ammonium-based PILs, phosphonium-based PILs,
imidazolium-based
quasi-solidified PILs. Each
ionic liquid membranes of these[64].
(QSILMs) typesTwo
of PILs has its unique
approaches properties,
to utilize ionic and
searchers are working to optimize the CO 2 binding capacity and selectivity of each typ
liquid as membrane material for CO absorption are shown in Figure 4.
2

Figure 4. Utilizing ionic liquid as membrane material [64]. © 2016, copyright permission, Elsevier.
Figure 4. Utilizing ionic liquid as membrane material [64]. © 2016, copyright permission, Elsev
In addition, PILs are highly reusable, which is a major advantage over traditional
4.3. Nano Sorbent
amine-based absorbents that must be replaced after a certain number of usages. PILs
4.3.1. Nanofluids
can be regenerated by heating for them
CO2 Absorption
to release the absorbed CO2 , which can then be
captured and stored. This process can
Nanotechnology is a novel be repeated multiple
technology times,
that which makes
is broadly utilizedPILs
in anumerous
cost- ene
effective option for CCS. Several different types of PILs have been
systems to produce energy in an energy-friendly, economical way. In recent studied for use as CO 2 years, C
absorbents. absorption
These include via quaternary
nanofluids has ammonium-based PILs, phosphonium-based
attracted much attention due to this methodPILs,having a hig
and imidazolium-based
capacity for CO PILs. Each of these
2 absorption. Thetypes
conceptof PILs has its unique
of nanofluids properties,
was first proposed andby Choi,
researchers are workingnanofluids
he defined to optimizeasthe CO2 binding
dispersed capacitymaterials
nano-sized and selectivity
into theof soluble
each type.
base mate
Numerous nanomaterials, such as nanorods, droplets, nanowires, nanoparticles, and n
4.3. Nano Sorbent
ofibers, can be applied to prepare nanofluids, whereas water-soluble or non-water-solu
4.3.1. Nanofluids for CO2 Absorption
liquids (Al2O3, TiO2, SnO2) can be employed as base materials [65].
Nanotechnology is a novel technology that is broadly utilized in numerous energy
systems to produce energy in an energy-friendly,
4.3.2. Nano-Emulsions for CO2 Captureeconomical way. In recent years, CO2
absorption via nanofluids
Nano-emulsions are amuch
has attracted new attention due to thisthat
class of materials method
have having
recently a higher
been proposed
capacity for CO2 absorption. The concept of nanofluids was first proposed by Choi, and
potential absorbents for carbon dioxide (CO2) capture. These materials are composed
he defined nanofluids as dispersed nano-sized materials into the soluble base material.
small droplets of one liquid suspended in another liquid, and they have unique proper
Numerous nanomaterials, such as nanorods, droplets, nanowires, nanoparticles, and
that make them well-suited for CO2 capture applications. One of the main advantage
nanofibers, can be applied to prepare nanofluids, whereas water-soluble or non-water-
nano-emulsions as CO2 absorbents is their high CO2 uptake capacity. The small drop
soluble liquids (Al2 O3 , TiO2 , SnO2 ) can be employed as base materials [65].
of the nano-emulsion have a large surface area to volume ratio, which allows them
efficiently capture
4.3.2. Nano-Emulsions for CO2 CO 2. Additionally, nano-emulsions can be formulated to have a h
Capture
selectivity for CO2, meaning that they can effectively capture CO2 while leaving ot
Nano-emulsions are a new class of materials that have recently been proposed as
gases such as N2, O2, and CH4 behind. Another advantage of nano-emulsions is their
potential absorbents for carbon dioxide (CO2 ) capture. These materials are composed of
bility. These materials are thermodynamically stable and can remain stable over a w
small droplets of one liquid suspended in another liquid, and they have unique properties
range of temperatures and pressures. This makes them well-suited for use in CCS ap
that make them well-suited for CO2 capture applications. One of the main advantages of
nano-emulsionscations, which
as CO often involve capturing CO2 at high temperatures and pressures.
2 absorbents is their high CO2 uptake capacity. The small droplets
of the nano-emulsion Nano-emulsions
have a large can be prepared
surface area to by a variety
volume of which
ratio, methods, including
allows high-press
them to
efficiently capture CO2 . Additionally, nano-emulsions can be formulated to have a highof the na
homogenization, ultrasonication, and micro-fluidization. The properties
emulsion
selectivity for can bethat
CO2 , meaning tailored
they canby adjusting
effectivelythe composition
capture CO2 while andleaving
concentration of the differ
other gases
such as N2 , O2 , and CH4 behind. Another advantage of nano-emulsions is their stability. of the m
components, such as the type of oil, surfactant, and co-surfactant used. One
common
These materials types of nano-emulsion
are thermodynamically stableused
andascanCOremain
2 absorbent is oil-in-water (O/W) nano-em
stable over a wide range
sion, which is composed of small droplets
of temperatures and pressures. This makes them well-suited for use in CCS of oil suspended in water. The oil droplets
applications,
be formulated to have a high CO 2 uptake capacity, and the water can act as a solvent
which often involve capturing CO2 at high temperatures and pressures.
the CO2. Research
Nano-emulsions has shown
can be prepared by athat O/Wofnano-emulsion
variety methods, includingcan adsorb up to 40 times m
high-pressure
CO than bulk oil, due to the high surface area
homogenization, ultrasonication, and micro-fluidization. The properties of the
2 to volume ratio of thenano-
droplets (ref
emulsion can Another typeby
be tailored of nano-emulsion that has been
adjusting the composition andproposed as a potential
concentration CO2 absorbent is
of the different
components,water-in-oil
such as the(W/O) type ofnano-emulsion,
oil, surfactant, and which is composed
co-surfactant of small
used. One of droplets
the most of water s
common types pended in oil. These nano-emulsions
of nano-emulsion used as CO2 absorbenthave a higher density than(O/W)
is oil-in-water the O/W nano-emuls
nano-
emulsion, which is composed of small droplets of oil suspended in water. The oil droplets
can be formulated to have a high CO2 uptake capacity, and the water can act as a solvent
Sustainability 2023, 15, 5827 12 of 33

for the CO2 . Research has shown that O/W nano-emulsion can adsorb up to 40 times more
CO2 than bulk oil, due to the high surface area to volume ratio of the droplets [1]. Another
type of nano-emulsion that has been proposed as a potential CO2 absorbent is the water-
in-oil (W/O) nano-emulsion, which is composed of small droplets of water suspended
in oil. These nano-emulsions have a higher density than the O/W nano-emulsion, which
makes them more suitable for use in CCS applications where the CO2 is being captured at
high pressures. W/O nano-emulsions have been reported to have a high CO2 adsorption
capacity and stability at high pressures and temperatures. A third type of nano-emulsion
that has been studied for CO2 absorption is the multiple emulsion, which is composed
of droplets of one liquid (typically water) suspended within droplets of another liquid
(typically oil), which are then suspended in a third liquid (typically water). These emulsions
have been found to have high CO2 adsorption capacity and stability, as well as the ability
to separate CO2 from other gases, making them promising candidates for CCS applications.
Despite the promising properties of nano-emulsions for CO2 absorption, there are
still some challenges that need to be overcome before they can be used in real-world
applications. One of the main challenges is the cost of producing nano-emulsions on a
large scale. The methods used to prepare nano-emulsions are often energy-intensive, which
can make them more expensive than other CO2 absorbents. Additionally, nano-emulsions
can be difficult to separate from the captured CO2 , which can also add to the cost of the
overall process.

4.4. Amino Acid Solution

Amino acid salt solutions are being recognized as promising absorbents to be used in
CO2 capture technologies due to their several amenities, such as higher surface tension,
minor absorbent loss, lower oxidative degradation, and evaporation rate while being envi-
ronmental [37–39]. AASs are normally produced through the reaction of alkaline substances
and amino acids, while there are more than 20 standard amino acids for which diverse
AASs can be observed. The most common amino acids that are employed to produce
AASs include arginine, glycine, glutamine, lysine, and taurine, while potassium is the most
common ingredient used to produce the AAS solution. Potassium lysine was recognized as
a more effective absorbent solution due to having a higher CO2 loading capacity compared
to MEA and other AASs. In the case of AASs, the environmental pollution is lower because
of the easy disposal of absorbed solutions due to their natural reaction, ionic nature, and
extraordinary biodegradation possess. Ramazani et al. experimented with a MEA+PL
solution to investigate the effect of the addition of PL and MEA on CO2 loading capacity
and found that with an increasing PL/MEA ratio, the loading capacity and corrosion rate
of the blend solution increased [66].

4.5. Hydroxide Absorbent

4.5.1. Potassium Hydroxide
The aqueous solutions of alkali metal hydroxides have gathered much attention from
scientists and researchers for employing CO2 capture technologies due to their lower
energy consumption and minimal negative influence on the environment and ecosystem.
Rastegar et al. [67] experimented with aqueous KOH to investigate the effects of stirring
and temperature on CO2 absorption, and their results revealed that just by increasing the
stirring of the mixer from 50 to 150 rpm, the absorption rate raised by 32%, whereas when
they increased the temperature from 22 ◦ C to 65 ◦ C, CO2 absorption dropped by 2.4%.
Mourad et al. experimented with different effects on CO2 absorption, such as CO2 inlet
concentration, KOH concentration, temperature, gas flow rate, pressure, and found that
KOH concentration did not influence CO2 absorption [68]. However, Firman et al. found
different results from their experimental investigation, and their results revealed that an
increment occurred in CO2 absorption when KOH concentration was increased [69].
Sustainability 2023, 15, 5827 13 of 33

4.5.2. Sodium Hydroxide-Based Hybrid Absorbents

Sodium hydroxide (NaOH) has been an extensively studied topic for CO2 absorption
Sustainability 2023, 15, x FOR PEERsince
REVIEWthe 1940s; however, at that time, NaOH was not studied for CO2 capture 13 of 33
[70,71].
Sodium hydroxide is considered an alkaline absorbent of acidic gases due to its ability to
capture CO2 at ambient temperature as well as its fast kinetics, availability, and reasonable
price [72].CO
capture Traditional
2 at ambient procedures
temperature followed a water
as well as its fastwash with
kinetics, caustic solutions
availability, of NaOH
and reasonable
pricewt%).
(5–15 [72]. Traditional
Since the wasted procedures followed
caustic creates aa water
disposal wash with caustic
problem solutions
as expected, of NaOH
it must first be
(5–15 wt%).with
neutralized Since the before
acid wastedbeing
caustic creates adisposed
properly disposal of problem
followingas expected, it must first
current environmental
be neutralized
and hazardous waste with acid before beingAside
requirements. properly
fromdisposed
that, theofannual
following current environmen-
production of hundreds of
tal and hazardous waste requirements. Aside from
millions of tons of sodium carbonate and sodium bicarbonate makes that, the annual production of hun-
the CO2 absorption
from flue gases commercially relevant. However, NaOH is generated as waste inCO
dreds of millions of tons of sodium carbonate and sodium bicarbonate makes the some
2

absorption from flue gases commercially relevant. However, NaOH

chemical processes, such as the manufacturing of chlorine, which lowers the price of CO2 is generated as waste
in some from
removal chemicalflueprocesses,
gases [73]. such as the manufacturing
According of chlorine,
to study findings, which lowers
increasing the price
the concentration
ofofNaOH
CO2 removal
reducesfrom flue loss
energy gasesbut
[73]. According
increases to studyrate
corrosion findings, increasing
and solution the concen-
viscosity. Alkali
tration
metal of NaOH such
hydroxides reduces energy loss
as sodium but increases
hydroxide corrosion arate
are considered and solution viscosity.
well-established technology
Alkali
for CO2 metal hydroxides
capture; however, such
theas sodium
major hydroxide
drawbacks of are considered
it are a well-established
its high corrosion rate andtech-
energy
nology for CO 2 capture; however, the major drawbacks of it are its high corrosion rate and
loss. In this regard, glycerol can be applied as a potential element in aqueous NaOH
energy loss.
solutions which Inintensifies
this regard,theglycerol can beperformance
mass transfer applied as aaspotential element inthe
well as minimizes aqueous
offensive
NaOH solutions which intensifies the mass transfer performance
wastes and related pollutions. Sheyda et al. experimented with such NaOH-Gly solution as well as minimizes the
offensive wastes and related pollutions. Sheyda et al. experimented with such NaOH-Gly
to investigate the effect of glycerol on the CO2 capture performance of aqueous NaOH
solution to investigate the effect of glycerol on the CO2 capture performance of aqueous
solution and found that the presence of glycerol in the solution caused CO2 absorption
NaOH solution and found that the presence of glycerol in the solution caused CO2 absorp-
efficiency of more than 97%, as shown in Figure 5 [74].
tion efficiency of more than 97%, as shown in Figure 5 [74].

Figure 5. CO2 outlet concentration and CO2 absorption efficiency versus time under different
Figure 5. CO2 outlet concentration and CO2 absorption efficiency versus time under different NaOH
NaOH concentrations (Qg = 200 mL/min, C-Gly = 8 wt%, T = 25 ◦ C) and glycerol concentration at
concentrations (Qg = 200 mL/min, C-Gly = 8 wt%,◦ T = 25 °C) and glycerol concentration at at (Qg =
(Qg
200=mL/min,
200 mL/min,
C NaOH C= NaOH = 0.5T M
0.5 M and = and T [74].
25 °C) = 25Copyright
C) [74]. Copyright
permissionpermission © 2022by
© 2022 Published Published
Elsevier by
Elsevier
Ltd. Ltd.

4.6. Carbonate Absorbent

4.6. Carbonate Absorbent
4.6.1. Potassium Carbonate
4.6.1. Potassium Carbonate
Potassium carbonate (K2 CO3 ) is a favorable CO2 absorbent due to it having some
Potassium carbonate (K2CO3) is a favorable CO2 absorbent due to it having some at-
attractive properties such as relevant cost, low degradation rate, minimal toxicity, solubility
tractive properties such as relevant cost, low degradation rate, minimal toxicity, solubility
in carbonate/bicarbonate solution, and less energy consumption. Zhao et al. caused the
in carbonate/bicarbonate solution, and less energy consumption. Zhao et al. caused the
formation of sesqui-hydrated potassium carbonate crystal (K2 CO3 .1.5H2 O) for the lower
formation of sesqui-hydrated potassium carbonate crystal (K2CO3.1.5H2O) for the lower
energy requirement of K2 CO3 [75]. Moreover, Thee et al. stated that 37% of the energy
energy requirement of K2CO3 [75]. Moreover, Thee et al. stated that 37% of the energy
requirement
requirement can beminimized
can be minimizedbyby utilizing
utilizing potassium
potassium carbonate
carbonate duringduring regeneration
regeneration pro-
processes [76]. Despite having such advantages, this absorbent also brings
cesses [76]. Despite having such advantages, this absorbent also brings a limitation, a limitation,
which
which
is its is its poor
poor mass mass transfer
transfer rate.
rate. To To overcome
overcome this limitation,
this limitation, researchers
researchers have c have
arriedc out
arried
out several
several experiments
experiments to investigate
to investigate thethe effect
effect of different
of different promoters
promoters thatthat
cancan
be be applied
applied
with K CO to increase its performance.
with K2CO3 to increase its performance.
2 3
The overall reaction of CO2 and potassium carbonate is as follows:
+ + ⇌ 2 (10)
Bicarbonate formation:
Sustainability 2023, 15, 5827 14 of 33

The overall reaction of CO2 and potassium carbonate is as follows:

CO2 + H2 O + K2 CO2 2KHCO3 (10)

Bicarbonate formation:

CO2 + OH − HCO3− (Fast reaction) (11)

Carbonate formation:

HCO3− + OH − CO23− + H2 O (12)

Hu et al. discussed detailed information on different promoters for potassium car-

bonate to be applied in CO2 capture technologies [77]. Zheng et al. proposed a novel
[Cho] [Pro] + K2 CO3 absorbent for CO2 capture and experimented with different weights
of the solution to investigate CO2 absorption and desorption performances [78]. Choi et al.
experimented with K2 CO3 -based absorbents for CO2 absorption to study the influences of
promoters, evaluating the mass transfer coefficient for a specific absorbent [79]. Their ex-
perimental results revealed that the mass transfer rate was highest for K2 CO3 +CL-4 among
other promoted K2 CO3 solutions, and it was quite similar to commercial MEA solutions.

4.6.2. Sodium Carbonate

Sodium carbonate, also known as soda ash, is a chemical compound that has been
used as a CO2 absorbent in various industrial processes. Sodium carbonate has been
found to be an effective and low-cost option for removing CO2 from gas streams. The CO2
capture process using sodium carbonate typically involves the reaction of CO2 with sodium
carbonate to form sodium bicarbonate and carbonate ions. This reaction is exothermic,
which means it releases heat, and it can be represented by the following equation:

Na2 CO3 + CO2 + H2 O → NaHCO3 + Na2 CO3 (13)

Na2 CO3 is a strong base, and it can effectively capture CO2 from flue gas, biogas, and
other sources. It has been found to be highly selective for CO2 over other gases, which
makes it an efficient absorbent. Additionally, sodium carbonate can be easily regenerated
for reuse, which makes it an attractive option for CO2 capture. Research has shown that it
can capture up to 90% of CO2 from flue gas and biogas. It has also been found to be stable
and durable, with a service life of up to 10 years. There are several ways to regenerate
sodium carbonate after it has been used to capture CO2 . One method is to heat the sodium
bicarbonate, which releases CO2 and regenerates the sodium carbonate. This process is
known as thermal regeneration, and it can be represented by the following equation:

NaHCO3 → Na2 CO3 + CO2 + H2 O (14)

Another method for regenerating sodium carbonate is to treat it with an acid. This pro-
cess is known as chemical regeneration, and it can be represented by the following equation:

NaHCO3 + HCl → NaCl + CO2 + H2 O (15)

Both methods are effective for regenerating sodium carbonate, but thermal regenera-
tion has been found to be more energy-efficient. Sodium carbonate has also been used in
combination with other absorbents to enhance its CO2 capture capabilities. For example,
research has shown that sodium carbonate can be used in combination with amines to
capture CO2 from flue gas. The use of amines in combination with sodium carbonate has
been found to increase the CO2 capture capacity and improve the efficiency of the process.
Valluri et al. [80] carried out experiments to investigate the performance of several reagents
with frother-assisted NaCO3 slurry in a pilot gas–liquid column. The added surfactant
Sustainability 2023, 15, 5827 15 of 33

increased the surface area of NaCO3 to absorb more CO2 . Hornbostel et al. experimented
with CO2 absorption with Na2 CO3 -filled capsules and compared them with the amine
solvent capsules [81]. They found that for similar dimensions, Na2 CO3 -filled capsules
caused less energy penalties, and they also revealed that carbonate-based solvents could
compete with fast-reacting amine-based solvents.

4.6.3. Calcium Carbonate

Calcium carbonate (CaCO3 ) is a naturally occurring compound that is commonly
found in rocks such as limestone, marble, and chalk. It has recently been proposed as a
potential CO2 absorbent for carbon capture and storage (CCS) technologies. The use of
calcium carbonate as a CO2 absorbent is attractive due to its low cost, non-toxicity, wide
availability, and environmentally friendly characteristics. Another advantage of calcium
carbonate is its high CO2 uptake capacity. Calcium carbonate can effectively capture CO2
through a process known as carbonation. When CO2 comes into contact with calcium
carbonate, it reacts to form calcium bicarbonate and water. This reaction is exothermic,
meaning it releases heat, which can be recovered and used to generate electricity. Moreover,
the reaction is reversible, which means that the captured CO2 can be released from the
calcium carbonate by heating it, making it a potential option for CO2 utilization. Hong
et al. [82] studies CaCO3 polymorphs to evaluate their performance as CO2 absorbents and
compared this absorbent with several amine-based absorbents such as MEA, MDEA, and
DEA. They found that CaCO3 had a 100% recovery rate in the MEA solution. They reported
that when amine solutions came into contact with the CaCO3 surface, they restricted crystal
growth. Calcium carbonate can be used in different forms as a CO2 absorbent. The most
common form is precipitated calcium carbonate (PCC), which is made by reacting calcium
oxide (CaO) and CO2 . PCC can be used in a variety of industrial applications, including
as a filler and coating material, and it can be produced in a variety of particle sizes and
shapes. Additionally, there are other forms of calcium carbonate, such as ground calcium
carbonate (GCC), which is made by grinding natural limestone and marble, and nano
calcium carbonate (NCC) which is made by grinding PCC to a very fine powder. Each form
has its unique properties and applications.
One of the main challenges in using calcium carbonate as a CO2 absorbent is its
low reactivity and solubility compared to some other absorbents. This means that it
requires a longer contact time and higher concentrations of CO2 to effectively capture the
gas. Researchers are working to overcome this challenge by developing new methods to
increase the reactivity of calcium carbonate, such as by modifying its surface or adding
catalysts. However, CaCO3 takes many acids for CO2 absorption. To find out a suitable
solution for this limitation Chen et al. reported adding a molten carbonate electrolyzer to
capture CO2 without using other additives such as lithium salts [83]. They also stated that
their study provided a guideline to utilize CaCO3 as a mediator to get pure O2 from CO2 in
an enviro-economically friendly way.

4.7. Amine-Based Absorbents

4.7.1. Primary Amines
Primary amines have been studied as a potential solution for capturing carbon dioxide
(CO2 ) from industrial flue gases. These amines are a class of organic compounds that
contain a nitrogen atom with a lone pair of electrons and at least one hydrogen atom
bonded to it. They are effective CO2 absorbents due to their chemical reactivity towards
CO2 . One of the main advantages of using primary amines for CO2 capture is their high
capacity for CO2 absorption. They can effectively remove CO2 from gas streams at low
concentrations, making them suitable for use in industrial settings. Additionally, primary
amines are relatively low-cost and easily available, making them a cost-effective option
for CO2 capture. The process of CO2 capture using primary amines typically involves the
use of an amine solution, such as monoethanolamide (MEA), which is passed through the
flue gas stream. As the flue gas comes into contact with the amine solution, the CO2 reacts
Sustainability 2023, 15, 5827 16 of 33

with the amine, forming a carbamate species. This carbamate species can then be separated
from the flue gas stream, allowing the CO2 to be captured. The captured CO2 can then
be further processed and utilized for various industrial purposes. Akram et al. studied a
30% MEA solution and found the absorption efficiency to be 90%, which was decreased to
89.6% due to an increase in the MEA at 40 wt% [84]. They also reported that the energy
requirement was increased by 12.3%, and the solvent degraded thermally due to increasing
the regeneration temperature.
One of the main challenges in using primary amines for CO2 capture is their tendency
to degrade over time. This can lead to a decrease in their effectiveness as CO2 absorbents,
as well as the formation of byproducts that can be harmful to the environment. To mitigate
this, research is ongoing to develop more stable amine solutions and to improve the
overall process of CO2 capture using primary amines. Another challenge is the energy
consumption associated with the regeneration of the amine solution. Amine solutions used
in CO2 capture are typically heated to high temperatures to release the CO2 , which can be
energy-intensive. However, researchers are exploring various methods to reduce energy
consumption, such as the use of membrane separation or pressure swing adsorption.

4.7.2. Secondary Amines

Secondary amines, like primary amines, have been studied as a potential solution
for capturing carbon dioxide (CO2 ) from industrial flue gases. These amines are a class of
organic compounds that contain a nitrogen atom with two hydrogen atoms or alkyl groups
bonded to it and they are effective CO2 absorbents due to their chemical reactivity towards
CO2 [85]. One of the main advantages of using secondary amines for CO2 capture is their
high selectivity towards CO2 . They can selectively remove CO2 from a gas stream contain-
ing other gases such as nitrogen, oxygen, and water vapor. Additionally, secondary amines
have higher thermal stability compared to primary amines, which means they are less prone
to degradation and can have longer lifetimes. The process of CO2 capture using secondary
amines typically involves the use of an amine solution, such as diisopropanolamine (DIPA)
or methyl diethanolamine (MDEA), which is passed through the flue gas stream. As the flue
gas meets the amine solution, the CO2 reacts with the amine, forming a carbamate species.
This carbamate species can then be separated from the flue gas stream, allowing the CO2
to be captured. The captured CO2 can then be further processed and utilized for various
industrial purposes. For example, it can be used as a feedstock for producing chemicals
and fuels, or for enhanced oil recovery. Wang et al. carried out experimental investigations
to compare the performance of n-methyl-2-hydroxy ethylamine (MAE)+H2 O with several
water-soluble alcohols [86]. They reported that the MAE/n-butanol/H2 O system with a
ratio of 3:4:3 possessed great potential to be an attractive phase change absorbent for CO2
capture, and they found excellent absorbent stability during CO2 absorption.
One of the main challenges in using secondary amines for CO2 capture is their rel-
atively high cost compared to primary amines. However, secondary amines are more
selective and have better thermal stability, which can lead to reduced costs associated with
amine degradation and fewer emissions of byproducts. Another challenge is the energy
consumption associated with the regeneration of the amine solution. Amine solutions
used in CO2 capture are typically heated to high temperatures to release the CO2 , which
can be energy-intensive. However, researchers are exploring various methods to reduce
energy consumption, such as the use of membrane separation or pressure swing adsorption.
A comprehensive list depicting the advancements of alkanol amines during the period
from 1970 to 2022 can be observed from Table 3.

4.7.3. Tertiary Amines

Tertiary amines have been studied as a potential solution for capturing carbon dioxide
(CO2 ) from industrial flue gases. They are a class of amines that contain a nitrogen atom
with three alkyl or aryl groups bonded to it and these amines are effective CO2 absorbents
due to their chemical reactivity towards CO2 [87]. One of the main advantages of using
Sustainability 2023, 15, 5827 17 of 33

tertiary amines for CO2 capture is their high selectivity towards CO2 . They can selectively
remove CO2 from a gas stream containing other gases such as nitrogen, oxygen, and water
vapor. Additionally, tertiary amines have higher thermal stability compared to primary
and secondary amines, which means they are less prone to degradation and can have
longer lifetimes.
The process of CO2 capture using tertiary amines typically involves the use of an
amine solution, such as trimethylamine (TMA), which is passed through the flue gas
stream. As the flue gas comes into contact with the amine solution, the CO2 reacts with the
amine, forming a carbamate species. This carbamate species can then be separated from the
flue gas stream, allowing the CO2 to be captured. The captured CO2 can then be further
processed and utilized for various industrial purposes. For example, it can be used as a
feedstock for producing chemicals and fuels or for enhanced oil recovery.
Sharif et al. [88] conducted a study to compare the intermolecular reaction of single
DMAE, 2EAE, and blended solvent (2DMAE/PZ, 2EAE/PZ) with CO2 . They used a
material studio application to carry out molecular dynamic simulations, and their results
revealed that the mixture of secondary and tertiary amines showed better intermolecular
reaction with CO2 compared to single amines where PZ acted as a promoter on 2DMAE
and 2EAE with carbon dioxide.
One of the main challenges in using tertiary amines for CO2 capture is their relatively
high cost compared to primary and secondary amines. However, the higher selectivity
and thermal stability of tertiary amines can lead to reduced costs associated with amine
degradation and fewer missions of byproducts. Another challenge is the energy consump-
tion associated with the regeneration of the amine solution. Amine solutions used in
CO2 capture are typically heated to high temperatures to release the CO2 , which can be
energy-intensive. However, researchers are exploring various methods to reduce energy
consumption, such as the use of membrane separation or pressure swing adsorption.

Table 3. Development of several alkanol amines as CO2 absorbents (from 1970–2022).

Temperature, The Partial Pressure CO2 Loading,

Year Alkanolamine Amine Concentration, % Ref.
K of CO2 , kPa α
1972 DEA 323 7–3370 19.2 0.45–1.13 [89]
1976 DEA 338.5–366.9 32–767 25 0.4–0.79 [90]
1977 DIPA 313–373 2.7–5888 33.63 0.07–1.11 [91]
1978 DGA 323–373 1.58–4720 60 0.13–0.62 [92]
1988 AMP 313.2 1.25–144 28 0.4–0.9 [93]
1990 AMP 313, 343 0.16–5279 18.8 0.03–1.65 [94]
AMP 293–353 1.59–94 18.76, 28.14 0.13–0.94 [95]
1991
MDEA 313 0.18–92.8 22.9 0.04–0.84 [96]
1992 MDEA+MEA 313.15–373.15 1.12–2080 MDEA: 12–24/MEA: 6–18 0.188–1.015 [97]
AMP 313–353 3.94–336.6 30 0.28–0.9 [98]
1996
DEA 313–353 4.85–357.3 30 0.4–0.73 [98]
DEA+AMP 313.15–373.15 22–2838 DEA: 20–25/AMP: 5–10 0.337–1.2 [99]
1998 DEA: 10–32.5/MDEA:
DEA+MDEA 313.15–393.15 0.4–2833.6 0.038–1.119 [99]
10–35
Sustainability 2023, 15, 5827 18 of 33

Table 3. Cont.

Temperature, The Partial Pressure CO2 Loading,

Year Alkanolamine Amine Concentration, % Ref.
K of CO2 , kPa α
MDEA 297.7 0.02–1.64 23.63 0.02–0.26 [100]
2000 313–343 0.03–40 4.7 0.16–0.96 [101]
PZ
313–343 29–40,200 4.7 0.6–0.96 [101]
2001 MDEA 298–373 0.78–140.4 50 0.01–0.49 [102]
MDEA 298–348 2.7–4559.5 48.88, 25.73 0–1.3 [103]
2004
DEA 298–348 4.85–357.3 47.78 0–1.09 [103]
2006 DEA 323–366 0.4–3798 25 0.1–1.13 [104]
AMP 313.2 0.89–151.9 28 0.4–0.9 [105]
313 0.28–89.9 22.9 0.06–0.80 [106]
MDEA
2010 323 6–434 50 0.1–0.89 [106]
PZ 313 5800–7500 15–60 0.34–0.86 [107]
TEA 313–353 1.43–153.4 26.5 0.03–0.53 [106]
PZ 354–464.8 28–2583 29.8–40.59 0.23–0.45 [108]
298–328 0.41–1449 23.5–46 0.19–1.1 [109]
2011 AMP
303–328 0.31–1472 40,50 0.24–1.04 [109]
MEA 303–323 0.9–335.9 6.7–19 0.35–1.16 [110]
2012
AMP 313–393 6–983.5 30 0–0.97 [111]
2013 AEEA 303–323 1.11–794.67 15 0.06–1.4077 [112]
2014 DIPA 313–343 107–4064 45 0.52–1.05 [113]
NH3 335–395 0.01–1000 20.4 1 [114]
DIPA 313–343 91.2–3826.6 30 0.89–1.14 [113]
DIPA + AEEA 313.15–343.15 105–3819.7 DIPA: 20.25/AEEA: 5–10 0.5837–1.251 [113]
2017
MDEA:22.6–47.6/PZ:
MDEA + PZ 313–375.15 0.033–95.78 0.027–0.37 [115]
0.4–21.3
DIPA + AMP + DIPA: 24–36/AMP:
313.15–343.15 112.9–3709.7 0.502–1.091 [113]
PZ 7–13/PZ: 2–8
2019 MEA 303–353 0–50.65 12–15 0.017–0.577 [116]
AMP + PZ 293.15–323.15 0.127–140.4 AMP: 8.9–38/PZ: 0.87–8 0.1511–0.9405 [117]
MEA + DAP 315.15–333.15 13.24–215.46 MEA: 10–12.5/DAP: 2.5–5 0.22–0.711 [118]
MEA 307.9 - 24.9 2.5–32.5 [119]
2021 MEA + DEA 308 - 24.8 2.5–32.5 [119]
MEA + TEA 308 - 25 2.5–32.5 [119]
DA2MP/AMP/
2022 313.15–383.15 - 20 0.91–0.95 [120]
PrOH

4.8. Triethylenetetramine (TETA)/Ethanol Solution as Absorbent

Triethylenetetramine (TETA) is a type of tertiary amine that has been studied as a po-
tential CO2 absorbent as when dissolved in ethanol, it forms a solution that has been found
to be effective at capturing CO2 from industrial flue gases. One of the main advantages
of using TETA/ethanol solution for CO2 capture is its high selectivity towards CO2 [121].
The TETA/ethanol solution can selectively remove CO2 from a gas stream containing
other gases, such as nitrogen and oxygen [122]. Additionally, the TETA/ethanol solution
has a high absorption capacity for CO2 and can be regenerated easily by heating, which
Sustainability 2023, 15, 5827 19 of 33

allows for the captured CO2 to be released and reused. The process of CO2 capture using
TETA/ethanol solution typically involves passing the flue gas through a TETA/ethanol
solution. As the flue gas encounters the TETA/ethanol solution, the CO2 reacts with the
TETA, forming a carbamate species. This carbamate species can then be separated from the
flue gas stream, allowing the CO2 to be captured. The captured CO2 can then be further
processed and utilized for various industrial purposes, such as being used as a feedstock for
producing chemicals and fuels. Additionally, TETA/ethanol solution has a high potential
for enhanced oil recovery. From several experiments, it was found that TETA/ethanol
solution having lower concentration polyamine showed better CO2 desorption and cycle
loading compared to higher concentration polyamines [123].
One of the main challenges in using TETA/ethanol solution for CO2 capture is its
relatively high cost. However, the high selectivity and absorption capacity of TETA/ethanol
solution can lead to reduced costs associated with amine degradation and fewer emissions
of byproducts. Another challenge is the energy consumption associated with the regen-
eration of the TETA/ethanol solution. TETA/ethanol solutions used in CO2 capture are
typically heated to high temperatures to release the CO2 , which can be energy-intensive.
However, researchers are exploring various methods to reduce energy consumption, such
as the use of membrane separation or pressure swing adsorption. One typical challenge for
applying this technology is the generation of gelatinous products during absorption. To pro-
vide a potential solution to this problem, Zhifang et al. introduced an amine AMP into
the TETA-based SLPCAs which successfully ignored such solid byproducts via yielding
crystalline powders that could be easily separated [124].

4.9. Amino Acid Salt as Liquid-Solid Phase Changes Absorbent

Amino acid salt as a liquid-solid phase change absorbent of CO2 is a novel technology
that utilizes amino acid salts as a material for CO2 capture [125]. These absorbents can
capture CO2 through a liquid-solid phase change process. When in contact with CO2 ,
the absorbent changes from a solid state to a liquid state, allowing for the absorption of
CO2 . Amino acid salts have been found to have high selectivity and capacity for CO2
capture. They can also be regenerated and reused, making them a sustainable option
for CO2 capture. The absorbents can capture CO2 at a lower energy cost compared to
traditional absorbents, which can lead to a more cost-effective CO2 capture process. One
of the advantages of using amino acid salts as liquid-solid phase change absorbents is
that they can be used in a wide range of applications, such as power plants, industrial
facilities, and transportation. Additionally, they can be used in post-combustion CO2
capture processes, which is a common method for capturing CO2 from power plants and
other large emitters [126].
Li et al. [127] developed novel biphasic absorbent based on water-lean amino acid salt
to enhance CO2 capture performance and energy efficiency. They employed potassium
prolinate and potassium sarcosinate with a secondary amino group as active elements,
while 2-alkoxy ethanol possessing low specific heat, volatility, and viscosity were employed
as physical antisolvents and accelerated the generation of the solid phase during CO2
absorption. They used 13 C NMR and XRD to characterize the phase change behavior and
separation of CO2 in the solid and liquid phases and to identify the product species in
CO2 -rich solid phase. Their experiment revealed that 50–80% CO2 could be captured via
the solid slurry with 2.5 to 3.5 mol kg−1 CO2 loading.
Amino acid salts have also been found to have high thermal stability, which allows
them to be used in high-temperature CO2 capture processes [128,129]. They are also non-
toxic and biocompatible, making them a safe and environmentally friendly option for CO2
capture. In addition, amino acid salts can be produced from renewable sources such as
waste or byproducts of agriculture, food, and the chemical industry. This makes them a
sustainable alternative to traditional absorbents, which are usually produced from fossil
fuels. Research on amino acid salt as liquid-solid phase change absorbents is ongoing, and
further advancements are expected in the future. For example, researchers are working
Sustainability 2023, 15, 5827 20 of 33

on developing new amino acid salts with improved properties and finding new ways to
integrate them into CO2 capture processes [130,131].

4.10. Encapsulated Absorbents

Encapsulated absorbents for CO2 capture are technologies that involve enclosing an
absorbent material within a protective shell or capsule. The main benefit of this technology
is that it improves the stability, selectivity, and overall performance of the absorbent material
by protecting it from degradation and chemical reactions. This can lead to a more efficient
and cost-effective CO2 capture process. Encapsulation can be achieved through a variety of
methods, such as coating, impregnation, and entrapment. Coating involves applying a thin
layer of protective material on the surface of the absorbent, impregnation involves filling
the pores of the absorbent with a protective material, and entrapment involves enclosing
the absorbent within a protective shell or capsule. The selection of any approach from
these usually depends on the application and the properties of the absorbent. Polesso
et al. experimented with a novel CO2 absorption system where they investigated
Sustainability 2023, 15, x FOR PEER REVIEW the33
20 of
performance of encapsulated poly-ionic liquids (PIL) as green solvents [132]. From this
experiment, by employing the nano spray dryer B-90, encapsulated ionic liquids Emim
[X],
Emim and[X],
capsules of water-based
and capsules PIL P[DADMA][BF
of water-based 4 ] were
PIL P[DADMA][BF achieved, and the stability
4] were achieved, and the sta-
of this system and high CO
bility of this system and high selectivity emphasized its potentiality
2 CO2 selectivity emphasized its potentiality for CO absorption.
for2 CO 2 absorp-
Other encapsulated PILs were also compared, and P[DADAM]/BF4 showed the highest
tion. Other encapsulated PILs were also compared, and P[DADAM]/BF4 showed the high-
performance based on CO2 absorption rate and CO2 /N2 selectivity, which is clear from
est performance based on CO2 absorption rate and CO2/N2 selectivity, which is clear from
Figure 6.
Figure 6.

Figure6.6.CO
CO2sorption
sorption(mg
(mgCO
CO2/g)
/g) and
and CO
CO22/N
/N2 selectivity [132].
Figure 2 2 2 selectivity [132].

Oneof
One ofthe
thesignificant
significantadvantages
advantagesof ofencapsulated
encapsulatedabsorbents
absorbentsisisthatthatthey
theycan
canimprove
improve
thestability
the stability of the
the absorbent.
absorbent.The Theprotective
protective shell or or
shell capsule
capsulecancan
shield the absorbent
shield from
the absorbent
environmental
from environmental factors, suchsuch
factors, as heat andand
as heat light, which
light, whichcancan
cause degradation.
cause degradation.This
Thiscancan
in-
crease the
increase theservice
servicelife ofof
life thethe
absorbent
absorbent andandreduce
reducethethe
need for for
need frequent replacement.
frequent Ad-
replacement.
Additionally, encapsulationcan
ditionally, encapsulation canalso
alsoprotect
protectthe
theabsorbent
absorbent from
from chemical reactions
reactionsthat
thatmay
may
occur
occurduring
duringthe absorption
the absorption process,
process,which
whichcancanimprove
improveits performance.
its performance. Encapsulation can
Encapsulation
also
can improve the selectivity
also improve of theof
the selectivity absorbent material
the absorbent for COfor
material 2 . For
CO2example, encapsulating
. For example, encapsu-
amines within awithin
lating amines porousamaterial
porous can increase
material cantheir selectivity
increase their for CO2 over
selectivity forother
CO2gases. This
over other
leads
gases.toThis
a more efficient
leads to a moreandefficient
specific CO captureCO
and2 specific process. Some
2 capture encapsulated
process. absorbents
Some encapsulated
are also designed
absorbents to be
are also regenerated
designed and reused after
to be regenerated the CO2after
and reused is captured.
the CO2 is This can reduce
captured. This
the cost of the CO
can reduce the cost capture process and make it more sustainable.
2 of the CO2 capture process and make it more sustainable.

4.11. Enzymatically Catalyzed Absorbent Systems

Enzymatically catalyzed absorbent systems for CO2 capture are novel technologies
that utilize enzymes to catalyze the capture and conversion of CO2. Enzymes are biomol-
ecules that act as catalysts, increasing the rate of a chemical reaction without being con-
sumed in the process. This technology has been studied as a way to improve the efficiency
Sustainability 2023, 15, 5827 21 of 33

4.11. Enzymatically Catalyzed Absorbent Systems

Enzymatically catalyzed absorbent systems for CO2 capture are novel technologies that
utilize enzymes to catalyze the capture and conversion of CO2 . Enzymes are biomolecules
that act as catalysts, increasing the rate of a chemical reaction without being consumed
in the process. This technology has been studied as a way to improve the efficiency and
sustainability of CO2 capture processes.
Enzymes that have been studied for CO2 capture include carbonic anhydrase, which
catalyzes the conversion of CO2 and water to bicarbonate and protons, and formate de-
hydrogenase, which catalyzes the conversion of CO2 and formate to carbon monoxide
and water. These enzymes are typically immobilized on a support material, such as a
polymer or a nanoparticle, to increase their stability and reuse. Hannaneh et al. proposed a
hybrid enzymatic CO2 absorption process integrated with carbonic anhydrase II enzyme in
a membrane, and they found from their experiment that such enzymatic approaches im-
prove CO2 absorption [133]. In another article, Hannaneh reported a promising enzymatic
CO2 absorption approach in a packed bed reactor, and based on their experimental and
theoretical approaches, they reported a packed bed bioreactor with immobilized carbonic
anhydrase enzyme on magnetic nanoparticles and packing surface as an attractive process
for green CO2 capture process [134]. Several experiments have revealed that applying CA
with absorbents (such as MDEA) can enhance absorption performance significantly [135].
Mthias et al. experimented with an enzyme-enhanced CO2 absorption mechanism in which
they incorporated CA in the biocatalyst delivery system to investigate with an aqueous
MDEA solvent. Their result showed a sixfold improvement in the total absorbed CO2
moles [136].
One of the advantages of enzymatically catalyzed absorbent systems is that they can
improve the efficiency of CO2 capture. Enzymes can catalyze the conversion of CO2 to
a more easily captured and stored form, such as bicarbonate or formate. This can lead
to a more efficient and cost-effective CO2 capture process. Enzymatically catalyzed ab-
sorbent systems can also improve the sustainability of CO2 capture processes. Enzymes are
biocompatible and can be produced from renewable sources, such as bacteria or plants. Ad-
ditionally, enzymes can be recycled and reused after the CO2 is captured, reducing the need
for frequent replacement. Another advantage of enzymatically catalyzed absorbent systems
is that they can be used in a wide range of applications, such as power plants and industrial
facilities, and can be integrated into existing CO2 capture processes. This technology is an
active area of research, and further advancements are expected in the future.
The major barrier of this novel technology is the stability and activity of carbonic
anhydrase enzyme under typical flue gas operating conditions. To propose a solution to
this issue, Zhang et al. [137] proposed a novel CA/ZIF-L-1 composite by embedding CA
into ZIF-L (zeolitic imidazolate framework). They found that the novel composite caused
high enzyme activity retention and exhibited high thermal stability, which was improved
by 100% at 40 ◦ C, which can be observed from Figure 7.
 nology is an active area of research, and further advancements are expected in the future.
The major barrier of this novel technology is the stability and activity of carbonic
anhydrase enzyme under typical flue gas operating conditions. To propose a solution to
this issue, Zhang et al. [137] proposed a novel CA/ZIF-L-1 composite by embedding CA
into ZIF-L (zeolitic imidazolate framework). They found that the novel composite caused
Sustainability 2023, 15, 5827 22 of 33
high enzyme activity retention and exhibited high thermal stability, which was improved
by 100% at 40 °C, which can be observed from Figure 7.

Sustainability 2023, 15, x FOR PEER REVIEW 22 of 33

Figure 7. SEM images of (a) ZIF-L, (b) ZIF-L (5 min), (c) CA/ZIF-L-1, and (d) CA/ZIF-L-2. (e) The
stability
Figure 7.of SEM
the free enzyme
images and
of (a) CA/ZIF-L-1
ZIF-L, and
(b) ZIF-L (5reusability of CA/ZIF-L-1;
min), (c) CA/ZIF-L-1, and (f)
(d)Performance
CA/ZIF-L-2.of (e)CO
The
2
stability ofinto
absorption the 1M
freeMDEA
enzymesolution
and CA/ZIF-L-1 and reusability
with different of CA/ZIF-L-1;
604 concentrations at 40 ◦ C of
(f) Performance
of CA/ZIF-L-1 andCO
a2
absorption
CO into 1M MDEA
2 partial pressure solution
of 15 kPa with different
[137]. Copyright 604 American
© 2018, concentrations of CA/ZIF-L-1
Chemical Society. at 40 °C and a
CO2 partial pressure of 15 kPa [137]. Copyright © 2018, American Chemical Society.
4.12. Deep Eutectic Solvents (DESs)
4.12.Deep
Deep eutectic
Eutectic solvents
Solvents (DESs)
(DESs) are mixtures of two or more components that exhibit
lowerDeep
melting points and higher
eutectic solvents (DESs) solubility compared
are mixtures to individual
of two components.
or more components In recent
that exhibit
years,
lowerthey havepoints
melting gainedandattention as a potential
higher solubility solution
compared for CO2 capture
to individual and utilization
components. In recent
due to their
years, unique
they have properties,
gained such
attention as as low toxicity
a potential and high
solution for COCO 2 solubility.
2 capture Research
and utilization
advancements
due to their unique properties, such as low toxicity and high CO2 solubility. Researchthe
in the field of DESs for CO 2 absorption have focused on optimizing ad-
composition
vancements and properties
in the of DESs
field of DESs for to
CO increase their have
2 absorption efficiency andon
focused cost-effectiveness. For
optimizing the com-
example,
position studies have explored
and properties of DESstheto use of different
increase hydrogen
their efficiency bond
and donors and acceptors
cost-effectiveness. For ex-
ample, studies have explored the use of different hydrogen bond donors and acceptors to
improve CO2 solubility and selectivity. The Lewis or Bronsted acids and bases are used to
create the DESs as novel ionic solvents that can contain a wide range of anionic and cati-
onic species [138]. In more detail, DESs are created by combining a hydrogen-bond donor
(HBD) and a hydrogen-bond acceptor (HBA) at the proper molar ratio. These materials
Sustainability 2023, 15, 5827 23 of 33

to improve CO2 solubility and selectivity. The Lewis or Bronsted acids and bases are used
to create the DESs as novel ionic solvents that can contain a wide range of anionic and
cationic species [138]. In more detail, DESs are created by combining a hydrogen-bond
donor (HBD) and a hydrogen-bond acceptor (HBA) at the proper molar ratio. These
materials are inexpensive, reliable, and simple to make from a variety of readily accessible
beginning ingredients. The most typical and extensively utilized DESs are those based on
cholinium chloride (ChCl). ChCl-based DESs are well known for sharing characteristics
and behavior with traditional ILs, as well as having the same CO2 order. capability for
absorption [139–141]. Ruan et al. experimented with CO2 absorption using DES, which
was formed by superbase 1,5-diazabicyclo [4.3.0] non-5-ene (DBN) and 1,2,4-triazole (Tz),
and they investigated the impacts of molar ratios of DBN and TZ for CO2 absorption. Their
experimental results revealed that DES [2DBN:Tz] showed the highest performance [142],
which is clear in Figure 8.
DESs have potential applications in various industries, including the power generation
and chemical industries, where they can be used to capture and utilize CO2 emissions [138].
Additionally, they have been proposed to enhance oil recovery and as a solvent for chemical
reactions. DESs work by dissolving CO2 in the solvent mixture, forming a stable complex.
This allows for the efficient separation of CO2 from other gases, such as nitrogen and
oxygen, in flue gas streams. The captured CO2 can then be utilized or stored for later use.
The advantages of using DESs for CO2 capture include their low cost, low toxicity, and high
Sustainability 2023, 15, x FOR PEER REVIEW
CO2 solubility. Additionally, they can be regenerated and reused multiple times, making 23 of 33
them a sustainable solution for CO2 capture. However, there are also limitations to the use
of DESs for CO2 capture. For example, the efficiency of CO2 capture can be affected by
temperature and pressureand
affected by temperature changes, andchanges,
pressure the stability
andof DESs
the can be
stability ofimpacted
DESs canby beimpurities
impacted
in
by impurities in the gas stream. Additionally, the production and use of DESsimpact
the gas stream. Additionally, the production and use of DESs can also have an on
can also
the environment. Table 4 summarizes different absorbent materials with their
have an impact on the environment. Table 4 summarizes different absorbent materials advantages
and
withdisadvantages.
their advantages and disadvantages.

8. CO
Figure 8. CO22 gravimetric
gravimetric absorption
absorptioncapacity
capacityby
byDBN-Tz
DBN-TzDESs
DESs under
under different
different molar
molar ratios
ratios at ◦25
at 25 C
°C and
and 100100
kPakPa [142].
[142]. Copyright
Copyright permission
permission © 2022
© 2022 Elsevier.
Elsevier.

Table 4. Different absorbent materials with their advantages and disadvantages.

Solvent/Absorbent Advantages Limitations/Disadvantages Remarks

DEA is a liquid at room
temperature, making it
easy to handle and
Lower capacity. transport. It also has a
The absorption rate is high.
DEA Risk of corrosion due to the pres- high CO2 solubility, mak-
Less expensive.
ence of atmospheric oxygen. ing it an attractive option
for use in post-combustion
CO2 capture and air purifi-
cation applications.
Sustainability 2023, 15, 5827 24 of 33

Table 4. Different absorbent materials with their advantages and disadvantages.

Solvent/Absorbent Advantages Limitations/Disadvantages Remarks

DEA is a liquid at room temperature,
making it easy to handle and
Lower capacity.
transport. It also has a high CO2
The absorption rate is high. Risk of corrosion due to the
DEA solubility, making it an attractive
Less expensive. presence of atmospheric
option for use in post-combustion
oxygen.
CO2 capture and air purification
applications.
Degradation resistance is high,
Organic/inorganic salts are mixed to
less expensive, and less
K2 CO3 The mass transfer rate is low. enhance the mass transfer rate.
enthalpy is needed for this
Reactivity drops down at 40–200 ◦ C.
solvent.
Selectivity rate is high, simple
regeneration, good absorption Temperature decreases linearly with
AMP The absorption rate is low.
capacity, and degradation column height.
resistance.
Dipa is a tertiary amine that can react
Di-isopropylamine Less corrosion, cost-effective with CO2 to form a stable carbamate,
The absorption rate is low.
(DIPA) regeneration. making it a viable option for
removing CO2 from gas streams.
Lower capacity.
Among pilot, spray, and packed
The cost-effective, high Risk of corrosion due to the
MEA columns, pilot and spray columns
reaction rate presence of atmospheric
show superior performance.
oxygen.
Rigid thermal and chemical Its high volatility can result in
2PE has a high CO2 solubility and is a
behavior. significant losses of 2PE
2PE liquid at room temperature, making it
Non-toxic and during the CO2 capture
easy to handle and transport.
environmentally friendly. process.
Good corrosion resistance,
PZ less thermal degradation, -
cost-effective regeneration.
Include primary, secondary, and
The absorption capacity is
AEP - tertiary amines without formatting
high.
any carbamate.
Thermally and chemically
By decreasing temperature and
stable.
Ionic liquids - increasing concentration, an increase
Cost-effective and high carbon
in density and viscosity is possible.
capture selectivity.
Low cost, high selectivity.
Ammonia Absorption capacity is high Slip Pilot scale project.
and available.
Good corrosion and
Unlike primary and secondary
MDEA degradation resistance. The absorption rate is low
amines, it does not bind.
Low regeneration cost.
AMP + PZ - A pilot plant, packed bed.
MDEA is a tertiary amine that reacts
with CO2 to form a stable carbamate,
Available, less corrosion, low High cost.
MDEA + PZ while PZ is an acidic compound that
cost. High corrosivity.
can enhance the reaction kinetics of
the MDEA-CO2 reaction
Sustainability 2023, 15, 5827 25 of 33

Table 4. Cont.

Solvent/Absorbent Advantages Limitations/Disadvantages Remarks

MDEA is a tertiary amine that reacts
The solubility of carbon
Concentration and pressure with CO2 to form a stable carbamate,
dioxide can be decreased due
MDEA + glycerol are low. while glycerol is a hydrophilic
to high pressure and
Absorption capacity is high. compound that can act as a solvent
concentration of glycerol.
for the absorbent.
Huge loading of CO2 .
DETA Promote the absorption -
process.
A low volume required for
regeneration.
DETA/Sulfolane - Novel absorbent to capture CO2 .
Low total heat duty, improved
viscosity.
CO2 intake increased with Long-chain polymers have
increasing pressure and lesser mobility and CO2 PEG (polyethylene glycol) is a
PEG-dicholine temperature. loading. hydrophilic polymer that is combined
chlorides Non-toxic. High cost. with a dicholine chloride molecule to
Fast kinetics and high Form solid residue after form the PEG-dicholine chloride.
absorption capacity. absorption.
Selective CO2 absorption: a The PEG component provides
copolymer with an hydrophilic properties, while the
CO2 selectivity is moderate;
imidazolium chromophore PDMS component provides
PEG-PDMS copolymer severe foaming in the
was shown to be an effective hydrophobic properties, making the
presence of flowing gas
solvent for precombustion copolymer an attractive option for
CO2 capture. CO2 removal from gas streams
The high swelling capacity of the
Excessive water content Water has a low CO2 copolymers allows for efficient CO2
AAM-co-AAC porous
increased CO2 uptake solubility along polymer absorption, and the porous structure
hydrogel copolymers
capacity. scaffolds. increases the available surface area
for CO2 absorption.
DME-PEG copolymers are composed
Absorption of CO2 and H2 S of a combination of dimethanolamine
Process performance variation;
from syngas; recovery of (DME), a tertiary amine that can react
fluctuation in gas/solvent
DMEPEG co-absorbed H2 ; and with CO2 to form a stable carbamate,
processing capability within a
reduction of equipment size and polyethylene glycol (PEG), a
packed column.
through solvent saturation. hydrophilic polymer that increases
the solubility of DME in water.
Poly-ionic liquids based on
Procedures execution on an
Amino acid poly-ionic [Arg] exhibited the maximum
industrial scale or for
liquids (AAPILs) CO2 absorption and sorption
commercialization.
capability.
AIMGs are soft, hydrogel-like
Long-term robustness, particles that are infused with an
durability, and CO2 capturing amine-containing chemical, such as a
Amine-infused Increased CO2 intake and
capacity at high pressure and tertiary amine. The amine reacts with
microgels (AIMGs) absorbing kinetics.
temperature are required for CO2 to form a stable carbamate,
large-scale applications. which is trapped within the gel
structure of the AIMG.
Sustainability 2023, 15, 5827 26 of 33

Table 4. Cont.

Solvent/Absorbent Advantages Limitations/Disadvantages Remarks

Because of the positive
entropic impact, CO2 is
NOHMs are highly branched, organic
readily available. At high The high temperature has a
Novel Multiphase molecules that contain nitroxide
pressure, and high CO2 negative impact on CO2
Systems of NOHMs radicals, which can react with CO2 to
capture; CO2 has a high absorption.
form a stable carbamate.
selectivity over N2 O, O2 , and
N2 .
By incorporating POSS into NOHMs,
High thermal stability, More physical research and
the resulting materials can exhibit
POSS containing enhanced CO2 molecule polymer chain couplings are
unique properties, such as high
NOHMs assimilation due to high required to assess CO2
stability, low toxicity, and improved
canopy/core size ratio. capture efficacy.
CO2 uptake.
PDMS is often synthesized as a gel,
Highly soluble and thermally Decreasing CO2 solubility which can be cut into small pieces to
Polydimethylsiloxane
stable; efficient CO2 extraction with rising temperature increase the surface area and
(PDMS)
from H2 , H2 O, and H2 S. causes high-cost process. therefore the CO2 absorption
capacity.

5. Outlook and Prospects

Carbon capture and storage (CCS) is a technology that aims to capture carbon dioxide
(CO2 ) emissions from power plants and other industrial sources and store them under-
ground. The use of CO2 absorbents is a crucial component of CCS, as they are responsible
for capturing the CO2 from the emissions. The future of CCS and CO2 absorbents is of great
interest, as it is expected to play a significant role in reducing greenhouse gas emissions
and slowing down global warming. One of the main prospects of CO2 absorption and
absorbents is the increased use of CCS in power generation. As countries around the world
work to reduce their greenhouse gas emissions, the use of CCS in power generation is ex-
pected to grow. This will lead to an increased demand for CO2 absorbents, and researchers
are working to develop new and improved absorbents that are more effective and efficient
at capturing CO2 .
Another prospect is the use of CCS in industrial processes. Currently, many industrial
processes, such as cement and steel production, release large amounts of CO2 into the
atmosphere. CCS technologies, including CO2 absorbents, can be used to capture and
store these emissions, reducing their impact on the environment. As the demand for these
technologies grows, researchers are working to develop new absorbents that are specifically
designed for industrial applications. In addition, the use of CO2 absorbents in direct air
capture (DAC) is an emerging field. DAC technology captures CO2 directly from the
atmosphere rather than from industrial emissions [143]. This technology has the potential
to significantly reduce the amount of CO2 in the atmosphere and is seen as a promising
solution for addressing climate change. The use of CO2 absorbents in DAC is still in the
early stages of development, but it is expected to grow in the future as researchers work to
improve the efficiency and effectiveness of the technology [144].
Another prospect is the integration of CO2 absorption and utilization. While tradi-
tional CCS technologies focus on capturing and storing CO2 , there is also a growing interest
in utilizing the captured CO2 for various applications such as producing chemicals, fuels,
and even food. Researchers are working to develop new absorbents that can capture CO2
for utilization, which would increase the value of CCS and make it more economically
viable. Additionally, the development of sustainable and biobased absorbents is also an
area of research. Traditional CO2 absorbents are mostly based on fossil fuels, which are
non-renewable and environmentally damaging. There is a growing interest in the develop-
ment of absorbents that are based on renewable and sustainable resources such as biomass,
which can reduce the environmental impact of CCS.
Sustainability 2023, 15, 5827 27 of 33

Another prospect for CO2 absorption and absorbents is the development of hybrid
absorbents. These absorbents combine two or more different types of absorbents to improve
the overall efficiency and effectiveness of the CO2 capture process. For example, a hybrid
absorbent could consist of a traditional amine-based absorbent combined with a PIL (poly-
ionic liquid) absorbent. The amine-based absorbent would provide a high CO2 binding
capacity, while the PIL absorbent would provide improved thermal stability and selectivity
for CO2 . Researchers are also working on developing hybrid absorbents that integrate solid
and liquid absorbents to achieve better performance.
Another important area of research is the development of more cost-effective CO2
capture technologies. While CCS and CO2 absorbents have the potential to significantly
reduce greenhouse gas emissions, the cost of these technologies remains a major barrier to
their widespread adoption. Researchers are working to develop new absorbents that are
more cost-effective, such as those based on sustainable and biobased resources, to make
CCS more economically viable. There is also growing interest in the use of CO2 absorbents
in carbon mineralization, which is a process that converts CO2 into stable carbonates, such
as limestone. Carbon mineralization has the potential to permanently remove CO2 from the
atmosphere and researchers are working to develop new absorbents that can capture CO2
for mineralization. Lastly, the development of advanced monitoring and control systems
for CCS facilities is also an important area of research. These systems would allow the
real-time monitoring of the CO2 capture process and would enable facilities to optimize
the performance of their absorbents and improve the efficiency of their CCS systems.

6. Conclusions
In this paper, several absorption-based CO2 capture approaches have been studied
comprehensively, considering material and mechanism improvement as well as engineer-
ing aspects. The increased amount of CO2 has been considered a curse in recent years,
and researchers are working hard to develop materials for capturing CO2 to protect the
environment from its negative influences. Several catalysts can be utilized to ameliorate
the performance of different absorbent materials. Researchers are working to develop
different types of absorbents and technologies to synthesize absorbents while considering
cost, safety, absorption rate, stability, and durability. Different novel technologies such
as enzyme-based absorption and nanocomposite-assisted absorption, which improve the
potentiality of carbon capture, have been developed. Power plants and other GHG emitters
emit a massive amount of CO2 regularly, which is harming the environment. However,
these massive amounts of CO2 can be utilized for energy production by applying CO2 as
feedstock material as well as CO2 as a raw material to produce the alternative fuel methanol.
To properly utilize and store of CO2 , it is necessary to have comprehensive knowledge of
advanced CO2 capture technologies, and as absorption is the most widely applied CO2
capture technology, we reviewed and discussed previous research to assist researchers in
further research.

Author Contributions: All authors contributed equally. All authors have read and agreed to the
published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: No new data were created or analyzed in this study. Data sharing does
not apply to this article.
Conflicts of Interest: The authors declare no conflict of interest.

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