a review on solid adsorbents for carbon dioxide capture 2015 journal of industrial and engineering...

7/24/2019 A Review on Solid Adsorbents for Carbon Dioxide Capture 2015 Journal of Industrial and Engineering Chemistry

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Introduction

Global warming and carbon economy

Humans are endangered by global warming caused by the

greenhouse effect. The greenhouse effect can be attributed to an

increase in the emissions of greenhouse gases such as carbon

dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluor-

ocarbons, and sulfur hexafluoride (SF6) since the beginning of

industrialization. The average world temperature has increased by

0.74% in the past 100 years and is expected to increase by another

6.4% by the end of the twenty-first century [1,2].

Global warming leads to droughts, floods, heat waves, and

destruction of ecosystems, and the economic loss due to climate

change is expected to be 520% of the worlds gross domestic

product. CO2 is a major greenhouse gas and is the main cause of

global warming [3,4].

Major sources of CO2 emissions are thermoelectric power

plants and industrial plants (such as steel mills and refineries),

which account for approximately 45% of global CO2 emissions.

According to a report in 2013 by the International Energy Agency

(IEA), the global atmospheric CO2 concentration has increased

from a preindustrial value of 280 parts per million by volume

(ppmv) to 394 ppmv in 2012. Most of the CO2emissions into theatmosphere originate from the combustion of fossil fuels (99% of

global annual CO2emissions of approximately 32 Gt) [5]. The IEA

and the Organization for Economic Co-operation and Development

(OECD) forecast that CO2capture and storage (CCS) could take care

of approximately 14%, the expected reduction in volume of CO2emission potentials [6]. Furthermore, the Intergovernmental Panel

on Climate Change (IPCC) reported that it would be possible to

achieve a more than 50% reduction in CO2 emissions from

2009 levels by 2050. According to their model for estimating

CO2 capture potential, it is estimated that 30 billion tons ofCO2 can

be captured and stored within the European Union (EU) by

2050. Globally, 240 billion tons of CO2could be captured by 2050

[7]. Without CCS technology, the cost of meeting a 50% global

reduction target by 2050 will be 70% higher [8].Because of these global concerns, strict global regulations of

CO2 emissions to the atmosphere have been imposed. With

enforcement of the Kyoto Protocol, there has been increased

interest in the atmospheric residence time of CO2 and its

contribution to the greenhouse effect, and self-reduction techni-

ques for CO2 generation rates and post-treatment of CO2 have

received significant attention [9,10]. Various industries need to

deal actively with these regulations in order to survive. CCS has

great potential to be one of the more important green technologies

in the future.

There is an urgent need to develop CO2reduction technologies,

andwebelieve thatCCS is themain technology that can reduce CO2emissions from the energy sector. However, with the exception of

developed countries, there is very little concern about, and

investment in, developing CCS. Fortunately, the world is now

paying more attention and aggressive efforts are being made to

commercialize CCS [1113]. A global CCS market has begun to

develop, along with certified emission reductions. The IEA/OECD,

EU, and United States forecast that the CCS market will grow to its

full capacity by 2020, and there is a need to obtain better insights

into the techno-economic possibilities of CO2 capture.

CO2 capture and storage (CCS)

The basic concept of CCS is to capture CO2 emissions without

releasing them into the atmosphere; they include sequestration or

storage, as shown in Fig. 1. CCS is the process of capturing and

compressing CO2 generated by existing large sources of high-

density CO2, transporting and depositing it safely in the ground or

an ocean-bedrock sediment layer, and long-term monitoring [14

16]. Transportation of CO2 refers to transporting captured and

compressed CO2to a storage site via a pipeline or other means oftransport [17,18]. Storage of CO2 includes post-management: the

observation and prediction of the movement of the bedrock layer

after CO2storage and evaluation of its effect on the environment,

and evaluation of the bedrock layers characteristics for depositing

captured CO2(basins, oilfields, etc.) in the ground or below the sea

[19,20]. Technologies for capture, transportation, and storage of

CO2are summarized in Table 1.

All the above processes are called CCS. In addition, CCS

technologies allow the continuing usage of fossil fuels and

stabilize the density of greenhouse gases. Moreover, CCS may

decrease the total cost of CO2reduction and offer different ways to

reduce CO2emissions [2123]. CCS is a comprehensive technology

for direct reduction of CO2using the above-mentioned processes.

CO2 capture technology

Classification

of

CO2 capture technology

CO2 capture is a core technology and accounts for 7080% of the

total costs of CCS technologies. It is classified at (i) post-

combustion, (ii) pre-combustion, and (iii) oxy-fuel combustion

Fig.

1.

Concept

and

summary

of

CO2 capture

and

storage

(CCS).

S.-Y. Lee, S.-J. Park/Journal of Industrial and Engineering Chemistry 23 (2015) 1112


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technologies. Schematic diagrams of the three types CO2capture

process are presented in Fig. 2.

Post-combustion capture technology involves collecting CO2from the emission gases of a power plant [2426]. Pre-combustion

capture technology is mainly used in fertilizer and hydrogen

production and can be used in integrated gasification combined

cycle plants [2729]. Although the fuel conversion process (e.g.,

gasification

of

coal)

prior

to

combustion

is

more

complex

and

costsmore, the density and pressure of CO2 in the gases allow easy

separation. Oxy-fuel combustion, or oxygen-fired combustion, is in

a trial phase in a pilot, plant and uses high-purity oxygen instead of

air [3032]. Thus, the density of CO2 in the gas flow, i.e., the

emission gas, becomes high, resulting in easy separation of CO2;

however, the separation of oxygen from the air consumes more

energy.

Post-combustion capture technology

Among the currently available technologies, post-combustion

capture, a technology for capturing CO2 from post-combustion

emission gases, is the most easily applied technology for existing

sources of emissions. Post-combustion capture uses wet/dry

adsorbents, which are used for gas separation, and separates

and

collects

CO2 by

adsorption/desorption.

The

classification

oftechnologies for post-combustion capture of CO2 is shown in Fig. 3.

In general, post-combustion capture technologies include wet

absorption [3335], dry adsorption [3638], membrane-based

technologies [3941], and cryogenics [42,43]. Current research

focuses on dry adsorption systems using dry adsorbents. Wet

absorption is good for treating large emission volumes from

combustion and is very useful for changing the density of CO2;

Fig.

2.

Schematic

diagrams

of

three

types

of

CO2capture

process:

(a)

post-combustion,

(b)

pre-combustion,

and

(c)

oxy-fuel

combustion

technologies.

Table 1

Technologies for capture, transportation, and storage of CO2.

Technologies Classification Applied areas

CO2 capture technologies Post-combustion

Pre-combustion

Oxy-fuel combustion

Direct capture from (thermoelectric) power plants and

industries (steel mills, refineries, etc.)

CO2 transportation technologies CO2compression and transportation Transportation of captured CO2to a storage site via a

pipeline or other means of transport

CO2 storage technologies Characterization and evaluation of bedrock

layers Drilling and injection

Observation and prediction of movement of

bedrock layers

Evaluation of the effect on the environment

and post management

Deposit to a bedrock layer (basins, oilfields, etc.) in the

ground

and

below

the

sea

and

its

management

S.-Y. Lee, S.-J. Park/Journal of Industrial and Engineering Chemistry 23 (2015) 111 3


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however, it requires high energy for absorbent regeneration.

Additional drawbacks are (i) the use of heated absorbents, (ii)

erosion of materials, and (iii) slow solidgas reactions [44,45]. A

dry adsorption process has the following advantages: it requires a

simple device, is easy to operate, and has excellent environmental

effects and energy efficiency; however, its performance is poor in

the treatment of large emission volumes and its separation

efficiency is also poor [46,47]. A membrane-based process has the

following advantages: a simple device, easy operation, and low

energy consumption; however, it requires a high-cost module, isnot suitable for treating large volumes of emission gases, and is not

very durable [48,49]. Finally, cryogenics is a suitable process

because of its low investment cost and high reliability; however, it

is not suitable because of its energy consumption [50].

Recent research has focused on improving adsorbents and

developing efficient processes for cost-effective capture perfor-

mance. The capture and separation of CO2from a large volume of

combustion gases are still expensive and have high energy

consumption; therefore, more efficient adsorbents for relatively

concentrated streams need to be developed.

CO2 capture using dry adsorbents

The

process

of

capturing

CO2 using

a

dry

adsorbent

involvesselective separation of CO2based on gassolid interactions [51]. In

general, universal dry adsorbents such as activated carbons and

molecular sieves are used in packed columns [52]. The important

variables in a dry adsorption process are the (i) surface tension and

pore size of the adsorbent and (ii) temperature andpartialpressure

during theadsorptionprocess [53]. Theprocess involvesa repeated

cycle of adsorption and desorption (regeneration).

The different types of adsorption are as follows: (i) pressure-

swing adsorption (PSA) [5456]; (ii) temperature-swing adsorp-

tion (TSA) [57,58], where two processes are combined, i.e.,

adsorption at a low temperature followed by desorption/regener-

ation by heating or lowering the pressure; (iii) electric-swing

adsorption (ESA) [59], i.e., adsorption/desorption by varying the

electricity

supply,

with

a

low-voltage

current

passing

through

the

adsorbent; and (iv) vacuum swing adsorption (VSA) [60]. Further-

more, adsorption-based technologies such as pressure/vacuum-

swing adsorption (PVSA)have frequently been investigated because

of their low energy requirements and relative simplicity [6163].

It is essential to improve the CO2 capture selectivity and

adsorptive capacity of adry adsorbent for treating large volumes of

combustion emissions from many different sources. The CO2adsorption can be improved and stabilized by introducing

functional groups (mainly amine groups) with high affinities for

CO2onto the surface of the adsorbent material to react with CO2,which can then be selectively adsorbed using the wide specific

surface area and pore structure of the adsorbent [6469].

To achieve high capture performance and high selectivity, the

incorporation of various amine groups into solid materials used as

CO2capture sorbents is expected to improve polarization and CO2capture. Such adsorbents have several advantages such as

potential elimination of corrosion problems and lower energy

costs for regeneration. The interactions between CO2 and amine

functional groups produce ammonium carbamates under anhy-

drous conditions as follows [70]:

1CO22RNH2 ! RNHCOORNH3

2CO22R2NH ! R2NCOOR2NH2

The adsorption process is significantbecause it is reversible and

the adsorption efficiency can be improved by modifying the

structure of the adsorbent materials. The CO2 adsorption efficiency

can therefore be improved by selecting an appropriate adsorbent

material. At present, the main commercially available adsorbents

are activated carbons [71,72], zeolites [73], hollow fibers, and

alumina. Each material has a different pore structure, specific

surface area, and surface functional groups, and their application

fields are highly specific. Some typical non-carbonaceous dry

adsorbents for CO2 capture are listed in Table 2 and will be

described

in

the

following

section.

Fig. 3. Classification of application technologies for post-combustion capture of CO2.



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Non-carbonaceous

dry

adsorbents

Zeolites

Zeolites are typical naturally occurring microporous crystalline

silicate framework materials; they can also synthesize in the

laboratory. They have uniform pore sizes of 0.51.2 nm, forming

networks of interconnecting channels or cages for adsorbing/

capturing gas molecules [74]. They have therefore been widely

used for gas separation and purification [7577]. Zeolites have

been extensively investigated for CO2 capture because of their

molecular sieving effect and the strong dipolequadrupole(electrostatic) interactions between CO2 and alkali-metal cations

in the zeolite frameworks [78]. The cations in zeolites, such as Li,

Na, and Al, influence the heat of adsorption of CO2; the heat of

adsorption increases with increasing monovalent charge density

(of negative charges) [79,80].

Many groups have reported the synthesis and characterization

of zeolites for CO2 capture, mainly zeolite 13X and zeolite 5A,

which gave CO2 capture performances of 325 wt.% at room

temperature and a CO2pressure of 100% [8184]; they also gave a

CO2capture performance of 212 wt.% at room temperature and a

CO2partial pressure of 15% [8587].

Cavenati et al. [88] investigated zeolite 13X as a solid adsorbent.

Zeolite 13X exhibited a CO2 adsorption capacity of 28.7 wt.% and

CO2/N2separation

capacity

of

3.65

at

298

K

and

10

bar.Jadhav et al. modified zeolite 13X via monoethanol amine

(MEA) impregnation to improve the CO2adsorption capacity. The

CO2 adsorption capacity was better than that of the unmodified

zeolite by a factor of approximately 1.6 at 303 K, and at 393 K, the

efficiency improved by a factor of 3.5. The higher capacity of MEA-

modified zeolite 13X at 393 K indicated that the chemical

interactions between CO2 and amine groups might play a key

role in CO2 sorption, despite the reduced pore volume and lower

surface area resulting from MEA impregnation [89].

In the case of zeolite 5A, a typical CO2 adsorbent, the CO2adsorption capacity and CO2/N2 selectivity were 20.8 wt.% and

8.45, respectively, at 298 K and 1 bar. At a higher pressure of

10 bar, the CO2 adsorption capacity of zeolite 5A was 22.3 wt.%

[90].

Recently, binderless zeolite NaX microspheres for CO2capture

were prepared using chitosan-assisted synthesis. The obtained

microspheres had auniformparticle size of approximately1.3 mm,

a specific surface area of 931 m2/g, and a moderate crushing

strength of 0.46 MPa. The CO2adsorption capacities were 22.7 and

31.2 wt.% at 1 and 10 bar, respectively, at 298 K [91].

Most of the researchusing zeolites forCO2 capture from flue gas

has focused on PSA or VSA processes [9294]. The CO2 capture

performances of zeolites are greatly influenced by the temperature

and pressure. Zeolites are typically used at pressures above 2 bar

and require very high regeneration temperatures, often above

573 K; therefore, CO2 regeneration results in huge energy loss[95,96]. In addition, the adsorption performances of zeolites

decrease because they absorb moisture easily; therefore, many

studies have been conducted improve this [9799].

Silica materials

Many studies on the synthesis and modification of silica

materials containing a large amount of mesoporous materials for

efficient CO2 capture have been reported [100111].

An amine-rich nano-silica adsorbent was synthesized using

poly(acrylic acid) as a multi-functional bridge for amine

immobilization on the silica nanoparticle surfaces, followed

by treatment with polyethylenimine (PEI) as the amine source.

The

CO2 adsorption capacity of the sample increased withincreasing temperature and reached a maximumCO2adsorption

capacity of 16.7wt.% at 313 K and 1 bar. The low adsorption

capacity at low temperatures was caused by kinetic limitations.

When the temperature was increased to 353 K, the adsorption

capacity decreased to only 4.4 wt.%, because the reaction

between CO2 and amines is an exothermic process (van t Hoff

behavior) [112].

Le et al. reported that tetraethylenepentamine (TEA)-function-

alized/monodispersed porous silica microspheres were success-

fully prepared using tetraethoxysilane as the precursor and

dodecylamine as the templating agent and hydrolysis catalyst,

followed by calcining at 873 K, and functionalization with TEA.

They found that for the optimal TEA loading of 34 wt.% on the silica

microspheres,

the

maximum

CO2 adsorption

capacity

was

Table 2

Comparison between major non-carbonaceous dry adsorbents for CO2capture.

Adsorbents Advantages Disadvantages

Zeolites, silica

materials

Low production cost

Large micropores/mesopores

Medium CO2adsorption

(at

298

K

and

1

bar)

Poor performance of CO2adsorption due to easy

moisture absorption

Heavy energy consumption during CO2desorption

(poor

economic

feasibility)

Renewal difficulties

Metal organic

frameworks

(MOFs)

Large specific surface area (over 10,000m2/g) and regularpore

distributions Ease of controlling pore sizes

Possible improvement in CO2selectivity according to various

combinations of metal clusters and organic ligands

Poor performance at the partial pressure of CO2

Poor

economic

efficiency

due

to

high

production

cost Complicated synthetic process

Moisture-sensitive (possible structure failure due to

moisture absorption during CO2 capture)

Unsuitable for use at high temperature. Mostly VSA

process (poor economic feasibility)

Alkali-based dry

adsorbents

(K-, Na-, etc.)

Possible adsorption and desorption at a low temperature, i.e.,

313-343

K

(similar

to

amine-based

absorption)

Possible CO2collection under wet conditions

Absorption and renewal under 473K and possible operation at

atmospheric

pressure

(high

economic

efficiency)

Low adsorption capability (311wt.%)

High-temperature reactions

Decrease in the collection ratio of CO2 because of stable

products (e.g., Na2CO3and NaHCO3)

Requires high temperatures during desorption

(high energy consumption)

Complicated operation

Metal oxides-based

adsorbents

(CaO, MgO, etc.)

Dry chemical absorbents

Adsorption/desorption at medium to high temperatures

(>673K)

Popular as a pre-combustion absorbent

High consumption of energy due to adsorption/desorption

at medium to high temperatures (>673K)

High cost for regeneration

Demand for continuous addition of absorbents

Complicated process



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18.8 wt.% at 348 K and 1 bar. The sample showed relatively good

thermal stability in CO2adsorption in the TSA process (desorption

temperature 393 K) [113].

Xu et al. reported that PEI-modified mesoporous MCM-41-type

molecular sieves (MCM-41-PEI) can serve as molecular baskets

for condensed CO2. MCM-41 has a synergetic effect on CO2adsorption by PEI. The CO2 adsorption capacities of MCM-41-PEI-

50 (PEI loading 50 wt.%) and MCM-41-PEI-75 (PEI loading 75 wt.%)

were 11.2 and 13.3 wt.%, respectively, at 348 K and 1 bar [114].

Hicks et al. synthesized covalently tethered hyperbranched

aminosilica-based SBA-15 materials capable of adsorbing CO2reversibly, with very high CO2 capacities of 13.6 wt.% at 298 K.

They suggested that the advantages of this adsorbent are its large

CO2 capacity and multicycle stability. The material was recycled by

thermally desorbing the CO2from the surface with essentially no

changes in the capacity [65].

Sanz-Perez et al. tested tetraethylenepentamine (TEPA)-

functionalized SBA-15 (SBA-15-TEPA) using a simulated gas

mixture similar to that from a coal-fired thermal power plant.

The CO2adsorption capacity of SBA-15-TEPA (50) after 10 adsorp-

tiondesorption cycles was 86% of the initial value, which was

11.35% under 100% CO2at 318 K; it also reached a CO2adsorption

capacity of 16.2% in a humid stream with 15% CO2 at 318 K and

1 bar. They concluded that the presence of 5% moisture enhancedthe CO2adsorption capacity [115].

Kjdary et al. suggested the incorporation of Cu, Fe, Ag, and Au

nanoparticles intomercaptopropyl-modified silica forCO2 capture.

The CO2adsorption capacities of the metal ions and nanoparticle

nanocomposites increased significantly, by 20 and 100%, respec-

tively. Cu-MP-S showed the maximum capacity, 22.9 wt.% at 323 K

and 1 bar [116].

Mello et al. synthesized amine-modified MCM-41 mesoporous

silica for CO2 capture. They showed that at a low pressures of

0.1 bar, the CO2 adsorption capacity of MCM-41-NH2 was

3.08 wt.%, which is much higher than that of 0.53 wt.% for

MCM-41. This is because of the great affinity of CO2, a Lewis acid,

for basic amine sites at low pressures. However, at high pressures

of 2.1 bar, the difference is not so pronounced; the CO2adsorptioncapacities of MCM-41-NH2 and MCM-41 were 5.06 and 4.4 wt.%,

respectively. From the results, they concluded that the most

reactive amine groups were bound to CO2 at low pressures, but the

available pore volume was filled at high pressures [117].

Besides the incorporation of amine-functional groups or metal

nanoparticles on silica supports, various silica materials with

controlled pore diameters and arrangements have been synthe-

sized and they showed significantly enhanced CO2 capture

performances [118122].

Metal-organic

frameworks

(MOFs)

and

porous

polymers

Numerous MOFs have been synthesized from combinations

of

inorganic (metal

clusters) and organic units (ligands) bystrong bonding (reticular synthesis). The flexibilitywith which

the geometries, sizes, and functionalities of the constituents

can be varied has led to more than 20,000 differentMOFs being

reported and studied; their specific surface areas typically

range from 1000 to 10,000m2/g [123,124]. They are popular

because of their excellent selectivities and superior adsorption

capacities, particularity for H2 adsorption [125127], CH4adsorption [128,129], and adsorption of some toxic gases

[130,131], and they are used as catalysts for fine chemical

synthesis [132,133].

The structures of MOF adsorbents for CO2 capture have

available spaces at the center because the molecules are

intertwined. Such a structure has a large body volume because

the

pores

are

attached

to

organic

molecules

and

knots

of

metallic

ions. MOFs are widely used as storage media for various gases

because the pore diameters can be easily controlled [134138].

McDonald et al. reported that N,N0-dimethylethylenediamine/

H3[(Cu4Cl)3(BTTri)8(CuBTTti; H3BTTri = 1,3,5-tri(1H-1,2,3-triazol-

4-yl)benzene] composites adsorbed 15.4 wt.% of CO2at 298 K and

1 bar and 9.5 wt.% of CO2 at 298 K and 0.15 bar CO2/0.75 bar N2,

with a selectivity of 327, determined using the ideal adsorbed

solution theory (IAST). They emphasized that amines tethered to

solid adsorbents have considerable advantages over aqueous

alkanolamines, because of their quick regeneration using mild

temperature swings [139].

Bao et al. reported that Mg-based MOF-74 had a very high CO2adsorption capacity of 37 wt.% at 298 K and 1 bar [140] and Caskey

et

al. reported that it had a considerable adsorption capacity of

23.6 wt.% at 296 K and 0.1 bar [141]; they investigated the

selectivities for CO2 and CH4, but not for CO2/N2. They suggested

that the exceptionally high performance of Mg-based MOF-74 for

CO2may be attributed to strong interactions between the oxygen

lone pair orbitals of CO2 with coordinatively unsaturated metal

cations, suggesting that it is a promising adsorbent for CO2 capture

[142].

MOFs show high CO2 storage capacities at 298 K and 42 bar.

Gravimetric CO2 isotherms for various MOFs such as MOF-2,

MOF-505, Cu3(BTC)2, MOF-74, IRMOFs-11, -3, -6, and -1, andMOF-177 were reported by Yaghis group. They confirmed that

the amine functionalities of the IRMOF-3 pores increased the

affinity for CO2. The highest CO2 adsorption capacity was

147 wt.%, for MOF-177 [143].

Yan et al. reported high enhancement of CO2 uptake by an

HKUST-1 MOF via a simple chemical treatment, to improve theCO2capture performance. It had a maximum CO2adsorption capacity

of 51 wt.% at 273 K and 1 bar, which is a significant increase of 61%

compared with the original MOF sample [144]. Llewellyn et al.

reported chemical treatment of MIL-101 with ethanol and then

NH4F aqueous solution; the CO2 adsorption capacity of the

chemically treated MIL-101 increased from 79.2 to 176 wt.% at

304 K and 50 bar [145].

More recently, conjugated microporous polymers (CMPs) haveattracted much interest because of their porous structures, formed

from organic functionalities; they have applications such as gas

storage, capture, and separation [146148]. In particular, the

incorporation of metal-organic species in CMPs provides new

materials capable of CO2capture and its simultaneous conversion

for cost-effective reduction of CO2. Xie et al. reported that the CO2sorptionproperties of theCo-CMP andAl-CMP were comparatively

high, although these materials have a relatively low specific

surface areas compared with other MOF materials. The CO2adsorption capacity and specific surface area of Co-CMP were

7.93 wt.% and 965 m2/g, respectively; those of Al-CMP were

7.65 wt.% and 798 m2/g, respectively. The co-catalysts Co/Al-

CMP displayed exceptionally high catalytic activity in the

conversion

of

propylene

oxide

and

CO2 to

propylene

carbonateat room temperature and atmospheric pressure [149].

MOFs have very high CO2capture capacity at high pressure.

However, most MOFs have poor CO2 capture performances

compared to other solid physical sorbents at a low partial

pressure of CO2, and maintenance of a high partial pressure is

not economically feasible (process unit costs). An economically

viable MOF needs to be developed for the efficient storage and

separation of large volumes of CO2, up to 1 ton/day. Further-

more, the synthesis of MOF, a metal complex, from an organic

ligand is in many cases very expensive, and the synthetic

process is complicated. MOFs suffer from durability and

mechanical strength problems because of moisture absorption

during CO2 capture. Theuse of MOFs in powerplants is therefore

limited.



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Alkali-metal-based materials

The use of alkali-metal (such as Na, K, Al) carbonates as

renewable dry adsorbents for capturing CO2 from flue gas at

operating temperatures below 473 K, relatively moderate condi-

tions, with independent supports has been reported [150152]. In

this process, the alkali-metal carbonates are added to various

inorganic supports such as carbon materials, zirconia, ceramics,

silica, and alumina; CO2 absorption is achieved by reaction with

moisture (carbonation; (Eq. (3))), and the absorbent is renewed by

decarbonation (Eq. (4)) [153,154]:

M2CO3H2O CO2@2MHCO3M Na;K (3)

DH = 135 kJ/mol and 141 kJ/mol for M = Na and K, respec-

tively.

2MHCO2@M2CO3H2O CO2 (4)

It is well known that CO2 and H2O normally react with a

carbonate sorbent at 333383 K to form alkali-metal bicarbonates

in an adsorption process, as in Eq. (3), and the bicarbonate

regenerates the alkali-metal carbonate at 373473 K, with release

of CO2. The theoretical CO2 adsorption capacities of Na2CO3 and

K2CO3 are 41.5 and 31.8 wt.%, respectively.Li-based zirconate (Li2ZrO3) and Li-based silicate (Li4SiO4) are

also promising CO2 captors and can be used for direct CO2separation from flue gas at high temperatures, 700900 K

[155,156]. Li4SiO4, in particular, has great potential as a CO2captor because of itshighCO2 sorption capacity of36.7 wt.% and its

small volume change during the CO2adsorptiondesorption cycle

[157,158].

Kato et al. investigated the CO2 sorption capacities of Li2ZrO3and Li4SiO4at low CO2 concentrations, i.e., 50 ppv, and concluded

that the CO2 sorption capacity of Li4SiO4 is more than 30 times

greater than that on Li2ZrO3. Moreover, silica materials are cheaper

than zirconia materials [159,160]. More recently, Seggiani

et

al. reported that Li4SiO4 with addition of 30 wt.% K2CO3 or

Na2CO3showed a CO2sorption capacity of 23 wt.%, at an optimumsorption temperature of 853 K and low CO2 partial pressure of

0.04 bar, corresponding to a Li4SiO4 conversion of about 80%

[161]. Duran-Munoz et al. proposed Li8SiO6 as an alternative dry

adsorbent for CO2capture, it showed a very high sorption capacity

of about 51.9 wt.% over a wide temperature range, with an

efficiency of 71.1% [162].

These alkali-metal-based materials are technically and eco-

nomically attractive for post-combustion CO2 capture at high

temperatures and low concentrations, because they do not need

further cooling processes; however, problems with the long-term

stabilities and sustained performances of these adsorbents under

real flue gas conditions in post-combustion applications need tobe

solved.

Metal

oxide

carbonate

materials

Mineral carbonation technology uses metal oxides such as CaO

and MgO as dry chemical adsorbents; a chemical reactionfixesCO2as an insoluble carbonate [163168]. MgO has been shown to be

good a particularly candidate for CO2capture adsorbents because

of its low cost, abundance, and low toxicity. However, MgO has a

very low sorption capacity of 0.57 wt.% at moderate temperatures

in dry environments [169].

Han et al. suggested novel MgO-based mesoporous composites

with concrete-like structures, which were synthesized using a

facile coprecipitation method, for trapping CO2 in flue gas in the

high temperature range 423673 K. Microcrystalline MgO present

in

the

alumina

frameworks

had

CO2 adsorption

capacities

of

7.7 and 13.1 wt.% in the absence or presence of water vapor,

respectively, at 473 K. Regeneration was performed at 873 K, and

stable cyclic adsorption was achieved [170].

Bhagiyalakshmi et al. synthesized mesoporous MgO using

mesoporous carbon, CMK-3, obtained from mesoporous SBA-15, as

an exotemplate. They confirmed that the pore structure of the

mesoporous MgO sample resembled those of SBA-15 and CMK-3.

The CO2adsorption capacities of mesoporous MgO were 8 wt.% at

298 K and 10 wt.% at 373 K, whereas that of non-porous MgO was

12 wt.% at 298 K [171].

Liu et al. reported that MgO sorbents doped with alkali-metal

carbonates had enhanced CO2sorption capacities at both low and

moderate operating temperatures, compared with pure MgO

sorbents. MgO-doped Cs2CO3 had a maximum CO2 sorption

capacity greater than 8.36 wt.% at 573 K [172].

However, the chemical reaction is too slow and requires high

consumption of energy. Moreover, the regeneration process is

performedat6731073 K, i.e., it alsohashighenergy consumption.

Therefore, although CO2capture using metal-oxide carbonates has

a high absorption volume, this technology may be inappropriate

for CO2 storage.

Carbonaceous

adsorbents

Although carbonaceous materials consist of a single element,

they have many advantages such as high thermal/chemical

stabilities, electrical and heat conductivities, strengths, elasticities,

and bio-affinities [173177]. They are particularly good for

applications for gas adsorption or storage, because they are

lightweight, and have very high specific surface areas and large

pore volumes [178180]. They also have advantages for CO2capture: (i) carbon materials are not moisture sensitive; (ii) the

cost of carbon materials is reasonable; (iii) the adsorption/

desorption temperatures are below 373 K; (iv) they can be used

at atmospheric pressure; and (v) the energy consumption is low;

all of these factors influence current studies in this field.

Activated carbonsCO2 adsorption capacity is strongly sensitive to the textural

properties and surface groups of carbon-based adsorbents [181

183]. The pore size distributions of activated carbons vary from

micropore to macropore; therefore, activated carbons are inap-

propriate for selective adsorption of a specific gas. Generally,

pristine carbon-based adsorbents have weak affinities for CO2,

with adsorption heats of less than 25 kJ/mol [184]. The typical CO2adsorption capacity of an activated carbon is 5 wt.% at 298 K and

0.1 bar [185,186].

Thepore structures of activated carbons canbe easily controlled

by varying the preparation and activation conditions [187

190]. Moreover, the functional groups on the activated carbon

surface can also be easily controlled using various treatments

[191194].

The

incorporation

of

various

basic

groups

on

activatedcarbon has been actively investigated for enhancing the CO2affinity by increasing the CO2 adsorption capacity [195,196].

Zhao et al. designed and prepared a novel microporous carbon

material (KNC-A-K) with a high N-doping concentration, above

10.5 wt.%, and extra-framework K+ cations, which gave CO2capture performances of 7.1 wt.% (0.1 bar) and 17.8 wt.% (1 bar)

at 298 K, and a CO2/N2 selectivity of 48, determined using IAST

method. They emphasized that its outstanding low-pressure CO2adsorption ability makes KNC-A-K a promising candidate for

selective CO2 capture from flue gas. The K+ ions played a key role in

promoting CO2 adsorption via electrostatic interactions [197].

Lee et al. prepared heat-treated carbons by pyrolysis of

poly(vinylidene fluoride) at various heat-treatment temperatures

and

evaluated

the

CO2 adsorption

capacities.

The

CO2 adsorption



8/11

capacity increased with increasing heat-treatment temperature up

to 873 K (15.5 wt.% at 298 K and 1 bar) and then decreased at

973 K; this correlated with their micropore volumes. Interestingly,

they emphasized that themicropore volumehad the greatesteffect

on CO2 adsorption at1 bar; meanwhile, the pore sizes also strongly

affected the CO2 adsorption behavior at pressures below 0.3 bar. In

addition, it was found that ultra-micropores (


9/11

adsorption capacities. They demonstrated that the ACF pore

structure can be designed to enhance the CO2adsorption capacity

by controlling the concentration of KOH intercalated into the

graphitic lattice. The highest CO2 adsorption capacity was

25.0 wt.% at 298 K and 1 bar, achieved using a materials heat-

treated with 3:1 KOH/ACF. It was also found that the CO2adsorption capacity was strongly affected by the ultra-micropore

size distribution rather than by the specific surface area or

micropore volume [230]. This is because ultra-micropores have a

high adsorption potential, which enhances the adsorption of CO2molecules [231,232].

Thiruvenkatachari et al. prepared large honeycomb-shaped

carbon fiber composite (HMCFC) adsorbents for large-scale CO2capture tests (the adsorbent mass in one column was 4.486 kg).

The average CO2 adsorption capacity was 11.9 wt.% for a simulated

fluegas consisting of13%CO2,5.5%O2, and thebalanceN2, at293 K.

They also showed that the CO2 capture efficiency of HMCFC

adsorbents is more effective in a combined vacuum and thermal

decomposition process [233].

Graphene

Graphene oxide (GO) is a derivative of graphene and can be

synthesizedwith various functional groups on thebasalplanes and

edges [234]. The surface modification of GO with variousfunctional groups and the synthesis of new types of GO-like

derivatives with lightweight frameworks has been widely

researched for applications such as gas storage and separation,

energy conversion, and sensors [235240].

Zhao et al. demonstrated that ethylenediamine (EDA)-interca-

lated GO had a CO2adsorption capacity of 4.65 wt.% at 303 K and

1 bar for CO2/N2 mixed gases [241].

Lee et al., prepared isothermally exfoliated GO using a new CO2pressure swing method. They suggested that the CO2 pressure

swing method exerted sufficient pressure to overcome the GO-

interlayer van der Waals binding energy and expand the GO, thus

promoting GO exfoliation by enabling large pores to develop. The

results showed that the best sample had a specific surface area of

547 m2

/g and total pore volume of 2.468 cm3

/g, resulting in a CO2adsorption capacity of 28.2 wt.% at 298 K and 30 bar [242].

Meng et al. reported thermally exfoliated graphene nanoplates

as novel high-efficiency sorbents for CO2 capture. The prepared

graphene nanoplates had high capture capacities, 248 wt.%, at

298 K and 30 bar. The improved CO2 capture capacity of the

graphene nanoplates was attributed to the larger inter-layer

spacing and high interior void volume [243].

Aminated GO for CO2 adsorption was synthesized by the

intercalation reaction of GO with amines such as EDA, diethylene-

triamine (DETA), and triethylene-tetramine (TETA). The results

showed that the adsorption performance of the sample with 50%

EDA covalently attached to GO was better than those of GO

modified with DETA or TETA. The EDA-modified sample had a CO2

adsorption

capacity

of

4.65

wt.%

at

303

K

in

15%

CO2/N2 mixedgases at a flow of 40 mL/min [241].

Conclusions

The performances of currently available adsorbents for CO2capture technologies need to be improved in terms of working

adsorption capacity, cycle lifetime, and multicycle durability. The

development of new highly efficient adsorbents for CO2capture is

necessary to obtain systems that are techno-economical. It is

therefore very important to acquire data on adsorption reactors,

regeneration processes, and overall process integration of capture

systems in power plants.

Furthermore, through thedevelopment of advanced adsorbents

for

CO2 capture,

highly

techno-economical

systems

will

be

achieved by combining advanced CO2 capture technologies with

related systems such as hydrogen generation in the steam

reforming reaction, water gas conversion, electricity generation

using hydrogen, fuel cells, and water treatments.

Acknowledgements

This work was supported by the Carbon Valley Project of the

Ministry

of

Knowledge

Economy

and

the

Energy

Efficiency

&Resources of the Korea Institute of Energy Technology Evaluation

and Planning (KETEP) Grant funded by the Ministry of Commerce,

Industry, Trade and Energy, Korea.

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