Carbon Dioxide Capture Technology

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Conversion of Carbon Dioxide to Chemicals and Fuels
Carbon Dioxide Capture Technology
Hariprasad Narayanan1
, Honey Mary Joseph2
, Soumya Gopinath2
1
School of Environmental Studies, 2
Department of Applied Chemistry
Cochin University of Science and Technology, CUSAT-682022
Image Courtesy: Angew. Chem. Int. Ed. 2010, 49, 6058 – 6082

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ASSIGNMENT ON CARBON CAPTURE TECHNOLOGY
SUBMITTED TO NATIONAL CENTRE FOR CATALYSIS RESEARCH
IIT MADRAS
Submitted by
HARIPRASAD NARAYANAN
HONEY MARY JOSEPH
SOUMYA GOPINATH

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This short writing highlights the challenge for capture technologies, especially
post combustion, precombustion capture and natural gas sweetening. The recent
developments such as solvent absorption, chemical and physical absorption, and
membranes, will be talked about.
1. Introduction
The increasing level of carbon dioxide in the atmosphere is the single of the most
environmental problem the land faces. The drastic rise has been ascribed to an
increasing dependence on the burning of fossil fuels like coal, natural gas and
gasoline, which accounts for nearly 86% of anthropogenic greenhouse gas
emissions, the remaining from land usage change and chemical processing.
Schemes to reduce global atmospheric concentration of GHG’s has received a
great heap of attention in recent times, especially from governments and
industries, and a routine of high profile collaborative programs has been installed,
including IPCC (Interogovernal Panel on Climate Change) & GCCI (Global
Climate Change Initiative). The capture and sequestration of carbon dioxide is
the main strategy in these initiatives. CCS (Carbon Capture & Sequestration)
include a group of technologies for the capture of carbon dioxide from various
sources followed by compression, transport and permanent storage. CCS also
focusing to improve the energy efficiency, shifting to less carbon intensive fuels
(natural gasoline) and phasing in the role of alternative energy resources like solar
energy, wind and biomass.

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A critical point is the development of CCS scheme which is a multifaceted
problem. It requires shared vision and worldwide collaborative efforts from
governments, industries, policy makers and economist, as well as scientists,
engineers and venture capitalist. From these views, it is clear why the carbon
dioxide capture is regarded as one of the grand challenges for the 21st
century.
The existing methods such as amine scrubbers and cryogenic coolers are energy
intensive and are not cost effective for carbon emissions decrease. This is the
primary cause for the dense deployment of fully integrated commercial CCS
schemes.
Several parts of the CCS process chain including compression, transportation (by
pre-existing pipelines for Instance), and storage of CO2 are technologically
mature and Available, and a growing number of fully integrated CCS Projects are
reaching the pilot and demonstration phases prior to commercialization.
In accession to three large-scale demonstration projects which are presently
underway in the Sleipner West (Norway), Weyburn (Canada), and In Salah
(Algeria), several smaller projects have started out on the Dutch continental shelf
(Netherlands), Snøhvit (Norway), La Barge (Wyoming, United States), Fenn Big
Valley (Canada), Ketzin (Germany), and Schwarze Pumpe (Germany). All of the
current projects demonstrate carbon storage or reuse in enhanced coal-bed
methane recovery systems, although one project at Schwarze Pumpe in Germany,

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captures CO2 at a coal-based plant. A further 40 CCS projects have already been
proposed worldwide between 2008 and 2020.
One explanation for the dense deployment of fully integrated commercial CCS
schemes is the considerable cost of the capture phase, which represents
approximately two thirds of the total cost of CCS.
A recent comprehensive report on post combustion CO2 capture technologies has
seen that the regeneration energy, succeeded by the capital cost of capture specific
equipment are the two variables contributing most significantly to the monetary
value of CO2 capture. One significant contributor to the regeneration energy is
the maximum separation efficiency, which can be achieved by a given capture
material. Enhancing this efficiency, will possess the greatest potential for turning
down the overall monetary value of capture systems in near-term, with
improvements in the capture phase for new materials representing one of the first
challenges.
As recorded in Figure 1, there exists a serious demand for research on innovative
new fabrics in order to shorten the time to commercialization.
It is apparent that a consideration of the process design and economics costs are
needed to evaluate fully the potential of any given new material.
Here, we concentrate on the important challenge of CO2 capture and highlight
recent improvements in materials and emerging concepts. The stress is on

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designing materials in which a molecular level of command can be achieved as
a means of tailoring their performance in separating relevant gas mixtures.
Figure 1. Advanced concepts in CO2 capture and their cost reduction benefits versus their
remaining development needs/time to commercialization.
Adopted from Chem.Rew.2014, 114,1413-1492
In this respect, special care is pointed towards the latest developments in CO2
separations using porous solids or metal–organic frameworks.
2. Key Challenges for CO2 capture
The central challenge for gas separations materials is that the differences in
properties between the gases that have to be separated are relatively small.

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Novel concepts for capture therefore require a molecular level of control that can
take advantage of differences in the chemical reactivity of the gas molecules. A
further challenge is that the selectivity of a separate procedure is influenced by a
combination of adsorption and diffusion selectivity, which are partnered off with
most materials. For instance, the first appearance of a functional group that
specifically binds one species, and improves on the adsorption selectivity, will
simultaneously decrease the diffusion of these particles. The need, therefore
exists to design materials in which one can independently tune the diffusion and
adsorption selectivity at the molecular level. The capture of CO2 from ambient air
has also been adduced as a strategy to reduce atmospheric CO2 levels, particularly
with respect to small and mobile emission sources. Distinctly, the low
concentration of CO2 in air (0.04 %) presents a significantly higher
thermodynamic barrier to get compared with post combustion methods, while the
expense of running great masses of air through an absorbing material presents a
further challenge. Although current post combustion capture technology is
suitable for the retrofitting of existing plants, the energy penalty for a coal-fired
power station using traditional aqueous monoethanolamine (MEA) for capture
has been estimated at 25 to 40 %.
3. Conventional Chemical Absorption
One technology suggested to get rid of CO2 from the air, is the soda lime process.
This calls for drawing air through a sodium carbonate solution and then calciner,
to separate out CO2, which can be stacked away. But the scale that would be

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needed to lessen the concentration of CO2 in the atmosphere by simply a few parts
per million is almost inconvincible because of the depleted degree of CO2 and the
vast intensity of atmosphere that would need to be treated.
Most of the world’s existing fossil fuel power plant operates by firing the fuel in
the melodic line and then practicing the vitality produced by that combustion to
drive turbines. To capture CO2 from these power plants, it is necessary to split up
the CO2 from exhaust gases after the combustion process, through a process
called post combustion capture or PCC.
“Wet-scrubbing” CO2 capture technologies have been used industrially for over
50 years and are based predominantly on the industrially important primary
alkanolamine MEA. The procedure requires the transition of an aqueous amine
solution (typically 25–30. %) down the crest of an absorption tower, while a
gaseous stream of flue gas containing CO2 is put in at the tail end. A blower is
needed to pump the gas through the absorber. At a temperature of around 40O
C,
the reaction of CO2 with the amine occurs through a zwitterion mechanism to
form carbomates, a response that has been extensively analyzed.

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SCHEME 1: General reaction schemes for the chemical absorption of CO2 by a)
primary or secondary and b) tertiary amine-containing solvents
Adapted from Angew. Chem. Int. Ed.2010, 49, 6058 – 6082.
The liquid amine CO2-rich solvent passes from the absorber column to a stripping
tower where the mix is heated with steam to release the CO2.The regeneration of
the chemical solvent is carried out at elevated temperatures (100–140O
C) and
pressures not much higher than atmospheric pressure. The high heat of formation
associated with carbamate production leads to a considerable energy penalty for
regeneration of the solvent. Following positive feedback, the amine solution is
cycled back to the absorption tower for additional CO2 absorption.

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The principal advantages of the post combustion method are that it is
commercially mature and can be easily retrofitted into existing power plants, they
have a number of drawbacks. These include the considerable energy requirements
for solvent regeneration and the necessary use of inhibitors to control corrosion
and oxidative degradation due to residual oxygen in the flue stream. The
sensitivity of the solvents to chemical degradation from other by-products in the
flue gas streams, such as SOx and NOx, also lead to reduced efficiencies and
increased costs for electricity production. Improved strategies for postcombustion
capture include the use of liquids with lower heats of adsorption, increasing the
concentration of the adsorbent molecules and improving the mass transfer and
reaction kinetics. The use of other amine-based molecules with lower
regeneration temperatures have thus been considered for chemical absorption.
Secondary amines such as diethanolamine (DEA) possess a lower heat of reaction
compared with primary amines: the lower stability of the carbamate formed upon
CO2 absorption gives rise to a more economical regeneration step for the
secondary amines.
The CO2 loading capacity for primary and secondary amine lies in the range 0.5-
1 mol of CO2 per mol of amine, since a fraction of the carbamate species is
hydrolyzed to form hydrogen carbonates. The reaction of CO2 with tertiary
amines such as N-methyldiethanolamine (MDEA) occurs with a higher loading
capacity of 1 mol of CO2 per mol of amine, albeit with a relatively lower reactivity
towards CO2 compared with the primary amines. The carbamation reaction

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cannot proceed for tertiary amines. MDEA is commonly utilized for natural gas
treatment and exhibits lower solvent degradation rates in addition to a low energy
penalty for regeneration of the solvent in the stripper. Addition of a small amount
of primary and secondary amine enhances the CO2 absorption rates for tertiary
amines. The steric hindrance and basicity of the amine are the major factors
controlling the efficiency of CO2 capture reactions. Sterically hindered amines
such as 2-amino-2-methyl-1-propanol (AMP) which contain bulkier substituents
have been identified as the most promising absorption solvents due to the lower
stability of their carbamates.
(carbamate stability constant: AMP, 0.1 <DEA, 2.0< MEA, 12.5 at 303 K).
Inorganic solvents such as aqueous potassium and sodium carbonate as well as
aqueous ammonia solutions have also been considered for chemical absorption.
The chilled ammonia process for CO2 capture involves the reversible formation
of ammonium hydrogen carbonate, with the forward reaction to capture CO2 as
solid NH4HCO3 occurring at temperatures below 200
C.
Despite the strongly absorbing nature of the solutions, the large energy demands
of the regeneration step present a comparable problem with that taken in post
combustion CO2.
4. Emerging Methods for CO2 capture
Three strategies for incorporating capture into power generation scenarios are of
primary focus today: post-, pre- and oxy-combustion capture.

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Figure 2: Post-, oxy-, and precombustion concepts and separation system
integration into power plants.
Adopted from Environ. SCI. Technol.2011, 45, 8597–8604
4.1. Physical Absorbents
Monoethanol amine is a commonly used physical adsorbent, which selectively
binds CO2 at high partial pressure and low temperatures, in accordance with

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Henry law. Selexol and Rectisol are industrially used physical adsorbent for 40
years for natural gas sweetening and the treatment of synthesis gas. Selexol is a
mix of dimethyl ethers of polyethylene glycol and rectisol, a methanol chilled to
-400
C. The advantage of this case is the lower heat consumption in the solvent
regeneration step and the stripping process can be driven by heat or pressure
reduction, called flash distillation. Physical solvents are suitable for CO2 capture
from high pressure streams, hence mainly used in the post combustion method.
Ionic liquids are another class of physical solvents which are also known to be
selective towards CO2 adsorption. These contain a mix of organic cations and
inorganic anions and are viscous liquids at room temperature. These are
nonflammable, environmentally begin and can exhibit thermal stability and low
vapour pressure.
The mechanism of CO2 capture is physisorption with the heat of adsorption
around -11KJ Mol-1
. Hence solvent regeneration is energy economic. For this
reason, ionic liquids usually employed for precombustion applications. Some
ionic liquids adsorb CO2 in a chemisorption mechanism. In addition to these,
ionic liquids serve as SO2 polishing agent. It simultaneously removes both SO2
and CO2 since the solubility of SO2 is 8-25 times greater than that of CO2 at the
same partial pressure. Hence an additional step is required to remove SO2 from
CO2.

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The viscosity of ionic liquids minimizes the solvent loss from gas streams, limit
mass transfer, hence they suffer from low rates of adsorption. To surmount these
shortcomings, task specific ionic liquids have been developed.
Introduction of amine groups into TSIL increases the rate of CO2 adsorption, at
pressures relevant to flue streams (ca. 1bar). Polyaminized ionic liquids also show
high selectivity towards CO2/N2 and enhanced CO2 solubility than the monomeric
ionic liquid itself.
4.2. Adsorption Materials
A variety of physical adsorbents have been considered for CO2 capture which
includes microporous and mesoporous materials, metal oxides, and hydrotalcite
compounds.
Metal oxides such as CaO and MgO are promising materials for carbon dioxide
capture because of their ability to retain high adsorption capacities above 3000
C.
The reaction of CO2 with calcium oxide produce CaCO3 at 600-6500
C. The
reverse calcination reaction regenerates the oxide at 800-8500
C. Adsorbent
degradation has been observed after several cycles. Lithium based perovskites
have attracted attention for their high carbon dioxide adsorption capacities.
Hydrosilicates and derivatives are also suitable for carbon dioxide capture.
Usually these compounds are used in precombustion at temperature above 4000
C.
These compounds belong to a family of layered double hydroxide (LDH’S) with
the general formula

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M2+
1-x M3+
x (OH)2 Am-
x/m. yH2O where the M2+
is typically Mg2+
, M3+
is Al3+
and
the anion A may be chloride, nitrite or carbonate ions. Recent studies indicates
that Ga substituted hydrotalcites adsorb more carbon dioxide than naturally
occurring hydrotalcite Mg-Al-CO3.
“Dry carbonate process” is currently in the stages. In this, flue gas mixes with a
solid dry powered carbonate sorbent (Na2CO3 or K2CO3) in a fluidized bed to
form corresponding hydrogen carbonate salt. The regenerative decarboxylation
reaction can be achieved at a relatively low temperature of 1200
C.
4.2.1.Zeolites
Zeolites are the most widely reported physical adsorbate for CO2 capture. The
main functions that significantly influence the carbon dioxide capture
performance involved basicity, pore size distribution, electric field strength
stemming from exchangeable cation of zeolites, water vapor in gaseous streams
(hence they need a high regeneration temperature) and Si-Al Ratio.
Normally used zeolites are X and Y Faujasite system. Some synthetic zeolite such
as 5A and 15X are also used as physical adsorbents.
We can improve the adsorption capacity of zeolite by incorporating alkyl amine
on to their internal surfaces, which in turn improves the gas sorption properties
for low pressure capture applications. Hence we can use this method for capturing
carbon dioxide from the atmosphere.
Common zeolites used for carbon dioxide capture are given below

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 Polyethylamine impregnated MCM-41 (24 fold enhancement in the CO2
adsoprtion)
 Amines immobilized in support such as poly (methyl methacrylate) - suffer
from lack of stability over repeated cycles
 Alkyl amine covalently tethered to the surface of mesoporous support. For
example, Aziridine polymerization at silica, which exhibit reversible
carbon dioxide binding and multi-cycle stability under simulated flue gas
conditions using a temperature swing adsorption (TSA) approach.
Type of amine tethered or surface density of an amine group influences the
adsorption capacity. SBA-15 grafted with monoamino,diamino, and tetraamino
ethoxysilanes will give an adsorption of 0.52,0.87, and 1.10 mmol CO2/g
adsorbent, respectively.
4.2.2. Carbonaceous Adsorbents
Porous carbon are playing an important role in CO2 sorption and separation for a
long time due to their porosities, excellent stabilities, wide availabilities.
Numerous studies are going on adsorption studies of different types of carbon
forms like activated carbon, molecular sieve carbon, hecteroatom doped porous
carbons and nanocarbons. Out of which, hectroatom-decoration continues to
dominate in this area. Another marked trend is the vast range of nanostructured
carbons (graphite and CNT) as CO2 sorbents which are being created. The
carbonaceous materials are inexpensive compared to other solid adsorbants like
zeolite 13X.

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Figure: The synthesis of molecular sieve carbon through nanocasting pathway: the sacrificial
template including crystalline zeolites and MOFs.
Adopted from Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem., 2013,109, 484-503
4.2.3. Metal Organic Frameworks (MOFs)
Are microporous crystalline solids which composed of organic bridging to metal
base nodes to make three dimensional extended network with a uniform pore
diameter typically in the range 3 to 20Ao
.The nodes generally consist of one or
metal ions to which the organic bridging ligands coordinate through a specific
functional group like carboxylate and pyridyl.
Unique structural properties of MOFs include robust, high thermal and chemical
stabilities, unprecedented internal surface areas, high void volumes and low
densities (from 0.21 to 1.00 g cm). It takes in an ability to modulate systematically
the pore dimensions and surface chemistry within metal–organic fabrics. The
high surface area-to-weight ratio of MOFs is such that they have enhanced

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capacities for CO2 capture at moderate pressures. Compared with activated
carbon and zeolites, it delivers a high active surface area per unit weight (1500-
4500 m2
g-1
).
The framework [Zn4O(btb)2] with a surface area (SA) of 4500 m2
g-1
exhibits the
highest capacity for CO2, taking up 33.5 mmol g-1
at 32 bar. By comparison, the
benchmark material zeolite 13X adsorbs 7.4 mmolg-1
at 32 bar.
The isotherms of terephthalate based materials exhibited an interesting S-shaped
profile in which the presence of inflection points indicated electrostatic
interactions between the CO2 molecules themselves, which dominate as the
pressure increases.
Measurements on the adsorption kinetics for CO2 in [Zn4O(bdc)3] showed that
the activation energy for diffusion is directly proportional to the ratio of the
kinetic diameter of the diffusing molecule to the pore opening of the structure.
Grounded on this rationale, frameworks with smaller pores poses a higher barrier
to diffusion. A molecular sieving effect can be exploited for the separation of gas
mixtures in cases where the pore sizes are sufficiently small, and are in the range
of the kinetic diameters of the adsorbates. In accession to such kinetic size effects,
favorable electrostatic interactions between CO2 and the framework also provide
enhanced capture, as in the case of [Al(OH)(ndc)] (2.3 mmolg-1
, 10 wt % ,303 K,
5 bar). Gas uptake measurements revealed that while CO2 (kinetic diameter 3.3
Ao
) is excluded from entering the 3.0 Ao
diameter pores, favorable interactions
between the hydroxy groups of the framework protruding toward the 7.7 Ao

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diameter channels give rise to preferential CO2 adsorption over N2 (by a factor of
ca. 10).
The majority of adsorption studies has reported selectivities based on single
component adsorption isotherms, IAST (Ideal Adsorbed Solution Theory) serves
as the bench mark for the simulation and computational analysis of binary
mixture adsorption from the experimental isotherms.
Zeolite Imidazolate Frameworks (ZIFs) constitute a subclass of metal–organic
fabrics that can adopt zeolite structure types based on the replacement of (1)
tetrahedral Si4+
and Al3+
ions with tetrahedral transition metal ions such as Zn2+
or Co2+
and (2) bridging O2-
ions with bridging imidazolate-based ligands. High-
throughput synthetic methods have been used with great utility to produce ZIFs
with a range of pore aperture metrics. Functionalization of the imidazolate and
benzimidazolate linkers was also established to permit fine-tuning of the
interactions between the pore walls and guest molecules, thereby varying the
selectivity of adsorption. Significantly, in contrast to many metals–organic
frameworks, ZIFs exhibit high thermal stabilities and chemical stability in
aqueous and organic media, which are needed for practical separations processes.
In especial, the stability of a framework toward long-term exposure to water
vapor is a vital event in determining its suitability for CO2 capture from flue gas.

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4.2.4. Membranes
Membranes, acting like a very fine sieve, can preferentially remove CO2 from a
stream of mixed oxide gases. They are normally made up of polymer or ceramics
and can likewise be utilized in conjunction with liquids. Membranes are in many
industries, including for natural gas separation, for the production ammonia and
hydrogen and for the separation of nitrogen gas from the atmosphere. Yet at
present they are not practiced on a big scale for separation of CO2 from flue gases.
There are two main type of membrane systems that can be used to capture CO2.
One separate out the CO2 from other gases (gas separation membranes); the other
allows CO2 to be adsorbed from a gas stream into a solvent via a porous
membrane barrier (membrane gas absorption).
Gas separation membrane system has a key advantage over solvent systems: the
equipment is smaller, more versatile and involves no solvent. This cuts material
cost and reduces environmental concerns. The membrane acts as a semipermeable
barrier through which CO2 passes through more easily than do other gases. The
CO2 is driven across the membrane by a pressure difference. The energy required
to maintain these pressure difference represents a large proportion of the
operating cost of the technology at the present time.
Early membranes for CO2 separation were made from cellulose acetate, a natural
plastic made from wood or cotton. Generally glassy polymers are good at
separating CO2 from other gases, but overtime, they tend to become less effective,
which is a challenge when using membranes with wet flue gases. Composite

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membranes, with a combination of glassy and rubbery polymers segments,
combines the structure of the glassy polymer (giving good selectivity for CO2)
with the higher permeability of a rubbery polymer (allowing more gas to flow
through). But like all polymeric membranes, they do not operate well at high
temperatures; ceramic membranes, which are able to handle higher temperature’s,
represent an alternative approach.
Membranes can be used in conjunction with solvents, with the membrane
separating the flue gas from the liquid solvent, thereby reducing flow problems
that occur when the liquid and gases meet. The CO2 through the membrane and
is the absorbed by the solvent. However this may lead to a smaller contact surface
area between the liquid and the gas, limiting the absorption efficiency compared
to other solvents absorption techniques. The CO2 is then removed from the
solvent by heating. Using membranes in conjunction with solvent can reduce the
size of the equipment required to absorb the CO2.
Depending on the purity required, membrane separation at a coal powered station
may need to be a multistage process, with different membranes needed to separate
different gases. The separation process need a pressure or concentration
difference to be maintained between the two sides of membrane, which requires
energy. More research and development is required to enable membranes to be
used at higher temperature’s, with lower energy requirements and to enable them
to be upscaled to the necessarily massive scale that will be required. Nonetheless,
membranes are a promising technology for CO2 capture.

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References
1) Deanna M. DAlessandro, Berend Smit, Jeffrey R. Long, Carbon Dioxide
Capture: Prospects for New Materials, Angew. Chem. Int. Ed, 2010, 49,
6058 – 6082.
2) An-Hui Lu, Guang-Ping Hao, Porous materials for carbon dioxide capture,
Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem., 2013,109, 484-503.
3) Marc Pera-Titus, Porous Inorganic Membranes for CO2 Capture: Present
and Prospects, Chem. Rev., 2014, 114, 1413−1492.
4) Carbon dioxide-a themed issue, Energy Environ. Sci., 2012, 5, 7238.
5) Peter J Cook, Clean Energy, Climate and Carbon, CSIRO PUBLISHING,
Australia, 2012.
6) Philip H. Stauffer, Gordon N. Keating, Richard S. Middleton, Hari S.
Viswanathan, Kathryn A. Berchtold, Rajinder P. Singh, Rajesh J. Pawar,
and Anthony Mancino, Greening Coal: Breakthroughs and Challenges in
Carbon Capture and Storage, Environ. Sci. Technol., 2011, 45, 8597–8604.
For this assignment, we took outline of the article [1]. Rather than copying
the article we add some more details and figure to it and verbalize it in our
own words.

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