extensive list of references is provided.

solutions are often employed to separate carbon dioxide from various gas matrices.

Fuel Processing Technology 86 (2005) 14231434

www.elsevier.com/locate/fuprocT Corresponding author. Tel.: +1 412 386 4607; fax: +1 412 386 6004.Published by Elsevier B.V.

Keywords: Carbon dioxide; Separation; Electrochemical pumps; Membrane; Chemical looping

1. Introduction

The President recently announced the U.S. Global Climate Change Initiative (GCCI) to

reduce greenhouse gas (GHG) intensity by 18% by 2012 [1]. The GCCI proposal would

slow the growth in U.S. emissions of carbon dioxide. Alkaline sorbents and scrubbingReview of novel methods for carbon dioxide

separation from flue and fuel gases

Evan J. GraniteT, Thomas OBrien

United States Department of Energy, National Energy Technology Laboratory,

P.O. Box 10940, MS 58-106, Pittsburgh, PA 15236-0940, United States

Abstract

This paper reviews some of the more novel methods for carbon dioxide separation from flue and

fuel gas streams. These methods include electrochemical pumps, membranes, and chemical looping

approaches to CO2 separation. Electrochemical pumps discussed include carbonate and proton

conductors. Selective membranes for hydrogen separation are discussed as a method for carbon

dioxide concentration in fuel gas streams. The selective membranes include mixed ionicelectronic

(solid electrolytemetal) films as well as palladium-based materials. Chemical looping combustion is

briefly discussed. The fundamental mechanisms behind these techniques will be explained, and

recent advances in these methods are emphasized. Future research directions are suggested and an0378-3820/$ -

doi:10.1016/j.

E-mail address: evan.granite@netl.doe.gov (E.J. Granite).see front matter. Published by Elsevier B.V.

fuproc.2005.01.001

result in low currents. The current represents the flux of carbonate anions across the

E.J. Granite, T. OBrien / Fuel Processing Technology 86 (2005) 142314341424membrane. This necessitates the use of huge stacks to obtain a significant flux of carbonate

ions across the device [914]. Sulfur dioxide present in flue gas poisons the cell, resulting

in sulfate formation [914]. There are also severe problems with electrolyte segregation

and electrode degradation in the severe high temperature flue gas environment [914].

A solid electrolyte membrane for the separation of carbon dioxide from flue gas could

solve many of the problems associated with molten carbonate [1517]. Solid electrolytesCryogenic separation of carbon dioxide is also commonly used. A comprehensive review

of carbon dioxide capture methods is given by White [24]. Novel methods have the

potential to reduce the cost of carbon dioxide sequestration. These methods include

electrochemical, membrane, enzymatic, photosynthesis, catalytic routes, and chemical

looping combustion for CO2 separation or conversion [26].

2. Electrochemical pumps for separation of CO2 from flue gas

2.1. Carbonate ion pumps

The molten carbonate and aqueous alkaline fuel cells have been studied for use in

separating carbon dioxide from both air and flue gases [714]. Operation of the molten

carbonate fuel cell in a closed circuit mode (with application of an external emf) will result

in the transport of carbonate ions across the membrane [714]. The molten carbonate

electrochemical separator requires oxidizing conditions for the formation of carbonate

from carbon dioxide, and is less applicable for direct separation of CO2 from fuel gases.

This pioneering work was first performed by Winnick [79]. Winnick proposed the use of

molten carbonate fuel cell membranes for separation of carbon dioxide from air for space

flight (Sky Lab) [79]. Winnick first examined molten carbonate membranes for

separation of CO2 from power plant flue gas [79]. More recently the Osaka Research

Institute in Japan, British Petroleum in the UK, and Ansaldo Fuel Cells in Italy have also

experimented with molten carbonate electrochemical systems for CO2 capture from flue

gas [1014].

There are several advantages of molten carbonate electrochemical cells for CO2separation. A large knowledge base exists for the use and application of molten carbonate

from its use in fuel cells. Molten carbonate is nearly 100% selective for the transport of

carbonate anions at elevated temperatures. It exhibits high conductivity of around 1 S/cm

at 600 8C, or equivalently a diffusivity of 105 cm2/s for the carbonate anion [79]. Theparasitic power requirements for the separation of carbon dioxide from power plant flue

gases are low and estimated to be less than 5% [9]. The cost for CO2 capture from flue gas

was estimated at $20/ton in 1990 by Winnick [9].

Unfortunately there are major disadvantages in the application of molten carbonate

electrochemical cells for separation of carbon dioxide from power plant flue gas. Molten

carbonate is a corrosive paste. The high temperatures, along with the extremely corrosive

nature of the molten carbonate, make fabrication and handling extremely difficult. The

small applied voltages allowable in order to avoid decomposition of the molten carbonateare solid ionic conductors, and are used in sensors, catalyst studies, and fuel cells [18].

readily available hW alumina, nafion, and barium cerium oxide solid electrolytes [1823].

E.J. Granite, T. OBrien / Fuel Processing Technology 86 (2005) 14231434 1425An analysis of the electrochemical hydrogen pump, similar to the analysis of the carbon

dioxide pump above, follows.

The application of an external EMF (voltage) to a proton-conducting solid electrolyte

through a closed circuit will cause hydrogen to be selectively transported (pumped) to/or

from a metal electrode. The rate of hydrogen transport is given by the current (flux of

protons) across the membrane as:

flux of H2 I=2F 1where F is Faradays constant, 96,487 C/mol.

There are several factors which limit the transport of hydrogen (current) across the

membrane, and these are the conductivity of the electrolyte, thermodynamic stability of the

electrolyte, and the electrode kinetics.

For an ohmic membrane (V=IR), current is inversely proportional to the resistance orMillions are used in automobiles as the oxide airfuel ratio sensors. Most solid electrolytes

are ceramics. Solid electrolytes often operate at lower temperatures than molten carbonate

cells. A solid electrolyte would be easier to handle, have far reduced corrosion problems,

and would possess a longer operating life than its molten carbonate counterpart. The

development of a highly conductive carbonate ion solid electrolyte is an area of active

research [1517]. The possibility of using alkali carbonate or alkaline earth carbonate solid

electrolytes for the separation of carbon dioxide from flue gas is discussed by Granite [15

17]. Granite and Pennline propose doping alkaline earth carbonates in order to increase the

ionic conductivity by orders of magnitude [1517].

There are many solids that are excellent conductors of oxygen anions or protons. The

carbonate anion is significantly larger than the oxygen anion or proton, suggesting that it

will be difficult to obtain mobile carbonate anions within a solid electrolyte matrix.

3. Electrochemical pumps for concentrating CO2 from fuel gas

3.1. Proton pumps

Hydrogen gas can be produced by IGCC plants, steam gasification reactions, the

pyrolysis of coal, the electrolysis of water, and biochemically. Each of these processes

generates hydrogen with associated impurities, such as carbon monoxide, carbon dioxide,

methane, nitrogen, water, and oxygen. Oxygen must be removed from hydrogen streams in

order to avoid explosion hazards.

Previous hydrogen purification processes involved the step-wise removal of the

impurities through absorption processes such as MEA (ethanolamine) washing to remove

the carbon dioxide, TEG (glycol) dehydration of the gas, as well as the catalytic (Pt)

removal of trace oxygen. A far simpler and potentially more cost-effective means of

hydrogen separation and carbon dioxide concentration can be accomplished in one step by

an electrochemical pump [1823]. These electrochemical pumps are efficient, silent, and

non-polluting. Electrochemical pumps for hydrogen separation can be based upon theproportional to the conductivity. The doped barium cerium oxide, BaCe0.9Y0.1O2.95, has a

E.J. Granite, T. OBrien / Fuel Processing Technology 86 (2005) 142314341426conductivity of 101 V1 cm1 at 1000 8C [1921]. The conductivity of hW alumina is106 V1 cm1 at 100 8C [18,22,23]. Nafion (polymer electrolyte) is typically employedat temperatures of around 100 8C. The higher conductivity of the doped barium ceriumoxide allows a higher flux of protons to be pumped to a metal electrode.

The maximum value for the applied external voltage is determined by the

decomposition potential of the solid electrolyte. This is found from the thermodynamic

stability, the free energy of formation, DGf, of the material, namely:

DGf nFE 2where E is the maximum applied voltage.

More stable electrolytes can be subjected to higher applied voltages. Typically a 12 V

potential can be applied to a ceramic membrane. Therefore, the rate of transport of protons

across the membrane is limited by both the conductivity (resistance) and stability of the

electrolyte.

Electrode kinetics also plays a key role in the rate of separation of hydrogen. Consider

the example of a closed hW alumina electrochemical cell having palladium electrodes:Hydrogen must be first adsorbed from the impure gas stream on to the palladium

electrode. Hydrogen then dissociates to hydrogen atoms. Charge transfer occurs, with

protons forming. The protons migrate across the electrolyte under the influence of the

externally applied field (voltage). Finally, the protons reverse the earlier steps, and form

hydrogen gas at the opposite palladium electrode. At the interface between the gas, metal

electrode, and solid electrolyte (the three-phase boundary), the overall reaction can be

written as:

H2 gas 2H 2e 3Therefore an electrode must efficiently adsorb and dissociate hydrogen so as to not

limit the rate of hydrogen transport across the electrolyte. The electrode must be porous to

allow gas to diffuse in and out, yet continuous, to avoid short circuits. Electrodes can be

formed by thermal decomposition of organometallic precursor compounds on the

electrolyte surface or with metal pastes.

The overall resistance of the pump is the sum of the resistances due to the electrode,

electrolyte, and the electrode and electrolyte contact. Overall resistance can be minimized

by: (1) use of a thin electrolyte and electrodes, (2) having intimate contact between

electrode and electrolyte, and (3) selecting an electrolyte with high conductivity.

The power consumed by the membrane separator is given by:

Power VI I2R 4For the lab-scale separator, with an applied voltage of around one volt, and a resulting

current in the milliamp range, the power requirements will be on the order of milliwatts.

Granite and Jorne applied external voltages to hW alumina and barium cerium oxidemembranes to obtain the selective transport of deuterium from D2N2 mixtures [18,22,23].

Balachandran recently applied an external voltage to an yttria-doped barium cerium oxide

membrane to obtain the selective transport of protons [1921]. The mechanism of protonconduction by SrCe0.95Y0.05O3d is discussed by Balachandran [21].

E.J. Granite, T. OBrien / Fuel Processing Technology 86 (2005) 14231434 14274. Membranes for separation of CO2

The development of a membrane separator for the selective removal of CO2 in the

presence of CO, H2, H2O, and H2S (fuel gas) or N2, O2, H2O, SO2, NO, and HCl (flue gas)

would be of tremendous economic value. A membrane separation process requires far less

expenditure for maintenance and energy than does a comparable absorption (alkanolamine

or alkaline salt solution) process for the purification of concentrated CO2 streams [24]. A

membrane material which either allows the selective transport (diffusion) or selective

exclusion of CO2 is desired.

Membranes can separate carbon dioxide from a gas stream by size exclusion or by

chemical affinity [2527]. Chemical affinity membranes are often impregnated with a

scrubbing solution or chemical functional group (e.g., amine) selective for carbon dioxide,

and will not be discussed here. The scrubbing solutions, as well as functional groups on

the surface of sorbents, are discussed in the critical review papers [24].

A large body of research has been conducted on the properties of carbon dioxide

selective membranes based upon inorganic materials such as zeolites, alumina, carbon, and

silica. Some recent work on these membranes has been presented at an NETL-sponsored

conference. Noble and Falconer examined CO2 separations from simple two component

mixtures using zeolite membranes [28]. Shih suggests that the gas permeabilities of zeolites

are limited by the tortuous pore structures, and the selectivity between carbon dioxide and

nitrogen is low because of the similar molecular diameters [29]. Andrews proposed the

separation of carbon dioxide from flue gases by carbon-multiwall nanotube membranes

[30]. His experimental work used a packed bed of carbon nanotube sorbent and CO2N2mixtures. The carbon selectively adsorbed carbon dioxide from the simple two component

mixture at temperatures ranging from 30 8C to 150 8C. This result prompted the author tofurther examine the potential of carbon nanotube materials for membrane separation of

carbon dioxide [30]. Inorganic membranes have been reviewed by Shekhawat et al. [27].

Substantial advances in the selectivity, permeability, and chemical stability of inorganic

membranes are needed for successful application to flue or fuel gas mixtures.

Polymer membranes have been successfully applied for the separation of carbon dioxide

from natural gas streams [3142]. The harsh chemically reactive and high temperature

matrix that is flue and fuel gas makes the use of polymer membranes unlikely for these

applications. Hendriks recently examined the use of polymer membranes for the separation

of carbon dioxide from flue gas [43]. He treated flue gas as a two component mixture (CO2

N2). This simplification essentially divides flue gas into two groups consisting of fast

permeating and slow permeating compounds. Hendriks concluded that a polymer membrane

would require a CO2N2 selectivity greater than 200 and a permeability of more than

5109 m3/m2 Pa s [43]. The available polymer membranes are not economicallycompetitive with other separation methods for flue gas. A review on polymer membranes for

carbon dioxide separation was recently written by Shekhawat et al. [27].

4.1. Palladium-based membranes for concentrating CO2 from fuel gas

Palladium membranes have been extensively studied and used for the separation ofhydrogen from various gas streams [44,45]. The metal can absorb enormous quantities of

E.J. Granite, T. OBrien / Fuel Processing Technology 86 (2005) 142314341428hydrogen; at 1800 8F it absorbs enough to correspond to the formula PdH0.6 [44]. Anexcellent reference on the properties of palladium relevant in the separation of hydrogen is

provided by Buxbaum and Kinney [45]. Palladium is readily fabricated into a variety of

shapes and sizes, and has excellent chemical resistance to carbon monoxide, steam, and

hydrocarbons [45]. Palladium membranes have been studied for potential application for

fuel gas mixtures for hydrogen separation, leaving a gas residue enriched in carbon

dioxide. Palladium is an active oxidation catalyst and therefore not an appropriate

membrane for hydrogen separation from streams that contain oxygen.

Palladium is not inert; it chemisorbs and reacts with sulfur and chlorine species present

in fuel gas. There are a rich variety of compounds which can form between palladium and

chlorine. Hydrogen embrittlement can occur in palladium. Undesirable phase changes also

occur in palladium at elevated temperatures. Therefore alternative metal membranes are

necessary for hydrogen separation from fuel gas mixtures.

Govind prepared composite palladium membranes for selective hydrogen separation at

high temperature [46]. He formed a thin palladium film on a silver substrate, and found the

resulting membrane had promising mechanical strength and selectivity for hydrogen from

argon streams at 370 8C to 407 8C.Buxbaum proposed the use of tubular membranes of palladium-coated tantalum and

niobium for the separation of hydrogen from hot gas streams [45]. He demonstrated

continuous separation of hydrogen from argon at 420 8C over a 31-day period using thetubular membranes. Buxbaum suggests that the economics of hydrogen separation using

the palladium-coated refractory metals are promising [45].

Morreale obtained an Arrhenius type expression for the hydrogen permeability of

palladium and examined the mechanism of hydrogen transport within palladium at

elevated temperatures and pressures [47]. Ciocco examined the permeability of hydrogen

through thin-film palladium membranes [48]. Palladium films, palladium coated tantalum,

and palladium coated stainless steel membranes were examined for their hydrogen

transport characteristics [48]. Thin films of palladium were utilized in order to minimize

the material cost of the precious metal. The tantalum and stainless steel substrates were

employed in an attempt to improve the material properties of the membrane.

Damle studied the use of thin palladiumsilver alloy films deposited on a ceramic

substrate for hydrogen separation from carbon dioxide [49]. Alloying palladium with

silver increases the permeability for hydrogen, and reduces hydrogen embrittlement.

Commercially available alumina ceramics were used as the substrate.

Way and McCormick recently examined the use of palladiumcopper alloy composite

membranes for high temperature hydrogen separation from fuel gas streams [50]. The

palladiumcopper alloy was used in order to improve resistance to poisoning by hydrogen

sulfide, enhance the hydrogen permeability, and reduce hydrogen embrittlement. Further

improvements in the material properties of metal membranes are needed for their use in

fuel gas mixtures.

4.2. Mixed ionicelectronic conducting membranes for concentrating CO2 from fuel gas

The use of proton-conducting membranes with application of an external emf in aclosed circuit was discussed in the section on electrochemical separations. A

hydrogen flux through the most promising membrane exhibited no reduction over a 190 h

E.J. Granite, T. OBrien / Fuel Processing Technology 86 (2005) 14231434 1429test at 900 8C in a simulated syngas consisting of 66% H2, 33% CO, and 1% CO2. Furtherresearch is needed to develop highly selective and permeable mixed ionicelectronic

membranes which are also chemically stable in hot fuel gas streams.

5. Chemical looping

Chemical looping combustion is a novel method to facilitate carbon dioxide

separation from flue gases [5476]. Ishida has championed the potential of chemical

looping for simplifying the separation of carbon dioxide from flue gases [5475]. The

oxygen for combustion of the fuel is provided by a regenerable metal oxide catalyst. The

use of lattice oxygen from a metal oxide catalyst as the oxidizing agent, in lieu of gas-

phase oxygen from air, results in a flue gas that is not diluted by nitrogen. This results in

a flue gas that is concentrated in carbon dioxide. Because air is not used in the

combustion of the fuel, NOx emissions are low. The chemical looping scheme can be

presented in the general form:

HCmetal oxideYCO2 H2O lower oxideand=or metal 5

Lower oxideand=or metal O2Ymetal oxide 6Reaction (5) is the combustion of the fuel (typically natural gas) in the absence of gas-

phase oxygen using a reactive oxide oxidation catalyst. Step (6) is the regeneration of

the catalyst for reuse in step (5). Reactions (5) and (6) take place in separate vessels.

Nickel oxide has been extensively examined as a candidate oxygen carrier oxidedisadvantage of the use of proton pumps for hydrogen separation (and carbon dioxide

concentration) is the need for an externally applied voltage. The rate of proton transport

across a hydrogen solid electrolyte ceramic membrane is determined by diffusion and

will be small in the absence of an external emf. One method proposed to avoid the need

for an externally applied voltage is through the use of mixed ionicelectronic conducting

membranes [5153].

Gade studied the use of a proprietary proton-conducting ceramic sandwiched between

outer nickel films [51]. The initial work has focused on the fabrication of the novel

membranes. This design is envisioned to be incorporated within a monolithic, multi-

membrane hydrogen separation module for Vision 21 plants.

Eltron recently employed cerates doped with transition metals to impart electronic

conductivity for hydrogen separation [52]. Roark found that a large flux of hydrogen

passed through this membrane without the application of an external emf [52]. Data is

provided for hydrogen separation from helium at temperatures of 650 8C to 950 8C by theproprietary Eltron membranes.

Rothenberger examined dense cermet membranes for hydrogen separation from

simulated syngas at temperatures of 600 8C to 900 8C [53]. Barium cerium yttrium oxideceramic was doped with proprietary metals to obtain high permeabilities for hydrogen.

Hydrogen fluxes through the membranes of varying thickness were measured. Thecatalyst for the chemical looping combustion of methane. Alumina and yttria-stabilized

NiO 400 Ni reference [111]

CaO 530 no reduction

E.J. Granite, T. OBrien / Fuel Processing Technology 86 (2005) 142314341430zirconia (YSZ) have been suggested as supports for nickel oxide catalysts to be employed

within chemical looping combustion schemes. For nickel oxide, reactions (5) and (6) can

be written as:

CH4 4NiOYCO2 2H2O 4Ni 7

4Ni 2O2Y4NiO 8Nickel oxide is readily reduced to nickel by methane at temperatures as low as 400 8C[77].

Granite conducted an X-ray diffraction study of metal oxide reduction in methane [18].

Several oxides were exposed to a methane atmosphere for 6 h. The phase changes were

determined via X-ray diffraction, and are summarized in Table 1. Nickel oxide and

lithium-doped nickel oxides were readily reduced to nickel, whereas other oxides were

more difficult to reduce. It is noted that chemical looping combustion would be conducted

at higher temperatures than those reported in Table 1, which should facilitate reduction of

MoO3 535 major MoO3 minor Mo2O5Bi2O3 515 major Bi2O3 trace Bi

Cu2O 530 major Cu2O minor Cu

YSZa 400 no reduction

Reducing atmosphere: 100% CH4 for 6 h, unless noted.a Reducing atmosphere: 2% CO in He for 21 h.Table 1

X-ray diffraction study of metal oxide reduction in methane [18]

Sample Temperature (8C) Resulting phases

20% Li-doped NiO 700 major Ni

20% Li-doped NiO 515 major Nithe oxides.

The economics of chemical looping depend upon robust and active oxide oxidation

catalysts that can be regenerated for numerous cycles. Catalyst attrition and deactivation

are obviously important issues for the technical success of the process. Catalyst

deactivation by carbon deposition has been investigated by Jin and Ishida [73].

Preliminary cost analyses of chemical looping as a strategy for carbon dioxide capture

from flue gases have been very promising [76].

6. Conclusions

The novel CO2 capture technologies have advantages and disadvantages when installed

in a power-generation system. Initial evaluations by Herzog et al. [78] indicate that certain

technologies for CO2 control from flue gas from conventional PC-fired power plants create a

substantial thermal efficiency power loss. A study by Kosugi et al. [79] investigated the

Hoffman, Rich Hargis, Dave Luebke, and Dan Fauth for helpful suggestions. This paper is

epahome/headline2_021402.htm.

E.J. Granite, T. OBrien / Fuel Processing Technology 86 (2005) 14231434 1431[2] C.M. White, B.R. Strazisar, E.J. Granite, J.S. Hoffman, H.W. Pennline, Separation and capture of CO2 from

large stationary sources and sequestration in geological formationscoalbeds and deep saline aquifers, J.

A&WMA 53 (2003) 645715.

[3] C.M. White, B.R. Strazisar, E.J. Granite, J.S. Hoffman, H.W. Pennline, Separation and capture of CO2 from

large stationary sources and sequestration in geological formationsa summary of the 2003 critical review,

EM (2003 June) 2934.

[4] J.C. Chow, J.G. Watson, A. Herzog, S.M. Benson, G.M. Hidy, W.D. Gunter, S.J. Penkala, C.M. White,

Critical review discussionseparation and capture of CO2 from large stationary sources and sequestration in

geological formations, J. A&WMA 53 (2003) 11721182.

[5] E.J. Granite, A review of novel methods for carbon dioxide separation from flue and fuel gases, 20th Int.

Pitt. Coal Conf., paper 29-1, Pittsburgh, PA, Sept., 2003.

[6] E.J. Granite, Review of novel methods for carbon dioxide separation from flue and fuel gases, 227th ACS

National Meeting, Fuel Preprint 105, Anaheim, CA, Mar., 2004.

[7] J. Winnick, H. Toghiani, P. Quattrone, Carbon dioxide concentration for manned spacecraft using a molten

carbonate electrochemical cell, AIChE J. 28 (1) (1982) 103111.

[8] J.L. Weaver, J. Winnick, The molten carbonate carbon dioxide concentrator: cathode performance at high

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[9] J. Winnick, Electrochemical membrane gas separation, Chem. Eng. Prog. (1990) 4146.

[10] K. Itou, H. Tani, Y. Ono, H. Kasai, K.-I. Ota, High efficiency CO2 separation and concentration system bydedicated to Philip Granite on the occasion of his 75th birthday.

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Review of novel methods for carbon dioxide separation from flue and fuel gasesIntroductionElectrochemical pumps for separation of CO2 from flue gasCarbonate ion pumps

Electrochemical pumps for concentrating CO2 from fuel gasProton pumps

Membranes for separation of CO2Palladium-based membranes for concentrating CO2 from fuel gasMixed ionic-electronic conducting membranes for concentrating CO2 from fuel gas

Chemical loopingConclusionsAcknowledgementsReferences