INTERNATIONAL JOURNAL OF ENERGY RESEARCH

Int. J. Energy Res. 2011; 35:741–764

Published online 13 December 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.1798

REVIEW

A review of recent developments in carbon captureutilizing oxy-fuel combustion in conventional and iontransport membrane systems

M. A. Habib1, H. M. Badr1,�,y, S. F. Ahmed1, R. Ben-Mansour1, K. Mezghani1, S. Imashuku2,G. J. la O’2, Y. Shao-Horn2, N. D. Mancini2, A. Mitsos2, P. Kirchen2 and A. F. Ghoneim2

1Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia2Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, U.S.A.

SUMMARY

Fossil fuels provide a significant fraction of the global energy resources, and this is likely to remain so for severaldecades. Carbon dioxide (CO2) emissions have been correlated with climate change, and carbon capture is essentialto enable the continuing use of fossil fuels while reducing the emissions of CO2 into the atmosphere therebymitigating global climate changes. Among the proposed methods of CO2 capture, oxyfuel combustion technologyprovides a promising option, which is applicable to power generation systems. This technology is based oncombustion with pure oxygen (O2) instead of air, resulting in flue gas that consists mainly of CO2 and water (H2O),that latter can be separated easily via condensation, while removing other contaminants leaving pure CO2 forstorage. However, fuel combustion in pure O2 results in intolerably high combustion temperatures. In order toprovide the dilution effect of the absent nitrogen (N2) and to moderate the furnace/combustor temperatures, partof the flue gas is recycled back into the combustion chamber. An efficient source of O2 is required to make oxy-combustion a competitive CO2 capture technology. Conventional O2 production utilizing the cryogenic distillationprocess is energetically expensive. Ceramic membranes made from mixed ion-electronic conducting oxides havereceived increasing attention because of their potential to mitigate the cost of O2 production, thus helping topromote these clean energy technologies. Some effort has also been expended in using these membranes to improvethe performance of the O2 separation processes by combining air separation and high-temperature oxidation into asingle chamber. This paper provides a review of the performance of combustors utilizing oxy-fuel combustionprocess, materials utilized in ion-transport membranes and the integration of such reactors in power cycles. Thereview is focused on carbon capture potential, developments of oxyfuel applications and O2 separation andcombustion in membrane reactors. The recent developments in oxyfuel power cycles are discussed focusing on themain concepts of manipulating exergy flows within each cycle and the reported thermal efficiencies. Copyright r2010 John Wiley & Sons, Ltd.

KEY WORDS

carbon capture; oxy-fuel combustion; ion transport membrane; environment; global warming

Correspondence

*H.M. Badr, Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia.yE-mail: badrhm@kfupm.edu.sa

Received 3 May 2010; Revised 29 September 2010; Accepted 30 September 2010

1. INTRODUCTION

Greenhouse gas emissions and carbon dioxide (CO2) in

particular have become an increasing concern in the

power generation industry. Energy production from

fossil fuel combustion results in the emission of

greenhouse gases, the dominant contributor being

CO2. Public awareness and the potential threat of

climate change have led to proposing policies for

reducing greenhouse gas emissions, with the regula-

tions partially driven by (international) initiatives

such as the Kyoto protocol and more recently the

Copenhagen summit [1]. Greenhouse gas emissions

from energy production can be reduced by the use

of alternative energy sources such as nuclear power

and renewable energy as well as through efficiency

Copyright r 2010 John Wiley & Sons, Ltd.

improvements. Until alternative energy sources can

reliably and cost effectively produce significant

amounts of energy, immediate energy demands are

likely to continue to be met by combustion of

conventional fossil fuels.

To reduce greenhouse gas emissions from fossil fuel-

fired power generation, carbon capture technologies

are being investigated. CO2 capture technologies that

are being developed for combustion and gasification

include, among others, oxyfuel technology. The de-

velopment of CO2 capture technologies for hydro-

carbon-fired power plants can be divided into three

broad categories, see Figure 1 which also shows the

components of this review article. The first is separa-

tion of CO2 from waste gas, that is, post-combustion

decarbonization. The second is combustion in oxygen

(O2) instead of air, oxyfuel combustion, and the third is

production of a carbon-free fuel as proposed in pre-

combustion decarbonization. Each of these approa-

ches has some advantages and disadvantages, while all

technologies decrease the power generation efficiency

and increase the production cost. Post-combustion

decarbonization separates CO2 from the flue gases and

thus requires minimal modifications to the power cycle,

but large gas quantities must be treated because CO2 is

diluted by the nitrogen that entered with the combus-

tion air. The O2 for oxyfuel combustion is typically

produced by a conventional (cryogenic) air separation

unit. Fuel combustion in pure O2 results in very high

combustion temperatures. In order to reduce these

temperatures, part of the flue gas is recycled back to

the combustion chamber. Oxyfuel cycles are typically

based on near-stoichiometric combustion, where the

fuel is burned in O2 produced in an air separation unit

diluted by recycled flue gas. Thus, combustion takes

place in the absence of nitrogen. Oxyfuel combustion

produces only CO2 and H2O. The CO2 separation is

accomplished by condensing water (H2O) from the flue

gas, providing a byproduct of high-purity H2O, while

other impurities are removed from the CO2 stream

before compression and storage. However, a relatively

large amount of energy is needed for the O2 produc-

tion. Based on a quantitative comparison of the three

carbon capture strategies, those based on oxyfuel

combustion generally have higher thermodynamic ef-

ficiencies than those based pre- or post-combustion

decarbonization [2].

Several technologies have been developed for O2

production. Currently, the most common is the cryo-

genic technique that is expensive and energy intensive.

An alternative method is to separate O2 using mem-

brane technology either at ambient or at high tem-

peratures. For ambient-temperature O2 separation,

polymer membranes have been developed, but their

stability and production rate are very limited. For

high-temperature applications, ceramic membranes are

generally considered and used because of their high

selectivity and thermochemical stability under harsh

conditions. The high-temperature O2 separation, using

ceramic membranes, has received increasing interest

because of the possibility to reduce the O2 production

cost and energy penalty. These membranes can be

integrated into the oxyfuel reactor where simulta-

neously O2 will be separated from air at one side of the

membrane and react with fuel at the opposite side. The

separation of O2 molecules from air at high tempera-

tures is based on a dense ceramic layer where each O2

molecule, in contact with the ceramic surface, is

decomposed into two ions that are integrated in the

ceramic structure. The formation and stability of these

ions require pair of electrons, supplied by the ceramic

surface. These O2 ions are driven from the higher-

O2-pressure (air side), to the lower-O2-pressure (fuel

side) of the membrane whereas electrons are driven

in the opposite direction. The transport of these ions

in the dense membrane is accomplished by vacancy

diffusion. These ceramic membranes are known as

ion-transport membranes (ITM) or sometimes as

mixed-conducting membranes (MCMs) because of

their involvement of ions and electrons in the electric

conductivity. At the fuel side of the dense layer of the

ITM, each pair of O2 ions is joined together to form O2

molecules leaving extra electrons, which will travel

toward the air side of the ITM. The selectivity of an

ITM is generally optimized for a target separation by

choosing a certain microstructure and chemical com-

position suitable for the operating conditions. Among

the MCMs, perovskite ITMs exhibit relatively high O2

permeation fluxes [3]. These perovskite membranes

can selectively separate O2 from the air at elevated

temperatures in the range of 700–9001C. In addition,

these ITMs can be fabricated in tubular or planar

configurations, which enable compact and efficient gas

separator equipment designs. These ITMs are pro-

mising for use in many industrial processes that requireFigure 1. Elements of the present review.

A review of recent developments in carbon captureM. A. Habib et al.

Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

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a continuous supply of pure O2. A limiting factor for

the implementation of ITMs for power plant applica-

tions is the O2 flux through the membrane, which must

be increased considerably. One very promising means

of doing this is by coupling the O2 separation and fuel

conversion in an ITM reactor.

To illustrate the suitability of the oxy-fuel combus-

tion process for clean power generation, this review

will present an overview and comparison of potential

CCS power cycles, followed by a review of works

considering the oxy-fuel combustion process itself,

with a particular regard for those using ITMs for O2

separation. Because of new challenges presented by

oxy-fuel combustion, as opposed to air-fuel combus-

tion, works pertaining to the analysis of oxy-fuel

combustion in real systems are also discussed. Finally,

a detailed discussion focusing on the use ITMs for O2

separation will be presented, including the current state

of the art for the material selection, as well as mem-

brane design and characterization. This paper is in-

tended to provide a review of previous work done on

the performance of combustors utilizing oxyfuel with

special emphasis on potential for carbon capture and

combustion in membrane reactors. It also includes a

review of key oxyfuel power cycles operating on

natural gas together with a discussion of the thermo-

dynamic implications of the overall design strategies

implemented in each.

2. CO2 CAPTURE

Numerous different power plant structures are avail-

able for CO2 capture from power plants, with the

dominant approaches being those based on decarbo-

nization of the fuel, post-combustion carbon removal,

and oxy-fuel combustion. An overview of the state of

the art for CO2 capture is presented below with

consideration of both coal and natural gas fuels.

Examples of relevant quantitative analyses of Inte-

grated Gasification Combined Cycles (IGCC), and

Chemical Looping Combustion (CLC) are also briefly

presented as these have a significant presence in the

literature. A particular focus will be placed on oxy-

combustion in a subsequent section.

2.1. Previous reviews on carbon capture

Many research investigations have been performed in

the area of CO2 capture [4,5]. A review of the

technologies used and proposed during the last two

decades for coal-based power generation closest to

commercial application and involving carbon capture

was presented by Wall [6]. Most carbon capture and

storage (CCS) technologies are based on adaptations

of conventional combustion systems, with one or more

additional unit operations for O2 supply or CO2

capture by absorbents, and CO2 compression and

storage. The research and development challenges were

identified to be those of design, optimization and

operational aspects of new combustion and gasifica-

tion plants, controlling the gas quality required by

CCS units. Our review indicates the need for funda-

mental research that includes fuel reactions at pres-

sures higher than that used in IGCC, and in O2/CO2

atmospheres, as few studies have been carried out in

these areas.

Currently, technologies proposed have significantly

lower power generation efficiencies due to the added

processes required for carbon capture. Therefore, the

challenges involved with the design and operation of,

e.g. IGCC, and CCS should be considered. Wall [6]

identified the need for new designs of combustion

equipment for the next generation of technologies and

concluded that there are significant challenges involved

with the design and operation of these integrated sys-

tems. The impact of capture of CO2 from fossil fuel

power plants on the emissions of nitrogen oxides and

sulfur oxides has been studied by Tzimas et al. [7].

They estimated the emissions from natural gas com-

bined cycle (CC) and pulverized coal plants, equipped

with post-combustion carbon capture technology, and

compared them with the emissions of similar plants

without CO2 capture. Moreover, the reduced efficiency

of power plants with CCS is likely to result in an

increase in Nox emissions from the power generation

sector [7].

Armor [8] discussed the global problem of increasing

emission of CO2 as a result of the expanding use of

hydrocarbons as a source of energy. Based on the

available figures, hydrocarbons will provide much of

the fuel needs for the next 20 years and will meet more

than 60% of the world’s energy demand. A further

increase in atmospheric CO2 levels will result from the

growth of both power generation and the size of the

transportation fleet since renewable sources cannot

cover the entire energy demand. Various means for

reduced CO2 emissions by improving process efficien-

cies have been discussed. These include lowering

operating temperatures and pressures, reducing

energy-intensive separation steps, reducing the number

of unit operations, and improving chemical selectivities

for separation [8]. Accordingly, the CO2 capture and

storage technologies require further developments.

Catalysis is expected to play an important role in

developing such technologies, e.g. due to the low

temperature limitation of ceramic membranes. Armor

also emphasized that the problem of excessive CO2

emissions will not be solved with one approach but

rather that multiple solutions should be developed.

Moreover, the solution suitable for a particular geo-

graphical region will be influenced by the available

resources.

Different CO2 capture techniques have been studied

and evaluated recently. Bolland and Undrum [9]

evaluated three concepts for capturing CO2 from

A review of recent developments in carbon capture M. A. Habib et al.

743Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

natural gas-fired combined gas/steam turbine power

plants, namely (1) post-combustion separation of CO2

from exhaust gas of a standard gas turbine power plant

using chemical absorption; (2) gas turbine CC using a

semi-closed gas turbine using O2 as an oxidizing agent,

resulting in CO2 and H2O vapor as combustion pro-

ducts (the gas turbine working fluid is mainly CO2);

and (3) decarbonization, which consists of a reforming

reactor for air-blown catalytic partial oxidation of

natural gas, a H2O-gas-shift reaction and a high-

pressure CO2 capture process. In the last concept, the

hydrogen-rich reformed fuel gas was combusted in a

gas turbine CC, which was integrated with the de-

carbonization process. A novel approach that takes

into consideration CO2 capture and compression was

used to evaluate the fuel-to-electricity conversion effi-

ciency for each of the three concepts. In comparison

with the conventional combined gas/steam turbine

power plant (giving 58% total fuel-to-electricity effi-

ciency without CO2 capture), Bolland and Undrum

estimated efficiencies of (1) 49.6; (2) 47.2; and (3)

45.3%, respectively. They indicated that the plant

efficiency may not be the major decisive factor due to

the very high investment costs, predicted to be more

than double that of a standard CC power plant

without CCS. Moreover, they noted that increased

operating costs are expected as well as a decreased

availability.

A simplified model of a power plant with two

methodologies of CO2 capture has been developed [10]

with the aim of defining second law efficiency targets

for CO2 separation technologies. Their study has

demonstrated the effect of CO2 separation and se-

questration on a hydrocarbon-based power plant using

the two most prevalent separation techniques, oxyfuel

combustion and post-combustion separation. A com-

parison between these two separation techniques was

carried out based on an exergy analysis, performed

using black-box models of the various components.

They analyzed multiple scenarios to determine the

impact of plant configuration and separation unit

efficiency on the overall plant performance. Based on

their results, they concluded that the approach can be

used to determine the most efficient investments for

separation technology. In another study, the exergic

performance of a high-temperature solid oxide fuel cell

(SOFC) combined with a conventional recuperative

gas turbine plant has been investigated [11]. Individual

component models (for the SOFC CC and a down-

stream combustor) were developed. In addition, an

actual system has been assessed and the results were

compared with previously published data. They indi-

cated that increasing the turbine inlet temperature

results in decreasing the exergy and thermal efficiencies

of the cycle, whereas it improves the total specific

power output. Furthermore, an increase in either

turbine inlet temperature or compression ratio leads

to a higher rate of exergy destruction of the plant.

A comparison between the proposed combined gas

turbine and fuel cell plant and a conventional gas tur-

bine cycle, based on identical operating conditions, was

also conducted. It was found that the proposed cycle had

superior performance, in terms of thermal and exergy

efficiencies, over a conventional gas turbine cycle [11].

2.2. System level analysis of CO2 capturecycles

Various means of CO2 capture from IGCC systems

have been developed to reduce costs and energy

penalties. These include membrane reactors and gas

separation processes. Different models of IGCC pro-

cesses were presented [12]. They first considered an

IGCC process without CO2 capture and then presented

different types of gasifiers and performance data of the

existing power stations. Second, the concept of an

IGCC process with an integrated O2 transport mem-

brane reactor and CO2 capture was proposed. Process

simulations of the stand-alone O2 transport membrane

reactor and the same reactor as part of the IGCC

process were carried out. The results showed that the

operating conditions of the membrane reactor affect the

overall performance of the IGCC process tremendously.

A change in pressure from 2 to 15bar on the permeate

side of the O2 transport membrane was found to

increase the efficiency by 5%. The concept of the

integrated gasification zero emission plant was pre-

sented [13]. The objective of the study was to use O2

produced in a MCM reactor in an integrated gasifica-

tion zero emission plant. The core of the membrane

reactor was a ceramic membrane, which separates O2

from air exiting the gas turbine compressor. The reactor

was operated at temperatures around 9001C and is

driven by a difference in O2 partial pressure. Integration

of membranes into an IGCC process was investigated

[14], where computational simulations of an O2 trans-

port membrane were carried out. The operating

conditions of the membrane were varied and their

impact on the membrane performance with respect to

O2 permeation flow as well as implications regarding the

steam power cycle were discussed. In this process,

indirect combustion takes place with no mixing of

fuel and air so that CO2 can be easily obtained for

sequestration. Two separate reactors were used; one for

air and the other for fuel. O2 transfer between the

reactors occurs by a metal oxide. A preliminary reactor

design was carried out in order to demonstrate the

feasibility of the process. It was found that the

temperature in the reactors can be kept below 9001C.

Besides CO2 capture, the proposed process was found to

have the potential to achieve higher selectivity toward

H2 than conventional steam reforming plants because of

the low temperature in the reactors and the high heat-

transfer coefficient on the outside of the reformer tubes.

A pre-combustion natural gas-fired power plant

concept with an O2 transfer membrane reactor and a

A review of recent developments in carbon captureM. A. Habib et al.

Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

744

H2-fired CC was developed [15]. The ITM reactor

consisted of three channels: one steam reformer chan-

nel that produces synthesis gas, one combustion

channel that supplies heat to the steam reforming

channel, and an air channel with the ITM that feeds

the combustion channel with pure O2 through the

ITM. A pressure swing adsorption separated a shifted

synthesis gas into a hydrogen fuel stream and a purge

gas, which was fed to the combustion channel in the

ITM reactor. The hydrogen fuel was diluted with

nitrogen in a catalytic pre-burner by reacting partially

with the O2 lean air from the air channel. After the pre-

burner, the hydrogen was fed to a H2-fired CC. It was

indicated that an efficiency of 50% can be achieved by

increasing the pressure swing adsorption purge gas

pressure and implementing an even higher degree of

heat integration.

Sander et al. [16] presented modeling of an ITM

together with the integration of the membrane model

into power cycle models. Two different power pro-

duction processes were discussed in this work, where

an ITM reactor was integrated. First, an IGCC pro-

cess, where CO2 can be separated from a syngas before

combustion, was proposed. Hard coal was gasified in

the presence of O2 and steam. After gas treatment, CO2

capture was allowed to take place before the synthesis

gas is supplied to the burner. Besides the IGCC pro-

cess, a lignite-fired oxyfuel boiler cycle was proposed

with an integrated ITM reactor where CO2 is captured

by cooling the cleaned exhaust gases. A part of the

CO2 rich stream was re-circulated to the boiler to

control temperature; the excess CO2 was compressed

for sequestration. The ITM model considered heat and

mass transfer through the membrane for different op-

erating pressures, temperatures, pressure differences,

and partial pressure differences. The membrane was

modeled by using the finite difference method. The

performance of the membrane and its impact on dif-

ferent power cycles were presented and various con-

figurations of the membrane were analyzed.

2.3. Chemical looping technology

A potentially attractive alternative to ITM-based

systems for carbon capture is chemical looping

technology, in which a metal oxide is used to transfer

O2 extracted from air to the fuel, as shown in Figure 2.

In chemical-looping combustion (CLC), direct contact

between air and fuel is avoided and the power loss

inherent in CO2 separation is reduced [17,18]. The

advantages of using CLC include the high CO2 capture

efficiency and the absence of NOx formation. The

authors indicated that some companies such as

ALSTOM and CES are currently building power

plants using CLC technology. In a study by Naqvi

and Bolland [19], an application for utilizing the CLC

technology in natural gas-fired CCs for power genera-

tion with CO2 capture is presented. The CC consisted

of a single CLC-reactor system that includes an air

turbine, a CO2-turbine together with a steam cycle that

was designated as the base–case cycle (introduced by

Naqvi et al. [20]). They found that the base–case cycle

could achieve a net plant efficiency of about 52% at an

oxidation temperature of 12001C. In order to achieve a

reasonable efficiency at lower oxidation temperatures,

they proposed to introduce reheating into the air

turbine by employing multi CLC-reactors. The results

indicated that the single reheat CLC-CC could achieve

a net plant efficiency of above 51% at oxidation

temperature of 10001C and above 53% at the

oxidation temperature of 12001C including CO2

compression. They concluded that a marginal effi-

ciency improvement could be attained by utilizing the

double re-heat cycle. The CLC-cycles were also

compared with a conventional CC with and without

post-combustion capture in amine solution. It was

concluded that all CLC-cycles show higher net plant

efficiencies with close to 100% CO2 capture as

compared to a conventional CC with post-combustion

capture.

3. OXYFUEL POWER CYCLES

Oxyfuel combustion of fossil fuels for CCS has been

developed with a multitude of potential power cycle

concepts, each having distinct operating conditions,

components and thermodynamic design philosophy.

The method used to obtain the required pure O2 varies

depending on the cycle as some concepts use a more

Figure 2. Schematic of CLC using a metal that is oxidized to uptake O2 and reduced under hydrocarbon atmospheric conditions for

combustion with the fuel (http://www.wku.edu/ICSET/chemloop.htm).

A review of recent developments in carbon capture M. A. Habib et al.

745Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

conventional cryogenic air separation unit; whereas

others use new separation technology such as the ITM

reactor that combines separation and combustion.

Each of the concepts has unique ways of manipulating

the exergy flows within the system under a variety of

operating conditions resulting in a wide spectrum of

reported thermal efficiencies.

Kvamsdal et al. [2] compared various natural gas-

fired power cycle concepts including six oxyfuel cycles

(oxyfuel CC, H2O cycle, Graz cycle, advanced zero-

emission power plant, SOFC/gas turbine and CLC) in

addition to one post-combustion (amine absorption)

and two pre-combustion cycles (auto-thermal reform-

ing and hydrogen membrane reactor). They concluded

that the oxyfuel with cryogenic O2 production shows

the lowest efficiency, while the SOFC/gas turbine

SOFC/GT has the highest efficiency, followed by the

membrane-based cycles giving the second best, al-

though the technology availability of the SOFC/GT

will be considered in the future.

In the following, a review of key oxyfuel power cy-

cles operating primarily on natural gas is presented and

the thermodynamic implications of the overall design

strategies implemented in each are discussed.

3.1. Advanced zero-emission power plant

The Advanced Zero-emissions Power plant (AZEP),

Figure 3, is an oxyfuel cycle using methane or natural

gas that essentially replaces the traditional combustor

with an ITM reactor, resulting in a much smaller

penalty for the separation processes of O2 from air, or

CO2 from exhaust compared with conventional carbon

capture technologies [2]. Specifically, this concept

reports a thermal efficiency loss of approximately 5%

compared to a 500MW natural gas CC, whereas

conventional post combustion capture processes ex-

hibit a penalty of roughly 9% [21]. Thermal efficiencies

reported for the AZEP cycle range from 50 [2], to

52.5% [21] for simple, non-optimized flowsheets.

However, it should be noted that the AZEP turbine

inlet temperature for the efficiencies cited above is

12001C, 2001C above the reported limit for the ITM

reactor required for membrane stability [22,23]. The

AZEP concept consists of a Brayton cycle combined

with a bottoming steam cycle in addition to either an

unconventional CO2, steam-like turbine, or a heat

recovery steam generator that utilizes the availability

of the combustion products exiting the ITM reactor

[2]. In this cycle, Figure 3, pressurized air and natural

gas enter the ITM reactor separately in addition to a

portion of the exhaust gases from the turbine that is

recycled to moderate reactor temperature [21]. The O2

is separated through the membrane and the fuel is

oxidized, producing a CO2-rich exhaust which is then

routed to either a heat recovery steam generator, or to

a specially designed steam-like turbine that can operate

with large amounts of CO2 [2]. Additionally, since the

heat transfer between the combustion side stream and

the O2-depleted air inside the ITM reactor is likely to

be high, an additional gas turbine is used to develop

work from the air residual stream. The H2O is then

condensed out and the remaining CO2 is processed for

sequestration [24].

The AZEP cycle offers many advantages and di-

rectly implements the ITM technology for air separa-

tion and oxidation of the fuel. One of the strongest

features of this cycle is the opportunity to use the

conventional power plant equipment for the turbines

and compressors, avoiding additional costs already

incurred by using the ITM reactor. Specifically, com-

pared to a conventional natural gas CC with post-

combustion capture, the CO2 capture costs could be

reduced by as much as 50%, with significantly lower

investment costs [24]. Further, the cycle can be con-

structed on a modular basis due to the flexibility of the

ITM reactor shape and size, and thus can be scaled

according to the desired power rating of the plant.

A drawback of this cycle is that it is limited by the

outlet temperature of the ITM reactor itself, which is

significantly lower than the maximal inlet temperature

of state-of-the-art gas turbines [25], and thus the

maximum attainable thermal efficiency of the cycle is

limited due to the second law of thermodynamics.

However, the actual thermal efficiency attained is not

necessarily lower than cycles that operate at maximal

turbine inlet temperatures, since the second law effi-

ciency, or simply the performance of the cycle relative

to the ideal is also significant. A proposed remedy is to

install an air-operated afterburner downstream of the

Figure 3. A simplified presentation of the AZEP concept [2].

A review of recent developments in carbon captureM. A. Habib et al.

Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

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ITM reactor, resulting in net CO2 emissions, but still

giving the plant flexibility between emissions avoidance

costs and operational costs associated with the reduc-

tion in efficiency [2]. Further, fabrication, testing and

modeling of key ITM components under practical

operating conditions has bolstered support for the

feasibility of the plant [21]. Overall, the AZEP cycle is a

promising concept that both mitigates the penalty

associated with O2 and CO2 separation, but also pro-

vides a simple and cost-effective integration of the ITM

reactor with a highly efficient traditional CC.

3.2. Graz and water cycle

The Graz cycle, Figure 4, is a unique oxyfuel concept

developed over that last decade at the Graz University

of Technology that offers a high efficiency, zero-

emission plant using natural gas, syngas, or even coal

[26]. The Graz cycle consists of a high-temperature

Brayton cycle integrated with a low-temperature

Rankine-like cycle. Various designs have been pre-

sented with power production in the range of

400–600MW, and efficiencies ranging from 54.1 [26]

to 60.1% [27]. Sanz et al. [28] presented a modified

Graz concept called the S-Graz cycle that achieved a

thermal efficiency of 57% with syngas. As shown in

Figure 4, high-pressure O2 supplied by a cryogenic air

separation unit and fuel are fed to a combustor

operating at typically 40 bar, with a turbine inlet

temperature of 14001C along with recycled CO2 and

H2O to moderate the temperature [26]. The H2O- and

CO2-rich combustion products are expanded in a high-

temperature turbine with recycled steam injection,

cooled by the bottoming Rankine cycle via regenera-

tion, and then the H2O is condensed where the CO2

separation process occurs [2]. The separated H2O is

then pumped at a high pressure, typically 200 bar,

heated by the Brayton cycle before being expanded in a

high-pressure steam turbine, and then ultimately is

recycled back into the Brayton cycle through the

combustor and high-pressure turbine [26]. A net

amount of H2O and CO2 is then removed from the

cycle for make-up steam and storage, respectively.

Variations of this concept have been presented, but the

key characteristic of this cycle is the thermodynami-

cally effective heat integration scheme between the

Brayton-like and Rankine-like cycles that is primarily

responsible for the high efficiencies.

It should be noted that there exists a related, more

basic concept called the Water Cycle that is quite

similar to the bottoming Rankine cycle of the Graz

concept [2]. The only difference is that the heat is

added directly to the Rankine-like cycle at a signi-

ficantly lower turbine inlet temperature of 9001C in-

stead of to a topping Brayton cycle ranging from 1200

to 15001C. Consequently, the thermal efficiencies

reported are relatively low, i.e. 44.6% compared to the

other oxyfuel cycles analyzed, illustrating the impor-

tance of a well-designed heat integration scheme and

heat addition at a maximal temperature. There are

many advantages of using the Graz cycle for oxyfuel

carbon capture and sequestration. Specifically, heat is

added to the cycle at an overall high average tem-

perature [26], resulting in low entropy transfer into the

cycle and consequently a higher potential for work.

Additionally, since the bottoming Rankine cycle

pumps the H2O up to high pressure in the liquid

form, the work requirements are relatively small [29].

Further, most of the components, except for the tur-

bines, are standard power plant equipment and thus

the component development can be focused on the

unconventional high-pressure turbine operating with

primarily CO2 and H2O as the working fluid [26].

Though the developers of the cycle claim to have

significant research experience pertaining to this novel

turbine [29], the costs and implementation feasibility

are still relatively unknown. Further, the Graz cycle

requires O2 production via a cryogenic air separation

unit, implying higher capital costs compared with the

AZEP [2]. Additionally, the cycle has relatively high

pressures throughout, i.e. 40 bar in the combustor and

180 bar in the Rankine cycle, which has materials and

operating implications for the plant [24]. Finally, the

turbine inlet temperature exceeds the limit of an ITM

reactor, and thus the cycle would need significant

modification to be integrated with this air separation

technology.

3.3. MATIANT and Feher supercritical CO2

cycles

The MATIANT cycle named for the two inventors of

the concept, Mathieu and Iantovski, is a novel oxyfuel

cycle comprised of a Rankine-like CO2 supercritical

cycle feeding a bottoming regenerative Brayton-like

CO2 cycle [30]. The cycle uses mainly CO2 as the

working fluid, with natural gas as the fuel, and pure O2

Figure 4. A simplified presentation of the principle scheme of

Graz Cycle power plant [2].

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DOI: 10.1002/er

is provided by a cryogenic air separation unit. CO2

sequestration is performed by injecting high pressure,

supercritical CO2 into the ground. The flow rate of the

circulating CO2 was varied to maintain a high-pressure

turbine inlet temperature of 13001C. The cycle employs

both regeneration and reheating to enhance its

efficiency, and CO2 compression is conducted by a

staged, inter-cooled compressor train that serves as

both the primary compressor of the cycle and the CO2

delivery system. Additionally, a net amount of

condensed H2O is delivered by the cycle. A sensitivity

analysis and optimization study by Mathieu indicates

that the cycle could achieve nearly 50% thermal

efficiency [31] compared with the initial value of

44.3% given in the original proposal by Mathieu and

Nihart [30].

The MATIANT cycle appears to be based on the

much older Feher supercritical CO2 cycle. In this

concept, a supercritical cycle was proposed that sought

to capture the advantages of conventional Brayton and

Rankine cycles without their associated disadvantages

[32]. Specifically, supercritical cycles offer high thermal

efficiency, a low power to volume ratio that is im-

portant for plant sizing, no blade erosion in the tur-

bines or cavitation in the pumps, and relative

insensitivity to the compression process. The pseudo-

supercritical cycle proposed by Feher shows high effi-

ciencies due to low pumping work requirements, along

with the opportunity for significant regeneration as in

the MATIANT cycle. Finally, CO2 was chosen as the

working fluid since the critical pressure is reasonably

low compared with H2O, considerable property data

exists for thermodynamic design, and CO2 is relatively

inexpensive and abundant, especially in light of carbon

capture and sequestration applications as it is a pri-

mary component of fossil fuel combustion products.

The relatively low efficiency of the MATIANT

concept compared with some of the cycles described

previously should not be taken to mean that it is ne-

cessarily inferior to them. Ultimately, the distinction of

a superior power cycle concept depends on many

considerations in addition to the thermal efficiency.

Some advantages of the MATIANT cycle are due to

the unique properties of CO2, namely the non-ideal

compressibility characteristics near the saturation line

resulting in far less work for compression compared

with that of an ideal gas [30]. Additionally, since the

working fluid is almost entirely CO2, separation and

sequestration become far easier than traditional post

combustion capture processes, and possibly relative to

alternative oxyfuel cycles as well. However, a dis-

advantage of this cycle is that it would need significant

modification to be integrated with an ITM reactor in

order to reduce the air separation thermodynamic

penalty and costs. Furthermore, the high pressures

required for the cycle, namely up to 300 bar for the

high-pressure turbine, could have severe implications

for materials and design considerations. Additionally,

components that can handle the negative properties of

supercritical fluids, namely the corrosion issues, or

simply that can handle concentrated CO2 streams in

general, would need significant research and develop-

ment before implementation in an actual power plant.

In conclusion, the possible benefits of utilizing super-

critical CO2 as the main working fluid in an oxyfuel

cycle for carbon capture and sequestration should not

be overlooked, as this concept provides unique advan-

tages that conventional working fluids in the usual

thermodynamic states simply cannot offer.

3.4. The zero-emission power plant and thezero-emissions ITM oxyfuel plant

The zero-emissions power plant (ZEPP), Figure 5, a

term coined by Yantovski et al. [33], has been

developed by different researchers working indepen-

dently. The term covers a broad range of oxyfuel

power cycles [24,26,30,33–36]. The difference between

the ZEPP and a conventional post-combustion capture

plant is that the combustion products exit the plant in

the liquid form rather than as flue gases to be scrubbed

[33]. Yantovski et al. [33] presented a history of ZEPP

plants reviewing some of the early developments of the

cycles by tracking the changes in concept design and

the resulting increases in thermal efficiency. Further,

they presented a zero-emission ITM oxyfuel plant

(ZEITMOP) with thermal efficiencies ranging from 46

to 55% and corresponding turbine inlet temperatures

between 1300 to 15001C. In this cycle, as shown in

Figure 5, the ITM is used as an air separation unit

exclusively using re-circulated CO2 as a sweep gas to

maintain a low partial pressure on the permeate side.

Air is preheated after compression using exhaust gas

regeneration to between 800 and 9001C in order to

provide sufficient O2 flux for the combustion process

downstream. H2O is condensed from the CO2-rich

exhaust gases and then removed from the cycle. The

remaining CO2 is compressed to high pressure for

sequestration, executing a quasi-CC, whereas the air

stream forms a simple Brayton cycle. Overall, this cycle

Figure 5. A simplified presentation of the ZEPP concept [35].

A review of recent developments in carbon captureM. A. Habib et al.

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748

is quite similar to the AZEP concept [2], except that it

uses the ITM to separate O2 only, rather than simul-

taneously oxidize the fuel as in a manner similar to the

cycle presented by Anantharaman et al. [36]. While

combining the fuel conversion and O2 processes in an

ITM reactor increases the O2 flux through the

membrane, the maximum membrane outlet tempera-

ture (900–10001C) is considerably lower than typical

turbine inlet temperatures (approximately 12001C),

resulting in a potential loss in cycle efficiency.

Another ZEPP cycle was proposed [35] where a

simple Rankine-like cycle is utilized with an external

cryogenic air separation unit. This relatively simple

cycle, in terms of plant components, is designed using

more accurate manufacturers’ steam turbine data,

severely limiting the operating temperatures and pres-

sures. H2O is condensed out from the system, resulting

in easy CO2 capture, as well as producing H2O.

Additionally, a detailed second law analysis was

performed to identify the effects of using a cryogenic

air separation unit and also implementing turbine

blade cooling. The thermal efficiency for a 400MWe

cycle with a high-pressure turbine inlet temperature of

8171C was 46.5%, a value on par with the MATIANT

cycle [30], despite the lower turbine inlet temperature,

and more conventional plant design [35]. An additional

advantage to this cycle is that other fuels besides

natural gas could be used such as syngas, coal, or

biomass [35]. In essence, this concept provides a sim-

ple, cost-effective cycle that could be used to demon-

strate the feasibility of oxyfuel carbon capture and

sequestration systems. Further, the irreversibility ana-

lysis indicates significant exergy destruction due to the

external cryogenic air separation unit, second only to

the combustion process itself, thus substantiating the

need to develop ITM technology.

3.5. Other novel cycles

Additional cycles have been proposed in the literature

that attempt to combine advantageous aspects of

various concepts in order to obtain higher overall

thermal efficiencies. In one such cycle proposed by

Gabbrielli and Singh [34], a bottoming Rankine cycle

is combined with a topping Brayton cycle incorporat-

ing novel aspects such as chemical recuperation and

steam compression and injection. This concept uses

natural gas as the fuel to form syngas, which is

subsequently combusted with pure O2 produced by an

external cryogenic air separation unit with recycled

steam to moderate the combustion temperature. The

cycle offers the same advantages for CO2 separation as

the previous cycles, and also produces relatively pure

H2O. Alternative flow-sheets following the same design

philosophy were analyzed by Gabbrielli and Singh [34],

and the highest thermal efficiency reported was 52.3%

with a turbine inlet temperature of 13751C. Overall, the

penalty associated with the compression of saturated

steam, as well as the exergetic losses during the syngas

production process conspire to make this cycle as

efficient as the AZEP cycle [2], despite the higher

turbine inlet temperature, and less efficient than the

optimized Graz cycle [27]. However, the chemical

recuperation and steam injection processes could prove

to be valuable design modifications for other oxyfuel

concepts.

Another novel cycle was proposed by Anantharaman

et al. [36] that utilizes the ITM reactor as an O2 se-

parator only with a separate combustor, along with

auto-thermal reforming and a sequential burner to

reach maximum turbine inlet temperatures. The design

philosophy is quite similar to the AZEP, with important

subtle differences, namely that combustion takes place

in a separate unit. In this concept, the ITM reactor is

comprised of a high and low temperature heat ex-

changer along with a separation section that transports

O2 across the membrane to a secondary sweep gas loop,

where fuel is then mixed in and combusted. The turbine

inlet temperature is assumed to be limited to 12751C,

and thus the authors proposed a sequential burner to

obtain an overall turbine inlet temperature of 14251C.

A bottoming steam cycle is used to develop additional

work from the exhaust gases, and the CO2 separation

process is conducted via H2O condensation as in the

usual oxyfuel cycle. Since the sequential air-burner

scheme proposed results in net CO2 emissions of 15%,

an auto-thermal reformer is installed as an alternative to

avoid emissions. Results of the various flow-sheets

proposed give cycles with thermal efficiencies ranging

from 49.3% to 55.1%. This demonstrates the flexibility

of the ITM reactor and shows that it can be used as a

separation unit without simultaneous combustion.

Two system configurations for oxyfuel (natural gas)

turbine systems with integrated steam reforming and

CO2 capture and separation units have been developed

[37]. The heat required for steam reforming was ob-

tained from the available turbine exhaust heat, and the

produced syngas was used as fuel with O2 as the oxi-

dizer. H2O was used as the main working fluid and the

turbine exhaust was mainly a mixture of H2O and CO2.

Combustion in this case resulted in a very high tem-

perature. A thermodynamic simulation was performed

and the efficiency was predicted to be approximately

50%, taking into account the energy needed for O2

separation. In addition, Zhang and Lior [37] predicted

a near 100% CO2 capture. Normann et al. [38] investi-

gated new possibilities and synergy effects for an

oxyfuel-fired polygeneration scheme (transportation

fuel and electricity) with carbon capture and co-firing

of biomass. A process scheme of a polygeneration

plant for sub-stoichiometric oxyfuel combustion with

co-production of electricity and dimethylether was

presented and synergy effects with the scheme were

discussed in relation to a reference process. Moreover,

a process simulation of the proposed power plant was

performed and compared to the literature references

A review of recent developments in carbon capture M. A. Habib et al.

749Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

with respect to the oxyfuel technology and the

dimethylether process. It was concluded that the pro-

posed process resulted in a more effective oxyfuel

process through a sub-stoichiometric combustion

between normal combustion and gasification. In

addition, the proposed O2 lean combustion process

constitutes an improved oxyfuel carbon capture pro-

cesses with production of dimethylether in a poly-

generation process.

In their overview of oxyfuel technologies for CO2

capture, Simmonds et al. [39] examined the potential

application of oxyfuel technologies to both heat

and power production systems. Oxyfuel combustion

resulted in a 30% cost savings, compared with post

combustion separation using amine scrubbing. They

also found that a boiler utilizing oxyfuel with OTR

would be 40% more expensive than a conventional

unit and excluded commercialization before 2010.

Using a cryogenic O2 separation, the cost of CO2

capture is expected to lie in the range of $35–40/ton

(equivalent to CO2 avoided cost in the range of

$40–45/ton). Sho-Kobayashi and Van-Hassel [40] have

shown that standalone O2 generation plant consumes

10–20% of the power plant output. Oxyfuel combus-

tion without expensive O2 separation penalty, e.g.

cryogenic, appears promising for CO2 sequestration

A comparison of a boiler using oxyfuel combustion

with one using air combustion was carried out by Shah

and Christie [41]. For the same net power output of

452MW, they found that oxyfuel combustion resulted

in a doubling in capital cost, an efficiency reduction

from 44 to 35% and a reduction of the CO2 emissions

from 8.6 to 1.1 ktons per day. They also found that

increasing the O2 purity results in an increase in both

power consumption and capital costs.

4. OXY-FUEL COMBUSTION INCOMBUSTION SYSTEMS

While the preceding discussion has demonstrated

the promise of oxy-fuel-based power cycles, there

are considerable differences between air and O2-

CO2-based combustion processes, most notably the

influence of the CO2 on the radiative heat transfer and

combustion process. Numerous experimental and

numerical works have focused on elucidating the

difference between the two combustion types, both

quantitatively and qualitatively. Presented below is a

summary of these works, first by considering the

general difference between air and oxy-fuel combus-

tion and then the more detailed numerical and

experimental analyses.

4.1. Oxyfuel combustion studies

There are many features that affect the efficiency of

oxyfuel combustion [42], including air separation unit

load, compression of CO2 and air, working fluid

temperature and turbine cooling load. Buhre et al. [40]

provided a comprehensive review of the existing

research on oxyfuel combustion technology. They

mainly investigated oxyfuel combustion of pulverized

coal with the production of relatively pure CO2 for

enhanced oil recovery. They indicated that oxyfuel

combustion research should include the development

of new plants with the advantage of smaller flue

gas cleaning equipment, as well as retrofits of the

existing plants.

The interest in oxyfuel combustion has led to many

laboratory-scale and pilot-scale studies by various

groups that covered many scientific and engineering

fundamental issues. Buhre et al. [43] identified four

issues, namely heat transfer, gaseous emissions, ash-

related issues, and ignition and flame stability of the

combustion process. They also identified the research

needed for a more fundamental understanding of the

changes from conventional air-fired combustion to

oxyfuel combustion. These include the heat transfer

performance of new and retrofitted plants and the

impact of O2 feed concentration and CO2 recycle ratio,

assessment of retrofits for electricity cost and cost of

CO2 capture, the combustion of coal in an O2/CO2

atmosphere, including ignition, burn-out, and emis-

sions. They concluded that oxyfuel combustion is a

cost-effective method of CO2 capture. More impor-

tantly, the studies indicated that oxyfuel combustion is

technically feasible with current technologies, reducing

the risks associated with the implementation of new

technologies.

There are several differences between regular

combustion (using air) and oxyfuel combustion. In

order to achieve the same mass fraction of O2 in a

O2/CO2 mixture as in atmospheric air, the O2 and CO2

volume fractions must be 30 and 70%, respectively. In

addition, the CO2 and H2O combustion products have

higher emissivity that produces more radiation if the

temperature is held constant. Also, CO2 and H2O

vapor have high heat capacities compared to nitrogen.

This increase in heat capacity increases heat transfer in

the convection sections. In a recent study, Kakaras

et al. [44] investigated the issue of the radiative heat

transfer in oxyfuel combustion for low-grade quality

fuels. The same thermodynamic H2O/steam para-

meters and similar combustion temperatures were

maintained in both cases. Owing to the absence of N2,

flue gas recirculation was required in the combustion

process to moderate the furnace temperature. In their

study of the design requirements of the heat exchange

surfaces for the oxyfuel steam boiler, Kakaras et al.

found that the dominant factors for dimensioning the

oxyfuel boiler are the higher radiative heat transfer

(due to the high concentrations of CO2 and H2O in the

flue gas) and the flue gas mass flow rate. They for-

mulated a modified design of heat exchange surfaces of

the oxyfuel boiler and compared it to a conventional

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air-fired boiler in a typical modern air-fired power

plant. For the determination of the thermodynamic

cycle characteristics, they used a thermodynamic-cycle

calculation software and found that the oxyfuel con-

cepts resulted in higher gross power output, but lower

net power output due to the power consumption of

auxiliary equipment. The total efficiency decreased

by 8.5% compared to the air-fired unit. Moreover,

Kakaras et al. concluded that the capital cost of the

oxyfuel power plant would be significantly higher than

the reference plant.

Most of the previous work was focused on coal

fuels. Andersson et al. [45] carried out a comparison of

the radiative heat transfer in oxyfuel flames to corre-

sponding conditions in air-fuel flames during com-

bustion of lignite in the Chalmers 100 kW oxyfuel test

facility. They used the same stoichiometry for all cases,

by adjusting the flue gas-recycling rate in the oxyfuel

combustion processes. They carried out measurements

of the radial profiles of gas concentration, temperature

and total radiation intensity in the furnace. In another

study, Andersson et al. [46] presented their findings on

the differences in soot-related radiation intensity be-

tween two different oxyfuel flames and an air-fired

flame. The study was conducted in a 100 kW oxyfuel

test unit firing propane while keeping an O2-to-fuel

ratio of 1.15, relative to stoichiometric. They indicated

that CO2 not only increases the gas radiation, it can

also drastically change the radiation originating from

soot during oxyfuel combustion. Yamada [47] made a

direct comparison between air and oxy-firing of coals

through pilot-scale experiments, keeping the same heat

transfer in oxyfuel and air combustion. He found that

the flame with oxyfuel combustion mode was blown

out at a lower load and resulted in reduced NOx

emissions by 60–70% as well as reduced SO2 emissions

by 30%. Finally, Yamada found that the carbon-

in-ash content is 30% lower in oxyfuel combustion

compared to air combustion.

The fundamentals of the oxyfuel combustion for gas

and coal firing have been studied experimentally and

theoretically in a Chalmers 100 kW oxyfuel test facility

(Figure 6) [48]. Results showed that the temperature of

the flame (using 27% O2 and 73% CO2 and a swirl

number of 0.79) drops drastically and leads to a

delayed burn-out compared to the normal flame with

air. However, despite the lower flame temperature, the

radiation intensity of this flame increases by approxi-

mately 20–30%. This was attributed to the increase in

gas band radiation as well as the increase in soot

volume fraction. Moreover, small differences in CO

emissions were found between air and oxy-cases and

significantly reduced SO2 and NOx. de-Jager [49] in-

vestigated combustion and noise phenomena in tur-

bulent alkane flames and found that dilution with

steam strongly diminishes the formation of both CO

and NO. It was concluded that a global reaction me-

chanism predicts the same CO2, CO and NO emissions

as the detailed reaction mechanism for various oper-

ating conditions. Another study was carried out by

Monckert et al. [50] for combustion with lignite under

both air and oxyfuel conditions including (wet) flue-gas

recirculation in a modified 500 kW-pulverized fuel

combustion facility. The facility was tested toward safe

handling and continuous operation, and optimized

toward minimum air in-leakage. The experiments were

carried out under continuous oxyfuel operation with

an O2 concentration of 27% (dry basis) and under flue

Figure 6. CHALMERS 100 kW oxy-fuel test facility [48].

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gas recirculation conditions. Moreover, switching from

single-pass combustion to recirculation mode has been

performed. The combustion behavior and ignition of

individual particles of coal (106–125mm) in 6–36% O2

in N2 and in CO2 at 1400–1800K have also been in-

vestigated [51]. It was found that for combustion in

CO2 environments, flame temperatures were reduced

significantly (from 2226 to 1783K). Both O2 and CO2

concentrations have been observed to affect single-

particle ignition, devolatilization, and char combustion

processes, with O2 resulting in much stronger effects.

Furthermore, the O2/CO2 effects on ignition and

devolatilization approximately balance each other for a

30% O2 mass fraction. It has been concluded that CO2

appeared to decrease char burning rate. Similar results

have been found in other experimental and theoretical

investigations of ignition of oxyfuel flames [52].

In oxyfuel combustion, the high concentration of O2

in the vicinity of the burner can enhance flame stabi-

lity, which is advantageous in high velocity burners.

Sarofim [53], using laboratory- and pilot-scale studies,

concluded that the near-term commercial implemen-

tation of oxy-combustion for CO2 capture is feasible.

Moreover, it has been indicated that the required

percentage of recirculated flue gases depends on the

furnace size, O2 purity, fuel type and temperature of

recycle. In another work, the thermo-acoustic in-

stabilities were studied in a CO2-diluted oxyfuel com-

bustor [54]. It was observed that additional control of

CO2 dilution/O2 enrichment allowed more possibilities

for ‘zoning’ the flame. It was also found that the in-

crease in burner stability by O2 enrichment can lead to

an increase in dynamic instability. Therefore, staging

O2 to enhance combustor performance should be per-

formed carefully. It was also concluded that knowledge

of flame properties in CO2/O2 system was necessary

to predict the flame structure. Investigation of the

feasibility of hydroxyl-fuel combustion for the

oxyfuel systems was performed by Zanganeh et al. [55].

Modeling and pilot-scale experiments, Figure 7, were

carried out in order to study the reduction in size and

capital cost of equipment of such systems. In addition,

the possibility of using H2O/steam to moderate the

flame temperature was considered.

4.2. CFD Calculations of oxyfuel combustion

CFD-based combustion models include sub-models

such as turbulence, combustion and radiation. CFD

calculations typically provide detailed results including

velocity, temperature and concentrations fields that are

not easily obtained through experimental measure-

ments. Mahesh et al. [56] studied the numerical tools

required to perform large-eddy simulation (LES) of

turbulent flows in realistic engineering configurations

with particular interest in flow inside gas-turbine

combustors. They discussed the progress made in

developing numerical algorithms and solvers for this

purpose, and validated their solver for a variety of

steady and unsteady flows. Eriksson [57] adapted air-

combustion CFD software packages for studying

oxyfuel combustion with the objective of developing

gas phase reaction schemes, measurement of soot in

oxyfuel combustion and development of soot models

and coal characterization under oxyfuel conditions. He

indicated that the development and validation of the

sub models were necessary for oxyfuel applications and

that detailed measurements in well-defined flames at

different scales are needed for the purpose of model

validation.

CFD calculations for fuel-air mixtures were con-

ducted by Mahesh et al. [56] Mohanraj et al. [58],

Habib et al. [59,60]. Mahesh et al. [56] developed a

numerical method using LES for simulating turbulent

Figure 7. Oxyfuel pilot-scale experiments performed by Zanganeh [55].

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flows in complex geometries. They investigated the

performance of Reynolds turbulence models for the

numerical simulation of non-reacting gas turbine

combustor swirl flow. Mean velocities and Reynolds

shear stresses and their distributions in the computa-

tional domain were presented. Mahesh et al. [56]

presented LES simulations of gas turbine chambers.

Mohanraj et al. [58] provided simulations of swirl

stabilized liquid fuel model gas turbine combustion

system. Mongia [61] presented progress in compre-

hensive modeling of gas turbine combustion. Compu-

tational Combustion Dynamics (CCD) codes were

developed to construct empirical/analytical design

methodology for low-emission combustion systems.

4.3. Measurements in gas turbine combustors

Many investigations have studied the combustion

process in gas turbine combustors. Yaga et al. [62]

performed a number of experiments inside a furnace of

diameter of 0.2m and a length of 0.8m. Methane was

burned at a flow rate of 0.2Nm3 h�1 with air at a rate

of 1.9Nm3h�1 at stoichiometric equivalence ratio.

They presented axial and radial mole fraction and

temperature distribution measurement, as well as

simulated results obtained using large-scale eddy

simulation. Tan et al. [63] carried out experimental

studies investigating the combustion of natural gas in

air and in mixtures of recirculated flue gas and O2. The

main objective was to enrich the flue gas with CO2 to

simplify its capture and sequestration. In this study,

gas composition, flame temperature and heat flux

profiles were measured inside a down-fired vertical

combustor fitted with a pilot-scale burner. The effect of

various parameters controlling burner operation on

the flame characteristics and NOx emission was also

investigated. The results indicated that the oxy-

combustion techniques based on O2/CO2 combustion

with recycled flue gas have excellent potential for use in

conventional boilers with simple processes of CO2

capture and the elimination of NOx emissions. Other

benefits include improved plant efficiency due to lower

gas volume and better operational flexibility. The

experimental data can be used for CFD modeling

validation.

An experimental study on O2/CO2 flame character-

istics with a focus on the radiation and the burn-out

behavior was presented by Andersson and Johnsson

[64,65]. An 80-kW test unit that facilitates O2/CO2

combustion with flue gas recycle was considered in this

investigation. The tests consisted of a reference test in

air and two O2/CO2 test cases with different recycle

feed gas mixture concentrations of O2 (case A: 21% O2

and 79% CO2, by volume and case B: 27% O2 and

73% CO2, by volume). In-furnace gas concentration,

temperature and total radiation profiles were presented

and discussed. The results showed that the fuel burn-

out was delayed for the case in which O2 concentration

was 21% (by volume) compared to air-fired conditions,

as a consequence of reduced temperature levels.

However, the 27% O2 case resulted in similar com-

bustion behavior to the reference conditions in terms

of in-flame temperature and gas concentration levels,

but with significantly increased flame radiation inten-

sity. The information obtained from the radiation and

temperature profiles showed that the flame emissivity

for case A and case B both differ from air-fired con-

ditions. The results showed that the 27% O2 flame

yields a higher radiative contribution from in-flame

soot compared to the air-fired flame in addition to the

known contribution from the elevated CO2 partial

pressure and its associated higher emissivity. The

measurements showed that the temperature levels of

the 21% O2/CO2 flame were significantly lower than

that in the reference conditions (in air) due to the

dilution by the recycled CO2. Compared to the 21% O2

case, the fuel burn-out of the 27% O2 case was favored

by the increased temperature level and improved

mixing conditions between fuel and O2. The 27% O2

case exhibited similar overall combustion behavior as

the air-fired reference case in terms of gas concentra-

tion and temperature profiles. In a study using a 20kW,

coal fired once-through reactor, the NOx and SO2

emissions were seen to be similar for air and 27% O2

cases [66].

Another work done by Maier and co-workers [67,68]

presented results of experiments carried out during

oxyfuel combustion in a once through, electrically

heated 20 kW test facility with flue-gas recirculation.

Investigations were carried out for combustion with air

as well as oxy-coal combustion with different con-

centrations of O2. Emissions at the end of the furnace

with different concentration of O2 during O2/CO2

firing were compared with air-firing. In-flame profile

measurements during air combustion and oxy-coal

combustion with 27% O2/73% CO2 were performed to

investigate and compare the combustion progress and

emission formation of the different technologies.

Another research topic addressed in the paper was the

fuel NOx reduction potential during oxy-coal com-

bustion with the application of oxidant-staging by

utilizing over-fire oxidant. The results showed that

oxidant staging during oxy-coal combustion with 27%

O2/73% CO2 demonstrated higher effectiveness in

terms of NOx reduction when compared to air-staged

combustion.

5. OXY-FUEL COMBUSTION INMEMBRANE REACTORS

Several authors consider oxyfuel combustion to be the

most effective way of capturing CO2 in power plants

[69,70]. However, due to the high cost of O2 produc-

tion by cryogenic processes [71], new technologies to

separate O2 from air with reduced cost are needed for

A review of recent developments in carbon capture M. A. Habib et al.

753Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

oxyfuel technologies [72–74]. One of these technologies

is the ITM. These are dense, mixed-ionic electronic

conducting (MIEC) ceramic membranes with typical

operating temperatures in the range of 700–9001C. The

crystalline structure of the ITM incorporates O2 ion

vacancies through which O2 ions diffuse. Armstrong

and Fogash [75] pointed out that ITMs enable a step-

change reduction in the cost of O2 production

compared to other methods like cryogenic air separa-

tion and separation by distillation. In ITMs, O2 is

separated from air using a dense ceramic membrane, as

shown in Figure 8. O2 from the ‘feed’ with high O2

partial pressure (p0O2) side permeates to low O2 partial

pressure side (pO2). In the case of an ITM reactor, the

O2 then oxidizes the fuel present in the sweep gas

(for example CH4), which decreases pO2and influences

the membrane temperature. It has been seen that using

a reactive sweep gas results in an increase in the O2

flux. The main advantages of this technology are: 1)

the potential to achieve up to 100% CO2 capture; 2)

reducing the power consumption by nearly 70%

compared to the currently existing methods of O2

production and 3) increasing the power generation

efficiency by 4% compared to a conventional oxyfuel

process. In catalytic combustion, the membrane acts as

a reactor for a partial oxidation of hydrocarbons [69],

which produces syngas composed of CO and H2 from

CH4 and O2 [71,74]. Thus, ITM catalytic combustion

allows for an efficient combustion of methane at

concentrations and temperatures lower than those used

in flame combustion without undesired byproducts

such as unburned hydrocarbons, carbon monoxide,

and oxides of nitrogen. Enhancement of O2 permea-

tion flux through the membrane is a key factor not

only for decreasing the overall membrane size and

reducing the cost of oxyfuel technology, but also for

enabling the use of membrane catalytic combustion in

a wide range of industrial applications.

The improvement of O2 permeation flux (JO2) and

the reduction in installation cost form the challenges of

commercializing this ITM technology. The geometry

and design configuration of ITM in an O2 transport

reactor (OTR) to be used in a power plant represents

another challenge because of several technical diffi-

culties. The relatively large area of ITM needed may be

reduced by improving the rate of O2 permeation. Other

difficulties are due the performance of ITM under

actual operating conditions. For ceramic O2 mem-

branes, the membrane thickness and the interfacial

processes are key parameters for achieving high O2

permeation rates. In addition, the membrane must

have a satisfactory chemical and mechanical stability

when exposed to elevated temperatures and, poten-

tially, pressures. In light of these demanding require-

ments for the membrane, the following discussion

presents the state of the art for ITMs using both

inert (separation only ITM) and reacting (ITM reactor

coupling combustion and O2 separation) sweep gases.

5.1. Materials selection for O2 transportmembranes

ITMs made from MIEC oxides have received increas-

ing attention because of the possibility to cut O2

production cost, thus providing potential to advance

clean combustion technologies. For high O2 permea-

tion flux ðJO2Þ, ITMs are required to have key

properties such as: fast O2 ion diffusion in the lattice,

fast surface O2 exchange kinetics, high electronic

conductivity, and excellent thermodynamic stability

under reducing and carbon containing environment.

Among the MIECs, perovskites with the chemical

formula of (Ln1�xAex)(TM1yTM21�y)O3�d where

Ln5La, Pr, Nd, Sm, Gd, Ae5Ca, Sr, Ba, and

TM15Cr, Mn, Fe, Co, and TM25Co, Ni, Cu, ex-

hibit excellent O2 permeation fluxes, whose structure is

shown as Figure 9(a). Recent studies have shown that

Ln2TMO41d (Ln5La, etc.; TM5Ni, Nd, Pr, etc.)-

based materials that may also be used for ITMs

[76–78]. The structure of Ln2TMO41d consists of an

alternate sequence of Ln2O2 bilayers and TMO

monolayers (K2NiF4 structure type) as shown in

Figure 9(b). The source of fast O2 transport in the

K2NiF4 materials is via interstitial O2 transport

through the Ln2O2 bilayers [79]. These perovskites can

selectively separate O2 from air at elevated temperatures

typically higher than 7001C, as shown in Figure 9(c)

[3,80,81].

High O2 ion diffusivity (D in cm2 s�1) is critical for

thick ITM designs while high surface O2 exchange

coefficient (k in cm s�1) is important for enhancing O2

permeation flux with ITMs below the characteristic

thickness Lc (at Lc, D/k5 1; in cm). Reduction in

thickness considerably below the characteristic thick-

ness will result in no further enhancement of JO2, as

shown in Figure 10. Increasing the kinetics of gas/solid

interface reaction becomes essential to improving the

performance of supported thin-film ITM designs. Fast

O2 ion transport has been widely demonstrated in the

La1�xSrxCo1�yFeyO3�d (LSCF) perovskite system byFigure 8. Operating schematic of an O2 membrane forming the

wall of an OTR 2007 [41].

A review of recent developments in carbon captureM. A. Habib et al.

Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

754

numerous researchers and is frequently used as an ITM

material with composition of La0.6Sr0.4Co0.2Fe0.8O3�d

(LSCF6428) [82–88]; recently, a novel perovskite

compound Ba1-xSrxCo1-yFeyO3�d (BSCF), in parti-

cular the composition Ba0.5Sr0.5Co0.8Fe0.2O3�d

(BSCF5582), was reported to have significantly im-

proved O2 permeation flux [89,90].

To compare the performance of these two perovskite

structures, we summarized the reported properties of

LSCF6428, BSCF5582 and La2NiO41d (LNO) for

membranes with 1–1.85mm thickness in Figure 11.

The values for JO2[81,89,91] and tracer O2 diffusion

coefficients (D�) [92–94] are plotted in Figures 11(a)

and (b), respectively for LSCF6428, BSCF5582 and

LNO. JO2values for these three materials are found to

be in the order of BSCF55824LNO4LSCF6428 and

follows the same trends for D�. This result indicates

that bulk diffusion is the rate-limiting step for JO2and

the thickness of these ITMs are above Lc. Calculating

the Lc for these three compounds using the tracer

surface exchange coefficient (k�) at �5501C reveals

values ranging from 300 mm, 100 mm and 3 mm for

BSCF5582, LNO, and LSCF6428, respectively. The

nature of the surface O2 exchange is currently not well

understood and insights into the rate-limiting step for

surface exchange will be critical to improving JO2with

thin membranes. On the other hand, electronic con-

ductivity is found to have negligible effect on JO2as

this is significantly much faster than the rates of all

other reaction steps, as shown in Figure 11(d).

Figure 9. (a) Structure of the perovskite crystal (ABO3). (b) Structure of the double perovskite crystal (A2BO4) and (c) O2 permeation

flux for ABO3 and A2BO4 materials as a function of temperature and varying composition.

Figure 10. Schematic of O2 permeation flux as a function of

membrane thickness.

A review of recent developments in carbon capture M. A. Habib et al.

755Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

Although JO2is a key factor in selection of ITM

materials, the ITM must also show robustness and

stability under large O2 partial pressure (pO2) gradients.

During operation, ceramic membranes are subjected to

pO2gradients ranging from 1 bar on the ‘feed’ or high

p0O2side to 10�6 bar along the ‘permeate’ or low pO2

side. For LSCF and BSCF, stability has been demon-

strated at pO2gradients as much as p0O2

43004300pO2of

1 bar//10�6 bar at 9501C [89,95]. In addition to varying

pO2, the O2 permeation membrane may also be exposed

to significant concentrations of nitrogen, carbon and

hydrogen containing gases in the permeate. For ex-

ample, during oxyfuel combustion, the pO2side of the

membrane is exposed to CO2, H2O vapor, and CH4

[96]. The combination of two or more gases, such as

H2O and CO2 can have a catalytic effect on membrane

degradation and hence render the O2 membrane im-

permeable to oxide ions, thereby cutting off O2 flux

[97]. LSCF6428 has been shown to have negligible

degradation under 1 bar of CO2 at 7501C for 1wk [93].

In contrast, BSCF5582 reacts readily with CO2 at

concentrations 410 000 ppm above 4001C [98]. This

makes BSCF5582 unsuitable for direct exposure to the

permeate side during oxyfuel combustion because CO2

is produced when hydrocarbon reacts with O2. Also,

La2NiO4 has been found to be fairly stable against

CO2, as the formation of lanthanum carbonates or

oxycarbonates was observed to be minimal [79].

5.2. Microstructure of oxygen transportmembranes

The ability to fabricate thin membranes to achieve high

oxygen permeation rates, typically 10–50mm, on

porous mechanical supports is thereby often of vital

importance. The fabrication and O2 permeation rates

of mixed conducting CaTi0.9Fe0.1O3�d membranes

were reported by Fontaine et al. [99]. Submicron

powder of CaTi0.9Fe0.1O3�d was prepared by spray

pyrolysis, and shaped into symmetrical membranes,

using conventional powder pressing with sintering, and

asymmetric membranes, using tape casting. The O2

permeation rates of a 30mm membrane were char-

acterized as a function of operating temperature

(800–10501C).

When ITM thickness is considerably smaller than

Lc, surface morphology has been found to be an im-

portant factor in improving JO2, as the surface ex-

change reaction depends on surface area of the ITM.

Tan et al. [74] have demonstrated that addition of

porous layers of silver or LSCF6428 on the high

pO2side of the LSCF6428 membrane can increase

Figure 11. Comparison of BSCF5582, LSCF6428 and LNO (a) O2 permeation flux JO2[81,89,91], (b) tracer diffusion coefficient D�

[81,92,94], (c) tracer surface exchange coefficient k� [92–94] and (d) electronic conductivity sel [95,96]. In (a), O2 permeation flux of

LSCF6428 and LNO were measured under p0O2==p00O2

gradient of 0.2//10�3 bar and that of BSCF5582 was measured under p0O2==p00O2

gradient of 0.2//10�5 bar. The unit d indicates the thickness of the samples in mm.

A review of recent developments in carbon captureM. A. Habib et al.

Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

756

permeation flux of LSCF6428 membranes by 2–20

times due to a combination of higher surface area and

enhanced surface exchange kinetics. They concluded

that the O2 dissociation on the high pO2side could play

an important role in determining O2 flux through un-

modified membranes. In another study, Jin et al. [100]

have reported an asymmetric ITM of LSCF6428 was

successfully prepared by coating a slurry containing

LSCF6428 powder dispersed in H2O together with a

nitric acid dispersant, and exhibited�3–4 times higher

an O2 permeation flux in comparison to dense sintered

LSCF6428 discs. It is evident that altering membrane

surface morphology porosity and roughness provides

enhancement of O2 permeation flux.

To enable the use of high-flux materials such as

BSCF in oxy-combustion applications, the addition of

a thin surface layer, such as LSCF6428, on the

permeate side of the ITM to prevent direct contact

with CO2 containing environment may be a feasible

approach, as shown in Figure 12.

5.3. Experimental methods for measuringO2 flux, surface exchange coefficient andbulk O2 ion conductivity

Conventional techniques for measurement of ITM

require a two-chamber setup with a p0O2side containing

air or pure O2 and pO2side, typically containing

a majority of helium (He) or N2 gas, as shown in

Figure 13. To measure the O2 flux JO2, gases are

pumped in the two chambers at known flow rates and

ceramic glass sealant is used between membrane and

tube enclosure to prevent leaking. However, ITM

experiments are never leak-free, therefore it is neces-

sary to compensate for O2 leakage into the two

chambers. To compensate for this, the O2 permeation

flux can be expressed [70] as:

JO2¼

FpermeationyO2� Fleakage

Að1Þ

where Fpermeation is the total flow rate of the permeate

side (cm3 s�1), yO2is the O2 concentration in permeate

side measured by gas chromatography (GC), Fleakage is

the O2 flow rate of the leakage, and A is membrane

surface area (cm2). Fleakage can be determined by

measurements that flow only argon (Ar) and He gases

in the feed and permeate sides, respectively. Once the

leakage rate is known, it is then necessary to determine

whether the O2 permeation is limited by either bulk

diffusion or surface exchange, as shown in Figure 12.

In the case where the membrane is above the

characteristic thickness, O2 permeation flux is limited

by bulk diffusion and can be determined by the

following equation [101]:

JO2¼

RT

42F2L

Z ln p00O2

ln p0O2

si d½ln pO2�

JO2¼

RT

42F2Lsi ln

p0O2

p00O2

ð2Þ

where L and s are sample thickness and O2 ionic

conductivity, respectively. Thus, when bulk diffusion

limits the O2 permeation flux, the O2 permeation flux

Figure 12. Addition of a second ITM layer (dark blue region on

right side) on the low p00O2side or permeate side to prevent

material decomposition and exposure gas atmospheres con-

taining CO2. This second layer is critical for BSCF materials to

prevent reaction with CO2.

Figure 13. Typical two-chamber configuration for measure-

ment of O2 permeation flux (JO2) using a dense ceramic disk for

the ITM at the center, mass flow controllers (MFCs) to deliver

gases and gas chromatograph to monitor JO2. The entire two-

chamber configuration is placed inside a furnace to control

temperature.

A review of recent developments in carbon capture M. A. Habib et al.

757Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

has linear dependence on sample thickness and

temperature and an exponential dependence on O2

partial pressure.

On the other hand, in the case where the membrane

is considerably below the characteristic thickness, the

O2 permeation flux becomes [95]

JO2¼

k0exkr

kf

p0O2

p00O2

!0:5

�1

24

35 ð3Þ

or

JO2¼

k00exkr

kf1�

p00O2

p0O2

!0:524

35 ð4Þ

where k0ex and k00ex are surface exchange coefficient at

feed side and permeation side, respectively. Also,

kf and kr are the forward and reverse reaction rate

constants for the following reaction:

12O21V��O ,

kf=krOx

O12h� ð5Þ

The nature of the surface exchange is currently not

well understood. However, its magnitude and pO2

dependence, can been measured using techniques such

as 18O2 tracer based on ex-situ secondary-ion mass

spectrometry (SIMS) analysis [102,103], EIS at varying

O2 partial pressures and temperatures [89,104,105],

conductivity relaxation methods following a sudden

change in pO2[42,106], and direct measurements of

permeation through a dense membrane [107,108].

5.4. Performance of O2 separation mem-branes

Recently, there has been increasing interest in using the

ITM to improve the performance of CH4 conversion

processes by enhancing O2 permeation through per-

ovskite hollow fiber membranes utilizing methane

activation. Liu et al. [3] manufactured hollow fiber

membranes of mixed conducting perovskite via the

combined phase inversion and sintering technique. The

fibers were tested for air separation with a custom-built

reactor under the O2 partial pressure gradient gener-

ated by the air/He streams. Some fibers were activated

in situ by introducing methane in the He sweeping gas

at high temperatures. The activated membranes with

new morphology were created by transforming the

inner densified surface layer to a porous structure

causing a decrease in the O2 ionic transport resistance.

Compared to the original membranes, the activated

membranes were found to give appreciable higher O2

fluxes. At 8001C, the O2 fluxes were increased by a

factor of 10 times after activation was carried out at

10001C for 1 h. During the permeation process under

O2 gradient provided by air/He, it was observed, that

the O2 fluxes through the prepared perovskite hollow

fiber membranes were greatly improved by introducing

methane inside the sweep gas for a short activation.

Perovskite LSCF6428 hollow fibers were synthesized

via a modified phase inversion and sintering technique.

The prepared membrane had multi-densified layer

structure. O2 permeation through the fibers was

measured under O2 concentration gradient of air/He

with He as sweep gas in the fiber lumen. However, as

indicated by Liu et al. [3], these membrane reactor

performances are not ideal because of the following

two disadvantages: (i) perovskite materials are apt to

deteriorate due to the unstable phase structure under

reducing CO2 containing or low O2 partial pressure

atmospheres and (ii) the disk-shaped membranes with

symmetric structure usually suffer high ion-transport

resistance.

Oxyfuel combustion using a catalytic ceramic

membrane reactor was investigated by Tan et al. [73].

It was indicated that membrane catalytic combustion is

an environmentally friendly technique for heat and

power generation from methane. LSCF6428 hollow

fiber membranes were prepared by the phase-inversion

spinning/sintering technique. The prepared hollow

fiber membranes packed with catalysts were assembled

in reactors, which were suitable for the catalytic com-

bustion of methane. The work demonstrated the per-

formances of a perovskite hollow fiber membrane

reactor for the catalytic combustion of methane.

A simple mathematical model that combined the local

O2 permeation rate with approximate catalytic reac-

tion kinetics was developed and was used to predict the

performance of the membrane reactor for methane

combustion. The effects of operating temperature and

the flow rates of methane and air feed flow rates on the

performance of the membrane reactor were investi-

gated both experimentally and theoretically. It was

found that both the methane conversion and O2 per-

meation rate can be improved by coating platinum on

the air side of the hollow fiber membranes. It was

concluded that during operation, both the methane

and the air feed flow rates should be chosen carefully in

order to control the product composition and to im-

prove combustion efficiency.

6. CONCLUSIONS

Carbon capture enables the use of fossil fuels while

reducing the emissions of CO2 into the atmosphere.

The oxyfuel technology provides a promising CCS

option applicable in power and steam generation

systems. One of the problems of oxyfuel combustion

is the need for O2 that is expensive when obtained

using cryogenic separation from air. Ceramic mem-

branes have received increasing attention because of

the possibility to reduce O2 production costs, thus

providing potential to advance the clean oxyfuel

energy technology. Major advances in ITMs have

been made in the past decade and this technology

has the strong potential to reduce greenhouse gas

A review of recent developments in carbon captureM. A. Habib et al.

Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

758

emissions in a wide range of conventional combustion

systems and improve energy generation efficiency.

The development of mixed-ionic electronic-conducting

materials, in particular with the perovskite material

system such as LSCF and BSCF, has shown large

enhancement in O2 permeation flux and catalytic

combustion. Next generation ITMs will require

significantly higher permeation flux at reduced operat-

ing temperatures to lower the cost of the technology

and increase the probability of commercialization

success. This paper reviews research on the develop-

ment of oxyfuel combustion membranes, reactors and

systems. The review includes work done on carbon

capture potential, developments of oxyfuel applica-

tions and O2 separation and combustion in membrane

reactors. The recent developments in oxyfuel power

cycles have been discussed focusing on the main

concepts of manipulating exergy flows within each

cycle and the reported thermal efficiencies.

7. SUGGESTED RESEARCH NEEDS

Most of the previous work done in the area of oxyfuel

combustion has focused on coal combustion and,

hence, it is recommended that more efforts are needed

for the characterization of the oxycombustion of

natural gas. Extensive efforts are also required to

improve the performance of fuel gas conversion

processes through combining air separation and

oxyfuel combustion into a single step. In order to

improve O2 permeation in O2 transport membranes,

one promising approach is to fabricate supported

submicron thickness ITMs on porous substrates to

eliminate bulk O2 transport loss. This approach will

limit O2 permeation at the solid/gas interface. Recent

studies have also shown significant enhancement of

surface exchange kinetics via modified surface chemis-

tries and/or microstructures that may pave the way for

increased permeation flux in these submicron mem-

branes. Recently, Sase et al. [109] have reported

�1000� improvement in surface exchange at the

interface of La0.6Sr0.4CoO3 and LaSrCoO4 although

the mechanism for enhancement is not clearly under-

stood yet. These improvements in k with surface

modification signal a novel approach for increasing O2

permeation flux in thin-film membranes below the Lc.

In addition, this development will allow for reduced

operating temperature for O2 membranes. Another

approach to improving O2 permeation flux is to utilize

double-perovskite (i.e. La2NiO41d) materials that have

higher O2 transport properties and surface exchange

kinetics. The successful development of these new

approaches for ITMs will make commercial O2

separation systems a reality. At present, oxyfuel

combustion using membrane reactors is still in the

stage of early development. Extensive research is

currently ongoing for possible solution of technical

difficulties facing this new technology. It may be too

early for a complete evaluation of the feasibility of

using this technology for carbon capture. Finally,

integration of this new technology into the power

generation sector requires detailed systems-level stu-

dies that assess the impact of ITM operational

requirements and costs.

NOMENCLATURE

A 5membrane surface area

ASU 5 air separation unit

AZEP 5 advanced zero emissions power plant

BSCF 5Ba1�xSrxCo1�yFeyO3�d

BSCF5582 5Ba0.5Sr0.5Co0.8Fe0.2O3�d

CLC 5 chemical-looping combustion

D 5 bulk oxygen ion diffusivity

D* 5 tracer oxygen diffusion coefficient

Fleakage 5 oxygen flow rate of the leakage

Fpermeation 5 total flow rate of the permeation side

HAT 5 humid air turbine

HRSG 5 heat recovery steam generator

IGCC 5 integrated gasification combined

cycle

JO25 oxygen permeation flux

k0ex 5 surface exchange coefficient at feed

side

k00ex 5 surface exchange coefficient at

permeation side

k 5 surface oxygen exchange coefficient

k* 5 tracer surface exchange coefficient

kf 5 forward reaction rate constant

kr 5 reverse reaction rate constant

L 5 sample thickness

Lc 5 characteristic thickness

Ln 5 lanthanoid

LNO 5La2NiO41dLSCF 5La1�xSrxCo1�yFeyO3�d

LSCF6428 5La0:6Sr0:4Co0:2Fe0:8O3�d

MAST 5moist air steam turbine

MCM 5mixed conducting membrane

MFCs 5mass flow controllers

MIEC 5mixed ionic and electronic conducting

oxides

NG 5 natural gas

p0O25 oxygen partial pressure on feed side

p00O25oxygen partial pressure on permeation

side

SIMS 5 secondary-ion mass spectrometry

SOFC 5 solid oxide fuel-cell

SR 5 steam Reforming

STIG 5 steam injected gas turbine

TM 5 transition metal

yO25oxygen concentration in permeation

side measured by gas chromatograph

ZEITMOP 5 zero-emission ITM oxyfuel plant

A review of recent developments in carbon capture M. A. Habib et al.

759Int. J. Energy Res. 2011; 35:741–764 r 2010 John Wiley & Sons, Ltd.

DOI: 10.1002/er

ZEPP 5 zero-emission power plant

sel 5 electronic conductivity

ACKNOWLEDGEMENTS

The financial support of KFUPM under the KFUPM-MIT collaboration program during the course of thiswork is greatly appreciated. The support and colla-boration of MIT mechanical engineering department isalso appreciated.

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