1836
Energy & Fuels 2006, 20, 1836-1844
Investigation of Chemical Looping Combustion by Solid Fuels. 1.
Process Analysis
Yan Cao and Wei-Ping Pan*
Institute for Combustion Science and EnVironmental Technology, Western Kentucky UniVersity,
Bowling Green, Kentucky 42101
ReceiVed July 24, 2005. ReVised Manuscript ReceiVed December 17, 2005
To concentrate CO2 in combustion processes by efficient and energy-saving ways is a first and very important
step for its sequestration. Chemical looping combustion (CLC) could easily achieve this goal. However, only
limited references are available that use coal as the energy resource in a CLC process even though the
development of CLC of solid fuels follows the trend of energy utilization. This paper is the first in a series of
two, where we present the concept of a CLC process of solid fuels using a circulating fluidized bed with three
loop seals. The riser of this circulating fluidized bed was used as the oxidizer of the oxygen carrier; one of the
loop seals was used as the reducer of the oxygen carrier and the separator for ash and oxygen carrier, and the
other two loop seals were used for pressure balance in the solid recycle process. Pressure profiles of recycled
solids using this process are presented in detail. For the development of an oxygen carrier, we focused on the
establishment of a theoretical frame of oxygen transfer capability, reaction enthalpy, a chemical equilibrium,
and kinetics. Analysis results indicated that Cu-, Ni-, and Co-based oxygen carriers may be the optimum
oxygen carriers for the CLC of solid fuels. Mn-based oxygen carriers have several disadvantages in their
lower oxygen transfer capability, thermodynamic limitations of purifying the CO2 stream, or a larger endothermic
reduction enthalpy. Fe-based oxygen carriers have the disadvantage of a larger endothermic enthalpy in the
reducer and lower reactivity. Thermodynamic analysis indicated that CO2 can be concentrated and purified to
at least 99% purity for the gas-solid reaction mode (reduction of the oxygen carrier by gasification products
such as CO and H2) or even higher for the solid-solid reaction mode (reduction of the oxygen carrier directly
by solid fuels) on the basis of the selected oxygen carriers. A Cu-based oxygen carrier is the choice that has
the potential to make the reducer self-sustaining or autothermal because of its exothermic nature during reduction.
This would be beneficial for simplifying the operation of the reducer. The tendency of the Cu-based oxygen
carriers to agglomerate can be eliminated by decreasing the operating temperature in the CLC system (600900 °C). In the second part of the series, we will evaluate the reduction kinetics of selected Cu-based oxygen
carriers by coal and other “opportunity solid fuels” using a simultaneous differential scanning calorimetrythermogravimetric analysis to simulate a microreactor, using an X-ray diffractometer and a scanning electron
microscope to characterize the solid residues, and a thermogravimetric analysis coupled with mass spectra to
characterize the evolved gas compositions.
1. Introduction
It has been known for more than 100 years that CO2 is a
greenhouse gas.1 Fossil fuel combustion is a major source of
CO2 emission. Worldwide fossil-fuel-based power production
contributes about one-third of the total CO2 emissions annually.2
In a conventional combustion system, fuel is directly mixed with
air and burned. This results in a low partial pressure of CO2 in
the presence of a majority of nitrogen. To dispose of the diluted
CO2 emissions will create a significant energy penalty.3 The
estimated cost of the CO2 disposal, ∼$4-8 U.S. per ton of
carbon,4 is much smaller when compared to the costs for the
separation and purification of the diluted CO2 stream, which is
typically in the range of $100-200 U.S. per ton of carbon.5
For a coal-fired power plant, roughly one-fifth of the electricity
* To whom correspondence should be addressed. Fax: 270-745-2221.
E-mail: wei-ping.pan@wku.edu.
(1) Arrhenius, S. On the Influence of Carbonic Acid in the Air upon the
Temperature of the Ground. Philos. Mag. (1798-1977) 1896, 41, 237277.
(2) Herzog, H.; Drake, E.; Adams, E. CO2 Capture, Reuse, and Storage
Technologies for Mitigating Global Climate Change. A White Paper; U.S.
Department of Energy: Washington, D.C., January, 1997, U.S.DOE/DEAF22-96PC01257.
(3) Herzog, H.; Eliasson, B.; Kaarstad, O. Capturing Greenhouse Gases.
Sci. Am. 2000, 282 (2), 72-79.
produced will be lost to CO2 separation and compression because of its low concentrations in flue gases.6 Faster CO2 sequestration by chemical measures at combustion sources with
high CO2 concentrations (i.e., stationary power plants) would
be highly desirable.
Among available or proposed technologies7,8 involving CO2
purification in the combustion process, chemical looping
combustion (CLC) is the most promising technology to combine
(4) Yu, J.; Corripio, A. B.; Harrison, D. P.; Copeland, R. J. Analysis of
the Sorbent Energy Transfer (SETS) for Power Generation and CO2 Capture.
AdV. EnViron. Res. 2002.
(5) Riemer, P. Greenhouse Gas Mitigation Technologies. An Overview
of the CO2 Capture, Storage and Future Activities of the IEA Greenhouse
Gas R& D Programme. The IEA Greenhouse Gas R&D Programme, 1998.
http://www.ieagreen.org.uk/paper2.htm (accessed Dec 2005).
(6) Freund, P. Abatement and Mitigation of Carbon Dioxide Emissions
from Power Generation. Powergen 98 Conference, Milan, June, 1998. http://
www. ieagreen. org. uk/pge98. htm (accessed Dec 2005).
(7) Lyngfelt, A.; Leckner, B. Technologies for CO2 Separation. In
Minisymposium on CO2 Capture and Storage; Lyngfelt, A., Azar, C., Eds.;
Publisher: Location, 1999; pp 25-35. Chalmers University of Technology
and University of Gothenburg, Goteborg, Sweden, October 22, 1999.
Available on http://www.entek.chalmers.se/∼anly/symp/sympco2.html (accessed Dec 2005).
(8) Croiset, E.; Thambimuthu, K. Coal Combustion with Flue Gas
Recirculation for CO2 Recovery. In Greenhouse Gas Control Technologies;
Riemer, P., Eliasson, B., Wokauun, A., Eds.; Elsevier: New York, 1999;
pp 581-586.
10.1021/ef050228d CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/11/2006
Chemical Looping Combustion by Solid Fuels
Energy & Fuels, Vol. 20, No. 5, 2006 1837
fuel combustion and pure CO2 production in situ allowing for
CO2 sequestration. In a general chemical looping combustion
process, a metal (Me) or the reduced metal oxide (MexOy-1)
associated with its oxidized form (MexOy) is circulated between
two reactorssthe oxidizer and reducer. In the reducer, the metal
oxide (MexOy) reacts with the fuel to produce CO2, H2O, and
metal (Me) or the reduced metal oxide (MexOy-1), as illustrated
in eqs 1-1 and 1-2, respectively. Pure CO2 is ready for
subsequent sequestration in the exit gas stream from the reducer
after H2O is condensed. In the oxidizer, the metal (Me) or the
reduced metal oxide (MexOy-1) reacts with air to form metal
oxide (MexOy), as illustrated in eqs 2-1 and 2-2. The advantage
of CLC compared to the normal combustion process is that CO2
is not mixed and diluted with nitrogen without any energy
needed for separation. Other benefits include a large elimination
of NOx emission9-11 and high thermal efficiency.12
(2n + m)MexOy + yCnH2m f (2n + m)xMe + myH2O +
nyCO2 (1-1)
or
(2n + m)MexOy + CnH2m f (2n + m)MexOy-1 + mH2O +
nCO2 (1-2)
or
xMe + y/2O2 f MexOy
(2-1)
MexOy-1 + 1/2O2 f MexOy
(2-2)
The metal oxides with their reduced metal oxides or metals,
which are used as oxygen carriers in the CLC, must have
sufficient reactivities in reduction and oxidation and enough
strength to limit particle breakage and attrition. A number of
(9) Richter, H.; Knoche, K. Reversibility of Combustion Processes. ACS
Symp. Ser. 1983, 235, 71-86.
(10) Podolski, W. F.; Swift, W. M.; Miller, S. A. Air Emissions from
Pressurized Fluidized Bed Combustor, Pressurized Fluidized Bed Combustion; Cuenca, M. A., Anthony, E. J., Eds.; Chapman & Hall: London, 1995.
(11) Jin, H.; Okamoto, T.; Ishida, M. Development of a Novel ChemicalLooping Combustion: Synthesis of a Looping Material with a Double Metal
Oxide of CoO-NiO. Energy Fuels, 1998, 12 (6), 1272.
(12) Ryu, H. J.; Bae, D. H.; Han, K. H.; Lee, S. Y.; Jin, G. T. Reactivity
Study on Ni-based Oxygen Carrier Particle in a Fixed Bed Reactor for
Chemical-Looping Combustor. Theor. Appl. Chem. Eng., KIChE 2002, 8,
1329.
(13) Hatanaka, T.; Matsuda, S.; Hatano, H. A New-concept Gas-Solid
Combustion System ‘MERIT’ for High Combustion Efficiency and Low
Emissions. Proc. Intersoc. Energy ConVers. Eng. Conf. 1997, 30, 944948.
(14) Herzog, H.; Eliasson, B.; Kaarstad, O. Capturing Greenhouse Gases.
Sci. Am. 2000, 282 (2), 72-79.
(15) Ishida, M.; Jin, H. A Novel Combustor Based on Chemical-looping
Reactions and its Reaction Kinetics. J. Chem. Eng. Jpn. 1994, 27, 29630.
(16) Ishida, M.; Jin, H. Novel Chemical-looping Combustor without NOx
Formation. Ind. Eng. Chem. Res. 1996, 35, 2469-2472.
(17) Ishida, M.; Jin, H.; Okamoto, T. A Fundamental Study of a New
Kind of Medium Material for Chemical-looping Combustion. Energy Fuels
1996, 10, 958-963.
(18) Ishida, M.; Jin, H.; Okamoto, T. Kinetic Behaviour of Solid Particle
in Chemical-looping Combustion: Suppressing Carbon Deposition in
Reduction. Energy Fuels 1998, 12, 223-229.
(19) Ishida, M.; Yamamoto, M.; Saito, Y. Experimental Works on
Innovative Chemical-looping Combustor. ECOS ‘99, Proceedings, International Conference on Efficiency, Costs, Optimization, Simulation and
Environmental Aspects of Energy Systems, Tokyo, June 8-10, 1999; pp
306-310.
metals have been discussed in the literature,13-34 such as Fe,
Ni, Co, Cu, Mn, and Cd, as well as some metal blends. The
investigation of the oxidation and reduction kinetics of selected
metal oxides has been intensively carried out in either a
thermogravimetric analysis or lab-scale fixed bed and fluidized
bed using gaseous fuels such as H2, CO, or CH4.13-34 The
kinetics of reactions vary widely depending upon the type of
metal oxide, particle size (70 µm to 2 mm), reduction gas (H2,
CO, and CH4), and temperature (600-1000 °C). Generally, Cu,
Ni, and Co and their oxides showed higher oxidation and
reduction reactivities and greater durability after repeated
oxidation and reduction cycles than those of Fe.21-24,28-31 It
was also found that an impregnated type of oxygen carrier could
increase the reactivity and durability, even with its particle size
being as large as about 2 mm in diameter.21,23-27,30-32 An almost
full conversion of the reactants could be achieved in minutes
for the impregnated type of oxygen carriers. The candidates for
the inert support materials could be SiO2, Al2O3, yttria-stabilized
zirconium, TiO2, and MgO. However, different combinations
of active materials of oxygen carriers with inert support materials
showed different crushing strengths and sintering temperatures.28,29 During the experiments with CH4, carbon deposition
may occur, which could cause a dramatic loss of reactivity.
However, a high concentration of water vapor available in the
reducer can help to eliminate carbon deposition.23,28,31
Starting from the introduction of CLC in 1983, the majority
of studies and process development has been with gaseous fuels
(20) Ishida, M.; Yamamoto, M.; Ohba, T. Experimental Results of
Chemical-looping Combustion with NiO/NiAl2O4 Particle Circulation at
1200C. Energy ConVers. Manage. 2002, 43, 1469.
(21) Jin, H.; Ishida, M. Reactivity Study on Natural-gas-fueled Chemicallooping Combustion by a Fixed Bed. Ind. Eng. Chem. Res. 2002, 41, 40044007.
(22) Jin, H.; Okamoto, T.; Ishida, M. Development of a Novel Chemicallooping Combustion: Synthesis of a Looping Material with a Double Metal
Oxide of CoO-NiO. Energy Fuels 1998, 12, 272-1277.
(23) Jin, H.; Okamoto, T.; Ishida, M. Development of a Novel Chemicallooping Combustion: Synthesis of a Solid Looping Material of NiONiAl2O4. Ind. Eng. Chem. Res. 1999, 38, 126-132.
(24) Cho, P.; Mattisson, T.; Lyngfelt, A. Comparison of iron-, nickel-,
copper- and manganese-based oxygen carriers for chemical looping combustion. Fuel 2004, 83, 1215-1225.
(25) Mattisson, T.; Johansson, M.; Lyngfelt, A. Multi-cycle reduction
and oxidation of different types of iron oxide particles. Application to
chemical-looping combustion. Energy Fuels 2004, 18, 628-637.
(26) Johansson, M.; Mattisson, T.; Lyngfelt, A. Investigation of Fe2O3
with MgAl2O4 for chemical-looping combustion. Ind. Eng. Chem. Res. 2004,
43, 6978-6987.
(27) Mattisson, T.; Järdnäs, A.; Lyngfelt, A. Reactivity of some metal
oxides supported on alumina with alternating methane and oxygens.
Application for chemical-looping combustion. Energy Fuels 2003, 17, 643651.
(28) Adanez, J.; de Diego, L. F.; Garcia-Labiano, F.; Gayan, P.; Abad,
A. Selection of Oxygen Carriers for Chemical-Looping Combustion. Energy
Fuels 2004, 18, 371-377.
(29) de Diego, L. F.; Garcia-Labiano, F.; Adanez, J.; Gayan, P.; Abad,
A.; Corbella, B. M.; Palacios, J. M. Development of Cu-based Oxygen
Carriers for Chemical-Looping Combustion. Fuel 2004, 83, 1749-1757.
(30) Mattisson, T.; Johansson, M.; Lyngfelt, A. The use of NiO as an
oxygen carrier in chemical-looping combustion. Fuel 2005, submitted for
publication.
(31) Cho, P.; Mattisson, T.; Lyngfelt, A. Carbon Formation on Nickel
and Iron Oxide-Containing Oxygen Carriers for Chemical-Looping Combustion. Ind. Eng. Chem. Res. 2005, 44, 668-676.
(32) Mattisson, T.; Lyngfelt, A.; Cho, P. The use of iron oxide as an
oxygen carrier for chemical-looping combustion of methane with inherent
separation of CO2. Fuel 2001, 80, 1953-1962.
(33) Wolf, J.; Anheden, M.; Yan, J. Performance Analysis of Combined
Cycles with Chemical Looping Combustion for CO2 Capture. Proceedings
of the 18th Pittsburgh Coal Conference, New South Wales, Australia, Dec.
4-7, 2001 [CD-ROM, session 23].
(34) Mattisson, T.; Jarders, A.; Lyngfelt, A. Reactivity of some Metal
Oxide Support on Alumina with Alternating Methane and Oxygenapplication for Chemical-looping Combustion. Energy Fuels 2003, 17, 643651.
1838 Energy & Fuels, Vol. 20, No. 5, 2006
such as methane and natural gas.35-37 However, the methane
and the natural gas supply cannot fully support the energy needs
of the country’s electricity demand for the long term. Solid fuels
such as coal and biomass have been seldomly used in the
concept of chemical looping combustion because of their
technical problems.41-43 Although the adaptation of CLC to the
combustion of solid fuels presents many challenges, this
adaptation is very attractive because of rich coal deposits. The
control of other pollutants emitted in coal combustion, such as
sulfur oxides and trace metals, can be conducted in an efficient
way because of the very low volume of nitrogen-free flue gas
that is generated in the reducer. The meaning is significant for
sustaining the long-term utilization of rich coal resources in an
economical, efficient, and environmental way.
This paper is the first in a series of two, where we present
the concept of a CLC process of solid fuels using a circulating
fluidized bed (CFB) with three loop seals and the establishment
of a theoretical frame of oxygen transfer capability, reaction
enthalpy, chemical equilibrium, and kinetics for oxygen carriers
of interest. In the second part of the series, we will evaluate the
reduction reaction of selected Cu-based oxygen carriers by coal
and other “opportunity solid fuels” using simultaneous differential scanning calorimetry-thermogravimetric analysis to
simulate a microreactor, an X-ray diffractometer and a scanning
electron microscope as the characterization methods for the solid
reaction residues, and thermogravimetric analysis coupled with
mass spectrometry to characterize the evolved gas compositions.
2. Technical Approach for CLC of Solid Fuels
2.1. Process Analysis and Technical Issues. There are two
approaches to applying CLC to solid fuel combustion. The first
approach is to gasify solid fuels in a separate gasifier with pure
oxygen to produce a syngas of CH4, CO, and H2 without
nitrogen. Syngas is supplied for the CLC system, which is
similar to the CLC process using a natural gas with an additional
gasifier. However, the production of pure oxygen and the
fabrication of an additional gasifier will be required and, thus,
dramatically increase the capital cost of the CLC system.
Approach 2, which is the proposed CLC process for solid
fuels in this study, is to directly supply solid fuel into the reducer
(35) Lyngfelt, A.; Leckner, B.; Mattisson, T. A fluidized-bed combustion
process with inherent CO2 separation; application of chemical-looping
combustion. Chem. Eng. Sci. 2001, 56, 3101-3113.
(36) Zafar, Q.; Mattisson, T.; Gevert, B. Integrated Hydrogen and Power
Production with CO2 Capture Using Chemical-Looping Reformings. Redox
Reactivity of Particles of CuO, Mn2O3, NiO, and Fe2O3 Using SiO2 as a
Support. Ind. Eng. Chem. Res. 2005, 44 (10), 3485-3496.
(37) Mattisson, T.; Lyngfelt, A. Applications of chemical-looping
combustion with capture of CO2. Second Nordic Minisymposium on CO2
Capture and Storage, Göteborg, Sweden, 2001.
(38) U.S. DOE Project Fact Sheet, Hybrid Combustion-Gasifictaion
Chemcial Looping Coal Power Technology Development. www.netl.doe.gov/coal (accessed Dec 2005).
(39) Andrus, H. E., Jr.; Chiu, J. H.; Stromberg, P. T.; Thibeault, P. R.
Alstom Power Inc, Alstom’s Hybrid Combustion-Gasifictaion Chemcial
Looping Coal Power Technology Development. Twenty-Second Annual
International Pittsburgh Coal Conference, Pittsburgh, PA, Sept 12-15, 2005.
(40) Bedick, R. C. Advancements in Coal-fired Power Generation
Technologies. Coal Utilization Technologies Workshop, National Research
Center for Coal & Energy, Morgantown, WV, Sept 22, 2004.
(41) Cao, Y.; Cheng, Z.; Meng, L.; Riley, J. T.; Pan, W.-P. Reduction
of Solid Oxygen Carrier (CuO) by Solid Fuel (Coal) in Chemical Looping
Combustion. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 2005, 51, 99100
(42) Cao, Y.; Pan, W.-P. Chemical Reversal Cycle of Solid Oxygen
Carrier for Producing Pure Oxygen or Oxygen Rich Gas Stream. Prepr.
Pap.sAm. Chem. Soc., DiV. Fuel Chem. 2005, 51, 415-416
(43) Cao, Y.; Riley, J. T.; Pan, W.-P. Application of a circulating fluidized
bed process for the chemical looping combustion of solid fuels. Prepr.
Pap.sAm. Chem. Soc., DiV. Fuel Chem. 2004, 49 (2), 815-816.
Cao and Pan
in the CLC system. It has economic advantages, but several
technical problems. There are two reaction paths in this
approach, direct reduction of the oxygen carrier by solid fuels
and indirect reduction by syngas from solid fuel gasification in
the reducer. There is actually a mixed mechanism of these two
reaction paths mentioned above. The actual reaction modes
occurring in the chemical looping combustion of solid fuels are
dependent on the physical properties, reaction thermodynamics,
and kinetics of the selected solid fuels and oxygen carriers. For
the first path occurring in the reducer, the primary technical
concern is the lower reactivity between solid fuel and the oxygen
carrier due to low solid-solid contact efficiency. For the second
path, the technical concern is the lower solid fuel gasification
rate compared with its combustion rate at the same evaluated
temperature. Such issues need to prolong the residence time of
solid fuels inside the reducer to fulfill the higher carbon
conversion efficiency. Therefore, reduction mechanisms of
oxygen carriers by solid fuels and their kinetics in the reducer
of the chemical looping combustion process should be examined
more closely. Other technical issues pertaining to the development of the chemical looping combustion of solid fuels also
include (1) the separation of the oxygen carrier from unburned
carbon and fly ash, (2) the prevention of unburned carbon
particles in the oxidizer, (3) the energy distribution between
the oxidizer and reducer in the presence of an endothermic
gasification process, (4) the recycling of solid phase materials
and system pressure balance, and (5) the prevention of gas
leakage between the reducer and oxidizer. All technical issues
will be addressed and discussed in this study, except issue 5,
which has been intensively discussed in ref 44.
Presently, the only available technology to use the concept
of chemical loops to combust the solid fuel is the Alstom Power
Inc.’s Hybrid Combustion-Gasification Chemical Looping Coal
Power Technology.38-40 It applies two chemical loops of
CaSO4-CaS and CaCO3-CaO and also one thermal loop with
bauxite. In the oxidizer of the first chemical loop of CaSO4CaS, coal was reacted with CaSO4 to produce CO, not CO2
directly. CO needs to be further converted into CO2 through a
shift reaction and then concentrated for sequestration through
the second chemical loop of CaCO3-CaO. In this process, one
more chemical loop, one more thermal loop, and one shift
reaction step are introduced into the whole system to conduct
solid fuel combustion and CO2 concentration simultaneously.
Thus, its economic competitiveness and process complexity need
to be further improved.
2.2. The Concept of the Proposed Process. To overcome
the technical difficulties mentioned above in an economical way,
we propose a CFB with a combination of several loop seals for
the chemical looping combustion of solid fuels, as briefly
illustrated in Figure 1. In this approach, the facility will consist
of three major componentssa high-velocity riser acting as an
oxidizer of the oxygen carrier, a down-flow moving bed or a
bubbling fluidized bed acting as a reducer of the oxygen carrier
in which solid fuels are fed, and a low-velocity bubbling bed
or a turbulent fluidized bed acting as a deep reduction reactor
and separator for the oxygen carrier from fly ash as well as
unburned carbon. Among them, the reducer and separator are
combined together to assemble a larger loop seal. The other
two small loop seals will act as connectors of solid recycling
between the oxidizer and the reducer with the separator. The
gas velocity in the riser provides the driving force for the
(44) Johansson, E.; Lyngfelt, A.; Mattisson, T.; Johnsson, F. Gas Leakage
Measurements in an Interconnected Fluidized Bed for Chemical-looping
Combustion. Powder Technol. 2003, 134, 210-217.
Chemical Looping Combustion by Solid Fuels
Energy & Fuels, Vol. 20, No. 5, 2006 1839
Figure 1. System configuration for the pilot-scale chemical looping combustion process of solid fuels.
circulation of particles between the three components. The loop
seals balance the system pressure and allow solids to flow from
locations of lower static pressure to those of higher static
pressure. A heat exchanger can be installed in the air oxidizing
riser to produce steam for power generation.
In the riser, air is supplied as the fluidizing agent to oxidize
the metal or the reduced metal oxide to its oxidized form where
oxygen is transferred from the air to the oxygen carrier. After
being separated by a cyclone, the oxidized oxygen carrier returns
to the reducer. Steam or recycling CO2 is used as the aeration
or transport media and gasification agent in the reducer. Metal
oxide is reduced to elemental metal or the reduced metal oxide
in the reducer by direct or indirect reaction with solid fuels,
which is dependent on the properties and reaction mechanisms
of the solid fuel and oxygen carrier. In the bubbling bed or the
turbulent fluidized bed, deep reduction of the oxygen carrier,
deep conversion of the solid fuels, and full separation of the
reduced oxygen carrier with fly ash will occur simultaneously.
The reduced oxygen carrier will return to the riser (oxidizer)
via another small loop seal. The remaining unburned carbon is
recycled to the reducer or directly combusted in another facility
because of its lower reactivity. Fresh oxygen carrier probably
has to be added to maintain the reactivity of the oxygen carrier.
The volume of the gas flow in the oxidizer is much larger
than that in a reducer because a large amount of nitrogen is
carried in by the air. The oxidizing rate of the oxygen carrier is
much faster than the rate of the reducing reaction. Thus, a high
gas velocity has been chosen in the oxidizing reactor to keep a
reasonable size of reactor in consideration of the capital cost.
In the reducer, relatively long residence times, on the order of
tens of minutes, are needed for the reduction of the oxygen
carrier directly by solid fuels or indirectly by syngas from
gasification of the solid fuels in the same reactor. Only a small
amount of gas is needed for fossil fuel gasification, so a downflow moving bed or a bubbling fluidized bed is selected. The
ash with unburned carbon produced is much lighter (800-1200
kg/m3 in particle density) compared to the oxygen carrier
(generally above 5000 kg/m3 in particle density, which is
dependent on its porosity). Thus, the oxygen carrier and fly ash
can be separated on the basis of density difference. By
controlling the operating velocity, fly ash will be entrained out
of a bubbling fluidized bed or a turbulent fluidized bed with
“flue gas 2” and enter “cyclone 2” for cleanup. The flue gas
exiting the bubbling bed is a nitrogen-free gas with CO2, H2O,
and possibly CO and H2, which depends on the properties of
the selected oxygen carriers.
The system pressure balance is essential to the buildup of
solid recycling and the adjustability of the recycle rate. The
actual pressures at specific locations of the reactor for several
operating conditions form pressure profiles, which are shown
in Figure 2. In this figure, points 4-8, points 8-12, and points
12-16 represent the upper loop seal, the middle loop seal
1840 Energy & Fuels, Vol. 20, No. 5, 2006
Cao and Pan
Figure 2. Pressure profiles of the chemical looping combustion system of solid fuels.
(reducer and separator), and the down loop seal, respectively.
The pressure difference is distributed in these loop seals to fulfill
the task of transporting recycled solids from a lower pressure
at point 4 to a higher pressure at point 8, then from point 8
(lower pressure) to point 12 (higher pressure), and further from
point 12 (lower pressure) to point 17 (higher pressure). Generally
speaking, the higher recycle rate of solids will make the pressure
differential inside the riser increase; therefore, a higher pressure
difference created by the loop seals must be made in the CLC
system. To adjust the pressure difference, one needs to adjust
the gas velocity inside the loop seals so that the flow pattern
inside the loop seals will be changed, especially in the middle
loop seal. The capability of loop seals for adjusting pressure
differences to adapt to changes in operation conditions is
dependent on the operation modes of the loop seals and their
sizes. For the same solid recycle rate, the larger loop seals or
that made with the fluidized bed mode is always better than
the smaller loop seals or that made with the moving bed.
In the one design mode, the middle loop seal is made up of
a moving bed on one side, which acts as the reducer, and is
combined with a bubbling fluidized bed, which acts as a
separator for ash and the oxygen carrier. The pressure difference
in the moving bed is always smaller, as shown in points 8′ and
9′ in Figure 2 (red for the high mass flow rate and green for
the low mass flow rate operating conditions). If solids flow is
intended to be operated at the higher mass flow rate condition,
the upper and the down loop seals must work at the larger
pressure difference. However, these two loop seals are both
small and cannot create the large pressure difference. The static
pressure at the point 5′ location, which is expected, will actually
be at the point 5′′ location in the dashed line, which may actually
be lower than that at the point 8′ location. The normal pressure
profiles in the upper loop seal may collapse. Then, the solids
flow may back-flow into the upper loop seals, which would
cause the cycle system of the CLC to be unsustainable. If
recycling of the solid is operated at a lower mass flow rate
condition, as shown in the pressure profile marked with green,
it is very difficult to control and adjust the system under such
narrow pressure differences between loop seals.
In the preferable design mode, the pressure profile of the
middle loop seal was indicated by the black line in Figure 2.
The reducer works under fluidized flow conditions, and the
separator works under turbulent flow conditions. In this configuration, the larger loop seal can work at larger pressure
differences (points 8-9 in black) and, thus, let small loop seals
(upper and down) work reasonably at a smaller pressure
difference. This modified system also will provide the flexibility
of control of the mass flow rate (higher or lower). Thus, it is
suitable for system setup, startup, stabilization, and condition
testing to occur at the pilot-scale stage. An additional benefit is
that bubble fluidization could provide good solid-to-solid contact
between solid fuels and the oxygen carrier in the reducer, rather
than in the moving bed. The operating gas velocity at the
different reactors could be 5 times the terminal velocity of the
selected oxygen carrier in the riser to maintain the fast
fluidization state, 5 times the minimum fluidization velocity of
Chemical Looping Combustion by Solid Fuels
Energy & Fuels, Vol. 20, No. 5, 2006 1841
Table 1. Physical Properties and Oxygen Transfer Capability of Oxygen Carriers45
No.
reduction reaction
melting
point of
the reduced
metal form,
°C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2CuO + C ) 2Cu + CO2
2Cu2O + C ) 4Cu + CO2
2NiO + C ) 2Ni + CO2
2Co3O4 + C ) 6CoO + CO2
1/ Co O + C ) 3/ Co + CO
2
3 4
2
2
2CoO + C ) 2Co + CO2
6Mn2O3 + C ) 4Mn3O4+ CO2
2Mn2O3 + C ) 4MnO + CO2
2/ Mn O + C ) 4/ Mn + CO
3
2 3
3
2
2Mn3O4 + C ) 6MnO + CO2
1/ Mn O + C ) 3/ Mn + CO
2
3 4
2
2
2MnO + C ) 2Mn + CO2
6Fe2O3 + C ) 4Fe3O4 + CO2
2Fe2O3 + C ) 4FeO + CO2
2/ Fe O + C ) 4/ Fe + CO
3 2 3
3
2
2Fe3O4 + C ) 6FeO + CO2
1/ Fe O + C ) 3/ Fe + CO
2 3 4
2
2
2FeO + C ) 2Fe + CO2
2PbO + C ) 2Pb + CO2
2CdO + C )2Cd + CO2
1083
1083
1452
1480
1480
1480
1564
1650
1260
1650
1260
1260
1538
1420
1275
1420
1275
1275
327.5
320.9
melting
point of
the oxidized
metal form,
°C
specific
density of
the reduced
metal form
FR, kg/m3
specific
density of
the oxidized
metal form
FO, kg/m3
moles of metal
per mole
of oxygen
transfer
(N), mol/mol
(NxFR)M/
(NxFR)Cu
in reducer
(NxFO0.5)M/
(NxFO0.5)CuO
in oxidizer
1026
1235
1452
895
895
1800
1080
1080
1080
1564
1564
1650
1560
1560
1560
1538
1538
1420
886
900
8920
8920
8900
8900
8900
8900
4856
5180
7200
5180
7200
7200
5200
5700
7030
5700
7030
7030
11340
8650
6450
6000
7450
6070
6070
5680
4810
4810
4810
4856
4856
5180
5120
5120
5120
5200
5200
5700
8000
8150
1
2
1
3
0.75
1
6
2
0.67
3
0.75
1
6
2
0.67
3
0.75
1
1
1
1.0
2.0
1.0
3.0
0.7
1.0
3.3
1.2
0.5
1.7
0.6
0.8
3.5
1.3
0.5
1.9
0.6
0.8
1.3
1.0
1.0
1.9
1.1
2.9
0.7
0.9
5.2
1.7
0.6
2.6
0.7
0.9
5.3
1.8
0.6
2.7
0.7
0.9
1.1
1.1
the selected oxygen carrier in the reducer to maintain the
bubbling fluidization state, and the terminal velocity of the fly
ash in the separator to maintain the turbulent fluidization state.
2.3. The Oxygen Carriers for the CLC of Solid Fuels and
Thermodynamic Analysis. On the basis of the oxygen transfer
capability, an energy balance analysis, and a thermodynamics
analysis, copper (Cu) seems to be the better choice as an oxygen
carrier for the CLC system of solid fuels.
2.3.1. Physical Properties of Oxygen Carriers. As described
in the section on process analysis in this paper and in previous
studies,35,41-43 the interconnection of the fast fluidized bed with
the another fluidized bed or the moving bed is the most possible
reactor configuration to conduct the CLC process. For a reactor
in which solid particles are flowing, moving, and recirculating,
the possibility of particle agglomeration should be avoided. For
metal-based oxygen carriers, the melting points of the selected
metals, reduced metal oxides, and metal oxides are important
parameters in evaluating their agglomeration tendencies. The
melting of oxygen carriers may also resul in a loss of reactivity.
The melting points of candidate metals, their reduced metal
oxides, and metal oxides are shown in Table 1.45 The majority
of metals and their metal oxides have very high melting points,
which are always higher than 1200 °C, except those with melting
points around 1050 °C, which are within the operation temperature range of the CLC process, such as Cu, CuO, and Mn2O3,
which are in italics. Some metals and their metal oxides have
very low melting points such as PbO, CdO, and Co3O4, which
are in bold and must be removed from the candidate list to be
oxygen carriers in the CLC process. However, the operating
temperature of the CLC process is also dependent on the
acceptable reactivity of the oxygen carriers. Previous studies
indicated that Cu-based oxygen carriers had a reactivity of 100%
reduction within minutes at low temperatures (600-900 °C)19,34-36
in the fixed bed or fluidized bed testing facilities. When copper
oxide was doped on the substrate, it was indicated that most of
its oxygen was active for reaction with methane, and the highest
efficiency could be achieved with reduction rates up to 100%/
min and oxidation rates up to 25%/min.24,28-29
2.3.2. Oxygen Transfer Capability of the Oxygen Carrier. The
utilization efficiency of an oxygen carrier is dependent on its
oxygen transfer capability, which is listed in Table 1 as the
(45) Perry, R. H.; Green, D. W. Perry’s Chemical Engineers’s Handbook,
7th ed.; The McGraw-Hill Companies, Inc.: New York, 1997
moles of metal per mole of oxygen transfer (N). The recirculation of the oxygen carrier required energy consumption, which
is proportional to the pressure drop across the reactor.35 In the
oxidizer, which is a fast fluidization bed, the pressure drop is
proportional to the critical velocity labelled as the terminal
velocity Ut.46 In the reducer, which is a bubble fluidization bed
or a moving bed, the pressure drop is proportional to the critical
velocity labelled as the minimum fluidizing velocity Umf.46 As
shown in eqs 3 and 4, Fp, which is the particle density of the
oxygen carriers, is the most important parameter affecting Ut
and Umf. All other parameters in eqs 3 and 4 can be controlled
during the manufacturing of the oxygen carriers and system
operation parameters. In eq 3, the factor value of Fp is 0.5, and
in eq 4, the factor value of Fp is 1. These two parameters can
be multiplied by N relative to the same parameters of the
CuO-Cu system, which are (NxFp)/(NxFp)Cu-CuO or (NxFp)/
(NxFp0.5)Cu-CuO, to evaluate the relative energy consumption by
using different oxygen carriers when the oxygen carriers are
directly manufactured by their metal or metal oxide. Some
oxygen carriers were manufactured as an impregnated type. In
this case, N can be used as the relative energy consumption by
using different oxygen carriers.
Ut ) [4dp(Fp - Fg)g/(3FgCD)]1/2
(3)
As shown in Table 1, N values and (NxFp)/(NxFp)Cu-CuO or
Umf ) [dp2(Fp - Fg)g(mf3Φs2)]/[(150µ)(1 - mf)]
(4)
(NxFp)/(NxFp0.5)Cu-CuO values follow the same trend, so we can
focus on the N value to evaluate energy consumption. The best
candidates for oxygen carriers should be those with a smaller
N value. The minimum value of N is 0.67 for reactions 9 and
15 and the next is 0.75 for reactions 5, 11, and 17 for Co-,
some Mn-, and some Fe-based oxygen carriers. N values of other
reactions are close to 1 for Cu-, Ni-, and some Co- and Febased oxygen carriers. In Table 1, we listed reactions in bold if
the N value was greater than 3 and in italics for N values of
1-3. By evaluating the parameters of (NxFp)/(NxFp)Cu-CuO and
(NxFp)/(NxFp0.5)Cu-CuO, the same conclusion can be made.
(46) Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.;
Butterworth-Heinemann: Woburn, MA, 1990; Butterworth-Heinemann
Series in Chemcial Engineering, ISBN: 0-409-90233-0.
1842 Energy & Fuels, Vol. 20, No. 5, 2006
Cao and Pan
Table 2. Enthalpies of Reduction Reaction by Carbon at 1000 °C and 1 atm (Calculations Based on Data from ref 45)
C + O2 f CO2, -392.75 kJ/mol
endothermic
exothermic
2NiO + C f 2Ni + CO2, 75.21kJ/mol
2CoO + C f 2Co + CO2, 73.92 kJ/mol
1/ Co O + C f 3/ Co + CO , 53.9 kJ/mol
2
3 4
2
2
2/ Mn O + C ) 4/ Mn + CO , 239.61 kJ/mol
3
2 3
3
2
6Mn3O4 + C ) 4MnO + CO2, 54.21 kJ/mol
1/ Mn O + C f 3/ Mn + CO , 296.65 kJ/mol
2
3 4
2
2
2MnO + C f 2Mn + CO2, 378.98 kJ/mol
6Fe2O3 + C f 4Fe3O4 + CO2, 83.56 kJ/mol
2Fe2O3 + C f 4FeO + CO2, 158.40 kJ/mol
2/ Fe O + C f 4/ Fe + CO , 146.37 kJ/mol
3 2 3
3
2
2Fe3O4 + C ) 6FeO + CO2, 195.78 kJ/mol
1/ Fe O + C ) 3/ Fe + CO , 151.27 kJ/mol
2 3 4
2
2
2FeO + C ) 2Fe + CO2, 136.44 kJ/mol
2CuO + C f 2Cu + CO2, -96.51 kJ/mol
2Cu2O + C f 4Cu + CO2, -61.04 kJ/mol
6Co3O4 + C ) 6CoO + CO2, -8.63 kJ/mol
6Mn2O3 + C ) 4Mn3O4 + CO2, -216.63 kJ/mol
2Mn2O3 + C ) 4MnO + CO2, -36.07 kJ/mol
Any candidate in italics or bold is unsuitable for use as an
oxygen carrier in a CLC system; therefore, any candidates not
in italics or bold have passed the selection criteria. They include
Ni-based (NiO-Ni), Co-based (CoO-Co), Mn-based (Mn3O4Mn and MnO-Mn), and Fe-based (Fe2O3-Fe, Fe3O4-Fe, and
FeO-Fe) oxygen carriers. Because of the perfect reactivity of
Cu-based oxygen carriers at lower temperatures, reaction 1 is
still included in the candidates list for the CLC process of solid
fuels. It was reported that Fe-based oxygen carriers have a lower
reactivity than those of other selected oxygen carriers. Aluminadoped manganese-based oxides could produce stable nonreactive
compounds of spinel (MnAl2O4 and Mn2AlO4), which may
result in a considerable loss of the oxygen transfer capability
of oxygen carriers.34 Ni-based (NiO-Ni), Co-based (CoO-Co),
and Cu-based (CuO-Cu) carriers are expected to be good
candidates for oxygen carriers.
Energy Balance Analysis. The oxygen carrier reduction
process by solid fuel is far more complicated compared to that
by gaseous fuels. The process is governed by the prevailing
chemical thermodynamics and kinetics. If the indirect path (the
gasification of solid fuels followed by the reduction of metal
oxides by gaseous gasification products) is dominant, the
properties of pyrolysis and gasification of the solid fuels should
be considered. All reactions related to the pyrolysis and
gasification of fossil fuels are endothermic, as illustrated by eqs
5-7. All enthalpy data are based on 1 atm and 25 °C.45 In
conventional gasification technologies, the self-combustion of
solid fuels is needed to provide the heat requirement of the
endothermic pyrolysis and gasification processes. This is called
the autothermal process. In the proposed CLC system, the
gasification process for solid fuel occurs simultaneously with a
reduction process for the oxygen carrier in the reducer where
no oxygen exists. There are two ways to supply heat for the
solid fuel gasification process. One is the reduction of the
oxygen carrier indirectly by solid fuels or their product gases,
and the other method is to use a heat-transfer material with a
high heat capacity to transfer heat from the oxidizer to the
reducer. Obviously, the previous one is the better choice to
prevent additional energy consumption by recycling of the heattransfer material.
CnHmOp f aCO2 + bH2O + cCH4+ dCO + eH2 + f(C2C5), 20.9 kJ/mol (5)
Until now, just a few metal oxides that have been examined
C + H2O f CO + H2, 118.3 kJ/mol
(6)
C + CO2 f 2CO, 160.5 kJ/mol
(7)
as oxygen carriers for the CLC process have shown exothermic
properties when reacted with carbon or syngas. The possible
reactions related to oxygen carrier candidates and carbon in the
reducer are shown in Table 2. There are two steps (oxidizing
and reducing) in the process of chemical looping for solid fuel
combustion. Thus, the enthalpy of solid fuel combustion in the
chemical looping process is equal to the sum of the enthalpies
of the two steps. For the different oxygen carriers shown, there
is a different distribution of enthalpies in two steps, and even
the endothermic and exothermic reactions may be changed. Cubased and CoO-Co oxygen carriers are the only choices whose
reduction is exothermic due to the smaller enthalpy of the
oxidation compared to the direct combustion of solid fuel. The
Mn3O4-MnO oxygen carrier should be removed from the
suggested candidate list because of the reason (N ) 6) previously
mentioned. Despite endothermic properties in the reducer,
oxygen carriers such as NiO-Ni, CoO-Co, Co3O4-Co,
Mn3O4-MnO, and Fe2O3-Fe3O4 have smaller reaction enthalpies, so they can be included as candidates for oxygen carriers
if heat-transfer materials will be used for heat transfer in the
reducer. Because of the reason previously mentioned (melting
points and N values), Co3O4-Co, Mn3O4-MnO, and Fe2O3Fe3O4 can be removed from the suggested candidate list.
Thermodynamics Analysis. Chemical reaction thermodynamics are important for the control of CO2 purity. From the standard
Gibbs free energy changes, the equilibrium constants can be
calculated for the various reactions of metal oxide reduction
and solid fuel gasification for a wide range of operating
temperatures. The phase diagrams for the reduction reactions
using the reducing agents CO and H2 produced from the solid
fuel gasification process at atmospheric pressure are shown in
Figures3 and 4, respectively.
Figure 3 shows an equilibrium gas ratio of PCO2/PCO as a
function of the temperature for the reduction of various metal
oxides. It shows that the ratio varies from approximately 105
for the reduction of CuO to Cu, Cu2O to Cu, Mn3O4 to MnO,
and Fe2O3 to Fe3O4 to values on the order of 10-5 or less for
the reduction of MnO to its elemental state of Mn. Information
related to the Boudouard reaction, as illustrated in eq 5, is also
presented in Figure 3. For the reduction of a metal oxide with
CO in the absence of solid carbon, the oxides of copper and
nickel will be reduced to their elemental forms at gas ratios
between 105 and 102. Therefore, the completeness of the reaction
could be achieved and a highly concentrated CO2 stream will
be obtained in the proposed CLC system. On the other hand,
the reduction of MnO requires a CO2-free environment, which
is practically impossible for the CLC of solid fuel to achieve.
Because carbon is also present in the reaction mixture in the
proposed reducer, the reduction of metal oxide and carbon
gasification to CO occurs simultaneously. As shown in Figure
3, above the simultaneous equilibrium temperature, where two
Chemical Looping Combustion by Solid Fuels
Figure 3. Variation of the thermodynamic equilibrium factor for
MexOy-CO as a function of temperature (calculations based on data
from ref 45).
Figure 4. Variation of the thermodynamic equilibrium factor for
MexOy-H2 as a function of temperature (calculations based on data
from ref 45).
curves for the reduction of metal oxides and the Boudouard
reaction intersect, Fe3O4 will be converted to FeO above 650
°C and then to Fe above 700 °C at a low PCO2/PCO ratio of
∼10. In contrast, the curves for CuO, Cu2O, NiO, and Fe2O3
do not intersect with the carbon curve, even at the temperature
range of interest (600-1200 °C). The simultaneous reactions
are not limited by thermodynamics; they are determined entirely
by kinetics. In this case, the gas constituents produced will have
an intermediate impact on the value of the PCO2/PCO ratio.
Generally, the reduction reaction of the metal oxide is faster
than the solid fuel gasification in the CLC process.
H2 is another product from the solid fuel gasification process
using H2O as the gasification agent. Figure 4 is the thermodynamics diagram for the PH2O/PH2 equilibrium system, similar to
the PCO2/PCO system presented in Figure 3. The reduction of
metal oxides with H2 is less exothermic than its corresponding
reaction with CO. Moreover, equilibrium of the water-gas
reaction will occur and shift to the right at lower temperatures.
This shows that H2 at high temperatures is a better reducing
agent than CO for oxygen carriers. Solid fuel pyrolysis and
gasification may produce some CH4 in the reducing stream.
Thermodynamics shows that CH4 could be a better reducing
agent than either CO or H2 at high temperatures. The tendency
Energy & Fuels, Vol. 20, No. 5, 2006 1843
of CH4 decomposition to result in oxygen carrier deactivation
by carbon deposition may be largely eliminated under higher
partial pressures of H2O and CO2 in the reducer.
The main constituent of solid fuels is carbon. The possibility
of directly reducing metal oxides with carbon can also be
calculated using thermodynamic theory.45 All of these reduction
reactions with carbon are heterogeneous reactions with only one
gaseous species included. On the basis of thermodynamics
theory, if the gas (CO2) is ideal, the equilibrium constant
expressions for these reactions cannot include the solid phase
because of the fact that pure solid phases are nearly equal to
unity for moderate pressures. Thus, a phase diagram relating
the partial pressure of CO2 and the temperature can be
constructed as shown in Figure 5. In this figure, the equilibrium
line for each oxygen carrier divides the 2-D area of temperature
and CO2 partial pressure (PCO2) into two zones in the temperature
range of interest. Metal oxides, reduced metal oxides or metals,
carbon, and CO2 are present simultaneously only at the
equilibrium line correlating the reaction temperature and CO2
partial pressure. Above this equilibrium line, metal oxides and
carbon can have a stable existence. Under this equilibrium line,
metal or reduced metal oxides can have a stable existence. The
temperature and CO2 partial pressure are the parameters that
control the extent of the reaction and its direction. Figure 5
indicates that the CO2 partial pressure (PCO2) of Cu-based, Nibased, and Co-based oxygen carriers can reach above 103 so
that 99.999% purity of the CO2 stream can be assured in view
of the reduction of a metal oxide directly by carbon in the
temperature range of interest. Above 1000 °C, Fe-based oxygen
carriers can only ensure the PCO2 to be above 10 with a CO2
stream purity of 90%. Generally, Mn-based oxygen carriers lack
practicality because of a very low PCO2, at about 10-7 for the
MnO-Mn reaction system and 10-3 for the Mn3O4-MnO
reaction system. It is interesting to find that Mn2O3-Mn3O4
has a higher equilibrium PCO2 at about 1017, but it still needs to
be rejected as an oxygen carrier because of a high N value,
which was mentioned previously.
3. Conclusion
In this study, a new CLC process of solid fuels, which is
based on one chemical loop by metal and its oxide, is proposed,
and its technical issues are intensively discussed, including the
selection of an optimum reactor and oxygen carriers.
On the basis of the requirement of recycling the oxygen
carrier between the oxidizer and the reducer and the properties
of the reaction occurring inside the reactors, an interconnected
CFB with three loop seals was envisioned. The riser of the CFB
was used as the oxidizer, and a larger loop seal was used as the
reducer and separator, which was operated in bubbling fluidization or moving bed mode for the reducer and in turbulent
fluidization mode for the separation of ash and the oxygen
carrier. The other two small loop seals were used for system
pressure balance.
On the basis of the physical properties of oxygen carriers
and a thermodynamics analysis of the reduction reaction between
oxygen carriers and solid fuels, CuO-Cu-, NiO-Ni-, and
CoO-Co-based oxygen carriers were determined to be the
optimal oxygen carriers for development of the CLC of solid
fuels. They all have larger oxygen transfer capabilities. Cubased oxygen carriers are the only choice that may make the
reducer self-sustaining or autothermal with its exothermic
properties during reduction in these three candidates of oxygen
carriers. The exothermic enthalpy property of the Cu-CuO
oxygen carrier in its reduction process would be beneficial for
1844 Energy & Fuels, Vol. 20, No. 5, 2006
Cao and Pan
Figure 5. Variation of the thermodynamic equilibrium factor for MexOy-C as a function of temperature (calculations based on data from ref 45).
simplifying the operation of the reducer. The agglomeration
tendencies of Cu-based oxygen carriers can be eliminated by
decreasing the operating temperature because of its high
reactivity even at the temperature range of interest in the reducer
(600-900 °C). Thermodynamic analysis indicated that CO2 can
be concentrated and purified to at least 99% purity for the gassolid reaction mode (reduction of the oxygen carrier by
gasification products such as CO and H2) and even higher for
the solid-solid reaction mode (reduction of the oxygen carrier
directly by solid fuels) on the basis of the selected oxygen
carriers.
Generally, Mn-based oxygen carriers have several disadvantagesslower oxygen transfer capability (Mn2O3-Mn3O4, Mn2O3MnO, and Mn3O4-MnO), thermodynamic limitations of purify
ing the CO2 stream (Mn3O4-MnO and MnO-Mn), a lower
melting point (Mn3O4), or a larger endothermic reduction
enthalpy (Mn2O3-Mn, Mn3O4-Mn, MnO-Mn, and Mn3O4MnO). Fe-based oxygen carriers have the disadvantage of a
larger endothermic reduction enthalpy and lower reactivity.
Acknowledgment. This paper was prepared by the Institute for
Combustion Science and Environmental Technology of Western
Kentucky University with support, in part, by grants made possible
by the U.S. Department of Energy SBIR program (DE-FG0204ER84036). The authors also acknowledge Dr. Kunlei Liu of the
University of Kentucky (CAER) for providing good suggestions
and comments in this study.
EF050228D