AIChE Journal Perspective Article

Chemical Looping Technology Platform

Liang-Shih Fan*, Liang Zeng1

and Siwei Luo

Department of Chemical and Biomolecular Engineering

The Ohio State University

Columbus, Ohio 43210

U. S. A.

To whom correspondence should be addressed (fan.1@osu.edu)

1Current address: School of Chemical Engineering and Technology, Tianjin University, China

Introduction

The term “chemical looping” was first used by Ishida et al. (1987) in their description of

a high exergy efficiency combustion process that engages redox reactions employing metal

oxides as a reaction intermediate.1

The use of metal oxides as an intermediate in the redox

reactions as applied to processes that are of non-combustion in nature, however, has not been

uncommon and can be traced to the work in early 1900s.2,3

In this perspective article, the term

“chemical looping” is used in a broad sense to describe the nature of a reaction scheme in which

a given reaction is decomposed into multiple sub-reactions using chemical intermediates that are

reacted and regenerated through the progress of sub-reactions.4 The chemical intermediates

considered are limited to metal oxides. Further, the sub-reactions are ideally configured in such a

manner that the exergy loss of the process as a result of this reaction scheme can be minimized.

This reaction scheme concept is rooted on the second law of thermodynamics as applied to the

reduction of the process irreversibility and hence the enhanced exergy efficiency of the process.

The application of this exergy concept for combustion systems was reported earlier by Knoche

and Richter (1968, 1983).5,6

In the work of Ishida et al. (1987), they employed metal oxides

like NiO, CuO, Mn2O3, and Fe2O3 as chemical intermediates for combustion reaction

applications.1 The unique feature of the metal oxide combustion scheme is that CO2 becomes an

easily separable combustion product, and thus the requirement of the parasitic energy in

separating it is low. Such a CO2 separation feature has been deemed important in recent years

particularly in consideration of the world concerns over the climate change.

The growing interest in the chemical looping technology has been significantly

increasing as evidenced from a sharp increase in the number of publications on this topic from

Google Scholar since early 2000s to date as given in Figure 1. The increase in the interest in

chemical looping applications using metal oxides beyond combustion is also apparent due to the

potential economic benefits and the commercial implications of the chemical looping processes.

Specifically, the chemical looping concept can evolve into platform technologies that yield major

impact on the formation of a variety of products that influence the fuel and chemical industries

beyond the utility industry.

In this article, the types of the chemical looping reactions are described along with the

evolution of the chemical looping technology. It is followed by a discussion on the advantages of

the chemical looping scheme compared to the conventional scheme in process applications. The

formulation, physical and chemical properties, morphological variations and the ionic diffusion

phenomena of the pure and composite metal oxide materials used as oxygen carriers during their

redox conditions are elaborated. Of equal importance to the oxygen carrier behavior in the

chemical looping technology development are the gas-solid contact modes and types of reactors

used in the processing of these oxygen carriers. Several Ohio State University platform

technologies developed in recent years are introduced and their operational efficiencies,

economics and process applications are also illustrated.

Two Groups of Chemical Looping Reactions

There are two groups of chemical looping reactions using metal oxides that can be

categorized based on the current applications: Group A refers to chemical looping reactions

involving CO2 generation or separation as given in Figure 2a. Group B refers to chemical

looping reactions involving non-CO2 generation as given in Figure 2b. Comparing to Group A,

Group A has been relatively extensively studied.4,7

It includes two types of operation. Type A-I

can be represented by the reactions between metal sulfate and metal sulfide or typically

represented by the reactions between metal oxide and metal. In the process of metal oxide

reduction reactions that take place in the reducer, the carbonaceous feedstock in gas, liquid or

solid form is fully oxidized to CO2. The reduced metal oxide is then regenerated in the

oxidizer/combustor. Depending on the oxidizing agent used for reduced metal oxide regeneration,

different chemical looping products can be generated. For example, when the oxidizing agent is

air, heat and/or electricity can be produced. When the oxidizing agents are steam and/or CO2,

hydrogen and/or CO can be produced. The regenerated metal oxide is then recycled to the

reducer with the chemical looping reactions repeated. In Type A-II, the metal in the metal oxide

is typically calcium. The Type A-II reaction is represented by that between metal carbonate (e.g.,

CaCO3) and metal oxide (CaO), characterized by the carbonation and calcination reactions. That

is, the metal oxide undergoes carbonation reaction with CO2 in the carbonator to form metal

carbonate. Through heat addition in the calcination process, metal oxide is regenerated from

metal carbonate after CO2 is released. The regenerated metal oxide is then recycled to the

carbonator with the chemical looping reactions repeated.

While the metal oxide regeneration process is similar to that in Group A reactions, in

Group B, the reduction process is different in that it does not produce CO2. In Group B, there are

three types of the reduction reactions in which Types B-I and B-II perform partial oxidation

reactions with the carbonaceous feedstock to form, respectively, syngas, i.e., CO and H2, and

chemicals, e.g., ethylene. The Type B-III reaction does not involve the use of any carbonaceous

feedstock; rather, it uses heat from solar or nuclear energy to decompose metal oxide to metal

and oxygen in the reducer. The metal generated then moves to the oxidizer/combustor for

regeneration to metal oxide, which is then recycled back to the reducer. Water splitting to

produce hydrogen can be performed when the regeneration step uses steam as the oxidizing

agent. The fact that the Type B-III operation does not involve the use of fossil feedstock reflects

that the chemical looping systems can be operated in a fully renewable fashion.

In the following section, some notable technology advances using the chemical looping

principles for Group A and Group B reactions are highlighted to indicate the major milestones of

this technology development.

Evolution of Chemical Looping Technology

The use of the chemical looping concept for Group A reactions as a technology was seen

in the work of Howard Lane as early as 1904.3,8-10

The Lane Process, shown in Figure 3, was

operated with periodic switching of syngas derived from coal gasification and steam over fixed

bed reactors. Syngas was first used to reduce iron ores in the fixed bed, after which the inlet gas

stream was switched to steam and the bed was then performing the steam-reduced iron reaction

to produce hydrogen. Hydrogen plants based on such redox process were initially applied for

filling hydrogen balloons. They were then constructed throughout Europe and the United States

to produce 24 million m3 of hydrogen annually by 1913. However, there were two major issues

that hampered the efficient applications of this steam-iron process, i.e., incomplete conversion of

reducing gas and low recyclability of the iron ores. With availability of oil and natural gas in the

1940s, the steam-iron process for hydrogen production became less competitive and was then

phased out.

In the 1950s, a chemical looping scheme was proposed by Lewis and Gilliland using iron

and copper oxides as the looping particles.11,12

The purpose of their study was to examine the

feasibility of the chemical looping scheme for CO2 generation for the beverage industry. There

were two fluidized bed reactors used for this scheme. The first reactor was a CO2 generator, in

which metal oxide was reduced by solid fuels such as coal with steam and/or CO2 used as

fluidizing gas. The reduced metal oxide was subsequently regenerated in the second reactor by

air. There was no plant data reported that substantiated this process. Other types of reactors such

as multi-tray fluidized bed and moving bed were mentioned as a CO2 generator with copper

oxide as an oxygen carrier. They proposed gas-solid countercurrent flow pattern with iron oxide

swinging between Fe2O3 and Fe3O4.11

The swinging between CuO and Cu2O was also indicated,

which was noted as the chemical looping with oxygen uncoupling (CLOU) in the recent

literature.12

In the 1970s, the Institute of Gas Technology (IGT) developed the HYGAS Process

which converts coal and hydrogen into synthetic natural gas (SNG) by the methanation

reaction.13

The hydrogen used in the HYGAS Process was generated by the high pressure steam-

iron process using the syngas as feedstock produced from coal gasification. Two-stage,

countercurrent fluidized bed reactors were used for both the reducer and the oxidizer in order to

enhance the fuel gas and iron oxide conversions as well as mass and heat transfer. The efficiency

of the steam-iron process in the HYGAS was much higher than that of Lane Process.

Synthesized iron oxide particles consisting of 4% silica, 10% magnesia, and 86% hematite ore

were used. Both the reactivity and the strength of particles for this synthesized iron oxide were

improved over the iron ore used in the Lane Process. The HYGAS Process was demonstrated at

pilot plant scale but was not commercialized. Drawbacks of the HYGAS process include low

metal oxide conversions, low redox rates, and incomplete syngas conversions.

In 1960s and 1970s, the CO2 Acceptor Process was developed by the Consolidation Coal

Company and later Conoco Coal Development Company with pilot plant operation conducted.14

It was envisioned also to produce the synthetic natural gas from pulverized lignite or

subbituminous coal. In the gasifier, syngas or hydrogen was produced through the reactions of

coal and steam with CaO added to the reactor to remove CO2 for enhancing hydrogen generation.

Like the HYGAS Process, the CO2 Acceptor Process was not commercialized also in part due to

the economic reason caused by low market price of natural gas and high capital cost requirement

for the processes. Further, bituminous coals were difficult to handle and major agglomeration

problems caused by the low temperature eutectic formation among fly ash and calcium

compounds in the gasifier and the regenerator were evidenced, but could be resolved with the

operational temperature and solid composition control. It was also noted that the CaS-CaSO4

redox cycle observed in the CO2 Acceptor Process was different from the typical redox cycles

based on metal/metal oxide. Following the concept of the CO2 Acceptor Process, the HyPr-Ring

Process was developed in Japan for the hydrogen production from coal in 2000s.15,16

In the HyPr-

Ring Process, coal was fed along with calcium oxide, steam, and oxygen to the gasifier. Calcium

oxide captured CO2 generated in the water-gas shift reaction and drove the water-gas shift

reaction forward, resulting in a product gas stream of ~90% H2 mixed with methane. The solids

from the gasifier consisted of mostly spent CaO sorbents (CaCO3) and some unconverted carbon

was introduced to the regenerator along with oxygen. The heat generated from combustion of the

unreacted carbon allowed the calcination reaction to be carried out for CaO regeneration while

releasing a high purity of CO2 for sequestration.

For Group B chemical looping reactions, in the 1980s, Keller and Bhasin produced

ethylene and ethane from methane in a redox process known as oxidative coupling of methane

(OCM).17

In the OCM approach, metals oxides were sequentially and separately reduced by

methane and re-oxidized by air. In major efforts on adopting the chemical looping concept for

producing chemicals en route to producing liquid fuels via OCM followed by oligomerization,

ARCO Chemical in 1980s conducted research and developed a pilot scale system, using

manganese composite metal oxides to convert natural gas to olefins.18,19

The performance of the

oxygen carrier materials and the looping reactor design are key to good product selectivity and

reactant conversions. In this system, low acidity materials such as silica when used as a support

for manganese formed braunite Mn7SiO12 that increased the product selectivity. It was observed

that an addition of alkali components such as sodium pyrophosphate further improved the

selectivity. These OCM reactions took place in fluidized-bed reactors with a short gas-solid

contact time that was found to avoid further oxidation of the desired product to COx. The ARCO

activities were halted in 1990 due to in part the economic reason caused by the increased price in

natural gas.

The maleic anhydride production from butane catalyzed by oxidized vanadium

phosphorous oxide (VPO) which also serves as an oxygen carrier was developed by DuPont in

the 1980s and 1990s.20

The VPO particles looped around the circulating fluidized bed (CFB)

reactors and swang between oxidized and reduced states. This process was developed from the

bench scale to the pilot scale, and finally to the commercial scales. Its commercial operation,

however, was not successful due to the scale-up effects of the CFB reactor on the reactant

contact time and the oxygen carrier holdup in the CFB system.21

Specifically, the insufficiency

of the oxygen carrying capacity of the VPO particles coupled with their attrition or breakage

behavior due to high particle through flow requirements in the CFB system contributes to the

inadequacy for sustainable operation of this process at the commercial scale. The oxidation

reaction of butane then relied in part on molecular oxygen introduction exteriorly. The outcome

of the DuPont commercial operation underscores the importance of the viability of the oxygen

carriers in the CFB system operation for the chemical looping process applications.

In 1990s, the partial oxidation of methane to syngas using CeO2 in a chemical looping

scheme was attempted.22

CeO2 was found to be an oxygen carrier with a thermodynamically

favored selectivity for full CH4 oxidation towards CO and H2. Its slow kinetics and high carbon

deposition properties could be resolved by the use of various types of dopants.23-25

The solar

assisted metal oxide thermochemical looping processes was also initiated that coproduced iron

and syngas using partial oxidation of methane with Fe3O4 in a high-temperature solar reactor.26

It

was found that although fluidized particles were effective in absorbing the solar radiation, the

high cost in the solar energy conversion, the sintering and recrystallization of metallic iron, and

low methane conversion contributed to the slow development of such solar based chemical

looping process. Metal oxide redox pairs such as ZnO-Zn and Fe3O4-FeO/Fe were used for water

splitting solar thermochemical cycles.27, 28

Recently, more elaborate materials such as

Co0.85Fe2.15O4 and Fe2O3 composite thin films supported on ZrO2 synthesized using the atomic

layer deposition (ALD) indicated encouraging activities for isothermal water splitting reactions

in solar-based redox cycle.29

Since early 2000s, several pilot-scale units of chemical looping processes were

constructed for renewed large-scale demonstration of the technology. Notably, the chemical

looping combustion is being demonstrated at 1 MWth at Technical University of Darmstadt in

Germany with ilmenite as the oxygen carrier using solid fuels/coal as feedstock. The reactors for

both the reducer and the combustor are circulating fluidized beds.30

The high pressure pilot

Syngas Chemical Looping (SCL) Process of the Ohio State University (OSU) are being

demonstrated at National Carbon Capture Center (NCCC) at 250 kWth – 3MWth to produce

hydrogen using syngas derived from coal gasification or natural gas as the feed.31

The reducer

and oxidizer are moving bed reactors while the combustor is a fluidized bed. Two pilot plants are

being developed for calcium looping operations to capture CO2 from flue gas stream. One is a

1.7 MWth demonstration pilot plant in Spain; operated by a consortium led by the mining

company HUNOSA.32

Both the calcination and carbonation reactors are a circulating fluidized

bed. The other is a 2 MWth plant in Taiwan, operated by ITRI (Industrial Technology Research

Institute) and is conducted using both the calcination-carbonation and the calcination-

carbonation-hydration schemes.33

This is a demonstration of the Ohio State University’s CCR

(Calcination and Carbonation Reactions) Process technology.34

ITRI’s CO2 capture operation is

linked to a cement production plant nearby. Further, the installation at the Technical University

of Darmstadt used for the chemical looping combustion is also being used for operation in the

calcium looping mode with one of the circulating fluidized beds reactor performing carbonation

while the other circulating fluidized bed performing calcination, in a manner similar to the pilot

unit in Spain.35

Motivation in Modern Chemical Looping Process Development

The modern chemical looping approach is motivated by three factors. They are 1) the

pressing needs in mitigating the climate change due to CO2 emissions with efficient and

cost/energy effective CO2 capture method in the combustion of fossil fuels;36-39

2) the

optimization of reaction schemes that can minimizes the exergy loss involved in the

chemical/energy conversion system;5,6,40,41

3) the requirement of novel process intensification

strategy for reduction of capital and operating costs.42

These factors are elaborated below:

CO2 Capture

Current coal combustion power plants require considering the capture of CO2 in the flue

gas besides the removal of such environmentally hazardous air pollutants as sulfur oxides,

nitrogen oxides, fine particulates, and mercury. The total amount of CO2 produced from coal-

based power plants accounts for more than 40% of all anthropogenic CO2 emissions.43

The CO2

management involves three steps: capture including separation and compression, transportation,

and sequestration. Among the three steps, the CO2 capture is the most energy-consuming one.44

The CO2 represents ~15% of the flue gas stream from coal combustion power plants and the

existing techniques for CO2 capture from PC power plants include the well-established

monoethanolamine (MEA) scrubbing technology and chilled ammonia process.42

Similar to solvent based CO2 scrubbing techniques, high temperature sorbents such as

limestone, potassium carbonates, lithium silicates, and sodium carbonates can be used to capture

CO2 in the flue gas at elevated temperatures.45,46

With good heat integration, these solid sorbents

can be effective in the CO2 capture. One heat integration scheme is of particular relevance,

which is based on the Type A-II chemical looping as discussed earlier using hydrated lime,

natural limestone, or reengineered limestone sorbents at 600–700°C for CO2 capture.47

In the

process, both CO2 and SO2 in the flue gas are captured by the calcium-based sorbents in the

carbonator operated at ~650 °C, forming CaCO3 and CaSO3/CaSO4. The carbonated sorbent,

CaCO3, is then regenerated to calcium oxide (CaO) sorbent in the calciner at 850–900°C,

yielding a pure CO2 stream. In the Type A-I chemical looping, as discussed earlier, the oxygen

carrier particle is reduced by the fuel in the first reactor (reducer or fuel reactor), producing CO2

and H2O. The reduced oxygen carriers are then moved to the second reactor (oxidizer, combustor,

or air reactor) where combustion takes place with air and the reduced oxygen carrier is converted

to its original oxidation state. During the oxidation step, a large amount of heat is generated,

which can be used for electricity generation. Since air and fuels are converted in separate

reactors, the CO2 generated from the reducer is only mixed with steam and is readily separable.

The capital and energy-intensive CO2 separation step can be avoided.

Energy Conversion Efficiency

The indirect fuel conversion in two separate reactors is advantageous from a

thermodynamic point of view. The exergy of a system refers to the maximum amount of usable

work in bringing the system into equilibrium.48,49

The ambient environment is generally used as

the reference state. The exergy loss in the system could be reduced by recuperating the low grade

heat while producing a larger amount of high grade heat. Other than heat recovery, a proper

choice of chemical intermediates for a chemical looping process can further decrease the exergy

loss for an energy conversion process.4

As an example, Scheme I in Figure 4 represents a

traditional hydrogen production route where carbon is gasified and coupled with the water-gas

shift reaction. There is a 12% exergy loss in the gasification/partial oxidation step. The water-gas

shift reaction leads to another 8.8% exergy loss. An alternative approach using the Fe-Fe3O4

chemical looping method is considered in Scheme II which consists of reactions as follows:

C + 0.395Fe3O4 + 0.21O2 1.185Fe + CO2 (1)

3Fe + 4H2O Fe3O4 + 4H2 (2)

A small amount of O2 is used in the first reaction to neutralize the heat of reaction. Due to

the generation of Fe, a medium exergy rate chemical, from Fe3O4, a low exergy rate chemical,

much less exergy loss occurs in Step 1 (23.1 kJ) compared to the traditional gasification step

(48.8 kJ). The steam-iron reaction in Step 2 of Scheme II only needs little amount of low grade

heat and a zero exergy loss is achievable in this step of the scheme. As a result, the exergy loss

for H2 production is reduced to one fourth using the chemical looping process in Scheme II than

using the traditional method in Scheme I. It should be pointed out that the results presented

above are based on a set of assumptions. Nevertheless, due to the common assumptions used in

assessing the process options, the results obtained serve as a good guide for relative comparisons

of these options.

Process Intensification and Cost Reduction

The existing coal gasification and natural gas reforming processes for coal to liquid (CTL)

application involve elaborate unit operations, which incur significant amount of auxiliary energy

and capital costs. Simplification of these processes through process intensification is thus desired.

The chemical looping approach offers a viable process intensification example for such

applications. Figure 5 represents a two-reactor chemical looping gasification of coal to syngas

(CTS) system consisting of a co-current moving bed reducer and a fluidized bed combustor.50

In

the reducer, coal, natural gas, water/steam and the light hydrocarbons from the liquid fuel

synthesis system are introduced together with composite iron based oxygen carrier. Both gas

and solid streams flow downward along the reducer where oxygen carrier partially oxidizes the

fuel to syngas while it is reduced from Fe2O3 to a mixture of Fe and FeO. The combustor, a

fluidized bed reactor, regenerates the oxygen carrier to Fe2O3 and combusts a small amount of

coal using air. The heat released from these reactions is carried to the reducer by the regenerated

particles to compensate the heat needed for coal gasification and natural gas reforming. High

quality hydrogen rich syngas can be directly obtained from fuel gasification in the reducer and

suitable for liquid fuel synthesis. The CTS process eliminates the need for air separation unit,

water gas shift reactor, and acid gas removal unit from conventional CTL process, and thus,

significantly reduces the capital and operational costs for hydrogen rich syngas production.

The above discussion indicates that the chemical looping processes possess particular

advantages from the viewpoints of carbon capture, exergy loss minimization, and process

intensification. While extensive research is on-going globally to substantiate the commercial

potential of the chemical looping processes, two key aspects of the processes that dictate the

successful applications of this technology have received considerable attention: one is concerned

with the oxygen carriers51, 52

and the other is concerned with the reactor configuration4,7

as given

below.

Oxygen Carriers

For Type A-I and Type B chemical looping systems, in performing the reduction reaction,

the lattice oxygen in the metal oxygen carrier is transferred from the carrier lattice to oxidize the

fuels in either full oxidation or partial oxidation condition. For either oxidation condition, the

selection of the carrier materials and of the method for their synthesis requires a thorough

knowledge of the performance characteristics required of the oxygen carrier particles including

equilibrium phase behavior of metal oxides, kinetics on redox reactions, temperature and

pressure effects, mechanical properties such as attrition, recyclability, heat transfer properties,

process configuration, intended desired reaction products and formation of unintended products

as in carbon deposition. Furthermore, for looping particles to be commercially viable, the

materials and synthesis costs require to be low.

Selection of metal oxides can be based on the Ellingham diagram where the Gibbs free

energy of reaction for metal oxides varies with the temperature.4 Based on the Ellingham

diagram, metal oxide materials can be grouped into several zones depending on their applications

as shown in Figure 6. In the figure, it is seen that the zones are bounded by three key lines

represented by the following reactions:

Reaction line 1: 2CO+O2=2CO2 (3)

Reaction line 2: 2H2+O2=2H2O (4)

Reaction line 3: 2C+O2=2CO (5)

Specifically, materials in Zone A given in the figure have strong oxidizing properties and can

work as oxygen carriers for both full oxidation and partial oxidation. The input molar ratio of the

reacting oxygen that can be transferred from the oxygen carrier to the reacting fuel species

determines whether the reaction is at the full or partial oxidation condition. At a given feedstock

flow rate, less oxygen is required for partial oxidation than for full oxidation. For materials in

Zone B given in the figure, the intended reaction is partial oxidation and an excessive amount of

input oxygen carrier does not yield full oxidation. Metal oxides in Zone C cannot be used as

oxygen carriers and are considered as inert materials. Although the Ellingham diagram was

developed for single metal oxide, a similar analysis can be applied to complex metal oxides. It

should be noted that the Ellingham diagram only provides the thermodynamic indication as to

the possibility of metal oxides that can serve as oxygen carriers. Other factors such as reaction

kinetics as indicated earlier need also to be considered.

Most pure oxygen carrier materials deteriorate over multiple reduction-oxidation reaction

cycles.53,54

Pure oxygen carrier material like CaSO4 with low cost and high oxygen carrying

capacity has been extensively tested. However, portion of sulfur in CaSO4 will leak during the

reduction reaction of CaSO4 to CaS, resulting in sulfur emissions, CaO accumulation in the

system, and decreased CaSO4 reactivity.55

Most common pure oxygen carrier materials, however,

are metal oxides where the deactivation over multiple reduction-oxidation reaction cycles is

often accompanied by deterioration of morphological properties. In the reduction reaction for

metal oxides, the oxygen carrier is reduced with the formation of the oxidized gaseous product

that leaves the solid surface to the bulk gas phase. Macroscopically, as the reduction reaction

induces the volume shrinkage of oxygen carrier particles, it leaves behind porosity that gives

direct exposure of the gas phase reactants with the reduced solid materials. At the grain level,

further reduction reaction can proceed with the ionic diffusion for the gas-solid reaction. In the

oxidation reaction for metal oxides, on the other hand, it involves volume expansion with

dominated transport processes being ionic diffusion. Indeed, despite observed low surface area

and pore volume for the reactive oxygen carrier, ionic diffusion processes for the dominant

diffusion species are responsible for the desired redox kinetics in the chemical looping

reactions.56

The ionic diffusion can take place in three modes: 1) outward diffusion mode; 2) inward

diffusion mode; and 3) mixed diffusion mode. During the oxidation reaction of pure Fe to Fe2O3,

in the wüstite phase and the magnetite phase, the outward diffusion of iron cations is the

dominant mode; while in the hematite phase, the inward diffusion of oxygen anion is the

dominant mode.57

The diffusion processes can be altered by the addition of a support material

and anti-sintering dopant.58-60

The addition of TiO2 to iron oxide facilitates the diffusion of

oxygen anions resulting in the superior reactivity over multiple redox cycles.56

The oxygen ion

diffusion barrier is significantly lower in FeTiO3 compared to that in FeO. This behavior

illustrates the faster kinetics of ilmenite compared with FeO recognizing that ilmenite ores are

regarded as nature-occurring TiO2 support materials.56,61,62

The faster kinetics were attributed to

the superior ionic diffusivity of the solid materials.

The type of the support on the ionic diffusion behavior can further be elaborated by

examining the redox effects on the morphological structure of the two metal oxide particle

systems made up by Fe2O3/TiO2 and Fe2O3/Al2O3. Initially, two metal oxides in each of the

particle systems are well mixed. At the end of fifty cyclic reduction-oxidation reactions, a core-

shell structure is formed in the Fe2O3/Al2O3 system while in the Fe2O3/TiO2 system, a well-

mixed condition for the metal oxides remains. In the Fe2O3/Al2O3 system, the redox reaction

proceeded with diffusion of iron cations, from the particle interior to the particle surface, forming

a shell with Al2O3 in the core.63

On the other hand, in the Fe2O3/TiO2 system, the redox reaction

proceeded with diffusion of oxygen anions and the active oxygen carrier material expanded and

shrunk locally during the redox reactions, leading to the Fe2O3/TiO2 particle maintained in the

mixture form. In chemical looping processes, it is desirable to have oxygen carriers that continue

to be highly reactive over a long term of reactions. The formation of the core-shell structure

manifests a phase separation which is undesirable for recyclability of metal oxide carriers.

Further to the supported iron oxides, a series of ferrites, MxFe3-xO4 (M=Ni, Co, Zn, Cu,

0<x<1.5) have been tested as oxygen carriers. Most of the ferrites have a spinel structure. The

NiFe2O4 synthesized from Fe2O3 and NiO by ball milling and high temperature calcination

exhibited improved reactivity and recyclability for hydrogen production from the steam iron

reaction.64

Nickel ferrite has also been tested as oxygen carrier and catalyst for the partial

oxidation of methane to syngas. Since nickel is known to be an effective catalyst for steam

methane reforming, the idea of using the metal oxides in the nickel ferrites as oxygen carriers in

the first stage and the metallic nickel generated in the first stage as a catalyst for the second stage

steam methane reforming is interesting. However, the long term use of these materials is of

concern when considering ion diffusion and volume expansion during cyclic reduction-oxidation

reactions. In well mixed active metals of Fe and Ni, a Fe2O3-rich shell and NiO-rich core

structure forms after the oxidation reaction in each redox cycle.65,66

It is because that Ni has a

faster oxidation rate and smaller volume expansion rate than Fe during oxidation. That is, Ni ions

react with oxygen first and remain in the core, whereas Fe ions tends to diffuse out of the core

driven by the ionic concentration gradient of Fe and a high volume expansion rate of Fe to

Fe2O3.

Iron oxide has been extensively studied also microscopically as oxygen carrier materials.

The changes in its morphology at the nano or micro scale during the redox reactions are expected

to affect its macroscopic properties, reactivity and recyclability in the chemical looping systems.

Oxidation of pure Fe particles occurs via inward diffusion of oxygen ions and outward diffusion

of Fe ions forming oxides. The outward diffusion of the Fe ions is faster as compared to the

inward diffusion of oxygen ions.67,68

This ionic diffusion pattern creates a porous center shown

in Figure 7(b) relative to the random porosity initially seen as given in Figure 7(a), and about 25%

volume expansion.66

As shown in Figures 7(c) and (f), during Fe oxidation, two types of

nanostructures – nanopores and nanowires – are developed on the particle surface.66

The

formation of these structures is due to a stress-driven mass transport, caused by the volume

expansion during oxidation.69

These nanostructure formations are heavily dependent on the

positive or negative grain/crystallite surface curvature. They disappear when oxidation is

followed by reduction, which occurs with outward diffusion of oxygen ions, leading to the

formation of vacancies. Aggregation of vacancies creates micropores throughout the particle

volume and causes the disappearance of these nanostructures. The voids created inside the

particles during the first oxidation-reduction cycle lower the surface stress during the subsequent

cycles. Hence, these nanostructures are only observed during the first cycle. Fe particles having

undergone multiple redox cycles often form irreversible agglomerates because of inevitable

sintering effects. This effect causes pure Fe particles to be deactivated over time during redox

chemical looping processes.

In the case of a binary system like an Fe-Ti alloy, increased number of redox cycles

increases the porosity of the particles as shown in Figure 8. The addition of Ti assists in reducing

the sintering effect, enhancing the porosity which promotes the pore diffusion, and enhancing

lattice vacancies which promote the ionic diffusion within the particle. Hence, a binary system of

Fe and Ti can undergo multiple redox cycles while maintaining its reactivity over time. Under

high pressure oxidation-reduction cycles, high particle porosity within the metal oxides is

observed. These porosity effects corroborate the faster redox kinetics observed at high pressures

as compared to atmospheric pressures.

In certain oxygen carriers, molecular oxygen can be released from the oxygen carriers

with sufficient concentrations that can provide effective oxidation reactions for the fuels. The

metal oxides possess this property is noted as the CLOU (chemical looping with oxygen

uncoupling) materials. A high reaction rate for the oxidation reaction of the fuel is an advantage

of these CLOU materials.70, 71

Typical CLOU materials include CuO, Co3O4, and Mn2O3, which

have disadvantages in terms of melting point, environmental health and oxygen carrying capacity,

respectively. In recent years, binary metal oxides CLOU materials are studied.72,73

For example,

copper ferrites CuxFe3-xO4, with 0.3<x<1.5 were tested for methane partial oxidation. It had a

lower CH4 conversion and selectivity compared to iron oxides, but carbon deposition was

eliminated. The reduced phases consisted of Cu, Cu2O, CuFeO2, and/or Fe3O4, depending on the

reaction conditions. For these materials, the phase migration, together with the formation of Cu2S

and Fe2SO4 during the reaction with high sulfur content fuel, may lead to irreversible structure

changes and deteriorate the recyclability.

As indicated earlier, the variations in the ionic diffusivity and the morphological

properties play an important role in determining the recyclability of the oxygen carriers during

the reduction-oxidation cycles. As the structure of the perovskite materials is known to provide

excellent electron and ionic conductivities, considerable studies have been devoted to employing

perovskite materials, either alone or in combination with other active metal oxides or support for

use as oxygen carriers. Perovskite materials have a general formula of A(II)B(IV)O3, and are of

oxygen nonstoichiometry and with high oxygen diffusivity.74-78

For example, CaMnO3-δ that can

be synthesized from calcium oxides and manganese oxides was found to have desirable

reactivity, strong mechanical strength, and high melting point. However, the perovskite structure

of CaMnO3-δ is not stable and tends to decompose to CaMn2O4 and Ca2MnO4-δ in reducing

environment. To enhance the stability of perovskite structure in order to improve its performance,

several types of transitional metals including Fe, Ti, Mg, Cu and La are used as partial

substitution of atoms in Mn sites.79,80

Substitution of part of Mn with Ti has indicated improved

oxygen carrier performance. It was found that the substituting metal ions should be of a similar

size as Mn; otherwise, segregation of metal ions could take place in the particle. Thus, Mg was

found to be an undesirable candidate as substituting atom for B site.

In addition to the type of CaMnO3-δ perovskite materials, the type of AFeO3 (A = La, Nd,

Eu) perovskite materials has also been synthesized and tested.81

Two stages of the reaction

process were observed during the reaction of AFeO3 with methane. A significant amount of CO2

was formed at the first stage while a high purity syngas was generated at the second stage.

Among these three perovskite materials, the highest yield of syngas was obtained with LaFeO3

as the oxygen carrier. By adding a small amount of other components, the performance could be

further improved. More metals can be included in the substitution for perovskite materials as in

La1-xSrxMyFe1-yO3 (M = Ni, Co, Cr, Cu).82

Complex metal oxides materials have also been used

for chemical looping reactions with specific applications to hydrogen generation.83

The

fundamental nature concerning the interplay of the various metal ions for complex materials is

more difficulty to be probed aside from the complexity involved in the synthesis and the cost

associated with their practical use.

Reactor Configurations

Reducer and oxidizer/combustor are main reactors for chemical looping systems.

Considering Type A-I and Type B-I chemical looping systems with iron-based metal oxides as

oxygen carriers for the discussion of reactor configurations, it is recognized that the combustor

should be conducted in a fluidized bed mode because of its good mass and heat transfer

properties. Relative to the reducer reactions, the reactions in the combustor are intrinsically fast

and thermodynamically favored. The reduction reactions are relatively slow and limited by

thermodynamics. The heat of reactions for reduction is also relatively small in most cases of the

chemical looping applications. Therefore, for optimum design of the reducer, other types of

reactors than the fluidized bed also exist that can yield the high solid and gas conversions

through a thermodynamically favored gas-solid contacting pattern. Thus, the design of chemical

looping reducers can mainly be classified on the basis of the fuel-oxygen carrier contacting

patterns, including mixed contact, co-current contact, and countercurrent contact. The flow

pattern in the mixed contact can be represented by a single fluidized bed, while that in the co-

current contact and countercurrent contact can be represented by a moving bed or a series of

fluidized beds. It is noted that fixed beds can also be used for gaseous feedstock when proper

control is made of the feeding point and product discharging point for the reducer operation.

As indicated, fluidized bed reducers with the mixed flow pattern and accompanied good

heat and mass transport properties have been adopted widely for chemical looping combustion.

Small scale systems studied include IFP 10-kWth CLC system (France), Chalmers University of

Technology 10- and 100-kWth CLC system (Sweden), Instituto de Carboquimica 1.5-, 10-, and

50-kWth CLC system (Spanish), Vienna University of Technology 120-kWth CLC System

(Austria), Korea Institute of Energy Research 50-kWth CLC system (Korea), Southeast

University 1-, 10-, and 50-kWth Solid Fuel CLC system (China), Western Kentucky 10-kWth

CLC system (U.S.), and University of Utah 100-kWth CLC system (U.S.).84-96

Among these

studies, the feedstock of primary interest has been solid fuels such as coal and gaseous fuels such

as syngas and natural gas. Factors that challenge the design of a reducer for a solid-fueled

chemical looping system include the kinetics of the solid fuel gasification and the solid fuel

residence time. In fluidized bed reducers, the fluidizing gas is usually CO2 and/or steam, which

also serves as an enhancing gas to promote solid fuel gasification kinetics. In order to extend the

residence time of the solid fuel such as char, the contact pattern in fluidized bed reducers can be

altered. For example, a spouted bed can be used for providing a longer residence time for solid

fuel reactions.92-94

In some systems, a carbon stripper was added to perform the separation and

recycling of unconverted char discharged from the reducer.87,88

However, this is a remedial step

in overcoming the char discharge to the combustor directly from the fluidized bed and hence

enhancing the carbon capture efficiency. The step is also to assist in the ash separation from the

oxygen carriers before their flow to the combustor. The well mixed and channeling behavior also

provides a direct path for leakage of the volatiles from the reducer which then requires a separate

volatile treatment outside the reducer.

There is a thermodynamic constraint in the oxygen carrier conversion using a fluidized

bed reducer. To illustrate it, consider the conversions of iron oxides as the oxygen carriers in the

fluidized bed reducer and the countercurrent moving bed reducer for CO combustion. The

comparisons can be made by examining the phase diagram of iron oxides in the presence of CO

and CO2 as shown in Figure 9. With a well-mixed fluidized-bed reducer the gas inside the bed is

rich in CO2 as in the bed outlet and thus, the oxygen carrier cannot be sufficiently converted as

reflected by the iron oxide phase diagram. As an example, in order to achieve a 99.99% CO

conversion from the fluidized bed reducer, the iron oxides cannot be reduced to an oxidation

state lower than Fe3O4 that corresponds to a solid conversion of no more than 11.1%. In contrast,

in a moving bed reducer with a countercurrent gas-solid contacting pattern, oxygen carrier

particles can exit from the reducer with lower oxidation states with a fresh CO feed

countercurrently flowing to it. Under the countercurrent moving bed operating condition, >99.99%

of CO can be converted and the solids can be reduced to FeO and Fe, corresponding to a

conversion of about 50%.97

The same principle can be applied to other gaseous fuels including

H2, CH4, and coal/biomass volatiles. For a given fuel input, the oxygen carrier circulation rate in

a countercurrent moving bed reducer is less than ¼ of a fluidized bed reducer. Further, ash

separation in a solid fueled countercurrent moving bed reducer can be readily achieved due to the

significant size difference between oxygen carrier particles and ash, which reflect the advantage

of the countercurrent moving bed operation for Type A-I applications.4

Thus, improved gas and

solids conversions over the fluidized bed can be achieved when a countercurrent moving bed is

used as the reducer for combustion applications.

For Type B-I applications for syngas production, a well-mixed fluidized bed reducer

possesses a widely varied residence time distribution, and hence yielding a mixed extent of

particle conversions with different oxidation states of the oxygen carriers. For iron based oxygen

carriers, high oxidation states give rise to lower syngas purity, while the low oxidation states

give rise to higher syngas purity, but tend to catalyze the reactions with carbon deposition. The

gas channeling in a fluidized bed can leak the unreacted fuel mixed with syngas at the reactor

outlet. In an alternative moving bed reactor with a co-current gas-solid flow pattern, the

residence time and the extent of the oxygen carrier conversion can be accurately controlled while

preventing carbon deposition. 50

At the inlet of the reactor, the high concentration of fuel feed

reacts with the high oxidation state of metal oxides that are highly oxidative. At the outlet of the

reducer, although reduced oxygen carriers may contain reduced metal, carbon deposition can be

prevented or minimized due to a low concentration of unreacted fuel and of the lattice oxygen in

the remaining metal oxides. When the reduced oxygen carriers are consisted of a redox pair in

Zone B given in Figure 6, a full oxidation of the fuel can be inhibited thermodynamically and a

high purity of syngas can be generated with a partial oxidation product. Thus, significantly

improved gas and solids conversions over the fluidized bed can also be achieved when a co-

current moving bed is used as the reducer for gasification applications.

Ohio State University Chemical Looping Technologies

The Ohio State University (OSU) has engaged in chemical looping technology

development for more than 20 years. One central feature of the OSU technology for Types A-I

and B-I is the use of the moving bed reducer coupled with the versatility under which gas and

solid reactants are introduced. When this feature is applied with iron, other metal oxide or their

composites-based oxygen carriers, the OSU technology is evolved readily as a technology

platform that is capable of taking various types of feedstock including coal, natural gas, liquid

hydrocarbon, biomass, petroleum coke, and char and converting them directly to such products

as heat and electricity, syngas, hydrogen, chemicals and/or their combination. The CO2 capture

can take place in this technology platform. There are six OSU chemical looping technologies

that have been developed and successfully tested at the sub-pilot scale or pilot scale operation

including Calcium Looping Process (CLP), Calcination-Carbonation Reaction (CCR) Process,

Syngas Chemical Looping (SCL) Process, Coal Direct Chemical Looping (CDCL) Process,

Shale Gas-To-Syngas (STS) Process, and Coal-To-Syngas (CTS) Process.31,34,97-103

In the

following, four processes are introduced:

Calcium Looping Process CLP – Type A-II

The Calcium Looping Process (CLP) using gaseous fuels simplifies the production of

hydrogen (H2) by integrating various process steps in a single stage reactor including the water-

gas shift (WGS) reaction with in situ carbon dioxide (CO2), sulfur, and halide removal from the

synthesis gas at high temperatures. In the CLP Process, the calcium sorbent is used to overcome

the equilibrium limitation of the WGS reaction, thus reducing the amount of steam required to

near stoichiometric levels. In addition to removal of CO2, the process achieves simultaneous

removal of sulfur (H2S + COS) and halide (HCl/HBr) impurities by forming calcium carbonate

(CaCO3), sulfide (CaS) and halide (CaX) respectively. The CO conversion is flexible to produce

H2:CO ratios varying from 0.5 to 20, while reducing the sulfur and halide impurities in the

product gas stream to levels of parts per billion, suitable for fuels/chemical synthesis by the

Fischer-Tropsch reaction. The process can produce a sequestration-ready CO2 stream through

spent sorbent regeneration at high temperatures (700–1100°C). The sorbent regeneration requires

calcining the carbonated sorbent (CaCO3) using oxygen (O2) and/or steam to generate calcium

oxide (CaO). The CaS sorbent can be partly regenerated to CaO by treatment with steam and

CO2.98

Hence, this process provides a “one-box” mode of operation for the production of high-

purity H2 through WGS reaction, along with integrated CO2, sulfur, and halide capture and H2

separation in one consolidated unit. In addition to generating a high purityH2 stream, CLP is also

capable of adjusting the H2:CO ratio to a required level at the reactor outlet while reducing sulfur

in the syngas to a very low concentration for the catalytic Fischer-Tropsch synthesis of liquid

fuels. This integrated one-box process yields higher system efficiencies and lower overall

footprint by combining different process units in one stage. A high-purity CO2 stream from

regeneration can be obtained by careful control of the calcination atmosphere. Such regeneration,

however, requires the calcination temperature to be above 950°C, which results in significant

sintering of CaO. The performance of CLP depends on the reactivity and recyclability of the

calcium sorbent. Maintaining the reactivity of sorbent over multiple cycles requires overcoming

the high temperature sintering which occurs during calcination step.

The OSU CLP specifically utilizes sorbent hydration post-calcination as means of

regenerating the sintered sorbent, resulting in a three-step calcium loop. The calcium hydroxide

(Ca(OH)2) sorbent is found to have superior reactivity towards CO2 and higher recyclability than

CaO produced by the conventional two-step cyclic process. The process concept has been

verified on the lab-scale fixed bed reactor and on a sub-pilot scale entrained bed reactor for the

integrated WGS, and CO2 capture, and sulfur and halide removal. 34

The continuous flow of

Ca(OH)2 and simulated flue gas has been tested at the sub-pilot scale. The isothermal

experiments conducted at 600°C in the absence of any WGS catalyst at atmospheric pressure at

sub-pilot scale resulted in ~70% H2 product purity (on a steam-free basis), and complete removal

of CO2 on the order of seconds. Ca(OH)2 sorbent also holds an advantage over conventional CaO

sorbent, in that it aids the WGS reaction by providing additional source of steam upon

decomposition at the reaction temperature. At high pressures (~21 atm), fixed bed results have

obtained high CO conversion with >99% H2 purity at near stoichiometric steam to carbon

ratios.99

Syngas Chemical Looping (SCL) Process – Type A-I

The Syngas Chemical Looping (SCL) Process converts gaseous fuels like syngas and

natural gas by high temperature cyclic reduction and oxidation of iron oxide based oxygen

carriers. The process can be designed to cogenerate heat and H2 while producing a sequestration-

ready CO2 stream. Clean syngas from a conventional gasifier is introduced into a countercurrent

moving bed reducer. In the reducer, the syngas is converted to steam and CO2 and the oxygen

carriers are reduced. The reduced oxygen carriers are first partially oxidized using steam in the

“oxidizer”, thereby producing H2 and then completely reoxidized using air in the “combustor”.

The oxidation reactions are highly exothermic and the heat generated is partially used to generate

electricity via steam turbines. Thus, CO2, H2 and heat are generated in three separate reactors

obviating the need for any downstream product separation.

A comprehensive analysis at OSU has established that iron oxide is an ideal choice for

oxygen carrier materials both from the process performance and economic standpoints. The

composite iron oxide based oxygen carrier developed by OSU exhibits good reactivity and

recyclability, which are desirable for the SCL Process.31

The countercurrent moving bed reducer

performance was studied in the 2.5 kWth reactor with >99.9% syngas conversion and >99.95%

H2 purity.97

The reducer design based on comprehensive kinetic modelling and thermodynamic

analysis.100

The SCL Process has been successfully operated over 300 hours in an integrated 25

kWth sub-pilot unit for the cogeneration of >99.99% purity H2 and heat with 100% carbon

capture and nearly 100% syngas conversion.101

Figure 10 presents results from a 3-day runs of

the sub-pilot unit exhibiting steady, continuous high purity CO2 and H2 generation from the

reducer and oxidizer gas outlets, respectively. Sub-pilot studies with methane as the feed has also

been tested with nearly complete methane conversion, thus confirming the feasibility of the SCL

process for natural gas conversion.

With the successful operation of the sub-pilot unit, OSU was awarded a project under the

United States Department of Energy (USDOE) Advance Research Project Agency – Energy

(ARPA-E) to design, construct, and operate a pilot scale SCL demonstration facility at the

National Carbon Capture Center (NCCC) in Wilsonville, Alabama. Figure 11 is a photo of the

250 KWth – 3MWth pilot plant which has been in operation under pressurized (up to 15 atm)

conditions with syngas as a feed from a Kellogg Brown & Root (KBR) transport gasifier. The

NCCC unit is the basis for the next scale commercial demonstration unit which is being planned.

Coal Direct Chemical Looping (CDCL) Process – Type A-I

The Coal Direct Chemical Looping (CDCL) Process converts solid fuels like coal or

biomass by circulating iron oxide based oxygen carriers between a moving bed reducer and a

fluidized bed combustor.102

Coal is injected into the reducer where it is divided into two sections

as illustrated in Figure 12. The coal decomposes into volatiles and char in the reducer after being

injected. In the upper section, the volatiles moving upwards react with the downward-flowing

oxygen carriers. In the lower section, the slow solid-solid reaction between char and the oxygen

carriers is facilitated using an enhancing gas like steam and/or CO2. The enhancer gas gasifies

the char which is then oxidized to CO2 and H2O by the oxygen carriers. The moving bed reducer

design ensures full fuel conversion while achieving a high oxygen carrier conversion.

Different types of coal including lignite, bituminous, sub-bituminous, and anthracite have

been tested in a 2.5 kWth bench scale reactor with nearly complete volatile and char conversions.

The bench scale reactor has been operated with solid fuel loading ranging from 0.6 to 3.35 kg/hr

and with coal, metallurgical coke and pelletized woody biomass.103

Furthermore, a 25 kWth fully

integrated sub-pilot chemical looping facility with complete zone sealing and non-mechanical

valves for gas and solid circulation control has also been operated. A successful 200-hour

continuous sub-pilot run achieved nearly complete coal conversions to CO2 and steam with no

solid circulation or gas flow issues observed throughout the run.102

This long continuous CDCL

operation marks an important milestone for a CLC system with solids feedstock. Table 1

summarizes results from various bench unit and sub-pilot unit tests. In every case, the reducer

outlet contains an almost pure stream of CO2 with negligible concentrations of H2, CO and CH4,

indicating full volatile conversion.

Shale Gas-To-Syngas (STS) Process – Type B-I

The Shale Gas-to-Syngas (STS) Chemical Looping Process was developed for the

production of high purity syngas from shale gas.50

The H2:CO molar ratio of the syngas product

is ~2:1 and is ready for use in downstream synthesis processes for valuable products such as

methanol, dimethyl ether, gasoline, and other fuels and chemicals. The cyclic reduction

(endothermic) and oxidation (exothermic) reaction of composite metal oxide forms a reaction

and heat integrated process loop that can perpetuate.

The STS Process is characterized by the co-current gas-solid moving bed contact

pattern for the reducer operation. The co-current reactor design allows the gaseous fuel to

achieve complete conversion while reducing the composite metal oxide based oxygen carriers to

an oxidation state that provides high quality syngas product. The metal oxides conversion is

controlled to ensure high CO/CO2 and H2/H2O ratios in the product syngas and to avoid carbon

deposition. In addition to shale gas, other carbonaceous fuels like coal, biomass, petroleum coke,

naphtha, residual oil, C2-C4 light hydrocarbons, and any combination thereof can also be used as

feedstock for this system. For the different types of feedstock, the CO/H2 ratio formed in the

product syngas can be adjusted to the ratios desired for downstream processes by injection of

H2O and/or CH4. For example, with biomass as feedstock, CO/H2 ratio could be adjusted to 1:1

by co-injection of the biomass with methane.

The STS Process has been tested successfully in a sub-pilot moving bed reactor system

using methane as the feed. The steady operation under isothermal conditions was achieved, with

a methane conversion of >99%, CO:CO2 molar ratio of ~9:1, and H2:CO molar ratio of ~2:1.

Little carbon deposition was detected on the oxygen carrier materials. Figure 13 shows the

conversion and gas composition results from the sub-pilot tests.

STS Process Simulation – An Example

An ASPEN simulation of a chemical looping system that converts shale gas/natural gas

to liquid fuel is shown in Figure 14 to illustrate quantitatively an industrial application of

chemical looping technology. In the simulation, the STS Process is used to produce syngas

which is then used to produce liquid fuel at 50,000 bpd using the Fischer-Tropsch Process. The

Fischer-Tropsch and refining complex are modelled following the DOE baseline report analysis,

utilizing syngas as feed input and a gasoline pool and a diesel pool as products.104, 105

. The

system downstream of the syngas polishing unit is kept similar in capacity to the DOE report,

which minimizes the errors due to scale variation. The STS reducer converts the natural gas feed

into a high-quality syngas via partial oxidation by the iron-based metal-oxide. The reduced

metal-oxide is re-oxidized with air in the combustor reactor. The re-oxidation reaction is

exothermic and the heat extracted is used to generate electricity to offset the parasitic energy

requirements for the entire plant. The H2 production plant also uses an iron-based chemical

looping system to increase the natural gas utilization efficiency over the steam methane system

used in the baseline case. The overall process simulation is balanced for material and energy

flow based on standard modelling assumptions.106-108

The base case uses a part of the recycle flue

gas (5%) to produce electricity in a combustion turbine. The chemical looping air reactor

(combustor) produces enough electricity to bypass the need for a flue-gas based separate

combustion turbine. The entire flue gas stream is recycled to the reducer along with fresh natural

gas feed.

The system simulation results are given in Table 2 with a comparison of the STS based

system with the conventional system comprised of an auto-thermal reforming unit and a steam-

methane reforming unit. The STS based system uses a smaller amount of steam per mole of

carbon processed compared to the conventional system. The oxygen for a STS based system

comes from the iron-oxide lattice as compared to the molecular oxygen produced from an Air

Separation Unit (ASU). The lattice donation of oxygen leads to a higher selectivity in natural gas

oxidation. It can be seen that for an equivalent amount of syngas specification (H2/CO ratio), the

chemical looping system has a higher selectivity as seen from the stoichiometric number, (H2-

CO2)/(CO+CO2), comparison.

The higher efficiency of syngas production indicates that for the same amount of syngas,

a chemical looping system uses less natural gas. This correspondingly increases the carbon

utilization value from 81.8% for the baseline case to 86.68% for a chemical looping system as

shown in Table 2. The heat produced is adequate to offset the parasitic energy demands of the

system. The higher intrinsic selectivity coupled with lower steam consumption and no molecular

oxygen requirement makes the chemical looping system a promising economical alternative to

conventional systems.

Outlook, Challenges and Concluding Remarks

The chemical looping concept was practiced as early as 1900s. The early interest had

been mainly in the production of hydrogen. It was discontinued a short while later until 1960s –

1970s when it was applied for generation of synthetic natural gas. The discontinuity and the shift

in interest had a close relevancy to the effectiveness of the earlier practice and the availability

and prices of the feedstock and products that were produced. An interest in the chemical looping

applications resurfaced in the 1980s in view of this meritorious concept in enhancing the exergy

efficiency in solid fuel combustion and subsequent benefits in effective CO2 capture for curbing

climate change. With now an abundant supply of the shale gas/natural gas in the U.S., the

economic attractiveness in converting natural gas for combustion and gasification applications

using chemical looping processes over the traditional processes has brought a new excitement on

the interest in this technology.

Chemical looping operation embodies all elements of particle science and technology

including particle synthesis, reactivity and mechanical properties, flow stability and contact

mechanics, gas-solid reaction and system engineering. Extensive recent research and

development efforts on chemical looping technology have been focusing on the design and

synthesis of the oxygen carrying particle, ionic transport phenomena in the redox reactions, and

large-scale chemical looping unit demonstration. The physical and chemical properties of the

metal oxide particles used in the reduction and oxidation reactions or carbonation and calcination

reactions play a key role in the successful operation of chemical looping processes. Ionic

diffusion in crystal grains is identified as a rate determining step in the oxygen carrier redox

reaction while the ionic diffusion consumes or creates lattice vacancies that could aggregate and

form cavities or micro-pores, where pore diffusion occurs, upon multiple redox and sintering

processes. Much research is carried out to probe the oxygen uncoupling materials that can

release molecular oxygen and complex ferrites and perovskite materials that can enhance the

reaction kinetics through augmented electron and/or ionic transfer.

Fluidized beds and moving beds are two main types of reactors considered for the reducer

operation although fixed beds as a reducer can also be possible. Much progress has been made in

the operation of fluidized bed reducers with large pilot demonstration on-going. Although either

type of reactors in a single or an assembly reactor arrangement can achieve the same reactant

conversion, process economics eventually comes into play in determining the optimum reactor

configuration. In this regard, considering the use of a single reactor for a given reactor volume,

the analysis indicates higher gas and solids conversions can be achieved using a moving bed

reducer compared to using a fluidized bed reducer. Through the 20 years of research and

development activities, OSU has developed a chemical looping technology platform that

comprises six processes generating a variety of products including electricity, syngas, hydrogen,

chemicals and liquid fuels, with CO2 capture. These processes are based on moving bed reducers

and are at the sub-pilot or pilot demonstration stage with flexible feedstock such as coal, biomass,

natural gas, liquid hydrocarbon and any of their combinations. The total demonstration time has

exceeded 1,000 hours. With extensive on-going research and demonstration efforts elsewhere,

the outlook of the chemical looping technology for larger scale applications is promising.

The reported information in the 1980s and 1990s on commercial development and

operation of the DuPont butane oxidation process based on the chemical looping concept for

production of maleic anhydride is valuable as it reflects the challenges involved in scaling up a

chemical looping process. With cases on chemical looping combustion and gasification

applications in discussion, the key factor that challenges the timing for the commercial

deployment is the economic readiness of the oxygen carrier for viable, long-term commercial

chemical looping operation. The “long-term” refers to the time scale on the order of one year

while the “viable” refers to the chemical (e.g., oxygen carrying capacity and recyclability) and

physical (e.g., attrition resistance and unconfined yield strength) properties that can sustain in

this long-term time scale. The other factor that can be challenging is concerning the operation of

a large scale circulating system or a circulating fluidized bed system that involves a massive

solids flow well exceeding that of the conventional FCC system. These challenges, however,

should not be unsurmountable with further research and development. It is projected that the first

commercial chemical looping process is to be from shale gas/natural gas based applications. The

prospect of its timing can likely be the near future.

Acknowledgements

The work was supported in part by the C. John Easton Professorship endowed funds at

The Ohio State University.

Tables and Figures

Table 1. Summary of OSU’s bench and small pilot-scale unit testing results using different fuels

Bench Scale (2.5 kWth) Small Pilot (25 kWth)

Type of Fuel Coal Volatile Coal Char Coal Coal

CO, H2 CH4 Lignite Bituminous PRB Illinois #6 Anthracite Metallurgical coke PRB Illinois #6

Fuel Conversion, % 99.9 99.8 94.9 95.2 >97 >95 95.5 81 99 70

CO2 purity, % 99.9 98.8 99.2 99.1 -* - 97.3 99.85 98 97

Table 2: Carbon efficiency and overall production for the chemical looping case and the base-line DOE

case.

Component Base Case1,2

Chemical Looping

H2O/C 0.68 0.2

H2/CO 2.19 2.16

Stoichiometric number {S =

(H2-CO2)/(CO+CO2)} 1.59 1.84

Nat gas feed flow (kg/hr) 354365 334427

Butane feed flow (kg/hr) 18843 18843

Diesel Fuel( bbl/day) 34543 34543

Gasoline ( bbl/day) 15460 15460

Total Liquids, bbl/day 50003 50003

Carbon Efficiency 81.80% 86.68%

Figure 1. Trend of Publications with “Chemical Looping” as Key Words in the “Google Scholar”

Search

0

200

400

600

800

1000

1200

1986 1989 1992 1995 1998 2001 2004 2007 2010 2013

Nu

mb

er o

f P

ub

licat

ion

s

Year

Type A-I Type A-II

Figure 2a. Group A Chemical Looping Reactions Involving CO2 Generation

Type B-I Type B-II Type B-III

Figure 2b. Group B Chemical Looping Reactions Involving No CO2 Generation

Figure 3. Steam Iron Processes Using Fixed Bed8

Figure4. Exergy Recovering Scheme for Two H2 Production Schemes with the Reaction

Temperature for Scheme II to be 1,123 K

Figure 5. Schematic Flow Diagram of Chemical Looping Gasification (CLG) Process for Liquid

Fuel Synthesis

Figure 6. Reaction Zone Division Based on the Ellingham Diagram for Metal Oxide Materials

Applications for Chemical Looping Combustion and Gasification

Figure 7. Characterization of Fe particle. (a) SEM Image of Cross-Section of Fresh Fe Particle;

(b) SEM Image of Cross-Section of Fe2O3 Particle after Fe Oxidation; (c) and (f) SEM Image of

Top Surface of Fe2O3 Particle; (d) EDS Mapping of O from (b); (e) EDS Mapping of Fe from (b)

Figure 8. Characterization of FeTi Particle after One Oxidation-Reduction Cycle. (a) SEM Image

of Surface; (b) SEM Image of Cross-Section; (c) EDS Mapping and Spectrum of Surface.

Characterization of FeTi Particle after Five Oxidation-Reduction Cycles. (d) SEM Image of

Surface; (e) SEM Image of Cross-Section; (f) EDS Mapping and Spectrum of Surface

Figure 9. Equilibrium Phase Diagram for Fe/FeO/Fe3O4/Fe2O3 System for Redox Reactions with

CO2/CO (Solid Lines) and H2O/H2 (Dashed Lines).

Figure 10. Hydrogen and Carbon Monoxide Concentrations at (a) Reducer Gas Outlet; (b)

Oxidizer Gas Outlet for OSU Syngas Chemical Looping Sub-Pilot Plant for Hydrogen

Generation and CO2 Capture

Figure 11. Photo of the OSU High Pressure Syngas Chemical Looping Pilot Plant at the National

Carbon Capture Center (NCCC) in Alabama for Hydrogen Generation and CO2 Capture

Figure12. Moving Bed Reducer Layout for the OSU Coal-Direct Chemical Looping (CDCL)

Process.

Figure 13. Performance of the OSU Shale Gas (Methane) to Syngas (STS) Sub-Pilot Unit

Operation (a) Syngas Compositions; (b) Methane Conversion and Hydrogen and Carbon

Monoxide Ratios

Figure 14: Gas-to-Liquid Process Block Diagram Using OSU STS Chemical Looping System for

Syngas and H2 Production Coupled with the Fischer-Tropsch (F-T) Complex Modelled Based on

National Energy Technology Laboratory (NETL) Gas-to-Liquids Report for Production of

50,000 bpd of Liquids Using a Co-Based F-T Catalyst

Reference

1. Ishida M, Zheng D, Akehata T. Evaluation of a chemical-looping-combustion power-

generation system by graphic exergy analysis. Energy. 1987;12:147-154.

2. Messerschmitt A. Process for producing hydrogen. U.S. Patent 971,206, 1910.

3. Lane H. Process for the production of hydrogen. U.S. Patent 1,078,686, 1913.

4. Fan L-S. Chemical looping systems for fossil energy conversions. John Wiley & Sons,

2010.

5. Knoche KF, Richter H. Improvement of reversibility of combustion processes.

Brennstoff-Warme-Kraft. 1968;20:205.

6. Richter H, Knoche K. Reversibility of combustion processes. In Efficiency and Costing,

ACS Symp Ser. 1983;235:71-86.

7. Fan L-S, Zeng L, Wang W, Luo S. Chemical looping processes for CO2 capture and

carbonaceous fuel conversion - prospect and opportunity. Energy Environ Sci.

2012;5:7254–7280.

8. Hurst S. Production of hydrogen by the seam-iron method. J Am Oil Chem Soc.

1939;16:29-36.

9. Gasior S, Forney A, Field J, Bienstock D, Benson H. Production of synthesis gas and

hydrogen by the steam-iron process. Bureau of Mines Report of Investigation 5911, 1961.

10. Teed PL. The chemistry and manufacture of hydrogen. Longmans, Green and Co., 1919.

11. Lewis WK, Gilliland ER. Production of pure carbon dioxide. U.S. Patent 2,665,971, 1954.

12. Lewis WK, Gilliland ER. Production of pure carbon dioxide. U.S. Patent 2,665,972, 1954.

13. Institute of Gas Technology. Development of the steam-iron process for hydrogen

production. U. S. Department of Energy EF-77-C-01-2435, 1979.

14. Dobbyn RC, Dobbyn RC, Ondik HM, Willard WA, Brower WS, Feinberg IJ, Hahn TA,

Hicho GE, Read ME, Robbins CR, Smith JH. Evaluation of the performance of materials

and components used in the CO2 acceptor process gasification pilot plant. U.S.

Department of Energy, DOE-ET-10253-T1, 1978.

15. Lin SY, Suzuki Y, Hatano H, Harada M. Developing an innovative method, Hypr-Ring,

to produce hydrogen from hydrocarbons. Energy Convers Manag. 2002;43:1283-1290.

16. Lin, SY, Harada M, Suzuki Y, Hatano H. Process analysis for hydrogen production by

reaction integrated novel gasification (Hypr-Ring). Energy Convers Manag.

2005;46:869-880.

17. Keller GE, Bhasin MM. Synthesis of ethylene via oxidative coupling of methane. J Catal.

1982;73:9-19.

18. Jones CA, Leonard JJ, Sofranko JA. The oxidative conversion of methane to higher

hydrocarbons over alkali-promoted MnSiO2. J Catal. 1987;103:311-319.

19. Jones CA, Leonard JJ, Sofranko JA. Fuels for the future: remote gas conversion. Energy

Fuels. 1987;1:12-16.

20. Contractor RM. Dupont's CFB technology for maleic anhydride. Chem Eng Sci.

1999;54:5627-5632.

21. Dudukovic MP. Frontiers in reactor engineering. Science. 2009;325:698–701.

22. Otsuka K, Wang Y, Sunada E, Yamanaka I. Direct partial oxidation of methane to

synthesis gas by cerium oxide. J Catal. 1998;175:152-160.

23. Wang S, Kobayashi T, Dokiya M, Hashimoto T. Electrical and ionic conductivity of Gd-

doped ceria. J Electrochem Sco. 2000;147:3606-3609.

24. Gupta A, Hegde M, Priolkar K, Waghmare U, Sarode P, Emura S. Structural

investigation of activated lattice oxygen in Ce1-xSnxO2 and Ce1-x-ySnxPdyO2-δ by EXAFS

and DFT calculation. Chem Mater. 2009;21:5836–5847.

25. Salazar-Villalpando MD, Berry DA, Cugini A. Role of lattice oxygen in the

partial oxidation of methane over Rh/zirconia-doped ceria. Isotopic studies. Int J

Hydrogen Energy. 2010;36:1998-2003.

26. Steinfeld A, Kuhn P, Karni J. High-temperature solar thermochemistry - production of

iron and synthesis gas by Fe3O4-reduction with methane. Energy. 1993;18:239-249.

27. Fletcher EA. Solarthermal processing: a review. J Sol Energy Eng. 2001;123:63-74.

28. Steinfeld A. Solar thermochemical production of hydrogen- a review. Sol Energy.

2005;78:603-615.

29. Muhich CL, Evanko BW, Weston KC, Lichty P, Liang X, Martinek J, Musgrave CB,

Weimer AW. Efficient generation of H2 by splitting water with an isothermal redox cycle.

Science. 2013;341:540-542.

30. Ströhle J, Orth M, Epple B. Design and operation of a 1MWth chemical looping plant.

Appl Energy. 2014;113:1490-1495.

31. Tong A, Bayham S, Kathe MV, Zeng L, Luo S, Fan L-S. Iron-based syngas chemical

looping process and coal-direct chemical looping process development at Ohio State

University. Appl Energy. 2014;113:1836-1845.

32. Arias B, Diego M, Abanades J, Lorenzo M, Diaz L, Martínez D, Alvarez J, Sánchez-

Biezma A. Demonstration of steady state CO2 capture in a 1.7 MWth calcium looping

pilot. Int J Greenhouse Gas Control. 2013;18:237-245.

33. Chang MH, Huang CM, Liu WH, Chen WC, Cheng JY, Chen W, Wen TW, Ouyang S,

Shen CH, Hsu HW. Design and experimental investigation of calcium looping process

for 3-kWth and 1.9-MWth facilities. Chem Eng Technol. 2013;36:1525-1532.

34. Wang W, Ramkumar S, Li S, Wong D, Iyer M, Sakadjian BB, Statnick RM, Fan L-S.

Subpilot demonstration of the carbonation−calcination reaction (CCR) process: high-

temperature CO2 and sulfur capture from coal-fired power plants. Ind Eng Chem Re.

2010;49:5094–5101.

35. Kremer J, Galloy A, Strohle J, Epple. Continuous CO2 capture in a 1-MWth carbonate

looping pilot plant. Chem Eng Technol. 2013;36:1518–1524.

36. Hossain MM, de Lasa HI. Chemical-looping combustion (CLC) for inherent CO2

separations - a review. Chem Eng Sci. 2008;63:4433-4451.

37. Jin HG, Ishida M. A new type of coal gas fueled chemical-looping combustion. Fuel,

2004;83:2411-2417.

38. Johansson E, Mattisson T, Lyngfelt A, Thunman H. Combustion of syngas and natural

gas in a 300W chemical-looping combustor. Chem Eng Res Des, 2006;84:819-827.

39. Ishida M, Jin HG. A new advanced power-generation system using chemical-looping

combustion. Energy. 1994;19:415-422.

40. Dewulf J, Van Langenhove H, Muys B, Bruers S, Bakshi BR, Grubb GF, Paulus DM,

Sciubba E. Exergy: its potential and limitations in environmental science and technology.

Environ Sci Technol. 2008;42:2221-2232.

41. Anheden M, Svedberg G. Exergy analysis of chemical-looping combustion systems.

Energy Convers Manag. 1998;39:1967-1980.

42. Li F, Fan L-S. Clean coal conversion processes – progress and challenges. Energy

Environ Sci. 2008;1:248-267.

43. Blockstein DE, Shockley MA. Energy for a sustainable and secure future: a report of the

sixth national conference on science, policy and the environment. National Council for

Science and the Environment, 2006.

44. Chatel-Pelage F, Varagani R, Pranda P, Perrin N, Farzan H, Vecci SJ, Lu Y, Chen S,

Rostam-Abadi M, Bose AC. Applications of oxygen for NOx control and CO2 capture in

coal-fired power plants. Therm Sci. 2006;10:119-142.

45. White CM, Strazisar BR, Granite EJ, Hoffman JS, Pennline HW. Separation and capture

of CO2 from large stationary sources and sequestration in geological formations--

coalbeds and deep saline aquifers. J Air Waste Manag Assoc. 2003;53:645-715.

46. Aaron D, Tsouris C. Separation of CO2 from flue gas: a review. Sep Sci Technol.

2005;40:321-348.

47. Iyer MV, Gupta H, Sakadjian BB, Fan L-S. Multicyclic study on the simultaneous

carbonation and sulfation of high-reactivity CaO. Ind Eng Chem Res. 2004;43:3939-3947.

48. Kotas TJ. The exergy method of thermal plant analysis. Butterworth-Heinemann, 1985.

49. Tsutsumi A. Exergy recuperative gasification technology for hydrogen and power co-

production. Presented at: 231st ACS National Meeting, 2006.

50. Fan L-S, Luo S, Zeng L. Methods for fuel conversion. PCT application

PCT/US2014/014877, 2013.

51. Adánez J, de Diego LF, García-Labiano F, Gayán P, Abad A, Palacios J. Selection of

oxygen carriers for chemical-looping combustion. Energy Fuels. 2004;18:371-377.

52. Adánez J, Abad A, Garcia-Labiano F, Gayan P, de Diego LF. Progress in chemical-

looping combustion and reforming technologies. Prog Energy Combust Sci.

2012;38:215-282.

53. Luis F, Adánez J, Gayán P, Abad A, Corbella BM, Palacios JM. Development of Cu-

based oxygen carriers for chemical-looping combustion. Fuel. 2004;83:1749-1757.

54. Lee J-B, Park C-S, Choi S-I, Song Y-W, Kim Y-H, Yang H-S. Redox characteristics of

various kinds of oxygen carriers for hydrogen fueled chemical-looping combustion. J Ind

Eng Chem. 2005;11:96-102.

55. Song Q, Xiao R, Deng Z, Zheng W, Shen L, Xiao J. Multicycle study on chemical-

looping combustion of simulated coal gas with a CaSO4 oxygen carrier in a fluidized bed

reactor. Energy Fuels. 2008;22:3661-3672.

56. Li F, Luo S, Sun Z, Fan L-S. Role of metal oxide support in redox reactions of iron oxide

for chemical looping applications: experiments and density functional theory calculations.

Energy Environ Sci. 2011;4:3661-3667

57. Edstrom J. Solid-state diffusion in the reduction of hematite. Jernkontorets Ann.

1957;141:809-836

58. Rubel A, Liu K, Neathery J, Taulbee D. Oxygen carriers for chemical looping

combustion of solid fuels. Fuel. 2009;88:876-884.

59. Adánez J, De Diego LF, García-Labiano F, Gayán P, Abad A, Palacios JM. Selection of

oxygen carriers for chemical-looping combustion. Energy Fuels. 2004;18:371-377.

60. Yu FC, Phalak N, Sun Z, Fan L-S. Activation strategies for calcium-based sorbents for

CO2 capture: a perspective. Ind Eng Chem Res. 2011;51:2133-2142.

61. Leion H, Lyngfelt A, Johansson M, Jerndal E, Mattisson T. The use of ilmenite as an

oxygen carrier in chemical-looping combustion. Chem Eng Res Des. 2008;86:1017-1026.

62. Cuadrat A, Abad A, Adánez J, de Diego LF, García-Labiano F, Gayán P. Behavior of

ilmenite as oxygen carrier in chemical-looping combustion. Fuel Process Technol.

2012;94:101-112

63. Sun Z, Zhou Q, Fan, L-S. Formation of core–shell structured composite microparticles

via cyclic gas–solid reactions. Langmuir. 2013;29:12520-12529

64. Kuo YL, Hsu WM, Chiu PC, Tseng YH, Ku Y. Assessment of redox behavior of nickel

ferrite as oxygen carriers for chemical looping process. Ceram Int. 2013;39:5459-5465.

65. Qin L, Fan JA, Sun Z, Fan L-S. Nano/micro scale redox phenomena of metals/oxides and

other chemical and electrical characteristics. Presented at: 248th

ACS National Meeting,

2014.

66. Qin L, Majumder A, Fan JA, Kopechek D, Fan L-S. Evolution of nanoscale morphology

in single and binary metal oxide microparticles during reduction and oxidation processes.

J Mater Chem A. 2014.

67. Chen WK, Peterson NL. Effect of the deviation from stoichiometry on cation self-

diffusion and isotope effect in wüstite, Fe1− xO. J Phys Chem Solids. 1975;36:1097-1103.

68. Amami B, Addou M, Millot F, Sabioni A, Monty C. Self-diffusion in α-Fe2O3 natural

single crystals. Ionics. 1999;5:358-370.

69. Zhao C, Li Y, Zhou J, Li L, Deng S, Xu N, Chen J. Large-scale synthesis of bicrystalline

ZnO nanowire arrays by thermal oxidation of zinc film: growth mechanism and high-

performance field emission. Cryst Growth Des. 2013;13:2897-2905.

70. Mattisson T, Leion H, Lyngfelt A. Chemical-looping with oxygen uncoupling using

CuO/ZrO2 with petroleum coke. Fuel. 2009;88:683-690.

71. Zhu Y, Mimura K, Isshiki M. Oxidation mechanism of Cu2O to CuO at 600-1050 °C.

Oxid Met. 2004;62:207-222.

72. Azimi G, Ryden M, Leion H, Mattisson T, Lyngfelt T. (MnzFe1-z)yOx combined oxides as

oxygen carrier for chemical-looping with oxygen uncoupling. AIChE J. 2012;59:582-588.

73. Azimi G, Leion H, Ryden M, Mattisson T, Lyngfelt T. Investigation of different Mn−Fe

oxides as oxygen carrier for chemical-looping with oxygen uncoupling (CLOU). Energy

Fuels. 2013;27:367-377.

74. Azad A, Hedayati A, Ryden M, Leion H, Mattisson T. Examining the Cu–Mn–O spinel

system as an oxygen carrier in chemical looping combustion. Energy Technol. 2013;1:59-

69.

75. Dai X, Wu Q, Li R, Yu C, Hao Z. Hydrogen production from a combination of the

water−gas shift and redox cycle process of methane partial oxidation via lattice oxygen

over LaFeO3 perovskite catalyst. J Phys Chem B. 2006;110:25856-25862.

76. Galinsky NL, Huang Y, Shafiefarhood A, Li F. Iron oxide with facilitated O2-

transport

for facile fuel oxidation and CO2 capture in a chemical looping scheme.

ACS Sustain Chem Eng. 2013;1:364-373.

77. Mihai O, Chen De, Holmen A. Catalytic consequence of oxygen of lanthanum ferrite

perovskite in chemical looping reforming of methane. Ind Eng Chem Res. 2011;50:2613–

2621.

78. Carter S, Selcuk A, Chater RJ, Kajda J, Kilner JA, Stelle BCH. Oxygen transport in

selected nonstoichiometric perovskite-structure oxides. Solid State Ionics. 1992;53:597-

605.

79. Hallberg P, Jing D, Ryden M, Mattisson T, Lyngfelt A. Chemical looping combustion

and chemical looping with oxygen uncoupling experiments in a batch reactor using

spray-dried CaMn1−xMxO3−δ (M = Ti, Fe, Mg) particles as oxygen carriers. Energy Fuels.

2013;27:1473-1481.

80. Arjmand M, Hedayati A, Azad AM, Leion H, Ryden M, Mattisson T.

CaxLa1−xMn1−yMyO3−δ (M = Mg, Ti, Fe, or Cu) as oxygen carriers for chemical-looping

with oxygen uncoupling (CLOU). Energy Fuels. 2013;27:4097-4107.

81. Dai X, Li R, Yu C, Hao Z. Unsteady-state direct partial oxidation of methane to synthesis

gas in a fixed-bed reactor using AFeO3 (A= La, Nd, Eu) perovskite-type oxides as

oxygen storage. J Phys Chem B. 2006;110:22525-22531.

82. Nalbandian L, Evdou A, Zaspalis V. La1-xSrxMyFe1-yO3-z perovskites as oxygen-carrier

materials for chemical-looping reforming. Int J Hydrogen Energy 2011;36:6657-6670.

83. Thursfield A, Murugan A, Franca R, Metcalfe IS. Chemical looping and oxygen

permeable ceramic membranes for hydrogen production - a review. Energy Environ Sci.

2012;5:7421-7459.

84. Mattisson T, Johansson M, Lyngfelt A. The use of nio as an oxygen carrier in chemical-

looping combustion. Fuel. 2006;85:736-747.

85. 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.

86. Abad A, Mattisson T, Lyngfelt A, Johansson M. The use of iron oxide as oxygen carrier

in a chemical-looping reactor. Fuel. 2007;86:1021-1035.

87. Berguerand N, Lyngfelt A. Design and operation of a 10 kWth chemical-looping

combustor for solid fuels–Testing with South African coal. Fuel. 2008;87:2713–2726.

88. Berguerand N, Lyngfelt A. The use of petroleum coke as fuel in a 10 kWth chemical-

looping combustor. Int J Greenhouse Gas Control. 2008;2:169-179.

89. de Diego LF, Garcia-Labiano F, Gayan P, Celaya J, Palacios JM, Adanez J. Operation of

a 10 KWth chemical-looping combustor during 200 h with a CuO-Al2O3 oxygen carrier.

Fuel. 2007;86:1036-1045.

90. Ryu HJ, Seo Y, Jin GT. Development of chemical-looping combustion technology: long-

term operation of a 50 KWth chemical-looping combustor with Ni- and Co- based oxygen

carrier particles. Proc Reg Symp Chem Eng. 2005.

91. Pröll T, Kolbitsch P, Bolhàr-Nordenkampf J, Hofbauer H. A novel dual circulating

fluidized bed (DCFB) system for chemical looping process. AIChE J. 2009;55:3255-3266.

92. Shen L, Wu J, Xiao J. Experiments on chemical looping combustion of coal with a NiO

based oxygen carrier. Combust Flame. 2009;156:721-728.

93. Shen L, Wu J, Xiao J, Song Q, Xiao R. Chemical-looping combustion of biomass in a 10

KWth reactor with iron oxide as an oxygen carrier. Energy Fuels, 2009;23:2498-2505.

94. Gu H, Shen L, Xiao J, Zhang S, Song T. Chemical looping combustion of biomass/coal

with natural iron ore as oxygen carrier in a continuous reactor. Energy Fuels.

2011;25:446-455.

95. Cao Y, Pan WP. Investigation of chemical looping combustion by solid fuels. 1. Process

analysis. Energy Fuels. 2006;20:1836-1844.

96. Eyring EM, Konya G, Lighty JS, Sahir AH, Sarofim AF, Whitty K. Chemical looping

with copper oxide as carrier and coal as fuel. Oil Gas Sci Technol. 2011;66:209-221.

97. Li F, Zeng L, Velazquez-Vargas, LG, Yoscovits Z, Fan L-S. Syngas chemical looping

gasification process: bench scale studies and reactor simulations. AIChE J.

2010;56:2186–2199.

98. Ramkumar S, Fan L-S. Calcium looping process (CLP) for enhanced noncatalytic

hydrogen production with integrated carbon dioxide capture. Energy

Fuels. 2010;24:4408-4418.

99. Ramkumar S, Fan L-S. Calcium looping process (CLP) for enhanced noncatalytic

hydrogen production with integrated carbon dioxide capture. Energy Fuels.

2010;24:4408-4418.

100. Zhou Q, Zeng L, Fan L-S. Syngas chemical looping process: dynamic modeling

of a moving bed reducer. AIChE J. 2013;59:3432-3443.

101. Tong A, Sridhar D, Sun Z, Kim HR, Zeng L, Wang F, Wang D, Kathe M, Luo S,

Sun Y, Fan L-S. Continuous high purity hydrogen generation from a syngas chemical

looping 25kWth sub-pilot unit with 100% carbon capture. Fuel. 2012;103:495-505.

102. Bayham S, Kim H, Wang D, Tong A, Zeng L, McGiveron O, Kathe M, Chung E,

Wang W, Wang A, Majumder A, Fan L-S. Iron-based coal direct chemical looping

combustion process: 200-h continuous operation of a 25-kWth subpilot unit. Energy

Fuels. 2013;27:1347−1356.

103. Luo S, Majumder A, Bayham S, Chung E, Xu D, Bayham S, Sun Z, Zeng L, and

Fan L-S. Conversion of woody biomass materials by chemical looping process – kinetics,

light tar cracking, and moving bed reactor behavior. Ind Eng Chem Res. 2013;52:14116-

14124.

104. Shuster E. Analysis of natural gas-to-liquid transportation fuels via Fischer-

Tropsch. U.S. Department of Energy, DOE/NETL-2013/1597, 2013.

105. Rath L. Assesment of hydrogen production with CO2 capture - volume 1: baseline

state-of-the-art plants. U.S. Department of Energy, DOE/NETL-2010/1434, 2010.

106. Turner MJ. Quality guidelines for energy system studies: capital cost scaling

methodology. U.S. Department of Energy, DOE/NETL-341/013113, 2013.

107. Chou V, Keairns D, Turner M, Woods M, Zoelle A. Quality guidelines for energy

system studies: process modeling design parameters. U.S. Department of Energy,

DOE/NETL-341/051314, 2014.

108. Myles P, Lanka S, Lu Y. Quality guidelines for energy system studies:

specification for selected feedstocks. U.S. Department of Energy, DOE/NETL-

341/011812, 2012.