Fan_Chemical Looping Technology and Its Fossil Energy Conversion Applications

Chemical Looping Technology and Its Fossil Energy Conversion Applications

Liang-Shih Fan* and Fanxing Li

Department of Chemical and Biomolecular Engineering, The Ohio State UniVersity, Columbus, Ohio 43210

The concept of chemical looping reactions has been widely applied in chemical industries, for example, theproduction of hydrogen peroxide (H2O2) from hydrogen and oxygen using 9,10-anthraquinone as the loopingintermediate. Fundamental research on chemical looping reactions has also been applied to energy systems,for example, the splitting of water (H2O) to produce oxygen and hydrogen using ZnO as the loopingintermediate. Fossil fuel chemical looping applications had been used commercially with the steam-iron processfor coal from the 1900s to the 1940s and had been demonstrated at a pilot scale with the carbon dioxideacceptor process in the 1960s and 1970s. There are presently no chemical looping processes using fossilfuels in commercial operation. A key factor that hampered the continued use of these earlier processes forfossil energy operation was the inadequacy of the reactivity and recyclability of the looping particles. Thisfactor led to higher costs for product generation using the chemical looping processes, compared to the otherprocesses that use particularly petroleum or natural gas as feedstock. With CO2 emission control now beingconsidered as a requirement, interest in chemical looping technology has resurfaced. In particular, chemicallooping processes are appealing because of their unique ability to generate a sequestration-ready CO2 streamwhile yielding high energy conversion efficiency. Renewed fundamental and applied research since the early1980s has emphasized on improvements over earlier shortcomings. New techniques have been developed fordirect possessing of coal or other solid carbonaceous feedstock in chemical looping reactors. Significant progressdemonstrated by the operation of several small pilot scale units worldwide indicates that the chemical loopingtechnology may become commercially viable in the future for processing carbonaceous fuels. This perspectivearticle describes the fundamental and applied aspects of modern chemical looping technology that utilizesfossil fuel as feedstock. It discusses chemical looping reaction thermodynamics, looping particle selection,reactor design, and process configurations. It highlights both the chemical looping combustion and the chemicallooping gasification processes that are at various stages of the development. Opportunities and challenges forchemical looping process commercialization are also illustrated.

1. Introduction

Energy is the backbone of modern society. A clean, relativelycheap, and abundant energy supply is a prerequisite for thesustainable economic and environmental prosperity of society.With the significant economic growth in the Asia Pacific regionand the expected development in Africa, the total world energydemands are projected to increase from 462.4 quadrillion BTUin 2005 to well over 690 quadrillion BTU by 2030.1 Theprojected energy supply through 2030 will be drawn from fossilfuels (i.e., oil, coal, and natural gas), renewable forms of energy(i.e., hydro, wind, solar, biomass, and geothermal), and nuclearenergy, in that order.

The impact of the global warming induced by the CO2

emissions from fossil energy conversion processes hasbecome an issue of international concern. An energy solutionprompted by the combination of ever-increasing energyconsumption and rising environmental concerns thus requiresa consideration of coupling fossil energy conversion systemswith economical capture, transportation, and safe sequestra-tion schemes for CO2. A long-term energy strategy for lowor zero carbon emission technologies would also includenuclear energy and renewable energy. Nuclear power iscapable of generating electricity at a cost comparable to theelectricity generated from fossil fuels.2 A variety of socialand political issues as well as operational safety andpermanent waste disposal concerns, however, could limitnuclear energy’s widespread utilization in overall energy

production.1,2 Renewable energy sources, although attractivefrom the environmental viewpoint, face complex constraintsfor large-scale application. Even when both the decrease inrenewable energy costs and the increase in fossil fuel pricesare taken into account, it is projected that only about 13.3%of the total energy consumption in 2030 will be fromrenewable sources. For primarily economic reasons, fossilfuels, including crude oil, natural gas, and coal, will continueto play a dominant role in the world’s energy supply for theforeseeable future.

The traditional carbonaceous fuel conversion technologiesfor combustion or gasification generate flue gas or syngasfrom which the separation of carbon dioxide requires anelaborative procedure. In contrast, the chemical loopingconcept allows a sequestration-ready CO2 stream to bedirectly generated through a different combustion or gasifica-tion path. This path is achieved using oxygen carryingparticles as the looping media. In this article, the historicaland current carbonaceous fuel conversion technologies basedon chemical looping concepts are presented. The loopingmedia employed in the processes are mainly in solid formwhile the carbonaceous fuels can be in solid, liquid, or gasform. Since the success of the chemical looping technologydepends strongly on the performance of the chemical loopingparticles, the subjects of particle properties and selection arediscussed. The looping processes can be applied for combus-tion and/or gasification of carbon-based material such as coal,natural gas, petroleum coke, and biomass directly or indirectlyfor steam, syngas, hydrogen, chemicals, electricity, and/orliquid fuels production. The test results obtained from the

* To whom correspondence should be addressed. E-mail: [email protected].

Ind. Eng. Chem. Res. 2010, 49, 10200–1021110200

10.1021/ie1005542 2010 American Chemical SocietyPublished on Web 07/30/2010

small pilot scale chemical looping combustion and gasifica-tion units are described. The various process configurationsand energy conversion efficiencies of the chemical loopingprocesses for combustion and gasification applications aregiven. Novel chemical looping applications, including thosefor steam-methane reforming and power generation using fuelcell, are also discussed. It is noted that this paper focusesmainly on the metal-metal oxide particle based chemicallooping processes in which the chemical looping particle actsas an oxygen carrier. The metal oxide-metal carbonatelooping processes,3 which capture carbon through the car-bonation reaction of a metal oxide based CO2 sorbent, arenot within the scope of this article.

2. History of Chemical Looping and Chemical LoopingBased Exergy Optimization Strategy

A given reaction can be decomposed into multiple subreac-tions in a reaction scheme using chemical intermediates thatare reacted and regenerated through the progress of thesubreactions. A reaction scheme of this nature is referred to aschemical looping. Two types of chemical looping systems, thatis, chemical looping gasification (CLG) and chemical loopingcombustion (CLC), are often used for fossil fuel conversions.The CLG process produces hydrogen from carbonaceous fuels.In most cases, heat is also produced. The CLC process indirectlycombusts the fuel to generate heat; no hydrogen is generatedin the CLC process.

The principles of chemical looping for carbonaceous fuelconversion were first applied for industrial practice between thelate nineteenth century and the early twentieth century. HowardLane from England was among the first researchers/engineerswho conceived and successfully commercialized the steam-ironprocess for hydrogen production using the chemical loopinggasification principle.4 With the aid of the iron oxide chemicalintermediate, the steam-iron process generates H2 from reducinggas obtained from coal and steam through an indirect reactionscheme:

The first commercial steam-iron process, based on the HowardLane design, was constructed in 1904. Hydrogen plants basedon the same process were then constructed throughout Europeand the U.S., producing 850 million cubic feet of hydrogenannually by 1913.5 The steam-iron process only partiallyconverts the reducing gas. Moreover, the iron based loopingmedium exhibited poor recyclability, especially in the presenceof sulfur.5-8 With the introduction of less costly hydrogenproduction techniques using oil and natural gas as feedstock inthe 1940s, the steam-iron process became less competitive andwas phased out.

In the 1950s, the chemical looping combustion (CLC) schemewas proposed for CO2 generation.9,10 A schematic flow diagramof the fundamental CLC scheme is given in Figure 1.

Following Figure 1, the Lewis and Gilliland Process usedtwo fluidized bed reactors, the CO2 generator or reducer andthe metal oxide regenerator or oxidizer, for continuous CO2

production. Oxides of copper or iron were used as the looping

particles and carbonaceous fuels such as coal, carbon monoxide,and syngas were used as the feedstock.9 The reaction scheme,which is somewhat similar to that in the steam-iron process, isgiven below:

In the early years, the adoption of a chemical looping strategywas mainly prompted by the lack of effective chemicalconversion/separation techniques for product generation. Thelack of understanding in oxygen carrier particles development,looping reactor and process design, and chemical reactionengineering renders the early chemical looping process far fromoptimized. The modern applications of chemical loopingprocesses are prompted by the need for developing an optimizedfossil fuel conversion scheme that minimizes the exergy lossinvolved in energy conversion and CO2 capture, thereby yieldingan overall efficient and economical process system.11-17 Asimplified exergy analysis given in Figure 2 illustrates thepotential application of the chemical looping gasification concept

Fe3O4 + 4CO (or H2) f 3Fe + 4CO2 (or H2O)

3Fe + 4H2O f 4H2 + Fe3O4

Net reaction: CO + H2O f CO2 + H2

Looping medium: Fe3O4 T Fe

Figure 1. Schematic flow diagram of the CLC process.

Figure 2. Exergy recovering scheme for carbon gasification/water gas shiftprocess, scheme I (top) and a simplified chemical gasification processscheme, scheme II (bottom; the reaction temperature is assumed to be 1123K for scheme II).

MeO (oxide of copper or iron) + CO/H2 f Me + CO2/H2O

Me + 1/2O2 f MeO

Net reaction: CO/H2 + 1/2O2 f CO2/H2OLooping medium: Oxides of copper or iron

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 10201

for efficient carbonaceous fuel conversion. The analysis involvesthe following key assumptions and definitions:

(1) The environmental temperature is T0 ) 273.15 K and theenvironmental pressure is P0 ) 101,325 Pa.

(2) The reference substance for C and CO is 400 ppm CO2,the reference substance for H2 is water, and reference substancefor Fe, FeO, and Fe3O4 is Fe2O3.

(3) Coal is considered as pure carbon.(4) Heat can be integrated with a 100% efficiency whenever

feasible.(5) The enthalpy of devaluation is defined as the enthalpy

change for a substance from the current state to its referencestate.

(6) The feedstock outside of the reaction loop is at theenvironmental state.

Scheme I represents a traditional route where carbon isconverted into syngas followed by the water gas shift reactionto produce hydrogen. As can be seen, the gasification step leadsto at least 12% exergy loss because of partial oxidation. Thewater gas shift reaction after the gasification step leads to another8.8% exergy loss because of the conversion of CO into H2 thathas both lower enthalpy of devaluation and a lower exergy rate.The exergy loss in separating hydrogen from CO2 is not takeninto account.

Scheme II illustrates an alternative chemical looping gasifica-tion approach. Here, Fe is used as a chemical intermediate toconvert carbon into hydrogen. There are also two steps involvedin scheme II:

As can be seen, much less exergy loss occurs in step 1 (23.1kJ) compared to the traditional gasification step (48.8 kJ).Moreover, a zero exergy loss is achievable in step 2 of schemeII through integration of a small amount of low grade heat. Asa result, the exergy loss per mole of H2 produced is reduced bymore than four times using the chemical looping approach.

The results presented in the examples are based on a set ofsimplified assumptions. Thus, they represent an upper boundto the energy conversion efficiencies in the processes. Neverthe-less, the results provide a guide for comparisons of theconventional option versus the chemical looping gasificationoption. It can also be shown, through simple exergy analysis,that the chemical looping combustion (CLC) process can bemore exegetically efficient than conventional power generationprocesses. To generalize, the exergy loss can be lowered througha well conceived chemical looping scheme comprising stepsthat are less irreversible. The irreversibility of the looping stepsis affected, to a large degree, by the type and performance ofthe chemical looping medium or particle, the chemical loopingreactions, and the chemical looping reactor and system design.

3. Chemical Looping Particles

A successful operation of chemical looping processes stronglydepends on the effective performance of the chemical loopingparticles or oxygen carrier particles. Many factors are involvedin the design of particles that possess desirable properties. Thissection discusses the desired characteristics for the chemicallooping particles and the role of thermodynamic analysis onthe selection of the particle and reactor design.

3.1. General Particle Characteristics. The chemical loopingparticles are usually composed of primary metal oxide, support,

and promoter or doping agents. The design and preparation ofthese composite particles involve, to date, a complex trial anderror procedure. Possible variables include types of metal oxideand support, weight percentage, and synthesis method andprocedure. Effective particle development requires comprehen-sive consideration of various desired properties of the loopingparticles including:

• Suitable thermodynamic properties.• Good oxygen carrying capacity.• Good gas conversions in both the reduction and oxidation

reactions.• High rates of reaction.• Satisfactory long-term recyclability and durability.• Good mechanical strength.• Suitable heat capacity and high melting points.• Ability to change the heat of reaction.• Low cost and ease in scale-up of synthesis procedure.• Suitable particle size.• Resistance to contaminants and inhibition of carbon

formations.• Minimal health and environmental impacts.Over 700 different particles have been tested with the focus

on redox reactivity, recyclability, and attrition behavior relatedto chemical looping combustion applications.18,19 Severalpromising particles have been identified. In terms of primarymetal/metal oxides, they are Ni/NiO, Cu/CuO, Fe/FeO, Fe3O4/Fe2O3, and MnO/Mn3O4 with the support of Al2O3, TiO2, Ni-,Co-, or Mg-Al2O4, bentonite, or ZrO2. The reactivities ofvarious metal/metal oxides with different inert support inreduction reactions with CH4 and oxidation reactions with airat different temperatures have been reported.16 It is concludedthat, in general, Ni and Cu have high activities. However, Cusinters at 950 °C, which limits its application at high temper-atures. Fe exhibits a moderate activity with the Fe2O3-Feconversion usually being incomplete.20 The Mn activity varieswith the active metal oxide content and type of inert support.With the ever expanding database on chemical looping particledevelopment, a rational particle design procedure should bedeveloped with consideration of the various physical andchemical characteristics of different primary metal oxide andsupport, previous testing results, and the specific looping processrequirements. A thorough thermodynamic analysis is almostalways required for the design of optimum chemical loopingparticle and reactor system.

3.2. Thermodynamic Properties of Metal and MetalOxides and Reactor Design Optimization. The capability tocapture CO2 is required for modern chemical looping processes.Thus, it would be desired that the primary metal oxide in thechemical looping particle can fully convert carbonaceous fuelsinto CO2 and H2O while the reduced metal can be fullyregenerated back to metal oxide through reaction with theoxidizing agents such as steam, air, or CO2. The maximumextents of the metal oxide reduction and oxidation reactionsare dictated by the thermodynamic relationship. Figure 3 showsthe thermodynamic phase diagrams of the Ni-H-O andNi-C-O systems (Figure 3a) and Ni-O system (Figure 3b).As can be seen in Figure 3a, a mixture of NiO and Niequilibrates with a steam and hydrogen mixture that contains∼99% steam at 900 °C. This phase diagram indicates that, whenpure hydrogen is used as the fuel, the presence of excess NiOcan lead to a maximum hydrogen conversion of ∼99% at 900°C. Similarly, the maximum CO conversion is also ∼99% at900 °C. Therefore, when used in the chemical looping reducer,NiO can convert nearly all the syngas fuel into a concentrated

Step 1 C + 0.395Fe3O4 + 0.21O2 f 1.185Fe + CO2

Step 2 3Fe + 4H2O f Fe3O4 + 4H2

10202 Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

CO2 and H2O mixture. The analysis on phase diagrams of nickelwith other fuels such as methane indicates that NiO is alsosuitable for oxidizing other carbonaceous fuels. The equilibriumphase diagram of Ni-H-O system shown in Figure 3a alsoreveals that Ni cannot convert a significant portion of steaminto hydrogen. Therefore, it is not suitable for chemical loopinggasification applications. Since Ni equilibrates with extremelylow partial pressure of oxygen as shown in Figure 3b, it can beeasily regenerated to NiO with air in the CLC oxidizer. Theabove analysis indicates that Ni/NiO particle is suitable to beused for chemical looping combustion; however, it is not suitableto be used for chemical looping gasification.

The thermodynamic analysis on metals with two oxidationstates such as nickel can be conducted with ease. The thermo-dynamic analysis on metals with more than two oxidation statessuch as Fe and Mn, however, is complex since different metaloxides may exhibit significantly different thermodynamicproperties. The information from the thermodynamic analysisnot only can reveal the suitability of certain types of metal forchemical looping reaction applications, it can also guide theoptimum strategy in designing the reducer or oxidizer reactorsfor the chemical looping reaction operation.

To illustrate the relationship between the thermodynamicproperties of the metal and metal oxide(s) and the reactor design,two types of oxygen carriers are considered, that is, NiO andFe2O3. With NiO as the looping particle in the reducer, themaximum fuel and NiO conversions can be achieved irrespectiveto the type of the multiphase flow reactor used from thethermodynamic standpoint. This is because the oxygen carrierhas only two oxidation states, that is, Ni and NiO. With Fe2O3

as the looping particle in the reducer, on the other hand, boththe extent of the fuel and the looping particle conversions woulddiffer depending on the types of the multiphase reactors used.This is because the oxygen carrier, Fe2O3, has multiple oxidationstates as shown in Figure 4. When a fluidized bed is used asthe reducer, the partial pressure ratio of CO2 to CO is expectedto be high throughout the reactor. Therefore, Fe2O3 can onlybe reduced to Fe3O4 as indicated in Figure 4. Thus, using afluidized bed, the extent of the conversion of Fe2O3 in thereduction reaction is to be low. In contrast, when a counter-current moving bed reactor is used, the extent of the conversionof Fe2O3 can be high because of a low partial pressure ratio ofCO2 to CO at the solids outlet, yielding a reduced product inthe form of Fe/FeO. Thus, to achieve a given extent of the fuel

conversions in a reducer, a significantly smaller amount of Fe2O3

particles is needed using a moving bed reactor as compared toa fluidized bed reactor. Similarly, a countercurrent moving bedreactor enhances the steam to hydrogen conversion during theregeneration of the reduced Fe/FeO particles.21 Since thermo-dynamic analysis reveals the intrinsic physical chemical proper-ties of the particle, it could be used as a screening step inselecting the type of looping particles to be used and the typeof reactor to be designed.

The development of an ideal chemical looping particlerequires also to consider such intertwining factors as the reactionkinetics, cost, recyclability, physical strength, ease in heatintegration, resistance to contaminants, and environmental andhealth effects. For metal oxides such as Fe2O3, a countercurrentfuel-particle contact mode conducted in a moving bed canachieve a maximum metal oxide reduction conversion, whichis not practically achievable by a fluidized bed. Although thereis no omni particle that possesses all the desired properties, asdescribed above, and can be utilized for all looping processapplications, a proper selection of a suitable primary metal oxidematerial, as well as its support and doping agent based on thetype of the fuel to be converted and the intended product to begenerated, is key to the ultimate development of a suitable

Figure 3. (a) Equilibrium phase diagram for Ni-NiO system for redox reactions with CO2/CO (solid line) and H2O/H2 (dashed line). (b) Equilibrium phasediagram for Ni-NiO and O2.

Figure 4. Thermodynamics of iron oxides and CO (solid line)/H2 (dashedline) reactions in fluidized bed and countercurrent moving bed reactors.


looping particle. Moreover, optimized reactor design is requiredto render the process operation economically feasible. Thefollowing section illustrates the primary metal selection for onespecific application.

3.3. Selection of Primary Metal for Chemical LoopingCombustion of Coal. A survey of several metal oxidecandidates for chemical looping combustion of coal is given inTable 1. Among the metal oxides, iron oxide is of a low costand has a high oxygen carrying capacity, favorable thermody-namics, a high melting point, and good mechanical strength.Further, iron induces low health and environmental effects. Thereactivity of iron particle is, however, relatively low. NiO isoften considered as a good oxygen carrier as it reacts fasterwith CO or H2

25 when compared to Fe2O3. However, it is notedthat, in the presence of H2O and/or CO2, the reaction ratebetween metal oxides and coal char is controlled by chargasification rather than the slow solid-solid reaction betweencoal char and metal oxide:26

CO2 or H2O acts as an enhancer which improves thesolid-solid reaction rate. For both NiO and Fe2O3, Reaction 2is much faster than Reaction 1. Therefore, in the presence ofCO2/H2O, the reaction rate between NiO and coal char shouldbe similar to that between Fe2O3 and coal char. Thus, thereactivity advantage of NiO with syngas is “silenced” whenusing coal in the chemical looping combustion system. Notingthat the iron based oxygen carrier particle is far less costly thanthe nickel based oxygen carriers; thus, iron is a more favorableprimary metal for chemical looping combustion of coal.

4. Chemical Looping Combustion Systems

As discussed in the introduction, the CLC concept can betraced back to the pioneering study of Lewis and Gilliland inthe 1950s when they proposed to use redox reaction of metaloxides to produce carbon dioxide from syngas. In their process,carbon dioxide was the desired product. In the late 1960s, theCLC process was proposed as a novel fuel conversion routethat reduces the irreversibility of fuel combustion for heat andpower generation.11,14 Verified to be advantageous by thermo-dynamic analysis especially under a carbon constrainedscenario,14,17,27 the chemical looping concept has been exten-sively explored during the last two decades. Studies carried outin the 1980s and 1990s focused on the development of thechemical looping particles and their applications in CLCprocesses using gaseous fuels such as methane and syngas.28-31

From the beginning of this century, the possibilities of utilizing

solid fuels such as coal and biomass in a CLC system havealso been considered.32-36 This section discusses the CLCreactor design and operational results.

4.1. Gaseous Fuel CLC System and Operational Results.The promising results obtained from the lab and bench scaleexperiments in the early 1990s prompted efforts for testing theCLC process at larger scales. Prior operational experiences inindustrial circulating fluidized bed (CFB) processes for FCCprocessing and coal combustion provide a logical basis and afundamental framework for their extension to CLC applications.As a result, nearly all the existing subpilot scale CLC systemsemploy a CFB design. The existing subpilot scale CLC testingunits include the Chalmers University 10 kWth CLC system,37

the Instituto de Carboquimica (CSIC) 10 kWth CLC system,38

the Korea Institute of Energy Research (KIER) 50 kWth CLCsystem,39 and the Vienna University of Technology (VUT) 120kWth CLC system.40,41 This section focuses on the design andoperational results obtained from the VUT 120 kWth CLCsystem.

As illustrated in Figure 5, the VUT CLC unit is a dualcirculating fluidized bed (DCFB). The DCFB system has adesigned fuel power of 120 kWth.

40,41 As can be seen fromFigure 5, the DCFB system consists of mainly a reducer (fuelreactor), an oxidizer (air reactor), an upper loop seal, a lowerloop seal, an internal loop seal, and two cyclones. The systemforms two particle circulation loops, the global particle circula-tion loop, and the local particle circulation loop. The cycloneand internal loop seal placed around the reducer allow for localparticles circulation within the reducer, independent of the globalparticles circulation between the oxidizer and the reducer. Thereducer is operated at near the turbulent regime, and the oxidi-

Table 1. Comparisons of the Key Properties of Different MetalOxide Candidates20,22-24

Fe2O3 NiO CuO CoO

costa + - - -oxygen capacityb (wt %) 30 21 20 21thermodynamicsc + + + +kinetics/reactivityd - + + -melting points + ∼ - +strength + - ∼ ∼environmental and health effects ∼ - - -

a +, positive; -, negative; ∼, neutral. b Maximum possible oxygencarrying capacity by weight. c Capability to fully oxidize C, CO, and H2

to CO2 and H2O according to thermodynamic principles. d Reactivityrefers to the rates of the reactions between metal oxides and syngas (COand H2).

H2O/CO2 + C f CO + H2/CO (1)

MeO + H2/CO f Me + H2O/CO2 (2)

Figure 5. Schematic diagram of the dual circulating fluidized bed (DCFB)system constructed at the Vienna University of Technology.


zer is operated at the dilute or dense transport regime. Thefluidization agents for the reducer and the oxidizer are gaseousfuel and air, respectively. Gaseous fuel is injected at the bottomof the reducer, and air is injected at both the bottom (primaryair inlet) and the middle portion (secondary air inlet) of theoxidizer. The staged injection of air enables effective controlover the global solids circulation rate. At a fixed overall airinjection rate, a larger primary air injection rate leads to a higherglobal solids circulation rate. As it is compatible with theoperation of both the oxidizer and the reducer, steam is used asthe fluidization gas in all the loop seals.

The VUT DCFB system was constructed in 2007. Varioustests have been performed since early 2008. Different types offuels such as hydrogen, syngas, natural gas, and propane areconverted in the DCFB with both ilmenite (FeTiO3) particlesand nickel oxide based particles. Ilmenite particles with a meandiameter of ∼220 µm have been used to convert H2, syngas,and natural gas. When the reducer is operated at 960 °C with asystem solids inventory of 70 kg, the syngas (1:1 of H2: CO bymole) conversion decreases from ∼85% at 40 kWth to ∼72%at 90 kWth. When methane is used as the fuel, the highest CO2

yield, which is defined by the fraction of carbon in the fuel thatis converted into CO2, was 67%. The corresponding methaneconversion was 72%. This was achieved at a low fuel processingcapacity (20 kWth) with the addition of olivine (ilmenite/olivine) 4.7:1 by weight).42 The low fuel gas conversion and CO2

yield were attributed to the low reactivity of the ilmeniteparticles.

NiO based oxygen carrier particles are also tested in the VUTDCFB system. The oxygen carrier particles used in the testsare a 50:50 mixture of NiAl2O4 supported NiO particles andMgAl2O4/NiAl2O4 supported NiO particles.43,44 The active NiOin the mixture of particles amounts to 40% of the particle weight.The mean particle size is 135 µm.43 Up to 140 kWth fuel processcapacity using natural gas was reached. Both the methaneconversion and CO2 yield were above 90%. Table 2 generalizesthe operational parameters and results of the VUT DCFB systemusing both ilmenite particles and NiO based particles. On thebasis of the test results obtained at a fuel processing capacityof 140 kWth, the solids inventory for a commercial DCFBsystem is estimated to be 240 kg/MWth. The correspondingglobal solids circulation rate is 11.4 kg/s ·MWth or 41,000 t/hfor a 1,000 MWth CLC plant. Such a high solids circulationrate results from the operating mode of the DCFB system, whichmaximizes the gas and solids reaction rates at the expense ofthe extents of the solids conversion.

Note that in all the tests performed with natural gas as thefuel, at least 5 vol % (dry basis) of the reducer exhaust gas isstill combustible fuel. Although a higher fuel conversion and aCO2 yield may be achieved in a scale-up unit, further treatmentto the reducer exhaust gas is likely to be necessary to avoid anenergy penalty and to minimize environmental impacts. Such

treatments include further combustion of the reducer exhaustgas with oxygen.

4.2. Solid Fuel CLC Systems and Operational Results.Solid fuels such as coal, petroleum coke, and biomass arenotably cheaper than gaseous fuels such as natural gas andsyngas. Therefore, it is desirable to convert solid fuels directlyin the CLC systems. Laboratory experiments using various typesof solid fuels reacting with oxygen carrier particles in circulatingfluidized bed systems revealed the possibility of converting solidfuels in a CLC scheme.25,33,35,36 This section reviews the designand operational results from the Chalmers University 10 kWth

solid fuel CLC unit.46,47 Testing results on another 10 kWth solidfuel CLC unit, which was designed and constructed by theSoutheast University in China, are available from a number ofpublications.48-50

The key difference between the solid fuel CLC unit and itsgaseous fuel counterpart lies in the design of the reducer orfuel reactor. To handle two solids, namely, the solid fuel andthe chemical looping particles, the reducer of the ChalmersUniversity solid fuel CLC unit is divided into three chambers,that is, a low velocity chamber for fuel devolatilization andconversion, a high velocity chamber for solids recirculation, anda carbon stripper chamber to recover the unconverted fuel fromthe oxygen carrier particles to be transferred to the oxidizer orair reactor. Auxiliary units added around the reducer were acoal feeder and an internal particle circulation system consistingof a small riser and cyclone. The schematic of the ChalmersUniversity solid fuel CLC system is given in Figure 6.

Two types of solid fuels of different characteristics, SouthAfrican coal46 and petroleum coke,47 were tested for 22 and11 h, respectively in the unit. Natural ilmenite particles wereused as the oxygen carrier to convert both fuels. The solid fuelconversion, defined by the fuel conversion in both the reducerand the oxidizer, varied from 50% to 80% for South Africancoal. The low conversion was attributed to a low efficiency ofthe cyclone used in the carbon stripper system. Modificationsto the carbon stripper system were made before the test usingpetroleum coke as the fuel.47 Around 70% conversion ofpetroleum coke was observed. The low fuel conversion wasattributed to the low reactivity of the fuel and the unsatisfactoryperformance of the modified carbon stripper cyclone. In bothtests, the CO2 concentration in the reducer exhaust ranges from70-80% (dry basis). In addition, a notable amount of carbonwas carried over to the oxidizer. As a result, the exhaust gasfrom the oxidizer is oxygen depleted air that contains CO2. Theoxygen carrier particles did not exhibit a tendency to agglomer-ate for experiments using both solid fuels. This observation isconsidered an initial indication of smooth operation with theseoxygen carriers. However, a short duration of testing renders itdifficult to conclusively assess the performance of the particles.The results obtained from the solid fuel tests indicate that thereducer solids inventories used in the experiment, ∼1,506 kg/

Table 2. Operational Results of the VUT DCFB Unit (Data with Air/Fuel Ratio <1 are Not Shown)42-45

type of particle

ilmenite NiO

parameter H2 syngas NG H2 CO NG propane

fuel processing capacity (kWth) 24-90 40-92 20-130 n/a n/a 60-140 127reducer temperature (°C) 894-966 878-976 839-969 850-900 850-900 800-950 900fuel conversion (%) 87.5-96 72-85 28-72 80-99+ 95-99 93-96 n/aCO2 yield (%) n./a 57-77 25-66 n/a 95-99 72-94 94system solids inventory (kg) 70-90 70-85 70-90 55-75 65-75 65 65olivine addition (kg) 0 0 0-15 0 0 0 0air/fuel ratio 1-1.2 1-1.2 1.1


MWth for South African Coal and ∼862 kg/MWth for petroleumcoke, are inadequate for an effective solid fuel conversion.Further testing with solid fuels for extended, stable, continuousoperation is desirable before the performance of the solid fuelCLC system can be comprehensively evaluated.

Apart from the two aforementioned solid fuel CLC systems,a 1 MWth coal fired CLC prototype is being developed underthe ECLAIR project sponsored by European Commission’sResearch Fund for Coal and Steel program. Numerical modelingof reactor system hydrodynamics, cold flow model testing, andreaction kinetics studies are being carried out. According toALSTOM, which is a leading partner of the project, the 1 MWth

prototype will be constructed and operated in 2010.51

Because of the various challenges in solid fuel conversions,the current solid fuel CLC systems yield a lower fuel conversionefficiency and a lower CO2 capture efficiency than the gaseousfuel CLC systems. Further research on oxygen carrier particlesynthesis, solid fuel conversion enhancement, improvements ofthe contact between coal char/volatile and oxygen carrier, anddesign of a reducer unit with effective residue char strippingsystem is required for the development of solid fuel CLCsystems. A process mass balance indicates that, for both gasand solid fuel CLC systems, a large air flow rate is required ina commercial CLC oxidizer. Considering the large solidscirculation rate resulting from the relatively low particleconversion in the CFB units, a commercial CLC system requires

either a low pressure oxidizer with an extremely large overallcross-sectional area or high pressure oxidizer with high solidsflux. Therefore, the design of the commercial CLC system mayface different challenges than those for the current test units.

5. Chemical Looping Gasification Processes

A modern chemical looping gasification process, which is exem-plified by the Syngas Chemical Looping (SCL) Process,21,23,52 theENI chemical looping process,53 and the Coal Direct ChemicalLooping (CDCL) Process,19 extends the concepts of earlierprocesses such as the steam-iron process5 and the chemical loopingcombustion process.9 A typical modern chemical looping gasifica-tion scheme consists of three steps:

This section discusses the chemical looping gasification processconfigurations and the results from bench and subpilot scale testswith the focus on the SCL and CDCL processes.

5.1. Syngas Chemical Looping Process (SCL). The SyngasChemical Looping (SCL) Process coproduces hydrogen andelectricity from syngas. A pure hydrogen stream, a concentratedcarbon dioxide stream, and heat are produced in three separatereactors, avoiding additional product and CO2 separation steps.A specially tailored iron oxide composite particle that canundergo multiple reduction-oxidation cycles is used in theprocess.23,52,54 The SCL process consists of two major com-ponents: a syngas generation and clean up system and a chemicallooping system. Figure 7 shows a simplified block diagram ofthe SCL process.

The syngas generation and cleanup system in the SCL processis similar to that in a conventional coal gasification process.Syngas with low to moderate levels of pollutants (i.e., HCl, NH3,sulfur, and mercury) is first produced from the syngas generationand cleanup units. This syngas stream is then used as the fuelin the chemical looping system, which consists of three chemicallooping reactors: the reducer, the oxidizer, and the combustoror combustion train. In the reducer, syngas is used to reducethe iron oxide composite particles. The syngas is converted intocarbon dioxide and water while the iron oxide compositeparticles are reduced to a mixture of Fe and FeO at 750-900°C. The Fe/FeO particles leaving the reducer are then introducedinto the oxidizer, which is operated at 500-750 °C. In theoxidizer, the reduced particles react with steam to produce agas stream that contains only H2 and unconverted steam. Thesteam can be easily condensed out to obtain a high purity H2

stream. Meanwhile, the Fe and FeO are regenerated to Fe3O4.The Fe3O4 formed in the oxidizer is further regenerated to Fe2O3

in an entrained flow combustor which also transports solidparticles discharged from the oxidizer to the reducer inlet,completing the chemical loop. A portion of the heat producedfrom the oxidation of Fe3O4 to Fe2O3 is transferred to the reducerthrough the particles. Spent air produced from the combustor,at high pressure and high temperature, can be used for powergeneration. In yet another configuration, a sub-stoichiometric

Figure 6. Schematic diagram of the modified Chalmers University 10 kWth

solid fuel CLC unit (A - Reducer; B - Oxidizer; C - Riser).46

Step 1: Fe2O3 + Fuel (gaseous, liquid, or solid) fFe/FeO + CO2/H2O

Step 2: Fe/FeO + H2O f H2 + Fe3O4

Step 3: Fe3O4 + Air f Fe2O3 + Oxygen Depleted Air

Net reaction: Fuel + H2O + O2 f CO2 + H2

Looping medium: Fe2O3 T Fe/FeO


amount of steam is introduced to the reducer if more heat orelectricity is desired. Hence, both chemical looping gasificationand chemical looping combustion concepts are applied in theSCL system, rendering it a versatile process for H2 and elec-tricity coproduction.

The SCL process has been tested in a 2.5 kWth bench unitand a 25 kWth subpilot unit (Figure 8) with combined operatingtime of 100+ hours and 40 h, respectively. Close to 50%reduction of Fe2O3 was achieved in both units. This extent ofreduction corresponds to a mixture of Fe and FeO in the reducedparticles. The syngas conversion in the bench unit was above

99.9% with the average hydrogen product purity of 99.95%.The subpilot unit achieved 99+% syngas conversion and93-99% hydrogen purity. The decrease in hydrogen purity wasdue to the carbon deposition from Boudouard reaction, whichwas catalyzed by the reduced Fe2O3 particle at a low temperaturezone in the reducer gas inlet. A syngas preheating unit wasinstalled for the subpilot system to prevent the carbon deposition.A 99.8% methane conversion was also achieved in the benchscale reducer. No significant carbon deposition was observed.The conversion of the Fe2O3 particle was also close to 50%.Both the gas and solid conversions achieved in the moving bedunits are close to the equilibrium conversions predicted bythermodynamic anlaysis.21 A 250 kWth scale SCL system iscurrently being designed and constructed under the sponsorshipof the Advanced Research Projects Agency - Energy (ARPA-E) of the U.S. Department of Energy.

Comprehensive process analyses have also been carried outusing ASPEN Plus. A common set of assumptions is used tocompare the performance of the SCL process with the Inte-grated Gasification Combined Cycle (IGCC) and the gasifica-tion based coal to hydrogen processes. Table 3 shows theprocess analysis results. As can be seen, the SCL processyields a higher efficiency than do the conventional gasifica-tion processes, especially under a carbon constrained scenario.

5.2. Coal-Direct Chemical Looping Gasification Processes.The Coal Direct Chemical Looping Process is another chemicallooping gasification process that can drastically simplify the coalconversion scheme. A simplified flow diagram of the CDCLprocess is shown in Figure 9. Here, a composite Fe2O3 particlesimilar to that used in the Syngas Chemical Looping (SCL)Process is used for converting coal to hydrogen and/or electric-ity. In this process, Fe2O3 particles are introduced into thereducer together with fine coal powder. By using suitablegas-solid contacting patterns, coal is gasified to CO and H2.The reductive gas will convert Fe2O3 particles to Fe and FeO,

Figure 7. Simplified schematic of the Syngas Chemical Looping Process for hydrogen production from coal.

Figure 8. Schematic flow diagram of the subpilot scale unit of 25 kWth forSCL process.

Table 3. Comparison of the Process Analysis Results55

IGCCprocess

SCLprocess

electricity

conventionalcoal to hydrogen

processSCL

process

coal feed (t/h) 132.9 132.9 132.9 132.9carbon capture (%) 90 100 90 100hydrogen (t/h) 0 0 14.4 15.6net power56 321 365 2.1 26efficiency (%HHV) 32.1 36.5 57.8 64.1


while generating a concentrated CO2 and H2O flue gas stream.H2O in the flue gas can be readily condensed, leaving sequestra-tion-ready CO2. The reduced Fe/FeO particles from the reducerenter the oxidizer and react with steam to produce hydrogenwhile being oxidized to Fe3O4. The resulting Fe3O4 exiting fromthe hydrogen production reactor is further oxidized before beingreturned to the reducer pneumatically in its original form ofFe2O3. Similar to the SCL process, more heat/electricty can begenerated from the CDCL process when a substoichiometricamount of steam is sent to the oxidizer.

The reducer is the most crucial unit for the CDCL process.Figure 10 shows a design of the CDCL reducer. In thisconfiguration, fresh Fe2O3 composite particles are fed from thetop of the reducer while pulverized coal is pneumaticallyconveyed to the middle section of the reactor using CO2. Asmall amount of CO2 or steam is also introduced from thebottom of the reducer to enhance char conversion. The coalinjection port divides the reducer into two sections. The intendedfunction for the upper section (Stage I) is to achieve a fullconversion of gaseous species to CO2 and H2O, while the lowersection (Stage II) is to maximize the char and iron oxideconversions.

The CDCL process has been tested in a bench scale movingbed unit. The operations of Stage I and Stage II of the reducerwere tested separately with either coal volatiles or coal char.Pittsburgh #8 coal volatiles, Pittsburgh #8 coal char, lignite coalchar, and anthracite coal have been tested. More than 90%conversions for all the aforementioned fuels were achieved. TheCO2 concentration at the reducer outlet exceeded 95%. Thestudies of the effects of sulfur and ash indicate that the reactivityof the particle was maintained after more than 10 h of operation.Since the composite Fe2O3 particles are significantly larger thancoal powders, coal ash can be readily separated from the Fe2O3

particles based on the size difference. The ASPEN Plussimulation performed on the CDCL process indicate that theCDCL process can be ∼79% efficient for hydrogen productionand nearly 50% efficient for electricity generation, both withclose to 100% CO2 capture. Thus, the energy conversionefficiency for the CDCL Process is almost 20% higher than thetraditional coal gasification WGS process for H2 production.The improved energy management (Figure 11) contributes tothe high efficiency for the CDCL Process.

Figure 9. Schematic diagram of the Coal-Direct Chemical Looping Process (---- steam).

Figure 10. Gas-solid contacting pattern of the reducer. Figure 11. Material flow and energy flow in a CDCL Process.


6. Novel Applications of the Chemical Looping Processes

The chemical looping strategy can be conveniently integratedwith other process systems to achieve improved energy conver-sion efficiency. For instance, the hydrogen generated in thechemical looping gasification system can be used in theFischer-Tropsch (F-T) reactor to enhance the liquid fuel yieldof an indirect coal to liquids process.57 Alternatively, thechemical looping system can be strategically integrated withexisting fuel cell systems for efficient power generation. Thefollowing describes two examples of the possible applicationsof such strategies.

6.1. Enhanced Steam Methane Reforming. Chemical loop-ing combustion integrated with the steam methane reformingprocess (CLC-SMR) was studied by Ryden and Lyngfelt atChalmers University.58 A schematic flow diagram of the CLC-SMR process is given in Figure 12. The CLC-SMR processutilizes an oxidizer that is identical to a CLC process given inSection 3. The uniqueness of the CLC-SMR process lies in itsreducer which is integrated with a steam methane reformer. Theintegrated reducer/reformer reactor is composed of a low-velocity bubbling fluidized bed with reformer tubes that areplaced inside the reactor. The tubes are filled with reformingcatalysts which convert steam and methane into syngas. Thesyngas from the reformer tubes is then shifted to a hydrogenrich stream in a water gas shift reactor before it is purified inthe pressure swing adsorption (PSA) units downstream. The tailgas from the PSA is used as the fuel for the reducer. Themethane reforming reaction scheme in the CLC-SMR processis practically identical to that in the traditional steam methanereforming (SMR) process with the exception of the heatintegration scheme for the reformer. In the traditional SMRprocess, the heat required for steam methane reforming isprovided by combustion of fuel exterior to the reformer tubes.In contrast, the CLC-SMR process uses high temperature oxygencarrier particles as a heat transfer medium. Theoretical analysisindicates that the CLC-SMR process has the potential to achievea higher H2 yield than the conventional SMR process. Therefore,it could offer a promising alternative for a reforming operation.However, a number of challenges including the heat require-ments for the reducer/reformer and potential erosion for thereformer tubes need to be addressed before the process can beconsidered viable.

6.2. Chemical Looping Gasification Integrated with FuelCells. An advanced electricity generation scheme includes theintegration of a coal direct chemical looping reducer and a directsolid fuel cell. As shown in Figure 13, by modifying theelectrochemical cell to oxidize supported Fe to supported Fe2O3,thereby generating electricity, a system that integrates a chemicallooping reducer and a direct solid fuel cell can be developed.In this system, reduced metal particles from the chemical loopingreducer are directly fed into a solid oxide fuel cell that canprocess solids directly. Particles are reduced in the reducer andthen introduced to the fuel cell to react with oxygen or air at500-1000 °C. The oxidized particles are recycled back to thereducer to complete the loop. It is desirable for particles to beconductive at both the reduced and the oxidized states.

The Ohio State University (OSU) and University of Akron

(UA) are developing chemical looping solid oxide fuel cellsystems. The preliminary study of the solid oxide fuel cell atUA indicates that the supported Fe particle, which produces 45mA/cm2 at 0.4 V and 800 °C, can be a more effective fuel thanpulverized coal. The resultant oxidized particle does not adhereto the anode surface of the fuel cell. The results suggest that anintegrated fuel cell with chemical looping could yield a highlyefficient electricity generation system from coal with integratedCO2 capture. Much work, however, is needed for successfuldevelopment of the chemical looping solid fuel cell system.

Figure 12. Schematic flow diagram of the chemical looping combustion integrate with the steam methane reforming process (CLC-SMR).58

Figure 13. Direct solid oxide fuel cell applications for chemical looping.


7. Concluding Remarks

Uniquely versatile, chemical looping has the potential to bean efficient and environmentally friendly fossil fuel conversionstrategy. Over the past century, many attempts had been madeon the utilization of the chemical looping strategy for fossil fuelconversions. At present, however, no chemical looping processesusing fossil fuels are in commercial operation. The key factorsthat hampered the continued use of the early chemical loopingprocesses was the high product cost resulting from the lowreactivity and recyclability of the chemical looping particles,the low conversions of both the fuel and the particles, and theinadequate understanding of process design and exergeticoptimization. The renewed fundamental and applied researchin chemical looping, which is particularly motivated by theglobal concerns over CO2 emissions and the high energy cost,resumes with a specific attention toward addressing the afore-mentioned shortcomings. Considerable R&D efforts, however,are still required for the eventual commercialization of the fossilfuel chemical looping processes.

An element that is key to the success of the chemical loopingprocess operation is the property of the metal oxide basedchemical looping particles. Synthesis of effective metal oxidesrequires consideration of the intricate interrelationships amongvarious reaction and process factors. The reaction factors ofimportance include types and thermodynamic properties of metaloxides and support materials, oxygen transfer capacity, gas andsolid conversions, rates in both reduction and oxidation reac-tions, heat capacity and heat of reactions, melting points,mechanical strength, long-term recyclability, ease in scale up,health and environmental effects, and particle cost. The processfactors of importance include intended products, reactor types,heat integration, and process intensification strategies, andoverall process efficiency and economics. Major metals con-sidered are Ni, Cu, and Fe supported by such metal oxides asAl2O3, TiO2, ZrO2, and SiO2. It is unlikely that a unique particlecan be developed that possesses all the requisite properties forall types of looping process applications. Thus, particle opti-mization should be evaluated in the context of the specificchemical looping application. A thermodynamic analysis coupledwith comprehensive consideration of the aforementioned factorsis necessary in the screening step for looping particle selection.

Chemical looping combustion of gaseous fuels and solid fuelsis being conducted in various small or subpilot scale units usinga circulating fluidized bed, similar to those used in commercialFCC or circulating fluidized bed combustion applications. Whengaseous fuel is used, both fuel conversion and CO2 yield canexceed 90%. However, a notable amount of unconverted gaseousfuels such as CO and H2 still exit from the reducer. Therefore,an additional flue gas polishing step, in which the unconvertedfuel is combusted with O2, is usually required. Chemical loopingcombustion of solid fuels is still at its early developmental stage.The solid fuel conversions are significantly lower than thoseachieved in gaseous fuel CLC systems and hence necessitatinga flue gas polishing step. A challenge to the solid fuel CLCsystems, in addition to the carryover of solid fuels from thereducer to the oxidizer, is the interaction between the particlesand the impurities in coal. Also, for all the CFB based CLCsystems, the simultaneous handling of a high flow rate of loopingparticles and air in reactors and/or risers in a plant of commercialscale, that is, on the order of 1000 MWth, poses important designand operational challenges. Specifically, the oxidizer/risers needto be designed with an extremely large diameter when operatedat a low pressure or to be designed for operation at a highpressure with a high solids density. For a high solids flow rate,

the chemical looping combustion system will likely use highrisers and high downcomers or low risers with a series of lowdowncomers that provide a large solids inventory capacity anda large pressure difference in the downcomers for the solidsflow.

The iron based chemical looping gasification processes havebeen practiced since the early 1900s as demonstrated by theLane and Messerschmitt steam-iron processes, the Bureau ofMines steam-iron process, and the Institute of Gas Technologysteam-iron process. Prompted by the urgent need for improvingthe energy conversion efficiency and CO2 control, the renewedefforts have led to the development of the Syngas ChemicalLooping (SCL) Process and the Coal Direct Chemical Looping(CDCL) Process. In the SCL and CDCL processes, the overallenergy conversion scheme and control strategy for the pollutantssuch as H2S, COS, and CO2 can be significantly simplified overthose in the traditional processes. These modern loopinggasification processes characterized by a countercurrent movingbed contact mode of the gas and the solid reactants revealoperational advantages in high iron oxide conversions, and hencea low solids circulation rate. Among the two processes indicted,the CDCL can be significantly more efficient; however, anumber of challenges including the fuel-oxygen carrier reactionenhancement and the effect of sulfur, ash, and other impuritiesin coal need to be overcome.

Acknowledgment

This work has been supported by the Ohio Coal DevelopmentOffice of the Ohio Air Quality Development Authority, the U.S.Department of Energy, the Ohio State University, and anindustrial consortium. The assistance of Liang Zeng in thepreparation of this manuscript is gratefully acknowledged.

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ReceiVed for reView March 8, 2010ReVised manuscript receiVed July 1, 2010

Accepted July 12, 2010

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