Periodic Operation of Chemical Reactors || Catalytic Gas-Solid Reactions

C H A P T E R

8

Catalytic Gas-Solid ReactionsPeter Lewis Silveston

Waterloo, Ontario, Canada

P

h

O U T L I N E

8.1 Partial Oxidation and OxidativeDehydrogenation of Hydrocarbons 2
06
8.2 Methane Cracking 2
09
8.3 Non-Catalytic Gas-Solid Reactions 2
098.3.1 Lime Recovery from Waste Gypsum 209 8.3.2 Regeneration of Calcium Oxide 211 8.3.3 Chemical Heat Pumps 212
8.4 Catalytic Gasification UnderModulation 2
16
eriodic Operation of Reactors

ttp://dx.doi.org/10.1016/B978-0-12-391854-3.00008-5 205

8.5 Gasification Employing a CirculatingSolid Oxygen Carrier 2
23
8.6 Combustion in Circulating FluidizedBeds 2
288.6.1 Methane and Syngas 228 8.6.2 CO2 Capture 230
8.7 Periodic Reaction Switching 2
33
Many of the modulated systems discussedheretofore have dealt with a solid phase func-tioning as a catalyst. Now attention turns tosystems in which the solid phase is a reactant.On one hand, it is a reactant that is consumedor converted into another substance, but onthe other, it is a reactant that provides oxygento a gaseous species and is not otherwiseconsumed. Indeed, that reactant is regeneratedin a second reaction step. Oxidation drawingO2 from a solid phase is commonly referred toas anaerobic oxidation. There are several partialoxidation or dehydrogenation examples of

anaerobic processes: O-xylene to maleic anhy-dride or phthalic anhydride, toluene to benzal-dehyde, butane to butadiene, the formation ofethylene or propylene oxides from hydrocar-bons and even syngas production from naturalgas. In all of these systems, the solid phase isreduced by the hydrocarbon and that phase isreoxidized in a separate step by air. The solidphase functions acts primarily as an oxygencarrier. Hydrocarbon partial oxidation andoxidative dehydrogenation have been discussedin Chapter 4. They will be revisited just brieflyin this chapter.

Copyright � 2012 Elsevier Inc. All rights reserved.

http://dx.doi.org/10.1016/B978-0-12-391854-3.00008-5

8. CATALYTIC GAS-SOLID REACTIONS206

Composition modulation has been appliedto gas-solid reaction systems also, such as thecatalytic gasification of coal, the apparentlynon-catalytic decomposition of gypsum andanhydrite, and the regeneration of sulfidedadsorbents. Interest in coal gasification hasdeveloped quite rapidly during the last twodecades as a process for producing more “envi-ronmentally friendly” fuels. Oxygen carriersystems appear to offer a means of economi-cally concentrating carbon dioxide, a necessityfor sequestering this ubiquitous greenhousegas.

Research and, recently, development havefocused on the design of circulating fluidizedbeds, discovery of natural or synthetic materialsthat offer high O2 capacity per weight of carrierand mechanical strength under time-varyingenvironments. Separation of the carriers fromash in the solid fuels used may also posea problem to be solved. This chapter is concernedprimarily with circulating fluidized beds. Theterm chemical looping or looping reactor is oftenused to describe such systems. Gas-solid reac-tions under modulation have been discussed inearlier chapters with reference to the oxidationof SO2 to SO3 in Chapter 3, methane conversionover metals, partial oxidation, epoxidation andoxidative dehydrogenation in Chapter 4, auto-motive exhaust catalysis in Chapter 6, and, ofcourse, combustion in Chapter 5.

8.1 PARTIAL OXIDATION ANDOXIDATIVE DEHYDROGENATION

OF HYDROCARBONS

Over the last several decades, surface scienceadvances have established that surface oxides oradsorbed oxygen species can react with hydro-carbons to yield different products. In mostsystems some sort of periodic operation willbe necessary to exploit these surface reactants.As a consequence, studies of periodically oper-ated gas-solid reactions have multiplied.

PERIODIC OPERATIO

Summaries of these studies have been given inTable 4-1 and Tables 4-3 to 4-7 in Chapter 4.

Chapter 4 dealt with the DuPont process forthe partial oxidation of butane to maleic anhy-dride using a vanadium phosphate oxygencarrier/catalyst which cycled between oxygen-rich and hydrocarbon environments (Contractoret al., 1988, 1990). More recent work on oxygencarriers by a Russian-Swiss team has dealtwith the mechanism of toluene partial oxidationto benzaldehyde over various vanadia catalysts(Bulushev et al., 2000c, 2004, 2005). Theyobserved that nucleophilic O2 associated witha sub-monolayer in a K-doped V2O5 phaseon a TiO2 support provided selective partialoxidation of toluene to the aldehyde througha Mars-Van Krevelen reaction scheme. Excesssurface vanadia as a pentoxide or a KVO3 phase,however, resulted in electrophilic O2 thatoxidized benzaldehyde all the way to CO2.A kinetic model devised by the team, based onthe nucleophilic and electrophilic oxygenspecies, reproduced their pulse measurementsclosely. Electrophilic oxygen, apparentlystrongly adsorbed on the VO2 surface, is prob-ably responsible for the fission of the Bz ring.Adsorbed nucleophilic and electrophilic oxygenspecies identified on the vanadium surfaceappeared to yield benzaldehyde and CO2, thelatter apparently resulting from further oxida-tion of the aldehyde through benzoic acid.Control of the oxygen species on the catalystsurface through separating surface reductionand hydrocarbon partial oxidation from surfacere-oxidation offered the prospect of significantlyimproving selectivity (Pyatnitsky and Ilchenko,1996). To do this requires some sort of periodicoperation if a single bed is used, or cycling ofthe oxygen carrier between reactors if multiplebeds are chosen.

Of course, highly selective partial oxidationemploying oxygen extracted from the catalystlattice has been studied under a compositionmodulation for over 30 years for producingoxides, such as ethylene oxide, from the lower

N OF REACTORS

8.1. PARTIAL OXIDATION AND OXIDATIVE DEHYDROGENATION OF HYDROCARBONS 207

molecular weight olefins (Renken et al., 1976; Parket al., 1983; Balzhinimaev et al., 1984: Li et al.,1992a, b). Producing acrolein or acrylic acidfrom propene has been explored in the labora-tory using oxygen extracted from antimony-tin and bismuth molybdate catalysts (Silvestonand Forissier, 1985; Labastida-Bardales et al., 1989;Saleh-Alhamed et al., 1992, 1993). Lang et al.(1989b, 1991) applied periodic operation to theproduction of maleic anhydride from C4’s.Several investigators (Fiolitakis et al., 1983;Fiolitakis and Hofmann, 1983; Cordova and Gau,1983) substituted benzene for butane. Thesemodulated partial oxidation processes are dis-cussed in more detail in Chapter 4. A muchmore exhaustive description as well as analysisand interpretation are given by Silveston (1998).

Limited industrial utilization of circulatingfluidized beds appears to have taken place atthe end of the 1970s. In several publications inthe technical press, the Lummus Company,now ABB Lummus Crest Inc., publishedsketchy descriptions of a process for producingisophthalonitrile, a herbicide-insecticide inter-mediate, from the ammoxidation of m-xylene(Sze and Gelbein, 1975, 1976). The processes arementioned in Chapter 4. More details are givenby Silveston (1998).

The application of modulation to propaneoxidative dehydrogenation has been exploredworldwide (Creaser et al., 1999a; Genser andPietrzyk, 1999; Grasselli et al., 1999; Grabowskiet al., 2002; Herguido et al., 2005). Usinga magnesium vanadate (MgO/V2O5) catalystand a bang-bang periodic operation (switchingbetween 6 vol% C3H8 and 6 vol% O2 in a Hecarrier), Creaser et al. (1999a, b, c) measureda 50% increase in the time-average selectivityto propene relative to a steady state operationfor a cycle duration of about 400 s, decreasingthe period reduced selectivity. The selectivityenhancement was accompanied by a lowerrate of C3

¼ production so that the net effect wasa large increase in propene yield at a cycleperiod of about 60 s as illustrated in Figure 8-1.

PERIODIC OPERATIO

The Creaser experiments are discussed also inChapter 4.

An industry-university team (Tsikoyianniset al., 1999; Grasselli et al., 1999, 2000) exploreda novel oxidative dehydrogenation processemploying a mixed catalyst made up of a 50:50mixture of 0.7wt%Pt-Sn/ZSM-5, adehydrogena-tion catalyst, and 42 wt% Bi2O3/SiO2, a selectivecatalyst for H2 combustion. They observed that at813K the time-average performance dependedstrongly on the duration of C3 contact with thecatalyst. Initially, selectivity to C3

¼ was 98%, butthen dropped. Re-oxidation of the catalyst wasrapid and could be carried out at 350�C. Oxygenconcentration was ramped from 2 to 21 vol%during the partial cycle to avoid overheatingand agglomeration of Pt in the catalyst mixture.The results of a second set of asymmetric experi-ments are shown in Figure 8-2 in which the C3

duration dropped to 2.8 min from 130 min whilethe O2 exposure remained at 10 min. Prior toa feed switch, 5 min inert flushes were used.

Addition of the selective Bi2O3 catalyst for H2

combustion greatly increased conversion andalso improved selectivity with respect to thePt-Sn catalyst alone under the modulationemployed. However, performance fell withsuccessive cycles. The researchers attributedthis to structural changes and loss of activephase dispersion to be caused by deep reduc-tion of the catalyst during hydrocarbon expo-sure. They concluded that the adoption ofmixed reducing-oxidizing catalysts dependson developing a stable composition and findingsuitable operating conditions.

In a different approach to finding a commer-cial process, Zagoruiko (2007) observed thatwith just a modest degree of propane total oxida-tion, the oxidative dehydrogenation of thatparaffin to propylene would be autothermal.He proceeded from this observation to explore,through simulation, an autothermal dehydroge-nation process employing a homogeneous, adia-batic plug flow model. The proposed processutilized periodic flow reversal with the hot

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0 50 100 150 200 250 300 350 40050

55

60

65

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75

80

3

3.5

4

4.5

5

5.5

6

Period (s)

Steady State

Time-Average

Propene Selectivity (%)

Tim

e-Av

erag

ePr

opan

e C

onve

rsio

n (%

)

0 50 100 150 200 250 300 350 400Period (s)

Steady State

Steady State

Tim

e-Av

erag

ePr

open

e Yi

eld

(%)

4

3.5

3

2.5

(a)

(b)

FIGURE 8-1 Performance of bang-bang cycling between 6 vol% C3H8 in He and 6 vol% O2 in He at 1 bar and 783 K

over a packed bed of MgO/Mg3(VO4)2 catalyst. Steady-state data for an equal mixture of the two streams is given on the

ordinate axis. (Figure from Creaser et al. (1999a), with permission of the authors.)


reduced Ti-supported vanadia catalyst re-oxidized by cold air flowing in one directionthrough the bed in one half-cycle and with coldpropane flowing through the hot catalyst in theopposite direction. Dehydrogenation and cata-lyst regeneration proceed at markedly differentrates at the catalyst temperature. Zagoruikohandled this problem by varying the gas velocityin the bed. Simulation indicated that for theconditions chosen, 85% selectivity to propylenecould be achieved, but at a conversion of just2% of the propane fed. This low conversion

PERIODIC OPERATIO

needs to be compared with conversion undera steady-state operation. Simulation of such anoperation suggests that at a maximum feasibletemperature of 750�C, conversion would be just5% at a propane selectivity of 50%. Zagoruiko’sinteresting autothermal process requires heatingthe catalyst bed to 400�C on start-up. Advan-tages, in addition to autothermal operation, arethe absence of an inert diluent in the feed andthe separation of hydrocarbon and oxidant.Chapter 4 also deals with the Zagoruikosimulation.

N OF REACTORS

FIGURE 8-2 Comparison of the performance of

a mixed dehydrogenation-oxidation catalyst with that of

a dehydrogenation catalyst alone for the oxidative dehy-

drogenation of propane under asymmetric modulation at

1 atm, 540�C and SV[ 2 hL1. (Figure adapted from Grasselliet al. (1999) with permission. � 1999 Elsevier Science B.V.)

8.3. NON-CATALYTIC GAS-SOLID REACTIONS 209

8.2 METHANE CRACKING

Production of hydrogen on a commercialscale employs a sequence of reaction and sepa-ration steps beginning with steam reforming ofnatural gas, successive water gas shift reactionsseparated by scrubbing with amine solutionsand usually ending with methanation. Partialoxidation or autothermal reforming of hydro-carbons or alcohols has also been considered,but, as with steam reforming, such a processrequires further conversion and separationsteps. A periodic process based on crackingmethane over metals avoids these further stepsand has been investigated by Zhang and Amiridis(1998), Otsuka et al. (2000, 2001) and later byMonnerat et al. (2001). These representativestudies are summarized in Table 8-1.

The Monnerat study employed a woven Nigauze which was treated to form a Raneynickel outer surface that increased the specificsurface area from 1 to 26.7 m2/g. Gauzestripes, tightly fitted into a quartz tube placedinside a temperature controlled furnace were

PERIODIC OPERATIO

used. In the Monnerat experiments, partialcycle feed pressures were 30 kPa for CH4 and12 kPa for the carbon gasifying O2 stream.Gauze temperatures varied, but most experi-ments were made at 773 K. Effluent composi-tion for a symmetrical cycle at scycle¼ 4 minare shown in Figure 8-3. Such a cycle gavethe maximum time average H2 flow rate. Smallamounts of carbon oxides appeared in theproduct gas during the first minutes onstream, but then vanished. Flushing the gauzewith O2 gave only CO and CO2; however, thefact that the gauze was partially oxidizedresulted in carbon oxide production on switch-ing to methane.

In Figure 8-4, effluent from a 20 min symmet-rical cycle is shown. After 3 min into a composi-tion switch, the effluent contains just ppm levelsof water and carbon oxides. Recovering justa portion of the effluent after 3 min producesa satisfactory fuel cell feed.

Cracking over the Ni catalyst resulted incarbon whisker formation on the metal surface.The phenomenon is widely observed over sup-ported metal catalysts and is well-studied(Trimm, 1977; Bartholomew, 1982; Snoeck andFroment, 1997a, b; Otsuka et al., 2000; Ogiharaet al., 2006). It is accompanied by the slow lossof metal through a nano-particle at the whiskertip which is lost when the whisker is gasified inthe regeneration step of the cycle.

8.3 NON-CATALYTIC GAS-SOLIDREACTIONS

8.3.1 Lime Recovery from WasteGypsum

Large quantities of low grade gypsum(CaSO4$2H2O) are generated in the productionof industrial acids, the treating of acidic waste-water and in some stack gas scrubbing systems.A phosphogypsum is generated also in vastamounts as a by-product in the production of

N OF REACTORS

TABLE 8-1 Methane Cracking

Authors Objective Reaction

Modulated

Variable

Reaction

Conditions Observations Comments

Zhang andAmiridis (1998);

ProcessDevelopment

CH4/Cþ 2 H2;CþO2/CO2

Asymmetriccycling between20 vol% CH4 inHe and steam or airover a 16.4 wt%Ni/SiO2 catalyst

1 bar and 823 Kin a packed bed ofcatalyst

Methane partial-cycles up to200 min atGHSV ¼15000 L/hwere possible.Catalyst retainedactivity over> 10cycleswith steamregeneration, butO2 regenerationdestroyed catalyst.

Carbon whiskerformation wasidentified: with steamregeneration,additional H2 wasproduced, but processwas no longerautothermal.

Otsukaet al. (2000);Ogiharaet al. (2006)

Development ofa transport -storage processfor CH4

CH4/Cþ 2 H2;Cþ 2 H2/CH4

over Ni/SiO2;CaNi5; and alloycatalysts, e.g.,Pd/Al2O3;Pd:Co/Al2O3

2 part cycleswitching betweenCH4 at 2.7 to67 kPa andH2 at 60 kPa

CH4 decompositionwas run at 523< 823K, but mostexperiments wereat 773 K; alloyexperimentsmainly at 973 K

Ni catalystsaccumulated> 200C/Ni, but 8C/Ni wererecovered onhydrogenation.Ni/SO2:CaNi5mixture showed thebest performance;Pd/Al2O3

accumulated> 450C/Pd, butPd-Ni/Al2O3>

10,000 C/metal.

Oxidation/hydrogenation torestore metal or alloysurface was notinvestigated.

Monneratet al. (2001)

Processoptimization

CH4/Cþ 2 H2;CþO2/CO2

over an Ni gauzewith a Raneynickel surface

Symmetrical andasymmetricalbang-bangswitching betweenCH4 at 30 kPa andO2 at 12 kPa

Quartzvesselþ packingwith surfacearea¼ 26.7 m2/g,T¼ 773 K, but683< T< 823 Kwas investigated

Optimal formationof H2 forsymmetricalcycling occurredat scycle¼ 4 min.Carbon combustionin the O2 half-cyclewas temperaturedependent.

Complete removal ofcoke was not possibleeven at the highesttemperatures used.Carbon wasdeposited as whiskerswith a nano-amountof Ni at tip.

8.CATALYTIC

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FIGURE 8-3 Hydrogen production from CH4 cracking

showing in (a) the imposed feed concentration cycle and

in (b) the off-gas composition in each partial cycle for

continuous cyclic operation at TReactor[ 773 K, Q[ 75

mL (NTP)/min, mCatalyst[ 207 mg, CCH4[ 4.67 mol/m3,

CO2[ 1.87 mol/m3. (Figure reproduced from Monnerat et al.(2001) with permission. � 2001 by Elsevier Science Ltd.)


phosphate fertilizers. Most of this waste ends upas landfill because processing to recoverbuilding grade gypsum or Portland cement, orphosphates in the case of the phosphogypsum,cannot be justified economically. Perhaps,because of the rising cost of landfill, there hasbeen an interest in reclaiming quicklime andsulfuric acid (via SO2) from gypsum wastes.

PERIODIC OPERATIO

Recovery is possible by reductive decomposi-tion at temperatures above 1270 K.

The primary reaction in reductive decompo-sition is the formation of calcium oxide andCO2 or H2O or both depending on the reductantused. Sulfur is recovered as SO2. The reductionproceeds through various mineral phases withsulfide as an intermediate. Strongly reducingconditions and temperatures below 1370Kfavor the sulfide rather than the oxide. Sinteringlimits reduction temperatures to 1470K orbelow. Endothermicity and the rather narrowpermissible temperature range dictate the useof a fluidized bed for reductive decomposition.Nevertheless, calcium sulfide persists as animpurity in the lime produced and affects theeconomics for some applications.

Wheelock and co-workers demonstrated thatthe sulfide impurity can be eliminated by usingfuel injection to create oxidizing and reducingzones in the fluidized bed, or by periodicallyswitching the air-fuel ratio in the fluidizing gasso that the entire bed becomes alternatelyoxidizing and then reducing. The modulatedfluidized bed, realized through switching theA/F ratio, was investigated on a bench scalebyWheelock and Riel (1991). An asymmetric cyclewas used. Composition Modulation of CatalyticReactors (Silveston, 1998) discusses this inter-esting research in some detail.

Table 8-2 summarizes this and the modula-tion of other non-catalytic processes.

8.3.2 Regeneration of Calcium Oxide

Jagtap and Wheelock (1996) applied theresearch results from Wheelock’s gypsumexperiments to the regeneration of calciumoxide, a potential sorbent in hot gas cleanup(removal of H2S from reducing gas streams).They demonstrated that under modulationcalcium oxide can be regenerated at 1225 to1375 K rather than at 1675 K used in a contin-uous process. Below the latter temperature, theCaS formed in hot gas cleanup reacts with O2

N OF REACTORS

FIGURE 8-4 Off-gas composition from a symmetrical cycle with scycle[ 20 min. Operating conditions are as given in

Figure 8-3. Only the first two minutes of the regeneration half-cycle are shown. (Figure reproduced from Monnerat et al.(2001) with permission. � 2001 by Elsevier Science Ltd.)


to form the sulfate. The sulfate is less dense andinterferes with regeneration. Jagtap and Wheelockreasoned that if exposure to O2 or air is followedby exposure to a reducing gas, some of thesulfate formed could be reduced to the oxideor back to the sulfide, thus opening the porestructure. To demonstrate their concept, theycarried out modulation experiments andobserved that essentially complete conversionof CaS to CaO was possible even at 1225 K.Further experiments used repeated sulfidationand regeneration cycles and verified that thelime produced did not lose its capacity forsulfur removal in successive cycles. Silveston(1998) provides further details of these studies.

Separating a gas-solid reaction into stepsand carrying each out in a near optimal envi-ronment is essentially what Wheelock andhis co-workers have done for gypsum decom-position. They were able in this way to avoidsintering and still minimize the impurities intheir solid product. However, a better solution

PERIODIC OPERATIO

may be to use a two-part circulating fluidizedbed with each part operated at steady state butwith different gas compositions and tempera-tures. It seems unlikely that the recovery oflime and SO2 from gypsum or spent limeregeneration are the only gas-solid reactionsthat can be separated into steps. Other indus-trial solid conversion reactions need to beexamined.

8.3.3 Chemical Heat Pumps

Periodically changing solid compositions areencountered in chemical heat pumps. Indeed,the familiar refrigeration cycle is a “loopingsystem” in which the refrigerant circulatesbetween an evaporator, where heat is with-drawn, and a condenser where heat is dis-charged. Industrial systems often employ anammonia cycle in which absorption anddesorption replace refrigerant condensationand vaporization. Adsorption and desorption

N OF REACTORS

TABLE 8-2 Non-Catalytic Fluid-Solid Reactions


Modulated

Variable

Reaction


FLUID-SOLID PROCESSES

Swift and Wheelock(1975); Wheelockand Morris (1986);Morris et al. (1987)

Regeneration oflime adsorbants

CaS(s)þO2/CaO(s)þ SO2

with an undesirableside reaction:CaS(s)þO2/CaSO4(s)

Employed solidscirculatingbetween oxidizingand reducing zonesin a fluidized bed

1370 K to 1480 Kin a 12-cm (i.d.)bed fluidized witheither CH4 orCOþ air andwith air injectionat bed midpoint,also a 25-cm (i.d.)fluidized bed andlarger i.d. pilotplant vesselwere used.

Sulfide content(0.4 to 2.6%) ofthe fine solidsentrained withthe fluidizing gaswas stronglyaffected by A/Fratio of thefluidizing gas.Sulfide content ofthe larger particlesin the bed overflowwas nil or verysmall.

SO2 in the reducingpartial cycle arisesfrom reduction ofCaSO4 by CO orCH4. Source in theoxidizing half-cycleis the oxidation ofCaS.

Wheelock et al.(1988)

As above As above usinggypsum, wastegypsum, anhydrite,phosphogypsum

Employed solidscirculatingbetween oxidizingand reducingzones in afluidized bed

1370< T< 1480with pilot plantbed fluidizedwith an air: CH4

mixture of 9.5 to12.6. Air injectedat bed midpoint

0 to 0.9 wt% CaS inlarger particles inbed overflow. Wt%in fines< 2.5 wt%

Wheelock and Riel(1991)

As above As above, butusinga phosphogypsum

Single zonefluidized bed withswitching of thefluidizing gasbetween air anda CH4: air mixture

1370 K with amolar CH4: CaSO4

ratio¼ 1.54;A/F¼ 9.1

CaO yields rangedfrom 86 to 91%,increasing witha 50 K rise intemperature.

Jagtap andWheelock (1996)

Regenerationof CaS from ahot gas cleanupprocess

CaS(s)þO2/CaO(s)þ SO2

with side reactionCaS(s)þO2/CaSO4(s)

Modulatedenvironment ina laboratory TGAbetween air and5 vol% CH4 in air

1230< T< 1370 Kfor scycle ¼ 2 min.Besides CH4, COand C3 wereused as reductants

At 1370 K, 98%conversion ofCaS to CaOrequired 40 min.At 1230 K, 90%conversionrequired 110 min.

H2S trappingcapacity wasmaintained throughmultiple cycles.

(Continued)

8.3.NON-C

ATALYTIC

GAS-SOLID

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213

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TABLE 8-2 Non-Catalytic Fluid-Solid Reactions (cont’d)


Modulated

Variable

Reaction


CHEMICAL HEAT PUMPS

Ogura et al.(2003)

Performancetesting

Low temperaturecycle: CaOþH2O4 Ca(OH)2; hightemperature cycle:H2O(v) 4 H2O(l)

Operation testing Performancetested for 273<TL< 426 K;683< TH<

773 K. Coefficientof performancemeasured¼ 1.4

Authors found theCaO-based systemoutperformed otherchemical heat pumpsand could deliverheat at highertemperatures thanmechanical heatpumps.

Huang et al.(2004)

Modeldevelopment

SrCl2$NH3þ 7NH3 4 SrCl2$8NH3

Investigationof cycle

Salt formation/decomposition.Formation:3< P< 5 bar,303< T< 313 K,decomposition:1 2< P< 15 bar,393< T< 423 K

Heat pump modelsatisfactorilyrepresentedexperimentalperformance.

Wang et al.(2008)

Improvedammonia trapsfor chemical heatpumps

Low temperaturecycle: CaCl2$2NH3þ2 NH3 4 CaCl2$4NH3; hightemperature cycle:MnCl2$2NH3þ 4 NH3

4 MnCl2$6NH3

Temperaturecycled betweenTM1 and TL for lowtemperature cycleand between TM2

and TH for hightemperature cycle

For heat inputTM ¼120�C anddischarge TL¼30�C, heat waspumped overDT¼ 22�C

Another salt,CaCl2$8NH3 ispossible by controlof TL and P. The DT

achieved dependson the lowtemperaturesalt at TL.

Li et al. (2009b);Oliveira andWang (2007, 2008)

Performancetesting andsystem simulation

Low temperaturecycle: MCl2$2NH3þ 2 NH3 4MCl2$4NH3;where M¼Ca, Ba,Mn; hightemperature cycle:NH3(v) 4 NH3(l)

BaCl2 and MnCl2system operatedbetween 283 K and453 K. CaCl2system operatedfor ice-making with253< TL< 263K;293< TH< 303 K

CaCl2 ice-makingsystem achieveda specific coolingpower> 1250W/kg salt atTL¼ 258 K andTH¼ 298 K

Specific coolingpower dependedon the thickness ofthe graphite-saltlayer in the reactor.

Performance resultsfor ice-makingapplication wereobtained from systemsimulation.

8.CATALYTIC

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or chemical formation and decomposition arealso exploited. The latter involve gas-solid reac-tions. The use of chemical reactions in heatpumps and the technology involved encompassa growing literature. Fortunately, a usefulreview (Wongsuwan et al., 2001) and a goodexplanation of system function (Goertz et al.,1993) exist.

The principle of a chemical heat pump isillustrated by the reversible dehydrogenationof 2-propanol to acetone: (CH3)2CHOH 4(CH3)2COþH2. Heat is supplied at a lowtemperature for endothermic dehydrogena-tion, the gas products are pressurized bya compressor and heat is released to a hightemperature sink by the exothermic hydroge-nation that then takes place. Expansion of2-propanol cools the gas and completes thecycle. Cycles are asymmetric and the periodmay vary depending on the heat source andsink temperatures.

Most of the current development effort onchemical heat pumps focuses on gas-solid reac-tions that do not involve a catalyst. Typicalof such systems are CaO/H2O/Ca(OH)2 orCaCl2/NH3/CaCl2$6NH3 (Wang et al., 2008;Oliveira and Wang, 2008). The presence of a solidcomplicates the operation of the chemical heatpump because of the problem of transport-ing a solid reactant. This is avoided usinga periodically operated reaction couple such asthe CaCl2/NH3/CaCl2$4NH3//MnCl2/NH3/MnCl2$6NH3 system or a salt formation/decomposition combined with NH3 vaporiza-tion and condensation. In the former, heatsupplied to CaCl2$4NH3 causes decompositionand release of NH3. The gas passes to anotherreactor where it reacts with MnCl2. The removalof low temperature waste heat leads to theformation of MnCl2$6NH3. Now at a higherpressure and temperature, that salt decomposesreleasing heat at a higher temperature to a targetsink. This is the heat pump step. The NH3

released in the high temperature decompositionreturns back to the CaCl2 system in the first

PERIODIC OPERATIO

reactor and initiates a new cycle. The operationis illustrated in Figure 8-5. Details are given byWang et al. (2008). Changes to the cycle toimprove the coefficient of performance havebeen proposed by Li et al. (2009b). Goertz et al.(1993) discuss the choice of salt pairs to matchheat source temperature and the temperaturelift desired.

Cyclic ammoniate formation and decomposi-tion take place in the same porous solid bed.Various designs are used, but the simplest,shown in Figure 8-6, illustrates the principle.The bed is heated in the decomposition stepby a heat transfer fluid circulating on theperiphery, while gas is withdrawn through thecentral tube. In the formation step, gas is fedunder pressure through the tube and the periph-eral heat transfer fluid cools the bed. Graphite isintimately mixed with the ammoniate salts toincrease the heat transfer rate (Oliveira andWang, 2007, 2008).

A heat pump can be operated as well withammonia condensation and vaporizationreplacing the salt formation/decompositionreactions. This operation has been studied andmodeled by Huang et al. (2004) for the forma-tion/decomposition of SrCl2$8NH3 and byOliveira and Wang (2008) for CaCl2$4NH3. Thediagram in Figure 8-5 applies but the part 2formation/decomposition is replaced by NH3

vaporization and condensation.Another version of the two reaction system

uses just formation and decompositionof CaCl2$8NH3/CaCl2$4NH3 þ 4NH3 andCaCl2$4NH3/CaCl2$2NH3 þ 2NH3 (Oliveiraand Wang, 2007, 2008). These contributions dealtwith the problem of improving the rate of heattransfer through the solid packing by mixingthe calcium salt with graphite powder. Transientbehavior of the solid decomposition and gas-solid synthesis reaction and their influence onheat transfer through the surface of the packedbed and the thermal sink are discussed byAidoun and Ternan (2004). These investigatorscautioned that the often used assumption of

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Heat input(part 1)

Heal input(part 1)

Heat output

Heat release

Heat release

Heat output

Heat input(part 2)

Heat input(part 2)

(a)

(b)

High temperature

Medium temperature Medium temperature

Low temperature

TL TM1 TM2 TH

PL

PH

SL SH

-1/T

W

W

InP

FIGURE 8-5 Operation of a chemical heat pump using salt transformations through temperature and pressure change:

CaCl2$xNH3/CaCl2$yNH3 and MnCl2$wNH3/MnCl2$zNH3: (a) process diagram, (b) representation on the lnP vs. T

plane. (Figure reproduced from Wang et al. (2008) with permission. � 2008 AIChE)


constant heat transfer coefficients during anoperating cycle is not justified. Dynamic modelsfor the gas-solid systems mentioned above havebeen developed byMbaye et al. (1998) andHuanget al. (2004).

A Beijing based research team (Lai et al., 1992,1993) exploredamethanolation-demethanolationcycle based on CaCl2 that employedCaCl2$MeOH and MeOH condensation andvaporization. Using a CaCl2 methanolation-demethanolation cycle, Lai and Li (1996) andLai (1999) proposed employing periodic flowdirection reversal in heating or cooling of thereactor block by their heat transfer fluid. Their

PERIODIC OPERATIO

simulation demonstrated higher COPs for bothheating and cooling applications.

8.4 CATALYTIC GASIFICATIONUNDER MODULATION

Gasification is a primary source of synthesisgas for industrial processes and a key step inintegrated combined cycle and carbon seques-tering systems now being developed to raisepower generating efficiency and reduce CO2

and SO2 discharge to the atmosphere. Modula-tion of the gasification process will be

N OF REACTORS

THERMAL FLUID IN

THERMAL FLUID OUT

INSU

LATI

ON

INSU

LATI

ON

GASIN/OUT

FIGURE 8-6 Reactor block used for ammoniate forma-

tion and decomposition in chemical heat pumps. Black-

ened region is the reactor bed. Central tube is open to the

solids bed. (Figure modified from Mbaye et al. (1998) and used

with permission. � 1998 by Elsevier Science Ltd.)

8.4. CATALYTIC GASIFICATION UNDER MODULATION 217

considered first on the scale of a coal particleand later on a process scale.

Investigations of gasification under modula-tion on the particle scale go back to the 1980s.All coals contain mineral matter, usually ironor alkali oxides that catalyze gasification. Earlyresearch dealt (see Table 8-3) with the mecha-nism of gasification in the presence of thesematerials. Indeed, adding oxide catalysts toa coal charge was considered seriously asa means of reducing combustion temperatureand thereby decreasing NOx generation andcoal ash slagging. A number of common,reducible metal oxides, such as Fe2O3, NiO,CaO, take part in carbon gasification throughthe mobility of oxygen on the oxide surface.The overall reaction cycle can be representedfor iron oxide as:

FenOmþ1ðsÞ þ C/FenOmðsÞ þ COðgÞ: (8.1)

2 FenOmðsÞ þO2ðgÞ/2 FenOmþ1ðsÞ: (8.2)

CO2 andwater exchange oxygen with the oxidesto yield CO and H2. Redox cycles of other metal-metal oxide couples also catalyze gasification.Of course, the gasification activity will differdepending on the oxides, as Ohtsuka et al.(1987) have demonstrated.

Early modulation studies used chars ratherthan coal because of complication introduced

PERIODIC OPERATIO

through volatilization during heating of thesamples. Thus, Suzuki et al. (1988, 1989)used a char prepared from Yallourn coal (anAustralian brown coal) impregnated with aniron nitrate solution. Symmetrical modulationwas between argon (Ar) and a CO2/Armixture. The operating temperature was1070 K. Results indicated a 60 s cycle maxi-mized the gasification rate. Up to 40% conver-sion, the gasification rate for this cycle periodis about three times the steady-state rate.Further experiments by Suzuki et al. (1990)used chars prepared from iron impregnatedMorwell and Loy Yang coals. These authorsfound about a 50% increase in gasificationrate over rates under steady state.

Zhang et al. (1994a, 1994b, 1995) extended theSuzuki studies, but used chars made from a For-estburg coal, a Canadian sub-bituminous coal.Modulation employed symmetrical cycles,switching between CO2 and N2. Improvementsin char burn-off, about 15 to 20%, were foundunder composition modulation at 1070K, butthey were much less than the rate enhance-ments reported above by Suzuki et al. Thesestudies are discussed in more detail by Silveston(1998).

The reactions proceeding in gasification aresuggested by the variation of the CO contentof the off-gas with time in a cycle. These areshown in Figure 8-7. In this figure, successivepulses of CO2 increase in duration and are sepa-rated by N2 exposures of constant duration(bottom portion of the figure). A burst of COevolution, seen as a sharp peak, occurs 5 to 10s after CO2 is switched on. Height and area ofthe peak are determined by the duration ofCO2 exposure only for durations under 30 s.For durations above 30 s, peak height and widthremain constant, but after 30 s a plateauappears. The height of this plateau is greaterthan the height of the CO signal under steadystate operation at the same degree of charconversion. The plateau height and the steady-state signal are strong functions of temperature.

N OF REACTORS

TABLE 8-3 Gasification of Carbonaceous Materials

Authors Objective Reaction Modulated Variable Reaction Conditions Observations Comments

GASIFICATION ON A PARTICLE SCALE

Suzuki et al.(1988, 1989)

Investigation ofCO2 gasificationof coal charundercompositionmodulation

CxHyOzþCO2/COþH2O catalyzed by Fe2O3.(Yallourn brown coal)

Symmetrical cyclingbetween CO2/Ar andAr, 30< scycle< 90 s

T¼ 1075 K, P¼ 1 bar;catalyst/coal¼ 0.6mmol FeO/gdry char

Up to 40%conversion,modulationat s cycle¼ 60 sincreasedgasificationrate 3 x, butenhancementdropped after40% conversion.

Damping of gascomposition dueto mixingdepressed rateenhancementthroughmodulation forscycle< 60 s.

Suzuki et al.,1990)

As above CxHyOzþCO2/COþH2O catalyzed by Fe2O3,NaOH and mixtures(Morwell and Loy Yangsub-bituminous)

As above T¼ 1075 K, P¼ 1bar; various levels ofchar impregnation

Up to a 50%enhancementof gasificationwas observed forrates undermodulation for60< scycle< 90 s.

Gasification ratesfor Loy Yang coalwere 10 x ratesfor Yallourn coal,while for Morwellcoal, rateswere 50 x.

Zhang et al.(1994a, 1994b,1995)

As above CxHyOzþCO2/COþH2O catalyzed by Fe2O3;also experimentswithout Fe2O3 and withdemineralized coal(Forestburg sub-bituminous,high ash coal)

Symmetrical cyclingbetween CO2 and N2

1075< T< 1175 K,P¼ 1 bar; 3 wt%Fe/C in char(some with 0 wt%Fe/C anddemineralized char)

15 to 20%yield orconversionincrease wasobserved atscycle¼ 60 sand 1075 K.Optimal scycledepended ontemperature.

Evolution ofCO gas withtime duringmodulationconfirms a redoxreaction and thatoxide reduction isthe slow reaction.Optimal cycling isasymmetric.

GASIFICATION/COMBUSTION USING A CIRCULATING SOLIDS OR A SIMULATED CIRCULATING SOLIDS OXYGEN CARRIER

Jin and Ishida(2002)

Testing of O2

carriers forcirculatingfluidized bedcombustion ofmethane

CH4þNiO/Ni/Al2O4/CO2þH2OþNi /Ni/Al2O4; Ni/Ni/Al2O4þO2/NiO/Ni/Al2O4

Step changeexperiments withsolids regenerationin air

873< T< 973 K,1< P< 3 atm.;experimentsextended over5 cycles

Conversions werenot given.NiO/NiAl2O4

gave reproducibleresults andmaintainedactivity.

Carbon depositionwas observed.

8.CATALYTIC

GAS-SOLID

REACTIO

NS

218

PERIO

DIC

OPERATIO

NOFREACTORS

Iyer et al.(2004)

Effect of SO2 onCO2 removalwith CaO

CaOþCO2 4 CaCO3;CaCO3þ SO2 4CaSO4þCO2

Symmetrical cyclingbetween 10 vol%CO2, 4 vol% O2 and3000 ppm SO2 in N2

and N2 for scycle¼ 60 min,600 min

T¼ 973 K, P¼ 1bar; Experimentsperformed in aTGA

Best resultwith CaO was 45wt% CO2

capture after40 cycles; thisresult dropped to36 wt% after 100cycles.

Sulfatedadsorbent can beregenerated atT> 1270 K.

Scott et al.(2006)

Developmentof circulatingsolids combustorfor solid fuels

CxHyOzþCO2/COþH2þH2O;COþ Fe2O3(s)/CO2þ Fe3O4þ FeO;Experiments usedHambach lignite or char

Asymmetrical cyclingbetween 27.5 vol%CO2 in N2 and airwith lignite added in thereducing step

T z 1173 K,P¼ 1 bar

Authors observed92% conversionof lignite char.

Observed thatgasification oflignite or lignitechar is the ratelimiting step.

Siriwardaneet al. (2007)

Investigationof O2 carrierperformancewith syngascombustion

NiOþCOþCO2þH2/NiþCO2þH2O:NiO supported onbentonite

4 part cycle:1) reduction with36 vol% CO,27 vol% H2, 12 vol%CO2 in N2 for 10 min,2) & 4) N2 or Arflush for 5 min, 3)re-oxidation inair for 60 min

Experiments wereconducted over 10cycles in a TGAand in a fixed bedof supported NiOat 973< T< 1173 Kand 0.67 MPa

100% of H2

was consumed inthe reduction step.Particle size andpressure hadlittle effect onrates. Particlemorphology wasunchanged over10 cycles.A small loss ofsurface areawas seen at1173 K.

Rate of reductionfor NiO wassignificantlygreater than there-oxidation rate.

Pfeifer et al.(2007)

Testing of pilotplant (100 kW)performanceusing biomass

CxHyOzþH2OþCaO/CaCO3þH2; CaCO3þO2 /CaOþCO2;Calcite oxygen carrier

Circulating solidsconsisting of abubbling fluid. bedþriser regenerator andbubbling fluid.bed gasifier

825< Tgasifier< 975 K;1075< Tregenerator< 1175K; experiments in a100 kW (fuel) pilotplant

H2 content of theproduct variedfrom 66 to 75vol%, and wasdependenton Tgasifier.

Off-gas containedtar. Tar productiondepended onTgasifier, the ratio ofsteam/biomass,and the carriercirculation rate.

(Continued)

8.4.CATALYTIC

GASIFIC

ATIO

NUNDER

MODULATIO

N219

PERIO

DIC

OPERATIO

NOFREACTORS

TABLE 8-3 Gasification of Carbonaceous Materials (cont’d)


Leion et al.(2008, 2009b)

Testing of thegasification ofdifferent fuelswith different Feand Ni oxygencarriers

CxHyOzþH2O/COþCO2þH2 catalyzed byilmenite, Fe2O3/MgAl2O4,NiO/NiAl2O4;fuel: bituminous orsub-bitum. coals orpetroleum coke

4 part cycle of 1) fuelfed with H2O in N2

fluidizing gas, for20 min, 2) N2 flushfor 3 min, 3) re-oxidationwith 5 vol% O2 inN2, 4) N2 flushfor 3 min

1123< T< 1273 K,but most experimentsdone at 1223 K, P¼ 1bar; conical fluidizedbed promoted mixing.In some experimentsCH4 or syngasreplaced steam

Experimentswere done over2 to 6 cycles.Gasificationrate depended on% steam in thefluidizing gas.No agglomerationor ash-oxygencarrier interactionwas observed.

When sandreplaced theO2 carrier,gasification ratewas reduced byca. 50%. Ilmeniteore performed aswell as Fe2O3/MgAl2O4, but NiOwas poisoned bysulfur in fuel.

Berguerandand Lyngfelt(2008)

Performancetesting ofa circulatingsolids coalgasificationpilot plant

CxHyOzþH2O/COþCO2þH2 catalyzed byilmenite (FeTiO2). Coalwas S. African sub-bituminous

Ilmenite circulatedbetween a fluidizedbed fed by H2OþN2

and a fluidized riserfed with coal and air

Tfluidized¼ 1223 K, gasvelocity> 0.1 m/s.Triiser¼ 1273 K, P¼ 1bar; coal flowwas 500 g/h

Pilot plant wasrun for 22 h with12 h of stableoperation. CO2

capture (¼ fuelcarbon convertedto CO2) rangedfrom 82 to 96%.

Song et al.(2008)

Evaluation ofCaSO4 as anoxygen carrierfor a circulatingsolids combustorfor methane

CaSO4þCH4/CaSþCO2þH2O;CaSþO2/CaSO4

4-part cycle: 1)reduction withCH4 for 600 min,2) & 4) N2 flush for40 min, 3) re-oxidationfor 340 min

Reduction step at1223 K, re-oxidationat 1123 K, P¼ 1 bar

Reduction stepwas slowest at1123 K. At 1273 K,SO2 in effluentindicated theformation ofCaO. MaximumCH4 combustionwas< 80%.

Shen et al.(2009a, b)

Performancetesting of abench-scale(10 kW)circulating solidsgasificationplant

CxHyOzþCO2/COþH2þH2O; COþNiO(s)/CO2þNi; experiments usedShenhua bitumen. coal

32.7 wt% NiO/NiAl2O4 oxygencarrier circulatedbetween a spoutedbed fuel reactor anda turbulent fluidizedbed oxidation reactor

Fuel reactor:1215< T< 1235 K;re-oxidizer:1245< T< 1310 K

Reactor achieved92.8% carbonconversionwith 80% CO2

capture efficiency.Efficiency waslowered by thecarryover of CO/CO2 from gasifierto re-oxidizer andelutriation of finecoal particles.

Slow activity lossof NiO oxygencarrier was relatedto sulfation of theoxide from sulfurin the coal.

8.CATALYTIC

GAS-SOLID

REACTIO

NS

220

PERIO

DIC

OPERATIO

NOFREACTORS

Acharyaet al. (2009)

Processdevelopmentfor H2

productionfrom biomass

CxHyOzþH2OþCaO/H2þCaCO3; CaCO3/CaOþCO2 with heatsupplied by burning H2:fuel or sawdust

Not cyclic; productionfollowed by limeregeneration withcontinuous biomassand CaCO3 feed

Tgasifier¼ 850 K;TRegenerator¼ 1070 K,P¼ 1 bar

Gasifier achieved71% H2 in theoff-gas.

Gasification stepand limeregeneration stepoperatedseparately.

Mattissonet al.(2009a, b)

Developmentof a gasification/combustionprocess forsolid fuels

CxHyOzþCuO/Al2O3/COþCO2þH2OþCu/Al2O3

2 part cycle: 1) airoxidation for 70 s,2) N2 for 30 s

T¼ 1258 K,ufluidization¼ 1.8umin. fluidiz.

Gasificationrate with CuOwas ca. 50 xrate obtainedwith a Fe2O3

carrier.

Single cycle wasused. Experimentsshowed highergasification ratesare possible usingCuO as the O2

carrier.

Kolbitschet al. (2009,2010)

Measurementof theperformanceof a 120 kWpilot plant

Reduction:H2þ FeTiO2/H2Oþ FeTiO2-x;re-oxidation:FeTiO2-xþO2/ FeTiO2;also used CH4 as fuelinstead of H2; artificialNIO carrier also used

Ilmenite circulationwas 10 to 88 kg/m2sfor 0.82<A/F< 1.10.Fuel reactor was afast fluidized bed(riser reactor) whilere-oxidationemployed a turbulentfluidized bed.

Fuel reactoroperated z 1173 Kand 1 bar. With NiOcarrier, 100%utilization of fuelwas observed forA/F � 1 and asufficient solidsloading

Pilot plantfunctionedsatisfactorily,but was notoptimized.Carbondeposition wasnot observedwith CO orCH4 fuels.

Oxygen carrieroperated at 10 to18% of oxygencapacity in fuelreactor and at20 to 25% in re-oxidation reactor.

Dennis et al.(2010),Dennis andScott (2010)

Investigation ofCO2 gasificationof coal in thepresence of aCuO oxygencarrier attemperaturesabove the Cumelting point

CxHyOzþCO2/COþH2O catalyzed by21 wt% CuO/Al2O3

active phase supportedon q-Al2O3

(Taldinskaya, Illinois#5 biuminous coal,Hambachlignite)

3 part cycle consistingof 1) coal feed withCO2 or H2O asfluidizing gas,2) fluidizedgasification of charge,3) re-oxidation ofCuO with air asfluidizing gas

T¼ 1203 K, P¼ 1 bar.78 mm (i.d.) fluidizedbed; scycle¼ 1650 to1700 s for bituminouscoals with s1¼570-600 s, s2¼700-800s, s3¼300-350s; forlignite, scycle¼ 1190 s

Lignite wasgasifiedcompletely incycle at 1203 K.Gasification ofthe bituminouscoals wasincomplete forscycle and T used,but the presenceof a CuOoxygen carrierenhancedgasification rates.

Reactive lignitefuels weresuccessfullygasified in thecycling system.Oxygen carriercycled betweenCu and Cu2O inexperiments.

(Continued)

8.4.CATALYTIC

GASIFIC

ATIO

NUNDER

MODULATIO

N221

PERIO

DIC

OPERATIO

NOFREACTORS

TABLE 8-3 Gasification of Carbonaceous Materials (cont’d)


Iggland et al.(2010)

Examination ofthe effect ofparticle size onchar gasificationrates

CxHyOzþH2O/COþCO2þH2 catalyzed byilmenite (FeTiO2). Coalwas a Colombianbituminous

5 part cycle: 1) coalfed to bed, 2) 92 vol%H2O in N2, 3< s2<30 min, 3) and 5) N2

purge s3¼ 3 min,4) 10 vol% O2 in N2

968< T< 980 K;conical fluidized bed

Coal particle sizedid not influencethe rate of coalgasification.

Xiao et al.(2010a, b)

Pressurizedsteam gasificationof coal with alow-cost ironore oxygencarrier

CxHyOzþH2O/COþCO2þH2 catalyzedby CVRD ironore. Experimentsused Xuzhoubituminouscoal

5 part cycle: 1) coalfed to bed, 2) 87 vol%H2O in N2, s2¼ 75 min,3) N2 purge,s3¼ 15 min, 4) 5 vol%O2 in N2, s4¼ 40 min,5) N2 purge, s5¼ 5 min

Downflow throughpacked coal bed;973< T< 1243 K,0.1< P< 0.5 MPa;iron ore¼ Fe3O4

Operation for20 cycles

CO2 in off-gasduringreducing phaseincreasedfrom ca. 80 vol%in 1st cycle to96% in 20th

cycle. Carbongasification at0.1 MPa was 76%and 85%at 0.5 MPa.

No agglomerationof ore particlesand no coal ash-iron ore fusionwere observed.

Li et al.(2009a, 2010a)

Development ofa process for H2

production fromsyngasþ testingof Fe basedoxygen carriers

Only countercurrentmoving bed reducerstudied: Fe2O3þCO/FeO or FeþCO2;oxygen carrier¼ 60 wt%Fe2O3 on inert support

Oxide in downflowat 12.9 g/min withsyngas containing43.8 vol% CO, 29.2vol% H2

1123< T< 1173 K,P¼ 1 bar; unitoperated for 13 h;tested Fe2O3 through100 oxidation/reduction cycles

Reducer achieved49.5% reductionfrom Fe2O3 toa mixture ofFeOþ Fe at99.5% conversionof CO to CO2.

Fe2O3 was shownto be a suitableoxygen carrier.

Zheng et al.(2010)

Investigation ofCaSO4 carrier forchemical looping

CxHyOzþH2OþCO2þCaSO4/CO2þH2OþCaS; Shenhuabituminous coal

Only the reducingstep was investigated:CoalþCaSO4 fluidizedby CO2 or H2O ormixtures

1123< TReactor<

1248 K; sreduction¼ 30min

High H2O/CO2 influidizing gasenhances theefficiency of CO2

capture, but coalconversion wasindependent ofthis ratio.

8.CATALYTIC

GAS-SOLID

REACTIO

NS

222

PERIO

DIC

OPERATIO

NOFREACTORS

TIME (min)

800°C3% Fe

2 minN2

CO

2

0

2

4

8

10

12

16N

OR

MAL

IZED

CO

CO

NC

ENTR

ATIO

N(S

igna

l/100

mg

carb

on)

FIGURE 8-7 Normalized IR signal for CO versus time in

the initial exposure of an iron loaded char (3 wt% Fe/g

carbon) from a Forestburg sub-bituminous coal for alter-

nating exposure to CO2 and N2 at 1070 K and 1 bar. Dura-

tion of the time exposure increases from left to right, while

the duration of the N2 exposure is constant. Time scale is

indicated in the schematic of the exposure sequence at the

bottom of the figure. (Figure reproduced from Fan, L.-S., Li, F.,Ramkumar, S., 2008 with permission. � 1994 Elsevier.)

8.5. GASIFICATION EMPLOYING A CIRCULATING SOLID OXYGEN CARRIER 223

There is no initial CO burst when char froma demineralized coal is used. Only the step-upto a plateau and the step-down are seen. Asmight be expected, the CO concentration corre-sponding to the plateau at any temperature islargest for char from iron impregnated coaland smallest for the char from a demineralizedcoal. The difference is large. On the switch toN2, a second burst of CO evolution is seen.The area under this peak is proportional to thearea under the first peak. These peaks or burstsof CO evolution are evidence for a redox cycle.The first peak represents gasification by CO2,while the second smaller peak arises fromreduction of iron oxide by the char (Eq. (8.1)).The existence of these bursts and their interpre-tation have been known for several years(Suzuki et al., 1988).

A consequence of the above observations isthat the cycle period is quite strongly tempera-ture dependent. Below 1070e1120K, wherecatalytic gasification would be commercially

PERIODIC OPERATIO

interesting, an asymmetric cycle must be usedwith the duration of the reducing partial cycleexceeding the oxidizing part. Asymmetric expo-sures are easily implemented using a circulatingfluidized bed.

8.5 GASIFICATION EMPLOYINGA CIRCULATING SOLID OXYGEN

CARRIER

Interest in non-steady-state gasification hasaccompanied the recent development of circu-lating fluidized beds as chemical looping reac-tors. Indeed, it has become a popular R & Dtopic judging from the large number of publica-tions that have appeared recently (see reviewsby Li and Fan (2008), Fan et al. (2008) andAnthony(2008) as well as a book, Chemical LoopingSystems for Fossil Energy Conversion, by Fan(2010)). These reviews offer broad comparisonsof competing technologies. They observe thatit is possible, as well as feasible, to separate airfor combustion into an O2-rich stream thatwould permit sequestering CO2 generated incombustion at a cost well below the cost forCO2 capture through scrubbing. In their view,the most attractive option is to transfer oxygenfor combusting fuels through a circulatingmetal/metal oxide material. This offers theopportunity of generating a nearly N2-free gaswithout prior air separation.

Circulating fluidized beds of oxygen carriersare fed with a gaseous fuel or coal, coal char orbiomass and a gasifying agent, such as steamor CO2, in one part of a cycle and air in a secondpart. Figure 8-8 shows a schematic of a doublefluidized bed version of a looping reactorsystem. The circulating solid is a readily reduc-ible oxide, such as NiO or Fe3O4 impregnatedinto an abrasion resistant support like g-Al2O3.

In the solid fuels version, coal is fed intothe first fluidized bed, operating at about1200e1240 K and under a slight pressure, whereit mixes with the hot circulating solid. Rapid

N OF REACTORS

Com

bustor

SteamAir

Compressor

ExpanderGenerator

Steam

SulfurByproduct

Hot Spent Air

Coal

Ash/Spent Particle

Air

Makeup Particle

Fe/FeO

O2

CO2

N2

H2H2

H2S Removal

Fe2O3

CO2 + H2O Hg Removal

BFW

Fe2O3

Fe3O4

Fe

Fe/FeO

FuelReactor

HydrogenReactor

FIGURE 8-8 Simplified schematicdiagram of a circulating fluidized bedsystem for coal gasification usingseparate fluidized beds for steam/CO2 gasification and air regenerationof the circulating solid oxygen carrier(Fe2O3). (Figure reproduced from

Fan, L.-S., Li, F., Ramkumar, S., 2008with permission.� 1994 Elsevier.)


heat exchange, characteristic of fluidized beds,pyrolyzes the coal converting it into a porouschar. The bed of hot solid particles andpowdered coal is fluidized by steam, carbondioxide or a mixture of these gases. Theseoxidizing gases react to some extent exothermi-cally with CH4, higher hydrocarbons and tardriven out of the coal through pyrolysis, butprimarily they gasify the char in an endothermicprocess. The heat demand of gasification ispartially met by the oxidation of the reducinggases.

The remainder of the heat requirement isdrawn from the hot solids that enter the gasifierat 1270e1340K. The residual finely divided ashfrom the gasified coal is carried over with thehot gas leaving the gasifier and is removed ina gas cyclone. The ash-free hot gas at1200e1240K, consisting primarily of H2O andCO2 with some H2 and CO, can be sent toa system of gas turbines, steam generators andsteam turbines for power generation. The solids,now partially reduced, are circulated back toa fluidized bed regenerator where they are re-oxidized with air and reheated. The hot gasleaving that regenerator is carbon-free and can

PERIODIC OPERATIO

be used to generate power in a system ofturbines and steam boilers. Such a process isnot suitable for all coals. Ash content andcomposition will be important because ashmelting must be avoided. Sulfur in the coal orash could contaminate the off-gas or react withthe metal oxide carrier to form sulfates.

Investigation of chemical looping for coal orchar combustion have generally used eithera single fluidized or a packed bed operated peri-odically (Scott et al., 2006; Leion et al., 2008, 2009b;Berguerand and Lyngfelt, 2008; Dennis and Scott,2010; Iggland et al., 2010; Xiao et al., 2010a, b).Brown et al. (2010) used such a system in theirstudy of the gasification of a lignite char,prepared from a Hambach lignite, with CO2 asthe gasifying agent and iron oxide as the oxygencarrier. They observed that the presence of anoxygen carrier increased the rate of char gasifi-cation by oxidizing CO formed in gasification.CO appeared to inhibit gasification. Denniset al. (2010) used a copper oxide and two bitumi-nous coal chars from Taldinskaya (Russia) andIllinois # 5. These were tested along with a Ham-bach lignite at 1200 K, 1 bar using CO2/N2 as thegasifying agent. The 1500 s, three-part cycle

N OF REACTORS


comprised gasification with continuous charaddition, gasification of the remainder of thechar with no further addition, and then a switchto air to reoxidize the copper oxygen carrier.Figure 8-9 shows the variation of the CO2

content during the three-part cycle with a lignitefuel. The fluctuating CO2 flow during thefeeding step reflects the pulsed feeding of coalchar to the bed. The sharp peak when air isadded results from O2 removed from theentering gas to re-oxidize the Cu/Al2O3 carrier.The Dennis experiments also found that thepresence of an oxygen carrier increased therate of gasification. The mechanism for thisappeared to be oxidation of the CO formed ingasification to CO2. Temperature spikes whenair is introduced cause dissociation of CuO toCu2O and O2. Some of the O2 released mayhave been available for gasification in thefollowing partial cycle when coal char was fed.

Leion et al. (2008) also used a periodic opera-tion consisting of a four-part cycle: Granularcoal addition with steam fluidization of themetal oxide-coal mixture, N2 fluidization forflushing (180 s), and air fluidization for re-

FIGURE 8-9 CO2 content of gas leaving the mixed fluidized

of time. TBed[ 1200 K, the N2-CO2 fluidizing gas fed at 600 cm

the pulsed, char feed rate was 0.25 g/min. Air flow was added

of about 1100 cm3/s. (Figure reproduced from Berguerand and L

PERIODIC OPERATIO

oxidation of the metal. Experiments were con-ducted at 1 bar and 1223 K and employedseveral bituminous coals or petroleum coke,thus, devolatilization took place. These volatileswere rapidly oxidized by the metal oxide at theoperating temperature. The residual char wasgasified by hot steam. Leion et al. used aMgAl2O3

support impregnated with hematite (Fe2O3) orilmenite (FeTiO3), a natural mineral. Cycleperiod was varied to allow for the different reac-tivities of the coals studied.

Several investigators have used the CaSO4eCaS cycle for O2 transport (Wang and Anthony,2008; Song et al., 2008; Zheng et al., 2010). Thelatter research team explored the kinetics ofthe sulfate reduction in the presence of CO2

gasification and re-oxidation of CaS as well asthe morphology changes of a particle duringmultiple cycles. Operating temperature wasthe major variable. Gasification rates and carbonconversion increased with temperature up to1220 K, but sintering appearred at highertemperatures and suppressed the rate. Asobserved by many investigators, the gasificationrate increases with an increasing H2O/CO2

bed of lignite char and CuO/Al2O3 particles as a function3/s during all parts of the cycle contained 16 vol% CO2 and

in the third part of the cycle at 530 cm3/s to give a total flow

yngfelt (2008) with permission. � 2008 by Elsevier Ltd.)

N OF REACTORS


ratio. Whisker and dendrite formation was seenfor CO2 gasification but not when H2O wasused. Small amounts of SO2 and H2S werefound in the gases released from the gasificationand re-oxidation steps.

A periodic operation was also used toexamine the effect of coal particle size on gasi-fication rates in the presence of oxygen carriersolids. Iggland et al. (2010) found no size effectin the range studied; devolatilization duringheating, however, was affected by particlesize.

Versions of the double fluidized bed system(Figure 8-8) have been tested in the laboratoryby several research teams (Cao et al., 2006;Berguerand and Lyngfelt, 2008; Shen et al.,2009a). Small scale demonstration units havebeen built in China and Sweden with a gasproduction rate capable of generating 3 to5 kW (Leion et al., 2008; Shen et al., 2009a, b).Figure 8-10 gives a schematic of the nominal10 kW Swedish system. Experiments with thissystem operating with ilmenite as the O2 carrierwere conducted at 1 bar using a South Africansub-bituminous coal, fed at 500 g coal/h (Ber-guerand and Lyngfelt, 2008). The gasifier oper-ated at about 1220K while the ilmeniteregenerator was held at 1270K. Continuousoperation over a 12-h period was achieved.Coal conversion ranged from 50 to 79% byweight, while carbon capture in terms of CO2

produced per unit of carbon in the fuel chargevaried from 82 to 96%. Design changes and pol-ishing of the gasifier off-gas with a postcombustion unit fed by oxygen would produce,as the authors suggest, a commercially accept-able process.

A Shenhua bituminous coal was used inexperiments on the Chinese 10 kWth unitreported by Shen et al. (2009a). This small scalepilot unit employed an up-flow fluidized bedsuch as that shown in Figure 8-10. However,in place of the segmented fluidized bed,a spouted bed served as the gasifier withpowdered coal fed along with a CO2eH2O

PERIODIC OPERATIO

mixture and/or recycled off-gas. The systemoperated at slightly above atmospheric pres-sure. Measurements in the gasifier indicatedthe gas temperature decreased with heightfrom 1230 to 1210 K. In the upflow regenerator,the NiO/Al2O3 oxygen carrier entered at about970�C and reached the cyclone at about 1035�C.Off-gas from the spouted bed was 94% CO2 byvolume. The CO2 capture efficiency was 80%.This is the ratio of CO2 leaving the gasifier tototal carbon as CO2, CO and carbon dust inthe off-gas from both gasifier and regenerator.Shen et al. reported that the two largest detrac-tors of capture efficiency were carbon in theash carried over from that unit and gasentrained with the solids passing from thegasifier to the regenerator with the recirculatingoxide.

Eliminating CO2 in the gasifier fluidizinggas reduces the CO2 content in the off-gas.Passing the off-gas separately through a gasturbine and steam boiler to cool the gas to ca.370K and recover further energy wouldprovide a feed to a MEA/DEA scrubber so asto remove almost all CO2 from this stream.After condensation, the gasifier off-gas wouldbe 95 to 98% H2 and suitable for process use.Indeed, depending on demand, such a processcould be switched between power and H2

generation. Either coal or biomass could fuelsuch a system.

A comprehensive review of gasificationusing chemical looping published by Fan andLi (2010) examines a variety of processes forchemical applications employing gasifier off-gas. Necessary properties for oxygen carriersare identified; perhaps of most interest is theauthors’ observation that moving bed contac-tors, rather than fluidized beds, wouldsubstantially reduce solids cycling by offeringmuch greater oxidation and reduction of theoxygen carrier. Advantages of countercurrentmoving-bed gasifiers have been tested bysimulation using the Aspen Plus designpackage, and assuming thermodynamic

N OF REACTORS

Steam Production UnitAIRFilters

To Chimney To Chimney

To Chimney

Water Seal

Insulation

Coal Feeding

Valves

Oven

d

b

a

Cooling

c

N2

H2ON2

N2

K-Tron

FIGURE 8-10 Schematic of the nominal 10 kW circulating fluidized bed coal combustion demonstration unit oper-

ating at the Chalmers University of Technology, Gothenburg, Sweden. In the schematic, “a” is the up flow fluidized bed

regenerator, “b” is a riser reactor, “c” is a cyclone for capturing the ilmenite oxygen carrier and “d” is a compartmen-

talized gasifier. (Figure reproduced from Acharya et al. (2009) with permission. � 2009 Amer. Chem. Soc.)


equilibrium with further parameters fromadditional experiments (Li et al., 2010a). Theprogram results show virtually completeconversion of the syn gas feed at almost 49%reduction of the Fe2O3 carrier at 30 atm and1170 K. Synthesis gas contained 43.8 vol%CO, 29.2% H2, 5.0% CO2 with N2 as thebalance. Fe2O3 conversion under fluidizedbed operation would be about 11% under thoseoperating conditions.

PERIODIC OPERATIO

In practice, char or coal gasification would becarried out under pressure, possibly up to 1e2MPa, so that the hot gases leaving the gasifierand regeneration beds could be sent to gasturbines for power generation. Several investi-gators have studied the effect of pressure onreduction and/or re-oxidation of oxygencarriers (Jin and Ishida, 2002; Garcia-Labiano,2006 and Siriwardane et al., 2007). Generally,they observed that increasing pressure

N OF REACTORS


suppressed rates. Xiao et al. (2010a, b) examineda pressurized process using a periodically oper-ated, 30 mm i.d. packed bed of a commercialiron ore. A powdered low rank, high ash bitumi-nous coal (Xuzhou) was fed to the bed with a N2

carrier gas. This was followed by 90 min exposureto 87% steam in N2, then a 5 min purge anda switch to 5% O2 in N2 for 10 to 15 min. Thiscycle was tested at 1240K and pressures up to0.6 MPa. Xiao et al. observed a higher CO2

concentration in the off-gas and increasedconversion of the iron oxide as pressureincreased; however, the devolatilization ratedecreased. Porosity of the iron oxide carrierincreased, without substantial loss throughattrition, for three to five cycles.

Power or hydrogen production frombiomass with CO2 sequestration has alsobecome a popular research topic in the lastdecade (Hanaoka et al., 2005; Ni et al., 2006; Caoet al., 2006; Cao and Pan, 2006; Mahishi and Gos-wami, 2007; Pfeifer et al., 2007). Acharya et al.(2009) proposed the biomass gasifier designshown in Figure 8-11, discussed the energybalance and reported separate batch perfor-mance tests on the gasifier and regenerator.They used CaO as a CO2 trap to separate H2

and CO2 in the process off-gas. To balance theheat demand of the endothermic gasificationstep, some of the H2 product was consumedto heat the lime regenerator. The cyclonefollowing the regenerator captured andrecycled the lime particles; fine dust from thesmall amount of ash in the biomass that wascarried over with the CO2 stream into the heatrecovery and steam generation section. Fora biomass feed rate of 1 kg/s at 100K,1.07 kg/s of 575K steam entered the gasifieralong with 2.41 kg/s of CaO at 1220 K. Endo-thermicity of gasification and heat loss drop-ped the temperature of the solids passing tothe regenerator to 1070 K. This was also thetemperature of the H2 stream. The authorsassumed, optimistically, that lime traps all ofthe CO2 produced by biomass gasification,

PERIODIC OPERATIO

so 4.30 kg/s of CaCO3 circulated to the regener-ator. The process consumed 1.07 kg/s of waterwhich was converted to steam using the hotoff-gases. These inputs generated 0.10 kg/s ofhigh purity H2 and 1.89 kg/s of 100 K CO2 forsequestering.

Acharya et al. (2009) reported that batch testson a bubbling bed gasifier using sawdust as thebiomass, supplemented by charcoal to generateextra heat, produced an off-gas containingabout 70% H2 at 580�C when CaO was fed tothe bed. Most of the remaining gas was CH4

and the CO2 content was 1 to 2%. Separatebatch experiments on the regenerator usingan upflow fluidized bed heated externally to1070K showed about 40% of the CaCO3 feedwas converted to CaO within a 1-h contacttime employing air as the fluidizing gas.

8.6 COMBUSTION INCIRCULATING FLUIDIZED BEDS

8.6.1 Methane and Syngas

Combustion systems allow for sequestrationof a CO2 target, besides natural gas, refineryand chemical plant discharges containinghydrocarbons, coal mine ventilation emissionsand gases generated in waste treatment. Thebasic process is identical to that shown inFigure 8-9, except that the powdered coal fedto the gasifier is replaced by a gaseous fuel.This input would be mixed with steam and/orCO2 as extra gas is necessary for fluidization.Additional steam and/or CO2 also reduces theamount of solid oxygen carrier that must becirculated to combust the fuel.

A number of laboratory studies, discussed byLi and Fan (2008) for example, have laid thefoundation for small scale pilot units such asthe 120 kWth dual circulating fluidized bedcombustor operated by the Vienna Universityof Technology on synthesis gas or on methane(Kolbitsch et al., 2009, 2010). A schematic of this

N OF REACTORS

CaCO 3

CaC

O3 =

CaO

+ C

O2

Steam

Screw feeder

Water

Heat Exchanger

Distributor plate

Biomass

H2

H2 for Applications

CO2 for Sequestration

CO2 for Fluidization

H2 for external heating

CO2

CaO

124

3

FIGURE 8-11 Schematic of a double fluidized bed system for steam gasification of biomass with capture of CO2 for

H2 production, or power generation or process application. “1” is the bubbling fluidized bed gasifier, “2” is a circu-

lating fluidized bed, “3” is a cyclone to separate gas and ash from the solid oxygen carrier and “4” is an external gas-

fired heater for the bottom of the circulating fluidized bed. (Figure reproduced from Acharya et al. (2009) with permission.

� 2008 American Chemical Society.)

8.6. COMBUSTION IN CIRCULATING FLUIDIZED BEDS 229

unit is given in Figure 8-12. The regenerationreactor is usually operated as an up-flow fluid-ized bed, while the gas oxidation reactor oper-ates as a bubbling fluidized bed, although itcould be run as an up-flow fluidized bed also.Circulating solids carry oxygen from the regen-erator to the oxidation reactor.

Several investigators have avoided theproblem of the slow gasification of carbona-ceous solids using circulating beds by employ-ing conventional gasifiers using O2 from an airseparator and processing the CO/CO2/H2/H2O/CH4 gas mixture produced in the fuelportion of a circulating fluidized bed. Thus,Li et al. (2009a, 2010a) undertook experimentalstudies of their looping systems with sucha synthetic gas. Their main interest was in the

PERIODIC OPERATIO

performance of the oxygen carrier in thecombustor and the regenerator.

Choice of the oxygen carrier is important:Requirements such as oxygen capacity, rate ofreduction and oxidation, thermal stability,mechanical strength and cost have been dis-cussed by several research teams (Adanezet al., 2004; Garcia-Labiano, 2005; Gupta et al.,2007). Iron oxides seemed to be preferred. Reac-tivity and mechanical performance of theoxides in the Fe2O3 4 Fe3O4 4 FeO cyclewere investigated by Li et al. (2009a). Iron oxidewas coprecipitated with an alumina precursor.A TGA and quasi-periodic tests in a fixed bedwere used to characterize the O2 capacity andoxidation/reduction reactivity. A hydraulicpress measured particle strength and an up

N OF REACTORS

AR exhaust FR exhaust

steam

fuelsteam

steam

sec.air

prim.air

air r

eact

or (A

R)

fuel

reac

tor (

FR)

FIGURE 8-12 Schematic of the 120 kWth Chemical

Looping Combustor used for burning either syngas, CO,

H2 or CH4 with air to generate a high purity CO2 stream

after water condensation. Both exhaust streams can be

used to generate power. (Figure reproduced from Kolbitschet al. (2010) with permission. � 2010 by Elsevier Ltd.)


flow fluidized riser determined the entrainmentrate. The Li experiments disclosed a strongincrease in the oxidation rate with the numberof cycles as may be seen in Figure 8-13. Reduc-tion rate increased just slightly. Alumina sup-ported iron oxide appeared to be an attractiveoxygen carrier. Measured compressive strengthwas high and attrition rate at 0.57 wt%/cyclewas low.

Song et al. (2008) have explored the operationof a methane combustor with separation of aN2-rich, CO2-free waste gas from a concentratedCO2 stream using the CaSO4/CaS oxygen

PERIODIC OPERATIO

carrier couple. Experiments were undertakenin a periodically operated fixed bed andfocused on the effect of operating conditions(temperature, gas flow rates, CaSO4 particlesize, time on stream) on capacity and reactivity.The authors suggested a 1220 K operatingtemperature for the combustor, but that theregenerator temperature should not exceed1270 K.

Under temperatures reached in the regener-ator, some oxide partially decomposedreleasing oxygen. Indeed, Mattisson et al.(2009a) have suggested that the oxygenreleased can be exploited to gasify chars. Theyidentified CuO or Mn2O3 as potential sources.The former is in equilibrium with Cu2O andgaseous O2 at 1186 K. At that temperature,gaseous O2 reacts exothermically with CO ora carbon char. This would raise bed tempera-ture and further dissociate CuO. Cuprousoxide, however, melts at 1508 K, so temperaturecontrol would be necessary. Mn2O3 dissociatesat 1048K, but both manganese oxides havemelting points above 1575 K so carrier agglom-eration would not be a problem. Mattisson’sexperiments found that periodic exposure ofa fluidized bed of CuO/Al2O3 at 120 K to air,N2 and CH4 showed almost complete conver-sion of CH4 to CO2 until the released O2 wasconsumed (Figure 8-14). A similar experimentwas undertaken at 1258 K with a petroleumcoke contained in a fluidized bed of CuO/ZrO2. Coke oxidation continued until the freeO2 was consumed whereupon diffusionbecame controlling. Details are given byMattis-son et al. (2009a, b).

8.6.2 CO2 Capture

Research on the application of chemicallooping to gasification and combustion dis-cussed above and summarized in Table 8-3has dealt with utilization of an oxide or a sulfateto introduce N2-free O2 into a reactor toproduce a gas for further processing or for

N OF REACTORS

OxidationReduction

1 22

5 10 20 30 40 50 60 70 80 90 100

3

4

5

6

7

8

9

1080%

R

ed

uc

tio

n &

O

xid

atio

n

Cycle

FIGURE 8-13 Time in minutes for

80% oxidation or reduction of a

single sample of Fe2O3/Al2O3,

particle size: 850e1000 mm, at 830�C.(Figure reproduced from Li et al. (2009a)with permission. � 2009 by American

Chemical Society).

8.6. COMBUSTION IN CIRCULATING FLUIDIZED BEDS 231

combustion that is not burdened by a highconcentration of nitrogen. An alternative useof looping employs the carbonation of lime.This was discussed earlier in Section 8.5 forH2 production from biomass. Researchers atthe Vienna University of Technology have alsodeveloped an H2 from biomass processinvolving carbonation and carbonate decompo-sition that has advanced to pilot plant testing(Pfeifer et al., 2007).

Materials used for CO2 capture are calcinedlimestone or, at lower temperatures,

00

100 200 300 400 500 600

20

40

60

80

100 Air Inert

Temperature

Methane

Time (s)

Con

cent

ratio

n of

CO

2, C

O a

nd C

H4

(%)

CO

CO2

CH4

O2

PERIODIC OPERATIO

a potassium promoted hydrotalcite or a sodiumpromoted alumina. Operation of these gaseouschemical looping combusters is illustrated inFigure 8-15.

The calciner would require a moderatelypure O2 stream to prepare a CO2 stream forsequestration. Decomposition of calciumcarbonate is endothermic so additional fuel ora part of the producer gas must be combustedwith O2 to provide heat for calcination.

In the carbonation bed, CH4 is oxidized and,with added steam, CO2 adsorption forces the

0

4

8

12

16

20

960

940

920

900

880

860

Oxy

gen

(%)

Tem

pera

ture

(°C

)

FIGURE 8-14 Exit gas composi-

tion and bed temperature for the

oxidation of methane by CuO/Al2O3

at 950�C and 1 bar in a periodically

operated laboratory fluidized bed.

Inert shown in the figure was N2.

(Figure reproduced from Mattisson et al.(2009a) with permission. � 2009 by

Elsevier Ltd.)

N OF REACTORS

Steam

Biomass

Bedmaterial

circulation

Air

Additionalfuel

CaO, heat

CaCO3, char

Flue gas(+ CO2)

Producer gas(H2-rich)

600…700 °C 800…900 °C

Gasification+ adsorption

Combustion+ desorption

FIGURE 8-15 Schematic ofa circulating adsorbent bed processfor H2 and CO2 separation usingcalcined lime as the trappingmedia. (Figure reproduced from Pfeiferet al. (2007) with permission. � 2007

The Berkeley Electronic Press.)


water gas shift reaction to H2. Use of anadsorber reduces substantially the steamdemand (Iyer et al., 2004). Although the loopingsystem can function at lower pressure,

HeaComGasBiom

Highly conducseparating w

Insu

Phase 1 Phase 1Phase 2 Phase 2 Phase 1

period τ period τ

Phase 1 (half-period 1)

Air Steam

CO, H2

CO2, H

2O

CH4

CO2

H2O

O2,N

2

1 2 3 4

FIGURE 8-16 Representation of temporal and location

a honeycomb of hexagonal channels filled with a fluidized

fuels (biomass in the diagram) and undergoing periodic s

et al. (2010) with permission. � 2009 Elsevier Ltd.)

PERIODIC OPERATIO

operating at 2 to 3 MPa would permit thecapture of power through gas turbines. Fanet al. (2008) report a laboratory scale demonstra-tion of H2-CO2 separation.

t fluxbustionificationass feed

tivealls

lation

Phase 2 Phase 1 Phase 1Phase 2Time

Phase 2 (half-period 2)

AirSteam

CO, H2, CO

2

H2O, CH

4

CO2

H2O

O2,N

2

al modulation of a solid fuel gasifier consisting of

solid oxygen carrier and intermittently fed with solid

witching of the fluidizing gas. (Figure adapted from Iliuta

N OF REACTORS

8.7. PERIODIC REACTION SWITCHING 233

8.7 PERIODIC REACTIONSWITCHING

In chemical looping, the circulating solidsencounter greatly different temperaturesand environments making laboratory experi-ments difficult and costly. This has led tosimulation of such systems using a stationarybed of an oxygen carrier and compositionmodulation of gases fed to the bed (seeTable 8-3).

Larachi and co-workers (Wang et al., 2009;Iliuta et al., 2010) have cleverly applied compo-sition modulation to perhaps solve problemsof incomplete gasification, the mixing ofdifferent gas streams and heat loss encoun-tered in chemical looping. They proposedundertaking gasification or combustion byarranging square or hexagonal bubbling fluid-ized bed cells containing the solids oxygencarrier, say Fe2O3/Al2O3, in a plane withcontinuous biomass feed, such as sawdust, toeach cell. Fluidizing gas, also to each cell,would periodically switch between steamand air so that the cell would shift from a pyro-lyzing and gasifying biomass to combustingthe char formed. Off-gases would be collectedseparately. Contiguous combusting and gasi-fying cells would allow heat transfer fromthe hotter combustion cells to the cooler gasifi-cation ones. Their concept is illustrated inFigure 8-16.

Simulating the proposed operation assumingadjacent beds, Iliuta et al. (2010) found, usingdata and fluidization models from the literature,that an operation producing O2- and N2-freehydrogen could be sustained for a switchingperiod, sswitch, of about a minute, even witha 20% loss of heat. Wang et al. (2009) consideredanother version of this design in which thebiomass particles are transported with the gasphase. They investigated just the hydrody-namics of the operation.

PERIODIC OPERATIO

Notation

A/F

N OF RE

= air/fuel ratio (-)
Bz = benzene Ci = concentration of species “I” (mol/m3) COP = coefficient of performance DEA = diethanolamine GHSV = gas hourly space velocity (h�1) i.d. = inner diameter (cm) l = liquid MEA = monoethanolamine m = mass of catalyst (g) P = pressure (kPa, MPa) PH = high pressure in heat pump cycle PL = low pressure in heat pump cycle PM = median pressure in heat pump cycle Q = volumetric flow rate (m3/h) SH = high temperature reaction couple in heat
pump cycle
SL = low temperature reaction couple in heat
pump cycle
SV = space velocity (s�1, h�1) s = cycle split (-) T = temperature (K) TH = high temperature in heat pump cycle TL = low temperature in heat pump cycle TM = median temperature in heat pump cycle TGA = thermogravimetric scale or analysis t = time (s, min) u = velocity v = vapour W = work
Greek

DT
= temperature lift or difference in heatpump cycle
g
= fraction or segment of a cycle (-) s = cycle period, partial period, time within
a period (s, min)
scycle = cycle period sswitch = time between a switch of a manipulated
variable
s1, s2, etc. = duration of partial cycles (s, min)
Subscripts

H
= high or higher L = low or lower M = median x, y, z = stoichiometric coefficients 1, 2, 3, etc. = segments or parts of a cycle
ACTORS

Periodic Operation of Chemical Reactors || Catalytic Gas-Solid Reactions

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