Mapping of the Range of Operational Conditions for Cu-, Fe-, and Nibased Oxygen Carriers in Chemical-Looping Combustion

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1 Submitted, accepted and publihed by: Chemical Engineering Science 62 (2007) Mapping of the Range of Operational Condition for Cu-, Fe-, and Nibaed Oxygen Carrier in Chemical-Looping Combution Alberto Abad, Juan Adánez 1, Francico García-Labiano, Lui F. de Diego, Pilar Gayán, Javier Celaya Intituto de Carboquímica (C.S.I.C), Department of Energy and Environment, Miguel Luema Catán 4, Zaragoza 50015, Spain Abtract Chemical-Looping Combution (CLC) i a two-tep combution proce that produce a pure CO 2 tream, ready for compreion and equetration. A CLC ytem i compoed by two reactor, an air and a fuel reactor, and an oxygen carrier circulating between the reactor, which tranfer the oxygen neceary for the fuel combution from the air to the fuel. Thi ytem can be deigned imilar to a circulating fluidized bed, but with the addition of a bubbling fluidized bed on the return ide. A mapping of the range of operational condition, deign value, and oxygen carrier characteritic i preented for the mot uual metal oxide (CuO, Fe 2 O 3, and NiO) and different fuel gae (CH 4, H 2, and CO). The preure operation of a CLC ytem i alo conidered. Moreover, a comparion of the poible ue of three high reactive oxygen carrier (Cu10Al-I, Fe45Al-FG, Ni40Al-FG) previouly characteried i carried out. It wa found that the circulation rate and the olid inventorie are linked, and the poible operating condition are cloely dependent on the reactivity of the oxygen carrier. The operational limit of the olid circulation rate, given by the ma and heat balance in the ytem, were defined for the different type of oxygen carrier. Moreover, a plot to calculate the olid inventorie in a CLC ytem, valid for any type of oxygen carrier and fuel ga, i propoed. The minimum olid inventorie depended on the fuel ga ued, and followed the order CH 4 >CO>H 2. Value of minimum olid inventorie in a range from 40 to 170 kg/mw f were found for the oxygen carrier ued in thi work. From the economic analyi carried out it wa found the cot of the oxygen carrier particle doe not repreent any limitation to the development of the CLC technology. 1 Correponding author: Tel.: ; fax: addre: jadanez@icb.cic.e (Juan Adánez) 1

2 Keyword: Deign; Environment; Fluidization; Kinetic; Chemical-Looping Combution; Oxygen Carrier 1. Introduction The problem of global warming caued by the greenhoue ga emiion i nowaday gaining increaing importance. Commercially available CO 2 capture technologie are high cot and their ue today will ignificantly increae the cot of electricity and heat generation. Thi fact ha produced an important development of CO 2 capture technologie to reduce the energy need for eparation and thereby, the cot per tonne of CO 2 avoided. Three main route can be explored to limit CO 2 emiion: pot-combution capture, precombution decarboniation, and denitrogenated converion. For each one of thee route a wide array of technologie i being preently evaluated from a technical and economical point of view. In thi ene, Chemical-Looping Combution (CLC) ha been revealed a an efficient and low cotly proce. Thi new type of combution, initially propoed by Ritcher and Knoche (1983), lead to beneficial exergy efficiencie in the ytem if CO 2 capture i conidered (Wolf et al., 2005). Wolf et al. (2001) reported a thermal efficiency a high a 52-53% in a combined cycle CLC plant operating at 1200 ºC in the air reactor and 13 bar, which repreent a 5 percent more efficient proce than a natural ga with combined cycle ytem with tate-of-the-art technology for CO 2 capture. The efficiency would be omething lower in an atmopheric CLC operating in a team cycle. However, different economic aement performed by the CCP (CO 2 Capture Project) (Kerr, 2005) and by the International Panel on Climate Change (IPCC, 2005) indicated CLC among the bet option for reducing the cot of CO 2 capture uing fuel ga. CLC i a two-tep ga combution proce that produce a pure CO 2 tream, ready for compreion and equetration. A olid oxygen carrier (OC) circulate between two reactor and tranport oxygen from the combution air to the fuel. Since the fuel i not mixed with air, the ubequent CO 2 eparation proce i not neceary. A gaeou fuel can be ued natural ga or ynthei ga from coal gaification. In thi ytem, the total amount of heat evolved from reaction in the two reactor i the ame a for normal combution, where the oxygen i in direct contact with fuel. Since the proce require a good contact between ga and olid a well a a flow of olid material between the fuel and air reactor, the ue of two interconnected fluidized bed have advantage over other deign (ee Figure 1). The bed material circulating between the two 2

3 fluidized bed i the oxygen carrier in the form of metal oxide particle. The volumetric ga flow in the air reactor i approximately 2.5 and 10 time larger than that of ynga and CH 4, repectively, becaue a large amount of nitrogen in the air i carried out. To keep a reaonable ize of the reactor a high velocity rier wa propoed for the air reactor (Lyngfelt et al., 2001) in a CLC plant uing CH 4 a fuel ga. In the air reactor (AR), oxygen i tranferred from the combution air to the oxygen carrier. The exit ga tream from the air reactor contain N 2 and ome unued O 2. The particle carried away from the rier are recovered by a cyclone and led to the fuel reactor (FR). In the fuel reactor (bubbling fluidized bed) oxygen i tranferred from the oxygen carrier to the fuel. The exit ga tream from the fuel reactor contain CO 2 and H 2 O, and almot pure CO 2 i obtained after H 2 O condenation. From the fuel reactor the particle are returned to the air reactor by gravity. Two particle eal prevent ga mixing between the two reactor. The technical rik of the ytem i therefore relatively low operating at atmopheric preure becaue all the procee are carried out in tandard proce equipment and uing element of proven technology (Copeland, 2001; Lyngfelt et al., 2001). Ideally, the number of reduction-oxidation cycle of the oxygen carrier would be infinite. However, the oxygen carrier material mut be renovated a a conequence of particle attrition/fragmentation or reactivity lo during the reduction/oxidation cycle and a makeup flow of new material (F 0 ) i neceary. Since the cot of thi new material i the main additional cot of the whole proce compared to a proce without CO 2 capture, the key iue in the ytem performance i the oxygen carrier material. Beide high reactivity toward the fuel ga and air during many reduction-oxidation cycle, the oxygen carrier mut fulfil other characteritic a high reitance to attrition, no preence of agglomeration, and no carbon depoition when uing carbon-included fuel. Other environmental and economical apect mut be alo conidered in the final election of the oxygen carrier. There are in the literature everal work related with deign of CLC reactor. Lyngfelt et al. (2001) preented the firt deign of an atmopheric CLC reactor including a high-velocity rier for the air reactor and a low-velocity fluidized bed for the fuel reactor. A conceptual deign of a 10 kw thermal power CLC working at atmopheric preure wa done by Kronberger et al. (2005a). Wolf et al. (2005) analyzed the feaibility of CLC in two interconnected preurized fluidized bed reactor for a capacity of 800 MW input of natural ga. Other author have analyed pecific apect of the CLC proce. Adánez et al. (2003) preented a model for the optimization of the fuel reactor operation. Johanon et al. (2003) analyed the ga leakage between the reactor, and Kronberger et al. (2004) ued a cold model to find critical deign parameter of the CLC ytem. 3

4 The CLC proce ha been uccefully demontrated in a 10 kw unit (Lyngfelt et al., 2004; Lyngfelt and Thunman, 2005) during 100 h of continuou operation burning natural ga and uing a Ni-baed oxygen carrier. Moreover, Ryu et al. (2004) preented the reult of 3.5 hour continuou run in a 50 kw Chemical-Looping Combutor. The following tep in the proce development will include a deign optimization, continuou operation during long time at pilot plant cale, and detailed engineering and cot analyi at higher cale. Moreover, the chemical and mechanical tability of the oxygen carrier mut be proved during long time. The objective of thi work i to determine the relation between uitable operating condition in a CLC proce and oxygen carrier characteritic for different Cu-, Fe-, and Ni-baed material and fuel gae (CH 4, H 2, and CO). Moreover, a comparion of the behavior in a CLC proce of three oxygen carrier previouly characteried i howed. 2. Oxygen Carrier Different metal oxide have been propoed in the literature (Ritcher and Knoche, 1983; Ihida et al., 1987; Mattion and Lyngfelt, 2001) a poible candidate for CLC proce: CuO, NiO, Mn 2 O 3, Fe 2 O 3, and CoO. In general, thee metal oxide are combined with an inert which act a a porou upport providing a higher urface area for reaction, a a binder for increaing the mechanical trength and attrition reitance, and, additionally, a an ion conductor enhancing the ion permeability in the olid. Among them, iron, nickel, and copper have been elected a the mot promiing candidate to be ued in a CLC proce (Adánez et al., 2004). Oxide of thee metal upported on everal inert howed low attrition rate and high reaction rate during many ucceive reduction-oxidation cycle (Mattion et al., 2003; Kronberger et al., 2004; Adánez et al., 2004, 2005) although their behavior during continuou operation mut be proved. Table 1 how the reduction reaction of thee metal oxide with CH 4, H 2, and CO, and the oxidation reaction with O 2. An important characteritic of the metal oxide i their oxygen tranport capacity, alo called oxygen ratio (Mattion et al., 2003), defined a R o =(m ox -m red )/m ox, where m ox and m red are the mae of the oxidied and reduced form of the metal oxide, repectively. The oxygen tranport capacity i the ma fraction of oxygen that can be ued in the oxygen tranfer. The maximum value correpond to the NiO and CuO and thi i lower for the Fe 2 O 3 in it tranformation to Fe 3 O 4. Although iron compound have different oxidation tate (Fe 2 O 3 -Fe 3 O 4 -FeO-Fe), only the tranformation from hematite to magnetite may be applicable for indutrial CLC ytem. Further reduction to FeO would produce a high decreae in the CO 2 purity 4

5 obtained in the fuel reactor becaue of the increae of the CO and H 2 concentration in the equilibrium (Copeland et al., 2001; Kronberger et al., 2005b). Obviouly, the tranport capacity of the oxygen carrier decreae due to the preence of the inert. In thi way, the oxygen tranport capacity of the oxygen carrier depend both on the active metal oxide content and on the type of metal oxide conidered, R o,oc =R o x MeO. In thi work, three oxygen carrier uing Al 2 O 3 a upport have been conidered: particle of copper (II) oxide prepared by impregnation (Aldrich Chemical Company), and iron- and nickelbaed particle prepared by freeze-granulation at Chalmer Univerity of Technology. The ample were deignated a Cu10Al-I, Fe45Al-FG, and Ni40Al-FG. In all cae, part of the metal oxide ued in the preparation react with the upport to give aluminate that, in the majority of the cae, wa not active for reaction. Table 2 how the main propertie of the material. For a better comparion of the effect of the type of metal oxide on their behavior in a CLC proce, oxygen carrier with the ame metal oxide content would be deirable. However, the Cubaed oxygen carrier with high CuO content undergo agglomerattion problem during their operation in a fluidized bed (de Diego et al., 2005). Therefore, the oxygen carrier Cu10Al-I preent a much lower MeO content than the other oxygen carrier Reactivity of the oxygen carrier The reactivity of the oxygen carrier i an important factor to be conidered in the deign of a CLC proce, becaue it i directly related with the olid inventory in the ytem. The oxygen carrier mut have enough reactivity to fully convert the fuel ga in the fuel reactor, and to be regenerated in the air reactor. The kinetic of the reduction and oxidation reaction of the three oxygen carrier baed on Cu. Fe, and Ni were determined at atmopheric preure in a thermobalance (CI Electronic Ltd.) at different operating condition. More detail about the experimental ytem and operating procedure can be found elewhere (García-Labiano et al., 2004). The temperature range wa varied from 600 ºC to 950 ºC for the Fe45Al-FG and Ni40Al-FG oxygen carrier and from 500 ºC to 800 ºC for the Cu10Al-FG. The compoition of the ga ued to determine the kinetic of the reduction reaction wa varied to cover the great majority of the ga concentration preent in the fluidized bed fuel reactor of a CLC ytem including the product gae (Fuel: 5-70 vol %; H 2 O: 0-48 vol %; CO 2 : 0-40 vol %). For the oxidation reaction, oxygen concentration from 5 to 21 vol % were ued. A an example, Figure 2 how the converion veru time curve for the reduction and oxidation of the three oxygen carrier uing a ga concentration of 15 vol %, correponding 5

6 aproximately to the average concentration exiting in a bubbling fluidized bed for the typical CLC reaction (ee eq. 19). The three oxygen carrier were very reactive, although the reactivity of the Cu10Al-I oxygen carrier wa higher than the reactivity of the other, Fe45Al-FG and Ni40Al-FG. However, thi difference in reactivity can not be directly tranlated into olid inventory data for the CLC proce, a a conequence of their different oxygen tranport capacity (ee Table 2). There are everal reitance that can affect the reaction rate of the oxygen carrier with the fuel ga or with the air. Previou calculation howed that the ma tranfer reitance in the ga film and in the product layer of the particle were not important in the experimental condition ued in thi work. To determine the kinetic parameter of thee oxygen carrier with repect to the reduction (CH 4, H 2, and CO) and oxidation reaction (O 2 ) the hrinking core model (SCM) with the reaction controlled by the chemical reaction in the grain wa ued. Two different geometrie were conidered taking into account the tructural difference of the oxygen carrier. Fe- and Ni-baed oxygen carrier prepared by freeze-granulation exhibited a granular tructure, and the SCM for pherical grain wa conidered. However, the SCM for plate-like geometry in the porou urface of the particle wa conidered for the Cu-baed oxygen carrier prepared by impregnation auming a uniform layer of metal oxide inide the pore covering the inert material. The equation that decribe thee model under chemical reaction control in the grain are the following: for pherical grain geometry t 1 (1 X ) 1/ 3 ρm rg τ (1) n n b k (C C ) i eq for plate-like geometry t X ρ L m τ (2) n bi k C where the kinetic contant follow an Arrheniu type dependence with temperature - E /RT k k 0 e (3) Eq. (1) conider the thermodynamic equilibrium for the reactant ga that occur in the Ni-baed oxygen carrier, epecially when working with H 2 or CO. For the other metal oxide the value of C eq i zero. Table 3 how the value of the kinetic parameter obtained at atmopheric preure for the different oxygen carrier and reaction ued in thi work (García-Labiano et al., 2005; Adánez, 2005). The activation energie determined were low, with value ranging from 7 to 78 kj/mol, and are imilar to thoe reported in literature (Ihida et al., 1996; Ryu et al., 2001). The reaction order depended on the reacting ga, and value from 0.2 to 1 were found. The ga product (CO 2 and 6

7 H 2 O) did not affect the reduction reaction rate. Only during the reduction of the Fe45Al-FG oxygen carrier with CH 4 a negative effect of the H 2 O on the direct reaction rate wa oberved. In thi cae, the total reaction time wa calculated a m g r,ch 4 C (4) 0.3 bch k (1-0.15CH O) CH 4 r 2 4 The effect of total preure on the reaction rate of the oxygen carrier Cu10Al-I, Fe45Al-FG, and Ni40Al-FG with H 2, CO, and O 2 wa analyed in a previou work (García-Labiano et al., 2005). The experiment howed that an increae in total preure had a negative effect on the reaction rate of all oxygen carrier and reaction. An apparent preexponential factor wa determined a a function of total preure and the preexponential factor obtained at atmopheric preure to fit the experimenatl data k 0 k 0,p (5) q P The value of the parameter "q" for each oxygen carrier and reaction are howed in Table Deign criteria for a CLC ytem According to Lyngfelt et al. (2001), the three main parameter for the deign of a CLC ytem are: 1) the amount of bed material in the two reactor, that mut be adequate for a ufficient converion of reacting ga, 2) the circulation rate between the reactor, that mut be high enough to tranfer the oxygen neceary for the fuel combution and the heat neceary to maintain the heat balance, and 3) the ga leakage between the reactor mut be minimied. The two firt are dependent on characteritic of the oxygen carrier ued, a olid reactivity, type of metal oxide, and weight content. Thi ection analye the main difference exiting in the ue of Cu-, Fe-, and Ni-baed oxygen carrier in relation with the deign of a CLC ytem. Moreover, the poibility of ue of the three oxygen carrier previouly characteried (Cu10Al-I, Fe45Al-FG, Ni40Al-FG) i included Ma Balance. The CLC concept i baed on the tranport of oxygen from the air to the fuel by mean of an oxygen carrier. Fig. 1 how the general cheme of a CLC ytem. Ideally, the oxygen carrier hould circulate in the ytem an infinite number of cycle. However, a makeup flow of new olid, F 0, i required to compenate for the natural decay of activity and/or olid loe by attrition/fragmentation during operation. 7

8 The ma balance in the ytem, baed on molar flow of oxygen carrier fully oxiided, i a follow, F F X F X b F X (6) 0 FR R 1,FR 0,F r f f AR FR X,AR co FO 2 XO (7) 2 where X,F0 i the converion variation of the freh particle fed into the ytem due to the reaction with the ga in the fuel reactor. For teady tate operation, F 1 =F 0, and if the oxygen carrier i valid for the ue in CLC, the condition F 0 <<F R i required to keep the orbent makeup cot within reaonable limit, and it can be aumed with low error that b X X (8) F 2 r O O Ff Xf d F 2 f co and,ar,fr f X X (9) where the parameter "d" i the toichiometric factor in the fuel combution reaction Circulation rate. The circulation rate, expreed a ma flow of oxygen carrier totally oxidied, depend on the type of oxygen carrier and fuel ued, a well a on the converion variation obtained in the oxygen carrier in the fuel and air reactor, and it can be obtained from Eq. (6) m OC 10 3 br M x MeO MeO Ff ΔX ΔX,FR f (10) Taking a reference 1 MW of fuel, and auming full converion of ga (X f =1), Eq. (10) can be rewritten a b M 1 mc m OC (11) x ΔX ΔX r MeO 0 MeO ΔHc,FR,FR where the characteritic circulation rate, m c, i a pecific parameter defined by the oxygen carrier-fuel pair. m c b x M r MeO 0 MeO ΔHc 2 d M R ΔH o,oc O 0 c (12) Although Eq. (11) give the circulation rate neceary to fulfill the oxygen ma balance in the CLC ytem, it i alo neceary to conider other apect related with the hydrodynamic behavior and with the heat balance in the ytem. 8

9 The circulation rate in a CFB ytem depend on the operation condition and configuration of the rier. There are everal work that have mapped poible combination of fluidiation velocitie, rier preure drop and external olid flux (Takeutchi et al, 1986; Lim et al., 1995). For typical condition of ga velocity (4-6 m/), temperature ( ºC), and exce of air (0-20%), value of rier area in a CLC proce from 0.12 to 0.35 m 2 /MW f are obtained conidering the combution of CH 4, H 2, or CO ( for CH 4, for CO, for H 2 ). Selecting a value of 0.2 m 2 /MW f a an average value of the cro ection area of the rier, S, the olid flow can be calculated a: G moc (13) S Fig. 3 how the circulation rate, G, for the different metal oxide (CuO, Fe 2 O 3, and NiO) and fuel ga conidered in thi work a a function of the metal oxide content in the oxygen carrier and on it converion variation. Obviouly, the value of G decreae with increaing the MeO content of the oxygen carrier. The Fe-baed oxygen carrier need much higher circulation rate than the Ni- and Cu-baed oxygen carrier, becaue of their low oxygen tranport capacity (ee Table 1). The difference in G a a conequence of the fuel ga ued are of lower importance. The olid flow i linked with the oxygen needed for the reaction of each fuel ga. Thu, to obtain 1 MW are neceary 1.25 mol CH 4-1, 3.53 mol CO -1, or 4.14 mol H 2-1 a a conequence of their different combution heat (ee Table 1), and the tranport of 5, 4.14, or 3.53 mol O -1, repectively. The limit of the circulation rate in a CLC plant i not clear. When the ga velocity in a circulating fluidized bed i increaed the circulation flow increae up to point where the recycling ytem i not dimenioned to accomodate a larger flow. Larger flow can be reached by redeigning of the recycling ytem. However, very large flow could generate increaed difficultie with entraining ga between reactor and increaing cot if they are outide the range of normal commercial experience. Although there i ome diperion in the experimental data available in the literature, value of G from 20 to 100 kg m -2-1 have been reported for CFB ytem (Smolder and Baeyen, 2001). Lyngfelt et al. (2001) yield value of G in the order of 50 kg m -2-1 for the combution of natural ga uing iron oxide a oxygen carrier in a CLC plant under atmopheric condition. Taking 80 kg m -2-1 a the maximum circulation rate feaible in a CLC plant without increaed cot and with commercial experience, it can be oberved that it would be impoible to work with Fe-baed oxygen carrier with Fe 2 O 3 content lower than 10 wt. % becaue the circulation rate would prevent the full converion of ga even with infinite olid inventory 9

10 (X =1). The higher tranport capacity of the other metal oxide (CuO, NiO) make poible the preparation of oxygen carrier with low MeO content. In thee cae, value about wt% could be enough for normal operation without high circulation rate. Other important apect to be conidered in a CLC ytem i the heat balance. The oxidation reaction of the metal oxide i alway exothermic. The reduction reaction can be endothermic or exothermic depending on the metal oxide and fuel ga combination, although the great majority of the heat, even for exothermic reduction, i generated in the air reactor (ee Table 1). For endothermic reduction reaction (NiO with CH 4, Fe 2 O 3 with CH 4 ), the fuel reactor i heated by the circulating olid coming from the air reactor at higher temperature. A previouly have been een (Fig. 3), the olid circulation rate depend on the MeO content and on it converion variation, X. Therefore, for a fixed temperature in the air reactor, the temperature in the fuel reactor will depend on the ame variable. Fig. 4 how the effect of the MeO content and the X value on the temperature in FR for the combution of CH 4 with a Ni-baed oxygen carrier for a fixed temperature in AR of 950 ºC. The heat extracted from the AR i neceary to fulfill the heat balance in the ytem, conidered a an adiabatic ytem. When the olid circulation rate decreae, which correpond to higher MeO content or higher X (ee Fig. 3), the temperature in FR alo decreae becaue le heat i tranfered from the AR. To avoid a large temperature drop in the FR, a high olid circulation rate i deired, which in practice mean a low X. In a real ituation, the energy loe in the ytem will make neceary even higher circulation rate than the above howed, and o lower X. Similar reult are obtained when a Fe-baed oxygen carrier i ued for the combution of CH 4. However, the temperature drop in FR i lower in thi cae becaue the Fe-baed oxygen carrier need a higher circulation rate to fulfill the ma balance a a conequence of their low oxygen tranport capacity (ee Table 1). In all thee ituation, it would be neceary to limit the temperature drop in the FR to maintain a high reduction reaction rate and to get full ga converion. Fig. 5 how the MeO content neceary to maintain a temperature difference of 50 ºC between the air and fuel reactor for the reduction reaction of Fe- and Ni-baed oxygen carrier with CH 4 a a function of X. It can be oberved that Ni-baed oxygen carrier with low NiO content or low X value mut be ued in the ytem; however, Fe-baed oxygen carrier with high Fe 2 O 3 content and high X can be ued to maintain a temperature difference lower than 50 ºC. It can be concluded that when the reduction reaction i endothermic, the circulation rate i limited between the maximum given by the rier capacity and the minimum given by the heat balance (ee Fig. 3). 10

11 When the reduction reaction i exothermic (ee Table 1), the olid circulation rate i not limited by the heat balance. In thi cae, the circulation rate will be elected to reach the deired olid converion in the air reactor. Fig. 5 how the combination between MeO content and circulation rate, expreed a X, that produce an increae of temperature of 50 ºC in FR. It i oberved that, for Cu-baed oxygen carrier, thi temperature increae i reached at low value of CuO content or low X. Similar ituation can be reached for the combution of ynga with Ni- or Fe-baed oxygen carrier, although in thi cae the difference of temperature i reached at high MeO content and high X. For the reaction of Fe 2 O 3 with ynga, a gradient of temperature of 50 ºC wa not obtained in any cae, even conidering a MeO content of 100 wt % and a X = Solid inventorie In a CLC proce, it i deirable to minimie the amount of bed material in the reactor becaue thi will reduce the ize and invetment cot of the ytem and alo le power will be needed by the fan that upply the reacting gae to the reactor, epecially to the air reactor. The olid inventory in the CLC ytem for any fuel converion can be obtained from a ma balance of the olid and ga in the fuel and air reactor. For preliminary etimation, it ha been conidered that the reaction between the ga fuel and air with the oxygen carrier take place in bubbling fluidized bed. Although the air reactor mut be normally a rier, it i very probable that the higher reidence time of the olid, and then the oxidation reaction, take place in the dene zone at the bottom of the rier. Therefore, in thi work, the fuel and air reactor are treated in a imilar way. For preliminary calculation, it wa aumed perfect mixing of the olid, ga plug flow in the bed, and no reitance to the ga exchange between the bubble and emulion phae. For the complete converion of the ga, the ma of oxygen carrier in each reactor per MW of fuel, expreed a ma of oxygen carrier totally oxidied, can be calculated a hown in García- Labiano et al. (2004) m OC m,fr m OC m,ar c c r FR o AR (14) (15) where the characteritic reactivity, FR or AR, are parameter defined a dx dt FR r (16) FR 11

12 dx dt AR o (17) AR The total olid inventory in the ytem, per MW of fuel, will be given by m OC,AR m m (18) FR OC,AR OC,FR The characteritic circulation rate m c wa defined by Eq. (12), and the pecific value for the oxygen carrier Cu10AL-I, Fe45Al-FG, and Ni40Al-FG are howed in Table 3. For the calculation of the olid inventory in the air reactor, the value of ytem mut be conidered. m c correponding to the fuel ued in the The value of r and o for the reduction and oxidation reaction were obtained at an average ga concentration. Auming perfect mixing of olid, ga plug flow in the reactor and no reitance to the ga exchange between bubble and emulion phae in the fluidized bed, the average concentration of reacting ga in their repective reactor can be obtained from the following equation (ee Appendix II): n ΔXgC n 0 C (19) n Xg,out 1 εgx g dxg Xg,in 1 Xg The parameter g in Eq. (19) conider the ga expanion a a conequence of the reaction, and it wa calculated a Vg,X g 1 Vg,X g 0 g (20) V g,x g 0 The value of g wa 2 for the reduction reaction with CH 4, 0 for the ue of H 2 and CO, and for the oxidation reaction. Table 3 how the average concentration conidering full converion of ga in the fuel reactor, and an air exce of 20 % in the air reactor. Auming perfect mixing of the olid in the reactor, the characteritic reactivity, j, can be expreed in each reactor (FR or AR) a a function of the olid converion at the inlet and the converion variation in the reactor (ee Appendix I), for pherical grain geometry 12

13 j 31 X 6X 2 j 2 2 / 3 o,inj exp 1 exp 1 X X 1/ 1 X 1/ X X 1/ 3 o,inj o,inj j 6X j j 1 X 1/ 3 o,inj exp X o,inj j (21) for plate-like geometry j 1 Xo,inj 1 exp j (22) X The characteritic reactivity, j, i very ueful to calculate the olid inventory becaue thi i only a function of the olid converion at the inlet and the converion variation reached in each reactor, and it i valid for any olid reactivity and oxygen tranport capacity. An important fact i that the value of thi term i limited between 0 and 3 for the ue of the SCM with pherical grain, and between 0 and 1 for plate-like geometry. Therefore, the olid inventory for a given oxygen carrier and fuel ga depend on the olid circulation rate and on the olid converion at the inlet to each reactor, a well a on their metal oxide content and reactivity. Fig. 6 how the total olid inventory, m, of oxygen carrier Ni40Al-FG per MW f of CH 4 in the CLC ytem a a function of the oxidation converion at the outlet of the air reactor, X. Obviouly, X o,out AR X o,out AR = X o,in FR OC,AR, and the variation of olid converion between the two reactor,. The level curve indicate the condition with a ame amount of oxygen carrier in the CLC ytem. From thi diagram it i poible to know the total olid inventory for a given olid recirculation a a function of the average olid converion from the air FR reactor X o,out AR. It can be oberved that, for a given X, the total olid inventory preent a minimum with Xo,out AR due to the effect of the average converion on the characteritic reactivity of each reactor, AR and FR, and therefore in the olid inventory. On the other hand, for a given X o,out AR, the olid inventory in the whole CLC ytem increae a X increae. The minimum olid inventory in the ytem i reached at X near to zero, in thi cae, 32 kg per MW f. Similar diagram can be obtained for the different oxygen carrier and fuel gae. However, thi type of graph i only valid for each pecific oxygen carrier and fuel ga, and can be very different depending on the reactivity of the material. To generalie thi tudy, Fig. 7 how a new deign plot valid for any oxygen carrier and fuel ga reacting under hrinking core model with chemical 13

14 reaction control for pherical grain. The optimum olid inventory in a CLC ytem can be obtained from the ue of thi plot and Eq. (14) to (18). The inlet variable are the type of oxygen carrier (R o,oc ) and the fuel ga, which define the value of m c, the reactivity of the oxygen carrier during reduction and oxidation, defined by the i value, and the circulation rate. Fig. 7 how the value of j a function of the olid converion at the inlet of the fuel reactor, X o,in FR, or in the air reactor, X o,in AR value ( X o,in FR, and X for the SCM with pherical grain. For a given pair of, X ) it i poible to know the value of FR from the plot. Similarly, for the air reactor, the value of AR i obtained from the pair of value ( X o,in AR, X ). The olid inventory in each reactor can be calculated then from Eq. (14) and (15) and the total olid inventory in the ytem by Eq. (18). Fig. 7 alo how the relation between the circulation rate, expreed a m / and X given by Eq. (11). It can be oberved that a decreae in the circulation rate OC m c produce a decreae in the value of the j parameter, both for the fuel and air reactor. Therefore, the olid inventory increae when the circulation rate decreae. For a fixed circulation rate of a given oxygen carrier, there i only a value of X that fulfil the ma balance in the ytem. A previouly hown, at thi condition the total olid inventory ha a minimum value a a function of X o,out AR (ee Fig. 6). Thi minimum inventory depend on the value of r and o, defined by the ga-olid reactivity. Fig. 7 how different curve to obtain the minimum olid inventory for everal value of r / o or o / r. The croing point of X and r / o line determine the FR value for the total minimum inventory at a fixed X, and the correponding value of olid converion at the inlet of FR. The olid inventory in FR can be directly calculated from Eq. (14). On the other hand, the croing point of X and o / r line determine the AR value and the olid inventory in AR can be calculated from Eq. (15). The value of the olid converion at the inlet of AR that give the minimum olid inventory in the ytem can be obtained directly from the above croing point. A example, for a X =0.3 and an oxygen carrier with value of r =30 and o = 6, the condition for the minimum olid inventory in FR are reached at the point (X =0.3, r / o =5), which give value of FR =2.2 and X o,in FR =0.75. For AR, the point (X =0.3, / r =0.2) give value of AR =2 and X o,in AR =0.45. Obviouly, o,in FR and AR can be calculated directly from Eq. (14) and (15). X = X o,in AR +X. The olid inventorie in FR The optimum operating condition in the CLC ytem are thoe that give the minimum olid inventory at a fixed X and get full ga converion. Thee condition will be conidered from now on. Fig. 8 how the minimum olid inventory and the circulation rate neceary for the full 14

15 converion of CH 4 a a function of X for the three oxygen carrier elected in thi work. Similar curve can be obtained for the fuel gae H 2 or CO although the difference in the value can be high, up to a decreae of 30% in G and a decreae of 70% in the the minimum olid inventory. Low value of X gave high circulation rate and low olid inventory. Oppoitely, high value of X gave low circulation rate, but high olid inventory. Auming a maximum circulation rate in the ytem of 80 kg m -2-1, the X value mut be higher than 0.04, 0.28, and 0.34 for the oxygen carrier Ni40Al-FG, Cu10Al-I, and Fe45Al-FG, repectively. The optimum value of X to get low circulation rate and low olid inventory could be about , becaue higher value of X highly increae the olid inventorie. Table 4 how the minimum olid inventory in the fuel and air reactor per MW f of CH 4, H 2 and CO for X cloe to zero and X =0.3. Although the oxidation rate i the ame independently of the ga ued for the reduction, the difference oberved among ga fuel in the air reactor inventory are due to the different oxygen flow neceary to fulfill the ma balance in the ytem. It i oberved that the ue of the oxygen carrier Fe45Al-FG for the combution of CH 4 in a CLC proce need the higher olid inventory, becaue although it i a very reactive olid, it low oxygen tranport capacity produce low reaction rate per kg of oxygen carrier. The Ni40Al-FG oxygen carrier need lower olid inventory becaue it high reaction rate and high oxygen tranport capacity. On the other hand, the value for the Cu10AL-I oxygen carrier wa between the above given for the other oxygen carrier, a a conequence of their low CuO content but high oxygen tranport capacity of the CuO. Similar curve can be obtained for the other fuel gae (H 2, CO). All thee value correpond to condition where the reitance to the ga exchange between the bubble and emulion phae ha been conidered negligible. In real ytem, if the reitance to the ga exchange between the bubble and emulion phae i important, the i value would increae and a a conequence, higher olid inventorie and o, higher reidence time of the olid in the reactor would be predicted. Etimation of the total inventory in a CLC ytem with different oxygen carrier and CH 4 a fuel ga can be found in the literature. Mattion et al. (2003) etimated that the amount of oxygen carrier wa 70 kg per MW in the fuel reactor, and 390 kg per MW in the air reactor, uing a 33 wt % CuO oxygen carrier prepared by dry impregnation. Cho et al. (2004) obtained that the bed ma in the fuel reactor wa 200 kg per MW with a 60 wt % CuO oxygen carrier produced by freeze granulation. However, for their calculu, they did not conider the olid reidence time ditribution and ue the reactivity obtained at a determined olid converion. The data howed by thee author were higher than thoe obtained in the preent work becaue of the lower reactivity of their material. 15

16 3.4. Relation between oxygen tranport capacity, olid reactivity and olid inventory A above howed in Eq. (14) and (15), the olid inventory in each reactor depend on the olid reactivity through the parameter r or o, on the oxygen tranport capacity of the oxygen carrier, R o,oc, through the parameter m c, and on X through the parameter j (ee Fig. 7). Therefore, for a given circulation rate, that i for a given X, there are pair of value R 0,carrier - i that generate the ame olid inventory. Fig. 9 how the olid inventorie, in FR or AR, obtained for the CH 4 combution. Thee value were obtained for X < 0.05 which correpond to minimum value of olid inventory in the ytem. It can be oberved that a ame inventory can be reached with an oxygen carrier with le oxygen tranport capacity but with higher reactivity. Fig. 9 alo how the point correponding to the oxygen carrier ued in thi work (Cu10Al-I, Fe45Al-FG, and Ni40Al-FG). For example, the reactivity of the Fe45Al-FG oxygen carrier with CH 4 ( r =53 ) i higher than the reactivity of the Ni40Al-FG ( r =60 ). However, thi increae i not enough to compenate their lower oxygen tranport capacity, and higher olid inventorie are neceary (m Fe45Al-FG =100 kg/mw CH4, m Ni40Al- FG =19 kg/mw CH4 ). Fig. 9 alo how the circulation rate, expreed a G X, a a function of the R o,oc. Metal oxide with low oxygen tranport capacity, a for example the Fe 2 O 3, need high metal oxide content, normally above 40%, to fulfil the ma balance in the ytem Effect of total preure in CLC ytem The ue of a preurized CLC ytem would have everal advantage with repect to the atmopheric operation. Firt, the efficiency of the cycle in the CLC power plant will be increaed (Wolf et al., 2001); econd, the recovering of the CO 2 a a high-preure ga require only a very mall amount of additional power for further compreing the CO 2 to pipeline (35 atm) or equetration preure (100 atm) (Copeland et al., 2001). The total preure can affect to important operating and deign parameter of the CLC ytem. A higher preure produce an increae in the ga molar concentration, thu an increae in the reaction rate of the oxygen carrier would be expected. Thi will produce a large decreae in the olid inventory of the CLC ytem a well a in the ize of the fluidized bed reactor. However, the experimental work carried out with the oxygen carrier Cu10Al-I, Fe45Al-FG, and Ni40Al-FG at total preure up to 30 bar howed that, oppoitely to the expected, an increae in operating preure did not produced the expected increae in the reaction rate (García-Labiano et al., 2005). 16

17 Table 4 how the value predicted for the different oxygen carrier at 10 bar, conidering that according to Wolf et al. (2001) the preure ha no ignificant impact on the efficiency of the plant at a range from 10 to 18 bar. For the ame ga linear velocity, the increae of the operating preure produce a decreae in the cro ection area of the reactor. Therefore, the higher value of the ratio between olid inventorie and cro ection area will tranlate into taller bed for preure CLC ytem in comparion to the atmopheric operation. Other important fact will be related with the olid circulation rate. It mut be conidered that the circulation flow need to be larger per cro ection area at increaing preure when compared to atmopheric operation. In thi cae, the value of G howed in Fig. 3 hould be multiplied by the operating preure, which would have important conequence on the CLC operation a well a in the election of the oxygen carrier. For example, the ue of Fe-baed oxygen carrier at a total preure of 10 bar i difficult, or the MeO content of the Cu- and Ni-baed oxygen carrier mut be increaed up to value above 60% in order to ue lower value of X, and conequently a lower olid inventorie a poible Economic analyi and performance of the oxygen carrier There are nowaday a wide range of technologie for the CO 2 capture and torage (CCS), which have different maturity level. Since any kind of power plant have yet built at a full cale with CCS, the cot of thee ytem cannot be tated with a high degree of confidence at thi time Therefore, it i beyond the cope of thi ection to do an exhautive economic analyi of the CLC in comparion with other CCS procee a done by IPCC (IPCC, 2005). The aim i only to etablih a link between the expected cot of an oxygen carrier and it expected performance in a CLC ytem. The main advantage of the CLC proce compared to normal combution i that CO 2 i not diluted with N 2 in the combution gae and almot pure CO 2 i obtained without any extra energy needed. Therefore, the cot of freh oxygen carrier cot added to the ytem i potentially the bigget cot per ton of CO 2 avoided in a CLC proce. There wa even a concern that the cot of the particle could be a how-topper for thi technology (Lyngfelt and Thunman, 2005). The oxygen carrier cot i related with the makeup flow, becaue of the natural decay of activity and/or loe during many reduction-oxidation cycle. Thu, the cot of the makeup flow of oxygen carrier per tonne of CO 2 avoided, OC, can be defined by OC , OC C (23) OC 17

18 where C OC i the unit cot of olid oxygen carrier ($/kg OC) and the flow of new oxygen 0, OC carrier added (kg/) per kg/ of CO 2 avoided. Fig. 10 how the value of OC a a function of the makeup flow, 0, OC, for everal cot of the oxygen carrier, C OC. A decreae of thi cot allow a decreae of the CO 2 capture cot and/or an increae of the makeup flow to maintain contant the OC value. The cot of the oxygen carrier, C OC, will be the um of everal factor including the cot of the metal oxide, the inert, and the manufacture cot, C OC x C (1 x )C C (24) MeO MeO MeO I m The cot of the metal, C MeO, for the oxygen carrier conidered in thi work were 0.03 $/kg Fe 2 O 3, 2.9 $/kg Cu and 13.8 $/kg Ni (US Geological Survey, 2005) conidered to be the raw material, and the cot of the inert, C I, wa 0.5 $/kg Al 2 O 3. According to Lyngfelt and Thunman (2005) the manufacture cot of the oxygen carrier were about 1$/kg OC. Thee cot are low, although the cot in the proce will depend on the lifetime of the particle. To have a general idea of the lifetime of the particle in the reactor, it ha been conidered a reference value the cot of CO 2 capture for a proven proce a it i the aborption with methanolamine, MEA. In thee ytem, the cot of CO 2 capture i about 60 $/tonne CO 2 avoided (Rao and Rubin, 2002). The mot of thi cot, 47 $/tonne CO 2 avoided, i aociated with the CO 2 capture proce, and from thi cot only 6 $/tonne CO 2 proceed from the makeup flow of MEA, becaue thi proce ha an important energy penalty. From Eq. (24) or Fig. 10(a) can be obtained the make-up flow of freh oxygen carrier, per kg/ of CO 2 avoided, that give a OC value of 6$ per ton CO 2 avoided. Thee value, per ton of CO 2 avoided, were kg/, kg/, and kg/ for the oxygen carrier Cu10Al-I, Fe45Al-FG, and Ni40Al-FG, repectively. On the other hand, the make-up flow of freh oxygen carrier together with the total olid inventory define the reidence time, or lifetime (LT), of the oxygen carrier in the CLC ytem, LT OC, ARFR 0, OC (25) where i the olid inventory per kg/ of CO 2 avoided OC, ARFR 18

19 H (26) 0 c OC, ARFR moc, ARFR em CO2 The lifetime of the oxygen carrier wa defined a the mean time that a particle of oxygen carrier mut be under reaction, reduction or oxidation, in the ytem without any reactivity lo or without uffer the attrition/fragmentation procee that produce the particle elutriation out of the ytem. m OC,AR+FR can be calculated with Eq. (14) to (18) and Fig. 7. The pecific emiion, e, i defined a the mole of CO 2 produced per mol of fuel ga. For example, e i 1 for CH 4 and 0.67 for a ynthei ga compoed by 67 vol % CO and 33 vol % H 2. Fig. 10(b) how the lifetime of the oxygen carrier a a function of the makeup flow for everal olid inventorie, OC, ARFR. Fig. 10 alo how the line correponding to the oxygen carrier ued in thi work for the combution of CH 4, and for a X value of 0.3. Conidering a cot of oxygen carrier, OC of 6 $/tonne of CO 2 avoided, it wa obtained a lifetime about 180 h for the Cu10Al-I and Fe45Al-Fg oxygen carrier, and about 290 h for the Ni40Al-FG. Thee data are not very high, and are even lower if we conider a reference the target range of $20-30 per tonne of CO 2 avoided propoed for future CO 2 capture procee (Kerr, 2005). In thi cae, the lifetime required for the particle would be under 100 h, which have been yet reached in a 10-kW CLC prototype located at Chalmer Univerity (Lyngfelt and Thunman, 2005). Therefore, the problem of ue particle with low lifetime arrive from the cot derived of the afe dipoal of the reidue generated in the proce a well a from the tronger filtration ytem required in the plant, epecially if CLC i ued in a ga turbine cycle. The unit cot of oxygen carrier, C OC, could be different to that aumed above. The conideration of the additional cot due to the afe dipoal of the material ued can increae the cot of the oxygen carrier in the proce. On the other hand, if the material lot in the proce can be ued a raw material in the production of new oxygen carrier, the cot will decreae. In any cae, the new lifetime of the oxygen carrier can be recalculated a C LT (27) OC,2 2 LT1 COC,1 Eq. (26) and Fig. 10 how that the neceary lifetime of the particle in the ytem increae with an increae of the olid inventory, OC, ARFR. Therefore, thoe deign variable affecting the olid inventorie a the oxygen carrier reactivity or circulation rate will alo affect the lifetime value. That i, an increae of the oxygen carrier reactivity or on the circulation rate will reduce the olid inventory in the whole CLC ytem, and o the cot of the CO 2 capture. 19

20 On the other hand, the conideration of other apect affecting the ga-olid reaction in the fluidized bed a the reitance to the ga exchange between bubble and emulion phae, or the bad olid mixing in the fluidized bed will produce an increae in the olid inventory and in the cot of the CO 2 capture. Moreover, a poible lo of reactivity of the oxygen carrier during cyclic reaction, or the chemical poioning of the active part of the material, increae the olid inventory needed in the ytem, and decreae the lifetime allowable in the ytem. Both factor give an increae in the makup flow of the oxygen carrier, 0, OC, and therefore in the cot, OC. Lyngfelt and Thunman (2005) reported the operational experience of the 10-kW CLC prototype uing Ni-baed particle during 100 h of combution condition, and 300 h conidering alo the time that the particle have been circulating at high temperature. No decreae in reactivity wa detected during the tet period and from their elutriation reult a lifetime of the particle of h wa inferred. Thi value i very much higher to the about obtained for different reference cot. Even auming a lifetime of the particle one order of magnitude lower, 4000 h, the cot of the particle will be about $/tonne CO 2. From the analyi carried out, it can be concluded that the cot of the particle doe not repreent a limitation to the technology development, and make of the CLC a very promiing proce for CO 2 capture if oxygen carrier with enough reactivity and high attrition reitance are produced. Thi agree with the economic aement performed by the CCP (CO 2 Capture Project) and by the IPCC (IPCC, 2005) which indicated CLC among the bet option for reducing the cot of CO 2 capture uing fuel ga (Kerr, 2005). Concluion The deign and operation condition of a CLC ytem highly depend on the type of oxygen carrier ued. Thi work how the range of uitable operating condition with repect to olid circulation rate, and calculate the olid inventorie needed to operate a CLC proce for the different Cu-, Fe-, and Ni- baed oxygen carrier and different fuel gae (CH 4, H 2, and CO). It wa found that both deign parameter are linked and depend on the reactivity and oxygen tranport capacity of the oxygen carrier. The circulation rate were defined for the different type of oxygen carrier and fuel gae a a function of the metal oxide content. The difference in the olid flux, G, a a conequence of the fuel ga ued were of low importance. When the reaction in the fuel reactor wa endothermic, a minimum circulation rate wa neceary to avoid a trong decreae in the fuel reactor temperature. 20

21 The Fe-baed oxygen carrier need much higher circulation rate than the Ni- and Cu-baed oxygen carrier, becaue of their low oxygen tranport capacity. Auming a maximum feaible circulation rate in the CLC ytem of 80 kg m -2-1 the ue of thee oxygen carrier with MeO content below 10 wt% i prevented, becaue the oxygen tranferred could be lower than oxygen needed. On the other hand, the higher tranport capacity of the other metal oxide (CuO, NiO) make poible the preparation of oxygen carrier with lower MeO content, and value about wt% could be enough for normal operation. The olid inventory for a given oxygen carrier and fuel ga depend on the olid circulation rate and on the oxygen carrier characteritic, a their metal oxide content and reactivity with repect to the reduction and oxidation reaction. In thi ene, a plot to calculate the olid inventorie in a CLC ytem, valid for any oxygen carrier and fuel ga, i propoed. For a typical CLC operation, the minimum olid inventorie per MW f depended on the fuel ga ued, and followed the order CH 4 >CO>H 2. Value of minimum olid inventorie, for X =0.3, in a range from 40 to 170 kg/mw f were found. Moreover, the relation between oxygen tranport capacity, olid reactivity and olid inventory wa defined. It wa found that a ame inventory can be obtained with an oxygen carrier with le oxygen tranport capacity but with higher reactivity. Some deign parameter of a preuried CLC ytem baed on kinetic data previouly obtained were alo explored. It wa found that the operation of a preuried CLC ytem will need higher olid inventorie that the expected by an increae in the ga concentration at higher preure. The preure operation alo affect to the circulation rate. It wa found that the operation with any Febaed oxygen carrier at a total preure of 10 bar could be quite difficult, becaue the upper limit for the tranport capacity of olid in the rier. On the other hand, the MeO content of the Cu- and Ni-baed oxygen carrier mut be increaed up to value above 60%. From the economic analyi carried out it wa found the cot of the particle doe not repreent any limitation to the development of the CLC technology. Conidering the makeup flow of the particle a the main cot aociated to the proce, a lifetime of the particle of about 300 h repreent the ame cot of material that the makeup of amine in a proven technology of CO 2 capture a it i the aborption with MEA. In addition, particle with lifetime under 100 h would fulfill the target range of 20-30$ per tone of CO 2 avoided propoed for future CO 2 capture procee. However, the cot of the afe dipoal of the reidue generated in thi cae and the filtration ytem required in the plant would be limitant apect to be conider in the CLC development when uing particle with low lifetime. 21