Seediscussions,stats,andauthorprofilesforthispublicationat:https://www.researchgate.net/publication/320463792

Usingahierarchically-structuredCuO@TiO2-Al2O3oxygencarrierforchemicalloopingairseparationinaparalleled....

ArticleinTheChemicalEngineeringJournal·October2017

DOI:10.1016/j.cej.2017.10.084

CITATION

1

READS

44

3authors,including:

Someoftheauthorsofthispublicationarealsoworkingontheserelatedprojects:

ChemicalLoopingCombustionViewproject

InterfacereactionandtransfermechanismwithincomplexporousstructureViewproject

XinTian

HuazhongUniversityofScienceandTechnol…

20PUBLICATIONS88CITATIONS

SEEPROFILE

HaiboZhao

HuazhongUniversityofScienceandTechnol…

180PUBLICATIONS1,682CITATIONS

SEEPROFILE

AllcontentfollowingthispagewasuploadedbyXinTianon06November2017.

Theuserhasrequestedenhancementofthedownloadedfile.

Contents lists available at ScienceDirect

Chemical Engineering Journal

journal homepage: www.elsevier.com/locate/cej

Using a hierarchically-structured CuO@TiO2-Al2O3 oxygen carrier forchemical looping air separation in a paralleled fluidized bed reactor

Xin Tian, Yijie Wei, Haibo Zhao⁎

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China

A R T I C L E I N F O

Keywords:Chemical loopingOxygen productionSelf-assemblyParalleled fluidized bed reactor

A B S T R A C T

Chemical looping air separation (CLAS) provides a novel route for O2 production, which can be coupled withintegrated gasification combined cycle (IGCC) or oxy-fuel combustion to address the O2 source issue. The de-velopment of regenerable and robust oxygen carrier, which should be able to release gaseous oxygen in O2-deficient atmosphere (usually CO2/H2O) and regenerate itself by oxidizing with O2 in air, is critical for thesuccessful deployment of CLAS systems. In this work, a CuO@TiO2-Al2O3 oxygen carrier with hierarchicalstructure (i.e., CuO-rich, TiO2-stabilized and Al2O3-supported), which was prepared by the self-assembly tem-plate combustion synthesis (SATCS) method, was adopted for continuous oxygen production in a paralleledfluidized bed reactor. Before which, isothermal experiments in a thermogravimetric analyzer (TGA) were firstconducted to analyze the effect of temperature on OC decoupling and regeneration characteristics. The influenceof inlet gas flow rate on the oxygen decoupling rate of the oxygen carrier was then investigated in a batchfluidized bed reactor. Appropriate reaction temperature as well as inlet gas flow rate for continuous O2 pro-duction in the paralleled fluidized bed reactor was ultimately attained. During a consecutive operation of 10 h,continuous stream of O2/CO2 with relatively stable and high O2 concentration (19.7–27.6 vol, steam free basis)was obtained. The superior reactivity and stability of CuO@TiO2-Al2O3 achieved during the continuous op-eration in the paralleled fluidized bed reactor indicate that it could be a promising oxygen carrier candidate forthe CLAS process.

1. Introduction

The increase of CO2 concentration in atmospheric environment,which is mainly due to the intensive human activities, has been con-sidered as a major contributor to global climate change [1]. To resolvethe global warming effect caused by large quantity of CO2 emission,many of CO2 capture and storage (CCS) technologies, i.e., pre-com-bustion (viz. integrated gasification combined cycle, IGCC), post-com-bustion and oxy-fuel combustion, have been proposed as possible so-lutions [2]. However, although high CO2 capture efficiency can beachieved for the IGCC and oxy-fuel combustion approaches, the re-quirement of large amount of O2 makes them highly dependent oneconomic oxygen production [3,4].

Presently, there are three routes available for sizable oxygen pro-duction, i.e., cryogenic air separation (CAS), membrane separation andpressure swing adsorption (PSA) [5,6]. Among these approaches, onlythe CAS technique is commercially operated for very large-scale oxygenproduction, which is required by oxy-fuel combustion, coal gasification,steel-making and other industrial processes. Nevertheless, CAS still

holds the drawbacks of high energy penalty and high investment cost[5]. To be more specific, for an oxy-fuel combustion power plant thatconsiders CO2 capture, the air separation unit accounts for 33% of theinitial capital investment and results in approximate 20% electricityconsumption of the gross generation in operation [7,8]. As a conceptrevolution, chemical looping air separation (CLAS) was proposed forhigh-efficient and low-cost oxygen production, which is expected toaddress the oxygen source issue faced by IGCC and oxy-fuel combustion[9].

The chemical looping concept, originated from a patent of Lewisand Gilliland in the 1950s for CO2 production [10], has been widelyinvestigated as chemical looping combustion (CLC) and chemicallooping with oxygen uncoupling (CLOU) during the past ten years[11–17]. CLC is identified as a novel technology for power generationwith distinguished advantages of inherent CO2 separation, high energyutilization efficiency, low energy penalty and low NOx emission. InCLC, the oxygen carrier (OC) reacts with fuel in the fuel reactor andthen the reduced OC regenerates itself by reacting with O2 in the airreactor. Consequently, the outlet gas from the air reactor is O2-depleted

http://dx.doi.org/10.1016/j.cej.2017.10.084Received 11 July 2017; Received in revised form 12 October 2017; Accepted 16 October 2017

⁎ Corresponding author.E-mail address: hzhao@mail.hust.edu.cn (H. Zhao).

Chemical Engineering Journal 334 (2018) 611–618

Available online 17 October 20171385-8947/ © 2017 Elsevier B.V. All rights reserved.

MARK

air, while the fuel reactor is concentrated CO2 with no N2 dilution and,the CO2 capture is thus inherently realized in the CLC process. As analternative of CLC, CLOU was proposed to effectively combust solidfuels by Mattisson et al. [18]. Gaseous O2 released by the OC in the FRis beneficial to attaining much higher fuel conversion rate and com-bustion efficiency during the CLOU processes.

As for CLAS, it can be regarded as a variant of the CLOU process.Fuel is no longer introduced into the reactor and the objective of theCLAS process is to produce high purity oxygen. A schematic view ofCLAS is illustrated in Fig. 1. As shown, two separate reactors, desig-nated as reduction reactor and oxidation reactor (presented as R2 andR1 in Fig. 1), are involved in the CLAS process. In the reduction reactor,CO2/H2O is used as carrier gas and oxygen decoupling of the oxidizedOC (MeOx) occurs at high temperature as Eq. (1). At the same time, thereduced OC (MeOx−1) regenerates itself in the oxidation reactor as Eq.(2). After the reactions in both reactors are completed, the introductionof carrier gas into the reduction and oxidation reactors is exchanged, soas the reactions in the two reactors. In this way, continuous stream ofO2/CO2 can be obtained after steam condensation of the flue gas[19,20]. Actually, O2-enriched flue gas with different O2 concentrations(or even pure O2) can be acquired by adjusting the CO2/H2O ratio in theinlet fluidizing gas. ASPEN-HYSYS calculations by Moghtaderi [9] de-monstrated a typical low average specific power (0.08 kW h/m3) for theCLAS process, which was only 26% of an advanced CAS system.

In the reduction reactor:

→ +−MeO MeO O2 2x x 1 2 (1)

In the oxidation reactor:

+ →−MeO O MeO2 2x 1 2 x (2)

Similar with other chemical looping processes, the development ofhigh-performance OC is a key issue for the CLAS process. The suitableOC should have the ability of releasing gaseous oxygen at appropriatetemperatures and oxygen deficient atmosphere, sufficient physical andchemical stability as well as low cost. Three metal oxide pairs, i.e.,CuO/Cu2O, Co3O4/CoO and Mn2O3/Mn3O4, have been evaluated asfavorable OC candidates for CLOU/CLAS [9,21]. Due to the highoxygen donating capacity, high reactivity and suitable temperaturewindows for oxygen decoupling, most of the previous publications havebeen focused on the CuO/Cu2O OC system [22–27]. Nevertheless, pureCuO is facing challenges of insufficient mechanical strength, reactivity

degradation and sintering problems at high temperatures. Tostrengthen the physical and chemical stability of CuO, addition of highmelting point inert support materials like SiO2, Al2O3, ZrO2, TiO2,CuAl2O4, MgAl2O4, as well as cement has been proven to be a goodsolution [28–34]. There are also plenty of synthetic methods, e.g., im-pregnation, freeze granulation, spray drying and mechanical mixing,proposed for the preparation of Cu-based OCs. CLOU performance ofCuO/ZrO2 OC manufactured by freeze granulation was first demon-strated by Mattisson et al. [22], using petroleum coke as fuel. NeitherOC agglomeration nor defluidization phenomenon was observed duringcyclic redox experiments in a lab-scale fluidized bed reactor. In theC.S.I.C. research group [35], a comprehensive study of more than 25kinds of Cu-based OCs with different copper contents, inert supportsand preparation methods was conducted. Experimental results of TGAand batch fluidized bed tests indicated that OCs of CuO supported on40wt% of MgAl2O4 and 60wt% of ZrO2 prepared by mechanicalmixing plus pressurized pelletizing method exhibited superior re-activity, mechanical strength as well as sintering resistance in CLOU.The feasibility of using MgAl2O4 and Al2O3 as inert support for CuO wasexperimentally compared by Arjmand et al. [36]. It was found that CuOwas prone to combine with Al2O3 at high temperatures to form CuAl2O4

spinel, resulting in a degraded oxygen donating capability. A kind ofCuO/CuAl2O4 OC derived by sol-gel method was proposed by Mei et al.[37], and its chemical looping properties were evaluated in a batch-scale fluidized bed reactor with three typical coals. Regardless of theslight decrease of OC reactivity with cycle numbers, agglomerationproblem did not occur to the OC particles during multi-redox tests. Cu-based OCs with combined supports, viz., MgAl2O4/TiO2, MgAl2O4/SiO2

and TiO2/SiO2, were investigated by Adánez-Rubio et al. [38]. Thesynthesized OCs showed combined advantages of low attrition rate,high mechanical stability and oxygen decoupling property.

For the OC preparation, chemical combination between the activecomponent and inert binder can be detrimental to the chemical loopingperformance of the OC. For example, formation of CuAl2O4 at hightemperatures for the CuO/Al2O3 OC system will eventually contributeto inferior oxygen transport efficiency, because the CuAl2O4 spinelexhibits much poorer oxygen decoupling property than CuO [39] andthe generated CuAlO2 phase after oxygen decoupling is difficult to bere-oxidized back to CuAl2O4 [35]. In this sense, the combination ofactive component with inert support should be effectively avoided. As astrategy, alkali metal ions (i.e., Na+, K+) decoration was proposed toinhibit the side reaction between CuO and Al2O3, and this method wasfound to be able to effectively hinder the formation of copper alumi-nates [30,40].

A kind of Cu-based OC with combined support, i.e., CuO@TiO2-Al2O3, was synthesized for the first time in our previous work [33,41],using the self-assembly template combustion synthesis (SATCS)method, where the aggregation between μm-Al2O3 and nm-TiO2 parti-cles spontaneously occurred due to the force of van der Waals andelectrostatic attractive effects. The combined support with core-shellstructure was found to be able to restrain the interaction between CuOand Al2O3, which eventually contributed to excellent oxygen decou-pling property as well as high mechanical strength of the OC duringhigh temperature cyclic tests. In this work, the feasibility of using theCuO@TiO2-Al2O3 OC to generate continuous stream of O2/CO2 in theframework of CLAS was validated in a paralleled fluidized bed reactor.Before which, isothermal experiments in a thermogravimetric analyzer(TGA, WCT-1D) were first conducted to analyze the OC decoupling andregeneration rates at different temperatures. The effect of inlet gas flowrates on OC decoupling characteristic was then investigated in a batchfluidized bed reactor. The appropriate operating parameters (i.e., re-action temperature, inlet fluidizing gas flow rate) of the paralleledfluidized bed reactor were ultimately attained, resulting in stable andcontinuous oxygen production of 10 h in the reactor.

Fig. 1. Schematic of the CLAS system, R1: oxidation reactor, R2: reduction reactor, W1and W2: four-way valves.

X. Tian et al. Chemical Engineering Journal 334 (2018) 611–618

612

2. Experimental

2.1. Materials

The Cu-based OC used in this work was synthesized following theSATCS method. Take the production of 200 g CuO@TiO2-Al2O3 OC asan example, the detailed synthesis process was as follows. (i): 35 g ofμm-Al2O3 (1–75 μm, Sinopharm, analytically pure) and 10 g of nm-TiO2

particles were dispersed in 500mL deionized water. The obtainedsuspension was then electrically stirred in water bath (80 °C) for 1 h.After which, dilute nitric acid or ammonia solution was added into thesuspension to adjust its pH level to 6, and core (Al2O3)-shell (TiO2)structure was gradually formed. (ii): 98 g of CO(NH2)2 (Sinopharm,analytically pure) was introduced into the slurry to stabilize the core-shell template, and also used as fuel during the subsequent combustionsynthesis process to burn with Cu(NO3)2. (iii): Upon complete stirring,471 g of copper nitrate (Cu(NO3)2·3H2O, Sinopharm, analytically pure)was further added into the slurry and the segregation of cupric ions(Cu2+) was avoided by forming {Cu[CO(NH2)2]2}2+ with urea mole-cules. (iv): The obtained wet gel was completely dried in a drying ovenat 80 °C for 24 h, and the dried precursor was then ignited in a muffleoven at 200 °C in air. Subsequently, the as-burned precursor was furthercalcined in a muffle oven at 950 °C for 2 h to improve its mechanicalstrength. (v): The calcined conglomeration was naturally cooled,ground and sieved. Finally, CuO@TiO2-Al2O3 OC particles with 77.5 wt% CuO, 5 wt% TiO2 and 17.5 wt% Al2O3 in the size range of0.125–0.18mm were obtained. X-ray diffractometer (XRD, ShimadzuX’Pert Pro) results of the fresh OC showed no phase of CuAl2O4, whichindicated that interaction between CuO and Al2O3 did not occur [33].

Note that, the nm-TiO2 particles were self-synthesized by flamesynthesis method and detailed description of the production process canbe found in our previous publications [33,42]. Additionally, the SATCSmethod was favored for zero-NOx emission due to the reaction (as Eq.(3)) in the combustion synthesis process. In this way, a rather en-vironmentally friendly synthesis route can be achieved.

+ → + + +3Cu(NO ) 5CO(NH ) 3CuO 5CO 10H O 8N3 2 2 2 2 2 2 (3)

2.2. Experimental procedure for TGA tests

Isothermal TGA experiments were conducted to investigate the OCdecoupling and regeneration characteristics at three different tem-peratures, i.e., 900 °C, 920 °C and 950 °C, which were within typicaltemperature windows of CLOU/CLAS. For each test, the gas aerationrate was maintained at 60mL/min and the sample weight was ap-proximate 10mg. High purity N2 (99.999 vol%) was used as the inertatmosphere for OC decoupling and air was adopted as the oxidizingagent for OC regeneration. For the test under each temperature con-dition, 5 cyclic OC decoupling/regeneration processes have been con-ducted to guarantee data reliability.

2.3. Pre-experiments in a batch fluidized bed reactor

For continuous oxygen production in a paralleled fluidized bed re-actor, the inlet gas flow rate matters significantly. The reasons can betwo-folds. On the one hand, a lower inlet gas flow rate will lead to ahigher O2 concentration in the outlet gas stream; however, the diffusionof gaseous O2 from the OC surface to its surroundings can be limited ata lower gas flow rate (due to the relatively high oxygen partial pressurearound the OC particles), which eventually results in slower oxygendecoupling rate of the OC. On the other hand, for a higher inlet gas flowrate, the limitation of oxygen diffusion to bulk gas can be weakened to acertain degree, so as to achieve relatively high oxygen decoupling rate;nevertheless, at higher inlet gas flow rates, the O2 concentration in fluegas will decrease due to the dilution effect, which is then difficult toachieve the target O2 concentration in outlet gas stream (e.g., for the

CLAS combined oxy-fuel system, the O2 concentration in the recycledO2/CO2 into the furnace should be higher than 30 vol%). Actually,when increasing the inlet gas flow rate to a certain value, the effect ofO2 diffusion on the OC decoupling rate can be eliminated. In this sense,oxygen decoupling experiments of the OC were conducted in a batchfluidized bed reactor to obtain the most suitable gas flow rate forcontinuous oxygen production. As the batch fluidized bed reactor andthe paralleled fluidized bed reactor exhibit very similar configuration inthe reaction zone, thus the appropriate gas flow rate obtained in thebatch fluidized bed reactor can be applied to the paralleled fluidizedbed reactor according to the similarity criterion. Detailed description ofthe batch fluidized bed reactor can be found in our recently publishedarticles [43,44].

For the experiments, the OC particles were first heated to 950 °C inair atmosphere. Upon full oxidation of the OC, the carrier gas wasswitched from air to high purity CO2, and oxygen decoupling of the OCproceeded gradually. When the O2 concentration in flue gas decreasedto zero, air was introduced again into the reactor for OC regeneration.Five different inlet gas flow rates, i.e., 100mL/min, 300mL/min,400mL/min, 500mL/min and 700mL/min, were considered, and eachtest has been repeated three times for accuracy concern. Note that,when the inlet gas flow rate was 100mL/min, the fluidization velocity(U) was smaller than the minimum fluidization velocity of the OCparticle (Umf), and the reactor was in the fixed bed regime; when theinlet gas flow rate was 700mL/min, the fluidization velocity was about3–5 times of Umf, and the reactor was in the bubbling bed regime.

2.4. Continuous oxygen production in paralleled fluidized bed reactor

Fig. 2 shows the schematic view of the paralleled fluidized bed re-actor. The continuous oxygen production system consists of a gas sup-plying unit, a paralleled fluidized bed reaction unit, an off-gas treat-ment system and analyzing unit. The carrier gas introduced into the twocoupled fluidized bed reactors was controlled by valve switching. Inthis way, oxygen uptake, N2 purging or oxygen decoupling occurs al-ternately in the two reactors. As a consequence, OC particles in theparalleled fluidized bed do not need to be circulated as that in an inter-connected fluidized bed reactor [45,46], but always remain in one re-actor to undergo oxygen decoupling/uptake cycles. The gas supplyingunit includes high-pressure cylinders, mass flow meter/controllers,constant flow pump (for steady water feeding), steam generator, pipe-lines and valves. Air, nitrogen or H2O/CO2 was introduced into thereactor to oxidize the OC, to decompose the OC or to purge the reactor,respectively. The two stainless steel reactors have the same internalstructure, and both being electrically heated by a separate furnace. Thereaction tube is 1030mm high and 30mm in diameter. A porous plateis located in the tube at 470mm high from the bottom of the reactor,and three K-type thermocouples are used to measure the temperature ofthe reaction zone as well as temperatures of 50mm above and belowthe plate. In this work, 100 g of OC samples were used in each reactor,and OC particles were added from the top of the reactor before ex-periments. For oxidation and purging stages, the off-gas was just led tothe air. While for the oxygen decoupling stage, the outlet gas after beingfiltered and condensed was introduced into an online gas analyzer(Gasboard Analyzer 3100) to determine the concentrations of CO2 andO2, and the data was automatically recorded by a connected computerwith Lab View 14.0 software.

To achieve simple control of the reactor system, the temperatureduring the oxygen uptake and oxygen decoupling processes was bothmaintained at 950 °C (which was pre-determined by TGA experiments).For the oxygen decoupling stage, mixture of CO2/H2O stream (with150mL/min of CO2 and 0.5mL/min of water feeding) was used as thefluidizing agent; for the purging stage, 900mL/min of high purity N2

was introduced to flush the reactor; and for the oxidation stage,800mL/min of air was employed for OC regeneration. Note that, beforethe continuous test, pre-experiments have been conducted to determine

X. Tian et al. Chemical Engineering Journal 334 (2018) 611–618

613

the duration for oxygen uptake, N2 purging and oxygen decouplingstages, respectively. It is worth noting that the purging step between theoxygen uptake stage and oxygen decoupling stage is essential to guar-antee both O2 yield and O2 purity. The purging duration was pre-de-termined by decomposing the CuO@TiO2-Al2O3 oxygen carrier in oneof the reactors of the paralleled fluidized bed, using pure N2 as flui-dizing agent and the outlet O2 concentration as the indicator. To bemore specific, when the oxygen carrier particles were fully oxidized inair, the fluidizing gas was switched from air to pure N2, and this was thestarting time of the measurement of the purging duration. Once the O2

concentration in outlet gas decreased to 4.3 vol% (the equilibrium O2

partial pressure of CuO at 950 °C [25]), that would be the ending pointof the measurement. The result indicated that about 5min was neededto purge out most of (if not all) the residual air in the reactor and de-crease the O2 level to below 4.3 vol%. There remains still a smallamount of air (O2 as well) in the reactor after the 5min purging stage.However, the O2 contained by the residual air in the reactor only ac-counted a very small part when compared with the total amount of O2

released by the oxygen carrier during the whole oxygen decouplingprocess. In this sense, the effect of residual air on the mass balance of O2

generation can be neglected.There was no a purging stage between the oxygen decoupling stage

and oxygen uptake stage in the paralleled fluidized bed test. Instead, airwas introduced directly to regenerate the decomposed oxygen carrier.Actually, we did not lay emphasis on the re-oxidation process of theoxygen carrier during the continuous tests in the paralleled fluidizedbed reactor. The spent air exhausted from the re-oxidation process wasjust led to the atmosphere without being utilized. From this perspective,it is no need to adopt a purging step after oxygen decoupling process todistinguish between the oxygen decoupling and re-oxidation processes,and consequently, the operation time can be reduced.

The switching of fluidization gas in the two reactors was realized bycontrolling the opening and closing of a series of valves. Fig. 3 showsthe operation states of the two reactors in a typical oxygen production/regeneration cycle. The duration from T0 to T1 (O2 uptake), T1 to T2(Purge), T0 to T2 (O2 decoupling), T2 to T3 (O2 uptake), T3 to T4 (Purge),and T2 to T4 (O2 decoupling) is 55min, 5min, 60min, 55min, 5minand 60min, respectively. The time for O2 decoupling is just equal to thetotal time of O2 uptake and purge. Moreover, the total time needed forone reactor to complete the oxygen uptake, N2 purging and oxygen

decoupling stages is 120min.Corresponding to the operation states of the two reactors shown in

Fig. 3, Table 1 displays the valves switching order within a typicaloxygen release/uptake cycle. Take T0 point as an example, when V1turns to Reactor 2# and closes V2 and V4, air is introduced into Reactor2#. At this point, Reactor 2# undergoes the oxygen uptake stage, andthe exhaust is directly led to the air by opening V6 and V7, whileclosing V8. At the same time, V3 is open and mixtures of CO2/H2O areintroduced into Reactor 1#, in which O2 decoupling process occurs. Inthis sense, V5 is closed to let exhaust of Reactor 1# go through a filterfirst. Subsequently, the gas flow goes through V9 (V10 closed) and theninto the online gas analyzer to measure the concentrations of CO2 andO2, respectively.

2.5. Data evaluation

For TGA experiments, the conversion of the OC within oxygen de-coupling process, XDec, and oxygen regeneration process, XOxi, are de-termined as,

= − −X m m m m( )/( )tDec 0 0 f (4)

= − −X m m m m( )/( )tOxi f 0 f (5)

where m0 is the weight of the OC at the beginning of the conversion, mt

is the instantaneous weight of the OC at time t and mf is the final weightof the OC after complete oxygen decoupling.

The oxygen decoupling rate, xDec, and oxygen regeneration rate,xOxi, of the OC are then calculated by,

=xX

td

dDec/OxiDec/Oxi

(6)

Additionally, the average conversion rate of the OC when achieving90% conversion during the oxygen decoupling, rDec,ave, and oxygenregeneration, rOxi,ave, processes are calculated for further comparison,

=−

×rt t

0.9 100%ave0.9 0 (7)

where t0 is the initial time of the conversion and t0.9 is the time whenthe OC achieves 90% decoupling/regeneration conversion.

For the batch fluidized bed experiments, the average oxygen de-coupling rate of the OC, xave, was calculated as,

Fig. 2. Schematic of the paralleled fluidized bed reactor.

X. Tian et al. Chemical Engineering Journal 334 (2018) 611–618

614

∫=

−x

Q y t

t t m

· d

( )·t

tout

aveout O ,

f 0 OC

0f

2

(8)

where yO2,out is the oxygen concentration in outlet gas stream, mOC isthe weight of OC in the reactor, t0 and tf represent the time point of thebeginning and the end of oxygen decoupling stage, respectively.Additionally, Qout is the molar flow rate of the outlet gas stream, cal-culated by CO2 balance,

=−

QQ y

y·

1in

outout

in CO ,

O ,

2

2 (9)

where Qin is the molar flow rate of the inlet gas stream and yCO2,in is theCO2 concentration in inlet gas stream.

3. Results and discussion

3.1. Effects of temperature on OC decoupling and regeneration in TGA

Fig. 4 shows the oxygen decoupling conversion (presented bysymbol) as well as oxygen decoupling rate (presented by line) of the OCin N2 at different temperatures. As seen, the increase of temperatureexerts a positive effect on the oxygen decoupling of the OC, in whichthe total time needed to achieve 90% oxygen decoupling conversiondecreased from 264 s at 900 °C to 158 s at 950 °C. Additionally, the peakvalue of the oxygen decoupling rate doubled when the reaction

temperature increased from 900 °C to 950 °C.The OC regeneration characteristics at different temperatures in air

were also investigated, as shown in Fig. 5. In contrast with the oxygendecoupling process, the increased temperature was unfavorable for theoxygen regeneration process of the OC. As seen, the time needed forcomplete OC regeneration was tripled when the temperature increasedfrom 900 °C to 950 °C. When comparing the peak values of the oxygenregeneration rate at different temperatures, the highest one was at-tained at 900 °C, i.e., 1.76 wt%/s, and the lowest one was attained at950 °C, i.e., 0.443 wt%/s.

For further comparison, the average conversion rate of the OC whenachieving 90% conversion during the oxygen decoupling, rDec,ave, andoxygen regeneration, rOxi,ave, processes at different temperatures werealso calculated, as shown in Fig. 6. For accuracy concern, the resultshown here was an average value of the last three cycles for each test.As seen, with the increase of temperature, the average oxygen decou-pling rate increased from 0.357 wt%/s at 900 °C to 0.596wt%/s at950 °C; while the average OC regeneration rate decreased from 1.2 wt%/s at 900 °C to 0.298wt%/s at 950 °C. The reasons for the oppositeeffects of temperature on the OC decoupling and regeneration processescan be explained by the combined effects of kinetic and thermodynamicdriving forces. For the OC decoupling process in N2, with the increase oftemperature, the O2 equilibrium partial pressure of the OC also in-creases (O2 concentration of bulk gas in TGA can be considered atconstant value of 0), which results in greater thermodynamic driving

Fig. 3. Operation states of the two reactors in a typicaloxygen production/generation cycle.

Table 1Valves switching order within a typical oxygen production/generation cycle.

Time point V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 Reactor 1# Reactor 2#

T0= 0min 2# C O C C O O C O C O2 decoupling O2 uptakeT1= 55min C 2# O C C O O C O C O2 decoupling PurgeT2= 60min 1# C C O O C C O C O O2 uptake O2 decouplingT3= 115min C 1# C O O C C O C O Purge O2 decoupling

C: valve closed; O: valve open; V1 and V2 are three-way valves, while the others (V3-V10) are globe valves.

Fig. 4. Oxygen decoupling conversion (in symbol) and oxygen decoupling rate (in line) ofthe OC in N2 at different temperatures.

Fig. 5. Oxygen regeneration conversion (in symbol) and oxygen regeneration rate (inline) of the OC in air at different temperatures.

X. Tian et al. Chemical Engineering Journal 334 (2018) 611–618

615

force for OC decoupling. Moreover, as revealed by Zhang et al. [39], thesurface O2 formation and O2 desorption stages are the rate limitationsteps for CuO decoupling. A higher temperature can be beneficial forthe activation of surface O2 formation and desorption, leading to largerkinetic driving force for OC decoupling. While for the OC regenerationprocess in air, the O2 concentration of bulk gas in TGA can be con-sidered at constant value (21 vol%). With the increase of temperature,the O2 equilibrium partial pressure of the OC increases, contributing todegraded thermodynamic driving force for OC regeneration. As can beconcluded, when using this kind of OC in the CLAS process, the ratelimitation step changes from OC decoupling stage at low temperature toOC regeneration stage at high temperature. In this sense, it is better tocarry out the OC decoupling process at relatively high temperature,while relatively low temperature for the OC regeneration process.However, for operation flexibility, the temperature adopted for con-tinuous oxygen production in the paralleled fluidized bed reactor in thiswork was the same, i.e., 950 °C, during the OC decoupling and re-generation stages.

3.2. Inlet gas flow rate selection for continuous oxygen production

Fig. 7 shows the average oxygen decoupling rate of the OC at fivedifferent inlet gas flow rates in the batch fluidized bed reactor. As seen,with the gas flow rate increased from 100mL/min to 500mL/min, theaverage oxygen decoupling rate of the OC increased promptly. Actually,the average oxygen decoupling rate at 500mL/min (0.034mol·-min−1·kg−1 OC) was over three times larger than that at 100mL/min(0.0095mol·min−1·kg−1 OC). However, further increase of the inlet gas

flow rate to 700mL/min did not contribute to any promotion of theoxygen decoupling rate. The reason has been explained in Section 2.3:the O2 diffusion is restrained to certain degrees at particular low gasflow rate, so as to result in relatively high oxygen partial pressurearound the OC particles, and eventually contribute to typical lowoxygen decoupling rate of the OC. With the increase of inlet gas flowrate to a certain extent, the restriction of O2 diffusion could be elimi-nated, which explains well the nearly same average oxygen decouplingrate at 500mL/min and 700mL/min conditions. Considering the factthat a smaller inlet gas flow rate means lower operation cost, the rate of500mL/min was thus selected out as the appropriate inlet gas flow ratefor oxygen production in the present work. Note that, the configurationof the batch fluidized bed reactor and paralleled fluidized bed reactor isvery similar, and the only difference is the inner diameter of the reac-tion tube, which is 26mm for the batch fluidized bed reactor and30mm for the paralleled fluidized bed reactor. In this sense, the de-termined optimum inlet gas flow rate in batch fluidized bed reactor canbe modified to 666mL/min (i.e., 500×302/262) when being applied tothe paralleled fluidized bed reactor. Actually, the similarity criterionadopted here is based on the consideration of the same fluidizing re-gime achieved in the two reactors and the appropriate inlet gas flowrate attained here corresponding to the fluidization ratio, U/Umf, of 4.4at 950 °C.

3.3. Continuous oxygen production in the paralleled fluidized bed reactor

Fig. 8 shows the O2 concentration evolution as a function of timeduring the first oxygen decoupling stage in Reactor 2#. The data of O2

concentration in dry basis (with CO2) was measured and collected di-rectly by the on-line gas analyzer, while the in-situ wet basis O2 con-centration at 950 °C with both steam and CO2 included was calculatedbased on the inlet CO2 gas flow rate, water feeding rate and the drybasis O2 concentration at room temperature. As seen, within the wholeoxygen decoupling process, the dry basis O2 concentration in flue gasfirst increased to a peak value of 26.2 vol% and then decreased gra-dually with the process proceeded. Similar evolution trend of the in-situwet basis O2 concentration at 950 °C was also observed, with the valuevaried from 4.28 vol% to 5.99 vol%. By integrating the total volume ofthe decoupled O2 during the whole oxygen production process, a valueof 2.4 L, accounting for 55% of the total amount that could be theore-tically released by the OC, was obtained. Relatively high O2 con-centration in the flue gas can be maintained during the oxygen pro-duction process, which realized the target of producing oxygenenriched O2/CO2 stream.

According to the experimental procedures described in Section 2.4,

Fig. 6. Average oxygen decoupling and regeneration rates of the OC at different tem-peratures.

Fig. 7. Effect of gas flow rate on the average oxygen decoupling rate of the OC.Fig. 8. O2 concentration evolution in Reactor 2# during the first oxygen decouplingprocess.

X. Tian et al. Chemical Engineering Journal 334 (2018) 611–618

616

10 h of continuous operation was conducted in the paralleled fluidizedbed reactor for oxygen production. Fig. 9 shows the CO2 and O2 con-centrations detected by the gas analyzer during the CLAS process. Asseen, the O2 concentration in flue gas varied between 19.7–27.6 vol%,and steady stream of O2/CO2 with high O2 concentration was obtained.To be noted, the O2 concentration in the O2/CO2 stream can be flexiblyadjusted by the inlet CO2/H2O ratio and the inlet gas flow rate. Andpure O2 can be generated when only steam is used as fluidizing agent.As known, the desired O2 concentration range for oxy-fuel combustionshould be 25–35 vol% to achieve similar combustion characteristics(adiabatic flame temperature, heat transfer, etc.) as in conventional aircombustion boilers [47]. From this perspective, the O2/CO2 streamproduced in CLAS process can be directly adopted in the oxy-fuelcombustion technology.

The observed O2 concentration fluctuation in continuous oxygenproduction can be explained as follows. Firstly, the fast increase of O2

concentration at the beginning of each decoupling process is attributedto the mixing and replacement of new product gas (O2) with previouspurging gas (high purity N2), and the O2 concentration quickly reachesits peak value due to thermodynamic equilibrium. Then, with theproceeding of OC decoupling, the fraction of undecomposed CuO in thereactor decreases and so as the O2 concentration. The relatively steadyand continuous oxygen production indicates that the CuO@TiO2-Al2O3

OC exhibited stable and high reactivity as well as robust physical sta-bility during successive redox process. No obvious OC reactivity de-gradation phenomenon occurred when judging from the result of con-tinuous operation.

To determine the oxygen generation capability of the CuO@TiO2-Al2O3 OC in the paralleled fluidize bed reactor, the average O2 pro-duction rate for each oxygen decoupling stage during the continuousoperation was calculated. As shown in Fig. 10, relatively stable O2production rate was attained after the 3rd cycle, which was about45mL/min. Note that, the O2 generation rate obtained here was relatedto the OC inventory in the reactor. When considering the 100 g of OCinventory in each reactor, an absolute average O2 generation rate of3.34×10−7 mol O2/s per gram CuO@TiO2-Al2O3 OC can be attained.The stable O2 production rate during continuous operation indicatedthe good reactivity and physical stability of the OC. Moreover, whentaking the results of Wang et al. [48] as reference, where the CLAS testswere conducted in a fixed bed reactor, and performance of four kinds ofCuO-based OCs prepared with 60wt% SiO2 (Cu4Si6), TiO2 (Cu4Ti6),ZrO2 (Cu4Zr6) and MgAl2O4 (Cu4Mg6) as support materials wereevaluated. For the oxygen decoupling tests at 950 °C, average O2 gen-eration rates of 13.2 mL/min, 21mL/min, 25.2 mL/min and 25.8 mL/min were attained for Cu4Ti6, Cu4Si6, Cu4Zr6 and Cu4Mg6,

respectively. Given the 70 g of OC inventory in their work, the highestO2 generation rate per gram OC was achieved as 2.74× 10−7 mol O2/s(using Cu4Mg6 as OC), which was smaller than that of the CuO@TiO2-Al2O3 OC in the present work. We should point out here that, the inletgas flow rate is known to significantly affect the oxygen decoupling rateof the OC, yet it has not been stated in the work of Wang et al. [48],which may make the comparison a little insufficient. Anyway, thecomparison results illustrated the distinguished performance of theCuO@TiO2-Al2O3 OC in the CLAS process, and this superior reactivitycan be mainly attributed to the rationally design of the OC particle aswell as the optimized operation condition.

4. Conclusion

In the current study, the feasibility of using a hierarchically-struc-tured CuO@TiO2-Al2O3 oxygen carrier in the chemical looping air se-paration process was systematically evaluated. Isothermal experimentswere first carried out in TGA to investigate the OC decoupling and re-generation characteristics at different temperatures. It was found thatthe increase of temperature can facilitate the oxygen decoupling rate ofthe OC in N2; yet the increased temperature was unfavorable for thereduced OC to regenerate itself in air. The totally opposite effects oftemperature on the OC decoupling and regeneration processes can beattributed to the combined effects of kinetic and thermodynamicdriving forces. Experiments were then conducted in a batch fluidizedbed reactor to acquire appropriate inlet gas flow rate for continuousoxygen production. The inlet gas flow rate during the oxygen decou-pling process was found not only affect the O2 purity in the generatedO2/CO2 stream, but also influence the oxygen decoupling rate of theoxygen carrier. Finally, continuous oxygen production was realized in aparalleled fluidized bed reactor. Stable and high O2 concentration (amaximum dry-basis value of 26.2 vol%) in the flue gas was attainedduring the 10 h continuous operation. To be noted, the O2 concentra-tion in the generated O2/CO2 stream can be adjusted by varying theCO2/H2O ratio in the inlet fluidization gas flow, and pure O2 can beattained when only steam is used as fluidization agent. An averageoxygen generation rate of 3.34× 10−7 mol O2/s per gram OC wasachieved, and no obvious OC reactivity degradation phenomenon oc-curred. The results of continuous operation indicate very stable andhigh reactivity as well as physical stability of the CuO@TiO2-Al2O3 OC,which can be a promising oxygen carrier candidate for CLAS.

Acknowledgments

This work was supported by “National Key R &D Program of China

Fig. 9. Evolution of CO2 and O2 concentrations (dry basis) during continuous test. Fig. 10. Average oxygen production rate during continuous test.

X. Tian et al. Chemical Engineering Journal 334 (2018) 611–618

617

(2016YFB0600801)” and “National Natural Science Foundation ofChina (51522603)”.

References

[1] V.G.R.C. Govindaraju, C.F. Tang, The dynamic links between CO2 emissions, eco-nomic growth and coal consumption in China and India, Appl. Energy 104 (2013)310–318.

[2] T.F. Wall, Combustion processes for carbon capture, Proc. Combust. Inst. 31 (2007)31–47.

[3] B. Moghtaderi, Review of the recent chemical looping process developments fornovel energy and fuel applications, Energy Fuels 26 (2011) 15–40.

[4] C. Kunze, H. Spliethoff, Assessment of oxy-fuel, pre- and post-combustion-basedcarbon capture for future IGCC plants, Appl. Energy 94 (2012) 109–116.

[5] W. Castle, Air separation and liquefaction: recent developments and prospects forthe beginning of the new millennium, Int. J. Refrig. 25 (2002) 158–172.

[6] Z. Yang, Y. Lin, Y. Zeng, High-temperature sorption process for air separation andoxygen removal, Ind. Eng. Chem. Res. 41 (2002) 2775–2784.

[7] B. Buhre, L. Elliott, C. Sheng, R. Gupta, T. Wall, Oxy-fuel combustion technology forcoal-fired power generation, Prog. Energy Combust. Sci. 31 (2005) 283–307.

[8] J. Xiong, H. Zhao, C. Zheng, Z. Liu, L. Zeng, H. Liu, J. Qiu, An economic feasibilitystudy of O2/CO2 recycle combustion technology based on existing coal-fired powerplants in China, Fuel 88 (2009) 1135–1142.

[9] B. Moghtaderi, Application of chemical looping concept for air separation at hightemperatures, Energy Fuels 24 (2009) 190–198.

[10] W.K. Lewis, E.R. Gilliland, Production of pure carbon dioxide, Google Patents, No.2665971, 1954.

[11] J. Adanez, A. Abad, F. Garcia-Labiano, P. Gayan, L.F. de Diego, Progress inChemical-Looping Combustion and Reforming technologies, Prog. Energy Combust.Sci. 38 (2012) 215–282.

[12] L.S. Fan, L. Zeng, W. Wang, S. Luo, Chemical looping processes for CO2 capture andcarbonaceous fuel conversion–prospect and opportunity, Energy Environ. Sci. 5(2012) 7254–7280.

[13] J. Bao, Z. Li, H. Sun, N. Cai, Experiment and rate equation modeling of Fe oxidationkinetics in chemical looping combustion, Combust. Flame 160 (2013) 808–817.

[14] A. Lyngfelt, Chemical-looping combustion of solid fuels – Status of development,Appl. Energy 113 (2014) 1869–1873.

[15] A. Cabello, A. Abad, F. García-Labiano, P. Gayán, L.F. de Diego, J. Adánez, Kineticdetermination of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier for usein gas-fueled Chemical Looping Combustion, Chem. Eng. J. 258 (2014) 265–280.

[16] H. Gu, L. Shen, Z. Zhong, Y. Zhou, W. Liu, X. Niu, H. Ge, S. Jiang, L. Wang,Interaction between biomass ash and iron ore oxygen carrier during chemicallooping combustion, Chem. Eng. J. 277 (2015) 70–78.

[17] X. Niu, L.H. Shen, S.X. Jiang, H.M. Gu, J. Xiao, Combustion performance of sewagesludge in chemical looping combustion with bimetallic Cu-Fe oxygen carrier, Chem.Eng. J. 294 (2016) 185–192.

[18] T. Mattisson, A. Lyngfelt, H. Leion, Chemical-looping with oxygen uncoupling forcombustion of solid fuels, Int. J. Greenh. Gas Con. 3 (2009) 11–19.

[19] K. Shah, B. Moghtaderi, T. Wall, Effect of flue gas impurities on the performance ofa chemical looping based air separation process for oxy-fuel combustion, Fuel 103(2013) 932–942.

[20] H. Song, K. Shah, E. Doroodchi, T. Wall, B. Moghtaderi, Reactivity of Al2O3-or SiO2-Supported Cu- Mn-, and Co-based oxygen carriers for chemical looping air se-paration, Energy Fuels 28 (2014) 1284–1294.

[21] T. Mattisson, Materials for chemical-looping with oxygen uncoupling, ISRN Chem.Eng. 2013 (2013) 19.

[22] T. Mattisson, H. Leion, A. Lyngfelt, Chemical-looping with oxygen uncoupling usingCuO/ZrO2 with petroleum coke, Fuel 88 (2009) 683–690.

[23] Y. Wen, Z. Li, L. Xu, N. Cai, Experimental study of natural Cu ore particles as oxygencarriers in chemical looping with oxygen uncoupling (CLOU), Energy Fuels 26(2012) 3919–3927.

[24] R.N. Harper, C.M. Boyce, S.A. Scott, Oxygen carrier dispersion in inert packed bedsto improve performance in chemical looping combustion, Chem. Eng. J. 234 (2013)464–474.

[25] I. Adánez-Rubio, P. Gayán, A. Abad, F. García-Labiano, L.F. de Diego, J. Adánez,Kinetic analysis of a Cu-based oxygen carrier: relevance of temperature and oxygenpartial pressure on reduction and oxidation reactions rates in Chemical Looping

with Oxygen Uncoupling (CLOU), Chem. Eng. J. 256 (2014) 69–84.[26] X. Tian, K. Wang, H. Zhao, M. Su, Chemical looping with oxygen uncoupling of

high-sulfur coal using copper ore as oxygen carrier, Proc. Combust. Inst. 36 (2017)3381–3388.

[27] S. Jiang, L. Shen, J. Wu, J. Yan, T. Song, The investigations of hematite-CuO oxygencarrier in chemical looping combustion, Chem. Eng. J. 317 (2017) 132–142.

[28] I. Adánez-Rubio, P. Gayán, A. Abad, L.F. de Diego, F. García-Labiano, J. Adánez,Evaluation of a spray-dried CuO/MgAl2O4 oxygen carrier for the chemical loopingwith oxygen uncoupling process, Energy Fuels 26 (2012) 3069–3081.

[29] L. Xu, J. Wang, Z. Li, N. Cai, Experimental study of cement-supported CuO oxygencarriers in chemical looping with oxygen uncoupling (CLOU), Energy Fuels 27(2013) 1522–1530.

[30] Q. Song, W. Liu, C.D. Bohn, R.N. Harper, E. Sivaniah, S.A. Scott, J.S. Dennis, A highperformance oxygen storage material for chemical looping processes with CO2

capture, Energy Environ. Sci. 6 (2013) 288–298.[31] Q. Imtiaz, M. Broda, C.R. Müller, Structure–property relationship of co-precipitated

Cu-rich, Al2O3-or MgAl2O4-stabilized oxygen carriers for chemical looping withoxygen uncoupling (CLOU), Appl. Energy 119 (2014) 557–565.

[32] X. Tian, H. Zhao, K. Wang, J. Ma, C. Zheng, Performance of cement decoratedcopper ore as oxygen carrier in chemical-looping with oxygen uncoupling, Int. J.Greenh. Gas Control 41 (2015) 210–218.

[33] Z. Xu, H. Zhao, Y. Wei, C. Zheng, Self-assembly template combustion synthesis of acore–shell CuO@TiO2–Al2O3 hierarchical structure as an oxygen carrier for thechemical-looping processes, Combust. Flame 162 (2015) 3030–3045.

[34] X. Tian, H. Zhao, J. Ma, Cement bonded fine hematite and copper ore particles asoxygen carrier in chemical looping combustion, Appl. Energy 204 (2017) 242–253.

[35] P. Gayan, I. Adanez-Rubio, A. Abad, L.F. de Diego, F. Garcia-Labiano, J. Adanez,Development of Cu-based oxygen carriers for Chemical-Looping with OxygenUncoupling (CLOU) process, Fuel 96 (2012) 226–238.

[36] M. Arjmand, A.M. Azad, H. Leion, A. Lyngfelt, T. Mattisson, Prospects of Al2O3 andMgAl2O4-supported CuO oxygen carriers in chemical-looping combustion CLC) andchemical-looping with oxygen uncoupling (CLOU), Energy Fuels 25 (2011)5493–5502.

[37] D. Mei, H. Zhao, Z. Ma, C. Zheng, Using the sol-gel-derived CuO/CuAl2O4 oxygencarrier in chemical looping with oxygen uncoupling for three typical coals, EnergyFuels 27 (2013) 2723–2731.

[38] I. Adánez-Rubio, M. Arjmand, H. Leion, P. Gayán, A. Abad, T. Mattisson,A. Lyngfelt, Investigation of combined supports for Cu-based oxygen carriers forchemical-looping with oxygen uncoupling (CLOU), Energy Fuels 27 (2013)3918–3927.

[39] Y. Zhang, H. Zhao, L. Guo, C. Zheng, Decomposition mechanisms of Cu-basedoxygen carriers for chemical looping with oxygen uncoupling based on densityfunctional theory calculations, Combust. Flame 162 (2015) 1265–1274.

[40] Q. Imtiaz, A.M. Kierzkowska, C.R. Muller, Coprecipitated, copper-based, alumina-stabilized materials for carbon dioxide capture by chemical looping combustion,ChemSusChem 5 (2012) 1610–1618.

[41] X. Tian, Y. Wei, H. Zhao, Evaluation of a hierarchically-structured CuO@TiO2-Al2O3

oxygen carrier for chemical looping with oxygen uncoupling, Fuel 209 (2017)402–410.

[42] Z. Xu, H. Zhao, H. Zhao, CFD-population balance Monte Carlo simulation and nu-merical optimization for flame synthesis of TiO2 nanoparticles, Proc. Combust. Inst.36 (2017) 1099–1108.

[43] W. Yang, H. Zhao, K. Wang, C. Zheng, Synergistic effects of mixtures of iron oresand copper ores as oxygen carriers in chemical-looping combustion, Proc. Combust.Inst. 35 (2015) 2811–2818.

[44] H. Zhao, L. Guo, X. Zou, Chemical-looping auto-thermal reforming of biomass usingCu-based oxygen carrier, Appl. Energy 157 (2015) 408–415.

[45] J. Ma, H. Zhao, X. Tian, Y. Wei, Y. Zhang, C. Zheng, Continuous operation of in-terconnected fluidized bed reactor for chemical looping combustion of CH4 usinghematite as oxygen carrier, Energy Fuels 29 (2015) 3257–3267.

[46] J. Ma, H. Zhao, X. Tian, Y. Wei, S. Rajendran, Y. Zhang, S. Bhattacharya, C. Zheng,Chemical looping combustion of coal in a 5 kWth interconnected fluidized bed re-actor using hematite as oxygen carrier, Appl. Energy 157 (2015) 304–313.

[47] M.B. Toftegaard, J. Brix, P.A. Jensen, P. Glarborg, A.D. Jensen, Oxy-fuel combus-tion of solid fuels, Prog. Energy Combust. Sci. 36 (2010) 581–625.

[48] K. Wang, Q. Yu, Q. Qin, Z. Zuo, T. Wu, Evaluation of Cu-based oxygen carrier forchemical looping air separation in a fixed-bed reactor, Chem. Eng. J. 287 (2016)292–301.

X. Tian et al. Chemical Engineering Journal 334 (2018) 611–618

618

View publication statsView publication stats