A Hydrodynamic Study of a Fast-Bed Dual Circulating Fluidized Bed for Chemical Looping Combustion

As a result of the industrial maturity of circulating fluidizedbed (CFB) reactors, the technology is widely deployed. In the power generation sector, the use of CFB boilers is particularly common, due to their ability to utilize low-grade, lowcalorific-value fuels, and maintain the capability for flexible operation. This flexibility lends itself well to other technologies, including high-temperature looping cycles for fossil fuel conversion and carbon capture systems. A high-temperature solid looping process known as chemical looping combustion (CLC) is the focus of this work. Such high-temperature looping systems must first be demonstrated at smaller scales using pilot-scale systems. CFB systems must consider the physical properties of the particles used (e.g., density, size, melting point) and the relevant reaction chemistry, plus decide the range of operating temperatures, residence times and, ultimately, the optimum reactor configuration to be used. Though CLC systems are based on CFB principles and can utilize reactor configurations such as CFB packed/moving bed and CFB bubbling-bed variants, the use of dual interconnected CFBs with fast fluidized-bed operation can allow for greater gas–solid contact and could be beneficial for the CLC process. This reactor configuration and mode of operation poses difficulties in accurate control of the solid transfer and circulation rates. Therefore, it is necessary to use a cold-flow system that can accurately determine the fluid-dynamic properties presented by the fluidized particles and a given reactor configuration, while conducting tests at ambient conditions. The operational and process considerations are explored here for a chemical looping combustor, for intended use with gaseous fuels and a copper(II) oxide-based oxygen carrier. The focus of this study is to investigate the solids handling and solid control behavior of a chemical looping reactor by using a 1:1 scaled cold-flow model (CFM). A comprehensive hydrodynamic study, including bed height sensitivity analysis, will provide data to predict if continuous circulation can be maintained with stable operation. This study explores the use of a dual interconnected circulating fluidized bed (CFB) for chemical looping combustion. This design can enhance gas–solid interactions, but it is difficult to control the solid transfer and circulation rates. With the use of a 1:1 scale cold-flow model, an investigation determining the hydrodynamic behavior of the dual CFB system has been conducted. The cold-flow system consists of two identical fast-bed risers, each with an internal diameter of 100 mm and a height of 7 m. The simplified cold-flow model is based on the chemical looping Pilot-Scale Advanced CO2 Capture Technology (PACT) facility at Cranfield. Here, we have determined the minimum fluidization and transport velocities, and we have assessed the solid density profiles, transport capacity, and potential for the dilution by air/N2 leakage into the CO2 stream exiting the fuel reactor. The experimental procedure uses two different bed materials, molochite (ceramic clay) and FE100 (iron particles), and it satisfies the dynamic scaling laws to model the bed inventory within the system. The results indicate that the two fast-bed risers share similar density and pressure profiles. Stable circulation can be achieved through pneumatic transport. The circulation rate of the system is flexible and can be adjusted by altering the fluidization velocity in the riser and by altering the bed inventory. The gas leakage from the loop seal to the cyclone was found to be sensitive to the bed height and fluidization velocity in the loop seal. However, by maintaining a loopseal bed height above 600 mm during operation, the outlet stream remains undiluted.


Introduction
As ar esult of the industrial maturity of circulating fluidizedbed (CFB) reactors,t he technology is widely deployed. In the power generation sector, the use of CFB boilersi sp articularly common,d ue to their ability to utilize low-grade,l owcalorific-valuef uels,a nd maintain the capability for flexible operation. [1] This flexibility lends itself well to other technologies,i ncludingh igh-temperature looping cycles for fossil fuel conversion and carbon capture systems. [2] Ah igh-temperatures olid loopingp rocess known as chemical looping combustion( CLC) is the focus of this work. Such high-temperaturel oopings ystems must first be demonstrated at smaller scales using pilot-scale systems.C FB systems must consider the physical properties of the particles used (e.g., density,size,m elting point) and the relevant reaction chemistry,p lus decide the range of operating temperatures,r esidence times and, ultimately,t he optimum reactor configuration to be used. Though CLC systems are based on CFB principles and can utilize reactorc onfigurations such as CFB packed/moving bed and CFB bubbling-bed variants,t he use of dual interconnected CFBs with fast fluidized-bed operation can allow for greater gas-solid contacta nd could be beneficial for the CLC process.T his reactorc onfiguration and mode of operation poses difficulties in accurate control of the solid transfer and circulation rates.T herefore,i ti snecessary to use ac old-flows ystem that can accurately determine the fluid-dynamic properties presented by the fluidized particlesa nd ag iven reactor configuration, while conducting tests at ambientconditions.
Theo perational and process considerations are explored here for ac hemical loopingc ombustor,f or intended use with gaseous fuels and ac opper(II) oxide-based oxygenc arrier. [3] Thef ocus of this study is to investigate the solids handling and solid control behavioro fachemical loopingr eactor by using a1 :1 scaled cold-flow model (CFM). Ac omprehensive hydrodynamic study,i ncluding bed height sensitivity analysis, will provide data to predict if continuous circulation can be maintained with stable operation.
This study explorest he use of ad ual interconnected circulating fluidized bed (CFB)f or chemical looping combustion. This design can enhance gas-solid interactions,b ut it is difficult to control the solidt ransfer and circulation rates.W ith the use of a1:1 scale cold-flow model, an investigation determining the hydrodynamic behavior of the dual CFB system has been conducted. Thec old-flows ystem consists of two identical fast-bed risers,e ach with an internal diameter of 100 mm and ah eight of 7m.T he simplified cold-flow model is based on the chemical looping Pilot-Scale Advanced CO 2 Capture Te chnology (PACT) facility at Cranfield. Here, we have determined the minimum fluidization and transport velocities, and we have assessed the solid density profiles,transport capacity,a nd potential for the dilution by air/N 2 leakage into the CO 2 stream exiting the fuel reactor. Thee xperimental procedure uses two different bed materials,molochite (ceramic clay) and FE100 (iron particles),and it satisfies the dynamic scaling laws to model the bed inventory within the system. Ther esultsi ndicate that the two fast-bedr isers share similar density and pressurep rofiles.S table circulation can be achievedt hrough pneumatic transport. Thec irculation rate of the system is flexible and can be adjusted by altering the fluidization velocity in the riser and by altering the bed inventory.T he gas leakage from the loop seal to the cyclone was found to be sensitive to the bed height and fluidization velocityi nt he loop seal. However, by maintaining al oopseal bed heighta bove 600 mm during operation, the outlet stream remainsundiluted.

Chemical looping combustion and fluidized beds
Lewis and Gilliland [4] were the first to study the concepto f producing pure CO 2 from carbon-containing fuels by their reduction using as olid copper oxide as an oxygens ource.I t was later developed as am eans for carbonc apturea nd responsible fossil fuel conversion,a nd thus,apotential contributing strategy for climate change mitigationw as developed by Ishidaetal. [5] Lyngfelt et al. [6] developed the first chemical looping reactor schemeb ased on the CFB principle.T he technologys major benefit centers on its ability to convert fuel to combustion products in the absence of air, avoiding any post-combustion flue-gas treatment to separate the CO 2 along with its associated energy penalties. [7] CLC is at wo-stage process by which oxygen is absorbed from air by as olido xygenc arrier (typically at ransition metal oxide).T he exothermico xidation reaction takes place in af ast-bed riser known as the air reactor (AR). Theh igh-velocity gas stream enters ac yclone where the oxygen carriers are separated from the oxygen-depleted air and sent to the fuel reactor( FR) to provide the oxygenr equired for combustiono ft he fuel, typically by an endothermic reaction process. [8] Ther educed oxygen carrier is then returned to the AR to continuet he cyclic process (see Figure 1).
Thed evelopment and testingo fp otential oxygen-carrier materials is the most widelyr esearched topic in the CLC field. [10] Oxygen carriers are usuallyt ransition metal oxides, namely nickel, copper, iron, and manganese. [8] Oxygen carrier research has investigated materials ranging from ores [11,12] and industrial waste, [13] to combined metal oxides systems [14] and highly engineered nanostructured particles. [15] Thec urrent magnitude of chemical loopingr eactors installed ranges from 0.3-1000kW th and spans differentr esearch groups that have achieved more than 4000 ho fo perational experience collectively. [16] Them ajority of this operational experience came from the conversion of gaseousf uels,a lthough the use of solid and heavy liquid fuels is now the focus of current research. [17] Several configurations of reactor designh ave been considered for CLC applications,t he most commonb eing the dual circulating fluidized-bed system for gaseous fuels.I nsystems utilizings olid fuels,s uch as coal and/or biomass,at hird reactor will likely be added to the system to improve fuel conversion. Loop seals incorporated between the reactors prevent the reaction gases from mixing with one another. Adanez et al. [18] has reviewed variousr eactor configurationsi nd etail. Thec ommonly proposed reactor configurationsa nd correspondingoperating conditions are summarized in Table 1.

Dual fast-bed Cranfield design
TheC ranfield Pilot-Scale Advanced CO 2 Capture Te chnology (PACT) chemical looping reactor comprises two identical interconnected CFBs known as the air and fuel reactors.T he riser height of each reactor is 7.3 mw ith an inner diameter of 0.1 m. Thet wo reactors lead to primary and secondary cyclones, whichr eturns olids to the appropriate reactorb yw ay of ar eturn leg (down-comer) with an inner diameter of 0.04 m. Ther eactione nvironments are designed not to mix and are separatedb yaloop seal (LS);t his can help facilitate the solids throughput but is one-directional and, therefore, cannot be used as al ong-term strategy for controlling the solid circulation. Figure 2s hows ad iagram of the chemical looping reactor. Thei ntended goal of this facility is to study the conversion of gaseous fuel (methane) in the fuel reactor using ar e-circulating copper oxide-based oxygenc arrier to demonstrate pilot-scale CLC processes.

Requirement for operational strategy analysis
Thed esign of as ymmetricals ystem as described above has an inherent difficulty in operation with respect to controlling the solid circulation between the two reactors and heat management. With the assumption of no requirement for the additional make-up oxygenc arrier material, maintaining stable and balanced operation requires the solid flow transferred from the AR to the FR (m2 in Figure 3) to be equal to the solids flow transferred from the FR to the AR (m1 in Figure 3).T he difficulty in controllingt his arises from the two different reactions occurring in the AR and FR. The transfero fs olids would be determined by the temperatures of the reactors,s pecific particle properties (e.g.,s ize,d ensity, and sphericity), and the fluidizing gas properties such as density and viscosity.
To determine the best strategy for operation,ad escription of the heat and mass balancei sp resented below.T he reactions considered include the conversion of methane using ac opper-based oxygen carrier (60 wt %C uO and 40 wt % Al 2 O 3 ), which reduces to Cu 2 Ou nderc hemical looping with oxygenu ncoupling (CLOU) processes.T he followinga ssumptions are also made:t he formation of combustion products is stoichiometric with respect to the input fuel and, therefore,c omplete;a ll of the O 2 is consumed in the fuel re-actor;n oh eat is transferred by the oxygen carrier support material, which is inert and considered equal in terms of mass in and out;a nd the particle size distributions in the two columns are assumed to be the same,e ven thought he particle size may in fact changed uring the oxidation/reduction reactionsaswell as changing with time due to attrition.
With the pre-set design conditiono f5 0kWinput based on the current configurationo fe lectricalh eating and the heat of combustion of methane,m ass flowrates of 5.03 Nm 3 h À1 for methanea nd 47.6 Nm 3 h À1 for air are ideallyr equired. In ah ypothetical scenario assumingabed temperature of 800 8Ca nd 4% excess air, the corresponding gas velocity to be expected in the AR is 5.48 ms À1 .T aking into account the differences in density of the flue gases in the AR (majority N 2 )a nd in FR (CO 2 and H 2 O) it is estimated that an addi-tional2 2.74 Nm 3 h À1 of CO 2 is requiredt ob alancet he solid transferb etween the two reactors.I twould be ideal to recycle the flue gas from the FR, but in practical operation of the system we can simplys upply the additional required CO 2 to operate under equivalent fluidization conditions.A na dditional advantaget or ecycling of the CO 2 in this system is the decreasei nt he equilibrium partialp ressure of oxygeni nt he fuel reactor. This further enhances the CLOU effect, in terms of release of gaseouso xygen from the copper-based oxygenc arrier. With respectt ot he conversion ratio of the oxygenc arrier, ac irculation rate of 120-240 kg h À1 is requiredf or conversion ratios of 1( CuO/Cu) and 0.5 (CuO/ Cu 2 O), respectively.
In the first case,t he system as aw hole (AR + FR) was considered in the heat andm assb alance calculations.T he input streamsw ere air and methane, whereas the exit streamsc onsisted of 4vol %O 2 with the balancea sN 2 ,C O 2 , and H 2 O. Theb ed temperature was set at 900 8C. It wasc alculatedt hat approximately 30 kJ s À1 of heat is required to be removedfrom the system as awhole.
In the second case,t he heat and mass balance of the AR and FR were treateds eparately.I tw as calculated that without CO 2 recycle the FR will require heat removalo f 7.89 kJ s À1 .T his can be somewhat reduced as it is possible to supply CO 2 at room temperature.I twasc alculated that 12.1 kJ s À1 of heat would be required to heat up the CO 2 , which could be supplied by the reactor external heaters. The situation for the AR is less simple,due to the strong exothermic reaction of the oxygenc arrier oxidation, for which it was calculated that approximately 22 kJ s À1 of heat must be removed. Thed issipation of heat throught he reactor walls can only account for 1kJs À1 heat loss due to the two surrounding adiabatic furnaces.I nt he current configuration,t here is no available heat surface from which to extract heat. Thec omparisonsb etween the heat balanceo ft he system as aw hole and individually show an error of 1%.A saconsequenceo f the limitations in heat dissipation and management,afeasible strategy is,t herefore,t or educe the electrical heat input to 20 kW.T his in turn reduces the heat to be removed to manageable levels.T he approximate massf low rates required are 19.2 and 2.01 Nm 3 h À1 for air and methane, respectively. An excesso fa na dditional 9.1 Nm 3 h À1 of CO 2 is required to ach-  ieve as ufficient gas velocity( 2.66 ms À1 )t ob alance the solid transferb etween the two reactors.T his correspondst oa n oxygenc arrier circulation rate of approximately 3.4 kg m À2 s À1 .

Scaling and dimensionlessparameters
Thed esign of larger-scale fluidized beds can be investigated at smaller scales under ambient conditions through cold-flow modeling, permitting detailed fluid-dynamic investigations. Normally,t heir importance lies in the possibility of predicting the experimental conditions in the correspondingl argerscale system. [19] ACFM can be made of atransparent material (typically plastico ra crylic) that allowso ne to view the internal fluidization behavior and particle mixing. In the case of CLC,n otable studies include those conducted by Kronberger et al., [20] Prçll et al., [21] Shuai et al., [22] and Markstrçm and Lyngfelt. [23] Thep hilosophy of cold-flow modelingu tilizes non-dimensional analysis to provide scaling laws that can accurately represent dynamic similarity between as maller-scale CFM and ac orresponding larger reactor system. Then on-dimensional analysis of system similarity was adapteda nd developed for fluidized beds by Glicksman, [24] in which he proposed afull set of scaling laws,allowing ambienttemperature modelingo fs ystems operating at elevated temperatures. These scaling laws were later simplified in Glicksmans1 993 study, [25] which presented ar elaxed set of parameters [Eq. (1)].T heselaws are appropriate for both viscousa nd inertial dominated regionso ft he fluidized bed, and are valid over aw ide range of Reynolds numbers applicable to small particlesa tl ow fluidization velocities and large particles at high fluidization velocities,w hich was al imitationo fh is previouslyproposed scaling laws.
Glicksmanss caling laws are generally regarded as the standard methodology for scaling fluidized beds,a nd though many investigations have proven the applicability of these laws,l imitations still exist. Examples of such limitations are the studies conducted by Prçll et al. [21] who determined that neither gas and particle wall friction effects,nor solid particle acceleration are accounted for. Ar eview of scaling law limitationsi sp resented by Cotton et al. [26] Scalingl aws in CLC become challenging to apply,d ue to the typically high densities of fluidized oxygen carriers.T he use of higher-density particlesc an eliminate the requirement for scaling down ac old-flows ystem, while still allowing for sufficient dynamic similarity in comparison with aC LC reactor. Thea pplicability of these scaling laws for the modeling of the CLC reactor and CFM are discussed in the Experimental Section.

Minimum fluidization and transport velocities
Ther esistance co-efficient of the riser gas distributor was measured by determining pressure drops above and below the distributor with varying gas velocities.O nce this had been determined, the riser was filled with solid particles to ab ed height of 550 mm. As tep-wise increase in gas velocity (upstream)w as applied to the bed material until fluidization was observed and then decreased step-wise (downstream). Thep ressure curves for the molochitea nd the FE100p articles are shown in Figure 4a nd Figure 5, respectively.T he downstream curves were utilized to determinet he U mf values (variables are defined in the definition list at the endo ft he article),w hich were estimated as 0.11 and 0.023 ms À1 for the molochitea nd FE100p articles,r espectively.I tw as observed that there is ac lear and noticeable difference when comparing the upstreamc urves of the two particles.W hereas molochite displays smooth transition into fluidization, the FE100 particles show an overshoot of pressuredrop upon transitioning into the incipient fluidizing regime.T his wasa lso visually observed by as udden increase in bed height in the upstream, whereas as mooth transition was observed when decreasing the fluidizing gas velocity. Thep ronounced hysteresis shown in Figure 5i si ndicative of Geldartg roup Ap articles which  typically exhibit greater particle-particle cohesion. [27] The specificp ositions of the molochite and FE100p articles employed in this study in the context of their Geldart classifications are shown in Figure 6, and it was observed that althoughb oth classifications define the particles as group B, FE100 particles are locatedi nc lose proximity to the particle A/B border and exhibit group Ap roperties with respect to particlec ohesion. Thet ransportv elocity U tr was determined by visual observation of particles dropping from the cyclone to the return leg, coupled with the pressured rop of the bed; the transport velocities were determined to be 1.70 and 1.38 ms À1 for molochitea nd FE100, respectively.

Densityprofiles at varying velocities
Thed ensity profiles with varying fluidizationv elocities for molochitea nd FE100 are shown in Figure 7a nd Figure 8, respectively.T he generalt rends,w ith both particles employed, are that the solid concentration (x s )d ecreases as the height of the riser increases.I nt he case of molochite,a tv elocities below U tr ,t he solid concentration is near 0.55, similar to that in ab ubbling bed. As the gas velocityi ncreases,t he molochite particles are increasingly carried out of the bed and enter the freeboard. At velocities just above U tr ,( 1.7 and 1.9 ms À1 )a taheight of 0.45 m, the bed has ag reater solid concentration (0.1) than the solid concentration of 0.07 corresponding to increased fluid gas velocities (! 2.1 ms À1 ). At ah eight of 0.75 m, this solid concentration decreasesf or velocitiesc lose to U tr indicating that in this region most particles are already elutriated.
In the case of the FE100p articles,t he elutriation zone is highert han that of molochite.A taheight of 0.75 mt he bed has ag reater solid concentration for velocities greater than U tr ,a nd the particles are carried out of the dense phase at greater height between 0.75 and 2m.T his could be attributed to the shorter static bed height where the gas hold-up is  Thep ressurep rofiles of both the AR and FR risers under steady state conditions are shown in Figure 9. It was observed that for varying fluidizing velocities,t he pressuresi n both reactors are very similar. This indicates that the solid exchange between the two reactors can be maintained in as table operation despite the fact that the solid handlingi s controlled through pneumatic transport.

Solid circulation rate
Thee ffects of varying static bed height and fluidizingv elocity on the solid circulation ratew ere investigated for molochite and FE100 particles and are shown in Figure 10 and Figure 11,r espectively.T ypical behavior is observed,w ith the particle transfer rate increasing with greater gas velocity.I n the case of molochite,f or as tatic bed height of 500 mm and ag as velocity increasef rom 2.33 to 2.76 ms À1 ,t he transfer of solids increases from 1.66 to 4.32 kg m À2 s À1 .W ith higherf luidizing velocities (2.55-2.76 ms À1 )t he increasei ns olid transfer rate is fairly linear. Conversely,a tl ower velocities and ab ed height varying from 600 to 770 mm the increase in rate of solids transferred is minimal, with ar ise of 0.3 kg m À2 s À1 .A tavelocityi nt he range of 2.55-2.76 ms À1 the bed circulation appearst ob es ensitive,w hen the bed height is in the region of 430-600 mm, for which the difference in solids transferred is 1.4kgm À2 s À1 . TheF E100p articles followed the expected trend of increasing solids transfer with increasing fluidizing velocity.A tl ower velocity (2.33 ms À1 )t he difference between the solids transferred with increasing static bed height from 200-450mmw as minimal and exhibited ar ise of 1.8 kg m À2 s À1 .T he rise in solid transfer with greater static bed height is greater for fluidizing velocities between 2.33 and 2.97 ms À1 .T he dependence of increasing fluidizing gas velocity and al arger static bed yields an average rise of 6.4 kg m À2 s À1 .T heser esultsi ndicate that the circulation rate can be controlledi nt he dual   CFB system by adjusting the fluidizing velocity, thought he recirculation rate can also be increased by addingf urther bed material. This scenario is not ideal due to the thermal losses and energyp enalty of adding cold particles for "hot" operation and the requirement of an increased pressure head in the wind-box.

Gas bypass leakage
Fort he chemical looping fast-bed reactor design it is essential to ensure that almostp ure CO 2 is obtained at the exit of the FR cyclone.G as bypass or leakage that dilutes this CO 2 stream can potentially increase the cost of CO 2 purification and separation. Therefore, the potential for gas leakage and methods for its reduction are important considerations.I n this design, leakage can occur by gas passing in either ac ocurrento rc ounter-current flow to the solids stream between the loop seal and cyclone connection. As air is used as the primary fluidizing gas in the CFM, the leakage ratio [Eq. (2)] was determined by introducing CO 2 as at race gas to be monitored, where CO 2 replaced air as the fluidizing gas in the outlet of the FR loop seal. Thel eakage ratio can then be determined in terms of the amount of CO 2 detected in relation to the flowrate of CO 2 fluidizing the loop seal. Figure 12 shows the gas leakage ratio with FE100 particles, where the main considerations for investigation were the loop-seal fluidizing gas velocity and the bed height above the loop seal in the return leg. Ther iser flow rates were kept constant at 1000 Lmin À1 (2.12 ms À1 ). During maintained stable and balanced operation, the system shows that CO 2 concentration at the exit of the FR cyclone can be maintained at av ery low level. When the loop-seal fluidizing velocity is maintained below 0.05 ms À1 the system exhibits al eakage ratio of up to 2.5, correspondingt oaCO 2 concentration of 0.7 %w ith am inimal loop-seal bed height of 350 mm. Thel eakage ratio increases dramatically to 3.75-3.9, equivalent to aC O 2 concentration of 1.5-1.6 %, upon increasing the loop-seal fluidizing velocityf rom 0.04 to 0.055 ms À1 .T his indicates that, at this bed height, the system is sensitive to the fluidizing gas flow and velocity.T hese results indicate that the loop-seal bed height must be maintained above 600 mm to minimize the gas leakage from the riser. As presented in Figure 12, maintaining the loop-seal bed height above 600 mm results in CO 2 concentrations of 0.5 %, which cannot be accommodated given the limits of the analysis methods employed here,a nd indicates that it is criticalt oa void dilution of the stream exiting the FR reactor by maintaining an acceptable bed height.T he investigation provides acceptable indicators for gas leakage control for transfer to the dual fast-bed system.

Conclusions
This investigation centers on the design philosophy of a1 :1 scale cold-flowmodel (CFM)f or adual interconnected circulating fluidized-bed system for the chemical looping combustion of gaseous fuels.T he dual CFB system for chemical looping combustion allows for greater gas/solid contactc ompared to other reactorc onfigurations,b ut it is difficult to control the transfer and circulationr ate of the fluidized bed material. Theu se of aC FM allows the investigation of the fluidizing properties that can influencet he rates of circulation and transfer at ambientc onditions,w ith clear observations of the fluidizing behavior. TheC FM was operated with two different particles (molochite and FE100) as bed material for ac omprehensive understandingoft he system hydrodynamic characteristics.T he cold-flows ystem is modeled on the dual interconnected CFB fast-bed design of Cranfield's PACT facilityc hemical looping reactor. Them ajor findings there are detailed as follows: * Thes olids transfer between the dual CFB reactors can be controlled and maintained to ah igh level of stable operation in spite of the control philosophy governed by pneumatic transport.
* Thec irculation rate can be flexibly controlled by using the fluidizing gas velocityi nt he riser. Thee xperimental investigation determines that the recirculation rate can also be adjusted through solid make-up of the bed inventory.

Experimental Section
Cold-flowm odel design Cold-flow model reactors are typically reduced in size compared to the larger reactors,w hich they aim to simulate based on Glicksmansd imensionless scaling laws.T he CFM described in this investigation is of 1:1s cale to the Cranfield CLC fast-bed design. With this fast mode of fluid bed operation, it is necessary to reduce the possible gas/particle wall friction effects,w hich may have been unavoidable had the model been decreased in scale.T his approach of "like-for-like" scaling has been successfully employed previously by Bischi et al. [29,30] With the requirement for a1 :1 CFM plus the constraint of air being used as the fluidizing gas at this scale,i twasn ecessary to satisfy other dynamically similar properties in Glicksmanss implified scaling laws.T he conditions of the CLC unit with CFM are detailed in Table 2. Them ajor parameters that were used to simulate similar dynamic properties between the two systems were the particle density and particle diameter. Metallic iron particles (FE100) from William Rowland UK and molochite ceramic particles from Imerys supplied by Castree Kilns were determined to be suitable bed materials for use in this investigation. Thesimplified scaling laws shown in Table 3w ere applied using the conditions from Table 2, and it was determined that it was possible to obtain sufficient dynamic similarity and maintain reliability. This serves to allow one to utilize the CFM to control the solid flow,d etermine how much of the solids can be transferred in ag iven time,a nd determine how to avoid leakage.T hen, by using the scaling criteria, one can determine the circulation rate (G s )i nt he CLC reactor.T he G s of the CLC unit was calculated from the solids required, based on 20 kW operational input heat as ab ase case requirement for the operation of the CFM. For the modeling of CLC reactors,the AR and FR have different operational requirements.I nt he system described here,t he only feasible operational strategy requires the balance of solids transferred between both reactors,w hich is achieved through CO 2 re-cycle in the FR. As ac onsequence,t he AR and FR are maintained under the same conditions in this study.

Analysism ethods
Tw enty GMH-Greisinger GMUD MP-S MR-1 model transducers were used to measure the pressures in this investigation. The pressure outlet taps were located in the wind box, the inlet and outlet of the loop seals,t he outlet of the cyclone,t he outlet of the riser,a nd at every 0.3 mi nterval up the length of the riser. These are often interchanged throughout the course of the investigation, and the heights at which they are located relative to above the wind-box distributor are detailed. These measurements are recorded using aT C-08 data-logger from Pico Industries and accompanying software suite for process monitoring.

Experimental procedures
Thep ressure data provided by the pressure transducers located in the dense bed, transition zones,u pt he length of the risers,t he wind-box, and the outlet of the fluidizing gas inlet for the loop seals allowed for the determination of minimum fluidization velocity (U mf ), transport velocity/fast fluidization (U tr )a nd the density profiles in the system. Thes olid circulation rate (G s , kg m À2 s À1 )c alculated at varying static bed heights was measured during stable fluidization, and then the fluidizing gas was cut off from the loop seals.T he bed height of the accumulated solids in the return leg above the loop seal was measured over as hort period of time.T he reactor-to-reactor bypass leakage was determined by introducing CO 2 as at race gas to the inlet of the fuel reactor loop seal and measuring any corresponding trace CO 2 at the outlet of the fuel reactor cyclone.T his was measured by using ap re-calibrated ADC MGA 3000 multi-gas analyzer with 1.82 10 À5 particle diameter, dp ( 10 À6 m) 300 60 519 U mf (m s À1 )0.03 0.01 0.16 pressure (atm)111 inner diameter, D (m) 0.1 0.1 0.1 fluidization velocity, U 0 (m s À1 )222 U mf /(gD) 0 · 5 0.031 0.01 0.161 Fr = (U 0 ÀU mf )/(gD) 0 · 5 1.99 2.01 1.86 dp/D ( 10 3 )3.000 0.6 5.19 G s (kg m À2 s À1 )3. 3 2.56 10.7 1 g /1 s ( 10 4 )1. 7 2.1 8.6 Re = 1 g U 0 dp/m 4.2 7.1 68.5 Ar = 1 g (1 s À1 g ) g (dp 3 )/m 2 74 45 6961  (2). P CO 2 is the measured CO 2 at the analysis point (vol %), Q riser is the flowrate in the riser (L m À1 ), and Q LS is the flowrate of CO 2 in the loop seal (L m À1 ). Thel eakage ratio was used to measure any possible dependence on the leakage with loop-seal bed height.