Abstract:
Reactor for chemical looping combustion. The reactor includes a rotary wheel having a plurality of channels extending therethrough, each channel having a wall with a porous oxygen carrier layer disposed on a bulk layer having high thermal inertia and conductivity. A stationary feeding chamber is located proximate to a bottom portion of the rotary wheel, the feeding chamber partitioned into a plurality of sectors for delivery of a selected pressurized feed gas into the channels of the rotary wheel as it rotates through the sectors. A stationary exit chamber is located proximate a top portion of the rotary wheel, the exit chamber partitioned into at least two sectors through which separate gas streams emerge. A motor is provided for rotating the rotary wheel. In a preferred embodiment, the sectors In the feeding chamber are fuel, air, fuel purging and air purging sectors.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    This invention relates to a rotary reactor for gas dueled chemical-looping combustion that facilitates carbon capture. 
         [0002]    It has been widely acknowledged that emission of greenhouse gases is a primary contributor to global warming, and CO 2  is the most prevalent of these gas emissions. One possible approach to restrict anthropogenic CO 2  emissions, apart from improving conversion and utilization efficiency and expanding the use of alternative sustainable energy, is carbon capture and sequestration (CCS). So far, extensive research focus has been placed on three general processes for capturing CO 2  from combustion in power plants: pre-combustion capture, post-combustion capture, and oxy-combustion. One of the key issues that limits the applications of CCS approaches is the large energy penalty during the separation process which renders CCS inefficient. 
         [0003]    Recently, a new approach for CO 2  capture has been investigated. This approach was named “chemical-looping combustion (CLC)” by Richter and Knoche [1] and belongs to oxy-fuel combustion. In CLC, two steps of combustion are involved: fuel is oxidized by metal oxide in a fuel reactor to generate CO 2  and water steam. The reduced metal oxide is then regenerated by air in an air reactor. The flue gas from the fuel reactor contains only CO 2  and H 2 O where CO 2  can be readily captured after steam condensation. During this two-step CLC process, the looping metal oxide adsorbs oxygen in the air reactor and releases oxygen in the fuel reactor, that is, it acts as an “oxygen carrier” (OC) transporting oxygen while it is looping between the reactors. Since first proposed by Richter and Knoche in 1983 [1], the selection of the OC has been acknowledged as one of the most important aspects in chemical-looping combustion. Most of the work on CLC so far has been focused on the development and investigation of OCs configured in particle form using fixed or fluidized bed reactors. Some of the most commonly investigated oxygen carriers are nickel-, copper-, iron- and manganese-based oxygen carriers. 
         [0004]    Research on CLC reactor design has almost exclusively concentrated on an interconnected fluidized-bed reactor with oxygen carrier particles circulated throughout the reactors [2]. In this system, both reactors are fluidized beds. Oxygen carrier particles are fluidized and pneumatically transported continuously between the fuel reactor and the air reactor. A cyclone in the top and a loop-seal in the bottom are used to separate the oxygen carriers from gas streams. The reactor system based on a fluidized-bed design has several advantages such as perfect particle mixing, homogeneous temperature distribution and smooth, liquid-like particle flow inside the reactor with continuous, automatically controlled operations. Major challenges of this design are related to the particle circulation process [2, 3]; (1) extra energy is needed to fluidize the beds and hence the pressure drop throughout the reactor is usually high; (2) an efficient cyclone is critical to the recovery of oxygen carriers from the flue stream; (3) fine particles must be removed from the flue stream before entering a gas turbine; (4) agglomeration may happen at high operating temperatures; and (5) particle collisions will impair the lifetime of the reactors. Besides, nitrogen and CO 2  leakage can occur which leads to reduction of the capture efficiency or the need for extra separation downstream to purify the CO 2  stream. These issues are more severe under elevated pressures. Although a great variety of successful industrial applications using fluidized bed reactors have been built, the complexity of the multiphase, multi-scale reactive flow makes it difficult to design, optimize and scale-up such reactors. 
         [0005]    Alternative designs, such as the moving-bed reactor [4], the fixed packed-bed reactor [3] or the rotating packed-bed reactor [5], have also been proposed and investigated. Fan and co-workers [4] suggested utilizing a moving bed for the fuel reactor. As compared to fluidized-bed reactors, the moving-bed reactor has the advantages that the mixing of a gas (or solid) phase along the moving direction is small such that the dilution effect of product on the incoming fuel is limited. Similar technical difficulties still exist in the particle circulation process as in the fluidized-bed design. 
         [0006]    In order to avoid circulation issues, Noorman and co-workers [3] proposed a packed-bed reactor design. In this design, the oxygen carrier particles are packed into the reactor and are alternately exposed to reducing and oxidizing conditions via periodic switching of the gas feed streams. Two reactors in parallel are used alternately to assure a continuous high temperature gas stream supply to the downstream gas turbine [2]. The main advantages of the packed bed reactor are that the separation of gas and particles is intrinsically avoided, that the reactor design can be much more compact, and that the packed bed reactor allows for better utilization of the oxygen. Besides, since the operation is in stationary beds, no extra energy is needed for circulation. The use of such reactors, however, requires the effective control of large volumes of gas under high temperature and high pressure, in which the flow must be continually initiated and terminated. One potential challenge is that heating and cooling the packed particles may cause a large temperature fluctuation within the reactor [6]. Besides, fuel slip may occur during the switching period, which leads to safety issues. 
         [0007]    Dahl et al. [5] designed a rotating reactor. This rotating reactor is an extension of the packed bed reactor: an annulus packed bed containing OC particles rotates when fuel and air streams are introduced radially outwards through the reactor. Inert gas, (in this case, steam) is fed between air and fuel sectors, and separation walls on the outer and inner walls are used to avoid mixing. The advantages of the rotating bed reactor include the compactness of design with continuous operations, limited energy for circulation, and the feasibility of scale-up and commercialization. The main challenge for this design is to avoid the gas leakage and dilution between fuel and air streams, which at the moment are unavoidable [2]. 
       SUMMARY OF THE INVENTION  
       [0008]    The reactor for chemical looping combustion according to the invention includes a rotary wheel including a plurality of channels defined by channel walls extending therethrough, each channel wall including a porous oxygen carrier layer disposed on a bulk layer having high thermal inertia and conductivity. A stationary feeding chamber is located proximate to a bottom portion of the rotary wheel, the feeding chamber partitioned into a plurality of sectors for delivery of a selected pressurized feed gas into the channels of the rotating wheel as it rotates through the sectors. A stationary exit chamber is located proximate to a top portion of the rotary wheel, the exit chamber partitioned into at least two sectors through which separate gas streams emerge. A motor is provided for rotating the rotary wheel at a selected angular velocity. 
         [0009]    In a preferred embodiment, the sectors in the feeding chamber are fuel, air, fuel purging and air purging sectors. In this embodiment, the sectors in the exit chamber are for air and combustion products. It is preferred that the oxygen carrier layer be a metal oxide such as an oxide of cobalt, nickel, copper, iron or manganese. The gases may flow through the reactor in a co-current or counter-current pattern. A suitable material for the bulk layer is a ceramic such as Al 2 O 3 , YSZ, TiO 2  and BN. 
         [0010]    In a preferred embodiment, the channels are selected to be of the grid-type, honeycomb, plate-type or having corrugated shapes. In this embodiment, it is important that the sectors in the feeding chambers are separated by insulating walls to keep the gas streams from mixing. 
         [0011]    The wheel rotates continuously through four sectors: fuel, air, fuel purging and air purging sectors. Pressurized feed gas (fuel, air or steam) enters from the feeding chamber, reacts with the oxygen carrier (OC) as it passes through the wheel, and leaves the system from the exit chamber. As gas passes through each channel, the chemical energy from the surface reaction is transferred to the bulk flow by convection to heat up the streams from a low inlet temperature to a high outlet temperature. The rotary wheel consists of a large number of micro-channels with oxygen carriers coated onto their inner wall. The channel wall has two solid layers with one being a highly porous oxygen carrier layer and the other being a bulk dense layer with high thermal inertia and conductivity. Flue streams from a large number of channels merge into two separate streams from the fuel zone and the air zone, respectively. Advantages of the rotary design include the intrinsic separation between fuel and air streams, compactness, scale-up feasibility, and periodic and continuous operation without the need to transport particles at high pressures. 
         [0012]    In a preferred embodiment, the oxygen carrier is copper oxide and the supporting material is boron nitride. The pressurized gas may flow throughout the reactor in a co-current pattern and the channel of the reactor is square-shaped. A suitable fuel is methane. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING  
         [0013]      FIGS. 1   a, b, c,  and  d  are schematic diagrams of a rotary CLC system design according to one embodiment of the invention. 
           [0014]      FIGS. 2   a, b, c,  and  d  are schematic illustrations of the gas flow pattern through reactor and gas leakage through radial seals and peripheral seals. 
           [0015]      FIGS. 3   a ,  3   b  are the schematic layouts of individual channel structure and oxygen carrier coated on the surface. 
           [0016]      FIGS. 4   a ,  4   b  are the schematic profiles of gas species concentration at inlet and at the exit for two cycles. 
           [0017]      FIG. 5  is a simplified layout of the rotary chemical-looping combustion cycle. 
           [0018]      FIGS. 6   a, b, c  are graphs of the reactor performance in one cycle for different locations along the channel: (a) z=0.1 m; (b) z=0.5 m; and (c) z=0.9 m. 
           [0019]      FIG. 7  is a graph of the CH 4  concentration in the fuel and purge sectors (solid lines) and the periodic fuel conversion efficiency (dashed line) as a function of axial location. 
           [0020]      FIG. 8  is a graph of the CO 2  and O 2  flow rate at the outlet of the channel as a function of time for one cycle. 
           [0021]      FIG. 9  is a graph of the time-averaged temperature profile as a function of axial location for solid (lines) and flows (circles). The dashed line is the maximum temperature variation in one cycle. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    The rotary bed CLC reactor  10  according to the invention includes a rotary bed matrix wheel  12 , a driving motor  14 , and two stationary gas chambers  16  and  18  located at the top and bottom of the wheel  12  (inlet or outlet), as shown in  FIG. 1  ( a ). Pressurized feed gas  20 , both fuel and air, flows in a co-current pattern from the bottom feeding chamber  18 , reacts with the oxygen carrier as it passes through the rotary bed  12  and leaves the system from the top exit chamber  16 . The heat generated from the exothermic reactions is utilized to heat the passing gas to high temperature, which is ultimately used to drive turbines to generate electricity. The solid wheel  12  temporarily stores the heat of reaction and releases it to the flow. The rotary bed, powered by the driving motor  14 , rotates at a constant speed while the chambers  16  and  18  remain stationary. The entire reactor is surrounded by insulating walls  22  that can sustain high temperatures and high pressures (as shown in  FIG. 1  ( b )). For clarity, the insulating walls of the reactor wheel are not shown in  FIG. 1  ( a ).  FIG. 1  ( b ) shows the cross-sectional view of the reactor. The rotary bed matrix  12  consists of an array of identical long and narrow channels  24 . A typical channel  24  size is several millimeters wide depending on the cell density. 
         [0023]    Oxygen carriers are coated or impregnated onto the inner surfaces of each channel  24 . Two streams are admitted into the spinning channels from the feed side, and leave into two different zones divided by insulating walls in the exit chamber  16 . As the channel passes through the fuel zone, the gaseous fuel stream flows into the channel, reacts with the active metal oxide to generate CO 2  and H 2 O. As the same channel passes through the air zone, air flows into the channel to regenerate the oxygen carrier to its original state. The gas streams in the fuel and air zones are at the same pressure. The chemical energy from the continuous redox (reduction and oxidation) reactions is temporarily stored in the solid phase, and then transferred to the bulk flow by convection through the rotary matrix, which behaves in a similar way as in a rotary heat exchanger. The center of the rotary bed is a small hollow channel  26  through which a cylindrical bearing  28  is inserted to support the reactor construction and actuation. The design is not limited to the co-current flow pattern. For example, a counter-current flow pattern with fuel (or air) flowing from top chamber to the bottom chamber can be an alternative option. 
         [0024]      FIG. 1  ( c ) shows the bottom view of the gas feed chamber  18 . The feed chamber is divided into four sectors: a fuel sector (θ fuel ), an air sector (θ air ), a fuel purging sector (θ fuel     —     purge ) and an air purging sector (θ fuel     —     purge ). The fuel zone is divided into fuel and fuel purging sectors while the air zone is divided into air and air purging sectors. Fuel gas or air is fed into the fuel or air sector, respectively, while steam is used as a “sweeping” gas in the purging sectors to flush the reactor and hence avoid gas carry-over between sectors. The four sectors at the bottom chamber  18  are separated by insulating walls which remain stationary during operation.  FIG. 1  ( d ) shows the isometric projection of the reactor wireframe. The top exit chamber  16  consists of a fuel sector and an air sector. While one slug of feed gas passes through a channel, the reactor bed spins continuously. Accordingly, flue gas exits at the same radial location of the wheel at a slightly different angle. The two purging sectors act as “buffer zones” to account for this angle mismatch. Therefore, as one channel spins out of the fuel (or air) sector and enters the following purging sector, a feed steam continuously flushes the residual fuel (or unburned air) into the same zone at the top without diluting the other zone. Due to rotation, some steam from the fuel (or air) purging sector may also end up entering the following air (or fuel) sector. 
         [0025]      FIG. 2  shows the gas flow pattern through the reactor. The majority of the feed gas enters the reactor from the feeding chamber, flows through the different channels, and leaves the reactor from the exit chamber. Pressure drop along the channel is attributed to skin friction, which is generally small for a laminar channel flow. As a result, the pressure differences between different sectors are expected to be small and hence the pressure-driven gas leakage is limited. However, some gas leakage may occur due to the spinning motion of the reactor. For instance, as shown in  FIG. 2  ( a ) and ( b ), some gas may flow through the gap between the insulating walls and the peripheral reactor surface and leave the reactor without being reacted; some gas may bypass from the fuel zone to the air zone through the gap between the insulating separation walls and the rotary reactor, as shown in  FIG. 2  ( c ) and ( d ). Sealing systems similar to those used in the rotary regenerative heat exchanger can be utilized to reduce the gas leakage (as seen in  FIG. 2 ); peripheral seals will trap and force the flow into centrifugal motion and hence restrict the gas bypass from the inlet to the outlet side; radial seals with small clearance will restrict the gas leakage rate between stationary insulating walls and the moving reactor. 
         [0026]    The rotary bed matrix  12  consists of a large number of channels  24 . Each channel  24  consists of an inner gas passage and solid support material coated with the oxygen carrier, as shown in  FIG. 3  ( a ). Gas flows through the passages and reacts with the oxygen carrier on the inner surface. The oxygen carrier is coated or impregnated onto a porous layer of the solid support, as seen in  FIG. 3  ( b ). High porosity of the oxygen carrier layer enhances the surface area between the solid and the gas species and hence favors the heterogeneous surface reactions. The binder material in the porous layer acts as an oxygen-permeable material that helps improve the physical and chemical stabilities of the oxygen carrier and therefore maintains its reactivity after repeated cycles. In addition, as shown in  FIG. 3  ( b ), a bulk support layer is bonded to the porous layer. This bulk support layer is made of highly conductive materials with high heat capacity, which can effectively store the heat produced in the exothermic reaction, transfer it within the reactor, and heat the flowing gas. The utilization of the bulk support layer is critical to the temperature distribution of the reactor. Besides, the bulk layer helps avoid the gas mixing between the adjacent channels. Note that the support material in the porous layer and the bulk layer is not necessarily the same. 
         [0027]    As one channel travels through the fuel and air zones, following a sequence of fuel, fuel purging, air and air purging, the active oxygen carrier on the matrix surface continuously releases oxygen to oxidize the fuel, and adsorbs oxygen from air. Typical gas species profiles for two consecutive cycles at the inlet and the outlet of one channel are shown schematically in  FIG. 4 . The thermal and chemical state in one channel undergoes a transient process: gas species enters the channel  24 , adsorbs or releases oxygen from the oxygen carrier material, releases or absorbs energy with the reactor matrix, and leaves the reactor with a varying flow velocity and concentrations. Given constant inlet conditions during operation, as shown in  FIG. 4  ( a ), it is expected that after a number of cycles, the reactor will gradually converge to a periodic-stationary state: the physical and chemical processes within one cycle will go back to the original states after one cycle. The compositions at the exit of the reactor at stationary state are shown in  FIG. 4  ( b ). Therefore, the sum of a large number of transient flue streams exiting from the fuel sector (or air sector) will mix well to give one steady-state flue stream. The steady separate streams from the fuel sector and air sector can then be utilized to drive gas turbines  30  and  32 , as shown in  FIG. 5 , and CO 2  can be easily separated after water condensation. 
         [0028]    The reactor design is not limited by the materials or the preparation method described above. Any of the materials or preparation methods described by Adanez et al. [2] are potential candidates for this rotary design, as well as other materials studied by numerous other investigators, which include, for example, in reduced metal form, Fe, Cu, Mn, Co, Ni, etc. The support material can be any material conventionally utilized as ceramic insulators: Al 2 O 3 , YSZ, TiO 2 , BN, etc. The preparation method includes, for example, wet-impregnation, dry-impregnation, deposition-precipitation, wash-coating, etc. Surface treatment methods, such as surface-etching, can be utilized to enhance the surface porosity and improve the oxygen carrier loading. In addition, the channels can be formed in a variety of geometries or sizes, such as, grid-type, honeycomb geometry shapes, plate-types, any series of corrugated shapes, or any type of geometry that presents a high specific surface area. Besides, the reactor can be operated at different pressures, temperatures, flow velocities, and so on. 
         [0029]    A one-dimensional model was constructed to simulate the cyclic performance of a single channel with copper oxide used as the oxygen carrier and boron nitride as the binder. The model focuses on the reactive plug flow in each channel. At every point along the channel, one-dimensional conservation equations for mass and energy are solved for both the gas and solid phases. Kinetics from ref. [7] is utilized to describe the heterogeneous reactions. 
         [0030]    The model is used to simulate the operation of the rotary reactor. Simulations are conducted for repeated cycles until periodic operation is achieved. The output of the model consists of the gas flow velocity, the axial profiles of the temperature and gas composition, and the conversion of the oxygen carrier. As an example,  FIG. 6  shows the temperature and gas concentration profiles in one cycle as a function of the angle of rotation (from 0 to 2π) for three locations: near the inlet, in the middle, and near the exit of the wheel. As observed in  FIG. 6 , each curve (temperature or concentration) quickly reaches a quasi-steady state in the fuel sector after a short transition period from the previous purge sector. The fuel concentration gradually decreases from the inlet to the outlet. As seen in  FIG. 6  ( c ), at the wheel exit the methane concentration in zero all the time. Thus, the fuel is completely consumed before the exit of the reactor. As the channel enters the fuel purging sector, the residual fuel is quickly pushed towards the exit of the channel and reacts with the oxygen carrier near the outlet. All the methane is purged out of the channel before the channel enters the air sector. Therefore, no direct mixing between the fuel and oxygen is observed and thus the safety of the operation is ensured. The black lines in  FIG. 6  show the solid temperature profiles, which remain almost constant throughout the entire cycle. The maximum temperature variation with time is less than 20K. The limited temperature variation is mainly because of the high thermal inertia of the bulk dense layer in the solid phase which acts as a heat reservoir to match the energy transfer processes. Comparing  FIG. 6  ( a ), ( b ) and ( c ), it is observed that the solid temperature gradually increases from the inlet to outlet. The temperature variation at the outlet is within 0.1K. The gas temperature profile is directly determined by the solid temperature due to the large specific surface area of the channel and hence the high convective heat transfer rate between the solid phase and the flow. Consequently, the gas temperature fluctuation is limited and the maximum variation is generally within 20K. 
         [0031]      FIG. 7  shows the fuel conversion efficiency. As seen in  FIG. 7 , the fuel conversion efficiency gradually increases. At the outlet of the channel, the fuel conversion is unity. The majority of the methane is consumed in the fuel sector while around 15% of the methane is reacted in the purge sector. The methane concentration decreases monotonically in the fuel sector while a bell-shaped concentration curve is observed in the fuel purge sector with a maximum value of around 10% located close to 0.3 m of the channel. As purging gas flows through the channel, residual methane in the channel is pushed to the downstream end of the channel while the oxygen carriers continue oxidizing the fuel. Thus, the residual fuel in the purge sector is mostly oxidized within the same region of the channel as that in the fuel sector. As shown in  FIG. 7 , under the current operating conditions, 99.9% combustion efficiency is obtained at 0.75 m of the channel. The extra 0.25 m of the channel is utilized to ensure redundancy for the fuel conversion. If a higher mass flow rate of methane (e.g. higher operating pressure or lower feed stream temperature) is admitted into the channel or a less reactive metal oxide is utilized as the oxygen carrier, the concentration profile is expected to be shifted upwards. In these cases, the redundant length decreases. 
         [0032]      FIG. 8  shows the CO 2  and O 2  flux profiles at the outlet of the channel as a function of time. CO 2  flux is normalized by the inlet carbon flow rate in the fuel sector, including both methane and CO 2  while the O 2  flux is normalized by the inlet oxygen flow rate. Thus, at steady state the normalized species flow rate should be unity. As seen in  FIG. 8 , most of carbon is captured in the fuel zone while unreacted oxygen only exits in the air zone. Therefore, dilution between the oxygen and carbon dioxide is avoided. In the fuel sector, at about 7 seconds. CO 2  flow at the outlet reaches steady state while in the air sector it takes about 6 seconds to reach steady-state. Steady state in the air sector is indicated by the complete regeneration of the oxygen carrier. Because of the high velocity, the fuel purging sector has a spike of species flux. The residue CO 2  in the channel is rapidly flushed out at the beginning of the air sector, leading to a spike of the CO 2  flux. In the air purge sector, the oxygen flux drops to zero at about one second before the next cycle. Thus, under current operating conditions, the residence time of the channel in each purge sector is long enough to ensure complete separation. 
         [0033]    The temperature distribution within the channel is critical in determining the oxygen carrier reactivity as well as the overall energy balance in repeated cycles.  FIG. 9  compares the time-averaged temperature distribution of the solid and gas phases. The solid temperature increases monotonically from 1000K to 1350K. Thus the maximum temperature rise along the channel is within 350K. The gas temperature distribution is directly determined by the solid temperature except at the inlet where the feed gas is much cooler than the solid. Therefore, convective heating at the inlet is significant. The dashed line in  FIG. 9  shows the maximum solid temperature change with time in one cycle. This curve is generally less than 20K and therefore the temporal temperature variation is limited. 
         [0034]    The rotary reactor that we have developed intrinsically captures the carbon dioxide (CO 2 ) by dividing the combustion process into two separate zones. The utilization of the micro-channel structures with oxygen carrier coated onto the channel walls ensures the fuel conversion and the CO 2  separation. The reactor is intended for use with gaseous hydrocarbon fuels at elevated pressures. The intended application of this invention is for the combustors of continuous combustion system, such as power plants, industrial burners, power generation for small and large scale distributed generation, small and large scale boilers for heating and power. Continuous combustion systems continue to have some of the highest available power densities, but the chemical processes involved lead to the creation and emission of greenhouse gases. In particular, the invention is intended to help mitigate the emission of carbon dioxide (CO 2 ) from combustors. 
         [0035]    The references appended hereto are incorporated by reference in this application. 
         [0036]    It is recognized that modifications and variations of the invention disclosed herein will be apparent to those of ordinary skill in the art and it is intended that all such modifications and variations be included within the scope of the appended claims. 
       REFERENCES 
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         [0039]    [3] Noorman S, van Sint Annaland M, Kuipers H. Packed Bed Reactor Technology for Chemical-Looping Combustion. Ind Eng Chem Res. 2007;46:4212-20. 
         [0040]    [4] Fan L S. Chemical looping system for fossil energy conversions. John Wiley &amp; Sons, Inc: Hoboken, N.J., 2010. 
         [0041]    [5] Dahl I M, Bakken E, Larring Y, Spjelkavik A I, H{dot over (a)}konsen, S F, Blom R. On the development of novel reactor concepts for chemical looping combustion. Energy Procedia. 2009;1:1513-9. 
         [0042]    [6] Noorman S, van Sint Annaland M, Kuipers J A M. Experimental validation of packed bed chemical-looping combustion. Chem Eng Sci. 2010;65:92-7. 
         [0043]    [7] Garcia-Labiano F, de Diego L F, Adanez J, Abad A, Gayan P. Reduction and Oxidation Kinetics of a Copper-Based Oxygen Carrier Prepared by Impregnation for Chemical-Looping Combustion. Ind Eng Chem Res. 2004;43:8168-77.