Oxygen Uncoupling Research Articles

Examining the Cu-Mn-O Spinel System as an Oxygen Carrier in Chemical Looping Combustion

Chemical-looping combustion (CLC) and chemical-looping combustion with oxygen uncoupling (CLOU) are attractive alternatives to conventional combustion that provide efficient and direct separation of CO2. Both processes use metaloxides as oxygen carriers to transfer oxygen between two reactor vessels: the air and fuel reactors. Although monometallic oxides (such as Mn3O4, Fe2O3, NiO, and CuO) have been successfully employed as oxygen carriers, double oxides of the general formula CuxMn3_xO4 in the CuO–Mn2O3 system are examined in this work. The carrier was produced by mixing, extruding, and calcining a 1:1 molar (30.8:69.2 mass ratio) mixture of CuO and Mn2O3 at 950 8C for 6 or 12 h in static air. XRD analysis revealed that spinels of the general formula CuxMn3_xO4 were formed with 0.1_x_2.5 in which x=3Cu/(Cu+Mn). The chemical-looping performance was evaluated in a laboratory-scale fluidized-bed reactor from 800–850 8C over several alternating redox cycles using CH4 as the fuel. The oxygen carrier exhibited reproducible and stable reactivity behavior for both reducing and oxidizing periods in this temperature range. This characteristic makes the system an ideal oxygen-carrier material for CLOU. Moreover, the spinels in the CuxMn3_xO4 series are endowed with favorable physicochemical attributes (such as fast redox processes, high crushing strength, and demonstrated CLOU behavior) and may be viable alternatives to CuO–Cu2O and Mn2O3–Mn3O4 as potential CLOU materials.

Performance of CLOU process in the combustion of different types of coal with CO2 capture

Chemical-looping with oxygen uncoupling (CLOU) process is a chemical-looping combustion (CLC) technology that allows the combustion of solid fuels using oxygen carriers with inherent CO2 separation. The oxygen necessary for the fuel combustion is supplied by a solid oxygen carrier, which contains a metal oxide. The oxygen carrier circulates between two interconnected fluidized reactors: the fuel and the air reactor. In the CLOU process, the oxygen carrier releases gaseous oxygen in the fuel reactor which burns coal as in common combustion with air, so the CO2 generated is undiluted with N2. The reduced oxygen carrier is oxidized by air to the initial metal oxide in the air reactor, then being ready to start a new cycle. The aim of this work is to study the performance of the CLOU process using coals of different rank. Experiments were carried out in a continuously operated 1.5kWth unit. Particles prepared by spray drying containing 60wt% CuO were used as oxygen carrier. Four coals of different rank (anthracite, low volatile bituminous, medium volatile bituminous and lignite) were used as fuel. Besides, the temperature in the fuel reactor was varied between 900 and 950°C. In all the experiments there was complete combustion of the coal to CO2 and H2O, without any unburnt product. The carbon capture efficiency greatly depends on the coal rank and fuel reactor temperature. High carbon capture efficiencies were obtained for lignite and medium volatile bituminous coals. The maximum capture efficiency was 99.3% at 950°C with lignite. The analysis of the experimental results was used to evaluate the effect of the coal rank in a CLOU system when a carbon separation system is included. At 925°C, the solid inventory needed to reach 95% of CO2 capture efficiency with a carbon separation system of 90% of efficiency is 45kg/MWth using lignite, 85kg/MWth using MV bituminous, 140kg/MWth using LV bituminous and 490kg/MWth using anthracite. It must be pointed out the low solid inventories needed in the CLOU process for the different coal rank analyzed in this work.

Investigation of Different Mn–Fe Oxides as Oxygen Carrier for Chemical-Looping with Oxygen Uncoupling (CLOU)

The appropriate oxygen carrier for chemical-looping with oxygen uncoupling (CLOU) should be thermodynamically capable of being oxidized in the air reactor and also release gaseous O2 in the fuel reactor at appropriate temperatures and oxygen partial pressures. It should also be mechanically durable, cheap, and environmentally friendly. Iron–manganese oxides appear to be especially promising due to favorable thermodynamics. In this work, combined metal oxides of iron and manganese were investigated for the CLOU process. Particles with different ratios of Mn/Fe were produced using spray drying. The particles were calcined at 950 and 1100 °C for 4 h and then tested with respect to parameters important for CLOU. The crushing strength for these materials was between 0.1 to 1.7 N, depending on their composition and sintering temperature. The ability of the iron–manganese oxide particles to release oxygen in the gas phase was examined by decomposition of the material in a stream of N2. Moreover, the reaction with both methane and synthesis gas (50/50% CO/H2) was examined in a batch fluidized bed reactor. Here, the particles were alternately oxidized with 5% O2 and reduced in N2 or with fuel at 850 °C, 900 and 950 °C. From the results, it can be concluded that during the nitrogen period, the oxygen carriers with Mn3O4 content in the range from 20 wt % to 40 wt % release oxygen at 900 °C, whereas the materials with higher manganese content show no oxygen release. This is because they could not be oxidized to bixbyite. By decreasing the temperature from 900 to 850 °C, it was possible to oxidize oxygen carriers with manganese oxide content of 50 wt % and higher, and consequently, oxygen release during the nitrogen period was seen for these materials. This is in agreement with the phase diagram for this system. The reaction rate with methane follows the oxygen release trend very well. At the higher reaction temperature, 950 °C, oxygen carriers with manganese content in the range from 25% to 33% show the best gas conversion of methane. At 850 °C, on the other hand, high methane conversion is seen for particles with high manganese content. In fact, several particles had almost full conversion of methane to CO2 and H2O at 850 °C using a bed mass in the batch reactor corresponding to 70 kg oxygen carrier/MW.

Adipose-Specific Deletion of TFAM Increases Mitochondrial Oxidation and Protects Mice against Obesity and Insulin Resistance

Obesity and type 2 diabetes are associated with mitochondrial dysfunction in adipose tissue, but the role for adipose tissue mitochondria in the development of these disorders is currently unknown. To understand the impact of adipose tissue mitochondria on whole-body metabolism, we have generated a mouse model with disruption of the mitochondrial transcription factor A (TFAM) specifically in fat. F-TFKO adipose tissue exhibit decreased mtDNA copy number, altered levels of proteins of the electron transport chain, and perturbed mitochondrial function with decreased complex I activity and greater oxygen consumption and uncoupling. Asa result, F-TFKO mice exhibit higher energy expenditure and are protected from age- and diet-induced obesity, insulin resistance, and hepatosteatosis, despite a greater food intake. Thus, TFAM deletion in the adipose tissue increases mitochondrial oxidation that has positive metabolic effects, suggesting that regulation of adipose tissue mitochondria may be a potential therapeutic target for the treatment of obesity.

Oxygen Release and Oxidation Rates of MgAl2O4-Supported CuO Oxygen Carrier for Chemical-Looping Combustion with Oxygen Uncoupling (CLOU)

The chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) processes are novel solutions for efficient combustion with inherent separation of carbon dioxide. These processes use a metal oxide as an oxygen carrier to transfer oxygen from an air reactor to a fuel reactor, where the fuel reacts with the solid oxygen carrier. When solid fuel is used in CLC, the char must be gasified by, e.g., steam to form H2 and CO, that can be subsequently oxidized to H2O and CO2 by the oxygen carrier. In the case of CLOU, the oxygen carrier releases oxygen gas in the fuel reactor. This enables a high rate of conversion of char from solid fuels, because it eliminates the need for the gasification step needed in normal CLC with solid fuels. In this work, the rate of oxygen release and oxidation of an oxygen carrier consisting of CuO supported by MgAl2O4 (40/60 wt %) for the CLOU process is investigated. The oxygen carrier was produced by freeze-granulation, calcined at 950 °C, and sieved to a size range of 125–180 μm. The reaction rates were obtained in a laboratory-scale fluidized-bed reactor in the temperature range of 850–900 °C, under alternating reducing and oxidizing conditions. The rate of oxygen release was obtained using devolatilized wood char as the fuel in N2 fluidization. Care was taken to obtain reliable measurements not affected by the availability of the fuel and temperature increase in the bed during combustion of the fuel with the released oxygen from the carrier. The Avrami–Erofeev mechanism was used to model the rates of oxygen release and the values of ko and Eapp were estimated to be 2.5 × 105 s–1 and 139.3 kJ mol–1, respectively. The rates of Cu2O oxidation were investigated in a flow of 5% O2 at the inlet of the reactor. However, it was observed that the oxidation rate is limited by the oxygen supply, indicating rapid conversion of the oxygen carrier. From the obtained reaction rates, the minimum total amount of the investigated oxygen carrier needed in the air and the fuel reactor is estimated to be between 69–139 kg MWth–1.

Chemical-looping combustion and chemical-looping with oxygen uncoupling of kerosene with Mn- and Cu-based oxygen carriers in a circulating fluidized-bed 300 W laboratory reactor

The fuel conversion properties of two oxygen carriers, based on manganese- and copper-oxide, were investigated with sulfur-free kerosene in a chemical-looping reactor with continuous particle circulation. An injection system was used, in which kerosene was evaporated, mixed with superheated steam and fed directly into the chemical-looping reactor. The manganese-based oxygen carrier M4MZ-1200 was composed of 40wt.% Mn3O4 and 60wt.% MgO–ZrO2 and was used for chemical-looping combustion (CLC) experiments for 17h. The copper-based oxygen carrier C2Z-1050 was composed of 20wt.% CuO and 80wt.% ZrO2 and was used for 45h with fuel addition. With M4MZ-1200 oxygen carrier, 83–99.3% of the fuel carbon was converted to CO2 at temperatures between 800°C and 950°C and fuel flows equivalent to 144–462Wth. With C2Z-1050 oxygen carrier, 99.99% CO2 yield was achieved at 900°C and 144Wth fuel equivalent. The particles were analyzed before and after the experiments using XRD, SEM, BET surface area and particle size distribution. Whereas nearly the whole batch of M4MZ-1200 particles was disintegrated after the experiments, only about 5% of the C2Z-1050 particles turned to fines. This is the first time that combustion of liquid fuel using CLOU is demonstrated in a continuous unit.

Coprecipitated, Copper‐Based, Alumina‐Stabilized Materials for Carbon Dioxide Capture by Chemical Looping Combustion

Chemical looping combustion (CLC) has emerged as a carbon dioxide capture and storage (CCS) process to produce a pure stream of CO(2) at very low costs when compared with alternative CCS technologies, such as scrubbing with amines. From a thermodynamic point of view, copper oxide is arguably the most promising candidate for the oxygen carrier owing to its exothermic reduction and oxidation reactions and high oxygen-carrying capacity. However, the low melting point of pure copper of only 1085 °C has so far prohibited the synthesis of copper-rich oxygen carriers. This paper is concerned with the development of copper-based and Al(2)O(3)-stabilized oxygen carriers that contain a high mass fraction of CuO, namely, 82.4 wt %. The oxygen carriers were synthesized by using a coprecipitation technique. The synthesized oxygen carriers were characterized in detail with regards to their morphological properties, chemical composition, and surface topography. It was found that both the precipitating agent and the pH at which the precipitation was performed strongly influenced the structure and chemical composition of the oxygen carriers. In addition, XRD analysis confirmed that, for the majority of the precipitation conditions investigated, CuO reacted with Al(2)O(3) to form fully reducible CuAl(2)O(4). The redox characteristics of the synthesized materials were evaluated at 800 °C by using methane as the fuel and air for reoxidation. It was found that the oxygen-carrying capacity of the synthesized oxygen carriers was strongly influenced by both the precipitating agent and the pH at which the precipitation was performed; however, all oxygen carriers tested showed a stable cyclic oxygen-carrying capacity. The oxygen carriers synthesized at pH 5.5 using NaOH or Na(2)CO(3) as the precipitating agents were the best oxygen carriers synthesized owing to their high and stable oxygen transfer and uncoupling capacities. The excellent redox characteristics of the best oxygen carrier were interpreted in light of the detailed morphological characterization of the synthesized material and a synthesis-structure-performance relationship was developed.

Identification of operational regions in the Chemical-Looping with Oxygen Uncoupling (CLOU) process with a Cu-based oxygen carrier

Chemical-Looping with Oxygen Uncoupling (CLOU) is an alternative chemical-looping process for the combustion of solid fuels with inherent CO2 capture. The CLOU process demands a material as oxygen carrier with the ability to decompose with O2 release at suitable temperatures for solid fuel combustion, e.g. copper oxide. This article presents an experimental method to determine the maximum oxygen generation rate of an oxygen carrier as well as to determine the minimum solid inventory that must be used in the fuel reactor. The method here proposed can be used as basis for comparison of the use of different oxygen carriers or type of coals. In this work, the combustion of coal by using a promising Cu-based oxygen carrier prepared by the spray drying method was tested. The oxygen carrier (Cu60MgAl) was composed of 60wt.% CuO and MgAl2O4 was used as supporting material. Experiments were carried out in a batch fluidized-bed reactor at temperatures ranging from 900 to 980°C. Three different regions were identified depending on the oxygen carrier to coal mass ratio. For oxygen carrier to coal ratios higher than 50 (Region I), coal was fully converted to CO2 and H2O. In addition, an excess of oxygen was present in the flue gases, which was close to the equilibrium concentration. When this ratio was in the range 50–25 (Region II), the concentration of oxygen was decreasing whereas some CO was observed as the only unconverted gas. Further decrease in the oxygen carrier to coal ratio below 25 (Region III) caused the depletion of oxygen in the exhaust gases but CO remained as the only unconverted gas. CH4 or H2 were never detected at the reactor outlet in any case and agglomeration problems were never observed. These regions were related to the solids inventory in the fuel reactor by the rate of oxygen generation calculated in every case. A maximum rate of oxygen generation for the oxygen carrier was determined as kgO2/s per kg of oxygen carrier, which increased with the temperature from 2.1×10−3 at 930°C to 2.8×10−3 at 980°C. From these values, the estimated solids inventory in the fuel reactor was changed from 39 at 930°C to 29kg/MWth at 980°C. The results obtained in this work showed that in the CLOU process it is possible to reach full conversion of the solid fuel with very low solids inventory and avoiding the oxygen polishing step.

Rate Analysis of Chemical-Looping with Oxygen Uncoupling (CLOU) for Solid Fuels

Chemical-looping with oxygen uncoupling (CLOU) offers a promise to reduce energy penalty by facilitating the capture of CO2 emitted from power plants. It has a potential to lower the oxygen carrier inventory of the fuel reactor in contrast to chemical-looping combustion (CLC). The primary mechanism in CLOU for the combustion of solid fuels is their reaction with gaseous oxygen released by the decomposition of a metal oxide, which differs from CLC of solid fuels where the solid fuel has to be gasified first. The slower gasification reaction in CLC is subsequently followed by combustion of the fuel with a circulating oxygen carrier. The present study is concerned with the rate analysis from reported batch fluidized bed CLOU experimental data of Mexican petcoke particles by a CuO/ZrO2 oxygen carrier. The methodology to determine the kinetic parameters for CuO decomposition and solid fuel oxidation during the fuel reactor stage and for Cu2O oxidation in the air reactor stage have been discussed. The results of the study are expected to help in the development of a process model for CLOU, furthering the development of a pilot scale process.

(MnzFe1—z)yOx combined oxides as oxygen carrier for chemical‐looping with oxygen uncoupling

Oxygen carrier particles with the composition (Mn0.8,Fe0.2)2O3 were found to readily release gas phase oxygen at 850°C, and were capable to oxidize CH4 completely and convert wood char rapidly to CO2 during experiments in a batch fluidized bed reactor. The particles were able to release oxygen corresponding to more than 3% of their mass in less than 40 s. Because of the low price and favourable environmental properties of manganese and iron oxides, this finding could be of great importance for the development of chemical‐looping combustion with oxygen uncoupling. © 2012 American Institute of Chemical Engineers AIChE J, 59: 582–588, 2013

Experimental Study of Natural Cu Ore Particles as Oxygen Carriers in Chemical Looping with Oxygen Uncoupling (CLOU)

The possibility of using three natural Cu ore particles as oxygen carriers in the chemical looping with oxygen uncoupling (CLOU) was investigated. The reaction kinetics of Cu2O with O2 and the decomposition kinetics of CuO in a CO2 atmosphere at different temperatures and oxygen fractions were studied using thermogravimetric analysis (TGA). Both oxygen uptake and oxygen release exhibit a high reaction rate. Multiple oxygen uptake and oxygen release cycles indicated that three natural Cu ore oxygen carriers have high reactivity and cyclic stability. The high-temperature oxygen uptake and oxygen release process for producing O2–CO2 gas mixtures in a fluidized-bed reactor packed with Cu-based oxygen-carrier particles through air separation with carbon dioxide as the purge gas was investigated. Oxygen was absorbed, and heat is stored by the Cu-based oxygen-carrier particles with air being fed. An O2–CO2 stream can be obtained when the fluidized bed was regenerated with carbon dioxide as the fluidizing gas. Th...

Evaluation of a Spray-Dried CuO/MgAl2O4 Oxygen Carrier for the Chemical Looping with Oxygen Uncoupling Process

The chemical looping with oxygen uncoupling (CLOU) process is a chemical looping combustion (CLC) technology that allows for the combustion of solid fuels with inherent CO2 separation. As in CLC technology, in the CLOU process, the oxygen necessary for fuel combustion is supplied by a solid oxygen carrier, which is moving between two reactors: the fuel and air reactors. The CLOU technology uses the property of some metal oxides (CuO, Mn2O3, and Co3O4), which can generate gaseous oxygen at high temperatures. The oxygen generated by the oxygen carrier reacts directly with the solid fuel, which is mixed with the oxygen carrier in the fuel reactor. The reduced oxygen carrier is transported to the air reactor, where it is oxidized by air. In this work, a material prepared by spray drying containing 60 wt % CuO and 40 wt % MgAl2O4 as supporting material was evaluated as an oxygen carrier for the CLOU process using different installations. First, the oxygen release rate and the fluidization behavior, with regard to the agglomeration and attrition rate, were analyzed in a thermogravimetric analyzer (TGA) and in a batch fluidized bed, respectively. Then, the effects of the main operating conditions, such as the temperature, solids flow rate, and gas velocity in the fuel reactor, on the oxygen-carrier capability to release gaseous oxygen were analyzed in a continuous CLOU unit using N2 and CO2 as fluidization media. In addition, the effect of the oxygen concentration in the air reactor on the capability of the oxygen carrier to be regenerated was evaluated. The results obtained showed that this oxygen carrier has suitable characteristics for the CLOU process. Nevertheless, after 40 h of continuous operation at high temperatures, the particle integrity decreased significatively, indicating the need to improve the lifetime of this kind of material for use in an industrial CLOU process.

Development of Cu-based oxygen carriers for Chemical-Looping with Oxygen Uncoupling (CLOU) process

The Chemical-Looping with Oxygen Uncoupling (CLOU) process is a Chemical-Looping Combustion (CLC) technology that allows the combustion of solid fuels with inherent CO2 separation using oxygen carriers. This technology has low energy penalty for CO2 separation and thus low CO2 capture costs. The CLOU process is a new option, when the direct use of a solid fuel in a CLC technology is considered. The CLOU process uses oxygen carriers based on some metal oxides that have the capability to evolve gaseous oxygen at high temperatures. The oxygen generated by the metal oxide reacts directly with the solid fuel, which is mixed with the oxygen carrier in the fuel reactor. The selection of a suitable oxygen carrier is a key factor for the CLOU technology development. The aim of this work was to produce and characterize oxygen carrier materials based on CuO with high oxygen transfer capability, high oxygen generation rates and good fluidization properties. Several oxygen carriers were prepared with different CuO contents, inert supports and preparation methods (incipient wet impregnation, mechanical mixing following by pelletizing by extrusion, or pelletizing by pressure). The reaction rates for oxygen generation (reduction) and regeneration were determined carrying out successive cycles in a TGA system. In this way, it was determined the chemical suitability of the materials. Selected oxygen carriers were tested by redox cycles in a batch fluidized-bed reactor working at different temperatures and reacting atmospheres. The fluidization behavior against agglomeration and attrition during a high number of cycles was determined. Oxygen carriers with 60wt.% of CuO on MgAl2O4 and with 40wt.% CuO on ZrO2 prepared by mechanical mixing following by pelletizing by pressure were identified as suitable materials for CLOU process.

Insulin-Induced Generation of Reactive Oxygen Species and Uncoupling of Nitric Oxide Synthase Underlie the Cerebrovascular Insulin Resistance in Obese Rats

Hyperinsulinemia accompanying insulin resistance (IR) is an independent risk factor for stroke. The objective is to examine the cerebrovascular actions of insulin in Zucker obese (ZO) rats with IR and Zucker lean (ZL) control rats. Diameter measurements of cerebral arteries showed diminished insulin-induced vasodilation in ZO compared with ZL. Endothelial denudation revealed vasoconstriction to insulin that was greater in ZO compared with ZL. Nonspecific inhibition of nitric oxide synthase (NOS) paradoxically improved vasodilation in ZO. Scavenging of reactive oxygen species (ROS), supplementation of tetrahydrobiopterin (BH(4)) precursor, and inhibition of neuronal NOS or NADPH oxidase or cyclooxygenase (COX) improved insulin-induced vasodilation in ZO. Immunoblot experiments revealed that insulin-induced phosphorylation of Akt, endothelial NOS, and expression of GTP cyclohydrolase-I (GTP-CH) were diminished, but phosphorylation of PKC and ERK was enhanced in ZO arteries. Fluorescence studies showed increased ROS in ZO arteries in response to insulin that was sensitive to NOS inhibition and BH(4) supplementation. Thus, a vicious cycle of abnormal insulin-induced ROS generation instigating NOS uncoupling leading to further ROS production underlies the cerebrovascular IR in ZO rats. In addition, decreased bioavailability and impaired synthesis of BH(4) by GTP-CH induced by insulin promoted NOS uncoupling.

Demonstration of chemical-looping with oxygen uncoupling (CLOU) process in a 1.5 kWth continuously operating unit using a Cu-based oxygen-carrier

Chemical-looping with oxygen uncoupling (CLOU) process is a chemical-looping combustion (CLC) technology that allows the combustion of solid fuels with inherent CO2 separation. As in the CLC technology, in the CLOU process the oxygen necessary for the fuel combustion is supplied by a solid oxygen-carrier, which contains a metal oxide. The CLOU technology uses the property of the copper oxide which can generate gaseous oxygen at high temperatures. The oxygen generated by the oxygen-carrier reacts directly with the solid fuel, which is mixed with the oxygen-carrier in the fuel-reactor. The reduced oxygen-carrier is transported to the air-reactor where it is oxidized by air. The flue gases from the fuel-reactor are only CO2 and H2O, since fuel is not mixed with air. This work demonstrates the proof of the concept of the CLOU technology burning coal in a 1.5kWth continuously operated unit consisting of two interconnected fluidized-bed reactors. A bituminous coal was used as fuel. An oxygen-carrier prepared by spray drying containing 60wt.% CuO and MgAl2O4 as supporting material was used as oxygen-carrier. The effects of fuel-reactor temperature, coal feeding rate, and solids circulation flow rate on the combustion and on the CO2 capture efficiencies were investigated. Fast reaction rates of oxygen generation were observed with the oxygen-carrier and full combustion of coal was attained in the plant using a solids inventory≈235kg/MWth in the fuel-reactor. In addition, values close to 100% in carbon capture efficiency were obtained at 960°C. Results obtained are analyzed and discussed in order to be useful for the scale-up of a CLOU process fuelled with coal.

Prospects of Al2O3 and MgAl2O4-Supported CuO Oxygen Carriers in Chemical-Looping Combustion (CLC) and Chemical-Looping with Oxygen Uncoupling (CLOU)

The chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) processes are attractive solutions for efficient combustion with direct separation of carbon dioxide. In this work, the feasibility of CuO supported on Al2O3 and MgAl2O4 for CLC and CLOU processes is investigated. The oxygen carriers were produced by freeze-granulation and calcined at 950 and 1050 °C. The chemical-looping characteristics were evaluated in a laboratory-scale fluidized bed at 900 and 925 °C under alternating reducing and oxidizing conditions. Tendencies toward agglomeration, defluidization, and loss of active phase were analyzed by changing the experimental process variables, such as reaction time, temperature, and reducing and inert environments. Complete conversion of methane was obtained for all oxygen carriers investigated in this work. Three out of four oxygen carriers also featured the rapid release of oxygen in an inert environment (CLOU). In the case of Al2O3 as support, a CLC and a CLOU oxygen carrier were obtained depending on the calcination temperature. In addition, analyses of the CuO-Al2O3 phase equilibria system under oxidizing and reducing conditions have been carried out. At the investigated temperatures, it is inferred for the case of Al2O3 as support that part of the active phase (either CuO or CuAl2O4) is bound as CuAlO2 during incomplete reduction with slow kinetics for reoxidation. However, when complete reduction is attained, the original active phase composition is rejuvenated upon oxidation. As a result, the use of CuAl2O4 is suggested for CLC processes from the point of agglomeration and attrition-free functioning of the oxygen carrier. In the case of MgAl2O4 as support, the oxygen carrier exhibited a stable oxygen-releasing behavior due to the existence of relatively intact CuO. Together with the absence of agglomeration and major morphological changes, the use of MgAl2O4-supported CuO is suggested as a suitable oxygen carrier for CLOU processes.

Chemical Looping with Copper Oxide as Carrier and Coal as Fuel

A preliminary analysis has been conducted of the performance of a Chemical Looping system with Oxygen Uncoupling (CLOU) with copper oxide as the oxygen carrier and coal approximated by carbon as the fuel. The advantages of oxygen uncoupling are demonstrated by providing the energy balances, the circulation rate of oxygen carrier, the oxygen carrier mass loadings, the carbon burnout and oxygen partial pressure in the fuel reactor. Experimental data on the cycling of cuprous oxide to cupric oxide and kinetics for the oxidation and decomposition reactions of the oxides were obtained for use in the analysis. For this preliminary study unsupported oxides were utilized. The decomposition temperatures were rapid at the high temperature of 950°C selected for the fuel reactor. The oxidation kinetics peaked at about 800°C with the decrease in rate at higher temperatures, a decrease which is attributed in the literature to the temperature dependence of the diffusional resistance of the CuO layer surrounding the Cu<sub>2<sub/>O; the diffusion occurs through grain boundaries in the CuO layers and the rate of diffusion decreases as a consequence of growth of CuO grains with increasing temperature. The analysis shows the advantages of CLOU in providing rapid combustion of the carbon with carbon burnout times lower than the decomposition times of the oxygen carrier. For the full potential of CLOU to be established additional data are needed on the kinetics of supported oxides at the high temperatures (>850°C) at which oxygen is released by the CuO in the fuel reactor.

Chemical-looping with oxygen uncoupling using combined Mn-Fe oxides, testing in batch fluidized bed

Chemical-looping with oxygen uncoupling (CLOU) has been studied with gaseous and solid fuel in a small fluidized bed batch reactor, using new Fe-Mn-based oxygen carriers. CLOU is a development of chemical-looping combustion, using oxygen carriers with the ability to release oxygen, which can react directly with the fuel. The carbon dioxide from the combustion is inherently obtained as separated from the rest of the flue gases. In this work manganese is combined with iron oxides, giving new bimetallic oxide compounds with different thermodynamic properties compared to pure manganese oxides. Four different combinations of iron manganese oxide have been examined by decomposition in N2 and also reaction with methane and syn-gas. F3, a material with a molar ratio of Fe:Mn of 2:1, showed the best behaviour in terms of its release of oxygen, take up of oxygen, fluidizability and methane conversion. Therefore, F3 was also tested with solid fuel using inert fludization gas, N2. The char particles effectively remove oxygen released as it is converted to CO2. Thus, CO2 will represent the oxygen release. The tests show that the particles release oxygen corresponding to approximately 0.5% of their mass. Moreover, a test where steam was added in the fluidization gas showed high gas conversion, with essentially no unconverted gas. Thus, the tests indicate that the F3 particles, if used in chemical-looping of solid fuels, could contribute both to faster fuel conversion and to higher conversion of gas, as compared to a normal oxygen carrier that does not release oxygen.

Development of CuO-based oxygen-carrier materials suitable for Chemical-Looping with Oxygen Uncoupling (CLOU) process

The Chemical Looping with Oxygen Uncoupling (CLOU) process is a Chemical Looping Combustion (CLC) technology that allows the combustion of solid fuels with inherent CO2 separation using oxygen-carriers. This technology has low energy penalty and thus low CO2 capture costs. The CLOU process is a new option, when the direct use of a solid fuel in a CLC technology is considered. The CLOU process takes advantage of the capability of some metal oxides to evolve gaseous oxygen at high temperatures. The oxygen-carrier particles for CLOU process are based on those metal oxides. The oxygen generated by the metal oxide reacts directly with the solid fuel, which is mixed with the oxygen-carrier in the fuel-reactor. The selection of suitable oxygen-carriers is a key factor for the CLOU technology development. The aim of this work was to produce and characterize oxygen-carrier materials based in CuO with high oxygen transfer capability, reactivity and good fluidization properties. Several oxygen-carriers were prepared with different CuO contents, using different inert supports and using different preparation methods (incipient wetness impregnation, mechanical mixing + extrusion, and mechanical mixing + pelletizing by pressure). The reaction rate for oxygen generation (decomposition) and regeneration were determined carrying out successive cycles in a TGA system. In this way it was determined the chemical suitability of the materials. Selected materials were tested by redox decomposition-regeneration cycles in a batch fluidized bed reactor working at different temperatures and reacting atmospheres. The fluidization behaviour against agglomeration and attrition during a high number of cycles was determined. A window of suitable operating conditions for the CLOU process using CuO oxygen-carriers was found.

Combined manganese/iron oxides as oxygen carrier for chemical looping combustion with oxygen uncoupling (CLOU) in a circulating fluidized bed reactor system

Two kinds of particles consisting of mainly manganese and iron oxides have been examined as oxygen carrier for chemical-looping combustion with O2 uncoupling in a circulating fluidized-bed reactor. The first was produced by spray drying and consisted of 66.8 wt% iron oxide and 33.2 wt% manganese oxide. The second was a manganese ore which also contained iron oxide and silica oxide. During O2 uncoupling experiments, both materials were found to release O2 in gas phase at temperatures above 850 °C, when fluidized with CO2. 7–8 h of continuously operating experiments were recorded for each oxygen carrier, and it was found that the O2 release increased with increased reactor temperature. At 1000 °C, the O2 concentration in the outlet from the fuel reactor was in the order of 7.5 vol% for the synthetic particles. For the ore, the O2 concentration was roughly 0.7 vol% at 990 °C. Further, chemical-looping combustion experiments with natural gas as fuel were carried out. While the conversion of fuel to CO2 and H2O initially was very high (96%) for the synthetic particle and decent (75%) for the ore, both oxygen carriers were found to erode into dust during combustion experiments. Some of the ore particles also swelled greatly. The solids circulation stopped abruptly after 4 h of combustion experiments for the synthetic particle, and after 2 h for the ore. In both cases, the stoppage was likely associated with the physical breakdown of the particles. It is concluded that combined oxides of manganese and iron have very interesting thermodynamical properties and could potentially be suitable for chemical-looping applications. The physical and chemical stability of such materials will have to be further studied and improved though.