Oxygen Carrier For Chemical Looping Combustion Research Articles

Mechanism of phase segregation in ilmenite oxygen carriers for chemical-looping combustion

The solid particle of oxygen carrier (OC) plays a vital role in chemical looping combustion (CLC) for fossil fuel combustion and inherent carbon capture. Phase segregation or active metal surface layer formation upon redox cycle had been recently noticed to be a predominant factor causing complete failure of OCs. To explored and identified the mechanism behind rapid phase segregation and external iron-rich shell formation of ilmenite OCs during CLC process, this study investigated into OCs’ phase and element distributions, and surface layer morphologies at various redox cycles, and the cross-sectional microstructures of oxidizing OC particles with different exposure times. The detailed information on the exterior Fe-rich shell formation and the progression of phase segregation were visualized, which provided vital information for the new mechanism of rapid Fe phase migration. The new finding ruled out Kirkendall effects or the ionic diffusion mechanism. The immediate formation of surface layer was found to initiate the segregation of Fe2O3 form TiO2 phase. The rapid Fe phase migration was attributed to the driving force arising from the increased local pressure of particles and the violent exothermic oxidation on reaction front. The strong driving force could directly eject iron-containing phases from the reaction front to surface layer. In the new mechanism, different phases separated inside the reduced OC particles provided the vital intermediates for the starting points of deep phase segregation or rapid surface layer formation; O2 dissociation of on the metallic Fe outmost layer and the oxygen ion conduction in TiO2 phase seemed to provide a short-circuit diffusion pathway for particle oxidation. Finally, this study also explored the strategies that might succeed in improving the integrity of ilmenite OC particles.

Investigation of LD-slag as oxygen carrier for CLC in a 10 kW unit using high-volatile biomasses

A steel slag from the Linz-Donawitz process, called LD-slag, having significant calcium and iron-fractions, was investigated as an oxygen carrier in a recently developed 10 kWth chemical-looping combustor with three high-volatile biomass fuels. In order to improve operability, the LD-slag was found to require heat-treatment at high temperatures before being used in the unit. In total, operation with the biomasses was conducted for more than 26 h at temperatures of 870–980 °C. The fuel thermal power was in the range of 3.4–10 kWth. The operation involved chemical looping combustion (CLC), chemical looping gasification (CLG) and oxygen carrier aided combustion (OCAC). Around 12 h was in CLC operation, 13.3 h was conducted in CLG-conditions, while the remaining 0.7 h was OCAC. Here, the results obtained during the CLC part of the campaign is reported. Increased temperature in the fuel reactor and higher airflows to the air reactor both lead to better combustion performance. Steam concentration in the fuel reactor has little effect on the performance. The LD-slag showed higher oxygen demand (31.0%) than that with ilmenite (21.5%) and a manganese ore (19.5%) with the same fuel and normal solids circulation. However, with the LD-slag, there is possibility to achieve a lower oxygen demand (15.2%) with high solids circulation.

Operational experiences of chemical-looping combustion with 18 manganese ores in a 300W unit

Chemical-looping combustion is a novel combustion technology with inherent CO2 capture. The process uses oxygen carriers in the form of metal oxide particles to transfer oxygen from air to fuel. The particles make up the bed material in two fluidized-bed reactors, the air reactor and the fuel reactor, and circulate between the two reactors. Natural minerals of low cost are attractive as oxygen carriers in chemical-looping combustion (CLC), in particular when used for combustion of solid fuels. The presence of ash can restrict the effective lifetime of the oxygen carrier either by loss of bed material associated with the ash removal or by direct reactions between ash and oxygen carrier that impair its reactivity. Independent of the presence of ash, the oxygen carrier lifetime can be limited by attrition leading to loss of fines.Ores considered and used in chemical-looping combustion include ilmenite, iron ore and manganese ore. Manganese ore is the least tested of these, although several studies suggest manganese ores often have higher reactivity as compared to the other two. The present study compares data from operation of 18 different manganese ores in a 300 W chemical-looping combustor, involving 329 h of operation with fuel. Results for 10 of these, involving 148 h of operation, have previously not been published. Some of these manganese ores have also been used in larger pilots, as well as in a 10 MW circulating fluidized-bed boiler.Operational results indicate significant differences between the ores with respect to performance, with syngas conversion ranging between 80 and 100% and methane conversion ranging between 17 and 59% and attrition rates ranging from very high to as low as 0.05%/h. For a few ores formation of fines led to operational failure after only a short period with fuel and for one of the ores agglomeration led to failure.The correlation between performance data and oxygen-carrier characteristics, including elementary analysis, was assessed. Gas conversion for both syngas and methane were correlated to gas conversion in lab testing. However, neither jet cup attrition data nor crushing strength was correlated to attrition in 300 W. This suggests that the mechanisms causing attrition are different at hot conditions and with reactions taking place, which emphasizes the need for pilot testing in the screening of manganese ore oxygen carriers.Fortunately, the correlation between gas conversion and attrition was weak. Thus, high reactivity is not necessarily associated with low attrition assistance and vice versa and several ores show high reactivity in combination with low or moderate attrition. Consequently, screening of manganese ores is well worth while, in order to find materials that can give both high conversion and long life-time. The best four ores were the Chinese Guizhou, South-African UMK, Elwaleed B, and Sibelco´s Braunite having syngas conversion(%)/attrition rate(%/h) of 98.3/0.05, 100/0.33 100/0.5 and 96.7/0.12, respectively.

Exploring depolymerization mechanism and complex reaction networks of aromatic structures in chemical looping combustion via ReaxFF MD simulations

Uncovering the reaction mechanism of various fuels to chemical looping combustion (CLC), especially depolymerization of solids in a complex environment, is an important scientific issue. Aromatic structures are the backbone of solid fuels such as coal and biomass. However, their depolymerization mechanisms during CLC remain unclear, and few complex reaction networks have been established to date. This work presents molecular insights into reaction behaviors and mechanism for several aromatic structures in the fuel reactor of CLC. The distribution and evolution of reactants, important intermediates, and products in the CLC are studied in details by means of reactive force field molecular dynamics (ReaxFF MD) simulations. It is found that the reaction process of aromatic fuels reacted with Fe2O3 as oxygen carriers in CLC can be divided into four stages. The decomposition rate of fuel molecules is relevant to the reaction temperature and aromatic sizes. The higher of reaction temperature and the smaller aromatic sizes, the faster the conversion of fuel molecules. The amount of CO and H2O produced gradually increases during CLC process, and however shows a decreasing trend in the later stages. The higher the CLC temperature is, the more favorable the oxygen releases from the oxygen carrier lattice are, so that the fuel molecules can burn fully to produce more CO2. The fuel molecular structure has less effect on generation time of CO molecule, while more on that of H2O molecule. The initial reactions of aromatic structures in CLC involve competition between thermal cracking and oxidation reactions. Fuel molecules generally go through a thermal ring-opening step first, followed by continuous bonding dissociation and oxidation to produce numerous structural fragments. The reactions to aromatic structures become more complex with an increasing number of aromatic rings and a wider variety of intermediates. By visualizing the ReaxFF MD results, complex reaction networks of different aromatic molecules in CLC are systematically established.

Computational design and experimental validation of transition-metal doped CuFe2O4 as oxygen carriers in chemical-looping combustion

Owing to low cost and minimal energy penalty, chemical-looping combustion (CLC) is regarded as a promising combustion technology for fossil fuel utilization and CO2 capture. As oxygen carrier plays a key role in delivering oxygen and heat, the development of oxygen carrier with high performance is fundamental for the application of CLC. Here, a tradeoff between formation energy and transportation barrier of oxygen vacancy was found for the rational design of spinel CuFe2O4-based oxygen carriers by using a new descriptor (Φ, coupling electronegativity and atomic radius). The reactivity of CuFe2O4 is found to be strongly relevant to the proposed descriptor Φ. Compared with the undoped CuFe2O4, the formation energy and transportation barrier of oxygen vacancies in doped CuFe2O4 are reduced, which implies that oxygen migrates more easily in doped CuFe2O4. Cr and Co dopants with great potential are screened out, and they can reduce the energy barrier of CO oxidation over CuFe2O4 from 47.91 kJ/mol to 20.19 kJ/mol and 21.54 kJ/mol, respectively. Thermogravimetric analysis (TGA) experiments show that Cr dopant can improve the low-temperature reactivity of CuFe2O4, while Co dopant can facilitate the transition of CuFe2O4 to Fe3O4 and Cu (1.5CuFe2O4 + 2CO → 1.5Cu + Fe3O4 + 2CO2).

Selecting and Testing of Cement-Bonded Magnetite and Chalcopyrite as Oxygen Carrier for Chemical-Looping Combustion

Combining iron and copper ores can generate an oxygen carrier that has a synergic effect of high temperature resistance and high reactivity. In this work, typical cements available in the market were studied as binders to bind magnetite and chalcopyrite to develop a suitable oxygen carrier for chemical-looping combustion (CLC). A first selection step suggested that an aluminate cement, namely CA70, could favor the generation of oxygen carrier particles having good crushing strength, good particle yield, and high reactivity. The CA70-bonded oxygen carrier was then subjected to cyclic tests with CH4, CO, and H2 in reduction and in air oxidation at temperatures of 850, 900, and 950 °C with gas concentrations of 5, 10, 15, and 20% in a batch-fluidized bed reactor. The increase in temperature promoted the fuel conversion. At 950 °C, the conversions of CH4 and CO reached up to 80.4% and 99.2%, respectively. During more than 30 cycles, the oxygen carrier kept a similar reactivity to the fresh carrier and maintained its composition and physical properties. The oxygen transport capacity was maintained at 21–23%, and the phases were CuO, Fe2O3, Al2O3, and minor CaS. In the used sample, some grains were observed, but the morphology was not greatly changed. Agglomeration was absent during all the cycles, except for the deep reduction with H2.

A novel thermally stable Fe2O3/Al2O3 nanofiber-based oxygen carrier for chemical-looping combustion

A novel thermally stable Fe2O3/Al2O3 nanofiber-based oxygen carrier for chemical-looping combustion

Insights into reactive behaviors and mechanisms of nickel-based oxygen carriers doped by Fe/Co during chemical looping combustion from multiple-scale molecular modeling combined with experiments

Understanding reactive behaviors and mechanisms of oxygen carriers (OCs) is a key issue in chemical looping combustion (CLC), which can provide ideas and routes not only for constructing high-performance OCs, but also for designing reactors and systems intensification. However, various points about the reaction behaviors and oxygen release mechanism of iron/cobalt-doped nickel-based OCs in CLC remain unclear and highly desired to elucidate. Therefore, this work determines reactive behaviors and mechanisms of Fe/Co-doped nickel-based OCs using multiple-scale molecular modeling and experiments. The interaction between molecules and OCs surfaces was characterized using independent gradient modulus to directly visualize interaction region, type, and intensity; they were further quantified through energy decomposition analysis. The reaction path and coordinates in elementary reactions were explored based on DFT calculations, and the structures, energies, and electronic properties were obtained. According to calculated reaction energy barriers, Fe doping enhanced activity of nickel-based OCs, whereas Co doping reduced. Furthermore, nickel-based OCs were prepared and tested using thermo-gravimetric analysis (TGA) and temperature-programmed H2 reduction. Despite slight numerical differences, TGA-based apparent activation energies agreed well with DFT predications. Additionally, the effects of chemical bonding, surface structure, particle size and defects of OCs on the intrinsic and non-intrinsic reactive properties were revealed based on material characterizations.

Studies on the synergistically improved reactivity of spinel NiFe2O4 oxygen carrier for chemical-looping combustion

The investigation of the synergistically improved reactivity of spinel NiFe2O4 oxygen carrier during chemical-looping combustion was conducted by the thermogravimetric analysis (TGA) experiments and implemented within the density functional theory (DFT) calculations. TGA results demonstrated that spinel NiFe2O4 could be directly reduced into Ni–Fe alloy in the CO atmosphere. The reaction rate of NiFe2O4 showed two times faster as compared with Fe2O3. The increase of reaction temperature, CO concentration and heating rate can boost the reaction rate of NiFe2O4. The oxygen vacancy formation energy is a good indicator for the reactivity of lattice oxygen in NiFe2O4. DFT calculations indicate that the lattice oxygen coordinated with Ni atom shows higher reactivity than that with Fe atom. The reactivity of lattice oxygen in NiFe2O4 is primarily owing to the coordination environment of oxygen formed by different Ni/Fe atoms, which is not only related to the type and number of coordination metal atoms, but also correlated with the surface structure. It can be found that the calculated results are in good agreement with the improved reactivity of NiFe2O4 oxygen carrier that has been observed in the TGA experiments. These results are of importance to generally understand the synergistically improved reactivity in the spinel NiFe2O4 oxygen carrier.

Fluidized bed processes and catalyst systems for Fischer-Tropsch conversion

Some binary and ternary combined manganese oxides of Mn with one or two additional metals or metalloids of Fe, Si, Ca and Mg were investigated as oxygen carriers for chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU). More specifically the following systems were investigated: (1) MnyMgOx, (2) CaMnO3–(Fe0.25Mn0.75)2O3, (3) CaMnO3–(Fe0.67Mn0.33)2O3, (4) CaMnO3–MnMgOx, (5) MnMgOx–(Fe0.25Mn0.75)2O3 and (6) Mn2SiOx–Fe2SiOx. The general trend was that the binary systems, where two metals are used in the formulation showed the most promising results in terms of oxygen uncoupling and reactivity. However, there are several ternary combinations which show a combination of high oxygen uncoupling, reactivity with methane and reasonable strength. A pseudo first-order effective rate constant was evaluated for the investigated particles. The measured rates are lower than for benchmark nickel oxide and calcium manganites, but higher than for ilmenite. The ternary System 6, Mn2SiOx–Fe2SiOx was investigated in more depth, including solid fuel experiments to determine the rate of oxygen release. At 850 and 900 °C (Mn0.5Fe0.5)2SiOx had the highest average reactivity, with a maximum average yield of 91.5% at 850 °C. On the other hand, at higher temperatures, the particles with the highest Mn content showed best behavior, i.e. (Mn0.67Fe0.33)2SiOx. Reactivity experiments with char in the FB reactor with this OC showed that the oxygen capacity for CLOU was high, 3.5 wt% at 950 °C, with a maximum release rate of 0.2 g O2/kg OC,s. The low rate of uncoupling means that the experiments with gaseous fuel were likely dominated by the direct gas–solid reaction, and not CLOU. X-ray powder diffraction suggests that the main reaction path is via (MnxFe1−x)2O3 to (MnxFe1−x)3O4, although the reaction between Mn7SiO12 to MnSiO3 cannot be ruled out as a possible route of oxygen transfer. This was supported by thermodynamic calculations of this multi-component system.

Mercury removal by Co3O4@TiO2@Fe2O3 magnetic core-shell oxygen carrier in chemical-looping combustion

Mercury pollution has received much attention due to its severe harm to human and ecological system. The major source of mercury emission is from coal combustion. In the flue gas, elemental mercury (Hg0) is the most abundant and difficult form to be removed. In the chemical-looping combustion of coal (CLCC), oxygen carrier (OC) plays the role of oxygen storage and release, and it also has catalytic ability for Hg0 removal. In this study, a magnetic material Co3O4@TiO2@Fe2O3 (CTF) with a core–shell structure was prepared by combining sol–gel and impregnation methods. The Co: Ti: Fe ratio was obtained as 1:2:4, which was optimized to achieve strong magnetism and catalytic ability. The Hg0 removal efficiency increased with the increase in temperature in the temperature range of 800–900 °C. The mercury removal efficiency of Hg0 + CTF + HCl was higher than that of Hg0 + HCl, indicating that CTF promoted the conversion of Hg0 to Hg2+/HgP. Both homogeneous and heterogeneous reaction pathways are important for Hg0 removal. CTF promotes the oxidation of Hg0 in HCl atmosphere by reacting with HCl to produce more Cl2, which is more reactive towards Hg0. Furthermore, Hg0 is adsorbed on CTF to form adsorbed Hg0 and HgO. Mercury on CTF is mainly desorbed in the form of HgO, which is beneficial for mercury removal. Ten redox cycles were applied on CTF, which still maintained a high Hg0 removal efficiency and strong magnetism. CTF can be used as a recycled OC in CLCC.

Competition between reduction reaction and carbon deposition for CaSO4 oxygen carrier in chemical-looping combustion: A first-principle study and reaction equilibrium analysis

To understand the reaction mechanism of chemical looping combustion with calcium-based oxygen carrier, the competition mechanisms between reduction reaction and carbon deposition for CaSO4 oxygen carrier were calculated by density functional theory and simulated by chemical reaction equilibrium analysis. In this paper, the reaction paths of reduction reaction and carbon deposition at different CaSO4(010) surface outer oxygen content (100%, 75%, 50%, 25%) were studied, and the reversibility of the reaction was fully considered. Furthermore, based on the kinetic analysis of the transition state theory, the kinetic parameters of the elementary reaction were obtained. Moreover, the reaction equilibrium simulation revealed the competition of reactions under different conditions. The results show that the energy barriers of forward and reverse reactions decrease with the oxygen content decreasing on the CaSO4 (010) outer layer surface in the reduction reaction. Besides, the forward energy barriers at each stage are lower than the reverse energy. In the carbon deposition reaction, the energy barriers of the forward reaction are higher than the reverse reaction, and the energy barriers of the reverse reaction gradually increase with oxygen content decreasing. The kinetic analysis results show the rate-limiting steps of forward and reverse reaction in reduction reaction are 75% and 25% oxygen content stage. According to the reaction equilibrium, the conversion rate of CO oxidation to CO2 decreases with temperature increasing, and the conversion rate of carbon deposition increases. The rate of CO oxidation to CO2 increase and the occurrence of carbon deposition reaction decrease with oxygen carrier ratio increasing. Thus, when the oxygen carrier is insufficient or most of CaSO4 is reduced, the carbon deposition reaction occurs easily.

Innovative laboratory unit for pre-testing of oxygen carriers for chemical-looping combustion

Chemical-looping combustion (CLC) is a highly efficient CO2 separation technology with no direct contact between combustion air and fuel. A metal oxide is used as an oxygen carrier (OC) and acts in a dual fluidized bed as a separation tool and supplies the fuel with oxygen, which as an oxidation medium causes combustion to CO2 and H2O. The use of solid fuels, especially biomass, is the focus of current investigations. The OC plays a key role, because it must meet special requirements for solid fuels, which are different to gaseous fuels. The ash content, special reaction mechanisms, and increased abrasion make research into new types of OC essential. Preliminary testing of OC before their use in larger plants regarding their suitability is recommended. For this reason, this work shows the design and the results of a laboratory reactor, which was planned and built for fundamental investigation of OC. Designed as a transient fluidized bed, the reactor, equipped with its own fuel conveying system and an in situ solid sampling, is intended to be particularly suitable for cheap and rapid pre-testing of OC materials. During the tests, it was shown that the sampling device enables non-selective sampling. Different OC were tested under various operating conditions, and their ability to convert different fuels could be quantified. The results indicate that OC can be sufficiently investigated to recommend operation in larger plants.

The effect of Niobium oxide as a promoter on -Al2O3 based oxygen carrier for chemical looping combustion

Chemical looping combustion (CLC) is known as a low-cost strategy for the capture of carbon dioxide for fuel combustion. In CLC process, oxygen carriers (OCs) are the cornerstone and play a vital role in defining reaction process. The aim of the present work was to investigate the potential of niobium (Nb) as a promoter on the -Al2O3 and its behavior as an oxygen carrier for chemical looping combustion process. In this work, the Nb loading was varied between 5 – 15 wt. %. Their behavior in CLC was analyzed by TPR, TPO, SEM and TGA. From the TPR results, the reduction temperature for 5 wt.%, 10 wt.% and 15 wt.% Nb loading were 560 ℃, 529 ℃ and 545 ℃, respectively which indicated that reduction reaction occurs around 500 ℃ and above. SEM analysis showed that increasing of Nb loading resulted in some agglomeration and thus lowering the ability of metal oxide to gain and release oxygen. The redox characteristics were carried out using TGA with 5% CH4/N2 was used as the reducing gas, while air was used as oxidizing gas. The highest oxygen transfer capacity was 3.0% which is presented by 5 wt.% of Nb loading. Since the addition of Nb successfully improved the oxygen transport capacity, it can be concluded that Nb is the potential candidate for oxygen carrier in CLC.

Understanding the Interaction of Potassium Salts with an Ilmenite Oxygen Carrier Under Dry and Wet Conditions.

This study describes how potassium salts representative of those in bio ash affect the reactivity of the oxygen carrier ilmenite under moist and dry conditions. Ilmenite is a bench-mark oxygen carrier for chemical-looping combustion, a technique that can separate CO2 from flue gases with minimal energy penalty. Different potassium salts were mixed with ilmenite to a concentration of 4 wt % potassium. The salts used were K2CO3, K2SO4, KCl, and KH2PO4. Experiments were performed at 850 °C under alternately oxidizing and reducing conditions in a dry atmosphere or in the presence of steam. Analyses of the oxygen carrier regarding changes in reactivity, structure, and composition followed the exposures. This study showed that salts such as K2CO3, K2SO4, and KCl increase the reactivity of the ilmenite. For the samples mixed with KCl, most of the salt was evaporated. KH2PO4 decomposed into KPO3, forming layers around the ilmenite particles that lead to agglomeration. Additionally, the KPO3 layer was more or less nonpermeable for CO and decreased the reactivity toward H2 significantly in both dry and wet conditions. This decreased reactivity indicates that the concentration of phosphorus in biofuel may have a significant effect on oxygen carrier degradation. It was also observed that the presence of steam changed the chemistry drastically for the nonphosphorus-containing salts. Alkali salts may react with steam, forming volatile KOH that evaporates partly. KOH may also form K-titanates by reaction with the oxygen carrier, leading to segregation of iron and titanium phases in the ilmenite.

Hierarchical Structural Changes During Redox Cycling of Fe-Based Lamellar Foams Containing YSZ, CeO2, or ZrO2.

Several high-temperature energy conversion and storage technologies rely on redox cycling of Fe-based materials, including storage materials in solid-oxide Fe-air batteries and oxygen carriers in chemical-looping combustion. The materials' macroporosity necessary for gas flow is, however, irreversibly diminished during redox cycling due to (i) large volume changes during the redox transformations, (ii) foam sintering at elevated operating temperature (550-900 °C), and (iii) formation and growth of Kirkendall microporosity. To address these challenges, we use directional freeze-casting to create highly porous, lamellar, Fe-composite foams containing uniformly distributed sintering inhibitor (SI) particles-either Y2O3-stabilized ZrO2 (YSZ), CeO2, or ZrO2-at 0, 5, 10, or 15% of the solid volume. We characterize these foams before, during, and after redox cycling (Fe/FeO/Fe3O4, via H2O and H2) at 800 °C using operando synchrotron X-ray microtomography, metallography, and scanning electron microscopy. Shrinkage of the foam volume and formation of a gas-blocking shell surrounding the foam are reduced as the SI fraction increases. Volumetric shrinkage after the first five redox cycles is decreased from 66% (for pure-Fe foams) to 45% (for all Fe-composites containing 5 vol % SI). Foams containing 15 vol % YSZ show no volumetric shrinkage after five cycles, although, after 20 cycles, they have shrunk 53%. Post-cycling analysis reveals segregation of the SI particles to the cores of individual lamellae, surrounded by thick layers of sintered Fe on the lamellae surfaces. This segregation occurs due to Fe diffusion through FeO to the lamellae surfaces during oxidation, leaving behind the SI particles, which are then pushed into clusters by FeO/Fe3O4 contraction during reduction. The SI is thus rendered ineffective, which explains why foam densification is delayed (compared with pure-Fe foams), rather than fully prevented, after repeated cycling.

Evaluation of performance of copper converter slag as oxygen carrier in chemical-looping combustion (CLC)

In this study, copper converter slag sample was utilized as oxygen carrier (OC) for Chemical Looping Combustion (CLC) of synthetic syngas. The CLC cycles were substantiated in a bench-scale fluidized bed reactor by altering the inlet atmosphere between fuel and air consecutively. The tests were conducted at 4 times the minimum fluidization velocity, and 1123 K (850 °C) reduction half-cycle temperature (TRHC) for 50 cycles. The mean fuel gas conversion values obtained during 30 s of RHC time (tRHC) were as; 78% for 1:1 CO in nitrogen, 84–89% for syngas, and 97% for 1:1 H2 in nitrogen. Copper converter slag sample was stable and sufficiently reactive throughout the tests, moreover, the reactivity increased substantially by the consecutive CLC cycles. The increase of the porosity and the formation of the copper ferrite during the CLC cycles were found to be the main contributors to the activation of the copper converter slag. It is concluded that the evaluation of copper converter slag in the CLC process is both an economical option for OC supply (especially in coal-fueled CLC processes where some OC loss is inevitable), and an environmentally favorable alternative for recycling copper converter slag unless wasting the precious metals in it.

Performance of electric arc furnace slag as oxygen carrier in chemical-looping combustion process

The aim of this study was to assess the feasibility of an electric arc furnace slag (EAFS) sample from Turkey as oxygen carrier (OC) in syngas-fueled chemical-looping combustion (CLC) process. The CLC tests were performed at 800 °C fluidized bed conditions for 50 cycles using CO and H2 gases to constitute fuel atmospheres. EAFS exhibited an “activation behavior” which was characterized by a substantial increase on reactivity throughout the first 15 cycles of all tests. After that point, the reactivity of the OC became almost stable and more than 85% mean fuel gas conversion was obtained in all tests. Considering the abovementioned fuel conversion efficiency and the low cost of the EAFS it is concluded that, provided that the iron content can be increased by means of floating or magnetic separation techniques, the utilization of EAFS as OC may be both environmentally benign and favorable in terms of CLC technology.

Effect of Preparation Method on the Structural Characteristics of NiO-ZrO2 Oxygen Carriers for Chemical-looping Combustion

Chemical-looping combustion(CLC) offers an effective approach for power generation and CO2 capture. In this work, an NiO-ZrO2 oxygen carrier prepared by three methods was subjected to an optimal reaction temperature test, an optimal flow test and a cyclic redox reaction test to explore the most suitable reaction conditions. Through comparative analysis, it is noted that the coprecipitation method is not suitable for the preparation of this NiO-ZrO2 oxygen carrier, while the oxygen carrier prepared by the mechanical mixing method and solution combustion method obtained a higher CH4 conversion rate and CO2 selectivity. In addition, these two oxygen carriers also showed high stability during successive CLC testing. Therefore, both the mechanical mixing method and the solution combustion method can be used to prepare NiO-ZrO2 oxygen carriers.

Insights into the relationship between microstructural evolution and deactivation of Al2O3 supported Fe2O3 oxygen carrier in chemical looping combustion

The Fe2O3-Al2O3 oxygen carrier is regarded to a promising candidate for chemical looping combustion (CLC). However, Fe2O3-Al2O3 oxygen carrier was often subjected to a decrease of oxygen carrying capacity. The deactivation mechanism of Fe2O3-Al2O3 oxygen carrier in chemical looping combustion was investigated over consecutive 20 cycles. The Fe2O3-Al2O3 oxygen carrier exhibited relatively poor redox stability in a fixed bed reactor. The physico-chemical properties of cycled Fe2O3-Al2O3 oxygen carrier were characterized by different characterization methods. No carbon deposit was detected in the reduction process, indicating that carbon deposition was not the cause for Fe2O3-Al2O3 oxygen carrier deactivation under this experimental condition. The characterization results showed that the deactivation of Fe2O3-Al2O3 oxygen carrier in redox cycles was mainly attributed to thermal sintering. The cycled Fe2O3-Al2O3 oxygen carrier showed lower surface area, lower reducibility and oxygen mobility than that of fresh sample. After a certain number of redox cycles, Fe diffusion and enrichment at the surface of cycled oxygen carrier was observed. The surface Fe is unstable under high reaction temperature, leading to accelerate thermal sintering. A deactivation mechanism of Fe2O3-Al2O3 oxygen carrier in CLC was proposed based on the Fe diffusion and surface sintering.