Reactivity Of Oxygen Carrier Research Articles

Oxygen vacancy modulation for enhanced hydrogen production via chemical looping water-gas shift

Chemical looping water gas shift (CL-WGS)is prospective to generate high-purity hydrogen with integrated CO2 capture. However, this technology is impeded by the lack of active oxygen carriers at mid-temperatures. Here, we synthesized several Ni-doped CoFe2O4-δ to modulate oxygen vacancies and investigate its effect on promoting hydrogen production reaction via chemical looping water gas shift at 650 °C. The findings delineate that doping Ni considerably lowers the energy barriers associated with the oxygen vacancies formation, thereby augmenting their concentration. The underlying mechanism elucidates that within the CL-WGS process, the transfer of lattice oxygen acts as the rate-limiting step. NixCo1-xFe2O4 lowers the formation energy of oxygen vacancies and facilitates the bulk lattice oxygen diffusion through the bulk. Hence, Ni0.5Co0.5Fe2O4 demonstrates the most reduction depth and reversibility via redox reactions, resulting in an elevated hydrogen yield (∼15.5 mmol g−1) at 650 °C, which surpasses the yield from undoped CoFe2O4 by 1.4 times. This performance remains consistently high with only a minimal decline over 100 cycles. The findings introduce a promising approach to promote the reactivity of oxygen carriers, particularly for mid-temperature applications.

Effect of the Conversion Degree on the Apparent Kinetics of Iron-Based Oxygen Carriers.

The role of the oxygen carrier is important in energy conversion processes with fluidized beds, particularly chemical looping technology. It is necessary to establish the relevant kinetics of oxygen carriers that can be applicable for various chemical looping processes. In this study, we analyzed the apparent kinetics of three iron-based oxygen carriers, namely, ilmenite, iron sand, and LD slag, during the conversion of CO, H2, and CH4 in a fluidized bed batch reactor. The effect of both the oxidation degree, presented as the mass conversion degree, and temperature was considered. The results show that the changing grain size (CGS) model is generally applicable in predicting the apparent kinetics of reactions between the investigated iron oxygen carriers and gaseous fuels even at lower oxidation degrees (3-5 wt % reduction). The activation energies of the investigated materials in the conversions of CO, H2, and CH4 obtained from the fittings of the CGS model are about 51-92, 55-251, and 72-211 kJ/mol, respectively. Both the mass conversion degree and temperature influence the reactivity of oxygen carriers in a directly proportional way, especially at temperatures higher than 925 °C. The results of this study are useful for reaction engineering purposes, such as designing a reactor, in chemical looping units, or in any other processes that use oxygen carriers as a bed material.

Experimental and numerical investigation of chemical-loop steam methane reforming on monolithic BaCoO3/CeO2 oxygen

Chemical looping steam methane reforming (CL-SMR) is a promising approach to co-production of syngas and hydrogen. As the CL-SMR process relies on the redox reaction of the oxygen carriers, the reactivity of oxygen carriers is crucial in the performance of CL-SMR. In this study, a monolithic BaCoO3/CeO2 oxygen carrier was synthesized, and the reaction performance was evaluated in CL-SMR. The kinetic parameters were determined based on Sestak-Berggren model, an unsteady-state CFD model was established to predict the dynamic reaction processes in three reaction stages of CL-SMR. The result suggests that the flow rate of inlet gas in reaction stages could affect the mass transfer during gas-solid reactions, and then change the evolution and distribution of reaction rate in monolithic oxygen carrier. Increasing the flow rate could improve the uniformity of reaction rate distribution and decrease the time consumption for cyclic process, but decreased the concentration of gas productions and increased the burden of gas product separation at the same time. Among three reaction stages, the conversion of oxygen carrier was accelerated with the increase of temperature, and the varied half-life time of conversions suggested that the reactivity in RS stage is intense and the reactivity in AC stage is mild.

Test operation of microwave-assisted chemical looping gasification for water hyacinth with a lean iron ore as oxygen carrier

Chemical looping gasification has been considered as a promising biomass energy conversion technology for syngas production. In this work, a novel microwave-assisted chemical looping gasification (MACLG) reactor system is applied to water hyacinth with a lean iron ore as the oxygen carrier (OC). This approach is expected to achieve effective gasification performance with fast heating and small temperature fluctuation. MACLG can promote the proportion of syngas to 74.39%, lower heating value to 11.83 MJ/Nm3 and cold gasification efficiency to 80.94%. In addition, the increase of OC has a great effect on the conversion of water hyacinth. Favorable gasification performance, including total gas yield, H2/CO ratio, carbon conversion efficiency, and OC reactivity, can be achieved when the molar ratio of Fe2O3/C is in the range of 0.25–0.50. Furthermore, a moderate temperature is also vital for reaction effect. With a temperature of 900 °C, the highest H2/CO ratio of 1.37 of syngas is obtained.

Unraveling the atomic interdiffusion mechanism of NiFe2O4 oxygen carriers during chemical looping CO2 conversion

AbstractBy employing metal oxides as oxygen carriers, chemical looping demonstrates its effectiveness in transferring oxygen between reduction and oxidation environments to partially oxidize fuels into syngas and convert CO2 into CO. Generally, NiFe2O4 oxygen carriers have demonstrated remarkable efficiency in chemical looping CO2 conversion. Nevertheless, the intricate process of atomic migration and evolution within the internal structure of bimetallic oxygen carriers during continuous high‐temperature redox cycling remains unclear. Consequently, the lack of a fundamental understanding of the complex ionic migration and oxygen transfer associated with energy conversion processes hampers the design of high‐performance oxygen carriers. Thus, in this study, we employed in situ characterization techniques and theoretical calculations to investigate the ion migration behavior and structural evolution in the bulk of NiFe2O4 oxygen carriers during H2 reduction and CO2/lab air oxidation cycles. We discovered that during the H2 reduction step, lattice oxygen rapidly migrates to vacancy layers to replenish consumed active oxygen species, while Ni leaches from the material and migrates to the surface. During the CO2 splitting step, Ni migrates toward the core of the bimetallic oxygen carrier, forming Fe–Ni alloys. During the air oxidation step, Fe–Ni migrates outward, creating a hollow structure owing to the Kirkendall effect triggered by the swift transfer of lattice oxygen. The metal atom migration paths depend on the oxygen transfer rates. These discoveries highlight the significance of regulating the release–recovery rate of lattice oxygen to uphold the structures and reactivity of oxygen carriers. This work offers a comprehensive understanding of the oxidation/reduction‐driven atomic interdiffusion behavior of bimetallic oxygen carriers.

Density functional theory study on Support-Promoted CH4 reforming in Ni-Based oxygen carriers for chemical-looping conversion

Metal oxides play a pivotal role in the conversion of carbonaceous fuels. Nickel-based oxides, due to their affordability and high C–H activation activity, are regarded as promising oxygen carriers (OCs) for CH4 chemical looping technologies. Additives like Al2O3, ZrO2, and MgAl2O4 are used to boost the reactivity of OCs and mitigate agglomeration of OC particles at elevated temperatures. Despite extensive experimental studies on support selection, the influence of support materials on CH4 conversion remains unclearly. This research analyzed and compared the kinetic pathways of CH4 conversion on the supported and unsupported nickel-based OCs, utilizing the Density Functional Theory (DFT) model, to determine the rate limiting step (RLS) in the chemical looping process. The most promising support, NiAl2O4, weakens the Ni-C bond due to pronounced interaction between the OCs and supports, thereby leading to a reduced activation energy barrier for CH4 dehydrogenation. By probing the relationships between reaction kinetic barriers and critical electronic properties of OCs, a screening strategy is proposed to accelerate the development and optimization of novel OC/support materials for CH4 chemical looping conversion.

Interactions of Mg-Fe-Al-O oxygen carriers with rare earth dopants (Ce, Y, Sm, La, and Pr) in chemical looping steam reforming

Chemical looping steam reforming (CLSR) is a novel process for syngas and hydrogen co-production. In this work, rare earth metals (Ce, Y, Sm, La, and Pr) were doped to improve the reactivity and cyclic stability of the Fe-based oxygen carrier for CLSR. The cyclic tests indicate that the oxygen carrier doped with rare earth possesses a higher CH4 conversion rate, oxygen transport capacity, and hydrogen selectivity and yield. The crystal structure, surface morphology, and reactivity of the oxygen carrier were characterized by different analytical methods (e.g., XRD, H2-TPR, XPS, and SEM). The rare earth-doped oxygen carrier shows higher active site dispersion and smaller crystallite size owing to stronger metal-support interactions between the metallic Fe and the rare earth. XRD and SEM results revealed that the introduction of rare earth increases the oxygen transport capacity and oxygen vacancy concentration, promoting the reactivity of oxygen carriers. However, the rare earth doping reduces the CO purity, possibly because the high reactivity favors full CH4 oxidation. Among the prepared oxygen carriers, Fe10La exhibited the best performance and stability over cycles, with a high CH4 conversion rate (89.2%) and hydrogen purity (95 vol%).

Identification of HCl corrosion mechanism on Cu-based oxygen carriers in chemical looping combustion

In chemical looping combustion (CLC) of coal, HCl has been demonstrated as the primary gaseous products of the chlorine in coal. Therefore, a comprehensive characterization of Cu-based oxygen carriers (OC) by HCl corrosion was meticulously conducted, including the activity retention, composition distribution and apparent morphology. The experiment consisted of three parts: (I) evaluating of the OC performance for 100 redox cycles with 7.8 vol% CH4; (II) investigating HCl-corrosion of OC by adding 50 ppmv HCl with 7.1 vol% CH4 for the subsequent 50 cycles; (III) regenerating of OC without HCl addition during the last 50 cycles. The reactivity of OC was attenuating via the HCl addition during the 101st–150th cycles (part II) due to the competition adsorption between HCl and CH4, especially for the partly reduced OC. While, in part III, the reactivity performance of OC was partially recovered because of the abundant cracks on the surface of OC particles which ensured the sufficient gas–solid contact. It should be noted that the single particle hardness index and service life were irreversibly decreased for the HCl-treated OC. Moreover, OC particles were fragmented and the inside Al2O3 balls were exposed on the surface of the HCl-treated OC in SEM image. This work will contribute to understanding the mechanism of Cl-corrosive OC and designing the Cl-resistant OC.

Fluidized bed chemical looping gasification of sewage sludge with bituminous coal to produce H2 rich syngas: An examination on fuel synergy and reaction conditions

Chemical looping gasification (CLG) is an efficient and economical solution to convert organic solid waste into hydrogen-rich syngas for fossil fuel substitution. In this work, chemical looping co-gasification (CLCG) of sewage sludge (SS) and bituminous coal (BC) with hematite as the oxygen carrier was examined in a fluidized bed. The SS-BC CLCG exhibited significant fuel synergy compared to the SS/BC CLG, enhancing the gasification efficiency and carbon conversion. The effects of several key operating variables including SS blending ratio, reaction temperature, and steam mass flow rate on CLCG performance were investigated and the optimum conditions were 20 wt% SS, 900 °C with a steam mass flow rate of 0.6 g/min. Moreover, a cyclic CLCG test was also performed to evaluate the stability of oxygen carrier reactivity and gasification performance at optimized operating conditions. After 13 cycles, the hematite still presented good or even improved performance with gasification efficiency of 80.94 %, syngas yield of 2.10 Nm3/kgfuel, and carbon conversion of 93.73 %. The improved porous structure and the ash catalyst effect were responsible for the promoted performance of the natural hematite oxygen carrier with increasing cycles. It is concluded that CLG is a promising technology to provide high-quality hydrogen-rich syngas from SS and BC mixtures.

Study on the synergistic effect and oxygen vacancy of CeO2/Fe2O3 oxygen carrier for improving reactivity in carbon monoxide chemical looping combustion

Chemical looping combustion (CLC) is a promising way for CO2 separation and capture. The oxygen carrier (OC) plays a key role in CLC by delivering lattice oxygen for oxidation reaction. Herein, CeO2/Fe2O3 oxygen carrier with oxygen vacancies is successfully synthesized, and the synergistic effect of Ce and Fe in CeO2/Fe2O3 OC is investigated by thermogravimetric analysis (TGA) tests and theoretical calculations. The reactivity of OCs is also tested in a fixed bed. Experimental results show that the CeO2/Fe2O3 oxygen carrier exhibits higher CO oxidation efficiency than CeO2 and Fe2O3, which may be caused by the synergistic effect of Ce and Fe. Moreover, the CeO2/Fe2O3 composite OC shows a downward trend and achieved a weight loss rate of 4.1% which is higher than that of Fe2O3 and CeO2 in the TGA test. In addition, the electron paramagnetic resonance (EPR) spectra of oxygen carrier demonstrates that the typical signal peaks of oxygen vacancies around at g = 2.003 of CeO2, Fe2O3 and CeO2/Fe2O3 OC were observed, which can rapidly facilitate migration of lattice oxygen from interior to the surface of OC due to the existence of oxygen vacancies. Further, the oxygen vacancy formation energy was calculated by DFT, showing that the figure of the CeO2/Fe2O3 OC (3.95 eV) is lower than Fe2O3 (9.56 eV), which indicates that the addition of CeO2 can efficiently reduce the oxygen vacancy formation energy. Finally, based on the results of reactivity and characterization test of OCs, it is demonstrated that the CO CLC reaction with CeO2/Fe2O3 oxygen carrier follows Mars-van Krevelen (MvK) mechanism. This work reveals new insights into the synergistic improvement of the performance of Ce-Fe oxygen carriers with oxygen vacancies.

Lattice oxygen migration of iron-based oxygen carrier during chemical looping reaction process

Chemical looping combustion relies on high performance oxygen carriers (OCs). Iron-based OCs have excellent overall properties, but the disadvantage of low reduction reactivity limits their application prospects. Clarification of oxygen transfer pathways and bulk phase properties is essential to improve oxygen transfer rates and increase the reactivity of OCs. High-resolution transmission electron microscopy, electron energy loss spectroscopy and X-ray photoelectron spectroscopy had been used to study the lattice oxygen transport properties, the Fe and O elemental contents and Fe elemental valence changes at different positions on the radial surface of Fe2O3 OC, as well as the related elemental compositions, atomic valence and surface energy distribution on the surface of the OC during the reduction process of OC. The results show that during the reduction process, the active components at different locations of the OC are simultaneously reduced and lattice oxygen is released. Both internal and external lattice oxygen are involved in the reaction. Besides, reaction interface is believed to be fixed at a OC surface. This study reveals the migration direction of the reduction interface and the mechanism of oxygen release in the bulk phase, and would provide a theoretical basis for the structural design of Fe-based OCs.

Performance of red mud oxygen carriers in chemical-looping hydrogen production using different components of plastic waste pyrolytic gas

This study proposed plastic waste-fueled Chemical-looping Hydrogen Production (CLHP) based on the concept of three-reactor chemical-looping process for the purposes of sustainable treatment of plastic wastes while producing high purity H2 and separating CO2. Oxygen carrier prepared from red mud wastes (Bauxite residues) was suggested to be circulating materials to improve combustion efficiency and H2 yield. The feasibility of using pelletized oxygen carrier prepared from red mud wastes was experimentally investigated with major components (H2, CH4 and C2H4) of plastics pyrolysis gases as fuels. The red mud oxygen carrier composed of around 38 wt% Fe2O3 and 50 wt% inert materials, and CuO-promoted oxygen carriers composed of 8.0 wt% CuO and 92 wt% red mud wastes were used. The two oxygen carriers were examined by parallel experiments, focusing on the aspects of oxygen transport, stability, hydrogen productivity and carbon deposition. Temperature and different components of plastic pyrolysis gases had significant impacts on oxygen transport behaviors; reduction tests using CH4 or C2H4 fuels suggested that the operation temperature of Fuel Reactor should be higher than 800 °C for deep reduction and high H2 productivity. CuO addition significantly promoted oxygen carrier's reactivity and oxygen transport capacity with various fuel components. The cyclic CLHP experiments, performed over 10 redox cycles with C2H4 as fuels, showed that there was only a limited degradation in oxygen carriers caused by carbon deposition and element migration. The developed oxygen carriers can meet the requirements of long-term operation for CLHP process. Carbon deposition on the oxygen carriers derived from hydrocarbon fuel cracking seemed to be inevitable, which experimentally determined in the range of 0.01–0.043 g g−1 oxygen carriers as C2H4 was used as fuels. However, the reactivity of these carbon deposits was extremely low; their initial steam-gasification temperatures were around 750 °C, much higher than the temperature used for water splitting reaction. This provides the potential opportunities for an industrial CLHP process to produce high purity H2 (>97%) despite of carbon deposits presents in Fuel Reactor.

Improving attrition resistance of oxygen carriers by biomass ash in chemical looping process

Chemical looping combustion (CLC) relies on cyclic circulation of oxygen carriers (OCs) between fuel reactor and air reactor to transfer lattice oxygen and heat. OCs’ attrition is an important issue hindering large scale application of CLC technology. Attrition is caused by cracks and abrasion, and method to stop crack propagation is an effective way to mitigate attrition. Biomass ash contains inert and alkali metal components, and it may have the potential to increase the reactivity and reinforce the skeleton structure of OCs. The effects of rape straw ash, corn straw ash and bagasse ash on the reactivity, cumulative attrition rate of red mud OCs were evaluated, and their effects on skeleton structure were analyzed, and finally, the strengthening mechanism of OCs skeleton structure was proposed. The results showed that the rape straw ash rich in K and Ca components promoted the reactivity of OCs. Although the Si-rich bagasse ash decreased the reactivity of OCs, it improved the attrition resistance of OCs. Corn straw ash rich in K and Si components was an effective skeleton strengthening material, which improved both the reactivity and the attrition resistance. The cumulative attrition rate of red mud was 2.74%, and the cumulative attrition rate of CSA sample was 0.35%, which was 87.32% lower than that of OC sample, indicating fine attrition resistance. The fiber structure could strengthen the skeleton of OC, therefore, decreasing attrition rate.

Evaluation of oxygen vacancy and carbon deposition effect at the perovskite on methane adsorption

The perovskite with A-site doping can improve the reactivity of oxygen carrier and fuel so as to promote the performance of chemical looping hydrogen production (CLH). In this study, the effects of Sr doping in the A site of LaFeO3 perovskites on the adsorption of CH4, CH3, CH2 and CH are investigated and the oxidation reaction process on the perovskite surface is measured by means of the quantum chemistry calculation. The formation of oxygen vacancy and carbon deposition on the perovskite surface with A-site doping is evaluated. The results demonstrate that the increase of oxygen vacancy content can promote the charge transfer between the perovskite surface and CH4, CH3 and CH2. Whereas the formation of oxygen vacancy weakens the CH oxidation ability so as to reduce the adsorption energy of CH. Meanwhile, the carbon deposition does not only affect the charge transfer on the perovskite surface, but also alters the adsorption mode. The Sr doping lowers the energy barrier for the CH oxidation reaction, while the formation of carbon deposits makes the oxidation of CH difficult.

Chemical Looping Reforming with Perovskite-Based Catalysts for Thermochemical Energy Storage

The performance of a perovskite-based oxygen carrier for the partial oxidation of methane in thermochemical energy storage applications has been investigated. A synthetic perovskite with formula La0.6Sr0.4FeO3 has been scrutinized for Chemical Looping Reforming (CLR) of CH4 under fixed-bed and fluidized-bed conditions. Temperature-programmed reduction and oxidation steps were carried out under fixed-bed conditions, together with isothermal reduction/oxidation cycles, to evaluate long-term perovskite performance. Under fluidized-bed conditions, isothermal reduction/oxidation cycles were carried out as well. Results obtained under fixed-bed and fluidized-bed conditions were compared in terms of oxygen carrier reactivity and stability. The oxygen carrier showed good reactivity and stability in the range 800–1000 °C. An overall yield of 0.6 Nm3 of syngas per kg of perovskite can be reached per cycle. The decomposition of CH4 catalyzed by the reduced oxide can also occur during the reduction step. However, deposited carbon is easily re-gasified through the Boudouard reaction, without affecting the reactivity of the material. Fluidized-bed tests showed higher conversion rates compared to fixed-bed conditions and allowed better control of CH4 decomposition, with a H2:CO ratio of around 2 and CO selectivity of around 0.8. However, particle attrition was observed and might be responsible for a loss of the inventory of up to 9%w.

Development and performance of CeO2 supported BaCoO3−δ perovskite for chemical looping steam methane reforming

Chemical looping steam methane reforming (CL-SMR) is a promising approach to co-production of syngas and hydrogen, and the reactivity of oxygen carriers is crucial in the performance of CL-SMR. In this study, CeO2 supported BaCoO3-δ oxygen carriers with different component distributions (labeled as dry-mix, sol-gel, co-sol-gel samples) were prepared and evaluated in CL-SMR. The BaCoO3-δ was loosely attached on CeO2 in the dry-mix sample, resulting in a high resistance to oxygen mobility and a low gas production. The co-sol-gel sample possessed impurities of Co3O4, BaCO3 and BaCeO3, which decreased reaction capacity and enhanced the secondary reaction of methane decomposition, leading to an excessive H2/CO ratio of syngas and coke formation. The contact of perovskite and CeO2 in sol-gel sample favors the synergistic effect of perovskite and CeO2, where the H2/CO ratio was close to the ideal value of 2 at 850 °C with the produced syngas and hydrogen reaching 4.57 mol/kg and 1.47 mol/kg, respectively. In situ Diffuse Reflectance Infrared Fourier Transform spectroscopy was employed to investigate the reaction mechanism of partial oxidation of methane, indicating that CH4 is gradually dehydrogenated and partially oxidized by lattice oxygen, and the conversion of -O-CH3 into -CHO and H2 is the rate-controlling step.

Improved fuel conversion through oxygen carrier aided combustion during incineration of biomass-based solid waste in a rotary kiln

Solid waste (SW) incineration in a rotary kiln (RK) faces great CO fluctuations due to the batch-fired nature, resulting in incomplete SW combustion, which makes its optimization challenging. A method introducing an oxygen carrier (OC) material into the RK has been first proposed to improve SW conversion, lower the CO emission and reduce the fluctuation of emitted CO during the combustion. In this work, manganese and ilmenite ores have been investigated as OCs, and introduced into a lab-scale RK during the combustion of biomass-based SW. Results show that both the introduction of manganese and ilmenite ores have lowered the CO emission and alleviated the fluctuation of emitted CO under reduced oxygen supply. It has been demonstrated that the OC can increase the available oxygen for effective biomass-based SW combustion in the RK since it can continuously transfer lattice oxygen from the active layer to the passive layer with the bed rolling. Besides, the unconverted CO can be burnt more completely and evenly through the gas–solid redox reaction attributed to the reactivity of OCs over CO. In addition, the OCs can also react with the char through a solid–solid reaction at a high temperature over 1073 K. As a result, the carbon conversion can be increased in the presence of OCs.

Selective CO2 deoxygenation to CO in chemically looped reverse water–gas shift using iron-based oxygen carrier

Chemical-looped reverse water–gas shift reaction was investigated using transition metal/metal oxides as oxygen carriers. Iron is identified as the only promising oxygen carrier that shows compelling CO 2 splitting reactivity. A chemically looped reverse water–gas shift reaction was developed using an iron-based oxygen carrier. Compared with conventional catalytic conversion processes, the chemical looping method has the advantage of high selectivity and cheap materials cost due to the separation of CO2 splitting and H2 oxidation half-reactions that are enabled by earth-abundant transition metal oxygen carriers. However, for such process to be economically attractive, the operation temperature should ideally be low enough such that low-grade industrial waste heat can be utilized. In other words, the reactivity of oxygen carriers toward the aforementioned half-reactions is most critical. To address the materials challenge, four transition metal-based oxygen carriers, i.e., iron, nickel, manganese, and copper, are studied using temperature-programmed techniques under H2 and CO2. Iron is identified to be the only oxygen carrier reactive toward CO2 splitting and capable of completing the redox cycle at 450 °C with 100% reverse water–gas shift selectivity. Although the thermal stability of the iron oxygen carriers shows room for improvement, our work demonstrates the great potential of a scalable and economically viable route for CO2 conversion that is compatible with current industrial processes.Graphical abstract

Chemical looping combustion of lignite using iron ore modified by foreign ions: Alkaline-earth and transition metal ions

In the chemical looping combustion (CLC) of lignite, oxygen carrier reactivity is the key to combustion performance. In this paper, the effects of particle size and temperature upon the CLC of lignite are investigated in a fixed bed reactor using iron ore as the oxygen carrier. Under optimized working conditions, the iron ores are loaded with alkaline-earth metals (K, Na and Ca) and transition metals (Ni, Mn and Cu) in a mass ratio of 10%, and their effect upon its CLC reactivity are studied. The carbon conversion rate, carbon capture rate, CO2 capture rate, CO2 selectivity, and other parameters are obtained. The size of lignite particles had a direct effect on the carbon capture rate of the fuel reactor. The particle size with the best effect on carbon and CO2 capture for both the Yunnan and Yizhong lignite is 80–200 mesh. The increase in temperature effectively promotes the reaction between oxygen carrier and volatiles, thereby resulting in an increase in CO2 concentration. The capacity of the oxygen carrier to transform the lignite is effectively enhanced by the modification with Cu, K, Na, Ca and Mn. In particular, the CO2 selectivity is greatly improved (up to 94.84%) for the Cu-modified oxygen carrier. These results provide important references for the CLC process, especially for the design of the oxygen carrier.

Performance evaluation of rare earth (La, Ce and Y) modified CoFe2O4 oxygen carriers in chemical looping hydrogen generation from hydrogen-rich syngas

In this study, chemical looping cascade coupling hydrogen generation (CL-CCHG) process was proposed to achieve conversion from multi-component hydrogen-rich syngas to high purity hydrogen. CLHG experiments using simulated methane reforming gas (MRG, 64.89 % H2 + 24.47 % CO + 7.18 % CO2 + 3.46 % CH4) were carried out on a self-built fixed bed reactor to verify the feasibility of this process, and rare earth (La, Ce and Y) modified CoFe2O4 was selected as oxygen carriers. The crystal structure, surface morphology and properties, and reactivity of oxygen carriers were characterized by various analytical methods (e.g., XRD, SEM-EDS, BET, XPS, TPR). The fixed bed experimental results exhibited that rare earth modification significantly improved the fuel conversion capacity, carbon capture efficiency and hydrogen production capacity of CoFe2O4, with La0.1-CoFe possessing the highest overall fuel conversion rate (85.97 %) and hydrogen recovery efficiency (89.46 %) at 750 °C. This was attributed to the enhanced pore structure, surface properties and reduction performance of oxygen carriers after the rare earth modification. Moreover, experimental results and HSC thermodynamic data demonstrated that the reducing components showed different competing effects in the temperature range examined (650–850 °C). The high temperatures favored the conversion of low concentrations CH4 in MRG, but this would inhibit the consumption of CO and H2. The reduced La0.1-CoFe oxygen carrier achieved the best hydrogen production intensity at 800 °C (302.91 vs 335.15 mL H2·g−1 OC) with hydrogen recovery efficiency of 90.38 %. Finally, by adjusting the MRG flow rate, 94.36 % carbon capture efficiency was obtained, achieving efficient recovery of high purity hydrogen (>99.5 %) along with carbon capture.