Development of Stable Oxygen Carrier Materials for Chemical Looping Processes—A Review

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.

CuxMg1-xFe2O4-type spinels as potential oxygen carriers for waste wooden biomass combustion

Waste wood biomass is considered a renewable energy source. Combining biomass combustion with emerging clean combustion technologies such as chemical looping combustion (CLC) can yield effective and affordable carbon capture and, consequently, lead to negative net emissions of greenhouse gases. Oxygen carrier (OC) is a crucial material in CLC technology that must exhibit certain properties, such as high durability, good chemical stability during numerous red-ox cycles and, important for the combustion of solid fuels, the capability of spontaneously releasing oxygen in a process referred to as chemical looping with oxygen uncoupling (CLOU). In this work, a series of nine CuxMg1-xFe2O4 spinel-based materials were synthetized and evaluated for the first time as potential OCs for a waste biomass combustion. Their properties, such as oxygen transport capacity and reactivity with biomass (wood chips) as a fuel, were evaluated in a function of temperature (900-1000 °C). Tested oxygen carriers were characterized with an excellent oxygen transport capacity in CLOU process (up to 2.78 wt%) and good reaction rates with the fuel (up to 1.19 wt. %/min), and regeneration rates (up to 3.8 wt. %/min). High conversion of the waste biomass was also achieved (98.9 %). Moreover, new findings revealed a strong positive effect of magnesium addition on mechanical strength (crushing strength > 4 N for samples with Mg content above 0.5).

Theoretical Study of the Reactive Mechanisms of Li-Doped Ni-Based Oxygen Carrier during Chemical Looping Combustion

Ni-based oxygen carriers (OCs) are considered promising materials in the chemical looping combustion (CLC) process. However, the reactivity of Ni-based OCs still offers the potential for further enhancement. In this work, the Li doping method has been employed for the modification of Ni-based OCs. The reactivity and microreaction mechanisms of different concentrations of Li-doped Ni-based OCs with CO in CLC are clarified using density functional theory (DFT) simulation. The structures, energy, and density of states are obtained through computational investigation of the reaction path in elementary reactions. The results show that (1) the adsorption energies of CO molecules on NiO surfaces with 4, 8, and 12% Li doping concentrations are -0.53, -0.48, and -0.54 eV, respectively, demonstrating an enhanced reactivity compared to that of pure NiO (-0.41 eV); (2) the calculation of the transition state indicates that the most favorable pathway for CO oxidation takes place on the surface of NiO with an 8% Li doping concentration, exhibiting the lowest energy barrier of 0.51 eV; and (3) the oxygen vacancy formation energies on the surface of NiO are 3.05, 2.30, and 2.10 eV for 4, 8, and 12% doping concentrations, respectively. Additionally, the decrease in oxygen vacancy formation energies exhibits a gradual decline with an increasing Li doping concentration. By comprehensive analysis, 8% is considered to be the optimal doping concentration of NiO for chemical looping combustion.

Nanoscaled Oxygen Carrier-Driven Chemical Looping for Carbon Neutrality: Opportunities and Challenges

ConspectusClimate change poses unprecedented challenges, demanding efforts toward innovative solutions. Amid these efforts, chemical looping stands out as a promising strategy, attracting attention for its CO2 capture prowess and versatile applications. The chemical looping approach involves fragmenting a single reaction, often a redox reaction, into multiple subreactions facilitated by a carrier, frequently a metal oxide. This innovative method enables diverse chemical transformations while inherently segregating products, enhancing process flexibility, and fostering autothermal properties. An intriguing facet of this novel technique lies in its capacity for CO2 utilization in processes like dry reforming and gasification of carbon-based feeds such as natural gas and biomass. Central to the success of chemical looping technology is a profound understanding of the intricacies of redox chemistry within these processes. Notably, nanoscaled oxygen carriers have proven effective, characterized by their extensive surface area and customizable structure. These carriers hold substantial promise, enabling reactions under milder conditions.This Account offers a concise overview of the mechanisms, benefits, opportunities, and challenges associated with nanoscaled carriers in chemical looping applications, with a focus on CO2 utilization. We delve into the nuances of redox chemistry, shedding light on ionic diffusion and oxygen vacancy─two key elements that are crucial in designing oxygen carriers. This discussion extends to nanospecific factors such as the particle size effect and gas diffusivity. Through the application of density functional theory simulations, insights are drawn regarding the impact of nanoparticle size on syngas production in chemical looping. Interestingly, nanosized iron oxide (Fe2O3) carriers exhibit elevated syngas selectivity and constrained CO2 formation at the nanoscale. Moreover, the reactivity enhancement of mesoporous SBA-16 supported Fe2O3 over mesoporous SBA-15 supported Fe2O3 is elucidated through Monte Carlo simulations that emphasize the superiority of the 3-dimensional interconnected porous network of SBA-16 in enhancing gas diffusion, thereby amplifying reactivity compared to the 2-dimensional SBA-15. Furthermore, we explore prevalent nanoscaled carriers, focusing on their amplified performance in CO2 utilization schemes. These encompass the integration of nanoparticles with mesoporous supports to enhance surface area, the adoption of nanoscale core-shell architectures to enhance diffusion, and the dispersion of nanoscaled active sites on microsized carriers to accelerate reactant activation. Notably, our mesoporous-supported Fe2O3 nanocarrier facilitates methane dissociation and oxidation by reducing energy barriers, thereby promoting methane conversion. The Account proceeds to outline key challenges and prospects for nanoscaled carriers in chemical looping, concluding with a glance into future research directions. We also shine a spotlight on our research group's efforts in innovating oxygen carrier materials, supplemented by discussions on indispensable elements that are essential for successful scale-up deployment.

The behavior of organic sulfur species in fuel during chemical looping gasification

Uncoupling chemical looping gasification (CLG), the organic sulfur evolution was simulated and explored qualitatively and quantitatively using typical sulfur compounds on TG-MS and temperature-programmed fixed bed. The HS radical in the reductive atmosphere easier converted to H₂S and COS. H₂O activated the evolution of S which was stably bonded to carbon, and H₂ generated from gasification and oxidation of reductive Fe by H₂O contributed to the release of sulfur. The proportion of H₂S released from sulfur compounds was greater than 87% in steam gasification, and more than 60% during CLG. Oxygen carriers promoted the conversion of sulfur to SO₂ in the mid-temperature region (500 °C-700 °C), and H₂S in the high temperature region (700 °C-900 °C). Sulfur species played a pivotal role in sulfur evolution at low temperature of CLG. The organic sulfur in mercaptan and benzyl were more easily converted and escaped than in thiophene and phenyl. The thermal stability of sulfur species, the presence of steam and OC affected the initial temperature and peak concentration of gas sulfur release as well as sulfur distribution. Consequently, CLG strengthened the sulfur evolution, and made it possible to targeted restructure the distribution of sulfur by regulating process parameters, or blending fuel with different sulfur species for emission reduction, and selective conversion of sulfur.

Defect Chemistry and Chemical Looping Performance of La1-xMxMnO3 (M = Sr, Ca, (x = 0-0.5)) Perovskites

La_1-xM_xMnO₃ (M = Sr, Ca, (x = 0-0.5)) materials of the perovskite structure are synthesized by a co-precipitation method. They are subsequently investigated for their performance in a chemical looping process (fuel CH₄) using thermogravimetric analysis with simultaneous reaction. The goal of this work is to determine the relation between the defect chemistry of the materials and their behavior in chemical looping processes. A defect model is proposed that provides an explanation of the dependency of the Oxygen Transfer Capacity and of the CO₂/CO selectivity on composition. It appeared that the fuel may react with various types of oxygen available within the materials, generated by different mechanisms. The relative amounts of each oxygen type determine the CO₂/CO selectivity and depend on the material composition as well as on the partial pressure of oxygen used for regenerating the materials.

A Theoretical Study of the Oxygen Release Mechanisms of a Cu-Based Oxygen Carrier during Chemical Looping with Oxygen Uncoupling

The Cu-based oxygen carrier is a promising material in the chemical looping with oxygen uncoupling (CLOU) process, while its performance in the CLOU is significantly dependent on the oxygen release properties. However, the study of oxygen release mechanisms in CLOU is not comprehensive enough. In this work, the detailed oxygen release mechanisms of CuO(110) and CuO(111) are researched at an atomic level using the density functional theory (DFT) method, including the formation of O2, the desorption of O2 and the diffusion of O anion, as well as the analysis of the density of states. The results show that (1) the most favorable pathway for O2 formation and desorption occurs on the CuO(110) surface of O-terminated with energy barriers of 1.89 eV and 3.22 eV, respectively; (2) the most favorable pathway for O anion diffusion occurs in the CuO(110) slab with the lowest energy barrier of 0.24 eV; and (3) the total density of states for the O atoms in the CuO(110) slab shifts to a lower energy after an O vacancy formation. All of the above results clearly demonstrate that the CuO(110) surface plays a significantly important role in the oxygen release reaction, and the oxygen vacancy defect should be conducive to the reactivity of oxygen release in a Cu-based oxygen carrier.

Reactivity Study of Bimetallic Fe-Mn Oxides with Addition of TiO2 for Chemical Looping Combustion Purposes

The objective of the research was to prepare Mn-based materials for use as oxygen carriers and investigate their reactivity in terms of their applicability to energy systems. The family of Fe2O3-MnO2 with the addition of TiO2 was prepared by mechanical mixing method and calcination. Five samples with addition of Fe2O3 (20, 30, 35, and 50 wt.%) to MnO2 (65, 55, 50, 35, and 85 wt.%) with constant amount of inert TiO2 (15 wt.%) were prepared. The performance of TiO2 supported Fe-Mn oxides oxygen carriers with hydrogen/air in an innovative combustion technology known as chemical looping combustion (CLC) was evaluated. Thermogravimetric analysis was used for reactivity studies within a wide temperature range (800–1000 °C). Comprehensive characterization contained multipurpose techniques for newly synthesized materials. Moreover, post-reaction experiments considered morphology analysis by SEM, mechanical strength testing by dynamometry, and crystal phase study by XRD. Based on wide-ranging testing, the F50M35 sample was indicated as the most promising for gaseous fuel combustion via CLC at 850–900 °C temperature.

Extremely Stable and Durable Mixed Fe–Mn Oxides Supported on ZrO2 for Practical Utilization in CLOU and CLC Processes

This paper contains the results of research on a promising combustion technology known as chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU). The remarkable advantages of CLC are, among others, that concentrated CO2 stream can be obtained after water condensation without any energy penalty for its separation or significant decrease of NOx emissions. The objective of this work was to prepare a novel bi-metallic Fe–Mn supported on ZrO2 oxygen carriers. Performance of these carriers for the CLOU and CLC process with nitrogen/air and hard coal/air was evaluated. One-cycle CLC tests were conducted with supported Fe–Mn oxygen carriers in thermogravimetric analyzer utilizing hard coal as a fuel. The effects of the oxygen carrier chemical composition and process temperature on the reaction rates were determined. Our study proved that for CLOU, properties formation of bixbyite and spinel forms are responsible. Among iron ferrites, we concluded that iron-rich compounds such as Fe2MnO4 over FeMn2O4 spinel type oxides are more effective for CLOU applications.

Theoretical DFT Study on the Mechanisms of CO/CO2 Conversion in Chemical Looping Catalyzed by Calcium Ferrite

The CO/CO₂ conversion mechanism on the calcium ferrite (CFO) surface in chemical looping was explored by a computational study using the density functional theory approach. The CFO catalytic reaction pathway of 2CO + O₂ → 2CO₂ conversion has been elucidated. Our results show that the Fe center in CFO plays the key role as a catalyst in the CO/CO₂ conversion. Two energetically stable spin states of CFO, quintet and septet, serve as the effective catalysts. The presence of the triplet O₂ molecule caused the conversion of these two spin-state structures into each other along the catalytic reaction pathway. A double release of CO₂ was predicted following this reaction mechanism. The rate-determining step is the formation of the 2CO₂-CFO complex (P4) in the quintet state (19.0 kcal/mol). The predicted energy barriers for all the steps suggest that the proposed pathway is plausible.