Accurate Control of Cage-Like CaO Hollow Microspheres for Enhanced CO₂ Capture in Calcium Looping via a Template-Assisted Synthesis Approach

Herein we report the development of synthetic CaO-based sorbents for enhanced CO2 capture in calcium looping via a template-assisted synthesis approach, where carbonaceous spheres (CSs) derived from hydrothermal reaction of starch are used as the templates. Cage-like CaO hollow microspheres are successfully synthesized only using urea as the precipitant, and the formation mechanism of this unique hollow microsphere structure is discussed deeply. Moreover, cage-like CaO hollow microspheres possess an initial carbonation conversion of 98.2% and 82.5% under a mild and harsh conditions, respectively. After the 15 cycles, cage-like CaO hollow microspheres still possess a carbonation value of 49.2% and 39.7% under the corresponding conditions, exceeding the reference limestone by 85.7% and 148.1%, respectively. Two kinetic models are used to explore the mechanism of carbonation reaction for cage-like CaO hollow microspheres, which are subsequently proved to be feasible for analysis of chemical-controlled stage and diffusion-controlled stage in the carbonation process. It is found the unique hollow microsphere structure can significantly reduce the activation energy of carbonation reaction according to the kinetic calculation. Furthermore, the energy and raw material consumptions related to the synthesis of cage-like CaO hollow microspheres are analyzed by the life cycle assessment (LCA) method

Carbon Dioxide Capture and Hydrogen Production with a Chemical Looping Concept: A Review on Oxygen Carrier and Reactor

The large amount of greenhouse gases produced by the combustion of fossil energy has caused global warming and a series of climate problems. In order to reduce greenhouse gas emissions, captured carbon can be converted into high value-added products, thereby accelerating the transformation of the energy model from fossil fuels to clean energy. Chemical looping technology is considered an efficient and clean potential strategy for converting fuels into syngas and hydrogen. This work describes the chemical looping technology combined with CO2 or H2O reforming to produce CO and H2, respectively, and performance analysis such as thermodynamics, kinetics, oxygen transfer capacity, and heat balance were carried out to identify viable oxides. In view of the importance of high-performance, low-cost metal oxides as oxygen carriers (OCs) in this process, this work systematically reviews the classification of such OCs and their applications. In addition, reactors in the 0.2 kWth–1.0 MWth range currently used for chemical looping technology are discussed, which demonstrate their potential to enable the large-scale operation of chemical looping processes. Based on past research progress and additional aspects of chemical looping technology, we firmly believe that this process offers a promising commercial technology that will drive energy transition and carbon neutrality

Gasification Decoupling during Pressurized Oxy-Coal Combustion by the Isotope Tracer Method

As one of the crucial means of CO2 emission reduction in coal-fired power plants, oxy-fuel combustion has attracted widespread attention for years. Since oxy-fuel combustion and air combustion vary considerably in the gaseous environment, the gasification reaction has proved to be non-negligible. Exploring the promotion and competition mechanism between oxidation and gasification reactions will help regulate the combustion characteristics of char through pressure and atmosphere. Thence, it is critical to explore the influence of the gasification reaction on oxy-fuel combustion and decouple the contribution ratio of the gasification reaction. The experiments under different conditions are performed on a laboratory-scale pressurized fixed bed in the research. The C atom in CO2 gas is labeled with 5.95% 13C; thus, the contribution ratio of the gasification and oxidation reactions can be decoupled by tracking the 13C atom abundance in CO2 and CO. The results are apparent: the pressure, temperature, and oxygen concentration increase, the combustion process is promoted, the combustion rate increases significantly, and the burnout time (t80) decreases. In addition, the problem of the low combustion rate under oxy-fuel combustion can be solved by increasing the O2 concentration. The decoupling results indicate that the contribution ratio of the gasification reaction increases with increasing pressure and temperature. As the CO2 partial pressure increases, the gasification reaction is promoted. With the increase of temperature, the gasification reaction growth rate is faster than the oxidation reaction. In contrast, the contribution rate shows a decreasing trend due to the increased O2 concentration promoting the oxidation reaction

Construction of 2D/3D g‑C₃N₄/ZnIn₂S₄ Heterojunction for Efficient Photocatalytic Reduction of CO₂ under Visible Light

A 2D/3D g-C3N4/ZnIn2S4 heterojunction photocatalyst was constructed by a one-step hydrothermal method combined with a calcination process. This composite not only can use its heterostructure to improve the migration and separation of photogenerated electron–holes but also has stronger visible light utilization efficiency and CO2 adsorption capacity, thereby improving the severe charge recombination of g-C3N4 and ZnIn2S4 monomers. The g-C3N4/ZnIn2S4 heterojunction exhibits outstanding performance in CO2 photoreduction. A maximum CO production of 40 wt % g-C3N4/ZnIn2S4 composite can reach 82.26 μmol·g–1, which is 10.1 and 2.8 times as high as those of the g-C3N4 and ZnIn2S4 monomers, respectively. The heterojunction fabrication process and electronic changes were analyzed with respect to both experimental and theoretical aspects by means of photoelectrochemical measurements and density functional theory (DFT). Finally, we propose a feasible mechanism for the photocatalytic reduction of CO2 on the g-C3N4/ZnIn2S4 composite. This work could help to understand the structure regulation of carbon nitride-based materials and provides a certain guidance for the development of novel efficient and green heterojunction catalysts

Unraveling the Structure–Reactivity Relationship of CuFe₂O₄ Oxygen Carriers for Chemical Looping Combustion: A DFT Study

CuFe2O4 is an emerging high-performance oxygen carrier for chemical looping combustion (CLC), which is hailed as the most promising technology to reduce combustion-derived CO2 emission. CuFe2O4 oxygen carriers with minute structural differences could be largely divergent in the reactivity for the CLC process, which seems not to raise much concern by either experimental or computational studies. Herein, based on density functional theory (DFT) calculations, we compare the performance of three well-documented CuFe2O4 configurations as oxygen carriers in the CLC process and relate the reactivity difference to their structural nuances. The reaction mechanisms of representative CLC reactants (i.e., CH4, H2, and CO) over different CuFe2O4 configurations are explored in-depth. DFT calculations indicate that among different CuFe2O4 configurations, the distribution, orientation, and activity of the O/Cu/Fe sites vary largely over the respective CuFe2O4(100) surfaces, thus affecting the adsorption and oxidation of CLC reactants. Fe atoms, especially in configuration 3, are observed to exhibit a higher degree of exposure and afford lower steric hindrance to interact with CH4 and H2, thereby facilitating higher adsorption energies and lower dissociation energy barriers correspondingly. The Fe–Cu synergistic effect is revealed to promote the dissociation reaction of both CH4 and H2. CO exhibits direct oxidation to CO2 over the O sites, which generally exhibit higher CO binding energies than Cu/Fe sites. Particularly, O sites in configuration 3 are observed with generally lower oxygen vacancy formation energy as well as steric hindrance, thus affording the oxidation of CO in a more facile way. The structure–performance relationship revealed in this work is of positive significance for the design of high-performance spinel CuFe2O4 oxygen carriers

MgO-Based Granular Sorbent Pelletized by Using Ordered Mesoporous Silica as Binder for Low-Temperature CO₂ Capture

Cyclic CO2 adsorption by using MgO as a sorbent at low temperatures is considered a promising route for postcombustion CO2 capture. However, most MgO-based sorbents are in the form of fine powder and cannot be used in a fluidized bed reactor, and at the same time, suffer from a rapid loss in CO2 uptake capacity due to the decrease of surface area aroused by pore shrinking and grain sintering. In this study, mesoporous silicas with highly ordered pore structures have been used as binders, for the first time, to fabricate MgO-based sorbent pellets via a simple and scalable extrusion–spheronization approach. The obtained MgO-based pellets exhibit high porosity attributed to the nature of the mesoporous binder, leading to a significantly increased stability and CO2 uptake capacity. Especially for the low-concentration CO2 that is comparable to the flue gas from a coal-fired power plant, the results show that the ordered mesoporous silica binder provides a remarkable promotion effect and excellent stability in the capture performance. The CO2 uptake capacity of the best-performing sorbent, 20-KIT-6–100, displays a small decline of 6.86% (from 1.02 mmol of CO2/g in the first cycle to 0.95 mmol of CO2/g in the 10th cycle). It is envisaged that mesoporous materials hold great potential to be used as binders in reinforcing the metal oxide-based sorbents for flue-gas CO2 capture in practical applications

Solar–Wind–Bio Ecosystem for Biomass Cascade Utilization with Multigeneration of Formic Acid, Hydrogen, and Graphene

This paper describes the development of a 20MWth solar–wind–bio-distributed energy system and its viability of achieving biomass cascade utilization, water resource conservation, waste heat recovery, and CO2 mitigation while coproducing hydrogen, formic acid, and graphene. The proposed ubiquitous energy system model is developed by using ASPEN Plus for the operating parameter optimization and system-wide heat assessment. The system consists of (i) a biomass gasification module, (ii) a biochar utilization module, (iii) a chemical looping hydrogen generation module, (iv) an oxy-syngas combustion module, (v) a CO2 electroreduction module, and (vi) heat recovery units. Bioenergy is progressively converted through biomass gasification, chemical looping hydrogen generation, syngas combustion, and CO2 electroreduction. The role of solar energy and wind power is to induce the biomass gasification and CO2 electroreduction, respectively. The system has been proved to be water-saving with a water recycle efficiency of 70.1% from steam condensation and recovery from the outlets of i, iii, and iv modules. The synergistic effect of parameters such as the CO2/biomass mass ratio, operation temperatures, and oxygen-carrier/syngas mole ratio is optimized. Furthermore, the carbon migration pathway, water/steam consumption and conservation, energy transformation, and heat supplement of the system are investigated, achieving an optimized system energy efficiency of 59.2%