Excessive carbon dioxide (CO2) emissions have been subject to extensive attention globally, since an enhanced greenhouse effect (global warming) owing to a high CO2 concentration in the atmosphere ...could lead to severe climate change. The use of solar energy and other renewable energy to produce low‐cost hydrogen, which is used to reduce CO2 to produce bulk chemicals such as methanol, is a sustainable strategy for reducing carbon dioxide emissions and carbon resources. CO2 conversion into methanol is exothermic, so that low temperature and high pressure are favorable for methanol formation. CO2 is usually captured and recovered in the liquid phase. Herein, the emerging technologies for the hydrogenation of CO2 to methanol in the condensed phase are reviewed. The development of homogeneous and heterogeneous catalysts for this important hydrogenation reaction is summarized. Finally, mechanistic insight on CO2’s conversion into methanol over different catalysts is discussed by taking the available reaction pathways into account.
Condensed and converted: CO2 is usually captured and recovered in the liquid phase. In this Review, the emerging technologies of hydrogenating CO2 to methanol in the condensed phase are discussed. The development of homogeneous and heterogeneous catalysts for this important hydrogenation reaction is summarized.
Finite element modeling was performed to study the pyrolysis of centimeter‐sized biomass particles. The model proved to be accurate in predicting the intraparticle heat transfer during the pyrolysis ...of biomass particles at different operating conditions and for various particle properties. It was further applied to find the dominant factors of biomass physical properties influencing biomass pyrolysis, especially when the product distribution was considered. Simulation results indicate that biomass density and initial moisture content mainly affect the conversion time while have a limited effect on product distribution. With larger particle size the conversion time increases significantly. Higher tar yield with lower yields of char and non‐condensable gases were predicted at smaller particle size, indicating that the product distribution can be adjusted partially based on appropriate selection of particle size.
A finite element model was developed for the pyrolysis of a centimeter‐sized biomass particle. Experimental validation indicates that the model was accurate in predicting intraparticle heat transfer. The model was further applied to determine the dominant biomass physical properties in terms of density, size, and initial moisture content influencing its pyrolysis.
The influence of solid-phase wall boundary condition in terms of specularity coefficient and particle–wall restitution coefficient on the flow behavior of spouted beds was investigated using ...two-fluid model approach in the computational fluid dynamics software FLUENT 6.3. Parametric studies of specularity coefficient and particle–wall restitution coefficient were performed to evaluate their effects on the flow hydrodynamics in terms of fountain height, spout diameter, pressure drop, local voidage and particles velocity. The numerical predictions were compared with available experimental data in the literatures to obtain the suitable values of specularity coefficient and particle–wall restitution coefficient for spouted beds. The simulated results show that the solid-phase wall boundary condition plays an important role in CFD modeling of spouted beds. The specularity coefficient has a pronounced effect on the spouting behavior and a small specularity coefficient (0.05) can give good predictions, while the particle–wall restitution coefficient is not critical for the holistic flow characteristics.
► We used two-fluid model approach to simulate spouted beds hydrodynamics at different solid-phase wall boundary conditions. ► Specularity coefficient affects significantly spouting behavior. ► Small specularity coefficient gives good predictions. ► Particle–wall restitution coefficient plays minor role.
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•Tetragonal ZrO2 support promotes the Cu catalyst for ethyl formate hydrogenation.•A maximum yield of methanol, ∼ 53 %, was achieved over 20 wt% Cu/ZrO2-600.•The catalyst calcination ...temperature strongly affects the structure of Cu/ZrO2.•The change in Cu2+ content before and after hydrogenation affects catalyst stability.
Converting formate to methanol is an economic strategy to expand the production of formate from CO2 electro-reduction while satisfying the increasing market demand for methanol. For selective hydrogenation of formate esters to methanol, it is crucial to understand how the interaction between metal catalyst and support affects catalytic performance. In this paper, ZrO2 was employed as the support to promote the Cu catalysts’ activity and selectivity in liquid-phase hydrogenation of ethyl formate. To maximize the production yield of methanol, various catalyst properties and reaction condition were investigated. Under the optimized conditions, i.e., 160 °C and 400 psi H2, ∼ 53 % of methanol were yielded from the hydrogenation of ethyl formate in anhydrous ethanol solvent over the 20 wt% Cu/ZrO2 catalyst. The calcination temperature in the catalyst synthesis had profound effects on the Cu valence states, the structure of the ZrO2 support, and the interactions between Cu and ZrO2. A variety of characterization tools, including XRD, XPS, and HRTEM, were used to elucidate the structure-activity relationship of the Cu/ZrO2 catalysts. These findings will facilitate the next-generation catalyst design and elucidate the synergistic interaction between Cu and ZrO2 that promotes ester hydrogenation reactions.
Industrial CFB risers usually handle polydisperse mixtures with broad size distribution, which significantly influenced the performance of the reactors. However, traditional Computational Fluid ...Dynamics (CFD) models usually assumed that the particle followed the mono-disperse distribution. In the present work, the method of computational particle fluid dynamic (CPFD) was applied for simulating the complex hydrodynamics in the CFB riser with various particle size distributions (PSDs). Two kinds of PSDs, namely Gaussian and Lognormal distribution with various PSD widths, were implemented into the CPFD scheme. With the CPFD method, the present work extensively studied the effects of PSD on the hydrodynamics and on the solids back-mixing. The CPFD results showed that the PSD significantly affected particle's flow behaviors at the lower zone of the riser, while the PSD effects were negligible in the upper part of the riser. This is meaningful for the industrial riser reactors since most of the reaction and transport process occur in this lower zone of the riser. Besides, the simulation results showed that wider PSD dramatically weaken the particle's back-mixing behaviors in the riser. The significant effects of PSD predicted by the CPFD method imply that large errors will be introduced if the mono-disperse assumption is adopted to simulate the experimental CFB riser handling particles with broad size distribution.
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•We used the CPFD method to study the effects of particle size distribution (PSD).•The PSD significantly affects the particle flow behaviors.•Wider PSD is helpful to reduce the extent of solids back-mixing.
In view of the current status of catalytic cracking flue gas treatment, it is necessary to study the flow environment of desulfurization ash particles, which are a type of Geldart C particle, in a ...circulating fluidized bed (CFB) for semi-dry flue gas desulphurization using CFB technology. This study investigated the flow characteristics of desulphurization ash particles in a riser with an inner diameter of 70 mm and a height of 12.6 m, at a gas velocity of 4–7 m/s and a solids circulation rate of 15–45 kg/m2·s. The solids holdup in the axial distribution is relatively high near the bottom of the riser, and gradually decreases as the riser height increases, with a stable value from the middle to the top of the riser. In the radial distribution, the solids holdup of desulfurization ash particles is low in the center and high in the wall region. Within the above operating conditions, the solids holdup ranges from 0.008 to 0.025. The particle-based Archimedes number has a linear relationship with the solids holdup at all operating conditions.
In order to study the system hydrodynamics in a circulating fluidized bed (CFB), a 3D full‐loop simulation was conducted for a pilot‐scale CFB. The Eulerian‐Eulerian two‐fluid model with the kinetic ...theory of granular theory helped to simulate the gas‐solids flow in the CFB. The system hydrodynamics including pressure balance, vectors of gas and solids, distribution of solids holdup, and instantaneous circulating rates were obtained to get a comprehensive understanding of the system. It was predicted that the main driving force was the pressure drop of the storage tank. The storage height and valve opening were critical operating factors to control the riser operation. The effects of operating conditions including solids circulating rates and superficial gas velocity on the hydrodynamics were investigated to provide guidance for the stable operation of the CFB system.
A 3D full‐loop simulation was conducted for a pilot‐scale circulating‐fluidized bed to study the gas‐solids flow behavior. The system hydrodynamics such as pressure balance, distribution of solids holdup and velocity as well as instantaneous circulating rates were investigated, and the effects of operating conditions on the system were evaluated to get a comprehensive understanding of the system.
This study utilized a thermogravimetric analyzer to assess the thermal decomposition behaviors and kinetics properties of vacuum residue (VR) and low-density polyethylene (LDPE) polymers. The kinetic ...parameters were calculated using the Friedman technique. To demonstrate the interactive effects between LDPE and VR during the co-pyrolysis process, the disparity in mass loss and mass loss rate between the experimental and calculated values was computed. The co-pyrolysis curves obtained through estimation and experimentation exhibited significant deviations, which were influenced by temperature and mixing ratio. A negative synergistic interaction was observed between LDPE and VR, although this inhibitory effect could be mitigated or eliminated by reducing the LDPE ratio in the mixture and increasing the co-pyrolysis temperature. The co-pyrolysis process resulted in a reduction in carbon residue, which could be attributed to the interaction between LDPE and the heavy fractions, particularly resin and asphaltene, present in VR. These findings align with the pyrolysis behaviors exhibited by the four VR fractions. Furthermore, it was observed that the co-pyrolysis process exhibited lower activation energy as the VR ratio increased, indicating a continuous enhancement in the reactivity of the mixed samples during co-pyrolysis.
Single-atom alloys that consist of a catalytically active metal atomically dispersed in a less-catalytic metal host have recently attracted significant concern from worldwide researchers. The unique ...geometric and electronic properties of single-atom alloys enable both facile dissociation of reactants and weak adsorption of intermediates on the surface, representing two highly desirable elements for efficient and selective catalysis. This review article provides an overview of recent progress in the synthesis and properties of single-atom alloys in heterogeneous catalysis. First, we summarize recent advances in the controllable synthesis of single-atom alloys with various metal combinations. Second, we introduce key characterization techniques, especially
operando
spectroscopic techniques for the identification of the geometric and electronic structures of diluted active metal atoms in single-atom alloys. Then, we discuss how these geometric and electronic properties of single-atom alloys affect their catalytic performances and thoroughly comprehend the reaction mechanism. Finally, insights into the current challenges and future prospects of single-atom alloys are provided.
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