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Slugging represents one of the major regimes in fluidization, which occurs in small diameter beds with large bed height-to-diameter ratio or in large diameter beds with internals that resemble multiple small diameter fluidized beds. Slug types include round-nosed slug, wall slug and square-nosed slug. Studies of the slugs have been mainly focused on round-nosed or wall slugs known as half slug, typically occurring in Geldart group A particle fluidization. The square-nosed slug typically occurring for Geldart group D particles appears to be regarded as simple in its structure. The Electrical Capacitance Volume Tomography (ECVT) imaging of the square-nosed slugging phenomena conducted in this study reveals otherwise. That is the structure of the square-nosed slug is, in fact, complex, particularly with respect to its dynamic variation in fluidization. More broadly, this study examines experimentally the hydrodynamic characteristics of the square-nosed fluidization regime. Specifically, simultaneous measurements from multiple ECVT sensors provide non-invasive, continuous, 3-dimensional imaging of the entire flow region of the slugging bed and hence enabling the dynamic characterization of the evolution of the slugs. The analysis of the 3D images reconstructed for real-time gas–solid volume fraction profile of the slugging fluidized bed indicates that there are three different zones, namely, the bottom fluidization zone, the gas slug zone, and the solid slug zone, co-existing in the bed. The three zones present different hydrodynamic characteristics during the slug evolution. It is found that varying the gas velocity of the slugging bed mainly varies the maximum length of the gas slug zone, while it only has a minor effect on the lengths of the bottom fluidization zone and solid slug zone. It also has an insignificant effect on the solid volume fraction of the three zones.

A composite consisting of oxide-derived Bi embedded in the holes of reduced graphene oxide is synthesized for N2 electroreduction. The synergistic effect between the Bi defects and graphene vacancies leads to a high NH3 activity and selectivity.

In this study, interaction force of non-spherical particles in low Reynolds number gas-solid flow is investigated by neural network approaches. An artificial neural network (ANN) model is developed to correlate the non-spherical particle shape and the flow conditions with the interaction force. To define the particle shape, spherical harmonic expansion is applied. Furthermore, variational autoencoder model is then used to extract latent geometric features. The latent vector is utilized as an input with the Reynolds number for the ANN. The interaction force data, which is used as output data of the ANN, is obtained by particle resolved direct numerical simulation for 5200 non-spherical particles. The proposed model enables unsupervised extraction for non-spherical particle shapes and accurate predictions on the interaction force without heavy computation. This study provides the model that can explain complicated shapes of particles and be applied to a large scale, computational fluid dynamics simulation.

The chemical looping technology is an emerging technology for the efficient conversion of biomass to heat, power, fuel, and value-added chemicals. Chemical looping uses a metal oxide oxygen carrier to fully or partially oxidize the biomass feedstock, which avoids N2 dilution to the CO2 or syngas product. Thus, chemical looping technology can convert biomass to sequestration-ready CO2 or high purity syngas, which leads to the potential for negative carbon emission and enhanced energy efficiency in biomass conversions. Various chemical looping processes for biomass conversions, using different oxygen carrier materials and reactor designs, have been studied ranging from lab scale to pilot scale. This article reviews the current state of development in the scale-up of chemical looping technology for biomass conversions. It also provides perspectives on the chemical looping technology of relevance to various biomass conversion issues including CO2 capture, utilization and sequestration, biomass feedstock collection, handling and conversion enhancement, choice of reaction schemes, fuel and air reactor designs and operation, operational load variation, fate of pollutants and effect of ashes. Representative process configurations, reactor designs, oxygen carrier material selections, and the testing results are described along with the technology challenges, their potential solutions, and future research needs.

The climate models of the Intergovernmental Panel on Climate Change list black carbon (BC) as an important contributor to global warming based on its radiative forcing (RF) impact. Examining closely these models, it becomes apparent that they might underpredict significantly the direct RF for BC, largely due to their assumed spherical BC morphology. Specifically, the light absorption and direct RF of BC agglomerates are enhanced by light scattering between their constituent primary particles as determined by the Rayleigh–Debye–Gans theory interfaced with discrete dipole approximation and recent relations for the refractive index and lensing effect. The light absorption of BC is enhanced by about 20% by the multiple light scattering between BC primary particles regardless of the compactness of their agglomerates. The resulting light absorption agrees very well with the observed absorption aerosol optical depth of BC. ECHAM-HAM simulations accounting for the realistic BC morphology and its coatings reveal high direct RF = 3–5 W/m2 in East, South Asia, sub-Sahara, western Africa, and the Arabian peninsula. These results are in agreement with satellite and AERONET observations of RF and indicate a regional climate warming contribution by 0.75–1.25 °C, solely due to BC emissions., Environmental Science & Technology, 56 (12), ISSN:0013-936X, ISSN:1520-5851

Oxidative coupling of methane (OCM) is a compelling strategy for the direct conversion of methane to C2+ hydrocarbons in order to produce fuels and value-added chemicals. Nevertheless, it remains c...

Chemical looping partial oxidation (CLPO) is a novel technology for converting methane into high quality syngas that can be further converted into liquid fuels. In the present work, Ni-doped Ca2Fe2O5 oxygen carriers are employed as looping media wherein the Ni-doping concentration is varied from 0 to 10%. Thermogravimetric performance tests are carried out where the doped/undoped carriers are subjected to methane reduction in the first half cycle and subsequently the reduced carriers are regenerated using CO2 or air in the second half. The cyclic redox performance of Ni doping on the characteristics and the stabilities of these oxygen carriers are also investigated by means of X-ray diffraction and scanning electron microscopy. Based on the oxygen carrier characterization, an unwanted phase, NiFe2O4, is formed beyond 5% dopant concentration which exhibited weak methane interaction and inhibited CO2 regeneration thermodynamically. Moreover, doping Ni at 0–5% exhibited increased reactivity across the temperature range of 750–1000 °C as compared to the undoped sample, with the 5% Ni doped sample showing a substantial improvement of 1149% over the undoped sample at 750 °C. This explains its potential in an adiabatic process where a temperature gradient is generally observed across the syngas generation reactor. Density functional theory (DFT) calculations further reveal the role of the Ni doping effect on methane partial oxidation and CO2 conversion, wherein adding the Ni dopant lowers the oxygen vacancy formation energy and increases the CO2 adsorption energy which is favorable for CO2 activation and splitting. The findings of this study provide a fundamental insight into the reactivity enhancement of calcium ferrite-based oxygen carriers and open new avenues for designing a novel chemical looping system for simultaneous syngas generation and CO2 utilization.

As one of the possible alternatives to the Haber–Bosch process, electrochemical ammonia synthesis has been attracting considerable interest in recent years. However, most electrocatalysts are confr...

The global market size for acetic acid is expected to increase at a high compounded annual growth rate over the next few years due to an increased demand from the end-users such as the automobile, ...

The transformation of CO2 to chemicals and fuels offers a means of CO2 utilization while mitigating the global carbon footprint. An attractive strategy involves the CO2 reforming of methane with ox...

To meet the globally growing energy demands, it is essential to develop schemes with higher fuel conversion efficiency at temperatures

Production of various value-added chemicals through natural gas conversion with syngas as an intermediate is becoming increasingly popular because of the abundance of natural gas and maturation of ...

Chemical looping reforming (CLR) is a promising strategy of methane conversion to syngas with low cost and minimal environmental impact. A major challenge for chemical looping reforming systems is the development of oxygen carriers that have high reactivity, high oxygen carrying capacity, and long-term durability. We demonstrate that the addition of a low concentration of cobalt dopant to iron-based oxygen carriers can dramatically enhance the reactivity in CLR processes at low temperatures while maintaining the recyclability. The methane conversion increased by around 300% for Co-doped iron oxide compared to pure iron oxide oxygen carriers from 600 °C to 800 °C. It was found that 2% Co dopant concentration is optimal for methane conversion rate and cost of oxygen carriers at different temperatures. Density functional theory (DFT) calculations reveal that the Co dopant has a short-range effect on the formation of oxygen vacancies. The Co-doping-induced oxygen vacancy significantly reduces energy barriers of CH4 reforming on the surface of iron oxide oxygen carriers, leading to the reactivity enhancement. Our findings provide a pathway to lower the methane reforming temperature in chemical looping systems, while improving the syngas yield with desired oxygen carrier recyclability.

Chemical looping is a novel and promising technology that converts fossil fuels to electricity and high value chemicals with in-situ carbon capture without significant cost penalties. Smooth and controlled solid circulation is the key to the successful operation of the chemical looping system. Bridging or arching, which may occur in the moving bed standpipe of the system, caused by a combination of fines accumulation and gas flows, is a major impediment to the smooth solid circulation and can lead to the failure of the entire system operation. Thus, early detection of the tendency of bridging or arching is important. This paper describes a model that applies the long short-term memory based recurrent neural network scheme to detect the tendency of arching in the standpipe of a chemical looping system. The arching tendency can be ascertained by early detection of the bubble formation in the standpipe. The bubble movement or local fluidization is recognized to precede the arching formation due to local accumulation of fine particles generated from coarse particle attrition. The early detection of the bubbles thus renders it possible to prevent arching through the prompt action of fines removal from the system. In this study, the recurrent neural network model which detects the fines induced fault manifested as bubbles in the standpipe is developed. It is over the data generated from an experimental sub-pilot scale, cold-flow chemical looping unit. To improve the robustness of the diagnosis, a number of networks with different structures are considered, and an ensemble decision strategy is used to conduct the diagnosis. The recall value obtained of the diagnosed result, which represents the extent of the fraction of real bubbles that are detected, can reach higher than 86.7%. This result reflects a good accuracy of the recurrent neural network model in the fault detection for the chemical looping system.

Syngas production is highly critical to the manufacturing of many value-added products, and its economic prospects can be increased through the enhancement of fuel conversion and the syngas yield. ...

Chemical looping is an advanced material and energy conversion technology that can achieve both high-level process intensification and efficiency. To analyze chemical looping processes, it is essential to include process conditions that are realistic and comparable to those that are expected in industrial systems. Relevant variations in these conditions as occurred in bench versus industrial-scale systems include isothermal versus adiabatic operation of the reactors and local versus global process heat integration. Naturally, the types of reactors employed dictate how the reactor operation is to be conducted from the heat integration viewpoint in the overall process arrangement. As an example, in industrial applications, a fluidized bed reactor is operated near uniform temperature conditions. A fixed bed or a moving bed reactor, on the other hand, is typically operated adiabatically, and thus under the autothermal operation, the nonisothermal condition prevails, leading to different strategies for process simulations and heat integration requirements. This study presents the chemical looping process simulation based on a moving bed reactor used as a reducer for two H₂ generation process configurations under autothermal operating conditions. The two process configurations are represented by the two-reactor (reducer–combustor followed by the water–gas shift reaction) and the three-reactor (reducer–oxidizer–combustor with water splitting for H₂ generation in the oxidizer) chemical looping systems with each configuration producing H₂ in a different operating scheme. The simulation results are compared with the conventional steam methane reforming (SMR) system as a baseline case to underscore the attractiveness of the chemical looping configurations. Specifically, for each configuration, the parametric study under the adiabatic conditions is used to optimize the operating conditions that can satisfy the heat balance requirements and can achieve a maximum H₂ yield. The exergy analysis indicates that the two-reactor chemical looping and three-reactor chemical looping systems can achieve, respectively, a 4.0 and 11.4% increase in relative percentage in the overall process exergy efficiency over the conventional steam methane reforming system.

Coal is an essential primary energy source in modern industry. An important pathway of coal utilization is to gasify coal into syngas, which is then used for downstream production of numerous chemicals such as methanol and liquid fuels. Despite its wide applications, conventional coal gasification still faces challenges such as the use of a costly air separation unit and limited syngas H2/CO ratio. Chemical looping gasification technology is a promising alternative to replace conventional coal gasification with increased thermal efficiency and higher level of process intensification. This study presents the process analysis of chemical looping coal gasification and its integration with downstream production of dimethyl ether (DME) to reveal the advantages of the chemical looping scheme compared to conventional gasification schemes. This study discusses the simulation strategies to maintain adiabatic operation in the chemical looping system. The operating parameters of the chemical looping system are adjusted to achieve the maximum cold gas efficiency in one case study, while the operating parameters of the chemical looping system are adjusted to achieve the maximum effective thermal efficiency in another case study. These two chemical looping cases involve the integration of the chemical looping system with the DME synthesis in a process that considers overall process material and heat flows and power consumption and generation. An exergy analysis is also conducted for these two integrated chemical looping cases, revealing that they both can achieve around 10% increase in exergy efficiency over the conventional gasification system.

Alkanes are potential precursors to many value-added chemicals such as olefins and other petrochemicals. However, the conversion of light alkanes can be challenging due to their strong C–H bonds. Chemical looping technology based on cyclic redox schemes is an attractive platform that utilizes metal oxides as oxygen carriers for clean and effective fuel processing. Chemical looping systems can potentially be operated over a wide range of reaction conditions with a lower capital and operating cost under a reduced energy and environmental penalty. The reactivity and long-term performance of the oxygen carriers are key for the successful deployment of the chemical looping technology. This work reviews the recent advances in chemical looping alkane utilization including chemical looping partial oxidation (CLPO) of methane, chemical looping oxidative coupling of methane (CLOCM) and oxidative dehydrogenation (CLODH) of ethane, propane and butane. In particular, material design, mechanistic insights of the reactions, and associated reactor configurations for the reactions are discussed. The perspective viewpoints are also given on the viability of the cyclic redox schemes for shale gas dehydrogenation/reforming applications.

The methane-to-syngas (MTS) chemical looping process is an advanced methane reforming technology for the production of high purity syngas. The developed MTS process utilizes metal oxide oxygen carr...

Oxygen chemistry plays a pivotal role in numerous chemical reactions. In particular, selective cleavage of C-H bonds by metal oxo species is highly desirable in dehydrogenation of light alkanes. However, high selectivity of alkene is usually hampered through consecutive oxygenation reactions in a conventional oxidative dehydrogenation (ODH) scheme. Herein, we show that dual-functional Mo-V-O mixed oxides selectively convert propane to propylene via an alternative chemical looping oxidative dehydrogenation (CL-ODH) approach. At 500 °C, we obtain 89% propylene selectivity at 36% propane conversion over 100 dehydrogenation-regeneration cycles. We attribute such high propylene yield-which exceeds that of previously reported ODH catalysts-to the involvement and precise modulation of bulk lattice oxygen via atomic-scale doping of Mo and show that increasing the binding energy of V-O bonds is critical to enhance the selectivity of propylene. This work provides the fundamental understanding of metal-oxygen chemistry and a promising strategy for alkane dehydrogenation.