A novel cold reactor apparatus for in situ gasification chemical looping combustion (iG-CLC) is proposed and investigated. It is mainly comprised of a circulating fluidized bed (CFB) riser as the fuel reactor and a cross-flow moving bed as the air reactor. The direct hydraulic link between the two reactors brings inherent simplicity and stabilization of the whole system. The CFB fuel reactor provides favorable gas–solids contacts over the whole reactor height. The realization of high solids flux operation conditions greatly enhances the solids holdups, and the gas–solids contacts and reactions in the riser. The moving bed air reactor has advantages in terms of having a low pressure drop, continuous solids flow, and large gas–solids contact area except for the risk of plugging and particle leakage caused by excessive high cross-flow gas velocity. Independent pressure adjustments between the two reactors could control the gas flow direction and restrain the gas bypassing to ensure high CO2 concentration with little nitrogen dilution. The flexible adjustments of flow parameters (e.g., gas–solids residence time, solids holdups, and solids inventory) have been experimentally achieved with the cold model experimental system. Valuable data and operational experience for the further hot experimental system have also been obtained. The design process for the future hot experimental system has also been briefly discussed in this paper.

This study presents the results obtained from the operation of a 20 kWth in situ gasification chemical-looping combustion (iG-CLC) unit with a Chinese bituminous coal as the fuel and a natural iron ore as the oxygen carrier. This unit was built based upon the concept of a novel iG-CLC system evaluated through our previous cold-model tests. It is mainly composed of a high-flux circulating fluidized bed riser as the fuel reactor, a cross-flow moving bed as the air reactor, and a combination of an inertial separator and a cyclone separator as the separation system. The actual thermal power inputs from the coal for the reported tests in this study were about 11 kWth and the total duration reached about 70 h. The flow patterns and reaction characteristics of the whole system were investigated, and the effects of the fuel reactor temperature on the distributions of gas components in the two reactors were elucidated. At the fuel reactor temperature of 950 °C, the CO2 yield, gas conversion, carbon capture efficiency, and total solid fuel conversion reached high values at 90.47%, 92.77%, 97.12%, and 98%, respectively. During the reported tests, the iron ore exhibited adequate reactivity and oxygen transport capacity, good cyclic stability, low tendency for agglomeration and high resistance to attrition.

A series of chemical-looping combustion (CLC) tests were conducted in a thermogravimetric analysis (TGA) reactor to investigate the potential of a Chinese sulfur-containing lean iron ore as the oxygen carrier. Two main products of solid-fuel pyrolysis and gasification, namely, CH4 and CO, were selected as the reducing gases. Consecutive reduction–oxidation cycles were first carried out in the TGA reactor to evaluate the cyclic stability and agglomeration tendency of the oxygen carrier. The effects of the temperature, fuel gas concentration, and reaction gas composition on the reduction reaction were further investigated. Increasing the reaction temperature or fuel gas concentration enhanced the reduction rate and reaction degree of the oxygen carrier. Meanwhile, CO showed much higher reduction reactivity than CH4. A comparison of the rate index of the iron ore used with those of high-grade minerals indicated that the iron ore had adequate reactivity for its application in solid-fuel CLC technology. The side reaction of carbon deposition was also discussed. Finally, the shrinking-core model with chemical reaction control was adopted to determine the chemical kinetics.

In situ gasification chemical looping combustion (iG-CLC) is a promising coal combustion technology for implementing CO2 capture with a low energy penalty. A novel iG-CLC cold experimental system was developed in the authors’ previous work ( Ind. Eng. Chem. Res. 2013, 52, 14208). It mainly consists of a high-flux circulating fluidized bed (HFCFB) riser as the fuel reactor and a cross-flow moving bed as the air reactor. As an extension of that work, in this study, we further optimized the iG-CLC system by redesignung the air reactor to enhance the carrying capacity of the gas flow and developing a two-stage separation system by adding a second-stage cyclone to the original first-stage inertial separator. Stability in operation and flexibility in adjusting operating parameters were achieved with the improved system. In the riser (fuel reactor), higher solids fluxes and solids holdups were achieved, which should enhance the gas–solid contact and promote the complicated heterogeneous reactions. In the moving bed (air reactor), the carrying capacity of the gas flow was significantly enhanced, which should lead to a great increase in the system power capacity. The confirmation of the ability to control the gas flow directions in the two reactors means that the gas bypassing between the two reactors can be restrained so as to ensure a high CO2 concentration in the exhaust of the fuel reactor. The high global separation efficiency and selective separation efficiency of the new two-stage separation system for fine particles indicate that a high combustion efficiency of coal can be achieved with a hot iG-CLC system.

•A three-fluid model was developed to investigate hydrodynamics in a two-jet spout fluidized bed.•Simulations were performed in three-dimensional domains.•Mixing and segregation mechanism for binary particle mixtures was analyzed.This study presents a three-dimensional numerical study of the mixing and segregation of binary particle mixtures in a two-jet spout fluidized bed based on an Eulerian–Eulerian three-fluid model. Initially, the particle mixtures were premixed and packed in a rectangular fluidized bed. As the calculation began, the gas stream was injected into the bed from the distributor and jet nozzles. The model was validated by comparing the simulated jet penetration depths with corresponding experimental data. The main features of the complex gas–solid flow behaviors and the mechanism of mixing and segregation of the binary mixtures were analyzed. Moreover, further simulations were carried out to evaluate the effects of operating conditions on the mixing and segregation of binary particle mixtures. The results illustrate that mixing can be enhanced by increasing the jet velocity or enlarging the difference of initial proportions of binary particle mixtures.

In this study, we present the preparation of nickel oxide/carbon using sewage sludge as precursor and nickel nitrate as activating agent by one-step pyrolysis method. The nickel oxide could be self-assembled on the surface of sewage sludge based carbon materials. The surface modified carbon adsorbent was characterized by Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy (SEM), Fourier-transform infrared spectra (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and energy dispersive spectrum (EDS) to illustrate the successful deposition of nickel oxide on the carbon adsorbent. The effect of the surface activation treatment on the adsorption performance of the carbon adsorbents was investigated. The results indicated that the prepared nickel oxide/carbon adsorbents produced beneficial effects on sulfur dioxide adsorption performance due to the existence of nickel oxide and nickel particles. Furthermore, the Fourier-transform infrared spectra and thermogravimetric method are used to illustrate the possible adsorption mechanism.Highlights► The nickel oxide/carbon composite is prepared by novel direct preparation method. ► The effect of the surface modification on the adsorption performance was investigated. ► The SO2 adsorption capacities are comparable to commercial activated carbons. ► The carbon composite could improve adsorption performances by two possible ways.

Co-fluidization characteristics of irregular particles with bed material have been investigated. Experiments were carried out in a solid waste fluidized bed with cross section of 0.2 m × 0.2 m and height of 2 m. Four particles differing in shapes, sizes and densities were used as simulative solid waste, and silica sand was employed as fluidization medium. The pressure drop, flow pattern and minimum fluidization velocity (Umf) under different operating conditions were investigated by recording pressure differential signals and fluidization images. A correlation of Umf was also developed. The results showed that during the fluidization of irregular particles with the bed material of silica sand, the pressure drop curve measured with increasing fluidization gas flow rate was visibly fluctuant and always underestimated the Umf value; while the pressure drop curve measured with decreasing fluidization gas flow rate was smooth and adequate to determine the value of Umf. The Umf was found to be increased with increasing volume proportion and effective particle density, while the initial static bed height has no significant effect on the minimum fluidization velocity. By comparing the Umf values predicted by the developed correlation with the present experimental data and those from literature, it was found that the correlation was quite satisfactory on predicting the Umf value for the fluidization of irregular particles with the bed material of silica sand. The correlation was also found to be applicable to the prediction of Umf values of a solid waste fluidized bed with a single kind or multi-kinds of irregular particles.Co-fluidization characteristics of irregular particles with bed material were investigated. Particles differing in shapes, sizes and densities were tested and silica sand was employed as fluidization medium. The pressure drop, flow pattern and minimum fluidization velocity (Umf) under different operating conditions were tested. Correlation of Umf was also developed.Highlights► Co-fluidization of irregular particles with bed material has been carried out. ► Four kinds of particles with different shapes, sizes and densities were used. ► Pressure drop, flow pattern and minimum fluidization velocity were obtained. ► A correlation of Umf for multi-component irregular particles was developed.

In this work, a comprehensive three-dimensional numerical model was developed to simulate the coal-fired chemical looping combustion (CLC) process using ilmenite as oxygen carrier in a pressurized circulating fluidized bed (CFB) fuel reactor. Both gas–solid flow and chemical reactions were considered. The chemical reactions contain steam gasification of coal and subsequent reduction reactions of intermediate gasification products with the oxygen carrier. This model predicted the main features of the complex gas–solid flow behaviors from the velocity and voidage profiles. The concentrations of gas–solid components, the conversions of char and oxygen carrier, and the distributions of reaction rates were also obtained. Meanwhile, further simulations were performed to evaluate the effects of operating variables on the fuel conversion in the fuel reactor. The conversion of carbon in char is directly related to the extent of gasification, which is promoted by increasing the temperature or the fraction of steam in gasification agent. This work indicates a promising way to research the CLC process of solid fuels in a CFB fuel reactor.

Including the effects of particle–particle collision and particle rotation as well as the volume of particle occupied in the fluid, the Computational Fluid Dynamics (CFD)–Discrete Element Method (DEM) approach could properly predict the behavior of gas–solid flow with strong coupling and the motion of non-spherical particle, thus show good potential in simulations of alveolar region and fibrous particle transport and deposition. CFD–DEM has been developed to investigate the particle transport and deposition characteristics in human airway. An idealized pulmonary airway model of generations 3 to 5 has been established using the same geometric parameters as previous experiment conducted by Kim and Fisher (C. S. Kim and D. M. Fisher. 1999). The predicted deposition efficiencies are in good agreement with experimental data. Thus, CFD–DEM could properly simulate the particle behaviors in pulmonary airway. Based on the simulations, particle motions are studied by analyzing the particle positions at different time intervals. The results show that the initial position of particle would notably affect its trajectory. The near wall particles distribute evenly in the daughter tubes when they cross the airway. The most centered particles at inlet travel through the model via the inner side tubes of generation 5. The trajectories of other particles would shift from the inside tubes to the lateral ones of last generation as the distance between particle initial position and tube center increases.Graphical abstractComputational Fluid Dynamics (CFD)-Discrete Element Method (DEM) has been developed to investigate the particle transport and deposition characteristics in human airway. The predicted deposition efficiencies are in well agreement with experimental data, which indicates that CFD-DEM could properly simulate the particle behaviors in pulmonary airway. Qualitative and quantitative investigations of the particle motions have also been carried out.Highlights► CFD–DEM has been developed to investigate particle characteristics in airway. ► The particle transport in pulmonary 3–5 generation airway has been investigated. ► CFD–DEM could properly simulate the particle behaviors in pulmonary airway. ► The characteristics of particle motions have been obtained based on simulation.

A comprehensive three-dimensional numerical model including the kinetic theory of granular flow and complicated reactions was developed to simulate the chemical looping combustion process in the fuel reactor. The standard κ–ε model was used to simulate the gas-phase turbulence and the kinetic theory of granular flow to simulate the solid phase. The shrinking core model (SCM) with the reaction controlled by the chemical reaction in the grain was applied. The fuel reactor was designed as a bubbling fluidized-bed reactor with a height of 1.2 m and diameter 0.1 m. A Cu-based oxygen carrier was prepared with 14 wt % CuO on 86 wt % inert Al2O3. The inlet fuel gas was coal gas containing 44.5 vol % CO, 22.2 vol % H2, 22.2 vol % H2O, and 11.1 vol % CO2. The flow patterns, distributions of gas components, and conversions of gas reactants were obtained. The effects of the operating conditions (the initial bed height, bed temperature, and operating pressure) on fuel conversion were analyzed. The results show that the fuel conversion with the same inlet gas velocity would go up by modestly increasing the initial bed height and the temperature but would slightly decrease with an increase in the operating pressure. The high conversion of coal gas with a low solid inventory could be reached in a proper operating condition.

In this study, a detailed model was proposed to simulate the process characteristic of flue gas desulfurization with multi-level humidifying in the underfeed circulating spouted bed (UCSB). The model conducted a steady state one-dimensional model by means of infinitesimal analysis, incorporating with the models of droplet evaporation, shrinking core and sulfur absorption. The trends of moisture, temperature, and sulfur concentration along the axial direction are calculated. The predictions were also compared with the experimental results, and they are in very good agreement. The results indicate: The inlet temperature and jet water flow rate are found to be very important factors to the sulfur removal efficiency. Applying multi-level humidifying, the outlet temperature decreases. As a result, the droplet evaporation gets slower and the sulfur removal efficiency increases. Degressive distribution of jet water is also found beneficial to acquiring higher desulfurization efficiency.A detailed model was proposed to simulate the flue gas desulfurization in the underfeed circulating spouted bed. The model conducted a steady state one-dimensional model by means of infinitesimal analysis, incorporating with the models of droplet evaporation, shrinking core and sulfur absorption. The trends of temperature, and sulfur concentration along the axial direction are calculated.

Pressurized turbulent circulating fluidized-bed (CFB) gasification of bituminous coal is successfully realized in a lab-scale fluidized-bed gasifier, and the effects of the stoichiometric ratio, steam/coal ratio, and preheated temperature of the gasifying agent on coal gasification characteristics are studied in this work. Experimental results prove the feasibility of a high-temperature air/steam-blown gasification process. The lower heater value is increased by 21%, when the preheated temperature of the gasifying agent is increased from 400 to 700 °C. When the stoichiometric ratio and steam/coal ratio increased, the component of syngas has a regular change according to the gasification temperature. With the increase of the stoichiometric ratio, the gasification temperature is increased, the H2 concentration increased first and then decreased, the CH4 concentration decreased, and the CO and CO2 concentrations increased first and vary little in the second stage because of the steady gasification temperature. With the increase of the steam/coal ratio, the gasification temperature shows a decrease tendency, the CH4 concentration in gas remains basically unchanged, the CO concentration decreases appreciably, and the H2 concentration first increases and then decreases, while the CO2 concentration shows the opposite trend of H2. This indicates that the gasification temperature is the most important factor influencing coal gasification. Parameters of the equivalence ratio and steam/coal ratio exist at an optimum operating range, and at this range, the coal gasification shows advantages in lower heating value, cold gas efficiency, and carbon conversion for the bituminous coal CFB gasification process. In comparison to the two other representative types of gasifications using Chinese bituminous coal, pressurized CFB gasification decreases the carbon content of fly ash much better.

A Eulerian multiphase model with the kinetic theory of granular flow (KTGF) was studied for modeling the hydrodynamic behaviors of high-flux circulating fluidized beds (HFCFBs) with Geldart group A particles. The sensitivities of several key models (i.e., turbulence model, drag model, and granular shear viscosity model) and modeling parameters (particle−particle restitution coefficient, particle−wall restitution coefficient, and specularity coefficient) on the predictions have been tested systematacially. Experimental results of Pärssinen and Zhu [AIChE J. 2001, 47 (10), 2197−2205; Chem. Eng. Sci. 2001, 56, 5295−5303] were used as a numerical benchmark to assess the simulations quantitatively. The results show that particle−particle and particle−wall restitution coefficients are not critical for the holistic distribution trends of solid volume fraction and solid velocity. A small specularity coefficient, such as φ = 0, could give the well predictions. The Syamlal−O’Brien drag model displays better agreement with individual radial distributions of both solid volume fraction and solid velocity, in terms of microscopic features. While the Syamlal shear viscosity model fails to obtain right trends at the upper parts of the riser. For turbulence model, the mixture turbulence model and per-phase turbulence model could not predict reasonable trend of radial solid velocity distribution along the riser. As a result, a group of suitable models and modeling parameters—i.e., dispersed turbulence model, Gidaspow shear viscosity model, Syamlal−O’Brien drag model, a specularity coefficient of φ = 0, a particle−particle restitution coefficient of e = 0.9, and a particle−wall restitution coefficient of ew = 0.99—are proposed for modeling Geldart group A particle flow in HFCFB risers. Finally, further validations have been conducted to confirm this suggestion, by comparing simulated results with experimental data.

Experimental studies on flow patterns and transitions were carried out in a visible multiple-spouted bed. The bed combines three spouted bed cells, each with a cross-section of 100 × 30 mm, and each cell has an independent spout nozzle of 10 mm in width. Polypropylene beads with a density of 900 kg/m3 and mean diameter of 2.8 mm were used as bed materials. Six distinct flow patterns, i.e., fixed bed (FB), internal jet (IJ), internal jet with bubble (IJB), single spouting (SS), multi-spouting (MS), and internal jet with slugging (IJS), were determined on the basis of criteria as well as schematic diagrams and typical flow pattern images obtained from a high-resolution digital charge coupled device (CCD) camera. Typical flow regime maps at three static bed heights were plotted to describe the transitions of flow patterns with central and auxiliary spouting gases. Besides, some important flow characteristics associated with flow patterns and transitions, i.e., minimum spouted velocity and pressure drop, were studied. The results showed that the kind of flow pattern was dependent upon the static bed height; in particular, the flow pattern of IJS was only found at a high static bed height. The central minimum spouting velocity increased with an increasing static bed height, decreased with a low auxiliary spouting gas flow rate, but increased with a high auxiliary spouting gas flow rate. The total pressure drop increased first and then decreased gradually with the auxiliary spouting gas at a certain central spouting gas flow rate, while it increased first and then remarkably decreased with the central spouting gas at a given auxiliary spouting gas flow rate.

Modeling of the hydrodynamic behaviors of high-flux circulating fluidized beds (HFCFBs) with Geldart group B particles has been performed using a Eulerian multiphase model with the kinetic theory of granular flow (KTGF). The essential models involved are the dispersed k−ε turbulence model, the Gidaspow shear viscosity model, and the Syamlal−O’Brien drag model, and the boundary condition is the Johnson and Jackson wall boundary condition. The sensitivities of key model parameters (i.e., particle−particle restitution coefficient (e), particle−wall restitution coefficient (ew), and specularity coefficient (φ)) on the predicted gas velocity, solids velocity, and solids volume fraction were tested. It was found that e has remarkable dependence on the particle diameter. Large-sized particles experience a more sensitive effect of e on predictions. The particle−wall restitution coefficient ew has somewhat of an effect on the simulated values of gas velocity, solids velocity, and solids volume fraction; however, no critical changes in the trends of their radial distributions have been found. The specularity coefficient φ has a slight effect on the gas velocity and solids velocity distributions but a pronounced effect on the solids volume fraction distribution. An increase in specularity coefficient results in a reduction in the solids volume fraction near the wall. Based on the comparisons of simulated results with experiments, a group of suitable model parameters for modeling the flow of Geldart group B particles in HFCFB risers by a Eulerian multiphase model with KTGF was determined and verified. Besides, some interesting simulated results that are difficult to measure experimentally were presented under the suggested model parameters.

The particle mixing behavior has been investigated experimentally in a spout-fluid bed. The bed material used is polypropylene particles, and the tracer employed is chosen from silica gel, mung beans, and glass beads. During all experiments, the completely segregated arrangement of particles is adopted as the initial packing condition. The gas velocity varies to cover internal jet, minimum spouting, and fully developed spouting conditions, in order to establish a full mixing map about the effect of particle density. The mixing behavior is analyzed in terms of flow patterns, concentration profile, and mixing index. The results show that the degree and rate of mixing are significantly affected by the particle density, and heavier particles achieve a higher mixing rate but a poorer mixing quality. The mixing map reveals that to reach the same mixing index, the spouting gas velocity needs to increase with the increase of particle density. Besides, provided that the jet gas velocity does not exceed the minimum spouting gas velocity, a segregation phenomenon can always be observed by simply adjusting the ratio of the fluidizing to spouting gas velocity.

In this study, a detailed model was proposed to simulate the process characteristic of flue gas desulfurization with multi-level humidifying in the underfeed circulating spouted bed (UCSB). The model conducted a steady state one-dimensional model by means of infinitesimal analysis, incorporating with the models of droplet evaporation, shrinking core and sulfur absorption. The trends of moisture, temperature, and sulfur concentration along the axial direction are calculated. The predictions were also compared with the experimental results, and they are in very good agreement. The results indicate: The inlet temperature and jet water flow rate are found to be very important factors to the sulfur removal efficiency. Applying multi-level humidifying, the outlet temperature decreases. As a result, the droplet evaporation gets slower and the sulfur removal efficiency increases. Degressive distribution of jet water is also found beneficial to acquiring higher desulfurization efficiency.A detailed model was proposed to simulate the flue gas desulfurization in the underfeed circulating spouted bed. The model conducted a steady state one-dimensional model by means of infinitesimal analysis, incorporating with the models of droplet evaporation, shrinking core and sulfur absorption. The trends of temperature, and sulfur concentration along the axial direction are calculated.Download full-size image