Modeling study for the effect of particle size on char gasification with CO2

A high temperature stage microscope to investigate the temperature effect caused by particle size on char gasification is applied in this study. Experiments were carried out with different particle sizes for raw chars and chars on molten slag surface, respectively. Heat transfer models were built for the raw char of two temperature distributions and char particle on molten slag, respectively. Results showed that reaction layer temperature of raw char decreased in the reaction dominant while char on molten slag had higher temperature. Temperature difference between two distributions increased with the initial particle size, indicating the temperature effect on large particles was obvious. Shrinking core model was applied and modified herein coupled with the modification of reaction layer temperature and reaction area. Model prediction and experimental data showed good agreements of carbon conversion and reactivity index for raw char and char on molten slag, respectively. © 2016 American Institute of Chemical Engineers AIChE J, 63: 716–724, 2017     

Effect of carbonization temperature on the yield and porosity of char produced from palm shell

Several batches of chars were prepared from palm shell by carbonization in a flow of nitrogen using a fixed‐bed reactor. Palm shell was carbonized at temperatures of 500, 600, 700, 800 and 900 °C for 1 h to study the effects of carbonization temperature on char yield and its porosity. The prepared chars were characterized for the micropore volume using CO₂ adsorption while the meso‐ and macropore volumes were analyzed using a mercury porosimeter. The char yield was around 25% and is comparable with yields reported from other lignocellulosic materials. The results show that carbonization temperature has a significant effect on the micro‐ and mesopore volumes. However, it has negligible effect on the macropore volume. © 2001 Society of Chemical Industry     

Pore‐network modeling of particle retention in porous media

Transport and filtration of micron and submicron particles in porous media is important in applications such as water purification, contaminants dispersion, and drilling mud invasion. Existing macroscopic models often fail to be predictive without empirical adjustments and a more fundamental approach may be required. We develop a physically‐representative, 3D pore network model based on a particle tracking method to simulate particle retention and permeability impairment in polydisperse particle systems. The model includes the effect of hydraulic drag, gravity, electrostatic and van der Waals forces, as well as Brownian motion. A converging‐diverging pore throat geometry is used to capture the mechanism of interception. With the analytical solution of fluid velocity within a pore throat, the trajectory of each particle is calculated explicitly. We also incorporate surface roughness and particle–surface interaction to determine particle attachment and detachment. Pore throat structure and conductivity are updated dynamically to account for the effect of deposited particles. Predictions of effluent concentration and macroscopic filtration coefficient are in good agreement with published experimental data. We find that the filtration coefficient is dependent on the relative angle between fluid flow and gravity. Particle deposition by interception is significant for large particle/grain size ratios. Brownian diffusion is the primary cause of retention at low Peclet numbers, especially for small gravity numbers. Particle size distribution is found to be a cause of hyperexponential deposition often observed in experiments. Permeability reduction was small for strong repulsive forces because particles only deposited in paths of slow velocity. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3118–3131, 2017     

A branched pore kinetic model applied to the sorption of metal ions on bone char

A slightly modified form of the branched pore model of Peel, Benedek and Crowe was successfully applied to describe the batch sorption kinetics of three metal ions—cadmium, copper and zinc—on bone char. In comparison with an analytical film‐surface solution, the additional parameters of the branched pore model were observed to produce a significant improvement in correlating the experimental results. The ranges of the values of the model parameters derived were deemed reasonable and the branched pore sorption capacities of two of the three metal ions were comparable (ca 0.16 mmol g^?1). Given that the surface diffusivities of the metal ions were observed to vary with averaged surface loading, a number of correlations were examined for their accuracy in describing this behaviour. The exponential expression of Neretnieks resulted in the smallest total error when the data for all three metal ions were considered together. Copyright © 2005 Society of Chemical Industry     

Significance of char active surface area for appraising the reactivity of low- and high-temperature chars

Contemporary char reactivity studies have focussed primarily on coal chars prepared under severe (high-temperature) conditions. In this study, the reactivity of chars prepared under mild (low-temperature) conditions has been addressed. A thermogravimetric analysis system (TGA) was used to determine the reactivity of chars in oxidizing atmosphere using isothermal or non-isothermal techniques. Coal chars were prepared in a TGA or in a slow heating rate organic devolatilization reactor (SHRODR) at a temperature range between 500 ° and 950 °C. The chars prepared by mild pyrolysis of coal at 500 °C are shown to be highly reactive. Comparison of reactivities of low- and high-temperature chars shows that the low-temperature chars exhibit higher reactivity than either the parent coals or the high-temperature chars. Correlation between isothermal reactivity results (e.g. time) and non-isothermal reactivity data (e.g. temperature) has been obtained. Hydrogen contents of chars correlate well with the reactivity of the chars. The study confirms the importance of oxygen chemisorption capacity as a significant reactivity parameter for both low- and high-temperature chars. A new approach has been used for calculating the oxygen chemisorption capacity of chars by accounting for the carbon surface sites occupied by hydrogen (and, therefore, these sites were unavailable for oxygen chemisorption). The occupied sites are readily freed during reactivity measurements and thus were available for participation in carbon-oxygen reactions.     

Modeling the influence of changes in aliphatic structure on char surface area during coal pyrolysis

The influence of changes in aliphatic structure on char surface area during coal pyrolysis was modeled, and the effect was introduced to a previous char surface area model for lignite pyrolysis established based on the chemical percolation devolatilization (CPD) model. The modified model can predict not only the N₂ and CO₂ char surface area during rapid pyrolysis of three lignites but also the CO₂ char surface area of two high-volatile bituminous coals; the agreement of the modified model with experiments is improved at high temperatures. The decrease in aliphatic chain length can reduce adsorption positions around aromatic core, and decrease char surface area. When mass release is more than 55% at about 1,100 K, the predicted N₂ char surface area starts to decrease with further generation of volatiles, and the increase of predicted CO₂ char surface area with increasing generation of volatiles also become slow at the end of mass release.     

Numerical simulation analysis of the induced thrust on a char particle in reaction process

The force exerted on particles is of great significance to the flow and reaction characteristics of particles in gasifier. In this study, the unbalanced thrust, especially its magnitude, of a single char particle induced by chemical reactions during combustion process is investigated numerically, based on the random distribution of active sites. It is revealed that the nonuniform distribution of active sites directly leads to the nonuniform absorption of reactants and release of products, which accounts for the net induced thrust on particles. The effects of active site ratio, ambient gas temperature and particle diameter on the induced thrust of reacting particles were investigated. The results show that the induced thrust on particles could be equal to the magnitude of particle gravity. The induced thrust is determined by the nonuniformity of carbon distribution on char surface. The net thrust is enhanced with the increase of specific carbon consumption rate.     

Coupled fluid‐particle modeling of a slot die coating system

This work seeks to develop a fundamental understanding of particle motion in the slot die coating process through studying the interaction of forces between particles, with the die walls and the fluid phase. Coupled computational fluid dynamics and the discrete element method is employed for evaluating the motion of individual suspended particles near moving surfaces in a complex three‐dimensional flow field, motivated by the flow of particle laden fluid in a slot die coating system, including the presence of free surfaces. Overall, the particles follow the flow streamlines and their final position in the coating depends on the initial entry region of the particles. Particles experiencing adhesion with each other agglomerate in the low velocity regions of the coating gap, and have long residence times near the edge of the die at the end of the feed slot in the coating gap. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1933–1939, 2016     

Evolution of pore structure and active surface areas of coal and char during hydrogenation

A Belgian coal (Beringen) has been used in a fundamental study of the changes in pore structure and active surface area during hydrogenation. The coal was devolatilized under N₂ and then hydrogenated at constant temperature up to various conversion yields. Reactivities of the chars were determined by isothermal thermogravimetric analysis. Changes in total surface area, calculated from CO₂ adsorption isotherms at 298 K, and in macropore distribution, obtained by mercury penetration, were found to lead to the same conclusion. In the first reaction stage, the preponderant phenomenon was the opening of previously closed micropores, induced by the material consumed by the reaction. This stopped completely when conversion yield exceeded 55 wt% when the maximum surface area was observed. Then removal of material from pore walls became predominant, pores were enlarged and pore walls disappeared gradually with increasing hydrogenation yields. Oxygen chemisorption capacities were measured at 383 K and 0.1 MPa air to give a relative indication of the concentration of carbon active sites. Active surface area results were compared to total surface area values and correlated with reaction rates.     

Fully resolved simulations of single char particle combustion using a ghost‐cell immersed boundary method

A novel ghost‐cell immersed boundary method for fully resolved simulation of char particle combustion has been developed. The boundary conditions at the solid particle surface, such as velocity, temperature, density, and chemical species concentration, are well enforced through the present method. Two semiglobal heterogeneous reactions and one homogeneous reaction are used to describe the chemical reactions in the domain, and the Stefan flow caused by the heterogeneous reactions is considered. A satisfactory agreement can be found between the present simulation results and experimental data in the literature. The method is then used to investigate the combustion property of a char particle and the interaction between CO₂ gasification and O₂ oxidation. Furthermore, combustion effect on the exchange of mass, momentum and energy between gas‐ and solid‐ phase is explored. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2851–2863, 2018     

Characteristics of single petcoke particle during the gasification process at high temperatures

Particle concentration significantly affected the gasification of petcoke particles according to our previous studies. In this work, gasification characteristics and morphological evolution of single petcoke particle were investigated using a high temperature stage microscope experimental setup. The results showed that the reaction temperature significantly affected the reactivity of petcoke in the temperature range of 1200-1300℃. While the promoting effect on gasification reactivity decreased with further increasing the reaction temperature, the SEM analysis demonstrated the pore development during the gasification process, which attributed to the increase of reaction rate with conversion. The Raman analysis, HRTEM and SEM-EDX analysis showed that the heterogeneous graphitization of petcoke and non-uniform distribution of catalytic elements in petcoke attributed to the development of surface pores with limited depth. The gasification mechanism of petcoke particle can be briefly described as the reaction rate mainly contributed from the fast-reaction area. Besides, the pore development in fast-reaction area also enlarged the surface area of petcoke particle.     

Modeling char surface area during coal pyrolysis: Validation of relationship between pore structure and polymer network

The relationship between pore structure and polymer network during coal pyrolysis was studied by analyzing the evolution of microcrystal, pore structure, and functional groups of char prepared from Naomaohu subbituminous coal by a drop-tube furnace reactor at 600–1000°C. The char specific surface area changes little with aliphatic bridge cleaving at temperatures lower than 700°C; starts to increase at 800°C with the beginning of side chain cleaving; then reaches the maximum with the methyl and methylene content together at 900°C; finally decreases with the further cleaving of aliphatic structures at higher temperatures. Moreover, the lattice stacking height is minimum at 900°C, indicating aliphatic structure can reduce the lattice order degree. These phenomena support the assumption in the CPD-PS model that the side chain cleaving generates open pores and meanwhile reduces the adsorption sites in them, making the char specific surface area first increase and then decrease.     

Characterization of porous structure of coal char from a single devolatilized coal particle: Coal combustion in a fluidized bed

The combustion characteristics of coal char are highly dependent on initial pore structure of devolatilized char as well as on the structural evolution during the combustion of char. The development of pore structure also throws light on the mechanism of the combustion process. In the present work evolution of pore structure of partially burnt coal char of Indian origin has been investigated experimentally in a batch-fluidized bed and analyzed. The BET surface area, micropore surface area and porosity of char at various levels of carbon burn-off have been determined. Experimental specific surface area has been found to agree well with theoretical prediction using random pore model. Modified random pore model is used to determine the active surface area. Char combustion mechanism based on shrinking unreacted core and shrinking reacted core models are delineated during the course of reaction at various bed temperatures. This is substantiated with the proportional representation of ash and carbon matrix in scanning electron microscope images. It is also concluded that in the present investigation the mean pore size is much smaller and hence the Knudsen diffusion predominates. Analysis based on similar experimental observations and models for pore structure evolution to investigate char combustion reaction regime has not been reported in literature.     

Retracted: Three‐dimensional computational fluid dynamic modeling and simulation of fine particle separation from water by a hydrocyclone

The following article from AIChE Journal, entitled “Three‐Dimensional Computational Fluid Dynamic Modeling and Simulation of Fine Particle Separation from Water by a Hydrocyclone,” by Vivek Kumar, Mohd Asif, Sudipto Chakraborty and Bhim C. Meikap, published online on Nov. 7, 2011 in Wiley Online Library (wileyonlinelibrary.com), has been retracted by agreement between the authors, the journal Editor in Chief, Michael P. Harold and Wiley Periodicals Inc. on behalf of the American Institute of Chemical Engineers. The retraction has been agreed due to overlap between this article and the following article, “Large Eddy Simulation of Hydrocyclone‐Prediction of Air‐Core Diameter and Shape,” by Narashima, Brennan, and Holtham published in International Journal of Mineral Processing.     

Nitric oxide emissions and temperature evolution during the char grate‐fired process

Understanding the char grate‐fired process is key to developing a low‐nitric oxide (NO) technology for industrial boilers. In this work, char combustion and NO emissions during a grate‐fired process were studied in a small‐scale one‐dimensional fixed‐bed system by adjusting the char/oxygen (O₂) ratio. Evolution of the surface temperature of the char bed was measured using an infrared temperature measurement system. As the char/O₂ ratio increased, a reaction layering of the char bed occurred. The char bed can be divided into oxygen‐absent and ‐present parts in time, and into reduction and oxidation layers in space. This kind of division was determined by the complete oxidation layer that could deplete all O₂. The reduction layer could reduce NO emissions well. With the increase of the char/O₂ ratio, the char mass proportion of the oxygen‐absent part increased, while that of the oxygen‐present decreased; and the NO emissions and conversion rate of char nitrogen decreased. When combustion began, char started to burn and released a large amount of heat, and the surface temperatures of both the oxidation and reduction layers increased, with a larger rise of the former of about 260 °C. As the reaction proceeded, the surface temperature of the oxidation layer gradually decreased, while that of the reduction layer increased until the char bed was burnt through.     

Three‐dimensional color prediction modeling of single‐ and double‐layered woven fabrics

In this research, the three‐dimensional structural and colorimetric modeling of three‐dimensional woven fabrics was conducted for accurate color predictions. One‐hundred forty single‐ and double‐layered woven samples in a wide range of colors were produced. With the consideration of their three‐dimensional structural parameters, three‐dimensional color prediction models, K/S‐, R‐, and L*a*b*‐based models, were developed through the optimization of previous two‐dimensional models which have been reported to be the three most accurate models for single‐layered woven structures. The accuracy of the new three‐dimensional models was evaluated by calculating the color differences ΔL*, ΔC*, Δh°, and ΔE_CMC(2:1) between the measured and the predicted colors of the samples, and then the error values were compared to those of the two‐dimensional models. As a result, there has been an overall improvement in color predictions of all models with a decrease in ΔE_CMC(2:1) from 10.30 to 5.25 units on average after the three‐dimensional modeling.     

Three‐dimensional direct simulation of a droplet impacting onto a solid sphere with low‐impact energy

In this paper, a numerical model is developed for direct simulation of droplet impinging onto a spherical surface on a fixed Eulerian mesh. The model couples the level‐set method and the interfacial cell immersed boundary method to the single‐fluid formulation of the Navier–Stokes equations which are solved by a finite‐volume projection technique. Moving contact lines are modelled here with a simple static contact angle model. The model is shown to converge, and to agree with previous work in the literature. The model is then applied to investigate the impact behaviour of a droplet onto solid sphere of different diameters at low Weber number and low Reynolds number. The simulation results show that the droplet used in present study seems to deposit on different spherical surfaces through oscillating. The simulated results also suggest that the impacted‐sphere size has a significant effect on the impact dynamics of the droplet. A local breakage phenomenon may be found in the centre of the droplet collision with a smaller sphere during the first recoiling stage. A regime map is then established to provide quantitative analysis for the breakage mode of the current impacting process.     

Three‐dimensional simulations of biofilm growth in porous media

Biofilm growth occurs in a variety of random porous media in a range of industrial processes; prediction of its growth and subsequent influence on hydrodynamics is hence desirable. In this study, we present the first numerical 3D pore‐scale model of biofilm growth in porous media, based on a lattice Boltzmann simulation platform complemented with an individual‐based biofilm model (IbM). We use it to explore the coupled interaction between nutrient mass transport, biofilm growth, and hydrodynamics. Biofilm is shown to be very effective at reducing the permeability of porous media, particularly under nutrient limited conditions. We conclude with a direct comparison of 3D and 2D biofilm growth simulations in porous media and show the necessity of performing the simulations in 3D. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

A study of the three‐dimensional particle size segregation structure in a rotating drum

Three‐dimensional particle size segregation structures of binary mixtures of six different size ratios (SR = 1.42–3.37) in a rotating drum are studied. The formation of two smaller particle satellites around the central smaller‐particle rich band after the band formation core‐thickening mechanism is reported for the first time. The binary mixtures of six SRs show three satellite shapes, including the small bump shape, the axe shape, and the hemisphere shape. Except for the binary mixture of SR = 2.01 with the axe satellite shape, the satellite size increases with the increasing of the SR value at the same bed depth. The degrees of mixing of binary mixtures of six different SRs at different bed depths are analyzed using the Lacey mixing index. The degree of mixing at the bed surface and close to the drum cylindrical wall can be explained by the drift‐diffusional model of Savage (1993). © 2011 American Institute of Chemical Engineers AIChE J, 2012