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While in scientific literature much focus is directed toward model validation,comparison, and even parametric investigations on individual model parameters, the possible effects of the used computational fluid dynamics (CFD)software on the results are largely neglected. In this article, CFD modeling ofcirculating fluidized beds (CFB) are performed with Ansys Fluent and OpenFOAM to investigate the effect of software implementation on the simulationresults. Transient Eulerian–Eulerian simulations are performed of two different laboratory-scale CFB cold models in turbulent and circulating fluidized bedconditions. The same mesh and as identical models and settings as possible areutilized on both software. The obtained time-average profiles of pressure, velocity, and solids volume fraction are compared between the software and withavailable measurements. A difference in the granular energy equation was identified between the software, and a modification was made to achieve the sameformulation. The effect of boundary conditions was also investigated. It wasfound that depending on the case, the software could have a notable effect onthe results. These differences are found especially in particle distribution, visiblein the vertical pressure and solids volume fraction profiles as well as in externalcirculation mass flow rates.

A cold flow circulating fluidized bed (CFB) reactor is simulated under three fluidization velocities with the coarse grain discrete element method (DEM) using two different polydisperse particle systems namely glass beads and slightly coarser sand particles of Geldart A-B range. Particle velocities and particle concentration were measured by capacitance probe for the validation of the numerical model. The simulations were carried out using a homogenous drag model and a structure dependent drag model using the theory of energy minimization multiscale method (EMMS). Numerical parameters like grid resolution and computational time were investigated for the coarse grain CFD-DEM model, suggesting a cell uniformity criteria that might lead to more mesh independent results. The simulated macroscopic quantities such as pressure profile are generally in good agreement for all simulated cases using the EMMS model. Microscopic quantities such as particles velocities and solids concentration are partially matched well with the experimental data. The qualitative profiles of particle velocity and particle concentration are in better agreement for the EMMS model than for the homogenous drag model. The simulated reactor outflux using glass beads is well matched with experiment. The simulated reactor outflux with sand material is overestimated with EMMS model, although not that strong as for the Gidaspow model, in comparison to experimental measurements. One reason for the discrepancy is due to the cluster diameter correlation that require further development to be applicable in turbulent fluidization flow regime. Further model improvements are discussed and solutions are provided.

Currently waste incineration is a widely used method of waste management in China. Effective incineration requires understanding and tools to analyze the incineration process leading to good incinerator performance and efficiency as well as lower emissions. In this work, a Chinese circulating fluidized bed (CFB) incinerator firing municipal solid waste (MSW) and coal is modeled with a three-dimensional CFB furnace model to evaluate the incineration process. First, the modeling results are verified with measurement data from the incinerator. Then, the furnace model is applied in the simulation of a case with more dry MSW without coal. The objective of the research is to provide insight and increase understanding of the MSW incineration process. The simulation case of MSW with the lower moisture content highlights the possible reductions of fossil carbon emissions associated with the utilization of coal. To achieve this, a moderate reduction in the moisture content of MSW is required. A comparison between the MSW and coal versus only the MSW shows minor differences in the performance of the incinerator. Utilization of modeling in incinerator studies can aid in development of more efficient CFB incinerators, improving the waste management and reducing the utilization of fossil coal. Post-print / Final draft

Gas-solid fluidized beds have heterogeneous flow structures, which affect the reactions due to uneven distribution and limited mixing of reactants. Some model approaches neglect these heterogeneous structures and use a homogeneous assumption instead. This work demonstrates the effects of neglecting this heterogeneity. Eulerian multiphase modeling is utilized to model fuel conversion in a fluidized bed riser. The effects of the chemical reactivity of the fuel, the amount of fuel in the reactor and the fluidization regime are investigated. The model data are time- and space-averaged into 1D results and compared with a 1D modelling which assumes homogeneous distributions in radial direction. Based on this study, the negligence of the heterogeneous flow structures leads to significantly higher local and overall reaction rates and conversion in the system. © 2020 Elsevier Ltd

The hydrodynamic characteristics of gas and solid flows in circulating fluidized beds (CFB) can also influence other processes. To investigate various features of flows and mass circulation in CFB, a computational study was performed with the support of experiments measured in the riser section of a lab-scale apparatus of CFB. The fluidization velocity was slightly below the terminal velocity to avoid a major solid entrainment. The Gidaspow's model was used to calculate the gas-solid drag term in computational cells, which led to a good agreement with the experimental data of pressure profile. The gas inlet conditions were shown to be uninfluential in the hydrodynamics of major part of the upper riser, and the solid inlet (return) imposed non-uniformity in radial and tangential directions. The time-averaged profiles presented a good indication of the wall layer formation. The rate of circulating mass in the riser from simulations was obtained 25% lower than the experiments. The coefficient of restitution was a parameter with notable influence on the hydrodynamics and the present study suggests that a variable coefficient of restitution in Eulerian models can describe the behavior of CFB system more accurately.

The study analyzes a potential to improve the local waste and wastewater management in a Finnish community. The Lappeenranta wastewater treatment plant (WWTP) is used as a case study. Two different technological setups are considered for improving the wastewater treatment plant. These are used to construct four alternative wastewater reclamation scenarios. The mass and energy balances for the considered scenarios are developed and used as input for the profitability evaluation. The utilization of the sewage sludge from the WWTP and municipal solid waste fraction that cannot be recycled for the generation of heat and electricity at the CHP plant is investigated with the aim to improve the economic performance of the wastewater treatment facility. The studied scenarios are initially compared based on their investment and operational costs. The cost of water treatment increases significantly in the case of the investigated tertiary treatment systems: higher amounts of chemicals and electricity are needed to improve the water quality. At the same time, the study indicates that the profitability of a WWTP integrated with a CHP plant can be reasonably high in a wide range of likely price scenarios for alternative wastewater purification systems. The results of the analysis showed a significant potential for the investigated wastewater reclamation systems to improve the efficiency of solid waste and wastewater management in the community.

Significant effort has been exerted on developing fast and reliable numerical models for modeling particulate flow; this is challenging owing to the complexity of such flows. To achieve this, reliable and high-quality experimental data are required for model development and validation. This study presents the design of a novel test-rig that allows the visualization and measurement of particle flow patterns during the collision of two particle streams. Valuable data sets are provided for the validation of numerical models dedicated to granular flows. The experimental work was conducted for three particle distributions and different configurations of the test rig setup. Additionally, a standard discrete element method for modeling particle transport was applied to the test-rig configuration and the effects of the material spring constant on the predicated flow patterns are investigated. An additional purpose of these simulations was also to collect necessary data for further collision model validation, developed based on a reduced-order technique. Post-print / Final draft

Spent coffee grounds (SCGs) and tea leaf residues are investigated as a potential source of bioenergy and biochemicals and compared to several woody biomasses. The effect of initial processing (coffee roasting and tea oxidizing) on feedstock composition and thermal behavior is assessed experimentally and analytically. In addition to proximate and ultimate analyses, the materials are examined by combustion tests coupled with Fourier-transform infrared spectroscopy and thermogravimetric analysis coupled with mass spectrometry in oxidizing and inert atmospheres. The biggest differences in composition of the samples are found in the high elemental N and S contents of SCGs (2.9 wt% of N and 0.1 wt% of S on average) and especially tea leaf residues (4.5 wt% of N and 0.2 wt% of S on average). The elevated N and S values lead to NO and SO2 release during combustion. Overall, the obtained levels of reactivity and heating values (with a maximum higher heating value of 24 MJ kg−1 for the Arabica-Robusta coffee blend) confirm the significant energy potential of both the coffee and tea samples. Mass spectrometry analysis of the studied materials reveals higher heterogeneity of macromolecular composition of coffee and tea samples compared to woody biomasses. Pyridine, pyrazine, phenols and acetic acid are the most abundant among detected species and have demonstrated beneficial characteristics for a broad spectrum of human activities. The composition characteristics and reactivity test results presented in this work promote more effective valorization and usage of these currently little used biomass residues.

The composition, heating value, and reactivity of different municipal and industrial solid waste materials were studied and compared with biomass and coal samples. Reactivity characterization was performed in a vertical tube reactor by dropping a pelletized sample on a porous grid and monitoring the conversion of the sample by Fourier-transform infrared spetroscopy. Samples were tested in ambient oxygen atmosphere at three temperatures typical to incinerator applications, and in reduced oxygen atmospheres, that better represents conditions near the fuel feeding points in incinerators. Thermogravimetrical analysis was used to analyze selected samples and reactivity information was compared with the vertical tube reactor results. This research provides information on the material composition and reactivity for utilization of munical solid waste and sewage sludge in incineration applications. Comparison of two characterization methods illustrates that the characterization method has a significant effect on the reaction rate results, with the implication that the reaction characterization method should be selected to represent the actual conditions of the application. Post-print / Final draft

In this work, computational fluid dynamics with Eulerian-Eulerian modeling approach is used in the investigation of the effect of the solid phase closure models on the hydrodynamics of a circulating fluidized bed riser. The studied models are the kinetic theory of granular flow and the powder modulus model. Computational performance and model prediction accuracy are investigated by performing transient simulations for three different cases and for different mesh sizes. The modeling results are compared with measured vertical pressure profile, external circulation mass flow rates, and particle velocity and concentration profiles obtained from a laboratory scale circulating fluidized bed unit. Additionally the computational times are compared to estimate the computational performance and to investigate if a simpler set of closures is computationally more affordable. The results indicate that both closure models can give good predictions for some quantities, while lacking in other quantities. The results indicate that the closure models for the solid phase have a noticeable effect on the model predictions, such as the vertical pressure profile and external circulation rate of the particles. Post-print / Final draft

In modeling of large-scale circulating fluidized bed (CFB) furnaces, semi-empirical 3D process models are used in furnace analysis, design and optimization as their accuracy versus computational times make them more favorable than high resolution computational fluid dynamics (CFD) in many engineering type of studies. One of the key aspects in determining the furnace performance and efficiency is the correct modeling of the fuel flow as it determines where the thermochemical reactions take place in the furnace, affecting the temperature and gaseous species distributions. In modeling, this requires the suitable approximation of the momentum exchange between the fuel and the gas phase as well as the fuel and the bed material. To model the momentum exchange with current models presented in the literature, information on the material properties, namely the fuel particle size has to be given. The determination of particle size is affected by the particle shape and for irregular, non-spherical material, such as biomass, currently there are no readily applicable methods to determine the effective momentum exchange based on the particle geometry. In this work, a new gas–fuel momentum exchange model is developed for the fuel flow, which acknowledges the effect of physical properties of the fuel, namely the shape and size. Due to impracticality of determining a representative fuel particle size, characterization data is utilized to directly obtain information on the gas–fuel momentum exchange and the particles are classified based on their elutriation behavior, rather than particle size. The new momentum exchange model is utilized with an existing semi-empirical 3D process model and applied in simulation of a utility-scale CFB furnace firing biomass. The simulation results are compared with measurements and are found to be in good agreement, which demonstrates the applicability of the modeling approach. The presented method improves the modeling of fuel flow with CFB process models, especially with irregular, non-spherical fuels such as biomass. The improved modeling of fuel flow within large-scale CFB furnaces enables faster and more reliable design and analysis of CFB furnaces.

To study the mixing and flow of fuel inside commercial scale circulating fluidized bed (CFB) furnaces, a new model was introduced to solve the flow of fuel in an existing semi-empirical, steady state, three-dimensional Eulerian model frame. The fuel flow model is a simplified momentum equation which is based on the momentum balance considering the fuel inertia, gravity, and drag force from gaseous and solid phase. The model improves the prediction capabilities of fuel flow patterns in the existing model frame for conventional and especially renewable fuels, such as biomass. The fuel flow model improves the analysis of CFBs and can be utilized in the design and development of commercial CFB units.

The shape and size of particles have a major effect on the fluidization behavior of the particles through the drag force from the fluid phase. Several methods to estimate the particle size exist, but without the consideration of the particle shape of irregular particles these methods can produce varying results. The particle shape can be described in numerous ways, but currently no method can successfully describe irregular particles without ambiguity. Many computational models consider all particles as monodisperse perfect spheres, although the real particles often have size and shape distribution. Especially for biomass materials these distributions are usually wide. Disregarding this in modeling might lead to inaccurate results. Another way to describe material drag force is through experimental testing and without detailed knowledge about the material particle size and shape distribution. This paper describes a characterization method of an average drag force between gas and different materials. Material sizes and shapes are defined with optical measurements and the averaged values are combined with the characterization method for comparison with standard drag curves of single particles.

The combined ion exchange-membrane filtration system, whereby an active filtration layer is formed using a slurry flow of resin beads and water, has the potential to reduce energy usage and increase the quality of purified wastewater. The hydrodynamic design of these systems is in its infancy and there is a need for numerical methods to analyze such systems. In this work, computational fluid dynamics coupled with the discrete element method (CFD-DEM) is utilized to analyse the hydrodynamics of the combined ion exchange-membrane filtration system, and the formation of the filtrating bed and the parameters affecting it are studied by varying the flow conditions. As a result, the details of the bed formation with insights on the parameters affecting these phenomena and suggestions for an improved hydrodynamic design of the combined ion exchange-membrane filtration system are presented. Post-print / Final draft

The discrete element method (DEM) is utilized to calculate the drag force on big spherical objects exerted by the cloud of small particles as they are subjected to an initial relative velocity. The drag force is calculated as the average over single collisions within a certain time interval. Then it is comparable to the calculated values obtained from Eulerian simulations, which are performed on the same particulate system by keeping the big spherical objects in discrete form and turning the cloud of small particles into a continuum phase. The DEM simulations indicate that the average drag force on a big particle in a cluster is lower than that on a single big particle. The decrease in drag force is even larger when the big particles in the cluster are arranged in an ordered configuration such as a simple cubic array. In Eulerian simulations, it is found that the model for kinetic viscosity plays an influential role in the calculated drag force. The two models known as Syamlal–O’Brian and Gidaspow models ...

Current paper presents the results of proximate analysis and heating value measurements for several different municipal solid waste (paper, cardboard, waste wood, textile, juice box carton and sewage sludge) and industrial waste fractions (bark, sawmill residue and industrial sludge). The values are compared with the fuel samples (forest residue, eucalyptus wood chips, peat and coal). Municipal solid waste fractions exhibit quite similar composition, comparable on dry basis with the industrial wood wastes, woody fuels and peat. Post-print / Final draft