CFD-DEM simulation of biomass pyrolysis in fluidized-bed reactor with a multistep kinetic scheme

The pyrolysis of biomass in a fluidized-bed reactor is studied by a combination of a CFD-DEM algorithm and a multistep kinetic scheme, where fluid dynamics, heat and mass transfer, particle collisions, and the detailed thermochemical conversion of biomass are all resolved. The integrated method is validated by experimental results available in literature and a considerable improvement in predicting the pyrolysis product yields is obtained as compared to previous works using a two-fluid model, especially the relative error in the predicted tar yield is reduced by more than 50%. Furthermore, the evolution of light gas, char and tar, as well as the particle conversion, which cannot easily be measured in experiments, are also revealed. Based on the proposed model, the influences of pyrolysis temperature and biomass particle size on the pyrolysis behavior in a fluidized-bed reactor are comprehensively studied. Numerical results show that the new algorithm clearly captures the dependence of char yield on pyrolysis temperature and the influence of heating rate on light gas and tar yields, which is not possible in simulations based on a simplified global pyrolysis model. It is found that, as the temperature rises from 500 to 700 \ub0C, the light gas yield increases from 17% to 25% and char yield decreases from 22% to 14%. In addition, within the tested range of particle sizes (<1 mm), the impact on pyrolysis products from particle size is relatively small compared with that of the operating temperature. The simulations demonstrate the ability of a combined Lagrangian description of biomass particles and a multistep kinetic scheme to improve the prediction accuracy of fluidized-bed pyrolysis

Molecular dynamics simulation of flow around a circular nano-cylinder

In this study, the wake flow around a circular nano-cylinder is numerically investigated with molecular dynamics simulation to reveal the micro/nano size effect on the wake flow. The cavitation occurring when Reynolds number (Re) > 101 can effectively influence the wake flow. The Strouhal number (St) of the wake flow increases with the Re at low Re, but steadily decreases with the Re after the cavitation appears. The dominant frequency of the lift force fluctuation can be higher than that of the velocity fluctuation, and be drowned in the chaotic fluctuating background of the Brownian forces when Re {\geq} 127. Also because of the strong influence of the Brownian forces, no dominant frequency of the drag force fluctuation can be observed. The Jz number, which is defined as the ratio between the mean free path {\lambda} of the fluid molecules and the equilibrium distance of potential energy {\sigma}, is newly introduced in order to consider the internal size effect of fluid. The St of the wake flow increases with the Jz until it falls to zero sharply when Jz {\approx} 1.7. It denotes the discontinuity of the fluid can eventually eliminate the vortex generation and shedding. Meanwhile, the St decreases with the Kn because of the intensification of the cavitation.Comment: 17 pages, 17 figures, 37 conference

High-temperature pyrolysis modeling of a thermally thick biomass particle based on an MD-derived tar cracking model

Biomass pyrolysis in the thermally thick regime is an important thermochemical phenomenon encountered in many different types of reactors. In this paper, a particle-resolved algorithm for thermally thick biomass particle during high-temperature pyrolysis is established by using reactive molecular dynamics (MD) and computational fluid dynamics (CFD) methods. The temperature gradient inside the particle is computed with a heat transfer equation, and a multiphase flow algorithm is used to simulate the advection/diffusion both inside and outside the particle. Besides, to simulate the influence of intraparticle temperature gradient on the primary pyrolysis yields, a multistep kinetic scheme is used. Moreover, a new tar decomposition model is developed by reactive molecular dynamic simulations where every primary tar species in the multistep kinetic scheme cracks under high temperature. The integrated pyrolysis model is evaluated against a pyrolysis experiment of a centimeter-sized beech wood particle at 800 to 1050 \ub0C. The simulation results show a remarkable improvement in both light gas and tar yields compared with a simplified tar cracking model. Meanwhile, the MD tar cracking model also gives a more reasonable prediction of the species yield history, which avoids the appearance of unrealistically high peak values at the initial stage of pyrolysis. Based on the new results, the different roles of secondary tar cracking inside and outside the particle is studied. Finally, the model is also used to assess the influence of tar residence time and several other factors impacting the pyrolysis

A new reduced model for the moments of droplet size distribution in condensing flow

Purpose - The purpose of this paper is to create a computationally efficient reduced model (RM) for the moments of droplet size distribution (DSD) in condensing flow. Design/methodology/approach - The kinetic equation (KE) exactly describes the time dependence of the DSD and can be regarded as the most rigorous representation of a system with condensation. Because of the typical wide range in droplet size, the KE requires excessive computational time and is not attractive for most practical applications. To reduce the overall computational efforts, a novel set of moment equations, derived from the KE has been proposed. Findings - To demonstrate the simplicity and accuracy of the model, the authors employ a typical nucleation pulse experiment for which benchmark KE-solutions have also been computed. Comparison of predicted moments from both the RM and the KE approach reveals that the RM is capable of capturing the evolving feature of moments with reasonable accuracy. Originality/value - The authors have created a novel reduced method for numerical computations of the lower-order moments of the DSD in condensing flow. Unlike the typical method of moments, the RM eliminates the need for assumptions on the shape of the distribution function and could estimate the moments at very low computational cost

Eulerian–Lagrangian Simulation of Biomass Gasification Behavior in a High-Temperature Entrained-Flow Reactor

In this paper, a multiscale Eulerian–Lagrangian CFD model based on OpenFOAM has been constructed, which takes into account heat and mass transfer, pyrolysis, homogeneous and heterogeneous reactions, radiation, as well as the interactions between the continuous gas phase and discrete particles. The proposed model is validated and applied to a laboratory-scale biomass entrained-flow reactor. The operating temperatures are high (1000–1400 °C) and influences of five operating parameters (reactor temperature, steam/carbon molar ratio, excess air ratio, biomass type, and particle size) on the gasification behavior are explored. Results show that an increase in the reactor temperature has a positive effect on both the H2 and CO productions; increasing the steam/carbon ratio increases the H2 production but decreases the CO production; increasing the excess air ratio decreases both the H2 and CO productions; the variations in the gas product for the four biomasses studied are not so significant, because of similar biomass nature and, hence, one type can be replaced by another without any major consequences in the gasification performance; and both the CO and H2 productions and carbon conversion decrease with an increase in particle size. Moreover, the predicted results follow the same trend as the experimental data available in the literature. Quantitative comparisons are also made, and the agreement is good.acceptedVersion© American Chemical Society 2014. This is the authors accepted and refereed manuscript to the article

Influence of drag force correlations on periodic fluidization behavior in Eulerian-Lagrangian simulation of a bubbling fluidized bed

In this paper, an Eulerian–Lagrangian approach, in which the gas flow is solved by the volume-averaged Navier–Stokes equation and the motion of individual particles is obtained by directly solving Newton's second law of motion, is developed within the OpenFOAM framework to investigate the effects of three well-known inter-phase drag force correlations (Gidaspow, 1994, Di Felice, 1994 and EHKL, 2006) on the fluidization behavior in a bubbling fluidized bed reactor. The inter-particle and particle–wall collisions are modeled by a soft-sphere model which expresses the contact forces with the use of a spring, dashpot and friction slider. The simulation results are analyzed in terms of particle flow pattern, bed expansion, bed pressure drop and fluctuation frequency. Qualitatively, formation of bubbles and slugs and the process of particle mixing are observed to occur for all the drag models, although the Gidaspow model is found to be most energetic and the Di Felice and EHKL models yield minor difference. The flow behavior also shows a strong dependency on the restitution coefficient e and the friction coefficient μ and no bubbling and slugging occur at all for the ideal-collision case (e=1, μ=0). Quantitatively, the mean pressure drops predicted by the three models agree quite well with each other and the amplitudes of the fluctuations measured by the standard deviation are also comparable. However, a significant difference in fluctuation frequency is found and the Gidaspow model predicts a lowest fluctuation frequency whereas the Di Felice model gets a highest one. Finally, effects of the spring stiffness and the discontinuity in the Gidaspow model are studied. The results show that both mean bed pressure drop and fluctuation frequency slightly decrease as the spring stiffness increases for all the three drag models and no significant differences are observed in the mean bed pressure drop and fluctuation frequency between the Gidaspow model and the linear continuous model

CFD-DEM simulation of biomass gasification with steam in a fluidized BEd reactor

A comprehensive CFD–DEM numerical model has been developed to simulate the biomass gasification process in a fluidized bed reactor. The methodology is based on an Eulerian–Lagrangian concept, which uses an Eulerian method for gas phase and a discrete element method (DEM) for particle phase. Each particle is individually tracked and associated with multiple physical (size, density, composition, and temperature) and thermo-chemical (reactive or inert) properties. Particle collisions, hydrodynamics of dense gas-particle flow in fluidized beds, turbulence, heat and mass transfer, radiation, particle shrinkage, pyrolysis, and homogeneous and heterogeneous chemical reactions are all considered during biomass gasification with steam. A sensitivity analysis is performed to test the integrated model׳s response to variations in three different operating parameters (reactor temperature, steam/biomass mass ratio, and biomass injection position). Simulation results are analyzed both qualitatively and quantitatively in terms of particle flow pattern, particle mixing and entrainment, bed pressure drop, product gas composition, and carbon conversion. Results show that higher temperatures are favorable for the products in endothermic reactions (e.g. H2 and CO). With the increase of steam/biomass mass ratio, H2 and CO2 concentrations increase while CO concentration decreases. The carbon conversion decreases as the height of injection point increases owing to both an increase of solid entrainment and a decrease of particle residence time and particle temperature. Meanwhile, the calculated results compare well with the experimental data available in the literature. This indicates that the proposed CFD–DEM model and simulations are successful and it can play an important role in the multi-scale modeling of biomass gasification or combustion in fluidized bed reactor

CFD-DEM simulation of biomass gasification with steam in a fluidized BEd reactor

A comprehensive CFD–DEM numerical model has been developed to simulate the biomass gasification process in a fluidized bed reactor. The methodology is based on an Eulerian–Lagrangian concept, which uses an Eulerian method for gas phase and a discrete element method (DEM) for particle phase. Each particle is individually tracked and associated with multiple physical (size, density, composition, and temperature) and thermo-chemical (reactive or inert) properties. Particle collisions, hydrodynamics of dense gas-particle flow in fluidized beds, turbulence, heat and mass transfer, radiation, particle shrinkage, pyrolysis, and homogeneous and heterogeneous chemical reactions are all considered during biomass gasification with steam. A sensitivity analysis is performed to test the integrated model׳s response to variations in three different operating parameters (reactor temperature, steam/biomass mass ratio, and biomass injection position). Simulation results are analyzed both qualitatively and quantitatively in terms of particle flow pattern, particle mixing and entrainment, bed pressure drop, product gas composition, and carbon conversion. Results show that higher temperatures are favorable for the products in endothermic reactions (e.g. H2 and CO). With the increase of steam/biomass mass ratio, H2 and CO2 concentrations increase while CO concentration decreases. The carbon conversion decreases as the height of injection point increases owing to both an increase of solid entrainment and a decrease of particle residence time and particle temperature. Meanwhile, the calculated results compare well with the experimental data available in the literature. This indicates that the proposed CFD–DEM model and simulations are successful and it can play an important role in the multi-scale modeling of biomass gasification or combustion in fluidized bed reactor.acceptedVersion© 2015. This is the authors’ accepted and refereed manuscript to the article. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0

Pyrolysis Simulation of Thermally Thick Biomass Particles Based on a Multistep Kinetic Scheme

A new Eulerian-Lagrangian pyrolysis model is developed in the thermally thick regime to study the pyrolysis yields of large biomass particles based on a multistep kinetic scheme. The integrated pyrolysis model is first validated with experiments of centimeter-sized wood particles under both low- and high-temperature conditions. Good agreement is obtained between simulations and experimental measurements for low-temperature (375 and 470 \ub0C) pyrolysis. However, for 800 \ub0C pyrolysis, the predicted char, tar, and gas yields are not as accurately predicted as those of the low-temperature situation, while the overall gas and tar yields show an improvement when secondary tar decomposition is considered. Moreover, simulation results reveal that different sized particles generate similar char, tar, and gas yields, which account for about 30, 52, and 12% of the initial mass, respectively. Besides, a significant increase of the gas yield (∼70%) and a moderate decrease of the char yield (∼25%) are observed when the pyrolysis temperature increases from 400 to 700 \ub0C, while the tar yield only changes slightly with first an increasing trend and then a decreasing trend with raising the pyrolysis temperature. The biomass type also has an important impact on both light gas and tar yields as a result of different components in each particle. Finally, effects of tar cracking and alkali metal catalyst on pyrolysis are also discussed

Poiseuille flow-induced vibrations of two cylinders in tandem

Laminar flows past two tandem cylinders which are free to move transversely in a parallel-wall channel were studied numerically by the lattice Boltzmann method. With fixed Reynolds number Re=100, blockage ratio,beta=1/4 and structural damping xi=0, the effect of streamwise separation between two cylinders at a range of S/D=[1.1, 10] on the motions of cylinders and fluids was studied for both mass ratios of m*=1 and m*=0.1. A variety of distinct vibration regimes involving periodic, quasi-periodic and non-periodic vibrations with corresponding flow patterns were observed. A detailed analysis of the vibration amplitudes, vibration frequencies and relative equilibrium positions for both mass ratios demonstrated that as S/D increases, the interaction of the two cylinders first enhances and then reduces. In the strong coupling regime, both cylinders oscillate periodically around the centerline of the channel with large vibration amplitudes and high vibration frequencies. By comparing with the case of an isolated cylinder, a further study indicated that the gap flow plays an important role in such a dynamic system, and the vortex cores formation behind the front cylinder causes the interaction of the cylinders decouple rapidly. Based on the present observations, such a dynamic model system can be considered as a novel type of vortex-induced vibrations (VIV) and is expected to find applications in fluid mixing and heat transfer. (C) 2013 Elsevier Ltd. All rights reserved