1,721,032 research outputs found
Mixing and segregation in fluidized bed thermochemical conversion of biomass
No full textFluidized bed thermochemical conversion of biomass (combustion, gasification, pyrolysis) displays a long record of successes, spanning from lab- to industrial scales, and stems out as the most viable pathway for the exploitation of biogenic fuels, either alone or in combination with fossil fuels. In spite of its diffusion, there are still open design and operational issues that are largely related to segregation and mixing of solid and gas phases in fluidized beds and effectiveness of multiphase contacting patterns. The common claim of fluidized beds being well stirred/well controlled environments for heterogeneous and gas-phase reactions falls short when applied to processing of biomass fuels.This study aims at providing a comprehensive framework of fundamental phenomena affecting mixing/segregation of phases during thermochemical processing of biomass and of their interlinks. The basic processes include patterns and kinetics of biomass devolatilization, particle and volatile matter (VM) segregation along and across the reaction chamber, particle attrition/fragmentation and generation of fine particulates, the diversity of gasification patterns and rates, as related to chemical composition and morphology of the parent biogenic fuels. Segregation brings about important consequences in terms of temperature uniformity, of proper control of heterogeneous and gas-phase reaction pathways, of ash behaviour, of pollutant emissions, of plant operability and dependability. Measures to counteract segregation, including pre-processing of biomass and/or appropriate control of bed hydrodynamics, will also be surveyed from the fundamental and applied standpoints
Flow structures and mixing patterns in the freeboard of gas-fluidized bed reactors
No full textFlow structures and gas mixing patterns establishing in the freeboard of a bubbling gas-fluidized bed have been investigated by means of a planar laser light scattering (PLLS) technique. Nondiffusive tracing of the flow structures has been accomplished by injection of an aerosol of micrometer-sized scattering particles at a preset axial position in the splash zone of the reactor. A digital image acquisition and analysis technique has been developed and used to characterize the macroscopic and microscopic features of the flow structures. Experiments have been carried out in the freely bubbling regime at gas superficial velocities ranging from incipient fluidization to U/Umf = 2.6. Bed solids consist of glass beads of two different size ranges belonging to group B of the Geldart classification of powders. Results are analyzed with the aim of determining effective gas dispersion coefficients and characteristic time-scales for gas micromixing in the splash zone as a function of the excess gas superficial velocity
Multiphase flow patterns in entrained-flow slagging gasifiers: Physical modelling of particle-wall impact at near-ambient conditions
No full textParticle–wall interaction phenomena relevant to multiphase flow in entrained-flow slagging coal gasifiers have been investigated. The micromechanical patterns of particle impingement on the reactor walls have been characterized in a model system by high speed imaging and tracking of wax particles impacted onto a flat surface at near-ambient conditions. The solid/plastic versus fluid state of the wax particles was controlled by proper selection of the particle, ambient and target temperatures. Particle–wall collision was described in terms of normal and lateral restitution coefficients and capture efficiency. The influence of the particle stickiness, impact velocity and angle, and surface properties and structure of the target on the rebound patterns was studied. Results indicate that the elastic–plastic adhesive model provides an adequate representation of the non sticky particle–wall collisions. Moreover, the presence of a powder layer on the target favours energy dissipation and accumulation of particles close to the surface. This pattern promotes the establishment of a dense-dispersed phase in the near-wall zone of entrained-flow slagging gasifiers. Increasing the temperature, particles shift from the solid/plastic to the fluid state and the coefficient of restitution drops to vanishingly small values, confirming that deposition is the prevailing phenomenon during the collision of sticky particles on a wall
Gas and solid flow patterns in the loop-seal of a circulating fluidized bed
No full textGas and solid flow patterns in a loop-seal operated as solid re-injection device between the downcomer and the riser of a lab-scale cold CFB apparatus were characterized under a broad range of operating conditions by a combination of methods, including measurement of pressure time series along the CFB loop, solid mass flux along the riser and solids and gas tracing into the loop-seal. Analysis of results from solids and gas tracing tests highlights the role of both solid downflow in the supply chamber and gas cross-flow between the loop-seal chambers on the fluidization patterns. An approximate criterion is given for the onset of gas "leakage" between the two loop-seal sections, relevant to loop-seal applications in dual interconnected fluidized beds (DIFB)
Modeling gasification of waste-derived fuels in a rotary kiln converter operated with oxygen staging
No full textThermal conversion of waste-derived fuels is gaining a clear role in the general frame of the circular economy as one pathway to close the recycle loop when a material or chemical recycle is impossible or economically unfeasible. Sewage sludge derived from the treatment of urban wastewaters is currently facing rapidly increasing production volumes and severe restrictions of the conventional disposal options: thermal conversion stems out as the most viable strategy, entailing large reduction of the sludge volume and thermal destruction of the toxic organic constituents. In the frame of thermochemical processing of waste-derived fuels, pyrolysis/gasification presents several advantages over the direct waste-to-energy combustion path, mostly related to the generation of syngas and condensable species which can be easily transported, burned or even exploited in gas-to-liquid fuel or chemical processes. The present study addresses the development of a process for oxy-pyrolysis of sewage sludge in a rotary kiln converter. The aim of the process is the production of syngas from devolatilization of a waste-derived fuel, with oxygen playing the role of promoting autothermal operation of the pyrolyzer by controlled oxidation of volatile compounds. The specific concern of the study is the assessment of the effectiveness of staged oxygen feeding, as opposed to localized feeding at the reactor inlet, as a tool to selectively promote desired secondary reactions occurring in gas phase, like partial oxidation of tars. The converter consists of a rotary kiln in which the oxidizer is fed at multiple coordinates along the reactor axis, so as to obtain a reactant contacting pattern resembling that of a Zwietering reactor. The reactor is modelled at steady state using a 1.5D frame. Material and energy balances are set up considering a semi-lumped kinetic mechanism that was purposely developed to represent the complex chemical pathways of the solid fuel, of the gaseous compounds, of different tar components and of soot. Model results are analyzed with a focus on the effect of axial staging of the oxidizer on the quality of the produced gas and on the performance of the reactor
Eulerian Modeling of Lateral Solid Mixing in Gas-fluidized Suspensions
No full textAbstractWe used the Eulerian-Eulerian modeling approach to investigate lateral solid dispersion in fluidized beds. To estimate the lateral dispersion coefficient (Dsr) we fitted the void-free solid volume fraction radial profiles obtained from the numerical simulations of multifluid models with those obtained analytically by solving Fick's law. The profiles match very well. The values of Dsr obtained numerically are larger than the experimental ones, but the two do have the same order of magnitude. We believe that the overestimation is due to how we modeled the frictional solid stress; we used the kinetic theory of granular flow (KTGF) model for the frictional solid pressure and the model of Schaeffer[20] for the frictional solid viscosity. To investigate how sensitive the numerical results are on the constitutive model used for the frictional stress, we ran the simulations again using a different frictional stress model, and changing the solid volume fraction at which the bed is assumed to enter the frictional flow regime (ϕmin). We observed from the results that Dsr is quite sensitive to ϕmin. This is because the latter influences the size and behavior of the bubbles in the bed. We obtained the best predictions for Dsr when ϕmin is 0.50. The results show that accurate prediction of lateral solid dispersion in fluidized beds depends on adequate understanding of the frictional flow regime, and accurate modeling of the parameters that characterize the latter, in particular the frictional pressur
Particle-wall impact experiments for entrained-flow coal gasifiers
No full textThe present study addresses particle–wall interaction phenomena relevant to entrained-flow coal gasifiers. The dynamics of coal, char and ash particles as they are impacted onto a flat surface in hot conditions has been characterized by means of high speed imaging and tracking. Particle–wall collisions were described in terms of normal, tangential and global restitution coefficients. The influence of carbon conversion, impact velocity and surface properties and structure of the target on the dynamical pattern of rebound has been scrutinized. The results indicate that the restitution coefficients decreased as carbon conversion increased. This feature was more pronounced at large carbon conversion, confirming the criticality of the char/slag transition to particle deposition on the wall
Experimental investigation of Directly Irradiated Fluidized Bed Autothermal Reactor (DIFBAR) for thermochemical processes
No full textThe integration of Concentrating Solar Thermal technology (CST) with thermochemical processes is regarded as a frontier innovation, with potential applications to energy storage and chemical industry. In this context, the development of novel multiphase reactors for CST becomes a strategic goal. This study aims to investigate the potentiality of a Directly Irradiated Fluidized Bed Autothermal Reactor (DIFBAR), that incorporates a solar particle receiver/reactor and a solid-solid heat exchanger for heat recovery. A laboratory prototype is tested with a high-flux solar simulator. The bed inventory is continuously recycled to a reservoir, that can also be operated as a secondary reactor. Black proppant, mullite and doped silica are used for inert experiments. Solid circulation rates in the range 0.5–2.5 g/s and heat transfer coefficients in the range 300–900 W/(m2 K) are estimated. Calcium Looping process is chosen for reactive experiments, using a mixture of limestone and black proppant
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