CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors, Part A: Eulerian computation of momentum transport in bubbling fluidised beds

The fluid-particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase, according to the literature. FLUENT 6.2 has been used as the modelling framework of the simulations with a completely revised drag model, in the form of user defined function (UDF), to calculate the forces exerted on the particle as well as its velocity components. 2-D and 3-D simulations are tested and compared. The study is the first part of a complete pyrolysis model in fluidised bed reactors.     

Computational modelling of the impact of particle size to the heat transfer coefficient between biomass particles and a fluidised bed

The fluid-particle interaction and the impact of different heat transfer conditions on pyrolysis of biomass inside a 150 g/h fluidised bed reactor are modelled. Two different size biomass particles (350 μm and 550 μm in diameter) are injected into the fluidised bed. The different biomass particle sizes result in different heat transfer conditions. This is due to the fact that the 350 μm diameter particle is smaller than the sand particles of the reactor (440 μm), while the 550 μm one is larger. The bed-to-particle heat transfer for both cases is calculated according to the literature. Conductive heat transfer is assumed for the larger biomass particle (550 μm) inside the bed, while biomass-sand contacts for the smaller biomass particle (350 μm) were considered unimportant. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Biomass reaction kinetics is modelled according to the literature using a two-stage, semi-global model which takes into account secondary reactions. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of User Defined Function (UDF).     

3D simulation of the effects of sphericity on char entrainment in fluidised beds

The paper presents a 3-dimensional simulation of the effect of particle shape on char entrainment in a bubbling fluidised bed reactor. Three char particles of 350 μm side length but of different shapes (cube, sphere, and tetrahedron) are injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. Due to the fluidising conditions, reactor design and particle shape the char particles will either be entrained from the reactor or remain inside the bubbling bed. The sphericity of the particles is the factor that differentiates the particle motion inside the reactor and their efficient entrainment out of it. The simulation has been performed with a completely revised momentum transport model for bubble three-phase flow, taking into account the sphericity factors, and has been applied as an extension to the commercial finite volume code FLUENT 6.3.     

Numerical simulation of coal gasification at circulating fluidised bed conditions

This paper presents modelling results for a new pressurised fluidised bed gasifier concept, called the Power High-Temperature Winkler gasifier (PHTW gasifier). The numerical simulation of the steam/oxygen blown and lignite-fuelled power plant gasifier is performed on the 4800 t/day (1000 MW) at a pressure of 33 bar. The formation of flow pattern, turbulence, product gas composition, temperature, and radiation heat transfer were investigated. Influence of diameter variation on the flow patterns at constant operating conditions is presented. A comparison between the calculation and literature data of similar fluidised bed systems shows good conformance. To anticipate the solid's behaviour, particle concentration, particle size change due to pyrolysis and surface reactions, and particle tracks were modelled using an Eulerian–Lagrangian approach. While varying the total particle mass flow, the pressure drop as a function of reactor height was observed.     

Measurement and simulation of bubbling fluidised beds

The effective control of systems requires the formulation of suitably robust models of their behaviour. The work described in this paper describes the simulation and modelling of the behaviour of a bubbling fluidised bed. A simple system is investigated consisting of a vertical planar bed. The performance of the bed is characterised by measuring the proportion of the bed occupied by the voids associated with bubbles. From these measurements it is possible to evaluate the response of the bed to changes in the gas flow rate into it in the time domain and through transformation into the frequency domain. These techniques allow a simulation of the bed based on the work of Clift and Grace [R. Clift, J. Grace, Coalescence of bubbles in fluidised beds, A.I.Ch.E. Symp. Ser. 67 (116) (1970) 23–33.] to be validated. The simulation can then be used to evaluate a simple but effective physical model of a bubbling fluidised bed which treats it as being primarily a temporary store of gas. The model represents the dynamics of the bed well and in the form of a transfer function which can be used successfully as a basis for controlling the bed.     

Mixing and segregation in a bidisperse gas–solid fluidised bed: a numerical and experimental study

In many industrial applications of dense gas–solid fluidised beds, mixing and segregation phenomena play a very important role. The extent of mixing and segregation is strongly influenced by the bubble characteristics. Therefore, the extent of mixing and segregation, induced by a single bubble injected in a monodisperse and bidisperse fluidised bed at incipient fluidisation conditions and in freely bubbling fluidised beds has been studied both with well-defined experiments and with a 3D Euler–Lagrangian model. Particle image velocimetry (PIV) was successfully applied to obtain the ensemble averaged particle velocity profile in the vicinity of the bubble in dense gas–solid fluidised systems.

The bubble size of a single injected bubble in a fluidised bed at minimum fluidisation conditions calculated with a 3D discrete particle model (DPM) depended strongly on the selected gas-particle drag model. The widely used Ergun equation combined with the Wen and Yu [Powder Technol. 98 (1998) 38; Chem. Eng. Sci. 47 (1992) 1913] relations overpredicted the bubble size due to an overprediction of the drag force. The DPM with the drag model proposed by Koch and Hill [Annu. Rev. Fluid Mech. 33 (2001) 619], based on Lattice–Boltzmann simulations, gave much better agreement with the experimental findings.

The segregation rates in a bidisperse freely bubbling fluidised bed predicted by the DPM agreed very well with the experimentally measured segregation rates by Goldschmidt [M.J.V. Goldschmidt, Hydrodynamic modelling of fluidised bed spray granulation, PhD thesis, Twente University, 2001].     

Co-combustion of coal and olive oil industry residues in fluidised bed

Recently new environmental regulations of fossil fuels have further increased interest in the use of waste and biomass for energy generation. Co-combustion is generally viewed as the most cost-effective approach to biomass and wastes utilisation by the electric utility industry.The aim of this paper is to assess the feasibility of co-firing coal and a very specific biomass waste from the olive oil industry: foot cake, in a fluidised bed. This waste is quite difficult material to be used in combustion process, due to its high moisture content and alkaline content in ashes.Two different Spanish coals were selected for this study: a lignite and an anthracite. The combustion tests were carried out in the CIEMAT bubbling fluidised bed pilot plant. In order to study the effect of different parameters on the emissions and combustion efficiency, the tests were done using different operating conditions: furnace temperature, share of foot cake in the mixtures and coal type.The pilot plant tests show that the combustion of foot cake/lignite or anthracite mixtures in bubbling fluidised bed is one way to utilise this biomass residue in energy generation. The presence of foot cake in the mixtures has not any significant effect on the combustion efficiency. SO₂ and NO_x emissions decrease when the amount of foot cake in the mixtures increases, while N₂O emission increases.     

CFD study of droplet atomisation using a binary nozzle in fluidised bed coating

An Eulerian–Lagrangian computational fluid dynamics (CFD) model was built to describe two-fluid atomisation in a tapered fluidised bed coater using the air-blast/air-assisted atomiser model. Atomisation was modelled both with and without the inclusion of the solid phase (i.e. gas–liquid and gas–solid–liquid multiphase modelling). In addition, a multi-fluid flow model (Eulerian–Eulerian framework) combined with a population balance model was used as an alternative approach for modelling the spray produced by a two-fluid nozzle. In this approach, the CFD solver couples the population balance equation along with the Navier–Stokes equations for predicting the droplet diameter and mass fraction distribution. Comparison between simulated spray pattern (gas–liquid model) and that experimentally visualised by means of UV illumination was made and a good agreement was obtained. Parametric studies were done in order to investigate the effects of operating conditions on spray cone and liquid mass fraction inside the reactor. Furthermore, comparison of time-averaged fluidised bed behaviour with the inclusion of sprays obtained by both gas–solid–liquid multiphase modelling methods is presented.     

Coarse grained computational fluid dynamic simulation of sands and biomass fluidization with a hybrid drag

The bubbling fluidized bed reactor is widely used in fast pyrolysis of biomass. Discrete simulation of this reactor is challenging due to many sand particles and lack of accurate drag corrections accounting for the interaction of two different solid particles with different properties. In this research, the computational cost is reduced by using the coarse-grained computational fluid dynamic-discrete element method, where many sand particles are lumped into a larger numerical parcel. The Syamlal–O'Brien drag model is used for sand, while Ganser correction coupled with Gidaspow model is used for the nonspherical biomass particles. This hybrid approach shows superior behavior over other drag models using pressure drops as a benchmark. The predicted bed height and pressure fluctuating frequencies compare well with experiment. The mixing of biomass is close to perfect if the superficial velocity is larger than four times the minimum fluidization velocity.     

Measurements and numerical predictions of gas vortices formed by single bubble eruptions in the freeboard of a fluidised bed

Gas vortices generated in the freeboard of a bubbling fluidised bed have become the centre of increasingly more research due to the advances in experimental technology. The behaviour of gas flow in the freeboard of a bubbling fluidised bed is of interest for applications such as the gasification of coal where reactions of gas mixtures, as well as gas–particle heat and mass transfer take place. Knowledge of the hydrodynamics of the gas within the freeboard can be hard to characterise, especially the detailed behaviour of gases escaping from bubbles that erupt at the bed surface. In the present study, experiments were conducted on a rectangular three-dimensional gas–solid fluidised bed. The experiments used a particle imaging velocimetry (PIV) measurement technique to visualise and measure the gas flow within the freeboard after a single bubble eruption. A computational study was carried out using Eulerian–Eulerian, kinetic theory of granular flow approach with a quasi-static flow model and with LES used to account for gas turbulence. Results from a three dimensional simulation of the experimental fluidised bed were compared with experimental velocity profiles of gas flow in the freeboard of the gas–solid fluidised bed after a bubble eruption. The CFD simulations showed a qualitative agreement with the formation of the gas vortices as the bubble erupted. Consistent with experimental findings the CFD simulations showed the generation of a pair of vortices. However, the simulations were unable to demonstrate downward flow at the centre of the freeboard due to particles in free fall after a bubble eruption event was observed in the experiments. Velocity profiles from the CFD data are in reasonably good agreement with the characteristic trends observed in the experiments, whereas the CFD model was able to predict the gas vortices phenomena and the velocity magnitudes were over-predicted.     

Computational fluid dynamic simulation of ethylene hydrogenation in a fluidised bed of porous catalyst particles

Computational fluid dynamics (CFD) is used to study the flow behaviour and conversion in a freely bubbling bed of porous cracking catalyst particles fluidised by a mixture of ethylene and hydrogen on the in‐house code FLOTRACS‐MP‐3D. The solid phase viscosity and pressure are modelled on the basis of kinetic theory of granular flows (KTGF). An effective solid density is calculated to account for the inherent porosity of particles. The cohesive inter‐particle forces are incorporated into the CFD model by using an empirical approach proposed in literature. Qualitatively, the CFD model captures the flow behaviour and heat transfer in the bed quite well. On the quantitative front, the variation of conversion with gas velocity is predicted fairly well with the deviation between the predicted and measured conversion remaining within 20%. © 2011 Canadian Society for Chemical Engineering     

Modelling and simulation of coal gasification process in fluidised bed

A one-dimensional steady state mathematical model and a numerical algorithm have been developed to simulate the coal gasification process in fluidised bed. The model incorporates two phases, the solid and the gas. The gaseous phase participates in the emulsion (with the solid phase) and forms the bubble. The solid phase is composed of carbonaceous material, limestone and/or inert bed material. The model can predict temperature, converted fraction, and particle size distribution for the solid phase. For the gaseous phase, in both emulsion and bubble, it can predict profiles of temperature, gas composition, velocities, and other fluid-dynamic parameters. In the feed zone, a Gaussian distribution for the solid particle size is considered. This distribution changes due to attrition, elutriation, consumption and drag inside the reactor. A system of 29 differential and 10 non-linear equations, derived from the mass, energy and momentum balances for each phase, at any point along the bed height, are solved by the Gear and Adams Method. Experimental data from the Universidad de Antioquia and Universidad Nacional-Medellin have been used to validate the model. Finally, the model can be used to optimise the gasification process by varying several parameters, such as excess of air, particle size distribution, coal type, and geometry of the reactor.     

A circulating fluidised bed

Induced particle circulation was studied in a 0.3 m diam. air fluidised bed of sand with central draught tubes of 0.2 m and 0.15 m diam. and 0.6 m and 1.2 m in length. A “two-dimensional” bed, 0.3 m in width, of similar cross-section, was also used to study catalyst particle circulation. Superficial gas velocities of up to 0.4 m/s of air were supplied to the base of the draught tube to induce particle circulation rates in the annular downcomer of up to 400 kg/m² s. The circulation rate was shown to be affected by the gap height between the distributor and the draught tube, but was not affected by the draught tube length of height of bed above it. A model was developed to predict the circulation rate, assuming that the driving force for circulation was the density difference between draught tube and annulus and that energy was dissipated by particle shear at the walls. The theory is in reasonable agreement with the experimental results. A tentative model for predicting the shear stress at the wall of a flowing fluidised bed is presented.     

Modelling of batch fluidised bed drying of pharmaceutical granules

This paper presents a mathematical model based on a three-phase theory, which is used to describe the mass and heat transfer between the gas and solids phases in a batch fluidised bed dryer. In the model, it is assumed that the dilute phase (i.e., bubble) is plug flow while the interstitial gas and the solid particles are considered as being perfectly mixed. The thermal conductivity of wet particles is modelled using a serial and parallel circuit. The moisture diffusion in wet particles was simulated using a numerical finite volume method. Applying a simplified lumped model to a single solid particle, the heat and mass transfer between the interstitial gas and solid phase is taken into account during the whole drying process as three drying rate periods: warming-up, constant rate and falling-rate. The effects of the process parameters, such as particle size, gas velocity, inlet gas temperature and relative humidity, on the moisture content of solids in the bed have been studied by numerical computation using this model. The results are in good agreement with experimental data of heat and mass transfer in fluidised bed dryers. The model will be employed for online simulation of a fluidised bed dryer and for online control.     

Circulating fluidised bed co-combustion of coal and biomass

Circulating fluidised bed combustion (CFBC) is receiving wide research attention in view its potential as an economic and environmentally acceptable technology for burning low-grade coals, biomass and organic wastes, and thereby mixtures of them. Designs of the existing fluidised bed boilers for biomass combustion are mainly based on experience from coal combustion because the mechanism of combustion of biomass in fluidised beds is still not well understood. A good understanding of the combustion and pollutant formation processes and the modelling of the combustor can greatly avoid costly upsets of the plants.In this paper, the performance of CFBC burning coal and biomass mixtures was analysed. Experimental results were obtained from the combustion of two kinds of coal with a forest residue (Pine bark) in two CFB pilot plants (0.1 and 0.3 MW_th). The effect of the main operating conditions on carbon combustion efficiency was analysed. Moreover, a mathematical model to predict the behaviour of the co-combustion of coal and biomass wastes in CFB boilers has been developed and validated. The developed model can predict the different gas concentrations along the riser (O₂, CO, CH₄, etc.), and the carbon combustion efficiency. The experimental results of carbon combustion efficiencies were compared with those predicted by the model and a good correlation was found for all the conditions used.     

Control of the fluidised bed in the pellet softening process

Multiple fluidised bed reactors for water softening (crystallisation of calcium carbonate) have been in operation at drinking water treatment plants since the late 1980s. Research on the operation of these pellet reactors has been focussed on investigating crystallisation under constant fluidised bed composition. However, in practice the bed composition varies frequently, despite large effort of plant operators. An improvement in the control of the fluidised bed can be achieved by using model-based multivariable control. Due to the nonlinear behaviour of the reactor, caused by water temperature variation during the year, a nonlinear control approach is used. A particle filter, based on a first-principles model, estimates the state of the softening reactor and a nonlinear model-predictive controller determines the values of the manipulated variables. We show in a simulation experiment, that it is possible to keep the reactor at desired operational parameters (pellet size and bed height) under varying operational conditions. In this way, the cost of pellet softening can be reduced and irregularities prevented.     

A new version of CSFB,comprehensive simulator for fluidised bed equipment

A comprehensive simulation program for fluidised bed equipment (CSFB) has been improved and is now able to predict operational conditions of bubbling and circulating fluidised bed equipment such as boilers, gasifiers, dryers, shale-retorting reactors, and pyrolysers with various designs and consuming a wide range of feedstock. This paper concentrates on the improvements on the original bubbling model. Comparisons between simulation results and real equipment operational conditions under bubbling fluidised bed techniques are presented. Since those comparisons showed relatively low deviations, it is believed that CSFB is useful for improving operational conditions of existing industrial units and as auxiliary tool for designing of new equipment.     

The combustion of polymer pellets in a bubbling fluidised bed

Wastes burned in incinerators usually contain polymers, whose combustion can be associated with noxious emissions, unless the conditions are properly selected. This paper investigates how polymers burn in a fluidised bed; in fact, the combustion of a number of polymers, including several types of polyethylene, polystyrene and a polyamide, was studied in a laboratory-size, bubbling, fluidised bed, filled with quartz sand, with no external heating. Pellets of a polymer were mostly thrown into such a bed of sand, fluidised and maintained hot by a fuel-lean mixture of propane, methane or hydrogen in air, which burned soon after entering the bed. In addition, polymers were also used as the only fuel, i.e., added to a hot bed fluidised by only air. Visual observations of burning polymer pellets up to 240 mg were made, as well as video records obtained and the flue gas composition monitored, when the combustor was run at 800–1000 °C with 1.1–2.0 times more O₂ than required for complete combustion. It is clear that a polymer burns as if its volatile content were 100%. The polymer pellet first melts at a rate controlled by heat transfer. However, the melt and the gaseous products of thermal decomposition are dispersed, albeit sometimes slowly, in a fluidised bed. Although the high U/U_mf of above 10 caused some back-mixing of the gas leaving the bed and the combustion efficiency was high (assessed from O₂ consumption and CO₂ production), long streaks or plumes of fuel-rich gases (from each polymer pellet) did reach the freeboard, i.e., these plumes burned as transient diffusion flames at a rate controlled by mixing. By increasing the temperature and the residence times of gas in the bed and freeboard, the observed emissions of CO and hydrocarbons could be considerably reduced. The concentrations of NO were low, except when the polymer contained chemically-combined nitrogen, as in a polyamide. It is concluded that bubbling fluidised beds can be good for incinerating polymers, possibly together with other wastes.