CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors. Part B: Heat, momentum and mass 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 heat, momentum and mass 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. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. 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). The study completes the fast pyrolysis modelling in bubbling fluidised bed reactors.

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.

The flow behaviour of shallow fluidised beds

The flow behaviour of shallow gas-fluidised beds was studied. experimentally using a rotational viscometer, and an inclined open channel. Initially, tests were carried out with the viscometer in order to establish qualitative trends in the flow properties of a variety of materials over a wide range of fluidising conditions. Also, a technique was developed which enabled quantitative viscosity data to be extracted from the experimental results. The flow properties were found to be sensitive to the size, size-range and density of the fluidised material, the type of distributor used, and the moisture content of the fluidising gas. Tests in beds up to 120 mm deep showed that the fluidity of the bed improves with reduction in depth; and indicated a range of flow behaviour from shear-thinning to Newtonian, depending chiefly on fluidising velocity .. Later, an apparatus was built which provided for a steady, continuous flow of fluidised material down an inclined open channel of 3m length x 0.15m square, up to a mass flowrate of 10 kg/s (35 ton/hr). This facility has enabled data to be obtained that is of practical value in industrial applications; which is otherwise difficult in view of the present limited understanding of the true mechanism of fluidised flow. A correlation has been devised, based on analogy with laminar liquid flow, which describes the channel flow behaviour with reasonable accuracy over the whole range of shear-rates used. 1he channeI results indicated that at low fluidiising velocities the flow was adversely affected by settlement of a stagnant layer of particles on to the distributor, which gave rise to increased flow resistance. Conversely, at higher fluidising velocities the resistance at the distributor appeared to be less than at the walls. In view of this, and also because of the disparity in shear-rates between the two types of apparatus, it is not possible as yet to predict exactly the flow behaviour in an open channel from small-scale viscometer tests.

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. © 2010 Elsevier B.V.All rights reserved.

Advances towards a pressurised rotating fluidised bed combustor

Rotating fluidised Beds offer the potential for high intensity combustion, large turndown and extended range of fluidising velocity due to the imposition of an artificial gravitational field. Low thermal capacity should also allow rapid response to load changes. This thesis describes investigations of the validity of these potential virtues. Experiments, at atmospheric pressure, were conducted in flow visualisation rigs and a combustor designed to accommodate a distributor 200mm diameter and 80mm axial length. Ancillary experiments were conducted in a 6" diameter conventional fluidised bed. The investigations encompassed assessment of; fluidisation and elutriation, coal feed requirements, start-up and steady-state combustion using premixed propane and air, transition from propane to coal combustion and mechanical design. Assessments were made of an elutriation model and some effects of particle size on the combustion of premixed fuel gas and air. The findings were: a) more reliable start-up and control methods must be developed. Combustion of premixed propane and air led to severe mechanical and operating problems. Manual control of coal combustion was inadequate. b) Design criteria must encompass pressure loss, mechanical strength and high temperature resistance. The flow characteristics of ancillaries and the distributor must be matcheo. c) Fluidisation of a range of particle sizes was investigated. New correlations for minimum fluidisation and fully supported velocities are proposed. Some effects on elutriation of particle size and the distance between the bed surface and exhaust port have been identified. A conic distributor did not aid initial bed distribution. Furthermore, airflow instability was encountered with this distributor shape. Future use of conic distributors is not recommended. Axial solids mixing was found to be poor. A coal feeder was developed which produced uniform fuel distribution throughout the bed. The report concludes that small scale inhibits development of mechanical design and exploration of performance. future research requires larger combustors and automatic control.

A quasi-stationary approach to drying kinetics of different shaped food particulates in a heat pump assisted by fluid bed dryer

Experiments were undertaken to study drying kinetics of different shaped moist food particulates during heat pump assisted fluidised bed drying. Three particular geometrical shapes of parallelepiped, cylindrical and spheres were selected from potatoes (aspect ratio = 1:1, 2:1, 3:1), cut beans (length: diameter = 1:1, 2:1, 3:1) and peas respectively. A batch fluidised bed dryer connected to a heat pump system was used for the experimentation. A Heat pump and fluid bed combination was used to increase overall energy efficiency and achieve higher drying rates. Drying kinetics, were evaluated with non-dimensional moisture at three different drying temperatures of 30, 40 and 50o C. Due to complex hydrodynamics of the fluidised beds, drying kinetics are dryer or material specific. Numerous mathematical models can be used to calculate drying kinetics ranging from analytical models with simplified assumptions to empirical models built by regression using experimental data. Empirical models are commonly used for various food materials due to their simpler approach. However problems in accuracy, limits the applications of empirical models. Some limitations of empirical models could be reduced by using semi-empirical models based on heat and mass transfer of the drying operation. One such method is the quasi-stationary approach. In this study, a modified quasi-stationary approach was used to model drying kinetics of the cylindrical food particles at three drying temperatures.

The coefficient of restitution of different representative types of granules

Gas fluidised beds have many applications in a wide range of industrial sectors and it is important to be able to predict their performance. This requires, for example, a deeper appreciation of the flow of the particles in such systems using both empirical and numerical methods. The coefficient of restitution is an important collisional parameter that is used in some granular flow models in order to predict the velocities and positions of the particles in fluidised beds. The current paper reports experimental data involving the coefficients of restitution of three different representative types of granule viz. melt, wet and binderless granules. They were measured at various impact velocities and the values were compared with those calculated from different theoretical models based on quasi-static contact mechanics. This required knowledge of the Young's moduli and yield stresses, which were measured quasi-statically using diametric compression. The results show that the current theoretical models for the coefficient of restitution explored here lead to either an over- or an under-estimation of the measured values. The melt granules exhibited the greatest values of the coefficient of restitution, Young's modulus and yield stress. The differences in these values were consistent with the nature of the interparticle bonding for each of the three granule types. A new model for the calculation of the coefficient of restitution of granular material was developed that takes account of the work hardening of the granules during impact. Generally, this model provides an improved prediction of the measured values. (c) 2006 Elsevier Ltd. All rights reserved.

Solids mixing and segregation in a gas spout-fluid bed-effect of the distributor design

This work is concerned with the assessment of a newer version of the spout-fluid bed where the gas is supplied from a common plenum and the distributor controls the operational phenomenon. Thus the main body of the work deals with the effect of the distributor design on the mixing and segregation of solids in a spout-filled bed. The effect of distributor design in the conventional fluidised bed and of variation of the gas inlet diameter in a spouted bed were also briefly investigated for purpose of comparison. Large particles were selected for study because they are becoming increasingly important in industrial fluidised beds but have not been thoroughly investigated. The mean particle diameters of the fraction ranged from 550 to 2400 mm, and their specific gravity from 0.97 to 2.45. Only work carried out with binary systems is reported here. The effect of air velocity, particle properties, bed height, the relative amount of jetsam and flotsam and initial conditions on the steady-state concentration profiles were assessed with selected distributors. The work is divided into three sections. Sections I and II deal with the fluidised bed and spouted bed systems. Section III covers the development of the spout-filled bed and its behaviour with reference to distributor design and it is shown how benefits of both spouting and fluidising phenomena can be exploited. In the fluidisation zone, better mixing is achieved by distributors which produce a large initial bubble diameter. Some common features exist between the behaviour of unidensity jetsam-rich systems and different density flotsam-rich systems. The shape factor does not seem to have an affect as long as it is only restricted to the minor component. However, in the case of the major component, particle shape significantly affects the final results. Studies of aspect ratio showed that there is a maximum (1.5) above which slugging occurs and the effect of the distributor design is nullified. A mixing number was developed for unidensity spherical rich systems, which proved to be extremely useful in quantifying the variation in mixing and segregation with changes in distributor design.

Hospital beds : A primer for counting and comparing

There is considerable public, political and professional debate about the need for additional hospital beds in Australia. However, there is no clarity in regard to the definition, meaning and significance of hospital bed counts. Relative to population, there has been a total decline in bed availability in Australia over the past 15 years of 14.6% (22.9% for public hospital beds). This decline is partly offset by reductions in length of stay and changes to models of care; however, the net effect is increased bed occupancy which has in turn resulted in system-wide congestion. Future bed capability needs to be better planned to meet growing demands while at the same time continuing trends for more efficient use. Future planning should be based in part on weighted bed capability matched to need.

Terrestrial invertebrates of dry river beds are not simply subsets of riparian assemblages

Dry river beds are common worldwide and are rapidly increasing in extent due to the effects of water management and prolonged drought periods due to climate change. While attention has been given to the responses of aquatic invertebrates to drying rivers, few studies exist on the terrestrial invertebrates colonizing dry river beds. Dry river beds are physically harsh and they often differ substantially in substrate, topography, microclimate and inundation frequency from adjacent riparian zones. Given these differences, we predicted that dry river beds provide a unique habitat for terrestrial invertebrates, and that their assemblage composition differs from that in adjacent riparian zones. Dry river beds and riparian zones in Australia and Italy were sampled for terrestrial invertebrates with pitfall traps. Sites differed in substrate type, climate and flow regime. Dry river beds contained diverse invertebrate assemblages and their composition was consistently different from adjacent riparian zones, irrespective of substrate, climate or hydrology. Although some taxa were shared between dry river beds and riparian zones, 66 of 320 taxa occurred only in dry river beds. Differences were due to species turnover, rather than shifts in abundance, indicating that dry river bed assemblages are not simply subsets of riparian assemblages. Some spatial patterns in invertebrate assemblages were associated with environmental variables (irrespective of habitat type), but these associations were statistically weak. We suggest that dry river beds are unique habitats in their own right. We discuss potential human stressors and management issues regarding dry river beds and provide recommendations for future research.