137 research outputs found
A novel sensor measuring local voidage profile inside a fluidised bed reactor
Liquid-solid fluidisation is frequently encountered in drinking water treatment processes, often to obtain a large liquid-solid interfacial surface area. A large surface area is crucial for optimal seeded crystallisation in full-scale softening reactors. Due to crystallisation, particles grow and migrate to a lower zone in the reactor which leads to a stratified bed. Larger particles adversely affect the surface area. To maintain optimal process conditions in the fluidised beds, information is needed about the distribution of particle size, local voidage and available surface area, over the reactor height. In this work, a sensor is developed to obtain the hydraulic state gradient, based on Archimedes’ principle. A cylindrical heavy object is submerged in the fluidised bed and lowered gradually while its weight is measured at various heights using a sensitive force measuring device. Based on accurate fluidisation experiments with calcite grains, the voidage is determined and a straightforward empirical model is developed to estimate the particle size as a function of superficial fluid velocity, kinematic viscosity, suspension density, voidage and particle density. The surface area and specific space velocity can be estimated accordingly, which represent key performance indicators regarding the hydraulic state of the fluidised bed reactor. The prediction error for voidage is 5 ± 2 % and for particle size 9 ± 4 %. The newly developed soft sensor is a more time-effective method for obtaining the hydraulic state in full-scale liquid-solid fluidised bed reactors
Dynamic behaviour of liquid-solid systems:Modelling and experiments applied to the blast furnace hearth
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The behaviour of gas-emitting particles in fluidised beds
This dissertation concerns solid spheres, with diameters ~ 6 - 10 mm, densities between ~700 - 1500 kg m-3 and emitting gas at various peripheral velocities, Ud, and their tendency to float or sink when introduced into gas-fluidised beds of Geldart Group B particles. This is relevant, for example, to the fluidised bed combustion of biomass, and the apparent tendency of the fuel to devolatilise predominantly near the upper surface of the bed with the attendant undesirable complications of unconverted volatile matter (VM) entering the freeboard.
Inert spheres (viz. where Ud = 0) in a bubbling fluidised bed can sink, even if less dense than the fluidised medium, owing to the additional weight of bed particles which tend to settle on top of them forming a defluidised hood. A 2-D fluidised bed, at room temperature, was used to investigate the structure of the fluidised bed in the vicinity of a cylinder emitting gas, as a mimic of a 3-D system. It was found that if Ud is more than 0.7, then the gas emitted can fluidise the bed particles in the entire defluidised hood. Consequently, it was inferred for a 3-D system that gas emitting spheres are not burdened by a defluidised hood and will rise to the surface more rapidly than inert spheres, which are burdened.
The hypothesis that a gas-emitting sphere forms a pocket of high pressure around its underside sufficient to enable it to hover above the surface of the fluidised bed, was investigated, in a mechanism akin to the Leidenfrost effect exhibited by liquid drops on a hot plate. Experimentation showed that this hypothesis could be rejected. In fact, by observing the structure of the bed and measuring the pressure around a gas-emitting cylinder close to the surface of a 2-D fluidised bed, it was found that the emission of gas from a freely-floating sphere decreases the net upthrust of the bed on its underside thereby causing the sphere to sink lower into the bed than buoyancy alone would suggest. However, it was also discovered that the emission of gas from a sphere sunk deep within a fluidised bed caused the net upthrust from the bed to increase, causing the sphere to rise more rapidly to the surface than an inert sphere. This suggests that there exists a stable depth at which gas-emitting spheres reach dynamic equilibrium just beneath the surface of the bed where the bed’s upthrust matches the weight of the sphere. An interesting aside of investigating the Leidenfrost mechanism was that, as far as Geldart Group B solids are concerned, experiments showed the two-phase theory of fluidisation holds exactly.
To simulate spheres of devolatilising biomass, spheres of dry ice, sublimating in a hot fluidised bed were used, because dry ice emits a single, readily detectable gas. The spheres of dry ice were, however, much denser than any biomass fuel and so only segregated once the rate of sublimation was very high. The external heat transfer coefficient for the spheres of dry ice was measured at a variety of bed temperatures and bed particle sizes. Unlike inert particles, gas emitted by the dry ice particles caused the heat transfer coefficient to a) decrease as the bed material size was decreased and b) decrease as the bed temperature increased. For the first time, a heat transfer model, which accounted for the change in structure of the bed material near the gas-emitting particle, was developed to predict the rate of gas emitted from the dry ice particles and gave good agreement with the experimental results.
A novel method for finding the peripheral velocity of VM, emitted by spheres of biomass during devolatilisation in a fluidised bed, was developed and validated experimentally. The mean molar mass and composition of the VM was measured, with the result that measuring the concentration of the combustion products of the VM alone could be used to find the molar flowrate of the VM. Using this method, values of Ud, for spheres of beech, devolatilising in a hot fluidised bed, were measured and, simultaneously, the depth of the spheres in the bed was determined using X-radiography. The simultaneous measurements of gas velocity and depth allowed the behaviour of freely floating, devolatilising spheres to be compared with the calculations obtained with the 2-D fluidised bed. The spheres of beech remained just beneath the surface of the bed throughout devolatilisation and were less influenced by the mixing motions of the bed than inert spheres, even when the fluidisation velocity was increased. The devolatilising beech behaved much as anticipated by the results of the 2-D bed experiments.
Tentatively, a dimensionless plot was made which, brings the variables Ud, the incipient fluidisation velocity Umf, the densities of the gas-emitting particles and the bed material, and the depth at which a particle will neither rise nor sink in the bed, together. The plot shows under what conditions a gas-emitting particle is likely to have a sinking or rising tendency in a fluidised bed. The plot is a tool for predicting if segregation of a particular fuel particle is likely to occur in any bubbling fluidised bed.
Overall, this dissertation concludes that the emission of VM from a devolatilising particle of biomass not only draws the particle to the surface of the bed but acts to keep it there, even at low rates of gas emission. To eliminate the segregation of biomass during combustion in a bubbling fluidised bed, the biomass must be denser than the emulsion phase of the fluidised bed and the velocity of VM leaving the biomass must be as low as possible. An impracticable degree of pre-processing of the biomass would be required to achieve these conditions.EPSR
Dynamic behaviour of liquid-solid systems:Modelling and experiments applied to the blast furnace hearth
Non-invasive and non-intrusive diagnostic techniques for gas-solid fluidized beds – A review
Gas-solid fluidized-bed systems offer great advantages in terms of chemical reaction efficiency and temperature control where other chemical reactor designs fall short. For this reason, they have been widely employed in a range of industrial application where these properties are essential. Nonetheless, the knowledge of such systems and the corresponding design choices, in most cases, rely on a heuristic expertise gained over the years rather than on a deep physical understanding of the phenomena taking place in fluidized beds. This is a huge limiting factor when it comes to the design, the scale-up and the optimization of such complex units. Fortunately, a wide array of diagnostic techniques has enabled researchers to strive in this direction, and, among these, non-invasive and non-intrusive diagnostic techniques stand out thanks to their innate feature of not affecting the flow field, while also avoiding direct contact with the medium under study. This work offers an overview of the non-invasive and non-intrusive diagnostic techniques most commonly applied to fluidized-bed systems, highlighting their capabilities in terms of the quantities they can measure, as well as advantages and limitations of each of them. The latest developments and the likely future trends are also presented. Neither of these methodologies represents a best option on all fronts. The goal of this work is rather to highlight what each technique has to offer and what application are they better suited for
Assessment of experimental methods for measurements of the horizontal flow of fluidized solids under bubbling conditions
Dual fluidized bed systems are indispensable for future energy systems that require solids cycling between different atmospheres. However, controlling the residence time of solids in the reactor, which is crucial for controlling the heat and mass transfer of the fuel, is a significant challenge. This study investigates four experimental techniques to quantify the horizontal flow of solids fluidized under bubbling conditions: integral mass accumulation; differential mass accumulation; thermal tracing; and magnetic solids tracing. Integral mass accumulation entails collecting bed material using a defluidized box within a given time period. Differential mass accumulation measures the material accumulation rate in a section of the bed that is monitored using pressure measurements. Thermal tracing calculates the solids flow rate by solving the heat balance to match the temperature field captured by a thermographic camera. Magnetic solids tracing involves injecting a batch of magnetic tracer solids into the reactor and then measuring the residence time distribution using impedance coils. The experiments were conducted under down-scaled conditions that resemble large-scale operations with a length scaling factor of 0.12. For this study, three operational parameters were varied: the fixed bed height; the volumetric flow rate of the conveying air; and the fluidization velocity in the bed. The horizontal solids circulation rates achieved ranged from 1.7
710−4 to 10 kg/m\ub7s, corresponding to 1.2
710−3 to 70 kg/m\ub7s on a hot up-scaled basis, which is a relevant range to indirect biomass gasification in an industrial setting. The three selected operational parameters led to increases in the horizontal solids flow. While all four methods replicated the trends, quantitative variations in the measured circulation rates occurred due to the inherent characteristics of the methods. High circulation rates resulted in a continuous decrease in the solids inventory, leading to an underestimation of the circulation rate when using the integral mass accumulation method. The accuracy of the differential mass accumulation method relied on transient pressure measurements, which were less-effective at low solids flow rates. Conversely, the accumulation time required for pressure measurements was reduced at high circulation rates, resulting in uncertainties in the analysis. The accuracy of the thermal tracing method decreased drastically with higher solids circulation, resulting in an overestimation of the circulation rate. Moreover, low circulation rates adversely affected the accuracy of the magnetic solids tracing by producing barely discernible tracer concentration gradients
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The Investigation of Aspects of Chemical Looping Combustion in Fluidised Beds
Chemical looping combustion (CLC) is a promising fossil fuel combustion technology, which is able to separate CO2 from the flue gases without a large consumption of energy. In this thesis, the study was extended to look at the use of chemical looping materials within traditional fluidised bed combustion and investigation of the interaction between the fuel, the supplied air and the chemical looping agent. Three topics of chemical looping combustion are discussed, including 1) the Sherwood number in the fluidised bed; 2) properties of different oxygen carriers, Fe2O3 and CuO (with supporting materials), were tested in the fluidised bed reactor; 3) the simulation of a steady state and a dynamic model of a coal-fired CLC power plant using Fe2O3 as oxygen carriers.
The Sherwood number, which represents the mass transfer rate, is important in the calculation of CLC process. With Sherwood number, the mass transfer rate kg around the acting particle can be calculated using correlation Sh=kg∙d/D, where d is the diameter of acting particle, and D is the diffusivity around the acting particle. Hayhurst and Parmar (Hayhurst and Parmar 2002) calculated the Sherwood number in the fluidised bed by using the CO/CO2 ratio, which was measured by the temperature difference between the carbon particle and the bulk phase (Hayhurst and Parmar 1998). However, the temperature of the particle could be overestimated, so the CO/CO2 ratio could be underestimated. In this thesis, a universal exhaust gas oxygen (UEGO) sensor was employed, which could measure the actual carbon consumption rate in the fluidised bed by oxidizing CO in the sample gas into CO2 and. Fe particles of the same size of the char particle is used to measure the O2 consumption rate, and thus eliminate uncertainty in the Sherwood number. The CO/CO2 ratio was calculated by using the carbon consumption rate and the O2 consumption rate. In contrast to Hayhurst and Parmar (Hayhurst and Parmar 2002) who assumed CO2 was the main product, for this char the actual ratio of CO/CO2 was almost zero. The measurement here is in agreement with Arthur. This more accurate determination of CO/CO2 allows a better estimate of the mass transfer coefficient and leads to a correction of the Hayhurst and Parmar’s (Hayhurst and Parmar 2002) correlation by a factor of ½. Interestingly, very small fluidised beds have mass transfer coefficients which are about twice that expected in a large bed (owing to the very different flow and indeterminate flow pattern). This means the correlation of Hayhurst and Parmar (Hayhurst and Parmar 2002), by fortuitous coincidence works wells for beds with diameters < 30 mm., without the correction factor, should be ignored.
In the fluidised bed in a typical CLC process, different fluidising material could have different influence on the reactions. Thus, it is worth discussing different kinds of fluidising materials. The char combustion in the fluidised bed was simulated by using inert (sand) and active (Fe2O3 or CuO) fluidising materials, and air as fluidising gas. The results indicated that 1) CO combustion in the boundary layer leads to smaller carbon consumption rate and larger oxygen consumption rate; 2) Using Fe2O3 particles as fluidising materials slows down the carbon consumption rate, since the diffusivity of CO2 is smaller than CO; 3) CuO particles slow down the carbon consumption rate at large Sherwood number (Sh=2 or 2.5). The influence of using CuO as fluidising material is further discussed experimentally by using low O2 fluidising gas. The results indicated that since the amount of CuO used in the experiment is small, when the O2 concentration in the bulk phase is lower than the equilibrium concentration, the O2 concentration in the bulk phase gradually decreases, and the O2 concentration in the bulk phase has large influence on the char particle combustion.
A steady state model of a coal-fired CLC power plant was simulated. The aim of the model was to test the suitable operating conditions of the power plant, such as recycle rate of oxygen carriers, for the power plant design. In the steady state model, the power plant consists of a combustor and a steam cycle. Hambach lignite coal, Polish bituminous coal and natural gas were tested as fuels. The results indicated that: (1) The effect of the fuel is largely due to the amount of oxygen required per GJ released; (2) Preheating is important, but seems to have a minor effect since the most of the heat is released at temperatures well above the pinch point; (3) since the temperatures of heat source in this research is well above the pinch point, all heat are usable for the steam cycle. In this case, the steam cycle and the chemical looping plant could be optimised separately; (4) As long as the preheat temperature of the air flow into the air reactor is higher than the temperature of turbines, in most of cases the power output is unaffected by the choice of variables, leaving the designer free to choose the most convenient.
With the conclusions above, a dynamic model of a coal-fired CLC power plant using Fe2O3 as oxygen carrier is then simulated. The aims of this simulation include: 1) explaining the kinetics of Fe2O3 oxygen carriers at high temperature (1223K) in a fluidised bed reactor using Brown’s data (Brown 2010); 2) a 1GWth dynamic power plant was simulated to test different cases including changing power supply and power storage. In the dynamic model, a chemical looping power plant using Hambach lignite char is tested, and the parameters of the system are adjusted so as to simulate the operations of a real chemical looping power plant. The two-phase model is employed for the fluidised bed reactors. Experimental data from Brown (Brown 2010) was simulated using this model first to test its validity. Then the model is scaled up to simulate a 1GWth dynamic power plant. The ideal operation conditions are found, and a char stripper is found helpful for carbon capture.EPSR
Experimental, numerical and analytical study for the improvement of biomass fluidized bed gasifiers
The gasification of biomass is considered one of the most important sources of renewable
energy due to the sustainability of agriculture waste around the world. There are many types
of gasification systems depending on the mechanism of gasification. BFBG is one of the
powerful gasifiers due to the mixing mechanism between the solid materials (biomass and the
inert material) and the gas phase (air). Gasification process in the BFBG involves three main
interactive factors: hydrodynamics, heat transfer and chemical reaction.
The present work focuses on improving the hydrodynamic performance and the product gas
quality of a new BFBG developed at Cardiff University. Hydrodynamics has been analysed
experimentally and numerically using four different distributors designed to improve the
fluidized bed fluidic patterns. The tests have been performed experimentally using a
representative perspex prototype, while an isothermal 3D unsteady-state CFD simulation by
using OpenFOAM software based on multiphase resolution was employed in order to select
the optimal design that can improve the system performance. The post improving of the BFBG
product gas with catalyst has been analysed numerically by using ASPEN PLUS software.
The hydrodynamic behaviour of the BFBG with four different air distributors was studied
experimentally in terms of pressure drop and bubble formation. Two design factors were
observed as the major contributors towards the impact on the BFBG performance, i.e. the
orifice size and the distribution of orifices. Small orifices with triangular arrangement have
demonstrated superior performance than large orifice size with square arrangement. Similar
findings were obtained from the CFD simulation of the BFBG with the four distributors with an
accepted comparison with the experimental results and literature.
Regarding the post -gasification improvement, ASPEN PLUS analysis showed the using of
BFBG product gas with suitable amount of N2 and Ar can increase the H2 and CO selectivity,
H2/CO ratio and decrease the heat duty. The analysis results were compared with literature
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