Mixing of Large Solids in Fluidized Beds

Fluidization is a technology that is widely used in systems in which particulate solids are to be transported, mixed, and/or reacted with gases. In fluidized bed applications, the lateral mixing rate of the solids and the heat and mass transfer with their surroundings play important roles in process performance. These transport mechanisms are affected by the solids axial mixing, as particles immersed in the dense bed will experience higher levels of heat transfer, lower mass transfer, and lower rates of lateral mixing than they would if floating on the bed surface. However, there is a lack of knowledge regarding the effects of the solids properties and operating conditions on the solids mixing. As a consequence, there is a lack of predictive tools that can be used for optimizing the design and operation of fluidized beds.This work focuses on advancing the current understanding of the mixing of large solids (typically fuels) in fluidized beds, with the aims of promoting the design of new applications and improving the scale-up and operation of commercial units. While a generic approach is adopted in terms of considering a wide range of solid particle properties (size and density), the focus is on biomass particles, for which thermochemical conversion fluidized beds are especially suited, due to their: high-level fuel flexibility (being able to convert efficiently low-grade fuels); ability to control emissions with in-bed methods; and inherent capability to capture CO2 with looping dual fluidized bed systems. This work combines semiempirical modeling with experiments that apply magnetic particle tracking in a fluid-dynamically downscaled bed, enabling the closure as well as the validation of the model. By deriving a mechanistic description of the motion of a spherical object, the model identifies key parameters that are crucial for describing the mixing. Among these, the effective drag of the bed emulsion acting on the fuel particle is further studied in dedicated experiments with falling and rising tracers in various types of beds at minimum fluidization. The stress patterns observed in these rheological experiments reveal a non-Newtonian behavior of the drag between the bed emulsion and immersed larger objects. This is then implemented in the model for further upgrading of the mechanistic description. The model is shown to describe ably both axial mixing and the lateral mixing of different fuel types under conditions applicable to industrial-scale hot units.The combination of modeling and experimental work shows that while axial mixing is fostered by increasing the fluidization velocity, bed height, distributor pressure drop, or fuel particle density and decreasing the fuel particle size, only a higher fluidization velocity exerts a clear influence on the lateral dispersion. This can be explained in terms of the influence of the fluidization velocity on the width of recirculation cells, which are found to play a major role in the lateral mixing of fuel particles and warrant further study

Metal-Metal Thermoelectric Harvester

A 3-couples proof-of-concept harvester (55 76 mm2) was assembled by spot welding 0.1 mm thick molybdenum foil and 0.15 mm thick nickel foil together. To insulate the foils from each other at the hot side a 0.1 mm thick glass fiber sheet was placed between the foils. At the cold side the harvester was insulated with 0.1 mm polyimide tape for easier handling and fabrication of the harvester. The load resistance measurement gave impedance match of approximately 0.24 Ω at 28\ub0C, which slowly decreased to approximately 0.1 Ω as the temperature increased to 172\ub0C. With a temperature gradient of 172\ub0C (0-172\ub0C) and 0.125 (\ub10.025) Ω load resistance, a power output was measured to 450 (\ub185) μW at 7.2 mV

Energy Harvesting for Wireless and Less-Wired Sensors in Gas Turbines

Four types of energy harvesters aimed for gas turbine applications were developed during this thesis. The unique gas turbine environment shaped the design- and material choices. A semiconductor thermoelectric harvester was built for a location in the gas turbine with active cooling at 600\ub0C, with 800\ub0C wall temperature. The thesis covers the material synthesis, design, assembly and proof-of-concept tests of this harvester at 800\ub0C. A metal thermoelectric harvester was also built, but instead for locations without active cooling. The harvester design is long metal strips, capable of reaching active cooling far away. This harvester was successfully used to power wireless sensors and reached 290 μW power output after power management electronics. Two different types of piezoelectric harvesters were developed, both consisting of coupled off-the-shelf cantilevers. The development included simulations, analytic models and assembly/measurements on harvesters. The first design was a 2-degree-of-freedom folded coupled harvester which after optimizations achieved a minimum of 2.75 V in the frequency range 92-162 Hz with peak power output of 1.80 mW. The second design was a 4-degree-of-freedom self-tuning harvester, showing increased 3 dB-bandwidth from 8 Hz to 12 Hz with the use of a sliding weight

Energy Harvesting and Energy Storage for Wireless and Less-Wired Sensors in Harsh Environments

Engineering requires sensors to control and understand the environment. This is particularly important in harsh environments. The drawbacks, especially in gas turbines is the complexity of installing a wired sensor and the weight of the wires. This makes wireless sensors attractive.A wireless sensor requires a power source for transmission of data. Batteries have previously taken the role of power source for most wireless sensors, but is unfortunately not suitable for all applications. Lately, with energy harvesting and supercapacitors in the picture, sensor applications in high temperature environments, with high power requirements or with long life requirements have the possibility of wireless interface.A supercapacitor can handle higher temperatures, higher power output and can have a cycle life that exceeds batteries by a factor of 10000. The lower energy density and high self-discharge makes it unsuitable to power a wireless sensor without power source. However, connected to an energy harvester converting waste energy into electricity makes this a powerful combination.Energy harvesters thrives in environments where waste energy is plentiful and low conversion efficiencies can be enough to power both the sensor and the transmitter. A thermoelectric harvester is designed and fabricated for the middle to rear part of a gas turbine. The temperature in this region can reach 1600 ◦ C and require extensive cooling. In the cooling channels the wall temperature reach 800-950 ◦ C when the cooling air is 450-600 ◦ C. In this location a thermoelectric harvester will have access to high thermal gradients and active cooling.To harvest the vibrations a piezoelectric energy harvester was built. To harvest enough energy the resonance frequency of the energy harvester is frequency-matched with the high energy vibrations. In many applications these frequencies drift and thus require a broad bandwidth harvester. Simulation and assembly of a broadband coupled piezoelectric energy harvester is presented in the thesis.A piezoelectric harvester require electronics and energy storage to gather enough energy to power up and run a wireless sensor. The thesis covers the fabrication of a high temperature supercapacitor capable of temperatures up to 181 ◦ C

Axial Mixing of Large Solids in Fluidised Beds – Modelling and Experiments

Fluidisation is a technology commonly found wherever particulate solids are to be transported, mixed and/or reacted with a gas. At present, it is a widespread technology with applications ranging from the production of carbon nanotubes in the manufacturing industry to the conversion of solid fuels in the heat and power sector. As for the latter, fluidised beds are well received for their fuel flexibility (being able to efficiently convert low-grade fuels) and for their ability to control emissions with in-bed methods. In most applications, like solid fuel conversion, the heat and mass transfer between the gas and the solids (e.g.\ua0fuel particles) play an important role in the process performance. In turn, these transfer mechanisms are affected by the axial solids mixing, as solids immersed in the dense bed will experience higher heat transfer and lower mass transfer than otherwise. This work focuses on the axial mixing of large solids in fluidised beds with the aim to advance current knowledge on in-bed mixing with an emphasis on biomass particles. As the latter typically have a high content of moisture, volatile and ash and are larger and lighter than conventional fuels like e.g. coal or lignite, they are even more prone to segregate axially in the bed in a flotsam fashion. Yet, the effect of fuel density and size as well as the effect of fluidisation conditions on the axial mixing of fuel has not been fully understood. To enhance the understanding of solids mixing, this work combines a one-dimensional semi-empirical model with experiments applying magnetic particle tracking (MPT) in a fluid-dynamically down-scaled fluidised bed. The model is used to identify governing mechanisms and the respective key parameters to be studied with dedicated experiments which, in their turn, contribute to the continuous upgrading of the model.The key parameters in the axial mixing of larger solids in a fluidised bed are found to be: i) the apparent viscosity of the emulsion, for which MPT measurements confirmed its Newtonian character, and ii) the bubble flow, which experiments revealed to have a higher upwards velocity and fuel-to-bubble velocity ratio than shown in previous literature not accounting for hot conditions

Modeling Axial Mixing of Fuel Particles in the Dense Region of a Fluidized Bed

A semiempirical model for the axial mixing of fuel particles in the dense region of a fluidized bed is presented and validated against experimental magnetic particle tracking in a fluid-dynamically downscaled fluidized bed (K\uf6hler et al. Powder Technol., 2017, 316, 492-499) that resembles hot, large-scale conditions. The model divides the bottom region into three mixing zones: a rising bubble wake solid zone, a zone with sinking emulsion solids, and the splash zone above the dense bed. In the emulsion zone, which is crucial for the mixing, the axial motion of the fuel particle is shown to be satisfactorily described by a force balance that applies experimental values from the literature and an apparent emulsion viscosity of Newtonian character. In contrast, the values derived from the literature for key model parameters related to the bubble wake zone (such as the upward velocity of the tracer), which are derived from measurements carried out under cold laboratory-scale conditions, are known to underestimate systematically the measurements relevant to hot large-scale conditions. When applying values measured in a fluid-dynamically downscaled fluidized bed (K\uf6hler et al. Powder Technol., 2017, 316, 492-499), the modeled axial mixing of fuel tracers shows good agreement with the experimental data.\ua0\ua9 2020 American Chemical Society

A holistic framework for designing for structural robustness in tall timber buildings

With the ever-increasing popularity of engineered wood products, larger and more complex structures made of timber have been built, such as new tall timber buildings of unprecedented height. Designing for structural robustness in tall timber buildings is still not well understood due the complex properties of timber and the difficulty in testing large assemblies, making the prediction of tall timber building behaviour under damage very difficult. This paper discusses briefly the existing state-of-the-art and suggests the next step in considering robustness holistically. Qualitatively, this is done by introducing the concept of scale, that is to consider robustness at multiple levels within a structure: in the whole structure, compartments, components, connections, connectors, and material. Additionally, considering both local and global exposures is key in coming up with a sound conceptual design. Quantitatively, the method to calculate the robustness index in a building is presented. A novel framework to quantify robustness and find the optimal structural solution is presented, based on the calculation of the scenario probability-weighted average robustness indices of various design options of a building. A case study example is also presented in the end

Determination of the Apparent Viscosity of Dense Gas-Solids Emulsion by Magnetic Particle Tracking

When designing fluidised bed units a key to ensure efficient conversion is proper control of the mixing of the fuel in both lateral and axial directions in the bed. In order to mechanistically describe the mixing of fuel particles in a fluidised bed, there is a need to determine the apparent viscosity of thegas-solids emulsion, which determines the drag on the fuel particles. In this work the apparent viscosity of a bed of spherical glass beads and air at minimum fluidisation was determined by means of the falling sphere method. Hereto the drag of the bed on a single immersed object was obtained by measuring the velocity of a negatively buoyant tracer with magneticparticle tracking (MPT). MPT allows for highly temporally and spatially resolved trajectories (10-3 s and 10-3 m, respectively) in all 3-dimensions. The bed consisted of glass beads with a narrow size distribution (215 to 250 μm) and tracers with a size from 5 to 20 mm and densities from 4340 to 7500kg/m3 were used. Hence, the literature, which typically covers data for velocities lying within or just above the Stoke flow regime (0.002 < Re < 2.0) could be expanded to Re numbers (53 to 152) well within the transition flow regime. The drag and apparent viscosity was compared to different fluidmodels and agreed well with the Newtonian model, when taking into account possible effects of the bed walls. Comparing the drag coefficient of data of free falling spheres and data of spheres falling with controlled velocities, the latter showed a dependence on the product of tracer diameter andfalling velocity, dput, while the former was constant over dput. This indicates the method with controlled falling velocities to be intrusive and influencing the result of the apparent viscosity of the bed. Using the free falling sphere method this work obtained an apparent viscosity of 0.24 Pa s, which isconsistent with values found in earlier literature for an emulsion of air and sand of similar size and density

Chiral Alcohols from Alkenes and Water: Directed Evolution of a Styrene Hydratase

Enantioselective synthesis of chiral alcohols through asymmetric addition of water across an unactivated alkene is a highly sought-after transformation and a big challenge in catalysis. Herein we report the identification and directed evolution of a fatty acid hydratase from Marinitoga hydrogenitolerans for the highly enantioselective hydration of styrenes to yield chiral 1-arylethanols. While directed evolution for styrene hydration was performed in the presence of heptanoic acid to mimic fatty acid binding, the engineered enzyme displayed remarkable asymmetric styrene hydration activity in the absence of the small molecule activator. The evolved styrene hydratase provided access to chiral alcohols from simple alkenes and water with high enantioselectivity (>99 : 1 e.r.) and could be applied on a preparative scale

Rheological effects of a gas fluidized bed emulsion on falling and rising spheres

To enable the mechanistic description of the mixing of larger particles in gas-fluidized beds in models (e.g. fuel particles in combustors), knowledge about the rheology of the bed emulsion is required. Here, it is crucial to determine the drag on large fuel-alike particles. This work presents the experimental work on the fate of 13 different solid spheres falling or rising through a bed of air and glass beads at minimum fluidization. The trajectories of the tracer are highly resolved (sampling rate of 200 Hz) by means of magnetic particle tracking, this previously unmet accuracy allows disclosing the complex rheological behavior of gas-solids fluidized bed emulsions in terms of drag on immersed objects. The trajectories reveal that none of the tracers reach terminal velocity during their fall and rise through the bed. The shear stress is obtained through the drag force by solving the equation of motion for the tracer. The data reveal particularities of the bed rheology and clear differences of its effect on rising and falling particles. When studying the shear stress over the characteristic shear rate of each tracer, it can be seen that the stress of the bed on the tracers is dominated by a yield stress, with a somewhat smaller contribution of the shear stress. For rising tracers this last contribution is almost negligible. The falling tracers show strong interaction with the bed emulsion, resulting in a fluctuating shear stress, which increases with tracer size and density. The stagnation of some tracers at low shear rates reveals a viscoplastic behavior of the bed emulsion, exhibiting a typical yield stress that showing a clear dependence on the tracer diameter and buoyant density. The concept of yield gravity is used in order to introduce a normalized shear stress which provides additional verification of the experimental observations in relation to the influence of tracer size and relative density on the shear stress