Role of Sound Vibration during Aeration of Nano-Sized Powders

The behaviour of two different nano-sized powders, Al2O3 (40 nm) and SiO2 (15 nm), during aeration has been investigated in a laboratory scale fluidized bed. The fluidization quality of both powders is very poor without application of acoustic fields even if some bed expansion has been found. The application of acoustic fields of intensities larger than 135 dB and frequencies close to 120 Hz is able to increase the fluidization quality of both powders. Sound is also able to promote an apparent self-fluidization of a relatively thin portion of the upper part of the bed. The possibility that there is an efficient mixing between aggregates during aeration has been highlighted by experiments using a tracer powder

Modeling Mercury Capture by Powdered Activated Carbon in a Fluidized Bed Reactor

A steady state model of mercury capture on activated carbon in a bubbling fluidized bed of inert material is presented. The model takes into account the fluidized bed fluid-dynamics, the presence of both free and adhered carbon in the reactor as well as mass transfer limitations and mercury adsorption equilibrium. The activated carbon adsorption parameters and the relative amount of free versus adhered carbon in the reactor have been estimated with purposely designed experiments. Model results are compared with results from mercury capture experiments conducted with commercial powdered activated carbon at 100°C in a lab-scale pyrex fluidized bed of inert particles. The role of free versus adhered carbon in determining the overall mercury capture efficiency is discussed

Hydrodynamics of compartmented fluidized beds for concentrated solar power applications

Application of fluidized beds to collection and thermal storage of solar radiation is beneficial in Concentrated Solar Power (CSP) systems thanks to their well-known inherently good thermal performances. Non-conventional design and operation of fluidized beds based on uneven or unsteady (pulsed) fluidization (1), may further enhance their thermal performances improving the potential for applications in the very demanding CSP systems. Dense gas-fluidized beds have the potential to effectively accomplish three complementary tasks: the collection of incident solar radiation; the heat transfer of the incident power to immersed tube bundles of high-efficiency steam and/or organic Rankine cycles (ORC) and the thermal energy storage equalizing the inherent time-variability of the incident radiation for stationary CHP generation. A novel concept of solar receiver for CHP (combined heat and power) generation consisting of a compartmented dense gas fluidized bed has been proposed (2). The present study addresses the hydrodynamics of a dense gas-fluidized bed operated at ambient conditions and equipped with a compartmented windbox. Figure 1 outlines the experimental apparatus which consists of a nearly-2D fluidization column (2850x1860x200mm) equipped with two sparger-type gas distributors extending along 20 and 80% of the fluidized bed width. The regions of the bed above the two spargers were marked as compartments A and NA, respectively. The bed material was fine silica sand with a mean Sauter diameter of 145 µm. A pressure measurement system was used to monitor pressures and pressure gradients at different locations inside the fluidized bed. A pressure gradient exceeding a threshold of 0.11mbar/mm was assumed to mark the onset of local fluidization. A procedure was developed to draw the separation boundary (dashed line) between fluidized and non-fluidized regions for different bed heights (0.55, 0.95, 1.39 and 1.85m) as the gas superficial velocity was varied in either regions of the bed (UA and UNA=0-4Umf). Selected fluidization maps are shown in figure 2 where separation boundaries at different values of UA and UNA for a static bed height of 1.85 are reported. Figure 3 reports the fractional extension of the fluidized region at a level of 400mm for different values of UA e UNA as the static bed height was varied. Results indicate that a perfectly compartmented fluidized bed cannot be obtained simply using a compartmented windbox, but a proper choice of the operating conditions enables good control of the local fluidization conditions and of the gas cross-flow between the compartments. Please click Additional Files below to see the full abstract

Hydrodynamics of a Loop-seal Operated in a Circulating Fluidized Bed: Influence of The Operating Conditions on Gas and Solid Flow Patterns

Hydrodynamic features of a loop-seal operated as solids re-injection device in a labscale cold CFB apparatus are studied. Gas flow patterns are characterized by means of gas tracing experiments with continuous injection of CO2 in the loop-seal chambers. Solids flow patterns are characterized by impulsive injection of dyecoloured particles into the supply chamber, followed by particle tracking

Binary mixtures of biomass and inert components in fluidized beds: experimental and neural network exploration

Considering the little understanding of the hydrodynamics of multicomponent particle beds involving biomass, a detailed investigation has been performed, which combines well-known experimental and theoretical approaches, relying, respectively, on conventional pressure drop methods and artificial neural network (ANN) techniques. Specific research tasks related to this research work include: i. to experimentally investigate by means of visual observation the mixing and segregation behavior of selected binary mixtures when varying the biomass size and shape as well as the properties (size and density) of the granular solids in cold flow experiments; ii. to carry out a systematic experimental investigation on the effect of the biomass weight and volume fractions on the characteristic velocities (e.g., complete fluidization velocity and minimum slugging velocity) of the investigated binary mixtures in order to select the critical weight fraction of biomass in the mixtures beyond which the fluidization properties deteriorate (e.g., channeling, segregation, slugging); iii. to analyze the results obtained in about 80 cold flow experiments by means of ANN techniques to scrutinize the key factors that influence the behavior and the characteristic properties of binary mixtures. Experimental results suggest that the bed components’ density difference prevails over the size difference in determining the mixing/segregation behavior of binary fluidized bed, whereas the velocities of minimum and complete fluidization increase with a growing biomass weight fraction in the bed. The training of ANNs demonstrated good performances for both outputs (Umf and Ucf); in particular, the best predictions have been obtained for Umf with a MAPE1 <4% (R2=0.98), while for Ucf the best ANN returned a MAPE of about 7% (R2=0.93). The analysis on the importance of each individual input on ANN predictions confirmed the importance of particle density of the bed components. Unexpectedly, results showed that morphological features of biomass have a limited importance on Ucf

HYDRODYNAMICS OF UNCONVENTIONAL FLUIDIZED BEDS: SOLIDS FLOW PATTERNS AND THEIR INFLUENCE ON MIXING/SEGREGATION OF A LARGE FLOTSAM PARTICLE IN A BED OF FINER SOLIDS

Gross solids circulation of solid phase and its influence on mixing/segregation of a large flotsam particle in beds of finer solids in unconventional fluidized beds has been investigated. A tapered two-dimensional fluidization column and a fluidization column equipped with a diverging cone as gas distributor have been adopted. The hydrodynamics of the gas-solid suspension in the two apparatus has been qualitatively assessed by visual observation and the trajectories of the centre-of-gravity of large flotsam particles have been evaluated to assess the extent of mixing/segregation

Thermal behaviour of fluidized beds directly irradiated by a concentrated solar radiation

Directly-irradiated fluidized bed reactors are very promising in the context of concentrated solar power applications as they can be operated at process temperatures high enough to perform thermochemical storage with high energy density. The present study aims at experimentally investigating the direct interaction between a concentrated simulated solar radiation and a fluidized bed by measuring the time-resolved bed surface temperature with an infrared camera under different fluidization gas velocities. The effect of a localized generation of bubbles was investigated too, by injecting a chain of bubbles through a nozzle located just at the centre of the concentrated solar beam. The obtained results encourage the localized generation of bubbles, just at the larger value of the impinging radiative heat flux, as a strategy to reduce the overheating of the bed surface and, as a consequence, the energy losses related to fluidizing gas and radiative re-emission

Influence of bubble bursting on heat transfer phenomena in directly irradiated fluidized beds

Concentrating Solar Power (CSP) is a fast-growing technology in which several hundreds/thousands of optical mirrors concentrate the solar energy onto a receiver. The received energy can be used to produce electricity through thermodynamic cycles or to drive an endothermic chemical reaction for chemicals production (e.g. solar fuels) or for chemical storage. The receiver is a crucial part of the whole system, as it owns the severe task of collecting and transferring the concentrated solar energy minimizing the heat losses. Dense fluidized beds have been proposed as CSP receivers thanks to their large heat-transfer and thermal diffusivity coefficients, and their use is currently under investigation (1-2). Directly-irradiated fluidized bed receivers are very promising in the context of solar chemistry and CSP applications, but they can undergo to extensive bed surface overheating. Tailoring bed hydrodynamics close to the region where the incident power is concentrated may disclose effective measures to improve the interaction between the incident radiative flux and the bed in order to maximize the receiver efficiency and mitigate the bed surface overheating. The present work addresses the study of the interaction between a concentrated solar radiation and bed surface. The experimental apparatus schematically reported in figure 1a mainly consists of: i) a fluidized bed reactor (square 0.78 x 0.78 m bed column, 0.6 m tall); ii) a simulated solar radiation source, consisting of a short‑arc Xe lamp coupled with an elliptical reflector, whose spatial flux distribution map is shown in Figure 1b; iii) a Bubble Generation System (BGS), able to produce bubbles with a minimum diameter of 0.045 m at a maximum frequency of 2 Hz, connected to a submerged nozzle aligned with the focal point of the simulated solar beam. SiC particles (127 mm) were used as solid bed material. The main diagnostic tool is represented by an infrared camera used to map the bed surface temperature. Tests were performed at incipient fluidization condition injecting through the nozzle bubbles with different diameters and at different frequencies keeping constant the gas flow rate. Two snapshot sequences of the bubble eruption phenomena are reported in Figure 2. It can be observed that increasing bubble diameter the lateral dispersion heat at bed surface is more efficient, as the hot particles are shifted towards a larger annular region. Nevertheless, the long delay between two successive bubble eruption events brings to a larger bed surface overheating, which could result into a fluidized particles degradation or into higher heat losses due to re-irradiation. A trade-off between these two-fold results has to be found to optimize bed hydrodynamics in CSP applications. Please click Additional Files below to see the full abstract

Combustion of lignin-rich residues with coal in a pilot-scale bubbling fluidized bed reactor

The deployment and the exploitation of bioethanol as automotive fuel became more and more relevant to reduce the emissions of greenhouse gases and to limit the dependence on countries supplying fossil fuels. However, the production of second-generation bioethanol, i.e. using lignocellulosic biomass or scraps of agricultural crops as feedstock, generates a waste stream consisting of lignin-rich residues whose fate has to be found (1-2). This work aims at investigating the combustion of lignin-rich residues (in the following simply called lignin), coming from a second-generation bioethanol production plant, with coal in a pilot-scale bubbling fluidized bed combustor (FBC). The pilot-scale 200kWth FBC schematically shown in Fig. 1 basically consist of a AISI 310 stainless steel fluidization circular column (370 mm ID for 5.05 m and 700 mm ID for 1.85m in the upper part of freeboard), a continuous over-bed feeding system, two cyclones for flue gas de-dusting, a propane premixed burner for the start-up and different heat removal devices located along the fluidization column. On-line gas analyzers (ABB AO2020) measured flue gas composition sampled at the exhaust. Fuels were the lignin-rich residue, a bituminous coal and wood chips. Silica sand (0.8-1.2mm) was used as bed material. An experimental campaign was carried out to study gaseous and particulate emissions and thermal regimes during the co-combustion of different mixtures of coal-lignin varying the percentage of lignin fed with coal, the bed temperature, the excess air and the fluidization velocity. Figure 2 reports the main results in terms of normalized emissions of NO, SO2, particulate and carbon in particulate as a function of the O2 concentration measured at the exhaust obtained during the steady state operation of the pilot-scale FBC. A large part of the investigated experimental conditions regarded the operation using a mixture lignin-coal at 30%w in lignin. Experiments with coal, with a mixture at 40%w in lignin and with a mixture coal-wood chips at 20%w in wood chips were carried out for comparison. The analysis of the experimental results mainly highlights that: 1) the gaseous emissions do not significantly change with respect to coal or to reference biomass-coal mixture at least until the mixture content of lignin is 30-40%w; 2) the particulate emissions increase with the percentage of residues content, but, at the same, the carbon content is significantly reduced. Bottom bed particles were analyzed at the end of each experiments highlighting the absence of agglomerates but a significant enrichment of metals like Fe, Mg, Na, Ca and K coming from lignin ash when the FBC was operated for long time and at high temperature. Please click Additional Files below to see the full abstract

Fluidized Bed Design and Process Calculations for the Continuous Torrefaction of Tomato Peels with Solid Product Separation

This work reports the Authors’ concept idea and the gross design of a plant system capable of continuously separating the torrefied solids from the inert bed material downstream from a fluidized bed reactor, where biomass torrefaction is performed in a continuous operation mode. It is constituted of three units that process solids: i. a bubbling fluidized bed, equipped with a heat exchanging tube bundle, acting as a torrefaction reactor; ii. an inclined plate sieve separator for collection of the torrefied product as oversize solids; iii. a loop-seal for reinjection of undersize particles, i.e., the inert solids, back into the bed.A simple model of the torrefaction reactor as a well-stirred system has been devised to predict the conversion of feedstock (i.e., tomato peel particles) on the basis of an empirical correlation previously established by the Authors under batch conditions; the variability of biomass particle residence time in the bed as induced by the fluidization of inert solids has been accounted for by introducing a distribution function of the biomass residence time, and this latter has been suitably incorporated within the equations yielding the bed inventory of biomass. The recycling system of undersize inert solids back into the bed through a standpipe and a loop-seal for reinjection has been simply designed according to literature.The resulting set of equations is easily handled and smoothly provides the plant design variables and the relevant process calculations