Numerical simulation and structural optimization of multi-compartment fluidized bed reactor for biomass fast pyrolysis

<p>Condensing heat exchangers play a key role in the waste heat recovery process, which can be of<br>various designs depending on the primary heat source, its pollution level, the place where the heat<br>exchanger needs to be installed, etc. In order to be able to evaluate various influencing parameters, it<br>is most appropriate to use numerical simulation, which can be used to quickly evaluate the efficiency<br>of a heat exchanger of a specific design in the case of different flow regimes. It is the most difficult to<br>model the condensation process, because in this case two phases appear, the interaction of which<br>requires a lot of theoretical and practical knowledge. In order to be able to analyze the interaction of<br>two-phase flows, it is necessary to approach this task step by step, starting from the analysis of single-<br>phase flow to understand the physical processes taking place in the heat exchanger, which usually<br>contains variously arranged tube bundles of different shapes. Therefore, in this paper, numerical<br>studies of hydrodynamics and convective heat transfer were performed to better understand the fluid<br>behavior in a serpentine type condensing heat exchanger. This research study is related to the EU<br>Horizon 2020 iWAYS project (Innovative WAter recoverY Solutions through recycling of heat,<br>materials, and water across multiple sectors) activities. The presented analysis was conducted in<br>parallel on two different well established CFD softwares. One with Ansys Fluent and the other with<br>Siemens Star-CCM+ CFD softwares. Both simulations considered monophase currents of hot air<br>flowing into the serpentine heat exchanger thermodynamically coupled with the cooling water flowing<br>into the pipes. The flows were organized in counterdirections. The metallic pipes were included and<br>characterized by means of thermal conductivity and specific heat of the considered steel material. The<br>cooling water, due to the relatively small increase in temperature, was characterized by a constant<br>density approach. The calculation of the inflowing air Reynolds number led to the intermittent regime<br>to that a turbulent model is required to the calculation. The flowing air than was characterized by<br>realistic thermodynamic properties by means of tabular values. Both the simulations were considered<br>as steady-state. The realized meshes followed two diverse methods. Ansys Fluent case implemented a<br>fully tetrahedral mesh, thus exploiting the ability of this kind of cells to fully adapt to peculiar<br>geometries. Star-CCM+ then implemented both automated and directed meshes methods. The<br>automated method resembles the classic meshing method where the entire volume is meshed by<br>means of macro parameters and user defined refinements or modifications. The directed mesh was<br>fully employed in the regular extrusion of the surface mesh for the cooling water region starting from<br>an automated surface mesh and a patched regular mesh used for the metallic pipes. The hot air region<br>was generated by means of polyhedral cells with the automated method to exploit the same<br>tetrahedral adaptiveness to irregular geometries but reducing the total amount of the employed cells.<br>The Ansys Fluent simulation then resulted in 10 milion cells and Star-CCM+ in a reduced 7 milion<br>cells. The temperature field was recorded by means of numerical probes realized in correspondence of<br>experimental thermocouples, recording a three point average value for the inflowing air, metallic<br>pipes and cooling water. The comparison between the experimental and the simulated results<br>confirmed the good agreement between both the numerical and the experiment. Little divergences<br>were recorded but resulted in being perfectly acceptable based on current differences unavoidable<br>between experimental and numerical setups.</p> <p>This work was partly funded by the European Union H2020 programme project iWAYS under grant agreement number <a href="https://www.sciencedirect.com/science/article/pii/S2451904924004323?via%3Dihub#gp010">958274</a>.</p>

Waste tires contain abundant zinc (Zn), which could cause severe environmental issues during waste tire pyrolysis. This study presents a systematic investigation into Zn release and transformation behavior during the pyrolysis of waste tires at 400–900 °C using various reactors, including a fixed-bed reactor for slow pyrolysis (Type A), a drop-tube/fixed-bed reactor for fast pyrolysis with continuous feeding (Type B) or pulse feeding (Type C), and a fluidized-bed reactor for fast pyrolysis (Type D). In the Type A reactor, over 90% of Zn was retained in char during waste tire pyrolysis at 400–700 °C, but the Zn release dramatically increased from ∼9.8 to ∼47.0% as the pyrolysis temperature increased from 700 to 900 °C, possibly due to the reduction of ZnO to volatile Zn and the dissociation of ZnS. Compared to the Type A reactor, the high heating rates in the Type B, C, and D reactors caused rapid melting and volatile release, as proven by the micromorphology analysis, resulting in more Zn release at all temperatures. The highest Zn release of ∼67.6% was obtained in the Type D reactor at 900 °C. Using a multistep extraction process, our results clearly showed the transformation of acid-soluble to acid-insoluble Zn at temperatures higher than 700 °C (especially for the Type B and D reactors), likely due to the formation of zinc silicate. The Zn mass balance analysis showed that Zn species in the volatiles were mainly in the inorganic form, which could be easily captured by a bubbler system with diluted HNO3 solution.

Electricity production from concentrated solar power (CSP) plants has been more commonplace in the last decade since the sun is one of the most abundant, renewable energy sources. The heat transfer fluid temperature in a CSP plant may go up to 1000°C; however, most of the current power plants operate on temperature ranges between 220°C and 565°C due to decomposition of molten salts in high temperatures. Since the sun is not available at nights and cloudy days, an important consideration is how to store the energy received by the sun to use at other times. In this study, a three‐dimensional borehole heat exchanger model is developed to store solar energy underground using concrete and molten salt as a storage medium and heat transfer fluid, respectively. While molten salt is circulating through a pipe, which is placed into the concrete, heat is transferred from the molten salt to the concrete or vice versa during the charging and discharging processes. Numerous simulations are conducted using ANSYS Fluent, with varying borehole diameters, mass flow rates, and thermal resistances of the borehole wall. Average concrete temperature, outlet heat transfer fluid temperature, and energy and exergy efficiencies are investigated for each case. It was found here that while concrete temperature increases with increasing mass flow rate, the increasing trend is minimal after the mass flow rate increases beyond 6 kg/s. There exists a negative relation between the borehole diameter and average concrete temperature during the charging process, and vice versa in discharging. Energy and exergy efficiencies varied from 0.2% to 98.1% and 0.1% to 77.9%, respectively. While the most efficient system was found at a borehole diameter of 550 mm for adiabatic cases, it was found to be 750 mm when heat leakage is taken into consideration. Borehole diameters of 2000 mm performed the worst among all cases due to low heat transfer rates. Heat leakage was found to have a significant impact on energy and exergy efficiencies, especially in energy efficiencies for higher borehole diameters and low mass flow rates in the discharging process.

Reaction of volatiles – A crucial step in pyrolysis of coals

Fundamental particle fluidization behavior and handling of nano-particles in a rotating fluidized bed

The mechanism of gas-water two-phase percolation was previously characterized based on percolation experiments. However, the changes in the gas-water interface and the volume fraction of the two phases in the process of gas-water percolation in porous media cannot be accurately described by experiments. This paper uses core casting thin sections and AutoCAD software to establish a digital core. Based on the idea of level set, a two-dimensional gas-water seepage model in microscopic pores is established. The finite element numerical simulation software is used to solve the model to describe the gas. The changes of the water two-phase interface are visually displayed. At the same time, Darcy’s law is used to compare the relative permeability curve obtained from the gas-water two-phase seepage experiment with the relative permeability curve obtained from the gas-water two-phase seepage finite element simulation. The results show that the gas-water two-phase seepage experiment and the gas-water two-phase seepage finite element numerical simulation have basically the same characterization of the seepage ability of porous media. There is obvious fingering phenomenon in the process of gas flooding, and the gas preferentially passes through the large pore throat. The smaller the pore throat, the greater the velocity of gas passing through the pore throat. The gas-water two-phase interface change described by the numerical simulation of the gas-water two-phase flow based on the digital core is more reliable.

Biomass energy has attracted considerable attention as a renewable and clean energy source that can be stored and transported effectively. A critical component in this context is the fluidized bed, which plays a vital role in biomass energy utilization. An in-depth understanding of the flow characteristics of non-spherical biomass particles within the fluidized bed is essential for improving energy efficiency. This study utilizes a combination of numerical simulations and experimental analysis to explore the flow patterns of biomass particles under eight distinct drag correlations. The investigation covers various flow regimes in the fluidized bed, concentrating on critical parameters such as flow evolution, gas-phase pressure distribution, particle phase velocity vectors, bed pressure drop, particle mixing characteristics, bed expansion height, and the energy-dynamic behavior of the particles. The results indicate that the Ergun resistance model closely matches the experimental data, particularly regarding the flow evolution of the particles. This model shows a more uniform mixing of the particles and greater stability in the magnitude and frequency of pressure fluctuations at the bed surface. This stability is reflected in the average pressure drop, highlighting the enhanced performance of the fluidized bed. Overall, this research sheds light on the intricate dynamics of non-spherical biomass particles in fluidized beds and provides valuable insights for optimizing particle design. By focusing on design enhancements, it is possible to achieve higher combustion efficiency, supporting the broader objective of sustainable energy utilization. In summary, this study emphasizes the significance of precise modeling and understanding of flow characteristics in the fluidized bed, which is crucial for advancing biomass energy technologies.

A technique has been developed to give a quantitative measure of the contacting efficiency of liquid feed onto particles in a hot fluidized bed for petroleum applications. Copper naphthenate was used as a tracer to measure the coating of atomized bitumen feed onto solid coke particles in a fluid coking pilot plant. The pilot plant consisted of a reactor vessel, where bitumen was injected into the reactor as feed, and a regeneration vessel, where coke particles were heated and circulated back to the reactor. Atomized bitumen droplets were injected through a feed nozzle into the reactor section. A small concentration of copper naphthenate tracer, pre-diluted with bitumen, was injected upstream of the nozzle, following a step function with time. As the bitumen reacted, the copper formed solid deposits on the surface of the particles, effectively labeling the particles that had been contacted by the bitumen droplets. Frequent coke samples from the solids exiting and entering the reactor were taken during and after the injection of the tracer.Two types of measurements were conducted on the coke samples; bulk copper concentration in the samples, using Inductively Coupled Plasma (ICP), and copper concentration on individual coke particles using energy-dispersive X-ray (EDX) analysis. The data for bulk copper concentration confirmed that the solids were mixed rapidly in the reactor, approximating a Continuous Stirred Tank Reactor (CSTR). The EDX analysis of individual coke particles demonstrated, however, that a significant fraction of the coke particles within the samples were not labeled with copper. This result indicated that many of the particles in the fluidized bed were not contacted by bitumen in the reactor. Consequently, the tracer technique was used to compare the coating efficiency of bitumen on coke particles by operation of different nozzle designs.

Pulsating flow effect on the segregation of binary particles in a gas–solid fluidized bed

Collison of sand particles in fluidized bed plays a very important role in determining hydrodynamics of fluidized bed. Effect of coefficient of restitution on hydrodynamics on 300 µm particles in gas-solid fluidized bed was investigated. Eulerian-Eulerian model was used for simulating hydrodynamics of fluidized bed. For investigation, different values of coefficient of restitution which correspond to different values of elasticity of particles were used. No bubbles were found for perfectly elastic collisions. With decrease in the value of coefficient of restitution from 1 to 0.8, bubble formation shows a positive effect.

The overall objectives of this project are to develop experimental techniques for measuring the forces of fluidized particles, and to predict the solid-gas performance in fluidized beds by using data analysis system, and by elucidating the intrinsic mechanism of erosion and attrition phenomena in fluidized beds. The reduction of erosion and attrition rates is one of the critical engineering problems for the design and operation of fluidized bed combustors. Specifically, the objectives are to: (1) develop the experimental techniques to measure the forces of solid particles prevailing in fluidized beds: (2) measure and characterize the forces of solid particles in various types of fluidized beds with various configurations (conventional and spouted fluidized beds) and with different scales (10, 20, and 30cm) under various fluidization conditions (particle size, bed aspect ratio and gas velocity); (3) find and verify the mechanism of erosion rates of in-bed tubes and attrition rates of fluidized particles by forces of solid particles in fluidized beds. We developed three different kinds of measurement methods, i.e., fracture sensitive sensor, piezoelectric sensor and gas pressure fluctuation method. By using these methods the exact forces of solid particles, including the transient corporate in fluidized beds, were systematically measured. Simultaneously, the erosion rates of in-bed tubes and attrition rates of fluidized particles were measured. 69 figs., 9 tabs.

The present work details the three‐dimensional numerical simulation of single‐phase and two‐phase flow (air‐water) in a venturi scrubber with an inlet and throat diameters of 250 and 122.5 mm, respectively. The dimensions and operating parameters correspond to industrial applications. The mass flow rate conditions were 0.483 kg/s, 0.736 kg/s, 0.861 kg/s, and 0.987 kg/s for the gas only simulation; the mass flow rate for the liquid was 0.013 kg/s and 0.038 kg/s. The gas flow was simulated in five geometries with different converging and diverging angles while the two‐phase flow was only simulated for one geometry. The results obtained were validated with experimental data obtained by other researchers. The results show that the pressure drop depends significantly on the gas flow rate and that water flow rate does not have significant effects neither on the pressure drop nor on the fluid maximum velocity within the scrubber.

Axial mixing of fine particles in fluidized beds

A numerical investigation of balcony geometry impact on single-sided natural ventilation and thermal comfort

Heat recovery from hot solid particles in a shallow fluidized bed