| 1. |
- Dal Belo Takehara, Marcelo, et al.
(författare)
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Effect of acoustic perturbation on particle dispersion in a swirl-stabilized pulverized fuel burner: Cold-flow conditions
- 2022
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Ingår i: Fuel processing technology. - : Elsevier. - 0378-3820 .- 1873-7188. ; 228
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Tidskriftsartikel (refereegranskat)abstract
- Inter-particle distance and particle dispersion during gasification of biomass have been found to significantly affect soot emission. Consequently, enhanced particle dispersion decreases energy losses and the risk for blockages of downstream equipment, increasing the efficiency and reliability of entrained flow reactors (EFRs). In this work, we investigated the interactions between imposed acoustic oscillations and particle dispersion under non-reacting conditions in a co-axial burner for a lab-scale EFR. A flow of air, laden with pulverized stem wood particles (Norwegian Spruce) of three different sizes (63–112 μm, 200–250 μm, and 500–600 μm), was forced axially through the burner center tube at Reynolds numbers ranged from 800 to 1700, and loading ratio of 0.7–4.2. The influences on particle dispersion from variations of the Strouhal number (0.12–0.6), the pressure amplitude at synthetic jet cavity (0.5–4.0 kPap-p), the swirl number (0–2.3), and the center jet velocity (1.9–3.9 m s−1) were investigated. Post-processed shadowgraph images revealed the influence of acoustic perturbations, which generate large structures with high particle concentration for both swirling and non-swirling conditions. Time-averaged contour maps showed a significantly higher particle dispersion, quantified as dispersion angle, for higher values of forcing amplitude and swirl numbers, with a stronger influence from the forcing amplitude, especially at lower Stokes number.
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| 2. |
- Guo, Ning, et al.
(författare)
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Computational fluid dynamic simulations of thermochemical conversion of pulverized biomass in a dilute flow using spheroidal approximation
- 2020
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Ingår i: Fuel. - : Elsevier. - 0016-2361 .- 1873-7153. ; 271
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Tidskriftsartikel (refereegranskat)abstract
- A drag force model for spheroids, referred as the spheroid model, was implemented in OpenFOAM, in order to better predict the thermochemical conversion of pulverized biomass. Our previous work has found that the spheroid model predicts more dispersed results in terms of particle velocities and local concentrations comparing to other conventional particle models under non-reactive conditions. This work takes the spheroid model one step further, by validating against experiments performed under reactive conditions with a newly implemented heat transfer model for spheroids as well as updated devolatilization kinetic parameters. In addition, simulations were conducted in a configuration similar to a pilot-scale entrained flow gasifier for more realistic scenarios. Particle mass and axial velocity development were compared accordingly using four different modelling approaches with increasing complexity. When compared with models of spheroidal shape assumptions, the sphere and simplified non-sphere model predict 61% and 43% longer residence times, respectively. The combination of the spheroid shape assumption with the heat transfer model for spheroids tends to promote drying and devolatilization. On the other hand, the traditional spherical approach leads to longer particle residence times. These opposing effects are believed to be a major contributing factor to the fact that no significant differences among modelling approaches were found in terms of syngas production at the outlet. Furthermore, particle orientation information was reported in both experiments and simulations under reactive conditions. Its dependency on gas velocity gradient under reactive conditions is similar to what was reported under non-reactive conditions.
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| 3. |
- Llamas, Angel David Garcia
(författare)
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Fast devolatilization of biomass : An experimental study using high-speed imaging, relevant for suspension firing technologies
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We have performed experiments using imaging techniques in a flame-assisted, drop-tube reactor with optical access, to extract information relevant to the improvement of multi-particle devolatilization models. These models could be of interest for different suspension firing techniques, such as entrained flow biomass gasification, (EFBG) and oxyfuel combustion.Reactor-scale CFD simulations usually disregard the implications of the elongated shape of biomass particles, (high aspect ratio: AR) and their morphological changes during devolatilization for their heat-transfer and fluid-dynamics behavior. In addition, solid-fluid interactions are prone to take place in highly seeded flows, potentially causing heat and mass transfer limitations during thermochemical conversion. Although there is sufficient experimental evidence supporting that particle morphology affects the devolatilization rate and the motion of the individual particles in a particle-laden flow, there is a lack of experimental data documenting the morphological transformations during biomass devolatilization and the heat and mass transfer limitations due to solid-fluid interactions. In this work, we have documented the morphological transformations and fluid dynamics behavior during biomass devolatilization in relation to the initial particle shape and operation conditions relevant to suspension firing technologies. In addition, we have observed an interesting phenomenon related to the fast release of volatile matter during pyrolysis that affected significantly the particle fluid dynamics. We have also performed CFD simulations to observe whether taking into account the phenomenon described in this work can help to reproduce better the experimental results. Future work should aim to provide a better model for this phenomenon, and to complement the current results with thermometric and spectroscopic measurements, which can draw a better insight on the transformations taking place during devolatilization.
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| 4. |
- Llamas, Angel David Garcia, et al.
(författare)
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Morphology and volume fraction of biomass particles in a jet flow during devolatilization
- 2020
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Ingår i: Fuel. - : Elsevier. - 0016-2361 .- 1873-7153. ; 278
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Tidskriftsartikel (refereegranskat)abstract
- Particle size, aspect ratio (AR, defined here as major over minor dimension), orientation and volume fraction have been measured for a stream of pulverized biomass particles undergoing devolatilization. Milling of raw biomass for thermochemical conversion yields elongated particles with high AR. Particle shape affects the heat and mass transfers and motion of particles within a jet, potentially shifting the particle group regimes. Therefore, the effects of carrier gas flow and fuel AR on the devolatilization behavior of biomass particles streams have been addressed experimentally. Two shapes of dried Norwegian Spruce have been used: one nearly equant (AR = 1.8 ± 0.64) and the other elongated (AR = 3.8 ± 2.9), both derived from the same sieve size of 200–250 μm. Experiments were performed in a laboratory-scale flat-flame assisted laminar drop tube reactor, where similar mass flows of particles (10–16 g⋅h−1) were injected with two different flow rates of CO2 to a high temperature flame zone (methane flame at O2-to-fuel equivalence ratio of λ = 0.63). Time and space-averaged measurements of particle morphology and velocity during conversion were obtained with 2D particle tracking velocimetry (PTV) together with image analysis. Carrier gas flow acted as thermal ballast, affecting the heating rate to the gas and particles. Heterogeneity in morphological changes was observed, and the behavior was affected by heating rate, particle shape and carrier gas flows. This paper describes phenomena relevant for the understanding of biomass devolatilization under very fast heating rates, such as shrinking, transient swelling, spherodization and lateral migration, and relates them to differences in heating rate and particle shape.
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| 5. |
- Llamas, Angel David Garcia
(författare)
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Particle dynamics during biomass devolatilization : Momentum exchange and particle dispersion
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Devolatilization is a heat-driven thermochemical process in which a liquid or a solid fuel releases mass in the form of volatile compounds after drying, as a result of the combination of endothermic and exothermic reactions. It differs from pyrolysis in that it does not require an inert atmosphere and that the reactant must be either solid or liquid. Devolatilization is present in every industrial process involving high temperature thermochemical conversion of solid fuels, such as combustion or gasification.Biomass devolatilization is a complex process which entangles dynamic changes in internal heat transfer with phase change and chemical kinetics. The higher the heating rate and the temperature at which devolatilization takes place, the more uncertain the outcome of these processes and the more challenging to measure. Furthermore, since devolatilization involves heat and mass transfer to its surroundings, this can also have an effect on the external conditions, for example by altering the surrounding gas temperature or composition, or by transferring momentum to the particle or the surrounding gas. The inherent uncertainty in biomass thermochemical properties and composition difficult the fundamental understanding even further.Suspension firing of pulverised fuels is a technique applied to high temperature thermochemical conversion processes, in which a particle-laden gas flow is injected into a hot atmosphere. Due to the extreme heat transfer conditions, the particles exhibit a very fast heating rate and achieve quickly a very high temperature during the stage at which they devolatilize. Measurements of mass loss and composition under these conditions are difficult to achieve using laboratory equipment, such as thermogravimetric analysers.The aim of this PhD thesis is to investigate the devolatilization of biomass particles under high heating rate conditions, by measuring their morphology and velocity dynamics while reacting in a hot laminar gas flow. The measurement techniques applied, allow a very fast sampling rate and, while they prevent a fundamental understanding of the underlying mechanisms of high temperature devolatilization. They provide valuable and practical knowledge that can be applied to realistic conditions and allow the introduction of uncertainties in biomass size, shape and composition. The work carried for this thesis provides experimental measurements of particle size, shape and velocity for a devolatilizing stream of biomass particles, using high-speed imaging diagnostics. An interesting phenomenon related to the interaction between devolatilization reactions and particle momentum is investigated in detail. Additional modeling and simulation work is provided, to assess the model’s performance and the importance of this phenomenon. Particle dispersion caused by this phenomenon is compared to the one achievable by active flow manipulation techniques, such as swirling and vortex generation.This work provides important information regarding the complex fluid-solid interactions caused by the dynamics of biomass devolatilization from a stochastic and modelling and simulation point of view. Although these results are not directly applicable to industrially-realistic conditions, the methodology for this investigation can be applied to more complex flows and further work must be conducted to understand the mechanisms behind the phenomenon observed and the consequences for devolatilization.
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| 6. |
- Llamas, Ángel David García, et al.
(författare)
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Rapid change of particle velocity due to volatile gas release during biomass devolatilization
- 2022
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Ingår i: Combustion and Flame. - : Elsevier. - 0010-2180 .- 1556-2921. ; 238
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Tidskriftsartikel (refereegranskat)abstract
- Our earlier study showed significant differences in average particle velocity between simulation and experimental results for devolatilizing biomass particles in an idealised entrained flow reactor [N. Guo et al., Fuel, 2020]. This indicates that the simulations do not accurately describe the physicochemical transformations and fluid dynamic processes during devolatilization. This article investigates the reasons for these discrepancies using time-resolved analyses of the experimental data and complementary modelling work. The experiments were conducted in a downdraft drop-tube furnace with optical access, which uses a fuel-rich flat flame (CH4 O2 CO2) to heat the particles. Gas flow was characterized using particle image velocimetry, equilibrium calculations and thermocouple measurements. High-speed images of devolatilizing Norway spruce (Picea Abies) particles were captured and analysed using time-resolved particle tracking velocimetry methods. The data were used to estimate the balance of forces and fuel conversion. Thrust and “rocket-like” motions were frequently observed, followed by quick entrainment in the gas flow. Rocketing particles were, on average, smaller, more spherical and converted faster than their non-rocketing counterparts. These differences in conversion behaviour could be captured by a particle-size dependent, 0-D devolatilization model, corrected for non-isothermal effects. The results from this investigation can provide a basis for future modelling and simulation work relevant for pulverized firing technologies.
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| 7. |
- Trubetskaya, Anna, et al.
(författare)
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Effect of fast pyrolysis conditions on biomass solid residues at high temperatures
- 2016
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Ingår i: Fuel processing technology. - Amsterdam : Elsevier. - 0378-3820 .- 1873-7188. ; 143, s. 118-129
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Tidskriftsartikel (refereegranskat)abstract
- Fast pyrolysis of wood and straw was conducted in a drop tube furnace (DTF) and compared with corresponding data from a wire mesh reactor (WMR) to study the influence of temperature (1000-1400)°C, biomass origin (pinewood, beechwood, wheat straw, alfalfa straw), and heating rate (103 °C/s, 104 °C/s) on the char yield and morphology. Scanning electron microscopy (SEM), elemental analysis, and ash compositional analysis were applied to characterize the effect of operational conditions on the solid residues (char, soot) and gaseous products. The char yield from fast pyrolysis in the DTF setup was 3 to 7% (daf) points lower than in the WMR. During fast pyrolysis pinewood underwent drastic morphological transformations, whereas beechwood and straw samples retained the original porous structure of the parental fuel with slight melting on the surface. The particle size of Danish wheat straw char decreased in its half-width with respect to the parental fuel, whereas the alfalfa straw char particle size remained unaltered at higher temperatures. Soot particles in a range from 60 to 300 nm were obtained during fast pyrolysis. The soot yield from herbaceous fuels was lower than from wood samples, possibly due to differences in the content of lignin and resin acids.
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