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Computational fluid dynamic simulations of thermochemical conversion of pulverized biomass in a dilute flow using spheroidal approximation
Department of Energy and Process Engineering, Faculty of Engineering, NTNU – Norwegian University of Science and Technology, Trondheim, Norway.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.ORCID-id: 0000-0002-1445-4121
Department of Energy and Process Engineering, Faculty of Engineering, NTNU – Norwegian University of Science and Technology, Trondheim, Norway.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.ORCID-id: 0000-0001-6081-5736
Vise andre og tillknytning
2020 (engelsk)Inngår i: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 271, artikkel-id 117495Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

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.

sted, utgiver, år, opplag, sider
Elsevier, 2020. Vol. 271, artikkel-id 117495
Emneord [en]
Spheroidal particle, Pulverized biomass, CFD, Entrained flow gasifier, OpenFOAM
HSV kategori
Forskningsprogram
Energiteknik
Identifikatorer
URN: urn:nbn:se:ltu:diva-78298DOI: 10.1016/j.fuel.2020.117495ISI: 000522876500018Scopus ID: 2-s2.0-85082014379OAI: oai:DiVA.org:ltu-78298DiVA, id: diva2:1421141
Merknad

Validerad;2020;Nivå 2;2020-04-02 (alebob)

Tilgjengelig fra: 2020-04-02 Laget: 2020-04-02 Sist oppdatert: 2023-09-04bibliografisk kontrollert
Inngår i avhandling
1. Particle dynamics during biomass devolatilization: Momentum exchange and particle dispersion
Åpne denne publikasjonen i ny fane eller vindu >>Particle dynamics during biomass devolatilization: Momentum exchange and particle dispersion
2022 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

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. 

sted, utgiver, år, opplag, sider
Luleå: Luleå University of Technology, 2022
Serie
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Emneord
Biomass, devolatilization, rocketing, PIV, PTV
HSV kategori
Forskningsprogram
Energiteknik
Identifikatorer
urn:nbn:se:ltu:diva-88689 (URN)978-91-8048-005-5 (ISBN)978-91-8048-006-2 (ISBN)
Disputas
2022-03-17, E231, Luleå, 09:00 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2022-01-10 Laget: 2022-01-10 Sist oppdatert: 2022-02-28bibliografisk kontrollert

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