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Morphology and volume fraction of biomass particles in a jet flow during devolatilization
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap. (Thermochemical conversion)ORCID-id: 0000-0002-1445-4121
Department of Energy and Process Engineering, Faculty of Engineering, NTNU - Norwegian University of Science and Technology, Trondheim, Norway.
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: 0000-0002-6958-5508
Vise andre og tillknytning
2020 (engelsk)Inngår i: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 278, artikkel-id 118241Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

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.

sted, utgiver, år, opplag, sider
Elsevier, 2020. Vol. 278, artikkel-id 118241
Emneord [en]
Biomass, Devolatilization, PTV, Morphology, Aspect ratio
HSV kategori
Forskningsprogram
Energiteknik
Identifikatorer
URN: urn:nbn:se:ltu:diva-79441DOI: 10.1016/j.fuel.2020.118241ISI: 000559054900002Scopus ID: 2-s2.0-85086384548OAI: oai:DiVA.org:ltu-79441DiVA, id: diva2:1439316
Merknad

Validerad;2020;Nivå 2;2020-06-15 (alebob)

Tilgjengelig fra: 2020-06-12 Laget: 2020-06-12 Sist oppdatert: 2022-01-10bibliografisk 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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