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Rapid change of particle velocity due to volatile gas release during biomass devolatilization
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-1445-4121
Department of Energy and Process Engineering, Faculty of Engineering, NTNU - Norwegian University of Science and Technology, Trondheim, Norway.ORCID iD: 0000-0002-0536-2655
Department of Energy and Process Engineering, Faculty of Engineering, NTNU - Norwegian University of Science and Technology, Trondheim, Norway;RISE Fire Research, Tiller 7092, Norway.ORCID iD: 0000-0002-4248-8396
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-6958-5508
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2022 (English)In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 238, article id 111898Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Elsevier, 2022. Vol. 238, article id 111898
Keywords [en]
Biomass devolatilization, TR-PTV, In-situ measurements, Rocket effect, Non-isothermal modelling
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-88625DOI: 10.1016/j.combustflame.2021.111898ISI: 000735744600001Scopus ID: 2-s2.0-85121437396OAI: oai:DiVA.org:ltu-88625DiVA, id: diva2:1623773
Note

Validerad;2022;Nivå 2;2022-01-01 (johcin)

Available from: 2021-12-30 Created: 2021-12-30 Last updated: 2022-01-28Bibliographically approved
In thesis
1. Particle dynamics during biomass devolatilization: Momentum exchange and particle dispersion
Open this publication in new window or tab >>Particle dynamics during biomass devolatilization: Momentum exchange and particle dispersion
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
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. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2022
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Biomass, devolatilization, rocketing, PIV, PTV
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-88689 (URN)978-91-8048-005-5 (ISBN)978-91-8048-006-2 (ISBN)
Public defence
2022-03-17, E231, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2022-01-10 Created: 2022-01-10 Last updated: 2022-02-28Bibliographically approved

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Llamas, Ángel David GarcíaGebart, RikardUmeki, Kentaro

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