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Biomass pellet combustion: cavities and ash formation characterized by synchrotron X-ray micro-tomography
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Swedish University of Agricultural Sciences, Department of Forest Biomaterials and Technology, Umeå, Sweden.
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. (Thermochemical Energy Conversion Laboratory (TEC-Lab))ORCID iD: 0000-0002-5777-9241
Swedish University of Agricultural Sciences, Department of Forest Biomaterials and Technology, Umeå, Sweden.
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2018 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 176, p. 211-220Article in journal (Refereed) Published
Abstract [en]

Ash formation during thermochemical conversion of biomass-based pellets influences both char conversion rates and ash-related operational problems. The objective of the present study was to provide detailed insights into changes in fuel and ash properties during fuel conversion. Pellets of poplar wood and wheat straw were used as model biofuels, representing vastly different compositions of ash-forming elements. Pellet samples at different char conversion phases were analyzed by synchrotron-based 3D X-ray micro-tomography, to map and visualize the development of cracks, internal cavities, and ash layers during conversion. The analysis of ash layers was complemented by scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. The results provide new insights into how large cracks and internal cavities are developed already during devolatilization, for example, the poplar wood pellets had a 64% void fraction after the devolatilization stage. As expected, there were large variations between the ash layer properties for the two fuels. A porous, low density, and calcium-rich ash was formed from the poplar fuel, whereas the wheat straw ash was a high-density silicate melt that developed into bubbles on the surface. As the conversion proceeded, the wheat straw ash covered parts of the active char surface area, but without blocking the gas transport.

Place, publisher, year, edition, pages
Elsevier, 2018. Vol. 176, p. 211-220
Keywords [en]
ash composition, pellet, thermochemical conversion, wheat straw, poplar, SEM-EDS
National Category
Chemical Process Engineering Bioenergy
Identifiers
URN: urn:nbn:se:umu:diva-146673DOI: 10.1016/j.fuproc.2018.03.023OAI: oai:DiVA.org:umu-146673DiVA, id: diva2:1198233
Available from: 2018-04-17 Created: 2018-04-17 Last updated: 2018-06-09Bibliographically approved
In thesis
1. Fuel conversion and ash formation interactions: a thermochemical study on lignocellulosic biomass
Open this publication in new window or tab >>Fuel conversion and ash formation interactions: a thermochemical study on lignocellulosic biomass
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomass is considered to be CO2 neutral, and to be able to reduce the dependency on fossil fuels the need for expanded and sustainable biomass feedstock is increasing. Ash-related problems are some of the most important aspects of this increasing use of new biomass assortments in thermal energy conversion systems. An improved basic understanding of fuel conversion, ash formation, ash transformation and ash interactions with the converting fuel is therefore important.

In the present thesis, the main objective was to provide new knowledge on thermochemical fuel conversion, specifically on how ash formation interacts with fuel conversion for lignocellulosic biomasses. The main methods used were experimental characterization of decomposition behavior and analysis of morphology and elemental composition of samples, using different appliances, analytical methods and fuels. Multivariate data analysis was successfully used on thermogravimetric data for prediction of compositional data and fuel properties.

New, detailed explanations of structural changes in char morphology and ash properties during conversion were provided including descriptions of the influences of ash formation on fuel conversion rates under different conditions. The influences were found different depending on both particle size and ash composition. One implication of these findings is that for fuels with low temperature melting ash, the diffusion barrier formed causes difficulties for typical thermogravimetric experiments aiming at determination of reactivity in the kinetically controlled regime. This is recommended to carefully consider for future studies. On a single pellet level, char encapsulation was not found to dominate and limit gas transport and conversion for any of the fuels tested. In practical applications, however, the situation may be different with thick ash layers accumulating on a fuel bed surface. Another important finding was the extensive formation of cracks and internal cavities during combustion of pellets, providing new insights in the fundamentals of fuel conversion.

Clean woody fuels, rich in calcium, formed a porous ash layer with no sign of limiting char conversion rates. The phase chemical transformations involving carbonate and oxide formation from poplar pellets was studied in detail. For grassy fuels, on the other hand, low melting point silicates are expected to form. The physical properties of K-Ca-silicates from silicon rich straw fuels were also characterized, providing new insights on ash formation on micrometer scale resolution; at high temperature, the silicate melt formed bubbles on the surface that partially covered the char, while for lower temperature a more rigid net structure was formed.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet, 2018. p. 66
Keywords
Char conversion, pyrolysis, devolatilization, ash transformation, biomass, fuel characterization, fuel composition, ash composition, silicate formation, carbonate formation, thermogravimetric analysis, micro-tomography
National Category
Energy Systems Chemical Process Engineering Inorganic Chemistry
Identifiers
urn:nbn:se:umu:diva-147532 (URN)978-91-7601-871-2 (ISBN)
Public defence
2018-06-01, Carl Kempe salen (KBE 303), KBC-huset, Umeå, 10:00 (Swedish)
Opponent
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Available from: 2018-05-09 Created: 2018-05-07 Last updated: 2018-06-09Bibliographically approved

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Strandberg, AnnaThyrel, MikaelSkoglund, NilsBroström, MarkusBackman, Rainer

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