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Biomass pellet combustion: cavities and ash formation characterized by synchrotron X-ray micro-tomography
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik. Swedish University of Agricultural Sciences, Department of Forest Biomaterials and Technology, Umeå, Sweden.
Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik. (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.
Vise andre og tillknytning
2018 (engelsk)Inngår i: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 176, s. 211-220Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
Elsevier, 2018. Vol. 176, s. 211-220
Emneord [en]
ash composition, pellet, thermochemical conversion, wheat straw, poplar, SEM-EDS
HSV kategori
Identifikatorer
URN: urn:nbn:se:umu:diva-146673DOI: 10.1016/j.fuproc.2018.03.023OAI: oai:DiVA.org:umu-146673DiVA, id: diva2:1198233
Prosjekter
Bio4EnergyTilgjengelig fra: 2018-04-17 Laget: 2018-04-17 Sist oppdatert: 2019-09-02bibliografisk kontrollert
Inngår i avhandling
1. Fuel conversion and ash formation interactions: a thermochemical study on lignocellulosic biomass
Åpne denne publikasjonen i ny fane eller vindu >>Fuel conversion and ash formation interactions: a thermochemical study on lignocellulosic biomass
2018 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
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.

sted, utgiver, år, opplag, sider
Umeå: Umeå Universitet, 2018. s. 66
Emneord
Char conversion, pyrolysis, devolatilization, ash transformation, biomass, fuel characterization, fuel composition, ash composition, silicate formation, carbonate formation, thermogravimetric analysis, micro-tomography
HSV kategori
Identifikatorer
urn:nbn:se:umu:diva-147532 (URN)978-91-7601-871-2 (ISBN)
Disputas
2018-06-01, Carl Kempe salen (KBE 303), KBC-huset, Umeå, 10:00 (svensk)
Opponent
Veileder
Tilgjengelig fra: 2018-05-09 Laget: 2018-05-07 Sist oppdatert: 2018-06-09bibliografisk kontrollert

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