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Effects of pyrolysis conditions and ash formation on gasification rates of biomass char
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.
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
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2017 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 31, no 6, p. 6507-6514Article in journal (Refereed) Published
Abstract [en]

Pyrolysis conditions and the presence of ash-forming elements significantly influence char properties and its oxidation or gasification reactivity. In this study, intrinsic gasification rates of char from high heating rate pyrolysis were analyzed with isothermal thermogravimetry. The char particles were prepared from two biomasses at three size ranges and at two temperatures. Reactivity dependence on original particle size was found only for small wood particles that had higher intrinsic char gasification rates. Pyrolysis temperature had no significant effect on char reactivity within the range tested. Observations of ash formation highlighted that reactivity was influenced by the presence of ash-forming elements, not only at the active char sites but also through prohibition of contact between char and gasification agent by ash layer formation with properties highly depending on ash composition.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2017. Vol. 31, no 6, p. 6507-6514
National Category
Other Chemical Engineering
Identifiers
URN: urn:nbn:se:umu:diva-136565DOI: 10.1021/acs.energyfuels.7b00688ISI: 000404691900079OAI: oai:DiVA.org:umu-136565DiVA, id: diva2:1112132
Available from: 2017-06-19 Created: 2017-06-19 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
Supervisors
Available from: 2018-05-09 Created: 2018-05-07 Last updated: 2018-06-09Bibliographically approved

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Strandberg, AnnaHolmgren, PerWagner, David R.Broström, Markus

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