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Fuel conversion and ash formation interactions: a thermochemical study on lignocellulosic biomass
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. (Thermochemical Energy Conversion Laboratory)ORCID iD: 0000-0003-0895-3474
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 [en]
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: urn:nbn:se:umu:diva-147532ISBN: 978-91-7601-871-2 (print)OAI: oai:DiVA.org:umu-147532DiVA, id: diva2:1204166
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
List of papers
1. Predicting fuel properties of biomass using thermogravimetry and multivariate data analysis
Open this publication in new window or tab >>Predicting fuel properties of biomass using thermogravimetry and multivariate data analysis
2017 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 156, p. 107-112Article in journal (Refereed) Published
Abstract [en]

Simple and reliable characterization methods for determining fuel properties of biomass are needed for several different applications. This paper describes and demonstrates such a method combining thermogravimetric analysis with multivariate data analysis, based on the thermal decomposition behavior of the fuel. Materials used for the tests were milled samples of wood chips thermally pretreated under different conditions in a torrefaction pilot plant. The predictions using the multivariate model were compared to those from a conventional curve deconvolution approach. The multivariate approach showed better and more flexible performance, with error of prediction of 2.7% for Mass Yield prediction, compared to the reference method that resulted in 29.4% error. This multivariate method could handle samples pretreated under more severe conditions compared to the curve deconvolution methods. Elemental composition, heating value and volatile content were also predicted with even higher accuracies. The results highlight the usefulness of the method and also the importance of using calibration data of good quality. (C) 2016 Elsevier B.V. All rights reserved.

Keywords
Thermogravimetric analysis, Multivariate data analysis, Fuel characterization, Fuel composition, Torrefaction
National Category
Bioenergy Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-130216 (URN)10.1016/j.fuproc.2016.10.021 (DOI)000390078200014 ()
Available from: 2017-02-02 Created: 2017-01-14 Last updated: 2018-06-09Bibliographically approved
2. Effects of pyrolysis conditions and ash formation on gasification rates of biomass char
Open this publication in new window or tab >>Effects of pyrolysis conditions and ash formation on gasification rates of biomass char
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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
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-136565 (URN)10.1021/acs.energyfuels.7b00688 (DOI)000404691900079 ()
Available from: 2017-06-19 Created: 2017-06-19 Last updated: 2018-06-09Bibliographically approved
3. Biomass pellet combustion: cavities and ash formation characterized by synchrotron X-ray micro-tomography
Open this publication in new window or tab >>Biomass pellet combustion: cavities and ash formation characterized by synchrotron X-ray micro-tomography
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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
Keywords
ash composition, pellet, thermochemical conversion, wheat straw, poplar, SEM-EDS
National Category
Chemical Process Engineering Bioenergy
Identifiers
urn:nbn:se:umu:diva-146673 (URN)10.1016/j.fuproc.2018.03.023 (DOI)
Available from: 2018-04-17 Created: 2018-04-17 Last updated: 2018-06-09Bibliographically approved
4. Time-dependent studies of silicate slag formation during combustion of wheat straw
Open this publication in new window or tab >>Time-dependent studies of silicate slag formation during combustion of wheat straw
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-147530 (URN)
Funder
Swedish Research Council
Available from: 2018-05-07 Created: 2018-05-07 Last updated: 2018-06-09
5. Ash formation and transformation during combustion of poplar wood pellets
Open this publication in new window or tab >>Ash formation and transformation during combustion of poplar wood pellets
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences Engineering and Technology
Identifiers
urn:nbn:se:umu:diva-147531 (URN)
Funder
Swedish Energy Agency
Available from: 2018-05-07 Created: 2018-05-07 Last updated: 2018-06-09

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