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Fluidized-Bed Combustion of Mixtures of Rapeseed Cake and Bark: The Resulting Bed Agglomeration Characteristics
Åbo Akad Univ, Proc Chem Ctr, Inorgan Chem Lab, Turku, Finland.
Luleå Univ Technol, Dept Engn Sci & Math, S-95187 Luleå, Sweden.
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics, Energy Technology and Thermal Process Chemistry.ORCID iD: 0000-0002-5777-9241
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics, Energy Technology and Thermal Process Chemistry.
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2012 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 26, no 4, 2028-2037 p.Article in journal (Refereed) Published
Abstract [en]

The bed agglomeration characteristics resulting from the combustion of 11 mixtures of rapeseed cake and spruce bark were studied in a bench-scale bubbling fluidized-bed reactor (5 kW). The objective was to determine the defluidization temperatures and the prevailing bed agglomeration mechanism as functions of the fuel mixture. Controlled fluidized-bed agglomeration tests were performed for each mixture with quartz sand as the bed material. The total defluidization temperatures and the initial defluidization temperatures were determined based on the measured pressure and temperature profiles in the bed. After combustion, bottom ash samples, agglomerates, and fly ash samples were analyzed by means of scanning electron microscope combined with energy dispersive X-ray detector (SEM-EDX). The composition of the ash-forming matter produced by the combustion of rapeseed cake is significantly different from that produced by the combustion of bark, resulting in different bed agglomeration tendencies. Bark contains ash-forming matter dominated by calcium, with some silicon and potassium, whereas rapeseed cake is rich in phosphorus, potassium, and sodium. The total defluidization temperature for pure bark was above 1045 degrees C, whereas, for rapeseed cake, defluidization occurred during combustion (800 degrees C). During the combustion of bark, the formation of a potassium-rich layer on the silica-bed grains was found to be a crucial for the formation of agglomerates. The low defluidization temperature for the rapeseed cake can be attributed to the formation of sticky ash, which is dominated by phosphates. Two main phosphate forms were observed in the neck between the silica grains: calcium-potassium/sodium phosphates, and magnesium potassium phosphates. As the proportion of bark increased, the Ca/P ratio increased in the fuel mixture, and the formation of high-temperature melting phosphates in the ash was favored. However, the addition of bark also favored the formation of a potassium-rich layer on the silica bed material, leading to the coexistence of both bed agglomeration mechanisms. In the present work, mixtures with a minimum of 60 wt % bark resulted in significantly increased defluidization temperatures and reduced bed agglomeration tendencies, compared to what occurs in rapeseed cake monocombustion.

Place, publisher, year, edition, pages
Washington, DC: American Chemical Society (ACS), 2012. Vol. 26, no 4, 2028-2037 p.
National Category
Engineering and Technology
URN: urn:nbn:se:umu:diva-55521DOI: 10.1021/ef300130eISI: 000302924400005OAI: diva2:528682
Available from: 2012-05-28 Created: 2012-05-21 Last updated: 2014-05-14Bibliographically approved
In thesis
1. Ash chemistry and fuel design focusing on combustion of phosphorus-rich biomass
Open this publication in new window or tab >>Ash chemistry and fuel design focusing on combustion of phosphorus-rich biomass
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomass is increasingly used as a feedstock in global energy production. This may present operational challenges in energy conversion processes which are related to the inorganic content of these biomasses. As a larger variety of biomass is used the need for a basic understanding of ash transformation reactions becomes increasingly important. This is not only to reduce operational problems but also to facilitate the use of ash as a nutrient source for new biomass production.

Ash transformation reactions were examined in the present work using the Lewis acid-base concept. The model presented in Paper I was further extended and discussed, including the definition of tertiary ash transformation reactions as reaction steps where negatively charged molecular ions, Lewis bases, other than hydroxides are present in the reactants. The effect of such reactions for bonding of various metal ions, Lewis acids, were discussed. It was found that the formation of various phosphates through secondary and tertiary ash transformation reactions is important for the behaviour of biomass ash in combustion. The suggested model was supported by findings in Papers II-VIII.

The experimental findings in Papers II-VIII were discussed in terms of ash transformation reactions. The fuel design choices made to investigate the effect of phosphorus in particular on ash transformation reactions were high-lighted. Addition of phosphoric acid to woody-type and agricultural biomasses showed that phosphate formation has a large influence on the speciation of Si, S, and Cl. Co-combustion of a problematic agricultural residue with other biomasses showed that the relation between phosphorus, alkali and alkaline earth metal content is important. Co-combustion of biosolids with wheat straw was shown to greatly improve the combustion properties of wheat straw.

It was suggested that fuel analyses should be presented using molar concentration (mole/kg) in diagrams based on ash transformation reactions and elements forming Lewis acids or bases. This may facilitate the assessment of the combustion behaviour of a fuel. Some comments were made on fuel design and additives, specifically pointing out that phosphorus content should always be carefully considered in relation to alkali and alkaline earth metals in fuels and fuel blends.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2014. 50 p.
phosphorus, biomass, combustion, ash chemistry, fuel design, ash transformation, phosphorus-rich, ash-forming elements, fuel fingerprint, ash transformation reactions, Lewis base, Lewis acid
National Category
Inorganic Chemistry Energy Engineering
Research subject
Inorganic Chemistry
urn:nbn:se:umu:diva-88505 (URN)978-91-7601-070-9 (ISBN)
Public defence
2014-06-05, N430, Naturvetarhuset, Umeå universitet, Umeå, 10:00 (English)
Available from: 2014-05-15 Created: 2014-05-08 Last updated: 2014-05-15Bibliographically approved

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