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Bed agglomeration characteristics in fluidized quartz bed combustion of phosphorus-rich biomass fuels
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics, Energy Technology and Thermal Process Chemistry. (Thermochemical Energy Conversion Laboratory)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.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
2011 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 25, no 3, 937-947 p.Article in journal (Refereed) Published
Abstract [en]

The bed agglomeration characteristics during combustion of phosphorus-rich biomass fuels and fuel mixtures were determined in a fluidized (quartz) bed reactor (5 kW). The fuels studied (separately and in mixtures) included logging residues, bark, willow, wheat straw, and phosphorus-rich fuels, like rapeseed meal (RM) and wheat distillers dried grain with solubles (DDGS). Phosphoric acid was used as a fuel additive. Bed material samples and agglomerates were studied by means of scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX), in order to analyze the morphological and compositional changes of coating/reaction layers and necks between agglomerated bed particles. Furthermore, bed ash particles were separated by sieving from the bed material samples and analyzed with SEM/EDS and powder X-ray diffraction (XRD). For logging residues, bark, and willow, with fuel ash rich in Ca and K but with low contents of P and organically bound Si, the bed layer formation is initiated by reactions of gaseous or liquid K compounds with the surface of the bed material grains, resulting in the formation of a potassium silicate melt. The last process is accompanied by the diffusion/dissolving of Ca into the melt and consequent viscous flow sintering and agglomeration. The addition of high enough phosphorus content to convert the available fuel ash basic oxides into phosphates reduced the amount of K available for the reaction with the quartz bed material grains, thus preventing the formation of an inner bed particle layer in the combustion of logging residues, bark, and willow. Some of the phosphate-rich ash particles, formed during the fuel conversion, adhered and reacted with the bed material grains to form noncontinuous phosphate−silicate coating layers, which were found responsible for the agglomeration process. Adding phosphorus-rich fuels/additives to fuels rich in K and Si (e.g., wheat straw) leads to the formation of alkali-rich phosphate−silicate ash particles that also adhered to the bed particles and caused agglomeration. The melting behavior of the bed particle layers/coatings formed during combustion of phosphorus-rich fuels and fuel mixtures is an important controlling factor behind the agglomeration tendency of the fuel and is heavily dependent on the content of alkaline earth metals in the fuel. A general observation is that phosphorus is the controlling element in ash transformation reactions during biomass combustion in fluidized quartz beds because of the high stability of phosphate compounds.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2011. Vol. 25, no 3, 937-947 p.
National Category
Natural Sciences Chemical Engineering Inorganic Chemistry
Research subject
Inorganic Chemistry
URN: urn:nbn:se:umu:diva-60947DOI: 10.1021/ef101451eISI: 000289613300010OAI: diva2:564924
Available from: 2012-11-05 Created: 2012-11-05 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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