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Combustion of biosolids in a bubbling fluidized bed part 1: main ash forming elements and ash distribution with a focus on phosphorus
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. (Thermochemical Energy Conversion Laboratory)ORCID iD: 0000-0002-5777-9241
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
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. (Thermochemical Energy Conversion Laboratory)
2014 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 28, no 2, 1183-1190 p.Article in journal (Refereed) Published
Abstract [en]

This is the first in a series of three papers describing combustion of biosolids in a 5-kW bubbling fluidized bed, the ash chemistry, and possible application of the ash produced as a fertilizing agent. This part of the study aims to clarify whether the distribution of main ash forming elements from biosolids can be changed by modifying the fuel matrix, the crystalline compounds of which can be identified in the raw materials and what role the total composition may play for which compounds are formed during combustion. The biosolids were subjected to low-temperature ashing to investigate which crystalline compounds that were present in the raw materials. Combustion experiments of two different types of biosolids were conducted in a 5-kW benchscale bubbling fluidized bed at two different bed temperatures and with two different additives. The additives were chosen to investigate whether the addition of alkali (K2CO3) and alkaline-earth metal (CaCO3) would affect the speciation of phosphorus, so the molar ratios targeted in modified fuels were P:K = 1:1 and P:K:Ca = 1:1:1, respectively. After combustion the ash fractions were collected, the ash distribution was determined and the ash fractions were analyzed with regards to elemental composition (ICP-AES and SEM-EDS) and part of the bed ash was also analyzed qualitatively using XRD. There was no evidence of zeolites in the unmodified fuels, based on low-temperature ashing. During combustion, the biosolid pellets formed large bed ash particles, ash pellets, which contained most of the total ash content (54%–95% (w/w)). This ash fraction contained most of the phosphorus found in the ash and the only phosphate that was identified was a whitlockite, Ca9(K,Mg,Fe)(PO4)7, for all fuels and fuel mixtures. With the addition of potassium, cristobalite (SiO2) could no longer be identified via X-ray diffraction (XRD) in the bed ash particles and leucite (KAlSi2O6) was formed. Most of the alkaline-earth metals calcium and magnesium were also found in the bed ash. Both the formation of aluminum-containing alkali silicates and inclusion of calcium and magnesium in bed ash could assist in preventing bed agglomeration during co-combustion of biosolids with other renewable fuels in a full-scale bubbling fluidized bed.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014. Vol. 28, no 2, 1183-1190 p.
Keyword [en]
sewage-sludge; chemical fractionation; organic contaminants; crystal-structures; rapeseed cake; biomass ashes; wheat-straw; fly ashes; cocombustion; metals
National Category
Inorganic Chemistry Chemical Engineering Bioenergy
URN: urn:nbn:se:umu:diva-84975DOI: 10.1021/ef402320qISI: 000331861800052OAI: diva2:690589
Bio4EnergySwedish Research CouncilSwedish Energy Agency
Available from: 2014-01-24 Created: 2014-01-24 Last updated: 2014-06-02Bibliographically 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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Skoglund, NilsBoström, Dan
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