| 1. |
- Öhman, Marcus, et al.
(författare)
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Slagging tendencies of wood pellet ash during combustion in residential pellet burners
- 2004
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Ingår i: Biomass and Bioenergy. - Oxford : Elsevier BV. - 0961-9534 .- 1873-2909. ; 27:6, s. 585-596
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Tidskriftsartikel (refereegranskat)abstract
- Ash related problems have more than occasionally been observed in pellet burners during the last years. These problems can lead to reduced accessibility of the combustion systems as well as bad publicity for the market. The objectives of the present work were to; (i) evaluate how different raw materials for pellets affect the accessibility of the existing burner equipment, (ii) determine which of the ash forming element(s) that could be responsible for the deposit/slagg formation and, (iii) estimate the critical slagging temperature for the different raw materials. Stored and fresh materials from sawdust, logging residues and bark were used as raw material in three different pellet burners. The results showed that the slagging properties were relatively sensitive to the variations in total ash content and ash forming elements of the fuel. It is therefore recommended that ash rich fuels like bark and logging residues should not be used in the existing residential pellet burners. Both fuel and burner type affected the amounts of ash deposit produced. The degree of sintering (i.e. the strength of the deposits) was mostly affected by the fuel composition. Subsequent controlled sintering test of the produced deposits/slags showed critical slagging temperatures of about 850-900°C for stored bark and about 1000°C for fresh bark and stored and fresh materials from sawdust and logging residues. The results further indicated that the Si-content in the fuel correlated (relatively) well to the sintering tendencies in the burners. Chemical equilibrium models were used to interpret the experimental findings, and good quantitative agreements between modelling and experimental results were generally obtained. © 2004 Elsevier Ltd. All rights reserved.
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| 2. |
- Brus, Elisabet, et al.
(författare)
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Bed agglomeration characteristics of biomass fuels using blast-furnace slag as bed material
- 2004
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Ingår i: Energy & Fuels. - Washington, D.C. : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 18:4, s. 1187-1193
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Tidskriftsartikel (refereegranskat)abstract
- Agglomeration of bed material may cause severe operating problems during fluidized bed combustion. The attack or coating layers that are formed on the bed particles during combustion play an important role in the agglomeration process. To reduce bed agglomeration tendencies, alternative bed materials may be used. In this paper, bed agglomeration characteristics during the combustion of biomass fuels using a relatively new bed material (iron blast-furnace slag) as well as ordinary quartz sand were determined. Controlled agglomeration tests lasting 40 h, using five representative biomass fuels (bark, olive residue, peat, straw, and reed canary grass) were conducted in a bench-scale fluidized bed. The bed materials and agglomerates were analyzed using SEM/EDS and X-ray diffraction. Chemical equilibrium calculations were performed to interpret the experimental findings. The results showed that blast-furnace slag had a lower tendency to agglomerate than quartz sand for most of the fuels. The quartz particles showed an inner attack layer more often than did the blast-furnace slag. The blast-furnace slag had a lower tendency to react with elements from the fuel. The outer coating layer had similar characteristics and thickness for both bed materials when the same fuel was combusted. However, the inner attack layer thickness was larger for quartz particles. SEM/EDS analyses of the agglomerates showed that the inner Ca-K-silicate-rich attack layer was responsible for the agglomeration of quartz sand. The composition of blast-furnace slag agglomerate was similar to the outer coating layer. Chemical equilibrium calculations showed that the original composition of the blast-furnace slag was close to the equilibrium composition, and hence there was no major driving force for reactions between that bed material and K and Ca from the fuel. The homogeneous silica-rich attack layer (with a low melting temperature) was not formed to the same extent for blast-furnace slag, thus explaining the lower bed agglomeration tendency.
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| 3. |
- Öhman, Marcus, et al.
(författare)
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Ash transformations during combustion of meat-, bonemeal, and RDF in a (bench-scale) fluidized bed combustor
- 2003
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 17:5, s. 1153-1159
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Tidskriftsartikel (refereegranskat)abstract
- Following the recent Bovine spongiform encephalopathy (BSE) experiences, thermal treatment of meat- and bonemeal (MBM) in existing fluidized bed combustion (FBC) plants for refuse-derived fuels (RDFs) has evolved as an interesting disposal and disintegration method. However, only a limited number of studies have previously been performed for combustion of MBM in fluidized beds. The objectives of the present work were, therefore, to determine the bed agglomeration tendencies of these materials during combustion in fluidized beds and to evaluate the effects of dolomite and kaolin addition to the fuel mix, as well as to elucidate the overall ash transformation mechanisms governing the potential bed agglomeration and fouling processes. By controlled agglomeration experiments in a 5 kW bench-scale fluidized bed reactor, the fuel-specific critical agglomeration temperatures in normal quartz bed material were determined for the different fuel/additive mixtures. All collected samples of bed materials, final bed agglomerates, and cyclone ashes were analyzed using SEM/EDS and XRD. The results indicated that the MBM fuels could be expected to be problematic concerning bed agglomeration in normal quartz beds, while kaolin and possibly dolomite addition could be used to reduce this risk to moderate levels. A significant elemental fractionation between the bed material and the cyclone ash was obtained. Apatite (Ca5(PO4)3(OH) or potentially some other calcium phosphates are elutriated from the bed and enriched in the fly ash, while sodium and potassium are enriched in the bed material. The characteristics and the corresponding melting behavior estimations of the necks formed between agglomerated bed particles suggest that silicate melts are responsible for the bed agglomeration. Results from XRD analysis of the fly ash formed from the fuels used in the present study indicated that the risk for melt-related fly ash problems seem relatively small.
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| 4. |
- Öhman, Marcus, et al.
(författare)
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Bed agglomeration characteristics during fluidized bed combustion of biomass fuels
- 2000
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 14:1, s. 169-178
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Tidskriftsartikel (refereegranskat)abstract
- The in-bed behavior of ash-forming elements in fluidized bed combustion (FBC) of different biomass fuels was examined by SEM/EDS analysis of samples collected during controlled agglomeration test runs. Eight fuels were chosen for the test. To cover the variations in biomass characteristics and to represent as many combinations of ash-forming elements in biomass fuels as possible, the selection was based on a principal-component analysis of some 300 biomass fuels, with respect to ash-forming elements. The fuels were then combusted in a bench-scale fluidized bed reactor (5 kW), and their specific agglomeration temperatures were determined. Bed samples were collected throughout the tests, and coatings and necks formed were characterized by SEM/EDS analyses. On the basis of their compositions, the corresponding melting behaviors were determined, using data extracted from phase diagrams. The bench-scale reactor bed samples were finally compared with bed samples collected from biomass-fired full-scale fluidized bed boilers. In all the analyzed samples, the bed particles were coated with a relatively homogeneous ash layer. The compositions of these coatings were most commonly constricted to the ternary system K2O-CaO-SiO2. Sulfur and chlorine were further found not to `participate' in the agglomeration mechanism. The estimated melting behavior of the bed coating generally correlated well with the measured agglomeration temperature, determined in the 5 kW bench-scale fluidized bed reactor. Thus, the results indicate that partial melting of the coating of the bed particles would be directly responsible for the agglomeration.
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| 5. |
- Öhman, Marcus, et al.
(författare)
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Effect of kaolin and limestone addition on slag formation during combustion of wood fuels
- 2004
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Ingår i: Energy & Fuels. - Washington, D.C. : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 18:5, s. 1370-1376
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Tidskriftsartikel (refereegranskat)abstract
- Ash-related problems have more than occasionally been observed in wood-fuel-fired boilers and also recently in wood-pellet burners. These problems can lead to reduced accessibility of the combustion systems as well as bad publicity for the market. The objectives of the present work were, therefore, to determine the effects of kaolin and limestone addition on the slagging propensities of problematic and problem-free wood fuels during combustion in residential pellet appliances (burners), thus contributing to the understanding of the role of kaolin and limestone in preventing slagging on furnace grates. Pellets with additive-to-fuel ratios between 0 and 0.7 wt %d.s. were combusted in three different types of burner constructions (10 kW): over-, horizontal-, and under-feeding of the fuel. The collected slag deposits from the under-fed burner as well as the corresponding deposited fly ash in the boiler were characterized with X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The initial sintering temperatures of the formed slags were also determined. By adding limestone with an additive-to-fuel ratio of 0.5 wt %d.s. to the problematic stemwood raw material (Si-enriched probably because of contamination of sand/soil), the severe slagging of the fuel could totally be eliminated. Adding kaolin to the problematic raw material gave a minor decrease in slagging tendency of the problematic raw material and a major increase in slagging tendency of the problem-free stemwood raw material. When adding limestone to the problematic raw material, the composition of the formed slag was changed from relatively low temperature melting silicates to high temperature melting silicates and oxides. On the other hand, kaolin addition to the problematic raw material changed the content of the slag from mainly Ca-Mg silicates to be dominated by K-Al silicates which have relatively low melting points. When introducing kaolin to the problem-free raw material, the high temperature melting Ca-Mg oxides react to form lower temperature melting Ca-Al-K silicates. Chemical equilibrium model calculations were used to interpret the experimental findings, and generally good qualitative agreements between modeling and experimental results were obtained. © 2004 American Chemical Society.
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| 6. |
- Öhman, Marcus, et al.
(författare)
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Reasons for slagging during stemwood pellet combustion and some measures for prevention
- 2004
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Ingår i: Biomass and Bioenergy. - Oxford : Pergamon Press. - 0961-9534 .- 1873-2909. ; 27:6, s. 597-605
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Tidskriftsartikel (refereegranskat)abstract
- Ash related problems have more than occasionally been observed in pellet burners during the last years. These problems lead to reduced accessibility of the appliances and also bad publicity for the pellet market. The objectives of the present work were therefore to: (i) determine the critical levels of the problematic ash components in stemwood pellets regarding slagging, (ii) document the variations of these problematic elements in the outgoing pellets from two pellet-mills during one operational season, (iii) determine how frequently these elements exceed the critical levels, (iv) determine how different sub-processes in the pelletising process (especially the dryer) effect the slagging properties of the pellet, and if possible (v) suggest some measures for prevention. A significant number of wood pellets reported to be problematic and problem-free, regarding slagging in ordinary residential pellet burners, were collected from the Swedish market. The ash compositions of these fuels were analysed and the results compiled in a database. Partial Least-Squares Discriminant Analysis (PLS-DA) and F-tests were used to statistically identify both the critical ash components and the critical levels of these components that separated the two reported classes. In addition, chemical equilibrium model calculations were used to interpret the findings. The variations of these elements in the in-going raw material and in the produced pellets were determined during one season in two pellet mills equipped with exhaust gas dryers. The results showed that the problematic wood-pellets had a significantly higher amount of Si, but also Al and Fe, in the fuel ash. The critical level of Si (given as SiO2) was about 20-25 wt% of the fuel ash, i.e. pellets with levels in or over this range resulted in slagging problems in residential burners. This critical Si content was exceeded once and twice for the analysed samples in the two studied pellet mills. In one of the studied mills, this was because of contamination by sand of the raw material during storage and handling, and in the other mill the reason was found to be contamination of the raw material by elutriated particles from the dryer fuel. The major conclusion of the work is that both raw materials and drying fuels/processes should be carefully treated to avoid mineral contamination, and an additional cyclone separator could potentially also be used to improve the pellet quality.
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| 7. |
- Sangiorgi, Daniela, et al.
(författare)
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Toward a Participatory Design Approach to Service Design
- 2004
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Ingår i: PDC 2004 - Proceedings of the Eighth Conference on Participatory Design. - Toronto, Canada : ACM Press. ; , s. 148-151
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Konferensbidrag (refereegranskat)abstract
- This paper introduces service design as a potential Participatory Design (PD) theme. It proposes that the success of designing good services can be increased by including the perspectives and practices of the future service participants into the design process. Through the description of an action-research project, the paper explores the potentials and limitations of using an ethnographic approach and Activity Theory to frame the service design process and interpret the complexity of services.
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