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Fine particle emissions and slag formation in fixed-bed biomass combustion: aspects of fuel engineering
Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. (Thermochemical Energy Conversion Laboratory)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

There is a consensus worldwide that the share of renewable energy sources should be increased to mitigate climate change. The strive to increase the renewable energy fraction can partly be met by an increased utilization of different biomass feedstocks. Many of the "new" feedstocks puts stress on certain challenges such as air pollution emissions and operation stability of the combustion process. The overall objective was to investigate, evaluate, and explain the effects of fuel design and combustion control - fuel engineering - as primary measures for control of slag formation, deposit formation, and fine particle emissions during biomass combustion in small and medium scale fixed-bed appliances. The work in this thesis can be outlined as having two main focus areas, one more applied regarding fuel engineering measures and one more fundamental regarding the time-resolved release of ash forming elements, with particular focus on potassium.

The overall conclusion related to the abatement of particle emissions and slag formation, is that the release of fine particle and deposit forming matter can be controlled simultaneously as the slag formation during fixed-bed biomass combustion. The methodology is in this perspective denoted “fuel engineering” and is based on a combined approach including both fuel design and process control measures. The studies on time-resolved potassium release showed that a Macro-TG reactor with single pellet experiments was a valuable tool for studying ash transformation along the fuel conversion. The combination of dedicated release determinations based on accurate mass balance considerations and ICP analysis, with phase composition characterization by XRD, is important for the understanding of potassium release in general and time-resolved data in particular. For wood, the results presented in this work supports the potassium release mechanism from "char-K" but questions the previously suggested release mechanism from decomposition of K-carbonates. For straw, the present data support the idea that the major part of the potassium release is attributed to volatilization of KCl. To further explore the detailed mechanisms, the novel approach developed and applied in this work should be complemented with other experimental and analytical techniques.

The research in this thesis has explored some of the challenges related to the combined phenomena of fuel conversion and ash transformation during thermochemical conversion of biomass, and has contributed with novel methods and approaches that have gained new knowledge to be used for the development of more effective bioenergy systems.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2015. , 73 p.
Keyword [en]
Renewable energy, biomass, thermochemical fuel conversion, combustion, fine particle emissions, slag formation, fixed-bed, ash chemistry, fuel engineering, release
National Category
Chemical Process Engineering Energy Engineering Environmental Engineering
Identifiers
URN: urn:nbn:se:umu:diva-102687ISBN: 978-91-7601-274-1 (print)OAI: oai:DiVA.org:umu-102687DiVA: diva2:809070
Public defence
2015-05-25, KBC-huset, sal KB3A9, Umeå Universitet, Umeå, 13:00 (Swedish)
Opponent
Supervisors
Available from: 2015-05-04 Created: 2015-04-30 Last updated: 2015-05-08Bibliographically approved
List of papers
1. Influence of Peat Ash Composition on Particle Emissions and Slag Formation in Biomass Grate Co-combustion
Open this publication in new window or tab >>Influence of Peat Ash Composition on Particle Emissions and Slag Formation in Biomass Grate Co-combustion
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2014 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 28, no 5, 3403-3411 p.Article in journal (Refereed) Published
Abstract [en]

Co-combustion by fuel blending of peat and biomass has shown positive effects on operational problems. However, peat ash compositions vary considerably, and this has been shown to affect the potential for operational problems in different fuel-blending situations. The present work used three different peat types with the objective to elucidate how the variation in peat ash composition influences both particle emissions and slag formation during co-combustion with three different biomasses in a small-scale pellet boiler. Estimations of potassium release and slag formation were performed and discussed in relation to fuel composition in the (K2O + Na2O) (CaO + MgO) (SiO2) system. All tested peat types reduced the fine particle emissions by capturing potassium into the bottom ash as one or several of the following forms: slag, sulfates, chlorides, and alumina silicates. However, there were considerable differences between the peat types, presumably depending upon both their content and mineral composition of silicon, calcium, aluminum, and sulfur. Some general important and beneficial properties of peat type in co-combustion situations with biomass are defined here, but the specific blending proportion of peat should be decided on an individual basis for each scenario based on the relative contents in the fuel mixture of the most relevant ash-forming elements.

National Category
Bio Materials
Identifiers
urn:nbn:se:umu:diva-90439 (URN)10.1021/ef4023543 (DOI)000336199300063 ()
Available from: 2014-07-09 Created: 2014-06-23 Last updated: 2017-12-05Bibliographically approved
2. Slagging in fixed-bed combustion of phosphorus-poor biomass: critical ash-forming processes and compositions
Open this publication in new window or tab >>Slagging in fixed-bed combustion of phosphorus-poor biomass: critical ash-forming processes and compositions
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2015 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 29, no 2, 894-908 p.Article in journal (Refereed) Published
Abstract [en]

Slagging in combustion facilities is not welcomed, because it may cause technical and operational problems, as well as extra costs. Increased understanding of the critical slagging subprocesses makes it easier to suggest semiempirical models and fuel indexes for predicting the slagging tendencies of different fuels. That could open the biomass market for potentially more troublesome raw materials. The objective of this work was to determine critical ash-forming processes and compositions in the fixed-bed combustion of phosphorus-poor biomass fuels. This was achieved by performing a systematic review of data and experience gathered from combustion experiments in a small grate burner of 36 different biomasses, as well as chemical analysis of their bottom ashes and slags. The paper presents a discussion of the slagging tendency in phosphorus-poor biomass by combining three different slagging classifications, culminating in a proposed starting point for a new slagging index. The slag (ash particles >3.15 mm in size) formed during the combustion experiments has been described according to the fraction of fuel ash that forms slag (expressed in terms of weight percent), the visual sintering category (1-4), and the viscosity predictions. The results explain that both the fraction of melt and its viscosity are critical for the slag formation process in phosphorus-poor biomasses. In addition, fuels with low Si/K ratio along with a higher Ca concentration may form a low viscous carbonate melt that is not prone to form slag. Increased Si and lowered Ca concentration will increase the amount of formed silicate melt formed, as well as its viscosity, thus resulting in a more sticky melt.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2015
National Category
Energy Engineering Chemical Engineering
Identifiers
urn:nbn:se:umu:diva-101613 (URN)10.1021/ef502531m (DOI)000349943300050 ()
Available from: 2015-04-10 Created: 2015-04-07 Last updated: 2017-12-04Bibliographically approved
3. Alkali transformation during single pellet combustion of soft wood and wheat straw
Open this publication in new window or tab >>Alkali transformation during single pellet combustion of soft wood and wheat straw
2016 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 143, 204-212 p.Article in journal (Refereed) Published
Abstract [en]

Controlling slag and deposit formation during thermochemical fuel conversion requires a fundamental understanding about ash transformation. In this work, a macro-TGA reactor was used to determine the release of ash forming elements during devolatilization and char combustion of single pellets. Soft wood and wheat straw were combusted at two temperatures (700 °C and 1000 °C) and the residual ashes were collected and analyzed for morphology, elemental and phase composition. The results showed that the single pellet combustion exhibit similar release character as in grate boilers. The temporal release was found to be both temperature and fuel dependent. For wood, the release of potassium occurred mostly during char combustion regardless of furnace temperature. Similar results were found for straw at 700 °C, but the temperature increase to 1000 °C implied that the release occurred already during devolatilization. The differences are presumably explained by different fuel phase compositions. The residual ash were composed of three different categories of phases; crystalline compounds, molten ash (glass) and char, and the work concludes that K was captured by crystalline K/Ca-carbonates as well as in amorphous glassy silicates for wood, and by almost fully molten ash of glassy silicates for straw. The fuel conversion processes occurring on a grate influence the fuel combustibility in terms of e.g. burnout, slag formation and release of fine particle and deposit forming matter, and the present work has given novel insights into the specific alkali behavior during biomass fuel conversion.

Keyword
Biomass, Combustion, Ash, Alkali, Release, Single pellet
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-102732 (URN)10.1016/j.fuproc.2015.11.016 (DOI)000369455300023 ()
Available from: 2015-05-04 Created: 2015-05-04 Last updated: 2017-12-04Bibliographically approved
4. Influence of kaolin additive on the release of ash forming elements during biomass combustion
Open this publication in new window or tab >>Influence of kaolin additive on the release of ash forming elements during biomass combustion
(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-102733 (URN)
Available from: 2015-05-04 Created: 2015-05-04 Last updated: 2015-05-04
5. Semi time-resolved release of ash forming elements during single pellet combustion of biomass
Open this publication in new window or tab >>Semi time-resolved release of ash forming elements during single pellet combustion of biomass
(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-102734 (URN)
Available from: 2015-05-04 Created: 2015-05-04 Last updated: 2015-05-04
6. Control strategies for reduction of alkali release during grate combustion of woody biomass: influence of process parameters and fuel additives
Open this publication in new window or tab >>Control strategies for reduction of alkali release during grate combustion of woody biomass: influence of process parameters and fuel additives
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2015 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188Article in journal (Refereed) Accepted
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-102735 (URN)
Available from: 2015-05-04 Created: 2015-05-04 Last updated: 2017-12-04

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