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Fate of Cu, Cr, As and some other trace elements during combustion of recovered waste fuels
Umeå University, Faculty of Science and Technology, Applied Physics and Electronics.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increased use of biomass and recovered waste fuels in favor of fossil fuels for heat and power production is an important step towards a sustainable future. Combustion of waste fuels also offers several advantages over traditional landfilling, such as substantial volume reduction, detoxification of pathological wastes, and reduction of toxic leaches and greenhouse gas (methane) formation from landfills. However, combustion of recovered waste fuels emits more harmful trace elements than combustion of other fuels. These elements are distributed between bottom ash, fly ash and flue gas, depending on the elements partitioning and enrichment behavior. Volatilized harmful trace elements are mainly enriched in the submicron fly ash fraction. If emitted to the atmosphere, submicron particles can penetrate deep into the alveoli of the lungs, causing severe impacts on human health. Consequently, to reduce ash related problems and to control the emissions to the atmosphere, there is an increased need for understanding the physicochemical processes involved in ash transformation, including particle formation.

The objective of this thesis was to carefully and systematically study the fate of trace elements during combustion, i.e. the chemical form of the elements and the partitioning behavior, by means of chemical equilibrium model calculations, X-ray diffraction, microscopy techniques and various spectroscopy methods. The influence of some fuel additives was also analyzed. Primarily, the elements copper, chromium and arsenic were studied.

An initial review and evaluation of the content of thermodynamical data in commercial thermochemical databases used for chemical equilibrium model calculations showed that there was a significant difference in number of included phases and species between databases. Thermodynamical data also differed between databases, although in general less for condensed phases than for gaseous species. A state-of-the-art database for Cu, Cr and As was compiled and used for further chemical equilibrium model calculations. The fate of Cu, Cr and As was determined in combustion experiments on wood impregnated with copper, chromium and arsenic (CCA) in a bench scale reactor (15 kW). The results showed that global chemical equilibrium model calculations predicted the overall fate of Cu, Cr and As in bottom ash and ash particles quite well. However, compared to the experimental results the global model overpredicted the formation of refractory calcium arsenates, thus the arsenic volatilization was found to be higher then the predicted volatilization. In terms of chromium volatility, copper was found to be an important refractory element forming stable CuCrO2(s) and CuCr2O4(s) that suppressed the formation of CrO2(OH)2(g). The retention and speciation of Cu, Cr and As in bottom ash was further determined from combustion experiments of CCA wood fuel particles in a single particle reactor. Local chemical equilibrium model calculations were performed to simulate the combustion stages of a burning CCA treated wood fuel particle: drying, devolatilization, char burning and post-combustion. The results from the work showed that a mix of global and local chemical equilibrium model calculations is needed to describe the reality and that the combustion stages are partially overlapping. The fate of harmful trace elements, including Cu, Cr and As, was finally studied in full scale (65 MW) combustion experiments. Particles from the raw flue gas emissions were sampled and analyzed. The comparison with chemical equilibrium model calculations showed that the model explained the results well, but due to lack of thermodynamic data for K2ZnCl4(s), the formation of this phase could not be predicted.

Place, publisher, year, edition, pages
Umeå: Tillämpad fysik och elektronik , 2007. , 42 p.
Series
ETPC Report, ISSN 1653-0551 ; 07-07
Keyword [en]
combustion, chemical equilibrium model calculations, trace elements, impregnated wood, waste, thermodynamic data, Cu, Cr, As
National Category
Other Chemistry Topics
Identifiers
URN: urn:nbn:se:umu:diva-1132ISBN: 987-91-7264-338-3 OAI: oai:DiVA.org:umu-1132DiVA: diva2:140325
Public defence
2007-06-08, KB3A9, KBC-huset, Umeå Universitet, Umeå, 13:00
Opponent
Supervisors
Available from: 2007-05-11 Created: 2007-05-11Bibliographically approved
List of papers
1. Trace element speciation in combustion processes: review and compilatons of thermodynamic data
Open this publication in new window or tab >>Trace element speciation in combustion processes: review and compilatons of thermodynamic data
2007 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 88, no 11-12, 1061-1070 p.Article in journal (Refereed) Published
Abstract [en]

Chemical equilibrium calculations are often used to determine the fate of trace metals in combustion processes and to study the effects of different process variables and varying fuel compositions. In the present report, thermodynamic data on compounds containing the trace elements As, Cd, Cr, Cu and Pb from different database sources are compared. The results showed significant differences between existing databases in both number of compounds included in the databases and thermodynamic data. The differences also significantly affected the outcome of the equilibrium calculations.

Place, publisher, year, edition, pages
Amsterdam: Elsevier, 2007
Keyword
combustion, thermodynamic data, chemical equilibrium model calculations, thermochemical databases
National Category
Chemical Engineering Energy Engineering
Identifiers
urn:nbn:se:umu:diva-2361 (URN)10.1016/j.fuproc.2007.06.032 (DOI)000250950100009 ()
External cooperation:
Conference
Conference on Impacts of Fuel Quality on Power Production, OCT 29-NOV 03, 2006, Snowbird, UT
Available from: 2007-05-11 Created: 2007-05-11 Last updated: 2017-12-14Bibliographically approved
2. Fate of Cu, Cr and As during combustion of impregnated wood with and without peat additive
Open this publication in new window or tab >>Fate of Cu, Cr and As during combustion of impregnated wood with and without peat additive
2007 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 41, no 18, 6534-6540 p.Article in journal (Refereed) Published
Abstract [en]

The EU Directive on incineration of waste regulates the harmful emissions of particles and twelve toxic elements, including copper, chromium, and arsenic. More information is critically needed on the speciation and behavior of these trace elements during combustion, including the effects of different process variables, as well as of different fuels and fuel mixtures. Using a 15 kW pellets-fueled grate burner, experiments were performed to determine the fate of copper, chromium, and arsenic during combustion of chromate copper arsenate (CCA) preservative wood. The effects of co-combustion of CCA-wood with peat were also studied since peat fuels previously have proved to generally reduce ash related problems. The fate and speciation of copper, chromium, and arsenic were determined from analysis of the flue gas particles and the bottom ash using SEM-EDS, XRD, XPS, and ICP-AES. In addition, chemical equilibrium model calculations were performed to interpret the experimental findings. The results revealed that about 5% copper, 15% chromium, and 60% arsenic were volatilized during combustion of pure CCA-wood, which is lower than predicted volatilization from the individual arsenic, chromium, and copper oxides. This is explained by the formation of more stable refractory complex oxide phases for which the stability trends and patterns are presented. When co-combusted with peat, an additional stabilization of these phases was obtained and thus a small but noteworthy decrease in volatilization of all three elements was observed. The major identified phases for all fuels were CuCrO2(s), (Fe,Mg,Cu)(Cr,Fe,Al)O4(s), Cr2O3(s), and Ca3(AsO4)2(s). Arsenic was also identified in the fine particles as KH2AsO4(s) and As2O3(s). A strong indication of hexavalent chromium in the form of K2CrO4 or as a solid solution between K3Na(CrO4)2 and K3Na(SO4)2 was found in the fine particles. Good qualitative agreement was observed between experimental data and chemical equilibrium model calculations.

Place, publisher, year, edition, pages
American Chemical Society, 2007
Identifiers
urn:nbn:se:umu:diva-2362 (URN)10.1021/es0630689 (DOI)
Available from: 2007-05-11 Created: 2007-05-11 Last updated: 2017-12-14Bibliographically approved
3. Fate of Cu, Cr and As during the combustion stages of burning CCA treated wood fuel particles
Open this publication in new window or tab >>Fate of Cu, Cr and As during the combustion stages of burning CCA treated wood fuel particles
Show others...
Manuscript (Other academic)
Identifiers
urn:nbn:se:umu:diva-2363 (URN)
Available from: 2007-05-11 Created: 2007-05-11 Last updated: 2010-01-13Bibliographically approved
4. Trace element speciation in a 65 MW MSW fired combined heat and power plant
Open this publication in new window or tab >>Trace element speciation in a 65 MW MSW fired combined heat and power plant
Manuscript (Other academic)
Identifiers
urn:nbn:se:umu:diva-2364 (URN)
Available from: 2007-05-11 Created: 2007-05-11 Last updated: 2010-01-13Bibliographically approved
5. Chromium volatility by formation of CrO2(OH)2(g)
Open this publication in new window or tab >>Chromium volatility by formation of CrO2(OH)2(g)
Manuscript (Other academic)
Identifiers
urn:nbn:se:umu:diva-2365 (URN)
Available from: 2007-05-11 Created: 2007-05-11 Last updated: 2010-01-13Bibliographically approved

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