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Chemical characterization of combustion deposits by TOF-SIMS
Applied Surface Science 203±204 (2003) 669±672
Chemical characterization of combustion deposits by TOF-SIMS P. SjoÈvalla,*, J. Lausmaaa, C. Tullina, J. HoÈgbergb a
SP Swedish National Testing and Research Institute, P.O. Box 857, SE-50115 BoraÊs, Sweden b Vattenfall Development, SE-81426 AÈlvkarleby, Sweden
Abstract We have investigated the potential usefulness of TOF-SIMS for chemical analysis of deposits formed in combustion reactors. By using TOF-SIMS, it was possible to (i) identify inorganic chemical compounds in the deposits, (ii) semi-quantitatively estimate the relative concentrations of the main constituents and (iii) obtain images showing the lateral distribution of the main constituents, on the surface and in cross-sections of deposit samples. It was found that the main components in the deposit samples were KCl and K2SO4, while K2CO3, NaCl, Na2SO4, Ca(OH)2 and CaCl2 were present in smaller concentrations. In addition, deposits from combustion of recycled wood chips contained considerable amounts of ZnCl2, PbCl2, ZnO and PbO. Large variations in the chemical composition were observed for different samples and throughout the cross-section of a single sample. The chlorides, in particular NaCl, were present mainly as particles, while the sulfates were more homogeneously distributed in the deposit. The results from this study show that TOF-SIMS analysis of combustion deposits can contribute signi®cantly to an increased understanding of the formation and growth of deposits in combustion reactors. # 2002 Elsevier Science B.V. All rights reserved. Keywords: TOF-SIMS; Combustion; Deposits; Inorganic; Compound
1. Introduction Deposits formed on surfaces inside the reactor of combustion power plants constitute a severe and costly problem. The deposits give rise to reduced energy extraction ef®ciency and to accelerated corrosion of metal components inside the reactor. In order to understand the deposit formation and to develop methods to minimize their growth rate, it is important to obtain information about the chemical composition of the deposits [1]. The deposits consist mainly of the inorganic constituents of the fuel, and the composition depends on several factors, such as (i) the chemical composition of the fuel, (ii) the combustion parameters *
Corresponding author. Tel.: 46-33-165299; fax: 46-33-103388. E-mail address: [email protected] (P. SjoÈvall).
and (iii) the position inside the reactor and the reactor geometry. Combustion deposits are commonly analyzed by SEM/EDX which provides valuable information about the morphology and elemental composition of the deposits. This method does not, however, provide information about the chemical compounds in which the elements exist. In the present work, the possibility to use TOF-SIMS for characterization of combustion deposits was explored. 2. Experimental Two types of samples were studied in this work, both of which were collected in a power plant at normal production conditions. A thin ®lm (ca. 2±5 mm) sample was collected on a steel ring inside the
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 7 8 8 - 2
670
P. SjoÈvall et al. / Applied Surface Science 203±204 (2003) 669±672
Table 1 Characteristic fragments and sensitivity factors of compounds studied in reference samples Compound
Positive spectra Characteristic ion fragment
KCl K2SO4 K2CO3 NaCl Na2SO4 Na2CO3 CaCl2 Ca(OH)2 PbO PbCl2 ZnO
K2Cl K3 SO4 K3 CO3 Na2Cl Na3 SO4 Na3 CO3 CaCl CaOH Pb2O PbCl ZnOH
reactor (T 350 8C) during combustion of recycled wood chips. The second sample was ca. 1 mm thick deposit ¯ake taken from the tube wall of the super heater during combustion of wood chips (T 510 520 8C). A cross-section of this sample, perpendicular to the underlying metal surface, was prepared by grinding. Reference samples, each consisting of equal amounts (by mass) of three different compounds, were prepared by mixing the pure crystalline forms of the compounds and grinding (in a mortar) the mixture into a ®ne powder. Using double-sided tape (3 M, type 665), the powder mixture was then attached to a metal block and analyzed. From the spectra of the reference samples, characteristic peaks were identi®ed and relative sensitivity factors (RSFs) were obtained for a number of different compounds (see Table 1). The RSFs were calculated from the reference sample spectra as the ratio between the signal intensity of the characteristic peak of the compound in question and the signal intensity of the characteristic peak of KCl (K2Cl and KCl2 , respectively). In reference spectra which did not contain KCl, the RSF was ®rst calculated relative to another compound, for which, in turn, an RSF relative to KCl was obtained from a spectrum containing KCl. Semi-quantitative estimates of the relative composition of the deposit samples were made using the RSFs. The procedure to use RSFs for quantitative analysis can be justi®ed only if the chemical
Negative spectra RSF
Characteristic ion fragment
RSF
1 0.069 0.18 0.56 0.097 0.023 1.48 10.9 0.017 0.057 0.11
KCl2 KSO4 KCO3 NaCl2 NaSO4 NaCO3 CaCl3
1 0.031 0.017 0.66 0.073 0.012 0.30
PbO2 PbCl3 ZnO
1.02 0.98 2.51
environment of the compound, which gives rise to the characteristic peak, is the same in the sample as in the reference sample. For example, if the sample primarily contains mixed compounds, one expects considerable differences in the RSFs between the sample and the reference sample (which primarily contains pure compounds on the scale of the sputtering process, ca. 10 nm), making a quantitative analysis impossible. However, the present results indicate that the deposit samples primarily contain pure compounds (on the scale of the sputtering process), as judged both from the image analysis and the relatively low signal intensity from mixed fragments (e.g. KNaCl, etc.). The semi-quantitative analysis presented below can therefore be justi®ed, however, only for providing rough estimates of the relative concentrations of the different compounds in the samples. TOF-SIMS analysis was carried out using a TOFSIMS IV instrument (ION-TOF GmbH) with a primary beam of 25 kV Ga ions. High mass resolution (m=Dm 7000 8000) spectra were collected over an analysis area of 300 mm 300 mm. Before analysis, the sample surfaces were sputtered using a 3 kV Ar beam, typically over an area of 500 mm 500 mm for 1000 s (IAr 35 60 nA), in order to remove surface contamination. It was found that the initial sputtering (<60 s) introduced considerable changes in the fragmentation patterns of the analyzed compounds, while after these initial changes the fragmentation patterns remained relatively stable during additional sputtering.
P. SjoÈvall et al. / Applied Surface Science 203±204 (2003) 669±672
671
Fig. 1. Positive TOF-SIMS spectrum from deposit ®lm sample.
3. Results Fig. 1 shows a positive TOF-SIMS spectrum from the thin ®lm deposit sample. A number of compounds was identi®ed from this and the corresponding negative spectrum, such as KCl, NaCl, ZnCl2, PbO, PbCl2 and BaCl2. A semi-quantitative analysis of the composition, using the RSFs obtained from the analysis of the reference samples, showed that the alkali content of the deposit consists of approximately 60% KCl and
30% NaCl, 4% K2SO4, 2% Na2SO4 and 1% K2CO3 (by weight). The PbCl2 content was estimated to 16% relative to KCl (by weight), while the ZnCl2 content could not be determined due to uncertainties in the sensitivity factors obtained from the reference samples. High resolution images (lateral resolution 0.5± 1 mm) of the thin ®lm deposit sample after sputtering showed detailed information about the lateral distribution of the main compounds, e.g. the NaCl and KCl
Fig. 2. TOF-SIMS image of deposit cross-section. Analysis area 300 mm 300 mm.
P. SjoÈvall et al. / Applied Surface Science 203±204 (2003) 669±672
672
Table 2 Semi-quantitative analysis of different areas of the deposit cross-section. The relative concentrations (by weight) have been normalized so that the sum of the ®ve included compounds is 1 Area
[KCl]rel
[K2SO4]rel
[K2CO3]rel
[NaCl]rel
[Ca(OH)2]rel
A/Ca(OH)2 B/KCl C/K2SO4
0.17 0.49 0.024
0.60 0.45 0.90
0.026 0.027 0.050
0.004 0.008 0.008
0.19 0.024 0.019
is distributed in 1±10 mm diameter particles, separate or agglomerated, and that the particles mainly consist of the pure compounds and not mixtures thereof. Fig. 2 shows TOF-SIMS images (recorded at high mass resolutionÐlow lateral resolution) of the deposit cross-section sample, showing evidence for a layered structure. It is possible to distinguish at least three different areas in the images: (A) Ca(OH)2-rich layer in the lower right part of the images; (B) KCl-rich layer in the middle and (C) K2SO4-rich layer on top (closest to the deposit surface). A semi-quantitative analysis (see Table 2) indicates large variations in the chemical composition of the three different layers. This sample did not contain signi®cant amounts of Znor Pb-containing compounds, as expected in deposits formed from combustion of pure wood chips.
identi®cation of chemical compounds, (ii) semi-quantitative analysis of the chemical composition and (iii) lateral distribution of compounds on the surface and in cross-sections of deposit samples. This type of information is likely to contribute signi®cantly to the understanding of combustion deposits and to the development of methods to control their growth. Additional work is required to improve the reliability of the semi-quantitative analysis. Careful sample handling and storage is essential due to the hygroscopic nature of many of the compounds present in the deposits.
4. Discussion and conclusions
References
The results from this study show that TOF-SIMS may be a very useful method for obtaining important information about combustion deposits, such as (i)
Acknowledgements This project was ®nanced by VaÈrmeforsk.
[1] A.F. Saro®m, J.J. Helble, in: J. Williamson, F. Wigley (Eds.), The Impact of Ash Deposition on Coal Fired Plants, Taylor & Francis, Washington, 1993, pp. 567±582.
Chemical characterization of combustion deposits by TOF-SIMS P. SjoÈvalla,*, J. Lausmaaa, C. Tullina, J. HoÈgbergb a
SP Swedish National Testing and Research Institute, P.O. Box 857, SE-50115 BoraÊs, Sweden b Vattenfall Development, SE-81426 AÈlvkarleby, Sweden
Abstract We have investigated the potential usefulness of TOF-SIMS for chemical analysis of deposits formed in combustion reactors. By using TOF-SIMS, it was possible to (i) identify inorganic chemical compounds in the deposits, (ii) semi-quantitatively estimate the relative concentrations of the main constituents and (iii) obtain images showing the lateral distribution of the main constituents, on the surface and in cross-sections of deposit samples. It was found that the main components in the deposit samples were KCl and K2SO4, while K2CO3, NaCl, Na2SO4, Ca(OH)2 and CaCl2 were present in smaller concentrations. In addition, deposits from combustion of recycled wood chips contained considerable amounts of ZnCl2, PbCl2, ZnO and PbO. Large variations in the chemical composition were observed for different samples and throughout the cross-section of a single sample. The chlorides, in particular NaCl, were present mainly as particles, while the sulfates were more homogeneously distributed in the deposit. The results from this study show that TOF-SIMS analysis of combustion deposits can contribute signi®cantly to an increased understanding of the formation and growth of deposits in combustion reactors. # 2002 Elsevier Science B.V. All rights reserved. Keywords: TOF-SIMS; Combustion; Deposits; Inorganic; Compound
1. Introduction Deposits formed on surfaces inside the reactor of combustion power plants constitute a severe and costly problem. The deposits give rise to reduced energy extraction ef®ciency and to accelerated corrosion of metal components inside the reactor. In order to understand the deposit formation and to develop methods to minimize their growth rate, it is important to obtain information about the chemical composition of the deposits [1]. The deposits consist mainly of the inorganic constituents of the fuel, and the composition depends on several factors, such as (i) the chemical composition of the fuel, (ii) the combustion parameters *
Corresponding author. Tel.: 46-33-165299; fax: 46-33-103388. E-mail address: [email protected] (P. SjoÈvall).
and (iii) the position inside the reactor and the reactor geometry. Combustion deposits are commonly analyzed by SEM/EDX which provides valuable information about the morphology and elemental composition of the deposits. This method does not, however, provide information about the chemical compounds in which the elements exist. In the present work, the possibility to use TOF-SIMS for characterization of combustion deposits was explored. 2. Experimental Two types of samples were studied in this work, both of which were collected in a power plant at normal production conditions. A thin ®lm (ca. 2±5 mm) sample was collected on a steel ring inside the
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 7 8 8 - 2
670
P. SjoÈvall et al. / Applied Surface Science 203±204 (2003) 669±672
Table 1 Characteristic fragments and sensitivity factors of compounds studied in reference samples Compound
Positive spectra Characteristic ion fragment
KCl K2SO4 K2CO3 NaCl Na2SO4 Na2CO3 CaCl2 Ca(OH)2 PbO PbCl2 ZnO
K2Cl K3 SO4 K3 CO3 Na2Cl Na3 SO4 Na3 CO3 CaCl CaOH Pb2O PbCl ZnOH
reactor (T 350 8C) during combustion of recycled wood chips. The second sample was ca. 1 mm thick deposit ¯ake taken from the tube wall of the super heater during combustion of wood chips (T 510 520 8C). A cross-section of this sample, perpendicular to the underlying metal surface, was prepared by grinding. Reference samples, each consisting of equal amounts (by mass) of three different compounds, were prepared by mixing the pure crystalline forms of the compounds and grinding (in a mortar) the mixture into a ®ne powder. Using double-sided tape (3 M, type 665), the powder mixture was then attached to a metal block and analyzed. From the spectra of the reference samples, characteristic peaks were identi®ed and relative sensitivity factors (RSFs) were obtained for a number of different compounds (see Table 1). The RSFs were calculated from the reference sample spectra as the ratio between the signal intensity of the characteristic peak of the compound in question and the signal intensity of the characteristic peak of KCl (K2Cl and KCl2 , respectively). In reference spectra which did not contain KCl, the RSF was ®rst calculated relative to another compound, for which, in turn, an RSF relative to KCl was obtained from a spectrum containing KCl. Semi-quantitative estimates of the relative composition of the deposit samples were made using the RSFs. The procedure to use RSFs for quantitative analysis can be justi®ed only if the chemical
Negative spectra RSF
Characteristic ion fragment
RSF
1 0.069 0.18 0.56 0.097 0.023 1.48 10.9 0.017 0.057 0.11
KCl2 KSO4 KCO3 NaCl2 NaSO4 NaCO3 CaCl3
1 0.031 0.017 0.66 0.073 0.012 0.30
PbO2 PbCl3 ZnO
1.02 0.98 2.51
environment of the compound, which gives rise to the characteristic peak, is the same in the sample as in the reference sample. For example, if the sample primarily contains mixed compounds, one expects considerable differences in the RSFs between the sample and the reference sample (which primarily contains pure compounds on the scale of the sputtering process, ca. 10 nm), making a quantitative analysis impossible. However, the present results indicate that the deposit samples primarily contain pure compounds (on the scale of the sputtering process), as judged both from the image analysis and the relatively low signal intensity from mixed fragments (e.g. KNaCl, etc.). The semi-quantitative analysis presented below can therefore be justi®ed, however, only for providing rough estimates of the relative concentrations of the different compounds in the samples. TOF-SIMS analysis was carried out using a TOFSIMS IV instrument (ION-TOF GmbH) with a primary beam of 25 kV Ga ions. High mass resolution (m=Dm 7000 8000) spectra were collected over an analysis area of 300 mm 300 mm. Before analysis, the sample surfaces were sputtered using a 3 kV Ar beam, typically over an area of 500 mm 500 mm for 1000 s (IAr 35 60 nA), in order to remove surface contamination. It was found that the initial sputtering (<60 s) introduced considerable changes in the fragmentation patterns of the analyzed compounds, while after these initial changes the fragmentation patterns remained relatively stable during additional sputtering.
P. SjoÈvall et al. / Applied Surface Science 203±204 (2003) 669±672
671
Fig. 1. Positive TOF-SIMS spectrum from deposit ®lm sample.
3. Results Fig. 1 shows a positive TOF-SIMS spectrum from the thin ®lm deposit sample. A number of compounds was identi®ed from this and the corresponding negative spectrum, such as KCl, NaCl, ZnCl2, PbO, PbCl2 and BaCl2. A semi-quantitative analysis of the composition, using the RSFs obtained from the analysis of the reference samples, showed that the alkali content of the deposit consists of approximately 60% KCl and
30% NaCl, 4% K2SO4, 2% Na2SO4 and 1% K2CO3 (by weight). The PbCl2 content was estimated to 16% relative to KCl (by weight), while the ZnCl2 content could not be determined due to uncertainties in the sensitivity factors obtained from the reference samples. High resolution images (lateral resolution 0.5± 1 mm) of the thin ®lm deposit sample after sputtering showed detailed information about the lateral distribution of the main compounds, e.g. the NaCl and KCl
Fig. 2. TOF-SIMS image of deposit cross-section. Analysis area 300 mm 300 mm.
P. SjoÈvall et al. / Applied Surface Science 203±204 (2003) 669±672
672
Table 2 Semi-quantitative analysis of different areas of the deposit cross-section. The relative concentrations (by weight) have been normalized so that the sum of the ®ve included compounds is 1 Area
[KCl]rel
[K2SO4]rel
[K2CO3]rel
[NaCl]rel
[Ca(OH)2]rel
A/Ca(OH)2 B/KCl C/K2SO4
0.17 0.49 0.024
0.60 0.45 0.90
0.026 0.027 0.050
0.004 0.008 0.008
0.19 0.024 0.019
is distributed in 1±10 mm diameter particles, separate or agglomerated, and that the particles mainly consist of the pure compounds and not mixtures thereof. Fig. 2 shows TOF-SIMS images (recorded at high mass resolutionÐlow lateral resolution) of the deposit cross-section sample, showing evidence for a layered structure. It is possible to distinguish at least three different areas in the images: (A) Ca(OH)2-rich layer in the lower right part of the images; (B) KCl-rich layer in the middle and (C) K2SO4-rich layer on top (closest to the deposit surface). A semi-quantitative analysis (see Table 2) indicates large variations in the chemical composition of the three different layers. This sample did not contain signi®cant amounts of Znor Pb-containing compounds, as expected in deposits formed from combustion of pure wood chips.
identi®cation of chemical compounds, (ii) semi-quantitative analysis of the chemical composition and (iii) lateral distribution of compounds on the surface and in cross-sections of deposit samples. This type of information is likely to contribute signi®cantly to the understanding of combustion deposits and to the development of methods to control their growth. Additional work is required to improve the reliability of the semi-quantitative analysis. Careful sample handling and storage is essential due to the hygroscopic nature of many of the compounds present in the deposits.
4. Discussion and conclusions
References
The results from this study show that TOF-SIMS may be a very useful method for obtaining important information about combustion deposits, such as (i)
Acknowledgements This project was ®nanced by VaÈrmeforsk.
[1] A.F. Saro®m, J.J. Helble, in: J. Williamson, F. Wigley (Eds.), The Impact of Ash Deposition on Coal Fired Plants, Taylor & Francis, Washington, 1993, pp. 567±582.