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Comparative Study of Deposits with Laser Ablation Inductively Coupled Plasma Mass Spectrometry and Scanning Electron Microscopy-Energy-Dispersive Spectrometry† Mirja H. Piispanen, Minna S. Tiainen, and Risto S. Laitinen* Department of Chemistry, Post Office Box 3000, 90014 UniVersity of Oulu, Finland ReceiVed NoVember 30, 2008. ReVised Manuscript ReceiVed March 31, 2009
Scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS) is a conventional method to study spatial compositional distribution in ash-related samples collected from various power plant boilers. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is an attractive alternative that can provide related information. In this work, we have analyzed deposits on seven water-cooled deposition probes by both SEM-EDS and LA-ICP-MS. The samples were collected in a 30 MW full-scale circulating fludized bed (CFB) power plant during its normal operation using solid recovered fuel. Calcium and silicon were the main components in all deposits and occurred throughout the deposit layers. Chlorine, potassium, sodium, and sulfur compounds seem to be concentrated mainly near to the interface between the deposits and the probe surfaces.
Introduction Combustion of biofuels and solid recovered fuel for energy production has increased in recent years as a consequence of environmental awareness. These fuels, however, contain components that render control of the combustion process challenging. Biofuels are considered environmentally benign, but their combustion often leads to the formation of various deposits on the surfaces of heat exchangers.1-5 The deposits usually contain chlorine and sulfur compounds and may lead to corrosion of the metallic materials.6-10 The formation of deposits is a complicated phenomenon, which depends on various factors such as the physical and chemical properties of the materials as well as on boiler design and conditions such as temperature and gas flow in the boiler.1,11-14 Deposit mechanisms have been divided into four categories: inertial impaction, thermophoretic deposition, condensation, and chemical reaction.1 The chemical composition of fuel naturally influences the growth of deposits and their properties. For example, high † Impacts of Fuel Quality on Power Generation and Environment. * Corresponding author: e-mail
[email protected]; fax +358 8 553 1608. (1) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47–78. (2) Frandsen, F. J. Fuel 2005, 84, 1277–1294. (3) Nutalapati, D.; Gupta, R.; Moghtaderi, B.; Wall, T. F. Fuel Process. Technol. 2007, 88, 1044–1052. (4) Strandstro¨m, K.; Mueller, C.; Hupa, M. Fuel Process. Technol. 2007, 88, 1053–1060. (5) Knudsen, J. N.; Jensen, P. A.; Dam-Johanssen, K. Energy Fuels 2004, 18, 1385–1399. (6) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. Prog. Energy Combust. Sci. 2000, 26, 283–298. (7) Knudsen, J. N.; Jensen, P. A.; Lin, W.; Frandsen, J.; Dam-Johansen, K. Energy Fuels 2004, 18, 810–819. (8) Davidsson, K. O.; Åmand, L.-E.; Leckner, B.; Kovacevik, B.; Svane, M.; Hagstro¨m, M.; Pettersson, J. B. C.; Pettersson, J.; Asteman, H.; Svensson, J.-E.; Johansson, L.-G. Energy Fuels 2007, 21, 71–81. (9) Aho, M.; Vainikka, P.; Taipale, R.; Yrjas, P. Fuel 2008, 87, 647654 and references therein. (10) Uusitalo, M. A.; Vuoristo, P. M. J.; Ma¨ntyla¨, T. A. Corros. Sci. 2004, 46, 1311–1331.
chlorine content in fuel leads to higher growth rates of deposition.15 Similarly, fuel that is rich in alkali metals and silicon results in the facile formation of low-melting alkali metal silicates.1,3 Potassium compounds easily form aerosols or fine fly ash particles due to condensation, sulfation, and carbonization.16 Potassium probably condenses as KCl (solid) or K2SO4 (solid) on coarse fly ash particles and form sticky deposits upon cooling.2,8 Mixtures of, for instance, potassium silicate and aluminosilicate or of potassium silicate and sulfate are formed if the fuel contains soil contaminants. The deposits are usually heterogeneous, complex, multiphase, and porous materials17 and they are normally studied by scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD).2,8-11,18-20 Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is also a promising tool for analyzing boilerrelated solid samples such as fly ash.21-23 In the present study we explored the suitability of LA-ICP-MS when studying spatial (11) Wigley, F.; Williamson, J.; Malmgren, A.; Riley, G. Fuel Process. Technol. 2007, 88, 1148–1154. (12) Salmenoja, K.; Ma¨kela¨, K.; Backman, R. In Proceedings of the 8th International Symposium on Corrosion in the Pulp & Paper Industry, Stockholm, Sweden, May 16-19, 1995; pp 198-206. (13) Uusitalo, M. A.; Vuoristo, P. M. J.; Ma¨ntyla¨, T. A. Mater. Sci. Eng. 2003, A346, 168–177. (14) Couch, G. Understanding slagging and fouling in PF combustion; International Energy Agency Coal Research: London, 1994; pp 50-61. (15) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Energy Fuels 1999, 13, 860–870. (16) Wei, X.; Schnell, U.; Hein, K. R. G. Fuel 2005, 84, 841–848. (17) Kweon, C. S.; Ramer, E.; Robinson, A. L. Energy Fuels 2003, 17, 1311–1323. (18) Michelsen, H. P.; Frandsen, F.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol. 1998, 54, 95–108. (19) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Hørlyck, S.; Karlsson, A. Fuel Process. Technol. 2000, 64, 189–209. (20) Henriksen, N.; Larsen, O. H. Mater. High Temp. 1997, 14, 227– 236. (21) Spears, D. A. Fuel 2004, 83, 1765–1770. (22) Spears, D. A.; Martinez-Tarrazona, M. R. Fuel 2004, 83, 2265– 2270.
10.1021/ef801039g CCC: $40.75 2009 American Chemical Society Published on Web 07/16/2009
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Table 1. Probe Materials and Temperatures and Deposit Thickness and Exposure Conditions probe I II III IV V VI VII
probe material
temp (°C)
10CrMo910 (painted) 10CrMo910 (painted) 10CrMo910 (sandblasted) X20CrMoV121 X20CrMoV121 (sandblasted) AISI 347 PM2000a
exposure time (h)
deposit thickness (µm)
500
2
230
350
12
380
350
2
240
350 400
12 12
550 260
400 400
2 1000
150 790
a This is test material constructed by Foster Wheeler Inc. by sintering a mixture of metals and oxides at high temperature and pressure.
Table 2. ICP-MS Operation Parameters laser parameters
ICP-MS parameters
output (%) 100 RF power (W) repetition rate 20 nebulizer gas (mL/min) (Hz) a b c spot size (µm) 80 /60 /95 auxiliary gas (mL/min) cooling gas (mL/min) hexapole bias (V) quadrupole bias (V) focus a 23Na, 39K. b 35Cl. c
1100 1.1a/1.1b/1.07c 1.0b/0.78a,c 12.0 -8.0a /-2.0b/1.3c -10a/-2.4b/-1.2c 5a/16b/8c
Other elements.
compositional distributions in fireside deposits in a circulating fluidized-bed boiler using solid recovered fuel. The results are validated with SEM-EDS, where appropriate. Experimental Section Samples. Tubular deposition probes were received from Foster Wheeler Energy, Inc.24 Tube materials and treatment, surface temperatures, and exposure times are shown in Table 1 together with the deposit thickness. Sample Preparation. Probe tubes were cut into suitable lengths to fit into the sample holders of both SEM-EDS and LA-ICP-MS. The cross sections thus obtained were cast in Struers Epofix resin. A sample of the fly ash standard NIST 2691 that was used as a reference was also cast in the resin. All mounted samples were ground by using 80, 240, and 600 Mesh SiC paper and polished by use of 1200 Mesh SiC paper. In order to enable electrical conduction, all samples were coated with a thin graphite layer. All samples were analyzed by both LA-ICP-MS and SEM-EDS. Laser Ablation Inductively Coupled Plasma Mass Spectrometry. A Thermo Elemental X7 ICP-MS with a laser unit, Thermo New Wave UP/213, was used for multielement analyses of deposits on the probes. The performance of the laser unit was verified daily with the glass standard NIST 612. ICP-MS operation conditions were optimized and a short-term stability test was measured every day to ensure the sensitivity of the instrument. Maximum sensitivity was ensured by using collision and reaction cell technology. The instrument parameters for the point determinations of 24Mg, 48Ti, 27 Al, 28Si, 31P, 34S, 40Ca, 23Na, and 39K are presented in Table 2. Peak jumping was used in the acquisition mode. 35Cl was analyzed without collision and reaction cell techniques (see Table 2). (23) Piispanen, M. H.; Arvilommi, S. A.; Van den Broeck, B.; Nuutinen, L. H.; Tiainen, M. S.; Pera¨ma¨ki, P. J.; Laitinen, R. S. Energy Fuels [Online early access]. DOI: 10.1021/ef801037a. Published Online: March 27, 2009. (24) The samples were collected from a full-scale 30 MW circulating fludized bed (CFB) boiler during its normal operation using solid recovered fuel. Proximate and ultimate analyses of the fuel have been given in Supporting Information. The bed temperature was 850-900 °C. The probes were installed before the superheaters. All collections were started at the same time in December 2005. The probes were removed at the end of the predetermined exposure times. For probe temperatures and exposure times, see Table 1.
Figure 1. LA-ICP-MS microscopic images of probes (a) I, (b) II, (c) III, (d) IV, (e) V, (f) VI, and (g) VII (for description of the probes, see Table 1). Magnification is 100×.
The laser module contains the video microscope including a highresolution charge-coupled device (CCD) camera. The microscope and camera were used to view the samples during ablation. Scanning Electron Microscopy-Energy-Dispersive Spectrometry. As described by Virtanen et al.,25 SEM-EDS analyses of the tube samples were carried out with a JEOL JSM-6400 scanning electron microscope equipped with an Inca energydispersive X-ray analyzer and Feature image processing software employing acceleration voltage of 20 kV and beam current of 390 × 10-9 A. The sample distance was 25 mm. Backscattered electron images (BE) were collected at a resolution of 1024 × 768 pixels. Elemental compositions were determined for selected points and ZAF correction was applied to the results. The X-ray maps were recorded with a beam current of 960 × 10-9 A and were calibrated with copper. The maps were visualized as so-called mix-images.
Results and Discussion The combustion of biofuels often leads to deposition on superheater surfaces and may occasionally lead to corrosion. This is a consequence of high alkali metal, chlorine, and sulfur contents of the deposits. In this work, we have investigated seven deposit probes made of different materials, some of which were painted or sandblasted (see Table 1). Whereas, probes I-VI were made of commercial metal alloys, probe VII was locally constructed by sintering a mixture of metals and oxides at high temperature and pressure. After exposure, each probe was visually examined by use of the microscope incorporated in the LA-ICP-MS system. Probes I-V proved to be more susceptible to corrosion than probes VI and VII (see Figure 1). Probes VI and VII were resistant (25) Virtanen, M. E.; Tiainen, M. S.; Laitinen, R. S. In 5th European Conference on Industrial Furnaces and Boilers; Porto, Portugal, 2000; Vol. II, pp 117-126.
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Figure 2. Dependence of deposit thickness (micrometers) on temperature (degrees Celsius), exposure time (hours), and probe material.
Figure 3. Deposits are mainly developed in an angle range of 0° ( 45° with respect to the gas flow.
even after long exposure times. The sharp border between probe and deposit can easily be seen from Figure 1f,g. Visual examination also shows that durability of the probe is enhanced by painting compared to sandblasting. Chemical compositions of deposits on the probes were investigated by LA-ICP-MS, and the results are compared to those by SEM-EDS when needed. The suitability of LA-ICPMS in the characterization of fly ash has been discussed in our previous paper.23 Formation and thickness of deposits expectedly depend on the conditions in the boiler, on the fuel, and on the exposure times (see Figure 2). Probe VII understandably shows the thickest deposit because of the extremely long exposure time of 1000 h. It can be seen from Figure 2 that there are no clear correlations between deposit thickness and probe material or temperature. The rate of formation of the deposits was also dependent on the direction of the probe surface with respect to the gas flow. The conditions most favorable for deposit formation are found on the “wind side” of the probe in the angle range of 0° ( 45° with respect to the gas flow, and consequently the thickest deposits are found here (see Figure 3). Calcium is the major component in most layers, together with silicon, sulfur, and chlorine. Only small amounts of sodium and potassium could be found in the deposits examined in this work. The highest calcium content was found in the sandblasted probes III and V, and also in probe VII, which was exposed for the longest time of 1000 h.
Figure 4. Normalized compositional distribution (weight percent) in the cross section of the deposit on probe IV analyzed with LA-ICPMS at an angle of 45° with respect to the gas flow (see Figure 3). Magnification is 100×.
The maximum silicon content in the deposits was found in probes I and VII. The highest sulfur contents were found in the deposits of probes V and VII. Elemental composition of the deposit layers is locationspecific. This is exemplified by probe IV. LA-ICP-MS determinations indicate that, at an angle of 45° with respect to the direction of the gas flow, sulfur is concentrated near the interface between the probe material and the deposit (Figure 4). The X-ray map mix-image in Figure 5 shows, on the other hand, that sulfur is mainly found in the outermost deposit layers directly against the gas flow (angle of 0°; see Figure 3). Sodium and potassium contents were generally low in deposits on probes I-VI. The deposit on probe VII, however, showed exceptionally high sodium and potassium levels. Interestingly, in this case both elements concentrate in the interface between tube and deposit (see Figure 6). It was also of interest to compare the effect of painting or sandblasting of the probe on the composition of the deposit. The comparison is shown for probes I and III in Figure 7. These two probes are made from the same metal alloy. Probe I was painted, and probe III was sandblasted. The deposit collection was carried out at different temperatures but for identical times. It can be seen from Figure 7 that the deposit on the sandblasted probe III is richer in calcium and chlorine but is poorer in potassium, silicon, and sulfur.
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Figure 5. X-ray map mix-image of the deposit on probe IV taken on the wind side at an angle of 0° with respect to the gas flow (see Figure 3). Magnification is 170×.
Figure 6. Spatial distribution of sodium and potassium contents of the deposit on probe VII (see Table 1), as determined by LA-ICP-MS. Image and analysis points have been taken on the wind side at an angle of -45° (315°) with respect to the gas flow (see Figure 3). Magnification is 100×.
Figure 8. Determination of chlorine with point analyses by LA-ICPMS and SEM-EDS. Probe I, one point; probe IV, two points; and probe VII, three points (for description of the probes, see Table 1). Locations of the points were randomly chosen in the deposit, but the same point was analyzed by both methods in each case.
Figure 7. Deposit composition of probes I and III (for description of the probes, see Table 1).
Figure 9. Spatial distribution of chlorine in the deposit on probe II, as determined by LA-ICP-MS and SEM-EDS (for description of the probe, see Table 1). Points are chosen on the wind side at an angle of 45° (see Figure 3). Magnification is 100×.
Since chlorine is one of the key elements in the development of corrosive damages, its content in probe deposits was explored by both SEM-EDS and LA-ICP-MS. Point analyses were carried out at randomly selected points in the deposits.26 Some illustrative results for probes I, IV, and VII are shown in Figure 8. It can be seen that there is good agreement in the results between the two methods, even though the laser spot size in LA-ICP-MS of 60 µm is significantly wider than the electron beam and the interactive volume in SEM-EDS. Since chlorine
content varied significantly from point to point, indicating that the deposits are heterogeneous, we carried out a more detailed investigation of the trend in chlorine content across the deposit-probe cross section, as exemplified for probe II in Figure 9. Both methods indicate that chlorine is concentrated near to the probe-deposit interface. Similar trends were also observed in other probes. Probe III showed the highest chlorine levels.
(26) Both SEM-EDS and LA-ICP-MS point analyses were carried out in the same locations of the deposits as accurately as possible.
It has been reported that chlorine occurs in superheater deposits as CaCl2, KCl, NaCl, MgCl2, or FeCl3 depending
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Figure 11. Spatial distribution of chlorine-, iron-, calcium-, and sulfurcontaining phases on the deposit layers.
Figure 10. Quasiternary diagrams of probes (a) I, (b) II, (c) III, (d) IV, (e) V, and (f) VI (for description of the probes, see Table 1).
conditions.27,28
on the boiler The probes studied in this work consistently contained chlorine together with calcium, potassium, iron, and chromium. Potassium chloride is reported to form a low-temperature eutectic with FeCl2 and CrCl2 at temperatures of 355-470 °C.6 It can therefore be expected that the innermost deposit layers consist of such phases, since the surface temperatures of the probes were low, ranging from 350 to 500 °C. Sodium was found only in small amounts in the deposits, except that probe VII showed significantly higher contents (see Figure 6). Since sodium, like chlorine, is concentrated near the surface of the probe, it probably occurs as sodium chloride. In some cases, relatively high sodium contents are also found in outermost layers of the deposits. SEM-EDS results of the point analyses of the deposits can also be illustrated as quasiternary diagrams25 and strengthen the deductions made above. Compositional distributions of the deposits on probes I-VI are shown in Figure 10. Direct comparison is facilitated by using the same corner definitions in each case. This simplified model is justified, since most of the analysis points are still accepted in the diagram according to the 80% rule.25 It can be seen that, for probe I, iron and chlorine compounds are more abundant than sulfur and calcium compounds (see Figure 10a). The increase in exposure time and temperature renders sulfur and calcium compounds more significant (see Figure 10b). The lack of sulfur and calcium compounds in the sandblasted probe III compared to their more abundant occurrence in probe II seems to depend on the short exposure time and the difference in the probe wall treatment. Probes IV and V are made of the same metal alloy (see Table 1). Probe IV, however, was untreated, while probe V was sandblasted. Whereas the exposure times were the same, probe IV had a surface temperature of 350 °C, and probe V, 400 °C. By comparison of Figure 10 panels d and e, it can be seen that (27) Xie, W.; Xie, Y.; Pan, W.-P.; Riga, A. Thermochim. Acta 2000, 357-358, 231–238. (28) Jensen, P. A.; Frandsen, F. J.; Dam-Johanssen, K. Energy Fuels 2000, 14, 1280–1285.
the deposit on the sandblasted probe V is less sulfur- or calciumrich than that on the untreated probe IV. Conclusion SEM-EDS is a conventional method to study compositional distribution in ash-related samples collected from various power plant boilers. LA-ICP-MS is an attractive alternative that can provide related information. Even though the location-dependent resolution of LA-ICP-MS is not as good as that of SEM and reliable results require suitable calibration, its strengths comprise a low detection limit enabling the determination of trace elemental contents. SEM-EDS instruments, on the other hand, contain versatile image processing software that facilitates automated data collection handling of more than 1000 points or domains. In this work, we have analyzed deposits on seven watercooled deposition probes by both SEM-EDS and LA-ICP-MS. The samples were collected in a 30 MW full-scale circulating fluidized-bed (CFB) power plant during its normal operation using solid recovered fuel. Calcium and silicon were the main components in all deposits and occurred throughout the deposit layers. Chlorine, potassium, sodium, and sulfur compounds seem to be concentrated mainly near the interface between deposits and probe surfaces. Deposits were examined visually and their compositional distributions were determined by LA-ICP-MS and SEM-EDS. The results from all methods were rather consistent. Quasiternary diagrams could be utilized to develop a qualitative model to illustrate the positional dependence of the chemical composition of the deposit. This trend is exemplified for iron-, chlorine-, sulfur-, and calcium-containing phases in Figure 11. It can be seen that whereas chlorine and iron-chlorine species are concentrated near the probe surface, sulfur and calcium species show a wider spatial distribution and seem to be enriched in the outermost layers of the deposits. Acknowledgment. Financial support from Academy of Finland, Fortum Foundation, and Finnish Concordia Fund is gratefully acknowledged. We are also grateful to Foster Wheeler Energy Inc. for the probe samples. Supporting Information Available: Proximate and ultimate analysis of solid recovered fuel used in the boiler test.This material is available free of charge via the Internet at http://pubs.acs.org. EF801039G
