Energy & Fuels 2002, 16, 871-877
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The Role of Amorphous Material in Ash on the Agglomeration Problems in FB Boilers. A Powder XRD and SEM-EDS Study Minna Tiainen, Jouni Daavitsainen, and Risto S. Laitinen* Department of Chemistry, University of Oulu, P.O. Box 3000, FIN-90014 Oulu, Finland Received November 14, 2001. Revised Manuscript Received April 6, 2002
Bed agglomeration is one of the most common problems in FB boilers. In this work the dependence of agglomeration tendency of peat and biomass ash on the content of amorphous material in ash has been investigated. The factors affecting the interactions between quartz bed particles and ash-forming elements have been modeled by involving mixtures of oxides that are commonly found in ash by utilizing X-ray powder diffraction for a semiquantitative determination of amorphous phases and SEM-EDS for compositional distributions. It is well-known that high iron or alkali metal contents increase the agglomeration tendency of ash. It can indeed be seen that the amount of amorphous material in ash increases with increasing iron or alkali metal contents. The results have been compared with those of two standard peat ashes, one of which is known to be unproblematic, while severe agglomeration has been observed in the case of the other.
Introduction The utilization of FB combustion for energy production involving various solid fuels has increased during the past decade, since it can be applied for the combustion of fuels of low heating value such as biomass or RDF. The FB technique, however, is sensitive to operational problems that are related to the formation and behavior of ash during combustion. Bed agglomeration is one of the most common problems in FB boilers. Certain phases in ash may have low melting points and act as an adhesive medium and attach bed particles to each other. The presence of agglomerates in bed therefore affects its degree of fluidization. Ultimately, the growth and increase of the agglomerates may lead to a complete defluidization of the bed and consequently to an unscheduled shutdown of the boiler with severe economic consequences. The occurrence of agglomeration is expectedly both fuel- and bed-dependent.1-10 If ash has a low melting * Corresponding author. (1) Ergudenler, A.; Ghaly, A. E. Biomass Bioenergy 1993, 4, 135147. (2) Grubor, B. D.; Oka, S. N.; Ilic, M. S.; Dakic, D. V.; Arsic, B. T. Fluidized Bed Combust. 1995, 515-522. (3) Manzoori, A. R.; Agarwal, P. K.; Lindner, E. R., Proceedings of the 10th International Conference on Fluidized Bed Combustion, 1989. (4) Manzoori, A. R.; Agarwel, P. K. Fuel 1994, 73, 563-568. (5) Mansaray, K. G.; Ghaly, A. E. Energy Sources 1998, 20, 631652. (6) Skirifvars, B.-J.; Sfiris, G.; Backman, R.; Widegren-Dåfgård, K.; Hupa, M. Energy Fuels 1997, 11, 843-848. (7) Skrifvars, B.-J.; O ¨ hman, M.; Nordin, A.; Hupa, M. Energy Fuels 1999, 13, 359-363. (8) Skrifvars, B.-J.; Zevenhoven, M.; Backman, R.; O ¨ hman, M.; Nordin, A. Effect of Fuel Quality on the Bed Agglomeration Tendency in a Biomass Fired Fluidised Bed Boiler; Tilla¨mpad fo¨rbra¨nningskemi, 684, A° bo Akademi University, Energy Technology Center, Piteå, Stockholm, 2000. (9) Werther, J.; Saenger, M.; Hartge, E.-U.; Ogada, T.; Siagi, Z. Prog. Energy Combust. Sci. 2000, 26, 1-27.
point, it is sticky by itself under the operating conditions of the boiler and can act as an adhesive between bed particles. These kinds of ashes are typical to fuels with a high iron(II) content, i.e., peat.11 Iron can act as a fluxing agent in silicates.12 Even if the melting point of ash is sufficiently high and it should therefore be unproblematic by itself, it can interact directly with the bed material and form an adhesive medium between the bed particles. Ashes from some biomass fuels, i.e., bark or forest residues, show this kind of behavior.1,10,13 Adhesive material can also be formed in reactions between the bed material and volatile alkali metal compounds that are released from fuels such as plywood waste or straw.6,13-15 Because of the economic significance of the unscheduled boiler shutdown, it is important to be able to understand the factors involved in the formation of the agglomerates with the ultimate goal to prevent the agglomeration from taking place. Several methods have been reported for the prediction of ash behavior during combustion. Agglomeration tendency of ash has been studied by use of ash fusion,16-19 compression strength sintering,20,21 and lab-scale FB combustion tests.7 (10) O ¨ hman, M.; Nordin, A.; Skrifvars, B.-J.; Backman, R.; Hupa, M. Energy Fuels 2000, 14, 169-178. (11) Heikkinen, R.; Virtanen, M.; Patrikainen, T.; Laitinen, R. Impact of mineral impurities in solid fuel combustion; Gupta, R. P., Wall, T. F., Baxter, L. L., Eds.; Kluwer Academic Plenum Publisher: New York, 1999; pp 333-339. (12) Mason, D. M.; Patel, J. G. Fuel Process. Technol. 1980, 3, 181. (13) Nuutinen, L.; Ollila, H.; Tiainen, M.; Virtanen, M.; Laitinen, R. Effects of Coal Quality on Power Plant Management: Ash Problems, Management and Solutions; Mehta, A. K., Benson S. A., Eds.; United Engineering Foundation Inc.: New York, and EPRI: Palo Alto, CA, 2001; 1001402, 4-53-4-60. (14) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13, 390-395. (15) Visser, H. J. M.; Hofmans, H.; Huijnen, H.; Kastelein, R.; Kiel, J. H. A. Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science: London, 2001; pp 272-286.
10.1021/ef010269j CCC: $22.00 © 2002 American Chemical Society Published on Web 06/21/2002
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Tiainen et al. Table 1. Compositions of Samplesa
a
b
c
d
e
f
component
1
2
3
4
1
2
3
4
1
2
3
1
2
3
4
1
2
3
4
1
2
3
4
SiO2 Al2O3 Fe2O3 CaO MgO P2O5 Na2SO4 K2SO4
70 15 15
15 70 15
15 15 70
35 30 35
70 15
15 70
15 15
35 30
70 15
15 70
15 15
70 15
15 70
15 15
35 30
70 15
15 70
15 15
35 30
70 15
15 70
15 15
35 30
15
15
70
35 15
15
70 15
15
70
35 15
15
70
35 15
15
70
35
a
The contents of components are presented as a wt %.
The prediction of the interaction between the bed particles and fuel ash, however, requires information at the molecular level. SEM-EDS together with automated image processing has proven to be a useful method, since it provides information about compositional distribution of ash, agglomerates, and coating layers on bed particles. Since molten phases play a role in the onset and development of agglomeration, it is also important to establish the phase-relationships in ash. The powder X-ray diffraction (XRD) offers a possibility to identify the crystalline phases and to estimate the content of amorphous material in ash.22-24 The presence of amorphous material in a sample creates a broad hump or halo in the diffraction pattern. The area under the halo depends on the amount of amorphous material in the sample and can therefore be used in its semiquantitative determination. The position and the width of the halo are dependent on the distribution of interatomic distances in the structure.25 In this paper we have studied for the dependence of agglomeration on the content of amorphous material in ash. Synthetic mixtures of oxides common in peat and biomass ash have been involved to model interactions between quartz bed particles and certain ash forming elements. The results have been compared with those of a standard peat ash. Experimental Section General. The formation of amorphous material in a silica sand bed was modeled with two distinct series of samples. Test series 1 was comprised of 23 three-component mixtures containing silicon dioxide, aluminum oxide, and a third (16) ASTM D 1857-68, Fusibility of Coal Coke Ash, Annual book of ASTM Standards, Part 19, 1970. (17) Wall, T. A.; Lowe, A.; Gupta, R. P.; Gupta, S.; Greelman, R.; Coin, C.; Ottrey, A. Application of Advanced Technology to Ash-related Problems in Boilers; Baxter, L., DeSollar, R., Eds.; Plenum Press: New York, 1996. (18) Wall, T. F.; Creelman, R. A.; Gupta, R. P.; Coin, C.; Lowe, A. Prog. Energy Combust. Sci. 1998, 24, 345-353. (19) Coin, C. D. A.; Reifenstein, A. P.; Kahraman, H. Application of Advanced Technology to Ash-related Problems in Boilers; Baxter, L., DeSollar, R., Eds.; Plenum Press: New York, 1996. (20) Skrifvars, B.-J.; Hupa, M.; Backman, R.; Hiltunen, M. Fuel 1994, 73, 171-176. (21) Nowok, J. W.; Benson, S. A.; Jones, M. L.; Kalmanovitch, D. P. Fuel 1990, 69, 1020-1027. (22) Daavitsainen, J.; Nuutinen, L.; Tiainen, M.; Laitinen, R. Progress in Thermochemical Biomass Conversion; Bridgwater, A., Ed.; Blackwell Science: London, 2001; pp 799-811. (23) Tiainen, M. S.; Ryyna¨nen, J. S.; Rantala, J. T.; Patrikainen, H. T.; Laitinen, R. S. Impact of mineral impurities in solid fuel combustion; Gupta, R. P., Wall, T. F., Baxter, L. L., Eds.; Kluwer Academic Plenum Publisher: New York, 1999; pp 217-224. (24) Huffman, G. P.; Huggins, F. H.; Dunmyre, G. R. Fuel 1981, 60, 585-597. (25) Nakamura, T.; Sameshima, K.; Okunaga, K.; Sugiura, Y.; Sato, J. Powder Diffraction 1989, 4, 9-13.
Figure 1. The area of the amorphous halo (Ab) in sample a4 (SiO2 35%, Al2O3 30%, Fe2O3 35%). Table 2. The Composition of the Standard Ashes (nonproblematic test series 2 and problematic test series 324) from Two Finnish Peat Types content (%) component Na2O MgO Al2O3 SiO2 SO3 P2O5
series 2
series 3
1.80 2.51 11.81 38.12 1.82 4.37
0.10 0.96 1.23 6.39 2.65 6.88
content (%) component K 2O CaO TiO2 Fe2O3 BaO
series 2
series 3
2.98 11.55 0.74 24.08 0.22
0.37 5.62 0.05 75.67 0.08
component that was either an oxide typically found in ash or that produced one upon heat treatment (see Table 1 for the composition and naming of the test mixtures). Test series 2 was comprised of five samples of synthetic ash, the composition of which was set to correspond to that of nonproblematic Finnish peat26 (see Table 2). The five samples in this series had different thermal history. The results were compared with those obtained for the standard ash of the above-mentioned peat. The ash in this case was produced according to ASTM (D 3174-89) standard procedure.27 X-ray Powder Diffraction. A Siemens D5000 diffractometer using zirconium-filtered Mo KR radiation (30 kV and 40 mA) and equipped with a 2θ/θ goniometer was used for the XRD measurements. The step size was 0.02° and the counting time was 0.3 s for each step. Diffraction patterns were recorded in the 2θ-range 5-60°. The content of amorphous material is a function of the ratio Ra ) Ab/At where Ab is the integrated area under the baseline of the diffraction diagram and At is the total area under the diffraction pattern (see Figure 1). The ratio Ra is calibrated for the amorphous content by use of selected standards as described below. (26) Heikkinen, R.; Laitinen, R. S.; Patrikainen, T.; Tiainen, M.; Virtanen, M. Fuel Process. Technol. 1998, 56, 69-80. (27) Standard Test Method for Ash in the Sample of Coal and Coke from Coal, Annual book of ASTM Standards, 05.05, 1989, D 3174-89, 302.
Agglomeration Problems in FB Boilers SEM-EDS. The compositional distribution of the particles in the samples was determined by use of a JEOL JSM 6400 scanning electron microscope equipped with a Link ISIS EDS analyzer. The same instrument was involved in the recording of X-ray maps. The acceleration voltage in all cases was 15 keV and the current was 1.2 nA. In the recording of X-ray maps the current of 6.7 nA was used. The sample distance was 15 mm. The magnification used for the SEM-EDS analysis was ×130 and for X-ray maps ×250. All SEM-samples were mounted with resin, cross-sectioned, polished, and coated with a thin carbon layer. About 1000 particles were analyzed for sodium, magnesium, aluminum, silicon, sulfur, phosphorus, potassium, calcium, titanium, or iron contents28 as appropriate. The image processing was performed with the IMQuant software incorporated in Link ISIS. The SEM-EDS results were visualized by use of quasiternary diagrams.29-31 Preparation of Samples. All samples in the test series 1 (see Table 1) were heated at 1200 °C for 1.5 h. After cooling to ambient temperature in a dry atmosphere they were ground in a WC/Co mortar (1500r/min) for one minute to obtain optimum particle size, and homogenized manually in an agate mortar. The test series 2 consisted of five samples all with the same composition (see Table 2) simulating that of peat ash that is known to be unproblematic during combustion. One sample was studied without heat treatment. The other four were heated for 1.5 h, each at different temperatures (700, 900, 1000, and 1200 °C). After cooling to ambient temperature the samples were ground in a WC/Co mortar for one minute and homogenized manually in an agate mortar. The test series 3 simulated the composition of the peat ash that is known to be problematic (see Table 2). The preparation of the samples in this test series have been reported previously.23 The heat treatments of the samples were carried out as described for test series 2. Calibration Standards. Eight calibration standards were prepared by using unheated synthetic ash of the test series 2 in order to eliminate the matrix effects. The content of amorphous material in each standard was controlled by adding different weight fractions of ground glass (0, 4, 11, 30, 42, 49, 53, 79%). The effect of the grinding time on the diffraction pattern was examined for all individual compounds that were used as starting materials for the test series 1 and 2 by recording the diffractogram for each material both with only manual grinding and after automatic grinding of each sample for one minute in a WC/Co mortar. The effect of a longer grinding time was investigated for a mixture of quartz (75%) and calcium oxide (25%). The sample was heated for 1.5 h in 1000 °C and then divided in two equal portions. One portion was ground for 1 min and the other for 5 min.
Results and Discussion Calibration. The calibration curve is shown in Figure 2. It can be seen that there is expectedly a (28) Virtanen, M.; Skrifvars, B.-J.; Heikkinen, R.; Hupa, M.; Patrikainen, T.; Laitinen, R. Impact of mineral matter in solid fuel combustion; Gupta, R. P., Wall, T. F., Baxter, L. L., Eds.; Kluwer Academic Plenum Publisher: New York, 1999; pp 147-154. (29) Virtanen, M. E.; Tiainen, M. S.; Laitinen, R. S., Proceedings of the 5th International Conference on industrial furnaces and boilers; Reis, A., Ward, J., Leuckel, W., Collin, R., Eds.; April 11-14.4.2000 Porto, Portugal; pp 117-126. (30) Virtanen, M.; Tiainen, M.; Nuutinen, L.; Pudas, M.; Laitinen, R. Effects of Coal Quality on Power Plant Management: Ash Problems, Management and Solutions; Mehta, A. K., Benson, S. A., Eds.; United Engineering Foundation Inc.: New York, and EPRI: Palo Alto, CA, 2001; 1001402, 2-117-2-121. (31) Virtanen, M. E.; Tiainen, M. S.; Pudas, M.; Laitinen, R. S. Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science: London, 2001; pp 671-677.
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Figure 2. Calibration curve of the ratio Ra in terms of the content of the amorphous material. The standards were prepared by mixing synthetic ash and silica glass in different proportions.
Figure 3. The effect of grinding time on the diffraction pattern of the mixture of SiO2 (75%) and CaO (25%).
correlation between Ra and the content of amorphous material in the sample. While the matrix in the samples of the test series 1 is different from that of the calibration samples that was based on unheated synthetic ash, we noted that the matrix effects were not significant and therefore the same calibration curve was utilized in all determinations. As discussed previously by Nakamura et al,25 the grinding time has an effect on the degree of crystallinity of the sample. The longer grinding time decreases the average particle size and increases the relative amount of microcrystalline material that resembles amorphous material in the X-ray powder diagram. Upon prolonged grinding the intensity of diffraction peaks is lowered and the area of the halo grows, as exemplified for the calcium oxide-quartz mixture in Figure 3. The effects of grinding on the development of amorphous halo were minimized by applying a standard oneminute grinding time for each sample. A desired homogenized particle size could be reached without overly distorting the results. Test Series 1. The effect of heating on the diffraction patterns of the samples in the test series 1 are exemplified by those of a1, a4, and e1 (for the numbering and composition of the samples, see Table 1). Diffraction patterns for the unheated samples a1, a4, and e1 are shown in Figures 4a, 4c, and 4e, respectively. Those shown in Figure 4b, 4d, and 4f are recorded for the same samples after the heat treatment at 1200 °C for 1.5 h. The samples a1-a4 contain quartz, aluminum oxide, and iron oxide in different rations, and the samples e1e4 contain quartz, aluminum oxide, and sodium sulfate. The intensity of the diffraction peaks of quartz decreased upon heating of each sample. It is probably a consequence of the formation of amorphous silicates. It is indeed seen in Figure 4 that the area under the halo is larger in all samples after the heat treatment. By use of the calibration curve shown in Figure 2, a semiquan-
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Figure 4. X-ray powder diffraction patterns of samples a1 (SiO2 70%, Al2O3 15%, Fe2O3 15%), a4 (SiO2 35%, Al2O3 30%, Fe2O3 35%), and e1 (SiO2 70%, Al2O3 15%, Na2SO4 15%). (a) unheated a1, (b) a1 heated at 1200 °C for 1.5 h, (c) a4 unheated, (d) a4 heated at 1200 °C for 1.5 h, (e) e1 unheated, and (f) e1 heated at 1200 °C for 1.5 h. Table 3. The Difference in Amount of Amorphous Material after Heating a 1 2 3 4
b
c
d
e
f
unheated
heated
unheated
heated
unheated
heated
unheated
heated
unheated
heated
unheated
heated
39 21
75 55
39
64
39 72 56 64
64 72 85 74
40 71 75 50
58 50 49 50
36 70 60 66
60 66 72 77
25 61 34 42
69 53 28 63
38 71 40 45
85 56 39 63
titative estimate of the increase in the content of the amorphous phases can be made, as shown in Table 3. The increase in the amorphous content can be explained by the presence of iron (a1-a4), sodium (e1e4), or potassium (f1-f4) in the mixtures. It is wellknown that iron causes severe slagging in the boilers when combusting coal32-35 or peat.26 The compositional (32) Bailey, C. W.; Bryant, G. W.; Matthews, E. M.; Wall, T. F. Energy Fuels 1998, 12, 464-469. (33) Bool, L. E.; Peterson, T. W.; Wendt, J. O. L. Combust. Flame 1995, 100, 262-270.
distribution diagrams of iron-containing samples a1 and a4 both before and after the heat treatment are shown in Figure 5. There are two distinct maxima visible in all quasiternary diagrams indicating the presence of two different silicate phases. Upon heating at 1200 °C the height of the distribution maximum due to iron aluminum silicate phases increases with respect to that of the aluminum silicate phase. (34) Kalmanovitch, D.; Sanyal, P. A.; Williamson, J. J. Inst. Energy 1986, 20-23. (35) Vorres, K. S. J. Eng. Power 1979, 101, 497-499.
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Figure 5. Quasiternary diagrams of a1 (SiO2 70%, Al2O3 15%, Fe2O3 15%) and a4 (SiO2 35%, Al2O3 30%, Fe2O3 35%) before and after heating at 1200 °C for 1.5h.
Figure 6. The combined X-ray maps (×130) of a coated quartz particle in a1 (SiO2 70%, Al2O3 15%, Fe2O3 15%).
The combined X-ray maps of the sample a1 shows that the quartz particles have been coated by material containing iron, aluminum, and silicon, as shown in Figure 6. Similar coatings have been found in bed particles collected from a full-scale FB boiler during its normal operation, when using peat as fuel.11 Such coatings probably initiate the agglomeration of bed particles. In FB combustion the alkali metals are known to play an important role in the agglomeration due to the formation of low-melting silicates.1,4,7,8,10,13,22,36-41 This can be exemplified by sample e4 (see Figure 7). The BE (36) Bapat, D. W.; Kulkarni, V.; Bhandarkar, V. P. Proceedings of the 14th International Conference on Fluidized Bed Combustion, Vancouver, Canada, May 11-14, 1997.
image shows that prior to the heat treatment mainly discrete particles are observed (see Figure 7a). After heating the sample at 1200 °C for 1.5 h, the particles appear to be glued together (Figure 7b). It is interesting to note that the compositional distribution in sample e1 became somewhat more compact (37) Daavitsainen, J.; Nuutinen, L.; Ollila, H.; Tiainen, M.; Virtanen, M.; Laitinen, R. Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science: London, 2001; pp 705-712. (38) Ergudenler, A.; Ghaly, A. Biomass Bioenergy 1992, 3, 419-430. (39) Lin, W.; Krusholm, G.; Dam-Johansen, K.; Musahl, E.; Bank, L. Proceedings of the 14th International Conference on Fluidized Bed Combustion, Vancouver, Canada, 1997. (40) Lin, W.; Dam-Johansen, K. Proceedings of the 15th International Conference on Fluidized Bed Combustion, Savannah, Georgia, May 1619, 1999. (41) O ¨ hmann, M.; Nordin, A. Energy Fuels 1998, 12, 90-94.
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Figure 7. BE- (backscattered electrons) image (×130) of a sample of e4 (a) unheated, (b) heated.
Figure 8. The quasiternary diagrams of a sample of e1 (SiO2 70%, Al2O3 15%, Na2SO4 15%) prior to the heat treatment and after heating at 1200 °C for 1.5 h.
upon heating, as shown by the quasiternary diagrams in Figure 8. The formation of sodium aluminosilicate phases is also indicated by the increase of the content of amorphous material 25% to 69%, as observed by XRD. The content of amorphous material of the samples c2-3, d2, e2-3, f2-3 (see Table 3) expectedly decreases upon heating. The main component in these materials are either Al2O3 or alkali metal or alkaline earth metal oxides that do not form low-melting phases. The Synthetic Ash (test series 2 and 3). Diffraction patterns of synthetic ash samples (test series 2, see Table 2) heated at different temperatures are shown in Figure 9. The most notable change in the diffraction patterns upon increasing temperature is the decrease of intensity of the SiO2 peaks, while those due to the other crystalline phases seem to remain unchanged. The growth of the amorphous halo can be seen to coincide the decrease of the intensity of SiO2 peaks. The content of amorphous material increased from 30 to 70%, as the temperature increased from ambient to 1200 °C. (see Figure 10) The major components in the synthetic ash of test series 2 are Si, Al, Fe, and Ca, and therefore it is conceivable that respective silicates are formed upon heating. The calcium and iron usually act as glass-forming agents and lower the melting point of the silicates.12 The most significant increase in the content of amorphous material of synthetic ash of test series 2 took place at temperatures that exceed the normal fluidized bed combustion temperature. This probably explains, why this peat ash has low agglomeration tendency.
The content of amorphous material in the ash of test series 3 exhibits a similar trend as a function of temperature.23 In this case, however, the formation of amorphous phases is initiated at significantly lower temperatures than in the case of test series 2. The synthetic ash of test series 3 contains almost 80% of Fe2O3 that is nearly a 3-fold increase to that in test series 2. It is well-known that iron-containing peat ash is severely agglomerating. The SEM-EDS confirmed the formation of a wide range of iron aluminosilicates that further explains the low melting temperatures of the material.23 Conclusions In this work we have investigated the role of ash composition in the bed agglomeration that is one of the most common problems in FB boilers. A systematic study was conducted by heating 24 three-component mixtures at 1200 °C for 1.5 h and monitoring the formation of amorphous phases by X-ray powder diffraction, as well as by SEM-EDS combined with automated image analysis. Each sample contained silicon dioxide and aluminum oxide, and a variable third component. The components and compositions were chosen to simulate the main constituents of biomass and peat ash. The content of amorphous material increased in most cases upon heat treatment. This behavior was expected for mixtures containing iron, sodium, or potassium in addition to quartz and aluminum oxide, since it is
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Figure 9. Diffraction patterns of synthetic ash (a) unheated and heated at (b) 700 °C, (c) 900 °C, (d) 1000 °C, and (e) 1200 °C.
known that the presence of iron in peat ash causes severe slagging and agglomeration problems. Alkali metals also play an important role in agglomeration due the formation of low-melting silicates. It was found by SEM-EDS that the quartz particles were coated with iron-rich material. Similar coating layers were also found in bed particles that were collected from full-scale test when combusting peat in a FB boiler.
Figure 10. The effect of the heat treatment temperature on the content of amorphous material in synthetic ash (series 2). The amorphous contents of standard peat ash that is known to be unproblematic has been included for comparison.
Acknowledgment. Financial support from The National Technology Agency, Academy of Finland, Tauno To¨nning Foundation, and IVO Foundation is gratefully acknowledged. EF010269J
