Energy & Fuels 1999, 13, 803-816
803
Characterization of Ashes and Deposits from High-Temperature Coal-Straw Co-Firing Lone A. Hansen, Flemming J. Frandsen,* and Kim Dam-Johansen Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark
Henning S. Sørensen Geological Survey of Denmark and Greenland, Thoravej 8, 2400 København NV, Denmark
Bengt-Johan Skrifvars Combustion Chemistry Research Group, Aabo Akademi University, Lemminkainengatan 14-18 B, 20520 Aabo, Finland Received September 29, 1998. Revised Manuscript Received April 5, 1999
Fly ashes, bottom ashes, and deposits collected on air-cooled probes at a PF-fired boiler cofired with coal and straw (0%, 10%, and 20% straw on an energy basis) have been studied with respect to chemical composition, mineralogy, sintering, and melting. The varying straw share was found not to influence the overall chemical composition of the fly ashes, which were quite alike on an oxide basis, whereas computer-controlled scanning electron microscopy data revealed a change in the species present, meaning that the more potassium that was available for reaction (i.e., the higher the straw share burned), the higher was the fraction of alumino-silicates having reacted to form potassium-alumino-silicates. Comparing compositions of fly ashes and deposits, it was found that K-, Ca-, Fe-rich silicates were concentrated in deposits, probably as an effect of relatively low viscosities of these particles. Based on simultaneous thermal analysis, STA, all ashes examined showed melting in the temperature range from 1000 to 1390 °C, and despite the mineralogical differences, no significant difference was found between the melting behavior of the different fly ashes and bottom ashes. When comparing results from the STA melting quantification method to results from the standard ash fusion test, moderate quantities of melt (1-36%) were found at the initial deformation temperature, IDT. Comparing the IDT to the onset of melting as determined by the STA, it was found that the first melting occurred as much as 150 °C below the IDT. This stresses that the standard ash fusion method should be used with care when determining melting behavior and thereby ash deposition propensity. Sintering experiments revealed that strength was built up in all ashes at temperatures below the first melt appearance. For the fly ash collected during coal combustion, high strengths were built up in the absence of a liquid phase, whereas for the ashes produced during coal-straw co-combustion, only low strengths were obtained without melt present. On the basis of viscosity calculations it was found that for all ashes the sintering onset was equivalent with an average viscosity of (13) × 106 P.
Introduction To reduce the CO2 emissions from power generation, the Danish Government has committed the Danish power industry to burn 1.2 million tons of straw, 0.2 million tons of wood chips, and 0.2 million tons of straw or wood chips by the year 2004. The power industry has proposed three possible concepts for the biomass burning: (1) combustion of (100%) biofuel in separate stokerfired boilers (built as additions to already existing coalfired boilers), (2) co-firing of straw, wood chips, and coal in circulating fluidized bed boilers, and (3) co-firing of straw (if necessary, washed straw) and coal in existing PF-fired boilers. * Author to whom correspondence should be addressed.
To investigate the concept of coal-straw co-firing in existing PF-fired boilers, a comprehensive measurement campaign was conducted in 1996 at the PF-fired Studstrup Power Station, Unit 1. The main purpose of the campaign was to evaluate whether the concept of coalstraw co-firing was competitive to other technologies, with respect to economical and technical feasibility. The measurement program included experiments of varying straw share (0-20% on an energy basis) and load (50100%) to demonstrate plant operational data and operating costs when co-firing coal and straw, as well as experiments to clarify the fuel and process chemistry. The program included, among other aspects, analyses of fuels, fly ash, bottom ash, deposits, and flue gas. Deposition and corrosion experiments were conducted
10.1021/ef980203x CCC: $18.00 © 1999 American Chemical Society Published on Web 05/20/1999
804 Energy & Fuels, Vol. 13, No. 4, 1999
Hansen et al. Table 1: Operational Conditions as a Function of Experiment Number expt no
1
2
3
4
5
straw [%]a load [%]
20 50
20 75
20 100
10 100
0 100
a
Straw share on an energy basis. Table 2: Chemical Composition of Fuels [%w/w]6 C
Coalb
O
H
S
N
Ash
61.3 6.2 4.2 1.0 1.3 13.8 Strawc 45.6 6.0 0.1 0.6 6.6 a
Water
Vol.
LHVa
10.5 6.4
31.6 ∼ 80
24.0 17.0
[MJ/kg]. b Basis: As received. c Dry basis.
The Studstrup Power Station, Unit 1 (MKS1) is a 150 MWe wall-fired unit commissioned in 1968 and equipped with 12 burners arranged in three burner rows. For the coal/straw co-firing experiments, the four burners in the middle burner row were converted to coal/straw co-firing with up to 50% straw (energy basis). The platen superheater and the secondary superheater are located in the top of the boiler, the reheater is located at the entrance to the second pass, further into which the primary superheater is located. Figure 1 shows a schematic drawing of the boiler with indication of sampling positions (numbered 1-3) where deposits were collected. Flue gas temperatures at the three positions varied
between 1220 and 920 °C, as a function of position and load.5 Deposition, in-situ experiments, and mass balance closures were performed at several operational conditions, reflected in a number of experiments of which some are given in Table 1. In this paper, focus will be drawn to ashes collected in experiments 3, 4, and 5, and for use in the sintering experiments, additionally, the fly ashes from experiments 1 and 2 have been examined. Fuel Composition. The coal burned in experiments 1-5 was a South American high-volatile bituminous coal, commonly used for power production in Denmark. The straw burned was primarily Danish wheat straw harvested in 1995. Chemical compositions of fuels and the laboratory ashes are shown in Tables 2 and 3. For the straw burned in experiments 3 and 4 (the full-load experiments), a total of four samples were ashed and analyzed. A quite high deviation was found between analysis results due to the highly inhomogeneous nature of straw, and for this reason both a minimum and a maximum value are given for the ash analyses. As typically seen, the straw is characterized by a lower content of carbon, sulfur, and ash, and a higher content of volatiles than the coal; for the ashes, the content of Al2O3 and Fe2O3 is much lower, and the content of CaO, K2O, Cl, SO3, and P2O5 is higher in the straw ash compared to the coal ash. The mineral content of the laboratory-prepared ashes of the two fuels were analyzed by computer-controlled scanning electron microscopy, CCSEM, at the Geological Survey of Denmark and Greenland.7,8 The results from the analyses are shown on a mineral category basis (% (w/w)) in Figure 2, illustrating a distinct difference between fuel ash minerals in coal and straw. The coal ash is characterized by high contents of quartz, K-Alsilicates, Ca-Al-silicates, Fe-Al-silicates and alumino silicates, whereas the dominant minerals in the straw ash are K-silicate, Ca-silicate, and KCl. For the straw ash the high content of unclassified particles should be noticed. These consist predominantly of K2O (∼35%), CaO (∼20%), SiO2 (∼15%), Cl2O7 (∼11%), and SO3
(1) Andersen, K. H.; Hansen, P. F. B.; Wieck-Hansen, K.; Frandsen, F. J.; Dam-Johansen, K. Proceedings of the 9th European Bioenergy Conference, Elsevier: Oxford, U.K., 1996; pp 1102-1107. (2) Hansen, P. F. B.; Andersen, K. H.; Wieck-Hansen, K.; Overgaard, P.; Rasmussen, I.; Frandsen, F. J.; Hansen, L. A.; Dam-Johansen, K. Paper presentated at the Engineering Foundation Conference, Snowbird, Utah, April 28-May 3, 1996. (3) Overgaard, P.; Hansen, P. F. B. Paper presented at POWER GEN s EUROPE ’97, Madrid, Spain, June 17-19, 1997. (4) Overgaard, P. Mks1-Straw Co-FiringsDemo Program, in Danish. Final Report, Midtkraft Power Company A/S, Denmark, 1998.
(5) Andersen, K. H.; Frandsen, F. J.; Hansen, P. F. B.; DamJohansen, K. Proceedings of the Eng. Found. Conference on Impact of Mineral Impurities in Solid Fuel Combustion, Plenum Press: New York, 1998. (6) Hansen, P. F. B. MKS1 Demoprogram. Stofbalancer og røggasemissioner, in Danish. Internal Report from I/S Midtkraft, 1997. (7) Laursen, K. Advanced Scanning Electron Microscope Analyses at GEUS. Geological Survey of Denmark and Greenland, Report No. 1997/1, 1997. (8) Sørensen, H. S. Proceedings of the Eng. Found. Conference on Impact of Mineral Impurities in Solid Fuel Combustion, Plenum Press: New York, 1998.
Figure 1. Schematic of the MKS1 boiler (modified from Andersen et al., 1996).
by means of probes, and a number of in-situ gas and particle sampling, temperature, and aerosol measurements were performed. Details concerning the measurement campaigns are presented in the literature.1,2,3,4 In this paper, a thorough characterization of the ashes collected during the measurement campaign will be provided. Results will be presented for three sets of fly ash, bottom ash, and deposits, and will include chemical composition, mineralogical composition as determined by means of computer-controlled scanning electron microscopy, CCSEM, ash melting behavior as determined by means of simultaneous thermal analysis, STA, standard ash fusion tests, and ash sintering behavior as determined by compression strength testing. The melting behavior of the ashes as determined by STA will be interpreted in terms of their chemical and mineralogical composition as determined by CCSEM, compared to results from standard ash fusion tests, and will be used for evaluation of sintering test results. Finally the melting behavior and sintering results will be correlated to the practical experience obtained at the plant. Plant, Measurement Positions, and Fuel Composition
Ashes and Deposits from Coal-Straw Co-Firing
Energy & Fuels, Vol. 13, No. 4, 1999 805
Table 3: Chemical Composition of Fuel Ashes [%w/w]6 coal straw min max
SiO2
Al2O3
P2O5
SO3
CaO
Fe2O3
MgO
Na2O
K2O
Cl
59.8
19.1
0.2
2.1
2.0
8.1
1.7
0.6
2.2
1500 1490 1490 1490 1460 1410
49.1 38.6 54.2
% meltc
% (w/w). b ∆T ) T0(STA) - IDT. c Not determined.
results indicate that the melting onset of the fly ashes decreases slightly with increasing straw share fired. The same trend s and more pronounced s is seen for the hemispherical temperatures, which increase in the following order: HT(MKS3FA) < HT(MKS4FA) < HT(MKS5FA). The measured fluid temperatures indicate, though, that MKS3FA (20% straw) may be completely melted at temperatures higher than MKS4FA and MKS5FA. For the bottom ashes three very similar melting behaviors are indicated by the standard AFT results, with the biggest deviation seen for the fluid temperatures, which indicate the melt completion occurs at the lowest temperatures for MKS5BA and at the highest temperatures for MKS3BA. Comparing standard AFT results to results from the STA, it is found that the STA detects melt formation at temperatures well below the IDT for all ashes. For the bottom ashes and MKS3FA, the temperature difference between melting onset as determined by STA and the initial deformation temperature is quite high, between 110 and 160 °C. The agreement is better for MKS4FA and MKS5FA for which the deviation in onset temperature is only 2040 °C. The fraction of melt present at IDT varies between 1% and 11%, except for MKS3FA which shows 36% melt at the IDT. For the hemispherical temperature, melt fractions between 38% and 65% melt is detected. For the fly ashes, the standard AFT finds a relative order of melting curves (for T < 1400 °C) of MKS3FA showing the highest degree of melting, MKS4FA showing the second highest and MKS5FA showing the lowest. This is equivalent with the STA results and predictions based on ash chemistry. For the bottom ashes, the standard AFT predicts similar onsets, and indicates approximately the same melting behavior below 1300 °C, whereas the melting process completion is indicated to occur at lower temperatures for MKS5BA compared to MKS4BA. The STA measurements found the melting behavior of the three bottom ashes to be almost similar, so the methods do not agree completely for temperatures between 1300 and 1400 °C. Overall, it is concluded, though, that a qualitative agreement between the standard AFT and the new STA-based melting test13,14 is found. Ash Sintering Sintering experiments were carried out in order to investigate the relationship between the melting behavior of a certain ash and the corresponding densification/strength developing process. The conducted sintering experiments implied strength testing of heattreated ash pellets, the procedure of which has been described earlier.11,18 The aim of the experiments was
812 Energy & Fuels, Vol. 13, No. 4, 1999
Hansen et al.
Figure 15. Strength of ash pellets as a function of heat treatment temperature.
to investigate whether it is possible to predict the temperatures at which the strength build-up process starts, on the basis of the STA-derived melting curves. For this purpose, the sintering behavior of four fly ashes, from experiment 1, 2, 3, and 5, were determined and compared to the melting behavior as determined by STA. Sintering Strength Results. In Figure 15, the influence of the heat-treatment temperature on the developed pellet strength is shown for a heat treatment time of 4 h. For each heat treatment temperature, four ash pellets were prepared and strength tested, and in the figure the vertical bar shows the variance on the strength measurement. For unsintered pellets or pellets sintered at temperatures below 850 °C, the strength was so low that pellets literally fell apart by the slightest touch. In Figure 15, it is seen that for MKS1FA, MKS2FA, and MKS3FA the first strength development is detected at 950 °C. Defining the sintering temperature as the highest temperature at which no strength increase is detected18 results in a sintering temperature of 900 °C for MKS1FA, MKS2FA, and MKS3FA, whereas for MKS5FA (0% straw), the sintering temperature is 1000 °C. In practice, this means, that if deposit surface temperatures can be kept below the sintering temperature, no strength will build up in the deposits after attachment of the individual ash particles, and the deposit will thus be quite easy to remove by sootblowing. At 1000 °C, two of the ashes show a lower strength compared to the strength at 950 °C, but for higher temperatures, the strength is generally seen to increase with increasing temperature. An exception to this are the last points at the curves for MKS2FA and MKS3FA, which exhibit lower strengths at 1150 °C compared to 1100 °C. Deposit sintering, i.e., strength buildup, is generally believed to be related to a porosity decrease. Since the (18) Skrifvars, B.-J. Sintering Tendency of Different Fuel Ashes in Combustion and Gasification Conditions. Academic Dissertation, Åbo Akademi University, Finland, 1994.
Figure 16. Relation between pellet strength and density.
pellet weight and dimensions were measured after heat treatment, an estimation of the pellet density could be made. Plotting the measured strengths as a function of the measured pellet densities (Figure 16), it appears that a qualitative relation between pellet strength and density is found; when the density increases, so does the strength, and vice versa. A density decrease, i.e., a porosity increase, is thus observed for the two abovementioned decreases in strength (MKS2FA and MKS3FA at 1150 °C), which is therefore believed to be caused by evaporation of material, leading to a higher pellet porosity. The influence of time on the strength development was also investigated. The data show that strength development occurs very fast for the first half hour of heat treatment, during which the ashes generally obtain between 64 and 100% of the finally obtained strength (∼12 h heat treatment). From 4 to 12 h, the strength of all ashes changed only slightly, with ratios between 1 and 1.17. This is in accordance with earlier findings.18 Relation between Ash Fusion and Sintering. To investigate the relation between melting and strength buildup in the ashes, Figure 17 shows corresponding values for strength and melt fraction in the ash at
Ashes and Deposits from Coal-Straw Co-Firing
Figure 17. Relation between strength and melt fraction in pellet.
different heat treatment temperatures. Generally, it is seen that strength build-up is initiated at temperatures lower than the melting onset temperatures (T0) of the ashes. This phenomenon has also been observed for another set of coal ashes investigated.19 For the fly ash from coal combustion (MKS5FA), the full strength buildup appears to occur without the presence of a melt phase, i.e., probably as a result of viscous flow sintering. Viscous flow sintering is normally the word used to describe strength buildup in ashes through the appearance of a liquid phase of high viscosity18,20,21. In this case no liquid phase has been detected in the ashes, but flow of viscous ash may occur (slowly) anyway, since these ashes are glasses which do behave as liquids with a very high viscosity. For the ashes produced during co-firing (MKS1FA, MKS2FA, MKS3FA), it appears that the initial strength buildup occurs without the influence of a melted phase, but that high strengths (in this case values above 4 N/mm2) are not obtained without the presence of a liquid phase. The further behavior shows no simple relation between melt fraction and strength obtained. The results from these experiments seem to indicate that ashes produced during straw firing sinter by a different mechanism than the pure coal-derived ashes, since the latter do not need the presence of melt to introduce high strengths. The straw share does not seem to have any systematic influence on the observed sintering behavior, though, and no simple relationship between strength and melt fraction could be found. The physical characteristics of the fly ash particles, which have not been considered in these experiments, may be a dominant factor in the observed strength buildup. SEM of Sintered Ash Pellets. To obtain information on the microstructure of the pellets at the different sintering stages, a selected number of pellets were cut through, and the cross sectional surface was studied by SEM. The pellets studied were those consisting of fly ash from experiments MKS2 and MKS5, which had been heat treated for 4 h at respectively 850, 950 (missing for MKS5FA), 1050, 1100, and 1150 °C. Photomicrographs showing the pellet structure of MKS2FA are presented in Figure 18. The images were obtained (19) Larsen, O. H.; Frandsen, F. J.; Hansen, L. A.; Vargas, S.; DamJohansen, K.; Laursen, K.; Yamada, T.; Teramae, T. Proceedings of the Eng. Found. Conference on Impact of Mineral Impurities in Solid Fuel Combustion, Plenum Press: New York, 1998. (20) Nowok, J. W.; Benson, S. A.; Jones, M. L.; Kalmanovitch, D. P. Fuel 1990, 69, 1020-1028. (21) Kingery, W. D.; Bowen, H. K.; Uhlman, D. R. Introduction to Ceramics; John Wiley and Sons: New York, 1976.
Energy & Fuels, Vol. 13, No. 4, 1999 813
with the backscattered electron detector (BSE); black areas represent epoxy-filled voids, whereas the varying gray shades represent ash particles with varying average atomic number. The figure shows that at 850 °C the pellets consist of individual ash particles, which do not show any significant interconnection. Even the largest fraction of the smallest particles (which are most prone to sinter) seem to occur as discrete particles. At 950 °C, a quite high degree of bonding has occurred between the ash particles, the main fraction of the smallest particles have been attached to other particles, and only a few of the original particles are observed as individuals. At higher temperatures, 1050, 1100, and 1150 °C, the ash particle “agglomerates” are larger in size and the present pores are fewer and larger. This is the classic structure development during sintering,21 which reflects the measured increases in strength. However, when comparing particle attachment as seen on the SEM images to the pellet strength values, it appears that on the basis of the few cases studied here, no simple relation between the degree of particle attachment and pellet strength is found. The porosities of the pellets were estimated as the ratio between black- and gray-tone areas. Results from these area comparisons are not very accurate porosity determinations and should therefore be interpreted with care. The obtained results show that porosity is reduced as the heat treatment temperature s and pellet strength s is increased, supporting the trend found from the density measurements. On some of the SEM images, it is possible to divide the structure into two parts: (1) original particles of unchanged form and (2) areas consisting of deformed material bonding discrete ash particles. The chemical composition of these two different material types was investigated by use of SEM-EDX. At 950 °C, the “binder phase” was found to distinguish itself from the “original particles of unchanged form” in terms of a lower Si content, a higher Al, Fe, Mg, Na, and Ca content, and a lower K content. Generally, an increased SiO2 content is known to increase the viscosity of a glass, Al2O3 may influence viscosity both positively and negatively, whereas increased contents of Fe2O3, MgO, K2O, Na2O, and CaO decreases glass viscosity.12,16 The observed differing chemical composition could thus indicate a “binder phase” with a lower viscosity than the surrounding particles, which explains the higher deformation tendency of this phase compared to the particles. The chemistry of the two types of materials was additionally characterized by an X-ray mapping of an area including both “original particles of unchanged form” and clearly deformed material. In this case, it was found that Si made up the “skeleton” of the particle structure, and that very high Si concentrations were found in the undeformed particles. Aluminum seemed to “follow” the Si, except for a few pure SiO2 particles. K was concentrated in areas/particles showing quite high deformation, and so was Ca, except for the Ca being present as phosphates. The Fe seemed to be evenly distributed, and so did the Na. In conclusion, the deformed particles binding together the matrix seem to be enriched in K and/or Ca compared to the bulk particles, which is equivalent with the K/Ca-rich mate-
814 Energy & Fuels, Vol. 13, No. 4, 1999
Hansen et al.
Figure 18. SEM pictures showing the structure of sintered MKS2FA pellets.
rial having a lower viscosity and thus a higher tendency to deform than more Si-rich or Al- and Si-rich material. Significant strength buildup and a large degree of particle attachment/agglomeration was observed in the ash pellets at temperatures below the melting onset temperature (as determined by STA). Since no melt has been detected, the particle attachment and the strength buildup must occur due to flow of viscous material. The viscosity of the sinter-tested ashes has thus been calculated on the basis of a model12 selected between many available viscosity models, since this model has
provided estimates being in good agreement with experimental values in a recent study of coal ash viscosities.22 In this connection, it is relevant to stress the importance of accurate ash viscosity models.16 The calculated viscosities are shown in Figure 19. As can be seen the ash viscosities decrease in the following (22) Vargas, S.; Frandsen, F. J.; Dam-Johansen, K. ELSAM-Idemitsu Kosan Cooperative Research Project: Performance of viscosity models for high-temperature coal ashes, CHEC Report No. 9719, Department of Chemical Engineering, Technical University of Denmark, August 1997.
Ashes and Deposits from Coal-Straw Co-Firing
Energy & Fuels, Vol. 13, No. 4, 1999 815
Figure 19. Calculated viscosities [Pa s]. Table 7: Measured Gas Temperatures [°C]10 expt. no.
1
2
3
4
5
position 1 position 2 position 3
1060 1030 755
1190 955
1225 1125 765
1275 1110 925
1265 1100 955
order (high-viscosity ash first): MKS5FA, MKS2FA, MKS3FA, and MKS1FA, with the largest deviation between MKS5FA and MKS2FA values. Comparing this to the strength measurements, it was found that for temperatures above 1000 °C, MKS1FA showed the highest strengths, followed by MKS3FA, then MKS5FA, and finally MKS2FA. It thus appears that the ashes which show the lowest viscosity (presumably causing ash particles to have a higher deformation/attachment tendency) show the highest strength; the ash with the second-lowest viscosity shows the second-highest strength, but the two ashes with the highest viscosities have switched positions, so that the ash with the highest strength shows only the second-lowest strength. This may indicate that a correlation between ash viscosity and strength buildup exists, generally meaning that lower ash viscosities result in higher strength buildup. By examining the data it also appears that the ash viscosity at the sintering temperature is almost alike for the four ashes: Ranging between 1.1 × 106 and 3.2 × 106 Pa s. Relating this to estimates reported in the literature, this (105-107 Pa s) corresponds to rapid sintering.23 Comparison to Operational Experience. In Table 7, gas temperature measurements conducted at positions 1, 2, and 3 are shown. Relating these temperatures to the melting curves shown in Figure 12 and assuming (1) that the fly ash collected in the electrostatic precipitator is equal to that present in the convective pass and (2) that the fly ash temperature is equal to that of the surrounding gas, the fraction of melt in the fly ash present at the different positions can be determined. Assuming that the attachment of fly ash particles onto the heat transfer surface requires a melt fraction larger than 10%, this leads to the conclusion that particle attachment will probably primarily occur at position 1 in experiments 2 (20% straw, 75% load), 3 (20% straw, 100% load), and 4 (10% straw, 100% load), and in position 2 in experiment 3. The operational experience from the campaign was in general that most severe deposits were formed in positions 1 and 2, and that the most problematic (23) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere Publishing Corporation: Washington, D.C., 1985.
deposits were formed in experiment 3.10 These practical experiences are thus in good agreement with the combination of high gas temperatures and the relatively high melt fractions present in the fly ash in experiment 3, and with the melting behavior determinations in general. However, the development of a problematic deposit is caused not only by particle attachment to the heat transfer surface, but also by subsequent strength buildup in the deposit. Relating therefore the gas temperatures to the sintering temperatures found from Figure 15 and considering worst casesa deposit surface temperature equal to the measured gas temperaturesit is seen that sintering may happen for all experiments (1, 2, 3, and 5) in positions 1 and 2, but not in position 3. This interpretation also agrees with the above-mentioned operational experience.10 To evaluate the deposit formation tendency in the furnace, the viscosities of the bottom ashes were calculated and the temperature corresponding to a viscosity of (1-3) × 106 Pa s was found. These temperatures vary between 870 and 980 °C, which are quite likely to occur at deposit surfaces in the furnace. Taking into consideration the melting onsets for the bottom ashes, deposits being melted at the surface are expected very strongly to form in the furnace. This is also in agreement with operational experience.10 Conclusions CCSEM analyses of three sets of fly ashes, bottom ashes, and deposits collected during co-combustion of respectively 0, 10, and 20% straw (on an energy basis) in a coal/straw PF-fired boiler revealed that by far the largest part of the ashes consisted of metal silicates. For the fly ashes from each of the three experiments, the chemical composition was quite alike on an oxide basis, but for the CCSEM data a clear trend was seen: when potassium was available for reaction (i.e., when straw was burned), a large part of the alumina-silicates had reacted to form potassium-alumino- silicates; and the larger the straw share, the larger the conversion of alumina silicates to K-Al-silicates. Comparing fly ash compositions to deposit compositions, it was found that the following species were concentrated in deposits: K-, Ca-, Fe-, and Fe-Alsilicates, mixed silicates, and iron oxide. Iron oxide may partly originate from the oxidized metal surface, whereas the concentration of K-, Ca-, and Fe-silicates indicate preferential deposition of ash particles with relatively low viscosity. The latter assumption is supported by the fact that more potassium-alumina-silicate particles with high potassium content were found in the deposits than in the fly ash particles, indicating that potassium-rich fly ash particles have a higher tendency for attaching to heat transfer surfaces than ash particles with low potassium contents. Based on simultaneous thermal analysis, all ashes examined showed melting in the temperature range from 1000 to 1390 °C, and despite the mineralogical differences, no significant difference was found between melting behavior of the fly ashes and the bottom ashes, respectively. When comparing results from the STA melting quantification method to results from the standard ash fusion test, moderate quantities of melt
816 Energy & Fuels, Vol. 13, No. 4, 1999
(1-36%) were found at the IDT. Comparing the IDT to the onset of melting as determined by the STA, it was found that the first melting occurred as much as 150 °C below the IDT. This stresses that the standard ash fusion method should be used with care when determining melting behavior. Sintering experiments were carried out to investigate the relationship between the melting behavior of a fly ash and the corresponding densification/strength developing process. Strength increase in all cases corresponded to a decrease in porosity, and strength was found to build up in all ashes at temperatures below the first melt appearance, probably due to the flow of viscous material. For the fly ash collected during coal combustion, high strengths were built up without the presence of a liquid phase, whereas for the ashes produced during partial straw combustion, only low strengths (
