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Energy & Fuels 2000, 14, 308-315
An Experimental Comparison of the Ash Formed from Coals Containing Pyrite and Siderite Mineral in Oxidizing and Reducing Conditions A. R. McLennan,† G. W. Bryant,*,† C. W. Bailey,† B. R. Stanmore,‡ and T. F. Wall† Cooperative Research Centre For Black Coal Utilisation, Department of Chemical Engineering, University Of Newcastle, Callaghan, NSW, Australia, 2308, and Department of Chemical Engineering, University of Queensland, St. Lucia, Qld, Australia, 4072
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Received May 20, 1999. Revised Manuscript Received December 1, 1999
Four coals containing iron mineral pyrite (FeS2) and siderite (FeCO3) were combusted in a laboratory drop tube furnace at temperatures of 1300, 1450, and 1600 °C under oxidizing and reducing conditions. Results for the behavior of pyrite mineral were in agreement with the established literature. The behavior of siderite mineral was determined and comparisons made. Coals containing pyrite minerals were determined to have potential to produce ash deposition and slagging at lower temperatures than coals containing siderite mineral. Reducing conditions were determined to lower the temperature at which ash deposition and slagging may occur for coals containing iron minerals compared to oxidizing conditions. With respect to ash deposition and slagging, it was determined that the iron levels in a coal are not definitive, but rather the iron mineral type (pyrite or siderite), mineral association (included or excluded), degree of association of included minerals, and the type of included alumino-silicate minerals have important roles.
Introduction Ash deposition on heat transfer surfaces remains a major problem in both the combustion and gasification of pulverized coal. The reduction in plant efficiency as a result of lower heat transfer rates, frequent maintenance, and unscheduled shutdowns to remove ash deposits can produce significant increases in the cost of power generation. In particular, iron-based minerals have been identified as contributing to ash deposition and slagging.1,2 To prevent such problems, an understanding of the mechanisms involved in the formation of the ash, and the effect of combustion conditions on these mechanisms, is critical. For conventional pulverized fuel (pf) combustion, pulverized coal and combustion air are injected into the boiler with stoichiometric ratio greater than one (i.e., oxidizing). Excess air is utilized to ensure complete combustion of coal to generate heat for raising steam, resulting in an oxidizing gaseous atmosphere composed primarily of N2, O2, CO2, and H2O. For air-staged low NOx combustion, pulverized coal and combustion air are injected into the lower regions of the boiler with stoichiometry less than one (i.e., reducing). This produces * Corresponding author. † University Of Newcastle. ‡ University of Queensland. (1) Raask, E. Mineral impurities in coal combustion, behaviour problems and remedial measures; Hemisphere Publishing: Bristol, PA, 1985. (2) Bryers, R. W. The physical and chemical characteristics of pyrites and their influence on fireside problems in steam generators. J. Eng. Power 1976, 517-527.
initial combustion in a reducing zone in the lower regions of the boiler with typical particle residence time of 1-2 s,3,4 after which burnout is completed in an oxidizing secondary zone in the upper regions of the boiler. Coal gasification employs limited air or oxygen (substoichiometric) to allow only partial combustion of coal to generate heat for the gasification reactions, producing a reducing gaseous atmosphere composed primarily of CO and H2 (+ N2 if air blown). The ash formation mechanisms for pyrite and siderite minerals, and the influences of reducing conditions on these mechanisms, have been discussed in a previous study.5 Excluded pyrite (Figure 1a6) was found to decompose to pyrrhotite, then oxidize from the surface inward to produce a molten FeO-FeS phase at 1080 °C, which oxidized to magnetite and hematite under oxidizing conditions, as established in numerous other studies.2,7-14 Under reducing conditions the FeO-FeS was found to persist. Excluded siderite (Figure 2a6) was determined to decompose to wustite melting at 1370 °C, (3) Spliethoff, H.; Greul, U.; Rudiger, H.; Hein, K. R. G. Basic effects on NOx emission in air staging and reburning at a bench scale test facility. Fuel 1996, 75, 560-564. (4) Wendt, J. O. L.; Pershing, J.; Lee, J. W.; Glass, J. W. Seventeenth Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, 1979; pp 77-87. (5) McLennan, A. R.; Bryant, G. W.; Bailey, C. W.; Stanmore, B. R.; Wall, T. F. Ash formation mechanisms during pf combustion in reducing conditions. Energy Fuels 2000, 14 (1), 150-159. (6) McLennan, A. R. Ash formation in reducing conditions. Ph.D. Thesis, University of Newcastle, Newcastle, Australia, 1998. (7) Groves, S. J.; Williiamson, J.; Sanyal, A. Decomposition of pyrite during pulverised coal combustion. Fuel 1987, 66, 461-466. (8) Srinivasachar, S.; Boni, A. A. A kinetic model for pyrite transformations in a combustion environment. Fuel 1989, 68, 829.
10.1021/ef990092h CCC: $19.00 © 2000 American Chemical Society Published on Web 02/25/2000
Ash Formed from High Iron Coals
Energy & Fuels, Vol. 14, No. 2, 2000 309 Table 1. Ash Analysis of Coal Samples ash SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O
Figure 1. Ash formation mechanisms for (a) excluded and (b) included pyrite.6
Figure 2. Ash formation mechanisms for (a) excluded and (b) included siderite.6
which oxidized to produce magnetite under oxidizing conditions,15,16 but remained as wustite under reducing (9) Huffman, G. P.; Huggins, F. E.; Levasseur, A. A.; Chow, O.; Srinivasachar, S.; Mehta, A. K. Investigations of the transformations of pyrite in a drop tube furnace. Fuel 1989, 68, 485. (10) Huffman, G. P.; Huggins, F. E.; Shah, N.; Shah, A. Behaviour of basic elements during coal combustion. Prog. Energy Combust. Sci. 1990, 16, 243-251. (11) Srinivasachar, S.; Helble, J. J.; Boni, A. A. Mineral behaviour during coal combustion : 1.-Pyrite transformations. Prog. Energy Combust. Sci. 1990, 16, 281-292. (12) Helble, J. J.; Srinivasachar, S.; Boni, A. A.; Bool, L. E.; Gallagher, N. B.; Peterson, T. W.; Wendt, J. O. L.; Huggins, F. E.; Shah, N.; Huffman, G. P.; Graham, K. A.; Sarofim, A. F.; Beer, J. M. Mechanisms of ash evolution: A fundamental study part II Bituminous coals and the role of iron and potassium. Proceeds of the Engineering Foundation Conference - Inorganic Transformations and Ash Deposition During Combustion; Benson, S. A., Ed.; 1992; pp 229248. (13) ten Brink, H. M.; Eenkhoorn, S.; Hamburg, G. Mineral matter behaviour in low-NOx combustion - a laboratory study. Proceeds of the E.P.R.I. Conference on Effect of Coal Quality on Power Plants; San Diego, CA, August 25-27, 1992. (14) Bool, L. E.; Peterson, T. W.; Wendt, J. O. L. The partitioning of iron during the combustion of pulverised coal. Combust. Flame 1995, 100, 262-270. (15) Bryant, G. W.; Bailey, C. W.; Mathews, E.; Gupta, R. P.; McLennan, A. R.; Stanmore, B. R.; Patterson, J. H.; Wall, T. F. Investigation of the High-Temperature Behaviour of Siderite Grains During Combustion. CRC for Black Coal Utilisation Conference Impact of Coal Quality on Thermal Coal Utilisation; Brisbane, Australia, Sept 1-4, 1996.
Coal A (%)
Coal B (%)
Coal C (%)
Coal D (%)
17.6 42.00 34.20 2.00 12.70 3.90 1.50 0.11 0.13
12.2 49.90 18.80 1.70 17.60 5.30 0.92 0.09 0.63
12.4 38.70 22.40 0.67 29.00 2.20 1.40 0.31 3.20
16.6 44.40 24.70 0.91 22.20 2.40 0.73 0.39 1.80
conditions. Included pyrite (Figure 1b6) was shown to behave as for excluded pyrite if there was no contact with alumino-silicates, as established in a previous study,14 though oxidation was delayed by char combustion. This study had indicated pyrrhotite or molten iron oxide derived from included pyrite could contact silicates to form a glass. This was confirmed, though it was shown that pyrrhotite-contacting silicates first formed a two-phase FeS/Fe-glass ash particle, with incorporation of iron into the glass proceeding as the FeS phase was oxidized. This delay in glass formation was expected to be accentuated by reducing conditions. Included siderite (Figure 2b) was determined to behave as for excluded siderite if there was no contact with aluminosilicates, though oxidation was delayed by char combustion. Included siderite that contacted alumino-silicate minerals was determined to directly form iron aluminosilicate glass ash particles. Iron alumino-silicate glass ash was formed with iron in the Fe2+ state in the locally reducing environment of the char, much of which subsequently transformed to Fe3+ state in oxidizing conditions, but remained primarily as Fe2+ under reducing conditions. Having reviewed previous literature for pyrite and siderite mechanisms, the objective of this study was to determine and compare the role of the iron minerals pyrite and siderite in ash deposition and slagging for coal combustion and gasification in reducing conditions. Experimental Section Experimental Setup. Coals were combusted in a drop-tube furnace at stoichiometries of 1.5 (oxidizing) and 0.6 (reducing), with temperatures of 1300, 1450, and 1600 °C. Particles were quenched rapidly with N2 gas in a water-cooled probe, passed to a cyclone for collection of coarse residual ash, and finally through an aerosol filter for collection of submicron ash. The experimental setup is described in greater detail in a previous study.5 Coal Sample Selection and Preparation. Four coal samples with relatively high iron contents (>10 wt % Fe2O3 in ASTM ash), as iron transformations were expected to be most influenced by reducing conditions, were chosen for the experimental program. All coal samples were received as pulverized fuel, and subsequently sized by sieving to -75/+45 µm size fraction. Coal Sample Characterization. Ash analyses of the coal samples utilized in this study are given in Table 1. Coal samples were also characterized by computer-controlled scanning electron microscopy (CCSEM), the results of which are summarized in Table 2. Ash Particle Collection and Analysis. Coarse particles collected in the cyclone consisted of ash particles derived from (16) Bailey, C. W.; Bryant, G. W.; Mathews, E. M.; Wall, T. F. Investigation of the high-temperature behaviour of siderite grains during pulverised fuel combustion. Energy Fuels 1998, 12, 464-469.
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McLennan et al.
Table 2. CCSEM Analyses of Coal Samples Showing Major Iron and Alumino-Silicate Minerals Coal A
Coal B
Coal C
Coal D
mineral
included (wt %)
excluded (wt %)
included (wt %)
excluded (wt %)
included (wt %)
excluded (wt %)
included (wt %)
excluded (wt %)
silicates alumino-silicates iron oxide/siderite pyrite pyrrhotite
0.4 23.0 1.5 0.1 0
1.7 52.1 10.7 0.2 0
10.2 12.4 0.8 0 1.1
10.5 16.1 14.2 2.5a 7.1a
0.9 20.3 0 17.9 0.3
4.4 17.0 0.4 13.2 0.3
1.4 24.4 0 11.4 0.7
2.7 24.0 2.0 12.7 0.2
a
Electron microprobe and Mo¨ssbauer spectroscopy analysis indicated minimal pyrite, pyrrhotite. Table 3. Mo1 ssbauer Analysis of Combustion Residues Showing Percentage of Total Iron in Each Phase phase
Coal A ash ox
1450 °C red
Coal B ash ox
1600 °C red
Coal C ash ox
1450 °C red
Coal D ash ox
1450 °C red
hematite magnetitea magnetiteb wustite pyrrhotite R-Fe γ-Fe Fe2+-glass Fe3+-glass Fe(II)/Fe(III)
0 39 41 4 0 0 0 5 11 0.42
0 27 31 31 0 3 0 0 8 0.92
0 37 47 0 0 0 0 9 7 0.48
0 23 20 19 0 5 8 25 0 1.64
8 22 30 9 11 1 6 13 0 1.07
0 14 24 14 25 4 5 14 0 2.5
6 18 25 0 19 2 0 16 14 0.94
0 9 14 0 36 4 0 26 10 2.6
a
Tetrahedral. b Octahedral.
coal minerals and uncombusted char particles, referred to as a whole as combustion residues. Loss on ignition measurements for combustion residues indicated significantly higher levels of burnout (defined as percentage of combustible mass consumed) for oxidizing stoichiometries compared to reducing stoichiometries. Contained within such unburnt char particles is ash derived from included minerals. Of the ash particles separate from the char, referred to as separated ash, there were several forms. Excluded minerals produced ash particles of comparable size to the feed (45-75 µm), unless fragmentation had occurred, and are hereafter referred to as ash derived from excluded minerals. Included minerals produced ash particles of two forms, depending on the association of minerals within a coal particle, both of which are referred to as ash derived from included minerals. If a coal particle (or fragment of) contained only one mineral, then the resultant ash was a product of this mineral. If, however, different minerals were closely associated within a coal particle, coalescence of these minerals produced glassy ash particles, typically aluminosilicate based. Ash particles derived from included mineral were generally of smaller size (typically 5-30 µm) compared to those derived from excluded minerals (typically 45-75 µm). Combustion residues were analyzed by Mo¨ssbauer spectroscopy as collected. Separated ash was analyzed by electron microprobe as polished sections of the combustion residue samples set in epoxy resin.
Results and Discussion CCSEM Analysis of Coal Samples. The major iron and alumino-silicate mineral forms and their associations, as identified by CCSEM analysis presented in Table 2, highlight some important characteristics of the coal samples. The primary iron forms were identified as siderite for Coal A and Coal B, and pyrite for Coal C and Coal D. The majority of the siderite mineral of Coal A and Coal B was determined to be of excluded form, though sink/float analysis indicated significant levels of included siderite (35 wt % and 20 wt %, respectively). CCSEM analysis indicated the pyrite mineral of Coal C and Coal D was of both included and excluded form. The major included alumino-silicate mineral forms were kaolinite for Coal A, kaolinite + quartz for Coal
Table 4. Burnout Data for Combustion Residuesa coal
temperature (°C)
atmosphere (SR)
burnout (%)
fraction of char consumed (%)
A 1.5 B 1.5 C 1.5 D 1.5
1450 95 1600 97 1450 78 1450 79
0.6 92 0.6 96 0.6 65 0.6 65
79
66
72
54
63
41
63
38
a
Determined from proximate analysis, using ash as a Tracer.
B, kaolinite + montmorillonite + potassium aluminosilicate for Coal C, and kaolinite + montmorillonite for Coal D. Burnout Implications. Ash particles analyzed by electron microprobe were separated from the char. As such, any ash particles derived from included minerals analyzed was the product of complete combustion of a char particle. Thus, burnout did not influence the results provided by electron microprobe analysis of ash. Under reducing conditions Mo¨ssbauer spectroscopy results (Table 3) indicated an increase in the proportion of Fe2+ glass for coal B, C, and D combustion residues. It is also apparent that compared to oxidizing conditions burnout (Table 4) is less complete for reducing conditions. Although lower burnout levels could be expected to imply less interaction and coalescence of included minerals, this does not appear to have been significant for iron and alumino-silicate minerals. As such, while it is conceded that lower burnout may have some effect on the results, it is considered insignificant compared to the effect of the reducing atmosphere. Electron Microprobe Analysis of Ash. The compositions of iron alumino-silicate ash particles, derived from the coalescence of included siderite or pyrite with quartz and clay minerals, were plotted on the FeOnSiO2-Al2O3 ternary equilibrium phase diagrams,17,18 as (17) Muan, A. J. Am. Ceram. Soc. 1957, 40 (12), 428. (18) Osborn, E. F.; Muan, A. Phase Equilibrium Diagrams for Oxide Systems; American Ceramic Society, Columbus, OH, 1960.
Ash Formed from High Iron Coals
Figure 3. Iron alumino-silicate glass ash particle compositions for Coal D combustion residue plotted on Fe3O4-SiO2Al2O3 phase diagram for oxidizing conditions (equilibrium with air).17
Figure 4. Iron alumino-silicate glass ash particle compositions for Coal D combustion residue plotted on FeO-SiO2Al2O3 phase diagram for reducing conditions (equilibrium with metallic iron).18
in Figures 3 and 4. Of these glass ash particles, only those with FeO + SiO2 + Al2O3 > 90 wt % were plotted, with FeO + SiO2 + Al2O3 normalized to 100 wt %. Mo1 ssbauer Analysis of Combustion Residues. Ash from combustion of Coal A, Coal B, Coal C, and Coal D under oxidizing and reducing conditions was analyzed by Mo¨ssbauer spectroscopy, the results presented in Table 3. The iron-containing glass detected was the result of reactions of included siderite and pyrite minerals with alumino-silicates. Crystalline phases hematite, magnetite, wustite, and pyrrhotite detected may have been derived from excluded siderite and pyrite, or included siderite and pyrite that did not coalesce with alumino-silicates. The magnetite spectrum normally contains two sets of absorption peaks, one for each of the octahedral and tetrahedral sites. Variation of octahedral-to-tetrahedral ratio from the stoichiometric value of 2:1 is attributed to poor crystallinity. For the oxidizing samples, oxidation of part of the Fe(II) giving a composition intermediate between that of magnetite (Fe2+Fe3+2O4) and maghemite (γ-Fe3+2O3) results, while for
Energy & Fuels, Vol. 14, No. 2, 2000 311
reducing conditions transfer of intensity from wustite toward magnetite results. Determination of ferric: ferrous ratios is straightforward, with the exception of magnetite. The iron on the octahedral site undergoes fast electron hopping, so is effectively Fe2.5+, and half the contribution of this iron should be assigned to each of Fe(II) and Fe(III). Approximate errors were estimated at (3% for the larger components and (2% for the smaller component.19 It has been well established in the literature20-24 that as conditions move from reducing to oxidizing, iron in glass transforms from the Fe2+ state to the Fe3+ state. Considering Coal B, Coal C, and Coal D, combustion residues exhibited an increase in the proportion of Fe2+ and total iron in glass for reducing conditions, the reducing figures for Coal A combustion residues are considered to be in error, and are not considered in this discussion. A possible explanation for this inconsistency is that Fe2+ iron in glass has crystallized as wustite as a result of ash quenching not being sufficiently rapid. It is noted that no wustite was detected for Coal D, which seems unusual as pyrrhotite oxidizes to wustite then magnetite, and both pyrrhotite and magnetite were detected. As wustite is unstable below ∼560 °C, the most likely explanation is that particles cooled to below this temperature before quenching, with poorly crystalline magnetite formed instead. Crystalline Phases. Mo¨ssbauer spectroscopy of the combustion residues identified crystalline phases pyrrhotite, wustite, magnetite, and hematite. These phases are the product of included and excluded pyrite and siderite minerals, and the proportion of total iron in each is shown in Table 3. The melting behavior of ash derived from pyrite and siderite minerals is illustrated by the FeS-FeO and FeO-Fe2O3 phase diagrams, shown in Figure 5.25,26,27 Pyrite. Included pyrite mineral will decompose to pyrrhotite, as established in the literature.2,7-14 Oxidation of pyrrhotite was considered to be minimal during combustion of the char due to the locally reducing environment, regardless of combustion stoichiometry. Thus, char combustion introduces a delay in the oxidation of included pyrite minerals, which is likely to be longer for reducing conditions where burnout is slower. If the pyrrhotite does not contact aluminosilicates (Figure 1b upper pathway),14 oxidation was found to proceed as established by the previous work for excluded pyrite (Figure 1a)2,7-14 when char combustion is complete and the ash particle enters the bulk gas atmosphere. The established behavior for pyrite was confirmedsdecomposition to pyrrhotite followed by (19) Cashion, J. Mo¨ssbauer Spectroscopy Analysis Report. Private communication, 1998. (20) Huffman, G. P.; Huggins, F. E.; Dumyre, G. R. Investigation of high-temperature behaviour of coal ash in reducing and oxidising atmospheres. Fuel 1981, 60, 585. (21) Nowok, J. W. Viscosity and structral state of iron in coal ash slags under gasification conditions. Energy Fuels 1995, 9, 534-539. (22) Slag Atlas, Verein Deutscher Eisenhu¨ttenleute (Ed), Verlag Stahleisen M. B. H., Du¨sseldorf, Germany, 1981. (23) O’Horo, M. P.; Levy, R. A. J. Appl. Phys. 1978, 49 (3), 1635. (24) Mysen, B. O.; Virgo, D. Am. Mineral. 1989, 74, 58-76. (25) Schu¨rmann, E.; von Hertwig, I. O. Gieβerei, techn.-wiss. Beih. 1960, 14, 31-36. (26) Oelsen, W. Mitt. K. Wilh. Inst. Eisenhu¨ ttenwes. 1961, 32, 741751. (27) Muan, A.; Osborn, E. F. Phase equilibria among oxides in steelmaking; Addison-Wesley: Reading, MA, 1965.
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Figure 5. Equilibrium phases for the progressive oxidation of pyrite and siderite.25-27
oxidation proceeding from the surface inward under diffusion control. An FeO-FeS melt phase is formed (reported as wustite and pyrrhotite in Table 3), which is progressively oxidized to wustite, magnetite, and hematite given sufficient time and oxygen partial pressure, as indicated in Figure 5. As the oxidation of pyrrhotite occurs under diffusion control, it is time dependent, so even under oxidizing conditions the FeO-FeS phase is present at short residence times. This is especially true for ash particles derived from included pyrite due to the delay of oxidation during char combustion. However, given sufficient residence time, under oxidizing conditions pyrite minerals will oxidize predominantly to magnetite and hematite,2,7-14 as indicated in Figure 5, with such ash particles melting at temperatures in excess of ∼1600 °C.27 Under reducing conditions, pyrite minerals will form significant proportions of FeO-FeS phase and wustite, as indicated in Figure 5, with such ash particles melting at temperatures from 910 to 1370 °C.25,26 However, even for reducing combustion stoichiometries there is initially oxygen present which may allow the oxidation of excluded pyrite to magnetite, as apparent in Table 3. Overall, the crystalline phases of ash particles derived from pyrite minerals under reducing combustion conditions will be molten at significantly lower temperatures, significantly increasing the potential for deposition and slagging compared to oxidizing combustion conditions. Siderite. Included siderite mineral will decompose to wustite.28 Oxidation of wustite will be minimal during combustion of the char due to the locally reducing environment, regardless of combustion stoichiometry. As for included pyrite, char combustion introduces a delay in the oxidation of included siderite minerals, which is likely to be longer for reducing conditions where burnout is slower. If the siderite does not contact (28) Warne, S. S. J. Differential thermal analysis of coal minerals. In Analytical Methods for Coal and Coal Products; Carr, C., Ed.; Academic Press: New York, 1979; Vol. 3.
McLennan et al.
alumino-silicates, as in the upper pathway of Figure 2a, oxidation will proceed similar to excluded siderite (Figure 2b) when char combustion is complete and the ash particle enters the bulk gas atmosphere. The wustite is progressively oxidized to magnetite and hematite given sufficient time and oxygen partial pressure, as indicated in Figure 5. Oxidation of siderite to magnetite and hematite is time dependent, but considerably faster than that of pyrite, where pyrrhotite must oxidize to wustite before forming magnetite and hematite. Ash particles derived from included siderite will require longer residence times to oxidize due to the delay of oxidation during char combustion. However, given sufficient residence time, under oxidizing conditions siderite minerals will oxidize predominantly to magnetite and hematite, as indicated in Figure 5, with such ash particles melting at temperatures in excess of ∼1600 °C. Under reducing conditions, siderite minerals, particularly those of included origin, will form significant proportions of wustite, as indicated in Figure 3, with such ash particles melting at temperatures from 1370 to 1430 °C. However, even for reducing combustion stoichiometries there is initially oxygen present which may allow the oxidation of excluded siderite to magnetite, as apparent in Table 3. Overall, the crystalline phases of ash particles derived from siderite minerals under reducing combustion conditions will be molten at moderately lower temperatures, increasing the potential for deposition and slagging compared to oxidizing combustion conditions. Iron Alumino-Silicates. Mo¨ssbauer spectroscopy of the combustion residues identified iron aluminosilicate glass with iron in the Fe2+ and Fe3+ states. These phases are the product of the coalescence of included pyrite or siderite and alumino-silicate minerals, as shown in Figures 1b and 2b, with the proportion of total iron in each shown in Table 3. The melting behavior of iron alumino-silicate ash is illustrated by the Fe3O4-SiO2-Al2O3 and FeO- SiO2-Al2O3 phase diagrams, shown in Figures 3 and 4, respectively. Included minerals in close proximity within a char particle may come into contact as the char is consumed.29,30 Temperatures within a burning char particle are generally such that ash derived from included minerals is molten and will coalesce on contacting. For the high-temperature and locally reducing conditions of the char during combustion, the coalescence of ash particles derived from pyrite, siderite, and aluminosilicate minerals will form molten iron alumino-silicate glass ash particles with iron in the Fe2+ state.20-24 On completion of char combustion, under reducing conditions iron will remain in the Fe2+ state, while under oxidizing conditions significant proportions of the iron will transform to the Fe3+ state.20-24 Pyrrhotite decomposed from included pyrite mineral may contact alumino-silicate minerals to form an iron (29) Graham, K. A.; Helble, J.; Kang, S. G.; Sarofim, A. F.; Beer, J. M. Mechanisms of the transformation of mineral matter to ash in coal and model chars. Proceedings of the Engineering Foundation ConferencesMineral Matter and Ash Deposition from Coal; Bryers, R. W., Vorres, K. S., Eds.; Engineering Foundation: New York, 1988; pp 165-186. (30) Sarofim, A. F.; Howard, J. B.; Padia, A. S. The physical transformation of the mineral matter in pulverised coal under simulated combustion conditions. Combust. Sci. Technol. 1977, 16, 187204.
Ash Formed from High Iron Coals
alumino-silicate glass ash particle.14 However, it was found that a two-phase FeS/Fe-glass ash particle is first formed (Figure 1b), and the FeS phase must oxidize to form FeO before the iron is incorporated into the glass. Due to the locally reducing conditions during char combustion, this will effectively delay glass formation, especially for reducing combustion conditions where burnout is slower. Included siderite mineral, however, will decompose to wustite,28 and on contacting aluminosilicate will form Fe-glass ash directly. Previous studies concerning the behavior of iron alumino-silicate glass formation have generally involved heating either ASTM ash20 of synthetically prepared mineral mixtures23,24 in crucibles under oxidizing or reducing conditions. Such techniques do provide useful information but cannot accurately reflect the properties of ash formed during pulverized coal combustion. Being bulk analysis techniques, they indicate the properties of the mean composition, whereas the composition of individual ash particles formed during the combustion of a coal may vary considerably (Figures 3 and 4). Consequently, a number of factors relating to the coal minerals and combustion conditions which are important to the formation of iron aluminosilicate glass ash have not been discussed by such studies. These factors include the proportions of included/ excluded minerals, the specific types of included iron, silicate, and alumino-silicate minerals, the association of the minerals within individual coal particles, and the variation of temperature and atmosphere throughout the combustion process. When considering the formation of iron aluminosilicates in ash, previous studies9,10,12,14,31 have generally not considered the degree of included mineral associations, or even the proportions of included and excluded iron minerals. The proportion of included iron mineral incorporated into the glass determined by Mo¨ssbauer spectroscopy (Table 3) varied considerably among combustion residues. Coal C was determined to have ∼55 wt % of pyrite mineral of included form, but only 1314% of total iron was incorporated into the glass (∼1/ 4). Coal D was determined to have ∼50 wt % of pyrite mineral of included form, with significantly more iron incorporated into the glass at 30-36% (∼2/3). This illustrates the importance of mineral associations within the coal, which will influence the degree of interaction of different minerals. The behavior of iron alumino-silicate ash particles in terms of melting temperatures is dependent on composition, and can be indicated by considering the appropriate ternary equilibrium phase diagram. Referring to Table 3, combustion residues formed under oxidizing conditions show iron present in both Fe3+ and Fe2+ forms, indicating the FeO.Fe2O3-SiO2-Al2O3 phase diagram is appropriate. Combustion residues formed under reducing conditions show iron present predominantly in Fe2+ form, indicating the FeO-SiO2-Al2O3 phase diagram is appropriate. Iron alumino-silicate glass ash particle compositions for Coal D combustion residues are plotted on the phase diagrams for oxidizing and reducing conditions in Figures 3 and 4, respectively. For all four coals, the ash particle compositions lie predominantly in a triangular area defined by the included alumino-silicate minerals
Energy & Fuels, Vol. 14, No. 2, 2000 313 Table 5. Liquidus and Solidus Temperatures of Iron Alumino-Silicate Glass Compositions Indicated on Phase Diagram oxidizing
reducing
composition
solidus (°C)
liquidus (°C)
solidus (°C)
liquidus (°C)
A B C D
1382 1382 1382 1382
1670 1510 1400 1465
1205 1083 1088 1148
1660 1460 1310 1315
kaolinite and montmorillonite extending to the iron corner of the phase diagram, as indicated in Figures 3 and 4. The presence of included quartz mineral produced some scatter toward the silicon corner of the phase diagram for Coal B combustion residues. As melting temperatures are dependent of particle composition, this illustrates the importance of determining the included alumino-silicate minerals present in a coal sample. As noted previously, Fe-glass ash particles are likely to form in a molten state. Char particle combustion temperatures are typically 200-300 °C higher than the gas temperature,32-34 so on completion of char combustion ash particles cool in the bulk gas atmosphere, which may be oxidizing or reducing. Thus, the temperature at which such ash particles solidify on cooling is relevant when considering ash deposition potential. The presence of oxidizing or reducing conditions exerts a strong influence on the solidification behavior of Fe-glass ash particles. Indicated on the phase diagrams for oxidizing and reducing conditions (Figures 3 and 4, respectively) are a number of typical ash particle compositions A, B, C, and D. The solidus and liquidus temperatures of these compositions determined from the phase diagrams for oxidizing and reducing conditions are presented in Table 5. The liquidus temperature is lower for reducing conditions, with the effect becoming more pronounced as the iron content increases. The solidus temperature, however, is significantly lower for reducing conditions. Thus, under reducing conditions Fe-glass ash particles are likely to be partially molten, and hence sticky, at considerably lower temperatures (200-300 °C) than for oxidizing conditions. In addition, comparison of the phase diagrams indicates that the 1400 °C isotherm for reducing conditions encloses a much larger range of compositions. It follows that at 1400 °C a greater proportion of Fe-glass ash particles will be molten for reducing conditions. Overall, the Fe-glass of ash particles derived from included siderite or pyrite and alumino-silicate minerals under reducing combustion conditions will be partially molten and sticky at significantly lower temperatures, increasing the potential for deposition and slagging compared to oxidizing combustion conditions. Metallic Iron. Mo¨ssbauer results (Table 3) for the combustion residues indicated some iron present in the form of metallic iron as R-Fe and γ-Fe for both oxidizing (31) Helble, J. J.; Srinivasachar, S.; Boni, A. A. Factors influencing the transformation of minerals during pulverised coal combustion. Prog. Energy Combust. Sci. 1990, 16, 267-279. (32) Seeker, W. R.; Samuelson, G. S.; Heap, M. P.; Trolinger, J. D. Symp. (Int.) Combust. [Proc.] 1981, 18, 1213-1224. (33) Cashdollar, K. L.; Hertzberg, N. SPIE 1980, 253. (34) Mackowski, D. W.; Altenkirch, R. A.; Peck, R. E.; Tong, T. W. Spring Meeting Combust. Inst., Western States Section, Salt Lake City, UT, 1982.
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and reducing conditions. An explanation for this result is that the highly reducing conditions within the char reducing pyrrhotite (FeS) from pyrite decomposition, and wustite (FeO) from siderite decomposition, to metallic iron. The chemical reduction of refractory metal oxides to metals during char combustion has been identified as a mechanism facilitating vaporization.35-39 For the pyrite containing Coal C and Coal D (less reactive than Coal A and Coal B), under oxidizing conditions char burnout was 78-79%, while for reducing conditions char burnout was 63%. Thus the metallic iron detected may be contained within the unburned char. Reducing conditions displayed slightly higher levels of metallic iron (9% and 4%) than oxidizing conditions (7% and 2%), which may be attributed to higher amounts of unburnt char. For the siderite-containing Coal A and Coal B, under reducing conditions, for which char burnout was ∼70-80%, some iron present in the form of metallic iron as R-Fe (5% and 3%) and γ-Fe (8% and 0%) was detected. For oxidizing conditions, however, burnout was ∼95%, so any metallic iron would have been oxidized on completion of char combustion and incorporated into crystalline or iron alumino-silicate phases. Implications for Coal Combustion and Gasification. A number of the effects of reducing conditions on ash formation identified will have significant implications on ash deposition for pulverized coal fired boilers and slagging gasifiers. Air Staged Combustion. Air staged combustion employs a reducing combustion zone that is fuel-rich in the lower regions of the boiler, and an oxidizing zone with excess air in the upper regions of the furnace. The reducing zone of the furnace is generally operated at stoichiometry of 0.7-0.9, with particle residence times of 1-2 s.3,4 As established in the discussion above, under reducing conditions crystalline phases of ash particles derived from pyrite mineral will be molten at significantly lower temperatures, crystalline phases of ash particles derived from siderite mineral will be molten at moderately lower temperatures, and iron aluminosilicate ash particles derived from included pyrite or siderite and alumino-silicate minerals will be partially molten at significantly lower temperatures, compared to oxidizing conditions. These effects are likely to apply in the lower regions of the furnace. It is also possible that some extension of these effects into the oxidizing regions of the furnace may occur due to the time required for sticky particles to oxidize sufficiently to become nonsticky. Hence, the results of this study would suggest the potential for ash deposition due to molten or partially molten sticky particles is increased for air (35) Quann, R. J.; Sarofim, A. F. Vaporization of refractory oxides during pulverised coal combustion. Symp. (Int.) on Combust. [Proc.] 1982, 19, 1429-1440. (36) Quann, R. J.; Neville, M.; Sarofim, A. F. A laboratory study of the effect of coal selection on the amount and composition of combustion generated submicron particles. Combust. Sci. Technol. 1990, 74, 245265. (37) Neville, M.; Quann, R. J.; Haynes, B. S.; Sarofim, A. F. Vaporization and condensation of mineral matter during pulverised coal combustion. Symp. (Int.) on Combust. [Proc.] 1981, 18, 1267-1274. (38) Senior, C. L.; Flagan, R. C. Ash vaporisation and condensation during combustion of a suspended coal particle. Aerosol Sci. Technol. 1982, 1, 371-383. (39) Nagelberg, A. S.; Mar, R. W.; Carling, R. W. Calculation of the role of coal constituent elements in the enhanced vaporization of silica. High-Temp. Sci. 1985, 19, 3-16.
McLennan et al.
staged combustion when utilizing coals with high contents of iron minerals pyrite and siderite. Slagging Gasification. Reducing conditions will have a number of important effects on slag formation and slag properties in a pf boiler or slagging coal gasifier. In all cases Mo¨ssbauer analysis indicated the Fe(II)-to-Fe(III) ratio was higher for reducing conditions, generally by a factor of greater than two. For ironcontaining glass, iron was present predominantly in the ferrous (Fe2+) state for reducing conditions, while significant proportions were present in the ferric (Fe3+) state for oxidizing conditions. This has a significant influence on the viscosity of the slag formed. In the ionic model of a silicate-based slag22 Fe2+ acts as a network modifier, breaking up the silicate network to decrease viscosity. On the other hand, Fe3+ ions in a silicatebased slag are amphoteric, and may act as either network modifiers or network formers. Amphoteric ions generally act as network formers to fit into the silicate network,22 unless there is a low level of modifier ions, in which case they act as modifier ions. In most cases Fe3+ ions will act as network formers, with increasing levels of Fe3+ increasing the slag viscosity. This will generally result in lower slag viscosity for reducing conditions, providing better slag flow. It was determined that for reducing conditions a significant proportion of the FeS phase from pyrite decomposition is likely to prevail, rather than oxidizing to magnetite or hematite. It was also determined that iron from the FeS phase is not incorporated into alumino-silicate glass until it is oxidized to FeO. The combination of these two effects indicates that for reducing conditions molten FeS phases, which are essentially immiscible in the slag, will be present. Since appreciable proportions of the iron remain in the FeS phase, rather than being incorporated into the slag to act as network modifiers, the viscosity of the slag is not lowered. Siderite minerals, however, decompose to wustite (FeO), which is directly incorporated into the slag, decreasing the slag viscosity. Hence, iron in the form of siderite mineral will more effectively flux the slag than will pyrite mineral for coal utilization in slagging coal gasification. Conclusions The following conclusions can be drawn concerning the role of iron mineral transformations for coal combustion and gasification in reducing conditions: (1) Although high iron levels in a coal have often been associated with ash deposition and slagging, they are not definitive with respect to potential for such behavior. (2) Whether iron mineral is predominantly in the form of pyrite or siderite, is of included or excluded nature, is closely associated with included silicate and aluminosilicate minerals, and the combustion conditions to which it is subject, are important factors when considering such minerals potential for ash deposition and slagging. (3) Coals containing pyrite mineral have the potential to produce ash deposition and slagging at lower temperatures than do coals containing siderite mineral. (4) Under reducing conditions coals containing iron minerals pyrite and siderite have the potential to
Ash Formed from High Iron Coals
produce ash deposition and slagging problems at lower temperatures than for oxidizing conditions. (5) For air staged combustion, where reducing conditions exist in the lower regions of the furnace, the potential for deposition and slagging due to molten ash particles will be greater than that for conventional combustion under oxidizing conditions. Based on the melting temperatures of the ash formed, the increase in ash deposition and slagging will be greatest for pyritecontaining coals, moderate for coals with a high degree of included mineral association, and slight for sideritecontaining coals.
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(6) For slagging coal gasification, iron in the form of siderite mineral will more effectively flux slag than iron in the form of pyrite mineral, which is retained as pyrrhotite (FeS) in reducing conditions. Acknowledgment. The authors acknowledge the financial support provided by the CRC for Black Coal Utilization, which is in part funded by the CRCs program of the Commonwealth Government of Australia. EF990092H
