Ash Formation Mechanisms during pf Combustion in Reducing

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Ash Formation Mechanisms during pf Combustion in Reducing Conditions A. R. McLennan,*,† G. W. Bryant,† B. R. Stanmore,‡ and T. F. Wall† Cooperative Research Centre For Black Coal Utilization, Department of Chemical Engineering, University of Newcastle, Callaghan, NSW, Australia, 2308, and Cooperative Research Centre For Black Coal Utilization, Department of Chemical Engineering, University of Queensland, St. Lucia, Qld, Australia, 4072 Received May 20, 1999. Revised Manuscript Received September 21, 1999

A range of pulverized coals were combusted in a laboratory drop-tube furnace at temperatures of 1573, 1723, and 1873 K under oxidizing and reducing conditions to determine the effect of combustion stoichiometry on ash formation mechanisms. As iron mineral transformations were expected to be most affected by combustion stoichiometry, two of the test coals chosen were of high pyrite (FeS2) content and two of high siderite (FeCO3) content. It was found that the ash formation mechanisms of excluded quartz, koalinite, and calcite were not affected by oxidizing or reducing combustion conditions. Excluded pyrite was found to decompose to pyrrhotite, which oxidized to produce an FeO-FeS melt phase which was stable under reducing conditions. Under oxidizing conditions oxidation continued, producing magnetite and hematite. Excluded siderite was found to decompose to wustite, which was stable under reducing conditions, but oxidized to produce magnetite under oxidizing conditions. Included pyrite and siderite were determined to behave as for excluded pyrite and siderite if there was no contact with alumino-silicates. Included pyrite that contacted alumino-silicate minerals was observed to form two-phase FeS/Fe-glass ash particles, with incorporation of iron into the glass proceeding as the FeS phase was oxidized. Included siderite that contacted alumino-silicate minerals was determined to directly form iron alumino-silicate glass ash particles. Iron alumino-silicate glass ash was determined to form with iron in the Fe2+ state, much of which subsequently transformed to the Fe3+ state in oxidizing conditions, but remained primarily as in the Fe2+ state under reducing conditions.

Introduction Coal is a mixture of combustible and noncombustible components. The noncombustible component of coal is made up of organically bound ions and mineral matter present as crystals (deposited from groundwater) or fragments of true minerals (blown or washed into coal deposits). The organically bound ions include Na, K, Ca, and Mg, and are found in significant proportions in low rank coals, but are minor to trace components in higher rank coals. Mineral matter can be classified as either included or excluded depending on its association with organic coal (Figure 11). Included minerals are contained within the coal and are generally finer than excluded minerals, which are liberated from the coal completely during milling. Mineral matter found in coal includes alumino-silicate clays, silicates, carbonates, and disulfides as major components, with sulfates, sulfides, and oxides as minor components. During coal combustion or gasification the coal minerals undergo many transformations to form ash. The ash can be classified in two ways depending on its size. Residual ash, formed from excluded minerals or coa* Author to whom correspondence should be addressed. † University of Newcastle. ‡ University of Queensland. (1) Couch, G. Understanding slagging and fouling in pf combustion; IEA Coal Research, London, 1994.

Figure 1. Organic coal and associated inorganic constituents.1

lescence of included minerals, ranges in diameter from 1 to 100 µm. Submicron ash, formed by vaporization of volatile components of the ash followed by condensation and coagulation, or extensive fragmentation of minerals, varies in diameter from 0.01 to 1 µm. Aside from combustion conditions, the association of minerals, as well as mineral type, influences ash formation processes and deposition characteristics of fly ash. Excluded minerals are unlikely to interact with other particles in the hot combustion gases due to low probability of collision,2 and, unless an exothermic chemical (2) Bool, L. E.; Peterson, T. W.; Wendt, J. O. L. Combust. Flame 1995, 100, 262-270.

10.1021/ef990095u CCC: $19.00 © 2000 American Chemical Society Published on Web 11/25/1999

Ash Formation in pf Combustion in Reducing Conditions

transformation takes place (e.g., pyrite oxidation), reach a temperature no higher than the gas temperature. Included minerals, however, may come into contact with each other to coalesce as the char is consumed, and reach char particle temperatures, which may exceed gas temperature by 200-300 K.3-5 The higher temperatures and locally reducing conditions, which may reduce minerals to more volatile forms, experienced by included minerals significantly increase the extent of vaporization when compared to excluded minerals. Combustion conditions such as temperature, oxygen concentration, and residence time have a significant influence on the ash formation process. Recent interest in low NOx combustion and coal gasification has led to research into ash formation processes in reducing conditions,6-11 which are known to lower melting temperatures12 and influence mineral transformations.7-9,12,13 Though reducing conditions may not have significant influence on included minerals during char combustion, as the char produces a locally reducing environment regardless of combustion stoichiometry, there may be major influences following char combustion. Reducing conditions may also have a notable impact on excluded minerals, particularly iron-based minerals. The objective of this study was to establish the effects of reducing conditions on ash formation mechanisms during coal combustion and gasification processes, with particular emphasis on the iron-containing minerals, the transformations of which were expected to be most affected.

Energy & Fuels, Vol. 14, No. 1, 2000 151

Figure 2. Schematic diagram of experimental setup.

Experimental Section Experimental Setup. A schematic diagram of the complete experimental setup is shown in Figure 2. Combustion stoichiometries of 1.5 (oxidizing) and 0.6 (reducing) were provided by gas mixtures of 3.5-7.5% O2, with the balance being N2. Gas flow rates were controlled by high precision rotameters. The coal feeder provided a stable feedrate of 8-10 g/h using minimal carrier gas to ensure maximum heating rates for particles entering the furnace through a water-cooled probe. The furnace was an ASTRO model 1000A unit, with a recrystallized alumina core of 53 mm nominal i.d. and 800 mm length, heated externally by a graphite heating element of 560 mm length. Experiments were conducted at furnace set temperatures of 1573, 1723, and 1873 K. The furnace set point (3) Seeker, W. R.; Samuelson, G. S.; Heap, M. P.; Trolinger, J. D. Symp. (Int.) on Combust. [Proc.] 1981, 18, 1213-1224. (4) Cashdollar, K. L.; Hertzberg, N. SPIE 1980, 253. (5) Mackowski, D. W.; Altenkirch, R. A.; Peck, R. E.; Tong, T. W. Spring Meeting, Combustion Institute, Western States Section, Salt Lake City, UT, 1982. (6) Groves, S. J.; Williiamson, J.; Sanyal, A. Fuel 1987, 66, 461466. (7) Helble, J. J.; Kang, S. G.; Srinivasachar, S. Proceedings of the Engineering Foundation Conference on the Impact of Ash Deposition on Coal Fired Plants; Williamson, J., Wigley, F., Eds.; 1993; pp 479485. (8) Ram, L. C.; Tripathi, P. S. M.; Mishra, S. P. Fuel Process. Technol. 1995, 42, 47-60. (9) ten Brink, H. M.; Eenkhoorn, S.; Hamburg, G. Mineral matter behaviour in low-NOx combustion - a laboratory study. Proceedings of the E. P. R. I. Conference on the Effect of Coal Quality on Power Plants; San Diego, CA, August 25-27, 1992. (10) ten Brink, H. M.; Hamburg, G.; Eenkhoorn, S. Proceedings of the Engineering Foundation Conference on the Impact of Ash Deposition on Coal Fired Plants; Williamson, J., Wigley, F., Eds.; 1993; pp 113122. (11) ten Brink, H. M.; Eenkhoorn, S.; Hamburg, G. Fuel 1996, 75, 952-958. (12) Huffman, G. P.; Huggins; F. E.; Dumyre, G. R. Fuel 1981, 60, 585.

Figure 3. Gas temperature profile for drop-tube furnace. and gas temperature profile is shown in Figure 3. Also shown in Figure 3 are the coal feed and combustion residue collection points. Furnace residence time was restricted by the axial distance between these points, and was of the order of 1-2 s. 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. Coal Sample Selection and Preparation. Four coal samples with a range of included and excluded minerals were selected for the experimental program. All coal samples contained relatively high iron contents (>10 wt % Fe2O3 in ash), as iron transformations were expected to be most influenced by reducing conditions. All coal samples were received as pulverized fuel, and subsequently sized by sieving to -75/+45 µm size fraction. Coal Sample Characterization. Proximate, ultimate, and ash analyses of the coal samples utilized in this study are given in Table 1. Coal samples were also characterized by computercontrolled scanning electron microscopy (CCSEM), the results of which are presented in Table 2. Ash Particle Collection and Analysis. Coarse particles collected in the cyclone consisted of ash particles derived from 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

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Table 1. Analysis of Coal Samples coal proximate analysis moisture ash volatile fixed C ultimate analysis carbon hydrogen nitrogen oxygen sulfur ash analysis SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O

A

B

C

D

7.6 17.6 28.5 46.3

2.9 12.2 34.3 50.6

4.0 12.4 31.4 52.2

2.5 16.6 32.1 48.8

56.3 2.70 0.80 14.8 0.19

68.6 4.52 1.60 9.2 1.04

67.1 4.14 1.46 7.3 3.61

64.0 4.22 1.34 8.0 3.70

42.00 34.20 2.00 12.70 3.90 1.50 0.11 0.13

49.90 18.80 1.70 17.60 5.30 0.92 0.09 0.63

38.70 22.40 0.67 29.00 2.20 1.40 0.31 3.20

44.40 24.70 0.91 22.20 2.40 0.73 0.39 1.80

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 a size comparable 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 alumino-silicate 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.

Thermodynamic Equilibrium Calculations Thermodynamic calculations to predict mineral transformations under oxidizing and reducing conditions were performed using the F*A*C*T package.14 F*A*C*T is a thermochemical software and database package which can compute multicomponent (24 elements, 2400 species), multiphase equilibria involving nonideal solutions of gases, alloys, ceramics, salts, mattes, slags, etc., drawn from critically evaluated and optimized nonideal solution databases for liquid and solid alloys, ceramics, salts, mattes, and slags. At specific temperature and pressure the desired solution (most stable products) is calculated by the software by systematically varying the moles of each species in a way that makes Gibbs energy most negative. If the existence of a particular solution phase does not assist in minimizing Gibbs energy, it will be dropped in the course of successive iterations. The maximum number of equilibrium phases cannot exceed (13) Nowok, J. W. Energy Fuels 1995, 9, 534-539. (14) Bale, C. W.; Pelton, A. D.; Thompson, W. T. F*A*C*T 2.1sUser Manual; Ecole Polytechnique de Montreal/Royal Military College, Canada, July, 1996.

the number of different elements in the reactants in order to respect the Gibbs phase rule. Calculations were performed over the temperature range 1000-2000 K, with the gas-phase compositions for oxidizing and reducing conditions and phases considered listed in Tables 3 and 4, respectively. The gas-phase composition was calculated using typical carbon, hydrogen, nitrogen, and oxygen values from coal ultimate analysis as input with additional oxygen at stoichiometry of 1.5 for oxidizing and 0.6 for reducing. Calculations were performed for excluded quartz, kaolinite, calcite, pyrite, and siderite, and for iron alumino-silicate formed from included minerals. Results and Discussion CCSEM Analysis of Coal Samples. The major 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 of these coals in another study15 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. Various alumino-silicate forms of included minerals, which have an important bearing on the composition of glassy ash formed by coalescence, were also identified by CCSEM. The major included alumino-silicate mineral compositions were kaolinite for Coal A, kaolinite + quartz for Coal B, kaolinite + montmorillonite + potassium aluminosilicate for Coal C, and kaolinite + montmorillonite for Coal D. Electron Microprobe Analysis of Ash. Particles analyzed by electron microprobe were separated ash. As such, any ash particles derived from included minerals analyzed was the product of complete combustion of a char particle. Iron-containing ash particles from pyrite containing Coal C and Coal D included ironoxysulfide particles derived from excluded or included pyrite, and iron alumino-silicates derived from coalescence of included pyrite and clay minerals. Iron-containing ash particles from siderite-containing Coal A and Coal B included iron oxide with varying levels of CaO and MgO impurities, and iron alumino-silicates derived from coalescence of included siderite and clay minerals. The electron microprobe was utilized for two forms of analysis of these particlessX-ray area maps and point compositional analysis. Mo1 ssbauer Analysis of Combustion Residues. Mo¨ssbauer spectroscopy of combustion residues, comprised of ash particles derived from included and excluded minerals, produces a spectrum with absorption peaks characteristic of different iron (valence) states. The proportion of total iron in each of the phases identified is determined from the areas of these peaks. Ash from combustion of Coal A, Coal B, Coal C, and Coal D under oxidizing and reducing conditions were ana(15) Bailey, C. W. Ash formation involving siderite. Ph.D. Thesis, University of Newcastle, Australia, 1999.



Table 2. CCSEM Analyses of Coal Samples Showing Weight Percentages of Major Mineral Forms 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 %)

quartz Si-rich kaolinite montmorillonite K-Al-silicate Fe-Al-silicate Ca-Al-silicate gypsum-Al-silicate gypsum iron oxide/siderite calcite dolomite ankerite pyrite pyrrhotite oxidized pyrrhotite unclassified

0.2 0.2 18.3 0.6 0 0.5 1.8 1.8 0 1.5 0 0 0 0.1 0 0 5.2

1.1 0.5 48.3 1.5 0.1 0.3 0.9 0.9 0 10.7 0.1 0.2 0 0.2 0 0.1 4.3

8.8 1.4 10.2 2.1 0 0.1 0 0 0.2 0.8 1.5 0 0 0 1.1 0.5 2.1

10.5 0 12.0 2.9 1.0 0 0.2 0.1 0.1 14.2 9.8 0.5 0 2.5a 7.0a 1.7a 7.6

0.7 0.2 4.5 10.9 4.7 0 0 0.2 0.6 0 0 0.2 0.1 17.9 0.3 0.3 16.8

4.4 0 2.5 5.4 8.8 0.2 0 0.1 0.6 0.4 0.1 0.2 0.4 13.2 0.3 0.3 5.6

1.2 0.2 14.2 6.6 1.3 0.30 0.6 1.4 2.5 0 0.1 0 0 11.4 0.7 0.2 9.0

2.4 0.2 11.1 6.9 3.5 1.1 0.4 1.1 0.9 2.0 0.2 0.1 0 12.7 0.2 0.2 7.0

a

Electron microprobe and Mo¨ssbauer spectroscopy analysis indicated minimal pyrite, pyrrhotite.

Table 3. Conditions for Thermodynamic Calculations Gas composition (vol %) stoichiometry oxidizing reducing

O2 32 0

CO2 50 20

CO 0 52

H2O 18 13

H2 0 15

lyzed at room temperature. As a consequence of some ambiguities in the room-temperature spectrum of Coal B combustion residue for reducing conditions, a low temperature (78 K) spectrum was also obtained for this sample. Both spectra were then used to provide the most accurate figures for each phase. The iron-containing glass detected was the result of reactions of included siderite and pyrite minerals with alumino-silicates. Hematite, magnetite, wustite, and pyrrhotite detected may have been 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. Poorly crystalline magnetite causes the peaks to broaden, with each cause having different effects on the octahedral and tetrahedral sites. Variation of the octahedral-to-tetrahedral ratio from the stoichiometric value of 2:1 may be due to substitution by divalent or trivalent atoms on the octahedral site, or poor crystallinity. Substitution with another ion, expected to be less magnetic than iron, results in the diminution of the hyperfine field values, which did not occur. Thus, variation of the octahedral-to-tetrahedral ratio from the stoichiometric value of 2:1 can be attributed to poor crystallinity. 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). Excluded Minerals. Quartz. The behavior of excluded quartz particles was not effected at all experimental conditions of temperature and atmosphere. All coals with appreciable levels of excluded quartz produced combustion residues with angular, nonporous high purity quartz particles. These experimental results were in agreement with thermodynamic calculations for

Table 4. Phases Considered for Thermodynamic Calculations mineral quartz SiO2

kaolinite Al2O3.2SiO2

calcite CaCO3 pyrite FeS2

siderite FeCO3

iron alumino-silicate FeO + SiO2 + Al2O3

phase name

model

quartz SiO2 tridymite SiO2 cristobolite SiO2 liquid slag kyanite Al2O3.SiO2 mullite 3Al2O3.2SiO2 corundum Al2O3 quartz SiO2 tridymite SiO2 cristobolite SiO2 liquid slag calcite CaCO3 lime CaO liquid slag pyrite FeS2 pyrrhotite FeS wustite magnetite hematite liquid slag solution FeS siderite FeCO3 wustite magnetite hematite liquid slag wustite FeO magnetite Fe3O4 hematite Fe2O3 quartz SiO2 tridymite SiO2 cristobolite SiO2 fayalite 2FeO.SiO2 hercynite FeO.Al2O3 iron-cordierite 2FeO. 2Al2O3.5SiO2 mullite 3Al2O3.2SiO2 liquid slag

stoichiometric stoichiometric stoichiometric quasi-chemical stoichiometric stoichiometric stoichiometric stoichiometric stoichiometric stoichiometric quasi-chemical stoichiometric stoichiometric quasi-chemical stoichiometric stoichiometric polynomial soln polynomial soln polynomial soln quasi-chemical polynomial soln stoichiometric polynomial soln polynomial soln polynomial soln quasi-chemical stoichiometric stoichiometric stoichiometric stoichiometric stoichiometric stoichiometric stoichiometric stoichiometric stoichiometric stoichiometric quasi-chemical

excluded quartz which indicated no melting up to 1873 K (the maximum experimental temperature), only phase changes from quartz to tridymite to cristobolite as temperature increases. Kaolinite. The behavior of excluded kaolinite particles was not effected at all experimental conditions of temperature and atmosphere. All coals with appreciable levels of excluded kaolinite produced combustion residues with angular, porous particles identified as meta-

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Figure 4. Ash formation mechanisms for excluded pyrite.17

kaolinite. These experimental results are in agreement with thermodynamic calculations for excluded kaolinite which indicated melting does not begin until a temperature of ∼1873 K (the maximum experimental temperature), above which partial melting occurs. Clays. Some excluded alumino-silicate clay particles present in Coal C and Coal D contained significant levels of FeO, K2O, and Na2O. Of these mineral particles, those with FeO + K2O + Na2O greater than ∼10 wt % exhibited some melting at 1723 K. These particles were rounded (but not spherical) with large pores, indicating limited melting had softened the angular edges and allowed assimilation of small pores to produce larger pores. Ash particles of lower FeO + K2O + Na2O content displayed the same angular porous morphology as the coal mineral. Calcite. Coal B contained significant levels of excluded calcite, identified by CCSEM analysis (Table 2) at 9.8 wt % of excluded minerals. Excluded calcite particles were found to decompose to CaO, but their angular nonporous particle morphology was not effected at all experimental conditions of temperature and atmosphere. These experimental results are in agreement with thermodynamic calculations for excluded calcite, which indicated no melting up to 1873 K (the maximum experimental temperature). Pyrite. CCSEM results (Table 2) identified significant proportions of excluded pyrite in Coal C (13.2 wt %) and Coal D (12.7 wt %). Based on the analysis of the combustion residues for these coals by SEM and electron microprobe, Mo¨ssbauer spectroscopy, and previous work,16 the transformation mechanisms of excluded pyrite are presented in Figure 4.17 These results are now discussed with reference to the mechanistic diagram. The excluded pyrite transformations diagram indicates that pyrrhotite oxidation occurs from the surface inward, as indicated by previous work.16 This is a result of the oxidation process being controlled by the diffusion of oxygen to the particle surface, resulting in an oxide shell and pyrrhotite core. This behavior is illustrated clearly in the X-ray maps of an FeO-FeS eutectic ash particle in Figure 5, which is typical of such particles in the early stages of oxidation. The oxygen X-ray map of Figure 4 shows oxidation is restricted primarily to the exterior region of the particle, confirming this step of the mechanistic diagram. Following oxidation of pyrrhotite to produce an FeO melt, the mechanistic diagram indicates that crystallization to magnetite occurs, with further oxidation resulting in the formation of hematite, as established by previous work.16 These transformations are depend(16) Srinivasachar, S.; Boni, A. A. A kinetic model for pyrite transformations in a combustion environment. Fuel 1989, 68, 829. (17) McLennan, A. R. Ash formation in reducing conditions. Ph.D. Thesis, University of Newcastle, Australia, 1998.

Figure 5. X-ray map of Fe-S-O eutectic ash particle of diameter ∼35 µm from Coal C oxidizing combustion residue: (a) iron; (b) sulfur; (c) oxygen, and (d) combination map.

Figure 6. Number of particles in FeO content ranges for FeO-S eutectic ash particles for Coal D combustion residue (131 particles analyzed for each of oxidizing and reducing).

ent on both temperature and oxygen concentration. First, sufficient oxygen concentration is required to oxidize the melt to magnetite, which will crystallize at ∼1863 K. For a given oxygen concentration, hematite becomes thermodynamically more stable than magnetite below a certain temperature, e.g., for [O2] ) 5% hematite forms below 1597 K.16 Obviously this transformation will be significantly influenced by oxidizing or reducing conditions. The extent to which these transformations proceed is indicated by Figure 6. For Coal D, there was a large number of oxidized particles of high FeO content for oxidizing conditions (42%), most likely present as magnetite and hematite. These particles are likely to be predominantly of excluded origin, as the oxidation of particles of included origin is delayed by the locally reducing environment of the char. For reducing conditions there is a significant reduction in the number of oxidized particles of high FeO content (11%). Such particles are likely to be derived from small excluded pyrite particles, which are oxidized quickly in the early stages of combustion, before oxygen depletion generates reducing conditions. The remainder of the



Figure 7. Percent of total iron in crystalline phases for Coal C combustion residues identified by Mo¨ssbauer spectroscopy.

excluded pyrite particles are not completely oxidized, and they account for the increase in the number of particles of high FeS content compared to oxidizing conditions. Hence, the transformations of excluded pyrite may not be followed to completion for reducing conditions. Mo¨ssbauer spectroscopy of Coal C combustion residues showing the proportion of iron in crystalline phases is presented in Figure 7. These results support the electron microprobe results discussed above. For oxidizing conditions, the small amount of wustite and pyrrhotite represents FeO-FeS eutectic. This is likely to be of included mineral origin, with oxidation delayed by the reducing environment of the char. A significant portion of the hematite and magnetite for oxidizing conditions must be of excluded pyrite origin. For reducing conditions, the proportion of FeO-FeS eutectic represented by wustite + pyrrhotite increases significantly, the proportion of magnetite is reduced, and no hematite is present. Thus, it would seem a significant proportion of the excluded pyrite has remained as FeOFeS eutectic rather than oxidizing to magnetite and hematite. Once again, the results indicate that for reducing conditions the transformations of excluded pyrite may not proceed to completion. Thermodynamic calculations for excluded pyrite (Figure 8) under oxidizing conditions predicted oxidation to hematite, with magnetite more stable at temperatures greater than 1690 K, and forming a molten slag at 1855 K. For reducing conditions, pyrite was predicted to decompose to pyrrhotite which melts to form a molten eutectic at ∼1220 K, which was determined to be stable for all reducing combustion stoichiometries. Thus, for reducing conditions a stable melt phase is formed at considerably lower temperature. These predictions reflect experimental results, which found magnetite to be the dominant phase for oxidizing conditions at 1723 K, and wustite (FeO) + pyrrhotite (FeS) phases to prevail under reducing conditions. Siderite. CCSEM results (Table 2) identified significant proportions of excluded siderite in Coal A and Coal B. Based on the analysis of the combustion residues for these coals by SEM and electron microprobe, Mo¨ssbauer Spectroscopy, and previous work,18 the transformation mechanisms of excluded siderite are presented in Figure 9.17 These results are now discussed with reference to the mechanistic diagram.

Figure 8. Proportions of phases as a function of temperature for excluded pyrite under (a) oxidizing and (b) reducing conditions.

Figure 9. Ash formation mechanisms for excluded siderite.17

Analysis of ash formed from siderite minerals in Coal A and Coal B combustion residues by electron microprobe indicated relatively low levels of calcium and magnesium impurities. Hence the influence of these impurities on melting behavior, producing the two pathways in the mechanistic diagram,19 was not able to be confirmed. Thus all analysis refers to the upper pathway of Figure 9. However, electron microprobe analysis was able to identify the physical character of excluded siderite-derived ash particles with low levels of MgO impurities at 1573, 1723, and 1873 K. At 1573 K excluded siderites produced angular porous ash particles for oxidizing and reducing conditions, indicating no melting had occurred. At 1723 and 1873 K, excluded siderites produced round ash particles for oxidizing and reducing conditions, as in Figure 10, indicating melting had occurred. As there is melting at 1723 K for oxidizing conditions, these results indicate siderite-derived ash particles melt as wustite at ∼1643 (18) 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, September 1-4, 1996. (19) 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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Figure 10. Excluded iron oxide from Callide oxidizing residue at 1873 K.

Figure 12. Proportions of phases as a function of temperature for excluded siderite under (a) oxidizing and (b) reducing conditions.

Figure 11. Percent of total iron in crystalline phases for siderite coal combustion residues identified by Mo¨ssbauer spectroscopy.

K. Following melting, the ash particles oxidize to magnetite and hematite (which melt at ∼1863 K and ∼1838 K, respectively), as indicated in the mechanistic diagram. CCSEM analysis (Table 2) of Coal A and Coal B indicated that the majority of siderite mineral was of excluded nature. Mo¨ssbauer spectroscopy of Coal A combustion residues represent both included and excluded ash particles, but mostly represent excluded ash. The results for crystalline phases of the sideritecontaining coal combustion residues are presented in Figure 11. For reducing conditions, wustite was detected in significant proportions. This confirms the first step of the mechanistic diagram, where siderite decomposes to wustite (FeO) and carbon dioxide (CO2). Coal A combustion residue for oxidizing conditions contained magnetite as the dominant crystalline phase. In this case wustite formed by thermal decomposition of siderite has oxidized to form magnetite, but no hematite has formed. This is most likely due to the experiments for these combustion residues being at temperatures of 1723 K, which is higher than the hematite formation temperature of 1688 K calculated by thermodynamic equilibrium. Similar to the case for excluded pyrite, the results indicate that for reducing conditions the transformations of excluded siderite may not proceed to completion. Thermodynamic calculations for excluded siderite (Figure 12) under oxidizing conditions, as for pyrite, predicted oxidation to hematite, with magnetite more stable at temperatures greater than 1690 K, and form-

Figure 13. Ash formation mechanisms for included pyrite.17

ing a molten slag at 1855 K. For reducing conditions, wustite was predicted to be the stable oxide phase, which melts at ∼1645 K. Thus, for reducing conditions the melt phase is formed at moderately lower temperature. These predictions reflect experimental results which found magnetite to be the dominant phase for oxidizing conditions at 1723 K, and the wustite phase prevailing under reducing conditions. Included Minerals. Pyrite. CCSEM results (Table 2) identified significant proportions of included pyrite in Coal C and Coal D. Based on the analysis of the combustion residues for these coals by SEM and electron microprobe, Mo¨ssbauer spectroscopy, and previous work,11 the transformation mechanisms of included pyrite are presented in Figure 13.17 These results are now discussed with reference to the mechanistic diagram. The high temperature of the burning char will induce decomposition of pyrite to pyrrhotite, releasing gaseous sulfur as in the first step of Figure 13. This decomposition occurs at relatively low temperature, and will be essentially completed during devolatilization, thus having little influence on ash formation.11 However, included pyrite minerals within the char will experience a locally reducing environment during the combustion of the char. Oxidation of included pyrrhotite will thus not proceed to any great extent until the completion of char combustion. This delay in the oxidation of included


pyrite may account for the significant number of FeO-S ash particles of high FeS content identified for oxidizing combustion stoichiometry. Approximately 47% of Fe-O-S ash particles from Coal D had FeS content greater than 60 wt % (Figure 6) for oxidizing conditions. These particles are typically spherical and molten. At longer residence times, under oxidizing conditions these ash particles would be expected to oxidize to produce magnetite and hematite, as in the upper pathway of Figure 13. For reducing stoichiometries, however, on completion of char combustion included ash particles are still subject to reducing conditions and thus are not likely to oxidize significantly to magnetite or hematite. Hence at longer residence times there is likely to be a distinct difference in composition of included pyritederived ash particles between oxidizing and reducing conditions. CCSEM analysis (Table 2) of Coal C indicated that 43 wt % of pyrite mineral was of excluded nature. Mo¨ssbauer spectroscopy of Coal C combustion residues includes both included and excluded ash particles. The analysis identified 60 wt % of total iron as magnetite and hematite. From these figures, it is apparent that some of the included pyrite has not contacted aluminosilicates, but has oxidized to form magnetite. This confirms that both crystallization and glass formation pathways of Figure 13 are followed. Mo¨ssbauer spectroscopy results for crystalline phases from pyritecontaining coal combustion residues were presented in Figure 7. For oxidizing conditions the wustite and pyrrhotite is likely to be present as FeO-FeS eutectic (as identified by electron microprobe) of included origin, and some of the magnetite is likely of included origin. The Mo¨ssbauer spectroscopy results of Figure 7 support the electron microprobe results discussed above. The large proportion of hematite and magnetite for oxidizing conditions reflects the large number of particles with 100 wt % FeOn detected by electron microprobe. For reducing conditions, the larger proportion of wustite and pyrrhotite reflects the larger number of FeO-FeS particles detected by electron microprobe. The lower pathway of Figure 13 (glass formation) indicates the result of pyrite-derived ash particles contacting silicates (quartz) or alumino-silicates (clays) as the char surface recedes during combustion, for which two possibilities exist. Pyrrhotite may contact the silicate/alumino-silicate, as in the first branch of the glass formation pathway, or alternately pyrrhotite may oxidize to an FeO melt before contacting the silicate/ alumino-silicate, as in the second branch. Both these pathways were indicated in a previous study.11 However, this study found that in the case of pyrrhotite contacting silicates/alumino-silicates an iron-containing glass is not formed directly. Rather, a two-phase FeS/ Fe-glass particle is formed, as depicted in Figure 14. A distinct boundary exists between the FeS phase and the Fe-glass phase, with compositional analysis indicating less than 2 wt % FeS in the glass phase. This clearly indicates FeS has very low solubility in aluminosilicates, as has been noted previously.11 As such, pyrrhotite must first oxidize to FeO before coalescence with alumino-silicate glass occurs. As mentioned previously, the oxidation of included pyrrhotite will not proceed to any great extent until complete burnout of


Figure 14. Two-phase Fe-O-S/Fe-alumino-silicate particle from Coal C reducing residue.

Figure 15. Percent of total iron in glass for Coal D combustion residues identified by Mo¨ssbauer spectroscopy.

the char particle. Thus the oxidation of pyrrhotite will delay the formation of iron-containing glass from included pyrite and alumino-silicate minerals, which may account for the limited amounts of iron detected in glass by Mo¨ssbauer spectroscopy in previous studies.20,21 This effect is likely to be accentuated for reducing combustion stoichiometries, where char burnout will be slower and oxidation of pyrrhotite following char burnout will be limited by low p[O2]. Iron-containing alumino-silicate glass is likely to form with the iron in an oxidation state of two as a result of the locally reducing environment of the char. However, following completion of char combustion, if conditions are oxidizing, the Fe2+ may transform to Fe3+ in the glassy ash particles, as indicated on the lower pathway of Figure 13. Mo¨ssbauer spectroscopy results indicating the proportion of total iron in glassy ash particles for pyrite-containing Coal D are presented in Figure 15. Coal D combustion residue exhibited a significant proportion of Fe3+ glass under oxidizing conditions, with reducing conditions producing mostly Fe2+ glass. The proportions of phases predicted by thermodynamic calculations for an iron alumino-silicate glass under oxidizing and reducing conditions are shown in Figure 16. The molten slag formed at high temperature (20) Helble, J. J.; Srinivasachar, S.; Boni, A. A. Factors influencing the transformation of minerals during pulverised coal combustion. Progress in Energy Combustion Science, 1990, 16, 267-279. (21) 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.

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Figure 18. Percent of total iron in glass for Coal B combustion residues identified by Mo¨ssbauer spectroscopy.

Figure 16. Predicted proportions of phases as a function of temperature for iron alumino-silicate glass composition 40 wt % FeO + 33 wt % SiO2 + 27 wt % Al2O3 under (a) oxidizing and (b) reducing conditions.

As discussed previously, iron-containing aluminosilicate glass is likely to form with the iron in an oxidation state of two as a result of the locally reducing environment of the char during combustion, but may subsequently oxidize, transforming some of the Fe2+ to Fe3+ in the glassy ash particles, as indicated on the lower pathway of Figure 17. Mo¨ssbauer spectroscopy results indicating the proportion of total iron in glassy ash particles for siderite containing Coal B are presented in Figure 18. Both Fe2+ and Fe3+ glass was identified for oxidizing conditions, while under reducing conditions only Fe2+ glass was identified, indicating the glassy ash particles remained as formed. In the case of siderite-containing coals, siderite decomposes directly to FeO, allowing immediate coalescence of iron with alumino-silicate glasses. There is no delay to glass formation, so there is then sufficient residence time for the glass to oxidize to form Fe3+ iron.

Figure 17. Ash formation mechanisms for included siderite.17

contains a large proportion of iron in the Fe3+ state for oxidizing conditions and almost exclusively in Fe2+ form for reducing conditions, as indicated by Mo¨ssbauer spectroscopy of combustion residues (Figure 15). The thermodynamic predictions also indicate that different solid phases are formed for oxidizing and reducing conditions, with commencement and completion of solidification at significantly lower temperature for reducing conditions. Siderite. CCSEM results (Table 2) identified some included siderite to be present in Coal A and Coal B. Based on the analysis of the combustion residues for these coals by SEM and electron microprobe, Mo¨ssbauer spectroscopy, and previous work,18 the transformation mechanisms of included siderite are presented in Figure 17.17 These results are now discussed with reference to the mechanistic diagram. Coal B combustion residues contained small particles (5-25 µm diameter) that were essentially iron oxide, with low levels of SiO2 and Al2O3 (

Ash Formation Mechanisms during pf Combustion in Reducing

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