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Energy & Fuels 1998, 12, 464-469
Investigation of the High-Temperature Behavior of Excluded Siderite Grains during Pulverized Fuel Combustion C. W. Bailey,* G. W. Bryant, E. M. Matthews, and T. F. Wall Cooperative Research Centre for Black Coal Utilisation, Department of Chemical Engineering, University of Newcastle, Callaghan, NSW, Australia 2308 Received July 3, 1997
An investigation of the behavior of excluded siderite samples derived from both pure mineral species and by density separation of pulverized coal samples was conducted at combustion temperatures of 1100 and 1600 °C. Analysis of quenched combustion residues collected from the combustion gases under oxidizing conditions was obtained via scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and X-ray fluorescence (XRF) spectroscopy. The FeO-MgO-CaO ternary phase diagram was used to predict the behavior of excluded siderite grains under combustion conditions and indicated that the siderite is unlikely to form sticky or completely melted particles at 1600 °C unless MgO < 5wt %, CaO < 37.5 wt % and (MnO + FeOn) > 60wt %. The extent of melting observed for collected particles having different compositions agreed with this prediction. By comparison with published work for excluded pyrite particles, residues from excluded siderite particles are found to be sticky at higher temperatures (1380 °C compared to ∼1080 °C for pyrite), and will generate less fines as fume or by fragmentation. Slagging indices based on iron in ash for coals containing pyrite will therefore not apply to coals containing siderite.
Introduction Fouling and Slagging. The deposition of ash can be broadly classified into two categories: slagging and fouling. Slagging results from the deposition of sticky ash particles on the walls of the high-temperature regions of a furnace. Sticky particles are of a chemical composition such that a molten phase is formed under the prevailing temperatures and gas phase conditions. These particles can impact upon furnace surfaces and be retained and may subsequently act as a base for further deposition. Other particle types that can be associated with slagging are fines and fume. Fine particles (450 (variable) >500 (variable)
stitutes for 8-18% of the iron and calcium substitutes for 8 to 15% of the iron in the siderite lattice. Although not classified for coal by Patterson, there is a complete solid solution between siderite and rhodochrosite (MnCO3) and as such manganese also substitutes for iron in the siderite lattice.13 The remaining carbonate minerals dolomite and ankerite also show variations in chemical composition. Dolomite, ((Ca,Mg)(CO3)2, where the normalized MgO content is 40-55%) occurs when calcium substitutes for 40-55% of the magnesium in the mineral lattice. Ankerite (Ca(Mg,Fe)(CO3)2) is variable in composition with up to 60% of the magnesium being substituted by iron in the dolomite lattice. Decomposition and Oxidation of Carbonates. The decomposition behavior of carbonates has been studied previously,14-16 differential thermal analysis (DTA) and thermogravimetric (TGA) analysis being the usual techniques. The general reaction for the decomposition of carbonates is17
MCO3 + heat f MO + CO2
(1)
where M ) Fe, Ca, Mg, Mn. Each carbonate species has a characteristic threshold temperature (depending on decomposition atmosphere and chemical composition) where thermal decomposition will commence; these are summarized in Table 1. From Table 1 it is evident that there is a wide range of temperatures for the decomposition of siderites. All of the decomposition temperatures listed are less than the temperatures encountered in commercial pf fired plants (1400-1600 °C) and the temperatures used in the experimental program. In an oxidizing environment the carbonate decomposition products can undergo further oxidation. The likely oxidation pathway for siderite under high temperatures has been determined by Vasyutinski18 and is shown in eq 2.
FeCO3 f FeO f Fe3O4 f γ-Fe2O3 f R-Fe2O3
(2)
It has been suggested that for siderite in equilibrium with graphite the decomposition proceeds directly to magnetite (Fe3O4).19 The overall rate of decomposition of siderite depends on many factors, such as the partial pressure of CO2, the temperature, particle size, and (13) Deer, W. A.; Howie, R. A.; Zussman, J. Rock Forming Minerals, 2nd ed.; London: Longmans, 1978. (14) Kulp, J. L.; Kent, P.; Kerr, P. F. Am. Mineral. 1951, 36 (9-10), 643. (15) Patterson, J. H.; Hurst, H. J.; Levy, J. H. Fuel 1991, 70, 1252. (16) Hurst, H. J.; Levy, J. H.; Patterson, J. H. Fuel 1993, 72, 885. (17) Warne, S. St. J. Differential Thermal Analysis of Coal Minerals in Analytical Methods for Coal and Coal Products 1979, 3. (18) Vasyutinski, N. A. Mineral Sb. 1969, 22 (4), 407. (19) French, B. M.; Rosenberg, P. E. Science 1965, 147, 1283.
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the regions corresponding to the compositions found for siderite and mixed siderites contained in Australian bituminous coals discussed earlier and the average grain compositions for the samples used in the experimental program (as determined by SEM EDS). By inspection of Figure 1, most siderite types contain less than 37.5 wt % CaO, and the determining factor for potentially slagging particles is therefore the MgO content. Experimental Section
Figure 1. Ternary phase diagram21 for excluded siderite mineral matter showing the composition regions for types identified by Patterson et al. (1991),12 together with isotherms. Letters refer to average compositions for siderite grains in experimental samples as determined by SEM EDS analysis (normalized to 100 wt %).
extent of diffusion of CO2 through the particle.20 The effect of impurities in the siderite lattice (Mn, Mg, Ca) on the oxidation of decomposition products has not been determined. In pulverized fuel-fired boilers siderite, ankerite and calcite have been reported to fragment when heated rapidly,1,9 yielding fines and fume particles. The fragmentation of siderite provides some controversy with a recent paper concluding that siderite does not fragment in the flame but melts to produce nonsticky particles when fired at 1400 °C under both oxidizing and reducing conditions with typical particle residence times of 30 ms.10 Whether the minerals were included or excluded was not specified. In the presence of sulfur, products of the decomposition of excluded calcite are converted to CaSO4 which has been suggested as being a slagging and fouling initiator.9 Ternary Phase Diagram. Excluded siderite can be considered independently of the other inorganic elements in the coal due to the low probability of interaction with other inorganic elements in the combustion gases. In this case, for the mixed siderite system where the major inorganic oxides are calcium, magnesium, manganese, and iron, the ternary phase diagram for the CaO-FeO-MgO sytem can be used with the iron and manganese contents combined. This grouping of elements is acceptable due to the similar atomic size, charge, and range of oxidation states encountered for these elements; also a comparison of the Mn and Fe ternary phase diagrams indicates FeO and MnO have similar properties in oxide systems. The phase diagram complete with isotherms21 is presented in Figure 1. The region where potentially sticky particles would form (at a liquidus temperature < 1600 °C) is MgO < 5 wt % and CaO < 37.5 wt %. Also presented in Figure 1 are (20) Bryers, R. W. Symp. Slagging Fouling Steam Generators 1987, 63. (21) Scheel, R.; Sprechsaal Keram., Glass. Baustoffe. 1975, 108 (2324), 685.
Siderite Samples. Coal samples were selected on the basis that iron occurred principally as siderite or mixed siderite. Three samples (samples A, B, and C) were derived from the mineral matter contained in three Australian bituminous coals. The coal siderite mineral fractions were prepared by a density separation at a specific gravity of 2.0, resulting in siderite-enriched mineral fractions. Sample A was predominately siderite, sample B magnesium calcium siderite with a proportion of siderite, and sample C siderite with elevated levels of manganese. These samples represent only a fraction of the siderite minerals contained in the coals; a fraction will be retained in the coal (as included mineral matter) and would be expected to behave differently. These particles will be examined in detail in a further study. Two pure mineral samples (samples D and E) were also used; these samples were prepared by crushing well-characterized rock samples. Sample D was siderite and sample E magnesium siderite. Sample Characterization. The bulk chemical compositions (expressed as weight percent elemental oxides) for the coal mineral samples were determined using XRF. Borate fusion was conducted as per Australian standards prior to the analysis.22 The results are shown in Table 2. The XRF analyses showed a large proportion of silica and alumina for samples A and B; sample C is mainly siderite with a proportion of calcite also present. A point count23,24 was performed for each of the coal mineral samples, and the results are presented in Table 2. These results show that the samples have a variety of minerals present other than the siderite. The most notable occurrence is the presence of pyrite in sample B. An illustrative average composition for the siderite grains present in each sample was obtained via SEM EDS analysis using a Joel 840 scanning electron microscope (SEM) with an Oxford energy-dispersive X-ray detection system. The compositions of the siderite minerals determined by EDS are shown in Table 2 for the coal mineral samples and Table 3 for the pure mineral samples. The CO2 content was calculated by difference. Combustion Studies. A laminar flow drop-tube furnace,25 using air as the carrier gas, was used in the investigation. Furnace set temperatures for the coal mineral sample experiments were 1100 and 1600 °C.The pure mineral samples were only investigated at a temperature of 1600 °C. The samples were fed into the furnace at a rate of approximately 7.5 g h-1 and the resultant combustion residues were rapidly cooled in a quench probe before passing through a cyclone and aerosol filter arrangement designed to separate coarse (>5 µm) and fine (60 wt %. This prediction has been supported by experiments. It was noted that increasing the combustion temperature from 1100 to 1600 °C had little effect on the fragmentation of excluded siderite or the combustion residue composition. Temperature was found to slightly increase the amount of iron fume formed. A comparison between excluded pyrite and excluded siderite indicated that the siderite should be less troublesome in respect to slagging issues principally due to composition and melting temperatures and second due to limited fragmentation and fume formation. Acknowledgment. The authors acknowledge the financial support provided by the Cooperative Research Centre for Black Coal Utilisation which is funded in part by the Cooperative Research Centres Program of the Commonwealth of Australia. The study is also supported by the Australian Coal Association Research Program (ACARP). The authors acknowledge Dr. H. J. Hurst of CSIRO Division of Coal and Energy Technology for provision of several siderite mineral samples and Dr. A. G. Tate for additional calculations. EF970107G
