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57Fe Mössbauer spectroscopic studies of fly ash from coal-fired power plants and bottom ash from lignite-natural gas combustion
57Fe Miissbauer spectroscopic studies of fly ash from coal-fired power plants and bottom ash from lignite-natural gas combustion M. D. Patil,
H. C. Eaton and M. E. Tittlebaum*
Department of Mechanical Engineering and Graduate Program in Materials Science and * Department of Civil Engineering, 1 ouisiana State University, Baton Rouge, Louisiana 70803, USA (Received 12 July 1982; revised 28 March 1983)
A series of four fly ashes, representing a variety of geological origins, and a bottom ash sample derived from the combustion of lignitenatural gas mixtures have been studied by !j7Fe Mossbauer spectroscopy. The ashes are separated into magnetic and non-magnetic fractions to facilitate a study of the chemical state of the iron contained in the ash. The bottom ash contains no magnetic fraction whereas the magnetic fractions of the fly ashes range from 1 .I to 7.3%. The magnetic fractions contained iron in the form of magnetite, FesO,. Iron in the non-magnetic fly ash fractions occur as Fe+’ and Fe+* mullites, and Fe+3 and Fe+* silicates. Only Fei3 silicates are found in the bottom ash. (Keywords:
fly ash; lignite;
natural
gas; combustion)
Bottom ash and fly ash are produced in significant quantities during the firing of coal combustion power plants. Technical interest in ash results primarily from the potential impact of the ash on the environment, the burden of disposal, and from several proposed ash utilization schemes. For example, particulates resulting from coal combustion contain a large respirable fraction which must be removed to satisfy Federal air pollution standards. The collected particulates are often disposed of on land. The discarded ash is, therefore, constantly washed by ground and surface waters. This leads to a concern for the possible generation of hazardous leachates which might contaminate existing water resources. Other interests in ash structure relate to suggested ash utilization schemes and to the indications ash may provide regarding the mechanism of incineration and combustion. For example, fly ash has been used as an admixture in Portland cement concrete and in many soil stabilization applications. Its behaviour therein has been empirically determined with little a priori knowledge of the relation between ash structure and the properties of the composite materials. Recently, combustion and incineration mechanisms have been of interest to technologists concerned with improving combustion or with devising advanced incineration methods, particularly for hazardous waste destruction. Regarding hazardous waste destruction, ash studies are interesting for two reasons. First, information is available about the behaviour of a variety of elements at high temperatures as the coal is such a complex conglomerate of organics and inorganics. Second, usually combustion is monitored by measuring global parameters such as temperature. NO, and CO concentrations. From these measurements combustion mechanisms are only inferred. The structure of the ash 0016-2361/84/060788~5$3.00 @ 1984 Butterworth & Co. (Publishers) Ltd.
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itself, however, reflects the local nature of the combustion process because it is formed in the high-temperature regions of the furnace. It should, therefore, provide a better fingerprint of what actually occurs during incineration. An economically important motivation for studying the iron content of ashes is the potential for information about the causes and cures for corrosion of metals in utility boilers. Although the exact corrosion mechanism is not understood, it is known that ash products can react at high temperatures to produce sodium-iron-trisulphate deposits on boiler surfaces’. An initial step in the formation of this deposit is the reaction of pyrite (FeS,) and organic sulphur with oxygen to form Fe,O,. Other reactions may involve iron-containing ash constituents; therefore, it becomes increasingly important to know the iron content of fly-ash. Various analytical methods2-6 have been used to analyse the chemical composition and understand the structure of ash. These methods have shown that almost all bottom ash and fly ash contain iron. The iron can occur in any of several chemical states, e.g. as complex 5‘Fe Miissbauer oxides, silicates, or sulphates. spectroscopy is perhaps the best technique available for speciating the iron compounds present in a variety of complex systems’. The method has been used for the analysis of iron-bearing minerals in naturally occurring samples, such as terrestrial and lunar rocks*, various types of ores, coal, and coal ash products’. However, the number of Miissbauer studies on coal ashes’O*” has been small even though its use can potentially yield important information about the composition and formation mechanism of ash. In the present paper, the “Fe Miissbauer studies of a series of fly ashes from power plants and bottom ash from
Mlissbauer studies of fly ash and bottom ash: M. 0. Patil et al.
lignite-natural gas combustion are reported. The sources of the ashes are given in Table 1. In each case the ashes were separated into magnetic and non-magnetic fractions for studying the nature of the iron. It can be seen from this Table that the ashes represent coals from a variety of geographical origins and, therefore, potentially reflect a variety of chemical compositions. EXPERIMENTAL Samples of ash, each weighing 2 g, were used for magnetic separation. The magnetic fractions were obtained by using a small laboratory magnet. The percentage of magnetic fraction is given in Table 2. All 57Fe Mijssbauer spectra were obtained with an Austin Science Associates (ASA) Mossbauer spectrometer operating in the constant-acceleration mode and spectra were collected with a Nuclear Data Series 2200 Multichannel Analyzer operating in the multiscaling mode. Velocity calibration of the spectrometer was accomplished by laser interferometry using a Spectra Physics He-Ne Laser System. The source was 20 mCi of 57Co in a host lattice of rhodium metal purchased from the Spire Corp., Bedford, MA. This source exhibited a line-width of 0.28 mm s-l for a K,[Fe(CN),] * 3H,O absorber. Mossbauer absorber samples were made using mylar windows on an aluminium plate with a hole of 15 mm diameter. The thickness of the absorber was 1 mm. All spectra were collected until approximately 5 x lo6 counts per channel in the baseline had been accumulated. The magnetic fractions were analysed at liquid nitrogen temperature and the non-magnetic fractions at room temperature. Mossbauer spectra were analysed by the use of Lorentzian-shaped multiplets which were fitted (using a least-squares algorithm) to the observed spectrum. A Perkin-Elmer Interdata 8/32 computer was used. Each iron absorption is described in terms of l-6 Lorentzian curves with the following parameters: isomer shift, I.S. Table 1 Ash designation Clinch River Amos NBS 1633a
G&H
Lignite-natural
Coal and ash source Eastern coal burned at Clinch River Power Plant, Carbo, Virginia Eastern coal burned at John Amos Power Plant, St. Albans, West Virginia National Bureau of Standards, Standard Reference Material No. 1633a. a product of Pennsylvania and West Virginia coals Western coal burned at power plant in Cason, Texas; obtained from Gifford and Hill Co., Dallas, Texas gas North Dakota lignite burned as a mixture with natural gas; the lignite contributed 62% of the total energy of the flame; the heating value of the lignite was 16.5 MJ kg-*
Table 2 Percentage of magnetic fraction
1 2 3 4 5
in coal ashes
Coal ash
Magnetic fraction
NBS 1633a Clinch River Amos G&H Lignite-natural
7.3 zt 0.36 3.1 i 0.15 2.0 * 0.10 1.1 zt 0.06 0.0
gas
(%I
(mms-‘); quadrupole splitting, Ee (mms-‘); and internal magnetic field (when present), H.I. (kG). All isomer shift valves are referenced to a National Bureau of Standards (NBS) metallic iron foil standard. RESULTS The composition and properties of a coal ash are mainly dependent on the type of coal and the method of burning’. The common chemical constituents of coal ash are SiOZ, Al,O,, Fe,OJ, CaO, MgO, Na,O, TiO,, and alkali or alkaline-earth metal sulphates. Eastern coals are characterized as high iron and low alkali/alkaline-earth in content whereas western coals are characterized as low iron and high alkali/alkaline-earth in content. Fly ash and bottom ash may contain iron in the form of both magnetic and non-magnetic oxides, as well as, mullite, silicates, and a variety of covalent compounds, e.g. FeS,. The western coal fly ash (G & H) had the lowest magnetic fraction and there was no magnetic fraction found in the bottom ash from the lignite-natural gas combustion. In comparison, fly ashes from eastern coals (Clinch River, NBS 1633a and Amos) exhibited sizeable magnetic fractions, and the NBS 1633a sample contained the largest. has been used 5‘Fe Miissbauer spectroscopy previously to understand the state of iron in fly ashes and bottom ash”. In the present study Miissbauer spectra were collected on magnetic fractions and non-magnetic fractions separately. The separation not only simplified the analysis of the spectra but also helped to facilitate identification of the iron species in different fractions. Mossbauer parameters of magnetic fractions and nonmagnetic fractions of coal ashes are given in Table 3 and Table 4, respectively. Iron oxides in fly ash contribute to the magnetic behaviour of particulates. Hematite (a-Fe,O,), magnemite (c+Fe,O,), magnetite (F304), geothite (CIFeOOH), and trevorite (NiFe,O,) all have high internal magnetic fields’. Metallic iron and iron sulphide, FeS, can also contribute to magnetic behaviour. For these oxides, metallic iron, and FeS, alI exhibit unique sets of Mlissbauer parameters such as isomer shift, quadrupole splitting, and internal magnetic field. The difference in Mijssbauer spectra allows an identification of the iron species in magnetic and non-magnetic fractions of fly ash. The Mijssbauer spectra of magnetic fractions of the coal ashes examined in the present study are shown in Figures 1 and 2. These results can be explained by the existence of two sets of hyperfine fields with parameters Hi =450+ kG with an isomer shift of 0.700.85 mm s- ‘, and H, = 500) 10 kG with an isomer shift of 0.33 mm s- i. These may be assignedi2*13 to the Fe+2 (octahedral site) and Fef3 (tetrahedral site) ions of the Fe,O, inverse spine1 phase. Magnetite is a black isometric mineral, Fe, _xO4, of the spine1 group, i.e. an oxide of iron. Mossbauer results of magnetic fractions of the coal fly ashes (NBS 1633a, Clinch River, Amos, G & H) strongly suggest that magnetite is the only iron species present. In addition, recent optical microscopic studies of these fly ashes support the presence of magnetite particlesi and have shown that all magnetic particles, whether they are cenospheres (hollow spheres) or plerospheres (spheres filled with other spheres), or any other shape, are black in colour.
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Mssbauer T&e
studies of fly ash and bottom ash: M. 0. Patil et al.
3 57Fe Mossbauer parameters of magnetic fraction of coal ashes
Peak no.
NBS 1633a fly ash
isomer shift 1 2 3 4 5 6 7 8 9 IS. Fe+* IS. Fe+3 H.I. (kG)
Clinch Aiver fly ash
Amos fly ash
8.22 4.96 1.67 -0.21 -cr.79 -3.60 4.28 -6.81 -7.80
8.22 4.95 1.67 0.05 -0.78 -3.55 4.27 -6.80 -7.81
8.22 4.95 1.66 0.08 -0.75 -3.53 4.28 -6.77 -7.81
0.70 0.33 450 500
0.73 0.33 450 500
0.73 0.33 450 500
G&H fly ash
Comments
8.32
Magnetite
5.51 1.67
Fe304
-0.68 -3.63 -4.37 -6.75 -7.91 0.85 f 0.03 0.42 450 500
IS. - Isomer shift in mm s-t; H.I. = internal magnetic field in kG calculated from Mossbauer magnetic splitting. The computed standard deviations for all peak positions, isomer shifts and internal magnetic fields are kO.1 mm s- 1, 0.2 mm s-l and 10 kG, respectively, unless otherwise indicated
Table 4 57Fe Mossbauer parameters ashes
Peaks (mm s-l)
Name
of non-magnetic
fraction
of coal
Isomer shift (mm s-l)
Quadrupole splitting, EQ(mm s-*1
Assignments
NBS 1633a
2.18 0.92 -0.12
1 .03 0.34
2.30 1.04
Fe+* mullite Fe+3 mullite
Clinch River
2.15 1 .lO -0.15
1 DO
2.30
Fe+* mullite
0.48
1.25
Fe+3 mullite
0.96
2.22
Fe+* mullite
0.24
0.79
Fe+3 mullite
0.20
1.18
Fe+3 silicate
0.31
0.99
Fe+3 silicate
2.07 0.64 -0.15 G&H
Lignitenatural gas
0.79 -0.39 0.81 -0.18
ash each show three Mijssbauer peaks. Analysis of these three peaks yields two sets of quadrupole split peaks. The first set of quadrupole split peaks has isomer shifts of splitting of 1.180.24-0.58 ,, s - ‘, and quadrupole 1.25 mm s-l. The second set of quadrupole split peaks has isomer shifts of 0.96-1.07 mm s- ‘, and quadrupole splitting of 2.14-2.30 mm s - ‘. The Mossbauer parameters of the first set of peaks correspond to Fe+3 and those for the second set of peaks correspond to Fe+‘. SoroczakW X-ray powder diffraction work shows the presence of mullite structures in all of these fly ashes. Thus, the forms of iron present in these fly ashes are Fe+3 and Fe+’ mullites, Fe +3 and Fe +’ silicates. DISCUSSION As discussed previously in this Paper, the composition of coal fly ash and bottom ash are complex functions of the geological origin of the coals and the combustion history. Consequently, general statements about ash chemistry
Error estimates are from computer least-squares fits to Lorentzian lines iO.01. Precision (reproducibility) from two mirror image spectra is r0.04 mm 5-l
At high temperature, magnetite is the more stable oxide of iron, while at < 1388°C hematite is more stabler5. The occurrence of ferrous iron present as magnetite is a reflection of the formation temperature of the ash. Thus, the formation temperature of all the fly ashes examined must have been > 1400°C. Mossbauer spectra of non-magnetic fractions of coal ashes are shown in Figures 3-5. Mijssbauer parameters are given in Table 4. All spectra have either two or three Mossbauer peaks. G & H fly ash and lignite-natural gas bottom ash each have two peaks. The isomer shift of these peaks ranges from 0.20-0.31 mm s-l whereas quadrupole splitting ranges from 0.99-l. 18 mm s - ‘. The iron state in these samples is Fe+3 mullite or both Fe+3 mullite and silicate’ 9 Soroczak’s’ 6 X-ray powder diffraction studies on this fly ash suggest that iron in G & H ash might be Fe+3 silicate. In the lignite-natural gas bottom ash, the iron might also be in the form of an Fe3 + silicate for the ash was formed under high temperature combustion conditions (approximately 1000°C’ ‘). NBS 1622a fly ash, Clinch River fly ash, and Amos fly
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FUEL, 1984, Vol63,
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z I
100
g
99
2 t-
98
k
3
97
x g
96
-12
-10
-0
-6
-4
DOPPLER
-2
0
VELOCITY
2
4
6
8
IO
IN MM/SEC
Figure 1 Miiaabauer spectra of (a) Amos and (b) Clinch River as-received fly ash samples (magnetic fractions)
I2
Mbssbauer studies of fly ash and bottom ash: M. 0. Patil et al.
ii!
I
L
-12
-10
I
-0
I
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I
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Figure 2 Mbssbauer spectra of (a) 1622a and (b) G Et Ii as-received fly ash samples (magnetic fractions).
II
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ray analyses of the non-magnetic fraction of the fly ashes confirmed the presence of mullite and silicates. The mullite and silicates were found, using Mi5ssbauer techniques, to contain both Fe+’ and Fe+3. This is in general agreement with the findings of Refs. 10 and 11. The analysis of bottom ash identifies trivalent iron as a constituent but reveals no oxides. Iron at this higher oxidation state can be the result of combustion at high temperatures. Hinckley et aI.” also examined a bottom ash and found that it contained only Fe2 + in amorphous silicates. Additionally, they found a bottom ash sample to be free of iron oxides. Finally, analyses of the magnetic fraction of Amos, Clinch River, G & H and NBS 1633a ashes confirm the presence of magnetite but do not suggest the presence of any hematite. Additional support for this contention is provided by the examination of the ash particles by optical microscopy. Under white light, the ash appeared black. Finally, a careful comparison was made between the spectra of the present analysis and those of mixed oxides by Saporeschenko et al.” The spectra differed, the present ones being characteristic of pure magnetite. In addition, the present Miissbauer experiments on magnetic fractions were carried out at liquid nitrogen temperature. Low-temperature spectra have a more narrow line-width than those obtained at room temperature. This also lends confidence to the interpretation of the results. As previously stated, the formation of magnetite occurs above approximately 1400°C. Iron in coal is normally in the form of iron pyrite and rarely are hematite or magnetite found in more than trace amounts. Consequently, it must be assumed that the magnetite found in the present study was a produce of the coal combustion.
I I2
100.00 99.60 99.20
5
-
2
90.80 90.40
-
i
90.00
-
2 F
lOO.00 99.80
-
I-
99.60
-
3 g
99.40 99.20
-
g
99.0098.60
P
I
-12
-10
I
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-6
Figure 4 fraction)
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I
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I
I
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DOPPLER
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-12
II
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I
I
I
I
I
I
I
0
2
4
6
8
IO
I2
-I
VELOCITY IN MM/SEC
MGssbauer spectra of Amos as-received (non-magnetic
DOPPLER VELOCITY IN MM/SEC Figure 3 Miissbauer spectra of (a) Clinch River, (b) NBS 1633a and (c) G & H as-received fly ash samples (non-magnetic fraction)
are difficult to make but nevertheless extremely important to energy technologists. The present investigation uses Mijssbauer spectroscopic methods in an attempt to identify the chemical state of iron in the ashes. MGssbauer has seen little use in fly ash research even though the method is one of the best for identifying iron-containing compounds. X-
Figure 5 Miissbauer spectrum of bottom ash from ligniteaatural gas combustion (non-magnetic) as-received
FUEL, 1964, Vol 63, June
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Mlissbauer studies of fty ash and bottom ash: M. D. Patil et al.
However, most combustion chambers obtain maximum temperatures which are considerably less than the required magnetite-formation temperature (typically a maximum operating temperature might be 1000’C). The temperature exceeds 140OC only in the flame zone. It must be concluded, therefore, that the magnetic oxides were produced by combustion in the high-temperature flame zone of the furnace. It may also be stated that, following formation, the magnetic particles did not reside for long periods of time in regions of the furnace where the temperature was less than 1400°C but simultaneously great enough for the transformation of significant quantities of magnetite into hematite.
LSU, Department of Mechanical Engineering, for providing the bottom ashes; and to Dr William Sack, Department of Civil Engineering, West Virginia University, for providing fly ashes.
REFERENCES 1
2 3
CONCLUSIONS Miissbauer techniques have been used in a study of several fly ashes and lignite-natural gas bottom ash. The analysis was aided by separation of the ashes into magnetic and non-magnetic fractions. The bottom ash was found to contain no magnetic fraction whereas the magnetic fraction ranged from 1.1 to 7.3% in the fly ashes. The fly ashes contained Fe+’ and Fe+3 silicates and mullite in agreement with previous investigations. However, the bottom ash was found to contain trivalent iron ions thereby suggesting formation under higher temperature conditions than bottom ashes previously analysed. It is suggested that the magnetite was formed in the flame zone of the furnace. Finally, the chemical and physical analyses reported herein suggest a more sensitive means of determining the local combustion characteristics than the measurement of the typical, more global, parameters.
10 11 12 13
ACKNOWLEDGEMENTS The authors gratefully acknowledge support of this work from the Louisiana Department of Natural Resources and the United States Department of Energy. Thanks are also extended to Dr Mary L. Good of UOP, Inc. for her helpful discussions; to Drs V. A. Cundy and D. Maples,
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FUEL, 1984, Vol 63, June
14 15 16 17
Singer, J. G. (Ed.) ‘Combustion: Fossil Power Systems’, 3rd Edn., Combustion Enar. Comoanv. 1000 Prosnect Hill Rd.. Windsor. CT 06095, 1981,~~. 3-i; 3233 Huffman, G. P., Huggins,F. E. and Dunmyre,G. R. Fuel 1981.60, 585 Fisher, G. L., Prentice, B. A., Silberman, D., Ondev, J. M., Biermann, A. H., Ragaini, R. C. and McFarland, A. R. Environ. Sci. Technol., 1978,12,447 Rothenberg, S. J., Dence, P. and Holloway, P. Appl. Spectrosc., 1980,34,549 Sckart, P. O., Amin, A., Defosse, C. and Rouxhet, P. G. J. Phys. Chem. 1981,&I, 1406 Germani, M. S., Gokmen, I., Sigleo, A. C., Kowlczyk, G. S., Olmen, I., Small, A. M., Anderson, D. L., Failey, M. P., Gulovali, M. C.,Choquette,C. E., Lelep, E. A., Gorden, G. E. and Zoller, W. H. Anal. Chem. 1980,52,240 Greenwood, N. N. and Gibb, T. C. ‘Mossbauer Spectroscopy’, Chapman and Hall, London 1971 Hafner, S. S. and Virgo, D. Proc. Apollo II Lunar Sci. Conj: 1970, 3,2183 Stevens, J. G. and Shenoy, G. K. (Eds.) ‘Miissbauer Spectroscopy and Its Chemical Applications’ Am. Chem. Sot. Adv. Chem. Ser. 194, Am. Chem. S&c., Washington DC, 1981 Hincklev. C. C.. Smith. G. V.. Twardowska. H., Sawroschenko. M., Shil&, R. H. and GritTen, R. A. Fuel 1980; 59: 161 Saporoschenko, M., Hinkiey, C. C., Smith, G. V., Twardowska, H., Shiley, R. H., Grilfen, R. A. and Russell, S. J. Fuel 1980,59,569 Banminger, R., Cohen, G. S., Morinov, A., Ofer, S. and Segal, E. Phys. Reu. 1961,122, 1447 Ito, A., Ono, K. and Ishikawa, Y. .r. Phys. Sot. Japan 1963,18, 1465 Patil. M. D.. Baton, H. C. and Tittlebaum, M. E., submitted for publication Deer, W. A., Howie, R. A. and Fussman, J. ‘Rock-Forming Minerals’. Lonaman Groun Ltd.. London. 1962. Vol. 5, P. 68 Soroczak~ M. M. M. SC. fhe&Lousiana State University, in preparation Cundy, V. A. personal communication.
H. C. Eaton and M. E. Tittlebaum*
Department of Mechanical Engineering and Graduate Program in Materials Science and * Department of Civil Engineering, 1 ouisiana State University, Baton Rouge, Louisiana 70803, USA (Received 12 July 1982; revised 28 March 1983)
A series of four fly ashes, representing a variety of geological origins, and a bottom ash sample derived from the combustion of lignitenatural gas mixtures have been studied by !j7Fe Mossbauer spectroscopy. The ashes are separated into magnetic and non-magnetic fractions to facilitate a study of the chemical state of the iron contained in the ash. The bottom ash contains no magnetic fraction whereas the magnetic fractions of the fly ashes range from 1 .I to 7.3%. The magnetic fractions contained iron in the form of magnetite, FesO,. Iron in the non-magnetic fly ash fractions occur as Fe+’ and Fe+* mullites, and Fe+3 and Fe+* silicates. Only Fei3 silicates are found in the bottom ash. (Keywords:
fly ash; lignite;
natural
gas; combustion)
Bottom ash and fly ash are produced in significant quantities during the firing of coal combustion power plants. Technical interest in ash results primarily from the potential impact of the ash on the environment, the burden of disposal, and from several proposed ash utilization schemes. For example, particulates resulting from coal combustion contain a large respirable fraction which must be removed to satisfy Federal air pollution standards. The collected particulates are often disposed of on land. The discarded ash is, therefore, constantly washed by ground and surface waters. This leads to a concern for the possible generation of hazardous leachates which might contaminate existing water resources. Other interests in ash structure relate to suggested ash utilization schemes and to the indications ash may provide regarding the mechanism of incineration and combustion. For example, fly ash has been used as an admixture in Portland cement concrete and in many soil stabilization applications. Its behaviour therein has been empirically determined with little a priori knowledge of the relation between ash structure and the properties of the composite materials. Recently, combustion and incineration mechanisms have been of interest to technologists concerned with improving combustion or with devising advanced incineration methods, particularly for hazardous waste destruction. Regarding hazardous waste destruction, ash studies are interesting for two reasons. First, information is available about the behaviour of a variety of elements at high temperatures as the coal is such a complex conglomerate of organics and inorganics. Second, usually combustion is monitored by measuring global parameters such as temperature. NO, and CO concentrations. From these measurements combustion mechanisms are only inferred. The structure of the ash 0016-2361/84/060788~5$3.00 @ 1984 Butterworth & Co. (Publishers) Ltd.
788
FUEL, 1984, Vol63,
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itself, however, reflects the local nature of the combustion process because it is formed in the high-temperature regions of the furnace. It should, therefore, provide a better fingerprint of what actually occurs during incineration. An economically important motivation for studying the iron content of ashes is the potential for information about the causes and cures for corrosion of metals in utility boilers. Although the exact corrosion mechanism is not understood, it is known that ash products can react at high temperatures to produce sodium-iron-trisulphate deposits on boiler surfaces’. An initial step in the formation of this deposit is the reaction of pyrite (FeS,) and organic sulphur with oxygen to form Fe,O,. Other reactions may involve iron-containing ash constituents; therefore, it becomes increasingly important to know the iron content of fly-ash. Various analytical methods2-6 have been used to analyse the chemical composition and understand the structure of ash. These methods have shown that almost all bottom ash and fly ash contain iron. The iron can occur in any of several chemical states, e.g. as complex 5‘Fe Miissbauer oxides, silicates, or sulphates. spectroscopy is perhaps the best technique available for speciating the iron compounds present in a variety of complex systems’. The method has been used for the analysis of iron-bearing minerals in naturally occurring samples, such as terrestrial and lunar rocks*, various types of ores, coal, and coal ash products’. However, the number of Miissbauer studies on coal ashes’O*” has been small even though its use can potentially yield important information about the composition and formation mechanism of ash. In the present paper, the “Fe Miissbauer studies of a series of fly ashes from power plants and bottom ash from
Mlissbauer studies of fly ash and bottom ash: M. 0. Patil et al.
lignite-natural gas combustion are reported. The sources of the ashes are given in Table 1. In each case the ashes were separated into magnetic and non-magnetic fractions for studying the nature of the iron. It can be seen from this Table that the ashes represent coals from a variety of geographical origins and, therefore, potentially reflect a variety of chemical compositions. EXPERIMENTAL Samples of ash, each weighing 2 g, were used for magnetic separation. The magnetic fractions were obtained by using a small laboratory magnet. The percentage of magnetic fraction is given in Table 2. All 57Fe Mijssbauer spectra were obtained with an Austin Science Associates (ASA) Mossbauer spectrometer operating in the constant-acceleration mode and spectra were collected with a Nuclear Data Series 2200 Multichannel Analyzer operating in the multiscaling mode. Velocity calibration of the spectrometer was accomplished by laser interferometry using a Spectra Physics He-Ne Laser System. The source was 20 mCi of 57Co in a host lattice of rhodium metal purchased from the Spire Corp., Bedford, MA. This source exhibited a line-width of 0.28 mm s-l for a K,[Fe(CN),] * 3H,O absorber. Mossbauer absorber samples were made using mylar windows on an aluminium plate with a hole of 15 mm diameter. The thickness of the absorber was 1 mm. All spectra were collected until approximately 5 x lo6 counts per channel in the baseline had been accumulated. The magnetic fractions were analysed at liquid nitrogen temperature and the non-magnetic fractions at room temperature. Mossbauer spectra were analysed by the use of Lorentzian-shaped multiplets which were fitted (using a least-squares algorithm) to the observed spectrum. A Perkin-Elmer Interdata 8/32 computer was used. Each iron absorption is described in terms of l-6 Lorentzian curves with the following parameters: isomer shift, I.S. Table 1 Ash designation Clinch River Amos NBS 1633a
G&H
Lignite-natural
Coal and ash source Eastern coal burned at Clinch River Power Plant, Carbo, Virginia Eastern coal burned at John Amos Power Plant, St. Albans, West Virginia National Bureau of Standards, Standard Reference Material No. 1633a. a product of Pennsylvania and West Virginia coals Western coal burned at power plant in Cason, Texas; obtained from Gifford and Hill Co., Dallas, Texas gas North Dakota lignite burned as a mixture with natural gas; the lignite contributed 62% of the total energy of the flame; the heating value of the lignite was 16.5 MJ kg-*
Table 2 Percentage of magnetic fraction
1 2 3 4 5
in coal ashes
Coal ash
Magnetic fraction
NBS 1633a Clinch River Amos G&H Lignite-natural
7.3 zt 0.36 3.1 i 0.15 2.0 * 0.10 1.1 zt 0.06 0.0
gas
(%I
(mms-‘); quadrupole splitting, Ee (mms-‘); and internal magnetic field (when present), H.I. (kG). All isomer shift valves are referenced to a National Bureau of Standards (NBS) metallic iron foil standard. RESULTS The composition and properties of a coal ash are mainly dependent on the type of coal and the method of burning’. The common chemical constituents of coal ash are SiOZ, Al,O,, Fe,OJ, CaO, MgO, Na,O, TiO,, and alkali or alkaline-earth metal sulphates. Eastern coals are characterized as high iron and low alkali/alkaline-earth in content whereas western coals are characterized as low iron and high alkali/alkaline-earth in content. Fly ash and bottom ash may contain iron in the form of both magnetic and non-magnetic oxides, as well as, mullite, silicates, and a variety of covalent compounds, e.g. FeS,. The western coal fly ash (G & H) had the lowest magnetic fraction and there was no magnetic fraction found in the bottom ash from the lignite-natural gas combustion. In comparison, fly ashes from eastern coals (Clinch River, NBS 1633a and Amos) exhibited sizeable magnetic fractions, and the NBS 1633a sample contained the largest. has been used 5‘Fe Miissbauer spectroscopy previously to understand the state of iron in fly ashes and bottom ash”. In the present study Miissbauer spectra were collected on magnetic fractions and non-magnetic fractions separately. The separation not only simplified the analysis of the spectra but also helped to facilitate identification of the iron species in different fractions. Mossbauer parameters of magnetic fractions and nonmagnetic fractions of coal ashes are given in Table 3 and Table 4, respectively. Iron oxides in fly ash contribute to the magnetic behaviour of particulates. Hematite (a-Fe,O,), magnemite (c+Fe,O,), magnetite (F304), geothite (CIFeOOH), and trevorite (NiFe,O,) all have high internal magnetic fields’. Metallic iron and iron sulphide, FeS, can also contribute to magnetic behaviour. For these oxides, metallic iron, and FeS, alI exhibit unique sets of Mlissbauer parameters such as isomer shift, quadrupole splitting, and internal magnetic field. The difference in Mijssbauer spectra allows an identification of the iron species in magnetic and non-magnetic fractions of fly ash. The Mijssbauer spectra of magnetic fractions of the coal ashes examined in the present study are shown in Figures 1 and 2. These results can be explained by the existence of two sets of hyperfine fields with parameters Hi =450+ kG with an isomer shift of 0.700.85 mm s- ‘, and H, = 500) 10 kG with an isomer shift of 0.33 mm s- i. These may be assignedi2*13 to the Fe+2 (octahedral site) and Fef3 (tetrahedral site) ions of the Fe,O, inverse spine1 phase. Magnetite is a black isometric mineral, Fe, _xO4, of the spine1 group, i.e. an oxide of iron. Mossbauer results of magnetic fractions of the coal fly ashes (NBS 1633a, Clinch River, Amos, G & H) strongly suggest that magnetite is the only iron species present. In addition, recent optical microscopic studies of these fly ashes support the presence of magnetite particlesi and have shown that all magnetic particles, whether they are cenospheres (hollow spheres) or plerospheres (spheres filled with other spheres), or any other shape, are black in colour.
FUEL, 1984, Vol63,
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Mssbauer T&e
studies of fly ash and bottom ash: M. 0. Patil et al.
3 57Fe Mossbauer parameters of magnetic fraction of coal ashes
Peak no.
NBS 1633a fly ash
isomer shift 1 2 3 4 5 6 7 8 9 IS. Fe+* IS. Fe+3 H.I. (kG)
Clinch Aiver fly ash
Amos fly ash
8.22 4.96 1.67 -0.21 -cr.79 -3.60 4.28 -6.81 -7.80
8.22 4.95 1.67 0.05 -0.78 -3.55 4.27 -6.80 -7.81
8.22 4.95 1.66 0.08 -0.75 -3.53 4.28 -6.77 -7.81
0.70 0.33 450 500
0.73 0.33 450 500
0.73 0.33 450 500
G&H fly ash
Comments
8.32
Magnetite
5.51 1.67
Fe304
-0.68 -3.63 -4.37 -6.75 -7.91 0.85 f 0.03 0.42 450 500
IS. - Isomer shift in mm s-t; H.I. = internal magnetic field in kG calculated from Mossbauer magnetic splitting. The computed standard deviations for all peak positions, isomer shifts and internal magnetic fields are kO.1 mm s- 1, 0.2 mm s-l and 10 kG, respectively, unless otherwise indicated
Table 4 57Fe Mossbauer parameters ashes
Peaks (mm s-l)
Name
of non-magnetic
fraction
of coal
Isomer shift (mm s-l)
Quadrupole splitting, EQ(mm s-*1
Assignments
NBS 1633a
2.18 0.92 -0.12
1 .03 0.34
2.30 1.04
Fe+* mullite Fe+3 mullite
Clinch River
2.15 1 .lO -0.15
1 DO
2.30
Fe+* mullite
0.48
1.25
Fe+3 mullite
0.96
2.22
Fe+* mullite
0.24
0.79
Fe+3 mullite
0.20
1.18
Fe+3 silicate
0.31
0.99
Fe+3 silicate
2.07 0.64 -0.15 G&H
Lignitenatural gas
0.79 -0.39 0.81 -0.18
ash each show three Mijssbauer peaks. Analysis of these three peaks yields two sets of quadrupole split peaks. The first set of quadrupole split peaks has isomer shifts of splitting of 1.180.24-0.58 ,, s - ‘, and quadrupole 1.25 mm s-l. The second set of quadrupole split peaks has isomer shifts of 0.96-1.07 mm s- ‘, and quadrupole splitting of 2.14-2.30 mm s - ‘. The Mossbauer parameters of the first set of peaks correspond to Fe+3 and those for the second set of peaks correspond to Fe+‘. SoroczakW X-ray powder diffraction work shows the presence of mullite structures in all of these fly ashes. Thus, the forms of iron present in these fly ashes are Fe+3 and Fe+’ mullites, Fe +3 and Fe +’ silicates. DISCUSSION As discussed previously in this Paper, the composition of coal fly ash and bottom ash are complex functions of the geological origin of the coals and the combustion history. Consequently, general statements about ash chemistry
Error estimates are from computer least-squares fits to Lorentzian lines iO.01. Precision (reproducibility) from two mirror image spectra is r0.04 mm 5-l
At high temperature, magnetite is the more stable oxide of iron, while at < 1388°C hematite is more stabler5. The occurrence of ferrous iron present as magnetite is a reflection of the formation temperature of the ash. Thus, the formation temperature of all the fly ashes examined must have been > 1400°C. Mossbauer spectra of non-magnetic fractions of coal ashes are shown in Figures 3-5. Mijssbauer parameters are given in Table 4. All spectra have either two or three Mossbauer peaks. G & H fly ash and lignite-natural gas bottom ash each have two peaks. The isomer shift of these peaks ranges from 0.20-0.31 mm s-l whereas quadrupole splitting ranges from 0.99-l. 18 mm s - ‘. The iron state in these samples is Fe+3 mullite or both Fe+3 mullite and silicate’ 9 Soroczak’s’ 6 X-ray powder diffraction studies on this fly ash suggest that iron in G & H ash might be Fe+3 silicate. In the lignite-natural gas bottom ash, the iron might also be in the form of an Fe3 + silicate for the ash was formed under high temperature combustion conditions (approximately 1000°C’ ‘). NBS 1622a fly ash, Clinch River fly ash, and Amos fly
790
FUEL, 1984, Vol63,
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z I
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Figure 1 Miiaabauer spectra of (a) Amos and (b) Clinch River as-received fly ash samples (magnetic fractions)
I2
Mbssbauer studies of fly ash and bottom ash: M. 0. Patil et al.
ii!
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Figure 2 Mbssbauer spectra of (a) 1622a and (b) G Et Ii as-received fly ash samples (magnetic fractions).
II
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ray analyses of the non-magnetic fraction of the fly ashes confirmed the presence of mullite and silicates. The mullite and silicates were found, using Mi5ssbauer techniques, to contain both Fe+’ and Fe+3. This is in general agreement with the findings of Refs. 10 and 11. The analysis of bottom ash identifies trivalent iron as a constituent but reveals no oxides. Iron at this higher oxidation state can be the result of combustion at high temperatures. Hinckley et aI.” also examined a bottom ash and found that it contained only Fe2 + in amorphous silicates. Additionally, they found a bottom ash sample to be free of iron oxides. Finally, analyses of the magnetic fraction of Amos, Clinch River, G & H and NBS 1633a ashes confirm the presence of magnetite but do not suggest the presence of any hematite. Additional support for this contention is provided by the examination of the ash particles by optical microscopy. Under white light, the ash appeared black. Finally, a careful comparison was made between the spectra of the present analysis and those of mixed oxides by Saporeschenko et al.” The spectra differed, the present ones being characteristic of pure magnetite. In addition, the present Miissbauer experiments on magnetic fractions were carried out at liquid nitrogen temperature. Low-temperature spectra have a more narrow line-width than those obtained at room temperature. This also lends confidence to the interpretation of the results. As previously stated, the formation of magnetite occurs above approximately 1400°C. Iron in coal is normally in the form of iron pyrite and rarely are hematite or magnetite found in more than trace amounts. Consequently, it must be assumed that the magnetite found in the present study was a produce of the coal combustion.
I I2
100.00 99.60 99.20
5
-
2
90.80 90.40
-
i
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-
2 F
lOO.00 99.80
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99.60
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99.40 99.20
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99.0098.60
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MGssbauer spectra of Amos as-received (non-magnetic
DOPPLER VELOCITY IN MM/SEC Figure 3 Miissbauer spectra of (a) Clinch River, (b) NBS 1633a and (c) G & H as-received fly ash samples (non-magnetic fraction)
are difficult to make but nevertheless extremely important to energy technologists. The present investigation uses Mijssbauer spectroscopic methods in an attempt to identify the chemical state of iron in the ashes. MGssbauer has seen little use in fly ash research even though the method is one of the best for identifying iron-containing compounds. X-
Figure 5 Miissbauer spectrum of bottom ash from ligniteaatural gas combustion (non-magnetic) as-received
FUEL, 1964, Vol 63, June
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Mlissbauer studies of fty ash and bottom ash: M. D. Patil et al.
However, most combustion chambers obtain maximum temperatures which are considerably less than the required magnetite-formation temperature (typically a maximum operating temperature might be 1000’C). The temperature exceeds 140OC only in the flame zone. It must be concluded, therefore, that the magnetic oxides were produced by combustion in the high-temperature flame zone of the furnace. It may also be stated that, following formation, the magnetic particles did not reside for long periods of time in regions of the furnace where the temperature was less than 1400°C but simultaneously great enough for the transformation of significant quantities of magnetite into hematite.
LSU, Department of Mechanical Engineering, for providing the bottom ashes; and to Dr William Sack, Department of Civil Engineering, West Virginia University, for providing fly ashes.
REFERENCES 1
2 3
CONCLUSIONS Miissbauer techniques have been used in a study of several fly ashes and lignite-natural gas bottom ash. The analysis was aided by separation of the ashes into magnetic and non-magnetic fractions. The bottom ash was found to contain no magnetic fraction whereas the magnetic fraction ranged from 1.1 to 7.3% in the fly ashes. The fly ashes contained Fe+’ and Fe+3 silicates and mullite in agreement with previous investigations. However, the bottom ash was found to contain trivalent iron ions thereby suggesting formation under higher temperature conditions than bottom ashes previously analysed. It is suggested that the magnetite was formed in the flame zone of the furnace. Finally, the chemical and physical analyses reported herein suggest a more sensitive means of determining the local combustion characteristics than the measurement of the typical, more global, parameters.
10 11 12 13
ACKNOWLEDGEMENTS The authors gratefully acknowledge support of this work from the Louisiana Department of Natural Resources and the United States Department of Energy. Thanks are also extended to Dr Mary L. Good of UOP, Inc. for her helpful discussions; to Drs V. A. Cundy and D. Maples,
792
FUEL, 1984, Vol 63, June
14 15 16 17
Singer, J. G. (Ed.) ‘Combustion: Fossil Power Systems’, 3rd Edn., Combustion Enar. Comoanv. 1000 Prosnect Hill Rd.. Windsor. CT 06095, 1981,~~. 3-i; 3233 Huffman, G. P., Huggins,F. E. and Dunmyre,G. R. Fuel 1981.60, 585 Fisher, G. L., Prentice, B. A., Silberman, D., Ondev, J. M., Biermann, A. H., Ragaini, R. C. and McFarland, A. R. Environ. Sci. Technol., 1978,12,447 Rothenberg, S. J., Dence, P. and Holloway, P. Appl. Spectrosc., 1980,34,549 Sckart, P. O., Amin, A., Defosse, C. and Rouxhet, P. G. J. Phys. Chem. 1981,&I, 1406 Germani, M. S., Gokmen, I., Sigleo, A. C., Kowlczyk, G. S., Olmen, I., Small, A. M., Anderson, D. L., Failey, M. P., Gulovali, M. C.,Choquette,C. E., Lelep, E. A., Gorden, G. E. and Zoller, W. H. Anal. Chem. 1980,52,240 Greenwood, N. N. and Gibb, T. C. ‘Mossbauer Spectroscopy’, Chapman and Hall, London 1971 Hafner, S. S. and Virgo, D. Proc. Apollo II Lunar Sci. Conj: 1970, 3,2183 Stevens, J. G. and Shenoy, G. K. (Eds.) ‘Miissbauer Spectroscopy and Its Chemical Applications’ Am. Chem. Sot. Adv. Chem. Ser. 194, Am. Chem. S&c., Washington DC, 1981 Hincklev. C. C.. Smith. G. V.. Twardowska. H., Sawroschenko. M., Shil&, R. H. and GritTen, R. A. Fuel 1980; 59: 161 Saporoschenko, M., Hinkiey, C. C., Smith, G. V., Twardowska, H., Shiley, R. H., Grilfen, R. A. and Russell, S. J. Fuel 1980,59,569 Banminger, R., Cohen, G. S., Morinov, A., Ofer, S. and Segal, E. Phys. Reu. 1961,122, 1447 Ito, A., Ono, K. and Ishikawa, Y. .r. Phys. Sot. Japan 1963,18, 1465 Patil. M. D.. Baton, H. C. and Tittlebaum, M. E., submitted for publication Deer, W. A., Howie, R. A. and Fussman, J. ‘Rock-Forming Minerals’. Lonaman Groun Ltd.. London. 1962. Vol. 5, P. 68 Soroczak~ M. M. M. SC. fhe&Lousiana State University, in preparation Cundy, V. A. personal communication.