- Email: [email protected]
Forms of potassium in coal and its combustion products
Forms of potassium in coal and its combustion products C. L. Spiro, J. Wong, S. H. Lamson
F. W. Lytle*,
R. B. Greegor”,
D. H. Maylotte
General Electric Corporate Research and Development, PO Box 8, Schenectady, 12301, USA * The Boeing Company, PO Box 3999, 2T-05, Seattle, WA 98124, USA (Received 22 April 1985; revised 18 July 1985)
and NY
High resolution X-ray absorption spectroscopy utilizing synchrotron radiation as a light source was used to probe the bonding and structure ofpotassium in coal and coal-derived products. The potassium sites in coals ofvarious ranks and in the products ofthermochemical processing were identified by comparison ofunknown spectra with those of a wide range of selected potassium model compounds and minerals. For most coals examined, potassium was found to occur in a site in illite, a layered clay related to muscovite. Such coals included subbutuminous, high and low volatile bituminous, cannel and anthracite types. The lower rank subbituminous and lignite coals show other forms of alkali, where potassium appears to be in a noncrystalline environment. Examination ofthe near-edge spectra fails to reveal similarities between potassium in low-rank coal and in any organically bonded potassium model compounds such as carboxylic, phenolic, benzoic, phthalimide or intercalated groups. It is possible that the exchangeable potassium is still inorganically associated on disordered clay surfaces, though this remains equivocal. Thermal treatment of potassium-bearing phases inherent to coals generally leads to formation of a potassium aluminosilicate glass. Samples include those obtained from flash pyrolysis, gasification, laboratory combustion and a large scale pressurized fluidized-bed combustor including bed material, scrubbing hot cyclones, and quench-cooled pins for vapour phase deposition. No potassium sulphates were observed in the deposits. (Keywords:coal;
potassium; combustion products; X-ray
absorption spectroscopy)
Potassium is a particularly pernicious impurity in coal’. The alkali metals are inherently corrosive toward hot
metal surfaces. They form low melting phases with iron and sulphate moieties. In addition, potassium fluxes other normally refractory materials which leads to extensive deposits in combustors. These fused deposits can intercept non-fluxing materials, further exacerbating deposition phenomena and corresponding corrosion. In an engine, fused sub-micron particulates may add to erosion or abrasion of contact surfaces. To remove potassium at the outset, or to somehow intercept potassium prior to the deposition/fusion/ corrosion cascade, an understanding of its inherent chemical environment is a desirable prerequisite. By identifying the fate of potassium during processing and utilization, key intermediate phases might be identified and circumvented or redirected by varying conditions or through the addition of chemical agents. Impurities, such as potassium, in coal are not readily amenable to study by conventional techniques such as chemical, microscopic, thermal d&action and spectroscopic techniques2. These techniques have been applied to coal3 with considerable aggregate success, but for trace elements present in concentrations of 1000 ppm or less in a matrix which is spectroscopically opaque over a wide range of wavelengths, and in non-crystalline, disordered, or non-unique sites, the results are often equivocal. High resolution X-ray absorption near-edge structure (XANES) measurements using well collimated and intense synchrotron radiation sources provide a 0016-2361/86/030327-10$3.00 ~3 1986 Butterworth & Co. (Publishers) Ltd.
novel method for characterizing the bonding and local atomic structure of trace elements in coa14-‘. The technique is atom selective8m12,has good sensitivity for trace constituents as low as 1OOppm in concentration4, and has been effectively applied to the analysis of coal trace elements4~5~7~13-16. In this study, the bonding and structure of potassium impurities in a number of coals of various rank have been determined with XANES using a selected range of potassium model compounds and minerals of known chemical structure. For bituminous coals, the potassiumcontaining combustion products have also been identified using a series of potassium-bearing glasses. For bituminous coals, the fate ofpotassium during the coking process and in blast furnace conditionslO~‘s*lh has recently been studied by Huffman et ul., with X-ray absorption spectroscopic and other techniques.
EXPERIMENTAL All potassium model compounds (acetate, benzoate, chloride, sulphate, etc.) used in this study were analytical reagent grade. The potassium-bearing minerals (illite, muscovite, biotite, orthoclase, leucite, sylvite, etc.) were loan samples from the New York Geological Survey and the New York State Museum at Albany, New York. The potassium aluminosilicate and sulphate glasses were laboratory synthesized. These, together with the coal samples, are listed in Table 1.
FUEL, 1986, Vol 65, March
327
Forms of potassium
in coal: C. 1. Spiro et al.
Table 1 Potassium samples employed in the study
_
Salts Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium
chloride sulphate zinc sulphate, K,Zn,(SO,), nitrate vanadate, K,,,,V,O, t-butoxide acetate oxalate thiocyanate benzoate sulphide superoxide sodium carbonate SO/SOeutectic metasilicate persulphate bicarbonate phthalimide hydroxide
Kentucky No. 9, hvA Bituminous Wyodak Subbituminous C Pittsburgh No. 8 floats from density beneticiation Pittsburgh No. 8 rejects from density beneliciation Acid leached lignite HF leached bituminous coal-Integrated Carbons, Inc. Ashes Bed material from PFB Combustor Ash from first cyclone, PFB Combustor Ash from second cyclone, PFB Combustor Ash from third cyclone, PFB Combustor Vapour phase deposits, on cooling pins, PFB Combustor Ash from Pittsburgh No. 8, 1ooo”C in air for 0.5 h Low-temperature ash, Pittsburgh No. 8 coal Low-temperature ash, Reading Anthracite Low-temperature ash, Navajo sub-C Low-temperature ash, Buelah lignite Low-temperature ash, Kentucky No. 9 Low-temperature ash, Oil agglomerated coal, Kentucky No. 9 Low-temperature ash, Winifrede, bottom of channel cut Low-temperature ash, Winifrede, middle of channel cut Low-temperature ash, Winifrede, top of channel cut Gasified Illinois No. 6 coal, isothermal packed bed reactor Graphite/KOH pyrolysate Sugar char/KOH pyrolysate Ash from Illinois No. 6 burn in turbine simulator; cooling pin probe, fuel filter, exhaust filter
Minerals Illite, Rochester, NY Illite, Fithian, IL Illite, heated to 400°C for 0.5 h Illite, heated to 6oQ”C for 0.5 h Illite, heated to SO&C for 0.5 h Illite, heated to 1OOOC for 0.5 h Illite, heated to 1200°C for 0.25 s Illite, heated to 1200°C for 1.Os Microcline feldspar, Olmsteadville, NY Orthoclase feldspar, Sing Sing, NY Muscovite mica Biotite mica Kaolinite Leucite feldspathoid Kalinite Montmorillonite Sylvite Dolomite (from PFB Combustor bed, unused) Glauconite
Miscellaneous
Coals Pittsburgh No. 8, hvA Bituminous Buelah Zap Lignite Cannel Coal, Paxeneses, PA Illinois No. 6, hvB Bituminous Navajo Subbituminous C Winifrede hvB Bituminous Reading Anthracite Oil agglomerated processed coal, Kentucky hvA-Otisca
Industries
Coal samples were generally mined by members of the research staff and stored in such a manner as to minimize degradation; i.e. in jars sealed under nitrogen, and in large lumps, sometimes under water where necessary. Table 2 gives analytical data on these coals, presented on an ‘as received’ basis. Analytical data should not be construed as representative of the mine’s output in general, but only of these specific lumps chosen for this study. Coal samples were ground to - 60 mesh (250 pm) and stored in air for a period of days in transit to Stanford Synchrotron Radiation Laboratory (SSRL). Deterioration of the coal was inevitable over this period of time, but the results described here indicate that the potassium bearing moieties are oxidatively stable. Processed coal samples in Table 1 include oil agglomerated Kentucky hvA coal provided by Otisca Industries, Syrcause NY, in which the agglomerating oil employed was freon 113. The HF leached bituminous coal was provided by Integrated Carbons, Inc, Tulsa, OK. Sink/float samples were obtained by ultracentrifugation in specific gravity 1.5 LiBr solution. The acid leached
328
FUEL,
1986,
Vol 65,
March
Glass, 25 mol% potassium sulphate in zinc sulphate Glass, 33 mol% potassium sulphate in zinc sulphate Glass, 42 mol% potassium sulphate in zinc sulphate Glass, 50 mol% potassium sulphate in zinc sulphate Glass, 58 mol% potassium sulphate in zinc sulphate Glass, 65 mol% potassium sulphate in zinc sulphate Glass, potassium titanium silicate Glass, potassium titanium aluminosilicate: K,O-8.33%; [email protected]%; TiO,-7.0%; SiO,-73.7x, by weight Glass, potassium titanium aluminosilicate: K,O-9.02%; A&O,-10.24%; TiO,-7.0%; SiO,-73.7x, by weight Glass, potassium titanium aluminosilicate: K,O-9.63%; [email protected]%; TiO,-7.0%; SiO,-73.7x, by weight Glass, potassium titanium aluminosilicate: K,O-9.85%; A&O,-9.41%; TiO,-7.0%; SiO,-73.7x, by weight Glass, potassium titanium aluminosilicate: K,O--11.23%; [email protected]%; TiO,-7.0%; SiO,-73.7x, by weight
lignite was prepared by packing ground lignite in a chromatography column, followed by exhaustive washings with 1N HCl and distilled water. Materials from the pressurized fluidized-bed (PFB) combustor are described in a later section and their analytical data are given in Table 3. All low temperature ashes were prepared by oxygen plasma treatment at z 120°C over a period of several days, or until weight loss was judged complete. Gasified Illinois No. 6 coali and char pyrolysates’s were prepared as described elsewhere. Materials from the small-scale gas turbine combustor were obtained by co-injecting 100 ppm micronized powder with a hot gas stream from General Electric’s 24 tpd fixed bed gasifier into a turbine combustor can. Particles experienced 7-10 atm pressures and peak temperatures of z 1900°C for 10ms. Deposits were collected from a 700°C air-cooled probe downstream, 50 ms after passing through the hot zone; at this time, the gas stream had cooled to 1100°C. Room temperature potassium K-edge spectra were obtained on beam line VII-3 wiggler side station at SSRL
Forms of potassium
in coal: C. 1. Spiro et al.
Table 2 Coal samples”
FM
Moisture
Ash
Volatile matter
Fixed carbon
(wt%)
(wt%)
(wt%)
(wt”/,)
Heating value (BTUlb-
49.1
12398
Coal
&%)
Pittsburgh No. 8 Lenhart Seam Heritage Resources, Inc. Pittsburgh, PA Lignite A Zap Mine, Beulah ND Cannel Coal Cannel Coal. Inc. Paxaneses, PA Illinois No. 6 hvB Island Steel Co. Sessor, IL Navajo Sub C Navajo Mine Fruitland, NM Winifrede hvB Sierra Coal Co. Lambric, KY Reading anthracite Pottsville, PA Kentucky No. 9 hvA Peabody Coal Co. Camp No. 2 Mine Wyodak Sub C Wyodak Resource Dev. Corp. Gillette. WY
68.3
4.8
1.3
0.1
2.9
2.1
43.5
3.0
0.8
0.0
0.7
31.1
8.1
45.2
15.6
6922
70.4
5.1
1.4
0.1
0.8
0.8
17.4
31.1
50.6
I2697
67.6
5.2
1.4
0.1
3.2
5.9
15.3
38.6
40.2
11284
57.0
4.3
1.3
0.0
0.6
11.7
15.0
39.9
9996
73.6
5.0
1.6
0.1
1.3
2.7
6.6
38.9
52.0
12870
78.3
2.1
0.6
0.0
0.3
3.7
13.7
4.3
78.3
11540
58.6
4.5
1.2
0.1
8.4
6.1
16.7
33.2
44.0
10981
50.7
4.0
0.7
0.0
0.5
24.8
7.2
36.0
32.1
8749
performed
by Commercial
’ All data presented
Table 3
Chemical
on ‘as received’ basis. Analysis
analysis
of bed and cyclone
Pfizer dolomite C
co* CaO MgO SiO, Al@, Fe,& K,O Na,O SO, Cl_
45.9 54.90 39.90 4.05 0.74 0.17 0.04 0.07 0.09 0.004
particulate
LTMT-285
Testing
and Engineering
Co., Chicago.
IL
h (wt%)
Bed material
First cyclone
Second
2.24 4.22 23.39 16.53 23.78 9.91 4.30 1.14 0.25 19.01
4.26 13.14 35.60 25.13 14.97 6.46 5.86 0.41 0.13 10.98 .OQ5
0.98 1.17 23.49 12.82 28.81 15.89 8.54 1.08 0.32 8.04 004
during a dedicated run of Stanford Positron Electron Accelerator Ring (SPEAR) at an electron energy of 3.0 GeV and an injection current of 75 mA. The beam was monochromatized with double Si(II1) crystals and a 1 mm entrance slit which yielded a resolution of approximately 0.5 eV22 at the potassium K-edge of 3607.4 eV19. Spectra of potassium in coal, coal-derived products and model compounds were measured using the fluorescence EXAFS technique2’. This technique monitors the potassium K, fluorescence intensity, which is proportional to the degree of absorption of the incident beam, and hence monitors the X-ray absorption spectrum. Spectral data were recorded in three energy regions about the potassium K-edge: - 100 to - 50 eV in 10 eV steps, -50 to +50 eV in 0.5 eV steps and 50 to 400 eV in 3 eV steps. A two second integration time was used for each data point and two scans were averaged for each of the coals and coal-derived specimens. This procedure yielded quality data for both pre-edge and post-edge EXAFS backgrounds for subsequent normalization of the XANES spectra. Calibrations of the
‘)
cyclone
Third cyclone 0.76 0.57 22.34 14.11 28.82 16.13 7.72 1.11 0.39 8.23 ,004
spectrometer were made periodically using a KC1 standard. A Stern-Heald type fluorescence detector was employed2’ with helium and nitrogen used as detector gases in the incident ion chamber and fluorescence detector respectively. The incident beam was detuned 50% to minimize harmonic contents. An all-helium path from the Be window to the sample was used to minimize absorption and scattering by air. Further details of the measurement procedure have been described elsewhere22. Spectral specimens were prepared by packing powdered samples into 6pm X-ray polypropylene envelopes sufficiently large that the X-ray beam impinged only on the sample. RESULTS AND DISCUSSION A list of selected potassium model compounds, minerals, coals, their combustion products, and potassium-bearing glasses employed in the present XANES investigation is given in Table 1. However, only spectra of the model compounds and minerals that are relevant to the
FUEL, 1986, Vol 65, March
329
Forms of potassium
in coal: C. L. Spiro et al.
identification of the forms of potassium in coal and coalderived products are presented in this paper. Experimental and normalized XANES
spectra
A typical raw experimental spectrum is given for the case of KC1 shown in Figure 1, plotted as I,/I, versus energy in eV, where IF and I, are the fluorescence and incident intensities respectively. To compare quantitatively various absorption features in the several potassium model compounds, minerals and coals, the experimental K-edge spectra were normalized as follows. The normalization procedure consisted of linearly fitting the pre-edge region in the range - 100 to - 10 eV and extrapolating just above the edge. The post-edge EXAFS background from 50 to 400 eV was determined by using a cubic-spline-fit procedure with three equal segments and extrapolating below the edge energy to determine the step jump at the edge. The zero ofenergy is taken with respect to the first inflection point ofKC1 in the derivative spectrum at 3609 eV, which marks the onset of photoejection of a Is electron from the K + ion in KCl. A derivative spectrum within + 70 eV of the K absorption .edge is obtained by drawing a curve through points given by: dA
A(E+ A)- A(E)
dE=
A
with A representing the energy step size of the absorption spectrum. A normalized XANES spectrum was obtained by subtracting the smooth pre-edge absorption from every point in the experimental spectrum shown in Figure 1 in the range + 70 eV and dividing by the step jump at E =0 eV. This spectrum is in excellent quantitative agreement with that reported by Parratt and Jossemz3. Further details of the normalization procedure has been described elsewhere24. Well resolved absorption features at 1.O,4.4,7.7,9.1 and 17.6 eV are recorded in the K-edge XANES spectrum of potassium in KCl. These absorption features are characteristic of K+ in a given coordination and ligand environment. While the interpretation of these features is complex, Satoko and Sugano2’ were able to match peak energies and intensities with a reasonable degree of I
I
I
I
I
I
I
I
I
58 I
t
.38
1
-100
I
I
I
-50
e
50
I
100
I
150
I
200
I
250
I
300
I
358
I 400
ENERGY (EU )
Figure 1 Experimental potassium K-edge absorption spectrum ofKC1 at room temperature plotted as IF/I, uersus energy. The zero ofenergy is taken with respect to the first peak in derivative spectrum shown in Figure Za at 3607.4 eV
330
FUEL, 1986, Vol 65, March
accuracy on theoretical grounds, using reasonable semiempirical parameters. Theoretical analyses are generally desirable, but only in highly symmetric systems are they practical. More important to this study is that the spectral features can be used to fingerprint unknown K+ sites in coal and its combustion products from spectra of model compounds of known chemical structure. Potassium-bearing minerals Figure 2 shows a series of normalized K-edge XANES spectra of K-bearing minerals. Illite represents a class of most abundantly occurring clay minerals26. It is structurally related to muscovite having an idealized formula of KA1,(A1Si30,,)(0H),, and consists of hexagonal network sheets of linked SiO, tetrahedra. An idealized formula for illite might be indicating that Ko.66Al,(Al,.,,Si,.33o~,)(OH),, 15-16x of the Si4+ in the tetrahedral sheets are replaced by A13+ compared with 25% in muscovite. Two of these sheets are placed together with vertices of the tetrahedra pointing inwards. These vertices are cross-linked by octahedral A13+ ions. The structure is a succession of tetrahedral-octahedral-tetrahedral or (t--t) sheets with K+ ions placed between them, occupying the large holes created by two opposite hexagonal rings in 12-fold coordination by oxygens at 2.77 to 2.86 A2’s2’ (see Figure 2b). The repeat distance between successive layers is 10 A. Considerable substitution into the octahedral site is observed. Overall charge balance is maintained by adjustment of potassium concentration. Thus, there is no single illite formula per se to describe the structural composition. The spectrum shown in Figure 2a is characteristic of K+ in 12-fold coordination by oxygens and exhibits a strong white line at 5.9 eV with a low energy shoulder at 2.6 eV, followed by a number of resolved features at 14.9, 20.0, 22.7 and 30.2 eV. As the potassium coordination decreases to 9 in orthoclase, KAlSi,O, (a feldspar)29, the white line decreases in intensity and shifts to higher energy as shown in Figure 2~. Orthoclase is monoclinic, C$, (C2/m), Z=4 and consists of a chain structure of four linked tetrahedra. The chains run parallel to the u-axis with alternate rings approximately perpendicular and parallel to the plane of the diagram shown in Figure 2d. One quarter of the atoms at the centres of tetrahedra are Al, the remainder Si. The chains are linked to each other by K+ lying on the reflection planes, fitting into cavities. The K + ion is coordinated by 9 oxygens, two each at 2.92, 2.95, 3.03 and 3.13 8, and one at 2.70.& In leucite, KAlSiO, potassium is 6-fold coordinated by oxygens. The observed white line decreases further in intensity to form a doublet, and the absorption features above 20eV become less prominent. The structure of leucite is similar to that of analcime, NaAlSi,O,. H,O and consists of a system of four- and six-membered rings of tetrahedra linked together to form a three-dimensional framework. The facile conversion of leucite into analcime by treatment with a salt solution of NaCl or NaHCO, substantiates the similarity ofthe tetrahedral frameworks in these two mineralsjO. Sylvite, the naturally occurring mineral KCl, has a spectrum identical to that shown in Figure 1 for pure KC1 in which the Kf ions are 6-fold coordinated by Cl- ions. In microcline, a triclinic form of KAlSi,O,, the K environment is very similar to that in orthoclase. The potassium K-edge spectrum can be
Forms of potassium
in coal: C. L. Spiro et al.
1.)
-
1.6
-
1.4
-
1.n
-
1.a
-
0.8
-
0.6
-
0.4
-
*.I!
-
(a
b-f2 91
b
rJ
/
0.
1
1
1
8.0
-
1.8
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1.8
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1.e
L
1.n 1.1
-
1.L
-
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@I
1.4
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1.8
-
1.0
-
1.0
0.8
-
1.4 0.8
0.6
-
0.6
0.4
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0.4
0.8
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o.i!
1
-20020
4000
Energy, w
1
1
1
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1
1
1 u i/G
-
1 I
-60-40
r
!
(0
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0.1 -.
l.
1
.
-60-40
-20020 Energy, eV
J
4060
Figure 2
Normalized potassium K-edge spectra of some potassium-bearing minerals: (a) illite Ko.66AI,(AI,,,,Ai,,,,0,,)(OH),; (c) Orthoclase KAlSi,O,; (e) leucite KA1Si20, and (f) sylvite KCl. The large full circles in (b) and (d) show the position and local coordination of the K ’ ion in illite2’ and orthoclase”, respectively
directly superimposed onto that of orthoclase shown in Figure 2c. Thus, the spectra are most diagnostic of the immediate coordination geometry about the atom of interest. The cross-linking Al atoms in the octahedral sheets of muscovite may be substituted by Mg and Fe forming the mineral biotite: K(MgFe)3(A1Si,0,0)(OH),. The environ-
ment of the K’ ions, however, remains essentially unperturbed from that in muscovite. As expected, the potassium K-edge spectra of both muscovite and biotite are almost indistinguishable from one another, showing a peak-for-peak match in the two spectra with only minor variations in intensity; only an absorption at 14.3 eV shows changes. For muscovite, this is a sharp band; for
FUEL, 1986, Vol 65, March
331
Forms of potassium
in coal: C. L. Spiro et al.
biotite and illite, it is less pronounced. In any event, the differences between the potassium spectra of these micas may be due to differences in the next nearest and further neighbours in the tetrahedral and octahedral sheets in these minerals. Nevertheless, these examples clearly demonstrate the power ofXANES to fingerprint the local atomic structure ofa given constituent atom in a material.
as a diagnostic probe for the determination of local geochemical metamorphism. No dramatic changes in the spectra along a channel cut were observed, however, though the top layer had a somewhat lowered intensity associated with the 14.3 eV band. . Based on a hydrothermal laboratory synthesis of a coal-like substance from lignin in the presence ofillite32 it has been speculated that illite catalyses the formation of coal. Without suggesting any specific mechanism, the fact that no potassium-bearing phyllosilicates were found in the lower rank coals, and the fact that the illite is ubiquitous in mid-rank coals, and is apparently modified in the highest rank coal, lends credence to the notion that the family of alkali aluminosilicates plays more than a passive role in coalilication. Certainly the acid/base behaviour and ion-exchange properties of phyllosilicates, their ability to reversibly absorb moisture and organic matter, the ability to undergo redox reactions, and their demonstrable catalytic properties also support this notion33. Also significant is that illites are the most finely divided mineral species in coa134. This is corroborated by the result obtained in this study that gravity beneficiation and selective agglomeration yield fractions which still show only an illite spectrum in the products. The existence of one K+ site in coal is contrasted with those for trace vanadium4s5 and titanium 35 for which at least two sites (or phases) exist in the coal.
Potassium in coal Figure 3 shows the potassium K-edge XANES spectra of four coals of various rank: Reading anthracite, Pittsburgh No. 8 HVA bituminous, Navajo subbituminous C coal and a cannel coal from Paxeneses, PA. In these and all untreated coals listed in Table 1, except the Wyodak subbutiminous coal from the Powder River Basin and the Beulah Zap lignite from North Dakota, the dominant form of potassium is in the layered aluminosilicate structure exemplified by illite. Note that only the Reading anthracite shows exactly the same feature at 14.3 eV observed in muscovite. It has been recently demonstrated 31 that correlation between illite crystallinity and coal rank is not good. From the X-ray absorption data it can be speculated that the coal’s organic metamorphism is tracked by the clay, whereby illite is converted to a muscovite structure. Muscovite is a reasonable product of illite metamorphism in the absence oforganic matter; possibly the 14.3 eV band could be used
I
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RECIDING fWTt!RACItE
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BITURINOUS
t
14 12' 10' oa06' 0.4 oz-
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Energy, eV Figure 3 Normalized potassium K-edge spectra of potassium Cannel coals. These spectra are identical to those of muscovite
332
FUEL, 1986, Vol 65, March
60
-60
-40
-20
0
20
40
60
Energy, eV impurities in Reading anthracite. Pittsburgh No. 8 bituminous, (for anthracite) and illite (all others) as discussed in the text
Navajo
sub C and
Forms
The lower rank coals yielded potassium spectra (Figure did not correspond to any of the model compounds measured. Though it is generally supposed’ that alkali metals in lower rank coals are bound in organic carboxylate groups, the spectra shown in Figure 4 bear no resemblance to the spectra of potassium acetate, benzoate or oxalate, nor to phthalimide or t-butoxide species, as well, shown in Figure 5. Leaching with HCl lowered the concentration of alkali in lignite, but the residue still showed an XANES spectrum unaltered from the raw coal other than in absolute intensity. HCl/HF leaching of a bituminous coal left residual potassium which also yielded the same spectrum as that found in the lower rank coals. One possibility is that the potassium is bonded to the surface ofa clay rather than inside a layered structure. Second, it may simply be adsorbed on a wide range of surfaces including mineral and organic, and features associated with the edge in model compounds are averaged. It is also possible that potassium is dissolved in water-filled pores or that the right model compounds have simply not yet been identified. 4) which
Potassium
in coal-derived
products
In addition to the study of indigenous potassium impurities in coal, several ash and char samples including low temperature ashes from illite-bearing coals, laboratory pyrolysates and gasification ashes were examined. In particular the fate of potassium in the Pittsburgh bituminous coal during a pressurized fluidized bed combustion cycle has been traced systematically by measuring the potassium spectrum in various residues from a demonstration unit. Several heat-treated illite samples prepared under various conditions were also measured as controls. For all low-temperature ashes prepared by an rf-generated oxygen plasma at 100°C the XANES spectra indicate that the potassium-bearing products were virtually indistinguishable from the original mineral species, i.e. illite. This is reasonable in that illite may only lose surface moisture at these temperatures (lOO”C), and other than anhydride formation by conversion of two surface hydroxyl groups into an 0x0 unit plus water36 at ~600°C. Major structural reorganization does not take place in illite until Z 1ooo”c.
1
I
I
I
I
I
1
1.4 1.2 1.0 s s ii 0.8 4: B 0.6 zm t g 0.4 I
ENERGY (EV)
Figure 4 Normalized potassium K-edge spectra of K impurities in a North Dakota Lignite (---) and Wyodak sub C young (---) coal
of potassium
in coal:
C. L. Spiro
et al.
In general, the products of combustion showed little variation among the resulting spectra. Figure 6 is typical of the POC spectra obtained. Initially, the raw Pittsburgh No. 8 coal (Figure 3b) was pulverized and combined with the dolomite bed material, and fluidized under 10 atmospheres pressure. The average residence time of the coal in the bed is 60 min, after which the ash either remains in the bed at 915°C or is entrained into the hot gas stream. The bed material is replaced every hour. The hot gas stream at 860°C passes through a series of three cyclones, each removing liner and finer products. The exit temperature ofcyclone 3 is 830 ‘C. The hot gas then passes through a series ofair cooled pins held at 610°C. Deposits were collected from the pins and the corresponding potassium spectrum taken. Compositional data on the deposits appears in Table 3. The primary variation among these spectra occurs in the relative intensity of two peaks constituting a doublet at 3.2 and 6.5 eV. The broad absorption at _ 34 eV did not vary significantly. More specifically, the spectra ofthe bed material, the second and third cyclones, and the pin scrapings down-stream are all virtually superimposable. More significant is that the spectra of these combustion products are also identical to the spectrum of illite heated by itselfto 1000 ‘C for 0.5 h (see Figure 6). This implies that the gaseous atmosphere, the presence of impurities such as sulphur and nitrogen, and even the carbon substrate do not dramatically impact the thermal transformations of illite in coal. The results suggest that the effect of potassium on deposition during PFB combustion can be studied to a reasonable degree ofaccuracy using pure illite rather than coal. Although the importance of alkali sulphates in the formation of fouling deposits during combustion is well known’, there was no evidence of potassium sulphates in the current deposits. To understand the nature of the coal-derived high temperature phases, the phase diagram of the pseudoternary system (Al,O,-SiO,-base) was developed by Huggins et ~1.~‘. In their scheme, an average basicity based on the combined alkali, alkaline earth and iron content is presumed which satisfactorily accounts for compositional effects on ash fusion temperature in coal. For pure illitic clays, Bohor 36 found that potassium had the dominant repressive effect on the formation of high temperature phases. A useful exercise is to consider Bohor’s model of illite decomposition in light of Huggins’ proposed phase diagram. For example, a typical illite composition might be K1,,Si6,~A11,,(Al,FeMg1,J O,,(OH),. Between 100 and 200°C surface moisture is desorbed, and between 475 and 600°C anhydride formation takes place, splitting off two water molecules from the four hydroxide units and leaving two 0x0 bridges on the surface of the tetrahedral-octahedral-tetrahedral aluminosilicate sandwich. Depending on the per cent base content, somewhere between 850 and 1000°C decomposition of the t-o-t system takes place, with the octahedral sites, Al,FeMg,,,O, in the example, possibly staying intact in the form of spinel. The remaining group, K,,,Si,,SAl,,50,6 partially melts, and ifquenched, would be expected to form a glass. Indeed, subsequent to experimental tenure at SSRL, a material of this composition prepared at 1730°C was confirmed to be amorphous by X-ray diffraction. At higher temperatures, the spine1 can redissolve into the melt, and phases such as mullite and cristobalite may form while gaseous alkali species can be released.
FUEL, 1986, Vol 65, March
333
Forms of potassium
(a)
::: 1.4
in coal: C. L. Spiro et al.
-
1.4 -
1.8 1.0
(W
1.8 I.8 -
1.8 -
-
1.0 -
0.8 -
0.8 -
0.6 -
0.6 -
0.4 -
0.4 -
0.8 -
e.1 -
0.
I
I
I
I
0.
8.0
-
8.8
-
1.0 -
I
I
I
I
I
I
I
I
I
0 20 40 Energy, eV
60
I
(a
1.4 1.8 1.0 0.8 0.8 0.4 0.8 0.
I
(e)
::: 1.4 I.8
1.8 -
-
1.0 -
1.0 -
0.8 -
0.8 -
0.8
-
0.8 -
a.4
-
8.4 @.I
0.8 0.
I
I
-60-40-20
I
1
I
0
20
1
I
40
60
Energy, eV Figure 5 sulphate
,
-60-40-20
,
Normalized potassium K-edge spectra ofa series of potassium salts: (a) acetate, (b) benzoate, (c) oxalate, (d) phthalimide, (e) t-butoxide, and (f)
This indicates that a glass would be expected based on the Huggins’ composition model. Several potassiumbearing glasses were synthesized’ and their spectra examined. In general, all the glasses tested in Table 1 exhibit a similar doublet feature at z 3.5 and 7.0 eV, and a broad absorption at 34 eV as those of the coah ash; but none corresponds precisely. The model glasses that best correspond to the coal-derived combustion products are the potassium titanium aluminosilicate glasses which showed little variation with composition.
334
-
0. -
FUEL, 1986, Vol 65, March
Clearly, most of the spectral features are obtained in these simulations, but the intensities do not correspond precisely to the unknown, and a more relined series of model compounds is required to assign all the transitions observed in the spectra. Even lacking a precise match at this stage, the form of potassium can be confidently assigned to a glass based on the spectral features, and based on the stoichiometry of the starting material, illite, and its thermochemistry at elevated temperatures. According to Huggins’ phase diagram approach, the
Forms of potassium
1.2 1
8.4 e.2 e. -20
I
I
1
I 20
0
I
1 40
Energy, eV
Figure 6 Comparison of cyclone deposit from PFB coal combustion (---) and illite heated to 1OOOC by itself(- - - - -) indicates spectator role played by sulphur and organics in illite thermochemistry
in coal: C. L. Spiro
et al.
between the layers of the aluminosilicate structure. The highest rank coal, anthracite, showed potassium in muscovite, a probable product of illite metamorphism. The lowest rank coal, lignite, showed potassium most likely in a non-unique environment, possibly as a of organic and mineral sorbates. combination Combustion products resulted in a potassium-bearing aluminosilicate glass whose XANES features are identical to those obtained by pyrolysis of pure illite at 1000°C. The results obtained in this study demonstrate the usefulness, and perhaps uniqueness, of the X-ray absorption technique using synchrotron radiation as a light source to probe the structure and bonding of trace constituents in coal and coal-derived products. Material characterization at an atomic level of such trace constituents is much needed to elucidate reaction mechanisms* in order to design more efhcient and cleaner use of coal as an energy source. ACKNOWLEDGEMENTS
product phase is assigned as a potassium-bearing aluminosilicate glass. This assignment is further borne out in Figure 6, comparing illite heated by itself (with no organics or sulphur present) with the cyclone products from the PFB combustion. The spectra are virtually superimposable. In a parallel programme to study coal-derived deposits, with synchrotron radiation, Huflinan et ~1.~~have made a preliminary identification of potassium bisulphate, KHSO,, as a component of a coal-fired boiler deposit, and potassium sulphate, K,SO,, as a component of a combustion rig deposit. In both cases the coal being fired was a North Dakota lignite. The potassium bisulphate spectrum appears similar to the potassium aluminosilicate glass spectra reported in this paper. However, the near perfect match between illite heated by itself and combusted coal is more consistent with the aluminosilicate glass assignment of the deposits. To further test this assignment, the deposits were leached for soluble alkali species, such as sulphate forms. The results show that of the total alkali in the deposit, only 0.3% was soluble, thus strengthening the conclusion. The discrepancy between the conclusions here and those of Huffman et al. most probably arises from the differences in reaction conditions and in the type of coal being burned, rather than from the data analysis. Indeed, Huffman et ~1.’ “I6 have also identified the potassium-bearing phase in coke samples prepared at 1050 and 1100°C as an aluminosilicate glass.
The US Department of Energy is acknowledged for supporting the Stanford Linear Accelerator Center and the Stanford Synchrotron Radiation Laboratory. The Staff at SSRL is also acknowledged for their considerable help during the tenure of the experiments and for their continued progress in providing and maintaining the SSRL facility. The authors greatly appreciate generous loans of samples from the New York State Geological Survey and to William Kelly, curator of the New York State Museum mineral collection, and to Jim Dickinson and Paul Hess, Geology Department, Brown University. Gordon Brown, Stanford University, is acknowledged for the use of his laboratory facilities and for helpful advice and G. P. Huffman of US Steel for reviewing the data and offering helpful suggestions. Finally the authors acknowledge research staff at General Electric including John Blanton, Russ McCarron and Robin Brobst and Ben Glover for providing combustion samples. REFERENCES I 2
3 4 5
CONCLUSION With intense and well-collimated synchrotron radiation, a high resolution X-ray absorption technique was used to investigate the chemical structure of potassium impurities in a variety ofcoals ofvarious ranks and their combustion products. A series of potassium-bearing inorganic and organic salts, minerals and their ashes, glasses and bronzes were also measured to aid identification of the unknown potassium sites in coal and combustion products. The model systems studied are given in Table I. For all coals (except the youngest coals from the western US) examined here, the dominant form of potassium was in the illite-like phase in which K+ ions are 1Zfold coordinated by oxygens and are situated at large cavities
6
Bryers, R. W. ‘Ash Deposits and Corrosion Due to Impurities in Combustion Gases’, Hemisphere Publishing Corp., Washington, 1978 Gluskoter, H. J. ‘Trace Elements in Coal: Occurrence and Distribution Circ. 499, Illinois State Geological Survey, Urbana, IL, 1977 Karr, C. Jr. (Ed.) ‘Analytical Methods for Coal and Coal Products’, Vol. 1-3, Academic Press, New York, 1978 Maylotte, D. H., Wong, Jr., St. Peters, R. L., Lytle, F. W. and Greegor, R. B. Science 1981, 214, 554 Wong, J., Maylotte, D. H., St. Peters, R. L., Lytle, F. W. and Greegor, R. B. ‘Process Mineralogy II: Proc. of the Metallurg. Sot. of AIME, 1982, p. 335 Sandstrom, D. R., Filby, R. H.. Lytle, F. W. and Greegor, R. B. Fuel 1982, 61, 195
* An example of a reaction mechanism under study by synchrotron radiation is the potassium catalysed gasitication of coal. Graphite (type UCP, - 325 mesh, Ultra Carbon, Inc.) and sugar charcoal were milled with 10% potassium carbonate in a Spex, Inc. shatterbox, and pyrolysed at 800°C for 2 h, below the melting point of potassium carbonate. The samples were then cooled to room temperature and stored in air for several weeks. The suaar char revealed exclusively potassium bicarbonate as the end product, while the graphite pyrolysate showed a good match for a linear combination of28.8% potassium carbonate and 71.2% potassium bicarbonate. A reasonable path for this process is the carbothennic reduction of the alkali carbonate to metallic potassium at elevated temperatures ‘* followed by facile reaction with atmospheric water vapour and carbon dioxide at room temperature
FUEL, 1986, Vol 65, March
335
Forms of potassium
10 11 12
13
14
15
-16 17
18 19 20
336
in coal: C. L. Spiro et al.
Spiro, C. L., Wong, J., Lytle, F. W., Greegor, R. B., Maylotte, D. H. and Lamson, S. H. Science 1984,2x,48 Lytle, F. W., Sayer, D. E. and Stern, E. A. Phys. Rev. B 1975, B15, 2426 Wong, J. ‘EXAFS Studies of Metallic Glasses’, chapter in ‘Topics in Applied Physics’, Vol. 46, ‘Glassy Metals I’ (Ed. H. J. Gunther and H. Beck), Springer-Verlag Publishers, Berlin, 1981,pp. 45-77 Huffman, G. P., Huggins, F. E., Greegor, R. B. and Lytle, F. W. ‘SSRL Report’ 84101, 1984, pp. 1X-31 Dickinson, J. E. and Hess, P. C. Geochim. Cosmo. Acta 1984, in press Greegor, R. B., Lytle, F. W., Dickinson, J. E. and Hess, P. C. ‘EXAFS/XANES Investigation of K and Ti Sites in Alkali Aluminosilicate Glasses’, 1lth SSRL Users’ Meeting, October 1984 Wong, J., Spiro, C. L., Maylotte, D. H., Lytle, F. W. and Greegor, R. B. ‘EXAFS and Near Edge Structure III’(Ed. K. 0. Hodgson, B. Hedman and J. E. Penner-Hahn), Springer-Verlag, BerlinHeidelberg, 1984, pp. 362-367 Spiro, C. L., Wong, J., Lytle, F. W., Greegor, R. B., Maylotte, D. H., Lamson, S. H. and Glover, B. L. ‘EXAFS and Near Edge Structure III’(Eds. K. 0. Hodgson, B. Hedman and J. E. PennerHahn), Springer-Verlag, Berlin-Heidelberg, 1984, pp. 361370 Huffman, G. P., Huggins, F. E., Cuddy, L. J., Schoenberger, R. W., Lytle, F. W. and Greegor, R. B. ‘EXAFS and Near Edge Structure III, Progress in Physics, Springer-Verlag, Vol. 2, 1984, pp. 371-373 HufIinan, G. P., Huggins, F. E., Schoenberger, R. W., Walker, J. S., Lytle, F. W. and Greegor, R. B. Fuel, submitted for publication Kosky, P. G., Lamby, E. J., Maylotte, D. H., McKee, D. W. and Spiro, C. L. ‘Coal Gasilication Catalysis Mechanisms’, Report DOE/MC/14591-1397, 1982 McKee, D. W., Spiro, C. L., Kosky, R. G. and Lamby, E. J. Fuel 1983, 62, 217 Bearden, J. A. and Burr, A. F. Rev. Mod. Phys. 1967, 39, 125 Jaklevic, J., Kirby, J. A., Klein, M. P., Robertson, A. S., Broan, G. S. and Eisenberger, P. Solid State Comm. 1977, 23, 619
FUEL,
1986,
Vol 65, March
21 22 23 24 25 26
27
28 29 30 31 32
33 34 35
36 37 38
Stern, E. A. and Heald, S. M. Rev. Sci. Instrum. 1979, 50, 1579 Lytle, F. W., Greegor, R. B., Sandstrom, D. R., Marques, E. C., Wong, J., Spiro, C. L., Huffman, G. P. and Huggins, F. E. Nucf. Instrum. Methods 1984, 226, 542 Parratt, L. G. and Jossem, E. L. Phys. Rev. 1955, 97, 916 Wong, J., Lytle, F. W., Messmer, R. P. and Maylotte, D. H. Phys. Rev. 1984, B30, 5596 Satoko, C. and Sugano, S. J. Phys. Sot. Japan 1973,3, 701 Murray, H. H. ‘Industrial Minerals and Rocks’, Seeley W. Mudd Series, 3rd Ed, Am. Institute Mining and Metallurgy, Petroleum Engineers 1960, Chapter 12,~. 259 Bragg, L., Claringbull, G. F. and Taylor, W. H. ‘Crystal Structures of Minerals’, Cornell University Press, Ithaca, NY, 1965, p. 254 Wychoff, R. W. G. Crystal Structure’, Wiley, 2nd Ed., 1968, Vol. 4, p. 346 Stern, E. A. and Heald, S. M. Rev. Sci. Instrum. 1979, 50, 310 Stern, E. A. and Heald, S. M. Rev. Sci. Instrum. 1979, 50, 345 Kisch, H. J. Eclogae Geol. Helv. 1980, 73(3), 753 Hayatsu, R., McBeth, R. L., Scott, R. G., Botto, R. E. and Winans, Study: Preparation and R. E. ‘Artificial Coalilication Characterization of Synthetic Macerals’, presented at the International Meeting on Organic Geochemistry, The Hague, 1983; Adv. Org. Geochem. 1984, 6 Grim, R. E. ‘Clay Mineralogy’, McGraw-Hill, New York, 1953 Palmer, C. A. and Filby, R. H. Fuel 1984,63(3), 318 Wong, J., Maylotte, D. H., Lytle, F. W., Greegor, R. B. and St. Peters, R. L. ‘EXAFS and Near-Edge Structure’, Springer Series in Chemical Physics, 1983, Vol. 27, p. 206 Bohor, B. F. Doctoral Dissertation, University of Illinois, 1959, also Bohor, B. F. Clays and Clay Minerals 1964, 19, 233 Huggins, F. E., Kosmack, D.A. and Huffman, G. P. Fuel 1981,60, 577 Huffinan, G. P., Huggins, F. E., Greegor, R. B. and Lytle, F. W. Observations of Potassium Sulfates in Fouling Deposits’, manuscript in preparation
F. W. Lytle*,
R. B. Greegor”,
D. H. Maylotte
General Electric Corporate Research and Development, PO Box 8, Schenectady, 12301, USA * The Boeing Company, PO Box 3999, 2T-05, Seattle, WA 98124, USA (Received 22 April 1985; revised 18 July 1985)
and NY
High resolution X-ray absorption spectroscopy utilizing synchrotron radiation as a light source was used to probe the bonding and structure ofpotassium in coal and coal-derived products. The potassium sites in coals ofvarious ranks and in the products ofthermochemical processing were identified by comparison ofunknown spectra with those of a wide range of selected potassium model compounds and minerals. For most coals examined, potassium was found to occur in a site in illite, a layered clay related to muscovite. Such coals included subbutuminous, high and low volatile bituminous, cannel and anthracite types. The lower rank subbituminous and lignite coals show other forms of alkali, where potassium appears to be in a noncrystalline environment. Examination ofthe near-edge spectra fails to reveal similarities between potassium in low-rank coal and in any organically bonded potassium model compounds such as carboxylic, phenolic, benzoic, phthalimide or intercalated groups. It is possible that the exchangeable potassium is still inorganically associated on disordered clay surfaces, though this remains equivocal. Thermal treatment of potassium-bearing phases inherent to coals generally leads to formation of a potassium aluminosilicate glass. Samples include those obtained from flash pyrolysis, gasification, laboratory combustion and a large scale pressurized fluidized-bed combustor including bed material, scrubbing hot cyclones, and quench-cooled pins for vapour phase deposition. No potassium sulphates were observed in the deposits. (Keywords:coal;
potassium; combustion products; X-ray
absorption spectroscopy)
Potassium is a particularly pernicious impurity in coal’. The alkali metals are inherently corrosive toward hot
metal surfaces. They form low melting phases with iron and sulphate moieties. In addition, potassium fluxes other normally refractory materials which leads to extensive deposits in combustors. These fused deposits can intercept non-fluxing materials, further exacerbating deposition phenomena and corresponding corrosion. In an engine, fused sub-micron particulates may add to erosion or abrasion of contact surfaces. To remove potassium at the outset, or to somehow intercept potassium prior to the deposition/fusion/ corrosion cascade, an understanding of its inherent chemical environment is a desirable prerequisite. By identifying the fate of potassium during processing and utilization, key intermediate phases might be identified and circumvented or redirected by varying conditions or through the addition of chemical agents. Impurities, such as potassium, in coal are not readily amenable to study by conventional techniques such as chemical, microscopic, thermal d&action and spectroscopic techniques2. These techniques have been applied to coal3 with considerable aggregate success, but for trace elements present in concentrations of 1000 ppm or less in a matrix which is spectroscopically opaque over a wide range of wavelengths, and in non-crystalline, disordered, or non-unique sites, the results are often equivocal. High resolution X-ray absorption near-edge structure (XANES) measurements using well collimated and intense synchrotron radiation sources provide a 0016-2361/86/030327-10$3.00 ~3 1986 Butterworth & Co. (Publishers) Ltd.
novel method for characterizing the bonding and local atomic structure of trace elements in coa14-‘. The technique is atom selective8m12,has good sensitivity for trace constituents as low as 1OOppm in concentration4, and has been effectively applied to the analysis of coal trace elements4~5~7~13-16. In this study, the bonding and structure of potassium impurities in a number of coals of various rank have been determined with XANES using a selected range of potassium model compounds and minerals of known chemical structure. For bituminous coals, the potassiumcontaining combustion products have also been identified using a series of potassium-bearing glasses. For bituminous coals, the fate ofpotassium during the coking process and in blast furnace conditionslO~‘s*lh has recently been studied by Huffman et ul., with X-ray absorption spectroscopic and other techniques.
EXPERIMENTAL All potassium model compounds (acetate, benzoate, chloride, sulphate, etc.) used in this study were analytical reagent grade. The potassium-bearing minerals (illite, muscovite, biotite, orthoclase, leucite, sylvite, etc.) were loan samples from the New York Geological Survey and the New York State Museum at Albany, New York. The potassium aluminosilicate and sulphate glasses were laboratory synthesized. These, together with the coal samples, are listed in Table 1.
FUEL, 1986, Vol 65, March
327
Forms of potassium
in coal: C. 1. Spiro et al.
Table 1 Potassium samples employed in the study
_
Salts Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium Potassium
chloride sulphate zinc sulphate, K,Zn,(SO,), nitrate vanadate, K,,,,V,O, t-butoxide acetate oxalate thiocyanate benzoate sulphide superoxide sodium carbonate SO/SOeutectic metasilicate persulphate bicarbonate phthalimide hydroxide
Kentucky No. 9, hvA Bituminous Wyodak Subbituminous C Pittsburgh No. 8 floats from density beneticiation Pittsburgh No. 8 rejects from density beneliciation Acid leached lignite HF leached bituminous coal-Integrated Carbons, Inc. Ashes Bed material from PFB Combustor Ash from first cyclone, PFB Combustor Ash from second cyclone, PFB Combustor Ash from third cyclone, PFB Combustor Vapour phase deposits, on cooling pins, PFB Combustor Ash from Pittsburgh No. 8, 1ooo”C in air for 0.5 h Low-temperature ash, Pittsburgh No. 8 coal Low-temperature ash, Reading Anthracite Low-temperature ash, Navajo sub-C Low-temperature ash, Buelah lignite Low-temperature ash, Kentucky No. 9 Low-temperature ash, Oil agglomerated coal, Kentucky No. 9 Low-temperature ash, Winifrede, bottom of channel cut Low-temperature ash, Winifrede, middle of channel cut Low-temperature ash, Winifrede, top of channel cut Gasified Illinois No. 6 coal, isothermal packed bed reactor Graphite/KOH pyrolysate Sugar char/KOH pyrolysate Ash from Illinois No. 6 burn in turbine simulator; cooling pin probe, fuel filter, exhaust filter
Minerals Illite, Rochester, NY Illite, Fithian, IL Illite, heated to 400°C for 0.5 h Illite, heated to 6oQ”C for 0.5 h Illite, heated to SO&C for 0.5 h Illite, heated to 1OOOC for 0.5 h Illite, heated to 1200°C for 0.25 s Illite, heated to 1200°C for 1.Os Microcline feldspar, Olmsteadville, NY Orthoclase feldspar, Sing Sing, NY Muscovite mica Biotite mica Kaolinite Leucite feldspathoid Kalinite Montmorillonite Sylvite Dolomite (from PFB Combustor bed, unused) Glauconite
Miscellaneous
Coals Pittsburgh No. 8, hvA Bituminous Buelah Zap Lignite Cannel Coal, Paxeneses, PA Illinois No. 6, hvB Bituminous Navajo Subbituminous C Winifrede hvB Bituminous Reading Anthracite Oil agglomerated processed coal, Kentucky hvA-Otisca
Industries
Coal samples were generally mined by members of the research staff and stored in such a manner as to minimize degradation; i.e. in jars sealed under nitrogen, and in large lumps, sometimes under water where necessary. Table 2 gives analytical data on these coals, presented on an ‘as received’ basis. Analytical data should not be construed as representative of the mine’s output in general, but only of these specific lumps chosen for this study. Coal samples were ground to - 60 mesh (250 pm) and stored in air for a period of days in transit to Stanford Synchrotron Radiation Laboratory (SSRL). Deterioration of the coal was inevitable over this period of time, but the results described here indicate that the potassium bearing moieties are oxidatively stable. Processed coal samples in Table 1 include oil agglomerated Kentucky hvA coal provided by Otisca Industries, Syrcause NY, in which the agglomerating oil employed was freon 113. The HF leached bituminous coal was provided by Integrated Carbons, Inc, Tulsa, OK. Sink/float samples were obtained by ultracentrifugation in specific gravity 1.5 LiBr solution. The acid leached
328
FUEL,
1986,
Vol 65,
March
Glass, 25 mol% potassium sulphate in zinc sulphate Glass, 33 mol% potassium sulphate in zinc sulphate Glass, 42 mol% potassium sulphate in zinc sulphate Glass, 50 mol% potassium sulphate in zinc sulphate Glass, 58 mol% potassium sulphate in zinc sulphate Glass, 65 mol% potassium sulphate in zinc sulphate Glass, potassium titanium silicate Glass, potassium titanium aluminosilicate: K,O-8.33%; [email protected]%; TiO,-7.0%; SiO,-73.7x, by weight Glass, potassium titanium aluminosilicate: K,O-9.02%; A&O,-10.24%; TiO,-7.0%; SiO,-73.7x, by weight Glass, potassium titanium aluminosilicate: K,O-9.63%; [email protected]%; TiO,-7.0%; SiO,-73.7x, by weight Glass, potassium titanium aluminosilicate: K,O-9.85%; A&O,-9.41%; TiO,-7.0%; SiO,-73.7x, by weight Glass, potassium titanium aluminosilicate: K,O--11.23%; [email protected]%; TiO,-7.0%; SiO,-73.7x, by weight
lignite was prepared by packing ground lignite in a chromatography column, followed by exhaustive washings with 1N HCl and distilled water. Materials from the pressurized fluidized-bed (PFB) combustor are described in a later section and their analytical data are given in Table 3. All low temperature ashes were prepared by oxygen plasma treatment at z 120°C over a period of several days, or until weight loss was judged complete. Gasified Illinois No. 6 coali and char pyrolysates’s were prepared as described elsewhere. Materials from the small-scale gas turbine combustor were obtained by co-injecting 100 ppm micronized powder with a hot gas stream from General Electric’s 24 tpd fixed bed gasifier into a turbine combustor can. Particles experienced 7-10 atm pressures and peak temperatures of z 1900°C for 10ms. Deposits were collected from a 700°C air-cooled probe downstream, 50 ms after passing through the hot zone; at this time, the gas stream had cooled to 1100°C. Room temperature potassium K-edge spectra were obtained on beam line VII-3 wiggler side station at SSRL
Forms of potassium
in coal: C. 1. Spiro et al.
Table 2 Coal samples”
FM
Moisture
Ash
Volatile matter
Fixed carbon
(wt%)
(wt%)
(wt%)
(wt”/,)
Heating value (BTUlb-
49.1
12398
Coal
&%)
Pittsburgh No. 8 Lenhart Seam Heritage Resources, Inc. Pittsburgh, PA Lignite A Zap Mine, Beulah ND Cannel Coal Cannel Coal. Inc. Paxaneses, PA Illinois No. 6 hvB Island Steel Co. Sessor, IL Navajo Sub C Navajo Mine Fruitland, NM Winifrede hvB Sierra Coal Co. Lambric, KY Reading anthracite Pottsville, PA Kentucky No. 9 hvA Peabody Coal Co. Camp No. 2 Mine Wyodak Sub C Wyodak Resource Dev. Corp. Gillette. WY
68.3
4.8
1.3
0.1
2.9
2.1
43.5
3.0
0.8
0.0
0.7
31.1
8.1
45.2
15.6
6922
70.4
5.1
1.4
0.1
0.8
0.8
17.4
31.1
50.6
I2697
67.6
5.2
1.4
0.1
3.2
5.9
15.3
38.6
40.2
11284
57.0
4.3
1.3
0.0
0.6
11.7
15.0
39.9
9996
73.6
5.0
1.6
0.1
1.3
2.7
6.6
38.9
52.0
12870
78.3
2.1
0.6
0.0
0.3
3.7
13.7
4.3
78.3
11540
58.6
4.5
1.2
0.1
8.4
6.1
16.7
33.2
44.0
10981
50.7
4.0
0.7
0.0
0.5
24.8
7.2
36.0
32.1
8749
performed
by Commercial
’ All data presented
Table 3
Chemical
on ‘as received’ basis. Analysis
analysis
of bed and cyclone
Pfizer dolomite C
co* CaO MgO SiO, Al@, Fe,& K,O Na,O SO, Cl_
45.9 54.90 39.90 4.05 0.74 0.17 0.04 0.07 0.09 0.004
particulate
LTMT-285
Testing
and Engineering
Co., Chicago.
IL
h (wt%)
Bed material
First cyclone
Second
2.24 4.22 23.39 16.53 23.78 9.91 4.30 1.14 0.25 19.01
4.26 13.14 35.60 25.13 14.97 6.46 5.86 0.41 0.13 10.98 .OQ5
0.98 1.17 23.49 12.82 28.81 15.89 8.54 1.08 0.32 8.04 004
during a dedicated run of Stanford Positron Electron Accelerator Ring (SPEAR) at an electron energy of 3.0 GeV and an injection current of 75 mA. The beam was monochromatized with double Si(II1) crystals and a 1 mm entrance slit which yielded a resolution of approximately 0.5 eV22 at the potassium K-edge of 3607.4 eV19. Spectra of potassium in coal, coal-derived products and model compounds were measured using the fluorescence EXAFS technique2’. This technique monitors the potassium K, fluorescence intensity, which is proportional to the degree of absorption of the incident beam, and hence monitors the X-ray absorption spectrum. Spectral data were recorded in three energy regions about the potassium K-edge: - 100 to - 50 eV in 10 eV steps, -50 to +50 eV in 0.5 eV steps and 50 to 400 eV in 3 eV steps. A two second integration time was used for each data point and two scans were averaged for each of the coals and coal-derived specimens. This procedure yielded quality data for both pre-edge and post-edge EXAFS backgrounds for subsequent normalization of the XANES spectra. Calibrations of the
‘)
cyclone
Third cyclone 0.76 0.57 22.34 14.11 28.82 16.13 7.72 1.11 0.39 8.23 ,004
spectrometer were made periodically using a KC1 standard. A Stern-Heald type fluorescence detector was employed2’ with helium and nitrogen used as detector gases in the incident ion chamber and fluorescence detector respectively. The incident beam was detuned 50% to minimize harmonic contents. An all-helium path from the Be window to the sample was used to minimize absorption and scattering by air. Further details of the measurement procedure have been described elsewhere22. Spectral specimens were prepared by packing powdered samples into 6pm X-ray polypropylene envelopes sufficiently large that the X-ray beam impinged only on the sample. RESULTS AND DISCUSSION A list of selected potassium model compounds, minerals, coals, their combustion products, and potassium-bearing glasses employed in the present XANES investigation is given in Table 1. However, only spectra of the model compounds and minerals that are relevant to the
FUEL, 1986, Vol 65, March
329
Forms of potassium
in coal: C. L. Spiro et al.
identification of the forms of potassium in coal and coalderived products are presented in this paper. Experimental and normalized XANES
spectra
A typical raw experimental spectrum is given for the case of KC1 shown in Figure 1, plotted as I,/I, versus energy in eV, where IF and I, are the fluorescence and incident intensities respectively. To compare quantitatively various absorption features in the several potassium model compounds, minerals and coals, the experimental K-edge spectra were normalized as follows. The normalization procedure consisted of linearly fitting the pre-edge region in the range - 100 to - 10 eV and extrapolating just above the edge. The post-edge EXAFS background from 50 to 400 eV was determined by using a cubic-spline-fit procedure with three equal segments and extrapolating below the edge energy to determine the step jump at the edge. The zero ofenergy is taken with respect to the first inflection point ofKC1 in the derivative spectrum at 3609 eV, which marks the onset of photoejection of a Is electron from the K + ion in KCl. A derivative spectrum within + 70 eV of the K absorption .edge is obtained by drawing a curve through points given by: dA
A(E+ A)- A(E)
dE=
A
with A representing the energy step size of the absorption spectrum. A normalized XANES spectrum was obtained by subtracting the smooth pre-edge absorption from every point in the experimental spectrum shown in Figure 1 in the range + 70 eV and dividing by the step jump at E =0 eV. This spectrum is in excellent quantitative agreement with that reported by Parratt and Jossemz3. Further details of the normalization procedure has been described elsewhere24. Well resolved absorption features at 1.O,4.4,7.7,9.1 and 17.6 eV are recorded in the K-edge XANES spectrum of potassium in KCl. These absorption features are characteristic of K+ in a given coordination and ligand environment. While the interpretation of these features is complex, Satoko and Sugano2’ were able to match peak energies and intensities with a reasonable degree of I
I
I
I
I
I
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I
I
58 I
t
.38
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I
-50
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50
I
100
I
150
I
200
I
250
I
300
I
358
I 400
ENERGY (EU )
Figure 1 Experimental potassium K-edge absorption spectrum ofKC1 at room temperature plotted as IF/I, uersus energy. The zero ofenergy is taken with respect to the first peak in derivative spectrum shown in Figure Za at 3607.4 eV
330
FUEL, 1986, Vol 65, March
accuracy on theoretical grounds, using reasonable semiempirical parameters. Theoretical analyses are generally desirable, but only in highly symmetric systems are they practical. More important to this study is that the spectral features can be used to fingerprint unknown K+ sites in coal and its combustion products from spectra of model compounds of known chemical structure. Potassium-bearing minerals Figure 2 shows a series of normalized K-edge XANES spectra of K-bearing minerals. Illite represents a class of most abundantly occurring clay minerals26. It is structurally related to muscovite having an idealized formula of KA1,(A1Si30,,)(0H),, and consists of hexagonal network sheets of linked SiO, tetrahedra. An idealized formula for illite might be indicating that Ko.66Al,(Al,.,,Si,.33o~,)(OH),, 15-16x of the Si4+ in the tetrahedral sheets are replaced by A13+ compared with 25% in muscovite. Two of these sheets are placed together with vertices of the tetrahedra pointing inwards. These vertices are cross-linked by octahedral A13+ ions. The structure is a succession of tetrahedral-octahedral-tetrahedral or (t--t) sheets with K+ ions placed between them, occupying the large holes created by two opposite hexagonal rings in 12-fold coordination by oxygens at 2.77 to 2.86 A2’s2’ (see Figure 2b). The repeat distance between successive layers is 10 A. Considerable substitution into the octahedral site is observed. Overall charge balance is maintained by adjustment of potassium concentration. Thus, there is no single illite formula per se to describe the structural composition. The spectrum shown in Figure 2a is characteristic of K+ in 12-fold coordination by oxygens and exhibits a strong white line at 5.9 eV with a low energy shoulder at 2.6 eV, followed by a number of resolved features at 14.9, 20.0, 22.7 and 30.2 eV. As the potassium coordination decreases to 9 in orthoclase, KAlSi,O, (a feldspar)29, the white line decreases in intensity and shifts to higher energy as shown in Figure 2~. Orthoclase is monoclinic, C$, (C2/m), Z=4 and consists of a chain structure of four linked tetrahedra. The chains run parallel to the u-axis with alternate rings approximately perpendicular and parallel to the plane of the diagram shown in Figure 2d. One quarter of the atoms at the centres of tetrahedra are Al, the remainder Si. The chains are linked to each other by K+ lying on the reflection planes, fitting into cavities. The K + ion is coordinated by 9 oxygens, two each at 2.92, 2.95, 3.03 and 3.13 8, and one at 2.70.& In leucite, KAlSiO, potassium is 6-fold coordinated by oxygens. The observed white line decreases further in intensity to form a doublet, and the absorption features above 20eV become less prominent. The structure of leucite is similar to that of analcime, NaAlSi,O,. H,O and consists of a system of four- and six-membered rings of tetrahedra linked together to form a three-dimensional framework. The facile conversion of leucite into analcime by treatment with a salt solution of NaCl or NaHCO, substantiates the similarity ofthe tetrahedral frameworks in these two mineralsjO. Sylvite, the naturally occurring mineral KCl, has a spectrum identical to that shown in Figure 1 for pure KC1 in which the Kf ions are 6-fold coordinated by Cl- ions. In microcline, a triclinic form of KAlSi,O,, the K environment is very similar to that in orthoclase. The potassium K-edge spectrum can be
Forms of potassium
in coal: C. L. Spiro et al.
1.)
-
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Figure 2
Normalized potassium K-edge spectra of some potassium-bearing minerals: (a) illite Ko.66AI,(AI,,,,Ai,,,,0,,)(OH),; (c) Orthoclase KAlSi,O,; (e) leucite KA1Si20, and (f) sylvite KCl. The large full circles in (b) and (d) show the position and local coordination of the K ’ ion in illite2’ and orthoclase”, respectively
directly superimposed onto that of orthoclase shown in Figure 2c. Thus, the spectra are most diagnostic of the immediate coordination geometry about the atom of interest. The cross-linking Al atoms in the octahedral sheets of muscovite may be substituted by Mg and Fe forming the mineral biotite: K(MgFe)3(A1Si,0,0)(OH),. The environ-
ment of the K’ ions, however, remains essentially unperturbed from that in muscovite. As expected, the potassium K-edge spectra of both muscovite and biotite are almost indistinguishable from one another, showing a peak-for-peak match in the two spectra with only minor variations in intensity; only an absorption at 14.3 eV shows changes. For muscovite, this is a sharp band; for
FUEL, 1986, Vol 65, March
331
Forms of potassium
in coal: C. L. Spiro et al.
biotite and illite, it is less pronounced. In any event, the differences between the potassium spectra of these micas may be due to differences in the next nearest and further neighbours in the tetrahedral and octahedral sheets in these minerals. Nevertheless, these examples clearly demonstrate the power ofXANES to fingerprint the local atomic structure ofa given constituent atom in a material.
as a diagnostic probe for the determination of local geochemical metamorphism. No dramatic changes in the spectra along a channel cut were observed, however, though the top layer had a somewhat lowered intensity associated with the 14.3 eV band. . Based on a hydrothermal laboratory synthesis of a coal-like substance from lignin in the presence ofillite32 it has been speculated that illite catalyses the formation of coal. Without suggesting any specific mechanism, the fact that no potassium-bearing phyllosilicates were found in the lower rank coals, and the fact that the illite is ubiquitous in mid-rank coals, and is apparently modified in the highest rank coal, lends credence to the notion that the family of alkali aluminosilicates plays more than a passive role in coalilication. Certainly the acid/base behaviour and ion-exchange properties of phyllosilicates, their ability to reversibly absorb moisture and organic matter, the ability to undergo redox reactions, and their demonstrable catalytic properties also support this notion33. Also significant is that illites are the most finely divided mineral species in coa134. This is corroborated by the result obtained in this study that gravity beneficiation and selective agglomeration yield fractions which still show only an illite spectrum in the products. The existence of one K+ site in coal is contrasted with those for trace vanadium4s5 and titanium 35 for which at least two sites (or phases) exist in the coal.
Potassium in coal Figure 3 shows the potassium K-edge XANES spectra of four coals of various rank: Reading anthracite, Pittsburgh No. 8 HVA bituminous, Navajo subbituminous C coal and a cannel coal from Paxeneses, PA. In these and all untreated coals listed in Table 1, except the Wyodak subbutiminous coal from the Powder River Basin and the Beulah Zap lignite from North Dakota, the dominant form of potassium is in the layered aluminosilicate structure exemplified by illite. Note that only the Reading anthracite shows exactly the same feature at 14.3 eV observed in muscovite. It has been recently demonstrated 31 that correlation between illite crystallinity and coal rank is not good. From the X-ray absorption data it can be speculated that the coal’s organic metamorphism is tracked by the clay, whereby illite is converted to a muscovite structure. Muscovite is a reasonable product of illite metamorphism in the absence oforganic matter; possibly the 14.3 eV band could be used
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Energy, eV Figure 3 Normalized potassium K-edge spectra of potassium Cannel coals. These spectra are identical to those of muscovite
332
FUEL, 1986, Vol 65, March
60
-60
-40
-20
0
20
40
60
Energy, eV impurities in Reading anthracite. Pittsburgh No. 8 bituminous, (for anthracite) and illite (all others) as discussed in the text
Navajo
sub C and
Forms
The lower rank coals yielded potassium spectra (Figure did not correspond to any of the model compounds measured. Though it is generally supposed’ that alkali metals in lower rank coals are bound in organic carboxylate groups, the spectra shown in Figure 4 bear no resemblance to the spectra of potassium acetate, benzoate or oxalate, nor to phthalimide or t-butoxide species, as well, shown in Figure 5. Leaching with HCl lowered the concentration of alkali in lignite, but the residue still showed an XANES spectrum unaltered from the raw coal other than in absolute intensity. HCl/HF leaching of a bituminous coal left residual potassium which also yielded the same spectrum as that found in the lower rank coals. One possibility is that the potassium is bonded to the surface ofa clay rather than inside a layered structure. Second, it may simply be adsorbed on a wide range of surfaces including mineral and organic, and features associated with the edge in model compounds are averaged. It is also possible that potassium is dissolved in water-filled pores or that the right model compounds have simply not yet been identified. 4) which
Potassium
in coal-derived
products
In addition to the study of indigenous potassium impurities in coal, several ash and char samples including low temperature ashes from illite-bearing coals, laboratory pyrolysates and gasification ashes were examined. In particular the fate of potassium in the Pittsburgh bituminous coal during a pressurized fluidized bed combustion cycle has been traced systematically by measuring the potassium spectrum in various residues from a demonstration unit. Several heat-treated illite samples prepared under various conditions were also measured as controls. For all low-temperature ashes prepared by an rf-generated oxygen plasma at 100°C the XANES spectra indicate that the potassium-bearing products were virtually indistinguishable from the original mineral species, i.e. illite. This is reasonable in that illite may only lose surface moisture at these temperatures (lOO”C), and other than anhydride formation by conversion of two surface hydroxyl groups into an 0x0 unit plus water36 at ~600°C. Major structural reorganization does not take place in illite until Z 1ooo”c.
1
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1.4 1.2 1.0 s s ii 0.8 4: B 0.6 zm t g 0.4 I
ENERGY (EV)
Figure 4 Normalized potassium K-edge spectra of K impurities in a North Dakota Lignite (---) and Wyodak sub C young (---) coal
of potassium
in coal:
C. L. Spiro
et al.
In general, the products of combustion showed little variation among the resulting spectra. Figure 6 is typical of the POC spectra obtained. Initially, the raw Pittsburgh No. 8 coal (Figure 3b) was pulverized and combined with the dolomite bed material, and fluidized under 10 atmospheres pressure. The average residence time of the coal in the bed is 60 min, after which the ash either remains in the bed at 915°C or is entrained into the hot gas stream. The bed material is replaced every hour. The hot gas stream at 860°C passes through a series of three cyclones, each removing liner and finer products. The exit temperature ofcyclone 3 is 830 ‘C. The hot gas then passes through a series ofair cooled pins held at 610°C. Deposits were collected from the pins and the corresponding potassium spectrum taken. Compositional data on the deposits appears in Table 3. The primary variation among these spectra occurs in the relative intensity of two peaks constituting a doublet at 3.2 and 6.5 eV. The broad absorption at _ 34 eV did not vary significantly. More specifically, the spectra ofthe bed material, the second and third cyclones, and the pin scrapings down-stream are all virtually superimposable. More significant is that the spectra of these combustion products are also identical to the spectrum of illite heated by itselfto 1000 ‘C for 0.5 h (see Figure 6). This implies that the gaseous atmosphere, the presence of impurities such as sulphur and nitrogen, and even the carbon substrate do not dramatically impact the thermal transformations of illite in coal. The results suggest that the effect of potassium on deposition during PFB combustion can be studied to a reasonable degree ofaccuracy using pure illite rather than coal. Although the importance of alkali sulphates in the formation of fouling deposits during combustion is well known’, there was no evidence of potassium sulphates in the current deposits. To understand the nature of the coal-derived high temperature phases, the phase diagram of the pseudoternary system (Al,O,-SiO,-base) was developed by Huggins et ~1.~‘. In their scheme, an average basicity based on the combined alkali, alkaline earth and iron content is presumed which satisfactorily accounts for compositional effects on ash fusion temperature in coal. For pure illitic clays, Bohor 36 found that potassium had the dominant repressive effect on the formation of high temperature phases. A useful exercise is to consider Bohor’s model of illite decomposition in light of Huggins’ proposed phase diagram. For example, a typical illite composition might be K1,,Si6,~A11,,(Al,FeMg1,J O,,(OH),. Between 100 and 200°C surface moisture is desorbed, and between 475 and 600°C anhydride formation takes place, splitting off two water molecules from the four hydroxide units and leaving two 0x0 bridges on the surface of the tetrahedral-octahedral-tetrahedral aluminosilicate sandwich. Depending on the per cent base content, somewhere between 850 and 1000°C decomposition of the t-o-t system takes place, with the octahedral sites, Al,FeMg,,,O, in the example, possibly staying intact in the form of spinel. The remaining group, K,,,Si,,SAl,,50,6 partially melts, and ifquenched, would be expected to form a glass. Indeed, subsequent to experimental tenure at SSRL, a material of this composition prepared at 1730°C was confirmed to be amorphous by X-ray diffraction. At higher temperatures, the spine1 can redissolve into the melt, and phases such as mullite and cristobalite may form while gaseous alkali species can be released.
FUEL, 1986, Vol 65, March
333
Forms of potassium
(a)
::: 1.4
in coal: C. L. Spiro et al.
-
1.4 -
1.8 1.0
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-
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-
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::: 1.4 I.8
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Energy, eV Figure 5 sulphate
,
-60-40-20
,
Normalized potassium K-edge spectra ofa series of potassium salts: (a) acetate, (b) benzoate, (c) oxalate, (d) phthalimide, (e) t-butoxide, and (f)
This indicates that a glass would be expected based on the Huggins’ composition model. Several potassiumbearing glasses were synthesized’ and their spectra examined. In general, all the glasses tested in Table 1 exhibit a similar doublet feature at z 3.5 and 7.0 eV, and a broad absorption at 34 eV as those of the coah ash; but none corresponds precisely. The model glasses that best correspond to the coal-derived combustion products are the potassium titanium aluminosilicate glasses which showed little variation with composition.
334
-
0. -
FUEL, 1986, Vol 65, March
Clearly, most of the spectral features are obtained in these simulations, but the intensities do not correspond precisely to the unknown, and a more relined series of model compounds is required to assign all the transitions observed in the spectra. Even lacking a precise match at this stage, the form of potassium can be confidently assigned to a glass based on the spectral features, and based on the stoichiometry of the starting material, illite, and its thermochemistry at elevated temperatures. According to Huggins’ phase diagram approach, the
Forms of potassium
1.2 1
8.4 e.2 e. -20
I
I
1
I 20
0
I
1 40
Energy, eV
Figure 6 Comparison of cyclone deposit from PFB coal combustion (---) and illite heated to 1OOOC by itself(- - - - -) indicates spectator role played by sulphur and organics in illite thermochemistry
in coal: C. L. Spiro
et al.
between the layers of the aluminosilicate structure. The highest rank coal, anthracite, showed potassium in muscovite, a probable product of illite metamorphism. The lowest rank coal, lignite, showed potassium most likely in a non-unique environment, possibly as a of organic and mineral sorbates. combination Combustion products resulted in a potassium-bearing aluminosilicate glass whose XANES features are identical to those obtained by pyrolysis of pure illite at 1000°C. The results obtained in this study demonstrate the usefulness, and perhaps uniqueness, of the X-ray absorption technique using synchrotron radiation as a light source to probe the structure and bonding of trace constituents in coal and coal-derived products. Material characterization at an atomic level of such trace constituents is much needed to elucidate reaction mechanisms* in order to design more efhcient and cleaner use of coal as an energy source. ACKNOWLEDGEMENTS
product phase is assigned as a potassium-bearing aluminosilicate glass. This assignment is further borne out in Figure 6, comparing illite heated by itself (with no organics or sulphur present) with the cyclone products from the PFB combustion. The spectra are virtually superimposable. In a parallel programme to study coal-derived deposits, with synchrotron radiation, Huflinan et ~1.~~have made a preliminary identification of potassium bisulphate, KHSO,, as a component of a coal-fired boiler deposit, and potassium sulphate, K,SO,, as a component of a combustion rig deposit. In both cases the coal being fired was a North Dakota lignite. The potassium bisulphate spectrum appears similar to the potassium aluminosilicate glass spectra reported in this paper. However, the near perfect match between illite heated by itself and combusted coal is more consistent with the aluminosilicate glass assignment of the deposits. To further test this assignment, the deposits were leached for soluble alkali species, such as sulphate forms. The results show that of the total alkali in the deposit, only 0.3% was soluble, thus strengthening the conclusion. The discrepancy between the conclusions here and those of Huffman et al. most probably arises from the differences in reaction conditions and in the type of coal being burned, rather than from the data analysis. Indeed, Huffman et ~1.’ “I6 have also identified the potassium-bearing phase in coke samples prepared at 1050 and 1100°C as an aluminosilicate glass.
The US Department of Energy is acknowledged for supporting the Stanford Linear Accelerator Center and the Stanford Synchrotron Radiation Laboratory. The Staff at SSRL is also acknowledged for their considerable help during the tenure of the experiments and for their continued progress in providing and maintaining the SSRL facility. The authors greatly appreciate generous loans of samples from the New York State Geological Survey and to William Kelly, curator of the New York State Museum mineral collection, and to Jim Dickinson and Paul Hess, Geology Department, Brown University. Gordon Brown, Stanford University, is acknowledged for the use of his laboratory facilities and for helpful advice and G. P. Huffman of US Steel for reviewing the data and offering helpful suggestions. Finally the authors acknowledge research staff at General Electric including John Blanton, Russ McCarron and Robin Brobst and Ben Glover for providing combustion samples. REFERENCES I 2
3 4 5
CONCLUSION With intense and well-collimated synchrotron radiation, a high resolution X-ray absorption technique was used to investigate the chemical structure of potassium impurities in a variety ofcoals ofvarious ranks and their combustion products. A series of potassium-bearing inorganic and organic salts, minerals and their ashes, glasses and bronzes were also measured to aid identification of the unknown potassium sites in coal and combustion products. The model systems studied are given in Table I. For all coals (except the youngest coals from the western US) examined here, the dominant form of potassium was in the illite-like phase in which K+ ions are 1Zfold coordinated by oxygens and are situated at large cavities
6
Bryers, R. W. ‘Ash Deposits and Corrosion Due to Impurities in Combustion Gases’, Hemisphere Publishing Corp., Washington, 1978 Gluskoter, H. J. ‘Trace Elements in Coal: Occurrence and Distribution Circ. 499, Illinois State Geological Survey, Urbana, IL, 1977 Karr, C. Jr. (Ed.) ‘Analytical Methods for Coal and Coal Products’, Vol. 1-3, Academic Press, New York, 1978 Maylotte, D. H., Wong, Jr., St. Peters, R. L., Lytle, F. W. and Greegor, R. B. Science 1981, 214, 554 Wong, J., Maylotte, D. H., St. Peters, R. L., Lytle, F. W. and Greegor, R. B. ‘Process Mineralogy II: Proc. of the Metallurg. Sot. of AIME, 1982, p. 335 Sandstrom, D. R., Filby, R. H.. Lytle, F. W. and Greegor, R. B. Fuel 1982, 61, 195
* An example of a reaction mechanism under study by synchrotron radiation is the potassium catalysed gasitication of coal. Graphite (type UCP, - 325 mesh, Ultra Carbon, Inc.) and sugar charcoal were milled with 10% potassium carbonate in a Spex, Inc. shatterbox, and pyrolysed at 800°C for 2 h, below the melting point of potassium carbonate. The samples were then cooled to room temperature and stored in air for several weeks. The suaar char revealed exclusively potassium bicarbonate as the end product, while the graphite pyrolysate showed a good match for a linear combination of28.8% potassium carbonate and 71.2% potassium bicarbonate. A reasonable path for this process is the carbothennic reduction of the alkali carbonate to metallic potassium at elevated temperatures ‘* followed by facile reaction with atmospheric water vapour and carbon dioxide at room temperature
FUEL, 1986, Vol 65, March
335
Forms of potassium
10 11 12
13
14
15
-16 17
18 19 20
336
in coal: C. L. Spiro et al.
Spiro, C. L., Wong, J., Lytle, F. W., Greegor, R. B., Maylotte, D. H. and Lamson, S. H. Science 1984,2x,48 Lytle, F. W., Sayer, D. E. and Stern, E. A. Phys. Rev. B 1975, B15, 2426 Wong, J. ‘EXAFS Studies of Metallic Glasses’, chapter in ‘Topics in Applied Physics’, Vol. 46, ‘Glassy Metals I’ (Ed. H. J. Gunther and H. Beck), Springer-Verlag Publishers, Berlin, 1981,pp. 45-77 Huffman, G. P., Huggins, F. E., Greegor, R. B. and Lytle, F. W. ‘SSRL Report’ 84101, 1984, pp. 1X-31 Dickinson, J. E. and Hess, P. C. Geochim. Cosmo. Acta 1984, in press Greegor, R. B., Lytle, F. W., Dickinson, J. E. and Hess, P. C. ‘EXAFS/XANES Investigation of K and Ti Sites in Alkali Aluminosilicate Glasses’, 1lth SSRL Users’ Meeting, October 1984 Wong, J., Spiro, C. L., Maylotte, D. H., Lytle, F. W. and Greegor, R. B. ‘EXAFS and Near Edge Structure III’(Ed. K. 0. Hodgson, B. Hedman and J. E. Penner-Hahn), Springer-Verlag, BerlinHeidelberg, 1984, pp. 362-367 Spiro, C. L., Wong, J., Lytle, F. W., Greegor, R. B., Maylotte, D. H., Lamson, S. H. and Glover, B. L. ‘EXAFS and Near Edge Structure III’(Eds. K. 0. Hodgson, B. Hedman and J. E. PennerHahn), Springer-Verlag, Berlin-Heidelberg, 1984, pp. 361370 Huffman, G. P., Huggins, F. E., Cuddy, L. J., Schoenberger, R. W., Lytle, F. W. and Greegor, R. B. ‘EXAFS and Near Edge Structure III, Progress in Physics, Springer-Verlag, Vol. 2, 1984, pp. 371-373 HufIinan, G. P., Huggins, F. E., Schoenberger, R. W., Walker, J. S., Lytle, F. W. and Greegor, R. B. Fuel, submitted for publication Kosky, P. G., Lamby, E. J., Maylotte, D. H., McKee, D. W. and Spiro, C. L. ‘Coal Gasilication Catalysis Mechanisms’, Report DOE/MC/14591-1397, 1982 McKee, D. W., Spiro, C. L., Kosky, R. G. and Lamby, E. J. Fuel 1983, 62, 217 Bearden, J. A. and Burr, A. F. Rev. Mod. Phys. 1967, 39, 125 Jaklevic, J., Kirby, J. A., Klein, M. P., Robertson, A. S., Broan, G. S. and Eisenberger, P. Solid State Comm. 1977, 23, 619
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1986,
Vol 65, March
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27
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33 34 35
36 37 38
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