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Sample preparation and electron microprobe examination of lignite ash deposits
Metallography
209
Sample Preparation and Electron Microprobe of Lignite Ash Deposits WALTER
W. FOWKES,*
THOMAS
D. WILHELM,b
Examination
AND WAYNE R. KUBEc
A suitable technique was developed for preparation of lignite ash deposits for electron microprobe examination. Prepared samples were analyzed for concentrations and distribution relationships of seven elements at four separate positions in the bulk deposit. Fortytwo discrete groups of these elements were identified, each of which may represent more than one chemical compound.
Introduction Varying degrees of fireside boiler-tube
fouling are experienced
in the com-
bustion of pulverized coal at power-generating stations. The heated ash entrained in the combustion gases contains material which first deposits in a thin layer on fireside surfaces of walls and tubes. Other ash particlcs which contact this initial layer tend to adhere physically or react chemically,
forming additional deposits
which grow into the gas stream. As the inner layers are gradually shielded by the growing thickness of the deposit, the temperature at any given point in the ash deposit drops, owing to the insulating effect of the ash. The inner layers, being cooled, develop the strength
necessary to support the increasing
mass of the
deposit. If ash deposition continues, operating efficiency of a boiler is significantly reduced, and it may be necessary to take the unit “off the line” for cleaning. This down-time
represents
a loss of approximately
company for a 200-megawatt availability.
generator,i
$500
emphasizing
per hour to the power the importance
of boiler
a Project coordinator, Grand Forks Coal Research Laboratory, Bureau of Mines, U.S. Department of the Interior, Grand Forks, North Dakota. b Chemical engineer (graduate student), Department of Chemical Engineering, University of North Dakota, Grand Forks, North Dakota. eProfessor of Chemical Engineering, University of North Dakota, Grand Forks, North Dakota; research chemical engineer, Grand Forks Coal Research Laboratory, Bureau of Mines, U. S. Department of the Interior, Grand Forks, North Dakota. Metallography,
2 (1969) 209-225
Copyright 0 1969 by American Elsevier Publishing Company, Inc.
210 The
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube strength
of the ash and, therefore,
its resistance
to cleaning varys in
relation to the mineral content of the lignite burned. A survey of the lignite mines in the Northern Great Plains Province has shown considerable variability in the percentage of ash and in the proportions of its constituents, both between different mines and also within the same mine.2 Tests have demonstrated that the degree of fouling caused by ash from these lignites may be related, at least in part, to the concentration of sodium in the coal, reported as the oxide in the ash.3-5 Excessive fouling in a commercial boiler may occur in a period of operation as short as 3 days when burning “high-sodium”
lignite; periods of up to 18 months,
or more, between shutdowns have been achieved by burning lignite of low sodium content. In examining the problem of ash and slag deposition, a detailed structural and elemental picture of the deposits themselves would be very helpful. A comparison of the nature of these deposits with that of the original mineral complement
of
the coal burned should permit the investigator some insight into the behavior of these minerals in the combustion zone and the character of the resultant materials responsible
for the initiation
formed on heat-exchanging
and development
Choice of the electron microprobe information on elemental composition structural
deposits
as a promising facility to furnish the and distribution posed the problem of
handling and preparing for examination configuration,
of the troublesome
surfaces within the boiler.
integrity,
samples of materials varying widely in
and homogeneity.
Limitations
on sample
size and necessity of a critically plane surface for examination indicated that an embedding procedure would be most useful. In addition, a satisfactory procedure must permit almost perfect impregnation and irregular outlines. One objective
of the investigation
of porous samples having odd
herein discussed was to devise a suitable
technique for preparation of samples of ash and fireside deposits for examination by microprobe;
another-development
of a plausible mechanism
of deposition-
will be dealt with elsewhere. This present report is based largely on a thesis submitted to the University of North Dakota by Thomas D. Wilhelm, working under a fellowship agreement between the University and the U.S. Department of the Interior,
Bureau of Mines.6
Experimental Six North content
Procedure
Dakota
lignites were chosen for testing
and mine location.
A high-sodium
on the basis of sodium
and a low-sodium
sample were
selected from each of two different mines. Two additional samples from other mines were selected because severe fouling characteristics had been observed with these lignites in commercial
use. Ash deposits were collected by burning
Microprobe of Lignite Ash
211
each of the selected coals in a laboratory pilot-plant combustor conditions
in a commercial
pulverized-coal-fired
which simulates
unit. Deposits
were formed
on air-cooled, &inch stainless-steel tubes at a tube temperature of 535” & 25°C. The collector tube was in the simulated superheater area (gas temperature 1100%).
Linear gas velocity was 30 fps, and the heat release rate was approx-
imately 13,000 Btu/f@ per hour, closely apprcximating mercial furnace.
For satisfactory
microprobe
examination,
conditions
vertical variations
in a com-
in the sample
surface must not exceed 2 to 3 microns. Ash samples collected from boiler-tube surfaces are usually a conglomeration
of fused and/or sintered solid and hollow
spheres and inclusions, which have highly irregular surfaces. Compressive strength of the deposit from lignite ash is usually too low to permit polishing in the original state; consequently method
of impregnating
polishing.
it was necessary
to develop a satisfactory
the ash with a suitable binder to allow cutting
Only then could the samples be satisfactorily
and
examined by using the
electron microprobe. The impregnating procedure of Procter and Taylor’ and the resin suggested by Rentons proved to be the most satisfactory for the deposits examined. Two sample holders were constructed (see Fig. 1): (1) a brass upright cylinder, l-inch I.D., for use with small samples apart from the collector tube, and (2) a form for horizontal use consisting of a cutaway section of l&-inch-I.D. steel pipe, 6 inches long, fitted with clamp-on ends to accommodate
samples adhering
to a cutoff section of the collector tube. In use, the inside of the sample holder was coated lightly with a layer of silicone
FIG. 1.
Impregnating
system and sample holders.
212
Walter
W. Fowkes,
Thomas D. Wilhelm and Wayne R. Kube
stopcock grease. The holder containing the sample was placed in a vacuum desiccator fitted with a dropping funnel. The system was then evacuated to about 1 mm Hg by means of a mechanical pump and maintained at this pressure for 3 to 4 hours to remove air from the pore spaces of the sample. Resin and activator were mixed as needed and admitted through the dropping funnel in portions sufficient to cover the sample while maintaining
the reduced pressure
by continued pumping. Reduced pressure was maintained until bubbling ceased, indicating essentially complete degassing of the sample. At this point, the pump was stopped and air was admitted slowly to the system, restoring atmospheric pressure in about 3 minutes. The pressure increase forces the resin into the pores of the sample. The resin was then cured, either by heating it in an oven at 70°C for 4 hours or by permitting it to stand at room temperature for 36 hours. The latter method gave better dimensional
stability.
The hardened resin, containing
the sample,
was then removed from the mold, and the silicone grease was removed by wiping. This technique produces a sample with characteristics satisfactory for polishing.
It is also suitable for embedding
content, as well as other coal substances Cutting, The
Grinding,
coal samples having full moisture
such as dried coal, char, or coke.
and Polishing
ash sample
completely
encased
in hardened
resin was cut without
difficulty on a band saw into lengths of less than 1 inch; excess resin was removed on a belt sander. Prior to grinding, the samples were mounted on microprobe sample holders, using Lakeside 70 as the bonding agent. The samples were surfaced on a metallographic
grinding wheel, starting with
120-grit grinding paper and following this with successively finer grades-180, 240, 320, 400, and 600 grit-rotating the sample 90° between each grade. Care was exercised on those samples that included a portion of the metal collector tube to not grind in a direction that would carry grinding particles from the tube to the ash. This procedure minimized metal carryover onto the ash sample. Each stage of grinding was terminated approximately 30 seconds after the grinding marks of the previous grade (at a 90” angle to the new grinding direction) were eradicated. As soon as the 600-grit
grinding was finished, the sample was ready
for polishing. Polishing was carried out in four steps with four sizes of diamond polishing compound-9-,
6-, 3-, and l-micron
particle sizes. The polishing
compound
(paste form) was distributed sparsely (dye from the polishing compound barely visible) on the polishing cloths. d Lapping oil was spread in a narrow band across d METCLOTH was used with 9- and 6-micron polishing compounds; TEXMET cloth with 3- and l-micron pastes (A. I. Beuhler, Inc., Evanston, Illinois). Use of trade names is to facilitate identification only and does not imply endorsement by either the Bureau of Mines or the University of North Dakota.
Microprobe of Lignite Ash
213
the radius of the rotating polishing wheel. The sample was polished by using the same technique as in grinding, except that in polishing it becomes increasingly difficult to determine when a given step is finished. The completion of each stage was verified by examination at 200 x under a microscope. On completion of the fourth step, the sample was ready for examination by microprobe.
Microprobe
Examination
Instrumentation
available
for this investigation
Electron Probe Microanalyzer for strip record,
digital readout,
display
output,
of x-ray
was the Norelco
AMR/3
fitted with one vacuum goniometer and arranged and automated
backscatter,
beam-scanning
and sample current
with visual
signals.
The
area
scanned by the beam was a square, 320 microns on a side. The signal from the x-ray detector was monitored by the grid of one oscilloscope while the sample current
was monitored
configurational Each
by a second,
resulting
in simultaneous
elemental
and
maps of the sample area being examined.
polished
surface
was coated under vacuum
with a layer of carbon
(estimated thickness 50 A) to improve conductivity of the surface. The coated sample was then inserted into the microprobe for examination by the following procedure. The sample was scanned over 320-micron-square
areas at each of positions 1,
2, 3, 4, and 5 (see Fig. 2). The tin layer of ash at positions to such a small percentage
FIG. 2.
1 and 2 contributed
of the area scanned that they were not considered
Positions examined by microprobe.
I
33.9
41.0 31.2 17.1 20.0 19.0 21.7 25.0
CA
INC
3.6
38.1 -7 24.4 -22 24.0 40 24.6 23 17.4 -8 27.0 24 25.9 8.3
CD
CaO CD
INC
-18.2
4.6 6.2 35 2.8 2.0 -29 10.3 4.6 -63 15.2 8.1 -47 7.8 7.6 -3 12.0 14.5 21 8.8 7.2 -14.3
CA
Fez%
Al,%
7.4 13.8 9.1 12.0 9.1 8.7 10.0
INC
39.0
9.5 28 13.0 -6 15.9 75 20.5 71 13.9 53 10.6 22 13.9 40.5
CA CD
* CA = percentage concentration in laboratory prepared ash. t CD = percentage concentration in deposit, as calculated from microprobe data. * INC = CD - CA/CA x 100 = percentage increase in concentration from CA to CD.
17.9
8.8 9.0 2 9.2 12.3 34 2.5 1.9 -20 5.8 5.3 -9 8.4 11.9 42 6.3 9.4 49 1.0 1.6 +60 5.1 8.6 69 20.2 23.5 16 4.1 5.8 41 9.5 11.5 21 5.2 6.3 21 8.4 9.9 0 5.9 7.9 34.2
HSV LSV HSB LSB N30 TS Average Increase in average concentration
CA CD INC
CA* CDtINC*
MgC
Coal title
Na,O
14.0 31.5 17.5 16.9 19.1 16.1 19.2
CA
26.7 48.8 27.5 24.4 30.6 28.0 31.0
61.0
91 55 57 44 60 74 63.5
CD INC
SiO,
CD
INC
-69.0
11.4 5.0 -56 6.2 3.8 -39 27.6 11.1 -60 27.1 11.6 -57 14.7 1.9 -87 24.9 1.3 -95 18.7 5.8 -65.7
CA
SO,
COMPARISON OF CONCENTRATIONS IN LABORATORY PREPARED COAL ASH WITH CONCFNTRATIONS IN TUBE DEPOSITS
TABLE
Microprobe of Lignite Ash
215
reliable data. Data taken at these two positions calculations. Seven elements routinely scanned calcium,
iron,
aluminum,
silicon,
and
sulfur.
were omitted in subsequent were: sodium, magnesium, Potassium
was
not
quantities greater than 1%. The counts on all scans were recorded of two sweeps-about 53 seconds. At the same time, a photograph the x-ray signal displayed The
output
of the
concentrations approximation
found
in
for the period was taken of
on the beam scanner.
microprobe
was standardized
in order
to estimate
the
of elements in the unknown samples. A semi quantitative to concentration of the different elements was obtained by
ash having a composition similar to that of the boiler a “synthetic” Intimate mixture was attained by pulverizing in a shatter box and
preparing deposits. pressing analysis
a 1-inch-diameter pellet at 30 tons per square inch. An x-ray fluorescent was performed on a 320-micron-square area (raster mode) of the pellet
in the microprobe, in deposit
analyses.
using parameters
selected
Each element
was counted
on the signal intensity. concentration, concentration
Assuming
the number (see Table
to duplicate
a linear relationship
between
of counts was converted I). Th is standardization
analysis
of each scan area covered
present.
The
resin-impregnated
by the beam
sample
those to be used later
for 10 to 50 seconds,
depending
counting
rate and
to counting rate per 1 o/0 of permitted an approximate scanner
of ash deposit
in terms
of elements
was then
placed
in the
microprobe, and an identical analysis was performed. The percentage of concentration of each element was estimated on the basis of values obtained on the synthetic
sample
the photographs
and the results
were normalized
of x-ray emissions
arrangements were derived,-that compounds or mineral species. This deposit
procedure
is subject
larger
than
matrix-effect
that
to possible
accelerating-voltage
probable
errors
From these data and
the relationships
as: (1) The
“standard,”
of chemical
occurrence
of chemical
surface
of the ash
owing to the many voids that
These voids are filled with resin and make it necessary to to 100%. (2) The particle size of the deposit is, in general,
of the synthetic
corrections
standard synthetic deposit specimen;
Results
is, the
is less dense than the prepared
occur in the former. normalize the results
to 100%.
from the sample,
ash, which
(enhancement
could
also generate
or absorption)
ash was not analyzed therefore, no correction of the microprobe
errors.
were applied.
(3) No
And (4) the
immediately following analysis of the arising from possible dirft in the high
could be applied.
and Discussion
Photographic Representation of Elements Two Figures 6
of the six sets of pictures 3 through
6 represent
deposits
taken
are presented
from a high-sodium
in Fig. lignite:
3 through
10.
Fig. 7 through
216
Walter
W. Fowkes,
Thomas D. Wilhelm and Wayne R. Kube
FIG. 3.
x. -ray
and sample current output patterns, N30, position 2.
FIG. 4.
X-ray
and sample current output patterns, N30, position 3.
217
Microprobe of Lignite Ash
FIG. 5.
FIG. 6.
X -ray and sample current output patterns, N30, P
X-ray
.tion 4.
and sample current output patterns, N30, position 5.
218
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube
FIG. 7.
X-ray
and sample current output patterns, LSV, position 2.
FIG. 8.
X-ray
and sample current output patterns, LSV, position 3.
Microprobe
of Lignite Ash
FIG. 9.
X.-1‘ay and sample current output patterns, LSV, positia m 4.
r::-k
FIG. 10.
219
X-ray
&_
‘_
c-
-3
and sample current output patterns, LSV, position 5.
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube
220 10 represent
those
from
a low-sodium
lignite.
Each
figure,
containing
eight
pictures, is the composite representation of the distribution of the seven elements at one position in the sample. The eighth picture represents the sample current pattern,
indicating
the actual configuration
in concentration
of the elements
of the deposit
of interest
area scanned.
in going from the inner
Changes
layer to the
outer layer were detected by taking data at three positions, covering the depth of each sample (see Fig. 2). Each position was square, the length of a side being 320 microns. three
Nine
positions
hundred
and sixty microns
perpendicular
to the tube)
(the sum of the lengths
represented
one-fifth
of the
to one-tenth
the
total thickness of the ash deposit. The relative concentrations of individual elements in the overall deposit were found to be unique for each lignite tested (see Table
I).
Figure
3 shows
adjacent
the presence
to the tube.
The
shows less sodium and pictures are representative high concentration Figures
of a sodium-sulfur
signal
of sodium
signals
silicon signals concentrations
Figure
at position
2,
4, at position
3,
less sulfur but a high concentration of iron. These of a high-fouling lignite. Original ash analyses show a and an average
7 and 8 show the x-ray pattern
and sulfur
compound
for iron is negligible.
are weak at both
to high concentration
typical
position
at a low-fouling
2 and 3. Calcium,
are strong. For this type of deposit, of both sodium and sulfur.
of iron. lignite.
Sodium
aluminum,
the original
and
ash shows
low
Relative Concentrations and Relationships of the Hements The
portion
varied
between
(voids
were
malized
of the 9%
filled with
to account
area
scanned
and 81%, epoxy
which
depending resin).
for this variation
contained
inorganic
on the void volume
The
calculated
(see Table
constituents of the deposits
concentrations
were
nor-
I). On the basis of these normal-
ized data, average concentrations of each of the seven elements were calculated for each position for the four high-sodium lignites and for the two low-sodium lignites.
These
arithmetic
means
changes between positions 3 and low-sodium group. A concentration if the data met all the following 1. Change
between
2. Minimum from
change
18% to 20%,
3. Minimum at position
positions
used
to detect
element
concentration
5 for the high-sodium group and was said to change between positions
arbitrary
criteria:
3 and 5 had to be consistent
in concentration
for the 3 and 5
was 2 percentage
in sign. units
(for example,
or from 7% to 5%).
change
in concentration
3 (for example,
thus, the concentration to meet criterion 3).
were
20%
at position
was 20%
of a 15%
of the concentration
concentration
at position
5 must be either less than 12 or greater
present 3 is 3%; than 18
Microprobe of Lignite Ash
FIG.
FIG.
12.
11.
Deposition
Elemental
concentration
selectivities,
percent
versus position.
concentration
versus position.
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube
222
On the basis of these
criteria,
the following
concentration (see Figs. lla, llb, coals decreased 37 y0 from position coals decreased
42%
from
elements
were said to change
in
and 12): (1) Sodium oxide in high-sodium 3 to position 5. (2) Iron oxide in low-sodium
position
3 to position
5. Calcium
oxide
narrowly
missed meeting the criteria, increasing consistently in concentrations by 18 %. The high concentration of sodium at position 3 may be caused by highly concentrated
sodium
next
to the tube,
which
was the case in four
of the
six ashes
examined (see Fig. 2 and Figs. 3 through 10). No other characteristics noted peculiar to either high-sodium or low-sodium lignites.
were
Comparisons with Ash in Original Coal A comparison determined
of the elemental
concentrations
on laboratory-prepared
following
selectivities
of the six samples
of deposition
showed
the laboratory-ashed
1. Magnesium
lignite
in the deposits
along the axes of the deposits.
only a loss (or gain) in the element
samples,
said to have occurred
in the original
ash, with those
positive
(or negative)
ashes,
indicated
as the
If at least five
in comparison
selectivity
to
of deposition
was
in the tube deposit
than
in the tube
than
(see Fig. 12):
oxide concentration
was 19 “/b greater
in the ash. 2. Aluminum in the ash. 3. Silicon the ash. 4. Sulfur
oxide concentration
dioxide
was 37%
concentration
concentration,
greater
was 53 o/o greater
deposit
in the tube deposit
as trioxide,
was 33%
as large in the tube
(Table
II)
showing
groupings
given
point
than
in
deposit
as
in the ash. A listing appeared
was
prepared
simultaneously
listing shows photographic
at any
An element members
picture
of elements
samples
which
examined.
was considered appeared
of the group,
the source
of the deposit-that
as part of a group
at any point as evidenced
is, low-sodium
if its x-ray pattern
that was also occupied by the pictures
leading,
since the method
among
FeS,
does not distinguish
FeSO, , and
more than
forty-two
Fe,(SO,),
. The
on the beam-
of the individual
compounds.
between list
Fe,O,
of groups,
or
by the rest of the elements.
By this method, forty-two different groups of elements were identified. respect to possible chemical combinations, this listing may be somewhat
represent
This
the total area occupied by each group, expressed as squares of a grid, and the area appearing at each of positions 3,4, and 5 in the
deposit, as well as designating high-sodium lignite. scanner
in the
and FesO, therefore,
With mis-
, nor could
TABLE
II
AREA OCCUPIED BY EACH GROUP OF ELEMENTS, INCLUDING ALL SIX LICNITFS
Total relative area
Area at position numbers
Total area in high sodium lignites *
Total area in low sodium lignites*
3
4
5
1 18 7 7 2
1 11 2 0 0
0 6 1 5 2
0 1 4 2 0
1 17 5 5 0
0 1 2 2 2
1 4 11 1 18 25 8 3 3 5
0 4 11 I 9 0 3 0 3 4
1 0 0 0 2 0 4 0 0 1
0 0 0 0 7 25 1 3 0 0
1 0 11 1 16 25 1 3 1 5
0 4 0 0 2 0 7 0 2 0
1 16 1 4 1 15 8 3 5 1 3 50 1
1 9 1 4 1 15 0 3 0 0 1 30 0
0 5 0 0 0 0 8 0 5 1 2 10 1
0 2 0 0 0 0 0 0 0 0 0 10 0
1 4 I 4 1 0 8 2 0 0 1 15 0
0 12 0 0 0 15 0 1 5 1 2 35 1
27 1 18 19
17 0 10 1
1 I 5 18
9 0 3 0
24 1 9 0
3 0 9 19
1 2 16 1 3 3
0 0 2 1 1 0
34 30 1
0 0 0
0 30 1
34 0 0
34 30 1
0 0 1
Na, Mg, Ca, Fe, Al, Si, S 128
0
58
70
108
20
146
171
189
339
167
Chemical
group
Na
Ca Fe Si S Na, Mg Na, Fe Na, S Mg, Fe Ca, Fe Ca, Si Ca, S Fe, Si Fe, S Al, Si Na, Mg, Fe
Na, Na, Mg, Mg,
Al, Si Al, S
Ca, Fe Ca, Al
Mg, Ca, S Mg, Fe, Al Mg, Fe, S Mg, Al, Si Mg, Ca, Ca, Ca,
Al, S Fe, Al Al, Si Al, S
Na, Na, Mg, Ca,
Ca, Al, Ca, Al, Ca, Al, Fe, Al,
Na, Na, Mg, Mg, Mg, Mg,
Mg, Mg, Ca, Ca, Ca, Fe,
Si S Si Si
Ca, Fe, S Ca, Al, Si Fe, Al, Si Fe, Al, S Al, Si, S Al, Si, S
Na, Mg, Ca, Fe, Al, Si Na, Mg, Ca, Al, Si, S Mg, Ca, Fe, Al, Si, S Totals * Four high-sodium
510 lignites;
1 2 16 1 3 0
I 0 14 0 0 3
two low-sodium
lignites.
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube
224 The identification
procedure
is the ever-present
problem
cannot
be considered
of elements
rigorously
in any given
definitive.
area overlapping
There into the
area of another group. Such overlap involves both discrete groups. The overlap area is considered as a third group, or fourth, if not all elements of the two groups overlap in the same place. No attempt was made to identify sodium
aluminosilicates
lignite
ash.3
Groups
(Table II). Areas of the elements blended distance given
specific
calcium
have
that not
to
contain been
compounds.
sulfate
corresponding
deposit
sought
phenomenon
and
these
of the
However,
well-known
compounds
a well-mixed
appreciably
of six or all seven
are
calcium
have
been
conglomeration
discussed
elements
in the
under
and
components
in found
of all the
literature.
The
consideration
being
together seems to be independent of the sodium content once the from a specific position in the deposit to the tube wall has reached a critical
although
magnitude.
it should
On the basis particle deposit
This
be a function
of the samples
magnitude examined,
that impinges upon it. The will vary with the inorganic
percentages
in the coal ash, which
the breaking
strength
may show random
has not
of the insulating
been
generally
properties
it is believed
that
physical and chemical chemical constituents
generally
of all seven
the ash holds properties and their
any
of the relative
differ for each coal. For example,
of one ash may be twice that of another, mixing
identified,
of the ash involved.
even though
both
elements.g
Summary A suitable technique for preparation of coal ash deposits electron microprobe was adapted. Six samples were prepared analyzed separate
for
elemental
positions
concentrations
in a tube deposit,
and
distribution
using the electron
to be examined by and systematically
relationships
microprobe.
at four
The samples
were collected on air-cooled tubes placed in the combustion-gas stream during tests burning four high-sodium lignites and two low-sodium lignites in a pilotplant
combustor.
Sodium-sulfur compounds were found in a layer completely surrounding the tube. The thickness of this layer was reduced where covered by the bulk of the deposit. Sodium
concentration
in deposits
tration in deposits from low-sodium decreased consistently toward the aluminum
concentrations
were
from high-sodium
lignites
and iron concen-
lignites were highest near the tube outer edge of the deposit. Silicon
consistently
higher
laboratory-prepared ash, while the concentration one-third that in the original ash.
in the deopsit of sulfur
than
in the deposit
and and
in the was
Microprobe of Lignite Ash Beam-scanner
photographs
225 demonstrate
the relationships
among elemental
constituents in the deposits. Forty-two discrete groups of elements were identified, each of which may represent more than one chemical compound.
References 1. Northern States Power Company, Annual Report, Minneapolis, Minnesota, 1966. 2. Everett A. Sondreal, Wayne R. Kube, and James L. Elder, U. S. Bw. Mines Rept. Invest., No. 7158 (1968), 94 pp. 3. Battelle Memorial Institute, A Review of Available Information on Corrosion and Deposits in Coal and Oil Fired Boilers and Gas Turbines. American Society of Chemical Engineers, New York, 1959, p. 111. 4. G. H. Gronhovd, R. J. Wagner, and A. J. Wittmaier, Trans. Sot. Min. Engrs., 238 (1967), 317-318. 5. H. R. Johnson and D. J. Littler (eds.), The Mechanism of Corrosion by Fuel Impurities, Butterworths, London, England, 1963, pp, 103, 143. 6. Thomas D. Wilhelm, Sample Preparation and Electron Microprobe Analysis of Lignite Ash Deposits. Unpublished thesis, University of North Dakota, 1968. 7. N. A. A. Procter and G. H. Taylor, J. Roy. Microscop. Sot., 85(3) (1966), 283-290. 8. J. J. Renton, PYOC. W. Vu. Acad. Sci., 37 (1965) 156-9. 9. M. L. Odenbaugh, Sintering Characteristics of Lignite Ash. Unpublished thesis, University of North Dakota, 1966.
Accepted July 9, 1969
209
Sample Preparation and Electron Microprobe of Lignite Ash Deposits WALTER
W. FOWKES,*
THOMAS
D. WILHELM,b
Examination
AND WAYNE R. KUBEc
A suitable technique was developed for preparation of lignite ash deposits for electron microprobe examination. Prepared samples were analyzed for concentrations and distribution relationships of seven elements at four separate positions in the bulk deposit. Fortytwo discrete groups of these elements were identified, each of which may represent more than one chemical compound.
Introduction Varying degrees of fireside boiler-tube
fouling are experienced
in the com-
bustion of pulverized coal at power-generating stations. The heated ash entrained in the combustion gases contains material which first deposits in a thin layer on fireside surfaces of walls and tubes. Other ash particlcs which contact this initial layer tend to adhere physically or react chemically,
forming additional deposits
which grow into the gas stream. As the inner layers are gradually shielded by the growing thickness of the deposit, the temperature at any given point in the ash deposit drops, owing to the insulating effect of the ash. The inner layers, being cooled, develop the strength
necessary to support the increasing
mass of the
deposit. If ash deposition continues, operating efficiency of a boiler is significantly reduced, and it may be necessary to take the unit “off the line” for cleaning. This down-time
represents
a loss of approximately
company for a 200-megawatt availability.
generator,i
$500
emphasizing
per hour to the power the importance
of boiler
a Project coordinator, Grand Forks Coal Research Laboratory, Bureau of Mines, U.S. Department of the Interior, Grand Forks, North Dakota. b Chemical engineer (graduate student), Department of Chemical Engineering, University of North Dakota, Grand Forks, North Dakota. eProfessor of Chemical Engineering, University of North Dakota, Grand Forks, North Dakota; research chemical engineer, Grand Forks Coal Research Laboratory, Bureau of Mines, U. S. Department of the Interior, Grand Forks, North Dakota. Metallography,
2 (1969) 209-225
Copyright 0 1969 by American Elsevier Publishing Company, Inc.
210 The
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube strength
of the ash and, therefore,
its resistance
to cleaning varys in
relation to the mineral content of the lignite burned. A survey of the lignite mines in the Northern Great Plains Province has shown considerable variability in the percentage of ash and in the proportions of its constituents, both between different mines and also within the same mine.2 Tests have demonstrated that the degree of fouling caused by ash from these lignites may be related, at least in part, to the concentration of sodium in the coal, reported as the oxide in the ash.3-5 Excessive fouling in a commercial boiler may occur in a period of operation as short as 3 days when burning “high-sodium”
lignite; periods of up to 18 months,
or more, between shutdowns have been achieved by burning lignite of low sodium content. In examining the problem of ash and slag deposition, a detailed structural and elemental picture of the deposits themselves would be very helpful. A comparison of the nature of these deposits with that of the original mineral complement
of
the coal burned should permit the investigator some insight into the behavior of these minerals in the combustion zone and the character of the resultant materials responsible
for the initiation
formed on heat-exchanging
and development
Choice of the electron microprobe information on elemental composition structural
deposits
as a promising facility to furnish the and distribution posed the problem of
handling and preparing for examination configuration,
of the troublesome
surfaces within the boiler.
integrity,
samples of materials varying widely in
and homogeneity.
Limitations
on sample
size and necessity of a critically plane surface for examination indicated that an embedding procedure would be most useful. In addition, a satisfactory procedure must permit almost perfect impregnation and irregular outlines. One objective
of the investigation
of porous samples having odd
herein discussed was to devise a suitable
technique for preparation of samples of ash and fireside deposits for examination by microprobe;
another-development
of a plausible mechanism
of deposition-
will be dealt with elsewhere. This present report is based largely on a thesis submitted to the University of North Dakota by Thomas D. Wilhelm, working under a fellowship agreement between the University and the U.S. Department of the Interior,
Bureau of Mines.6
Experimental Six North content
Procedure
Dakota
lignites were chosen for testing
and mine location.
A high-sodium
on the basis of sodium
and a low-sodium
sample were
selected from each of two different mines. Two additional samples from other mines were selected because severe fouling characteristics had been observed with these lignites in commercial
use. Ash deposits were collected by burning
Microprobe of Lignite Ash
211
each of the selected coals in a laboratory pilot-plant combustor conditions
in a commercial
pulverized-coal-fired
which simulates
unit. Deposits
were formed
on air-cooled, &inch stainless-steel tubes at a tube temperature of 535” & 25°C. The collector tube was in the simulated superheater area (gas temperature 1100%).
Linear gas velocity was 30 fps, and the heat release rate was approx-
imately 13,000 Btu/f@ per hour, closely apprcximating mercial furnace.
For satisfactory
microprobe
examination,
conditions
vertical variations
in a com-
in the sample
surface must not exceed 2 to 3 microns. Ash samples collected from boiler-tube surfaces are usually a conglomeration
of fused and/or sintered solid and hollow
spheres and inclusions, which have highly irregular surfaces. Compressive strength of the deposit from lignite ash is usually too low to permit polishing in the original state; consequently method
of impregnating
polishing.
it was necessary
to develop a satisfactory
the ash with a suitable binder to allow cutting
Only then could the samples be satisfactorily
and
examined by using the
electron microprobe. The impregnating procedure of Procter and Taylor’ and the resin suggested by Rentons proved to be the most satisfactory for the deposits examined. Two sample holders were constructed (see Fig. 1): (1) a brass upright cylinder, l-inch I.D., for use with small samples apart from the collector tube, and (2) a form for horizontal use consisting of a cutaway section of l&-inch-I.D. steel pipe, 6 inches long, fitted with clamp-on ends to accommodate
samples adhering
to a cutoff section of the collector tube. In use, the inside of the sample holder was coated lightly with a layer of silicone
FIG. 1.
Impregnating
system and sample holders.
212
Walter
W. Fowkes,
Thomas D. Wilhelm and Wayne R. Kube
stopcock grease. The holder containing the sample was placed in a vacuum desiccator fitted with a dropping funnel. The system was then evacuated to about 1 mm Hg by means of a mechanical pump and maintained at this pressure for 3 to 4 hours to remove air from the pore spaces of the sample. Resin and activator were mixed as needed and admitted through the dropping funnel in portions sufficient to cover the sample while maintaining
the reduced pressure
by continued pumping. Reduced pressure was maintained until bubbling ceased, indicating essentially complete degassing of the sample. At this point, the pump was stopped and air was admitted slowly to the system, restoring atmospheric pressure in about 3 minutes. The pressure increase forces the resin into the pores of the sample. The resin was then cured, either by heating it in an oven at 70°C for 4 hours or by permitting it to stand at room temperature for 36 hours. The latter method gave better dimensional
stability.
The hardened resin, containing
the sample,
was then removed from the mold, and the silicone grease was removed by wiping. This technique produces a sample with characteristics satisfactory for polishing.
It is also suitable for embedding
content, as well as other coal substances Cutting, The
Grinding,
coal samples having full moisture
such as dried coal, char, or coke.
and Polishing
ash sample
completely
encased
in hardened
resin was cut without
difficulty on a band saw into lengths of less than 1 inch; excess resin was removed on a belt sander. Prior to grinding, the samples were mounted on microprobe sample holders, using Lakeside 70 as the bonding agent. The samples were surfaced on a metallographic
grinding wheel, starting with
120-grit grinding paper and following this with successively finer grades-180, 240, 320, 400, and 600 grit-rotating the sample 90° between each grade. Care was exercised on those samples that included a portion of the metal collector tube to not grind in a direction that would carry grinding particles from the tube to the ash. This procedure minimized metal carryover onto the ash sample. Each stage of grinding was terminated approximately 30 seconds after the grinding marks of the previous grade (at a 90” angle to the new grinding direction) were eradicated. As soon as the 600-grit
grinding was finished, the sample was ready
for polishing. Polishing was carried out in four steps with four sizes of diamond polishing compound-9-,
6-, 3-, and l-micron
particle sizes. The polishing
compound
(paste form) was distributed sparsely (dye from the polishing compound barely visible) on the polishing cloths. d Lapping oil was spread in a narrow band across d METCLOTH was used with 9- and 6-micron polishing compounds; TEXMET cloth with 3- and l-micron pastes (A. I. Beuhler, Inc., Evanston, Illinois). Use of trade names is to facilitate identification only and does not imply endorsement by either the Bureau of Mines or the University of North Dakota.
Microprobe of Lignite Ash
213
the radius of the rotating polishing wheel. The sample was polished by using the same technique as in grinding, except that in polishing it becomes increasingly difficult to determine when a given step is finished. The completion of each stage was verified by examination at 200 x under a microscope. On completion of the fourth step, the sample was ready for examination by microprobe.
Microprobe
Examination
Instrumentation
available
for this investigation
Electron Probe Microanalyzer for strip record,
digital readout,
display
output,
of x-ray
was the Norelco
AMR/3
fitted with one vacuum goniometer and arranged and automated
backscatter,
beam-scanning
and sample current
with visual
signals.
The
area
scanned by the beam was a square, 320 microns on a side. The signal from the x-ray detector was monitored by the grid of one oscilloscope while the sample current
was monitored
configurational Each
by a second,
resulting
in simultaneous
elemental
and
maps of the sample area being examined.
polished
surface
was coated under vacuum
with a layer of carbon
(estimated thickness 50 A) to improve conductivity of the surface. The coated sample was then inserted into the microprobe for examination by the following procedure. The sample was scanned over 320-micron-square
areas at each of positions 1,
2, 3, 4, and 5 (see Fig. 2). The tin layer of ash at positions to such a small percentage
FIG. 2.
1 and 2 contributed
of the area scanned that they were not considered
Positions examined by microprobe.
I
33.9
41.0 31.2 17.1 20.0 19.0 21.7 25.0
CA
INC
3.6
38.1 -7 24.4 -22 24.0 40 24.6 23 17.4 -8 27.0 24 25.9 8.3
CD
CaO CD
INC
-18.2
4.6 6.2 35 2.8 2.0 -29 10.3 4.6 -63 15.2 8.1 -47 7.8 7.6 -3 12.0 14.5 21 8.8 7.2 -14.3
CA
Fez%
Al,%
7.4 13.8 9.1 12.0 9.1 8.7 10.0
INC
39.0
9.5 28 13.0 -6 15.9 75 20.5 71 13.9 53 10.6 22 13.9 40.5
CA CD
* CA = percentage concentration in laboratory prepared ash. t CD = percentage concentration in deposit, as calculated from microprobe data. * INC = CD - CA/CA x 100 = percentage increase in concentration from CA to CD.
17.9
8.8 9.0 2 9.2 12.3 34 2.5 1.9 -20 5.8 5.3 -9 8.4 11.9 42 6.3 9.4 49 1.0 1.6 +60 5.1 8.6 69 20.2 23.5 16 4.1 5.8 41 9.5 11.5 21 5.2 6.3 21 8.4 9.9 0 5.9 7.9 34.2
HSV LSV HSB LSB N30 TS Average Increase in average concentration
CA CD INC
CA* CDtINC*
MgC
Coal title
Na,O
14.0 31.5 17.5 16.9 19.1 16.1 19.2
CA
26.7 48.8 27.5 24.4 30.6 28.0 31.0
61.0
91 55 57 44 60 74 63.5
CD INC
SiO,
CD
INC
-69.0
11.4 5.0 -56 6.2 3.8 -39 27.6 11.1 -60 27.1 11.6 -57 14.7 1.9 -87 24.9 1.3 -95 18.7 5.8 -65.7
CA
SO,
COMPARISON OF CONCENTRATIONS IN LABORATORY PREPARED COAL ASH WITH CONCFNTRATIONS IN TUBE DEPOSITS
TABLE
Microprobe of Lignite Ash
215
reliable data. Data taken at these two positions calculations. Seven elements routinely scanned calcium,
iron,
aluminum,
silicon,
and
sulfur.
were omitted in subsequent were: sodium, magnesium, Potassium
was
not
quantities greater than 1%. The counts on all scans were recorded of two sweeps-about 53 seconds. At the same time, a photograph the x-ray signal displayed The
output
of the
concentrations approximation
found
in
for the period was taken of
on the beam scanner.
microprobe
was standardized
in order
to estimate
the
of elements in the unknown samples. A semi quantitative to concentration of the different elements was obtained by
ash having a composition similar to that of the boiler a “synthetic” Intimate mixture was attained by pulverizing in a shatter box and
preparing deposits. pressing analysis
a 1-inch-diameter pellet at 30 tons per square inch. An x-ray fluorescent was performed on a 320-micron-square area (raster mode) of the pellet
in the microprobe, in deposit
analyses.
using parameters
selected
Each element
was counted
on the signal intensity. concentration, concentration
Assuming
the number (see Table
to duplicate
a linear relationship
between
of counts was converted I). Th is standardization
analysis
of each scan area covered
present.
The
resin-impregnated
by the beam
sample
those to be used later
for 10 to 50 seconds,
depending
counting
rate and
to counting rate per 1 o/0 of permitted an approximate scanner
of ash deposit
in terms
of elements
was then
placed
in the
microprobe, and an identical analysis was performed. The percentage of concentration of each element was estimated on the basis of values obtained on the synthetic
sample
the photographs
and the results
were normalized
of x-ray emissions
arrangements were derived,-that compounds or mineral species. This deposit
procedure
is subject
larger
than
matrix-effect
that
to possible
accelerating-voltage
probable
errors
From these data and
the relationships
as: (1) The
“standard,”
of chemical
occurrence
of chemical
surface
of the ash
owing to the many voids that
These voids are filled with resin and make it necessary to to 100%. (2) The particle size of the deposit is, in general,
of the synthetic
corrections
standard synthetic deposit specimen;
Results
is, the
is less dense than the prepared
occur in the former. normalize the results
to 100%.
from the sample,
ash, which
(enhancement
could
also generate
or absorption)
ash was not analyzed therefore, no correction of the microprobe
errors.
were applied.
(3) No
And (4) the
immediately following analysis of the arising from possible dirft in the high
could be applied.
and Discussion
Photographic Representation of Elements Two Figures 6
of the six sets of pictures 3 through
6 represent
deposits
taken
are presented
from a high-sodium
in Fig. lignite:
3 through
10.
Fig. 7 through
216
Walter
W. Fowkes,
Thomas D. Wilhelm and Wayne R. Kube
FIG. 3.
x. -ray
and sample current output patterns, N30, position 2.
FIG. 4.
X-ray
and sample current output patterns, N30, position 3.
217
Microprobe of Lignite Ash
FIG. 5.
FIG. 6.
X -ray and sample current output patterns, N30, P
X-ray
.tion 4.
and sample current output patterns, N30, position 5.
218
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube
FIG. 7.
X-ray
and sample current output patterns, LSV, position 2.
FIG. 8.
X-ray
and sample current output patterns, LSV, position 3.
Microprobe
of Lignite Ash
FIG. 9.
X.-1‘ay and sample current output patterns, LSV, positia m 4.
r::-k
FIG. 10.
219
X-ray
&_
‘_
c-
-3
and sample current output patterns, LSV, position 5.
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube
220 10 represent
those
from
a low-sodium
lignite.
Each
figure,
containing
eight
pictures, is the composite representation of the distribution of the seven elements at one position in the sample. The eighth picture represents the sample current pattern,
indicating
the actual configuration
in concentration
of the elements
of the deposit
of interest
area scanned.
in going from the inner
Changes
layer to the
outer layer were detected by taking data at three positions, covering the depth of each sample (see Fig. 2). Each position was square, the length of a side being 320 microns. three
Nine
positions
hundred
and sixty microns
perpendicular
to the tube)
(the sum of the lengths
represented
one-fifth
of the
to one-tenth
the
total thickness of the ash deposit. The relative concentrations of individual elements in the overall deposit were found to be unique for each lignite tested (see Table
I).
Figure
3 shows
adjacent
the presence
to the tube.
The
shows less sodium and pictures are representative high concentration Figures
of a sodium-sulfur
signal
of sodium
signals
silicon signals concentrations
Figure
at position
2,
4, at position
3,
less sulfur but a high concentration of iron. These of a high-fouling lignite. Original ash analyses show a and an average
7 and 8 show the x-ray pattern
and sulfur
compound
for iron is negligible.
are weak at both
to high concentration
typical
position
at a low-fouling
2 and 3. Calcium,
are strong. For this type of deposit, of both sodium and sulfur.
of iron. lignite.
Sodium
aluminum,
the original
and
ash shows
low
Relative Concentrations and Relationships of the Hements The
portion
varied
between
(voids
were
malized
of the 9%
filled with
to account
area
scanned
and 81%, epoxy
which
depending resin).
for this variation
contained
inorganic
on the void volume
The
calculated
(see Table
constituents of the deposits
concentrations
were
nor-
I). On the basis of these normal-
ized data, average concentrations of each of the seven elements were calculated for each position for the four high-sodium lignites and for the two low-sodium lignites.
These
arithmetic
means
changes between positions 3 and low-sodium group. A concentration if the data met all the following 1. Change
between
2. Minimum from
change
18% to 20%,
3. Minimum at position
positions
used
to detect
element
concentration
5 for the high-sodium group and was said to change between positions
arbitrary
criteria:
3 and 5 had to be consistent
in concentration
for the 3 and 5
was 2 percentage
in sign. units
(for example,
or from 7% to 5%).
change
in concentration
3 (for example,
thus, the concentration to meet criterion 3).
were
20%
at position
was 20%
of a 15%
of the concentration
concentration
at position
5 must be either less than 12 or greater
present 3 is 3%; than 18
Microprobe of Lignite Ash
FIG.
FIG.
12.
11.
Deposition
Elemental
concentration
selectivities,
percent
versus position.
concentration
versus position.
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube
222
On the basis of these
criteria,
the following
concentration (see Figs. lla, llb, coals decreased 37 y0 from position coals decreased
42%
from
elements
were said to change
in
and 12): (1) Sodium oxide in high-sodium 3 to position 5. (2) Iron oxide in low-sodium
position
3 to position
5. Calcium
oxide
narrowly
missed meeting the criteria, increasing consistently in concentrations by 18 %. The high concentration of sodium at position 3 may be caused by highly concentrated
sodium
next
to the tube,
which
was the case in four
of the
six ashes
examined (see Fig. 2 and Figs. 3 through 10). No other characteristics noted peculiar to either high-sodium or low-sodium lignites.
were
Comparisons with Ash in Original Coal A comparison determined
of the elemental
concentrations
on laboratory-prepared
following
selectivities
of the six samples
of deposition
showed
the laboratory-ashed
1. Magnesium
lignite
in the deposits
along the axes of the deposits.
only a loss (or gain) in the element
samples,
said to have occurred
in the original
ash, with those
positive
(or negative)
ashes,
indicated
as the
If at least five
in comparison
selectivity
to
of deposition
was
in the tube deposit
than
in the tube
than
(see Fig. 12):
oxide concentration
was 19 “/b greater
in the ash. 2. Aluminum in the ash. 3. Silicon the ash. 4. Sulfur
oxide concentration
dioxide
was 37%
concentration
concentration,
greater
was 53 o/o greater
deposit
in the tube deposit
as trioxide,
was 33%
as large in the tube
(Table
II)
showing
groupings
given
point
than
in
deposit
as
in the ash. A listing appeared
was
prepared
simultaneously
listing shows photographic
at any
An element members
picture
of elements
samples
which
examined.
was considered appeared
of the group,
the source
of the deposit-that
as part of a group
at any point as evidenced
is, low-sodium
if its x-ray pattern
that was also occupied by the pictures
leading,
since the method
among
FeS,
does not distinguish
FeSO, , and
more than
forty-two
Fe,(SO,),
. The
on the beam-
of the individual
compounds.
between list
Fe,O,
of groups,
or
by the rest of the elements.
By this method, forty-two different groups of elements were identified. respect to possible chemical combinations, this listing may be somewhat
represent
This
the total area occupied by each group, expressed as squares of a grid, and the area appearing at each of positions 3,4, and 5 in the
deposit, as well as designating high-sodium lignite. scanner
in the
and FesO, therefore,
With mis-
, nor could
TABLE
II
AREA OCCUPIED BY EACH GROUP OF ELEMENTS, INCLUDING ALL SIX LICNITFS
Total relative area
Area at position numbers
Total area in high sodium lignites *
Total area in low sodium lignites*
3
4
5
1 18 7 7 2
1 11 2 0 0
0 6 1 5 2
0 1 4 2 0
1 17 5 5 0
0 1 2 2 2
1 4 11 1 18 25 8 3 3 5
0 4 11 I 9 0 3 0 3 4
1 0 0 0 2 0 4 0 0 1
0 0 0 0 7 25 1 3 0 0
1 0 11 1 16 25 1 3 1 5
0 4 0 0 2 0 7 0 2 0
1 16 1 4 1 15 8 3 5 1 3 50 1
1 9 1 4 1 15 0 3 0 0 1 30 0
0 5 0 0 0 0 8 0 5 1 2 10 1
0 2 0 0 0 0 0 0 0 0 0 10 0
1 4 I 4 1 0 8 2 0 0 1 15 0
0 12 0 0 0 15 0 1 5 1 2 35 1
27 1 18 19
17 0 10 1
1 I 5 18
9 0 3 0
24 1 9 0
3 0 9 19
1 2 16 1 3 3
0 0 2 1 1 0
34 30 1
0 0 0
0 30 1
34 0 0
34 30 1
0 0 1
Na, Mg, Ca, Fe, Al, Si, S 128
0
58
70
108
20
146
171
189
339
167
Chemical
group
Na
Ca Fe Si S Na, Mg Na, Fe Na, S Mg, Fe Ca, Fe Ca, Si Ca, S Fe, Si Fe, S Al, Si Na, Mg, Fe
Na, Na, Mg, Mg,
Al, Si Al, S
Ca, Fe Ca, Al
Mg, Ca, S Mg, Fe, Al Mg, Fe, S Mg, Al, Si Mg, Ca, Ca, Ca,
Al, S Fe, Al Al, Si Al, S
Na, Na, Mg, Ca,
Ca, Al, Ca, Al, Ca, Al, Fe, Al,
Na, Na, Mg, Mg, Mg, Mg,
Mg, Mg, Ca, Ca, Ca, Fe,
Si S Si Si
Ca, Fe, S Ca, Al, Si Fe, Al, Si Fe, Al, S Al, Si, S Al, Si, S
Na, Mg, Ca, Fe, Al, Si Na, Mg, Ca, Al, Si, S Mg, Ca, Fe, Al, Si, S Totals * Four high-sodium
510 lignites;
1 2 16 1 3 0
I 0 14 0 0 3
two low-sodium
lignites.
Walter W. Fowkes, Thomas D. Wilhelm and Wayne R. Kube
224 The identification
procedure
is the ever-present
problem
cannot
be considered
of elements
rigorously
in any given
definitive.
area overlapping
There into the
area of another group. Such overlap involves both discrete groups. The overlap area is considered as a third group, or fourth, if not all elements of the two groups overlap in the same place. No attempt was made to identify sodium
aluminosilicates
lignite
ash.3
Groups
(Table II). Areas of the elements blended distance given
specific
calcium
have
that not
to
contain been
compounds.
sulfate
corresponding
deposit
sought
phenomenon
and
these
of the
However,
well-known
compounds
a well-mixed
appreciably
of six or all seven
are
calcium
have
been
conglomeration
discussed
elements
in the
under
and
components
in found
of all the
literature.
The
consideration
being
together seems to be independent of the sodium content once the from a specific position in the deposit to the tube wall has reached a critical
although
magnitude.
it should
On the basis particle deposit
This
be a function
of the samples
magnitude examined,
that impinges upon it. The will vary with the inorganic
percentages
in the coal ash, which
the breaking
strength
may show random
has not
of the insulating
been
generally
properties
it is believed
that
physical and chemical chemical constituents
generally
of all seven
the ash holds properties and their
any
of the relative
differ for each coal. For example,
of one ash may be twice that of another, mixing
identified,
of the ash involved.
even though
both
elements.g
Summary A suitable technique for preparation of coal ash deposits electron microprobe was adapted. Six samples were prepared analyzed separate
for
elemental
positions
concentrations
in a tube deposit,
and
distribution
using the electron
to be examined by and systematically
relationships
microprobe.
at four
The samples
were collected on air-cooled tubes placed in the combustion-gas stream during tests burning four high-sodium lignites and two low-sodium lignites in a pilotplant
combustor.
Sodium-sulfur compounds were found in a layer completely surrounding the tube. The thickness of this layer was reduced where covered by the bulk of the deposit. Sodium
concentration
in deposits
tration in deposits from low-sodium decreased consistently toward the aluminum
concentrations
were
from high-sodium
lignites
and iron concen-
lignites were highest near the tube outer edge of the deposit. Silicon
consistently
higher
laboratory-prepared ash, while the concentration one-third that in the original ash.
in the deopsit of sulfur
than
in the deposit
and and
in the was
Microprobe of Lignite Ash Beam-scanner
photographs
225 demonstrate
the relationships
among elemental
constituents in the deposits. Forty-two discrete groups of elements were identified, each of which may represent more than one chemical compound.
References 1. Northern States Power Company, Annual Report, Minneapolis, Minnesota, 1966. 2. Everett A. Sondreal, Wayne R. Kube, and James L. Elder, U. S. Bw. Mines Rept. Invest., No. 7158 (1968), 94 pp. 3. Battelle Memorial Institute, A Review of Available Information on Corrosion and Deposits in Coal and Oil Fired Boilers and Gas Turbines. American Society of Chemical Engineers, New York, 1959, p. 111. 4. G. H. Gronhovd, R. J. Wagner, and A. J. Wittmaier, Trans. Sot. Min. Engrs., 238 (1967), 317-318. 5. H. R. Johnson and D. J. Littler (eds.), The Mechanism of Corrosion by Fuel Impurities, Butterworths, London, England, 1963, pp, 103, 143. 6. Thomas D. Wilhelm, Sample Preparation and Electron Microprobe Analysis of Lignite Ash Deposits. Unpublished thesis, University of North Dakota, 1968. 7. N. A. A. Procter and G. H. Taylor, J. Roy. Microscop. Sot., 85(3) (1966), 283-290. 8. J. J. Renton, PYOC. W. Vu. Acad. Sci., 37 (1965) 156-9. 9. M. L. Odenbaugh, Sintering Characteristics of Lignite Ash. Unpublished thesis, University of North Dakota, 1966.
Accepted July 9, 1969