/£ ?6 ~ X/3*jJt.

No. 3

MHD MATERIALS - SEED/SLAG INTERACTIONS AND EFFECTS

QUARTERLY PROGRESS REPORT

April - June 1980

Samuel J. Schneider

Project Manager

Center for Materials Science

U. S. Department of Commerce

National Bureau of Standards

Washington, D. C. 20234

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

MHD OFFICE

"This report was prepared as an account of work sponsored by the United

States Government. Neither the United States nor the United States

Department of Energy, nor any of their employees, nor any of their

contractors, subcontractors, or their employees, makes any warranty,

express of implied, or assumes any legal liability or responsibility

for the accuracy, completeness, or usefulness of any information,

apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights."

TABLE OF CONTENTS

Page

I. SUMMARY OF PROGRESS TO DATE 1

II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS 2

1. Thermochemistry of Seed and Slag 2

2. Electrical Conductivity and Polarization 11

3. Corrosion of Downstream MHD Components 21

i

I. SUMMARY OF PROGRESS TO DATE

1. Thermochemistry of Seed and Slag

Vapor pressure data of K(g) over two K2 0-Ca0-Al 2 0 3 -Si02 samples

are reported. Pressures over these compositions are orders of magnitude

greater than over a sample containing roughly the same amount of K2 0 but

lacking CaO.

Low temperature melting compositions in the quaternary system were

determined.

2. Electrical Conductivity and Polarization

The electrical conductivity of a slag containing 20% iron by weight has

been completed. The data reinforces the suggestion that the primary

conducting mechanism in iron-containing slags at high temperature is

iron ion conductivity. A paper giving the information leading to this

conclusion was presented at the 7th International Conference on MHD Electrical Power Generation, held at MIT, Cambridge, Mass., June 16 - 20, 1980.

A description of the conductivity measurement technique which was developed

over the past year in order to obtain the data is given.

3. Corrosion of Downstream Components

Type 316 and Type 304 stainless steel tubes were exposed for 4 hours

to various oxygen/fuel ratios and I^SO./KgCOj seed combinations. These

data indicate that seed could cause^tube Touting; fouling extent differs

under the various test conditions. Wastage evaluations, started last

quarter, indicate that Type 304 stainless steel corrodes (weight loss)

at the rate of about 4 mm per year.

1
II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS

1. Thermochemi stry of Seed and Slag'(E. R. Plante and L. P. Cook)

A. Vaporization Studies (E. R. Plante)

Measurements of the K and 02 vapor pressure on two K2 0-Ca0-Al 2 0 3 - Si02 samples were completed. As in prior work, vapor pressures were determined using a Knudsen-effusion , modulated beam mass spectrometric

method.

Both samples consisted of mechanical mixtures of potassium aluminosilicates with calcium silicates or calcium aluminosilicates.

Samples have been selected partly on the basis of their phase-equilibrium

behavior and partly in order to be able to vary the composition variables to determine their effect on the K2 0 activity. For this work, it

is advantageous if vaporization of K and 02 takes place from a homogeneous melt so that the bulk wt 1 K2 0 is an appropriate composition

variable. Since the vapor pressure of the other component oxides (CaO,

A1 2 0 3 , Si02 ) is low or their activity in the condensed phase is low,

only the K2 0 component was vaporized to any appreciable extent during

the current measurements.

Table 1 lists the sample designations, mineral components and

starting compositions for the two samples in which measurements have

been completed, KCAS-VP-1 and KCAS-VP-2. Similar data for KCAS-VP-3 on which measurements are just beginning are also included.

During the vaporization measurements, the K 2 0 composition for KCASVP-1 varied from its initial value of 14.8 wt% K2 0 to a final value of

1.6 wt%. Within experimental error this corresponds closely with the

complete vaporization of K2 0 from the sample. The measurements on KCASVP-2 covered the K2 0 composition range from 11.5 to 6.2 wt% at which

point they were terminated because a small droplet of sample lodged in

the orifice. Because sample preparation involves use of carbonates,

residual C02 dissolved in the sample at high temperature is expelled

with considerable force leading to sample spattering within the Knudsen

cell. Some of the vapor pressure data obtained are shown in figures 1-4

and discussed in more detail below.

Vapor pressures of K(g) from the KCAS-VP-1 sample over the range in

which the composition varied from 14.8 to 12.4 wt% K2 0 are shown in

figure 1. The line shown through the experimental points was obtained

by least squares without regard to the change in composition of the

condensed phase.

It should be noted that there is quite good linearity in the data over the range of the experiment even though the sample below the temperature at which a liquid phase first forms (1310 °C) , consists of a mixture of mineral oxides. However, there is a slight increase in the K pressure leading to positive deviation from the linear plot as the

temperature is increased above the melting point. At this point in the

experiment, the major part of the K2 0 is combined in the mineral KAlSi 04 , and this observation suggests that the effect of CaO on the K2 0 activity

at high concentration of K2 0 30 wt% in KA1 Si

0

4 ) is quite small. The

lower line in figure 1 shows the K pressure over a sample obtained from

KA1 S i

0

4 in which the K2 0 composition was reduced by vaporization to

2
14.7 wt%- K2 0. Although the concentrations of Si02 and AI 2 O 3 are significantly higher (46.1 and 39.1 wt%, respectively) than in the KCAS-VP1 sample they are in reality reduced somewhat by an indeterminate amount

because of the presence of crystalline mullite. Yet, the K pressure

over the calcia containing sample is 250 times higher, which is indicative of the large increase in K 2 0 activity due to the presence of CaO.

Figure 2 shows the last series of data using the KCAS-VP-1 sample. The

straight line through the points was fit using data in the composition

range 4. 8-4.0 wt% K2 0. The additional points show the rapid decrease in

K pressure as the K2 0 content becomes depleted. The upper curve shows

the original K pressure in the 14.8-12.4 wt% K2 0 range while the lower curve shows the data for the KAlSi 04 sample depleted to 14.7 wt% K2 0.

Comparison of pressures is a little tenuous for this case because of the

difference in slopes. At 1625 K (104 /T = 6.25) the pressure of K has

decreased to about 0.1 that in the original sample but is still 36 times

greater than that of the 14.7 wt% K2 0 sample derived from KAlSiOif.

Figure 3 is a comparison between the KCAS-VP-1 data in the 14.8-

12.4 wt% K2 0 range, the KCAS-VP-2 data in the 11.4-10.7 wt% K2 0 range

and the 14.7 wt °l K2 0 sample obtained from evaporation of K2 0 from

KAlSiOit. At 1 625 K the KCAS-VP-1 pressure is 4.5 times greater than

that of the KCAS-VP-2 data, which in turn is 65 times greater than the

KAlSi 04 pressure. Reference to Table 1 shows that the KCAS-VP-1 sample

initially has about ten percent greater CaO content and ten percent less

A1 2 0 3 content than the KCAS-VP-2 sample. The higher pressure over the

KCAS-VP-1 sample is consistent with its higher K2 0 content and the

expected behavior of CaO in increasing the K2 Q activity.

Figure 4 shows a comparison between the KCAS-VP-2 pressure data in

the 6 . 5-6.2 wt °l K2 0 range, the 11.4-10.7 wt% K2 0 range and the 14.7 wt%

K2 0 sample obtained from evaporation of KAlSi04

. At 1625 K, the pres- sure of the 11.4-10.7 wt% solution is 2.8 times that of the 6 . 5-6.2 wt%

K2 0 range while the pressure of the 6 . 5-6.2 wt% K2 0 solution is 22 times

that of the 14.7 wt% K2 0 sample derived from KA1 S i

4 .

While the pressure of K above a solution containing K2 0, CaO, A1 2 O 3 and Si02 will depend on the concentration of each of the component

oxides the major effect of CaO appears to be that of increasing the K2 0

activity at lower K2 0 concentrations.

Table 2 lists the experiment identification, K2 0 concentration

range, the slope and its standard error and the intercept and its standard error when the K pressure is fitted to the equation:

log P = A/T + B

It appears that the first two slopes of the KCAS-VP-1 series and

the first slope of the KCAS-VP-2 series differ from the remaining slopes

which average out to be 16800 ± 500. At the present time, it is not

certain whether the changes in slope are due to the gradual formation of

solutions from the original mineral constituents or result from ignoring

the change in K2 0 composition during the experiments. However, this

data requires further analysis and appears in this report primarily as a matter of record.

3
Table 1

Mineral Constituents and Composition of Vaporization Samples

Samp! e

Composition, vit%

Mineral Constituents K^Q CaO AloO^ Si0 9

KCAS-VP-1 KA1 Si0

4

Ca 9 Si0

4 14.8 27.5 22.2 35.5

Ca^Si^Oy, Ca ^A1 i Oy

KCAS-VP-2 KA1 S i

0

4

, KA1 S

i

2

0

6 11.5 17.6 33.7 37.2

Ca^Al^SiOy, CaAl^Si^Op

KCAS-VP-3 KA1 Si

0

4

, CaSi0

3 14.8 24.3 16.0 44.9

4
Table 2

Tabulation of Slope and Intercept Data for K Pressure Data

Expt. Id. wt% K

2 0 A CTtn 6

KCAS-VP-1

3-13-80 14.8-12.4 -20047 391 8.144 .255

3-14-30 12.4-9.3 -19071 524 7.278 .234

3-18-80 9. 3-7.

-17429 452 6. n 58 .289

3-18-30 6. 2-4.8 -16372 210 5.068 .129

3-21-80 4. 8-4.0 -1 6896 279 5.289 .178

KCAS-VP-2

5-14-80 11 .4-10.7 -14632 434 4.200 .278

5-15-80 10.7-8.6 -16366 180 5.097 .112

5-16-30 8. 6-6.

-17571 257 5.656 .158

5-19-80 6. 5-6.2 -16606 526 4.992 .321

5
B. Phase Equilibrium Studies in the System CaO-KpO-Al pO^-SiO?

( L. P. Cook)

The Determination of low temperature melting compositions within

the quaternary system continued. Results are shown in Table 3 below.

Table 3

Approximate Compositions (in mole %) of Binary Invariant Melts

Within System K

? 0-Ca0-Al

03 -S i 0^

KAlSi0

4 /CaSi03 35/65

KAlSiO^/CaAl^SioOg 19/81

KA1 S i 0^/ Ca ^ A1 pSiOy 37/63

KA1 Si0

4 /Ca2 Si04 17/83

KAlSipOg/CaSiOp 39/61

KA1 SipO^/Ca ? Al pSiOy 43/57

KA1 SipOg/CaAl pSi o 0

3 30/70

PI ans : Extend K pressure measurements to additional compositions in

the K2 0 -Ca 0 -Al 2 03 -Si 02 system. Attempt to establish correlations of the

effect of components on the K2 0 activity. Phase equilibrium studies in

the Ca 0 -K2 0 -Al 2 03 -Si 02 system will be continued.

6
Potassium

Pressure

(atm)

1

0

4 /T ,K

Potassium pressure over KCAS-VP-1 . Series 3-13-80, 14.8-12.4 wt%

K2 0. Mote slight positive deviation from straight line in presFigure 1 sures observed above the initial melting point. Comparison curve pressure from a sample initially consisting of KA1 S i

O

4 , whose K2 0

composition has been reduced to 14.7 wt% K2 Q by vaporization.

7
Potassium

Pressure

(atm)

Potassium pressure over KCAS-VP-1 . Series 3-21-80, 4. 8-4.0 wt% K2 0

Figure 2 (straight line plot) followed by pressures from 4. 0-1. 6 vit% K2 0 as

y K2 0 is depleted from the sample. Comparison curves: Upper curve,

K pressure with 14.8-12.4 wt% K2 0; Lower curve, KA1 S i

4 depleted to

14.7 wt% K2 0.

8
Potassium

Pressure

(atm)

io 4 /t,k

Potassium pressure over KCAS-VP-2. Series 5-14-80, 11.4-10.7 wt%

K2 0. Compared with KCAS-VP-1 pressures, 14.8-12.4 wt% K 2 0 and

Figure 3 KAlSi 04 depleted to 14.7 wt% K2 0.

9
Potassium

Pressure

(atm)

Figure 4

’otassium pressure over KCAS-VP-2. Series 5 - 19- 80 , 6.5-6 2 wtX

<2 0. Compared with KCAS-VP-2, 11.4-10.7 wt /, K 2 0 and KAlSiOe

depleted to 14.7 wt % K2 O.

10
2. Electrical Conductivity and Polarization (W. R. Hosier)

Progress

Slag Electrical Conductivity

In order to further investigate the effect of iron on the electrical

conductivity of coal slag in relationship to other slag components, and

in relationship to the redox conditions surrounding the material, an additional batch of slag from the Bow, NH steam generating plant (but

containing extra iron) has been prepared. Iron was added in the form of

Fe 3 0 4 to bring the total iron content to 20 mass percent. This

material is difficult to form in the measuring crucible and must be

formed in a reducing atmosphere before contacts are attached. (See

below for an explanation of contacting procedure and four probe DC

measurements.) The conductivity measurements on this material have been

completed and the data is given in Figures 1 and 2. As in all previous

measurements on this slag, there is a break point in the curve above

1385 °C (1/T(K) % 6 x 10 4

) and all samples show nearly the same slope of

conductivity vs. reciprocal temperature regardless of the iron or potassium content and regardless of the ambient oxygen pressure. It is

evident, however, that the magnitude of the conductivity in this temperature

range is larger than for the data on Bow, NH slag reported in previous

quarterly reports beginning April - June 1979 except for that slag

sample containing an even larger amount of iron (23.4%) reported in the

last quarterly report Jan. - March 1980. Table 1 gives the conductivity

at 1450 °C of all the samples measured to date. The analyses shown are based on the original analysis of this slag as received from the Bow, NH

steam plant. This original analysis is given in Table 1, page 33 of QR

April - June 1979.

Table 1

Based on Initial Analysis Conducti vi

1

:y At 1450 °C

Mass % Fe Mass % K Air 5 ppm 0^

14.7 0.43 0.066 0.062

12.8 8.4 0.058 0.066

23.4 7.5 0.31

20.0 0.40 0.16 0.16

11
The data shown in Figures 1 and 2 show the same general characteristics

below 1385 °C as the previous data on this slag shown in QR April - June

1979 when the data is taken according to a prescribed r^utin^ which was designed to freeze in a specific ratio of iron ions (Fe 2 /Fe 3

) below

1000 °C. The previous sample contained only 14.7% iron and the con- ductivity on the lower branch of the conductivity curves (below 1000 °C)

is about an order of magnitude lower than for the data shown in Figure 1.

The high conductivity branch of the curves, however, are similar in

magnitude as well as shape. A preliminary investigation using SEM

analyses of these branches of the curves (below 1000 °C) in<Jicat£s

that the conductivity is not primarily a function of the Fe 2 /Fe 3 ratio frozen in by the quench to 900 °C (see QR April - June 1979) but

is due mostly to the crystalline component of the solidified slag.

Scanning electron microscope micrographs show crystallite bridging

in various degrees depending on the history of the sample. It is possible

that this bridging accounts for the somewhat unpredictable conductivity

below 1000 °C particularly since there does not seem to be a primary

dependence of the conductivity on the oxygen pressure (vis. Fig. 1 and

2). This concept is being investigated and will be reported on at a later time. A paper was presented at the 7th International Conference

on MHD, held at MIT in Cambridge, June 16 - 20, 1980, Session 5, Paper

No. D - 7 which gives a possible explanation for the high temperature

behavior (above 1385 °C) of iron-containing slags. It can readily be seen from Table 1 that the conductivity at 1450 °C depends on the iron

content and not on the amount of potassium (for the several amounts

shown in the table) and not on the oxygen partial pressure in the range

from 0.2 to 10” 6 atmospheres. The conclusion drawn then is that for

this slag composition, the conductivity above 1385 °C is due primarily

to the iron ions.

Measurement Technique and Sample Preparation

Measurements of the physical properties of materials at high temperatures

are becoming increasingly important particularly with respect to emerging

energy technologies. A few examples include materials used in magnetohydrodynamics, fuel cells, and batteries for energy storage and conversion

of chemical to electrical energy. The electrical conductivity of some materials for these technologies is important as all require the passage

of current (ionic and/or electronic). For those materials that remain

solid with relatively simple chemistry in a device, the conductivity

measurement over the useful temperature range is not difficult. However,

for those materials that may be in a solid and/or liquid state, depending

on temperature, the measurement becomes more difficult. Measurements

are complicated further by chemical interaction of a material with the

containing crucible, by chemical/phase changes with temperature and

oxygen pressure, by thermal expansion or contraction problems upon

solidification or cooling, and by the requirement of electrical contacts

that will give reliable results without disintegration. A measurement

technique has been developed over the last year which can alleviate

these problems. A brief description of the sample preparation and

measurement technique both of which are extremely important for obtaining

valid results on high temperature materials such as coal slag follows.

12
Generally, only four probe DC conductivity measurements will be discussed.

Two probe DC measurements give erroneous results when ionic conductivity

is involved, especially when ion exchanging electrodes are not used. AC

measurements are useful for samples having relatively high resistances

where capacitive reactance components can be determined at reasonable

frequencies (< several megahertz). For most high temperature materials,

however, the electrical conductivities are high enough (^.01 to 10 ohm ^m’ 1

) so that the required frequencies are too high to resolve the capacitivereactive components of the total impedance which are due to contact

barrier layers or grain boundary effects. Four probe AC measurements,

however, are a viable option but usually require more instrumentation

than DC measurements.

For materials which can be cut to a suitable sample size and will not

flow over the temperature range of the measurement, contacts can be

applied directly to the material by drilling small holes (using a diamond

dental drill) in the sample to attach the contact lead wires. This is

done by peening spongy platinum around the leadout wires, which are usually

platinum to withstand the high temperature oxidizing atmospheres. A

schematic diagram of a typical contact is shown in Figure 3. Generally

the portion of the wire in the hole should have a non-symmetrical con- figuration on the end to prevent the wire from turning in the hole and

to prevent pull-out from the adjacent peened platinum. In the schematic,

an oval disk is shown. This was made by inserting the leadout wire with

a small ball on the end through a hole in a piece of metal and simply

hammering the ball flat. Any non-symmetrical shape formed in any way

will suffice. Measurements using this contacting method have been made

on many ceramic materials up to 1700 °C. No contact resistances were

apparent. For best results, the materials should be fully sintered up

to the highest measuring temperature to prevent further densification

during measurement. Figure 4 shows a radiograph of a sample of

Y 0 95 Ca 0 05Cr0 3 with contacts applied as described above.

For a material such as slag that becomes molten within the range of

temperatures involved in the measurement, a different procedure was

used. Alumina crucibles which are readily available and which show

minimal reaction with the slag were chosen for the experiments. A

crucible is carefully filled to just under one-half full by repeated

melting of additions of slag powder. This is necessary to prevent the

material with absorbed gases from bubbling over the top during the

initial filling process. Holes are then drilled through the crucible

sides and into the slag for the four probes. Leads are attached using

the same procedure as described above. Figure 5 shows a radiograph of

a typical crucible with leads attached and numbered for the discussion

to follow.

Spongy platinum is commercially available from several companies

specializing in platinum products. It can readily be made, however,

from powdered platinum which is sintered lightly at 1200 °C for 1/2

hour. The resulting spongy solid mass can be cut into pieces and can be

pressed into the holes surrounding the leadout wires. For small samples

such as described above, the contacting must be done under a binocular

13
microscope. The peening tools can be made from any hardened metal

material formed into a suitable tip for compaction of the spongy platinum.

This method yields a leak-tight seal.

It can be seen (Fig. 5) that the size and shape of the molten sample

remains fixed during the course of the measurement. This is essential

for accurate determination of a conductivity. Furthermore, the electrodes

do not penetrate a surface exposed to the ambient atmosphere so that

oxidation-reduction conditions at the surface do not affect the conductivity

value.

The measurements are made using a constant current power supply. In certain temperature ranges and under certain conditions the material is an ionic conductor. Under these circumstances, care must be taken

during data acquisition to avoid polarization on the conductivity probes

(two center probes). Since the current is maintained constant, the

voltage measured between the conductivity probes is a true measure of

the bulk conductivity of the material, i.e., a = 1/p = £l/AV23 where A is

the cross sectional area of the sample, z is the distance between the

conductivity probes 2 and 3, and V 23 is the potential developed between

the conductivity probes 2 and 3 at a constant current I. This conductivity

may be electronic and/or ionic depending on the chemical (bulk and

phase) and physical state of the material, but V 23 will remain constant

with time. If there is no potential drop at the current contacts

(contacts 1 and 4, Figure 5) due to a contact resistance and the conductivity

is entirely electronic, no polarization with time will be observed at a given temperature on all probe sets V 12 , V23 , V 34 , or V 14 . If the

conductivity is ionic or partially ionic and probe measurements are taken in a reasonably short time at low electric fields (< lV/cm) such

that polarization effects do not extend far beyond the current carrying

contacts 1 and 4, probe voltage V 23 will remain constant but V 12 , V 34

and V 14 will change with time during the short interval that current is

flowing for the measurement. With the data acquisition system (D.A.S.)

used in this experiment, collecting a set of data points (i.e., V 12 , V23 , V 34 , V ]4 , temperature, current, and time) required about 40 seconds

for one applied voltage polarity. Since the current is reversed for

each data point at a given temperature to eliminate permanent polarization

build up at any current carrying electrode and to eliminate any thermoelectric voltage present due to a temperature gradient along the length

of the sample, the total time to scan all voltages for both current

directions is approximately 80 seconds. The D.A.S. consists of a programmable calculator-based system interfaced with appropriate electrometers

and voltmeters to measure the necessary parameters. The data can be

stored and calculations are made automatically after the current is off.

All conductivity curves reported during the last year were obtained

using this method. These are a reflection of the probe voltage V 23 versus the temperature. Each point on the curve is representati ve of near equilibrium conditions for temperatures above 1000 °C. Since

diffusion of oxygen is relatively slow at temperatures below 1000 °C,

14
very long times are required, days or weeks, for complete stabilization

and data below this temperature have a quenched in Fe 2 /Fe 3 ratio and

crystalline content. In addition, for slag samples, precipitation and

re-solution of crystalline phases causes the conductivity to change with

time at a given temperature above 1000 °C. These processes generally

follow an exponentially decreasing curve. In order to be able to complete

the experiment in a reasonable time, a data point was considered close

enough to its final equilibrium value when the change in conductivity

was less than 1% over a ten minute period. At a given temperature, data

was taken at ten minute intervals with applied voltage off between

measurements. Above 1300 °C these conditions could be met in less than

1 hour while below 1300 °C times as long as 20 hours were required.

No polarization was observed in this voltage measurement (

V

2 3 ) at any

time during the run. Since polarization at current carrying contacts

can be expected for ionic conductors where the electrodes are not ion

exchanging, as is the case here, some information can be obtained by

comparing the voltages developed at the probe sets during the course of

the data acquisition. A complete description of the information that

can be obtained in this manner will be given in a later quarterly report.

For best results these scan voltages must be interpreted in conjunction

with x-ray diffraction and scanning electron microscope analytical

techniques.

PI ans

Electrical conductivity measurements of various slags will continue with

particular attention to analysis of the results pointing closely to

understanding the conductivity process. The analytical methods described

in this report will be extended and hopefully will be applied to four

probe AC conductivity data if the equipment on hand will lend itself to

this measurement.

In the last quarterly report, an electrochemical study was reported

on a slag at 1250 °C. This temperature is below the break point in

conductivity (1385 °C) as shown in Figures 1 and 2 of this report on slag containing no added potassium. Voltage breakdown was seen to occur

at this temperature (1250 °C) in about 2 minutes in a constant current

mode at a current density of 0.2A/cm2. This experiment should be repeated

at a temperature greater than 1385 °C to determine the effect of decreased

viscosity and hence, increased iron ion conductivity of slag on the

time for voltage breakdown.

15
00

o

CD

00

CO

,_(UJ0 LUMO)Al!AllDnaNO0

Figure 1. Electrical Conductivity of Bow N.H. slag containing 20% Fe

as a function of temperature in an air atmosphere.

16
h

(wo-uui|0) AllAllDflONCO

Figure 2. Electrical conductivity of Bow NH slag containing 20 % Fe as a function of temperature in an atmosphere of 5 x 10" b ppm

O

2 in Ng.

1

T(K)
LEAD

OUT

Schematic

of

typical

leadout

wire

assembly

for

high

temperature

electrical

conductivity

measurements
FIGURE 4. Radiograph of sample

and leadout wires mounted

as illustrated in Fig. 3
FIGURE 5. Radiograph of slag sample

contained in alumina crucible

with leadout wires (numbers)

extending through walls

20
4. Corrosion of Downstream MHD Components (J. Smit and C. D. Olson)

Progress : Type 316 and Type 304 stainless steel tubular specimens

were exposed to seeded oxygen-propane fired hot gas streams at various

seed compositions, fuel to oxygen ratios and tube wall temperatures.

Exposure time for all specimens, exclusive of apparatus warm-up and

cool-down time was four hours. Seeding took place for the first

twenty-five minutes of exposure during which approximately 250 grams

of seed was added to the hot gas stream. For all specimens the gas

stream temperature in the vicinity of the specimen was held at about

1300 °C. Tube wall temperatures, monitored by Pt/PtlORh thermocouples

welded into the midpoint of the specimens, were maintained at either

500 °C or 590 °C by internal air cooling.

Six specimens of Type 316 stainless steel were exposed in the manner

described above and to the specific conditions as indicated in Table

1 .

Table 1

.

Specimen No.

Wall Temp.

(°C) Seed Gas Stream State

5-6 500 k

2 so

4

fuel rich

6-6 590 ^2^4 fuel rich

7-6 500 k

2 so

4 + k2 co3

* 0

2 rich

8-6 590 k

2 so

4 + k2 co3

* 0

2

rich

9-6 590 k

2 so

4 + k2 co3

* fuel rich

10-6 500 K

2

S0

4 + K

2

CG

3

* fuel rich

Ratio of K

2

S0^ to k

2 co

3

is 1:4 by weight.

Following exposure, sections were taken from the specimens for optical,

SEM and EDX analysis. The sections, prepared using metallographic

techniques, were cut from a location 10 mm away from the midpoint position

on the specimen. Because of the sensitivity of the deposited coatings

to moisture the tubular specimens were encapsulated immediately upon

removal from the test rig. All cutting, grinding, and polishing

operations were conducted using non-aqueous media. In addition the

sections were stored in vacuum dissicators except during intervals of

processing. Figures 1-6 are optical micrographs of specimens 5-6 through

10-6 so handled. The geometry of the deposited coatings is consistent

with that observed on the Type 304 stainless steel tubes at the same conditions and reported on previously.

21
The nature, size and shape of the deposited coatings is of importance

to both the heat and seed recovery aspects of the bottoming steam

plant operation. These characteristics will effect not only the heat

transfer properties of the system but will also have a major influence

on fouling conditions particularly in the main heat exchanger. Further,

the removal of this material and its subsequent transport downstream

will prove an added burden to the seed recovery system.

Figures 7-18 are micrograph of sections of Type 304 stainless steel

reported on earlier showing the deposited coatings. The leading surface

of a specimen is at the top of each figure. These specimens were prepared in the same manner as the Type 316 stainless steel tubing described

in a succeeding paragraph. The conditions of exposure are given in

Table 2 and some conclusions regarding these coatings follow.

Table 2.

Figure No . Temp, of

Tube Wall, °C Seed Gas Stream Condition

7 400 °C K

2 so

4

0

2 rich

8 500 °C K

2 so4

0

2 rich

9 590 °C K

2

S0

4

0

2 rich

10 400 °C k

2 so4

fuel rich

11 500 °C k

2 so

4

fuel rich

12 590 °C K

2 S0

4

fuel rich

13 400 °C ^2^4 + K

2 C0

3

*

0

2 rich

14 500 °C k

2 so

4 + K

2 C03

*

0

2 rich

15 590 °C K

2 S04 + k2 co3

*

0

2 rich

16 400 °C k

2 so4 + K

2

C0

3

* fuel rich

17 500 °C K

2

S0

4 + K

2

C0

3

* fuel rich

18 590 °C K

2 S0

4 + K

2 C03

* fuel rich

Ratio of K^SO^ to K^CO^ is 1 : 4 by weight.

22
Generally a compact seed deposit forms on the leading surface of the tube,

being thinnest at the midpoint and broadest at the compact depositpowdery deposit merge zone. A thinner somewhat powdery deposit forms

on the trailing surface. The leading surface deposit is independent of

temperature (400 °C - 590 °C) for the case of K2 S0 4 in 0 2 rich hot gas

streams. For all other cases the deposit thickness decreases with

increasing tube wall temperature. Leading surface deposits are generally

thicker in 0 2 rich hot gas streams while trailing surface deposits are generally thicker in fuel rich hot gas streams. The combination of

K2 S04 with K2 C0 3 results in a thinner deposit at all temperatures for

the 0 2 rich hot gas stream case than for the K2 S04 alone.

From the preceeding then it may be argued that fouling in the heat

exchanger area would probably be least and the heat transfer efficiency

greatest at the higher temperature, i.e., above 500 °C, with a K2 S04 + K2 C0 3 mixture probably in an oxygen rich hot gas stream.

The gravimetric studies initiated in the last quarter were continued

during this reporting period. Three specimens of Type 304 stainless

steel tube were exposed at 590 °C to conditions indicated in Table 3.

Prior to exposure the specimens were cleaned with ethanol and weighed.

Immediately following exposure the specimens were rinsed with warm water

and then ethanol to remove the seed deposit. The specimens were then

cleaned in a ultrasonic bath with 1 0%HN0 3 at 50 °C for twenty minutes

and reweighed. The weight change is noted in Table 3. Even by the most

conservative treatment the weight loss translates to a wastage of 4 mm

per year which is not acceptable and would virtually eliminate Type 304

stainless steel as a candidate material.

PI ans : Specimens of Type 316 stainless steel previously exposed and

prepared will be analyzed by SEM and EDX techniques for signs of both

incipient and gross corrosion. In addition deposit samples obtained

from specimens exposed for gravimetric analysis will be analyzed using

x-ray diffractometer techniques. The purpose of this analysis is to obtain

more information on the corrosion process involved and on the possible

transformation of the seed compounds during deposition.

23
Table

e

•I- e

•I- </> <u E

CT)+J (d E -E E

rd CJ) CD o cu

CO i

f"-. «d" CO CM O o o

O o o

cu

e e

•r- CU

+->

4-> 4- 4-> ui CD

•I- l/l cu E 3 03E

CD

CUe

r-» cn i

CO co LO

CM CO

CO CO

CO CO CO

e o

4- CO o co

cu CO CM

+-> CO CM CO CO

-E +-> • • • cn CD CD I''. CO CO

•i

E CU CO CO CO

cu (d l— 3 E

CD

E c_> CUE* i

o o o o

a Q- cn cn cn

cu E LO LO LO

CL cu OO 1—

E

(TJ CUe cu

4-3 +-> OO fO

4-> CD O0

(d CD

-o

(U

cu CO

cu cu ^ =3 (XI 4-4—0

CO CO o o

o o CM CM

iZ xc + +

d- "d- o o o

OO CO CO CM CM CM

XC :Z iZ

CD E CD

<u CU E >— CU

•r- E Cl U -r- CU rd Q-+-> OO CO oCO«d- d" o o CO CO

CO e

CU

'd' u o

cu z: Q. OO

lo

«d" CO

«d"

I

24
Fig. 1. Section of Type 316

stainless steel tubing

after exposure to K 2 SO4

seeded fuel rich hot gas

stream. Note deposit on lower surface. Tube

temperature 500 °C. 6 X.

Fig. 2. Section of Type 316

stainless steel tubing a after exposure to K2 SO4

seeded fuel rich hot gas

stream. Note thin deposit

on upper surface. Tube

temperature 590 °C. 6 X.

25
Fig. 3. Section of Type 316 stainless steel tubing after exposure to

K

2 SO4 + K

2 CO3 seeded O

2 rich hot gas stream. Note thick

deposit on upper surface. Tube temperature 500 °C. 6 X.

Fig. 4. Section of Type 316 stainless steel tubing after exposure to

a K 2 SO4 + K

2 CO 3 seeded O 2 rich hot gas stream. Tube

temperature 590 °C. 6 X.

26
Fig. 5. Section of Type 316 stainless steel tubing after exposure to

a K2 SO4 + K2 CO 3 seeded fuel rich hot gas stream. Mote deposit

on lower surface. Tube temperature 500 °C. 6 X.

Fig. 6 . Section of Type 316 stainless steel tubing after exposure to

a K 2 SO4 + K

? C 03 seeded fuel rich hot gas stream. Tube temperature

590 °C. 6 X.

27
Fig. 7. Section of Type 304 stainless steel tubing after exposure

to K 0 SO4 SGe ^ed oxygen rich hot gas stream. Note formation

of thick deposit, 2 mm, on upper surface and thin powdery

deposit on lower surface. Tube temperature 400 °C. 6 X.

28
Fig. 8 . Section of Type 304 stainless steel tubing after exposure to

K 0 SO4

seec| ed oxygen rich hot gas stream. Note formation of

thick, 2 mm deposit on upper surface. Tube temperature 500 °C

6 X.

Fig. 9. Section of Type 304 stainless steel tubing after exposure to

K2 SO4 seeded oxygen rich hot gas stream. Note formation of

thick deposit on upper surface.. Tube temperature 590 °C. 6 X

29
Fig. 10. Section of Type 304

stainless steel tubing

after exposure to K

2 SO

4

seeded fuel rich hot gas

stream. Tube temperature

400 °C. 6 X.

Fig. 11. Section of Type 304

stainless steel tubing

after exposure to K 0 SO4

seeded fuel rich hot

gas stream. Tube

temperature 500 °C. 6 X.

30
Fig. 12. Section of Type 304 stainless steel tubing after exposure to

K 2 SO4 seeded fuel rich hot gas stream. Tube temperature

590 *C. 6 X.

Fig. 13. Section of Type 304 stainless steel tubing after exposure to

K^SO^ + K^CO^ seeded 0

^ rich hot gas stream. Note formation

of thick, 1.7 mm, depost on upper surface. Tube temperature

400 °C. 6 X.

31
Fig. 14. Section of Type 304 stainless steel tubing after exposure

to K

2 SO4 + K

2 CO

3 seeded oxygen rich hot gas stream. Note

formation of thick, 1 mm, deposit on upper surface. Tube

temperature 500 °C. 6 X.

Fig. 15. Section of Type 304 stainless steel tubing after exposure to

K 2 SO4 + K

2 CO 3 seeded oxygen rich hot gas stream. Note

formation of thinner deposit on upper surface. Tube temperature

590 °C. 6 X.

32
Fig. 16. Section of Type 304 stainless steel after exposure to K2 SO4 + K

2 CO3 seeded fuel rich hot gas stream. Note formation of

0.8 mm thick deposit on upper surface. Tube temperature

400 °C. 6 X.

Fig. 17. Section of Type 304 stainless steel tubing after exposure

to K

2 SO4 + K

2 CO

3 seeded fuel rich hot gas stream. Tube

temperature 500 °C. 6 X.

I

I

33
Fig. 18. Section of Type 304 stainless steel tubing after exposure

to K 2SO

4 + K

2 CO

3 seeded fuel rich hot gas stream. Note

deposit on lower surface. Tube temperature 590 °C. 6 X.

34

!