NBSiR 80-2064

Chemical Degradation of Castable

Refractories in Coal Gasification

Process Environments

Carl R. Robbins

Floyd A. Mauer

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

May 1980

Prepared for

The United States Department of Energy

Under Contract EA-77-A-01-6010

NBSIR 80-2064

CHEMICAL DEGRADATION OF CASTABLE

REFRACTORIES IN COAL GASIFICATION

PROCESS ENVIRONMENTS

Carl R. Robbins

Floyd A. Mauer

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

May 1 980

Prepared for The United States Department of Energy

Under Contract EA-77-A-01 -6010

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

FOREWORD

Since 1974, the National Bureau of Standards has been engaged in

research designed to address materials problems and needs pertinent to

coal conversion and utilization technologies. This work, sponsored in

part, at first by the Office of Coal Research, then ERDA and currently

the DOE, focuses on test method development, particularly accelerated

procedures for materials behavior and durability, on the determination

of the mechanisms of materials degradation; and on the development and

operation of materials data centers to provide evaluated information on properties and performance including failures occurring in operating

plants.

During the FY 77-79 period the NBS program, entitled "Materials

Research for the Clean Utilization of Coal" consisted of a number of

interrelated tasks including:

1 . Metal Corrosion

a. Constant Strain Rate Test

b. Pre-cracked Fracture Test

2. Ceramic Deformation, Fracture and Erosion

3. Chemical Degradation of Ceramics

a. Reactions and Transformations

b. Slag Characterization (viscosity)

c. Vaporization and Chemical Transport

4. Failure Prevention

a. Failure Information Center

b. Materials Properties Data Center

The results of the research have been disseminated through quarterly

reports of progress (available from NTIS; report designation EA-6010;

Dist. Category UC-90C) as well as numerous scientific publications in

technical journals. Further, as individual tasks are completed, an overall report is prepared detailing the results and accomplishments of

the project. This present publication is the final report for Task 3 a. Reactions and Transformations.

SAMUEL J. SCHNEIDER

Program Manager

Center for Materials Science

National Bureau of Standards
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TABLE OF CONTENTS

I. OBJECTIVE AND SCOPE OF WORK 2

II. TECHNICAL APPROACH 2

III. METHOD AND EQUIPMENT . 4

IV. RESULTS AND CONCLUSIONS 7

A-94 Exposure Tests 9

A-56 Exposure Tests 10

Conoco Pilot Plant Samples 12

Summary 13

Figures ........... 14

V. PUBLICATION AND TALKS 25

VI. REFERENCES 26
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ABSTRACT

Reactions and transformations that result in chemical degradation of

castable refractories used as liners for coal gasification reactors have

been studied. In addition to phase analysis of laboratory and pilot

plant specimens by conventional x-ray powder diffraction, a new test

method was developed that permits changes in the phase composition to be

observed without removing the specimen from the test atmosphere. Two

candidate castable refractories were included in the study, a high

purity Ca0-Al203 composition with tabular alumina aggregate (93.7 wt %

AI2O3, 5.6 wt % CaO) and a Ca0-Al203-Si02 composition with calcined

kaolinite aggregate (55 wt % A1203, 36 wt % Si02, 5 wt % CaO). Specimens

were exposed to steam, steam/C02, and a simulated coal gasification

atmosphere at a pressure of 7 MPa (1000 psig) and temperatures up to

1000 °C. Frequent changes in the bonding phases, and intervals in

which a bonding phase was in transition were observed in the case of the

high purity castable refractory. The silica-containing refractory, on the other hand, formed bonding phases which were stable in steam over a large range of temperatures and pressures.

1
CHEMICAL DEGRADATION OF CASTABLE REFRACTORIES

IN COAL GASIFICATION PROCESS ENVIRONMENTS

I. OBJECTIVE AND SCOPE OF WORK

Coal gasification processes require the handling and containment of

corrosive gases and abrasive particles at high temperatures and pressures. These severe environments cause materials failures which inhibit

successful operation and increase costs. The objective of the program

at the National Bureau of Standards is to provide test methods and data

for evaluating properties of candidate materials under the conditions

encountered in coal gasification process plants. The scope of this

report is limited to castable refractories for use as liners of reactor

vessels. It covers the reactions and transformations that result in

chemical degradation when these materials are subjected to typical coal

gasification process environments.

II. TECHNICAL APPROACH

Three different approaches have been used at NBS in studying

reactions and transformations in candidate refractory castables for

gasification reactor liners. The first, or post-exposure approach

involved exposure of test bars in hydrothermal pressure vessels, followed by strength tests and x-ray analysis of phase composition after

removal from the pressure vessel. A detailed discussion of the correlation of physical properties with phase assemblages observed after

exposure is given in the report on Task 2 - Ceramic Deformation, Fracture and Erosion^. In addition, some of the earlier results have been

summarized in a report by Wiederhorn et al^.

The second, or in situ approach was undertaken because the massive

nature of the pressure vessels used for hydrothermal exposure limits the

cooling rate so that conditions that prevail at high pressures and high

temperatures cannot be effectively quenched. Thus additional reactions

and transformations may occur while the specimen is being brought to

25 °C and one atmosphere for testing and examination. It was to eliminate any uncertainty caused by such changes that the decision was made

to carry out both the flexural strength measurements and the x-ray

analysis in situi that is, without removing the specimen from the

chamber. New and highly specialized equipment was designed and built at

NBS for this purpose. This report describes the apparatus, method, and

results of the in situ x-ray analysis. The measurements of flexural

strength will be described in a separate report.

The third, or pilot plant exposure approach was based on our

examination of approximately forty core samples from the refractory

lining of the Conoco Lignite Gasification Pilot Plant in Rapid City,

South Dakota. These refractories show the effects of 5.5 years of plant

operation, 0.76 years of which were at pressure and temperature (0.9 MPa

and 843 °C). The results of characterization of core samples from the

Conoco pilot plant by light microscopy, SEM and x-ray powder diffraction

have been reported by Dobbyn et aP. and have been summarized in a note

by Brower et al**. Some of the findings will be discussed here in an

2
attempt to correlate short term in situ test data with the effects of

much longer exposure under actual service conditions.

Two candidate castable refractories were included in the laboratory

study, a high-purity Ca0-Al203 composition with tabular alumina aggregate (NBS A-94) and a CaO-Al 203-Si02 composition with £alcined kaolinite

aggregate (NBS A-56). Samples of CAS2, CA2, CAs and CC* were also

prepared in order to examine the behavior of these important compounds

in more detail. The refractories were cast into bars 75 x 15 x 7.5mm

and cured for 24 hours in an environment of 100 percent relative humidity.

The bars were then dried at 110 °C for 24 hours and stored in a desiccator

for later use in in situ testing. Some were fired for five hours at

1010 °C before testing in order to compare the response of fired and

unfired specimens to the test atmosphere. Specimens were exposed to

steam, steam/C02 and to a simulated gasification atmosphere (Table 1) at

pressures up to 7 MPa" and temperatures up to 1000 °C. The composition of

Table 1. Nominal composition of the simulated coal

gasification atomsphere at approximately

850 ®C and 7 MPa.

Component Amount (Mole %)

CO 20

CO2 15

Hz 25

H2O (steam) 40

the atmosphere is difficult to predict if pressure as well as temperature is allowed to vary. It has been our policy to establish the

nominal composition shown in Table 1 for a pressure of 7 MPa and to

maintain this pressure throughout each series of experiments. Results

reported may be assumed to have been obtained at 7 MPa unless otherwise

noted. The change in composition with temperature has been examined

thermodynamically at NBS by Dr. W. S. Horton. The steam and CO content

each increase by roughly 6% while the CO2 and H2 content decrease by a similar amount in the temperature range from 600 ° to 1000 °C. The

composition shown in the table is, therefore, representative of the

middle of the range of experimental conditions covered and deviations

from this composition do not exceed ±5%.

C = CaO, A = AI2O3, S = Si02, H = H2O, C = CO2

'The International Standard (SI) unit of pressure is the pascal, or newton per square meter. One mega pascal (MPa) = 10® pascals. The

conversion to psig is given by p (psig) = (p (MPa) x 145) - 14.7.

3
«ET^D A.SD £QU:?«EMT

New arvi highly specialized ecuiprrent was needed in order to obtain

x-ray oi**raction oata wit.ncut rerwving the specimen from the pressure

vessel. The ecajipoent aesignec and built at the National Bureau of

Standards for this purpose is pased on the energy dispersive x-ray

di**raction (EDXD) tecnnique described by Giessen and Gordon^ in 1963.

Unlike conventional diffraction t>ethods, which use •nonochromatic

-aaiation ana a range of diffraction angles, tne EDXD -ethod employs

wnite (continuuB) radiation and a fixed diffraction angle, 2e . The

aiffractea tear, is analyzed with a solid state detector and nwltichannel

pulse neignt ar*alyzer to determine the energy distribution of the

pnotcns. Bragg's law in the form given by Sparks and Gedcke®

is usee

pnotons

cnannel

wnere a system,

and b =

The outstanding advantage of the EDXD Trethod for the present

aoDlication is that it requires only a single, narrow x-ray path through

tne pressure vessel. Tne entrance and exit windows can be as sinall as a puarter of an inch in diameter, and it is relatively easy to obtain

suitaole window raterial in the size and shape required. Using boron

caroice, BuC, it was possible to design for a pressure of 7 MPa with a suitable safety factor wnile keeping the windows thin enough for adequate

transmission of tne x-ray beam.

Tnere are sacrifices that nxjst oe tiade as a trade-off for the speed

ana convenience of the EDXD -netnod. The tost serious is the low resolution of the cetector, wnich results in peaks that are tore than an orcer of magnitude wider than in conventional diffractometry.

Details of the construction of the pressure vessel are shown in

Figure 1. The upper view is a section through the vessel in the plane

of li'-raction. Raciation from the x-ray source (A) passes through a 1/3 inen thick window of boron carbide (B) and, after being scattered by

t.ne specimen (C), passes tnrougn a second window (3) and slit (D) to

enter t,ne solid state detector (E). The bracket (F) which supports the

specir>en nas slots milled in it for the x-ray beam. These limit the

civerger.ee of the incident and diffracted beams to 2 An internal

"eater consisting of an alumina core (3) wound with approximately 5 feet

of 3.D22 inch aiameter 3C2 ?z-ZO% Rh wire (H) surrounds the specimen

^.kJi ^ 1Z.3981

2 sine

0 ) ‘hkx

to calculate the energy E (in kiloelectron volts) of x-ray

di*-racted by planes of spacing d. (in angstrom units). The

nucioer, N, is given by

^

N = (E - b)/a ( 2 )

and b are calibration constants of the solid state detector

For tne work reported here e = 12.00 a = 0.0353 keV/channel

,

0.30 keV.

A
bar. A concentric alumina tube (I) is -<fl"apped with platinum -cil (J) to

reduce heat transfer by radiation. The heater assembly is mounteci in a U-shaped bracket of 0.031 inch thick Inconel sheet (M) which forms three

sides of a box. The remaining three sides are fomed by a cover (L) of

0.010 inch thick Inconel. The space between the heater and the Pox is

packed with fibrous alumina insulation. Internal winoows of 0.002 inch

thick mica mounted in platinum disks (N) loosely cover the enas of the

heater core to reduce convection currents while allowing tne x-ray beam

to pass. The importance of these disks should not be underestimated, as operating temperatures without them are limited to about 500 =C. Three

thermocouples are provided. The first (0) is attached to the front edge

of the specimen at the top. The second (P) is at the bottom of the

vessel where it senses the temperature of the water that collects there.

The third (not shown) is on the outside of the vessel in contact with

the belt heater (not shown) that surrounds the vessel . Coned pressure

fittings in ports (Q) and (R) are used to connect the gauge assembly and

the water reservoir.

The pressure vessel, which is machined from 316L stainless steel,

is 4 1/2 inch in o.d. at the height of the x-ray beam and has 1/2 inch

thick walls. The flange is 6 inches in diameter. T’l^lve 3/8-16 hightensile-strength bolts are used to attach the lid, which is fitted with

a gold-plated K-shaped flange seal made of Inconel X-750. Tne enclosed

volume is approximately 435 ml. The side arms, which contain the x-ray

windows, are also made of 31 6L stainless steel and are welded in place

at an angle corresponding to 29=24 ®. The boron carbide windows are

sealed with gold plated V-gaskets of Inconel X-750. As the windows

appeared somewhat grainy even after they were polished, the surfaces in

contact with the gaskets were sputtered with gold to fill any irregularities and provide a softer seat. Threaded caps compress the gaskets

and support the load due to gas pressure.

In operation, the pressure vessel is mounted as shown in Figure 2.

A base assembly supports both the pressure vessel and the solid state

detector and provides the necessary adjustments for alignment. This

base is pivoted at a point directly below the focal spot of the x-ray

tube for ease in setting the take-off angle. Tnree leveling screws are

used for setting the height and for making small adjustments to the

angle of incidence. The base supports a heavy ring 8 1/4 inches in

diameter in which the pressure vessel is clamped by four 3/8 inch radial

set screws, the ends of which extend into holes in the six inch flange

of the vessel. In addition to supporting the pressure vessel the

mounting ring provides a means for counteracting the force applied in

tightening the lid bolts. There are 12 3/8 inch holes drilled vertically through the ring. A 10 inch lever is attached by means of pins

that fit any diametrical pair of holes. Equal and opposite forces

applied to this lever and the -wrench provide the necessary torque to

tighten the bolts without disturbing the alignment of the instrument.

Since the vessel must often be operate*d with its walls at temperatures as high as 285 ®C in order to maintain a steam pressure of 7 MPa,

it is equipped with an external belt heater visible just below the x-ray

3
ports in Figure 2. While heating the vessel, it is important to minimize

tne heat transferred to the base, the detector, and the x-ray tube.

With this aim, the i.d. of the mounting ring was made 1/4 inch greater

than the o.d. of the vessel flange. The four set screws comprise the

only path for the conduction of heat from the vessel to the mount. A

heavy copper sheet with a 1/8 inch aperture is mounted between the exit

slit and the detector window to aid in the dissipation of heat that

mignt otherwise be transferred to the detector. The only direct link

berween the vessel and the x-ray tube is a stainless steel beam tunnel

that fits inside the safety shutter. Although this tunnel has thick

walls (to reduce x-ray leakage) heat transfer to the x-ray tube has not

been a problem.

External fittings visible in Figure 2 include the gauge block

assemoly and the water reservoir. The gauge block provides, in addition

to the gauge, a pressure tubing connection for introducing gases, a needle valve, and a rupture disk. The water reservoir has a capacity of

40 ml. It is pressurized with nitrogen gas to ensure that water will

flow into the pressure vessel when the needle valve is opened.

The lid of the pressure vessel is fitted with two pressure seal

assemolies for electrical leads, one for the four thermocouple leads,

ano the otner for a single power lead rated at 40 amperes. The vessel

itself ser/es as the conductor for the grounded side of the low voltage

power supply of the internal heater.

A safety shield of 1/4 inch thick steel is installed before the

vessel is pressurized. A portion of this shield can be seen behind the

pressure vessel in Figure 2. The enclosure is connected to an exhaust

syscem in case of leakage of test atmospheres containing CO, H2 , or H2 S.

After installing the specimen bar with the thermocouple attached,

an initial charge of 20 ml of water is added to the vessel before

closing the lid. Approximately 15 ml would be required to pressurize

the vessel with saturated steam at 7 MPa. Excess liquid remains at the

botcom of the vessel and its temperature is controlled by means of the

external heacer to maintain the desired partial pressure of steam.

Other components of the atmosphere are added as gases to obtain the

desired total pressure. A reserve supply of water is kept in the 40 ml

reservoir to replenish any used in hydrating the sample or lost during

the run.

”he wall temoerature required to maintain a steam pressure of 7 MPa

is aporoximately 285 ^C. The external belt heater has a rating of 650

watts at 240 Y ac, but the power input during steady-state operation

does not exceed 400 watts.

Specimen temperature is separately controlled using the internal

heater. The maximum current required for control at 1000 in steam is

14 amoeres, corresponding to a power input of 390 -watts. As the power

incut to the internal heater is increased, that to the external heater

0
must be reduced to compensate. A small blower has been installed in

case additional cooling is required. It is not generally needed.

Specimen temperature is usually increased in increments of 50

Diffraction patterns are recorded after 6 hours, and again after 24 hours

A series of experiments covering the range to 1000 and 7 MPa requires

from 3 to 6 weeks.

Reference patterns of known phases occurring in refractory cements

are needed in order to recognize these phases when they are encountered

in the course of an experiment. Since energy dispersive x-ray diffraction patterns differ considerably from those prepared with a conventional diffractometer, a special set of reference standards had to be

prepared. Actually two sets of standards were prepared, one by obtaining

samples of the desired material and recording the pattern with the EDXD

apparatus, and the other by calculating the pattern from data given in

standard references such as the Powder Diffraction File^. An example of

an actual EDXD pattern for a-quartz is given in Figure 3. Calculated

patterns are given for AH, AH3 , C3AHS, C4A3H3, and CC (Figure 4), for

a-A, CAg, CA2, CA, C3A and S-A (Figure 5), and for ci-Sq (quartz), S^- (cristobal ite) , A3S2 (mullite) and low CAS2 (Figure 6). In addition to

being compact and convenient to reproduce, the computed patterns have

the advantage that a tabulation identifying all of the lines is available

When a peak is composed of a number of unresolved lines, the components

are clearly shown in the computed pattern.

Computer methods used to assist in the interpretation of the sixty

or more EDXD patterns resulting from each test included (1) automatic

plotting of tne patterns on a comnon axis system tor ease in comparison,

(2) subtraction of corresponding ordinates and plotting of the difference

to show subtle changes, and (3) plotting of the intensity of selected

lines for a series of patterns recorded over a range of pressure and

temperatures. Examples will be included in section IV - RESULTS AND

CONCLUSIONS.

IV. RESULTS AND CONCLUSIONS

The approximate bulk compositions of the castable refractories NBS

A-94 and NBS A-56 are shown by squares on the phase diagram of the

system Ca0-Al203-Si02^ (Figure 7). The chemical analyses are given in

Table 2.
Table 2. Chemical analyses (dried basis) of castable refractories

(weight percent)

NBS A-94 NBS

A1203 93.7 55.

CaO 5.6 5.

Si 02 0.1 36.

FezOs 0.3 1 . Others and loss

on ignition

0.3 2 .

In general, the in situ experiments were designed to monitor the

behavior of an unfired castable during its initial heating, during

exposure at 1000 °C and 7 MPa and during the cooling down phase of the

cycle. Specimens were examined before and after exposure tests by

conventional x-ray powder diffractometry. A list of the compounds

observed is given in Table 3. Some are metastable, others such as the

calcium aluminates may contain H2 O (although shown here as anhydrous),

and several of the hydrated calcium aluminates may be non-stoichiometric

with respect to H2 O.

Table 3. Compounds encountered in exposure studies of A-94 and A-56

CsA CzAHg 3-C2S a-A

C12A7

CA

CAHio

C3AHS

3 -A

CAz C4A3H3 CzAS AH

CA3

C4A3 CC

CASz (2 forms) AH3

CH S , S

c* q

A3S:

C = CaO, A = AI 2 O3 , S = Si 02 (Sq quartz or S^ cristobal ite) H = H2 O, C = CO2

Steam is usually the chemically dominant component of the test

atmosphere, as shown by the response of refractories to simulated or

actual gasification envirorments. In the experiments described below

the total pressure was maintained at 7 MPa, the design limit for our

apparatus, except for a few cases when the test temperature was so low

that the equilibrium vapor pressure of water was too low to sustain the

8
desired pressure. This condition occurred below 285 °C for the steam

atmosphere or 230 °C for the simulated gasification environment. The

estimated total pressure is given only when it is lower than 7 MPa.

A«94 Exposure Tests

The composition of A- 94 is represented by a square on the join CaOAI 2 O 3 of the Ca0 -Al 203 -Si 02 phase diagram (Figure 7). The compound

assemblage commonly observed in cast and dried (but unfired) test bars

includes: CA, CA2, a-A, CAHio» C2AH3, C3AHg , and AH3 . The aggregate is

tabular alumina.

With increasing temperature, the A-94 refractory passes through a sequence of phase assemblages consisting of hydrated calcium alumi nates,

hydrated alumi nates, and a-A. The sequence observed in steam, with

approximate equilibrium dissociation temperatures, is

20®C nO®C 240«C szs^c

CAHj 0+AH3 ^3AHg+AH3 CgAHg+AH C4A3H3+A^— CAj^^A

in general agreement with the pseudobinary diagram of Crowley for the

system Ca0-Al 203 -H20 at 100 MPa^. Above about 625 ®C none of the hydrated phases is stable. At this point CA2 forms as the bonding phase,

and the assemblage CA2 , a-A (with a trace of 8 -A) is observed in all

tests up to 1000 ®C. The reactions are reversible and have been fol-_

lowed by the in situ method. The bonding phases CaAHs and AH 3 dissociate

over a rather narrow temperature interval compared with C4 A3 H3 and AH.

C3 AH6 forms readily in saturated water vapor at 93 °C and dissociates

at 240 °C and 3.3 MPa. C4A3 H3 , which forms above 240 ®C and 3.3

MPa, persists to 625 ®C. It is the most difficult phase in the system

to monitor by the EDXD method because of the weak and poorly resolved x- ray pattern.

AH (boehmite) has been shown in NBS work under Task 2, Ceramic

Deformation, Fracture and Erosion, to be one of the most important

phases in the high purity castable refractory A-94. The observed loss

of flexural strength of this refractory has been correlated with the

decomposition of this compound. The "equilibrium" dissociation temperature of AH is 345 ®C. However, with rising temperature, this dissociation was repeatedly observed to occur over a range of 200 ®C.

Small amounts of AH are commonly present at 500 °C. The compound

decomposes rapidly above 500 ®C, and a refractory containing appreciable

amounts of the hydrate should not be heated rapidly through this temperature region.

The ability of the in situ method to follow the dissociation of AH

in real time is shown by the series of four x-ray patterns reproduced in

Figure 8 . The overall change is shown in a difference plot. Figure 9,

in which the negative peaks correspond to the AH phase consumed in the

reaction, while the positive peaks show the increase in a-A, the reaction

product.

9
Physical properties, such as strength and erosion resistance, which

are affected by the presence of AH, can be expected to follow trends

somewhat parallel to the formation or dissociation of this phase. For

example, the flexural strength of the test bar used in this series of

experiments might be expected to show the same trend as the intensity of

the x-ray pattern of the AH phase.

In Figure 10 the intensity of the 251 reflection of AH (channel

610) and the intensity of the 116 reflection of a-A (channel 492) have

been plotted for a sequence of fourteen experiments covering the range

from 247 °C and 3.2 MPa to 650 °C and 4.1 MPa in steam. There was a substantial loss of AH during 21 hours at 375 °C and 3.8 MPa (experiment

8) and at each subsequent temperature - pressure increment up to 650 °C

and 4.1 MPa (experiment 14). Under the conditions of these experiments

the flexural strength of the test bar would be expected to decrease at

temperatures from 375 °C to 650 °C at a. 4 MPa in steam.

In steam/C02, or in the simulated gasification environment, formation of hydrated calcium aluminates may be suppressed by the reaction of

CaO with CO2 to form CaCOs. This compound dissociates in the presence

of steam at 700 °C, which is generally above the stability range of

the hydrated phase.

Addition of dissolved silica to a test atmosphere of high pressure

steam results in formation of a reaction rim Inm thick formed on test

bars of high purity A- 94. The reaction rim was composed mainly of

C2AS, 3-CS, CA2, and a-A. Formation of hydrated calcium aluminates was

thus inhibited. Since silica impurities in coal would be expected to

contribute dissolved silica to the pressurized steam of a gasification

reactor, similar reactions between dissolved silica and an initially

high purity calcium aluminate castable can be expected to occur.

A-56 Exposure Tests

As may be seen from Figure 7, the composition of A-56 lies in the

compatibility triangle A3S2 (mullite) - CAS2 (anorthite) - S (quartz,

tridymite or cristobal ite) . The phase assemblage of the as-cast test

bars is quite complex. Hydrated compounds such as AH, AH3 , CAHiq*

C2AH8 and C3AH6 are commonly present as minor phases in addition to the

anydrous major phases S^, A3S2, CA and CA2. As the temperature is

raised, in the presence of steam or the gasification atmosphere, hydrated

phases gradually dissociate. The dominant reaction is in the main, a hydrothermal one, with high pressure steam interacting with CA, CA2, a-A

and to form the very strong bonding phase CAS2 (anorthite) and the

stable assemblage CAS2, A3S2, S. This assemblage is subject to leaching

in an open system such as a gasification reactor. Loss of Si02 to the

high pressure steam is commonly observed in the in situ experiments.

The dissolved silica is usually precipitated as cristobalite, or a mixture of cristobalite and quartz depending on temperature and time of

cooling.

10
Only the triclinic form of CAS2 was observed in the in situ, studies,

but a hexagonal (metastable) form was observed in some of the postexposure studies of the Ceramic Deformation and Erosion specimens (Task

2) and in some of the Conoco refractories. This phase may form hydrothermally, by crystallization from high-temperature melts in air, and by

solid state reactions. It rapidly transforms to triclinic CAS2 in the

presence of steam below 700 ®C^°.

For NBS A-56 and several commercial Ca0-Al203-Si02 castables

studied, there is a passage from complex to much simpler phase assemblages with rising temperature in high pressure environments containing

steam. By a variety of paths and reactions, the composition produces

the stable assemblage A3S2, S, CAS2 previously referred to in Figure 7.

The formation of new compounds at the expense of others is primarily a function of temperature at 7MPa. Alumina gels, a-A, CA, CA2, and the

various hydrated calcium aluminates react with to form the bonding

phase CA$2. CA$2 was observed forming in the 350-550 ® temperature

range and more rapidly at 700 ° - 1000 °C.

Compared with the unfired test bars, those fired in air at 1010 °C

for 20 hours show a large increase in CA2, a slight increase in A3S2,

the presence of poorly crystallized C2AS and CAS2, a decrease in a-A

and Sq, and the absence of hydrated forms. As before, with exposure to

the hydrothermal environment at 7 MPa this assemblage is changed to

CAS2, A3S2 and S with silica in the cristobalite form. The flexural

strength of the castable is markedly increased as these' reactions progress

in the gasification atmosphere.

Two supplementary experiments with test bars near CAS2 in composition

were conducted in a steam atmosphere. The unfired bars consisted initially

of CA, CA2 and silica, with an excess of A. After casting, the bars

were composed of Si02 (quartz form), a-A, CA, and AH3. X-ray analysis

gave no indication of a gel phase. Exposure times were 168 and 330

hours. In both series of experiments, some hydration and subsequent

dehydration of free alumina as well as the dissolution of free silica

was observed as temperatures and steam pressures were increased. Formation of hydrated calcium aluminates was not observed. At approximately

160 °C and 0.5 MPa CasSisOis, a phase not previously encountered in

these studies, was formed in preference to hexagonal or triclinic CAS2.

According to Speakman et al.^^ CasSisOis is metastable, and of variable

composition. The compound, along with free silica, persisted to 525 ®C.

In the temperature interval 525-625 °C, CasSisOia dissociated and/or

reacted with the Si02 to form triclinic CAS2. This is shown in the in

situ x-ray pattern of Figure 11 (a), (b), and (c). Pattern (a) is of

the unfired test bar at 25 °C and ambient pressures. Pattern (b) was recorded at 520 and 7 MPa steam and shows the presence of well

developed CaaSs as indicated by two prominent diagnostic peaks. Pattern

(c) was obtained one hour later after the temperature had been raised to

625 °C and 7 MPa steam. It shows that at this temperature, most of the

CaaSs had dissociated/decomposed, releasing CaO for the formation of

CAS2. The presence of CaaSisOia suppresses the formation of CAS2 by

incorporating available Ca. Its occurrence in a refractory would appear

to be undesirable.

n
Conoco Pilot Plant Samples

In addition to the in situ tests of A-56, results of exposure of

two similar commercial refractories to service conditions in the Conoco

Lignite Gasification Pilot Plant are available. Approximately 40 core

samples were examined by optical microscopy, SEM, and x-ray powder

diffractometry. The following generalizations are based on the examination of the Conoco samples as well as on the in situ tests.

1) The atmosphere of a coal gasification reactor is a complex

mixture of gases. Steam is the dominant component of this

mixture as shown by chemical and structural changes in the

CaO-Al 20a-Si02 refractory.

2) A hydrothermal reaction in which Si02 in the form of Cristobal ite (Sr) progressively reacts with free a-A, A3S2 and the

calcium aluminate binder (CA, CA2) to form the bonding phase

CAS2 (anorthite) is seen to be the dominant reaction in

laboratory and pilot plant studies.

3) Pilot plant specimens show a loss of Si02 from the calcined

kaolin aggregate (reaction rims). Some of the silica reacts

to form CAS2: At 900 °Z some silica reacts with free a-A

to form A3S2» and some is lost from the refractory, precipitating

in regions of lower temperature as cristobalite or a mixture

of cristobalite and quartz. The latter was frequently observed

in the in situ tests.

4) Evidence of the apparent fluxing action of alkalies, resulting

in surface melting at the working face of the refractories and

the formation of a residual crust 2-3mm in thickness was observed. Alkali reaction with the refractories was also

shown by the presence of (Nax^5Ko^5)Al 2Si20a (Nepheline,

idealized formula) near the hot of working face of refractory

specimens from the pilot plant.

5) The formation of hexagonal CAS 2 was not observed in the in

situ x-ray studies, but was obtained hydrothermal ly in work of

the NBS Fracture and Deformation Task group in selected high

pressure experiments. The compound may be formed by cooling

from the melt, by solid state reactions, or hydrothermal ly.

It is considered to be a metastable form and transforms

rapidly below 700 °C in the presence of steam to the triclinic

form. It should be noted that the 700 °C region would represent a temperature interval in which the primary bonding phase

of the refractory is in a state of transition. The hexagonal

form of CAS2 was observed as a minor phase in a number of

specimens of pilot plant refractories taken 3 to 6 inches from

the working face of the castable.

12
6) As previously described, when reactions in test bars approximating the composition CAS2 (composed of CA, CA2, and S in

the form of powdered quartz) were monitored by the in situ x- ray method, the metastable compound C3S5 was observed to form

quite rapidly at 160 ®C and 0.5 MPa. Formation of this compound suppressed the formation of CAS2 by incorporating the

available calcia. CsSs dissociates at ^ 525-625 °C at 7 MPa

and CAS2 then forms readily. In this temperature interval, a refractory might well suffer a loss of strength; the formation

of CsSs appears to be undesirable. Its presence has not been

observed in our work with other Ca0-Al203-Si02 castables, even when quartz has been used as a source of silica. Its formation

appears to be favored by the ready availability of all three

components of CAS2.

7 ) It is difficult to estimate the effect of leaching by a gasification environment on the CAS2 and A3S2 components of a Ca0-Al203-Si02 castable. However, available data suggest that

refractories of this type are a desirable choice for the

working face of a gasification reactor lining. As mentioned

previously, bonding phase CAS2 develops rapidly in the temperature interval 700-900 °C in the presence of steam. Free

a-A reacts at 900-1000 °C to form A3 $2

. Once formed, CAS2

persists when cooled from operating to ambient conditions of

temperature and pressure.

Summary

The in situ x-ray studies have shown that, in the presence of steam

and with changes in temperature, a high purity castable refractory is

subjected to frequent changes in bonding phases and to intervals in

which the main bonding phase is in a state of transition. These changes

are undesirable and should be minimized by avoiding exposure of the

refractories to steam below 750 °C, or preferably 900 °C. Reaction of

the high purity refractory with steam containing alkalis and silica from

impurities in the coal should be expected to occur at the working face

with formation of various silicates. These may alter the course of

hydration/dehydration reactions and the formation of bonding phases.

The Ca0-Al203-Si02 refractory has been shown (Task 2 ) to increase

markedly in flexural strength with exposure to either simulated or pilot

plant gasification atmospheres. The increase in strength is attributed

to the formation of CAS2 and A3S2. Once formed, these compounds are not

subject to hydration/dehydration and are stable in steam over a large

temperature range. Leaching of silica and other components from this

type of refractory has been observed, but may not pose a serious problem

during the service life of the material.

13
X 4J s-S

(U

g

msssmumm^

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"'

—' ro •—

cn

— 

E 4J _J

•• *r- <U O' > ^ D1 E T- (j c « Oi (O *f- C 2 U i. -O o O 0> ^ E ’r- &- *r— 4J cn 3 (0 E I— «— Q -f- S- 3

OJ V)

(fi E +J

C </) « 3 (O -p- (U E O <U >0)

(UiZ3

U)

E'

U (U <u _

00 V) (J •> <U (U O) (Q S-—' Q. S_ OJ ixo 00 o x:

>,^

— 

(TJ «U_ i. i. %. OO p oj a; 3 -M 40 X O *(0(0 O S- 0) <u OJ E O ^ ^ ^ -t- +J

+0 s , 0) cs

T

O j- OJ OQ T3 « • CO OO t. E--^ <U 4-> dJ •<- CQ 40 -P- >

(O r— o

(O 40 wo <J S- P OO -o OJu -o

>» i. -pU 0) <u s. o. 3

to W)

(/) s QJ o s.a i “ u

00 0

S'*. . o !. "O 4Jt> c s- 0 I •r 0 > 3 a;.'

01 10 P-S (/) o OO) E

'' ' * i. ^ 0) 0) ‘2 P- 4) a (0 3 i ouO 0

s<*-

s. (0 01 o

JD 44 0a

400) .Xo

<U 05

<J E E

02 o O S OO OO

<u

_ -

OO >>,

—< > <o OO (O UJ T— c

<C a>-p-o -f

O) 40 05 fO a r—

0) 3 a 3p*^ o 44 a

^44>^

Q.

O 3 42-0

OO c ^ (0 a a

OJw . M >) (O "r- a (U .0 .c <— = a 0) ^ 3 (fl sow w <j fl o " aj <U ^ Cl OO (U 3

(Q E 10 U 44 01

OUa

05

14
Figure 2. The pressure vessel and solid state detector mounted at the

x-ray tube.

1

5
CHANNEL

Figure 3. EDXD pattern of a-quartz (Si02). Diffraction peaks are

identified by their Miller indices. Four lines from the

characteri Stic spectrum of the tungsten x-ray tube are

also identified ( W La; W LBi, W LS 2 W Ly).

16
CHRNNEL

Figure 4. Calculated EDXD patterns of AH, AH3 , C3AH6 , C4A3H3, and CC.

17
a-auzot?

1

1

. 1 . . Il 1 .1.1 1,1 1 . 1 .1 .1 . il . II. 1 cae

1 -II 1 1 J . .il.

I J.i, 1 caz

iJil .1 ]jLMI _ 1 II III il i t iiiiiiii J1 CP

C5P

^ L_ 1 ^ 1 ». t III 1 iiJil 1 . In 1 J 1 ii< ^1 AilL

1 . II. 1 1. 1. 1 3_L_ iiJI u_j m_111 1 II Il 1 II 1 1 1 B-PLZO.?

Lii 1 J 1 . ill 1 1 1 i III 1 ' L_ Z 10Z 5ZZ 3ZZ 52Z 6ZZ 7Za 3ZZ 3ZQ 1ZZ0

CHANNEL

Figure 5 . Calculated EDXD patterns of a-A, CAg , CA2, CA, C3A, and 8-A.

18
a-sios

CRIST

MULLITS

jJ_L kJJl llll. L .. fi L -CaS3

I ll I: il liiilili nil ll 1S0 21^0 sues S00 6SR 70d srn? 30R 1000

CHANNEL

Figure 6. Calculated EDXD patterns of a-S (quartz), S (cri stobal ite)

,

A3 $2 (mull ite), and low CAS2. ^

19
Si02

1723*

Figure 7. Phase diagram of the system CaO-Al 203 -Si 02 showing the compositions

of NBS castable refractories (solid squares). A-56 lies in the compatibility

triangle A 3 S 2 -CAS2 -S, while A-94 is on the join Ca0 -Al 203 . The compositions

of two commercial refractories used in the Conoco pilot plant are

represented by open triangles.

20
CHRNNEL

Figure 8. A series of four EDXD patterns showing the dissociation of

AH (boehmite) and the formation of additional a- A in the

range from 247 °C and 3.2 MPa to 650 °C and 4.1 MPa.

21
I

(U <T3 x: CU o-

-M s_ z: 3

CT.+J ^ c <a a: CU <c1 •r- i. O < > C5 S CU +-> -f- O O. (U 4-> <^- £ CO "r” o to <u 4-> to

4-> >,

O c

rO Q. o

0) CU -M •r“

'd' -c +J CU -M M ra T3 ^ ca C E C -M s> S- C *r- o M CU -r- X Q. CU c M OtOt— CU +JCS-CU-I- o

ca o Q. s- ^ c

Q-t- Q. S_ S o

-t-» <t3 O

It- CJ « u

_J to 4->C

a

LU CM

O (O toO

to C3. ^ *r- -M E <-J <a +-> 2 C O 0 CU u 3

CE i.

U CL ra

CU

O CU

•rX +J CU LD CU 1- cu u 4-> to to > to

ra la T- CU ca • O.^ O -M J= CU -M

Q.-I-) ro 4J su

05 o 3 4-> C C_5 CU C c -o O T- 0 Ct- o <“

Q. OJ CU -a CU a. CT)^ ^ CU

O)ac(1) i-

(U 4-

-oc

a\

O)s-3

cn c C\i (O -= E a o

(T3 CU

S- CD

O) c > (T3 o s- 3

(/I • c E o

ra

u

CU +J O)

to to

(T3 c ^>- Q.

+J co S 'f- O 4J

-C o

to (O<u

to &-

(T3 OJ

CU J= cl+j

22 i
NET

COUNTS

500

1000

1500

2000

AH 4 fl

Figure 10. Computer plot of the intensity of the 251 reflection

of AH (solid line) and the 116 reflection of a-A (dashed

line) for a sequence of fourteen experiments. The

dehydration of AH is evident in the temperature range

from 375 °C to 650 °C at a pressure of approximately

4 MPa.

23
0 200 ' 400 600 800 1000

CHANNEL

Figure 11. EDXD patterns of an unfired test bar of CAS2 (a) at 25 °C,

(b) at 520 °C and 7 MPa steam, and (c) at 625 °C and 7 MPa

steam one hour later. Two lines of the CsSs phase are

identified with arrows.

24
V. PUBLICATIONS AND TALKS

F. A. Mauer and C. R. Robbins, "X-ray Powder Diffraction Measurements in

Reactive Atmospheres at 1000 °C and 7 MPa", Plenary Session, 27th Annual

Conference on Applications of X-ray Analysis, Denver, Colorado,

August 2, 1978

S, M. Wiederhorn, E. R. Fuller, Jr., J. Bukowski, C. R. Robbins, "Effect

of Hydrothermal Environments on the Erosion of Castable Refractories",

J. Eng. Mater. Technol . , 99_, 143-146 (1977).

F. A. Mauer and C. R. Robbins, "X-ray Powder Diffraction Measurements in

Reactive Atmospheres at 1000 °C and 7 MPa (1000 psig) Advances in X-ray

Analysis , Vol . 22, Plenum Press, New York (1979).

W. S. Brower, R. C. Dobbyn, and C. R. Robbins, "Laboratory Examination

and Evaluation of the Gasifier Lining Used in the Conoco Lignite Gasification Pilot Plant", Materials and Components in Fossil Energy Applications,

U. S. Department of Energy Publication, No. 24, June (1979).

R. C. Dobbyn, H. M. Ondik, W. A. Willard, W. S. Brower, I. J. Feinberg,

T. A. Hahn, G. E. Hicho, M. E. Read, C. R. Robbins, J. H. Smith and

S. M. Wiederhorn, "An Evaluation of the Performance of Materials and

Components Used in the Conoco Lignite Gasification Pilot Plant CO2

Acceptor Process", U. S. Department of Energy Report (1979) In Press.

25
VI. REFERENCES

1. "Materials Research for the Clean Utilization of Coal", Quarterly

Progress Reports for the period September 1975 - September 1979,

Center for Materials Science, National Bureau of Standards,

Washington, D. C. 20234 (available from NTIS; report designation

EA-6010; Dist. Category UC-90C).

2. S. M. Wiederhorn, E. R. Fuller, Jr., J. Bukowski, C. R. Robbins,

"Effect of Hydrothermal Environments on the Erosion of Castable

Refractories" , J. Eng. Mater. Technol., 99, 143-146 (1977).

3. R. C. Dobbyn, H. M. Ondik, W. A. Willard, W. S. Brower, I. J. Feinberg,

T. A. Hahn, G. E. Hicho, M. E. Read, C. R. Robbins, J. H. Smith,

and S. M. Wiederhorn, "An Evaluation of the Performance of Materials and Components Used in the Conoco Lignite Gasification Pilot

Plant CO 2, Acceptor Process", U. S. Department of Energy Report

(1979) In Press.

4. W. S. Brower, R. C. Dobbyn, and C. R. Robbins, "Laboratory Examination and Evaluation of the Gasifier Lining Used in the Conoco

Lignite Gasification Pilot Plant", Materials and Components in

Fossil Energy Applications, U. S. Department of Energy Publication,

No. 24, June (1979).

5. B. C. Giessen and G. E. Gordon, "X-ray Diffraction: New High Speed

Technique Based on X-ray Spectrography" , Science 159 , 973-975 (1968).

6. C. J. Sparks, Jr. and D. A. Gedcke, "Rapid Recording of Powder

Diffraction Patterns with Si (Li) X-ray Energy Analysis System: W

and Cu Targets and Error Analysis", Advances in X-ray Analysis, 1_^,

240-253 (1972).

7. Powder Diffraction File, JCPDS— International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, PA 19081.

3. E. M. Levin, C. R. Robbins, and H. F. McMurdie, Phase Diagrams for

Ceramists ", Figure 630, The American Ceramic Society (1964)

.

9. E. M. Levin, C. R. Robbins, and H. F. McMurdie, Phase Diagrams for

Ceramists - 1969 Supplement, Figure 4033, The American Ceramic

Society (1969)

.

10. G. L. Davis, 0. F. Tuttle, "Two New Crystalline Phases of the

Anorthite Composition, Ca0*Al 203 * 2Si 02 "

, Am. J. Sci., Bowen Vol .

,

107-114 (1952).

11. K. Speakman, H. F. W. Taylor, and (in part) J. M. Bennett and J. A.

Gard, "Hydrothermal Reactions of y-Oicalcium Silicate", J. Chem.

Soc. (A), 1052-1060 (1967).

26
HBS.114A iRgv- »-7«>

"

y.s. dept, of comm,

bibliographic data

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 80-2064

2oQo^L Aecessioo^ No.

\ title and su bti tl

e

CHEMICAL DEGRADATION OF CASTABLE REFRACTORIES IN COAL

gasification PROCESS ENVIRONMENTS

i Aoct^sjoR No;

/

5. Publication Date May 1980

S. Performins Organization Code

1. AUTH0R(5)

C. R. Robbins and F. A. Mauer

8. Performing Organ. Report No.

1. PERFORMING ORGANIZATION NAME AND ADDRESS 10.. Ptojecf/Taak/Wotk Unit No..

national bureau OF STANDARDS

department of commerce

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP) 13. Type of Report & Period Covered

14.. Spons^ng Agency Coda

15. SUPPLEMENTARY NOTES

1

' Document describes a computer program; SF-185, FIPS Software Summary, is attached.

literature survey, mention it here.) Reactions and _transfor!Tiations that result in chemical degradation of

castable refractories used as liners for coal gasification reactors have

been studied. In addition to phase analysis of laboratory and pilot

plant specimens by conventional x-ray pov/der diffraction, a new test

method was developed that permits changes in the phase composition to be

observed without removing the specimen from the test atmosphere. Two

candidate castable refractories were included in the study, a high

purity Ca 0 -Al 203 composition with tabular alumina aggregate ( 33.7 wt " AI 2 C 3 , 5.6 wt % CaO) and a- Ca 0 -Al 2 G<i-Si 07 composition with calcined

kaolinite aggregate (£5 wt % Al 203,'36 wt % Si 02 , 5 wt % CaO). Specimens were exposed to st?am, steam/C02 , ^ simulated coal gasification

atmosphere at a pressure of 7 MPa (1000 psig) and temperatures up to 1000 ”0. Frequent changes in the bonding phases, and intervals in

which a bonding phase 'was in transition were observed in the case of the

high purity castable refractory. The silica-containing refractory, on the other hand, formed bonding phases which were stable in steam over a large range of tamperatures and pressures.

~eV words r.i. .0 ai^haheticai order; capWa/iz, onlv r.»er o/ rWa .eK un/e,. a p«per na„.e..

separated by semicolona)

Coal; degradation; durability; environment testing; gasification, refractories.

18. AVAILABILITY rxi Unlimited

~~

For Official Distribution. Do Not Release to NTiS

c Order From Suo. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. $N003-0Q3-

3) Cfder From National Technical Information Service iNTlS), Springfieid, VA. 22161 — —

UNCLASSIFIED

20. SECURITY CLASS

iTHlS PAGE)

UNCL.ASSIFIED

30

22. Price

S6 .00

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