Failure Mechanisms in High Chrome Oxide

Gasifier Refractories

JAMES P. BENNETT and KYEI-SING KWONG

Gasification is a high-temperature, high-pressure chemical process used to convert a carbon

feedstock into CO and H2 (syngas) for use in power generation and the production of chemicals.

It is also a leading candidate as a source of hydrogen in a hydrogen economy and is one of

several technologies expected to see increased use in advanced fossil fuel power systems in the

future. Gasification is being evaluated because of its high efficiency, its ability to capture CO2 for

sequestration or reuse in other applications, and its potential for carbon feedstock fuel flexibility. At the heart of the gasification process is a gasifier, a high pressure chemical reaction

vessel used to contain the interactions between carbon and water in a shortage of oxygen,

producing syngas. The gasifier is lined with high chrome oxide materials to protect the containment vessel. Gasifiers are complex systems, and failure of the refractories used to line them

was identified by industry as a limitation to their reliability and availability and to their

increased use. NETL researchers have examined spent high-Cr2O3 (over 90 pct Cr2O3) refractories from numerous gasifiers to determine in-service failure mechanisms. This analysis

revealed that premature failure of the high chrome oxide refractories was related to ash in the

carbon feedstock, which liquefies during gasification and interacts with the refractories, leading

to wear by chemical dissolution or spalling (structural and chemical). A discussion of this

postmortem wear of spent refractory materials and of thermodynamic modeling used to explain

microstructural changes leading to wear are explained in this article. This information will serve

the basis to develop improved performance refractory materials.

DOI: 10.1007/s11661-011-0635-x

The Minerals, Metals & Materials Society and ASM International (outside the USA) 2011

I. INTRODUCTION

A. Gasification

GASIFICATION as a modern high-temperature,

high-pressure industrial process was first used by the

oil industry in the 1950s to process low-value petroleum

and by-products from petroleum refining into higher

value products. Since that time, gasification has

expanded to other uses, including processing low-cost

carbon materials for power generation and producing

CO and H2 feedstock for use by the chemical industry

(for applications from fertilizer to Fischer–Tropsch

liquids), and is considered a leading candidate to

produce H2 for a hydrogen-based economy. It is one

of several technologies expected to see increased use in

the future. Gasification is being evaluated, in part,

because of its high efficiency, ability to capture CO2 for

sequestration or reuse in other applications, and fuel

flexibility.

As a process, gasification converts a carbon feedstock

(typically methane, coal, or by-products from petroleum

processing such as petcoke) into CO and H2 (called

synthesis gas or syngas). The process of gasification

occurs in a facility similar to the general flow sheet

shown in Figure 1, with major downstream processing

and applications or proposed applications for the syngas

product shown. Gasification can occur using any of a

number of gasifier designs, including dry ash gasifiers

such as the Sasol–Lurgi* and Foster Wheeler designs

(slag typically does not fuse due to the low temperature

of operation), water-cooled entrained bed slagging

gasifiers such as the Shell and Siemens designs (gasification occurs at temperatures that create a molten slag

from ash which is contained by a water-cooled refractory liner), and the air-cooled entrained-bed slagging

gasifiers that include the ConocoPhillips and General

Electric designs (these gasifiers operate at temperatures

where ash liquefies into molten slag that flows down an

air-cooled refractory liner surface). Each gasifier design

has advantages and disadvantages that determine the

amount and type of carbon feedstock it can process and

the syngas composition produced.

Gasifiers are used to contain the severe service

reaction between carbon feedstock (typically coal,

petroleum coke, or biomass), water (or steam), and

oxygen in a reducing environment (shortage of oxygen,

with the oxygen partial pressure typically between 107

and 109 atm), producing a primary product of CO and

U.S. GOVERNMENT WORK

NOT PROTECTED BY U.S. COPYRIGHT

JAMES P. BENNETT and KYEI-SING KWONG, Ceramic

Engineers, are with the National Energy Technology Laboratory–

USDOE, Albany, OR 97321. Contact e-mail: james.bennett@netl.doe.gov

Manuscript submitted April 9, 2010.

Article published online March 3, 2011

*Disclaimer: The use of commercial products and brand names does

not imply endorsement by the DOE.

888—VOLUME 42A, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
H2 from the feedstock materials. The air-cooled slagging

gasifiers currently used by industry (shown in Figure 1)

are high-temperature, high-pressure reaction chambers,

typically operating between 1598 K and 1848 K

(1325 C and 1575 C) and at pressures between 2.07

and 6.90 MPa. A number of by-products occur during

gasification, with the overall process reaction shown as

follows:

C þ H2O gas ð Þþ O2 ð Þ shortage

! CO þ H2 þ CO2 þ minority gases

þ by-products þ heat

It should be noted that minority gases include H2S

CH4, NH3, HCN, and N2, and Ar by-products include

excess C, sulfur, and mineral impurities in the carbon

feedstock that become ash or slag.

The type, quantity, and amount of any minority gas

or impurity are determined by the composition of the

gasifier feedstock, the gasification temperature, or by

other gasification conditions such as the O2 content. For

most syngas applications, gas impurities are removed at

the gasification site. This is accomplished using a variety

of chemical processing techniques downstream from the

gasifier, as shown in Figure 1. Because of the reducing

environment of gasification, excess carbon from the

feedstock becomes a by-product of the process, which

can be recovered for reuse in the gasifier. Ideally, the

amount of excess carbon should be small, about

1.0 wt pct,[1] but is dependent upon variables such as

the gasifier type, carbon feedstock, and the C/O2 ratio.

In the reducing environment of gasification, between 20

and 30 pct of the O2 required for complete combustion

of the C and H2 in the carbon feedstock[2] is supplied.

Overall, gasification is considered a noncatalytic process

that involves a number of endothermic and exothermic

reactions, with the overall process being exothermic,[3]

as indicated in the preceding equation.

At the high-temperature and reducing environment of

gasification, organic matter in the carbon feedstock is

broken down, while inorganic materials (impurities)

remain as discrete particles (ash) that can fuse, coalesce,

and form slag, depending on the gasification temperature. When the ash becomes molten and coalesces as

slag, it can flow down the sidewalls of the gasification

chamber as liquid or leave the gasifier as individual

airborne particles. Molten ash particles that contact and

become attached to the gasifier sidewall that are not yet

fluid enough to move under the influence of gravity

(because of high viscosity) may require additives (fluxes)

introduced to the carbon feedstock. The purpose of

additives is to lower slag melting temperature and

viscosity, causing the slag to flow and exit the gasifier.

When slag additives are used, they can permit gasification to occur at a lower temperature. If a lower

gasification temperature is used, it will reduce corrosive

wear of protective liner materials in the gasification

chamber, assuming that the additives do not bring about

increased liner material corrosion. Ash quantity and

chemistry vary greatly with the carbon feedstock source,

with the quantity (percent of the overall carbon source)

being negligible for gas, very low for liquid petroleum,

Fig. 1—Schematic of the overall gasification process.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, APRIL 2011—889
and typically less than 1 pct for petcoke and 10 pct for

coal. Biomass, which is being considered as a future

carbon feedstock, has ash content varying from 1 to

20 pct and has widely varying chemistry. Containment

materials for the severe gasification environment of

temperature, ash chemistry, abrasion, and reducing

gases are called refractories, which are the focus of this

research.

B. Carbon Feedstock Slag Chemistry

Slagging gasifiers using air cooling can process

1250 metric tons/day or more of a carbon feedstock,

producing 90 metric tons or more tons per day of slag

waste, depending on the carbon feedstock and its ash

content. Ideal gasification occurs in a narrow temperature range that allows ash to liquefy into slag that flows

from the gasifier, yet is at a temperature low enough to

minimize chemical interactions with the refractory liner.

Because ash chemistry and quantity can vary greatly,

understanding and controlling slag chemistry for optimum gasifier operation (defined here as producing the

desired syngas composition with the longest refractory

service life) is critical. Two types of carbon feedstock are

commonly used in gasification, coal and petcoke, with

biomass feedstock considered in future applications and

not the subject of this article. A range of coal ash

chemical compositions for approximately 300 U.S.

coals[4] from across the United States are presented in

Table I and illustrate the wide chemistry variations that

exist. With regard to the compounds listed, Fe, Si, Ca,

and Al are the most common elements of concern

related to refractory corrosion and wear. These elements

also have a large impact on slag viscosity, which

influences slag penetration into the pores of the refractory lining and refractory corrosion. Iron oxide, for

example, influences slag viscosity, refractory penetration, and refractory corrosion, interacting with Cr2O3

and Al2O3 in the refractory grain to form either a solid

solution or a spinel, or causing its dissolution in the slag.

Other elements such as Na and K may be of concern

depending on their quantity in a slag.

Petcoke slags contain most major elements found in

coal slag but have higher levels of V and Ni. The average

range of chemistry for a limited number (nine) of

petcoke slags[5–7] is given in Table II. Slags with high

levels of V are a concern because of the large number of

valences the element can have, depending on gasification

temperature and oxygen partial pressure. In general,

petcoke slags are considered by gasifier operators to

have behavior similar to coal slags, although on a

microscopic level, differences exist between the refractory/slag interactions within the refractory microstructure. The focus of refractory research in this article will

not include vanadium compounds, which have limited

thermodynamic databases for high-temperature and low

oxygen partial pressure environments, making modeling

conditions in gasification difficult to impossible.

C. Impact of Refractory Lining Wear

The slag and gasifier environment are core issues

determining the gasifier lining’s refractory service life

and, thus, its on-line availability. The failure of a

refractory lining is expensive, both in terms of the

refractory replacement cost (‡$1 million U.S. dollars,

depending on gasifier size and the extent of rebuild

required) and also in terms of lost production time.

Relining a gasifier requires that it be completely shut

down and, under the best of circumstances, takes about

10 days for a partial rebuild and longer for a complete

rebuild. The process involves gasifier cooldown (up to

4 days), removal of the existing lining (1 to 3 days

depending on the extent of refractory repair necessary),

and repair/rebuild (3 days for a partial repair and 7 to

10 days or longer for a full rebuild, depending on the

extent of repairs necessary). Some gasification sites

maintain a second gasifier to ensure steady syngas

production. When one gasifier is shut down for maintenance, the second rebuilt gasifier, operating in standby

mode, is placed on line. This results in reduced system

downtime and increased on-line service and availability

to downstream operations. The second gasifier, however, increases capital and maintenance costs. Even with

a second gasifier, the time to switch to a second system

can vary from hours to days, depending on if the spare

gasifier is available and in the preheat mode.

Because of the long down times required for repair,

gasifier operators would like to install refractory linings

with a reliable life of at least 3 years and have identified

Table I. Range of Chemistry Found in over 300 U.S. Coal

Slags Due to Mineral Impurities[4]

Material

Weight Percent

Maximum Minimum Average

Standard

Deviation

SiO2 68.5 7.1 43.6 16.4

Al2O3 38.6 4.1 25.2 10.2

Fe2O3 69.7 2.1 17.0 11.2

CaO 45.1 0.5 5.8 6.6

MgO 8.0 0.1 1.2 1.1

K2O 3.5 0.2 1.4 0.7

Na2O 6.5 0.3 0.9 0.6

TiO2 3.7 0.4 1.4 0.8

Table II. Range of Chemistry Found in Nine Petcoke Slags

Due to Impurities[5–7]

Material

Weight Percent

Maximum Minimum Average

Standard

Deviation

SiO2 18.9 3.1 14.1 8.7

Al2O3 9.4 0.5 4.8 2.8

Fe2O3 31.6 1.2 7.2 9.3

CaO 11.9 2.0 5.4 3.8

MgO 5.1 0.3 1.0 1.6

K2O 0.7 0.3 0.5 0.4

Na2O 2.3 0.1 0.8 0.8

TiO2 0.6 0.2 0.3 0.2

NiO 11.4 2.9 8.4 3.2

V2O5 79.4 30.2 57.0 19.5

890—VOLUME 42A, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
refractory service life as a key barrier to widespread

commercialization of gasification technology.[8] The

current generation of refractory liners in slagging

gasifier systems has yet to meet this service life, failing

in as little as 3 months in high-wear areas while

approaching the desired 3-year service life in other

locations. Reliable refractory liners with improved

service life are needed to ensure that gasification fulfills

its potential as a clean and efficient means of generating

power and producing precursor gas streams. Gasifier

users need system availability of 85 to 95 pct for utility

applications and more than 95 pct in applications such

as chemical feedstock production.[9] Failure to meet

these goals has impacted the commercial use of gasification technology.

D. Gasifier Liner Technology

The material challenges for the refractory liner in an

air-cooled slagging gasifier are severe and include the

following: elevated temperature, large or rapid changes

in temperature, erosion by particulates, molten slag

attack, slags of variable composition, attack by hot

corrosive gases, and variable oxidizing or reducing

conditions.[10–12] As mentioned, refractory materials

that can withstand a gasifier environment for long

periods of time are necessary for a continuous, efficient,

and reliable gasification process. Two of the slagging

gasifier designs that are the focus of this research are

shown in Figure 2.

Refractory linings used in gasifiers are between two

and six layers,[13] with a typical air-cooled lining cross

section composed of four layers, as shown in Figure 3.

The hot face or working lining is designed for direct

contact with the gasification environment, with the

composition dependent not just on the gasifier

atmosphere and feedstock, but also on the temperature

and location in the gasifier where the material is used.

The most critical refractory properties are that it be

thermodynamically stable to hot gases such as H2, CO,

CO2, H2O, or H2S; that it withstand hot abrasion of

molten slag; that it be chemically resistant to liquefied

ash from the carbon feedstock; and that it withstand

thermal cycling. Because of the number of slag components and the complicated microstructure of gasifier

refractories, the use of existing phase diagrams or

thermodynamic data to predict refractory/slag interactions is of limited value. Few phase diagrams of

relevance beyond three components were created, and

thermodynamic databases for use in the high-temperature

oxidizing/reducing environment with the number of

compounds in a slag are incomplete or do not exist for

all interactions of interest. Limited information also

exists on the ability of a refractory material to withstand

the high-temperature particulate impact/abrasive associated with the injected feedstock. Other material

properties, such as the thermal conductivity of the

refractory lining, must be considered so gasifier shell

temperature is maintained at an acceptable internal

temperature for slag flow, yet keeps the steel shell and

refractory insulation above the acid dew point condensation temperature. If dew point condensation occurs

within the gasifier, it could result in shell corrosion and

catastrophic gasifier failure due to the high pressure

used in gasification.

Gasifier refractory designs fall into two types of

gasification systems: (1) gas and liquid feedstock gasifiers (typically low ash) and (2) solid feedstock gasifiers

(typically higher ash). The emphasis on liner materials is

on the hot-face refractory materials, which are exposed

to the most aggressive environment and, therefore,

exhibit the most aggressive wear/corrosion.

Fig. 2—Two types of air-cooled slagging gasifiers using refractory liners: (a) GE design and (b) ConocoPhillips design.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, APRIL 2011—891
E. Refractories for Low Slag Environment

(Gas or Liquid Feedstock)

In the low-slag environment of gas or liquid gasifier

feedstock, high-alumina refractories low in SiO2 and

FeO were found to perform best. The low SiO2 and FeO

levels are important because these components can

interact with syngas. Hydrogen reduction of SiO2 occurs

above 1253 K (980 C), causing the formation of SiO

vapor, which migrates to other locations in the refractory lining.[14]

The SiO2/H2 reaction becomes very likely

at temperatures above 1473 K (1200 C)[15] and is

impacted by brick porosity, gasifier pressure, and

feedstock throughput. Silica volatilization was found

to occur mainly at the hot face of a gasifier liner, where

refractory bond strength can be adversely impacted.

Iron oxide in alumina refractories used with gas or

liquid feedstock is kept low because it catalyzes the

Boudouard reaction (2CO fi CO2+ C),[14] leading to

the buildup of carbon in a refractory. The Boudouard

reaction occurs as low as about 783 K (510 C),

maximizes by 843 K (570 C), and nearly disappears

by 1003 K (730 C).[15] The carbon buildup leads to the

formation of free layers of C or Fe2C at joints, pores,

voids, or cracks, causing structural weakening of the

refractory, thermal expansion mismatches, or increased

or nonuniform heat transfer to the gasifier shell.

Typical failure mechanisms found in gasifiers having a

gas or liquid feedstock include thermal expansion or

irreversible creep deformation,[14] which occurs during

rapid thermal cycling and leads to surface spalling or

joint failure. Refractory linings in gasifiers with gas or

liquid feedstock last much longer than the 2 to 3 years

experienced with coal or petcoke (slagging) feedstock,

and are currently not viewed as areas requiring research.

When a monolithic lining is used with gas or liquid

carbon feedstock, anchors that attach the lining to the

shell can experience failure from mechanical stresses,

metal fatigue, or corrosion, leading to gaps between the

refractory shell and the lining. These gaps impact heat

transfer and can initiate other failure mechanisms due to

the heat buildup at those sites. Monolithic materials

such as castables, however, are seeing increased usage in

low slag gasifiers for many reasons, including fewer

bonds and increased speed of installation.[14]

F. Refractories for Solid Feedstock Gasifiers

(Typically Coal or Petcoke)

A number of refractory compositions were considered

or evaluated for slagging gasifier environments using

coal or petcoke feedstock before current liner materials

were selected, and include sintered or fused cast alumina-silicate, high alumina, chromia-alumina, chromemagnesia, alumina, and magnesia, as well as SiC

refractory materials.[16–20]

Laboratory research was used

to evaluate the performance of early liner materials by

simulating the gasifier environment to determine corrosive interactions between slag and refractory. This

research is traceable back to the 1970s and 1980s, and

was funded by the USDOE, the Electric Power Research

Institute (EPRI), and private industry.[10–12,16–19,21,22]

Refractory microstructures containing SiC and Si3N4,

for example, were found to react with components of the

gasifier slag, causing severe material wear. Silicon

carbide in the refractory and FeO in the slag react to

form reactive gases such as CO, the potential for SiO or

SiO2 formation, and metallic iron, all of which can lead

to rapid refractory degradation and material loss.

Fused-cast refractories (with little or no porosity)

interacted with slag at a slower rate, but had thermal

shock issues due to gasifier cycling or shutdown, leading

to higher wear rates than with porous materials.

Fig. 3—Type of air-cooled slagging gasifier with a cross section showing slag flow down hot-face refractories common to these types of gasifiers.

892—VOLUME 42A, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
Alumina refractories are not used in slagging gasifiers

with coal or petcoke feedstock because of potential

corrosive interactions between the liquid ash and the

refractory. Vanadium found in petcoke slags can

aggressively attack alumina refractories, rapidly

decreasing service life. When vanadium is present as

V2O3 (one of several stable phases present in the

reducing environment of a gasifier depending on its

oxygen partial pressure), it has a melting temperature of

about 2243 K (1970 C); however, V2O5 (a stable phase

present in atmospheric oxygen partial pressure) melts at

about 953 K (680 C). Initial liquid formation between

V2O3 and Al2O3 occurs at about 1993 K (1720 C),

while between V2O5 and Al2O3, it occurs at about 913 K

(640 C). Because of the low temperature at which liquid

occurs between V2O5 and Al2O3 phases, rapid reactions

between slag and refractory can occur in air, which can

lead to excessive refractory wear in a short period. In

practice, the valence of vanadium should not be an issue

except during gasifier preheat or cooldown, when high

oxygen partial pressures are present and V2O5 formation

can occur. Poor mixing of the carbon feedstock and

oxygen in a gasifier, however, could lead to isolated

environments and uneven refractory wear. With using

chrome oxide refractory liners in slagging gasifiers,

gasifier operators generally think that vanadium oxide

acts similar to iron oxide, with comparable wear rates.

Potential differences in the wear rate of high chrome

oxide refractories caused by slags rich in iron oxide or

vanadium oxide were observed in laboratory testing, but

have not been reported in commercially exposed samples; so they are not the subject of this article.[23]

On the basis of laboratory testing and hands-on

experience of gasifier users, chrome oxide refractories

were determined to have the best overall properties for

hot-face refractory materials, with compositions containing Cr2O3-Al2O3, Cr2O3-Al2O3-ZrO2, and Cr2O3-

MgO found to have the best performance.[10,20,24]

Examples of typical high chrome oxide compositions

used in industry are listed in Table III. Composition

‘‘C,’’ a formulation containing Cr2O3-MgO, has been

greatly limited or discontinued in the United States

because of concerns over possible hexavalent chrome

formation. Thermodynamic research has shown that

levels of alkali and alkaline earth materials normally

encountered in coal or petcoke, or that would occur in

potential mixtures of biomass and western coal slags

(materials naturally high in alkali and alkaline earth

compounds), would not cause the formation of hexavalent chrome oxide at a level to be a concern.[25] The

reason for this is related to oxygen partial pressure,

which causes hexavalent chrome oxide to form at very

low levels of the gasification process (106 to 109 atm).

Because of cost and performance considerations

associated with the high chrome oxide refractories,

zoning (the use of refractories with different compositions and properties in different areas of the gasifier) is

practiced at many gasifier sites. Originally, a minimum

of 75 pct Cr2O3 in a refractory was thought necessary to

give sustained lining performance.[26] Improvements in

refractory technology and a better understanding of

gasifier wear allowed Cr2O3 levels as low as 60 pct to be

used in gasifier locations experiencing low wear (dome

or some areas of the sidewall), with Cr2O3 levels as high

as 95 pct used in the severe wear areas (sidewall or slag

exit locations). Selection of a refractory is based, in part,

on the performance of the refractory (which is related to

the chrome content) and on the cost of the material

(with higher chrome oxide content typically costing

more). At many gasifier locations, the performance and

cost of the higher chrome oxide material does not justify

its use. As mentioned, even with improvements in

material technology, service life of the hot-face Cr2O3-

Al2O3 and Cr2O3-Al2O3-ZrO2 compositions is limited to

between 3 and 36 months.

The backup refractory lining (behind the high chrome

oxide hot face, as shown in Figure 3) is an alumina/

chrome refractory (approximately 90 pct Al2O3/10 pct

Cr2O3). The purpose of this lining is to slow slag

corrosion in case of primary lining failure. The backup

lining also controls the gasifier shell temperature during

periods of normal gasifier operation. The final brick

layer of the gasifier lining is a monolithic or firebrick

material, as shown in Figure 3. This layer, along with a

layer of fiber insulation, is located adjacent to the

gasifier steel containment shell where it provides insulation and controls the gasifier shell temperature.

Because the syngas environment is high in H2, which

has a thermal conductivity about 7 times that of air,

low-porosity linings are used as insulating material even

though it has a much higher density and thermal

conductivity in air than insulating firebrick.[14] Pores

of high porosity refractory become filled with syngas,

resulting in a thermal conductivity that can be 1.5 to 2

times that of air (assuming a 50 pct H2 syngas atmosphere). For this reason, low-porosity firebrick material

can have better insulating properties than traditional

high porosity insulating refractory liners. Super-duty

(mullite) or high-alumina refractory materials are typically used.[13]

The last layer of refractory lining material is located

between the insulating refractory brick and the steel

shell (Figure 3). This layer allows for expansion/movement of the refractory lining, in both the circumferential

and vertical dimensions. This layer is a fibrous ceramic

Table III. Chemical Composition and Physical Properties

of Commonly Used High Chrome Oxide Refractories Used

in Air-Cooled Slagging Gasifiers

Chemistry (Wt Pct)*

Brick Type

ABC

Cr2O3 89.0 87.3 81.0

Al2O3 10.2 2.5 0.4

ZrO2 — 5.2 NR

MgO NA NA 17.0

Bulk density* (g/cm3

) 4.21 4.07 3.95

Porosity* (vol pct) 16.7 16.5 12.0

CCS* (MPa) 48.3 66.9 40.0

*Data from manufacturer’s technical data sheets.

NR = not reported.

CCS = cold compressive strength.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, APRIL 2011—893
material, which absorbs expansion/contraction differences between the steel shell and the refractory lining. A

maximum 70 to 75 pct compression factor is used in this

layer to allow fiber flexibility, yet does not allow for hot

spots or permanent shaping of the fiber from overcompression. The fiber layer is important because it allows

for expansion of unsupported structure lengths in the

high-pressure vessels, limiting expansive stresses in the

gasifier shell.

High chrome oxide refractory liners currently used as

liners in slagging gasifiers were developed based on coal

and petcoke carbon feedstock. Future carbon feedstock

under consideration for slagging gasifiers includes biomass and coals from the western United States, materials that are high in alkali and alkaline earth oxides. If

used individually or combined in mixtures with coal or

petcoke, slags from these materials may have different

corrosion mechanisms and may require different refractory liners from the high chrome/aluminum/zirconium

oxide refractories currently used. An example of a

nontraditional carbon feedstock under consideration is

black liquor, a by-product from Kraft paper production.[27,28] This material was evaluated as a carbon

feedstock in gasification to recover potential heat from

the carbon it contains and to recover green liquor used

to regenerate pulping chemicals. Sodium carbonate and

sodium sulfide are generated in the gasifier (along with

other by-products) from the black liquor, which contact

the refractory liner, causing its wear and eventual

failure. Chrome oxide liners were one of several refractory compositions considered as liners in the experimental gasifier, but had performance related issues.[28]

In laboratory testing and in preliminary field testing, a

fusion-cast commercial refractory made of magnesiumaluminum spinel was found to perform best, and was

thought to provide the desired 2-year service life

necessary for this application. Unfortunately, research

on this type of gasifier was discontinued.

It is apparent that when using the current carbon

feedstock of coal or petcoke with the different refractory

layers of Figure 3, the hot-face lining experiences wear

issues that impact gasifier on line service. In order to

improve the performance of existing high-chrome oxide

refractory materials used in the hot-face lining or to

develop new, improved performance compositions,

postmortem analyses to determine the causes of failure

of existing refractory materials at several commercial

gasifier sites are necessary and were preformed.

II. ANALYSIS OF FAILURE HIGH

CHROME OXIDE REFRACTORIES

IN SLAGGING GASIFIERS

A. Refractory Failures

Based on discussions with gasifier users/operators,

observations of failed refractories at different gasifier

sites, and postmortem analyses of failed refractories, a

flow sheet categorizing the flow causes of refractory

failure (Figure 4) was developed. The causes of refractory failure include gasifier design (air vs water quench

in the lower cone/slope area), how the gasifier was

operated (material throughput, temperature, and number of thermal cycles per campaign), the composition of

the refractory and how it withstands chemical corrosion/

physical wear, the quality of the refractory (internal

flaws or exterior dimensions), and how well the refractory material was installed. Each of these varies over

time and at different gasifier sites. The most common

and catastrophic causes of refractory failure were found

to be chemical corrosion and spalling, although other

causes such as burner misalignment or refractory

installation can be problematic and can lead to rapid

lining failure.

As viewed from the interior of a gasifier, an example

of high chrome oxide refractory wear on the gasifier

sidewall, dominated by chemical corrosion and spalling,

is shown in Figure 5. Chemical corrosion involves the

dissolution of refractory material into the slag as it flows

Fig. 4—Refractory wear flow sheet based on discussion, observation, and postmortem analysis of spent air-cooled slagging gasifier refractories.

894—VOLUME 42A, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
over or penetrates refractory pores. Corrosion within

the pores can lead to the removal of refractory particles

or grains as the intergranular bond phase is weakened.

Chrome oxide, which can make up to 95 wt pct of a

refractory’s overall composition, is an excellent liner

material because it interacts with specific components of

the gasifier slag, forming high melting phases, and

because it increases the viscosity of the slag by interacting with components in it. Chrome oxide has a very low

solubility in the slag. Because of its very high chemical

resistivity and because of how it interacts with slag,

refractory bricks of this type have a longer refractory

service life than most other refractory oxides in gasifiers.

Hot-face chrome oxide refractories are sintered from

a mixture of controlled particle size grains (typically

from a maximum of 3 mm down to about 45 lm in size)

and have a porosity ranging from 14 to 17 pct. The

porosity aids in the thermal shock resistance of the

refractory, but is also the indirect cause of structural

spalling. The porous nature of chrome oxide refractories

and the small thermal gradient from the hot face to the

cold face of the brick (about 373 K (100 C)) allows slag

to penetrate deep within the refractory surface, setting

up the basis for structural spalling. Figure 6 illustrates

how corrosion and spalling influence overall refractory

wear,[26] with a gradual corrosion rate caused by

chemical dissolution (close to horizontal slope), rapid

lining wear caused by structural spalling or chemical

spalling (vertical slope areas), and the combined effects

creating a significantly higher wear rate vs by chemical

dissolution alone (combined slope area, which is a

dashed line). Hot-face spalling that occurs during a

gasifier operation is not uniform, as noted by the

irregular surface wear in Figure 5.

Wear and slag penetration in the cross section of

a 90 wt pct chrome oxide refractory liner material

removed from a gasifier (cross section goes from the

hot face (top) to the cold face (bottom)) caused by both

structural spalling and chemical corrosion is shown in

Figure 7. Note that slag penetrated deep within the

refractory and that cracks formed near the hot face of

the refractory where slag penetrated refractory pores

and close to the slag penetrated/nonpenetrated layer.

Crack formation is the basis for structural spalling and

typically originates in or near the slag penetrated/

nonpenetrated layer. Slag penetration is determined by

its chemistry, the ability of the slag to ‘‘wet’’ the

refractory, its viscosity, and the temperature profile

across the refractory (influenced by the thermal conductivity of the refractory). Once cracks form, they can

link (coalesce), resulting in refractory layers, up to

25-mm thick, that can spall off from the hot face.

Structural spalling is exacerbated by thermal cycling of

the gasifier, which occurs about every 30 to 45 days.Work

has been done to predict spalling wear[29]

using modeling.

Examples of spalled refractory flowing down the

sidewall of a gasifier and spalled refractory material

(after removal from a gasifier) are shown in Figure 8.

Note that the spalled refractory sliding down the

refractory sidewall (circled in yellow in Figure 8(a)) is

thin. It was removed from the gasifier and is shown in

Figure 8(b). This spalled refractory is approximately

5-mm thick, while the thick spalled refractory sample

shown in Figure 8(c) is approximately 20-mm thick and

came from a different gasifier. It is not known why the

thickness of these two spalled materials is different, but

factors such as the temperature of operation, the

length of uninterrupted service, the speed of a gasifier

Fig. 5—High chrome oxide sidewall refractories in a slagging gasifier

indicating surface corrosion and spalling.

Exposure Time (hours)

0 2000 4000 6000 8000

Wear (mm)

0

20

40

60

80

Spall, typical

0.005 mm/hr, typical

0.003 mm/hr

Average Wear rate

0.01 mm/hr

Fig. 6—Combined wear of chemical dissolution and structural spalling in high chrome oxide gasifier refractories (actual wear will vary

from what is shown).[26]

Fig. 7—Cross section of a refractory showing slag corrosion and

penetration leading to crack formation, a precondition for structural

spalling.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, APRIL 2011—895
shutdown/heatup, the number of heatup/cooldown

cycles, and at what stage spalled material is released

from the brick surface influence how spalling and

corrosion occur.

A refractory brick in the process of spalling was

removed from a gasifier lining and is shown in Figure 9.

Note that this brick has two locations in the process of

spalling: a thin layer on the surface (indicated by the

upper arrow) and a thicker area (indicated by the lower

arrow) that is a greater distance from the refractory hot

face. It is thought that the thin sample on the refractory

surface (Figures 8(a) and (b)) may have initiated as a

thicker specimen that gradually wore thinner by surface

corrosion before becoming dislodged from the surface

(through spalling). As long as a spalled refractory

remains in place, it will wear by corrosion, protecting

refractory material underneath it. Spalling is exacerbated by rapid gasifier cycling, which causes portions of

the refractory hot face to be continuously and rapidly

removed, decreasing overall refractory service life vs the

slow wear process brought about by only chemical dissolution.[26]

The mechanism of refractory slag attack on

a high chrome oxide refractory is shown in Figure 10.

An overview of air-cooled slagging gasifier wear based

on postmortem evaluations made in one type of

commercial gasifier is discussed in the following gasifier

sections: (1) dome, (2) sidewall, and (3) lower cone.

These areas of the gasifier are shown in Figure 3 and

correspond to where carbon feedstock is injected in the

gasifier (dome), the vertical sidewall, and the base where

slag and syngas exit the gasifier (lower cone).

B. Dome Lining Failure

The dome is located at the top of the GE type gasifier

(Figure 2(a)) and is where the carbon feedstock, oxygen,

and water used to produce syngas are injected into the

gasifier. The lining used in the dome can have a chrome

oxide content as high as 95 pct, with a service varying

Fig. 8—Examples of refractory wear caused by slag dissolution and

spalling. (a) Flowing slag is causing chemical dissolution of the

refractory sidewall and refractory material spalling (circled fragment). (b) Thin spalled refractory approximately 5-mm thick shown

in (a). (c). Thick spalled refractory approximately 20-mm thick.

Fig. 9—Refractory surface in the process of spalling (yellow arrows

point to spalled material).

Fig. 10—Stages of refractory wear in high chrome oxide refractories

used in slagging gasifiers.

896—VOLUME 42A, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
between 9 and 36 months. Refractory wear in the dome

area is shown in Figure 11 and can be caused by the

following: surface corrosion/erosion, structural spalling,

and spalling at refractory joints.

Much of the irregular surface in the dome area is

caused by structural spalling. An example of this

is shown in the side view of a dome refractory

(Figure 12(a)). A large depression at the refractory

brick joint, described as edge spalling, is also evident in

Figure 12(a). A number of possible causes exist for the

edge spalling and include hoop stresses, refractory

material that is slightly out of specification for the

dimensions of the dome, a gradual lowering (creep) of

the dome during use, or improperly installed refractory

materials.

Refractory wear due to the carbon feedstock combustion is noted at some of the refractory brick joint

intersections (indicated by an arrow in Figure 11) and

by the side view of refractories removed from the dome

(Figure 12). The joint defect occurs when carbon

feedstock and oxygen penetrate to the back of the dome

hot face, ‘‘combusting’’ there, vs reacting within the

gasification chamber. Slag from the carbon feedstock is

deposited on the refractory surface, which is red in

color. The red slag appearance in Figure 12 was caused

by an oxygen partial pressure close to theoretical

(around 101 vs the 108 in the gasifier), which lead to

the formation of thermodynamically stable Fe2O3. The

red slag is in contrast to the brown slag color in the

interior of the gasifier, which is due to the formation of

complex iron oxide, such as Fe3O4, which is the stable

thermodynamic state of Fe in the lower oxygen partial

pressure of the gasifier (between about 104 to 108

).

C. Sidewall Lining Failure

Refractory wear on the sidewall of the gasifier has two

main causes: chemical corrosion spalling from slag and

structural spalling. These two wear mechanisms cause

refractory service life on the gasifier sidewall to vary

between 9 and 30 months, depending on the gasifier site.

Worn gasifier sidewall refractories are indicated in

Figure 13 for refractory materials containing about

90 pct chrome oxide. These two photos are from

different gasifiers, with differences in surface wear due

to factors such as the carbon feedstock, the feed rate, the

Fig. 11—Dome of an air-cooled slagging gasifier showing the hot-face refractories and causes of wear.

Fig. 12—Dome hot-face refractories. (a) Side view showing spalling where bricks join and showing surface spalling in the process of occurring,

and (b) path of carbon feedstock that has entered between joints of the refractory, ‘‘combusting’’ between brick layers. Note the different shades

of color at the brick joints where slag is present, which is caused by slag iron in different oxidation states in (b). A combustion pathway is evident at the side and base of the bricks. In (b), the single brick on top is from the hot face and the two bricks underneath are from the backup

lining.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, APRIL 2011—897
temperature of gasifier operation, the number of gasifier

cycles, the heating/cooling rate, the brick design, and the

differences in how a material is installed. Hot-face

refractory samples taken from the surface of both

gasifiers had interior cracks or voids similar to Figure 7,

which led to spalling.

D. Lower Cone Failure

The lower cone (also called the throat) connects the

gasifier to the radiant syngas cooler or coolant quench

chamber. The radiant syngas cooler and the quench

chamber are designed to cool the syngas for processing

applications downstream, as shown in the gasification

flow sheet (Figure 4). The internal diameter of the lower

cone is much smaller than that of the gasification

chamber, which causes the molten slag exiting the

gasifier to have greater mass flow per unit surface area,

leading to greater erosive wear per unit time. The major

cause of refractory wear in the lower cone is chemical

dissolution, which occurs on the surface of the refractory (Figure 14(a)), although wear caused by other

mechanisms such as spalling and thermal shock exists.

Thermal shock in the lower cone layer closest to the

coolant water spray (Figure 14(b)) is thought to be

caused by coolant water splashing from the syngas

quench chamber. Thermal shock in the sample shown in

Figure 14(b) was so severe that cracking (and slag

penetration into the cracks) is evident throughout the

material. A white material buildup is also noted in the

sample, which was analyzed and found to be high in

silica. The long-term ramifications of the thermal shock

and cracking are not known, but should shorten service

life. Surface spalling at the corner/edge of the refractory

shown in Figure 14(c) is due to slag penetration and

interactions with the refractory. Exposed corners of

refractory that overhang underlying refractory layers

have a surface breakup, as shown in Figure 14(c), which

is about 3 mm in thickness. It is evident that this edge

spalling occurs throughout all levels of the lower cone

refractory shown in Figure 14(a.) Surface corrosion,

along with edge spalling and thermal shock, led to a

service life in the lower cone ranging from 3 to

18 months.

E. Analysis of Slag/Refractory Interactions

As mentioned, chrome oxide can comprise up to

95 wt pct of a hot-face refractory, and is used in gasifier

refractories because it interacts with several components

in the gasifier slag, forming high-melting phases (solid

solutions or spinels) and a slag of higher viscosity.

Fig. 13—Sidewall wear occurring on sidewall refractories containing approximately 90 pct chrome oxide. Figures (a) and (b) come from different

gasifiers run under different operational conditions and indicate two different types of surface wear: (a) spalling occurring in large sections not

limited to brick boundaries and (b) spalling of individual brick. Corrosion from slag occurs at the surface of both bricks.

Fig. 14—Hot face of lower cone (throat) refractory materials showing wear due to slag attack and spalling: (a) lower cone area, (b) thermal

shock and deposits occurring because of coolant spray, and (c) edge wear spalling on brick overhang between refractory layers.

898—VOLUME 42A, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
Alumina is often combined with chrome oxide to aid in

sintering or densification and because it also forms high

melting phases. Examples of how Cr2O3 and Al2O3

interact with specific slag compounds are shown in the

phase diagrams of FeO-Cr2O3 and FeO-Al2O3,

[30] as

shown in Figures 15(a) and (b). Because of the high iron

oxide content in gasifier slags (Tables I and II), the

reduction or removal of FeO from slag leads to a higher

viscosity, limiting the ability of a slag to penetrate the

refractory microstructure at a given temperature.

The depth of slag penetration into a refractory with

over 4000 hours of usage in a slagging gasifier (temperature generally between 1673 K and 1773 K (1400 C

and 1500 C)) is shown in Figure 16. A core sample

taken from the spent refractory was approximately 3 cm

in diameter and 8 cm in depth. The core extended from

the hot face (slag penetrated area) to the cold face (no

slag penetration) and was cut into discs parallel to the

hot face at approximately 4-mm intervals for analysis.

Slices were evaluated by SEM for microstructural

changes. Ground samples taken from individual slices

were also analyzed for chemistry, X-ray diffraction, and

thermal expansion measurements. Analyses are presented in Table IV. Thermal expansion samples were

also cored parallel to the hot face of the refractory in

two areas, where the slag had infiltrated (labeled hot

face), and at the cold face, where the slag had not

infiltrated (labeled cold face), as shown in Figure 16.

The crystalline phase analysis of the high chrome

oxide refractory vs depth from the hot face to the cold

face shown in Table IV indicates eskolaite (Cr2O3) was

the primary crystalline phase present in all layers except

near the hot face (hot face to 11.4 mm), which had an

iron/chrome spinel phase present. In evaluating the

chemistry of Table IV, it is of interest to note that iron is

predominately at the hot face of the refractory, although

smaller amounts are found to have penetrated the

porous refractory to a depth of 38.9 mm. The concentration of CaO in Table IV increases with depth from

the hot face, peaking in concentration from about 25 to

Fig. 15—Phase diagrams indicating solid and liquid phases present between (a) FeO-Cr2O3 and (b) FeO-Al2O3.

[30]

Fig. 16—Cross section of a 90 pct chrome oxide refractory removed from a gasifier sidewall using coal as a feedstock after approximately 4000 h

of usage between 1673 K and 1773 K (1400 C and 1500 C). Note the slag penetrated area on the hot face, along with the cracking occurring

at the slag penetrated/not penetrated interface, indicated by the arrow.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, APRIL 2011—899
39 mm, than decreasing to background concentrations

by 48.0 mm. SiO2 is also high near the surface and

decreases from the hot face to a depth of about 53 mm,

after which it is present at a background concentration.

These two compounds (CaO and SiO2) are carried as

part of the slag into the porous refractory where they

may react with refractory components or exist in the

glassy slag phase. Since there is little thermal gradient

through the hot-face refractory brick, it is thought that

the viscosity of the slag increases with depth of

penetration as reactions occur, limiting penetration.

Thermal expansion of samples taken from the hot

face (slag-infiltrated area) and cold face (nonslag-infiltrated area) of the refractory shown in Figure 16 had the

following average linear slopes between 373 K and

1573 K (100 C and 1300 C) when evaluated in an inert

He gas:

Hot face: 8:18  106 mm=mm=K mm=mm= ð Þ C

Cold face: 7:68  106 mm=mm=K mm=mm= ð Þ C

The difference between these two is 6.5 pct, which

could contribute to structural spalling, which could

contribute to cracking (and spalling) at the infiltrated/

noninfiltrated interface. No clear influence on expansion

was noted in the oxidation/reduction behavior of these

samples, which would have been caused primarily by

valence changes of iron in chromia/alumina. A discussion of stresses and their possible contribution to

structural spalling was previously presented.[29] The

formation of an iron/chrome spinel phase on the surface

of the high chrome oxide refractory is thought, however,

to contribute to chemical (micro)-spalling and refractory

wear at the refractory hot face/slag interface due to

volume changes associated with the formation of the

spinel phase. Chemical (micro)-spalling will be discussed

later.

Evidence of the surface reaction of iron is also shown

in a hot-face to cold-face cross section of the slag/

refractory interface analyzed by SEM (Figure 17), along

with phases identified by EDX chemical analysis at

specific points in the slag and in the refractory

(Table V). This refractory sample came from a gasifier

using coal as a feedstock in an oxygen partial pressure of

about 108 atm at temperatures from 1673 K to 1773 K

Table IV. Chemical Analysis of the Refractory Sample in Figure 16

Distance from Hot Face (mm)

Bulk Chemistry (Wt Pct)

Cr2O3 Al2O3 SiO2 CaO Fe X-Ray Crystalline Phases

Hot face to 2.3 80.0 10.8 5.4 0.3 1.6 P-Cr2O3

Tr-MCr2O4

6.9 84.2 10.2 3.9 0.3 0.4 P-Cr2O3

Tr-MCr2O4

11.4 83.9 10.7 3.2 0.4 0.4 P-Cr2O3

Tr-MCr2O4

16.0 87.7 10.3 3.5 0.5 0.3 P-Cr2O3

20.6 83.9 10.3 3.0 0.5 0.3 same

25.1 83.5 10.7 3.0 0.6 0.3 same

29.7 84.3 10.4 2.7 0.6 0.3 same

34.3 83.5 10.4 2.8 0.6 0.4 same

38.9 82.7 10.0 1.6 0.6 0.3 same

43.3 83.9 9.3 2.3 0.5 0.2 same

48.0 85.4 9.6 0.6 0.1 0.2 same

52.7 85.7 10.5 0.9 0.2 0.2 same

57.2 86.1 10.5 0.2 0.0 0.2 same

61.7 86.1 10.6 0.2 0.0 0.2 same

127 87.4 9.4 0.2 0.2 0.2 same

P = primary phase.

Tr = trace.

Fig. 17—SEM backscatter micrograph of a high chrome oxide

refractory slag/refractory interface exposed to a coal slag between

1673 K and 1773 K (1400 C and 1500 C) for approximately 2000 h

of exposure at an oxygen partial pressure of 108

. Markings 1

through 6 indicate points where EDX elemental analyses were taken

(chemistry is given in Table V).

900—VOLUME 42A, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
(1400 C to 1500 C) for about 2000 hours. The SEM

backscatter image of Figure 17 indicates that distinct

zones exist in the refractory where point EDX chemistry

was determined: point 1—slag near the refractory face,

point 2—phase buildup along the refractory surface,

point 3—diffusion layer that occurs parallel to the

refractory hot face, point 4—interior refractory grain,

point 5—pore slag, and point 6—refractory/slag interface phase in a pore. Interpretation of each point in

Figure 17 using the chemistries in Table V is as follows.

Point 1—slag near the refractory surface. The slag

area near the surface of the refractory had a fine

dendritic-type structure that occurred on gasifier

cooldown. Phases present could include hercynite

(FeAl2O4), fayalite (Fe2SiO4), or other compounds.

Specific phases depend on the ash composition of

the coal, dissolution from the refractory, the cooling

rate of the gasifier, and the gasifier atmosphere during cooldown.

Point 2—phase buildup along the refractory surface.

The phase grew from the surface of the refractory and

was determined to have chemistry near that of hercynite (FeAl2O4). Planar FeAl2O4 growth appeared to

have occurred during service, with dendritic growth

probably occurring during cooldown. The FeAl2O4

phase appeared to be stable during gasification at the

service temperature. It is not clear from where the aluminum in this phase originated (slag, refractory, or

both), but iron originated from the slag.

Point 3—diffusion layer that occurs parallel to the

refractory hot face. This layer was very uniform

along the slag/refractory grain surface. It is speculated that aluminum migrated from the refractory

surface, while iron originated from the slag. This

layer, along with the FeAl2O4, appeared to form a

barrier coating on the surface of the refractory

grain, impacting the diffusion of Fe into, or Al out

of, the refractory microstructure. At the slag/refractory interface, the FeCr2O4 layer had FeAl2O4

growth on its surface, while in the pore area where

the slag chemistry had changed (Fe depleted slag

chemistry), no FeAl2O4 growth occurred.

Point 4—interior refractory grain. The chemistry of

the grain contained Al and Cr, probably in a ratio

similar to the starting refractory grain chemistry. It

is a solid solution, with no fixed chemistry.

Point 5—pore slag. The chemistry of the slag did

not indicate crystallization growth as surface slag

did. Slag chemistry had significantly lower quantities of Fe and a higher amount of Si (comparison of

points 1 surface slag vs point 5 pore slag). Also note

that the static slag in the pore had slightly higher

quantities of Cr vs surface slag.

Point 6—refractory/slag interface phase in a pore.

The EDX chemistry of point 6 vs the internal

refractory grain (point 4) indicates higher amounts

of Si and Al, slightly higher Fe, and a pronounced

decrease in Cr. The slight buildup in Si may be

explained by thermodynamic modeling, which is discussed in Section II–F.

The microstructure illustrated in Figure 17 and the

chemistry of points 1 through 6 (listed in Table V)

evolved through a series of complex refractory/slag

thermodynamic and kinetic reactions. Iron-oxide in the

coal slag (point 1) reacts with chromium contained in

the chrome-alumina solid solution refractory grain

(point 4) to form a stoichiometric iron-chrome spinel

(point 3) containing approximately 24 wt pct Fe. As

mentioned, the spinel layer has a chemical composition

matching a fixed chemistry of FeCr2O4. Once formed,

the FeCr2O4 spinel layer becomes a barrier that must be

overcome by iron diffusing inward from the slag and

alumina diffusing outward from the chrome-alumina

grain. Note that the chemistry at point 3 indicated a

pronounced reduction in the amount of Al present from

that amount in the starting refractory grain (point 4). At

the surface, with a continual supply of iron oxide, a

layer of iron-aluminum spinel forms (point 2) on the

slag side of the iron-chromium spinel. The chemistry of

this point closely matches that for FeAl2O4. In both the

FeCr2O4 and the FeAl2O4 layers, there appears to be

some substitution of Al, Cr, and Fe in the structures.

With the formation of this double layer, protective

surface coating on the refractory surface, diffusion

appears to limit the iron-chromium layer to around 5

to 10 lm. An unexplained layer slightly higher in SiO2

(point 6) is noted in the pore interior. The reason for this

slight silica enrichment can be explained by thermodynamic modeling that will follow.

Without the protective FeAl2O4 and FeCr2O4 layers

on the surface to inhibit diffusion, the iron-chromium

spinel phase continues to grow as long as iron from the

slag is available. When the iron-chromium spinel crystals reach a certain size (thickness), internal stress

related to the associated volume change (from the

chromia/alumina solid solution grain to the FeCr2O4

spinel structure) causes microspalling (chemical spalling)

at the surface. This seems to occur once the iron-chrome

spinel layer exceeds 10 to 20 lm in thickness. The

FeCr2O4 structure seems to ‘‘seal’’ the surface of the

refractory from direct high iron slag contact. Ironchromium spinel crystal formation below the refractory/

slag interface is somewhat self-limiting because of the

much lower Fe content of the slag (point 5). Once a

static slag pool in a pore is depleted of iron, the slag

becomes silica rich, further complicating the formation

of either Fe/Al or Fe/Cr spinel phases. Some limited

Table V. EDX Chemistry of Points 1 through 6 in Figure 17;

Chemistries Are for a High Chrome Oxide Refractory

Exposed to a Coal Slag in a Gasifier at an Oxygen Partial

Pressure of About 1028 from 1673 K to 1773 K (1400 C

to 1500 C) for About 2000 Hours

Element

Point Chemistry (Wt Pct)

123456

Al 6.9 27.3 1.7 2.8 7.5 5.7

Si 23.9 0.2 0.1 0.1 40.2 3.8

Fe 20.8 31.7 23.6 0.2 1.5 0.5

Ca 1.5 — — — 0.5 —

Cr 0.1 1.5 42.7 62.1 1.5 53.0

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, APRIL 2011—901
dissolution of chrome into the reactive slag also occurs,

but never exceeds approximately 0.1 wt pct immediately

adjacent to the refractory surface. While this amount is

small, considering the volume of carbon feedstock slag

(at some gasifier sites, this can exceed 100 metric tons/

day), it could lead to measurable material loss.

It is important to note that Figure 17 and the

accompanying discussion are for a gasifier using coal

as the carbon source. When petcoke or biomass carbon

feedstock is used in place of or in addition to coal,

other reactions will occur depending on compositional

differences.

F. Thermodynamic Analysis of Slag/Refractory

Interactions

Thermodynamics can be used to predict stable phases

existing in a gasifier environment; however, it must be

remembered that thermodynamics do not predict reaction kinetics, and that thermodynamic calculations may

be limited by incomplete databases (such as with

vanadium compounds contained in petcoke slags) or

by complexities when calculating multicomponent phase

equilibrium associated with gasifiers. In the two examples that will be discussed related to Figure 17 (for a

coal slag), FACTSAGE** software was used to predict

possible phases present in the point chemistries given in

Table V, with the results plotted in Figures 18 and 19.

The gasification conditions used to predict the thermodynamic phases present were 1673 K (1400 C) and an

oxygen partial pressure of 108

. Interactions between

two phases were determined: between points 1 and 4

(slag and the refractory grain), and between points 1 and

3 (slag and the iron chromate compound on the surface

of the refractory). Results were calculated on the basis

of weight ratios of the mixture of points 1 and 4 (wt pt

4/(wt pt 4+ wt pt 1)), as shown in Figure 18, and the

weight ratio of points 1 and 3 (wt pt 3/(wt pt 3+ wt pt

1)), as shown in Figure 19. The chemistry weight ratio

goes from 100 pct slag phase (representing slag chemistry of point 1 in Figure 17) on the left x-axis of

Figure 18 (the sample is all slag, no refractory grain

within the slag) to 100 pct refractory grain (the mixture

is all refractory grain). A similar behavior exists in

Figure 19, where the x-axis goes from 100 pct slag phase

on the left to 100 pct point 3 spinel phase chemistry on

the right. In between either extreme are combinations of

slag and solid and refractory phases.

The thermodynamic phases predicted in Figure 18 for

different combinations of points 1 and 4 indicate that as

the slag begins to interact with chromia/alumina refractory grain (weight ratio 0 in Figure 18), a fixed

composition spinel (FeCr2O4) is predicted to form,

followed by a solid solution that is a mixture of varying

Cr and Al. The Fe/Cr phase noted to form in Figure 17

(at point 3) when the slag interacts with the surface of

the refractory grain has a Fe/Cr weight ratio of 0.553,

while the compound predicted by thermodynamics

(FeCr2O4) has a Fe/Cr weight ratio of 0.537. This

suggests that the phase formed by slag and refractory

interactions on the surface of the refractory grain in

Figure 19 is FeCr2O4. It is of interest to note that in

Figure 18, SiO2 was predicted to form as a stable phase

and that elevated levels of Si were found to exist at the

surface of interior slag/refractory grains in contact with

depleted slag (point 6 in Figure 17).

Once slag and chromia/alumina refractory grain

interact to form FeCr2O4 spinel on the refractory

surface, the spinel structure exists as a stable boundary

layer, slowing further interactions between the refractory chromia/alumina grain and slag. This is shown in

Figure 17, where contact between surface slag and the

chromia/alumina solid solution grain does not occur. In

Figure 17, note that slag (point 1) is in contact with a

Fe/Al phase (point 2), not with the Fe/Cr spinel

structure (point 3). It could not be determined from

the SEM micrographs whether the Fe/Al phase formed

on cooling or if it was formed at elevated temperatures

during use; however, it was speculated that the laminar

growth pattern exhibited at the original refractory-slag

interface occurred at high temperature, while the dendritic growth extended into the slag may have occurred

later during cooling. How this structure forms during

gasification will be clarified to some extent in the

following thermodynamic section. When plotting the

thermodynamic interactions between point 3 (FeCr2O4)

and point 1 (slag) in Figure 19, it is noted that a stable

FeCr2O4 phase can exist with slag and that increasing

slag additions result in the formation of a stable spinel

structure. In evaluating the thermodynamic data, this

stable spinel phase is made predominately of FeAl2O4,

although other spinel phases are present, and the ratio

of the phases varies depending on the weight ratio of

points 1 and 3. The results indicate that FeAl2O4 can be

Fig. 18—Thermodynamic prediction of stable weight phases for different weight ratio mixtures of coal slag chemistry at point 1 and

refractory grain chemistry at point 4 (wt pt 4/wt pt 4+ wt pt 1) at

1673 K (1400 C) and 108 oxygen partial pressure, based on SEMEDX compositions listed in Fig. 17. Point 4 is a refractory grain of

solid solution chemistry containing Cr2O3/Al2O3. Note that this diagram was produced using FACTSAGE. Chemistry is based on

points shown in Fig. 17, the chemical analysis of which are listed in

Table V.

**FACTSAGE is a trademark of Thermfact/CRCT (Montreal,

Canada) and GTT Technologies (Aachen, Germany).

902—VOLUME 42A, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
a stable phase that forms between the slag and FeCr2O4,

although with increasing slag concentration, it will

dissolve into the slag.

Thermodynamics can also be used to predict the

impact of gasification conditions (temperature and

oxygen partial pressure) on refractory dissolution,

something that is difficult to monitor during gasification.

FACTSAGE was used to model 100 g mixtures of slag

(point 1) and refractory grain (point 4) from Table V

(points indicated in the refractory/slag interaction in

Figure 17) between 1673 K and 1873 K (1400 C and

1600 C) and oxygen partial pressures from 100 to

1012 atm. The formation of CrO and Cr2O3 soluble in

the slag was predicted, and the combined total is shown

in Figure 20. Decreasing the oxygen partial pressure or

increasing the temperature was noted to increase the

chemical dissolution of chrome in the slag, especially

above 1773 K (1500 C) or at oxygen partial pressures

below 106 atm. The increase in chrome solubility in the

slag with temperature or a decrease in oxygen partial

pressure accounts for the slight amount of chrome

indicated in the surface slag (point 1) or pore slag

(point 5).

III. SUMMARY AND CONCLUSIONS

Because of their high efficiency, ability to capture CO2

for sequestration or reuse in other applications, and

potential for carbon feedstock fuel flexibility, air-cooled

slagging gasifiers are used by industry to produce CO

and H2 for power production, are used as a feedstock

material for the chemical industry, are considered a

possible source of H2 in a hydrogen-based economy, and

hold potential for use in the advanced fossil fuel power

systems of the future. Gasifiers are used commercially to

react a carbon feedstock, water, and oxygen (shortagereducing conditions) at temperatures between 1598 K

and 1848 K (1325 C and 1575 C) and at pressures

between 2.07 and 6.90 MPa. Impurities in the carbon

feedstock liquefy at the elevated gasification temperatures, interacting with the high chrome oxide liners that

protect the gasifier steel shell and lead to refractory

failure. Refractory liners may be exposed to 100 or more

tons of molten slag that is high in Si, Fe, Ca, and Al.

Failure of the refractory lining can occur between 3 and

30 months, a life that is dependent on variables such as

the ash chemistry and quantity, the operating temperature of the gasifier, gasifier maintenance, and the

frequency of gasifier cycling. Gasifier liners contain

between 60 and 95 pct chrome oxide, with the balance of

the composition being either alumina or alumina and

zirconia. The use of gasification as a process is limited

because it does not achieve the on-line availability or

reliability necessary for economical operation. Refractories were identified as the leading research need of the

gasification process. Refractory lining failure is caused

primarily by chemical dissolution of the chrome oxide at

the slag/refractory interface, chemical spalling at the

slag/refractory interface, and structural spalling caused

by molten slag penetration into the refractory. Using

postmortem analysis and thermodynamic modeling, slag

interactions at the refractory surface and interior were

determined. Chrome oxide was determined to have low

solubility in the slag, while slag FeO was determined to

interact with the refractory Cr2O3 to form FeCr2O4 as a

layer on the grain surface, which appeared to fracture

when it became thick (over 20 lm). A layer of FeAl2O4

was determined to form on the surface of the FeCr2O4.

Both layers would act to impact refractory wear and

corrosion by acting as a diffusion boundary. The

presence of FeAl2O4 and FeCr2O4 observed through

postmortem analysis and the presence of a high silica

area inside pores, along with a depletion of Cr2O3 in

these areas, were explained by the use of thermodynamic

modeling. The mechanisms of wear in high chrome

oxide gasifier refractories are complex; however,

through an understanding of them, improved refractory

liner materials can be developed.

Fig. 19—Thermodynamic prediction of stable weight phases for different weight ratio mixtures of coal slag chemistry at point 1 and

surface material chemistry at point 3 (wt pt 3/wt pt 3+ wt pt 1),

based on SEM-EDX compositions listed in Fig. 8. Point 3 area has

a fixed chemistry indicating a FeAl2O4 spinel composition. Note that

this diagram was produced using FACTSAGE. Chemistry is based

on points shown in Fig. 17, the chemcial analysis of which are listed

in Table V.

Fig. 20—Thermodynamic prediction of stable CrO and Cr2O3 in

gasifier slag using slag chemistry of point 1 and refractory grain

chemistry of point 4 in Table V. Note that this diagram was produced using FACTSAGE.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, APRIL 2011—903
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904—VOLUME 42A, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A