THE WEAR AND CORROSION OF HIGH CHROME OXIDE LININGS

USED IN AIR-COOLED SLAGGING COAL GASIFIERS

James P. Bennett 

NETL Office of Research & Development, 1450 Queen Ave., SW, Albany, OR 97321 

E-mail: james.bennett@netl.doe.gov; Telephone: (541) 967-5983; Fax: (541) 967-5845 

Kyei-Sing Kwong, Arthur V. Petty, Jr., Rick Krabbe, Hugh Thomas, and Cynthia Powell 

NETL Office of Research & Development, 1450 Queen Ave., SW, Albany, OR 97321 

ABSTRACT 

Gasifiers are high temperature, high pressure containment vessels used to convert carbon feedstock; such as coal or 

petcoke, into a synthesis gas (also called syngas) composed primarily of H2 and CO. Syngas produced by 

gasification is used as a fuel in energy production, as a raw material feedstock for chemical synthesis, and can be a 

source of H2 for a hydrogen based economy. Air cooled slagging gasifiers, one of several gasifier types used in 

industry, typically operate at temperatures between 1325-1575oC, and at pressures between 300-1000 psi. They are 

lined on the hot face with high chrome oxide refractory materials; which may contain up to 95 wt pct chrome oxide, 

with possible additives of zirconia and alumina also present. Impurities in the carbon feedstock exist in many 

forms, and can be oxides of Si, Fe, Ca, Al, Na, and S in coal; with additional elements, such as Ni and V, present in 

petcoke. Depending on the feedstock source, the quantities of impurities generally range from one to over ten pct; 

leading to ash waste quantities reaching 100 tons/day, or more, in the operation of a single gasifier. Because of the 

high temperatures used during gasification, the ash by-products liquefy and react with, or penetrate the refractory 

liner pores; leading to refractory wear and corrosion, and ultimately, premature brick failure. This paper will discuss 

interactions between slag components and gasifier refractories, with post-mortem analysis of refractory liner failure 

being presented from systems using coal as a carbon feedstock. 

INTRODUCTION 

Gasification as a modern high temperature, high pressure industrial process was first used by the oil industry in the 

50’s to process low value petroleum and by-products from petroleum processing into a usable product1

. Since that 

time, gasification has expanded to other uses; including power generation, the production of CO and H2 feedstock 

for use by the chemical industry (for applications including the production of fertilizer or Fischer Tropsch liquids), 

and as a leading candidate for H2 production in a hydrogen based economy. 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). Several types of gasifiers are used by industry for gasification; and include dry 

ash gasifiers (slag does not fuse due to the low temperature of operation), water cooled entrained bed slagging 

gasifiers (operate at temperatures that create a molten slag from ash this is contained by a water cooled refractory 

liner), and the air cooled entrained bed slagging gasifiers (these types operate at temperatures were ash liquidifies 

into molten slag that but slag flows on an air cooled versus a water cooled refractory liner surface). Variables in 

gasification include the type of carbon feedstock and the associated impurity levels, the required syngas 

composition, and operational parameters of the gasifier (i.e. related to the gasifier design). Two types of air cooled 

slagging gasifies used in industry are shown in figure 1; the GE* gasifier (1a) and ConocoPhillips* gasifier. Figure 

2 is a generic cross section of a gasifier showing molten slag (originating from mineral impurities) flowing down the 

hot face refractory lining and interacting with it. 

In practice, a gasifier is a high temperature reaction vessel used to contain reactions between carbon feedstock, 

oxygen, and water (steam). Gasification occurs in a pressurized environment (typically between 400 and 1000 psi) 

with a shortage of oxygen necessary for theoretical combustion (gasification occurs in a reducing environment with 
an oxygen partial pressure between 10-7 and 10-9 atm) at high temperature (typically between 1325 and 1575o

C). 

The general gasification reaction is as listed below: 

C + H2O (gas) + O2 → CO + H2 + CO2 + minority gases + by-products 

NOTE: by-products include mineral impurities in the carbon feedstock that become ash or slag 

 a) b) 

Figure 1: Two types of air cooled slagging gasifier used for syngas production. a) GE* design. b) ConocoPhilips* 

design. 

(*Use of manufacturer’s names in this paper does not constitute endorsement, and is for identification purposes 

only.) 

Figure 2: Cross section of a gasification chamber showing molten slag flow down the sidewall. 

The hot face of an air cooled slagging gasifier is lined with high chrome oxide refractory material of the types listed 

in table 1. This environment involves not only hot corrosive gases, but includes slag formed from impurities within 

the carbon feedstock that liquefy into slag at the elevated processing temperatures. The liners for air cooled slagging 

gasifiers are based on research conducted during the mid 70’s to mid 80’s that was a combined effort of DOE, EPRI, 

the refractory industry, and gasifier users2-5. This research identified high chrome oxide refractories as the liner 

material of choice. Unfortunately these materials have not met the service life requirements of the gasification 

industry, failing between 3 and 24 months, after which the gasifier is taken out of service for between 4 and 21 days 

while a new lining is installed or repairs are made. 

 

Gasifier slag originates as impurities in the carbon feedstock that become molten slag at the gasification 

temperature, and are the primary cause of refractory failure. Impurities typically compose less than one pct of 

petcoke, and can be 10 pct or more of coal. The carbon feedstock is ground to about 100 microns or less in particle 

size, after which it is injected with water and oxygen under high pressure into the gasifier. The air cooled slagging 

gasifier designs shown in figures 1a and b can process over 1400 tons/day of a carbon feedstock, producing large 

quantities of slag from the mineral impurities (up to 100 tons or more tons per day). This waste product must exit 

through the base of the gasifier as a liquid or it will clog the exit port, causing operations to cease. The molten slag 
generated by ash is a corrosive liquid that interacts with the refractory lining; producing new mineral phases, 

causing chemical dissolution of the refractory material, and penetrating the porous refractory lining; all contributing 

to premature refractory lining failure. 

Table 1: Chemical composition and physical properties of high chrome oxide refractories commonly used in air 

cooled slagging gasifiers 

Property Brick Type 

A B 

 Cr2O3 90.3 87.3 

 Al2O3 7.0 2.5 

 ZrO2 0.01 5.2 

Bulk Density (gms/cc) 4.21 4.07 

Porosity (pct) 16.7 16.5 

CCS (MPa) 48.3 66.9 

 *Data from manufacturer’s technical data sheet 

In general, the operational temperature of gasification in an air cooled slagging gasifier is dependent on the ash 

chemistry and its viscosity. Ideal operation is in a narrow temperature range that allows the slag to exit the gasifier 

as a fluid and not build up in the base exit port, yet at a temperature low enough to minimize chemical interactions 

with the refractory liner. An average chemistry of approximately 300 US6

 coal samples from across the US are 

listed in table 2, and data from a limited number of petcoke slags7-9 are listed in table 3. With interest growing in 

mixed feedstock of coal, petcoke, and/or biomass; understanding and controlling slag chemistry is important for 

optimal gasifier operation. From table 2, it is evident that coal contains varying but high percentages of oxide 

materials of Si, Al, Ca and Fe; and from table 3, that petcoke contain these oxides in addition to high levels of Ni 

and V oxides. Although not listed, biomass can vary extensively in ash percentage and chemistry depending on the 

source; but is typically high in silica, alkali, and alkaline earths. 

Table 2: Range of chemistry found in over 300 U.S. coal slags due to mineral impurities [6]. 

Gasifier operators have identified refractory service life as the most important barrier to widespread 

commercialization of gasification technology10. Failure of the refractory lining is expensive, both in terms of 

material replacement costs (as high as $1,000,000) and in lost production (due to gasifier shutdown). Downtimes 

can range from 5-21 days for lining repair. Gasifier users desire an availability of 85-95 pct for utility applications 

and more than 95 pct in applications such as chemical feedstock production, with service life of at least 3 years 

desired. Failure to meet these goals has impacted the utilization of gasification technology. 

 

Material Weight Percent

Max. Min. Avg. Std. dev.

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 
In this study, post-mortem examination of high chrome oxide refractory brick removed from service in air cooled 

slagging gasifiers using coal as a feedstock will be discussed, with emphasis on slag/refractory interactions that lead 

to hot face refractory failure. 

Table 3: Range of chemistry found in select petcoke slags due to impurities [7-9] 

POST-MORTEM ANALYSIS OF SPENT HIGH CHROME OXIDE REFRACTORIES 

BULK STRUCTURE 

Refractory wear in slagging gasifiers is caused by a number of factors, as shown in figure 3. Slag corrosion and 

spalling are the primary wear mechanisms, but operational issues; such as the frequency of gasifier cycling, 

gasification temperature, heating and cooling rates, and ash chemistry and quantity; interact to influence the rate of 

refractory wear. Spalling causes the most disruptive and least predictable wear to a gasifier operation. 

Figure 3 – Causes of wear in a refractory material when used in a air cooled slagging gasifier. 

The impact of spalling on refractory wear in a gasifier is shown in figure 4a, where different stages of spalling on 

the slope/throat intersection in a gasifier are shown. The high Cr2O3 samples shown had over 2,000 hours of service 

at temperatures in the range of 1400-1500o

C. Note the rough surface appearance of the sidewall refractories, with 

different thicknesses on the refractory surfaces. Figure 4b shows a cross section of a refractory take from a gasifier 

sidewall, where similar rough surface effects were noted. The cause of spalling is noted by an arrow, which points 

Material Weight Percent

Max. Min. Avg. Std. dev.

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 
to a large crack spanning the brick cross-section parallel to the hot face. Cracks form beneath the surface of the 

refractory in the slag infiltrated area, and link together. The surface pieces can separate from the refractory during 

use because of poor intergrain bonding. The thickness of spalled refractory material can range from the 2-5 to 30 

mm. 

 

a. b. 

 

Figure 4 – High chrome oxide refractory with over 2000 hrs of service from 1300-1500o

C. a) Surface wear is 

caused by corrosion and spalling. Note the cobbled appearance, which is due to spalling. b) Cross section of a high 

chrome oxide refractory showing spalling as it occurs. (arrow points to spalling crack, dashed line to maximum 

depth of slag infiltration.) 

Spalling rapidly removes protective refractory material from the gasifier interior, shortening the lining’s service life 

and leading to premature shutdown of the gasifier for repair. It is a process that repeats itself throughout a gasifier’s 

service. Spalling can originate from many different causes, as is shown in figure 3. In the air cooled slagging 

gasifiers studied, structural spalling is one of the most disruptive forms of spalling. It is caused by slag penetration 

into the porous microstructure (≈16 pct) of the weakly bonded high chrome oxide refractory material. During 

service, slag can penetrate to depths of 35 mm or more below the hot face (see figure 4b), with depth of penetration 

limited by compositional changes occurring in the slag that increase slag viscosity, and by the temperature drop in 

the refractory from the hot face to the cold face. The removal of spalled material from the surface of a refractory 

causes the highly irregular hot face visible on the refractory hot face shown in figure 4a. 

MICROSTRUCTURE ANALYSIS 

A SEM backscatter image of the slag/refractory interface in a 90 wt pct chrome oxide/10 wt pct alumina refractory 

after approximately 2000 hrs of exposure to a coal gasifier slag from 1400-1500o

C is shown in figure 5a, along with 

the chemistry and crystalline phases (or possible phases) at points 1-6 (figure 5b). Slag is located at the far right of 

the image in figure 5a (area around point 1), and has infiltrated the pores of the refractory (through point 5). The 

slag away from the refractory surface can form any of a number of crystalline components on cool down, with some 

types indicated in figure 5b. As the slag infiltrates pores, a decrease in Fe and an increase in Si are noted (point 1 

versus 5). 

A crystalline phase is identified on the surface of the refractory at point 2 (at the slag/refractory interface), which is 

high in Al and Fe (hercynite - FeAl2O4). It is not clear if this layer crystallizes from the slag on cool down or if it 

exists at gasification temperature; and what is the source of aluminum in the layer. Point 3, on the surface of the 

refractory grain, is in a diffusion zone, with high levels of Fe and Cr, and is lower in Al versus the starting 

chromia/alumina solid solution (point 4). FeO from the slag interacts with Cr2O3 at the surface of the refractory to 

form an iron-chrome spinel of fixed composition (FeCr2O4). The relationship between areas in points 2 and 3 

impacts surface wear. Indications are that the Al in the chromia/alumina refractory grain diffuses towards the grain 

surface when in contact with slag, interacting with slag Fe to form a specific compound (FeAl2O4). The flow of slag 

over the refractory surface provides a constant source of FeO for this interaction. If this mechanism occurs, the rate 

of Al, Fe, and Cr diffusion and compound formation in the surface layers impact layer thickness and refractory wear. 

It appears when the iron oxide/chromia spinel layer formed at point 3 becomes thicker than about 5-10 mm, it begins 
to experience chemical spalling, which may accelerate the breakup of the refractory surface into the slag (observed 

in the thicker areas of figure 5a), versus if chemical dissolution was the only factor in surface corrosion. The cause 

of chemical spalling may be volume changes associated with the formation of new spinel phases. Iron content is 

lower in the slag that has penetrated the refractory grain (point 5 versus point 1) because it has interacted with the 

refractory grain surface to form spinel as it penetrates the refractory, changing slag chemistry, which causes an 

increase in slag viscosity (due to higher Si and lower Fe content), and causes a decrease in the ability of the slag to 

chemically interact with the refractory grain (see point 6 chemistry). 

 a) 

 *Others can include Hercanite, Iron Sulfide, Iron Cordierite, or other crystalline phases 

b) 

Figure 5 – Microstructural analysis of a high chrome oxide refractory sample taken from an air cooled slagging 

gasifier after approximately 2000 hrs of exposure to a coal slag at about 1400-1500o

C. a) SEM micrograph 

(backscatter) image of the slag/refractory interface. b) WEDX chemical analysis of points in figure 5a. 

CONCLUSIONS 

Wear of high chromia/alumina refractories used to line air cooled slagging gasifiers is predominantly caused by 

chemical dissolution of the refractory and by spalling, although other factors impact wear. Two types of spalling 

commonly occur; 1) structural spalling of the refractory surface caused by deep (20-35 mm) slag penetration in the 

porous refractory, and 2) chemical spalling on the refractory grain surface caused by FeO interaction with chromia 

in the chromia/alumina solid solution of the refractory grain. Aluminium appears to diffuse from the 

chromia/alumina refractory grain, forming a diffusion layer of FeAl2O4 on the grain surface. A second source of Al 

may be from the slag. Iron oxide in the slag is depleted through interactions with the refractory, increasing slag 

viscosity as it penetrates within the refractory, and decreasing the ability of slag to interact chemically with the 

refractory grain. 

Zone Property Zones 

1 2 3 4 5 6 

Chemistry (Wt %) - 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 

Possible Crystalline Phases Fayalite, 

Others* 

Hercynite Fe/Cr 

Spinel 

Cr/Al 

Solid 

Solution 

Fe 

depleted 

slag 

High Cr, 

minor Al 

and Si 

50:m

1

2

3

4

5

6
REFERENCES 

1. N. Richter, Introduction to Gasification (ChevronTexaco); internet website: 

www.gasification.org/Docs/02Richter.pdf; August 2, 2005. 

2. Bonar, J.A., C.R. Kennedy, and R.B. Swaroop; “Coal-Ash Slag Attack and Corrosion of Refractories”; Amer. 

Ceram. Soc. Bulletin; Vol. 59, No. 4, 1980; pp 473-478. 

3. Bakker, W.T., S. Greenberg, M. Trondt, and U. Gerhardus, “Refractory Practice in Slagging Gasifiers,” 

Amer. Ceram. Soc. Bulletin, Vol. 63, No. 7, 1984, pp 870-876. 

4. Greenberg, S. and R.B. Poeppel; "The Corrosion of Ceramic Refractories Exposed to Synthetic Coal Slags by 

Means of the Rotation-Cylinder Technique: Final Report"; research sponsored under USDOE/FE AA 15-10-

10; April 1986; 66 pp. 

5. Starzacher, A.P., “Picrochromite Brick - A Qualified Material for Texaco Slagging Gasifiers,” RadexRundschau, Vol 1, 1988, pp. 491-501. 

6. W.A. Selvig and F.H. Gibson; Analysis of Ash from United States Coals; USBM Bulletin, Pub. 567; 1956; 33 

pp. 

7. S.V. Vassilev et al; “Low Cost Catalytic Sorbents for NOx Reduction. 1. Preparation and Characterization of 

Coal Char Impregnated with Model Vanadium Components and Petroleum Coke Ash”; Fuel; Vol 81, 2002; 

pp 1281-1296. 

8. R.E. Conn; “Laboratory Techniques for Evaluating Ash Agglomeration Potential in Petroleum Coke Fired 

Circulating Fluidized Bed Combustors”; Fuel Processing Technology; Vol 44, 1995; pp 95-103. 

9. R.W. Bryers; “Utilization of Petroleum Coke and Petroleum Coke/Coal Blends as a Means of Steam 

Raising”; Fuel Processing Technology; Vol. 44, 1995; pp 121-141. 

10. Gasification Markets and Technologies – Present and Future – An Industry Perspective, US DOE/FE Report 

0447, US DOE, (July, 2002), pp. 1-53.