Folie 1

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Alkali Corrosion of Refractories in Cement Kilns
2
Alkali Corrosion

• Topics
• Introduction to alkali corrosion of refractories
• Characterization of corroded industrial
  refractory materials
• Behavior of alkali salts and alkali salt mixtures
• Mechanisms of alkali corrosion
• Investigation methods
• Conclusions

3
Alkali Corrosion
Corrosion attack in cement rotary kilns
clinker burning
electrostatic filter
heat exchanger
rotary kiln
grate cooler
refractory lining high temperature thermal
insulation material metallic components
Deuna Zement GmbH, Informationsmaterial 2005
Introduction to Alkali Corrosion of Refractories
4
Alkali Corrosion

• Reason of alkali accumulation in the cement
  rotary kilns
• cement dust returns into the burning process
• implementation of raw meal preheating first with
  the Lepol grate
• improved preheating of cement raw meal in
  Humboldt air-suspension preheater and
• intensified due to alkali circulation
• use of secondary fuels, i.e. use of combustible
  waste instead of
• powdered coal ore oil
• Sources of corrosive substances
• alkali included in natural raw materials, coal,
  secondary fuels
• chlorine included in secondary fuels
• sulfur included in natural raw materials, coal,
  oil, secundary fuels

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Introduction to Alkali Corrosion of Refractories
5
Alkali Corrosion
The use of secondary fuels
P. Scur, Mitverbrennung von Sekundärbrennstoffen
wie heizwertreiche Abfälle und Tiermehl in der
Zementindustrie am Beispiel Zementwerk
Rüdersdorf. VDI-Berichte Nr. 1708, 2002, S. 189 -
20
Introduction to Alkali Corrosion of Refractories
6
Alkali Corrosion

• Combustion of secundary fuels
• The chlorine is particularly inserting in
  burning process
• ? chlorine containing compounds, not pure gas
• The chlorine is mainly included in ?
  polyvinylchlorid (PVC)
• ? used tires
• ? common salts of domestic waste
• The chlorine appearance tends to result ?
  changing of the reaction process
• ? intensification of the refractory corrosion
• Reasons for this behavior ? formation of low
  viscous and aggressive fused salts
• at relatively low temperatures
• ? high amount of the corrosive compound is
  gaseous
• ? gases an melts can simply pass trought pores
  and
• cracks of working refractory material to the
  metallic bars
• ? attack by chemical reaction and dissolution
  the
• fire-proof material behind
• ? condensate on the metallic components leads
  to
• excessive corrosion phenomena

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Introduction to Alkali Corrosion of Refractories
7
Alkali Corrosion
Effect of the combustion of secundary fuels in
cement rotary kilns
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Introduction to Alkali Corrosion of Refractories
8
Alkali Corrosion

• Post mortem investigations
• Roof of kiln hood of the DOPOL-kiln

Calcium silicate
Insulating brick
basic abrasion lining
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Characterization of corroded industrial
refractory materials
9
Alkali Corrosion

• Post mortem investigation
• Alkali corroded calcium silicate thermal
• insulating material in the chamber
• at 600 700 C
• ?    X-ray analysis
• Hot side area
• ? based on KCl and CaSO4
• ? residual NaCl,
• futher chlorides,
• Cr- and Fe-sulfates

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Characterization of corroded industrial
refractory materials
10
Alkali Corrosion

• Post mortem investigation
• Alkali corroded fireclay brick in the hot zone
• at 800 C
• ?    X-ray analysis
• Area around the crack
• ? based mainly on leucit (K2O?Al2O3?4SiO2)
• ? residual silica (SiO2),
• mullite (3Al2O3?2SiO2)
• corundum (Al2O3)

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Characterization of corroded industrial
refractory materials
11
Alkali Corrosion

• Post mortem investigation
• Alkali corroded fireclay insulating brick in
• the hot zone at gt 1000 C
• ?    X-ray analysis
• Hot side area
• ? based mainly on leucit (K2O?Al2O3?4SiO2),
• mullite (3Al2O3?2SiO2)
• ? residual silica (SiO2),
• kalsilit (K2O?Al2O3?2SiO2)
• larnit (2CaO?SiO2)

Cool side
Infiltration zone
Hot side
Fireclay insulating brick after 3 years in use in
a cement rotary kiln (feed end), front heat site
with a temperature gt 1000 C.
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Characterization of corroded industrial
refractory materials
12
Alkali Corrosion

• Post mortem investigation
• Alkali corroded magnesia brick in
• the sinter zone at gt 1100 C
• ?    X-ray analysis
• Hot side area
• ? based mainly on leucit (K2O?Al2O3?4SiO2),
• mullite (3Al2O3?2SiO2)
• ? residual silica (SiO2),
• kalsilit (K2O?Al2O3?2SiO2)
• larnit (2CaO?SiO2)

Hot side
Cool side
Magnesia brick after 2 years in use in a cement
rotary kiln (sinter zone), above on the heat site
with a temperature gt 1000 C.
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Characterization of corroded industrial
refractory materials
13
Alkali Corrosion

• Post mortem investigation
• Alkali corroded refractory concrete from the
• wall of a bottom cyclone of cement
• ? SEM-Analysis
• (pore size 100 to 200 µm)
• In pores and reacted layers
• ? A and B present deposit KCl
• ? bubbly microstructure of KCl-layer is an
• evidence for its primary liquid state
• ? B present cracks in the KCl-layer as a
• indication for differences of the thermal
• linear expansion coeffizients

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Characterization of corroded industrial
refractory materials
14
Alkali Corrosion

• Validation of the industrial refractory materials
  by alkali attack
• Refractories based on aluminum silicate
• ? formation of feldspar
• ? volume increase
• ? alkali bursting
• Refractories based on calcium silicate
• ? not stable in the exhaust
• ? disintegration to CaCO3, CaSO4, SiO2 without
  volume change
• Refractory bricks and concretes (based on alumina
  or magnesia)
• ? deposit of substances in pores
• ? spalling (spall in layers)
• The formation of feldspar, the alkali bursting,
  the cracks and the fractional dropout
• are caused due to alkali corrosion attack.

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Characterization of corroded industrial
refractory materials
15
Alkali Corrosion

• Alkali compounds in corroded refractory bricks
  and concretes
• The most of analyzed samples contained
• Feldspar,
• KCl,
• Alkali sulfate,
• NaCl,
• Other chlorides
• Other sulfates
• In summery, K and K-compounds are more common
  than Na and Na-compounds.

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Characterization of corroded industrial
refractory materials
16
Alkali Corrosion

• High temperature behavior of alkali salts and
  alkali salt mixtures
• Salts after heating at 1100C in crucibles
• Solid salt after 1100C
• Na2SO4, K2SO4 ? molten
• Na2CO3, K2CO3 ? molten
• NaCl, KCl ? evaporated
• CaSO4 ? sintered
• The solid salts as most reactive and corrosive
  mixtures after heating at 1100C
• in crucibles
• Salt mixtures after 1100C
• SM 1 K2SO4 / K2CO3 ? melting
• SM 2 K2SO4 / K2CO3 / KCl ? gas
• SM 3 K2SO4 / K2CO3 / KCl / CaSO4 ? solid

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Behavior of alkali salts and alkali salt mixtures
17
Alkali Corrosion

• High temperature behavior of alkali salts and
  alkali salt mixtures
• Thermal linear expansion
• coefficient (?lin) of solid salts and
• salt mixtures
• ? highest value K2SO4
• ? lowest value CaSO4
• ? is reflected in the value of the
• salt mixtures

Thermal linear expansion coefficient (?lin) of
solid salts and salt mixtures
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Behavior of alkali salts and alkali salt mixtures
18
Alkali Corrosion

• High temperature behavior of alkali salts and
  alkali salt mixtures
• Density of solid and molten salts (literature)
• ? density difference between liquid and solid
  salts
• ? volume increase during heating up
• Hygroscopicity
• ? K2CO3 are hygroscopic
• ? KCl, K2SO4, CaSO4 are
• not hygroscopic
• The volume expansion during heating up combined
  with the hygroscopicity (K2CO3)
• leads to the destruction of the refractory in
  humid atmospheres.

weight increase app. 15 after 4 days on normal
area (24 C, 60 rel. humidity)
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Behavior of alkali salts and alkali salt mixtures
19
Alkali Corrosion

• Behavior of satured water based solutions of
  alkali salts and alkali salt mixtures
• pH-values of satured water based
• salt solutions
• ? K2CO3-solution is high alkaline
• ? KCl-, K2SO4-, CaSO4-solutions
• are neutral to alkaline
• ? solutions of salt mixtures are
• mainly high alkaline
• The acid effect is not identifiable of the
• corrosion products of sheet-matall jacket
• of rotary kiln too.

pH-values of satured water based salt solutions
as a function of time at 21 C.
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Behavior of alkali salts and alkali salt mixtures
20
Alkali Corrosion

• Behavior of satured water based solutions of
  alkali salts and alkali salt mixtures
• Electrical conductivity of satured
• water based salt solutions
• ? K2SO4 is more soluble than CaSO4
• ? the value of electrical conductivity
• of CaSO4 is increased by a factor 16
• The corrosion due several micro
• processes is supported by Cl- and SO42-.
• One of the corrosion mechanisms is
• based on electrochemical corrosion.

Electrical conductivity in µS/cm of satured water
based salt solutions as a function of time at 21
C.
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Behavior of alkali salts and alkali salt mixtures
21
Alkali Corrosion
Melt formation
Change of density and volume of the solid phase
Expansion as a result of salt stored in pores
4 main alkali corrosion mechanisms
Corrosion due to water condensation
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
22
Alkali Corrosion

• Melt formation
• Alkali salt refractory material
• ? formation of melts at 750 1450 C
• (from literature)
• Alkali salt mixtures refractory material
• ? partially melt formation at 600 950 C
• ? completely melt formation at 700 1000 C
• (from phase diagrams)
• In addition presence of K2O and Na2O as
• reactive and corrosive substances at high
• temperature and water vapour

Temperature from the 1. melting for refractory
oxids or oxids mixturs with compounds of alkalis
from the phase diagrams.
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
23
Alkali Corrosion

• Melt formation
• Magnesia
• Phase diagram of the system K2SO4 MgO
• ? melt formation of eutectic at 1067 C
• Phase diagram of the system K2CO3 MgO
• ? melt formation of eutectic at 895 C
• similar behavior is due of the system
• KCl - MgO
• MgO based refractory materials
• are not alkali resistant because melt formation
• at 895 C.

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
24
Alkali Corrosion

• Melt formation
• SiO2-based refractories
• Phase diagram of the system Na2O SiO2
• ? melt formation at 782 C resp. 789 C
• ? complete melt of by 26 Na2O
• ? no strength of solid structure (25 melt)
• by 4 Na2O at 1300 C
• Phase diagram of the system K2O SiO2
• ? melt formation at 769 C
• ? complete melt of eutectic by 27 K2O
• ? no strength of solid structure (25 melt)
• by 4 K2O at 1300 C
• 25 eutectic melt by 6,5 Na2O or K2O at 800 C
• Strong effect of flux of the alkalis leads to
  damage of SiO2-based refractories
• at 700 and 800 C by a melt formation

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
25
Alkali Corrosion

• Melt formation
• Calcium silicate
• Phase diagram of the system Na2O CaO SiO2
• ? lower volume expansion of reaction products
• ? melt formation of eutectic at 720 C
• Phase diagram of the system K2O CaO SiO2
• ? melt formation of eutectic at lt 720 C
• Refractory materials based on wollastonite
• no alkali resistant, because melt formation
• at 700 C.

Alkali Corrosion of Therml Insulating Material
Based of Calcium Silicates
26
Alkali Corrosion

• Melt formation
• Applied Temperatures in presence of alkali lt 1300
  C,
• because of melt formation below 1100 C
• ? refractory oxides MgO, CaO, Cr2O3, TiO2 and
  SiO2
• ? binary combinations Al2O3/SiO2, CaO/SiO2,
  MgO/SiO2
• Applied Temperatures in presence of alkali gt 1300
  C
• ? refractory oxid Al2O3
• ? binary combinations Al2O3/MgO, Al2O3/CaO could
  be suitable
• (no dates of melt formation)

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
27
Alkali Corrosion

• Change of density and specific volume of the
  solid phase
• Alkali compounds unknown
• ? MgO, CaO
• Densities of refractory oxids
• ? gt 3 g/cm³
• (except SiO2, CaO?SiO2)
• Densities of new formed alkali
• compounds
• ? lt 3 g/cm³ (most frequently)
• The volume increase of solid phase
• of the refractory oxides containing
• alkali compounds leads to an
• attrition of microstructure and
• the damage of refractory lining.

Refractory oxids, possible alkali compounds
(cement chemistry notation) from the phase
diagrams, whose densities and change of volume
( expansion, - shrinkage).
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
28
Alkali Corrosion

• Change of density and specific volume of the
  solid phase
• Phase diagram of system
• MgO SiO2 K2O with forsterite
• ? formation of solids at 1100 1300 C
• 2MgO?SiO2, MgO, K2O?MgO?SiO2, K2O
• Change of densities e.g. specific volume
• by chemical reaction of forsterite with K2O
• ? expansion and shrinkage
• Refractory materials based on forsterite no
  alkali resistant, because volume increase leads
  to destruction of the structure

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
29
Alkali Corrosion

• Change of density and specific volume of the
  solid phase
• Phase diagram of system
• K2O Al2O3 SiO2 with mullite and fireclay
• ? formation of solids with lower densities
• at lt 1556 C
• mullite react to corundum
• fireclay react to alkali feldspar
• ? first eutectic melts appear at 1556 C
• similar behavior is due of the system
• Na2O Al2O3 SiO2
• Lower density of products by reactions of
• K2O and Na2O with mullite and fireclay
• leads to ? high volume expansion
• ? alkali bursting
• ? damage of refractories

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
30
Alkali Corrosion

• Change of density and specific volume of the
  solid phase
• Calculated volume expansion
• of mullite and fireclay depend
• on the content of K2O or Na2O
• (from phase components and
• densities)
• Mullit
• 22 volume increase with
• 8 linear expansion
• by formation of corundum
• Fireclay
• volume expansion decrease at a
• K2O/Na2O-content of gt 20

Volume expansion of mullite and fireclay by
reaction with K2O or Na2O
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
31
Alkali Corrosion

• Change of density and specific volume of the
  solid phase
• Phase diagram of system
• K2O CaO Al2O3 with hibonite
• ? formation of solids at 1100 C
• with high volume expansion
• Phase diagram of system
• Na2O CaO Al2O3 with hibonite
• ? more expansion of volume than with K2O
• Refractory materials based on hibonite
• are not alkali resistant, because the volume
• expansion at 1100 C leads to a damage of
• the structure (contrary to literature
  opinion)

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
32
Alkali Corrosion

• Change of density and specific volume of the
  solid phase
• Phase diagram of system
• Na2O Al2O3 with alumina
• ? formation of solids at lt 1300 C
• ? melt formation of eutectic at 1580 C
• Phase diagram of system
• K2O Al2O3 with alumina
• ? formation of solids at lt 1300 C
• ? melt formation of eutectic at 1910 C
• Refractory materials based on alumina
• are not alkali resistant, because the volume
• expansion up to 1000 C leads to a damage of
• the structure
• up to 1400 C destruction of the aluminates
  (NaAlO2, KAlO2) and evaporation of alkalis
• Exception ?-alumina with alkali resistant
  considerations

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
33
Alkali Corrosion

• Change of density and specific volume of the
  solid phase
• The increased volume of the solid phases to 52
  is leading to bursting of solid structures. Less
  known and in contrast to the general opinion are
  the following topics
• Alumina Al2O3 reacts to alkali aluminates with a
  volume increase to 52 and leads to a
  destruction of the products.
• Cr2O3 leads to expansion by reaction with
  alkalis.
• The density modifications of SiO2 and calcium
  silicates taking place by melting. The volume
  increase of solid parts by melting is not a
  problem, but the melt formation and the
  deformation of the products.
• Fireclay reacts to feldspars and shows a volume
  increase between 21 to 32 . This corrosion
  process is known as alkali bursting.
• Hibonite, known as alkali-resistant, reacts to
  ß-alumina, and presents a volume increase of
  about 22 .
• Spinel reacts to (Na2OMgOAl2O3)-compounds, like
  ß-alumina, and leads to volume increase of
  approximately 13 .
• Forsterite reacts to alkali compounds and shows a
  volume increase to 23 . Forsterite is also,(
  contrary to literature opinion), not alkali
  corrosion resistant.

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
34
Alkali Corrosion

• Expansion phenomena
• Salt storage in pores of refractories
• ? evaporation of salt at high temperatures
• ? condensation of salt in cooler range of
• refractory materials
• ? pores are filled entirely with liquid or
• solid salts
• Destruction mechanisms
• ? thermal linear expansion of salts
• 5- to 10-fold more than refractory
  materials
• ? thermal shock sensibility of refractory
• material is increased
• ? volume increase between solid and
• liquid salt (change of densities)
• ? hygroscopicity of salts and volume increase
• (destruction in humid atmosphere)

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
35
Alkali Corrosion

• Corrosion due to water condensation
• Satured water based salt solutions
• ? pH-values are neutral to alkaline (no acid!!)
• Metal corrosion
• ? pH-value lt 10
• ? electrochemical corrosion
• Investigations for the future

Alkali corrosion of a steel bar in a gradient
furnace after treatment at 1000C.
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
36
Alkali Corrosion

• Sumary of the alkali corrosion mechanisms
• physical-chemical high temperature melting
  processes associated
• with solution, sintering and shrinkage
• chemical material conversion under solid
  conditions and so modification
• of density of solid refractory phases causing
  bursting effects
• mechanical stresses/bursting between solid salt
  in the pores and the refractory material
• chemical material conversion followed by
  expansion and shrinkage due to
• water condensation and removal of water
  condensation products

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Mechanisms of alkali corrosion
37
Alkali Corrosion

• Investigations of the alkali resistance
  disc-test
• Disc-test
• pressed disc based on 70 refractory powder
• and 30 salt mixture (K2SO4, KCl, K2CO3)
• Change of sample diameter, weight and
• visual features of refractory/salt heat treated
• discs under periodic heating and cooling
• conditions
• Fireclay
• diameter increase from 50 to 53 mm
• ? linear expansion of 6 due to
• alkali bursting

Coating of solid raw material with solid salt
particles
U. Fischer, C.G. Aneziris, E. Schlegel Corrosion
Problems of Refractories due to the Use of
Secondary fuels
Investigation methods
38
Alkali Corrosion

• Investigations of the alkali resistance
  disc-test
• Change of diameter after 1100 C at 5 hours
• ? high value of expansion
• Zirconia mullite Z72
• Spinel MA 76
• Spinel AR 78
• Hibonite SLA-12
• Hibonite Bonite
• Forsterite Olivin
• Aluminium titanate
• ? high value of shrinkage
• Zirconia 3Y-TZP
• ? suitable materials
• Zirconia 3,5Mg-PSZ
• Na-aluminate
• ?-alumina
• Betacalutherm (dried, fired)

Salt mixtures SM 1 K2SO4 / K2CO3 SM 2 K2SO4 /
K2CO3 / KCl SM 3 K2SO4 / K2CO3 / KCl / CaSO4
Expansion and shrinkage of the different mixtures
after treatment at 1100 C and 5 h
U. Fischer, C.G. Aneziris, E. Schlegel Corrosion
Problems of Refractories due to the Use of
Secondary fuels
Investigation methods
39
Alkali Corrosion

• Investigations of the alkali resistance
  disc-test
• Change of diameter after 1300 C at 5 hours
• ? high value of expansion
• Zirconia mullite Z72
• Spinel AR 78
• Hibonite SLA-12
• Hibonite Bonite
• Forsterite Olivin
• Aluminium titanate
• ? high value of shrinkage
• Zirconia 3Y-TZP
• Zirconia 3,5Mg-PSZ
• Na-aluminate
• Spinel MA 76
• ? suitable materials
• ?-alumina
• Betacalutherm (dried, fired)

Salt mixtures SM 1 K2SO4 / K2CO3 SM 2 K2SO4 /
K2CO3 / KCl SM 3 K2SO4 / K2CO3 / KCl / CaSO4
Expansion and shrinkage of the different mixtures
after treatment at 1300 C and 5 h
U. Fischer, C.G. Aneziris, E. Schlegel Corrosion
Problems of Refractories due to the Use of
Secondary fuels
Investigation methods
40
Alkali Corrosion

• Investigations of the alkali resistance
  disc-test
• Change of diameter after 1300 C at 50 hours
• ? high value of expansion
• Spinel AR 78
• Forsterite Olivin
• ? suitable materials
• ?-alumina
• Betacalutherm (dried, fired)
• Spinel MA 76

Salt mixtures SM 1 K2SO4 / K2CO3 SM 2 K2SO4 /
K2CO3 / KCl SM 3 K2SO4 / K2CO3 / KCl / CaSO4
Expansion and shrinkage of the different mixtures
after treatment at 1300 C and 50 h
U. Fischer, C.G. Aneziris, E. Schlegel Corrosion
Problems of Refractories due to the Use of
Secondary fuels
Investigation methods
41
Alkali Corrosion

• Investigations of the alkali resistance
  disc-test
• Change of diameter after 1100 and 1300 C, 5 and
  50 hours hold time
• ? high value of expansion
• Zirconia mullite Z72
• Spinel MA 76
• Spinel AR 78
• Hibonite SLA-12
• Hibonite Bonite
• Forsterite Olivin
• Aluminium titanate
• ? high value of shrinkage
• Zirconia 3Y-TZP
• Zirconia 3,5Mg-PSZ
• Na-aluminate
• ? suitable materials
• ?-alumina
• Betacalutherm (dried, fired)

Samples for change of disc diameter after heating
at 1300 C and 5 h
U. Fischer, C.G. Aneziris, E. Schlegel Corrosion
Problems of Refractories due to the Use of
Secondary fuels
Investigation methods
42
Alkali Corrosion

• Investigations of the alkali resistance
  disc-test
• Influence of humidity of alkali-infiltrated
• used raw materials
• ? increase of sample weight
• 30 70
• The sample weight had increased
• because the humidity had condensed
• in the pores of the sample structure.

Salt mixtures SM 1 K2SO4 / K2CO3
Increase of sample weight after heat treatment
and storage time at 20 C and 100 rel.
humidity.
U. Fischer, C.G. Aneziris, E. Schlegel Corrosion
Problems of Refractories due to the Use of
Secondary fuels
Investigation methods
43
Alkali Corrosion

• Investigations of the alkali resistance
  disc-test
• Influence of humidity of alkali-infiltrated
• used raw materials
• ? volume increase
• lt 1
• ? volume decrease
• lt 1
• The water absorption of alkali infiltrated
• samples took place with out or minor
• changes in volume at high humidity
• across month.
• The alkali infiltrated Betacalutherm
• and ?-alumina take in humidity and
• dehumidify without change in volume
• again and no destruction of the structure.

Salt mixtures SM 1 K2SO4 / K2CO3
Change of sample volume after heat treatment and
2 and 3 months storage time at 20 C and 100
rel. humidity.
U. Fischer, C.G. Aneziris, E. Schlegel Corrosion
Problems of Refractories due to the Use of
Secondary fuels
Investigation methods
44
Alkali Corrosion

• Investigations of the alkali resistance
  crucible test according DIN 51069
• Crucibel test
• DIN 51069
• 1000 C for 5 hours
• salt mixture K2SO4, K2CO3
• Refractory concrete on the base of Fireclay
• ? completely infiltration of the salt mixture
• ? alkali bursting lead to critical cracks
• ? damage of the crucible at low temperature
• and short exposure time

E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Investigation methods
45
Alkali Corrosion

• Investigations of the alkali resistance
  crucible test according DIN 51069
• Crucibel test
• DIN 51069
• 700 C / 800 C for 5 hours
• salt mixture K2SO4, K2CO3 and
• salt mixture K2SO4, K2CO3, KCl, CaSO4
• Calcium silicate thermal insulating material
• ? infiltration with partly fluid salt melt at
  700 C
• ? damage the crucible at 800 C
• ? partly dissolving of the calcium silicate
• in the salt melt
• Melt formation at low temperature (720 C)

Calcium silicate thermal insulating material with
salt mixture K2SO4 and K2CO3 at 700 C for 5 h
Calcium silicate thermal insulating material with
salt mixture K2SO4, K2CO3, KCl and CaSO4 at 800
C for 5 h
Investigation methods
46
Alkali Corrosion

• Investigations of the alkali resistance test in
  a gradient furnace
• Gradient furnace
• ? gradient of temperature
• 100 - 1300 C
• ? alkali atmosphere
• Thermal insulation material
• ? Betacalutherm
• Refractory material
• ? refractory concrete
• Steel bar
• ? austenitic steel 1.48.28 with
• scaling resistance to 1000 C
• Salt mixtures
• ? K2SO4 / K2CO3 / KCl

Wall built-up for corrosion test in gradient
furnace
Investigation methods
47
Alkali Corrosion

• Investigations of the alkali resistance test in
  a gradient furnace
• Thermal insulation material
• ? Betacalutherm with out
• corrosion effects
• Refractory material
• ? refractory concrete with cracks,
• volume increase (2-3),
• formation of feldspar in the
• hot zone
• Steel bar
• ? scaling with volume increase
• (33-56 ) in the hot zone
• Verification of the post mortem investigations of
  the industrial refractory materials

Wall built-up after corrosion test in gradient
furnace left scaling of the steel bar in the
alkali corroded refractory material right
Betacalutherm without corrosion effects
Investigation methods
48
Alkali Corrosion

• Conclusions of alkali corrosion of the refractory
  materials
• Worst corrosion bursting effect
• ? salt mixture of K2SO4 / K2CO3
• No alkali resistant
• ? all refractory oxides
• ? all refractory mixtures
• alkali resistant considerations
• ? low alumina content materials
• (?-alumina doped material)
• ?-alumina
• ? alkali aluminate
• (5 to 11 mol Al2O3, 1 mol Na2O or K2O)
• ? melting point 1580 2053 C
• ?-alumina does not melt ore react
• with higher content of alkalis at
• temperatures below 1580 C

Sumary of phase diagrams
E. Schlegel, C.G. Aneziris, U. Fischer Alkali
Corrosion Resistance High-Temperature Insulation
Materials
Conclusions
49

Refractories for gasification process
50
Refractories for gasification process
Wear mechanisms of refractories in slagging
gasifiers
J.P. Bennett, Refractory liner materials used in
slagging gasifiers
51
Refractories for gasification process

• Potential Refractory Problems in Coal
  Gasification
• alkali attack
• carbon monoxide disintegration
• silica volatilization
• steam-related reactions
• thermoelastic stresses
• erosion due to solid particulates
• corrosion and erosion due to molten coal slag
  and/ or iron
• iron oxide bursting

dry ash gasifiers
slagging gasifiers
C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
52
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Corrosion and Erosion by Molten Coal
Slag and/or Iron

• High-purity alumina, chrome-magnesia,
  alumina-zirconia-silica, zirconia, SiC
• ? Grand Forks Energy Technology Center
  (GFETC) ? less than 10 h at 1550 C lifetime
• ? Ruhrchemie Texaco gasifier ? hundreds of
  hours at 1600 C lifetime
• ? lifetime depends on conditions (unique for
  single gasifier) and coal/ slag (e.g. CaO/SiO2 lt
  1 or CaO/SiO2 gt 1)
• major mechanisms of the corrosion process
  dissolution, penetration and disruption, and
  erosion
• higher velocity slag ? rate of corrosion ? ?
  dissolution and/or erosion ?

C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
53
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Corrosion and Erosion by Molten Coal
Slag and/or Iron

• dense high-chromia content refractories ?
  superior corrosion resistance to CaO/SiO2
  0.2-1.7
• high-iron oxide acidic coal slag at 1575 ?
  chrome-spinel (MgCr2O4)
• low solubility of Cr2O3 and MgCr2O4 in
  SiO2-Al2O3-CaO liquids
• refractories containing gt 30 Cr2O3
• reaction with all types of coal slags to form
  complex spinels (slowly dissolution)
• problems poor thermal-shock resistance and
  susceptible to iron oxide bursting
• high alumina refractory intermediate in
  performance in acidic slags and poor in basic
  slags
• SiC FexOy ? ferrosilicon alloy (low melting)
• magnesia-chromite refractories better in basic
  slags than in acidic slags (dissolution of MgO in
  all cases)

C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
54
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Thermal Shock Resistance of Brick
Linings

• only few data available (Fig.)
• dense high-chromia ( 80 wt) have significantly
  lower thermal shock resistance than sintered
  low-chromia bricks (e.g. 90 wt Al2O3-10 wt
  Cr2O3)
• improvement of the thermal shock resistance by
  microstructural alteration
• heating and cooling rates have to be carefully
  controlled to avoid spalling

C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
55
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Iron Oxide Bursting

• absorbed iron oxides leads to failures in spinel
  containing refractories
• ferrite spinels have larger unit-cell sizes than
  chromites or aluminates (Fig.)
• reactions with FexOy leads to internal stresses
  ? spalling
• Fe2/Fe3 ratio depends on partial O2-pressure
  (unit cell size alters)
• low porosity limits the penetration of iron
  oxides from the slag ? spalling occurs only in a
  thin surface layer (problem cracks due to
  thermal shock

C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
56
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Alkali Attack

• formation of low-melting low-viscosity liquids or
  dry alkali-alumino-silicate compounds
• problems occurs in the non-slagging regions of
  gasifiers
• most coal slags contain significant amounts of
  alkali (1-10)
• Na(g) atmosphere ? NaOH
• NaOH refractory (mullite) ? NaAlSiO4
  NaAl11O17 ( 30 volume expansion)
• minimizing the alkali attack by
• use of low-alkali coals
• lower process temperatures (decrease efficiency)
• higher density of refractories (limitation of the
  penetration)
• use of high-silica refractories (60 wt) ? react
  with alkali to produce glass ? sealing off of the
  surface

C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
57
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Carbon Monoxide Induced
Disintegration

• 2 CO C CO2 (400-700C, red. atm.)
• ? deposition of carbon ? refractory failure
  caused by internal stresses
• accelerated by metallic iron, free iron oxides,
  iron carbides
• no reported failures but laboratory experiments
  (Fig.)
• rate of attack increases rapidly as the pressure
  increases
• small amounts of iron (0.25 wt) affect the rate
  ? alumina castables loose strength in pure CO
• alkali compounds increase the attack rate
• H2S retard attack

C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
58
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Reduction of Silica by H2
SiO2 (s) H2 ? SiO(g) H2O

• (reducing and steam-containing atmosphere)
• loss of silica due to formation of volatile
  compounds
• e.g. 50 loss of silicate refractory in a
  secondary ammonia reformer after several years
• no changes of silica content at a depth of 10 mm
  from the hot face
• ? indicates extremely slow diffusion rate of SiO
  below 1200 C

C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
59
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Steam-Related Reactions

• coal gasification atmosphere containing high
  partial pressures of steam
• SiC disintegration
• strength loss in phosphate-bonded refractories
• no degradation of cement-bonded castables

Results applicable to low-temperature sections of
most gasifiers. (1000-1100 C)
C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
60
Refractories for gasification process
Refractory problems in coal gasification C
Physical wear spalling
J.P. Bennett, Refractory liner materials used in
slagging gasifiers
61
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Thermomechanical Degradation of
Monolithic Linings

• cracking during initial dryout and heat-up of
  monolithic refractory lining
• mechanical reliability of the lining can be
  improved by
• minimizing the amount of linear shrinkage of the
  refractory
• continuous, slow heat-up rate
• elimination of long hold periods during the
  heating and cooldown
• maintaining the vessel shell temperature as close
  to ambient as possible
• using incompressible bond barriers
• using anchor spacings greater than 1.5 times the
  lining thickness

C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
62
Refractories for gasification process
Potential Refractory Problems in Coal
Gasification Erosion of Refractory Materials

• Testing methods
• direct-impingement (dolomite and sand
  particles vs. refractory)
• chrome castable more erosion resistant than
  high-alumina and lightweight castable
• fluidized-bed (ambient temperature
  and 810 C with dead-burned dolomite)
• high- and intermediate-alumina castables more
  erosion resistant than chrome castable
• impingement-tube (simulates hot-gas transfer
  lines with dolomite)
• high- and intermediate-alumina castables
  performed well
• erosion occurs primarily in the softer matrix

C.R. Kennedy and P.E. Schlett, Refractories for
Coal Gasification
63
Refractories for gasification process
Corrosion Mechanisms
formation of an intermediate compound
dissolution
solid solution
Kwong, et al., Wear Mechanisms of Chromia
Refractories in Slagging Gasifiers
64
Refractories for gasification process

• oxygen partial pressure in a gasifier range from
  10-7 to 10-9
• oxygen potential affects
• valence state of transition oxides such as iron
  and vanadium oxides
• oxide basicity
• basicity of slags formed from iron and vanadium
  oxides
• melting point of the slags
• ? oxygen potential influences slag refractory
  reactions and the compounds formed

Kwong, et al., Wear Mechanisms of Chromia
Refractories in Slagging Gasifiers
65
Refractories for gasification process
Thermodynamic calculations - HSC Chemistry

• FeO with some Fe3O4 may be stable phase formed at
  oxygen partial pressure of 10-7 to 10-9

• V3O5 should be stable phase in gasifiers
  environments

Kwong, et al., Wear Mechanisms of Chromia
Refractories in Slagging Gasifiers
66
Refractories for gasification process
Material development from the 1970s until today
R. Dürrfeld, Refractories in Coal Gasification
Plants
67
Refractories for gasification process

• Evaluated materials in the 1970s and 1980s
• alumina-silicate
• high alumina
• chromia-alumina-magnesia spinels
• alumina and magnesia
• alumina and chrome
• SiC
• chrome materials with phosphate
• only materials with high chrome oxide
• content (min. 75 wt.-)
• (reaction between chromia and FeO)

J.P. Bennett, Low chrome/ chrome free
refractories for slagging gasifiers
68
Refractories for gasification process

• todays researches low /no chrome oxide
• alumina with ZrO2, MgO and additives
• alumina-zirconia with MgO, SiC and additives
• HfO2, HfSiO4
• ZrSiO4
• NiAl2O4
• researches still in progress

J.P. Bennett, Low chrome/ chrome free
refractories for slagging gasifiers M. Müller et
al., Corrosion behaviour of chromium-free
ceramics for liquid slag removal in pressurized
pulverized coal combustion
69
Thank you for your attention!