1 Alkali Corrosion of Refractories in Cement Kilns.
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Transcript of 1 Alkali Corrosion of Refractories in Cement Kilns.
1
Alkali Corrosion of Refractories in Cement Kilns
2
Alkali Corrosion
Topics
1. Introduction to alkali corrosion of refractories
2. Characterization of corroded industrial refractory materials
3. Behavior of alkali salts and alkali salt mixtures
4. Mechanisms of alkali corrosion
5. Investigation methods
6. Conclusions
3
Alkali Corrosion
Corrosion attack in cement rotary kilns
Deuna Zement GmbH, Informationsmaterial 2005
high temperature thermal insulation material
combustion of fuels
raw material preparation clinker burning clinker storage cement mill
clinker burning
heat exchanger electrostatic filter
grate cooler
rotary kiln
refractory lininghigh temperature thermal insulation materialmetallic components
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 ofpowdered 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
Introduction to Alkali Corrosion of Refractories
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
5
Alkali Corrosion
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
The use of secondary fuels
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
Introduction to Alkali Corrosion of Refractories
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
7
Alkali Corrosion
Secondary fuels
solid (plastic, rubber, battery, animal
residues; tyres, domestic waste...)
liquid (used oil, tar, chemical wastes...)
gaseous (landfill, pyrolysis gas)
Alkalibursting and chemical spalling of the
refractories
Gas corrosion (condensation) of the
metal components
Organic Compounds
alkalis
sulfates
chlorides...and other corrosive
compounds
fireclay insulating brick after 3 years in use in a cement rotary kiln (feed end)
Effect of the combustion of secundary fuels in cement rotary kilns
Introduction to Alkali Corrosion of Refractories
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
8
Alkali Corrosion
Post mortem investigations
•Roof of kiln hood of the DOPOL-kiln:
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Characterization of corroded industrial refractory materials
Hot side (refractory bricks or concrets)
Cool side (metal jacket)
Calcium silicate
Insulating brickbasic abrasion lining
9
Alkali Corrosion
Post mortem investigation
•Alkali corroded calcium silicate thermalinsulating material in the chamberat 600 – 700 °C:
X-ray analysis
•Hot side area:
based on KCl and CaSO4
residual NaCl, futher chlorides,Cr- and Fe-sulfates
Characterization of corroded industrial refractory materials
Calcium silicate thermal insulating material (thickness 25 mm) after 18 month in use in the chamber between the preheater and the rotary cement kiln.
Hot side (refractory concrete)
Cool side (metal jacket)
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
10
Alkali Corrosion
Post mortem investigation
•Alkali corroded fireclay brick in the hot zoneat 800 °C:
X-ray analysis
•Area around the crack:
based mainly on leucit (K2OAl2O34SiO2)
residual silica (SiO2), mullite (3Al2O32SiO2)corundum (Al2O3)
Characterization of corroded industrial refractory materials
Fireclay brick from the chamber between the preheater and the rotary tube of the cement kiln after use (18 month), left heat site with a temperature between 800 to 900 °C.
Hot side Cool side
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
11
Alkali Corrosion
Post mortem investigation
•Alkali corroded fireclay insulating brick in the hot zone at > 1000 °C: X-ray analysis
•Hot side area:
based mainly on leucit (K2OAl2O34SiO2),
mullite (3Al2O32SiO2)
residual silica (SiO2), kalsilit (K2OAl2O32SiO2)larnit (2CaOSiO2)
Characterization of corroded industrial refractory materials
Fireclay insulating brick after 3 years in use in a cement rotary kiln (feed end), front heat site with a temperature > 1000 °C.
Hot side
Cool side
Infiltration zone
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
12
Alkali Corrosion
Post mortem investigation
•Alkali corroded magnesia brick in the sinter zone at > 1100 °C: X-ray analysis
•Hot side area:
based mainly on leucit (K2OAl2O34SiO2),
mullite (3Al2O32SiO2)
residual silica (SiO2), kalsilit (K2OAl2O32SiO2)larnit (2CaOSiO2)
Characterization of corroded industrial refractory materials
Magnesia brick after 2 years in use in a cement rotary kiln (sinter zone), above on the heat site with a temperature > 1000 °C.
Hot side
Cool side
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Post mortem investigation
•Alkali corroded refractory concrete from thewall 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
13
Industrial refractory brick from the wall of a bottom cyclone of cement kiln after 1 year usage.
A
B
Alkali Corrosion
Characterization of corroded industrial refractory materials
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
14
Alkali Corrosion
Characterization of corroded industrial refractory materials
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
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
16Behavior of alkali salts and alkali salt mixtures
Alkali Corrosion
High temperature behavior of alkali salts and alkali salt mixtures
• Salts after heating at 1100°C in crucibles:
Solid salt after 1100°C
Na2SO4, K2SO4 molten
Na2CO3, K2CO3 molten
NaCl, KCl evaporated
CaSO4 sintered
• The solid salts as most reactive and corrosive mixtures after heating at 1100°C in crucibles:
Salt mixtures after 1100°C
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
17Behavior of alkali salts and alkali salt mixtures
Alkali Corrosion
High temperature behavior of alkali salts and alkali salt mixtures
• Thermal linear expansioncoefficient (lin) of solid salts and salt mixtures:
highest value: K2SO4
lowest value: CaSO4
is reflected in the value of the salt mixtures
Solid salt
lin
measured10-61/K(20/600 °C)
lin
literature10-61/K (0 °C)
KCl 52 66,2
K2SO4 90 44,6
K2CO3 58 43,3
CaSO4 16
SM 1 (K2SO4/K2CO3) 58
SM 2 (K2SO4/K2CO3/KCl) 50
SM 3 (K2SO4/K2CO3/KCl/CaSO4) 34
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
18Behavior of alkali salts and alkali salt mixtures
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.
Solid saltDensity of solid g/cm³
Density of meltg/cm³
Volumeincrease %
Hygroscopicity
KCl 1,99 1,52 31 no
K2SO4 2,66 1,89 41 no
K2CO3 2,43 1,96 24 hygroscopic*
CaSO4 2,96 no
*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
19Behavior of alkali salts and alkali salt mixtures
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 jacketof rotary kiln too.
Salt solutionpH-value
directlyafter
8 days
KCl 7,99 7,69
K2SO4 7,27 8,34
K2CO3 13,83 13,74
CaSO4 9,69 7,92
SM 1 (K2SO4/K2CO3) 12,10 12,23
SM 2 (K2SO4/K2CO3/KCl) 12,09 12,14
SM 3 (K2SO4/K2CO3/KCl/CaSO4) 12,08 12,14
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
20Behavior of alkali salts and alkali salt mixtures
Alkali Corrosion
Behavior of satured water based solutions of alkali salts and alkali salt mixtures
• Electrical conductivity of saturedwater 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 isbased on electrochemical corrosion.
Salt solution
Electrical conductivity
directlyafter
8 days
KCl 378 381
K2SO4 91 90
K2CO3 173 172
CaSO4 1560 1655
SM 1 (K2SO4/K2CO3) 161 161
SM 2 (K2SO4/K2CO3/KCl) 184 184
SM 3 (K2SO4/K2CO3/KCl/CaSO4) 178 178
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
21
Alkali Corrosion
4 main alkali
corrosion
mechanisms
Melt formation
Change of
density and
volume of the
solid phase
Expansion as a
result of salt
stored in pores
Corrosion due to water
condensation
Mechanisms of alkali corrosion
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
22Mechanisms of alkali corrosion
Alkali Corrosion
1. 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
Refractory oxid /melting point [°C]
Alkali compound Temperature of 1. melting [°C]
MgO / 2840
K2SO4
K2CO3
Na2OK2O
1067 895No miscibilityNo miscibility
CaO / 2580
KCl + NaClCaSO4
Na2OK2O
6451365No miscibilityNo miscibility
Cr2O3 / 2200KCl + K2OK2O
366 669
Al2O3 / 2050Na2OK2O
14101450
TiO2 / 1830K2SO4 + K2ONa2OK2O
804 986 950
SiO2 / 1713Na2OK2O
789 742
MgO + Al2O3 / 1925 No dates
Al2O3 + SiO2 / 1595Na2OK2O
732 695
MgO + SiO2 / 1543Na2OK2O
713 685
CaO + SiO2 / 1436Na2OK2O
725 720
CaO + Al2O3 / 1395 No dates
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
23Mechanisms of alkali corrosion
Alkali Corrosion
0 20 40 60 80 100
500
1000
1500
2000
2500
3000
Mol %
T, o
C
2850o
Liquid
MgO + Liq.
1067o
Hex-K2 SO4 + MgO
Ortho-K2 SO4 + MgO
588o
MgOK2 SO4
(2%)1069o
1. 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 materialsare not alkali resistant because melt formationat 895 °C.
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
24Mechanisms of alkali corrosion
Alkali Corrosion
1. 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 formationE. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
25
Alkali Corrosion
Alkali Corrosion of Therml Insulating Material Based of Calcium Silicates
1. 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 < 720 °C
Refractory materials based on wollastonite no alkali resistant, because melt formationat 700 °C.
26Mechanisms of alkali corrosion
Alkali Corrosion
1. Melt formation
• Applied Temperatures in presence of alkali < 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 > 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
2. Change of density and specific volume of the solid phase
• Alkali compounds unknown:
MgO, CaO
• Densities of refractory oxids:
> 3 g/cm³ (except SiO2, CaOSiO2)
• Densities of new formed alkali compounds:
< 3 g/cm³ (most frequently)
The volume increase of solid phase of the refractory oxides containing alkali compounds leads to anattrition of microstructure and the damage of refractory lining.
27Mechanisms of alkali corrosion
Alkali Corrosion
Refractory oxide
Densityg/cm³
New formedalkali compounds
Densityg/cm³
Volumechange %
Al2O3 3,99 (N,K)1…6A1…11 2,63…3,42 +17…+52
Cr2O3 5,25 NC 4,36 +20
SiO2 2,65 (N, K)1…3S1…4 2,26…2,96 -10…+17
3Al2O32SiO2 3,17(N,K)1…3AS1…6
N3CA3S6(SO4)2,40…2,62 +21…+32
CaO6Al2O3 3,69 (N,K)C0…14A4…11 3,03…3,31 +11…+22
MgOAl2O33,55… 3,70
NM0,8…4A5…15 3,28…3,33 +7…+13
2MgOSiO2 3,22 (N,K)1…2M1…5S3…12 2,56… 3,28 -2…+23
CaOSiO2 2,92 (N,K)1…2C1…23S1...12 2,72…3,36 -13…+7
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
K2OMgOSiO2
2. 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 2MgOSiO2, MgO, K2OMgOSiO2, K2O
• Change of densities e.g. specific volumeby 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
28Mechanisms of alkali corrosion
Alkali Corrosion
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
SolidDensity g/cm³
Specific volume cm³/g
2MgOSiO2 3,22 0,311
MgO 3,59 0,279
K2OMgOSiO
2
2,76 0,362
K2O 2,33 0,429
Mullite
Fireclay
1556 °C
29
Alkali Corrosion
2. 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 < 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 ofK2O and Na2O with mullite and fireclayleads to: high volume expansion
“alkali bursting”
damage of refractories
Mechanisms of alkali corrosion
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Volume expansion of mullite and fireclay by reaction with K2O or Na2O
30
Alkali Corrosion
2. 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 anddensities)
• Mullit:
22 % volume increase with8 % linear expansion by formation of corundum
• Fireclay:
volume expansion decrease at a K2O/Na2O-content of > 20 % Content of K2O or Na2O in % by weight
Mullite + K2O
Mullite + Na2O
Fireclay + K2O
Fireclay + Na2O
Mechanisms of alkali corrosion
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
31
Alkali Corrosion
2. 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)
Mechanisms of alkali corrosion
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
32
Alkali Corrosion
2. Change of density and specific volume of the solid phase
• Phase diagram of system Na2O – Al2O3 with alumina:
formation of solids at < 1300 °C melt formation of eutectic at 1580 °C
• Phase diagram of system K2O – Al2O3 with alumina:
formation of solids at < 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”
Mechanisms of alkali corrosion
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
2. 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 β-alumina, and presents a volume increase of about 22 %.
Spinel reacts to (Na2O MgO Al⋅ ⋅ 2O3)-compounds, like β-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.
33Mechanisms of alkali corrosion
Alkali Corrosion
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
34
Alkali Corrosion
3. 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)
Mechanisms of alkali corrosion
Industrial refractory brick from the wall of a bottom cyclone of cement kiln after 1 year usage.
A
B
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
35
Alkali Corrosion
4. Corrosion due to water condensation
• Satured water based salt solutions:
pH-values are neutral to alkaline (no acid!!)
• Metal corrosion pH-value < 10 electrochemical corrosion
Investigations for the future
Mechanisms of alkali corrosion
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Alkali corrosion of a steel bar in a gradient furnace after treatment at 1000°C.
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
Coating of solid raw material with solid salt particles
37
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
•Disc-test:
pressed disc based on 70 % refractory powderand 30 % salt mixture (K2SO4, KCl, K2CO3)
•Change of sample diameter, weight andvisual features of refractory/salt heat treateddiscs under periodic heating and cooling conditions
•Fireclay:
diameter increase from 50 to 53 mm
linear expansion of 6 % due to alkali bursting
Disc-test of fireclay salt briquette before and after heat treatment at 1100 °C for 5 hours
unfired 1100 °C / 5 hours
Investigation methods
solid raw material particle
layer of solid salt particles
mixture of solid raw material + alkali salts
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Expansion and shrinkage of the different mixtures after treatment at 1100 °C and 5 h
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 Z72Spinel MA 76Spinel AR 78Hibonite SLA-12Hibonite BoniteForsterite OlivinAluminium titanate
high value of shrinkage
Zirconia 3Y-TZP
suitable materials
Zirconia 3,5Mg-PSZNa-aluminate-aluminaBetacalutherm (dried, fired)
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
Salt mixturesSM 1 K2SO4 / K2CO3
SM 2 K2SO4 / K2CO3 / KClSM 3 K2SO4 / K2CO3 / KCl / CaSO4
Investigation methods
Expansion and shrinkage of the different mixtures after treatment at 1300 °C and 5 h
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 Z72Spinel AR 78Hibonite SLA-12Hibonite BoniteForsterite OlivinAluminium titanate
high value of shrinkage
Zirconia 3Y-TZPZirconia 3,5Mg-PSZNa-aluminateSpinel MA 76
suitable materials
-aluminaBetacalutherm (dried, fired)
Salt mixturesSM 1 K2SO4 / K2CO3
SM 2 K2SO4 / K2CO3 / KClSM 3 K2SO4 / K2CO3 / KCl / CaSO4
Investigation methods
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
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 78Forsterite Olivin
suitable materials
-aluminaBetacalutherm (dried, fired)Spinel MA 76
Salt mixturesSM 1 K2SO4 / K2CO3
SM 2 K2SO4 / K2CO3 / KClSM 3 K2SO4 / K2CO3 / KCl / CaSO4
Expansion and shrinkage of the different mixtures after treatment at 1300 °C and 50 h
Investigation methods
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
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 Z72Spinel MA 76Spinel AR 78Hibonite SLA-12Hibonite BoniteForsterite OlivinAluminium titanate
high value of shrinkage
Zirconia 3Y-TZPZirconia 3,5Mg-PSZNa-aluminate
suitable materials
-aluminaBetacalutherm (dried, fired)
Betacalutherm and -alumina are long-time and alkali resistant after that as the onlyfire-proof materials up to 1300 °C
Samples for change of disc diameter after heating at 1300 °C and 5 h
Investigation methods
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
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 increasedbecause the humidity had condensed in the pores of the sample structure.
Increase of sample weight after heat treatment and storage time at 20 °C and 100 % rel. humidity.
Salt mixturesSM 1 K2SO4 / K2CO3
Investigation methods
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
43
Alkali Corrosion
Investigations of the alkali resistance – “disc-test”
•Influence of humidity of alkali-infiltrated used raw materials:
volume increase < 1 %
volume decrease < 1 %
The water absorption of alkali infiltrated samples took place with out or minor changes in volume at high humidityacross month.
The alkali infiltrated Betacaluthermand -alumina take in humidity anddehumidify without change in volume again and no destruction of the structure.
Change of sample volume after heat treatment and 2 and 3 months storage time at 20 °C and 100 % rel. humidity.
Salt mixturesSM 1 K2SO4 / K2CO3
Investigation methods
U. Fischer, C.G. Aneziris, E. Schlegel: Corrosion Problems of Refractories due to the Use of Secondary fuels
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
Crucible test of castable gunning material according to DIN 51069, after heat treatment at 1000 °C for 5 hours
bottom crucible
upper crucible
sealing
alkali salt mixtur
alkali gas
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
Investigation methods
Wall built-up for corrosion test in gradient furnace
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
Investigation methods
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
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 attemperatures below 1580 °C
Conclusions
E. Schlegel, C.G. Aneziris, U. Fischer: Alkali Corrosion Resistance High-Temperature Insulation Materials
Refractory oxide
Alkali oxid
Damage by
SiO2
Na2O Melt up to 782 °C
K2O Melt up to 769 °C
Calcium silicate
Na2O Melt up to 720 °C
K2O Melt up to 700 °C
Al2O3
Na2O 10% volume expansion by 3% Na2O
K2O 10% volume expansion by 4% K2O
MulliteNa2O
10% volume expansion by 14% Na2O
K2O 10% volume expansion by 17% K2O
FireclayNa2O
10% volume expansion by 16% Na2O
K2O 10% volume expansion by 15% K2O
Forsterite K2O 10% volume expansion by 34% K2O
Spinel Na2O 10% volume expansion by 7% Na2O
HiboniteNa2O 10% volume expansion by 5% Na2O
K2O 10% volume expansion by 6% K2OSumary of phase diagrams
49
Refractories for gasification process
Introduction to Refractories for Gasification Processes 50
Refractories for gasification process
Wear mechanisms of refractories in slagging gasifiers
J.P. Bennett, Refractory liner materials used in slagging gasifiers
Introduction to Refractories for Gasification Processes 51
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
Potential Refractory Problems in Coal Gasification
1alkali attack
2carbon monoxide disintegration
3silica volatilization
4steam-related reactions
5thermoelastic stresses
6erosion due to solid particulates
7corrosion and erosion due to molten coal slag and/ or iron
8iron oxide bursting
dry ashgasifiers
slagginggasifiers
Introduction to Refractories for Gasification Processes 52
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
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 < 1
or CaO/SiO2 > 1)
• major mechanisms of the corrosion process: dissolution, penetration and disruption, and
erosion
• higher velocity slag rate of corrosion ↑ dissolution and/or erosion ↑
Introduction to Refractories for Gasification Processes 53
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
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 > 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)
Introduction to Refractories for Gasification Processes 54
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
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
Introduction to Refractories for Gasification Processes 55
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
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
• Fe+2/Fe+3 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
Introduction to Refractories for Gasification Processes 56
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
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
Introduction to Refractories for Gasification Processes 57
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
Potential Refractory Problems in Coal Gasification
Carbon Monoxide Induced Disintegration
2 CO = C + CO2 (400-700°C, 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
Introduction to Refractories for Gasification Processes 58
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
Potential Refractory Problems in Coal Gasification
Reduction of Silica by H2
(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
SiO2 (s) +H2 → SiO(g) + H2O
Introduction to Refractories for Gasification Processes 59
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
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)
Introduction to Refractories for Gasification Processes 60
Refractories for gasification process
Refractory problems in coal gasification
C – Physical wear – “spalling”
J.P. Bennett, Refractory liner materials used in slagging gasifiers
Introduction to Refractories for Gasification Processes 61
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
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:
(1) minimizing the amount of linear shrinkage of the refractory
(2) continuous, slow heat-up rate
(3) elimination of long hold periods during the heating and cooldown
(4) maintaining the vessel shell temperature as close to ambient as possible
(5) using incompressible bond barriers
(6) using anchor spacings greater than 1.5 times the lining thickness
Introduction to Refractories for Gasification Processes 62
Refractories for gasification process
C.R. Kennedy and P.E. Schlett, Refractories for Coal Gasification
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
Introduction to Refractories for Gasification Processes 63
Refractories for gasification process
Corrosion Mechanisms
dissolution formation of an intermediate compound
solid solution
Kwong, et al., Wear Mechanisms of Chromia Refractories in Slagging Gasifiers
Introduction to Refractories for Gasification Processes 64
Refractories for gasification process
Kwong, et al., Wear Mechanisms of Chromia Refractories in Slagging Gasifiers
• oxygen partial pressure in a gasifier range from 10-7 to 10-9
• oxygen potential affects:
(1) valence state of transition oxides such as iron and vanadium oxides
(2) oxide basicity
(3) basicity of slags formed from iron and vanadium oxides
(4) melting point of the slags
oxygen potential influences slag – refractory reactions and the compounds
formed
Introduction to Refractories for Gasification Processes 65
Refractories for gasification process
Thermodynamic calculations - HSC Chemistry®
Kwong, et al., Wear Mechanisms of Chromia Refractories in Slagging Gasifiers
• V3O5 should be stable phase in
gasifiers environments
• FeO with some Fe3O4 may be
stable phase formed at oxygen
partial pressure of 10-7 to 10-9
Introduction to Refractories for Gasification Processes 66
Refractories for gasification process
Material development from the 1970’s until today
R. Dürrfeld, Refractories in Coal Gasification Plants
Introduction to Refractories for Gasification Processes 67
Refractories for gasification process
Evaluated materials in the 1970’s and 1980’s
•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
Introduction to Refractories for Gasification Processes 68
Refractories for gasification process
today’s 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 progressJ.P. Bennett, Low chrome/ chrome free refractories for slagging gasifiersM. Müller et al., Corrosion behaviour of chromium-free ceramics for liquid slag removal in pressurized pulverized coal combustion
69
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