1 alkali corrosion of refractories in cement kilns

$: 1 Alkali Corrosion of Refractories in Cement Kilns$
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


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


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



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



8

Alkali Corrosion

Post mortem investigations

•Roof of kiln hood of the DOPOL-kiln:


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


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)


10

Alkali Corrosion


•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)


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


11

Alkali Corrosion


•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)


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


12

Alkali Corrosion


•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)


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



•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



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.


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.



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



Alkali Corrosion


• 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



Alkali Corrosion


• 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)



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.



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.


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


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.



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:


• similar behavior is due of the system KCl - MgO

MgO based refractory materialsare not alkali resistant because melt formationat 895 °C.



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


• 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.


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)


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.


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).


K2OMgOSiO2


• 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


Alkali Corrosion


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


• 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



Volume expansion of mullite and fireclay by reaction with K2O or Na2O

30

Alkali Corrosion


• 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



31

Alkali Corrosion


• 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)



32

Alkali Corrosion


• 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”




• 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.


Alkali Corrosion


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)


Industrial refractory brick from the wall of a bottom cyclone of cement kiln after 1 year usage.

A

B


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



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



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


•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)


Salt mixturesSM 1 K2SO4 / K2CO3

SM 2 K2SO4 / K2CO3 / KClSM 3 K2SO4 / K2CO3 / KCl / CaSO4



39

Alkali Corrosion




Zirconia mullite Z72Spinel AR 78Hibonite SLA-12Hibonite BoniteForsterite OlivinAluminium titanate


Zirconia 3Y-TZPZirconia 3,5Mg-PSZNa-aluminateSpinel MA 76

suitable materials

-aluminaBetacalutherm (dried, fired)





40

Alkali Corrosion




Spinel AR 78Forsterite Olivin

suitable materials

-aluminaBetacalutherm (dried, fired)Spinel MA 76






41

Alkali Corrosion


•Change of diameter after 1100 and 1300 °C, 5 and 50 hours hold time


Zirconia mullite Z72Spinel MA 76Spinel AR 78Hibonite SLA-12Hibonite BoniteForsterite OlivinAluminium titanate


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



42

Alkali Corrosion


•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.




43

Alkali Corrosion


•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.




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



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


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

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

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


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


FireclayNa2O

10% volume expansion by 16% Na2O


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


Wear mechanisms of refractories in slagging gasifiers

J.P. Bennett, Refractory liner materials used in slagging gasifiers



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





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 ↑





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)





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





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





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





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





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





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)



Refractory problems in coal gasification

C – Physical wear – “spalling”

J.P. Bennett, Refractory liner materials used in slagging gasifiers





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





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



Corrosion Mechanisms

dissolution formation of an intermediate compound

solid solution

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



Thermodynamic calculations - HSC Chemistry®


• 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



Material development from the 1970’s until today

R. Dürrfeld, Refractories in Coal Gasification Plants



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



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

Thank you for your attention!

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