TESTING PROCEDURES FOR CERAMIC REFRACTORY
MATERIALS IN BOTTOM GRID OF BIOMASS/WASTE-FIRED
CFBS
KAROL NICIA1*, MIKKO HUPA2, LEENA HUPA2, EDGARDO CODA ZABETTA1
1
2
Foster Wheeler Energia Oy, Varkaus, Finland
Process Chemistry Centre, Åbo Akademi University, Turku, Finland
*)phone: +48880631021, email: karol.nicia@gmail.com
ABSTRACT
Refractory ceramic materials are used mainly to limit heat transfer and to protect
sensitive components against aggressive environments in combustion devices.
However, in some cases a refractory material, used for example to protect the bottom
grid of biomass and waste fired CFBs, have failed at conditions where the refractory
material should be durable. In this work we have studied whether the observed changes
in the performance of the refractory materials depends on their corrosion due to
presence of alkali salts in the combustion devices at high temperatures. At first,
laboratory analyses and suitable procedures for indentifying the observed
underperformance of pre-cast refractory bricks were selected. Then, selected test
methods were utilized to evaluate the performance of the pre-cast bricks in laboratory
conditions corresponding to typical conditions in CFBs. Finally, observations from the
laboratory scale measurements of the most representative from seven different pre-cast
materials were compared with samples from boilers.
The mineralogical compositions of the refractories were analyzed using
Scanning Electron Microscope with Energy Dispersive X-Ray Analysis (SEM-EDXA)
and X-Ray powder Diffraction (XRD). For some surfaces also a topographic analysis
with spinning disc Confocal Optical Microscope (COM) was utilized. The performance
of each refractory was studied by plates (20x20x6mm) with 0.25g of an alkali salt,
either pure K2CO3 or a 90/10-mol% mixture of K2CO3/KCl put on the middle of the
plates. The plates with the salt were heat-treated in an electric laboratory furnace at two
temperatures, 500 and 700ºC for seven days. After the heat-treatment, changes in the
chemical composition of the surface and in the cross-section of the samples were
analyzed using SEM-EDX. For some samples, changes in the surface topography were
studied using COM. The SEM-EDX results were compared with the analyses of
samples from boilers.
The SEM-EDX analyses suggested that the observed weakening of the
refractories can at least partly be explained by refractory corrosion. In all materials
tested, some degree of penetration of potassium was observed. The penetration was
found to take place via the matrix phase, preferably through SiO2 in the refractory.
Thus, the results indicated that the laboratory testing method and the equipment used to
analyze the samples can be utilized to establish the performance and the causes of
corrosion of ceramic refractory materials in corrosive alkali salt containing
environments such as those in CFBs.
1
INTRODUCTION
Refractory materials are important components of nearly all combustion
devices, used mainly to limit the transfer of heat from the process and to protect more
sensitive components from the aggressive combustion environment. For most
applications refractory materials are well developed and are of no concern. However,
for fuels like biomass, wastes, demolition wood - with their impurities, large inert
fractions (stones or metallic debris) and high alkali content - better refractories may be
needed to address the demanding corrosion-erosion conditions.
As a concrete case, refractory materials used to protect the bottom grid of
biomass- and waste-fired circulating fluidized bed combustors (CFBs) have been known
to perform occasionally below expectation. In such applications the refractory material
consisted of pre-cast bricks of different materials and fabrications. The performance of
different bricks had been assessed based on samples from combustors. However, the
actual causes for the occasional underperformance of bricks could not be identified with
common analytical procedures. The purpose of this work was to:
select laboratory analyses and define suitable procedures to identify the causes for
the occasional underperformance of pre-cast bricks,
develop a test method for the evaluation of pre-cast bricks in laboratory prior their
utilization in real combustors, and
conduct a first screening on seven selected pre-cast materials, including
comparisons of laboratory samples with samples available from real boilers.
2
LABORATORY TESTS
In this work we have studied whether the observed changes in the performance
of the refractory materials depends on their corrosion due to presence of alkali salts in
the combustion devices at high temperatures. At first, laboratory analyses and suitable
procedures for indentifying the observed underperformance of pre-cast refractory bricks
were selected. Then, selected test methods were utilized to evaluate the performance of
the pre-cast bricks in laboratory conditions corresponding to typical conditions in CFBs.
Finally, observations from the laboratory scale measurements of the most representative
from seven different pre-cast materials were compared with samples from boilers.
Seven different materials (coded from “A” to “G”) was include to the experiment. The
experiment carried out of corrosion exposure at two different temperatures - to simulate
boiler conditions (500ºC and 700ºC) - with two different aggressive salts: K2CO3 and
90/10-mol % K2CO3/KCl. Exposure time was 7 days. The performance of each
refractory was studied by plates (20x20x6mm) with 0.25g of an alkali salt, either pure
K2CO3 or a 90/10-mol% mixture of K2CO3/KCl put on the middle of the plates. Also
whole bricks were investigated. Exact tests plan is showed in table 1.
From all materials four, most representative refractory types were analyzed
under the SEM/EDX and XRD. Also topographic appearance of samples after the tests
with corrosive salt mixtures inside the laboratory oven at 500 and 700 °C were analyzed
with the COM microscopy. All results from exposure, like also original material
composition, were compared with XRF test and private laboratory analysis. Moreover
materials extend from real boilers were analyzed under the SEM/EDX and results were
compared with others. In the end of the experiment erosion test were performed with
prior exposed refractory bricks and possible impact of corrosion for abrasion resistance
in the refractory material was checked.
Tab. 1. Scheme of campaigns in the laboratory furnace
Test number
Salt
Temperature [ C]
Exposure
time
Material tested
[h]
1
500
168
2x{A, B C, D}
coupons
A, C
bricks
2x{A, B, C, D, E, F}
coupons
A, C
bricks
2x{A, B, C, D, E, F}
coupons
A, C, E, F
bricks
K2CO3
2
700
3
500
168
168
K2CO3/KCl
4
700
168
2x{A, B, C, D, E, F, G} coupons
A, C, E, F, G
3
bricks
RESULTS
All seven unexposed materials (named thereafter alphabetically from A to G)
were analyzed by SEM-EDX and XRD. Selected backscattered SEM images and EDX
map analyses of materials B, C, F and G.
3.1
Original material
Table 2 summarizes and compares the composition of original (untreated)
materials as declared in the commercial certificates, as analyzed in earlier works, and as
analyzed during this work.
From table 2 is visible, that all material types used for testing can be in general
divided into alumina and silicon carbide based materials. High alumina materials
consist mainly on mullite, bauxite, corundum and silica (often as quartz or cristobalites).
Table 2. Comparison of refractory material compositions as by material certificates and SEM, XRF and
XRD analyses.
Type of material
A
B
C
D
E
F
G
Low cement;
Low
cement;
Ultra low
cement;
material
based on
silicon
carbide
aggregates
with
ceramic
and
organic
hard-bond
Low
cement;
Low cement;
high alumina
material
Low
cement;
low iron;
based on
silicon
carbide
material
Low
cement;
high
alumina
material
Type of analysis
high calcined
bauxite
Material certificate
Unspecified
analysis
method
high
calcined
bauxite
high
alumina
material
Al2O3 - 83
SiC -87
Al2O3 - 84
Al2O3 - 43.1
SiC -76.8
Al2O3 - 81.7
SiO2 - 11
Al2O3< 6
SiO2 - 8
SiO2 - 51.7
Al2O3-14.7
SiO2 - 12.2
Fe2O3 -1.5
SiO2 < 3
Fe2O3 - 0.8
Fe2O3 - 1.2
SiO2 - 5
TiO2 - 2.5
CaO
CaO - 2.3
TiO2 - 1.0
CaO - 1.3
Fe2O3 - 0.8
XRF
-(in wt-%)
Fe2O3 – 0.8
Fe2O3
P2O5
Al2O3 ~69
XRF
Earlier analyses
--
SiO2 ~20
--
--
--
--
--
--
--
--
--
--
(in wt-%)
CaO ~ 3
Corundum,
XRD
--
Mullite,
Cristobalite
SiC ~55
Al2O3 ~77
Al2O3 ~70
SiO2 ~ 55
Al2O3 ~78
SiO2 ~36
SiO2 ~14
SiO2 ~23
SiC
SiO2 ~12.5
Si3N4~ 7
CaO ~ 4
CaO ~ 4
Al2O3~7
TiO2 ~ 3.5
Al2O3<2
TiO2 ~ 3
TiO2 ~ 2.5
CaO ~ 0.5
Fe2O3 ~ 2
CaO ~0.5
Fe2O3 ~ 2
Analyses in this work
Al2O3 ~ 63
~37
SiO2 ~ 21
SEM-EDX
Fe2O3 ~ 2
(in wt-%)
CaO
~
TiO2 ~
--
2
1.5
CaO ~ 2
Aluminum
Silicon
carbide,
Aluminum
oxide,
Silicon
oxide,
Silicon
carbide,
Quartz,
oxide,
XRD
Mullite,
(700 C;
--
-Cristobalite
Corundum,
--
Haxonite,
Quartz,
Dolomite,
Almandine
Calcium
aluminum
oxide
7 days)
Quartz
3.2
Alkali exposed material
SEM imaging and XRD analyses were conducted on selected samples of four
different materials after exposure to corrosive material in the laboratory furnace.
3.2.1 SEM results
Key results from materials F is illustrated in Figure 1. The black area on the
backscattered SEM images shows the epoxy resin used to mold the sample.
Based on Figure 1 and other material results (Nicia, 2008), the following observations
can be made:
Penetration of corrosive salts was detected in every sample, as highlighted by the
XRD elemental maps of potassium (K).
Grains seemed unaffected by the corrosive salts.
Pure aggregates of silicon carbide (SiC) or alumina (Al2O3) showed higher
resistance to potassium penetration (K) than aggregates with impurities (Nicia,
2008).
Potassium penetration occurred in the matrix, preferably but not only along grain
boundaries.
Potassium penetration was favored through matrix with elevated SiO2 (e.g. Fig. 1),
thus suggesting (consistently with theory) that quartz and cristobalites would be less
resistant to chemical attack that other compounds present in refractories
Potassium penetration also occurred through mullite (Al6Si2O13), though less than
through SiO2 (Nicia, 2008).
F (90/10-mol % K2CO3/KCl; 700ºC; 7 days)
Al
O
Ca
Si
K
Figure 1. SEM backscattered images and corresponding EDX map analyses of ‘F’ material after exposure
to corrosive salts in laboratory heat furnace. Parameters declared nearby the figures: salt
composition, furnace temperature, and exposure time.
3.2.2 EDX results
EDX line analyses were performed on all investigated materials.
Typical line analyses of a selected sample are shown in Figure 2, where the line
selected for analyses is indicated on the backscattered image of the sample.
In this example, the penetration of potassium (K) correlates well with the silica
(Nicia, 2008) in the sample matrix, and deviates after the 400µm mark (Fig. 2), where
the silicon carbide grain presents.
C (90/10-mol % K2CO3/KCl; 700ºC; 7 days)
Figure 2. SEM backscattered image and EDX line analysis of material ‘C’ after 7 days exposure to 90/10mol % K2CO3/KCl at 700ºC
3.2.3 Comparisons
All data of four (B, C, F, G) analyzed materials were taken and test runs were
compared. Below (Tab. 3) comparisons of four test runs from C material is shown and
present in the Fig. 3.
Table 3. Estimated penetration of potassium in material ‘C’ after four different tests. Comparison of
estimates from EDX elemental maps and EDX line analyses.
Test number
1. K2CO3;
500ºC
Elemental maps
~ 200
3. K2CO3+
4. K2CO3+
90/10-mol %
90/10-mol %
KCl; 500ºC
KCl; 700ºC
~300
~750
~300
~80
~600
~400
2. K2CO3;
700ºC
Potassium
200
penetration depth
m)
Line analyses
(K presence
to 800)
Potassium penetration depth
800
700
Elemental
analysis
Line analysis
600
500
Depth [µm] 400
300
200
100
0
1
2
3
Test number
4
Figure 3. Estimated penetration of potassium in material ‘C’ after four different tests. Comparison of
estimates from EDX elemental maps and EDX line analyses, as by Tab.3.
Both estimates from elemental mapping and line analyses agree on that the
penetration of potassium was deeper in the tests 3 and 4, i.e. when the salt mix in the
samples included potassium as a chloride (KCl). Therefore, it is reasonable to speculate
that chlorides are appreciably more aggressive than carbonates, as expected.
Unexpectedly, however, both estimates also agree on that in the presence of chlorides
the penetration of potassium was deeper at 500ºC (test 3) than at 700ºC (test 4). One
explanation could be that at higher temperatures chlorine would volatilize more
effectively from the sample. However, other materials showed a more intuitive increase
of potassium penetration as a direct function of temperature (Nicia, 2008). Therefore,
the surprising behavior shown in Figure 3 is likely an artifact from the sample
preparation, which was improved in later tests.
Table 4. Estimated penetration of potassium in four materials exposed to salt mix 90/10-mol %
K2CO3/KCl at 700ºC. Comparison of estimates from EDX elemental maps and EDX line analyses.
Material type
Potassium
B
C
F
G
Elemental maps
~400
~300
~1000
~200
Line analyses
~330
~430
~1500
---
penetration depth
m)
Potassium penetration depth in four materials from test 4
1600
1400
Elemental
analysis
Line analys is
1200
1000
Depth
[µm]
800
600
400
200
0
B
C
F
Material type
G
Figure 4. Estimated penetration of potassium in four materials exposed to salt mix 90/10-mol %
K2CO3/KCl at 700ºC. Comparison of estimates from EDX elemental maps and EDX line
analyses, as by Tab.4.
Both estimates from elemental mapping and line analyses agree on that the
penetration of potassium was significantly deeper in material “F” than in any other
tested material. Both analyses also agree on than the penetration in materials “B” and
“C” is similar. This is an unexpected result, because the two materials are know to have
different performance in real combustors. Thus, the technique would need further
improvements to achieve superior discrimination capabilities. Furthermore, it appears
that material “G” could perform better than any other material. However, this result
must be still validated with samples from boilers.
4
CONCLUSIONS
Results from experiment show that is possible to induct corrosion in the
refractory material and see changes due to chemical attack in laboratory conditions. In
all of investigated materials corrosion was observed during experiment study, as
penetration of alkali (potassium in this study) inside the refractory material.
In all materials tested, some degree of penetration of potassium was observed.
The penetration was found to take place via the matrix phase, preferably through SiO2
in the refractory. Thus, the results indicated that the laboratory testing method and the
equipment used to analyze the samples can be utilized to establish the performance and
the causes of corrosion of ceramic refractory materials in corrosive alkali salt containing
environments such as those in CFBs.
Results proved that corrosion impact in the refractory depend mainly on
material composition and laboratory conditions, in which experiment was carried out.
Laboratory condition which have an impact of potassium penetration, were mainly
temperature and salt composition.
Corrosion inside the refractory pre-casts carried out inside the material matrix,
while aggregates were not attack by the salt. From different matrix composition the
most suffered for corrosion consist mainly of silica (SiO2) thus suggesting (consistently
with theory) that quartz and cristobalites would be less resistant to chemical attack that
other compounds present in refractories. Potassium penetration also occurred through
mullite (Al6Si2O13), though less than through SiO2
Opposite to matrix behavior, material aggregates (grains) were corrosion
resistant and no penetration was observed inside the grains. Pure aggregates of silicon
carbide (SiC) or alumina (Al2O3) showed higher resistance to potassium penetration (K)
than aggregates with impurities
Analysis of used material from real CFB boiler show biggest impact of calcium
and sulphure than potassium, what should be reconsider for a salt composition for future
corrosion studies.
In the end of the experiment erosion test were done with ‘A’ and ‘C’ material in
the industry laboratory. Both materials appeared to be extremely resistant against
erosion, and considering the accuracy of the method such resistance did not changed
significantly by either aggressive salts or temperature. Thus, during this study was
observed, no significant effect has corrosion attack to abrasion resistance.
Results from the same material after different condition exposure show that
exposure at 700ºC during 7 days with the 90/10-mol % K2CO3/KCl present most
valuable test results.
Further work is advisable in the following areas:
investigate further the role of calcium and sulfur penetrating the refractory material,
improve the laboratory procedures and test methods towards a superior capability to
discriminate and rank bricks according to their tested performance,
find conclusive proves that chemical aggression and erosion by hot bed material in
CFBs can be decoupled without artifacts; or alternatively develop a simple,
compact, and easily reproducible test rig that combines all these effects, and
test Electron Spectroscopy for Chemical Analysis (ESCA) and Time-of-Flight
Secondary Ion Mass Spectroscopy (ToF-SIMS) as alternative or complementary
techniques to SEM-EDX and XRD
REFERENCES
Nicia K., 2008, “Testing procedures for refractory material in bottom grid of
biomass/waste-fired CFBs” MSc Thesis, Varkaus, Finland