Testing procedures for refractory
material in bottom grid of
biomass/waste--fired CFBs
biomass/waste
1Karol
Nicia, Edgardo Coda Zabetta
Nicia,
2Mikko
Hupa, Leena Hupa
1Foster
Wheeler Oy, Varkaus , Finland
2 Process Chemistry Centre, Åbo Akademi University
University,, Turku, Finland
Naantali, 2009
Refractories
Refractor
ies in CFB applications
main goals
goals of ceramic refractor
refractories
ies in furnaces:
cross-over duct
• to control the heat
alongside the boiler
separator
combustion chamber
• to protect the more
sensitive metal
components against
thermal, erosive
and corrosive degradation
loop-seal
CFB boiler design for difficult fuels
• waste
• biomass
• demolition wood
Waste fuel in the fuel feeder
bottom ash chutes
refractory pre-casts
flat air nozzles
• Hydro beam grate system
• ‘Slopping floor’
• Step-grid
Step-grid design
Refractory prepre-cast bricks observed to weaken in
boiler operation
Thermal wear
Mechanical wear
• erosion
• chipping
Chemical attack
• corrosion
Aim
to select and define suitable laboratory analysis methods
and procedures for identifying primary causes for
occasional underperformance of prepre-cast bricks
to develop a test method for the evaluation of prepre-cast
bricks in laboratory conditions prior to their utilization in
real combustors
to conduct a first screening of seven selected prepre-cast
materials
to compare samples from laboratory tests with samples
from boilers
Corrosion Testing – Steel Corrosion
Techique to study molten salt induced corrosion of alloys at high
temperatures
Salt compositions of interest within (Na,K)2(CO3,SO4,Cl2)
Analysis with
SEM/EDX
before heat
treatment
after heat
treatment
polished coss-section of
sample in epoxy
Corrosion / degradation mechanism of refractory
in boilers using fuels with difficult ashes?
Refractories in CFB boilers typically high content of Al2O3 or SiC
refractory crystalline phases bonded with small amounts of silicates
Selected commercial compositions:
•
5 with Al2O3 (60…80 wt%) + silicate matrix
•
2 with SiC (<80 wt%) + silicate matrix
Corrosion of refractory complex process:
• Material does not have uniform composition
• Material is porous, thus spalling and liquid penetration is possible
Experimental
corrosion tests
erosion
ero
sion test
• laboratory tests of 7 commercial
refractories (A…G) at 4 conditions
• 2 temperatures (500 & 700ºC)
• 1 test with two refractories
A and C
• temperature (~25ºC)
• one week heat treatment
• test duration 8 minutes
• 2 salts:
100 % K2CO3
90/10 mol% K2CO3/KCl mixture
Test number
Time [days]
Temp [ºC]
Salt
1
7
500
K2CO3
2
7
700
K2CO3
3
7
500
90/10-mol% K2CO3/KCl
4
7
700
90/10-mol% K2CO3/KCl
Analyse
Analys
es using
SEM-EDX
+
XRD
+
COM
• phase composition of all (7) refractories
• four refractories from laboratory tests (‘B’, ‘C’, ‘F’, ‘G’)
• 2 refractory materials removed from boilers (‘A’ & ‘C’)
Results and discussions
SEM-EDX of exposed material
Sample F (SiC refractory) after exposure to 90/10 K2CO3/KCl at 700ºC, 7 d
Al
Potassium
penetrates
the matrix
phase
Si
K
Ca
SiC particles
O
Ca-Al-silicate
matrix
SEM-EDX line analysis of sample C
(Al2O3 –refractory)
after exposure to 90/10 K2CO3/KCl,
700ºC & 7 d
K
Si
Potassium diffuses along the silicate phase
Refractory C (Al2O3) from a full-scale CFB boiler firing
biomass and waste fuel
K
Si
Ca
Si
• increased K, Ca and S
contents within silicate
phase
• accumulation of K and S
at Alumina crystal
boundary
S
Si
Impact of test conditions on potassium diffusion
depth in Al2O3 –refractory (sample C)
Corrosion depth
800
700
600
Elemental
analysis
500
Corrosion depth
m]
400
Line analysis
300
200
100
0
K2CO3,
500C
K2CO3,
700C
K2CO3/KCl,
500C
K2CO3/KCl,
700C
• potassium diffusion depth higher for mixtures with chloride
• deeper diffusion at lower temperature (Cl evaporation?)
Corrosion depth
of different tested materials
Surface topography with confocal optical microscopy, COM
Refractory B (Al2O3), K2CO3/KCl, 700°C, 7 d
1544 x 1600 µm
B4
400
300
unexposed
surface
depth [µm]
200
100
0
-100 0
1000
2000
3000
4000
-200
-300
-400
lenght [µm]
5000
6000
7000
8000
Refractory erosion, sample C (SiC) after laboratory
corrosion and sand blasting
erosion
(8 min)
K2CO3, 500°C
K2CO3, 700°C
erosion
(8 min)
K2CO3/KCl, 500°C
K2CO3/KCl, 700°C
Salt exposure does not clearly affect abrasion resistance
CONCLUSIONS::
CONCLUSIONS
the laboratory procedure used for alloy corrosion can be
applied to study molten salt attack on refractory
molten salt attack was verified by potassium diffusion depth
in the sample crosscross-section
potassium diffusion mainly via the matrix phase
salt composition and furnace temperature affected the
diffusion depth
samples from CFB boilers indicated increased concentrations
of Ca, S and (K), thus suggesting that also other salt mixtures
should be considered in further studies