1
Sintering and Structure Development in Alkali Metal Salt
Deposits Formed in a Kraft Recovery Boiler
Jim FrederickChalmers University of Technology
Esa VakkilainenJaakko Poyry OY
Different Sized Particles that Plug Gas Passages in Recovery Boilers
50µm
1 µm1 µm
Sub-micron condensation aerosols (fume) Larger particles
2
Sintering and Hardening of Recovery Boiler Deposits
• Fume particles are sub-micron aerosols
• They begin to sinter at 300 to 350oC
• The rate of sintering increases rapidly with temperature
Tim
e
5 µm
5 µm
5 µmTechakijkajorn et al., 1999
Our Question
How do deposits sinter and harden in recovery boilers?
4
Dust Collected at ESP Entrance
1 µm
0
0.1
0.2
0.3
0.2 0.4 0.6 0.8 1.00
Particle size range, µm
Frac
tion
of p
artic
les
Median diameter = 0.40 µmMean diameter = 0.43 µm
Location of Probe Insertion Points
5
Time: 2 hours
Deposit from Superheater (A)Thermal load: 79%
Time: 10 min2 µm10 µm
Larger spherical particles at the surface of deposits from Superheater Location B
Thermal load: 122%Thermal load: 79%
6
10 µm
Cross-Sections of DepositsBoiler Bank (C)Load: 122%Time: 30 min
Upstreamsootblowers off
10 µm
Upstream sootblowerfired 1 min beforeprobe was removed
10 µm
Superheater (B)
Coarser structure
10 µm
Boiler Bank (C)
Finer structure
Cross-Sections of DepositsLoad: 122%; Time: 15 min
7
Sections of a Deposit fromthe Superheater (B)
Thermal load: 122%Outer surface(<1 minute)
10 µm
Mid-plane(5-10 minutes)
10 µm
Structure of Deposits fromBoiler Bank (C)
Thermal load: 122%, 10 min collection time
1 µm
1 µm
8
Structure of Deposits from Sinquefield/Sandia Study
1 µm
500 µm
Mid-boiler bank(FMT = 536oC)
Sandia MFR probe sample, (FMT ~ 536oC)
Structure of Joutsenodeposits are similar to Sandia probe deposits.
1 µm
1 µm
500 µm
500 µm
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The Sintering Mechanism(Baxter, circa 1998)
Deposit Surface
Boiler Tube
1 µm
Time
Analysis of Contacts
ParticlesNecks
Branch
Grain
10 µm
ParticlesNecks
Branch
Grain
10 µm10 µm
0
0.1
0.2
0.3
0.4
0.5
1 2 3 4 5 6Number of contacts (Nc)
Dis
trib
utio
n (fr
actio
n)
2-D Count3-D Estimate
Analysis for
0
0.1
0.2
0.3
0.4
0.5
1 2 3 4 5 6Number of contacts
Dis
trib
utio
n (fr
actio
n)
2-D Count3-D Estimate2-D Count3-D Estimate
Analysis for 2-D: 2.2±1.03-D: 2.6 ±1.4
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Sintering depends upon the coordination number, Nc…
• Nc = number of contact points between particles• Initially, Nc depends upon
– Structure– Packing density– Polydispersity
• Nc usually changes during sintering• Initial Nc must be
• >2 to generate chains• At least 4 to develop system rigidity
Sintering rate depends uponfirst melting temperature
(Frederick, Lien, Tran, 2000)
First melting temperature, oC
Normalized sinteringrateconstant
0
500
1000
1500
2000
2500
520 530 540 550 560 570
350oC400oC450oC500oC550oC
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First melting temperature increases with deposition time
510
520
530
540
550
0 50 100 150
Time for deposit formation, min
First meltingtemperature
oC
A, 79%A, 122%B, 79%B, 122%C, 79%C, 122%D, 79%ESP
Carbonate content decreases with deposition time
0
4
8
12
16
20
0
4
8
12
16
20
0 20 40 60 80 100 120 1400 20 40 60 80 100 120 140
Deposit formation time, min
AB
BC C D
A
B
C
C
C
ESP Entrance (79%)
79% 122%
Thermal load79% 122%
Carbonatewt-%
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Conclusions
• Fume deposits form with a very open structure
• Grains grow as they sinter, but with little early densification of the deposit
• Change in deposit composition with time slowed densification
1 µm
Conclusions
• Larger, intermediate size particles probably fuse together via – sintering of the dendritic, submicron fume
particles that deposit on their surfaces, and – sintering of the agglomerates of the finer
particles in contact with them.
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