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?

3

The Deposition Probe

EKV

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

9

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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Acknowledgments

• Metsa Botnia/Joutseno• The SEM photographs were prepared by

Dr. David Rothbard.• The first melting temperatures were

measured by Dr. Yulin Deng.• Anna Iisa assisted with analysis of the

structural connections.