Problems with Deposits

Transcription

1 Recovery Boiler Deposit Formation and Plugging Prevention Honghi Tran Pulp & Paper Centre University of Toronto Toronto, Canada Tappi Kraft Recovery Course St. Petersburg, Florida, January 7-10, 2008 Problems with Deposits Reduce boiler thermal efficiency Create a corrosive environment at tube surfaces Lead to unscheduled boiler shutdowns for deposit removal or tube repair 1

2 Presentation Outline Entrained Particles Deposit Properties Plugging in Different Regions Deposit Cleaning Plugging Prevention Entrained Particles Deposits Particles Dust Black Liquor 2

3 Particle Types Fume 0.5 μm (0.1-1 μm) ISP 20 μm (1-100 μm) Carryover 500 μm (100 μm - 3 mm) Deposits on Air Cooled Probe ISP Carryover Fume 3

4 Superheater Economizer Boiler Bank Presentation Outline Entrained Particles Deposit Properties Plugging in Different Regions Deposit Cleaning Plugging Prevention 4

5 Particle Composition (Wt%) K-salts 2.5% Other 1% NaCl 2% Other 0.2% K-salts 3.5% NaCl 4.0% Na 2 CO 3 6% Na 2 SO 4 45% Na 2 CO 3 50% Na 2 SO 4 87% Carryover Fume Deposit Composition Superheater Boiler Bank Na 2 CO 3 Na 2 SO 4 NaCl K-salts Other Economizer Precipitator Dust 5

6 Temperature ( o F) Appearance of a Deposit Cone Liquid Phase 0% ~1% First melting 15% Sticky 100% 70% Complete melting Flowing Temperature ( o C) Melting Temperatures First melting temperature is important for corrosion Sticky and flowing (slagging) temperatures are important for fouling/plugging These temperatures are related to liquid content, which is a function of: deposit composition (mainly chloride) temperature 6

7 Deposit Sticky Temperature Temperature ( o C) mole% K/(Na+K) mole% Cl/(Na+K) Chloride 0 mole% Cl/(Na+K)

8 Deposit Sticky Temperature Temperature ( o C) mole% K/(Na+K) mole% Cl/(Na+K) Particle Sintering Material transfer Surface Diffusion Bulk Diffusion Temperature 8

9 Dust Sintering Test (1 hour) Before After 380 o C (720 o F) Vol. Shrinkage 18% After 480 o C (900 o F) Vol. Shrinkage 58% Compressive Strength Comparison Glass Fired brick Cement Sintered dust Insulating brick Unsintered dust MPa <2 9

10 Presentation Outline Entrained Particles Deposit Properties Plugging in Different Regions Deposit Cleaning Plugging Prevention Plugging at the Superheater Inlet Superheater Screen Screen 10

11 Deposit Accumulation Pattern Carryover Superheater Screen Furnace Wall Plugging in the Superheater Caused by carryover deposition High gas temperature Large amount of sticky carryover Not by fume deposition Small amount of fume Deposits 11

12 Plugging at the GB Inlet Plugging at the GB inlet Narrow tube spacing (2 ) Carryover deposition upstream (superheater) High gas temperature Sticky carryover Fume deposition and sintering High gas temperature Large amount of fume Deposits 12

13 Cause of Plugging at the GB Inlet Clean Fouled Plugged Massive deposit buildup zone Time Plugging in the Generating Bank Fume deposition and sintering High gas temperature Large amount of fume Inadequate sootblowing Deposits Deposits 13

14 Temperature across a GB Tube and Deposit Layer 550 o C Steam 6200 KPa (900 psi) Tube o C (Hard) Deposit 300 o C (Soft) Flue Gas Deposits sinter rapidly if they are: jammed between tubes on stabilizing bars and baffles on sealed tubes (no steam inside) on mud drum (for bi-drum boilers) 14

15 Plugging in the Generating Bank Plugging in the Generating Bank of a Single Drum Boiler Flue Gas 15

16 Plugging in the Economizer Fume deposition and sintering Sticky fume (acidic sulphate formation) Baffle Deposits Na 2 SO 4 + SO 2 +½ O 2 +H 2 O = 2 NaHSO 4 Low bed temperature High sulphidity Inadequate sootblowing Presentation Outline Entrained Particles Deposit Properties Plugging in Different Regions Deposit Cleaning Plugging Prevention 16

17 Sootblower Lance Nozzle Throat Jet PIP 350 psig steam 60 to 100 sootblowers per boiler Consume 3 to 12% total boiler stream production Lance Tube Factors Affecting Sootblowing Efficiency - Jet Peak Impact Pressure (PIP) Nozzle design Steam flow rate and pressure Distance to deposit Blowing angle Blowing sequence and frequency 17

18 Factors Affecting Sootblowing Efficiency - Deposit Deposit adhesion strength Deposit composition (Cl, K) Deposit temperature Deposit size and thickness Deposit and tube temperature Tube Temperature and PIP Requirement Superheater o C: HIGH ( o F) Generating Bank o C: MEDIUM ( o F) Economizer o C: LOW ( o F) 18

19 Presentation Outline Entrained Particles Deposit Properties Plugging in Different Regions Deposit Cleaning Plugging Prevention Plugging due to Sticky Carryover Main Causes Excessive carryover High flue gas temperature Preventive Measures Minimizing carryover quantity Reducing carryover stickiness Lowering flue gas temperatures Increasing sootblowing efficiency 19

20 Plugging due to Fume Sintering Main Causes Excessive fume High flue gas temperature Ineffective sootblowing Preventive Measures Minimizing fume quantity (low bed temperature, coarse spray) Lowering flue gas temperatures Increasing sootblowing efficiency Plugging due to Sticky Fume Main Causes Low carbonate content in the fume High SO 2 /SO 3 conc. in the flue gas Ineffective sootblowing Preventive Measures Increasing bed temperature Lowering liquor sulphidity Increasing sootblowing efficiency 20

21 Summary Deposit formation and characteristics vary greatly with location within a boiler, and differ from boiler to boiler. It is important to identify the exact cause in order to devise appropriate solutions 21