Electrostatic Precipitators:

Transcription

1 Electrostatic Precipitators: Principles and Optimization Roger Lawton Southern Environmental Inc. TAPPI Kraft Recovery Course St. Petersburg, Florida, January 10, 2019

2 Overview Principles: How do they work Key Components Design Criteria Optimization: Latest Equipment Design Advances Troubleshooting Note: ESP stands for Electrostatic Precipitators in this presentation; it is NOT Emergency Shutdown Procedure! 2

3 Where in Pulp Mills Recovery Boilers = Saltcake Lime Kilns = Lime Dust Power Boilers = Flyash ESPs are important for reliable and cost effective pulp production, as well as environmental compliance! 3

4 FD Fan Recovery Boiler ESP (Courtesy of Andritz Inc.) ID Fan ESP 4

5 ESPs -How Does Process Work? Corona field developed between Discharge Electrodes (-) and Collecting Plates (grounded) Intense field ionizes molecules in dirty gas stream Ions attach to and negatively charge dust particles Dust (-) in electrostatic field attached to Collecting Plates (ground) Collected particles are cleaned off the Discharge Electrodes & Collecting Plates by Rapping & fall to bottom of chamber. 5

6 ESPs -How Do They Work? Electric Power Dust Hopper Dirty Gas In Electric Power In Clean Gas Out Dust Out in Hopper or Conveyor (Recovery boilers) 6

7 ESPs -How Do They Work? Establish Field /Current Charge Migrate Collect Rap 7

8 ESP Internal Components Rigid Discharge Electrodes (RDE) Collecting Plates 8

9 ESP External Components Power Supply Purge Air System Rappers Support Insulator 9

10 ESP Performance Calculation Modified Deutsch-Anderson Equation = where: W k = Modified migration velocity (ft/sec or m/sec) A/Q = Specific collection area or SCA (ft 2 /1000 acfmor m 2 sec/m 3 ) n = Empirically derived modifier size distribution (~0.5) 10

11 Particulate Migration Velocity Particle migration velocity is a function of the Power Density, Pc/A where: Pc = Average power input (Watts) A = ESP collecting area (ft 2 ) Pc is a function of voltage and current 11

12 ESP Performance Calculation Particulate Matter Collection Efficiency is a function of Specific Collection Area and migration velocity Higher migration velocity: Lower SCA needed for same efficiency -reduced capital cost Lower migration velocity: Higher SCA needed for same efficiency -increased capital cost 12

13 Discharge Electrodes Characteristics ESP Key Components Achieves optimum current versus applied voltage Maximizes field strength (kv/inch or kv/cm) Minimizes corona onset voltage (kv) Achieves optimum current distribution 13

14 Key Component -Collecting Plates Spacing: The distance between two adjacent plates The greater the distance, the higher the applied voltage (field strength) Latest trend uses 16 inch (406 mm) spacing 14

15 Power Supplies ESP Key Components Standard 60 Hz T-R Sets High Frequency Sets 15

16 Gas Flow Distribution in ESP Inlet Nozzle Collection Zone Outlet Nozzle Variable Porosity Plate 16

17 Physical Modeling Front View Scale 1 to 12 Institute Clean Air Companies ICAC -EP 7 Criteria for Flow distribution at Inlet and Outlet Smoke test 17

18 CFD Modeling Axial Velocity (ft/sec) 0 Object is to reduce flue gas velocity and make flow more uniform 18

19 Inlet and Outlet Transitions 19

20 Recovery Boiler Saltcake Typical salt cake inlet loading = 2 to 5 grains/acf(4.5 to 11.5 grams/m 3 ) Salt cake particle size = 0.5 to 6.0 µ The higher the black liquor solids firing percent, the higher the salt cake inlet loading and the finer the particle size Always a significant sub-micron fraction, always severe space charge in the ESP s first field(s) Salt Cake consistency from fine & dry to wet & sticky 20

21 Recovery Boiler ESP Win-Win Saltcakecollected gets recycled directly back to black liquor. Huge cost savings for chemicals Meet Environmental goals of Mill 21

22 Recent ESP Experience -Conditions OPERATING CONDITIONS DESIGN COMPLY TEST TEST CONDITION Out-of-service MF nos. None None 1, #4 No. Of electrical fields in service Inlet gas temperature (deg F) Flue gas flow rate(acfm) 435, , ,209 Collecting plate spacing (inches) Type energization SMPS SMPS SMPS Flue gas moisture (% by vol.) Flue gas O 2 (% dry) Inlet loading (gn/dscf@ 8% O2) ESP inlet loading (lb/hr) *** 8,540 10,047 BL liquor firing rate (MM lb/day) BL liquor solids (%)

23 Recent ESP Experience -Results PERFORMANCE TEST RESULTS DESIGN COMPLY TEST TEST CONDITION Stack emissions O2) Stack emissions (lb/hr) *** Stack emissions (kg/hr) *** ESP collection efficiency (%)

24 Plate spacing: Advanced ESP Design Narrow Plate Spacing (NPS) = 9 to 12 in. (230 to 305 mm) Wide Plate Spacing (WPS) = 15 to 17 in. (380 to 430 mm) Advantages of WPS: Stronger corona voltage fields: ~10 kv/inch (~3.9 kv/cm) from DE to plate with T-R sets ~12kv/inch (~4.7 kv/cm) from DE to plate with SMPS sets Superior migration and collection of fine particles 24

25 Advanced ESP Design Disadvantage of WPS: When compared to NPS, more particles pass by each Discharge Electrode, which may cause particle charging issues Conclusions: Narrow Plate Spacing = Ideal for 1 st Field of a Recovery Boiler Precipitator Wide Plate Spacing = Ideal for All Downstream Fields of a Recovery Boiler Precipitator 25

26 Lime Kiln Electrostatic Precipitator Collects Lime dust from Off-gases of Lime Kiln Cleaned Off-gases emitted to Stack thru Fan Collected Lime dust recycled to process 26

27 Kiln ESP Advantages Lower Pressure Loss versus Cyclones or Baghouses Lower Operating Cost for fan horsepower Dry product versus wet VenturiScrubber Eliminates need for bag change-out Reliably achieves High Collection Efficiencies 27

28 Lime Dust Characteristics Typical Dust inlet loading = 8 to 30 grains/acf(18 to 70 grams/m 3 ). Higher than Recovery boiler Typical Dust particle size = 5.0 µ. Good size for ESP Favorable electrical resistivity range allows high corona power densities. Dust cohesiveness in ideal range, reduces need for excessive rapping, which minimizes reentrainment Composition is consistent, due to stable operating nature of Kilns. 28

29 Factors Affecting Performance -All ESP s Dust Concentration Particle Size Type of Discharge Electrodes (V-I) Collecting Plate Area ESP Gas Velocity and Flow Distribution Air In-Leakage Gas Sneakage Power Supplies Rapping Ratio Internal Clearances Changes in Operating Variables 29

30 Gas Flow Distribution Minimize gas flow sneakage SIDE VIEW Minimize secondary flow in hopper END VIEW 30

31 ESP Problems -What to look for Check Operating data, compare to Normal Opacity/Continuous Emissions Monitoring System data Boiler Process data Upset condition T/R set readings for Voltage & Current Rapping performance 31

32 Typical Failures in RB ESP s Ash Deposits: Duct, Inlet, & Conveyor areas Tied to good gas flow distribution Moisture makes Saltcakesticky Insulator Failure: Quickest way to lose an ESP Separates charged DE s from grounded CE s & casing Locations: T/R bus duct, penthouse, lower frames Re-entrainment & In Leakage: Air from drag chain level, Access doors, ash valves Ineffective Rapping Air from Boiler and duct leaks 32

33 Mechanical Failures Discharge Electrode breaking: T/R power levels reduce, can swing in gas stream. Limited usable life for wire type DE s, before breaking. Snowballs on pipe & spike type. Collecting Electrode failure: Shake frame, listen for tapping metal. Structural failures: Look for alignment and out of level. Drag Chain SaltcakeRemoval Conveyors: Bearings, links, chain off sprocket. 33

34 Salt Cake Buildup at Inlet Plenum Problem is saltcakeaccumulating in the duct before the ESP inlet. If Flue gas velocity in ductwork is too low, saltcake can drop out. Or air in-leakage could allow moisture into the Flue gas stream to make the saltcake sticky. To increase velocity, baffles can be used in the top of the duct. 34

35 Washing Precipitators Difficult due to arrangement of plates & top frames Washing equipment not very effective For Recovery boilers, remaining wet salt cake can form Sulfuric acid and corrode internals Washing continues to be used on a routine basis 35

36 Conclusion Where are Electrostatic Precipitators: Recovery Boiler, Lime Kiln, & Power Boiler. How they work in Recovery boilers: Charge salt cake dust particles, collect on plates, rap to drop on conveyors, recycle salt cake. What can go wrong: Operational issues, Mechanical failures, troubleshooting & inspections. 36

37 Questions? Thank you. Southern Environmental, Inc. Pensacola, Florida - Columbus, Ohio