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Electrostatic Precipitators:
Principles and Optimization
TAPPI Kraft Recovery Course
St. Petersburg, Florida, January 10, 2019
Roger Lawton Southern Environmental Inc.
Overview
Principles:
� How do they work
� Key Components
� Design Criteria
Optimization:
� Latest Equipment
� Design Advances
� Troubleshooting
2
Note: “ESP” stands for Electrostatic Precipitators in this
presentation; it is NOT Emergency Shutdown Procedure!
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
Recovery Boiler ESP
4
ID Fan
(Courtesy of Andritz Inc.)
ESP
FD Fan
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
ESPs - How Do They Work?
6
� Dirty Gas In
� Electric Power In
� Clean Gas Out
� Dust Out in Hopper or Conveyor (Recovery boilers)
Electric Power
Dust
Hopper
ESPs - How Do They Work?
�Establish Field /Current
�Charge
�Migrate
�Collect
�Rap
7
ESP Internal Components
8
Rigid Discharge
Electrodes (RDE)
Collecting Plates
ESP External Components
9
Rappers
Purge Air
System
Support
Insulator
Power
Supply
ESP Performance Calculation
�Modified Deutsch-Anderson Equation
���������� = − ��� −�� ∗ �
�
�
10
where:
Wk = Modified migration velocity (ft/sec or m/sec)
A/Q = Specific collection area or SCA (ft2/1000 acfm or
m2 sec/m3)
n = Empirically derived modifier – size distribution (~0.5)
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 (ft2)
�Pc is a function of voltage and current
11
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
ESP Key Components
�Discharge Electrodes
�Characteristics
�Achieves optimum current versus
applied voltage
�Maximizes field strength (kV/inch or
kV/cm)
�Minimizes corona onset voltage (kV)
�Achieves optimum current
distribution
13
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
ESP Key Components
�Power Supplies
High Frequency Sets Standard 60 Hz T-R Sets15
Gas Flow Distribution in ESP
16
Inlet
Nozzle
Collection
Zone
Outlet
Nozzle
Variable
Porosity Plate
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
CFD Modeling
� Object is to reduce flue gas velocity and make flow more uniform18
0
1.6
3.2
4.8
6.4
8.0
Ax
ial
Ve
loci
ty (
ft/s
ec)
Inlet and Outlet Transitions
19
Recovery Boiler Saltcake
�Typical salt cake inlet loading = 2 to 5 grains/acf (4.5 to 11.5 grams/m3)
�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
Recovery Boiler ESP Win-Win
�Saltcake collected gets
recycled directly back to
black liquor.
�Huge cost savings for
chemicals
�Meet Environmental
goals of Mill
21
Recent ESP Experience - Conditions
22
OPERATING CONDITIONS DESIGN
COMPLY
TEST
TEST
CONDITION
Out-of-service MF nos. None None 1, #4
No. Of electrical fields in service 8 8 6
Inlet gas temperature (deg F) 421 481 486
Flue gas flow rate(acfm) 435,000 514,870 498,209
Collecting plate spacing (inches) 12 12 12
Type energization SMPS SMPS SMPS
Flue gas moisture (% by vol.) 22.5 18.4 18.9
Flue gas O2 (% dry) 4.0 7.1 7.0
Inlet loading (gn/dscf @ 8% O2) 8.7 4.4 5.0
ESP inlet loading (lb/hr) *** 8,540 10,047
BL liquor firing rate (MM lb/day) 3.75 3.94 3.92
BL liquor solids (%) 76.5 72 73
Recent ESP Experience - Results
23
PERFORMANCE TEST RESULTS DESIGN
COMPLY
TEST
TEST
CONDITION
Stack emissions (mg/dscm @8% O2) 60 6.8 33.4
Stack emissions (lb/hr) *** 6.2 30.0
Stack emissions (kg/hr) *** 2.8 13.6
ESP collection efficiency (%) 99.700 99.928 99.702
Advanced ESP Design
�Plate spacing:
�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
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 1st Field of a Recovery
Boiler Precipitator
�Wide Plate Spacing = Ideal for All Downstream Fields of a
Recovery Boiler Precipitator
25
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
process26
Kiln ESP Advantages
�Lower Pressure Loss versus Cyclones or
Baghouses
�Lower Operating Cost for fan horsepower
�Dry product versus wet Venturi Scrubber
�Eliminates need for bag change-out
�Reliably achieves High Collection Efficiencies
27
Lime Dust Characteristics
�Typical Dust inlet loading = 8 to 30 grains/acf (18 to 70 grams/m3). 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
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
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
Typical Failures in RB ESP’s
�Ash Deposits: Duct, Inlet, & Conveyor areas
�Tied to good gas flow distribution
�Moisture makes Saltcake sticky
�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
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 Saltcake Removal Conveyors: Bearings, links, chain off sprocket.
33
Salt Cake Buildup at Inlet Plenum
�Problem is saltcake accumulating 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
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
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
Questions?
Thank you.
Southern Environmental, Inc.
Pensacola, Florida - Columbus, Ohio
www.southernenvironmental.com -
850-944-4475
37