principles and optimization

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

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Page 1: Principles and Optimization

Electrostatic Precipitators:

Principles and Optimization

TAPPI Kraft Recovery Course

St. Petersburg, Florida, January 10, 2019

Roger Lawton Southern Environmental Inc.

Page 2: Principles and Optimization

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!

Page 3: Principles and Optimization

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!

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Page 4: Principles and Optimization

Recovery Boiler ESP

4

ID Fan

(Courtesy of Andritz Inc.)

ESP

FD Fan

Page 5: Principles and Optimization

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

Page 6: Principles and Optimization

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

Page 7: Principles and Optimization

ESPs - How Do They Work?

�Establish Field /Current

�Charge

�Migrate

�Collect

�Rap

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Page 8: Principles and Optimization

ESP Internal Components

8

Rigid Discharge

Electrodes (RDE)

Collecting Plates

Page 9: Principles and Optimization

ESP External Components

9

Rappers

Purge Air

System

Support

Insulator

Power

Supply

Page 10: Principles and Optimization

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)

Page 11: Principles and Optimization

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

Page 12: Principles and Optimization

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

Page 13: Principles and Optimization

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

Page 14: Principles and Optimization

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

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Page 15: Principles and Optimization

ESP Key Components

�Power Supplies

High Frequency Sets Standard 60 Hz T-R Sets15

Page 16: Principles and Optimization

Gas Flow Distribution in ESP

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Inlet

Nozzle

Collection

Zone

Outlet

Nozzle

Variable

Porosity Plate

Page 17: Principles and Optimization

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

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Page 18: Principles and Optimization

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)

Page 19: Principles and Optimization

Inlet and Outlet Transitions

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Page 20: Principles and Optimization

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”

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Page 21: Principles and Optimization

Recovery Boiler ESP Win-Win

�Saltcake collected gets

recycled directly back to

black liquor.

�Huge cost savings for

chemicals

�Meet Environmental

goals of Mill

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Page 22: Principles and Optimization

Recent ESP Experience - Conditions

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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

Page 23: Principles and Optimization

Recent ESP Experience - Results

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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

Page 24: Principles and Optimization

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

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Page 25: Principles and Optimization

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

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Page 26: Principles and Optimization

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

Page 27: Principles and Optimization

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

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Page 28: Principles and Optimization

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.

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Page 29: Principles and Optimization

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

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Page 30: Principles and Optimization

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Gas Flow Distribution

Minimize gas

flow sneakage

SIDE VIEW

Minimize secondary

flow in hopper

END VIEW

Page 31: Principles and Optimization

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

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Page 32: Principles and Optimization

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

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Page 33: Principles and Optimization

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.

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Page 34: Principles and Optimization

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.

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Page 35: Principles and Optimization

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

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Page 36: Principles and Optimization

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.

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Page 37: Principles and Optimization

Questions?

Thank you.

Southern Environmental, Inc.

Pensacola, Florida - Columbus, Ohio

www.southernenvironmental.com -

[email protected]

850-944-4475

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