Wet FGD Chemistry an

Performance Factors

Gordon Maller – URS Corporat

Presented at: 

2008 Power Gen Conference

December 1, 2008
Presentation Outline

• FGD Chemistry Overview• Effect of Key Process Variables o

FGD Operation and Performance

• Chemical Process Problems – How 

To Identify and Correct
What Are Chemical Reactions?

• Two or more reactants (molecules) combining 

(reacting) to form different products

• Reactants and products can be gas, liquid or 

solid

• Examples:

− SO

2 (g) + H

2O H

2SO

3 (aq)

− Ca++ + SO

4

= + 2H

2O CaSO

4 *2H

2

O

• The rate of a reaction depends on:

− Nature and concentration of the reactants and 

products

− Temperature
Types of Solutions

Gas-Liquid(CO

2 and Water)

Liquid-Liquid(Gin and Tonic)

Solid-Liq(Coffee a

Solid Sol(A crystacontains of molectwo diffesolids)
Solubility

Definition: Amount of substance required to produce saturated so

The substance is the solute. The liquid substance it is dissolving into is the solvent.

Temperature Effects: Sugar or Salt

Typical

Gypsum CaSO3

Limesto

Solubility

Temperature
Acids and Bases

pH

1 7 

Definition: Acid = pH < 7 Base = pH > 7

Stomach Acid

Sulfurous Acid

Vinegar

Typical Scrubber

Water

Limestone

Lime

Sulfuric Acid

Coca Cola

DBA

Blood

Draino
Buffers

Definition: Chemical species that can absor

H+ ion and moderate a pH change:

CaCO3 + 2H+ Ca++ + HCO3- + H+ Ca++ + 
Overall Chemistry of a Limestone Fo

Oxidation FGD System

Sulfur 

Dioxide

Limestone Oxygen Water Gypsum 

+ + +

+

SO

2 + CaCO3 + O

2 + H

2O CaSO

4 . 2H

2O 

Gas Solid Gas Liquid Solid 

Reactions involve gas, liquid, and solid pha
Chemical Reaction Steps in FGD 

Process

• SO

2 absorption in absorber

• Neutralization of the absorber SO

2 to keep 

SO

2 vapor pressure low so that more SO

2 

be absorbed

• Dissolution of the limestone or lime reage

provide alkalinity for neutralization and calcium ion for precipitation

• Oxidation of absorbed SO

2 to form sulfate 

(SO

4

)

• Precipitation of calcium and sulfate or sul

to form byproduct
Wet Limestone Forced Oxidized Pro

Chemistry

SO

2 Absorption:

SO

2 + H

2O H

2SO

3

H

2SO

3

H

+ + HSO

3

Limestone Dissolution:CaCO

3 + H

2O Ca++ + HCO

3 + OHOxidation:

HSO

3 + O

2

H

+ + SO

4

=

Precipitation:Ca++ + SO

4

= + 2H

2O CaSO

4 * 2H

2

O
Mass Transfer and Reaction Steps O

at Several Places in FGD System

• Absorber• Reaction tank• Solid-liquid separation

Boiler Sta

Dust 

Collector

Fuel

Air

ID 

Fan

Air Absorber

Water

Ash Removal

Waste Slurry S

Water Gypsum

Convertor

Belt Filter

Wash 

Water

Hydrocyclone

SO2 Absorber Bleed

Gypsum

Dewatering

R

R

Pre
Mass Transfer and Reaction Steps O

at Several Places in FGD System (co

Absorber• SO

2 absorption

• Reagent dissolution• CO

2 evolution or absorption

• Sulfite oxidation• Solids precipitation• HCl and HF absorption
Mass Transfer and Reaction Steps O

at Several Places in FGD System (co

Reaction Tank• Limestone dissolution• Scrubber solids precipitation• Sulfite oxidation
Mass Transfer and Reaction Steps O

at Several Places in FGD System (co

Solids-Liquid Separation• Solids concentration

−Gravity settling (e.g., thickener)− Hydroclones−Filtering− Centrifuging
Overview of Mass Transfer and Reac

Steps by Phase

Gas-Liquid Mass Transfer

Carbon Dioxide Desorption Carbonate

Sulfur Dioxide Absorption

Oxygen Absorption

HCl Absorption

Liquid-Solid Mass Tra

Calcium

Magnesium

Sodium

Carbonate

Dissolution 

Precipitation 

Sulfite

Sulfate

H+

Chloride

Fluoride

Carbonate + H

+ Bicarbonate

Sulfite + H

+ Bisulfite

Gas Phase Liquid Phase S
Acid-Base Equilibrium Reaction

SO2(g) + H

2O H

2SO3 

H

+ + HSO

3 2H

+ + SO

3

=

CO2(g) + H

2O H

2CO

3

H

+ + HCO

3 2H

+ + CO

3

=

Sulfurous

Acid

Bisulfite Sulfite

Carbonic

Acid

Bicarbonate Carbonate

Increasing pH

Decreasing pH

pK 1.81 pK 6.91

pK 6.37 pK 10.25
Liquid-Phase Alkalinity

• Concentration of liquid species that 

alkaline with respect to SO

2 absorpt

• Main source of alkalinity for inhibite

oxidation FGD process
Liquid-Phase Alkalinity (cont.)

Examples:Absorption SO

2 + H

2O H

+ + HSO

3

-

Gas

Neutralization SO

3

= + H

+ HSO

3

-

CO

3

= + H

+ HCO

3

-

Aqueous

Overall CO

3

= + SO

2 + H

2O HSO

3

- + 

Aqueous Gas
Solid-Phase Alkalinity

• Dissolution of limestone in absorber 

replenishes alkalinity and allows scrubbincontinue

• Main source of alkalinity for forced-oxidat

FGD process
Solid-Phase Alkalinity

Examples:Absorption SO

2 + H

2O H

+ + HSO

3

Gas

Dissolution CaCO3 Ca++ + CO

3

=

Solid

Neutralization CO

3

= + H

+ HCO

3

-

Aqueous

Overall CaCO

3

= + SO

2 + H

2O Ca++ + HSO

Solid Gas

H2O
Sulfite Oxidation Reactions

(Free Radical Chain Reactions)

SO

3

= + Fe+++ SO

3* + Fe++

where SO

3* is a free radical

SO3 

*

. + 

1

/2

O

2 SO

4

=

Ca++ + SO

4

= + 2H

2O CaSO

4

. 2H

2O (gypsu
Sulfite Oxidation

Depends On:• Ratio of O

2:SO

2

• Solution chemistry

− pH− [SO

3

=]

−Trace metals

• Temperature
Solid-Liquid Equilibrium

• Determines if limestone will dissolve• Determines if gypsum will precipitate• Dissolution refers to formation of ions in 

solution from a solid

• Precipitation refers to formation of solids 

ions in solution
What is Present in the Liquid Phas

Ions:Ca++, Na

+, Mg++, SO

4

=, Cl-, etc.

Ion Pairs:CaCO

3, CaHCO

3

+, MgHSO

3

-, MgSO

3, CaSO

4, etc

All liquid phase reactions are at equilibrium:Ca++ + CO

3

= CaCO

3

Ca++ + HCO

3

- CaHCO

3

+
Relative Saturation

Q: Will limestone dissolve?

R.S. =

A: Yes – when R.S. is < 1 (subsaturated)

No – when R.S. is > 1 (supersaturated)

[Ca++] . [CO

3

=]

Ksp

[Ca++] = Ca++ Activi

[CO

3

=] = CO

3

= Activ

Ksp = CaCO

3 So

prco
Q: Will Gypsum (CaSO

4 . 2H

2O) 

Precipitate?

[ Ca++] . [SO

4

=] . [ H

2O]

2

Ksp

A: Yes – when R.S. is > 1

No – when R.S. is < 1

R.S. =
Two Types of Gypsum Precipitatio

• Nucleation (spontaneous)• Crystal growth (controlled)

Safe Scaling

Crystal

Growth

Nucleation

Gypsum Precipitation Rate, rG

1.3 – 1.4

Gypsum Relative Saturation, RS

G
Factors that Determine Type of Gyp

Precipitation

• Relative saturation of the solution• The presence of sufficient amount of seed 

solids

• The rate at which the gypsum precipitates
Crystal Size, Shape, and Habit

• Affect dewatering and handling properties 

solids

• Determined by:

−FGD process or design −Type of crystal (gypsum or calcium sulfite)− Contaminants in the crystal− Crystal habit modifiers− Residence Time
Gypsum Crystal: Triclinic System

(No Axis of Symmetry)
Calcium Sulfite Crystal: Orthorhom

System (Three, 2-Fold Axes of 

Symmetry) 
FGD Gypsum Solids From Different Pro

*(Same Magnification)
Effect of Key Process 

Variables on FGD Operatio

and Performance
Performance Variables That Requ

Control

• SO

2 removal efficiency

• Reagent utilization• Scaling potential
SO2 Removal Efficiency

Key Chemical Factors• pH or alkalinity• Excess limestone in recycle slurry (key fo

forced oxidized process)

• Solution chemistry• Inlet SO2 loading (combination of 

concentration and load)
SO2 Removal Efficiency (cont.)

Key Mechanical Factors• Liquid-to-gas ratio (L/G)• Mass transfer characteristics of absorber 

trays, packing)

• Gas / liquid distribution• Flue gas bypass (where applicable)
Reagent Utilization

Key Factors• pH• Solution chemistry• Solids residence time• Surface area (grind size)• Limestone reactivity
Causes of Scaling

Key Factors• Oxidation (not usually a factor in forced 

oxidation process)

• Limestone utilization• Slurry density• Reaction tank volume• ME wash design or operation• ME wash water quality
Factors That Affect FGD Performan

and Reliability

• Direct Control

− pH**

− Slurry density**− L/G

• Limited Control

− Limestone properties**− Solution chemistry**− Mechanical factors− Water management

**To be discussed

• No Control

− Boiler load cycles− Coal S variations− Inlet flue gas

Operators have direct, limited or no control of key pa
pH / Utilization / SO

2 Removal 

Relationships

pH

Utilization

SO2 Removal

(lower) (higher)
Slurry Density (Slurry Solids 

Concentration)

• Minimum slurry density ensures adequate 

crystal surface area for precipitation. This important to prevent scaling

• Maintaining a higher slurry density can 

improve limestone utilization

• Slurry density affects solids residence tim

which, in turn, affects limestone utilization scaling potential
Explanation of Effect of Slurry Den

on Limestone Utilization

• Maintaining pH and SO

2 removal requires 

specific limestone loading (g-limestone/Lslurry) level for the system.

• At low density (low concentration of solids 

the slurry), a higher concentration of the solids will be limestone. Therefore limestoutilization will be lower.
Solution Chemistry

• Dissolved Cl- (Ca++) Concentration

− Inhibits dissolution due to common ion effect

• Forced oxidation vs. natural oxidation

−Stripping of CO

2 tends to enhance dissolution

• Aluminum fluoride blinding

• Sulfite blinding

−Problem during periods of incomplete oxidation

• All can affect the ease with which reagent 

dissolves and removal performance of scrubber
Effect of Dissolved Calcium 

Concentration

0 5 10 15 20 25 30 35 

Limestone Loading (g/L)

SO2 Removal (%)

7580859095

High Dissolved Ca

L

(Different Shaped Symbols Represent Different Limestone Tested
Effect of Oxidation Mode

0 5 10 15 20 25 30 35 

Limestone Loading (g/L)

SO2 Removal (%)

7580859095

Forced Oxidation Mode

Natural Oxidation Mode

(Different Shaped Symbols Represent Different Limestone Tested
Effect of Soluble Aluminum on 

Limestone Utilization

0

Aluminum (mg/L)

Utilization (%)

20

10 20 30 

30

40

50

60

70

80

90

100
Limestone Properties Affecting Scru

Performance

Properties will affect pH-Utilization-Remo

relationship

• Grind• Composition• Reactivity (dolomitic fraction)
Effect of Limestone Grind

0 5 10 15 20 25 30 3

Limestone Loading (g/L)

SO2 Removal (%)

708085909575

Course Ground Limestone (70% Pass 325 mesh)

Fine Ground Limestone (90% Pass 325 mesh)

(Different Shaped Symbols Represent Different Limestone Tested
Effect of Limestone Dolomitic Frac

Limestone With Low Dolomitic FraLimestone With High Dolomitic Fr

0

SO2 Removal (%)

7580859095

5 10 15 20 

Limestone Loading (g/L)
Chemical Process Problem

How To Identify and Corr
Problem Areas To Be Discusse

• Scaling and solids buildup• Mist eliminator pluggage• Limestone blinding• Poor SO

2 removal

• Poor reagent utilization• Poor gypsum byproduct 

quality
Scaling and Solids Buildup

Common areas for scaling:• Scrubber inlet• Absorber internal walls and 

support structure

• Absorber spray headers• Inside spray piping• Trays• Reaction tank walls
Causes of Scaling and Solids Bu

• Chemical (high gypsum relative satura

−Oxidation−Insufficient seed solids−Solid residence time that is too short−Poor wash water quality

• Wet-dry interface and recirculation of s• Poor reagent utilization
Causes of Scaling in FGD System

High Gypsum Relative Saturation:• Scaling can occur in all areas of absorber 

on mist eliminator (ME) as result of solids nucleation due to high relative saturation

• High relative saturation usually result of 

insufficient seed material or high precipitarates

• Scaling in ME may result from poor quality 

wash water (water saturated or nearly saturated with gypsum)
Causes of Scaling in FGD System

Wet/dry Interface:• Occurs in area of absorber where hot flue 

is quenched

• Scaling results from fluctuations in gas fl

and gas distribution. Causes areas along walls or other internal structures to vary fwet to dry

• If slurry is carried back into this area, it m

dry and accumulate as a hard scale compof calcium sulfate, and/or fly ash
Causes of Scaling in FGD Syste

Wet/dry Interface:• Scaling as result of a wet/dry interface is 

eliminated by:

− Modifying the physical design of the absorber i

to eliminate the flow disturbance

− Controlling the location of the wet/dry interface 

modifying the way flue gas is quenched
Scaling and Solids Buildup

Steps to Correct:• Control process chemistry within design 

limits

• Ensure that inlet flue gas is quickly and 

completely quenched

• Modify absorber inlet design to eliminate 

flow disturbances and gas recirculation

• Maintain reagent utilization within desig

range
Mist Eliminator Pluggage

• Chemical causes

−Poor reagent utilization−Poor quality wash water

• Mechanical causes

−Poor design of wash system−Broken or plugged wash system−High gas velocity−Non-uniform gas flow and liquid loading dis
Mist Eliminator Pluggage

• Indication

−Increase in pressure drop−Visual observation during inspection

• ME pluggage can:

−Result in increased gas velocity through 

ME and liquid carryover into outlet duct and stack

−Cause stack rainout and particulate 

emissions

−Damage ME due to weight of solids
Considerations For ME Wash Sy

• ME faces to wash• Wash coverage (150 to 200%)• Wash intensity and pressure (1.5 gpm/

ft, 30 to 45 psi)

• Wash duration and frequency• Recommended wash nozzles
Inhibited Dissolution of Limeston

• Limestone must dissolve in scrubber to 

provide alkalinity

• Certain dissolved chemical species can 

significantly slow or stop the dissolution olimestone

• Inhibition - Slowing of dissolution• Blinding - Significant slowing or stopping o

dissolution 

• High concentrations of dissolved chloride 

magnesium will inhibit dissolution

• The mechanism for this inhibition is called 

“common-ion” effect
Limestone Blinding

• In forced oxidation process, limestone 

blinding can result from high concentratioof dissolved sulfite or aluminum-fluoride complex

• Either sulfite or aluminum-fluoride comple

will react on surface of limestone particle block dissolving site

• Aluminum-fluoride blinding often initiated 

high concentration of inlet fly ash

• Sulfite blinding initiated by incomplete 

oxidation
Poor SO2 Removal as Result of Chem

Problem

Cause Analytical 

Indication 

Corrective Act

Insufficient Limestone In Scrubber 

Low carbonate measured in scrubber solids 

Increase pH set-point 

Malfunctioning pH Monitor Poor agreement during pH 

calibration check 

Make repairs to pH msystem 

Sulfite Blinding Poor reagent utilization 

along with elevated levels of soluble sulfite 

Verify operation of oxiair system. Correct arequired 

Aluminum Fluoride Blinding Poor reagent utilization 

along with elevated levels of aluminum and fluoride 

Improve particulate reupstream of scrubber 

Poor Limestone Quality or Off-Spec Grind Size 

Sieve analysis, composition analysis 

Modify grind circuit anobtain better limeston
Poor SO2 Removal as Result of 

Mechanical Problem

Cause Indication Corrective 

Plugged Spray Header(s) or Spray Nozzle(s) 

Low recycle flow As indicated by flow meter or motor amps 

Clean during out

Broken Spray Header Indication from flow meter 

or motor amps. Observed during inspection 

Repair during ou

Loss of Recycle Pump Capacity 

Low recycle flow as indicated by flow meter or motor amps 

Repair during ou
Poor Reagent Utilization

Cause Analytical 

Indication 

Corrective 

Malfunctioning pH Monitor Poor agreement during pH 

calibration check 

Make repairs to psystem 

Sulfite Blinding Poor reagent utilization 

along with elevated levels of soluble sulfite 

Verify operation oair system. Correrequired 

Aluminum Fluoride Blinding Poor reagent utilization 

along with elevated levels of aluminum and fluoride 

Improve particulaupstream of scrub

Poor Limestone Quality or Off-Spec Grind Size 

Sieve analysis, composition analysis 

Modify grind circuobtain better lime
Poor Gypsum Byproduct Qua

• High acid insoluble (inerts) fraction

−Verify operation of particulate control device upstre

scrubber

−Optimize operation of process hydroclones to max

concentration of inerts in overflow

− Increase liquid purge rate

• Poor reagent utilization

• Low sulfite oxidation

−Verify operation of oxidation air system
Poor Gypsum Byproduct Quality (

• High moisture

− Verify operation of primary and secondary dewateri

system

− Purge fines from process− Check filter cloth for blinding. Clean or replace as 

required

− Increase temperature of filtercake wash water− Verify system chemistry

• High level of limestone• High level of acid insolubles• Presence of crystal modifier