EPA/600/R-06/156 

September 2006 

EVALUATION AND MITIGATION OF VISIBLE ACIDIC 

AEROSOL PLUMES FROM COAL FIRED POWER 

BOILERS 

FINAL REPORT 

EPA Contract No. EP-C-04-056 

Work Assignment Numbers 0-3 & 1-4 

Southern Research Institute Project No. 11402 

November 14, 2006 

By: 

Peter M. Walsh, Joseph D. McCain, and Kenneth M. Cushing 

Southern Research Institute 

2000 Ninth Ave. South 

P.O. Box 55305 

Birmingham, AL 35255-5305 

For: 

C. Andrew Miller, Project Officer 

U.S. Environmental Protection Agency 

Office of Research and Development 

National Risk Management Research Laboratory 

Air Pollution and Prevention Control Division 

Research Triangle Park, NC 27711 
Notice 

The U.S. Environmental Protection Agency through its Office of Research and Development 

funded and managed the research described here. It has been subjected to the Agency`s review 

and has been approved for publication as an EPA document. Mention of trade names, products, 

or services does not convey, and should not be interpreted as conveying, official EPA approval, 

endorsement, or recommendation. 

ii 
Foreword 

The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting the 

Nation's land, air, and water resources. Under a mandate of national environmental laws, the 

Agency strives to formulate and implement actions leading to a compatible balance between 

human activities and the ability of natural systems to support and nurture life. To meet this 

mandate, EPA's research program is providing data and technical support for solving 

environmental problems today and building a science knowledge base necessary to manage our 

ecological resources wisely, understand how pollutants affect our health, and prevent or reduce 

environmental risks in the future. 

The National Risk Management Research Laboratory (NRMRL) is the Agency's center for 

investigation of technological and management approaches for preventing and reducing risks 

from pollution that threaten human health and the environment. The focus of the Laboratory's 

research program is on methods and their cost-effectiveness for prevention and control of 

pollution to air, land, water, and subsurface resources; protection of water quality in public water 

systems; remediation of contaminated sites, sediments and ground water; prevention and control 

of indoor air pollution; and restoration of ecosystems. NRMRL collaborates with both public and 

private sector partners to foster technologies that reduce the cost of compliance and to anticipate 

emerging problems. NRMRL's research provides solutions to environmental problems by: 

developing and promoting technologies that protect and improve the environment; advancing 

scientific and engineering information to support regulatory and policy decisions; and providing 

the technical support and information transfer to ensure implementation of environmental 

regulations and strategies at the national, state, and community levels. 

This publication has been produced as part of the Laboratory's strategic long-term research plan. 

It is published and made available by EPA's Office of Research and Development to assist the 

user community and to link researchers with their clients. 

Sally Gutierrez, Director 

National Risk Management Research Laboratory 

iii 
TABLE OF CONTENTS 

Page 

List of Figures................................................................................................................................. v 

List of Tables ...............................................................................................................................viii 

Nomenclature................................................................................................................................. ix 

Abstract...................................................................................................................................... xivv 

1. Introduction ............................................................................................................................... 1 

2. Evaluation of the Possible Extent of the SO3 Emissions Problem in the 

Electric Utility Industry............................................................................................................. 5 

3. Improvements in SO3 Measurement Technologies................................................................. 13 

4. Exploratory Study of SO3 Adsorption by Coal Fly Ash ......................................................... 20 

5. Formation of Sulfur Trioxide in the Convection Section of Coal-Fired 

Electric Utility Boilers ............................................................................................................ 30 

6. Development of a Estimator for Removal of SO3/H2SO4 Across Air Preheaters................... 47 

7. Evolution of Acid Mist in Wet Flue Gas Desulfurization Units and Stacks........................... 57 

8. References ............................................................................................................................... 84 

Appendix A. Correlations for sulfuric acid density, sulfuric acid surface energy, 

and the vapor pressure of water over sulfuric acid at 333.15 K (140 °F). ............................ A-1 

Appendix B. Computer code for the calculation of acid mist properties in the 

 scrubber and stack. ................................................................................................................ B-1 

iv 
LIST OF FIGURES 

Figure Page 

Figure 2.1. Measured SO3 concentrations versus temperature at the exit of an 

air heater and cold-side electrostatic precipitator. ................................................... 10 

Figure 2.2. SO3 emissions for states in the Mississippi Valley and Eastern U.S. 

emitting > 1% of total projected SO3 emissions for the region . ............................. 12 

Figure 3.1. Schematic diagram of the Apogee QSIS probe modified for 

use with the SO3 monitor. ........................................................................................ 14 

Figure 3.2. Comparison of SO3 concentrations measured by conventional controlledcondensation methods (CCM) with those measured using Apogee 

QSIS probes and a heated hose to deliver samples to a controlled- 

 condensation condenser. .......................................................................................... 16 

Figure 3.3. Comparisons of SO3 concentrations measured by the conventional controlledcondensation method (CCM) with those measured using the modified 

Apogee QSIS probe incorporating the hot gas dilution approach to 

evaporating condensed H2SO4. ................................................................................ 17 

Figure 3.4. Modified Apogee QSIS sampling probe.................................................................. 19 

Figure 4.1. Layout of ash samples in the exposure chamber as seen from above...................... 26 

Figure 4.2 Typical concentration versus time measurements at the inlet and outlet 

of the SO3 exposure chamber....................................................................................27 

Figure 4.3. Total soluble sulfate found in ash samples after exposure to SO3 at the 

conditions indicated versus ash base:acid ratios...................................................... 28 

Figure 4.4. Plot of SO4 on ash as predicted by the regression equation and the 

values measured in this study................................................................................... 29 

Figure 5.1. Surface temperature and SO3 formation in the superheater under the 

assumption of negligible fouling and tube scale exposed to flue gas. ..................... 32 

Figure 5.2. Surface temperature and SO3 formation in the superheater under the 

assumption of heavy fouling on the fire side, with ash deposits exposed 

to flue gas................................................................................................................. 33

v 
List of Figures (Continued) 

Figure Page 

Figure 5.3. Surface temperature and SO3 formation in the superheater under the 

assumption of moderate fouling on the steam side, with tube scale 

exposed to flue gas................................................................................................... 34 

Figure 5.4. Comparison of the calculated SO3 formation to measurements for the 

Hickling Station modeling. ...................................................................................... 35 

Figure 5.5. Equilibrium and kinetic constraints on SO3 formation. ........................................... 39 

Figure 5.6. Modeled temperature profiles through a 1300 MW pulverized 

coal-fired utility boiler. ............................................................................................ 43 

Figure 5.7 SO3 concentration profile through a 1300 MW pulverized coal-fired 

utility boiler as predicted by the model.....................................................................43 

Figure 6.1. Estimated SO3/H2SO4 losses across combustion air preheaters versus 

average air preheater exit temperature for a temperature offset of 35 °F. ............... 48 

Figure 6.2. Estimated air preheater exit SO3/H2SO4 concentration versus average 

air preheater exit temperature for a temperature offset of 35 °F.............................. 49 

Figure 6.3. Effect of flue gas moisture content on the estimated air preheater exit 

SO3/H2SO4 concentration for a temperature offset of 35 °F.................................... 50 

Figure 6.4. Estimated SO3/H2SO4 losses across combustion air preheaters versus 

average air preheater exit temperature for a temperature offset of 30 °F. ............... 51 

Figure 6.5. Estimated air preheater exit SO3/H2SO4 concentration versus average 

air preheater exit temperature for a temperature offset of 30 °F.............................. 52 

Figure 6.6. Comparison of predicted and measured SO3 concentrations at the exit 

of rotary air heater No. 1 for a temperature offset of 30 °F. ................................... 53 

Figure 6.7. Comparison of predicted and measured SO3 concentrations at the exit 

of rotary air heater No. 2 for a temperature offset of 30 °F. .................................... 54 

Figure 6.8. Comparison of predicted and measured SO3 concentrations at the exit 

of rotary air heater No. 3 for a temperature offset of 33 °F. .................................... 55 

Figure 6.9. Comparison of predicted and measured SO3 concentrations at the exit 

of rotary air heater No. 4 for a temperature offset of 30 °F. .................................... 56 

vi 
List of Figures (Continued) 

Figure Page 

Figure 7.1. Properties of acid mist as a function of time in the SO2 scrubber 

and stack—base case................................................................................................ 68 

Figure 7.2. Properties of acid mist as a function of time in the SO2 scrubber 

and stack—increased H2SO4 case............................................................................ 71 

Figure 7.3. Properties of acid mist as a function of time in the SO2 scrubber 

and stack—increased scrubber droplet size case. .................................................... 73 

Figure 7.4. Properties of acid mist as a function of time in the SO2 scrubber 

and stack—decreased scrubber droplet size case..................................................... 74 

Figure 7.5. Properties of acid mist as a function of time in the SO2 scrubber 

and stack—increased size of droplets passing the mist eliminator.......................... 76 

Figure 7.6. Measured size distributions of acid mist and other particulate components 

downstream of scrubber module “A.”...................................................................... 79 

Figure 7.7. Measured size distributions of acid mist and other particulate 

components in the stack downstream of a scrubber system..................................... 80 

Figure 7.8. Measured size distributions of acid mist in the stack as shown in 

Figure 7.7 after replacement of the scrubber system with one of 

a different type. ........................................................................................................ 81 

Figure 7.9. Comparison of droplet diameters extrapolated from measurements 

with those predicted by the model. .......................................................................... 82 

Figure 7.10. Comparison of predicted plume opacities versus H2SO4

concentration with those measured by a certified “smoke reader” 

for a 1300 MW unit with a pollution control system consisting of 

an SCR followed by a cold-side ESP and an SO2 scrubber..................................... 83 

vii 
LIST OF TABLES 

Table Page

Table 2.1. Ratio of SO3/SO2 based on 16 field tests...................................................................... 6 

Table 2.2. Number of units predicted to exceed selected values of SO3 concentration

under scenario assumptions. ......................................................................................... 7 

Table 2.3. Sulfur contents of coals fired in plants in the Mississippi Valley and Eastern U.S...... 8 

Table 2.4. SO3 removal efficiencies for control system components ............................................ 9 

Table 2.5. Comparisons of observed and predicted SO3 concentrations ....................................... 9 

Table 2.6. State-by-state tabulation of estimated SO3 emissions from individual 

power plant units in the Mississippi Valley and Eastern U.S..................................... 11 

Table 3.1 Comparison of measurements made using a conventional CCM probe 

with those made with the hot-gas dilution probe downstream of a 

full-scale utility scrubber. ........................................................................................... 18 

Table 4.1. Exposure conditions and results ................................................................................. 22 

Table 4.2. Properties of ashes used in adsorption study .............................................................. 23 

Table 4.3. Atomic concentrations of cations for ashes used in adsorption study ........................ 24 

Table 4.4. Results of multiple regression to obtain predictive equation for uptake of SO3......... 25 

Table 5.1 Conditions for the SO3 model run whose results are shown in 

Figures 5.6 and 5.7...................................................................................................... 42 

Table 6.1. Measured and predicted average exit SO3 concentrations for four 

 rotary air heaters. ........................................................................................................ 47 

Table 7.1. Values of the parameters used in the calculations for the base case........................... 70 

viii 
NOMENCLATURE 

cad mean speed of acid mist droplets, m/s 

cfg mean speed of flue gas molecules, m/s 

cw

Cad

Cad0

mean speed of water vapor molecules, m/s 

concentration of acid mist droplets, kg/m3 

initial concentration of acid mist droplets, kg/m3 

Cbl average gas concentration in the boundary layer of a tube or particle, kmol/m3 

Cc Cunningham correction factor, Eq. (7.9), dimensionless 

CD drag coefficient for scrubber spray droplets, dimensionless 

Cg

CH2SO4

total gas concentration, kmol/m3 

concentration of H2SO4, kg/m3 

Cs total gas concentration at a deposit or particle surface, kmol/m3 

Csd concentration of scrubber spray droplets, kg/m3 

Cw,ad concentration of water vapor in equilibrium with the surfaces of acid mist droplets, 

kg/m3 

Cw,sd

Cw,∞

concentration of water vapor in equilibrium with the surfaces of scrubber spray 

droplets, kg/m3 

concentration of water vapor in the free stream, far from droplet surfaces, kg/m3 

d characteristic length, diameter, m 

dad diameter of acid mist droplets, m 

dad0 initial diameter of acid condensation nuclei, m 

dp mean particle size, m 

dsd diameter of scrubber spray droplets, m 

dt tube diameter, m 

Dad diffusion coefficient for acid mist droplets, m2/s 

ix 
NOMENCLATURE (CONTINUED) 

Dw,fg pseudo-binary molecular diffusion coefficient for a mixture of water vapor with the 

other combustion products, m2/s 

DSO3 molecular diffusion coefficient of SO3 in combustion products, m2/s 

ESP electrostatic precipitator 

fcap fraction of initial H2SO4 captured by scrubber spray droplets, dimensionless 

FGD flue gas desulfurization 

kB Boltzmann constant, = 1.38 x 10-23 J/K 

k effective rate coefficient for the heterogeneous reaction between SO2 and O2, m/s 

ki global reaction rate constant for compound i 

kad rate coefficient for coagulation, m3/s 

kt effective rate coefficient for the heterogeneous reaction between SO2 and O2 at tube 

or ash deposit surfaces, m/s 

kp effective rate coefficient for the heterogeneous reaction between SO2 and O2 at 

particle surfaces, m/s 

ksa effective overall rate coefficient for transport of water vapor between scrubber 

spray droplets and acid mist droplets, Eq. (7.24), dimensionless 

k1 rate coefficient for Reaction (5.4), m/s 

k2 rate coefficient for Reaction (5.5), m/s 

Knad,ad Knudsen number for diffusion of droplets toward each other, dimensionless 

Knfg,ad Knudsen number for acid mist droplets in flue gas, dimensionless 

Knw,ad Knudsen number for water vapor diffusing to and from acid mist droplets in flue 

gas, dimensionless 

LG liquid to gas feed ratio in the scrubber, gal/1000 scf or dimensionless 

mad mass of acid mist droplets, kg 

Nad number concentration of acid mist droplets, m-3 

Nad0 initial number concentration of acid mist droplets, m-3 

x 
NOMENCLATURE (CONTINUED) 

Pw,ad partial pressure of water vapor in equilibrium with the surfaces of acid mist 

droplets, Pa 

Pw,af partial pressure of water vapor in equilibrium with a flat sulfuric acid surface, Pa 

Pw,sd partial pressure of water vapor in equilibrium with the surfaces of scrubber spray 

droplets, Pa 

rSO3 rate of sulfur trioxide formation, kmol/m3.s 

R universal gas constant, = 8314.51 J/(kmol·K) 

Re Reynolds number, dimensionless 

S effective area of catalytic surface per unit of gas volume, m-1 

Sad specific surface area of acid mist droplets, m2/kg 

Sp external surface area of fly ash particles per unit of gas volume, m-1 

Ssd specific surface area of scrubber spray droplets, m2/kg 

St surface area of tube or ash deposits on a tube, per unit of gas volume, m-1 

Sc Schmidt number, dimensionless 

SCR selective catalytic reduction 

Sh Sherwood number for mass transfer, dimensionless 

Shp Sherwood number for mass transfer through the external boundary layer of a 

particle, = 2, dimensionless 

Shsd Sherwood number for mass transfer to and from scrubber spray droplets, 

dimensionless 

Shw,ad Sherwood number for transport of water vapor to and from acid mist droplets, 

dimensionless 

Sht average Sherwood number for mass transfer between tube or deposit surface and 

the free stream, dimensionlesst time, s 

t residence time, s 

to residence time at the entrance to a row of tubes, s 

xi 
NOMENCLATURE (CONTINUED) 

T absolute temperature, K 

ufg velocity of flue gas in scrubber, m/s 

usd terminal fall velocity of scrubber spray droplets relative to the scrubber, m/s 

Wfg average molecular weight of flue gas, kg/kmol 

WH2SO4 molecular weight of H2SO4, kg/kmol 

WH2SO4·H2O molecular weight of H2SO4·H2O, kg/kmol 

Ww molecular weight of water, kg/kmol 

Xi mole fraction of species i, dimensionless 

XSO3 mole fraction of SO3 in the free stream, dimensionless 

XSO3,eq mole fraction of SO3 at equilibrium, dimensionless 

XSO3,eq,p mole fraction of SO3 at equilibrium at particle surfaces, dimensionless 

XSO3,eq,t mole fraction of SO3 at equilibrium at tube or ash deposit surfaces, dimensionless 

XSO3,s mole fraction of SO3 at catalyst surface, dimensionless 

XSO3,o initial mole fraction of SO3 at the entrance to a row of tubes, dimensionless 

Ya mass fraction of H2SO4 in sulfuric acid, dimensionless 

Ya0 initial mass fraction of H2SO4 in acid condensation nuclei, dimensionless 

YFe2O3 mass fraction of all iron oxides in deposit or particles, from ash analysis, 

dimensionless 

YFeOX mass fraction of higher iron oxide in deposit or ash particles, dimensionless 

YFeOX-1 mass fraction of reduced iron oxide in deposit or ash particles, dimensionless 

xii 
NOMENCLATURE (CONTINUED) 

Greek Symbols 

β factor matching droplet diffusion behavior in the continuum and free molecule 

regimes, Eq. (7.3), dimensionless 

Δt small increment of time, s 

λad mean free path of acid mist droplets, m 

λfg mean free path of flue gas molecules, m 

λw mean free path of water vapor molecules in flue gas, m 

μfg viscosity of flue gas, kg/(m·s) 

ρa density of sulfuric acid, kg/m3 

ρsd density of scrubber spray droplets, kg/m3 

σa surface energy of sulfuric acid, J/m

2 

Subscripts 

a acid 

ad acid mist droplets 

af flat acid surface 

c Cunningham (correction factor) 

cap captured (acid by scrubber spray droplets) 

D drag 

eq equilibrium 

fg flue gas 

sa scrubber spray droplets to acid mist droplets 

sd scrubber spray droplets 

w water vapor 

0 initial value at entrance to scrubber 

∞ free stream, far from droplet surfaces 

xiii 
ABSTRACT 

The formation of sulfur trioxide (SO3) during the combustion of sulfur-containing fuels, 

particularly coal, can increase significantly following the installation and operation of selective 

catalytic reduction (SCR) systems for reduction of nitrogen oxides (NOx). The increased SO3 

formation can in turn lead to adverse environmental impacts, including visible near-stack plumes 

and increased fine PM emissions, primarily in the form of sulfuric acid (H2SO4) aerosols. The 

potential extent of the problem in the electric utility sector is estimated based on the population 

of coal-fired utility boilers, the sulfur content of coal burned by each unit, and the likelihood that 

units will install SCR and flue gas desulfurization (FGD) systems. Of the 363 large (≥ 250 

MWe) generating plants in the eastern U.S., there is a significant potential that as many as 65 

could experience visible H2SO4 aerosol plumes or more serious problems after installation of 

SCR and FGD systems, based on the sulfur content of the coal historically used at those plants. 

As use of FGD systems increases, it is also likely that utilities will turn to higher sulfur coal, 

which can exacerbate this problem. This report describes the mechanisms of SO3 and acid 

aerosol formation and removal across boiler convection sections, air preheaters, and wet FGD 

systems, and presents information from an exploratory study of the absorption of SO3 onto coal 

fly ash. A model of SO3 formation and emissions based on these mechanisms is shown to 

accurately predict the stack concentration of SO3 for a 1300 MWe pulverized coal-fired boiler, 

indicating that the mechanisms described have captured the fundamental behavior of SO3 in 

utility combustion and flue gas treatment systems. This information can provide the basis for 

developing mitigation approaches to reduce the impacts of SO3 formation across SCRs and the 

subsequent formation and emission of acid aerosols. 

xiv 
1. INTRODUCTION 

Background

In 2006 the Environmental Protection Agency (EPA) promulgated the “Clean Air Interstate 

Rule” (formerly called the “Interstate Air Quality Rule”) with the goal of reducing sulfur dioxide 

(SO2) and oxides of nitrogen (NOx) emissions in the Eastern U.S. When fully implemented, the 

rule is projected to reduce SO2 emissions by over 70% from 2003 levels and reduce NOx 

emissions by over 60% from 2003 levels.1 Achieving these goals is expected to involve 

widespread use of selective catalytic reduction (SCR) for NOx control and flue gas 

desulfurization (FGD) scrubbers for SO2 control in coal-fired power plants. In fact, current usage 

of these technologies has already led to substantial reductions in emissions of NOx and SO2. 

However, the use of SCR for NOx control has the potential to enhance the formation of sulfur 

trioxide (SO3).2 At elevated concentrations, SO3 forms visible plumes and excessive local 

concentrations of sulfuric acid (H2SO4) aerosols. 

Elevated SO3 concentrations are of concern due in part to associated potential adverse health 

effects. Particularly in areas that can be affected by sinking plumes that fall to ground level near 

the stack, increases in PM smaller than 2.5 μm in aerodynamic diameter (PM2.5) caused by high 

H2SO4 concentrations are of concern. At typical ambient concentrations, the health impacts of 

H2SO4 aerosols are unclear. Some evidence of impaired mucociliary clearance and modest 

changes in lung function has been identified,3 but there is limited evidence of significant airway 

inflammation or altered bronchial responsiveness as a result of exposure to typical ambient levels 

of H2SO4 aerosol.4 In sensitive populations5 or in combination with other pollutants,6,7 exposure 

to H2SO4 may lead to more serious problems compared to exposure to H2SO4 alone. 

Where plumes intercept the ground, effects associated with the higher concentrations associated 

with occupational exposure levels may be more applicable than effects associated with ambient 

concentrations. A significant difference is that near-stack plume contact incidents can result in 

concentrations much higher than typical ambient levels, but much shorter in duration and less 

frequent than occupational exposures. At elevated occupational levels, exposure to SO3 can 

cause lung irritation, fluid build-up in the lungs (pulmonary edema), third-degree burns to the 

skin, and blindness. Long-term, lower level occupational exposure can lead to bronchitis, 

emphysema, chronic runny nose, tearing of the eyes, nosebleeds, headaches, nausea, dizziness, 

and erosion of the stomach and teeth.8 

There have not been any reported instances of the more serious effects associated with nonoccupational exposure to SO3 or H2SO4 aerosol, although physiological responses such as eye, 

nose, and throat irritation and breathing difficulty have been reported for episodes involving 

plumes containing sulfuric acid aerosols at ground level.9,10 In an incident documented in 2001 

involving the American Electric Power Gavin Power Plant, residents downwind of a plant having 

a plume that touched the ground complained of irritation of the eyes, nose, and throat; shortness 

of breath; and asthma-like symptoms. Measurements of H2SO4 levels in the area were found to 

be between 35 and 120 μg/m3

.

11 At the same time, ground-level SO2 concentrations were 

measured at 20-50 μg/m3, so it is not clear whether the effects were caused by H2SO4, SO2, or the 

combination of the two along with the other pollutants in the plume. In either case, the presence 

1
of increased levels of acid aerosols and SO3 is associated with adverse health impacts at 

concentrations that have been measured in plumes that have contacted the ground. 

In addition to the public health impacts of increased SO3 formation, plant components can be 

adversely impacted as well.12,13

 Increases in SO3 formation and emissions can clearly cause 

problems that need to be addressed for several reasons, but as noted above can also be a 

consequence of efforts to achieve significant reductions in SO2 and NOx. This report will discuss 

efforts to better understand SO3 formation processes, information that can lead to cost-effective 

mitigation approaches and minimization of the adverse impacts of SO3. 

Project Description

The purpose of this project was to conduct research that would improve EPA’s understanding of 

the formation mechanisms of SO3 and the means by which power plants can minimize the 

formation of sulfuric acid emissions. These included characterizing the potential significance of 

the problem and assessing interactions between coal type, pollution control system design and 

operation, and emissions of SO3, H2SO4, NOx and SO2. 

This research is important to efforts to achieve targeted reductions in emissions of NOx and SO2 

while minimizing adverse consequences of control technology application. Combined use of 

SCR for NOx control and FGD for SO2 control in coal-fired power plants has the potential to 

enhance the formation of SO3. At elevated concentrations, SO3 forms visible plumes and 

excessive local concentrations of sulfuric acid aerosols. Such indirect effects of SCR use must 

be more fully understood in order to meet regulatory goals of reducing ambient levels of SO2, 

NOx, and particulate matter (PM) in a way that does not create new air pollution problems. In 

particular, states need information related to SO3 emissions and its mitigation for the purpose of 

developing State Implementation Plans (SIPs). The tasks performed during this research were 

intended to provide that information to EPA and the states. 

Earlier work conducted by Southern Research Institute (SRI) for EPA produced an interim report 

that described the current state of understanding of SO3 formation and mitigation.2 The report 

concluded that “the difficulty faced by utility plant operators is that guidelines are not presently 

available to define the extent of the problem that may occur at a given site. In the absence of 

such guidelines, it is not possible to determine in advance the control strategies that are 

technically and economically feasible for a particular plant site.” This lack of guidelines is 

primarily the result of a lack of sufficient credible data on which to base models for control of 

SO3 emissions. In particular, SO3 collection by combustion air preheaters, electrostatic 

precipitators and scrubbers were poorly characterized. 

The current report describes research conducted by SRI to characterize methods to measure SO3, 

develop model plant specifications, estimate acid droplet growth rates, and characterize 

absorption and adsorption of SO3 by fly ash. The results reported here provide the basis for more 

accurate estimates of SO3 formation and emissions and identify research needs for further 

improvement in understanding SO3 formation and removal across different plant components. 

2
Report Overview 

Chapter 2 describes a survey of the potential extent of SO3 emissions in the electric utility 

industry under current regulations, including the Clean Air Interstate Rule. The chapter includes 

a state-by-state tabulation of potential SO3 emissions from individual power plant units and 

identified those areas where the greatest uncertainties exist (e.g., SCR conversion rates versus 

scrubber removal efficiencies). 

Chapter 3 addresses the methods used to measure SO3 in combustion flue gases, and describes 

efforts to develop improved measurement methods. In particular, Chapter 3 discusses tests of a 

modified sampling probe which permits extraction of a filtered gas sample without passing the 

sample through a layer of ash. Results are presented on exploratory tests of this probe for use 

with the controlled condensation method. The tests, conducted at SRI’s Coal Combustion 

Research Facility, demonstrate improvements in SO3 measurements using the modified probe. 

Additional, preliminary results are presented for tests at a full-scale operating power plant. 

Chapter 4 covers the absorption and adsorption of SO3 by coal fly ash for coals with significantly 

different ash properties. Absorption and adsorption of SO3 and H2SO4 can be a major controlling 

factor in the overall process. Chapter 4 reports on tests that passed an SO3 laden gas stream over 

an ash layer in a temperature and moisture controlled environment. Ashes from various coals 

were tested, and analyses are presented on the soluble sulfate levels for the untreated ashes and 

the ash specimens after exposure to SO3. Test conditions were chosen to be representative of 

those found in typical economizer and air preheater sections. 

Chapters 5 and 6 describe, respectively, the behavior of SO3 in boiler convection sections and air 

preheaters. Chapter 5 provides quantitative results on the formation and removal of SO3 across 

the convection sections (e.g., convective sections, superheaters, reheat sections, economizer). 

Chapter 6 presents results of estimates for SO3 removal across four preheaters and compares the 

estimates to measured SO3 concentrations at the preheater exits. These results can be used to 

model SO3 formation, removal, and emissions for plants with different configurations and 

operating conditions to more effectively estimate impacts of equipment, fuel, and operating 

changes on SO3 emissions and identify mitigation approaches. 

Chapter 7 presents the development of an algorithm to describe the evolution and behavior of 

acid mists in wet FGD units, including mist eliminators, and plant stacks. The algorithm is used 

to evaluate how changing parameters such as H2SO4 concentration, scrubber droplet size, and the 

size of droplets exiting the mist eliminator influence the mass fraction of H2SO4 in the stack 

aerosol and the scrubber droplets, as well as the acid droplet number concentration and volume 

fraction. These results are then used to estimate the impacts of changing scrubber type and are 

compared to measurements from a full-scale plant. 

The models presented in Chapters 5-7 are in the developmental stages and have had only 

minimal validation with measurements. Although they can be used to address performance at 

specific plants to identify where SO3 is most likely to be formed or removed, there has not been 

adequate comparison with measurements to allow significant certainty in the predictions. 

3
Additional measurements are needed along the entire flue gas path at several plants and with 

different coals to enable more certain estimates to be made. 

4
2. EVALUATION OF THE POSSIBLE EXTENT OF THE SO3 

EMISSIONS PROBLEM IN THE ELECTRIC UTILITY 

INDUSTRY 

With the promulgation of the Clean Air Interstate Rule (CAIR), the electric utility industry is 

beginning to install technologies for control of NOx and SO2. These newly installed technologies 

have the potential to increase emissions of SO3 above current levels. To determine whether such 

changes would be likely to impact a significant fraction of plants, and if so, the degree of such 

impacts, a simple spreadsheet-based model was developed to evaluate the possible extent of the 

issue. The model collected data on basic plant size and design parameters and on coal use 

patterns. These data were coupled with reasonable average rates of SO2 conversion to SO3 and 

removal of SO3 by common pollution control systems to develop an overall picture of what 

broad application of SCR and wet FGD scrubbers may mean for SO3 emissions. This evaluation 

can provide some overview information on the extent to which SO3 mitigation measures may be 

needed. 

However, these estimates are highly uncertain and cannot be used to develop estimates for a 

specific plant. The uncertainties stem from the use of “reasonable average” values for SO2 

conversion and SO3 removal, and particularly from an inability to predict how plants will work 

to meet the CAIR requirements. It is likely that the pattern of coal use will change as some 

plants meet the SO2 requirements by switching to lower sulfur coal, while others switch to 

higher sulfur coal following installation of FGD systems. Some plants may install either SCR or 

wet FGD, while others install both. Use of spray dryer systems or installation of fabric filters 

will also reduce a plant’s SO3 emissions compared to what would be estimated from the 

spreadsheet model. Finally, the trading approach used by CAIR will likely result in 

combinations of control efficiencies that cannot be incorporated into a simple spreadsheet model. 

Each of these factors combine to increase the uncertainties associated with the estimated results. 

Even so, the model does provide some order-of-magnitude estimate of the extent to which the 

SO3 issue may be significant for the electric utility industry. 

The evaluation conducted by Southern Research Institute is based on data from the Department 

of Energy (DOE) Federal Energy Regulatory Commission (FERC) Annual Steam-Electric Plant 

Operation and Design Database for 2002,14

 augmented when possible with additional 

information obtained from the U. S. EPA15 and an industry database16 to fill gaps in the FERC 

database. For each coal-fired power plant surveyed, SO3 emissions after anticipated technology 

additions for control of NOx by SCR and SO2 by wet scrubbers were predicted. These emissions 

are referred to here as “collateral SO3 emissions.” 

To compute the predicted SO3 concentrations, SO2 concentrations were calculated from the 

reported sulfur content of the coals being burned at each plant. The rate of oxidation of SO2 to 

SO3 depends on many factors, including the sulfur content of the fuel, the amount of excess air, 

and the presence of a catalyst. For this methodology, SO3 concentrations produced by the boilers 

were estimated using SO3/SO2 ratios developed by Hardman, Stacy and Dismukes for eastern 

bituminous, western subbituminous, and lignite coals.17 These ratios are based on measurements 

5
taken during 16 field tests and are shown in Table 2.1 below and are considered to be reasonably 

representative of existing information. The coal types used in the calculations were the 

predominant fuels for each unit as reported in the database. Subsequent production and removal 

of SO3 was calculated for the configuration of downstream controls at each site. 

Table 2.1. Ratio of SO3/SO2 Based on 16 Field Tests17 

Coals Burned SO3/SO2 Ratio 

Average Standard Deviation 

9 Eastern Bituminous Coals 0.004 0.003 

7 Western Subbituminous and Lignite Coals 0.0011 0.0005 

Table 2.2 summarizes the projected impact of collateral SO3 emissions from coal-fired power 

plant units in the Mississippi Valley and the Eastern U.S. after anticipated technology additions 

for control of NOx and SO2. The table summarizes the number of units exceeding the specified 

concentrations of SO3 in their stack emissions broken down by unit generating capacity. The 

column labeled “Missing Data” refers to units in the database for which critical information such 

as fuel sulfur was missing. The tabulations show the effect of retrofit of SCR and wet scrubbers 

on all units. Based on a simplified assumption that the proposed regulations would result in 

controls on approximately 75% of the total coal-fired generating capacity, all units larger than 

roughly 250 MW would operate with SCR/scrubber combinations. The total capacity for 250 

MW and larger plants is estimated here to be 200 GW. The more detailed EPA analysis done for 

the Clean Air Interstate Rule (CAIR) estimates 178 GW of capacity will be retrofit with SCR and 

slightly higher than that, 187 GW, will be retrofit with scrubbers.18 

Table 2.2 has columns for five SO3 emissions levels of increasing regulatory or health impacts. 

The rows in the table for unspecified unit loads, designated as Unknown, refer to units in the 

database for which unit load information was unavailable. Units with scrubbers having stack SO3 

concentrations of about 4 ppm and higher may have difficulty meeting 20% opacity limits if, as 

is becoming more common, the opacity limits are applied to the stack plumes. At stack 

concentrations of about 10 ppm and higher, visible “blue” plumes will exist even in the absence 

of a scrubber. Plumes from units equipped with scrubbers reach ground level much closer to 

stacks than those without scrubbers, given equal stack heights. Because stack gases are less 

diluted close to the stack than at greater distances, ground level SO3 concentrations near the stack 

will be higher for a unit with a scrubber than for the same unit without a scrubber.2 The Gavin 

Power Plant and the Cinergy/PSI Gibson Generating Station are two plants that have been faced 

with substantial difficulties with local citizens and regulatory agencies because of such ground 

level concentrations of SO3.

9,19 Potential health effect problems have led to concern at both the 

Gavin and Gibson plants where stack concentrations are approximately 13 ppm or higher. 

Another potential issue is complying with particulate mass emission rate standards. If the 

particulate emission standards were to change such that condensables (such as H2SO4 aerosols) 

are included as part of emissions of particulate matter, SO3 aerosol concentrations in some stacks 

could contribute significantly to the overall particulate loading (1 ppm is equivalent to about 4 

mg/dncm). 

6
Table 2.2. Number of units predicted to exceed selected values of SO3 stack concentration under scenario 

assumptions. Data cover all coal-fired plants in the Mississippi Valley and Eastern U.S. Note that each column listing 

the predicted number of affected plants is a subset of the columns to its right. 

Load 

Range 

(MW) 

Total # 

of 

Units 

in 

Range 

Approx. 

Total 

Capacity 

(MW) 

Approx. 

% of 

Total 

Capacity 

Included 

in Range 

SO3 ≥

25 ppm 

“Gavinlike” 

conc. 

SO3 ≥

20 ppm 

SO3 ≥

13 ppm 

“Gibsonlike” 

conc. 

SO3 ≥

10 ppm 

“Blue 

plume” 

w/o 

scrubber 

SO3 ≥

4 ppm 

Opacity 

problems 

with 

scrubber 

Missing 

Data 

1% SCR Conversion

≥1,200 14 18,989 7% 0 3 5 5 6 0 

900 – 1,199 13 13,378 5% 2 3 3 3 7 0 

700 - 899 67 52,803 20% 0 6 12 15 42 0 

400 - 699 136 74,345 28% 0 10 32 49 102 2 

250 - 399 133 40,690 15% 0 7 19 33 78 0 

<250 957 65,887 25% 23 62 163 266 590 17 

Unknown 30 0 3 7 11 20 2 

Totals 1,350 266,092 25 94 241 382 850 

Totals ≥

250 MW 363 200,205 2 32 78 116 255 

0.75% SCR Conversion 

≥1,200 14 18,989 7% 0 0 3 5 5 0 

900 – 1,199 13 13,378 5% 0 2 3 3 7 0 

700 - 899 67 52,803 20% 0 1 10 12 40 0 

400 - 699 136 74,345 28% 0 3 24 40 98 2 

250 - 399 133 40,690 15% 0 2 14 22 77 0 

<250 957 65,887 25% 8 26 124 197 564 17 

Unknown 30 0 0 5 7 20 2 

Totals 1,350 266,092 8 34 183 286 812 

Totals ≥

250 MW 363 200,205 0 8 59 89 247 

0.5% SCR Conversion

≥1,200 14 18,989 7% 0 0 3 4 5 0 

900 – 1,199 13 13,378 5% 0 0 3 3 5 0 

700 - 899 67 52,803 20% 0 0 6 10 32 0 

400 - 699 136 74,345 28% 0 0 13 26 85 2 

250 - 399 133 40,690 15% 0 0 9 15 69 0 

<250 957 65,887 25% 0 8 64 129 496 17 

Unknown 30 0 0 3 7 11 2 

Totals 1,350 266,092 0 8 101 194 703 

Totals ≥

250 MW 363 200,205 0 0 37 65 207 

In Table 2.2, SO3 from SCR conversion was calculated for conversion rates of 0.5%, 0.75%, and 

1.0%. This range of scenarios was selected to address the effects of catalyst aging as well as 

differences in production rate with new SCR installations. As SCR catalysts age, their NOx 

reduction efficiencies decline; however, their SO3 oxidation rates remain essentially constant. 

Thus, at some point in time additional catalyst volume will need to be installed on each unit in 

order to maintain the required NOx performance (the usual approach taken to extend the service 

7 
time of installed catalysts). This means that a unit with a 0.5% SO2 oxidation catalyst would be 

expected to eventually behave like the 0.75% catalyst in the table. Similarly, a 0.75% catalyst 

unit would be expected to eventually produce somewhat more SO3 than that shown for a 1% 

catalyst in the table. Because of the issue of SO3 formation, utilities are strongly pushing for the 

absolute minimum SO3 conversion, to avoid this problem. So while the additional beds will 

result in an increase in conversion, the competing trend is to begin with a catalyst that generates 

as little SO3 as possible. That pressure will push toward the 0.5% (or even lower) conversion 

catalysts. 

A summary of the coal sulfur contents in the databases used to generate Table 2.2 are provided in 

Table 2.3. It is worth noting that there are reports that several utilities are considering switching 

from low sulfur coals to higher sulfur coals after retrofitting units with scrubbers.20-25 Such 

actions would result in higher SO3 emissions than those shown in Table 2.2 for the same SCR 

conversion rates and would offset reductions otherwise achievable by use of lower conversion 

rate catalysts. 

Table 2.3. Sulfur contents of coals fired in plants in the Mississippi Valley and Eastern U.S. 

Load Range 

(MW) 

Total 

Number in 

Range 

Total 

Capacity 

(MW) 

Percentage 

of Capacity 

Included in 

Range 

S >3% S 2% to 

3% 

S 1% to 2% S < 1% Missing 

Data 

≥1,200 14 18,989 6% 3 2 0 9 0 

900 – 1,199 13 13,378 4% 3 0 0 10 0 

700 - 899 67 52,803 17% 4 6 14 43 0 

400 - 699 138 74,345 24% 23 15 42 56 2 

250-399 133 40,690 13% 12 6 42 73 0 

200 - 249 74 16,482 5% 11 7 26 30 0 

<200 882 65,887 21% 82 105 288 389 18 

Unknown 24 2 5 4 11 2 

Totals 1,345 282,574 140 146 416 621 

Totals ≥250 365 200,205 45 29 98 191 

Totals ≥ 200 439 216,687 56 36 124 221 

The estimated SO3 removal efficiencies provided by pollution abatement systems are 

summarized in Table 2.4, with comparison to previously reported reference values .

17,26

 Air preheaters were assumed to reduce SO3 by 30%. Units with fabric-filter fly ash collectors were 

credited with an additional 90% reduction in SO3. Units with cold-side electrostatic precipitators 

(ESP) were credited with an additional 25% reduction in SO3. For these calculations no effect 

was attributed to either hot-side electrostatic precipitators or wet scrubbers. 

Comparison of the columns in Table 2.4 shows the control efficiency values selected are more 

conservative than those used in the previous references. Air heaters are commonly estimated to 

remove 50% of the SO3 reaching them. However, when the above removal percentages were 

8
applied to three plants which have both cold-side precipitators and scrubbers, two plants with 

only cold-side precipitators, and two plants with only scrubbers, the predicted values for the SO3 

concentrations in the stack gases fell far below the observed values. This led to a reconsideration 

of these removal efficiencies to better match measured values. This is shown in Table 2.5. 

Table 2.4. Estimated SO3 removal efficiencies (in percent) for control system components

 Data Source 

Device HSD (a) MH (b) Used in This 

Report 

Air Heater 50 50 30 

Hot ESP 0 0 0 

Cold ESP 25 50 25 

Fabric Filter 90 90 90 

Wet FGD scrubber 0 50 0 

(a) Hardman, Stacey, and Dismukes17 

(b) Monroe and Harrison26 

Table 2.5. Comparisons of observed and predicted SO3 concentrations 

Location 

Percent 

Sulfur in 

Coal 

Measured 

SO3 (ppm) SO3 Predicted Here 

Predictions Based on 

MH(a) Removal 

Efficiencies 

Control System 

No SCR 0.5% SCR 1% SCR 1% SCR 

Plant 1 2.6 14 8.3 13 3.4 Cold-side ESP & Scrubber 

Plant 2 3.75 ~35 11.9 18.5 4.9 Cold-side ESP & Scrubber 

Plant 3 2.9 ~13 9.2 14.3 3.7 Cold-side ESP & Scrubber 

Plant 4a 0.92 7.7 3.9 6.1 3.1 Cold-side ESP 

Plant 4b 0.92 9.6 3.9 6.1 3.1 Cold-side ESP 

Plant 5a 0.99 4.0 3.2 4.9 2.5 Cold-side ESP 

Plant 5b 0.99 7.7 3.2 4.9 2.5 Cold-side ESP 

Plant 6 3.35 2.6 6.3 2.3 (No SCR) Scrubber 

Plant 7 3.6 10.7 6.8 2.4 (No SCR) Scrubber 

Plant 8 0.86 6.5 5.8 6.3 2.2 Cold-side ESP 

a. Monroe and Harrison26 

Review of data obtained at several installations tends to show air heater removal efficiencies in 

the range of 20% to 50% with typical values of about 25%. Removal efficiencies across coldside precipitators in these same data sets were typically about 25% as well. Similarly, removal 

efficiencies as low as 15% to 20% have been measured across some fabric filters and removal 

efficiencies across wet scrubbers have been measured as low as 12% to 15%. In the case of air 

heaters, the removal efficiencies would be expected to be higher for lower exit gas temperatures, 

as illustrated in Figure 2.1. Similarly, removal across precipitators and fabric filters increases 

substantially as the gas temperature decreases to and below the acid dew point. Conversely, if the 

9
SO3 Concentration, ppm 

35 

30 

25 

20 

15 

10 

5 

0 

250 260 270 280 290 300 310 320 330 340 350 

Exit Temperature, F 

ESP Outlet 

ESP Outlet 

AH Outlet 

AH Outlet 

AH Outlet 

AH Outlet 

gas temperature through a precipitator or fabric filter is substantially above the acid dew point, 

little removal will take place. 

It is clear that the commonly cited estimated removal efficiencies result in under-predicting stack 

SO3 concentrations. In Table 2.4, the value of zero for removal efficiency in scrubbers is not 

meant to imply that no removal takes place in them. Rather, it simply means that the removal by 

preceding components was over-estimated so much that the results would have been biased 

severely low if another 30 to 50 percent reduction were applied. The values in Table 2.2 are 

believed to be reasonable estimates of the emissions from units using their current particulate 

control systems followed by wet scrubbers. 

Figure 2.1. Measured SO3 concentrations versus temperature at the exit of an air heater and cold-side electrostatic 

precipitator. 

Table 2.6 and Figure 2.2 show state-by-state tabulations of estimated SO3 emissions from 

individual power plant units in the Mississippi Valley and Eastern U.S. Only units with 

capacities of 200 MW or more were included, as these are the units that will be most likely 

affected by the upcoming regulations. While not specifically related to the pertinent issues of 

opacity, ground effects, or local acidic deposition, these data demonstrate the potential for a 

significant increase in fine PM emissions relative to the filterable fraction (fly ash). The SO3 

emissions estimates were made using the SO3/SO2 ratios developed by Hardman, Stacy, and 

Dismukes for eastern bituminous and western subbituminous coals.17 In applying the 

correlations it was assumed that the fuels in the 2002 Annual Steam-Electric Plant Operation and 

10
Design Database14 are still in use and that the units operate at an average annual load of 75% of 

capacity (allowing for some reduced load operation and outages). Table 2.6 and Figure 2.2 show 

that of the 17 states included in the state-by-state tabulations, six states (OH, PA, IN, TN, KY, 

and FL) account for nearly 70% of the total projected SO3 emissions. 

Table 2.6. State-by-state tabulation of estimated SO3 emissions from individual power plant units in the Mississippi 

Valley and Eastern U.S. 

State 

1 % SCR Conversion 0.75 % SCR Conversion 0.5 % SCR Conversion 

SO3 

Emissions 

(kg/yr) 

State % of 

Total 

SO3 

Emissions 

(kg/yr) 

State % of 

Total 

SO3 Emissions 

(kg/yr) 

State % of 

Total 

AL 5.79E+06 2.70% 4.86E+06 2.71% 3.94E+06 2.72% 

AR 1.24E+05 0.06% 1.05E+05 0.06% 8.50E+04 0.06% 

CT 7.19E+05 0.34% 6.05E+05 0.34% 4.92E+05 0.34% 

DC 6.04E+05 0.28% 5.09E+05 0.28% 4.14E+05 0.29% 

DE 8.80E+05 0.41% 7.41E+05 0.41% 6.03E+05 0.42% 

FL 1.24E+07 5.76% 1.04E+07 5.78% 8.43E+06 5.82% 

GA 7.66E+06 3.57% 6.43E+06 3.58% 5.21E+06 3.60% 

IA 1.26E+06 0.59% 1.00E+06 0.56% 7.46E+05 0.52% 

IL 4.90E+06 2.29% 4.03E+06 2.25% 3.16E+06 2.19% 

IN 2.13E+07 9.92% 1.78E+07 9.89% 1.43E+07 9.85% 

KY 1.68E+07 7.85% 1.42E+07 7.89% 1.15E+07 7.96% 

LA 1.29E+06 0.60% 1.05E+06 0.58% 8.11E+05 0.56% 

MA 6.09E+04 0.03% 5.13E+04 0.03% 4.17E+04 0.03% 

MD 6.35E+06 2.96% 5.35E+06 2.98% 4.35E+06 3.00% 

ME 1.02E+05 0.05% 8.60E+04 0.05% 6.99E+04 0.05% 

MI 5.44E+06 2.54% 4.52E+06 2.52% 3.60E+06 2.49% 

MN 1.23E+06 0.57% 9.74E+05 0.54% 7.16E+05 0.49% 

MO 6.71E+06 3.13% 5.47E+06 3.04% 4.22E+06 2.92% 

MS 1.37E+06 0.64% 1.16E+06 0.64% 9.40E+05 0.65% 

NC 9.02E+06 4.21% 7.60E+06 4.23% 6.18E+06 4.27% 

NH 0.00E+00 0.00% 0.00E+00 0.00% 0.00E+00 0.00% 

NJ 1.02E+06 0.48% 8.56E+05 0.48% 6.92E+05 0.48% 

NY 6.82E+06 3.18% 5.74E+06 3.20% 4.67E+06 3.22% 

OH 4.00E+07 18.64% 3.35E+07 18.67% 2.71E+07 18.72% 

PA 3.09E+07 14.39% 2.60E+07 14.48% 2.11E+07 14.60% 

RI 0.00E+00 0.00% 0.00E+00 0.00% 0.00E+00 0.00% 

SC 4.86E+06 2.26% 4.09E+06 2.28% 3.33E+06 2.30% 

TN 1.75E+07 8.15% 1.47E+07 8.17% 1.19E+07 8.18% 

VA 5.11E+06 2.38% 4.30E+06 2.39% 3.50E+06 2.41% 

VT 0.00E+00 0.00% 0.00E+00 0.00% 0.00E+00 0.00% 

WI 4.28E+06 2.00% 3.50E+06 1.95% 2.72E+06 1.88% 

WV 5.37E+04 0.03% 4.52E+04 0.03% 3.68E+04 0.03% 

Summary 

More than half of the units expected to be affected by the new regulations could reasonably 

require mitigation of SO3 emissions to meet opacity requirements and about a quarter could 

require mitigation to alleviate problems related to ground level SO3 concentrations. Evidence in 

the trade press suggests that several utilities are considering returning to the use of high sulfur 

coals after they install scrubbers. If they do, SO3 emissions would be expected to increase to 

levels higher than those projected in this report. This study dealt with only coal-fired units. It 

should also be noted that similar SO3 emission problems would be experienced at heavy oil-fired 

11
units, if NOx control using SCR is applied to them. This analysis suggests that mitigation of SO3 

emissions will be needed in some cases to address plume visibility issues, increased acid aerosol 

emissions, and PM2.5 emission levels, particularly at plants that install SCR and FGD and use 

high sulfur coal. The remainder of this report discusses measurement methods and SO3 

formation and removal processes across different plant components. This information can 

provide useful guidance for plant operators who are interested in a range of approaches to 

avoiding excess collateral SO3 emissions. 

It should be noted that there is a significant lack of actual measurements of the conversion rate of 

SO2 to SO3 in the boiler. This lack makes predictions highly uncertain because of the inability to 

validate prediction results against actual measurements. Although the predictions made in the 

following chapters are in line with measured results, the scarcity of measurements makes it very 

uncertain whether such results can be repeated for the range of units in service. Further 

measurements of boiler exit SO3 concentrations are key to advancing our understanding of, and 

ability to predict, SO3 emissions. 

SO3 Emissions - 1% SCR Conversion 

Figure 2.2. SO3 emissions for states in the Mississippi Valley and Eastern U.S. emitting > 1% of total projected SO3 

emissions for the region (215,000 tonne/year total). 

OH 

PA 

IN 

TN KY 

FL 

NC 

GA 

NY MO MD AL MI VA IL SC WI 

0.00% 

2.00% 

4.00% 

6.00% 

8.00% 

10.00% 

12.00% 

14.00% 

16.00% 

18.00% 

20.00% 

12
3. IMPROVEMENTS IN SO3 MEASUREMENT TECHNOLOGIES 

Current SO3/ H2SO4 Measurement and Monitoring Methods

Method 8, the EPA promulgated method for measuring emissions of sulfuric acid mist,27 has a 

lower detection limit of about 50 mg/m3, thus it lacks the sensitivity needed for measurements at 

electric utility plants. The manual controlled condensation method (CCM) developed by Cheney 

and Homolya28 is generally recognized as the most reliable method for measuring SO3/ H2SO4 at 

the levels encountered at power plants.29 In the CCM, a sample gas stream is conveyed through 

a heated quartz-lined probe, through a quartz fiber filter and then through a condenser in which 

the acid vapor is removed. The probe and filter holder are held at a temperature at or above 

550 °F to evaporate condensed H2SO4 and ensure that the SO3/H2SO4 in the sample is entirely in 

vapor form before reaching the filter. The condenser is maintained at a temperature above the 

moisture dew point but well below the sulfuric acid dew point so that all of the acid is collected 

in it. A second filter downstream of the condenser ensures that any aerosolized acid is retained 

for analysis with that collected in the condenser itself. After each sample is collected, the 

condensor is washed to recover the collected acid. The amount of acid collected is later 

quantified in the laboratory by titration. Over the past few years several improvements have been 

made to the method as it is applied to coal-fired utility installations.30,31 The changes deal with 

minimizing problems arising from the acid vapor being adsorbed on or reacting with ash 

collected on the filter upstream of the condenser or the filter medium itself, together with the 

problem of ensuring that any sulfuric acid mist in the sampled gas is completely volatilized in the 

probe. The first problem is significant upstream of particulate control devices and the second is 

especially important downstream of a scrubber where the H2SO4 is, for practical purposes, 

entirely in the condensed phase. Both problems must be addressed in automated as well as 

manual systems. The CCM as described above was used as a manual reference measurement for 

comparison with the results of measurements reported here. 

Design and Fabrication of Sample Extraction and Transport System 

for SO3/H2SO4 

A semi-continuous SO3/H2SO4 monitor was developed by Southern Research Institute under 

Cooperative Agreement No. DE-FC26-02NT41593 with the U.S. Department of Energy. A 

significant part for that project addressed a major requirement for a field-useable monitor: a 

sample extraction system that separates SO3/H2SO4 vapor from in-stack particulate matter (PM) 

and transports it efficiently as vapor to the sensor.32 

The sample extraction and transport system (SETS) had to extract and maintain or heat the flue 

gas to >550 °F, transport the sample stream, separate SO3/H2SO4 from particulate matter, deliver 

SO3/H2SO4 to the analyzer, and return excess flue gas and PM to the duct. Heating of the sample 

gas is accomplished primarily by the injection of very hot dilution gas (at a dilution ratio of 

approximately 1:1) near the probe entrance as well as from contact heating. Flow through the 

SETS is driven by an eductor, with the hot sample gas being extracted from the probe through an 

annular filter upstream of the eductor. Because of proven performance in similar applications, a 

commercially-available probe designed for use in monitoring mercury emissions, the Apogee 

Scientific QSIS, was purchased and modified to include the hot dilution gas injection for heating 

13
the sample gas as illustrated in Figure 3.1. The QSIS probe and filter element are fabricated from 

stainless steel with a proprietary coating applied to all surfaces to minimize reactions with the 

sample gas. 

The exhausts of some wet FGD scrubbers contain significant quantities of droplets that are too 

large (on the order of tens of microns) to evaporate quickly which would lead to their being lost 

on internal surfaces of the sampling nozzle and transport line. Over time, such droplets deposited 

on surfaces would dry, leaving a residue that would accumulate and interfere with normal 

operation of the system. Therefore, when monitoring downstream of wet FGD scrubbers the 

SETS should incorporate an inertial collector at the probe inlet to remove these droplets. In the 

past this approach has been found necessary for sample extraction in wet streams. In particular, 

SRI developed a series of such procedures for the California Air Resources Board for making 

particle size measurements in wet process streams that have been successfully utilized for over 

10 years.33 Fortunately, the condensed acid tends to reside in droplets having diameters of a few 

micrometers and smaller. 

Venturi ΔP 

Heated Line to 

Monitor 

To Blow

 Back Venturi 

Pressure Lines 

Sample Inlet 

Gas Exhaust 

Connect to Compressed Air 

Eductor

 TC 

Apogee Control Unit 

Heated Air 

Figure 3.1 Schematic diagram of the Apogee QSIS probe modified for use with the SO3 monitor. 

The QSIS probe extracts a flue gas sample at a flow rate of approximately 10 to 12 acfm through 

a 3⁄4-inch diameter sample probe. The extracted gas then passes axially along the length of a 1⁄2­

inch ID cylindrical porous filter element. A slipstream of sample gas is pulled radially through 

the filter element at a flow rate of a few liters per minute and directed to the sample collection 

and measurement system. Due to the high axial velocities in the sample probe and core of the 

14
cylindrical filter element, particles entrained in the gas stream are prevented from depositing on 

the inner walls of the filter. However, fine particles are capable of being trapped in the filter 

element over time. As a result, the filter element must be periodically removed for cleaning by 

backwashing. As in the original SETS design, flow through the Apogee system is driven by an 

eductor, with the analyzer extracting about 2 slpm of hot sample gas upstream of the eductor. 

The excess gas not used for the actual measurement is returned to the host duct. 

Prior to proceeding with the purchase and modification of a QSIS probe for use with the monitor, 

a brief series of tests was performed to check for any obvious problems with the use of the probe 

for SO3/H2SO4 sampling. These tests were carried out in SRI’s Coal Combustion Research 

Facility (CCRF) in conjunction with an ongoing mercury measurement program which utilized 

two such probes, one sampling at a point in the CCRF at which the gas temperature was about 

550 °F and the second down stream of the first, following two heat exchangers, at which the gas 

temperature was about 325 °F. A heated hose like that to be used between the probe and 

condensor for the SO3 monitor was used at each location to convey a sample stream to a 

conventional controlled-condensation condensor setup. Data obtained with this setup were then 

taken for comparison with data obtained with conventional controlled condensation setups that 

were being operated to obtain information for the mercury emissions program. The results of 

these tests are shown in Figure 3.2. The agreement was excellent at the 550 °F location and was 

reasonably good at the 325 °F location although the results with the QSIS probe were slightly 

higher than those from the conventional probe at the 325 °F location, perhaps due to a bias 

introduced by particulate matter on the filter of the CCM sampler. (The differences in 

concentrations between the two locations results from losses in the heat exchangers.) These 

results were deemed satisfactory enough to proceed with the purchase and modification of a 

QSIS probe for use with the SO3/H2SO4 monitoring system. 

Tests conducted as part of the current project compared measurements made utilizing the 

modified probe with those obtained using a conventional CCM setup. These tests were 

conducted at two conditions: (1) with a flue gas stream in which the H2SO4 present would be 

entirely in the vapor phase and (2) a gas stream in which the majority of the H2SO4 present 

would be in the condensed phase. Condensation of the H2SO4 in the flue gas for the second test 

condition was induced by means of a water spray in the duct upstream of the sampling location. 

An Illinois Basin coal with a 3.5 % sulfur content was fired in the CCRF for these tests. 

Sampling was done at a location about 30 feet downstream of the 325 °F location of Figure 3.2. 

As is the case for air preheaters on full-scale utility boilers, the air preheaters of the CCRF 

remove significant fractions of the SO3/ H2SO4 formed in the furnace. The air preheaters were 

operated to provide less cooling than that used in normal CCRF operations in order to obtain 

higher H2SO4 concentrations and to provide assurance that the H2SO4 was entirely in the vapor 

phase for the first test condition. A nominal temperature of 400 °F was selected as the air 

preheater exit temperature based on the amount of cooling expected from the water spray to be 

used to induce condensation by cooling for the second test condition. A target temperature of 270 

°F was selected for the second test condition. The saturation vapor pressure of H2SO4 at 270 °F 

was low enough that the majority of the H2SO4 vapor could be expected to condense. 

15
SO3, ppm@ 3% O2 

25 

20 

15 

10 

Conventional CCM at 325F Location 

Apogee Probe at 325F Location 

Conventional CCM at 550F Location 

Apogee Probe at 550F Location 

5 

0 

8:00 10:00 12:00 14:00 16:00 18:00 

Figure 3.2. Comparison of SO3 concentrations measured by conventional controlled-condensation methods (CCM) 

with those measured using Apogee QSIS probes and a heated hose to deliver samples to a controlledcondensation condenser. 

Three Apogee probe operating conditions were used during the tests without the water spray with 

a flue gas temperature of 393 °F. During this phase of the testing the probe was operated without 

dilution and with dilution of flue gas with filtered hot air; first with the dilution air at 394 °F and 

then with the dilution air at 1113 °F. The mixed gas temperature was 559 °F at the higher 

dilution air temperature. The dilution ratio used in these tests was 1:1 on a mass basis. The 

dilution air temperatures cited here are those measured at the exit of the dilution air heater. The 

actual temperature of the dilution air as it enters the probe is somewhat lower. No change was 

noted in the dilution-corrected concentrations under these conditions as illustrated by the first 

block of data points in Figure 3.3. The temperatures of the dilution air and the mixed sample and 

dilution gas are given on the figure for each test condition. The initial rise in the SO3/ H2SO4 

concentration seen at the beginning of the test was the result of losses in the air preheater that 

diminished as an ash coating built up on the heat exchanger surfaces which were initially clean. 

The amount of cooling provided by the water spray was less than expected at the sampling 

location, perhaps because of incomplete evaporation in the transit time between the point of 

injection and the sampling location. Therefore the air preheater exit gas temperature was adjusted 

16
0:00 6:00 12:00 18:00 0:00

70 

Apogee Corrected to 3% O2 

Conventional CCM Corrected to 3%O2 

SO3 Concentration, ppmv 

60 

50 

40 

30 

20 

10 

0 

Baseline: Tgas = 393F Spray on: Tgas = Spray on Baseline: Tgas = 393F 273F with Hot w/o Hot w/o Hot Dilution with Hot Dilution Dilution Dilution 

Dilution Air Temp. Dil. Air Temp.

 394F 1113F 1079F Dilution Air Temp. 

1276F 1290F 869F 

 Mixed Temp.

 394F 562F

 Mixed Temp.

 559F

559F 

 Mixed Temp. 

559F 434F 

Reduce Tgas to ~ 360 F 

to obtain target Temp. 

with spray 

Time 

Figure 3.3. Comparisons of SO3 concentrations measured by the conventional controlled-condensation method 

(CCM) with those measured using the modified Apogee QSIS probe incorporating the hot gas dilution 

approach to evaporating condensed H2SO4. 

downward somewhat to about 360 °F in order to obtain an acceptably low gas temperature at the 

test location. This resulted in a noticeable decrease in the total SO3/ H2SO4 in the duct. 

The dilution air temperature had to be raised to about 1280 °F when the water spray was 

employed in order to obtain the target 560 °F temperature for the mixed gas. The Apogee probe 

was also operated with a nominal 870 °F dilution gas temperature, resulting in a mixed gas 

temperature of 434 °F. At the latter condition a very slight drop in the measured SO3/ H2SO4 

concentration was noted as compared to that measured with the 560 °F mixed temperature. 

Finally, the Apogee probe was operated without the hot dilution gas at which point the measured 

concentration fell to a value close to the saturation concentration at the flue gas temperature, 

reflecting the loss of the condensed phase SO3/ H2SO4 in the filter. The large drop in the value 

measured with the QSIS probe without the hot dilution air while the spray remained on at the end 

of the test series resulted from condensed acid no longer being evaporated in the probe. 

It appears to have taken about five hours after the water spray was turned on for the conventional 

CCM probe to reach steady-state conditions and provide reliable results. Evidently it took that 

long for the internal surfaces of the CCM probe to get hot enough to evaporate the condensed 

phase acid. Again, the lower steady-state values measured with the conventional CCM as 

compared to those obtained with the Apogee probe through most of this test series are believed 

17
to have resulted from adsorption onto particulate matter which collects on the CCM filter. The 

QSIS probe as adapted for SO3 measurements is shown in Figure 3.4. 

Following the tests conducted at the SRI pilot-scale Coal Combustion Research Facility, a short 

series of tests were conducted on a stack downstream of a scrubber on a large coal-fired utility 

boiler. Results of measurements using a standard controlled condensation probe and filter setup 

with results using the inertial-separation hot-gas-dilution probe are given in Table 3.1. All values 

have been corrected to 3% oxygen. The intended value of mixed gas temperature in the dilution 

probe, 550 °F to 600 °F, was never obtained during these tests. Too much of the transport tubes 

and mixing zone were in the flue where they were cooled beyond the capability of the dilution 

gas heater. A mixed gas temperature of 445 °F was as great a value as could be achieved on Test 

1. At that temperature the dilution probe results approached those of the conventional probe but 

were clearly low when the mixed gas temperature was only 419 °F. During Test 2, additional 

insulation was added to the dilution probe, but the target mixed gas temperature still could not be 

reached. A simple solution to the problem has been found: simply add a spool piece and probe 

extension so that the hot gas transport tubes and mixing zone can be located outside the flue. Had 

the mixed gas been as hot as intended, one could be reasonably certain that all of the condensed 

acid in the flue was evaporated and thus would have been measured. However, complete 

evaporation may or may not have been achieved during the tests when the temperature was 445 

°F to 490 °F, so it is uncertain as to whether all of the acid present in the sample was available 

for measurement downstream of the filter. 

Table 3.1 Comparison of measurements made using a conventional CCM probe with those made with the hot-gas 

dilution probe downstream of a full-scale utility scrubber. 

Standard Controlled Condensation Probe Hot-gas Dilution Probe 

SO3, ppm @ 3% O2 SO3, ppm @ 3% O2 

Test Run Average Range Mixed Gas 

Temperature, °F 

Average Range 

1 19.3 18.7 – 20.7 419 14.1 12.7 – 15.5 

445 18.4 18.4 - 18.4 

2 18.0 17.6 – 18.6 480 – 490 17.0 16.2 – 17.9 

Recommended future work includes: 

1. Performing tests utilizing the hot gas dilution system to check for artifact SO3 formation by 

oxidation of SO2. 

2. Performing tests downstream of a scrubber to verify performance when the predominant 

form of SO3 is condensed-phase H2SO4 in a low temperature gas stream. 

18
Waste Exhaust 

Dilution Gas 

Mixer 

Mounting Flange 

Main Flow 

Shut-off 

D

F

Dilution Gas 

Heater 

Sample Take-offs 

Eductor 

Venturi Taps 

Sample Inlet 19 

Figure 3.4. Modified Apogee QSIS sampling probe. Upper: probe with insulating jacket in place. Lower: Probe with jacket removed to sh

(The waste exhaust normally points opposite the direction shown.) 
4. EXPLORATORY STUDY OF SO3 ADSORPTION BY 

COAL FLY ASH 

Introduction 

Removal of SO3 or H2SO4 from the flue gas can take place through a number of 

mechanisms: by uptake by fly ash, by condensation on the heat exchanger surfaces in air 

preheaters, by reaction with reagents injected for control of vapor phase SO3/H2SO4, or 

by condensation and subsequent collection in the form of particles. Significant removal 

can result from uptake by fly ash but the mechanisms are not fully understood and at 

present cannot be well predicted. This study was undertaken to provide data which might 

be used to aid in the development of a predictive model for uptake of SO3/H2SO4 by fly 

ash from coal-fired utility boilers. 

Approach

Nine sets of ash samples from a wide range of coal types were selected from an inventory 

of ashes that had previously been characterized during studies related to resistivity 

modification by SO3 injection. These ashes had been collected either by EPA Method 

1734 directly from flue gas streams or as hopper samples from dry particulate control 

devices (either electrostatic precipitators or fabric filters). All had been previously 

exposed to flue gas streams having unknown concentrations of SO3 prior to collection. 

The ashes selected included ashes from high and low sulfur eastern bituminous coals, low 

and moderate sulfur western sub-bituminous coals, a Powder River Basin coal, and a 

North Dakota lignite. 

The ashes were exposed to synthetic flue gas atmospheres in an apparatus normally used 

for studies related to fly ash resistivity and ash resistivity modification by moisture and 

sulfur trioxide.35 The apparatus consisted of a continuous-flow generator which produces 

a synthetic flue gas stream which was passed through a bell jar containing the samples in 

a temperature controlled oven. The synthetic flue gas was humidified, and heated to a 

temperature above that at which SO3 and moisture combine to form H2SO4, after which 

controlled amounts of SO3 were added to obtain the desired SO3 concentration. Previous 

experience in resistivity work with this apparatus had indicated that over a 72 to 96 hour 

exposure time SO3 would permeate and apparently approach equilibrium in a 1 mm thick 

ash layer.36 Fresh ash samples were spread as 1 mm thick layers in the bottoms of 12 

petrie dishes and placed in the bell jar for each exposure condition. The layout of the 

samples in the bell jar is illustrated in Figure 4.1. The arrows shown in Figure 4.1 show 

the positions of the SO3 inlet and outlet gas streams. Duplicate samples of three of the 

nine ashes were included in each test to provide information on the reproducibility of the 

results. Experiments were performed at four temperatures (350 °F, 375 °F, 450 °F, and 

850 °F) and two SO3 concentrations (20 ppmv and 70 ppmv). In each test, prior to adding 

SO3 to the gas stream, the samples were cycled up to 850 °F and held at that temperature 

overnight. In addition to the exposure tests, one baseline run was made in which the 

samples were subjected to the 850 °F temperature cycle but not exposed to SO3. 

Following the exposure to SO3, the samples were removed from the bell jar as rapidly as 

20
possible and the covers were placed on the petrie dishes after which they were 

individually extracted for measurement of soluble sulfates. 

A typical set of SO3 concentrations at the inlet and outlet of the bell jar are shown over 

the course of a test in Figure 4.2. As can be seen, the uptake of SO3 by the ash was 

initially fairly high, but declined fairly rapidly over the course of about two days, after 

which it continued to decline but at a much slower rate, indicting that the limiting 

capacity of the samples was never reached. 

Results 

The exposure conditions and results of the measurements are provided in Table 4.1. The 

percentage of the total entering the bell jar that was taken up by the ash is provided in the 

table together with the SO3 mass balance for each set of exposures. The mass balances 

indicate that the recovery in the analyses of SO3 taken up by the ash was essentially 

complete. The results are shown plotted versus the base:acid ratios of the ashes in Figure 

4.3. The curves in Figure 4.3 are present only for purposes of tying the results for each 

test condition together and have no physical or chemical significance. Given the strong 

correlation between SO3 uptake and the base:acid ratios of the ashes, it appears that 

chemi-sorption rather than physical adsorption dominates the process and that 

temperature plays a strong role as well. Based on rather crude extrapolations of the 

concentrations of SO3 at the outlet of the exposure chamber, it is estimated that the 

limiting capacities of the ashes are perhaps twice the maximum uptakes found in these 

experiments with the uptake rates being concentration and temperature dependent. 

Following the general approach used in developing predictive equations for ash 

resistivity,36 ash compositions based on atomic percentages of cations were calculated 

from the weight percentages as oxides shown in Table 4.2. The resulting atomic 

percentages are given in Table 4.3. Multiple regression was then used to obtain fitting 

coefficients for use as a predictive tool for SO3 uptake by ash based on the ash chemistry. 

Results of the regression are provided in Table 4.4. A plot of values predicted by the 

regression equation versus those measured in this study is shown in Figure 4.4. 

21
22 

Table 4.1. Exposure conditions and results 

 Experiment Baseline Exposure Exposure Exposure Exposure  Duration, hours 12 96 74.75 74.72 93.4 

Oven Temperature, °C 454 177 190 232 232 

Oven Temperature, °F 850 351 374 450 450 

 Nominal SO3 Conc., ppm 0 20 20 20 70 

Percent Capture by Ash 21.5 26.7 26.3 19.2 

Mass Balance, % 96.7 96.1 94.4 100.9 

Ash ID Ash Type Post Exposure Soluble Sulfate, % 

301 Low S W SB (a) 0.44 2.16 1.95 2.46 3.62 

9896-1-68 E Bit (b) 1.58 4.05 3.67 4.51 7.31 

303 Low S W SB 0.88 2.82 2.55 2.94 4.73 143 Low S W SB 0.88 2.76 3.46 3.36 6.10 304 Low S E Bit (c) 0.10 0.86 0.95 1.32 1.78 

304 dup 0.83 0.91 1.24 1.81 

9896-1-69 W SB (d) 1.51 4.16 3.98 4.54 8.39 

129 ND Lignite (e) 4.04 6.55 6.19 6.56 14.36 

129 dup 6.15 6.01 6.47 12.91 

305 Hi S E Bit (f) 0.78 1.59 1.59 1.99 2.72 

305 dup 1.68 1.71 2.15 2.70 

9896-1-57 PRB (g) 2.49 4.69 4.54 4.99 10.22 

(a) Low sulfur western subbituminous coal (b) Eastern bituminous coal (c) Low sulfur eastern bituminous coal (d) Western subbituminous coal (e) North Dakota lignite (f) High sulfur eastern bituminous coal (g) Power River Basin coal 
23 

Table 4.2. Properties of ashes used in adsorption study 

Ash mineral analyses, wt% 

Ash ID. Coal Type Base/Acid 

Ratio(a) 

Li2O Na2O K2O MgO CaO Fe2O3 Al203 SiO2 

301 Low S W SB (b) 0.149 0.03 0.51 1.7 1.3 4.4 5.0 25.8 59 

9896-1-68 E Bit (c) 0.354 0.04 1.20 2.40 1.80 7.4 13.1 25.5 45.6 

303 Low S W SB 0.462 0.05 0.34 0.42 6.30 19.5 4.3 24.1 41.2 143 Low S W SB 0.535 0.01 1.13 0.70 4.00 22.7 4.8 21.6 38.8 304 Low S E Bit (d) 0.092 0.04 0.19 2.70 0.85 0.56 4.1 32.2 56.4 

9896-1-69 W SB (e) 0.698 0.01 4.00 1.90 5.70 19.9 9.0 14.4 42.7 

129 ND Lignite (f) 1.558 0.02 1.58 0.20 8.90 32.2 12.6 12.3 22.6 305 Hi S E Bit (g) 0.242 0.05 0.34 3.10 1.10 2.20 12.5 27.1 50.5 

9896-1-57 PRB (h) 0.901 0.02 1.70 0.52 4.40 27.9 9.9 19.3 27.4 

(a) Base/Acid ratio = (Na2O+K2O+MgO+CaO+Fe2O3)/(Al2O3+SiO2+TiO2) 

(b) Low sulfur western subbituminous coal (c) Eastern bituminous coal (d) Low sulfur eastern bituminous coal (e) Western subbituminous coal (f) North Dakota lignite (g) High sulfur eastern bituminous coal (h) Power River Basin coal 
Table 4.3. Atomic concentrations of cations for ashes used in adsorption study 

Ash ID Li Na K Mg Ca Fe Al Si Ti 

301 0.047 0.385 0.844 1.126 2.739 0.875 7.068 22.626 0.490 

9896-1-68 0.066 0.952 1.253 1.639 4.843 2.409 7.343 18.383 0.637 

303 0.075 0.245 0.200 5.219 11.612 0.719 6.315 15.112 0.414 143 0.015 0.830 0.338 3.370 13.745 0.816 5.755 14.471 0.533 304 0.065 0.149 1.395 0.766 0.363 0.746 9.178 22.505 0.690 

9896-1-69 0.015 2.839 0.887 4.643 11.651 1.480 3.710 15.398 0.250 

129 0.030 1.132 0.094 7.318 19.032 2.092 3.199 8.228 0.192 305 0.085 0.278 1.667 1.031 1.483 2.366 8.036 20.962 0.562 

9896-1-57 0.031 1.288 0.259 3.826 17.437 1.738 5.308 10.548 0.753 

24 
Regression Statistics 

Multiple R 0.973909 

R Square 0.948498 

Adjusted R Square 0.942555 

Standard Error 0.081164 

Observations 59 

Analysis of Variance (ANOVA) 

Regression df SS MS F Significance F 

Residual 6 6.308791 1.051465 159.6114 1.1E-31 

Total 52 0.342558 0.006588 

58 6.651349 

Coefficients Standard Error t Stat P-value Lower 95% Upper 95% 

Intercept -2.99121 0.512215 -5.83974 3.44E-07 -4.01904 -1.96337 

log(SO3 conc., mg/am3) 0.513845 0.062655 8.20119 6.13E-11 0.388119 0.639571 

log(T, R) 0.785485 0.185629 4.231475 9.45E-05 0.412993 1.157977 

Log(Li+Na)(a) 0.270711 0.077058 3.513064 0.000926 0.116082 0.42534 

Log(K) -0.1177 0.05934 -1.98347 0.052606 -0.23677 0.001375 

Log(Mg+Ca)(b) 0.292563 0.081426 3.592989 0.000725 0.12917 0.455956 

Log(Fe) 0.288818 0.056989 5.068003 5.43E-06 0.174462 0.403174 

(a) Li + Na = Sum of atomic concentrations of Li and Na (b) Mg + Ca = Sum of atomic concentrations of Mg and Ca 

Table 4.4. Results of multiple regression to obtain predictive equation for uptake of SO3 

25 
26 

1 

143 

0.81 

2 

304 

1.04 

3 

305 

0.98 

4 

308 

0.71 

5 

308 

1.47 

6 

304 

0.99 

7 

143 

1.46 

8 

308 

0.82 

9 

305 

0.90 

10 

305 

1.12 

11 

143 

0.72 

12 

304 

0.97 

Figure 4.1. Layout of ash samples in the exposure chamber as seen from above. Simulated flue gas enters as shown by the arrow a

exits as shown by the arrow at the top of the figure. The upper number is the sample position number. The center number is 

in that position during an initial check for reproducibility and positional dependence and the third value is the SO3 uptake by 

this initial check. 
60 

SO

3 Concentration, mg/m3 

50 40 30 20 

Inlet Concentration Outlet Concentration 

10 

0 

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3

Time, days 

27 

Figure 4.2. Typical concentration versus time measurements at the inlet and outlet of the SO3 exposure chamb
18 

16 14 12 10 

8 6 4 2 0 

Baseline SO4 Total SO4, post 350F 20 ppm Total SO4, post 375F 20 ppm Total SO4, post 450F 20 ppm ND LTotal SO4, post 450F 70ppm PRB Total SO4, Post 850F 70ppm Low S WSB 

E Bit 

W 

Low S WSB

E Bit

Low S E Hi S E 

% Soluble Sulfate 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 

Base:Acid Ratio 

28

Figure 4.3. Total soluble sulfate found in ash samples after exposure to SO3 at the conditions indicated versus ash base
29

% Soluble SO

4 from Regression

16 14 12 10 

8 6 4 2 0 

0 2 4 6 8 10 12 14 

Measured % Soluble SO4 

Line o f p erfect aerfe gr eement o p ct a r

Figure 4.4 Plot of SO4 on ash as predicted by the regression equation and the values measured in this study. 
5. FORMATION OF SULFUR TRIOXIDE IN THE CONVECTION 

SECTION OF COAL-FIRED ELECTRIC UTILITY BOILERS 

Formation of SO3 in the Convection Section 

Southern Research Institute, working in cooperation with the U.S. Environmental Protection 

Agency, Electric Power Research Institute, and Tennessee Valley Authority, is developing a 

model relating SO3 formation in the convection sections of pulverized coal-fired boilers to coal 

properties, convection section design, and operating conditions. The model is intended to provide 

electric utilities with guidance on coal specification, sootblowing schedule, and excess air 

adjustment to minimize the contribution of convection section processes to formation of sulfuric 

acid, subject to the other constraints on fuel selection and system operation. 

The processes incorporated in the model are: catalytic oxidation of SO2 to SO3 by oxide scale on 

convection section tubes, catalytic oxidation of SO2 by ash deposits on the tubes, catalytic 

oxidation of SO2 by suspended fly ash particles, and adsorption of SO3 and H2SO4 by fly ash. 

The model is a refinement and significant extension of one developed by Peter Walsh and 

coworkers at Pennsylvania State University, working with the Consolidated Edison Co. of New 

York, Electric Power Research Institute, Empire State Electric Energy Research Corp., and 

Florida Power & Light Co. on particulate emissions from residual oil-fired boilers, and with the 

New York State Electric and Gas Corp. on the blue plume at Hickling Station in Corning, NY. 

That work is documented in two reports and an article.37-39 

The basis for the calculation of catalytic oxidation of SO2 to SO3 by tube scale, ash deposits, and 

fly ash is a chemical mechanism in which SO2 is oxidized by iron oxide. The oxide, which is 

reduced in the process, is returned to its higher oxidation state by reaction with oxygen. This 

system of reactions, coupled with diffusion of SO3 from its point of formation on the ironcontaining surfaces of tubes, deposits, or suspended fly ash to the free stream, reproduces the 

complex dependence of SO3 formation on temperature, excess oxygen, coal sulfur content, heat 

exchanger surface area, extent of fouling of the tubes, and boiler load. The calculation includes 

flue gas, tube, and deposit surface temperatures; reaction rates, including approach to 

equilibrium; contact times; and transport of SO3. There is no diffusion limitation on the supply 

of the reactants, SO2 and O2, because they are present in great excess. The calculation is done 

for each row of convective section tubes from the furnace exit through the economizer. 

Measurements at an oil-fired boiler showed that a situation that leads to excessive SO3 formation 

was the growth of fouling deposits on the usually relatively cool surfaces of the primary 

superheater toward the back end of the convection section, where tube spacing is relatively close 

and the tube surface to gas volume ratio is high. The growth of deposits in this region eventually 

leads to deposit surface temperatures in the most active temperature range, and the combination 

of these temperatures with high surface to volume ratio generates high levels of SO3. 

A related problem is illustrated in Figures 5.1, 5.2, and 5.3, for the case of the stoker-fired boiler 

at Hickling Station.38 The top panel in Figure 5.1 shows the surface temperature distribution on 

clean superheater tubes without fouling deposits on either the steam or fire sides. Sulfur trioxide 

30
formation is most rapid in the range of catalyst temperatures from 900 to 1400 o

F. The catalyst, 

in the case of clean tubes, is iron oxides in tube scale, nearly 100% iron oxides on the SA213 

T11 and T22 tubing. Sulfur trioxide formation over clean tubes, shown in the bottom panel of 

Figure 5.1, is most rapid near the entrance to the superheater where the temperature is at or 

above 900 oF. At the lower temperatures further back, SO3 formation is negligible. 

The case of tubes heavily fouled on the fire side is shown in Figure 5.2. Surface temperatures 

through almost the entire superheater are in the most active range. The catalyst is now the 

relatively low concentration of iron oxides (~ 0.13 wt%) in the ash deposits. The combined 

effect of higher temperature and lower iron oxide concentration is a modest increase in SO3 

leaving the superheater, from about 25 to 30 ppmv. 

The case of tubes moderately fouled on the steam side but clean on the fire side is shown in 

Figure 5.3. Temperatures through much of the superheater are in the most active range and most 

of the surface exposed to flue gas is catalytic tube scale. This combination results in a marked 

increase in SO3 leaving the superheater, to approximately 75 ppmv. These examples illustrate 

some of the complexity of the SO3 formation process and the usefulness of a mechanistic model 

in providing rational explanation for observations in the field. 

Comparisons of model predictions in the Hickling report38 with measured values at various 

locations through the plant are shown in Figure 5.4. As can be seen, the model as it existed at the 

time appeared to provide plausible results. 

Mechanisms of SO3 Formation 

Sulfur trioxide is formed by oxidation of sulfur dioxide according to the overall reaction, 

SO2 + 1/2 O2 --> SO3 (5.1) 

Two distinct reaction mechanisms are possible under conditions in boilers: homogeneous (gas) 

and heterogeneous (solid-catalyzed). In the flame and postflame regions, above about 1200 K 

(1700 o

F), the steady-state concentration of SO3 is approximately described by formation and 

destruction reactions of SO2 and SO3 with oxygen atoms:40 

SO2 + O + M --> SO3 + M (5.2) 

SO3 + O --> SO2 + O2 (5.3) 

"M" is any molecule, such as N2, CO2, H2O, O2, etc., that collides with SO2 and the O atom at 

about the same time and carries away energy, stabilizing the product. All of the possible 

collision partners are not equally efficient, a refinement that a detailed treatment might take into 

consideration. Discussions of this homogeneous reaction system are given by Merryman and 

Levy,40 Cullis and Mulcahy,41 and Smith, Wang, Tseregounis, and Westbrook.42 

31
Figure 5.1. Surface temperature and SO3 formation in the superheater under the assumption of negligible 

fouling and tube scale exposed to flue gas.38 

32
Figure 5.2. Surface temperature and SO3 formation in the superheater under the assumption of heavy fouling on the 

fire side, with ash deposits exposed to flue gas.38 

33
. 

Figure 5.3. Surface temperature and SO3 formation in the superheater under the assumption of moderate fouling on 

the steam side, with tube scale exposed to flue gas.38 

34 
Figure 5.4. Comparison of the calculated SO3 formation to measurements for the Hickling Station modeling.38 The 

model calculations were done under the assumptions of moderate fouling and a mixture of tube scale 

and ash deposits exposed to flue gas. 

In a region where conditions are not changing too rapidly, the system of reactions (5.2) and (5.3) 

approaches a steady state in which the ratio of SO3 to SO2 is constant, independent of the oxygen 

atom concentration.40 At the furnace exit, far from the flame zone, where the oxygen atom 

concentration might reasonably be assumed to approach equilibrium at the local temperature, a 

useful approximation, in the absence of a direct measurement, may be to assume that SO2 and 

SO3 are also in equilibrium at that temperature. This would typically give an SO3 volume 

fraction on the order of 10 to 20 ppmv in combustion products from fuel containing 1 to 2 wt % 

sulfur at temperatures in the vicinity of 1560 K (2350 o

F). Measurements in a pilot-scale 

combustion tunnel firing residual fuel oil43 support the assumption that SO3 volume fractions in a 

boiler furnace and at the furnace exit are of roughly this magnitude. 

The equilibrium distribution of the sulfur oxides shifts toward SO3 as temperature decreases. 

Therefore, as combustion products cool, near the furnace exit and further downstream, 

35 
equilibrium favors additional conversion of SO2 to SO3, but the rate of the homogeneous 

reactions decreases with decreasing temperature, closing off this route for SO3 formation. The 

rate of SO2 oxidation may still be appreciable, however, in the presence of a heterogeneous 

catalyst. Solid-catalyzed reactions are thought to be the source of the high levels of SO3 

sometimes observed at the economizer outlet. 

The best-known catalyst for SO2 oxidation is vanadium pentoxide, V2O5, used to generate SO3 in 

the manufacture of sulfuric acid. V2O5 is a component of selective catalytic NOX reduction 

(SCR) catalysts and the ash constituent typically making the greatest contribution to catalytic 

SO3 formation in high-sulfur residual oil firing. Because of the importance of catalytic oxidation 

of SO2 in the manufacture of a major industrial chemical, this reaction has been the subject of 

numerous investigations.44 

A comparison of the catalytic activities of iron and vanadium oxides for SO2 oxidation, for the 

specific purpose of understanding SO3 formation in oil-fired boilers, was made in laboratory 

experiments performed by Wickert.45 His results demonstrated two important features relevant 

to boilers: (1) iron oxide was an active catalyst, only slightly less active than vanadium 

pentoxide, and with peak activity shifted to slightly higher temperatures than that for vanadium 

oxide, and (2) during a fixed residence time in contact with iron pentoxide, conversion of SO2 to 

SO3 was greatest over the range of temperatures from roughly 760 to 1030 K (900 to 1400 oF). 

The evident activity of iron oxide and the abundance of iron oxide in coal ash and tube scale 

suggest that it is likely to be the principal catalyst for SO3 formation in coal-fired boilers. Iron 

oxides are present in tube scale, in ash deposits on tubes, and in suspended fly ash. Significant 

conversion is observed only over a limited range of temperature because at higher temperatures 

formation of SO3 is limited by the equilibrium distribution of sulfur oxides and at lower 

temperatures by the rate of the solid-catalyzed reaction. 

Models for both homogeneous formation of SO3 and heterogeneous formation catalyzed by ash 

deposits in oil-fired boilers were developed by Squires.46 Squires treated the temperature 

dependence of conversion in the presence of catalyst by incorporating, in the rate coefficient for 

the heterogeneous reaction, a temperature dependence derived from the measurements of 

Wickert.45 This approach has the disadvantage that calculated SO2 oxidation could proceed to 

unrealistically high conversion in a system having long gas-catalyst contact time, regardless of 

the equilibrium constraint. 

Development of the model described below also began with a study of SO3 formation in the 

convection sections of residual-oil-fired boilers.37,39,47 The reactions were described using rate 

coefficients having Arrhenius temperature dependence, with conversion limited by the 

equilibrium between sulfur oxides. The oil-fired case did not require that a contribution from 

suspended ash be considered, because the ash concentration and its surface area per unit of gas 

volume are small. The model was applied to a coal-fired stoker boiler by Walsh, DeJohn, 

Bower, and Rahimi.

38 These authors included an approximate treatment of the contribution to 

SO3 formation from catalysis by fly ash, which was small, but significant. In pulverized coal 

firing, the fly ash concentration and contribution to SO3 formation are expected to be larger than 

in a stoker-fired boiler, therefore a more rigorous treatment of the contribution to SO3 formation 

36 
⎡

⎢

⎢⎣

1− Y S C X ⎤ Fe2O3 SO3 s ,s

⎥

⎥⎦

rSO3 = 1 1 X SO3 + ,eq

k1 X k SO 2 X 2 O2 

Sh DSO3 S C X SO3 ,s − X SO3 r ( ) SO3 = bl d 

by catalytic reaction on suspended fly ash has been implemented in the present version of the 

model. 

Model for Catalytic SO3 Formation in the Convection Section 

The kinetics of heterogeneous oxidation of SO2 were reviewed by Urbanek and Trela.44 The 

essential features of the system are oxidation of SO2 by a metal oxide, accompanied by reduction 

of the metal to a lower oxidation state having little or no catalytic activity. The active higher 

oxide is regenerated by reaction with oxygen. The cycle can be represented as follows: 

k1 

SO2 + FeOX ---> SO3 + FeOX-1 (5.4) 

k2 

FeOX-1 + 1/2 O2 ---> FeOX (5.5) 

Equations 5.4 and 5.5 are the overall reactions, not elementary steps, and k1 and k2 are global 

rate coefficients. The reactions are assumed to be first order with respect to the gaseous species 

and the iron oxides. The rate of formation of SO3 by reaction 5.4 is then (please see the list of 

nomenclature for definitions of the symbols): 

r k1 YFeO S C X (5.6) SO3 X =

When the catalyst composition is steady, the mass fraction of the higher oxide, FeOX, can be

written in terms of the total mass fraction of iron oxides, YFe2O3, from an ash analysis, or any 

other measure of total iron content, and the relative rates of formation and destruction of the 

higher and lower oxides: 

s SO2 

(5.7)

The factor in brackets is introduced to account for the approach of the rate to zero as the SO3 

concentration adjacent to the catalyst surface approaches its equilibrium value. At steady state, 

the rate of SO3 formation at the deposit surface equals its rate of transport from the surface to the 

free stream: 

(5.8)

=

Combining Eqs. 5.7 and 5.8 to eliminate the unknown concentration of SO3 at the deposit 

surface: 

rSO k S C 3 g (X SO3 ,eq − X SO3 ) (5.9) 

37 
1 k = C X ⎡ ⎤ 1 g SO3 ,eq 1 1 ⎢

⎢⎣

⎥

⎥⎦

+ + Sh D C Ys k1 X k SO3 C Fe 2 X bl 2O3 SO2 O

d Cg 

2 

in which the effective rate coefficient is: 

(5.10)

Under conditions of interest in boilers the extent of conversion of SO2 to SO3 is small, so the 

changes in SO2 and O2 concentrations accompanying SO3 formation may be neglected. The rate 

of SO3 production, Eq. 5.9, is equated to the rate of change in molar flux of SO3 with position in 

the convection section. Integration of the resulting expression, with the boundary condition XSO3 

= XSO3,o at t = to, gives an expression for the change in SO3 mole fraction with residence time in a 

row of tubes in which deposit composition and surface temperature are uniform: 

X SO3 X = ,eq − (X − X ) exp[−k S (t − t )] SO3 SO3 SO3 , eq , o o

Inspection of the rate expression, Eq. 5.9, and effective rate coefficient, Eq. 5.10, shows that 

there are four regimes of SO3 formation: (1) equilibrium; (2) control by diffusion through the 

concentration boundary layer adjacent to the deposit (near equilibrium at the surface); (3) control 

by O2; and (4) control by SO2. 

The temperature dependence of SO3 at equilibrium in the products of combustion of 1.41 wt % 

sulfur coal with 5 vol % excess oxygen is shown in Figure 5.5a and the temperature dependence 

of the effective rate coefficient and its components is shown in Figure 5.5b, as functions of the 

reciprocal of absolute temperature. The total volume fraction of sulfur oxides (SO2 + SO3) is 

1170 ppmv. The equilibrium volume fraction of SO3 (Figure 5.5a) is the ultimate value reached 

if the mixture were allowed to stand at the specified temperature, with or without catalyst, for a 

long period of time. Referring to the Fahrenheit temperature scale at the top in Figure 5.5a, we 

see that at equilibrium at 2200 o

F (1478 K), the gas would contain 10 ppmv SO3; at 1800 oF 

(1255 K) it would contain 40 ppmv SO3; at 1400 o

F (1033 K) it would contain 300 ppmv SO3; 

and at 1000 o

F (811 K) it would contain 1000 ppmv SO3. 

In the flow of combustion products through a convection section, in which the volume fraction of 

SO3 is rising from a low value but unable to keep up with the increase in equilibrium 

(5.11)

38 
Figure 5.5. Equilibrium and kinetic constraints on SO3 formation.38 (a) Equilibrium mole fraction of SO3 in the 

products of combustion of Lee coal (wt %, as received: moisture, 11.62; ash, 17.51; volatile matter, 

22.92; fixed carbon, 47.95; sulfur, 1.41; and 10,508 Btu/lb HHV, as received). (b) Rate coefficients for: 

diffusion of SO3 from the surface of a tube or deposit to the free stream; oxidation of SO2 by reaction 

with metal oxide, Reaction (5.4); reoxidation of reduced metal oxide to the active state, Reaction (5.5); 

and the overall reaction, when these three processes all occur simultaneously. 

concentration associated with the decrease in temperature, the equilibrium concentration at the 

local temperature of the point of formation (the catalyst surface) will not be exceeded. The curve 

shown in Figure 5.5a therefore represents the maximum possible SO3 that could be found in flue 

gas adjacent to a catalytic surface at the specified temperature. 

39 
Walsh, Mormile, and Piper39 proposed the rate coefficients shown in Figure 5.5b to simulate SO3 

formation in a residual-oil-fired boiler. The line labeled "diffusion" is the mass transfer 

coefficient for SO3 formed at the surface of a tube or ash deposit to the free stream. The dashed 

and dot-dash curves are effective rate coefficients for reactions (5.4) and (5.5). They are termed 

"effective" because the concentration of catalytic metal oxide on the surface and the distribution 

of the oxides between the higher and lower oxidation states are also incorporated in the 

coefficients. The solid curve is the overall effective rate coefficient governing the formation of 

SO3. The overall rate coefficient is never greater than the rate coefficient for the slowest process 

at a given temperature, which shifts from SO2 oxidation at low temperatures, to catalyst (metal 

oxide) regeneration at intermediate temperatures, to mass transfer at the highest temperatures. 

At high temperatures equilibrium also limits the conversion of SO2 to SO3. A connection 

between diffusion control and equilibrium at the catalyst surface is expected because, when both 

reactants (SO2 and O2) are present in excess, a limitation by mass transfer implies that the 

chemical reaction at the deposit surface is much faster than diffusion and, therefore, that the 

concentration of the product, SO3, at the surface approaches its equilibrium value. 

Combination of the upper limit imposed by equilibrium (Figure 5.5a) with the limit imposed by 

the rate coefficient for SO3 formation and the residence time of gas in contact with catalyst, 

results in the characteristic behavior observed by Wickert,45 i.e. conversion of SO2 to SO3 is 

greatest over a narrow range of temperatures. At lower temperature, the rates of the reactions are 

too slow and at higher temperatures the equilibrium concentration (the maximum possible 

concentration in the situation under consideration) of SO3 is low. 

The choice of active surface area is a question where little guidance is available. Several 

possible assumptions were examined by Shareef, Homolya, and Mormile.48 In the present work, 

the surface area of the convective tubes was used as the basis for the estimate of catalyst surface 

area, and the mass fraction of vanadium oxides in deposits provided an estimate of the fraction of 

the surface which was active. Measurements of the vanadium content of the outer surface of 

deposits from the boiler showed little variation in vanadium content through the convective 

section.49 

Simultaneous SO3 Formation on Ash Deposits and Suspended Particles 

The equations describing SO3 formation catalyzed by iron oxide in suspended fly ash particles 

and by iron oxide in stationary deposits or tube scale are the same, Eq. (5.11), but with their 

respective external surface areas per unit of gas volume, S, Sherwood numbers, Sh, characteristic 

lengths, d (tube and mean particle diameters), mass fractions of iron oxides, YFe2O3, and 

temperatures, T (flue gas temperature for fly ash and surface temperature for deposit or clean 

tube). The total rate of SO3 formation, including the reaction on tube surface or ash deposits, 

subscript "t," and the reaction on suspended fly ash, subscript "p," is the sum of the contributions 

from the two parallel processes: 

rSO3 = kt St Cg (X SO3 ,eq,t − X SO3 ) + k p S p Cg (X SO3 ,eq, p − X SO3 ) (5.12) 

40 
X = kt St X SO3 ,eq,t + k p S p X SO3 ,eq, p

SO3 kt St + k p S p 

−

⎣

⎢

⎡kt St X SO

k 

3 , 

t

eq

S 

,t

t 

+

+

k

k

p

p

S

S

p

p

X SO3 ,eq, p − X SO3 ,o ⎥⎦

⎥

⎤

exp[−(kt St + k p S p )(t − to )] ⎢

A derivation parallel to that for a single catalytic surface, above, gives the following expression 

for the mole fraction of SO3 in flue gas at the outlet from the space surrounding a row of tubes, 

in the presence of suspended fly ash, when the mole fraction at the inlet to the space is XSO3,o: 

(5.13) 

The first term on the right-hand side, which also appears as the first term inside the large 

brackets, is an average equilibrium SO3 mole fraction, weighted by the relative rates of formation 

of SO3 over deposit or tube surface and fly ash. 

Calculations 

Design data and operating conditions for the boiler of a large coal-fired electric generating unit 

were provided by the utility to which it belongs. The model described above was used to 

calculate SO3 formation in the convection section between the furnace exit and the inlet to the 

economizer at full load. This region included, in order, beginning at the furnace exit, the 

secondary superheater, reheat superheater, primary superheater, and horizontal reheat 

superheater, a total of 140 rows of tubes. For the present, preliminary calculations the flue gas 

temperature change from the furnace exit to the economizer inlet was divided into 140 equal 

increments, corresponding to a temperature change of approximately 5 K (9 o

F) across each row 

of tubes. The resulting distribution of gas temperatures through the region under consideration is 

shown in Figure 5.6. The temperature drop shown in the figure for the economizer was 

estimated simply by dividing the overall temperature change and residence time into four equal 

increments. 

The difference between steam inlet and outlet temperatures for each tube bank was divided into a 

number of increments equal to the number of rows of tubes in the bank to estimate the steam 

temperature in each row. The general direction of steam flow in each bank was taken into 

consideration, where an overall direction could be identified, but finer details of the flow were 

not considered, for example in some sections of the secondary and reheat superheaters, where the 

steam flow enters through tubes in the middle of a bank, then goes to upstream and downstream 

tubes in the same bank. The actual volumetric flow rate of flue gas was calculated at each tube 

row and used, with the tube diameter, to determine the residence time in the empty space 

associated with that row, the Reynolds, Nusselt, and Sherwood numbers for the tubes, and the 

tube surface area per unit of gas volume. The tubes were assumed to be covered with ash to the 

extent that the heat transfer resistance from the deposits plus tube metal plus steam film was 

equal to the external film resistance in the flue gas. The deposit surface temperature was 

assumed to be uniform around the perimeter, along the height or length, and over the width of a 

given row of tubes. 

41 
A mean particle size of 10 micrometers was assumed for the fly ash. Its concentration in the flue 

gas was calculated from the coal analysis and local flue gas volumetric flow rate, assuming that 

20% of the ash feed was retained in the boiler. Loss of ash by deposition and fall-out in the 

convection section was not considered. The fly ash temperature was set equal to the flue gas 

temperature at every point. The formation of SO3, catalyzed by fly ash, in the cavities within and 

between convective tube banks was included, as well as in the space surrounding the tubes. 

The calculation was begun at the furnace exit, with the equilibrium SO3 in flue gas containing 

SO2 and O2 corresponding to the coal analysis and typical excess air, at the furnace exit gas 

temperature. The calculated volume fraction of SO3 at this point is 12 ppm. The increment in 

the volume fraction of SO3 was calculated for each of the 140 rows of tubes using Eq. (5.13) and 

local values for the geometry, temperatures, and flow conditions, with the results shown in 

Figure 5.7, as the volume fraction of SO3 versus flue gas residence time. The rate coefficients, 

k1 and k2, for the iron-oxide-catalyzed reactions (5.4) and (5.5), respectively, were given the 

same values used by Walsh, DeJohn, Bower, and Rahimi

38 to explain their measurements in 

Boiler No. 1 at Hickling Station. The values assigned to the parameters used in the calculations 

are listed in Table 5.1. 

Table 5.1 Conditions for the SO3 model run whose results are shown in Figures 5.6 

and 5.7. 

Parameter Value 

Barometric pressure, inch Hg 29.50 

Excess air 0.24 

Stoichiometric air/fuel mass ratio, kg/kg 8.570 

Coal feed rate, kg/s 145.56 

Mass fraction sulfur in coal, kg/kg 0.03 

Mass fraction ash in coal, kg/kg 0.08 

Mass fraction iron oxides in ash, SO3 free 0.2 

Mass fraction iron oxides in tube scale 1.0 

Average mass fraction iron oxides presented to flow over tubes 0.2000 

Ratio of heat transfer coefficients, (steam+deposits)/gas 1 

Pressure at furnace exit, inch water column 0 

Furnace exit gas temperature, oF 2350 

Average molecular weight of flue gas, wet, kg/kmol 29.1 

External surface area-weighted mean particle size, m 0.000010 

Fly ash apparent density, kg/m3 2500 

Weight fraction of ash feed to furnace bottom 0.2 

Weight fraction of ash feed to economizer hopper 0.05 

Volume fraction O2 in furnace exit gas, wet 0.038 

Catalyst deactivation rate, activation energy, J/kmol 2.40E+08 

Catalyst deactivation rate, preexponential factor 4.794461E+14 

Catalyst regeneration rate, activation energy, J/kmol 5.00E+07 

Catalyst regeneration rate, preexponential factor 0.959806 

42 
43

Gas Temperature, F 

2500 2000 1500 1000 

500 

0 

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1

Time, seconds 

Gas temperature Deposit temperature 

Secondary Superheater 

Reheat Superheater 

Primary Superheater 

Horizontal Reheat 

Ec

Figure 5.6. Modeled temperature profiles through a 1300 MW pulverized coal-fired utility boiler. 
0 

10 20 30 40 50 60 70 80 90 

100 

SO3 Concentration, ppmv

Model Measured 

Secondary Superheater 

Horizontal Reheat 

Reheat EconoSuperheater 

Primary Superheater 

44 

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 

Time, seconds 

Figure 5.7. SO 3 concentration profile through a 1300 MW pulverized coal-fired utility boiler as predicted by the model. 
Results and Discussion 

The surface temperature of the deposit on the first row of secondary superheater tubes, at the 

furnace exit, was estimated to be 1130 K (1574 o

F), above the high end of the temperature range in 

which catalytic SO3 formation is thought to be most rapid. Nonetheless, the formation rate is 

significant. The discontinuities in the SO3 profile in the figure are cavities between tube bundles. 

The small positive slopes of the line segments in these regions are indicative of the contributions to 

SO3 formation by suspended fly ash. Its contribution is small throughout the convective section 

because the concentration and external surface area of fly ash are low. 

At the entrance to the outlet (third) section of the secondary superheater, the deposit surface 

temperature is 1068 K (1463 o

F), still above the most favorable range for SO3 formation, but the 

side-to-side spacing of the tubes has been reduced by half and the number of tube assemblies 

doubled, compared to that in the first two secondary superheater sections, so the surface area per 

unit of gas volume is doubled, with a corresponding increase in the rate of SO3 formation. 

At the entrance to the reheat superheater the deposit surface temperature is 1036 K (1405 o

F) and 

the tube density is the same as at the exit from the secondary superheater. The rate of SO3 

formation in this region remains high. In the second section of the reheat superheater, however, the 

rate is still high but beginning to decline. The deposit surface temperature at the exit from this 

section is 868 K (1103 o

F), near the temperature at which the highest conversion was observed by 

Wickert45 in the presence of iron oxide. Evidently, the temperature dependence of the activity 

obtained by fitting the Hickling Station measurements is not the same as that of the iron oxide 

samples examined by Wickert. 

In the short section at the entrance to the primary superheater (the entrance with respect to the flue 

gas flow, it is actually the outlet with respect to the steam flow) the rate of SO3 formation again 

shows a marked increase, due to another reduction in side-to-side tube spacing and near doubling of 

the surface area of ash deposits per unit of gas volume. From the slopes of the line segments 

between the following widely spaced (in the flow direction) tubes, one can see the contribution to 

SO3 formation from the heterogeneous reaction on suspended fly ash. In the last two sections of the 

primary superheater, the rate of SO3 formation remains high. 

The deposit surface temperature at the entrance to the horizontal reheat superheater is 794 K (970 o

F). At this point the rate of SO3 formation shows a large decline, due at least in part to a factor of 

two reduction in the side-to-side tube spacing. Also, surface temperature is evidently approaching 

the lower end of the most active range, because the rate of SO3 formation leaving this section has 

practically dropped to zero. The surface temperature is now 727 K (849 o

F), in agreement with the 

lower end of the active range of temperatures observed by Wickert.45 

Absorption of SO3 and H2SO4 by Fly Ash 

Walsh, DeJohn, Bower, and Rahimi

38 inferred a rapid decline in SO3 at temperatures below about 

700 K (800 o

F) in the flue of Boiler No. 1 at Hickling Station. This is approximately the 

temperature at which reaction with water vapor begins to convert SO3 to H2SO4. A preliminary 

treatment of this process was incorporated in the present calculation by considering the rate of 

diffusion of H2SO4 to suspended fly ash, in combination with the capacity of ash for SO3/H2SO4 

uptake described in another section of this report. For this calculation the temperature drop and 

45
residence time in the economizer were divided into four equal increments, as mentioned briefly 

above. The results of these calculations, also shown in Figure 5.7, agree favorably with the 

measured value at the economizer exit. The comparison of the predicted and measured values at the 

economizer outlet using reasonable values for the controlling parameters in this instance is 

encouraging. However, substantially more testing is needed for validation of the model before it can 

be relied upon as a useful tool for predicting SO3 emissions from coal-fired boilers. 

46
6. DEVELOPMENT OF AN ESTIMATOR FOR REMOVAL OF 

SO3/H2SO4 ACROSS AIR PREHEATERS 

Condensation of H2SO4 on the surfaces of combustion air preheaters can remove significant 

amounts of the incoming SO3/H2SO4, depending on the incoming concentrations and the air 

preheater exit temperatures. Although a detailed model of the processes related to the behavior of 

H2SO4 in air preheaters exists, an easily used, reasonably accurate means of estimating losses across 

rotary air preheaters would be useful. Such a method is described here. 

The surface temperatures of the heat exchange elements in the air preheater are cooler than the 

incoming flue gas. As the elements of the air preheater rotate the surfaces are heated by the 

incoming flue gas, resulting in a gradient in the outlet flue gas outlet temperatures. For the purposes 

of this estimator it is assumed that the effective surface temperature for H2SO4 condensation at any 

point in the rotation has some constant offset from the exit gas temperature at that point. The acid 

dewpoint at any point on the exit face is then taken to be equal to the flue gas temperature at that 

point less this offset and the H2SO4 concentration is taken to be the saturation concentration at that 

temperature. Typical ranges of gas temperatures across the exit faces of air heaters from the cold 

end to the hot end are 50 °F to 100 °F. 

Table 6.1 shows measured average concentrations at the exits of four rotary air heaters together 

with values predicted as described above using two offset temperatures. Figure 6.1 shows plots of 

estimated losses versus average exit temperature for a range of air heater inlet concentrations using 

a value of 35 °F for the temperature offset between the local exit gas temperature and the local exit 

saturation temperature. Curves are shown for two ranges of gas temperature variation across the exit 

faces of the preheaters: 50 °F and 100 °F. Figure 6.2 shows plots of the estimated average H2SO4 

exit concentrations for the same conditions used in generating Figure 6.1. The flue gas moisture 

concentration will have some affect on the exit H2SO4 concentrations as illustrated in Figure 3. 

Figures 6.4 and 6.5 show curves similar to those in Figures 6.1 and 6.2 resulting from the use of a 

30 °F offset temperature rather than the 35 °F offset. Figures 6.6 through 6.9 show plots of 

measured air heater exit H2SO4 concentrations together with concentrations predicted by the model 

using a 30 °F offset temperature. 

Table 6.1. Measured and predicted average exit SO3 concentrations for four rotary air heaters. 

Measured, ppm Predicted, ppm 

Site Inlet Outlet 30 degree offset 35 degree offset 

Rotary No. 1 3.1 0.9 1.7 1.4 

Rotary No. 2 50 23 30 26 

Rotary No. 3 42 18 17 13 

Rotary No. 4 50 24 30 26 

47
48

% Loss in AH 

100%

90%80%70%60%50%40%30%20%10%

0% 

280 290 300 310 320 330 340 

Average AH Exit Temperature, F 

Figure 6.1. Estimated SO3/H2SO4 losses across combustion air preheaters versus average air preheater exit temperature for a temper

first value of each pair in the legend is the preheater inlet SO3/H2SO4 concentration in ppm and the second value of the pa

temperature between the cold side and the hot side of the preheater exit. 
80 

70 60 50 40

Average AH Exit SO3 Concentration, ppm 

120/50 120/100 80/50 80/100 50/50 50/100 40/50 40/100 20/50 20/100 10/50 10/100 

30 20 10 

0 

280 290 300 310 320 330 3

Average AH Exit Temperature, F 

Figure 6.2. Estimated air preheater exit SO3/H2SO4 concentration versus average air preheater exit temperature for a temperature offse

of each pair in the legend is the preheater inlet SO3/H2SO4 concentration in ppm and the second value of the pair is the spre

between the cold side and the hot side of the preheater exit. 

49 
0 

5 

10 15 20 25 30 35 40 45 50 

Average Outlet SO

3 Conc., ppm 

5% H2O 8% H2O 10% H2O 15% H2O 

250 270 290 310 330 350 370 

Average Outlet Temperture, F 

50 

Figure 6.3. Effect of flue gas moisture content on the estimated air preheater exit SO3/H2SO4 concentration for a temperature offset o

calculated for an inlet concentration of 50 ppm with an air preheater exit temperature spread of 100 °F. 
% Loss in AH 

100%

90%80%70%60%50%40%30%20%10%

0% 

280 290 300 310 320 330 

Average AH Exit Temperature, F 

51

Figure 6.4. Estimated SO3/H2SO4 losses across combustion air preheaters versus average air preheater exit temperature for a tempe

first value of each pair in the legend is the preheater inlet SO3/H2SO4 concentration in ppm and the second value of the pa

temperature between the cold side and the hot side of the preheater exit. 
Average AH Exit SO3 Concentration, ppm 

80 70 60 50 40 

120/50 120/100 80/50 80/100 50/50 50/100 40/50 40/100 20/50 20/100 10/50 10/100 

30 20 10 

0 

280 290 300 310 320 330 340 

Average AH Exit Temperature, F 

52

Figure 6.5. Estimated air preheater exit SO3/H2SO4 concentration versus average air preheater exit temperature for a temperature offse

of each pair in the legend is the preheater inlet SO3/H2SO4 concentration in ppm and the second value of the pair is the spre

between the cold side and the hot side of the preheater exit. 
53 

1.0 0.5 0.0 

250 255 260 265 270 275 280 2

Gas Temperature, F 

Model Measured 

3.5 3.0 2.5 2.0 1.5 

SO

3 Concentration, ppm 

Figure 6.6. Comparison of predicted and measured SO3 concentrations at the exit of rotary air heater No. 1 for a temperature offset of 

concentration was 3.1 ppm. The measured average was 0.9 ppm while the predicted average was 1.7 ppm. 
54 

20 10 

0 

290 300 310 320 330 340 

Temperature, F 

Measured Model 

60 50 40 30 

SO

3 Concentration, ppm 

Figure 6.7. Comparison of predicted and measured SO3 concentrations at the exit of rotary air heater No. 2 for a temperature offset of 3

concentration was 50 ppm. The measured average was 23 ppm while the predicted average was 30 ppm. 
55 

20 15 10 

5 0 

280 290 300 310 320 330 

Temperature, F 

Measured Model 

50 45 40 35 30 25 

SO

3 Concentration, ppm 

Figure 6.8. Comparison of predicted and measured SO3 concentrations at the exit of rotary air heater No. 3 for a temperature 

concentration was 42 ppm. The measured average was 18 ppm while the predicted average was 17 ppm. 
56 

60 50 40 30 20 10 

0 

290 300 310 320 330 340 350 

Temperature, F 

Measured Model 

SO

3 Concentration, ppm 

Figure 6.9. Comparison of predicted and measured SO3 concentrations at the exit of rotary air heater No. 4 for a temperature 

concentration was 50 ppm. The measured average was 24 ppm while the predicted average was 30 ppm. 
7. EVOLUTION OF ACID MIST IN WET FLUE GAS 

DESULFURIZATION UNITS AND STACKS 

In the initial EPA report on work conducted by SRI on SO3 emissions, Farthing and 

coworkers assessed the effects of various pollution control technologies on sulfuric acid 

emissions and stack plume opacity.2

 Among the types of equipment and processes 

evaluated were ESPs, SCR, and wet FGD. Marked differences were noted in the 

behavior of buoyant, higher velocity plumes formed by hot stack gas in the absence of 

wet FGD and plumes formed by denser cold gas leaving a stack downstream from wet 

FGD equipment. 

In the evaluation of anticipated acid emissions and light scattering coefficients of plumes, 

it was found that very few data are available on the behavior of acid mist in wet FGD. A 

calculation was presented by Farthing et al. showing the expected growth of acid droplets 

by coagulation following nucleation and condensation of the acid on cooling at the 

entrance to a scrubber.2

 The calculation assumed a fixed acid composition, neglecting 

the possibility for growth of the acid droplets by absorption of water vapor from the high 

humidity flue gas in the scrubber and stack. 

To improve upon the accuracy of calculated acid mist properties, a more detailed analysis 

of acid mist evolution during wet FGD was conducted, considering coagulation of the 

acid droplets with each other, absorption of water vapor by the droplets, and coagulation 

of the acid droplets with spray droplets in the scrubber and with droplets carried over 

from the scrubber to the stack.50 The principal findings of that work are summarized 

below. 

Under the assumptions and conditions investigated, the principal role of a wet SO2 

scrubber, from the point of view of sulfuric acid mist, is to provide conditions for 

nucleation of acid droplets and a long residence time for their growth by coagulation and 

absorption of water vapor. Downstream from a wet FGD unit, the principal role of the 

stack, again from the point of view of sulfuric acid, is to provide additional residence 

time for growth of the acid mist by coagulation. Under all of the conditions examined, 

the mean size of acid droplets at the stack exit was predicted to have grown, through the 

combined effects of coagulation and absorption of water vapor, to sizes within the range 

able to scatter visible light. 

According to the calculations and within the ranges of the parameters investigated, the 

only factor having significant influence on the volume fraction of acid mist leaving the 

stack was the original H2SO4 content of the flue gas. When the initial H2SO4 

concentration was increased, the ultimate acid droplet volume fraction increased more 

rapidly than the H2SO4 concentration itself. Doubling the H2SO4 at the scrubber inlet 

from 10 to 20 ppmv increased the acid droplet volume fraction at the stack exit by a 

factor of 2.5, from 5.6 to 14 ppmv. The increased absorption of water vapor in the 

presence of higher acid concentration was due to the decrease in vapor pressure at the 

surfaces of the larger droplets that were formed, due to the Kelvin effect. The decrease in 

vapor pressure increased the rate of absorption of water vapor by the acid droplets. 

57
In spite of the fact that the mechanism for the increase in volume fraction of acid mist in 

the scrubber is vaporization of water from scrubber droplets and absorption of the vapor 

by the acid, variation of the size of scrubber spray droplets over the range from 0.7 to 2 

mm had hardly any influence on the properties of the acid. Transfer of water from the 

scrubber droplets to acid droplets appears to be regulated more by the dependence of the 

vapor pressure of the acid droplets on their H2SO4 content than on the specific surface 

area, mass transfer coefficient, or concentration of the scrubber spray. 

The principal effect of decreasing the scrubber spray droplet size was to increase capture 

of acid by spray droplets in the scrubber, although the fractions of acid captured were not 

large enough to have significant influence on acid mist droplet size and volume fraction. 

The fraction of the original H2SO4 retained in the scrubber increased from 0.33% to 7.6% 

on decreasing the spray droplet size from 2 to 0.7 mm, due to increased mass transfer to 

the smaller droplets and to the increase in spray droplet concentration as the terminal 

velocity of the spray droplets approached the gas velocity. Spray droplets smaller than 

0.7 mm were outside the range over which the model is applicable, because closer 

approach of the droplet terminal velocity to the gas velocity led to unrealistically high 

droplet concentrations characteristic of the "fluidized" regime.51 It was noted that most 

of the acid capture was predicted to occur at very early times, when the acid nuclei are 

still small, so the calculated capture may be influenced by the assumption that acid 

nucleates immediately to form H2SO4·H2O on entering the scrubber. 

According to the model, the source of water vapor that could be absorbed by acid mist in 

the stack is scrubber droplets carried over from the mist eliminator, not the water vapor 

present in the flue gas leaving the scrubber. This is because the acid at this stage is dilute 

(~ 1 wt% H2SO4), so the vapor pressure of water adjacent to acid mist droplets is not far 

from the vapor pressure over pure water. A continuous supply of water vapor is therefore 

needed if there is to be any significant uptake of water by the acid. Under the conditions 

investigated, an increase of 10 to 15% in acid mist volume fraction by absorption of 

water vapor in the stack was typical. Cooling of the stack gas, not taken into 

consideration in the calculation, is a possible mechanism by which water vapor could be 

maintained close to saturation in the presence of its absorption by acid mist. According 

to the model and calculations, the most important process occurring in the stack is the 

increase in droplet size due to coagulation. The acid droplet size was estimated to 

increase by 50% during 15 seconds of residence time in the stack. 

The model exhibits some unexpected and counter-intuitive behavior, such as (1) a 

decrease in H2SO4 concentration in acid droplets at the stack exit on increasing the H2SO4 

content of the flue gas, (2) insensitivity of acid volume fraction to scrubber spray droplet 

size, and (3) a decrease (though it is slight) in acid mist volume fraction on increasing the 

size of scrubber droplets passing the mist eliminator. The potential of such calculations 

to assist in the interpretation of observations of sulfuric acid behavior in the field would 

appear to justify further development of the model. To make the model a more powerful 

and useful simulation tool, the following refinements were proposed: 

• Evaluate the effects of scrubber and stack temperature. 

58
• Compare the results of the calculations with measurements of acid 

concentrations at the entrance and exit from wet FGD units. 

• Using the scattering efficiency for visible light versus particle size, determine 

relative opacities for the acid mist at the stack exit for comparison of the 

effects of scrubber and stack conditions on plume visibility. 

• Incorporate a droplet size distribution for the scrubber spray. 

• Locate experimental data to improve confidence in the assignment of the 

value for the size of scrubber droplets passing the mist eliminator. 

• Examine and refine, if necessary, the description of nucleation to provide a 

better model for droplet growth at short times and improve the simulation of 

capture of acid mist in the scrubber. 

• Allow for cooling of flue gas as a driver for transfer of water vapor to acid 

mist in the stack. 

Background

In a recent EPA report, Farthing et al. assessed the effects of various pollution control 

technologies on sulfuric acid emissions and stack plume opacity.2

 Among the types of 

equipment and processes evaluated were electrostatic precipitators (ESP), selective 

catalytic reduction (SCR), and wet flue gas desulfurization (FGD). Marked differences 

were noted in the behavior of buoyant, higher velocity plumes formed by hot stack gas in 

the absence of wet FGD and plumes formed by denser cold gas leaving a stack 

downstream from wet FGD equipment. 

In the evaluation of anticipated acid emissions and light scattering coefficients of plumes, 

it was found that very few data are available on the behavior of acid mist in wet FGD. A 

calculation was presented by Farthing and coworkers, showing the expected growth of 

acid droplets by coagulation following nucleation and condensation of the acid on 

cooling at the entrance to a scrubber.2

 The calculation assumed a fixed acid composition, 

neglecting the possibility for growth of the acid droplets by absorption of water vapor 

from the high humidity flue gas in the scrubber and stack. 

The present report describes calculations of acid mist evolution during wet FGD, 

considering coagulation of the acid droplets with each other, absorption of water vapor by 

the droplets, and coagulation of the acid droplets with spray droplets in the scrubber and 

with droplets carried over from the scrubber in the stack. 

The type of SO2 scrubber under consideration is a counterflow design, in which flue gas 

travels upward and scrubber spray droplets travel downward through the gas. The 

general outline of the model for the evolution of acid droplets in the scrubber and stack is 

as follows. Flue gas is cooled immediately to 333 K (140 o

F) on entering the scrubber, 

causing nucleation of sulfuric acid. The acid nuclei travel upward through the scrubber at 

the average gas velocity, growing by coagulation with each other and by absorption of 

water vapor from the flue gas. Some acid droplets are removed from the flow by 

coagulation with scrubber spray droplets. The acid and scrubber spray droplets are 

59
dNad 1 2 = − kad Nad dt 2 

β = 1+ Knad ,ad 

1+ 2Knad ,ad (1+ Knad ,ad ) 

characterized using single, volume mean sizes. Temperature, gas velocity, and droplet 

velocity are considered to be uniform throughout the scrubber and stack. 

At the outlet from the scrubber, the mist eliminator is assumed to remove all but 5 ppmv 

of the scrubber liquid flow. The droplets carried over are assumed to have a mean size of 

10 μm under the conditions considered as the base case. Absorption of water vapor by 

acid mist droplets, coagulation of acid droplets with themselves, and coagulation of acid 

droplets with the remaining scrubber droplets continue in the stack. 

In order to perform the calculations, we require the density and surface energy of sulfuric 

acid and the vapor pressure of water over sulfuric acid as functions of acid composition. 

The correlations for these properties, derived from data in the literature, are given in 

Appendix A. 

Immediately after their nucleation, acid droplets grow very quickly by coagulation and 

condensation. Because growth at early times is so rapid, an exact calculation of the 

initial size of the acid nuclei was not thought to be necessary. A useful approximation is 

to take the initial size of the nuclei to be the diameter of single molecules of H2SO4·H2O. 

This size was estimated by taking the diameter of a sphere having the volume occupied 

by the molecule, determined from the density of 84.5 wt % sulfuric acid, given by the 

correlation in Figure A1. Under this approximation, the initial number density of acid 

nuclei is simply the number concentration of sulfuric acid molecules in the flue gas. The 

initial diameter and values for the other constants used in the calculations are given in 

Table 7.1. 

Development of Mist Behavior Equations 

Coagulation of Acid Mist Droplets 

The coagulation of acid droplets is described approximately by the rate expression for the 

decrease in number concentration of a monodisperse aerosol (Friedlander, p. 193):52 

(7.1)

where Nad is the number concentration of acid mist droplets and kad is the rate coefficient 

for their coagulation. The rate coefficient is given, for uniform-size droplets, by 

(Seinfeld, p. 395;53 Friedlander, p. 19252): 

kad = 8π dad Dad β (7.2) 

in which dad is the diameter of the acid mist droplets and Dad is their diffusion coefficient. 

The factor, β, in Eq. (7.3) describes the transition of droplet diffusion behavior from the 

continuum, through the transition, to the free molecule regime (Seinfeld, p. 395):53 

(7.3)

60
Knad ,ad = 2λad 

dad 

1 

c Dad = ad λad 2 

⎛ 8kBT ⎞

1/ 2 

c = ⎜

⎜ ⎟

⎟ ad ⎝π mad ⎠

Knad ,ad = 2 2Dad 

cad dad 

Dad = kBT Cc 

3π μ fg dad 

where Knad,ad is the Knudsen number for diffusion of acid droplets toward each other, 

equal to the ratio of the mean free path of the droplets, λad, to the droplet radius, dad/2 

(Seinfeld, p. 395):53 

(7.4)

The mean free path of acid mist droplets in flue gas was evaluated using the form of the 

relationship between the mean speed of particles, their diffusion coefficient, and their 

mean free path required for consistency with the correction factor specified in Eq. (7.3), 

[Seinfeld (pp. 326, 335-338, 395)]:53 

(7.5)

in which cad is the mean speed of the droplets: 

(7.6)

The derivation by Seinfeld53 (pp. 395-398) is for different-sized particles, for which the 

mean velocity is defined as c12 = (c1

2 + c22 )1 / 2 . This introduces a factor of 2 in Eq. 

(7.5), when the particles are identical, and changes the equation from its more familiar 

form, D = 1⁄2 cλ. The Knudsen number is then (Seinfeld, p. 395):53 

(7.7)

The diffusion coefficient for the droplets is given by the Stokes-Einstein relation with 

Cunningham correction (Seinfeld, p. 324):53 

(7.8)

and the Cunningham correction factor, Cc, is (Seinfeld, p. 317) :

53 

Cc = 1 +{Kn fg ,ad [1.257 + (0.4 e 

−1.1 / Kn fg ,ad )]} (7.9) 

where Knfg,ad is the Knudsen number for acid droplets in flue gas, the ratio of the mean 

free path of the average flue gas molecule to the droplet radius (Seinfeld, p. 326):53 

61
Kn fg,ad = 2

d

λ fg 

ad 

Dw, fg = 1 

c fg λ fg 2 

⎛ 8RT ⎞

1 / 2 

c fg = ⎜

⎜ ⎟

⎟

⎝π W fg ⎠

Kn fg ,ad = 4 

c

D

fg

w

d

,

ad

fg

dC Sh D 

dt

sd = − sd

dsd

w, fg SsdCsd ( Cw,sd − Cw,∞ ) 

(7.10) 

The mean free path is determined from its relationship to the molecular speed and 

diffusion coefficient: 

(7.11)

The molecular diffusion coefficient, Dw,fg is estimated by treating the flue gas as a binary 

mixture of water vapor with the other combustion products. The mean speed of flue gas 

molecules is: 

(7.12) 

and the Knudsen number is then: 

(7.13) 

Absorption of Water Vapor from Flue Gas by Acid Mist Droplets 

All the while that they are coagulating, the acid mist droplets may also absorb water 

vapor from the flue gas. Simultaneous evaporation of water from scrubber liquor 

droplets maintains the flue gas at high relative humidity. The process can be represented 

as follows: 

H2O(scrubber spray droplets) = H2O(vapor) (R 7.1) 

H2O(vapor) = H2O(acid mist droplets). (R 7.2) 

The rate of change in concentration of scrubber liquor droplets, Csd (kg/m3

 gas), due to 

evaporation of water from the droplets, is given by the expression describing diffusion of 

water vapor from the surface of a droplet, through the concentration boundary layer 

surrounding the droplet, and into the free stream: 

(7.14) 

in which Shsd is the Sherwood number for mass transfer between scrubber droplets and 

flue gas; Dw,fg is the molecular diffusion coefficient in a pseudo-binary mixture of water 

vapor and flue gas, dsd is the mean scrubber liquor droplet size; Ssd is the specific external 

surface area (m2

/kg) of scrubber liquor droplets; and Cw,sd and Cw,∞ are the concentrations 

62
 dCad = Shw,ad Dw, fg S C ( C −C ). dt d ad ad w,∞ w,ad 

ad 

Shw,ad = 2

1+ 2Kn 

1 

w

+

,ad

Kn 

(1 

w

+

,ad

Knw,ad ) 

Knw,ad = 2λw 

dad 

1 Dw, fg = cwλw 2 

⎛ 8RT ⎞

1 / 2 

c w =

⎝

⎜

⎜

π Ww ⎠

⎟

⎟

(kg/m3 gas) of water vapor at the surface of scrubber liquor droplets and in the free 

stream, respectively. The Sherwood number was determined using the correlation of 

Frössling54 and Ranz and Marshall55 (please see the Nomenclature Section for definitions 

of symbols not mentioned here in the text): 

Shsd = 2 + 0.6 Re1/2 Sc1/3 (7.15) 

The rate of change in acid mist droplet concentration, Cad (kg/m3

 gas), due to absorption 

of water vapor, is similarly described by the rate of diffusion of water vapor from the free 

stream, through the concentration boundary layer surrounding an acid mist droplet, to the 

surface of the droplet: 

(7.16)

where the symbols have definitions analogous to those in Eq. (7.14), but now refer to 

properties of the acid mist rather than scrubber spray droplets. During evolution of the 

acid mist, its droplets grow from the free molecule, through the transition, to the 

continuum regime. To account for mass transfer at all three stages, the Sherwood number 

was based on the interpolation relation of Dahneke,56 given by Seinfeld (pp. 335-336):53 

(7.17) 

in which Knw,ad is the Knudsen number for water vapor molecules diffusing to acid mist 

droplets in flue gas, the ratio of the mean free path of H2O to the droplet radius: 

(7.18)

The mean free path of water vapor molecules in flue gas was evaluated using the same 

general relationship between mean speed, diffusion coefficient, and mean free path as 

before (Seinfeld, p. 338):53 

(7.19)

in which cH2O is the mean speed of water vapor molecules: 

(7.20)

The Knudsen number is then: 

63
Knw,ad = 4 

c

D

d

w, fg 

w ad 

Sh

d

w

ad

,ad SadCadCw,ad 

1 + Shsd Ssd Csd Cw,sd 

Cw,∞ = Cw,sd 

dsd 

Shw,ad Sad Cad 

1 +

dad 

Shsd SsdCsd dsd 

dCad = Cw,sd − Cw,ad 

dt 1 1

+ Shw,ad Dw, fg Shsd Dw, fg SadCad SsdCsd dad dsd 

dCad = ksa (Pw,sd − Pw,ad ) Ww 

dt RT 

Pw,ad = Pw,af exp ⎨

⎪⎧ 4σ aWw ⎢

⎡

1 +

Ya dρ a − 3 Ya dσ a ⎥

⎤

⎬

⎪⎫

⎪dad ρ a RT ⎣ ρ a dYa 2 σ a dYa ⎦⎪ ⎩ ⎭

(7.21) 

Assuming the system is close to its steady state throughout the scrubber, we can equate 

the net rate of change in water vapor concentration, represented by the sum of Eqs. (7.14) 

and (7.16), to zero and solve for the pseudo-steady concentration of water vapor in the 

free stream, Cw,∞: 

(7.22)

This expression is substituted for the free stream water vapor concentration in Eq. (7.16), 

which then becomes: 

(7.23)

Replacing the effective overall rate coefficient in Eq. (7.23) by the symbol, ksa, and 

changing from concentrations to partial pressures of water vapor, we have: 

(7.24)

Because the curvature of the surface of the small acid droplets is so high, especially at 

early times, the equilibrium pressure of water vapor at the surfaces of the droplets is 

significantly higher than that over a flat liquid surface. The effect of curvature on the 

vapor pressure was evaluated using the relation presented by Equation 7 in Yue:57 

(7.25)

The density, ρa, and surface energy, σa, of sulfuric acid, and the equilibrium vapor 

pressure of water over a flat sulfuric acid surface, Pw,af, are given by the correlations in 

Figures A1, A2, and A3, respectively, of Appendix A. The derivatives of the density and 

surface energy with respect to composition were also evaluated using their respective 

64
dCad = − Shsd Dad SsdCsdCad dt dsd 

dNad = − Shsd Dad SsdCsd Nad dt dsd 

dCad = ksa (Pw,sd − Pw,ad ) Ww − Shsd Dad SsdCsdCad dt RT dsd 

Cad (t + Δt) = Cad (t) +

⎧

⎨

⎩

ksa (t)[Pw,sd − Pw,ad (t)]

W

RT

w − Shsd

d

D

sd

ad (t) 

Ssd Csd Cad (t)

⎫

⎬

⎭

Δt 

correlations. The vapor pressure of H2SO4 over aqueous sulfuric acid is negligible at the 

temperatures of interest here. 

Coagulation of Acid Mist Droplets with Scrubber Liquor Droplets 

The last process to be considered is the coagulation of acid droplets with the much larger 

scrubber spray droplets, treated as a diffusion process analogous to the vapor transport 

processes discussed above: 

(7.26)

Coagulation of acid droplets with scrubber droplets changes the number density of the 

acid mist, as well as its mass concentration. The number density can be estimated by 

dividing the mass concentration by the average droplet mass, Nad = Cad/mad, therefore, 

from Eq. (7.26) we have, approximately, for uniform-size acid mist droplets: 

(7.27)

in which the Sherwood number for the scrubber liquor droplets, Shsd, is given by Eq. 

(7.15) and the diffusion coefficient for the acid mist droplets, Dad, by Eq. (7.8). 

Calculation Procedure 

The mass concentration of acid mist is increased due to absorption of water vapor by the 

droplets, but decreased due to loss of acid mist droplets to scrubber liquor. Adding the 

contributions from these two processes, according to Eqs. (7.24) and (7.26): 

(7.28)

This equation was integrated approximately over small increments of time, Δt, as follows: 

(7.29a) 

Cad = Cad 0 at t = 0 (7.29b) 

Coagulation of acid droplets with themselves does not change their mass concentration. 

The number density of acid droplets, on the other hand, is decreased both by coagulation 

of the droplets with themselves and by their coagulation with scrubber spray droplets. 

Adding the contributions from these two processes, according to Eqs. (7.1) and (7.27): 

65
dNad = − 1 kad N 2 − Shsd Dad SsdCsd Nad dt 2 ad dsd 

⎡1 2 Shsd Dad (t) ⎤ Nad (t + Δt) = Nad (t) − ⎢ kad (t)Nad (t) + SsdCsd Nad (t)⎥Δt

⎣2 dsd ⎦

mad (t) = Cad (t) 

Nad (t) 

fcap (t + Δt) = fcap (t) +

Shsd Dad (t) 

Ssd Csd Cad (t) Ya (t) Δt 

dsd CH 2 SO4 

CH 2 SO4 Ya (t) = [1− fcap (t)] Cad (t) 

(7.30)

This equation was integrated approximately over small increments of time, Δt, as follows: 

(7.31a)

Nad = Nad 0 at t = 0 (7.31b) 

Following the addition of an increment in time and calculation of new mass and number 

concentrations using Eqs. (7.29a) and (7.31a), the new average acid droplet mass is found 

from: 

(7.32)

The cumulative fraction of the original acid captured by scrubber spray droplets is 

found from: 

(7.33a)

fcap = 0 at t = 0 (7.33b) 

The new acid composition, resulting from its dilution by absorption of water vapor, is 

given by: 

(7.34a)

Ya = 0.845 at t = 0 (7.34b) 

where Ya = 0.845 is the mass fraction of H2SO4 in the first acid nuclei formed, assumed to 

have composition, H2SO4·H2O. 

Evolution of Acid Mist in the Stack 

The same processes of acid mist coagulation, absorption of water by acid droplets, 

evaporation of water from scrubber droplets, and coagulation of acid mist with scrubber 

droplets continue in the stack, downstream from the wet scrubber and mist eliminator. 

However, the characteristics of the system are changed in the following ways: (1) there 

are marked decreases in scrubber droplet mass concentration and droplet size across the 

mist eliminator, and (2) the remaining scrubber droplets have approximately the same 

velocity as the gas and therefore have Sherwood numbers for mass transfer equal 

66
approximately to 2. The effects of lower Sherwood number and smaller concentration on 

mass transfer are offset somewhat by the smaller droplet size so, depending upon the 

conditions in a particular situation, the net change in rates of evaporation of water vapor 

from scrubber droplets carried over and coagulation of acid droplets with the scrubber 

droplets may be less than might be expected. 

In contrast to the situation in the scrubber, where acid mist droplets coagulating with 

scrubber spray droplets are removed from the flue gas, acid captured by carried-over 

scrubber droplets in the stack is emitted as sulfate, though in a different particle size 

range from acid mist. 

Results and Discussion 

The results of the calculations of acid mist properties as functions of time through a wet 

SO2 scrubber and stack are shown in Figures 7.1 to 7.5. The results for a base case are 

presented in Figure 7.1, followed by figures showing the effects of changing individual 

parameters characterizing the H2SO4 concentration entering the scrubber, the scrubber 

spray quality, and mist eliminator performance. The parameters and their values were: 

H2SO4 concentration, 10 and 20 ppmv; scrubber spray droplet size, 0.7, 1, and 2 mm; and 

size of droplets leaving the mist eliminator, 10 and 20 μm. Droplet size was chosen as 

the indicator of mist eliminator performance because its influence is stronger than that of 

collection efficiency. The flue gas residence time in the scrubber was fixed at 5 seconds, 

followed by up to 15 seconds of residence time in the stack. Identical scales are used on 

the ordinates of the figures, for ease of comparison of acid properties under the various 

sets of conditions. 

Base Case 

The following conditions were assigned for the base case: H2SO4 volume fraction in flue 

gas entering the scrubber, 10 ppmv; scrubber spray droplet size, 1 mm; and size of 

droplets passing the mist eliminator, 10 μm. The values of the other parameters used in 

the calculations are given in Table 7.1. The evolution of acid mist properties in the 

scrubber and stack under the base case conditions is shown in Figure 7.1. The individual 

panels in the figure are, from the top: (a) the acid mist droplet size; (b) mass fraction of 

H2SO4 in the acid mist droplets; (c) the number concentration of acid mist droplets; (d) 

the volume fraction of acid mist (m3

 droplets/m3 flue gas); and (e) the cumulative fraction 

of the original acid captured by coagulation of acid mist with scrubber spray droplets. 

Both coagulation and absorption of water vapor contribute to the growth of acid mist in 

the scrubber, shown at the left in Figure 7.1a. The progress of coagulation is best seen in 

the decline in number concentration with time, shown on a logarithmic scale in Figure 

7.1c. Growth by coagulation is most rapid at early times, when the number concentration 

is high and the droplets are small. Absorption of water vapor has no direct effect on 

number concentration, though it influences the rate of coagulation through its effects on 

the volume fraction of acid mist and its droplet size. 

The progress of absorption of water vapor is best seen in Figure 7.1d, showing the 

increase in volume fraction of acid mist with time. Absorption of water vapor by the 

droplets is most rapid at early times, when the acid concentration in the droplets is high 

and the equilibrium vapor pressure of water at the surfaces of the droplets is low, so there 

67
Figure 7.1. Properties of acid mist as a function of time in the SO2 scrubber and stack—base case: 

H2SO4 entering scrubber, 10 ppmv 

mean spray droplet size, 1 mm 

mean droplet size passing the mist eliminator, 10 μm 

68 
is a large difference in water vapor concentration between the free stream and acid 

droplet surfaces. The importance of this can be seen by examination of Figure 7.1b, 

showing the decline in acid concentration in the acid mist droplets with time due to 

dilution by absorbed water vapor. The initial condensation nuclei are assumed to contain 

84.5 wt % H2SO4. During only 5 seconds of residence time in the scrubber, this 

concentration is reduced by absorption of water vapor to 0.71 wt%. Corresponding to 

this change in composition, the vapor pressure of water adjacent to the acid droplets rises 

from approximately 0 to 19954.8 Pa, compared with 19955.5 Pa at the surface of the 

scrubber spray droplets, which are the source of water vapor. At an acid concentration of 

0.71 wt % there is less than 1 Pa of partial pressure difference driving the transfer of 

water from scrubber droplets to acid mist so, by the time they reach the scrubber exit, the 

absorption of water vapor by acid droplets has become relatively slow. Coagulation of 

acid mist droplets with each other has no direct effect on the volume fraction of acid mist 

(m3

 acid droplets/m3 flue gas), though it influences the rate of absorption of water vapor 

through its effects on droplet size and the consequent effects on water vapor pressure at 

the droplet surfaces (Kelvin effect) and mass transfer coefficient. 

The evolution of the ability of the scrubber spray to capture acid mist is shown in Figure 

7.1e. According to the model, only 2% of the original sulfuric acid is retained in the 

scrubber. Most of the capture occurs at very early times, when the sizes of the acid 

condensation nuclei are still small and have large diffusion coefficients and high rates of 

mass transfer to the much larger scrubber droplets. Because the properties of the acid 

mist at short times are evidently so important to this process, the result may be sensitive 

to the assumption that all of the H2SO4 nucleates as H2SO4·H2O instantaneously on 

entering the scrubber. 

Evolution of acid mist in the stack, shown in the right-hand three-quarters of the panels in 

Figure 7.1, is dominated by coagulation. Because the mist eliminator removes most of 

the scrubber droplets and greatly reduces the source of water vapor, the free stream water 

vapor partial pressure drops to only 0.002 Pa above the partial pressure of water vapor 

adjacent to the dilute acid droplet surfaces, shutting off the transfer of water from 

scrubber droplets to acid. There is little increase in the volume fraction of acid mist in 

the stack, as shown in Figure 7.1d, and little further dilution of the acid in the mist, as 

shown in Figure 7.1b. Coagulation, however, continues in the stack, as shown by the 

factor of three decrease in number concentration (Figure 7.1c) and 50% increase in 

droplet size (Figure 7.1a) from stack entrance to exit. Coagulation of acid mist with the 

low concentration of scrubber droplets passing the mist eliminator is not significant in the 

stack (Figure 7.1e). At the stack exit, under the base case conditions, the acid mist 

droplets are estimated to have a mean size of 0.5 μm, are expected to contain 0.6 wt % 

H2SO4, and occupy 6 ppm by volume of the stack gas. 

Effect of H2SO4 Concentration 

Increasing the initial H2SO4 from 10 to 20 ppmv changes the properties of the acid mist 

as shown in Figure 7.2. The number concentration of acid droplets at the scrubber and 

stack exits is only slightly increased (Figure 7.2c), by 16 and 12%, respectively, because 

higher initial number concentration increases their coagulation rate. On the other hand, 

the volume fraction of acid mist at the scrubber outlet is increased from 5 ppm (base 

case) to 13 ppm, a factor significantly larger than the doubling in initial H2SO4 

69
__________________________________________________________________________________ 

Property Symbol Value Source 

________________________________________________________________________________ 

Temperature in scrubber T 333.15 K (140 o

F) typical 

Average molecular weight of flue gas Wfg 30 typical 

Flue gas velocity in scrubber ufg 3.7 m/s (12 ft/s) typical 

Scrubber spray droplet downward 

velocity, relative to the scrubber usd 1.5 m/s (5 ft/s) typical 

Liquid-to-gas ratio in scrubber LG 0.02 m3

/actual m3 

(150 gal/1000 acf) typical 

Drag coefficient for the scrubber 

droplets CD 0.54 a 

Scrubber liquor spray droplet diameter dsd 1 mm b 

Density of scrubber spray droplets ρsd 1225 kg/m3 15 wt% solids 

Initial mass fraction of H2SO4 in 

H2SO4·H2O, the condensation nuclei Ya0 WH2SO4/WH2SO4·H2O = 0.845 

Density of sulfuric acid ρa Figure A1 c 

Surface energy of sulfuric acid σa Figure A2 d 

Equilibrium vapor pressure of steam 

over a flat sulfuric acid surface Pw,af Figure A3 c 

Initial diameter of acid condensation 

nuclei dad0 0.597 nm e 

Viscosity of flue gas μfg

1.458x10−6 T1.5 

kg/(m·s) T +110.4 f 

Molecular diffusion coefficient in 

water vapor - flue gas mixture Dw,fg 1.87x10−10 T 2.072 m2

/s g 

Fraction of scrubber liquor flow rate 

passing the mist eliminator -- 5 x 10-6 h 

Mean size of scrubber liquor droplets 

passing the mist eliminator -- 10 μm estimate 

Flue gas residence time in scrubber -- 5 s typical 

________________________________________________________________________________ 

Table 7.1. Values of the parameters used in the calculations for the base case. 

a. Michalski, 2000.51 

b. Adjusted to give a terminal velocity for the scrubber spray droplets of 5.2 m/s (17 ft/s), equal to 

ufg + usd, using the drag coefficient of Michalski, 2000.51 

c. Perry et al., 1984, pp. 3-65, 3-66, and 3-237.58 

d. Morgan and Davis, 196959; Myhre et al., 1998.60 

e. Diameter of a sphere having the volume occupied by one molecule of H2SO4·H2O liquid at 333.15 K. 

f. Approximated using the viscosity of nitrogen from Hilsenrath et al., 1955.61 

g. Approximated using the binary diffusion coefficient for water vapor-nitrogen from Marrero and Mason, 

1972.62 

h. 300 MW unit, liquid-to-gas ratio of 150 gal/1000 acf, emission limit of 0.03 lb/106

 Btu, slurry is 15 wt% 

solids, and 1⁄2 of particulate matter in stack is scrubber solids. 

70
Figure 7.2. Properties of acid mist as a function of time in the SO2 scrubber and stack—increased H2SO4 

case: 

H2SO4 entering scrubber, 20 ppmv 

mean spray droplet size, 1 mm 

mean droplet size passing the mist eliminator, 10 μm 

71 
concentration. The increase in volume fraction, due to increased absorption of water 

vapor, is consistent with increased dilution of the acid, as shown by comparison of 

Figures 7.1b and 7.2b. The mass fraction of H2SO4 in the acid mist at the scrubber outlet 

is now only 0.55 wt %, compared with 0.71 wt % at the same location under the base 

case conditions. The higher absorption of water vapor in the present case is driven by the 

lower vapor pressure of water at surfaces having larger radii of curvature, characteristic 

of the larger acid droplets (Kelvin effect57). The combination of increased amounts of 

condensing species and small change in droplet number concentration result in a 

significant increase in mean acid mist droplet size at the scrubber outlet, from 0.33 μm in 

the base case to 0.44 μm on doubling the H2SO4 concentration. The fraction of the acid 

captured in the scrubber (Figure 7.2e) decreases in inverse proportion to the increase in 

initial H2SO4 concentration, from 2% in the base case (Figure 7.1e) to only 1%. 

As in the base case, there is little absorption of water by acid mist in the stack, because 

the mist eliminator removed of most of the scrubber droplets, which were the source of 

water vapor in the scrubber. Neither the droplet composition (Figure 7.2b) nor the 

droplet volume fraction (Figure 7.2d) changes significantly in the stack. Coagulation of 

the acid mist, however, continues in the stack, increasing the mean droplet size from 0.44 

μm at the scrubber outlet to 0.65 μm at the stack exit, a fractional increase similar to that 

observed in the base case. Little coagulation of acid mist with scrubber droplets passing 

the mist eliminator is expected (Figure 7.2e). 

Scrubber Droplet Size 

The effects of increasing the mean scrubber spray droplet size from 1 mm to 2 mm and 

decreasing it to 0.7 mm are shown in Figures 7.3 and 7.4, respectively. Compared with 

the base case, Figure 7.1, there are no large changes in acid mist droplet size, 

composition, number concentration, or volume fraction in either the scrubber or the stack. 

The properties of acid mist leaving the stack are practically the same in all three cases. 

According to the model, the only significant change in the behavior of the system is an 

increase in the capture of H2SO4 in the scrubber as the spray droplet size decreases, 

shown by comparison of Figures 7.1e, 7.3e, and 7.4e. The fraction of the original H2SO4 

captured by 2-mm spray droplets was 0.33%, the fraction captured by 1-mm droplets was 

2%, and the fraction captured by 0.7-mm droplets was 7.6%. A corresponding slight 

reduction in the volume fraction of acid mist leaving the stack can also be seen (Figures 

7.1d, 7.2d, and 7.3d). 

Further reduction in the size of the scrubber spray droplets below 0.7 mm is outside the 

range of application of the model. The terminal fall velocity of 0.5 mm droplets is 3.7 

m/s, equal to the upward gas velocity in the scrubber. As the spray droplet size 

approaches 0.5 mm, the calculated concentration of droplets increases to physically 

unrealistic levels. In practice this corresponds to the "fluidized" regime,

51 not described 

by the present model. The calculated volume fractions of droplets in the scrubber were 2, 

5, and 11% for the spray droplet sizes of 2, 1, and 0.7 mm, respectively. The increase in 

acid capture with decreasing size is due to the combined effects of increasing specific 

surface area and increasing concentration of the spray droplets. 

We note that, according to the calculation, capture of acid by scrubber spray droplets 

occurs primarily at early times when the acid nuclei are still very small and have high 

72
Figure 7.3. Properties of acid mist as a function of time in the SO2 scrubber and stack—increased scrubber 

droplet size case: 

H2SO4 entering scrubber, 10 ppmv 

mean spray droplet size, 2 mm 

mean droplet size passing the mist eliminator, 10 μm

73 
Figure 7.4. Properties of acid mist as a function of time in the SO2 scrubber and stack—decreased scrubber 

droplet size case: 

H2SO4 entering scrubber, 10 ppmv 

mean spray droplet size, 0.7 mm 

mean droplet size passing the mist eliminator, 10 μm 

74 
diffusion coefficients. This is shown by the rapid rise in the fraction of H2SO4 captured 

near time = 0 in Figure 7.4e. The present estimates of acid capture in the scrubber may 

be influenced by the assumption that nucleation of acid begins with instantaneous 

formation of H2SO4·H2O at the inlet. A more detailed description of the nucleation of 

acid droplets may be needed to achieve high accuracy in the calculation of acid removal 

by the scrubber. The presence of a distribution of droplet sizes in the scrubber spray also 

needs to be considered, including a mechanism for removal of droplets having terminal 

velocities near the gas velocity, so their concentrations are properly bounded. Possible 

mechanisms for removal of droplets near the critical size are coalescence of the droplets 

and a distribution of gas velocities in the scrubber. 

Performance of the Mist Eliminator 

The last case to be considered is the effect of spoiling the performance of the mist 

eliminator. We chose to simulate this by increasing the mean size of scrubber droplets 

passing the mist eliminator, rather than decrease the collection efficiency, because the 

base case already assumes that one-half of the particulate matter emissions limit is 

contributed by scrubber solids (Table 7.1, footnote h), so there is little room left for 

adjustment of the efficiency. The effect of increasing the mean size of scrubber droplets 

in the stack from 10 to 20 μm is shown in Figure 7.5. Interestingly, the most visible 

effect, compared with the base case, is a small decrease in the acid mist volume fraction 

at the stack exit with increasing droplet size. This is due to the decrease in rate of supply 

of water vapor to the free stream from the larger size scrubber droplets, limiting the 

transfer of water from the scrubber droplets to the acid mist. The acid concentration in 

the mist droplets at the exit from the stack is slightly higher, and the droplet size slightly 

smaller, than in the base case. With respect to acid mist, and under the conditions 

investigated, the function of scrubber droplets not collected by the mist eliminator is to 

provide a small supply of water vapor to increase the volume fraction of acid mist in the 

stack. An increase of 10 to 15% in acid mist volume fraction by absorption of water 

vapor in the stack was typical. The concentration of scrubber droplets is too low, and 

both acid mist and scrubber droplets are too large, for the scrubber droplets to be 

effective as scavengers of acid mist in the stack. Larger increases in the volume fraction 

of acid mist by absorption of water vapor or capture of acid mist by scrubber droplets in 

the stack might be possible under some conditions, for example in the presence of low 

efficiency of mist removal at the scrubber exit or unusually small size of droplets passing 

the mist eliminator. 

Comparisons of Predicted and Measured Acid Droplet Size 

Distributions 

Over the past several years Southern Research has developed a technique for analyzing 

the material collected on the stages of cascade impactors to separately ascertain the size 

distributions of the primary sources of the material collected in the various size fractions: 

fly ash, scrubber solids, and condensed H2SO4. Examples of these breakdowns by 

constituent are shown in Figures 7.6 through 7.8. As can be seen, the bulk of the 

condensed H2SO4 is found in particles having diameters of a few tenths of a micrometer. 

Comparisons of the volume mean diameter of the condensed acid from the model to 

extrapolations from the measured values of the diameters of the acid are shown in Figure 

7.9. The extrapolations were made by assuming constant number concentrations with the 

75
Figure 7.5. Properties of acid mist as a function of time in the SO2 scrubber and stack—increased size of 

droplets passing the mist eliminator case: 

H2SO4 entering scrubber, 10 ppmv 

mean spray droplet size, 1 mm 

mean droplet size passing the mist eliminator, 20 μm 

76 
diameters being proportional to the cube root of the SO3 concentration. As can be seen, 

the predictions compare favorably with values based on measurement. The validity of the 

predictions are further strengthened when predicted opacities in the plume from a 1300 

MW coal-fired using a wet scrubber for SO2 control are compared with those measured 

by a certified “smoke reader” as shown in Figure 7.10. The predictions and 

measurements in the latter case were made for the point downwind of the stack at which 

the condensed water fog had evaporated. 

Conclusions and Recommendations 

Under the assumptions and conditions investigated, the principal role of a wet SO2 

scrubber, from the point of view of sulfuric acid mist, is to provide conditions for 

nucleation of acid droplets and a long residence time for their growth by coagulation and 

absorption of water vapor. Downstream from a wet FGD unit, the principal role of the 

stack, again from the point of view of sulfuric acid, is to provide additional residence 

time for growth of the acid mist by coagulation. Under all of the conditions examined, 

the mean size of acid droplets at the stack exit is predicted to have grown, through the 

combined effects of coagulation and absorption of water vapor, to sizes within the range 

able to scatter visible light. 

According to the calculations and within the ranges of the parameters investigated, the 

only factor having significant influence on the volume fraction of acid mist leaving the 

stack was the original H2SO4 content of the flue gas. When the initial H2SO4 

concentration was increased, the ultimate acid droplet volume fraction increased more 

rapidly than the H2SO4 concentration itself. Doubling the H2SO4 at the scrubber inlet 

from 10 to 20 ppmv increased the acid volume fraction at the stack exit by a factor of 2.5, 

from 5.6 to 14 ppmv. The increased absorption of water vapor in the presence of higher 

acid concentration was due to the decrease in vapor pressure at the surfaces of the larger 

droplets that were formed, due to the Kelvin effect. The decrease in vapor pressure 

increased the rate of absorption of water vapor by the acid droplets. 

In spite of the fact that the mechanism for the increase in volume fraction of acid mist in 

the scrubber is vaporization of water from scrubber droplets and absorption of the vapor 

by the acid, variation of the size of scrubber spray droplets over the range from 0.7 to 2 

mm had hardly any influence on the properties of the acid. Transfer of water from the 

scrubber droplets to acid droplets appears to be regulated more by the dependence of the 

vapor pressure of the acid droplets on their H2SO4 content than on the specific surface 

area, mass transfer coefficient, or concentration of the scrubber spray. 

The principal effect of decreasing the scrubber spray droplet size was to increase capture 

of acid by spray droplets in the scrubber, although the fractions of acid captured were not 

large enough to have significant influence on acid mist droplet size and volume fraction. 

The fraction of the original H2SO4 retained in the scrubber increased from 0.33% to 7.6% 

on decreasing the spray droplet size from 2 to 0.7 mm, due to increased mass transfer to 

the smaller droplets and to the increase in spray droplet concentration as the terminal 

velocity of the spray droplets approached the gas velocity. Spray droplets smaller than 

0.7 mm were outside the range over which the model is applicable, because closer 

approach of the droplet terminal velocity to the gas velocity led to unrealistically high 

droplet concentrations characteristic of the "fluidized" regime.51 It was noted that most 

77
of the acid capture was predicted to occur at very early times, when the acid nuclei are 

still small, so the calculated capture may be influenced by the assumption that acid 

nucleates immediately to form H2SO4·H2O on entering the scrubber. 

According to the model, the source of water vapor that could be absorbed by acid mist in 

the stack is scrubber droplets carried over from the mist eliminator, not the water vapor 

present in the flue gas leaving the scrubber. This is because the acid at this stage is dilute 

(~ 1 wt % H2SO4), so the vapor pressure of water adjacent to acid mist droplets is not far 

from the vapor pressure over pure water. A continuous supply of water vapor is therefore 

needed if there is to be any significant uptake of water by the acid. Under the conditions 

investigated, an increase of 10 to 15% in acid mist volume fraction by absorption of 

water vapor in the stack was typical. Cooling of the stack gas, not taken into 

consideration in the present calculation, is a possible mechanism by which water vapor 

could be maintained close to saturation in the presence of its absorption by acid mist. 

According to the present model and calculations, the most important process occurring in 

the stack is the increase in droplet size due to coagulation. The acid droplet size was 

estimated to increase by 50% during 15 seconds of residence time in the stack. 

The model exhibits some unexpected and counter-intuitive behavior, such as the decrease 

in H2SO4 concentration in acid droplets at the stack exit on increasing the H2SO4 content 

of the flue gas (Figures 7.1b and 7.2b), the insensitivity of acid volume fraction to 

scrubber spray droplet size (Figures 7.3d and 7.4d), and the decrease (though it is slight) 

in acid mist volume fraction on increasing the size of scrubber droplets passing the mist 

eliminator (Figures 7.1d and 7.5d). The potential of such calculations to assist in the 

interpretation of observations of sulfuric acid behavior in the field would appear to justify 

some further development of the model. To make the calculation a more powerful and 

useful simulation tool, the following refinements are proposed: 

• Evaluate the effects of scrubber and stack temperature. 

• Compare the results of the calculations with measurements of acid 

concentrations at the entrance and exit from wet FGD units. 

• Using the scattering efficiency for visible light versus particle size, determine 

relative opacities for the acid mist at the stack exit for comparison of the 

effects of scrubber and stack conditions on plume visibility. 

• Incorporate a droplet size distribution for the scrubber spray. 

• Locate experimental data to improve confidence in the assignment of the 

value for the size of scrubber droplets passing the mist eliminator. 

• Examine and refine, if necessary, the description of nucleation to provide a 

better model for droplet growth at short times and improve the simulation of 

capture of acid mist in the scrubber. 

• Allow for cooling of flue gas as a driver for transfer of water vapor to acid 

mist in the stack. 

78
79 

0 

0.01 0.02 0.03 0.04 0.05 0.06 

Solids, less Carbon H2SO4 + 5H2O Carbon Total Volume 

dV/dlog D, cm 3 /dncm 

0.01 0.1 1 10 

Diameter, μm 

Figure 7.6. Measured size distributions of acid mist and other particulate components downstream of scrubber module “A.” 

Concentrations are shown on the basis of particle volume. The H2SO4 concentration in the flue gas was 2.6 ppmv. 
0 

0.01 0.02 0.03 0.04 0.05 0.06 

dV/dLogD, cm3 /dncm 

Acid Alone Solids Alone Sum Acid Alone + Solids Alone 

80 

0.01 0.1 1 

Diameter, μm 

Figure 7.7. Measured size distributions of acid mist and other particulate components in the stack downstream of a scrubber 

Concentrations are shown on the basis of particle volume. The H2SO4 concentration in the flue gas was 2.3 ppmv
0 

10 20 30 40 50 60 

dM/dLogD, mg/dncm 

2-7-06 H2SO4+H2O 1-04 H2SO4+H2O 

81 

0.1 1 10 

Diameter, μm 

Figure 7.8. Measured size distributions of acid mist in the stack as shown in Figure 7.7 after replacement of the scrubber system wit

Concentrations here are shown on the basis of particle mass. 
82 

Figure 7.9. Comparison of droplet diameters extrapolated from measurements with those predicted by the m

extrapolations were made by assuming constant number concentrations with the diameters being propocube root of the SO3 concentration. 
0 

10 20 30 40 50 60 70 80 90 

100 

Opacity, % 

Predicted Plume at dia. = 70m, Acid only Predicted Plume at dia = 70 m w Part. & NO2 Reported by VEE Smoke-reader 

83 

0 5 10 15 20 25

H2SO4, ppm

Figure 7.10. Comparison of predicted plume opacities versus H2SO4 concentration with those measured by a certified “smoke reader

MW unit with a pollution control system consisting of an SCR followed by a cold-side ESP and an SO2 scrubber.
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87
APPENDIX A. CORRELATIONS FOR SULFURIC ACID 

DENSITY, SULFURIC ACID SURFACE ENERGY, AND 

THE VAPOR PRESSURE OF WATER OVER SULFURIC 

ACID AT 333.15 K (140 °F). 
APPENDIX B. COMPUTER CODE FOR THE 

CALCULATION OF ACID MIST PROPERTIES IN THE 

SCRUBBER AND STACK.