United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85/038 Feb. 1986
H
SER& Project Summary
Modeling of S02 Removal in
Spray-Dryer Flue-Gas
Desulfurization System
Ashok S. Damle
This report presents a comprehen-
sive mathematical model of the SO2 re-
moval process in a spray-dryer flue-gas
desulfurization system. Simultaneous
evaporation of a sorbent droplet and
absorption/reaction of SO2 in the
droplet are described by the corre-
sponding heat- and mass-transfer rate
relations. Dissolution kinetics of lime
particles within a slurry droplet is in-
cluded in determining the overall SO2
removal rate. The model identifies sev-
eral important parameters which need
to be estimated or determined from ex-
perimental data. This report also in-
cludes a computer program,
"SPRAYMOD," written in Basic lan-
guage, to predict SO2 removal in a
spray dryer, based on the model devel-
oped. The program is user oriented and
easy to use. The contribution of particu-
late collection equipment, a baghouse
and an electrostatic precipitator, to-
ward overall S02 removal is also
discussed.
This Project Summary was devel-
oped by EPA's Air and Energy Engineer-
ing Research Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Spray-drying technology for S02 ab-
sorption/removal from flue gases has
been advanced for the past few years. In
spite of a large amount of pilot-plant
testing and a few full-scale commercial
applications, however, there is still a
lack of comprehensive predictive mod-
eling of this process. A review of quali-
tative mechanisms so far proposed was
published recently. Semi-empirical rela-
tionships have been developed to re-
lated S02 removal efficiency of the
spray-dryer system with stoichiometric
ratio and approach to saturation. Such
relationships, due to the empirical
parameters, tend to be specific for the
spray-dryer system used to obtain
them.
This report presents a simple mathe-
matical model describing various proc-
esses occurring in a spray-dryer flue-
gas desulfurization (FGD) system. The
overall process is subdivided into sub-
processes contributing to SO2 removal.
Various parameters required for such a
model are identified. An overall qualita-
tive picture is first described, followed
by modeling of the subprocesses.
Overall Process
In a spray-dryer S02 removal system,
a conventional spray dryer is typically
used to contact SO2-laden flue gas with
spray droplets of a slurry or a solution
of a suitable sorbent. Figure 1 is a typi-
cal schematic of a spray-dryer system.
Rotary or pneumatic atomizers are used
to inject the sorbent slurry/solution. The
amount of sorbent added depends on
the stoichiometric ratio to be used and
the inlet flue-gas SO2 concentration.
The amount of water added to the sys-
tem is controlled by inlet flue-gas tem-
perature and humidity, and the desired
approach to saturation at the spray-
dryer outlet. The sorbent may be intro-
duced as a slurry or solution, depending
on the sorbent solubility in water. Lime
slurry is typically prepared in a slaker to
-------
Flue Gas
Clean Gas to Stack
Makeup Water
Water
Fresh Sorbent
Spray
Dryer
Flue Gas
Paniculate
Collection
Equipment
Solids
Solids Disposal
System
Recycle
Discharge of
Solids
Recycle
Sorbent
Preparation
System
Recycle Solids
Preparation
Figure 1. Simplified flow diagram of a spray-dryer FGD system.
obtain a slurry of fine-grained lime par-
ticles. The flue-gas residence time is
typically about 10 seconds. After the
spray dryer, the flue gases, along with
flyash and dried sorbent/product parti-
cles, pass through particulate control
equipment, such as a baghouse or an
electrostatic precipitator (ESP). Some
particulate may also be collected in the
spray dryer itself.
In the spray chamber, two processes
occur simultaneously: water evapo-
rates from the droplet; and S02 is ab-
sorbed in, and reacts with, the alkaline
sorbent. The flue gas is typically humid-
ified adiabatically to within 10 to 35°C of
its saturation temperature. The amount
of water evaporated from a droplet is
determined by the operating conditions
of the dryer-inlet gas temperature, inlet
gas relative humidity, approach to satu-
ration temperature, and corresponding
equilibrium moisture content of the
solid. S02 may be removed by the sor-
bent both during and after drying of
droplets. Spray-dryer operation nearer
to flue-gas saturation condition and
higher stoichiometric ratios improves
S02 removal efficiencies. S02 removal
may continue through the particulate
collection equipment as gas passes
through filter cake in the baghouse or
over the deposits on the collection
plates of an ESP. A portion of the waste
particulate discharge from the spray
dryer and the particulate collection de-
vice may be recycled into the spray
dryer's feed slurry to increase sorbent
utilization.
Although droplet evaporation and
S02 absorption occur simultaneously,
the droplet drying process is more or
less independent of the S02 absorption
process. On the other hand, the S02 ab-
sorption/reaction process is strongly re-
lated to the drying process and droplet
moisture content.
Drying of Droplets
The drying behavior of a slurry
droplet with freely moving sorbent par-
ticles is similar to that of a solution
droplet. In line with conventional drying
theory, the evaporation from a slurry-
solution droplet proceeds in two
stages: (1) the rate of evaporation is de-
termined solely by the resistance of the
gas film surrounding the droplet to the
transfer of water vapor (this stage con-
tinues until the droplet moisture level
falls below a critical moisture content);
and (2) the solid's concentration re-
duces the rate of drying since the mois-
ture must diffuse through the solid ma-
trix. During stage (2), there is a change
from water as a continuous phase, as
initially in the droplet, to the solid ma-
trix as a continuous phase. The drying
continues until the droplet moisture
content reaches an equilibrium with the
surrounding gas atmosphere.
Constant Rate of Drying Period
The rate of droplet drying in this pe-
riod is determined by the simultaneous
heat transfer from the gas phase to the
droplet and water vapor transfer from
the droplet to the gas phase. The heat
and mass transfer processes between a
droplet and surrounding gas phase
have been studied extensively. The re-
spective gas-phase transfer coefficients
may be determined by widely used em-
pirical correlations. These correlations
take into account the effect of relative
velocity between the droplet and sur-
rounding gas phase. For droplets
smaller than 100 M.ITI, the relative veloc-
ity may be ignored, which leads to sim-
plified transfer coefficient correlations.
The effect of water evaporation on the
heat and mass transfer rates may also
be taken into account by a rigorous
analysis for a quiescent droplet-gas
system.
Falling Rate of Drying Period
The constant rate-drying process con-
tinues until the moisture content of the
droplet falls below a critical moisture
content at which point the solid's con-
centration begins to influence the dry-
ing rate. Critical moisture content of the
droplet depends on its solid's proper-
ties; e.g., hygroscopicity. The critical
moisture content may be considered as
that at which the solid particles begin to
touch each other and form a continuous
phase. The drop diameter then does not
change significantly during further dry-
ing. This period continues until the
moisture content reaches an equi-
librium value.
In this period, drying is controlled by
diffusion of moisture through the solid
matrix. The drying rate may be as-
sumed to fall linearly between the criti-
cal and equilibrium moisture contents.
The parameters, critical and equilibrium
moisture contents, depend on the
solid's contents and their properties
and must be determined experimen-
tally. For given solids, the equilibrium
moisture content varies linearly with re-
spect to relative humidity of the sur-
rounding gas phase.
SO2 Absorption/Reaction in
Spray Dryer
Absorption and reaction of SO2 in a
sorbent droplet occur both before and
after drying of the droplet up to an
equilibrium-moisture content. Presence
of moisture during the wet-droplet
stage is important: it provides an
-------
aqueous medium for absorption and
fast ionic reaction of S02. The lack of
moisture in the dry-particle stage con-
siderably reduces the rate of SO2 re-
moval because absorption and reaction
in the solid phase are slower. This is
especially true for the lime sorbent
which has a low reactivity in the solid
phase. The mechanisms of S02 removal
in the two stages are distinctly different
and, therefore, are considered
separately.
Wet-Particle Stage
During the wet stage, moisture in the
droplet participates actively in the over-
all S02 removal process. S02 is trans-
ported from the bulk-gas phase to the
droplet surface by the gas-phase diffu-
sion process. The dissolved S02 mi-
grates from the interface to the interior
of the droplet by liquid-phase diffusion
and reacts with the dissolved sorbent. If
a sparingly soluble sorbent (e.g., lime)
is used, dissolution of the sorbent also
becomes important. If the ionic reaction
between the sorbent and S02 is very
fast, both species may migrate to a reac-
tion plane or zone in the bulk liquid as
shown in Figure 2. The dissolution proc-
ess will not be present with a highly sol-
uble sorbent, but the overall process
may still be represented by Figure 2 by
replacing the equilibrium-solubility sor-
bent concentration by the bulk liquid-
phase sorbent concentration.
S02 removal during the wet stage by
the above mechanism continues until
the moisture content in the droplet falls
to the equilibrium moisture content.
Droplet
Surface
. so2
Bulk Gas
Phase
After that, the droplet may be consid-
ered a dry-porous solid to determine
further SO2 removal. Steps involved in
the removal of SC>2 during a wet-droplet
stage are:
1. Transfer of S02 from the bulk-gas
phase to the droplet/particle sur-
face. The rate of transfer in this
step is controlled by the resistance
of a gas film around the droplet.
2. Dissolution of S02 into the liquid
phase in the droplet, and transfer
of dissolved S02 from the droplet
surface to the interior liquid. The
transfer rate is controlled by the
liquid-film resistance.
3. Dissolution of lime into the liquid
phase.
4. Ionic reaction between dissolved
SO2 and the dissolved sorbent in
the liquid phase.
5. Transfer of reaction products to
precipitation states.
In the initial constant-rate drying
stage, water is the continuous phase in
the droplet with the suspended sorbent
particles free to move within the drop-
let. In such a situation, the resistance
offered by precipitation of the reaction
products may be ignored. Also, the
ionic reactions are very fast and may be
considered to be not controlling the
overall rate of S02 removal.
Gas-Film Resistance
Diffusion of SO2 from the bulk-gas
phase to the droplet surface is similar to
water evaporation from the droplet sur-
face, and similar correlations apply to
determine the gas-phase mass transfer
coefficient. Evaporation of water has a
Lime Particle
Surface
Gas Film Liquid Film
Figure 2. Schematic of SOz absorption/reaction in a wet droplet.
strong inhibiting effect on S02 transfer
and may be taken into account by a rig-
orous analysis for a quiescent droplet-
air system. The rate of S02 transfer
based on gas-phase mass-transfer re-
sistance alone depends strongly on the
S02 concentration in the bulk-gas
phase.
Liquid-Film Resistance
No good correlations are available to
estimate the liquid-phase resistance in
the droplet to the mass transfer; how-
ever, an order of magnitude estimate
may be obtained. The enhancement of
this liquid-film mass-transfer coefficient
should be considered because of the
very fast reaction of dissolved S02 with
dissolved lime in the bulk liquid. The
liquid-phase mass-transfer coefficient,
after considering this enhancement, is
about two orders of magnitude greater
than the corresponding gas-phase
mass-transfer coefficient. Therefore, the
liquid-phase resistance may be ignored
in model simulations.
Dissolution of Sorbent
This step is obviously required only
for sorbents (e.g., lime) which are spar-
ingly soluble in water. The rate of lime
dissolution may be estimated based on
the solubility of lime in water, diffusivity
of lime in water, and a mean distance
between sorbent particles. For fast reac-
tion between dissolved S02 and lime,
the bulk liquid-phase lime concentra-
tion may be assumed to be zero. The
interparticle distance depends on the
sorbent particle size and the slurry
concentration.
Effect of Product Precipitation
Precipitation of products, if it occurs
(e.g., with lime sorbent) may affect the
dissolution rate of sorbent. The reduc-
tion in sorbent dissolution rate would
be directly proportional to the surface
area of sorbent obstructed by product
precipitation. In the early stages of dry-
ing, this effect would be minimal be-
cause of the mobility of sorbent parti-
cles within the droplet. This effect
would increase considerably as the
solid phase becomes the continuous
phase. A simple way to account for this
effect in the model is to ignore it in the
early stages until moisture content falls
to the critical moisture content, and in
later stages consider the dissolution
rate to be proportional to the sorbent
fraction remaining in the solid phase.
-------
Dry-Particle Stage
As the droplet dries and the moisture
content approaches the equilibrium
moisture level, diffusion of S02 into the
solid matrix and the solid-phase reactiv-
ity become important compared to the
gas-phase mass-transfer resistance.
The S02 removal process becomes that
of S02 diffusion through the solid phase
with chemical reaction. SO2 absorption
and reaction during the dry-particle
stage may be analyzed in a rigorous ap-
proach of diffusion into a spherical par-
ticle with chemical reaction. For this ap-
proach, both the diffusivity of SO2 and
the reaction coefficient are needed.
Such rigorous analysis may be sim-
plified when either the diffusion process
in the solid matrix or the chemical reac-
tion in the solid matrix is dominant over
the other. When the chemical reaction
rate is much greater than the diffusion
coefficient, S02 is consumed by the
solid phase as soon as it diffuses in-
ward. This leads to a sharp reaction
front proceeding inward, and the S02
concentration profile in the solid phase
appears as a square wave moving in-
ward. On the other hand, when the dif-
fusion in the solid matrix is much faster
than the rate of chemical reaction, S02
concentration throughout the particle
will be uniform and equal to the gas-
phase concentration. The reaction will
then proceed throughout the volume of
the particle and may be expressed con-
veniently by a bulk volume reaction
coefficient.
Spray-Dryer Inlet and Operat-
ing Parameters
The rate of drying of a single droplet
and the rate of SO2 removal by a single
droplet are interrelated to bulk-gas and
droplet properties. In addition, the bulk-
gas properties also depend on gas-inlet
conditions, prescribed operating condi-
tions, and gas flow and mixing within
the spray dryer.
Inlet-Gas Specifications
The inlet-gas specifications required
are: 1) actual volumetric gas-flow rate;
2) flue-gas temperature; 3) flue-gas
composition and its molecular weight;
4) amount of water vapor present, or the
adiabatic saturation temperature; and
5) S02 concentration.
Operating Parameters
The operating parameters that influ-
ence the bulk-gas properties at the
spray dryer outlet and SO2 removal in
the spray dryer are: 1) approach to satu-
ration temperature at the spray dryer
outlet; 2) stoichiometric ratio of fresh
sorbent added in the atomized slurry to
the amount of SO2 in the inlet flue gas;
3) recycle ratio of solids collected in
spray dryer and paniculate collection
equipment in the spray dryer feed
slurry; 4) droplet size distribution in sor-
bent feed slurry/solution—method of
slurry atomization; and 5) method of
slurry preparation (slaking)—sorbent
particle size distribution in slurry.
In the spray dryer, the flue gas is
cooled and humidified adiabatically by
evaporation of the water from the
droplets (ignoring any heat losses).
Thus, prescribing the flue-gas inlet con-
ditions and the approach to saturation
at the spray-dryer outlet determines the
temperature and humidity of the flue
gas at the spray dryer outlet and also
the amount of water to be added to the
slurry. Alternatively, the amount of
water added to the slurry determines
the approach to saturation. The stoi-
chiometric ratio determines the amount
of fresh sorbent used per unit amount of
inlet flue gas. The recycle ratio further
determines the amount of recycled
solids accompanying the fresh sorbent
in the slurry. All three of the above
parameters thus determine the solids
concentration in the feed slurry-
Gas-Flow Pattern and Mixing in
the Spray Dryer
The inlet and operating conditions
specify the bulk-gas properties at the in-
let and outlet of the spray dryer. How-
ever, the rates of heat and mass transfer
depend on the local properties of the
bulk gas in contact with the spray
droplet. The local gas properties (e.g.,
temperature and concentrations) are
determined by the gas-flow pattern and
mixing within the spray dryer. With the
completely backmixed gas-flow pattern,
the bulk-gas properties throughout the
spray dryer are uniform and the same
as those at the spray dryer outlet. At the
other extreme, when the spray dryer is
considered to be a plug-flow system,
the bulk-gas properties change gradu-
ally with the local rate of change of a gas
property, depending on the local rate of
transfer processes. The model pre-
sented here considers both extremes.
Mass and Energy Balances
Evaporation of water from droplets,
and absorption and reaction of S02 in
droplets change the bulk properties and
composition of both the gas phase and
the droplets. The rates of heat and mass
transfers are coupled with mass and en-
ergy balances for both the gas phase
and the droplets to develop differential
equations to describe the rate of change
in the bulk properties (e.g., temperature
and composition). These differential
equations are then integrated over the
spray-dryer residence times of gas
phase and droplets to obtain the overall
removal of SO2 in the spray dryer.
After establishing the initial condi-
tions, the gas phase and the droplets
are "followed" from the spray dryer in-
let to the outlet to determine total
change in both. The overall mass and
energy balances on the gas phase deter-
mine the outlet temperature and humid-
ity of the gas phase. The differential
equations describing the rate of change
in droplet properties, with respect to
residence time, are derived using the
mass and energy balances around a
droplet. The droplets are assumed to be
uniformly mixed throughout the gas
phase. For plug-flow gas-flow patterns,
gas properties are assumed to vary con-
tinuously from its inlet to outlet proper-
ties. Thus, in this case, similar differen-
tial equations describing the rate of
change in gas-phase properties with re-
spect to residence time are derived
using local mass and energy balances
across a small section of the spray
dryer. No such gas-phase balance equa-
tions are required for a completely back-
mixed gas-flow pattern. The uniform
gas-phase temperature and humidity in
this case are determined by the initial
and operating conditions; however, the
outlet S02 concentration cannot be de-
duced. Therefore, trial-and-error is re-
quired to determine the efficiency of
SO2 removal for backmixed flow.
"SPRAYMOD" Computer
Program
To solve the material and energy bal-
ance equations described in this report,
using the rate relations developed, a
computer program, "SPRAYMOD," was
written in Basic language. The com-
puter program was developed on a
desk-top microcomputer.
The program basically has three sec-
tions. In section 1, a menu input format
is used to enter all the input data regard-
ing specifications and operating vari-
ables. After input, the data are printed
out for verification. In section 2, gas-
phase overall material and energy bal-
ances are carried out to establish all the
-------
initial conditions for both the droplet
and the gas phase. The dependent vari-
ables are then initialized.
In section 3, the differential equations
for the droplets and for the gas phase
(plug-flow option) are solved by a sim-
ple, explicit, finite-forward-difference
scheme. The time step is controlled so
that a maximum change during a time
step in the droplet temperature or the
droplet weight is less than 1% of the
function value. This criterion ensures
accurate solution in spite of the simple
numerical scheme used for the solution.
The program has two options for the
gas-flow pattern: backmixed flow and
plug flow. Backmixed flow requires
trial-and-error to determine 862 re-
moval efficiency. The backmixed-flow
assumption is often close to the real
situation.
The program essentially follows a sin-
gle spray droplet suspended in the bulk-
gas phase with time. The changes in
droplet and bulk-gas properties (e.g.,
droplet diameter) are related to rates of
mass and heat transfer. The bulk-gas
physical properties required in various
rate relations are evaluated at the mean
temperature of the gas film surrounding
the droplet. The properties of the liquid
phase (e.g., vapor pressure) are evalu-
ated at the droplet temperature, as-
sumed to be uniform throughout the
droplet. At each time increment, the
derivative functions are determined and
the time step, DT, is established. The
amount of water evaporated and S02
absorbed in time DT are then evaluated.
Corresponding changes in all gas-phase
and droplet variables during time step
DT are determined. The calculations
continue until no more SO2 is removed
by droplets or till the time reaches the
residence time value.
Program Simulation
Figure 3 shows results of a sample
simulation under assumed operating
conditions. The key parameters and op-
erating variables used in this model
simulation are:
Operating Variables:
• Inlet gas temperature = 160°C
• Amount of water in
inlet flue gas = 6.4% by
volume
• S02 concentration at
inlet = 800 ppm
• Approach to satura-
tion temperature at
the spray dryer outlet = 15°C
• Average droplet di-
ameter = 50 |im
100r
-i WO
Droplet Diameter
Approach to Saturation
Stoichiometric Ratio
Inlet SOi Concentration
No Recycle
4.0 5.0
Time, sec
9.0 10.0
Figure 3. Evaporation of water and SO2 absorption by a droplet in a spray dryer.
• Primary lime particle
size = 4 (Am
• Stoichiometric ratio =1.0
• Recycle ratio = 0
• Mass fraction of ac-
tive sorbent in feed = 1.0
Key Parameters Assumed:
• Critical moisture con-
tent = 50%
• Equilibrium moisture
content at 100% rela-
tive humidity = 3%
• Completely back-
mixed flow system in
spray dryer
• Volumetric reaction
coefficient at 2%
residual moisture in
solids = 1 x 107
cm3/
gmole-
sec
The simulation shows that, in this case,
the slurry droplet loses most of its mois-
ture in the first couple of seconds with
the moisture content eventually ap-
proaching its equilibrium value. Most of
the S02 is absorbed in the first stage of
drying with S02 removal continuing at a
slower rate in the dry stage.
For the operating conditions and key
parameters assumed, the S02 concen-
tration at the spray dryer outlet was de-
termined to be =215 ppm by trial-and-
error. This was equivalent to 73% SO2
removal efficiency in the spray dryer.
Comparison of Model Predic-
tions with Pilot-Plant Data
The predictions of S02 removal effi-
ciency of a spray-dryer system by the
model presented in this report were
compared with pilot-plant data under
various operating conditions. The pNot-
plant data set used for comparison, col-
lected by Cottrell Environmental Sci-
ences, Inc., at the Comanche Station of
Public Service Company of Colorado,
was chosen because of its availability
and extent of information.
In addition to various operating
parameters, some physical parameters
needed to be specified in model simula-
tions. The drying characteristics of
solids used were: critical-moisture con-
tent of 30% by mass and an equilibrium-
moisture content of 15% by mass at
100% relative gas-phase humidity. The
critical-moisture content value was con-
sidered to be that at which the solid
spheres start touching each other in the
droplet; whereas, the equilibrium-
moisture content value was approxi-
mately determined from the measured
moisture contents of spray-dryer solids
in this pilot-plant study. The informa-
tion on reaction coefficients was not
available in this report. For model simu-
lations, the solid-phase reactivity was
therefore considered to be zero. No
measured atomized droplet-size distri-
bution was available in this study, and
the mean-droplet size inferred from the
-------
measured spray-dryer outlet solid-size
distribution was taken as 50 urn
(monodispersed). The lime particle-size
distribution in slurry was available: it
was approximately monodispersed
with a mean size of 4 IJUTI. The gas-flow
pattern in model simulations was as-
sumed to be completely backmixed.
The model comparison with the data
is shown in Table 1 and Figure 4. The
agreement was very good: most of the
model prediction was within ±10% of
data values. With closer approach to
saturation, however, there was an
underprediction of S02 removal effi-
ciency. This is believed to be due to ne-
glecting the solid-phase reactivity,
which is expected to be significant at
closer approach to saturation, due to
the increased amount of equilibrium
moisture in solids.
SO2 Removal in Particulate
Control Equipment
As the spray-dried particles are car-
ried away by flue gas and collected in a
participate collection device, the unre-
acted sorbent is likely to remove addi-
tional S02. In a paniculate collection de-
vice, solids are accumulated for a long
time which effectively provides a very
high stoichiometric ratio. Since the
physical state of the particles in dust de-
posits is similar to that in the dry-
particle stage, related rate equations are
applicable. Due to the long exposure
times of the dust particles, the simple
approach of bulk volume reactivity
would be applicable to express SO2 re-
100r
o
I
CU
O
to
•a
£
t>
•8
I
40 -
20 •
Inlet Gas Temp.
Approach to Saturation
Stoichiometric Ratio
SOi Concentration
No Recycle
130°-160°C
8°-34°C
0.7-3.3
700-800 ppm
20 40 60
Observed SO? Removal Efficiency. '
80
100
Figure 4.
Comparison between predicted and observed SOz removal efficiencies under
various operating conditions.
moval by them. The reactivity of the
dust particles depends strongly on the
moisture content of the solids. It also
depends on particle size, available sur-
face area, and type of reagent.
Baghouse as a Particulate Con-
trol Device
As the flue gas passes through the
dust cake, S02 is transported from the
Table 1. Comparison of SO2 Removal Efficiencies Predicted by Model to Those Observed in Research Cottrell Pilot-Plant Data
Run
No.
101
104
105
107
109
111
112
115
116
118
120
125
126
130
131
Inlet Gas
Temperature
(°F>
334
340
301
300
307
301
307
256
261
340
341
262
264
340
342
Adiabatic
Saturation
Temperature
m
125
125
126
126
126
126
126
126
127
125
124
126
126
125
123
Approach to
Saturation
m
69
33
74
52
62
34
22
31
15
15
61
18
18
20
22
Stoichiometric
Ratio
3.26
1.69
2.36
2.70
1.28
1.62
1.55
1.56
1.30
1.31
1.35
0.71
0.99
1.17
0.72
Inlet SO2
Concentration
(ppm)
780
710
780
730
800
780
750
770
800
810
790
800
800
800
800
Observed
SO2 Removal
Efficiency
(%)
57.7
76.8
44.9
56.2
45.0
65.4
81.3
61.0
71.2
82.7
49.4
65.0
67.5
73.8
55.0
SO2 Removal
Efficiency
Predicted by
Model (%)
58.6
70.4
46.4
61.4
36.3
65.4
79.8
64.0
77.5
86.4
39.2
50.0
62.6
68.8
42.0
Note: Recycle ratio = 0.
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bulk-gas phase to the dust particle sur-
face. Due to intimate dust/gas contact,
the SO2 concentration at the surface will
be the same as the bulk-gas phase S02
concentration. The S02 removal by the
dust cake can thus be determined by
using the reactivity of the dust particles,
fraction of the unreacted sorbent, dust
cake thickness, and the surface area to
the volumetric flow ratio.
ESP as a Particulate Control
Device
In an ESP, S02 is transported from the
bulk-gas phase to the dust deposited on
collection plates by gas-phase diffusion.
Therefore, the gas-phase diffusion re-
sistance contributes to overall S02 re-
moval in addition to the dust particle
reactivity. ESPs have very well defined
flow geometry; therefore, experimental
mass transfer correlations reported in
the literature for similar flow ge-
ometries may be used to determine the
gas-phase mass transfer resistance to
SOj removal. S02 removal in an ESP
can then be determined by taking into
account both the gas-phase mass-
transfer coefficient and the dust-cake
reactivity.
Conclusions and Recommenda-
tions
1. A comprehensive mathematical
model was developed to describe
various subprocesses occurring in
the S02 removal process in spray-
dryer FGD systems. S02 removal,
both in the spray chamber and in the
particulate collection device, was
considered.
2. A computer program,
"SPRAYMOD," was written to pre-
dict S02 removal in a spray dryer
based on the mathematical model
developed in this report. The pro-
gram is listed in the report's
Appendix.
3. Key physical parameters influencing
the model were identified. Several
parameters (e.g., critical and equi-
librium moisture contents and the re-
action coefficient) need to be deter-
mined experimentally. These
physical parameters and process op-
erating parameters (e.g., stoichio-
metric ratio and approach to satura-
tion) determine the spray-dryer FGD
performance.
4. Comparison of model predictions
with one set of pilot-plant data
shows good agreement. The main
obstacle in such comparisons is lack
of availability of complete data and
the fundamental parameters needed
in the model. Future work, therefore,
could include procurement of com-
plete data and determination of un-
known parameters to allow further
validation of the model.
U. S. GOVERNMENT PRINTING OFFICE: 1986/646-116/20772
-------
Ashok S. Damle is with Research Triangle Institute. Research Triangle Park, NC
27709.
Louis S. Hovis is the EPA Project Officer (see below).
The complete report, entitled "Modeling of SQzRemovalin Spray-Dryer Flue-Gas
Desulfurization System,"(Order No. PB 86-136 165/AS; Cost: $11.95, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-85/038
00003Z9 PS
U S EWVIR PROTECTION AGENCY
REGION 5 LIBRARY
230 S DEARBORN STREET
CHICAGO IL 60604
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