United States
                    Environmental Protection
                    Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 277t 1
                    Research and Development
EPA/6QO/S7-90/011 June 1990
&EPA          Project  Summary
                    An  Evaluation of the E-SO
                    Process  on  the  EPA Pilot
                    Electrostatic  Precipitator
                  X
                    Louis S. Hovis
                     The E-SOX Process makes use of an
                   electrostatic  precipitator (ESP) for
                   combined sulfur  dioxide  (SO2)
                   removal and  particulate collection.
                   The concept of  spray drying  is
                   introduced to the inlet and/or first
                   section of the ESP in which electrical
                   components  are removed.  Because
                   of the many ESPs at coal-fired power
                   plants, the process  is well  suited to
                   retrofitting. The work  described  in
                   this report was a small pilot-scale
                   evaluation of  the  process  to obtain
                   the information  needed  to undertake
                   a  planned   5  MWe   field  pilot
                   demonstration. The results  from this
                   evaluation indicate that a  50 - 60%
                   removal of SO2 at a calcium to sulfur
                   ratio of 1.2 - 1.4  can  be  obtained.
                   Furthermore, this  reduction in SO2
                   can be  achieved without degrading
                   the particulate  emissions  even
                   though the  process  requires a
                   reduction in the collecting surface of
                   the ESP. The  utilization  of  a
                   temperature-controlled electrode
                   precharger to  compensate for loss of
                   collecting  surface is  also described.
                     This  Project  Summary  was
                   developed by  EPA's  Air and Energy
                   Engineering  Research Laboratory,
                   Research Triangle 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
                     E-SOX is a retrofit process for coal-fired
                   boilers, which combines in  a single unit
                   electrostatic  precipitator  (ESP)
technology for collecting particles  and
spray dryer technology .'for sulfur dioxide
(SO2)  removal. The process uses  a
modified existing ESP equipped with an
auxiliary system for preparation  and
injection of a lime slurry into the  ESP.
The front end of  an existing ESP is
converted to  a spray ..chamber where
contact is made between  gaseous  SO2
and lime slurry droplets.  Water  also
evaporates in this  converted section of
the  ESP so that the  reaction  product,
excess  lime, and fly  ash that enter the
remaining portion  of the ESP  are
sufficiently dry  for  efficient  ESP
operation. A  dry  solid waste product
containing reaction products, unreacted
lime, and fly ash is collected. That portion
of the  ESP  not converted to  a spray
chamber  is  left  with  electrical
components  intact  to operate as  a
particle collector. However, to provide the
contacting chamber sufficient space for a
finite drying  time of  1s or greater, the
ESP will lose 25 to 30% of its  collector
plate surface.  At  the same time, the
particulate load will increase by a factor
of 3 or more.  This particulate, a  large
fraction of which is calcium based, has an
extremely high electrical  resistivity at
normal ESP operating temperatures. The
sorbent reaction with  SO2 will eliminate
the effects of sulfur  trioxide (SO3)  that
would lower this resistivity. On the other
hand, lowering the gas temperature by
the  water  spray  will  more  than
compensate for resistivity increases  due
to ash  composition. Advanced  ESP
technology to aid  in  maintaining good
particulate collection performance is
available for E-SOX  retrofitting if required.
Cooled  pipe precharging, for  example,
can  be introduced in retrofit ESPs to

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compensate for less collector surface or
changes  in  resistivity characteristics of
the participate.
  The concept of E-SOX appears to offer
an  attractive  option  for acid  rain
mitigation. An economic evaluation  of E-
SOX  based on reasonable rates of SO2
removal and lime utilization has indicated
that it can be cost effective. A large pilot-
plant evaluation is underway  at  Ohio
Edison's Burger Station sponsored by the
U.S.  Environmental Protection  Agency
(EPA) and the  Ohio Coal  Development
Office. Preparation  for  that pilot  plant
evaluation started in 1987 and the actual
testing is being carried out in 1989. This
report covers work that was performed in-
house at EPA to verify the original results
and to define the parameters that control
SOg  removal. The work- reported was
completed  and  the technology
transferred for use in starting up the field
site evaluation.
  The E-SOX   concept   raises  two
fundamental  questions which  can  be
answered  only by  experiment.  The first
question  concerns  the feasibility of
removing  substantial SO2  by contacting
the  rapidly  moving  gas  with slurry
droplets and drying the droplets within
the  space of one ESP section. The
second  question has  to do  with
maintaining an acceptable level  of ESP
performance under a reduced  collector
area and an increased particulate loading.
The  results  of  experiments to  partially
answer these questions  are  reported
here.

Test Facility
  All the experiments were  conducted in
the ESP  pilot-plant located at EPA's Air
and  Energy  Engineering Research
Laboratory  (AEERL).  The pilot-plant
consists  of  a four-section, single-lane
ESP operating  at  a flue  gas  capacity
equivalent to 0.47 m3/s".Outside air is
heated to  149°C by a natural gas heater.
Gaseous SO2 is injected into the heated
air to the desired  concentration  (usually
1,500 to 2,500 ppm) to  simulate burning
of moderately high sulfur coal. The ESP
is operated under about 0.5 kPa negative
pressure so that no SO2 is released to
the  room. Fly ash is aspirated counter
currently  into the simulated  flue gas
stream just before  the cocurrent injection
of the lime slurry.  Lime slurry containing
10 - 20% solids is  pumped through a
spray nozzle designed to provide an oval
  "Readers mom familiar with nonmetric units may
  uso the conversion factors at the back.
spray pattern in  the 1.27  x  0.381  m
contact  chamber. The atomized  slurry
droplets have a 1.5 - 2 s residence time
in the chamber to evaporate most of the
water. The evaporation causes a flue gas
temperature drop  and results  in  a
relatively dry, powder-like product which
contains the unused  lime, the  absorbed
and reacted  SO2, and fly ash. The spray
chamber consists of the entrance section
and the first  of four ESP sections with all
the  electrical internals  (i.e., discharge
electrodes)   removed.  The electrical
configuration  of the  ESP is  flexible but,
for most experiments  reported, two cold
pipe prechargers were used: one in the
connecting  space  between  sections  1
and 2 and one between sections 3 and 4.
Conventional  wire-plate electrodes were
assembled in sections 2 and 4.			

Summary  of Results
  The primary objectives  of the  E-SOX
experiments  carried out at EPA using the
0.47 m3/s modified pilot ESP  were to
verify E-SOX as a  competitive retrofit
process for  S02  removal   and  to
determine the critical parameters which
influence the degree of SO2 removal.
Once the critical factors were determined,
they could be adjusted within limitations
of the process to give the best conditions
for SO2 removal and  sorbent utilization.
To  meet these objectives,  experiments
were  planned to investigate impacts of
indirect variables as well as those directly
influencing the lime slurry/SO2 reaction.

SO2 Removal Dependence on
Critical Factors
  A  number of tests were performed in
which only  concentration  and  rate of
injection of  slaked lime were varied. In
essence, this permitted an examination of
the  effect  of  the  two most  critical
parameters on  SO2-removal;  the
temperature  of approach  to saturation
(ATAS) and  the  stoichiometric ratio of
calcium to sulfur (Ca/S)  in the  slurry/gas
mixing. When other parameters are held
constant, including spray chamber
geometry, gas flow  rate,  S02  concen-
tration in the gas,  and the  inlet gas
temperature,  these  injection parameters
can be manipulated to give  a ATAS and
Ca/S combination. There is a lower limit
on ATAS for adequate droplet drying and
an upper limit on the solids concentration
for  consistent  spraying.  The fixed
conditions are listed  in Table 1.
  The removal of SO2 as a function of
approach  temperature  for  various
stoichiometric ratios is shown in  Figure 1.
This correlation between  approach to
 Table 1. E-SOX Fixed Conditions for SO2
        Removal Studies
 Air Flow

 Inlet temperature

 SO2
 concentration

 Fly ash
 concentration

 Nozzle
 configuration

 Inlet chamber
28 m3/min

149°C

2000 ppm


1.9 g/l
Single two-fluid CasterJet
oval spray pattern

1.5 m spray chamber
plus 1.2 m ESP section
"saturation"and  SO2' removal shows that
 good removal  is possible at  very low
 stoichiometric ratios,  but only at very
 close approach temperatures.  At these
 close approaches  the  droplets are not
 completely  evaporated and excess
 moisture will pass into  the  ESP.  The
 lower practical  ATAS  is  believed to be
 between 16  and  17°C.  Stoichiometric
 ratios of 1.3 to 1.4 produced a removal of
 50%  or better at  a 17°C approach  to
 saturation.  Results  indicate  a marginal
 improvement in SO2 capture at a ratio of
 1.4. The leveling off of removal rate with
 increasing  Ca/S is  also  reflected  in the
 plot of  percent removal as a function  of
 Ca/S in Figure 2.
 Particulate Removal
  The second  fundamental question
 about E-SOX as a  retrofit  concerns
 maintenance of  an acceptable level  of
 particulate removal.  In  conjunction  with
 the  SO2  removal  testing,  the  ESP
 electrical  configuration  was"varied  to
 determine  effects of the increased  load
 and change  in  characteristics  of the
 particulate matter collected by the ESP.
 With sections 2, 3, and  4 energized and
 containing 0.32 cm wires, the ESP has an
 18.8 s/m (96 SCA). With only two of the
 sections energized, the ESP was reduced
 to  12.5   s/m (64  SCA).  Cold   pipe
 prechargers, between sections 1  and  2
 and between 3 and 4, could be activated,
 but the SCA would remain the same. The
 data  in Table 2 "show  that high mass
 efficiencies were obtained during a 4-fold
 increase in particulate loading and a  50%
 reduction in the SCA. There appears  to
 be no significant change in efficiency with
 the amount of  particulate  as long as
 some  moisture   is  present.  The   ESP

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      60
      50
  1"
   o

   I
  DC
   w
  o
  CO
     40
     30
                6

                O
                                       S02
                                    2000 ppm

                                        Ca/S

                                      • 1.5

                                      O 1-4
                                      A 1.3

                                      A 1.2

                                    ...a.i.1
                                             n
       13.9
16.7            19.4            19.4
    Approach to saturation, °C
Figure 1. Effect of approach temperature on S02 removal at several stoichiometric ratios.
      55
      50
    as
    75
     (D
     o:
  t. * -..40
      35
        7.0
                   J.T
                              7.2         7.3
                                    Ca/S
                                                      7.4
                                                                 7.5
emission rates listed in Table 2 also
indicate  that  under  E-SOX
conditions the  ESP can meet or
exceed  the  NSPS standard  of 43
ng/J. For fly ash only, the efficiency
was reduced  severely  without
moisture addition.  In this case, the
38 kV could be maintained on the
cold pipe precharger, but not  on the
wires  because of  back  corona.
Evaporation of water  during the
drying step lowers the temperature
which   accounts  for the  low
resistivity  of the  lime  sorbent/fly
ash mixture in  the E-SOX process.
As  a consequence  of the  low
resistivity,  back corona  is  not a
problem.

Future Work         K
  The primary  immediate  E-SOX
follow-up will occur at  the Burger
Plant of Ohio  Edison  where the
EPA,  Ohio  Coal  Development
Office (OCDO),  Babcock & Wilcox
Research Division  and Southern
Research  Institute are  evaluating
the process.   The evaluation is
being carried out in a 5 MWe pilot
ESP which is  connected  by  a
slipstream  off of  the  main  ducts
between the boilers and the plant
ESP. The work  plan at this site has
been designed  to verify the SO2
removal results  and  the  ESP
efficiencies that  have been obtained
in the  in-house  process and
reported here.  The work, having
been done on a larger  unit, should
provide  experience in  design  that
will be more meaningful for  a full-
scale  demonstration.   The  pilot
evaluation is slated for completion
in late  1989 with  results to  be
reported in the spring of 1990.
Figure 2.  Effect of stoichiometric ratio on SO2 removal at two approaches to saturation.

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Table 2. E-SO, Particulate Removal Efficiencies for Various ESP Electrical Configurations
Sections Energized
38 kV
2,3,4
2,3,4
2,3,4
2,3
2,3
2,3,4
2,3,4
2.3.4
Cold Pipe
38 kV
Yes
No
No
No
No
No
Yes
Yes
Approach
Temperature (°C)
17
17
17
17
19
56
56
89*
Particulate
E-SOX & fly ash
Fly ash
E-SOX & fly ash
E-SOX & fly ash
Fly ash
Fly ash
Fly ash
Fly ash
ESP Efficiency
(%)
99.5
97.9
98.5
98.6
95.6
97.6
97.8
89.0
Emission Rate
(ngtJ)
7.319
13.776
12.915
37.884
24.969
12.915
16.359
55.104
*No moisture injection

NONMETRIC EQUIVALENTS

Readers more familiar with nonmetric units may use the following conversion factors:
                      Metric
                      °C
                      "C (app to sat.)
                      cm
                      g/i
                      kPa
                      m
                      MWe
                      nglJ
                 Multiplied by
                 9/5 x"C + 32
                 9/5 x °C
                 0.394
                 0.526
                 4.00
                 3.28
                 2128
                 3000
                 0.0023
Yields nonmetric
"F
"F (app to sat.)
in.
in. H2O
ft
cfm
acfm
lb/106 Btu
 The EPA author, Louis S. How's, also the EPA Project Officer (see below), is with Air and Energy Engineering Research
    Laboratory, Research Triangle Park, NC 27711.
 The complete report,  entitled "An Evaluation  of the E-SOX Process on the EPA Pilot Electrostatic Precipitator," (Order
    No. PB90-216 4411 AS; Cost: $17.00, 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/S7-90/011

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