H. Fujishima
                                  Y. Tsuchiya
                       Mitsubishi Heavy Industries, Ltd.
                1-1, Wadasaki-cho l-chorre, Hyogo-ku, Kobe, Japan

1.   Abstract

Dust emission level is recently required to be more and more severe in
advanced industrial countries.
In Japan, very low emission level which would realize an almost invisible
stack, is required especially for new installation of power station near
large cities.
In Europe and USA, meanwhile, there are problems of bluish smoke due to
SC1 mist.
As the countermeasure, wet type Electrostatic Precipitator(wet type ESP)
is applied in Japan.
The application of wet type ESP for boiler plants is described as follows.
2.    Wet Type ESP for 00- Fired Boilers

2.1   History

At the beginning of 1960's, no precipitators were considered to be
necessary for oil- fired boilers in Japan, because the flue gas from
oi1- fired boilers contains much less dust content than coal- fired
However the flue gas contains sulfuric acid (SO;;) which, not only causes
corrosion of equipments, duct and stack downstream air heater(AH) but
also produces acid smut.
  Furthermore, such SO;; exhausted from stack can be observed as bluish
smoke and thus the installation of precipitators had been considered to
prevent acid smut and bluish smoke.

In Japan, the first dry type ESP combined with ammonia (NlLjinjection
system in the world had been applied for Tokyo Electric Power Co.
Yokosuka Power Station Unit 5, in 1966. Since then, dry type ESP has
been extensively employed for utilities' boilers and industrial boilers
as well.
Flue gas desulfurization system (FGD) started to be installed to prevent
air pollution of 802 in Japan at the beginning of 1970's. It has begun
practical use of wet type ESP located downstream FGD to remove   sa mist
of flue gas from oi1- fired boiler.

2.2   :Mechanism of S<1Generation and its Behavior

In the exhaust gas from boilers, there exists  sa   converted from 802 in
boiler and flue gas denitrozen system with catalysis (DeNOx).
The quantity of generated S03 is proportional to sulfur content in fuel
oil and conversion rate of 802 ,
Fig - 1 shows the relationship of sulfur content in fuel oil and the
quantity of generated S03-
In the case of oi1- fired boilers, for example, 1% of sulfur in fuel
produces about 5",15 ppm of 803 , providing that conversion rate is 1-3%.
Fig - 2 shows the relationship of 803 quantity and sulfuric acid dew point
803 exists as gaseous condition in flue gas at AH inlet (400°C) _ As it
goes downstream, gaseous S03 becomes partially 803 mist, due to the
decrease of gas temperature, where the rate of SO;; gas and mist depends
on gas temperature.
Usually 803 exists as gas and mist mixture at AH outlet (140°C) and
exists as mist only atFGD outlet (50°C).
2.3     sa problems to be solved
S03 causes sulfuric acid corrosion at ESP, duct, and stack, etc. in
lower gas temperature zone downstream AB. Moreover it mostly becomes
SQ mist through the gas cooling process of FGD, and SQ mist can not be
completely removed in FGD of which principle of collecting mechanism is
based on impact among particles and water droplets.
As a result, sOa mist exhausted from stack causes the products of acid
smut and the generation of bluish smoke.

2.3.1     :Mrllanisnl of producing Acid Smut

In general, the soot from oil- fired boiler is originally fine particles
of micro - meter (11 m) order.
However, agglomeration occurs downstream AH, and as a result,
agglomerated soot falls in the neighbourhood as acid smut, which causes
pollution problem.
Acid smut is generally produced with following mechanism.

(I)   Acid Smut produced in gas stream through duct and stack
      When S03 mist exists in flue gas, fine particulates (boiler soot)
      are agglomerated as the medium of 803 mist and become, what is
      called, acid smut.
(2)   Acid Smut produced at the inside surface of duct and stack wall
      As the temperature at the surface of duct and stack wall is a little
      lower than flue gas temperature, sulfuric acid is condensed on the
      surface even at the flue gas temperature higher than acid dew point.
      As a result, agglomerated soot deposit increases on the surface of
      When deposited soot is worn off due to gas velocity fluctuation and
      dispersed, as acid smut.

2.3.2     Generation of Bluish Smoke

White smoke caused by water vapour can he easily eliminated by gas
Meanwhile, as S~ mist exhausted from stack is very fine and can not be
vapourized completely by gas reheating, it floats in the sky and is
observed as a trail of bluish smoke.
Therefore it is not possible to prevent bluish smoke due to 803 mist
without any countermeasure of SQ removal.
2.4 SQ Rerroval Teclmique

There are mainly three kind of flue gas treatment systems for oil - fired
boiler in Japan, as follows.
Fig - 3 shows the basic system flow and SC1 behavior in flue gas of each

1)   Dry type ESP without Nl1 injection and wet type FGD
     In the case of dry type ESP without NIL injection, gas temperature
     at AH outlet is kept higher than acid dew point temperature to
     prevent corrossion of ESP. That is, the average gas temperature at
     ESP inlet is usually kept at 15 - 20°C higher than the acid dew point
     temperature, considering the gas temperature distribution inside
     duct and ESP. The SC1 in flue gas passes through dry type ESP as
     gaseous condition and become mist in FGD.
     Some of sOa mist are removed in FGD and the others are exhausted
     from stack. This is insufficient as a countermeasure against SC1.

2)   Dry type ESP with Na injection and wet type ffiD
     In the case of dry type ESP with NH3 injection, 803 gas and mist
     react to NIt and produce ammonium sulfate (NR)2 SO.., which can be
     collected effectively together with boiler soot in dry type ESP.

3)   Wet ~ FGD and wet t~ f})P
     In the case of installing wet type ESP after FGD, the not - collected
     SO" mist and boiler soot in FGD can be removed by wet type .ESP.

2.4.1   Application of Dry type ESP with NIL Injection

For oil-fired boilers in Japan, the method of NIL injection upstream dry
type ESP is usually applied.
NIL injection system is originally employed to prevent corrosion of ESP
and come into wide use after following effects are confirmed.

1)   Prevention of acid smut
2)   Prevention of bluish smoke
3)   Stabilization of ESP collecting performance

There are lots of application examples among which more than a hundred
(100) plants have been already supplied only by us.
Concerning NHa injection, we put it to practical use in 1966 for the
first time in the world.
However, we experienced various unexpected troubles at the beginning.
It was found that most of them were- caused by shortage of NH3 injection
The following troubles occur when NH 3 injection quantity lacks.

1)     Corrosion of ESP casing and internal parts
2)      Sticky low fusion point compounds such as (NH 4 ) H 8 04 cause the
       following troubles
     · Discharge electrode enlarged by ash adhesion
     . Blockage of perforated plate at inlet duct
     • Choke of ESP hoppers and ash handling system

The following technical countermeasures are established.

1) Keeping a proper quantity of Nl1
2) NH3 injection distribution corresponding to gas temperature and
   sulfuric acid distribution in duct
3) NIl, injection nozzle structure, arrangement and purging method
4) Air dilution method considering the diffusion effect in duct
5) ESP design criteria corresponding to ash properties with NH 3
   injection     (collection    characteristics,  optimum    equipment
   specification. )
6) Optimum design of ash handling. system.

It is very important to design ESP, NH 3 injection and ash handling
system as a total treatment facility considering ash properties of NH 3 -
sa   compounds.

2.4.2 Application of Wet type ESP installed after Wet type
     DesuHurization System

As mentioned berore, 803 and boiler soot can not be removed completely
in FeD, and slurries carry over from FG-D.
These particles are so fine that they are observed as the trial of
bluish smoke, when exhausted from stack.
To cope with the bluish smoke, wet type ESP after FGD is to be applied.
The application of wet type ESP has some benefits as follows.

1)    Wet type ESP is generally so compact that it can be easily installed
      in the existing space.
2)    Waste water from wet type ESP can be utilized as supply water of wet
      type FGD.
2. 5   Problems to be solved in application of wet type ESP

Though wet type ESP was originally utilized in the steel industries,
there was no application of wet type ESP after FGD for boiler plant in
the world at that time.
Therefore the following points were needed to make clear and establish
the countermeasure.

(I)    Understanding of collection characteristics of boiler soot and 803
{2}    Optimum electrode structure to achieve a high collection performance
       without space charging
{3}    Preventing corrosion due tosa   mist and SQ gas
(4)    Establishment of rational cleaning method and waste water treatment
(5)    Preventing breakage of discharge electrode

Prior to the application, pilot tests in laboratory and in field had
been carried out since 1973.
However, at first, we experienced serious troubles which influenced
significantly on the applicability of wet type ESP. For example,
corrosion of stainless steel, frequent breakage of discharge electrode
and poor collection efficiency due to depressed lower electric current
caused by space charge, etc.
Appropriate improvement of technology has been implemented in each case,
and the standard specifications for wet type ESP have been established
finally. After that, stable operation results were obtained with the
plants delivered in 1981, in which big troubles were no longer reported

2.6     Examples of Application

Wet type ESPs for oil- fired boilers have been already applied at more
than 10 plants so far.
Some of them have been applied even for the plants, using high ash and
high S(h content residual oil such as petroleum coke and asphalt.
Fig - 4--.6 show the typical application examples for oil- fired boiler.

1)     A-Plant
       This is a typical application for oil- fired boilers.
       Technical improvement was added to this plant, reflecting from our
       experience of wet type ESP for the past plants. Since the start-
       up in 1981, stable operation has been continued without any serious
       troubles and high performance was obtained. The measured dust
       concentration at ESP outlet shows 4mg/ rriN which satisfies enough
       the design value 20 mg/ni N,
2)    B-Plant
      This is the newest application for the oil- fired power plant and now
      under construction, aiming at start - up in 1997.
      Outlet dust concentration less than lOmg/       rrrNdry is determined
      against inlet dust 145mg/ rrf N dry (including    sa mist 115mg/ rri N),
      expecting that this plant will be effective for prevention of S03
      mist which FGD can hardly remove.

3)    C-Plant
      This is an application for industrial private power plant in Japan
      and now under designing, aiming at start-up in 1998.
      In the case of the new facilities' installation, the total allowable
      emission should be kept for existing and new facilities in
      accordance with environment assessment in Japan. Therefore, a
      lower ernisson is required for new installation.
      In this case, a wet type ESP is installed after FGD in order to keep
      very low emisson.
      It is considered that in Japan such a high performance flue gas
      treatment system which consists of dry type ESP, wet type FGD and
      wet type ESP will be needed to meet the coming severe request for
      environmental condition.

3.    Application for Other Fields

Wet type ESP has the following features.

       -Collecting performance is independent of electric dust
      . The lowest dust concentration can be obtained because of no
         dust - reentrainment.
       - Sub - micron particulates including S03 mist and small
         infinitesimal element can be effectively collected.
Considering the most of the above features, wet type ESP is used for
various process in these days.
We would like to introduce some new applications of wet type ESP.

3.1    Application to Coal- fired BOilers

In recent utility coal- fired power plant in Japan, emission level in the
specified area is required to attain the same level as current oil-
fired power plant in terms of environmental conservation.
Strict requirements are imposed to lower total emission levels for the
territory and the almost invisible smoke.
In order to satisfy these requirement, the high performance flue gas
treatment system has been put to practical use.
This system consists of wet type ESP and non -leakage type gas gas heater
(GGH) along with dry type ESP and wet type FGD, and low emission not
more than lOrng/ rrfN can be realized.
The recent application examples of this system are 700 MW x 3 coal- fired
utility plants, which have continued stable operation with good
performance since the start - up of commercial operation in 1991.

3.2    Application to Fluid Craking Process (FCC) of Oil Refinery

The wet type ESP is applied for FCC process of oil refinery.          Try
type ESP was originally applied for this process, however some did not
keep stable performance because of high dust resistivity.
Recently, FGD is installed in the process to remove SOX and wet type ESP
is applied downstream the system for the purpose of removing particles
and SCh.
Fig - 8 shows an example of system flow and design eondi tion.
In addition, since this process needs a continuous operation for two (2)
years without stoppage. Wet type ESP win also be required to have a
high reliability to realize such continuous operation for a long time.

3.3    Application to Blast Furnace Gas (BFG) - fired Gas Turbine Combined
      Cycle Power Generation

In BFG-fired Gas Turbine Combined Cycle Power Plant, wet type ESP is
applied to prevent abrasion of gas compressor and gas turbine.
Wet type ESP removes the dust in fuel gas (BFG), which contains 5-10 mg/
nfN of dust concentration.
Fig-9 shows the example for the plant process and design condition.
Seven plants (7) are already constructed by us in Japan and now under
construction in Europe either.

4.    Conclusion

The application of the wet type ESP to remove S03 mist have been
described based upon our experience in Japan.
There is the tendency that an interest is mounting on how to reduce the
emission level of fine particulates such as submicron particles and .
harmful infinitesimal materials.
From the point of view, the author considers that wet type ESP will be
more and more applied to the various fields such as boilers using many
kind of fuels, and incinerators, etc.
                        Fig -1        Relationship of Sulfur ·Content in Fuel Oil and S03

                                      in Flue Gas
                                 (Excess Air Rate at AH outlet 1.2)



             20                                                                               '7P'


              Owc::::=..-             ..l..-                                       -L-                   .-..l-                                         ---L         --L.                --'

                                     0.5                                      1.0                      1.5                                              2.0           2.5                3.0
                                                                             Sulfur in fuel Oil (%)

                          Fig -2        Relation of                                          S~   and Sulfuric Acid Dew Point Temperature


                                                                                                                                                                                 V vl/ vV'
                                                                                                                                                                                   V     l/v
                                                                                                                                                                                 V V vV'
      (J     150                                                                                                                                                            /"

                                                                                                                                                                 ~ ~ /Y
                                                                                                                                                v   V
                                                                                                                                                                 V VV
                                                                                                             V t> r) v V V                              I,.-V      V
                                                                                                     V         t- ~v                                    I,..-V   V
                                                                                                                                t> L>v~
                                                                                                                                                        ,,,v -:
      ~                                                                                                      V                        t-
      <:     130                                                                              /       ->          ..........-

                                                                                             ~~      ~
                                                                           ..,..   .......                   V
       ::J                                                         v                                                                            VV
      IU                                                    ,.,/           ..,..   .......                                            V
       ::J                                ...............          /v                                             ..........-
      U)     120
                                 1/            V
                                         V v vV                            v
             110                 t-: ~
                        ~                                          /v

                   0.1                                                                                                                                     10                             100

                                                                                                  so) in flue gas (ppm)
Fig-3                    Basic Flow without/with S03 Removal Technique
                         and S03 Behavior in                                                       Flue Gas

         Fig3-A              Dry type ESP without NH J Injection                      ~   FGD

          I BOller f----:-i             AH       H          Dry ESP             H           FGD         H    Reheating     r-Jl
         Gas Tellp(OC) 400                        170                            170                    50                 100

                                              I SOJ   gas

                  Boi ler/DeNO.         AH               Dry type ESP                     fGD                Rchea II ng
 L-.-                                                                                                                                   ------'

r----TI~l                  Dry type ESP with NIl. Inject ion                i   FGD

         I~                            All
                                              till'.iniect ion
                                                                  Dry ESP
                                                                         .  =r-1                  GGH

        Gas TelllPt"C)      400                  140                                  140                    90                   100

        so.•   cone

                               I SOJ    gas

                 Boiler/DeNO.          AH         NIl.     Dry type ESP                           GGH              FGD
                                                   inject ion

        FiI:3-C             fGD i Wet tYlle ESP (without dry-type ESP)

               Doi lef   J----:-1L-__-JHr--FC-D-'H Wet
                                    AH     -                                           type ESP Hr-R-eh-e-at-i      n-I:---'~
        Gas TempCC)         400                   140                 50                                50                 100
        SO, cone

                 Be i ler fDeNO.       All                  FGD                 Wet trne ESP                 Reheating
        Fig -4    _~~ation                         Example 01 Wet ESP lor _Q!l:J.l!~J~2_~!~~                                                                                                                                                                                                         Fig - 5~p..pJJg!lJion-E...!~m.plu-'-                              We_~_,-q.c.. Oi!.:ll!'.!£~oiler

                                                                                             (A plant)                                                                                                                                                                                                                                                               (B plant)

   Flow                                                                                                                                                                                                                                                                                      Flow

                                                                                                                                                                                                                                                                                     I   Boil"        f---El--G-j                                              FOD    Hl~Wil)K~r'!H On           R,h..,,,   ~
                                                                                                                                                                                                                                                           Stack                                                                                                                                            Stack

   Design Condition and Operation Data

                                                             Design Condition                                                                                                                                             Operation Data                                                     Design Condition

Fu.el                                                                     Heavy oil                                                                                                                                                   ~                                                                           Fuel                                                           Heavy oil

Desulfuri:l:8tion system                               Limestone· Gypsum Process                                                                                                                                                          ~                                                Oesulfurization system                                                       Limestone- Gypsum Process

Gas volume                                                350,000 mlN/h • wet                                                                                                                                         329,000 mlN/h • wet                                                  Gas volume                                                                      505,600 mlN/h • wet

Gas temperature                                                                    60 ·C                                                                                                                                                            60 "C                                  Gas temperature                                                                        53 'C

                                                            200 mg/mlN • dry                                                                                                                                             00 rrg/m'N• dry                                                                                                                                  145 mg/mlN • dry
Inlet dust durden                                                                                                                                                                                                                                                                          Inlet dust durden
                                                   ( Boiler soot:90 mg/mlN )                                                                                                                      ( Boiler                                soot:53mg/m lN                      )                                                                               (Boiler soot:30 mg/mlN. SOl mist:115 mg/m)N)
                                                     SOl mist:110 mg/mlN                                                                                                                            SOl mist:37mg/m lN

                                                                                                                                                                                                                                                                                           Outlet dust durden                                                             10 mg/mlN • dry
Outlet dust durden                                          20 mg/mlN. dry                                                                                                                                                    4 rrg/rrlN • dry
                                                                                                                                                                                                                                                                                         -~-~--_.   ~-'~"-""""-----'~~~"*-"""'~'~--'.--'--- --~---       ~-                                  .~~------
                                      __                        .__ ..__
                                                       __.___.___                          ____                    _
                                                                                                          .__.__ ...       ...                                   ..                                                 ___•.
                                                                                                                                                                                                                  ... _.... ..                     _
                                                                                                                                                                                                                                                  .. _..       "   . "._...

                                                                                                                                                                                                                                                                                           Operation Start                                                                          1997
Operation Start                                                                        -_._-----                                                                                                                                                     198I
                                                                                                                                                                                          I                                                                                                                                                          I
       Fig - 6      Application Example. of Wet ESP for Oil· fired Boiler                       Fig.7 ~Iication Exal!Ple of Wet E~P for-1Jtili!Y~.!:J.Lr!d ~I.!t_
                                            (C plant)

  Flow                                         Non -leakage heat exchange                                                                                                    Non -leakage heal exchange

                                                                                    {J      ~~EJ~[:]~~~~~EJ~~~~ijl

  Design Condilion                                                                                     pesign Condifion

                 Fuel                                   Asphalt                                       Power Generation                                               700MW X 3

 Desulfurization system                        Llmestone- Gypsum Process                              Fuel                                                                  Coal

 Gas volume                                       671.000 mJN/h • wet                                 Gas volume                                                  2.500.001 m'N/h • wet
                               -                                                                    ______.__   ._~c_~_     ~~   _____ . . __..
                                                                                                                                        ~      _ft       TW

 Gas temperalure                                        55 'C                                         Treated gas temperature                                             49 ·C
                                                                          *,   *-                   r~~_~~~       ____   "_.*__"_'__'_·'__  ~~._   ..

                                                 43mg/m 'N • dry
 Inlet dust du rden                                                                                   Inlel dust durden                                             j1   rrglrrlN • dry
                                   (Boiler soot:7 mg/m'N. Ammonium Sulfate:36mg/rrlN)

                                                                                                                                                                      nol more Ihan
 Outlet dust durden                              2mg/m JN. dry                                        Oullet dust burden
                                                                                                                                                                     tl rrglrrlN • dry

                                                                                                                                                                      Un I 1       1991
. Operation Start· up                                     1998                                        Opera lion start                                                Un t 2       1992
                                                                                                                                                                      Un t 3       1993
       Fig -8   !'.-eplic8tion Exam...ele of Wet ESP for Fluid Catalistie              Fig·9                       _~.p'plication Exam~J         Wet ;'pP for Gas Turbine...fp.!!}.bined

                fracking Process of Oil         Refiner~                                                           ~ystem t~!j ..!.!_J!.last Furnace Gas

 Flow                                                                            Flow

No.1               Waste
Regenerator        Boiler
                                                                                                                                             !   Gas Compressor

                                                                                     ~ COG            j.                                          Air Compressor Gas Turbine
                                                                                     ' ..... ~    ....'
No.2                 CO
Regenerator          Boiler                                                      Steam
                                                                                                                                -...     •   W          LO
 .Design Condition
                                                                                                    ;                     G.n"",,       I
                                                                                                                                                         IFu,.... I B.II."
 Desulfurization system                   Magnesium Hydroxide Process

                                                                                     Design Condition
 Gas volume                                   230.000 m'N/h • wet

                                                                                 Fuel                                                                        BFG (LOG. COG)
 Gas temperature                                     65 ·C

                                              235 tTlg/m!N • dry                 Gas volume                                                               310.000 m!N/h • dry
 Inlet dust durdsn
                                    (FCC dust:80 mg/m!N. SO! mist:155 mg/m!N)
                                                                                 Gas temperature                                                                  40 ·C
 Outlet dust durden                           30 mg/m!N • dry

                                                                                 Inlet dust durden                                                        10 mg/mlN • dry
                                                                                _.     •....   _..._--   .......   _...
                                                                                                                      _~~   ..~~----~. ---

                                                                                 Outlet dust durden                                                       t mg/mlN • dry

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