ABSORPTION

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ABSORPTION

• FLUE GAS DESULFURIZATION

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CREATED BY

• ZELIHA SALDIR
• ITIR SARI
• TUGBA BEGENDI
• MUSTAFA ÖZGIRAY
• YAKUP TURGUT

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INTRODUCTION

• Absorption, or gas absorption, is a unit
  operation used in the chemical industry to
  separate gases by washing or scrubbing a gas
  mixture with a suitable liquid .
• The fundamental physical principles underlying
  the process of gas absorption are the solubility
  of the absorbed gas and the rate of mass
  transfer. One or more of the constituents of the
  gas mixture dissolves or is absorbed in the
  liquid and can thus be removed from the mixture.
  In some systems, this gaseous constituent forms a
  physical solution with the liquid or the solvent,
  and in other cases , it reacts with the liquid
  chemically.

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The purpose of such scrubbing operations may be
any of the following gas purification (eg ,
removal of air pollutants from exhausts gases or
contaminants from gases that will be further
processed) , product recovery , or production of
solutions of gases for various purposes.
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Gas absorption is usually carried out in vertical
counter current columns as shown in figure 1.The
solvent is fed at the top of the absorber ,
whereas the gas mixture enters from the bottom
.The absorbed substence is washed out by the
solvent and leaves the absorber at the bottom as
a liquid solution . The solvent is often
recovered in a subsequent stripping or desorption
operation . This second step is essentially the
reverse of absorption and involves counter
current contacting of the liquid loaded with
solute using and inert gas or water vapor .
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• The absorber may be a packed column , plate
  column , spray column , venturi scrubbers ,
  bubble column , falling films , wet scrubbers
  ,stirred tanks

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PACKED COLUMN
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The packed column is a shell either filled with
randomly packed elements or having a regular
solid structure designed to disperse the liquid
and bring it Dumped-type packing elements come
in a great variety of shapes and construction
materials, which are intended to create a large
internal surface but a small pressure drop.
Structured ,or arranged packings may be made of
corrugated metal or plastic sheets providing a
large number of regularly arranged channels ,but
a variety of other geometries exists. Packing
materials may be classified as follows,
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• rock
• 3-coke
• 4-stonaware shapes
• 4a-raching rings
• 4b-berl saddle
• 4c-sprial rings 1-wood slats
• 2-broken
• 4d-grid bloks
• 5-miscalloneous material

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Rashing rings are the most widely used form of
tower packing. They are cylindrical rings, of
the some length as the diameter of the cylinder
and with the walls as thin as the material will
permit. Rashing rings are almost always dumped
into the tower at random and not stacked
regularly. They offer the best combination of low
weight per unit volume,free volume,free cross
section and total surface of any type of packing.
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A packed bed column contains a support plate, a
liquid distributor, and a mist eliminator. Mist
eliminators are used to condense any vaporized
scrubbing liquid. Support plates hold the packing
in place.
The advantages of packed columns include simple
and,as long as the tower diameter is not too
large,usually relatively cheaper construction.
These columns are preferred for corrosive gases
becuase packing, but not plates, can be made from
ceramic or plastic materials. Packed columns are
also used in vacuum applications because the
pressure drop, especialli for regularly
structured packings, is usually less then through
plate columns.
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Usage examples Packed columns are used mostly in
air pollution control.. The water soluble
ethylene gas ishydrolyzed to ethylene gylcol.
Packed columns are also used in the chemical
,petrochemical,food, pharmaceutical,paper, and
aerospace industries.
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TRAY COLUMN
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Tray absorbers are used in applications where
tall columns are required, because tall,
random-type packed towers are subject to
channeling and maldistribution of the liquid
streams. Plate towers can be more easily cleaned.
Plates are also preferred in applications having
large heat effects since cooling coils are more
easily installed in plate towers and liquid can
be withdrawn more easily from plates than from
packings for external cooling. Tray columns have
got some disadvantage. These are slow reaction
rate processes, higher pressure drops than packed
beds and plugging and fouling may be occur.
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Tray absorbers are used in applications where
tall columns are required,because
tall,random-type packed towers are subject to
channeling and maldistribution of the liquid
streams. Plate towers can be more easily cleaned.
Plates are also pereffered in applications having
large heat effects since cooling coils are more
easily installed in plate towers and liquid can
be withdrawn more easily from plates than from
packings for external cooling.
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Usage Examples

• Tray columns are used in a refinery dehexanizer
  to decrease the benzene content in the naptha
  feed to the process. This results in lower
  automobile exhaust emissions.

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STIRRED TANKS

• If the absorbtion process includes a slow
  liquid-phase chemical reaction, or close control
  of the process is needed, stirred tanks are
  used.the gas is introduced directly into the
  liquid and mixed by the stirred in a stirred tank.

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Usage examples

• Stirred tanks can be used in lime slurry
  carbonation,paper stock chlorication, regular oil
  hydrogenation,fermentation broth
  aeration,penicilin production, citric acid
  production,and aeration of activated sludge.

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BUBBLE COLUMN

• Structured catalytic bubble columns are new, very
  promising types of multiphase reactors. Their
  configuration lies basically between slurry
  reactors and trickle bed reactors. The solid
  phase, consisting of catalyst particles, is
  enclosed in fixed wire gauze wraps, which are
  mounted along the height of the column. The gas
  phase is dispersed into the liquid phase and it
  flows in the empty passages between adjacent
  envelopes. The liquid phase may be operated in a
  batch manner or it may also circulate in
  co-current or counter-current manner to the gas
  flow.

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The main advantages of this reactor type with
respect with the conventional slurry bubble
column are 1.no problems for separating
catalyst from the liquid 2.improved
conversion and selectivity due to staging of the
liquid phase 3.no scale up problems because
the hydrodynamics is dictated by the size of the
open channels of the catalytic structure.
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Usage Examples

• Bubble columns can be used to purify
  nitroglycerin with water, in the chemical
  industry for hydrogenation, oxidation,
  chlorination, and alkylation, and in the
  biotechnological field for effluent treatmet,
  single-cell protein productin, animal cell
  culture, and antibiotic fermentation. Bubble
  columns can be used for radioactive elements
  because there are no moving parts.

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Venturi Scrubbers
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Adjustable-throat venturi scrubber with movable
plate Venturi scrubbers can be used for removing
gaseous pollutants however, they are not used
when removal of gaseous pollutants is the only
concern. The high inlet gas velocities in a
venturi scrubber result in a very short contact
time between the liquid and gas phases. This
short contact time limits gas absorption.
However, because venturis have a relatively open
design compared to other scrubbers, they are very
useful for simultaneous gaseous and particulate
pollutant removal, especially when
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• Scaling could be a problem
• A high concentration of dust is in the inlet
  stream
• The dust is sticky or has a tendency to plug
  openings
• The gaseous contaminant is very soluble or
  chemically reactive with the liquid

To maximize the absorption of gases, venturis are
designed to operate at a different set of
conditions from those used to collect particles.
The gas velocities are lower and the
liquid-to-gas ratios are higher for absorption.
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For a given venturi design, if the gas velocity
is decreased, then the pressure drop (resistance
to flow) will also decrease and vice versa.
Therefore, by reducing pressure drop, the gas
velocity is decreased and the corresponding
residence time is increased. Liquid-to-gas ratios
for these gas absorption applications are
approximately 2.7 to 5.3 l/m3 (20 to 40 gal/1000
ft3). The reduction in gas velocity allows for a
longer contact time between phases and better
absorption. Increasing the liquid-to-gas ratio
will increase the potential solubility of the
pollutant in the liquid.
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Flooded elbow
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• Venturi scrubbers can have the highest particle
  collection efficiencies (especially for very
  small particles) of any wet scrubbing system.
• They are the most widely used scrubbers because
  their open construction enables them to remove
  most particles without plugging or scaling.
  Venturis can also be used to absorb pollutant
  gases however, they are not as efficient for
  this as are packed or plate towers.

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• Venturi scrubbers have been designed to collect
  particles at very high collection efficiencies,
  sometimes exceeding 99. The ability of venturis
  to handle large inlet volumes at high
  temperatures makes them very attractive to many
  industries consequently, they are used to reduce
  particulate emissions in a number of industrial
  applications.

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• This ability is particularly desirable for cement
  kiln emission reduction and for control of
  emissions from basic oxygen furnaces in the steel
  industry, where the inlet gas enters the scrubber
  at temperatures greater than 350 C (660 F).
• Venturis are also used to control fly ash and
  sulfur dioxide emissions from industrial and
  utility boilers.

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Falling film

• With high efficiency in absorbing HCl
  (hydrochloric) gas, H2S, HF, SO2, NH3 gas,
  graphite falling film absorbers comprise of
  absorption liquid distributor, cooling and
  absorption section and Gas-Liquid separator.

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• Absorption liquid distributor is for film forming
  and flow into absorption tube in cooling and
  absorption section.
• On request, cooling and absorption section has
  two models basis of its heat transfer unit -Shell
  and Tube and Block. Gas-Liquid separator is to
  separate tail gas and product. 

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Its convincing advantages and disadvantages is
following -

• Advanges
• High efficiency of absorption drop
• Low outlet temperature
• No need after-cooling
• Low flow resistance
• Easy maintenance 

• Disadvanges
• Restricted by pressure
• Film breakup
• Flooding

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SPRAY COLUMN
Spray columns are differetal contactors. The
liquid stream enters the coloumn through one or
more spray nozzles at different heights in the
column. The droplets formed provide a large
surface area for exposure to the gas stream,with
smaller droplets resulting in a greater Exchange
area. The liquid and gas streams can flow
counter-currently or in paralel. An optimum
droplet velocity is essential because low
velocity will lead to low contact or turbulence
and high velocity may cause flooding.
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• A mist eliminator is used to separate any liquid
  that is entrained into the gaseous phase. Spray
  columns are used to absorb SO2 from coal-fired
  boiler exhaust gases.

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WET SCRUBBER

• Wetted packed towers are the simplest and most
  commonly used approaches to gas scrubbing. The
  principle of this type of scrubber is to remove
  contaminants from the gas stream by passing the
  stream through a packed structure which provides
  a large wetted surface area to induce intimate
  contact between the gas and the scrubbing liquor.
  the contaminant is absorbed into or reacted with
  the scrubbing liquor.

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• The packing of the tower is normally a
  proprietary loose fill random packing designed to
  encourage dispersion of the liquid flow without
  tracking, to provide maximum contact area for the
  'mass transfer' interaction and to offer minimal
  back pressure to the gas flow. The reactivity
  between the contaminant and the scrubbing liquor
  influences the system designer's determination of
  gas and liquor flow and the height and diameter
  of the packed bed.
• A demister is fitted at the top of the tower to
  prevent entrainment of droplets of the scrubbing
  liquor into the extraction system or stack.

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• Wetted packed towers can be designed for very
  high efficiencies with relatively low capital and
  running costs. The low pressure drop associated
  with packed bed scrubbers permits the use of
  smaller more economical fans. Although efficiency
  may be affected, a packed tower will usually
  function when gas or liquor flows vary from its
  original design parameters.
• Usage examples
• Wet scrubbers are used by the food industry,such
  as in cheese proessing for dust and ambient
  moisture removal.

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FLUE GAS DESULFURIZATIONSYSTEM

• Gas desulfurization can be accomplish by wet,
  dry, or alkali scrubing.These methods are covered
  in this section.
• THE WET FLUE-GAS DESULFURIZATION SYSTEM
• The wet FDG system, also called a wet
  scrubber, is cammonly based on low-cost
  lime-limestone in the form of an aqeous
  slurry.this slurry, brought into intimate contact
  with the flue gas by various technique, absorbs
  the SO2 in it.

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• The wet scrubbing process was orriginaly
  developed in the 1930s by Imperial Chemical
  Industries (ICI) in England.In the modern version
  of the process, the flue gas is scrubbed with a
  slurry that contains lime (CaO) and limestone
  (CaCO3) as well as the salts calcium sulfite
  (CaSO3 .2H2O)and calcium sulfate (in hydrate
  form, naturel gypsum, CaSO4 .2H2O).The SO2 in
  the flue gas reacts with the slurry to form
  additional sulfite and sulfate salts, which are
  recycled with the addition of fresh lime or
  limestone. The chemical reaction arent known
  with certainty but are thougth to be

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CaO H2O -----------? Ca(OH)2 Ca(OH)2
CO2 ---------? CaCO3 H2O CaCO3 CO2
H2O-------? Ca(HCO3)2 Ca(HCO3)2 SO2
H2O --------?CaSO3 .2H2O 2CO2 CaSO3 .
2H2O 1/2O2 ------------? CaSO4 .2H2O
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• One technique employs a spray tower downstream of
  the particulate-removalsystem (electrostatic
  precipitator or fabric fitler). The flue gas is
  drawn into the spray tower by the main
  steam-generator induced-draft fan where it flows
  in countercurrent fashion to the limestone-slurry
  spray. A mist eliminator at the upper exit of the
  tower removes any spray droplets entrained by the
  gas. The gas may have to be slightly reheated
  before it enters the stack to inprove atmospheric
  dispersion.
• The sprayed limestone slurry collects in the
  bottom of the tower and is recirculated back to
  the spray nozzles by a pump. A system of feed
  and bleed charges a fresh slurry, under pH
  control , and discharges an equivalent amount
  from the circulating slurry. The fresh slurry is
  prepared by mixing the lime-limestone with water
  in a slaker-grinder and stirred in a slurry
  tank. The bled slurry is sent to a dewatering
  system, which is in the form of thickeners and
  filters or centrifuges, where water is removed
  from the calcium-sulfur salts. The reclaimed
  water is used to help make fresh slurry.

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• The wet scrubber has the advantages of high SO2
  removal efficiencies, good reliability, and low
  flue-gas energy requirements.In addition, it is
  capable of removing from the flue gases residual
  particulates that might have escaped the
  particulate-removal system.

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A main disadvantages is the build up of scale in
the spray tower and possibilitiy of plugging. The
prevention of such scale is essential to the
reliable operation of the tower. Scaling occurs
because both calcium sulfite and calcium sulfate
have low water solubility, normally around 30
percent, and can therefore form supersaturated
water solutions. A minimum liquid-to-gas ratio
must therefore be used, its value depending upon
the SO2 content of the flue gas and the expected
extent of sulfite oxidation. Precipitation occurs
at a finite rate, which necessitates holding the
SO2-absorbing liquar in a delay tank after each
pass. An insufficient delay time increases
supersturation and promates scalling. Another
tecnique for controlling scale is the use of seed
crystals. These are calcium sulfite and sulfate
precipitate crystals, in a supersaturated
solution, that are maintained in the
SO2-absorbing liquor. They provide sites around
which preferential precipitation takes place and
enhance the precipitation rate.
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• Other disadvantages of the wet scrubers are the
  reheating of the flue gas, a larger gas pressure
  drop requiring higher fan power requirements than
  the dry FGD system (below), and typicallyhigher
  capital and operating costs.
• The waste material from wet scrubbers is a water-
  logged sludge that poses difficult and costy
  disposal problems.

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THE DRY FLUE-GAS DESULFURIZATION SYSTEM

• Like the wet scrubber, above, the dry FGD
  system, also called a dry scrubber, utilizes an
  aqueous sliurry of lime, CaO, to capture flue gas
  SO2 by forming calcium sulfites and sulfates in
  spray absorbers. The slurry in the case, however,
  is atomized, usually by a centrifugal atomizer,
  into a fine spray that promotes the chemical
  absorption of SO2 and, because of the small spray
  paticle size, is quickly dried bye the hot flue
  gases themselves to a particulate suspension that
  is carried along with the desulfurized gas
  stream. The reaction particulates as well those
  carried by the flue gases (fly ash) are then
  removed, mainly by a fabric fitler, before the
  gas is drawn by the induced-draft fan to the
  stack.

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• A major component of this system is the
  slurry-generating system. A slaker meters lime
  and water into an agitated tank to prepare a
  slaked lime slurry which, in turn, is diluted by
  additional water and processed to remove inert
  impurities called grits, which are disposed of.
  The lime slurry is pumped to the spray absorber
  with the flow controlled by the amount of SO2 in
  the flue gas.
• Particulates both coming in with the flue gas and
  generated in the FGD are collected from the
  absorber and fabric-filter hoppers and sent to a
  recycling silo for disposal or for recycling of a
  portion of it with the slurry (depending upon the
  extent of original utilization of the reactant in
  the absorber). The recycled slurry is enriched by
  an alkaline material, such as CaO, MgO, K2O, or
  Na2O.

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The main advantages of the dry system are
he dry, powdery nature of the waste material,
which poses fewer and less costly disposal
problems then the wet waste from the wet FGD
system (thought these problems are still large),
and the mechanical simplicity of the system.
The main disadvantage is that the efficiency of
SO2 removal is lower than that of the wet
scrubber. 1979 NSPS (New Source Performances
Standards) regulations, which specify only 70
percent SO2 removal in new plants, have
encouraged the developmed of the system, however.
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Other disadvantages are the need for careful
design optimization of the spray absorber and the
slaker, and the storng dependence of collection
efficiency on absorber outlet temperature, which
neccessitates opereting as close as is safe to
the saturation temperature that corresponds to
the partial presure of the water vapor in the gas
in order to avoid condensation (below the
coresponding dew point). This poses problems with
fitler-bag performance.
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SINGLE ALKALI SCRUBBING

• Clear water solutions of either sodium (usually
  in the form of sodium hydroxide, NaoH, or sodium
  sulfite, Na2So3) or ammonia (NH3) are excellent
  absorbers or SO2. The advantages of alkali
  scrubbing is tahat it avoids the scaling and
  plugging problems of slurry scrubbing by using
  alkaline earth. Ammonia scrubbing has the
  advantage that the scrubber product, ammonium
  sulfate, can be sold as a fertilizer, but the
  disadvantage that the process produces
  troublesome fumes.

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• A well-developed sodium scrubber is the
  Welman-Lord SO2 recovery process, which has found
  use in powerplants, refineries, sulferic acid
  plants, and other industrial installations in the
  USA and Japan. The process utilizes a water
  solution of sodium sulfite (Na2SO3) for scrubbing
  and generates a concentrated SO2 (about 90), in
  effect removing the SO2 gas from other flue
  gases.
• The flue gas from fossil powerplants (or
  nonferrous smelters) is first pretreated by
  cooling and removal of particulate matter, such
  as by electrostatic precipitators, prior to being
  sent to the absorber. In the absorber the water
  solition of sodium sulfite absorbs the SO2 in the
  pretreated flue gas to produce sodium bisulfite
  NaHSO3 according to
• SO2 Na2SO3 H2O -----------? 2NaHSO3

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• The desulfurized gas is reheated before going to
  the stack in order to improve atmospheric
  dispersion.
• The sodium bisulfite is sent to a
  forced-circulation evaporator-crystallizer via a
  surge tank. The evaporator-crystallizer is the
  herth of the system. The surge tank allows steady
  flow rates into it despite gas flow and
  concentration fluctuations. Through the
  application of low-pressure steam, such as from a
  turbine exhaust, the sulfite is regenerated in
  the form of a slurry according to
• 2NaHSO3 -----------? NaSO3 SO2 H2O

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• The product SO2 may be utilized to produce liquid
  SO2 or sulfuric acid, on
• site or in a satellite plant, or to produce
  elemental sulfur. A well-known process for doing
  this is called the Claus process, which is based
  on the addition of H2S according to
• SO2 2 H2S -----------? 3S 2 H2O

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NO REMOVAL

• A process for the removal of NO, also by the
  addition of H2S, is proposed. It is given by
• NO H2S -----------? S 1/2N2 H2O
• The combined removal of SO2 and NO is under
  study. In both reactions, the H2S must be
  completely consumed as it is a pollutant itself.
• In 1977 the system was estimated to add an
  additional 120/kW, or some 12 to 15 percent to
  the base capital cost of a powerplant. It was
  said operating costs would increase by about
  60/MBtu.

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• Most scrubbers in use by 1981 have been of the
  wet type. There is not sufficient experience with
  the dry type to establish which of the two may be
  selected by utilities in the future. Presently
  all scrubber systems are large and occupy a
  sizable area of a powerplant, have capital costs
  that run in the tens of millions of dollars for
  500-to 1000-MW plants, and consume a sizable
  fraction of the gross electrical output of these
  plants. They also require a lot of maintenance ,
  which results in the doubling of operation and
  maintenance personel and causes, consequently,
  larger operation and maintenance costs. In
  addition, they generate huge amounts of waste
  that has to be disposed of.

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• There are two types of disposal of FGD wastes
  wet disposal, called ponding, and dry disposal in
  landfills, which are getting scarce. In general
  utilities are not always eager to build these
  disposal systems. Nevertheless, some 19000 MW of
  FGD and sludge disposal systems were in
  operation, and 26000 MW were under construction
  or planned, in 1981. The Electric Power Research
  Institute (EPRI) has published the FGD Sludge
  DIsposal Manual, which incorporates the latest
  waste-disposal technology and regulations and
  describes how to design an environmentally
  acceptable waste-disposal system and the options
  available for processing and disposal of the
  wastes.