Absorbtion Flue Gas

7/30/2019 Absorbtion Flue Gas

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ABSORPTION

FLUE GAS DESULFURIZATION
http://en.wikipedia.org/wiki/Image:Air_.pollution_1.jpg


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CREATED BY

ZELHA SALDIR

ITIR SARI

TUBA BEEND MUSTAFA ZGRAY

YAKUP TURGUT


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INTRODUCTION

Absorption, or gas absorption, is a unit operation usedin the chemical industry to separate gases by washing orscrubbing a gas mixture with a suitable liquid .

The fundamental physical principles underlying theprocess of gas absorption are the solubility of theabsorbed gas and the rate of mass transfer. One ormore of the constituents of the gas mixture dissolves oris absorbed in the liquid and can thus be removed fromthe mixture. In some systems, this gaseous constituentforms 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 anyof the following : gas purification (eg , removal of airpollutants 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 verticalcounter current columns as shown in figure 1.The

solvent is fed at the top of the absorber , whereas thegas mixture enters from the bottom .The absorbedsubstence is washed out by the solvent and leaves the

absorber at the bottom as a liquid solution . Thesolvent is often recovered in a subsequent strippingor desorption operation . This second step isessentially the reverse of absorption and involves

counter current contacting of the liquid loaded withsolute 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 withrandomly packed elements or having a regular solidstructure designed to disperse the liquid and bring itDumped-type packing elements come in a great

variety of shapes and construction materials, which

are intended to create a large internal surface but asmall pressure drop. Structured ,or arranged packingsmay be made of corrugated metal or plastic sheetsproviding 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 thesome length as the diameter of the cylinder and

with the walls as thin as the material will permit.

Rashing rings are almost always dumped intothe tower at random and not stacked regularly.They offer the best combination of low weightper 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, aliquid distributor, and a mist eliminator.

Mist eliminators are used to condense any vaporizedscrubbing liquid. Support plates hold the packing in

place.

The advantages of packed columns include simple and,aslong as the tower diameter is not too large,usually relativelycheaper construction. These columns are preferred forcorrosive gases becuase packing, but not plates, can be

made from ceramic or plastic materials. Packed columnsare also used in vacuum applications because the pressuredrop, especialli for regularly structured packings, is usuallyless then through plate columns.


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Usage examplesPacked 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 tallcolumns are required, because tall, random-type packedtowers are subject to channeling and maldistribution ofthe liquid streams. Plate towers can be more easilycleaned. Plates are also preferred in applications having

large heat effects since cooling coils are more easilyinstalled in plate towers and liquid can be withdrawnmore easily from plates than from packings for externalcooling. Tray columns have got some disadvantage.

These are slow reaction rate processes, higher pressuredrops than packed beds and plugging and fouling maybe occur.


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Tray absorbers are used in applications where tall

columns are required,because tall,random-typepacked towers are subject to channeling andmaldistribution of the liquid streams. Platetowers can be more easily cleaned. Plates are alsopereffered in applications having large heateffects since cooling coils are more easilyinstalled in plate towers and liquid can be

withdrawn more easily from plates than frompackings for external cooling.


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Usage Examples

Tray columns are used in a refinery dehexanizer todecrease the benzene content in the naptha feed to theprocess. This results in lower automobile exhaust

emissions.


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STIRRED TANKS

If the absorbtion process includes a slow liquid-phasechemical reaction, or close control of the process isneeded, stirred tanks are used.the gas is introduced

directly into the liquid and mixed by the stirred in astirred tank.


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Usage examples

Stirred tanks can be used inlime slurry carbonation,paperstock chlorication, regular oilhydrogenation,fermentation

broth aeration,penicilinproduction, citric acidproduction,and aeration ofactivated sludge.


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BUBBLE COLUMN

Structured catalytic bubble columns are new, verypromising types of multiphase reactors. Theirconfiguration lies basically between slurry reactors andtrickle bed reactors. The solid phase, consisting ofcatalyst particles, is enclosed in fixed wire gauze wraps,

which are mounted along the height of the column. Thegas phase is dispersed into the liquid phase and it flowsin the empty passages between adjacent envelopes. The

liquid phase may be operated in a batch manner or itmay also circulate in co-current or counter-currentmanner to the gas flow.


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The main advantages of this reactor type with respectwith 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 isdictated by the size of the open channels of the

catalytic structure.


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Usage Examples

Bubble columns can be usedto purify nitroglycerin with

water, in the chemicalindustry for hydrogenation,oxidation, chlorination, andalkylation, and in thebiotechnological field foreffluent treatmet, single-cellprotein productin, animal cell

culture, and antibioticfermentation. Bubblecolumns can be used forradioactive elements because

there are no moving parts.


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Venturi Scrubbers


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Adjustable-throat venturi scrubber with

movable plateVenturi scrubbers can be used for removing gaseouspollutants; however, they are not used when removal of

gaseous pollutants is the only concern.

The high inlet gas velocities in a venturi scrubber resultin a very short contact time between the liquid and gasphases. This short contact time limits gas absorption.

However, because venturis have a relatively open design

compared to other scrubbers, they are very useful forsimultaneous 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 streamThe dust is sticky or has a tendency to plug openingsThe gaseous contaminant is very soluble or chemically

reactive with the liquid

To maximize the absorption of gases, venturis aredesigned to operate at a different set of conditions from

those used to collect particles. The gas velocities arelower and the liquid-to-gas ratios are higher forabsorption.


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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, byreducing pressure drop, the gas velocity is decreasedand the corresponding residence time is increased.

Liquid-to-gas ratios for these gas absorptionapplications are approximately 2.7 to 5.3 l/m3 (20 to40 gal/1000 ft3). The reduction in gas velocity allowsfor a longer contact time between phases and better

absorption.Increasing the liquid-to-gas ratio will increase thepotential solubility of the pollutant in the liquid.


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Flooded elbow


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Venturi scrubbers can have the highest particlecollection efficiencies (especially for very smallparticles) of any wet scrubbing system.

They are the most widely used scrubbers because theiropen construction enables them to remove mostparticles without plugging or scaling. Venturis can alsobe used to absorb pollutant gases; however, they arenot 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 ofventuris to handle large inlet volumes at hightemperatures makes them very attractive tomany industries; consequently, they are used toreduce particulate emissions in a number ofindustrial applications.


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This ability is particularly desirable for cement kiln

emission reduction and for control of emissions frombasic oxygen furnaces in the steel industry, where theinlet gas enters the scrubber at temperatures greater

than 350 C (660 F).

Venturis are also used to control fly ash and sulfurdioxide 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 andabsorption section and Gas-Liquid separator.


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Absorption liquid distributor is for filmforming 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 isto separate tail gas and product.


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Advanges

High efficiency ofabsorption drop

Low outlet temperature No need after-cooling

Low flow resistance

Easy maintenance

Disadvanges

Restricted by pressure

Film breakup

Flooding

Its convincing advantages and

disadvantages is following: -


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SPRAY COLUMN

Spray columns are differetal contactors. Theliquid stream enters the coloumn through oneor more spray nozzles at different heights inthe column. The droplets formed provide a

large surface area for exposure to the gasstream,with smaller droplets resulting in agreater Exchange area. The liquid and gasstreams can flow counter-currently or inparalel. An optimum droplet velocity is

essential because low velocity will lead to lowcontact or turbulence and high velocity maycause flooding.


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A mist eliminator is used toseparate any liquid that is

entrained into the gaseousphase. Spray columns are usedto absorb SO2 from coal-firedboiler exhaust gases.


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WET SCRUBBER

Wetted packed towers are the simplest and mostcommonly used approaches to gas scrubbing.The principle of this type of scrubber is to

remove contaminants from the gas stream bypassing the stream through a packed structurewhich provides a large wetted surface area to

induce intimate contact between the gas and thescrubbing liquor. the contaminant is absorbedinto or reacted with the scrubbing liquor.


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The packing of the tower is normally a proprietaryloose fill random packing designed to encouragedispersion of the liquid flow without tracking, toprovide maximum contact area for the 'mass transfer'interaction and to offer minimal back pressure to thegas flow. The reactivity between the contaminant andthe scrubbing liquor influences the system designer'sdetermination of gas and liquor flow and the height anddiameter of the packed bed.

A demister is fitted at the top of the tower to prevententrainment of droplets of the scrubbing liquor into theextraction system or stack.


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Wetted packed towers can be designed for very highefficiencies with relatively low capital and running costs.

The low pressure drop associated with packed bed

scrubbers permits the use of smaller more economicalfans. Although efficiency may be affected, a packedtower 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 moistureremoval.


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FLUE GAS DESULFURIZATION

SYSTEM

Gas desulfurization can be accomplish by wet, dry, oralkali scrubing.These methods are covered in thissection.

THE WET FLUE-GAS DESULFURZATON

SYSTEM The wet FDG system, also called a wet scrubber, is

cammonly based on low-cost lime-limestone in theform 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) inEngland.In the modern version of the process, the fluegas 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 hydrateform, naturel gypsum, CaSO4 .2H2O).The SO2 in theflue gas reacts with the slurry to form additional sulfiteand sulfate salts, which are recycled with the addition of

fresh lime or limestone. The chemical reaction arentknown 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

O h i l d f h


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One technique employs a spray tower downstream of theparticulate-removalsystem (electrostatic precipitator or fabricfitler). The flue gas is drawn into the spray tower by the mainsteam-generator induced-draft fan where it flows in

countercurrent fashion to the limestone-slurry spray. A misteliminator at the upper exit of the tower removes any spraydroplets entrained by the gas. The gas may have to be slightlyreheated before it enters the stack to inprove atmosphericdispersion.

The sprayed limestone slurry collects in the bottom of the towerand is recirculated back to the spray nozzles by a pump. A systemof feed and bleed charges a fresh slurry, under pH control , anddischarges an equivalent amount from the circulating slurry. Thefresh slurry is prepared by mixing the lime-limestone with waterin a slaker-grinder and stirred in a slurry tank. The bled slurry is

sent to a dewatering system, which is in the form of thickenersand filters or centrifuges, where water is removed from thecalcium-sulfur salts. The reclaimed water is used to help makefresh slurry.


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The wet scrubber has the advantages of high SO2removal efficiencies, good reliability, and low flue-gasenergy requirements.In addition, it is capable of

removing from the flue gases residual particulates thatmight have escaped the particulate-removal system.


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A main disadvantages is the b ild p of scale in the spray tower


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A main disadvantages is the build up of scale in the spray towerand possibilitiy of plugging. The prevention of such scale isessential to the reliable operation of the tower. Scaling occursbecause both calcium sulfite and calcium sulfate have low watersolubility, normally around 30 percent, and can therefore formsupersaturated water solutions. A minimum liquid-to-gas ratiomust therefore be used, its value depending upon the SO2content of the flue gas and the expected extent of sulfite

oxidation. Precipitation occurs at a finite rate, whichnecessitates holding the SO2-absorbing liquar in a delay tankafter each pass. An insufficient delay time increasessupersturation and promates scalling. Another tecnique for

controlling scale is the use of seed crystals. These are calciumsulfite and sulfate precipitate crystals, in a supersaturatedsolution, that are maintained in the SO2-absorbing liquor. Theyprovide sites around which preferential precipitation takes placeand enhance the precipitation rate.


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Other disadvantages of the wet scrubers are thereheating of the flue gas, a larger gas pressure drop

requiring higher fan power requirements than the dryFGD system (below), and typicallyhigher capital andoperating costs.

The waste material from wet scrubbers is a water-

logged sludge that poses difficult and costy disposalproblems.

THE DRY FLUE-GAS


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THE DRY FLUE GASDESULFURZATON SYSTEM

Like the wet scrubber, above, the dry FGD system, also called adry scrubber, utilizes an aqueous slurry of lime, CaO, to captureflue gas SO2 by forming calcium sulfites and sulfates in sprayabsorbers. The slurry in the case, however, is atomized, usuallyby a centrifugal atomizer, into a fine spray that promotes the

chemical absorption of SO2 and, because of the small spraypaticle size, is quickly dried bye the hot flue gases themselves to aparticulate suspension that is carried along with the desulfurizedgas stream. The reaction particulates as well those carried by theflue gases (fly ash) are then removed, mainly by a fabric fitler,before the gas is drawn by the induced-draft fan to the stack.

A j f hi i h l


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A major component of this system is the slurry-generating system. A slaker meters lime and water intoan agitated tank to prepare a slaked lime slurry which, in

turn, is diluted by additional water and processed toremove inert impurities called grits, which are disposedof. The lime slurry is pumped to the spray absorber withthe flow controlled by the amount of SO2 in the fluegas.

Particulates both coming in with the flue gas andgenerated in the FGD are collected from the absorberand fabric-filter hoppers and sent to a recycling silo fordisposal or for recycling of a portion of it with the slurry(depending upon the extent of original utilization of thereactant in the absorber). The recycled slurry is enrichedby an alkaline material, such as CaO, MgO, K2O, orNa2O.


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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 thenthe 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 ofSO2 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 thedevelopmed of the system, however.


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Other disadvantages are the need for careful design

optimization of the spray absorber and the slaker, andthe 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 vaporin the gas in order to avoid condensation (below the

coresponding dew point). This poses problems with

fitler-bag performance.


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SNGLE ALKAL SCRUBBNG

Clear water solutions of either sodium (usually inthe form of sodium hydroxide, NaoH, or sodiumsulfite, Na2So3) or ammonia (NH3) are excellent

absorbers or SO2. The advantages of alkaliscrubbing is tahat it avoids the scaling and pluggingproblems of slurry scrubbing by using alkaline earth.

Ammonia scrubbing has the advantage that the

scrubber product, ammonium sulfate, can be sold asa fertilizer, but the disadvantage that the processproduces troublesome fumes.

A ll d l d di bb i h W l


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A well-developed sodium scrubber is the Welman-Lord SO2 recovery process, which has found use inpowerplants, refineries, sulferic acid plants, and other

industrial installations in the USA and Japan. Theprocess utilizes a water solution of sodium sulfite(Na2SO3) for scrubbing and generates a concentratedSO2 (about 90%), in effect removing the SO2 gas fromother flue gases.

The flue gas from fossil powerplants (or nonferroussmelters) is first pretreated by cooling and removal ofparticulate 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 thepretreated flue gas to produce sodium bisulfiteNaHSO3 according to

SO2 + Na2SO3 + H2O ----------- 2NaHSO3


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The desulfurized gas is reheated before going to thestack in order to improve atmospheric dispersion.

The sodium bisulfite is sent to a forced-circulationevaporator-crystallizer via a surge tank. The evaporator-crystallizer is the herth of the system. The surge tankallows steady flow rates into it despite gas flow and

concentration fluctuations. Through the application oflow-pressure steam, such as from a turbine exhaust, thesulfite is regenerated in the form of a slurry accordingto

2NaHSO3 ----------- NaSO3 + SO2 + H2O


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The product SO2 may be utilized to produceliquid SO2 or sulfuric acid, on

site or in a satellite plant, or to produce

elemental sulfur. A well-known process fordoing this is called the Claus process, which isbased 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 additionof 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 completelyconsumed 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 capitalcost of a powerplant. It was said operating costs wouldincrease by about $60/MBtu.

Most scrubbers in use by 1981 have been of the wet


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Most scrubbers in use by 1981 have been of the wettype. There is not sufficient experience with the dry typeto establish which of the two may be selected by utilities

in the future. Presently all scrubber systems are largeand occupy a sizable area of a powerplant, have capitalcosts 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 alsorequire a lot of maintenance , which results in thedoubling of operation and maintenance personel andcauses, consequently, larger operation and maintenancecosts. In addition, they generate huge amounts of wastethat has to be disposed of.

Th f di l f G


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There are two types of disposal of FGD wastes: wetdisposal, 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 sludgedisposal systems were in operation, and 26000 MW

were under construction or planned, in 1981. The

Electric Power Research Institute (EPRI) haspublished the FGD Sludge Dsposal Manual, whichincorporates the latest waste-disposal technology andregulations and describes how to design an

environmentally acceptable waste-disposal system andthe options available for processing and disposal of thewastes.

Absorbtion Flue Gas

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