Absorb Ti On Flue Gas

8/3/2019 Absorb Ti On Flue Gas

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ABSORPTIONABSORPTION

FLUE GAS DESULFURIZATIONFLUE GAS DESULFURIZATION


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CREATED BYCREATED BY

ZELHA SALDIRZELHA SALDIR

ITIR SARIITIR SARI

TUBA BEENDTUBA BEEND MUSTAFA ZGRAYMUSTAFA ZGRAY

YAKUP TURGUTYAKUP TURGUT


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INTRODUCTIONINTRODUCTION

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

The fundamental physical principles underlying theThe fundamental physical principles underlying theprocess of gas absorption are the solubility of theprocess of gas absorption are the solubility of theabsorbed gas and the rate of mass transfer. One orabsorbed gas and the rate of mass transfer. One ormore of the constituents of the gas mixture dissolves ormore of the constituents of the gas mixture dissolves or

is absorbed in the liquid and can thus be removed fromis absorbed in the liquid and can thus be removed fromthe mixture. In some systems, this gaseous constituentthe mixture. In some systems, this gaseous constituentforms a physical solution with the liquid or the solvent,forms a physical solution with the liquid or the solvent,and in other cases , it reacts with the liquid chemically.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 vertical

counter current columns as shown in figure 1.Thesolvent 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 COLUMNPACKED COLUMN


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The packed column is a shell either filled with

randomly 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 oftower 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 aerospaceindustries.


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TRAY COLUMNTRAY COLUMN


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Tray absorbers are used in applications where tall

columns 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 tallcolumns are required,because tall,random-typepacked towers are subject to channeling andmaldistribution of the liquid streams. Plate

towers 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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STIRRED TANKSSTIRRED TANKS

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

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


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Usage examplesUsage examples

Stirred tanks can be used inStirred tanks can be used inlime slurry carbonation,paperlime slurry carbonation,paperstock chlorication, regular oilstock chlorication, regular oilhydrogenation,fermentationhydrogenation,fermentationbroth aeration,penicilinbroth aeration,penicilinproduction, citric acidproduction, citric acidproduction,and aeration ofproduction,and aeration ofactivated sludge.activated sludge.


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BUBBLE COLUMNBUBBLE COLUMN

Structured catalytic bubble columns are new, veryStructured catalytic bubble columns are new, verypromising types of multiphase reactors. Theirpromising types of multiphase reactors. Theirconfiguration lies basically between slurry reactors andconfiguration lies basically between slurry reactors and

trickle bed reactors. The solid phase, consisting oftrickle bed reactors. The solid phase, consisting ofcatalyst particles, is enclosed in fixed wire gauze wraps,catalyst particles, is enclosed in fixed wire gauze wraps,which are mounted along the height of the column. Thewhich are mounted along the height of the column. Thegas phase is dispersed into the liquid phase and it flowsgas phase is dispersed into the liquid phase and it flowsin the empty passages between adjacent envelopes. Thein the empty passages between adjacent envelopes. Theliquid phase may be operated in a batch manner or itliquid phase may be operated in a batch manner or itmay also circulate in comay also circulate in co--current or countercurrent or counter--currentcurrentmanner to the gas flow.manner 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 is

dictated by the size of the open channels of thecatalytic structure.


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Usage ExamplesUsage Examples Bubble columns can be usedBubble columns can be used

to purify nitroglycerin withto purify nitroglycerin withwater, in the chemicalwater, in the chemicalindustry for hydrogenation,industry for hydrogenation,

oxidation, chlorination, andoxidation, chlorination, andalkylation, and in thealkylation, and in thebiotechnological field forbiotechnological field foreffluent treatmet, singleeffluent treatmet, single--cellcellprotein productin, animal cellprotein productin, animal cell

culture, and antibioticculture, and antibioticfermentation. Bubblefermentation. Bubblecolumns can be used forcolumns can be used forradioactive elements becauseradioactive elements because

there are no moving parts.there are no moving parts.


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Venturi ScrubbersVenturi Scrubbers


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Adjustable-throat venturi scrubber with

movable plateVenturi scrubbers can be used for removing gaseous

pollutants; however, they are not used when removal ofgaseous 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 problemA high concentration of dust is in the inlet stream

The 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 elbowFlooded elbow


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

They are the most widely used scrubbers because theirThey are the most widely used scrubbers because theiropen construction enables them to remove mostopen construction enables them to remove mostparticles without plugging or scaling. Venturis can alsoparticles without plugging or scaling. Venturis can alsobe used to absorb pollutant gases; however, they arebe used to absorb pollutant gases; however, they arenot as efficient for this as are packed or plate towers.not as efficient for this as are packed or plate towers.


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Venturi scrubbers have been designed to collectVenturi scrubbers have been designed to collect

particles at very high collection efficiencies,particles at very high collection efficiencies,sometimes exceeding 99%. The ability ofsometimes exceeding 99%. The ability of

venturis to handle large inlet volumes at highventuris to handle large inlet volumes at high

temperatures makes them very attractive totemperatures makes them very attractive tomany industries; consequently, they are used tomany industries; consequently, they are used toreduce particulate emissions in a number ofreduce particulate emissions in a number ofindustrial applications.industrial applications.


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This ability is particularly desirable for cement kilnThis ability is particularly desirable for cement kiln

emission reduction and for control of emissions fromemission reduction and for control of emissions frombasic oxygen furnaces in the steel industry, where thebasic oxygen furnaces in the steel industry, where theinlet gas enters the scrubber at temperatures greaterinlet gas enters the scrubber at temperatures greaterthan 350than 350 C (660C (660 F).F).

Venturis are also used to control fly ash and sulfurVenturis are also used to control fly ash and sulfurdioxide emissions from industrial and utility boilers.dioxide emissions from industrial and utility boilers.


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Falling filmFalling film

With high efficiency in absorbing HClWith high efficiency in absorbing HCl(hydrochloric) gas, H2S, HF, SO2, NH3 gas,(hydrochloric) gas, H2S, HF, SO2, NH3 gas,graphite falling film absorbers comprise ofgraphite falling film absorbers comprise ofabsorption liquid distributor, cooling andabsorption liquid distributor, cooling andabsorption section and Gasabsorption section and Gas--Liquid separator.Liquid separator.


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Absorption liquid distributor is for filmAbsorption liquid distributor is for filmforming and flow into absorption tube informing and flow into absorption tube incooling and absorption section.cooling and absorption section.

On request, cooling and absorption section hasOn request, cooling and absorption section hastwo models basis of its heat transfer unittwo models basis of its heat transfer unit --ShellShell

and Tube and Block. Gasand Tube and Block. Gas--Liquid separator isLiquid separator isto separate tail gas and product.to separate tail gas and product.


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AAdvangesdvanges

High efficiency ofHigh efficiency ofabsorptionabsorption dropdrop

Low outlet temperatureLow outlet temperature No need afterNo need after--coolingcooling

Low flow resistanceLow flow resistance

Easy maintenanceEasy maintenance

DisadvangesDisadvanges

Restricted by pressureRestricted by pressure

Film breakupFilm breakup

FloodingFlooding

Its convincing advantagesIts convincing advantages andanddisadvantagesdisadvantages is following:is following: --


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SPRAY COLUMNSPRAY COLUMNSpray 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 toA mist eliminator is used toseparate any liquid that isseparate any liquid that is

entrained into the gaseousentrained into the gaseousphase. Spray columns are usedphase. Spray columns are usedto absorb SO2 from coalto absorb SO2 from coal--firedfiredboiler exhaust gases.boiler exhaust gases.


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WET SCRUBBERWET SCRUBBER

Wetted packed towers are the simplest and mostWetted packed towers are the simplest and mostcommonly used approaches to gas scrubbing.commonly used approaches to gas scrubbing.

The principle of this type of scrubber is toThe principle of this type of scrubber is toremove contaminants from the gas stream byremove contaminants from the gas stream bypassing the stream through a packed structurepassing the stream through a packed structure

which provides a large wetted surface area towhich provides a large wetted surface area to

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


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The packing of the tower is normally a proprietaryThe packing of the tower is normally a proprietaryloose fill random packing designed to encourageloose fill random packing designed to encourage

dispersion of the liquid flow without tracking, todispersion of the liquid flow without tracking, toprovide maximum contact area for the 'mass transfer'provide maximum contact area for the 'mass transfer'interaction and to offer minimal back pressure to theinteraction and to offer minimal back pressure to thegas flow. The reactivity between the contaminant andgas flow. The reactivity between the contaminant and

the scrubbing liquor influences the system designer'sthe scrubbing liquor influences the system designer'sdetermination of gas and liquor flow and the height anddetermination of gas and liquor flow and the height anddiameter of the packed bed.diameter of the packed bed.

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


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

The low pressure drop associated with packed bedThe low pressure drop associated with packed bedscrubbers permits the use of smaller more economicalscrubbers permits the use of smaller more economicalfans. Although efficiency may be affected, a packedfans. Although efficiency may be affected, a packedtower will usually function when gas or liquor flowstower will usually function when gas or liquor flows

vary from its original design parameters.vary from its original design parameters. Usage examplesUsage examples Wet scrubbers are used by the food industry,such as inWet scrubbers are used by the food industry,such as in

cheese proessing for dust and ambient moisturecheese proessing for dust and ambient moistureremoval.removal.


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FLUE GAS DESULFURIZATIONFLUE GAS DESULFURIZATIONSYSTEMSYSTEM

Gas desulfurization can be accomplish by wet, dry, orGas desulfurization can be accomplish by wet, dry, oralkali scrubing.These methods are covered in thisalkali scrubing.These methods are covered in thissection.section.THE WET FLUETHE WET FLUE--GAS DESULFURZATONGAS DESULFURZATONSYSTEMSYSTEM

The wet FDG system, also called a wet scrubber, isThe wet FDG system, also called a wet scrubber, iscammonly based on lowcammonly based on low--cost limecost lime--limestone in thelimestone in theform of an aqeous slurry.this slurry, brought intoform of an aqeous slurry.this slurry, brought into

intimate contact with the flue gas by various technique,intimate contact with the flue gas by various technique,absorbs the SO2 in it.absorbs the SO2 in it.


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The wet scrubbing process was orriginaly developed inThe wet scrubbing process was orriginaly developed inthe 1930s by Imperial Chemical Industries (ICI) inthe 1930s by Imperial Chemical Industries (ICI) inEngland.In the modern version of the process, the flueEngland.In the modern version of the process, the fluegas is scrubbed with a slurry that contains lime (CaO)gas is scrubbed with a slurry that contains lime (CaO)and limestone (CaCO3) as well as the salts calciumand limestone (CaCO3) as well as the salts calcium

sulfite (CaSO3 .2H2O)and calcium sulfate (in hydratesulfite (CaSO3 .2H2O)and calcium sulfate (in hydrateform, naturel gypsum, CaSO4 .2H2O).The SO2 in theform, naturel gypsum, CaSO4 .2H2O).The SO2 in theflue gas reacts with the slurry to form additional sulfiteflue gas reacts with the slurry to form additional sulfiteand sulfate salts, which are recycled with the addition ofand sulfate salts, which are recycled with the addition of

fresh lime or limestone. The chemical reaction arentfresh lime or limestone. The chemical reaction arentknown with certainty but are thougth to be;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 theOne technique employs a spray tower downstream of theparticulateparticulate--removalsystem (electrostatic precipitator or fabricremovalsystem (electrostatic precipitator or fabricfitler). The flue gas is drawn into the spray tower by the mainfitler). The flue gas is drawn into the spray tower by the mainsteamsteam--generator inducedgenerator induced--draft fan where it flows indraft fan where it flows incountercurrent fashion to the limestonecountercurrent fashion to the limestone--slurry spray. A mistslurry spray. A misteliminator at the upper exit of the tower removes any sprayeliminator at the upper exit of the tower removes any spraydroplets entrained by the gas. The gas may have to be slightlydroplets entrained by the gas. The gas may have to be slightlyreheated before it enters the stack to inprove atmosphericreheated before it enters the stack to inprove atmosphericdispersion.dispersion.

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

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


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The wet scrubber has the advantages of high SO2The wet scrubber has the advantages of high SO2removal efficiencies, good reliability, and low flueremoval efficiencies, good reliability, and low flue--gasgasenergy requirements.In addition, it is capable ofenergy requirements.In addition, it is capable of

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


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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 occurs

because 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 sulfiteoxidation. 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 theOther disadvantages of the wet scrubers are thereheating of the flue gas, a larger gas pressure dropreheating of the flue gas, a larger gas pressure drop

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

The waste material from wet scrubbers is a waterThe waste material from wet scrubbers is a water--

logged sludge that poses difficult and costy disposallogged sludge that poses difficult and costy disposalproblems.problems.

THE DRY FLUETHE DRY FLUE GASGAS


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THE DRY FLUETHE DRY FLUE--GASGASDESULFURZATON SYSTEMDESULFURZATON SYSTEM

Like the wet scrubber, above, the dry FGD system, also called aLike the wet scrubber, above, the dry FGD system, also called adry scrubber, utilizes an aqueous slurry of lime, CaO, to capturedry scrubber, utilizes an aqueous slurry of lime, CaO, to captureflue gas SO2 by forming calcium sulfites and sulfates in sprayflue gas SO2 by forming calcium sulfites and sulfates in sprayabsorbers. The slurry in the case, however, is atomized, usuallyabsorbers. The slurry in the case, however, is atomized, usuallyby a centrifugal atomizer, into a fine spray that promotes theby a centrifugal atomizer, into a fine spray that promotes thechemical absorption of SO2 and, because of the small spraychemical absorption of SO2 and, because of the small spraypaticle size, is quickly dried bye the hot flue gases themselves to apaticle size, is quickly dried bye the hot flue gases themselves to aparticulate suspension that is carried along with the desulfurizedparticulate suspension that is carried along with the desulfurizedgas stream. The reaction particulates as well those carried by thegas stream. The reaction particulates as well those carried by theflue gases (fly ash) are then removed, mainly by a fabric fitler,flue gases (fly ash) are then removed, mainly by a fabric fitler,before the gas is drawn by the inducedbefore the gas is drawn by the induced--draft fan to the stack.draft fan to the stack.


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

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

Particulates both coming in with the flue gas andParticulates both coming in with the flue gas andgenerated in the FGD are collected from the absorbergenerated in the FGD are collected from the absorberand fabricand fabric--filter hoppers and sent to a recycling silo forfilter hoppers and sent to a recycling silo fordisposal or for recycling of a portion of it with the slurrydisposal or for recycling of a portion of it with the slurry

(depending upon the extent of original utilization of the(depending upon the extent of original utilization of thereactant in the absorber). The recycled slurry is enrichedreactant in the absorber). The recycled slurry is enrichedby an alkaline material, such as CaO, MgO, K2O, orby an alkaline material, such as CaO, MgO, K2O, orNa2O.Na2O.


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The main advantages of the dry system arehe

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 arestill large), and the mechanical

simplicity of thesystem.

The main disadvantage is that theefficiency 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 thespray absorber and theslaker, and

thestorng dependence of collection efficiency on

absorber outlet temperature, which neccessitates

opereting as close as issafe to thesaturation 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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SNGLE ALKAL SCRUBBNGSNGLE ALKAL SCRUBBNG

Clear water solutions of either sodium (usually inClear water solutions of either sodium (usually inthe form of sodium hydroxide, NaoH, or sodiumthe form of sodium hydroxide, NaoH, or sodiumsulfite, Na2So3) or ammonia (NH3) are excellentsulfite, Na2So3) or ammonia (NH3) are excellentabsorbers or SO2. The advantages of alkaliabsorbers or SO2. The advantages of alkaliscrubbing is tahat it avoids the scaling and pluggingscrubbing is tahat it avoids the scaling and pluggingproblems of slurry scrubbing by using alkaline earth.problems of slurry scrubbing by using alkaline earth.

Ammonia scrubbing has the advantage that theAmmonia scrubbing has the advantage that the

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


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

industrial installations in the USA and Japan. Theindustrial installations in the USA and Japan. Theprocess utilizes a water solution of sodium sulfiteprocess utilizes a water solution of sodium sulfite(Na2SO3) for scrubbing and generates a concentrated(Na2SO3) for scrubbing and generates a concentratedSO2 (about 90%), in effect removing the SO2 gas fromSO2 (about 90%), in effect removing the SO2 gas fromother flue gases.other flue gases.

The flue gas from fossil powerplants (or nonferrousThe flue gas from fossil powerplants (or nonferroussmelters) is first pretreated by cooling and removal ofsmelters) is first pretreated by cooling and removal ofparticulate matter, such as by electrostatic precipitators,particulate matter, such as by electrostatic precipitators,prior to being sent to the absorber. In the absorber theprior to being sent to the absorber. In the absorber the

water solition of sodium sulfite absorbs the SO2 in thewater solition of sodium sulfite absorbs the SO2 in thepretreated flue gas to produce sodium bisulfitepretreated flue gas to produce sodium bisulfiteNaHSO3 according toNaHSO3 according to

SO2 + Na2SO3 + H2OSO2 + Na2SO3 + H2O ---------------------- 2NaHSO32NaHSO3


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

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

concentration fluctuations. Through the application ofconcentration fluctuations. Through the application oflowlow--pressure steam, such as from a turbine exhaust, thepressure steam, such as from a turbine exhaust, thesulfite is regenerated in the form of a slurry accordingsulfite is regenerated in the form of a slurry accordingtoto

2NaHSO32NaHSO3 ---------------------- NaSO3 + SO2 + H2ONaSO3 + SO2 + H2O


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

site or in a satellite plant, or to producesite or in a satellite plant, or to produceelemental sulfur. A wellelemental sulfur. A well--known process forknown process fordoing this is called the Claus process, which isdoing this is called the Claus process, which isbased on the addition of H2S according tobased on the addition of H2S according to

SO2 + 2 H2SSO2 + 2 H2S ---------------------- 3S + 2 H2O3S + 2 H2O


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NO REMOVALNO REMOVAL

A process for the removal of NO, also by the additionA process for the removal of NO, also by the additionof H2S, is proposed. It is given byof H2S, is proposed. It is given by

NO + H2SNO + H2S ---------------------- S +1/2N2 + H2OS +1/2N2 + H2O

The combined removal of SO2 and NO is under study.The combined removal of SO2 and NO is under study.In both reactions, the H2S must be completelyIn both reactions, the H2S must be completelyconsumed as it is a pollutant itself.consumed as it is a pollutant itself.

In 1977 the system was estimated to add an additionalIn 1977 the system was estimated to add an additional

$120/kW

, or some 12 to 15 percent to the base capital$120/kW

, or some 12 to 15 percent to the base capitalcost of a powerplant. It was said operating costs wouldcost of a powerplant. It was said operating costs wouldincrease by about $60/MBtu.increase by about $60/MBtu.


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Most scrubbers in use by 1981 have been of the wetMost scrubbers in use by 1981 have been of the wettype. There is not sufficient experience with the dry typetype. There is not sufficient experience with the dry typeto establish which of the two may be selected by utilitiesto establish which of the two may be selected by utilitiesin the future. Presently all scrubber systems are largein the future. Presently all scrubber systems are largeand occupy a sizable area of a powerplant, have capitaland occupy a sizable area of a powerplant, have capitalcosts that run in the tens of millions of dollars for 500costs that run in the tens of millions of dollars for 500--

to 1000to 1000--MW

plants, and consume a sizable fraction ofMW

plants, and consume a sizable fraction ofthe gross electrical output of these plants. They alsothe gross electrical output of these plants. They alsorequire a lot of maintenance , which results in therequire a lot of maintenance , which results in thedoubling of operation and maintenance personel anddoubling of operation and maintenance personel and

causes, consequently, larger operation and maintenancecauses, consequently, larger operation and maintenancecosts. In addition, they generate huge amounts of wastecosts. In addition, they generate huge amounts of wastethat has to be disposed of.that has to be disposed of.


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There are two types of disposal of FGD wastes: wetThere are two types of disposal of FGD wastes: wetdisposal, called ponding, and dry disposal in landfills,disposal, called ponding, and dry disposal in landfills,

which are getting scarce. In general utilities are notwhich are getting scarce. In general utilities are notalways eager to build these disposal systems.always eager to build these disposal systems.Nevertheless, some 19000 MWof FGD and sludgeNevertheless, some 19000 MWof FGD and sludgedisposal systems were in operation, and 26000 MWdisposal systems were in operation, and 26000 MW

were under construction or planned, in 1981. Thewere under construction or planned, in 1981. TheElectric Power Research Institute (EPRI) hasElectric Power Research Institute (EPRI) haspublished the FGD Sludge Dsposal Manual, whichpublished the FGD Sludge Dsposal Manual, whichincorporates the latest wasteincorporates the latest waste--disposal technology anddisposal technology andregulations and describes how to design anregulations and describes how to design an

environmentally acceptable wasteenvironmentally acceptable waste--disposal system anddisposal system andthe options available for processing and disposal of thethe options available for processing and disposal of thewastes.wastes.

Absorb Ti On Flue Gas

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