Modern Gas Cleaning TechniquesFor TRS and SO2 Control in thePulp and Paper Industry

The control of SO2 and total reducedsulfur (TRS) emissions in the pulpand paper industry can be effectivelyachieved using the RotaBed™

Fluidized Bed Scrubber*. As the US pulp and paper industrydevelops and implements strategiesto meet the Cluster Rule require-ments, particular attention will bepaid to the control of both sulfurdioxide (SO2) and total reducedsulfur compounds (TRS).

Sources of SO2emissions may be:■ Power Boilers

■ Waste Sludge Boilers (Incinerators)

■ Noncondensible Gas (NCG) Thermal Oxidizers

■ Lime Sludge Kilns burning NCGs

■ Recovery Boilers

Sources requiring TRScontrol may include:■ Low Concentration High Volume

NCG Sources

■ High Concentration Low Volume NCG Sources

■ Dissolving Tank Vents

■ Miscellaneous Mill Odorous Sources

To meet the Cluster Rules’ MACT Iand MACT II guidelines, odorousmercaptan and hydrogen sulfidesources are collected and scrubbedor are thermally oxidized either instand-alone oxidizers or are injectedinto the combustion zone of a lime-kiln or boiler. In some cases, a wetscrubber using a chemical oxidant(such as sodium hypochlorite orchlorine dioxide) is used to oxidizewater-soluble TRS compounds.Emissions from thermal oxidationtype devices are typically controlledusing a wet scrubber with traditionalalkaline chemistry to scrub out theSO2. Many paper mills are alreadytreating their odorous emissionssources using these techniques.

Until recently, packed towers, traytowers, and spray towers werecommonly used to control SO2 and RotaBed ™ and ScrubPac ™ are trademarks of Bionomic Industries, Inc. Patent Pending.

TRS emissions. These designsoperate by increasing the absorptionof contaminant gases by extendingthe scrubbing liquid surface.

Comparison ScrubberTypes for AbsorptionCommon absorbers are usually oneof the following type:

1. Spray Tower

2. Tray Tower

3. Packed Tower

4. Venturi Scrubber

5. Fluidized Bed Scrubber (Swirling or Ebullient)

Table 1 provides a comparison ofthese various scrubber types.

The common purpose of all of these absorbers is to place a largesurface area of scrubbing liquid intodirect contact with the flue gas. Thegas diffuses to the liquid surface,penetrates into and through the liquidfilm, and is absorbed. Once the gasis absorbed into the liquid, thechemicals in the liquid react with theabsorbed gas usually forming a lowvapor pressure salt. These devicesall accomplish the same desiredresult using vastly differenttechniques.

Traditional ScrubberTechnologySpray TowerThe spray tower may be a hydrauli-cally atomized unit or may use steamor air atomized spray. The atomizeddroplets create the liquid surfacearea. This technology requires thatthe scrubbing liquid be elevated in pressure, passedthrough a small orifice spray nozzle, and then flashed back down to atmospheric pressure in thescrubber vessel. The energyimparted into the liquid is primarilyused to create the liquid droplets.The gas pressure drop, in contrast, islow. Spray towers have been used fordecades on utility power boiler fluegas desulfurization (FGD) systemsusually by administering a spray oflime or limestone.

TechnicalBulletinNo. 2

To generate smaller droplets ofgreater surface area per unitvolume to enhance the masstransfer, more energy input isneeded.Some designs introducethis extra energy in the form ofcompressed air or steam.The fan,liquid pump, and air compressor orsteam source contribute the totalenergy input. Each increase inenergy input increases the powerconsumption of the device andincreases the system’s complexityand maintenance challenge. Inaddition to high-energy consumption(usually in the fo rm of pump energy),these designs are also limited byscaling and plugging problems inthe nozzles.

At any given instant, there must beenough scrubbing liquid in contactwith the gas in the spray tower toallow that gas to be absorbed.Thisis particularly important under theCluster Rule where consistently lowoutlet loadings are required eventhough the inlet loading may vary.Therefore, spray towers typicallyallow for retention of the spray inthe vessel or for the use of excessspray in multiple spray zones.Vertical counterflow spray towersoperate typically at 6-8 feet/secondvertical velocity and may have twoor more spray zones. Air atomizedor “fog” type scrubbers may useeven more spray zones in either acounterflow or concurrent gas/liquidpath.The fog, once created, mustbe removed prior to the gasesleaving the tower.This requires highefficiency droplet control since thedrops created are quite small(typically below 50 microns diameter).

Tray TowerThe tray tower creates the liquidsurface area by bubbling the fluegas through small orifices in ahorizontal tray.These holes aretypically less than 3/16” diameterand can be a source of plugging.A froth of liquid and gas is createdabove the tray as the gas passesthrough the orifice.To create moreliquid to gas surface area, additionaltrays are generally used in stages.The energy input is mostly derivedfrom the fan (i.e. fan pressure).These towers typically operate at 8-12 feet/second through the traybut the tray occupies only about 50-70% of the available area of thevessel.The rest is consumed bytray supports, liquid distributionweirs and other internal devices.

Packed TowerIn a packed tower, the surface areais created by extending the area ofthe liquid making the liquid passover some media inside the tower.This media, or packing, may be inthe form of hundreds of individualpieces that are dumped into thetower (“dumped type” packing) ormay be in the form of corrugated,formed sheets (“structural type”packing).The latter is similar tocooling tower fill.Only the liquidsurface away from the packing is available for mass transfer,therefore, packing designers try to maximize the wetted packingsurface in their designs. Packedtowers can unfortunately act asliquid filters and can plug with solids.T h ey are used pri m a rily where thegas and liquid are free of solids.

The energy input to a packed toweris part fan energy (pressure drop)and part pump energy. Since thepacking occupies space in thetower, the resulting packing depthcan be a significant contributor tothe pressure drop.The higher thepacking height, the greater thepumping power required to deliverthe liquid to that elevation.

Packed towers typically operate atgas velocities of 6-9 feet/secondvertical. As gas speed and/or liquidrates increase, the packed sectioncan flood and become turbulentwith a resulting loss of efficiencyand tower damage.Therefore,packed towers are typicallydesigned to operate well below the flooding point.

Venturi ScrubbersThe venturi scrubber is primarilyused for particulate removal but can also absorb gases.It works by atomizing droplets through the use of the difference in speed(shearing action) of the gases andthe injected liquid.The minimumpressure drop for suitableabsorption is typically 8-12” w.c.The liquid breaks up into tinydroplets.The size of these dropletsare related to the total energy inputof the device and the smaller thedroplet the greater the surface areaper unit volume.As codes havetightened, venturi scrubbers aloneare only infrequently used for gasabsorption since their efficiency islimited given the very low contacttime (milliseconds) in the device.The fan energy required in a venturiwhen used as an absorber may be

Table 1

Pressure Gas Liquid Footprint Dia.Drop Velocity Pressure for 60,000 acfm,

Scrubber Type (“ w.c.) (feet/sec.) (psig) (feet-inches)

RotaBed™ Scrubber (Fluidized Bed) 3 to 5 16 to 30 3 to 5 8-6

Spray Tower 2 to 4 6 to 8 20 to 60 12-6 to 14-6

Tray Tower 4 to 6 6 to 8 5 to 10 12-6 to 14-6

Packed Tower 3 to 4 6 to 8 10 to 20 12-6 to 14-6

Venturi with Separator 8 to 12 ~200 at throat 5 to 10 11-6

3-5 times higher than that of otherdevices. Its use is typically avoidedon anything but low volume sources(under 50,000 acfm).

Drawbacks of TraditionalControl DevicesAll of these devices operate atrelatively low gas velocity, typicallyin the order of 6-8 feet per second.The result is a tower that can be ofconsiderable size that is both costlyto build and can consume valuablereal estate (pad or roof space). Inmany cases in older mills where thescrubber must be retrofitted to anexisting source, there is insufficientroom for such a large tower.

Also, scrubber operating history todate proves that the scrubbingliquid can often plug the packed,tray, and spray towers.Packedtowers use media that is in directcontact with adjacent pieces.Openings of less than 1/8” canresult. Solids in the liquid or gasstream can become lodged in thesesmall spaces and plug the packing.The media also presents a largesurface area upon which scale(usually calcium carbonate) canbuild. Since the tower “tricklesdown” it is very hard to acid washsuch designs. Unfortunately, thepacking must be removed, cleaned,and be replaced or discarded.Spray towers can have spray nozzleorifices of less than 1/8” which canalso plug.The spray scrubberperformance is directly related tothe integrity of the spray pattern andif the latter fails, the efficiency fails.The high liquid velocity through thespray nozzles is typically erosiveand spray pattern performance candecrease over time.Tray scrubbersalso have perforations of typicallyless than 3/16” and can similarlyplug.The latter uses internal weirsand downcomers which are all outof sight and require shutdown formaintenance.

The RotaBed™

Fluidized Bed ScrubberA reliable, plugging resistant, yetefficient gas/liquid contact device isa real need that has not been trulysatisfied until the invention of thedeep fluidized bed wet scrubber.

This design produces a random“bubbling” or ebullient fluidized bedthrough which the contaminant gasis forced to pass. A major break-through in these designs is thepatent pending RotaBed scrubberwhich uses a stabilized, swirling,fluidized bed. Figure 1 below showsa typical RotaBed.Operating at upto five (5) times the velocity of apacked tower, the RotaBedscrubber conserves space andreduces the capital cost of installingand operating the emissions controlsystem.The RotaBed scrubberbalances the fan and liquid energyby passing the flue gas through ahighly open grid and uses theCoriolis effect to impart a stabilizing“swirl”to the bed.The grid istypically 60-90+% open.Gasvelocities are 16-30 feet/second.The liquid is usually injected freeflow (below about 3 psig) usingopen horizontal headers.The gridpressure drop is typically 2” per gridwith multiple grids used to createadditional gas/liquid mass transfersurface as required for theapplication.This design can use Eofiltrate, white liquor, weak wash, andcertain alkaline “waste” chemicalstreams that would plug traditionalscrubbers.The net result is a lowerchemical and operating cost.

Range of ScrubberChemistriesA variety of scrubbing chemistriescan be used in the RotaBed scru bb e rgiven its large liquid recycle pathand resistance to plugging.

The most common scrubberchemistry is the use of an alkalinescrubbing solution.These unitstypically scrub the contaminantsfrom the flue gases using a solutionsuch as:

■ Caustic

■ Eo Filtrate/Caustic Extract

■ Soda Ash

■ White Liquor (Kraft mill)

■ Weak wash

■ Lime Slurry

■ Alkaline Supplements

The reaction products are typicallysulfurous compounds as a sulfide,sulfite, bisulfite, and sulfate at abasic pH (usually above pH 8) andare returned to the process or aretreated in the water treatment plant.

The RotaBed scrubber can operateat various liquid to gas ratios (L/G)so that even a waste liquor streamthat may be low in alkalinity can still be used by increasing itsvolume that is retained in thescrubber. A case in point is the use of Eo filtrate to remove SO2.A high volumetric rate of Eo can be passed through the scrubberconverting a waste stream into a supplemental process stream.This conserves valuable alkali.

For high sulfidity mills that do notwant to return the sulfur salts to theprocess, the RotaBed scrubber canbe integrated with a wastewatertreatment system to convert thesulfurous waste to a solid, crystal-lized salt. Specialized chemistriesand processes are also available to convert the liquor to elementalsulfur, or even produce sulfuric acid that can be removed from the process loop. In this case, thereaction byproducts are separatedfrom the system.

No matter what chemical systemis selected, the uniqueness of theRotaBed scrubber lends itself to handling a greater variety of chemistries than any othercontrol device.

Figure 1

Direct TRS Gas ControlThe RotaBed™ scrubber design canbe used as either an SO2 controldevice to scrub thermal oxidizerexhaust gas, or when employing a chemical oxidant, a stand-aloneTRS scrubber as a backup to the oxidizer.

The low solubility of dimethyldisulfide makes it difficult to controlwith a wet scrubber unless the highliquid to gas ratio of the RotaBedscrubber is used.Though thescrubber will have a lower NCGdestruction efficiency than theoxidizer, it can serve as a valuabletemporary backup.The scrubbercan be designed to use caustic fornormal SO2 scrubbing and anoxidant, such as sodiumhypochlorite or hydrogen peroxide,as an emergency mercaptan/sulfidecontrol scrubber.The liquid circuitsare arranged so that the scrubberchemistry may be switched on therun.This requires primary pHcontrol in either mode and ORPcontrol in backup (oxidant) mode.This flexibility and the elimination ofa backup thermal oxidizer can makethis arrangement very attractive.

Since fluidized bed type scrubbersrequire the gas motion to create theextended liquid surface, the designmust provide for a minimum gasmovement through the scrubber atall times.This is often accomplishedusing a simple gas reflux system asshown in Figure 2. Using the NCGsystem draft as the control point, forexample, a modulating damper canregulate the return flow of scrubbedgases back to the scrubber inlet.This also serves to keep the NCGsystem draft constant if the thermaloxidizer goes off line.The NCG draftbecomes a mill set point for controlrather than a variable.This helpsmaintain a constant draft on theNCG ventilation system, a plus forworker safety.

E x t e n s i ve testing shows conclusive l ythat an SO2 removal efficiency of over 99% is obtainable using the RotaBed scrubber as the gas absorber when scrubbing with Eo filtrate. As a backupscrubber, it has proven its efficiencyon soluble organic odors usingoxidants such as sodiumhypochlorite and chlorine dioxide.

On dissolving tank vent applica-tions, the gas contains both TRSp a rticulate and gaseous compounds.

The RotaBed Plus™ using a specialprecleaning section is able toremove TRS compounds withgreater removal efficiency than wet dynamic and venturi scrubbers.

TRS Removal byConversion to SO2in a Kiln or BoilerSome Kraft pulp mills control total reduced sulfur compounds(TRS) by accumulating and burningthese gases in the lime-kiln, orblack liquor recovery boiler.Thecombustion process converts theTRS to SO2, some of which iscollected in the kiln’s gas cleaningsystem.Many mills use electrostaticprecipitators for particulate control.Though excellent particulate controld ev i c e s, they are poor gas absorbers.

The RotaBed scrubber can beapplied for use after a precipitatorfor SO2 control.A fluidized limeslurry up to 25% wt. can be used in the RotaBed scrubber, ofteneliminating the dewatering step that would be required for scrubberslimited to operating at lowsuspended solids such as spray,tray, or packed towers.The fluidizedslurry can be recycled and a portionbe bled back to the process.

Using lime, removal efficiencies inexcess of 98% are attained on SO2.

ConclusionNew regulations such as the paperindustry Cluster Rule are causingplant process and environmentalpersonnel to recognize more thanever that their plant’s productionand environmental processes areintegrated.This is not necessarily

bad news.The portion of the newrules concerning the control of TRSand SO2 can be most economicallyaccomplished using modern gascleaning techniques like the RotaBedscrubber such that the resultingsystem provides high removalefficiencies along with the operatingflexibility and process compatibilitythat mill engineers demand.

The RotaBed scrubber has thefollowing advantages over othertraditional technologies:

■ High gas and liquid throughput

■ Exceptional plugging resistance

■ Stabilized Coriolis inducedfluidized bed

■ Lower installed cost

■ Flexibility in operation

■ Compatible with proven scrubbing chemistry

■ Simplicity of operation

■ Lower chemical/operating costs

Bionomic Industries, Inc.777 Corporate DriveMahwah, NJ 07430

201/529-1094FAX: 201/529-0252

E-mail: bionomic@bellatlantic.netWeb site: www.bionomicind.com

Engineered Air Pollution ControlSystems with Unequaled

Performance

©Copyright, Bionomic Industries, Inc. 2/99 Printed in USA

Figure 2