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Environmental Friendly Technologies for Heat Recoveryfrom Waste Fuels and Process/Industrial Wastes

Jalindar Gaikwad

Heat recovery from low-grade waste fuels and various industrial/process wastes assumes great importancein the present scenario with increase in global prices for oil and coal due to increasing demand and limitedsupplies. Firing of low-grade waste fuels/process wastes gives double benefit, firstly the available energyis recovered efficiently and secondly the waste material is disposed off in environmental friendly manner.In both of these cases the energy which was otherwise going to waste is recovered and put to proper useand conserves the main source of energy which would have been otherwise used.

This paper discusses on technologies, operating experience boilers for firing various waste fuels foreffective and economical generation of power/steam.

INTRODUCTION

Manufacturing and processingindustries generate many types of solid,semisolid and liquid wastes (papersludge, packing waste, palm sheIIlkernel, DOB, spent grain, etc.)Frequently these wastes containconsiderable amount of combustiblemater, which can be burned to recoverthe heat for use. Similarly various typesof fuels are used in industries and partof the fuel is rejected in the process (e.g.washery rejects, char fines, coke fines,run off mines, off gases, refinery wasteoils, etc.), which are not suitable for thatprocess. These fuels can be put to usewith better technologies and the heatcan be recovered for use.

Thermax Limited has wide experiencein firing the various fuels employingdifferent firing technologies. Thermaxis offering unique hopper bottom AFBCboiler technology and internalcirculating CFB boiler technology forheat recovery from these wastes.

Waste charecteristics related to use

Research and DevelopmentThermax Babcock & Wilcox, Division ofThermax Limited01 Block, MIDC, ChinchwadPune- 411 019

as fuel in boiler

Hetrogenous nature of the waste andvariation in the properties is the majorconsideration for using these materialas fuel for boiler. The following are theconcern areas,

Higher moisture,

Low heat content

High inert content

Presence of Potassium and sodiumin fuel

Presenceof corrosive elements likeCI, S etc.

High variation in the fuel properties

Irregular and difficult sizing of thefuel

Materials not easy to convey ffeed

Emission of pollutants after burning

Fouling nature

Many of the wastes and waste fuelscontain Potassium and Sodium. Whenthey are to be used as fuel in the boilersfor steam generation. the maincharacteristic of the ash contents highamount of alkalies mainly potassiumand sodium oxides. These alkaliescauses agglomoration and sintering of

ash and leads to clinker formation whenfired on grates where combustiontemperatues exceeds more than lOOO°C.

In the Fluidised bed combustion, thebed temperaures are maintained in therange of 700°C to 850°C The lowtemperature operation greately helps inminimising the agglomeration process(below ash fusion temperature of ash).

Further with controlled draining of bedmaterial & make up with suitable bedmaterial. ensure the alkali concentrationin the bed at a lower level & thusreduces agglomeration & Foulingcharacteristics of ash.

HOPPER BOTTOM AFBC TECH·NOLOGY

Hopper bottom AFBC is unique designdeveloped by Babcock & Wilcox USAthat can handle difficult. agglomerating.high ash low GCV fuels (high foulingbiomass. paper sludge. lignite. char.washery rejects. etc.). This design hasopen hopper at bottom of the combustorinstead of bedplate, with wide pitchedair nozzles. Open hopper allows uniformdraining of agglomerate f clinkers. Thisdesign has ease on multiple fuel firingthrough over bed and under-bedfeeding.

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There are may units in operation firngvarious fuels as listed below suppliedby babcock & Wilcox USA andThermax in India and south east Asia.

Paper Mill Sludge,

Bark,

Sewage Sludge

Used packaging and waste plastic

Industrial and domestic wastes

Coffee waste

Spent grain

Washery rejects

Char

Agricultural and forest wastes, etc.

Design feature Hopper bottom AFBCTechnology

The schematic of this design is shownin Figure-I, In the fluidised bedcombustor airflows upwards in thesuspended stream of bed material(mixure offuel ash + crushed refractory)at a velocity of approx. 2 meter persecond, ensures emense mixing of fueland air. The uniform temperaturedistribution in the bed and agitatingcharacteristic of fluid bed ensuresoptimum combustion.

Figure I : Schematic view of OpenHopper Bottom Fluid Bed

In the conventional fluid bed boilerwith distributor plate, the air nozzles arefixed on a carbon steel plate parting theplenum chamber and the combustor. Inthe Open Hopper Bottom design,bubble caps are fixed on large diameter

pipes. This arrangement allowsparticles to settle down in the hopperswhich can be removed from the bottomdrain. A table shows the advantagesof Open Hopper Bottom design overconventional Distributor plate Fluid bedfor partucular application of Biomassfiring.

Comparision of Open Hopper Bottomdesign with regular bed plate design

Comparison is given in Table-I and thearrangement of both is as shown inFigure-2

Table -1 : Design Comparison

Operating experience:

There are many installation of hopperbottom design to name few are

JOCL,

SARO Power

Nestle

Bhushan

Figure below shows generalarrangement for biomass waste firedOpen Hopper Bottom BFB Boilers.

Sr. Open Hopper Bottom AFBC Distributor plate AFBC

1. Large particles / stone which comesalong with fuel due to large volumehandling can settle into the hopper

2. Continuous slow draining of bed ashpossible hence alkali concentrationcan be maintained at minimum level.

3, The ash retention time in hopper ishigh resulting into cooling of ash henceno separate ash cooler is required.

4. Gives better advantage of slupmingof bed . Due to continuous slowdraining possible, accumulation of fuelover the slump bed is avoided.

Large particle / stones remains onthe bed plate and tend to defluidise

Draining cycle is intermittent-highquantity bed ash drain is requiredto mainatin low concentration ofalkai

Separate ash cooler due to suddendraining of bed.

Intermittent f1uidisation is requiredduring ash draining.

Open Hopper Bottom Design Bed Plate Design

Inbed coils

Effective bed ash drain system Bed plate

Figure- 2: Schematics of both designs

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Figure- 3: General Arrangement of Open hopper bottom AFBC Boiler

THE INTERNAL CIRCULATINGBOILER DESIGN (IR-CFBC)

This is another technology for efficientburning of low grade fuels for achievinghigh combustion efficiency and lowpollution. It has wider fuel firing range,

Volatile matter - 4 - 40%

Ash 0-60%

> 1500 Kcal/kg(2700 BTUllb)

<55%

Heating value -

Moisture

Two-Stage Particle Separation forSuperior Combustion Efficiency

Babcock & Wilcox's unique IR-CFBboiler design employs a patented two-stage particle separation system toprovide high-solids loading and auniform furnace temperature profile(U.S. Patent Number 5,363,812, issuedNov. 15, 1994). The benefits of thistechnological break-through includesuperior combustion efficiency, lowemissions and improved overall plantperformance.

Our two-stage system includes aprimary U-beam impact separator and asecondary ESP dust collector (Ist field)which work together to providecombined particle collection efficiencyin excess of99.8 percent. The U-beams,a staggered array of stainless-steelchannels at the furnace exit plane, (referto figure-4) capture nearly all of the

solids suspended in the flue gasleaving the furnace and internally re-circulate these solids to the lowerfurnace. ECO/APH hopper and ESP 1stfield captures the finer material thatpasses through the U-beam particleseparator and returns this material tothe lower furnace in a controlled mannerthrough recycle ash hopper. Being ableto regulate the secondary recyclesystem provides the operator withunprecedented control over furnacetemperature, resulting in improvedboiler performance and load response.

Compact, Economical Design

B&W's two-stage particle separationsystem results in a compact, simplified

r: Sidewell Membrane Panel- - - - - - - -" < ~

s•••t·1/- \':1Flue -. --Gas:JI \ --I r.Plus "-Solids ,,--""

Flow '::J,\....=:=I~ .....•.~ -U-Beam

Plan Vltw of tht U-btam Impact S¥JIStor.

Plan view the Li-beam impact separatorFig. 4 : U-beam Particle Separator

boiler arrangement. The entire U-beamparticle separator is tucked into cavityat the furnace exit. Compared to cycloneCFB designs, the IR-CFB requiressignificantly less building volume andby relying on internal re-circulation, theIR-CFB design eliminates J-valves, loopseals and high-pressure blowers whichare required with other CFB designs.With the IR-CFB, compact and simplemeans economical.

lR-CFB technology offers• High combustion efficiency• Compact, economical design• Higher reliability & availability• Lower maintenance costs• Reduced erosion• Fuel flexibility• Low emissions

Figure-5 : IR CFB general arrangement

Higher Availability,Maintenance

For the last 50 years, one goal of boilermanufacturers has been to eliminatethick, uncooJed refractory and hotexpansion joints from their designs, inorder to reduce the expense and losttime associated with refractorymaintenance. B&W's engineers werethe first in the CFB industry to achievethis goal through the development ofthe IR-CFB boiler. The furnace, U-beamseparator and super- heater enclosures

Lower

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Figure 6: KCIL IR-CFB

are constructed entirely of top-support,gas-tight, all-welded membrane tubewalls which do not require hotexpansion joints. The small amount ofrefractory that is used in the IR-CFB isapplied to selected areas of the water-cooled enclosure surface in a thin layerwhich is only 16-25 mm thick in thelower furnace (refer figure-4) and nevermore than 76 mm thick elsewhere. As aresult, B&W's IR-CFB requires lessthan one-fourth of the total refractoryfound in a hot cyclone CFB design andless than one-half of the refractory usedin a water-cooled or steam- cooledcyclone CFB unit. This constructionhas significantly reduced the need forrefractory maintenance in B&W'soperating CFB units

CFBC boiler experience

There are many units in operationsupplied by B&W and Thermax. TBWcommissioned first IR-CFB in 1997 atKanoria chemicals Ltd. (Renukoot, UP,INDIA). This boiler is designed to fire

high ash low sulfur coal to generate 105TPH steam at 66Kg/cm2 and, 485deg.cel.

Secondary ash is recycled from APHIESP hoppers to furnace. It has two fuelfeed points on front wall and foursecondary solids re-injection points onrear wall in the primary zone. Boiler hastwo furnace bed drains and two fluidbed bottom ash coolers. Sand feedsystem is provided to make-up (foremergency use only) and start-upinventory in the furnace. Boiler has two15.12 MkcaIlhr over-bed burners (oil-fired) located at the boiler rear wall, forstart-up on coal.

The bubble caps for primary airdistribution are installed between theboiler tubes that form the bottom of thefurnace. The balance of combustion airis admitted as over fire air throughnozzles at two levels in the front andrear walls of the furnace for stagedcombustion. The primary andsecondary air is supplied by separate

fans. KCIL boiler is balance draft withan ID fan installed.

Boiler has vertical pendant typesuperheater banks located within thewater- cooled, gas-tight membraneenclosure. A horizontal economizer islocated downstream of the superheater.Underneath the economizer, is a three-pass tubular air heater with flue gasoutside the tubes. An electrostaticprecipitator for is installed for finalparticulate control. Boiler is equippedwith a DCS system to monitor andoperate the unit.

CONCLUSION

Open hopper bottom AFBC and theinternal circulating CFB are well-demonstrated technologies and areimplemented for firing various low gratefuels, wastes etc. in economical manner.These technologies can burn the wasteto generate steam or power, saving theconsumption of captive power fromgrid.

REFERENCES :

1. SF Belin, M .. Maryamchik, D.J.Walker, D,J. Wietzke, B&W USA. "Babcock & Wilcox. CFB boilers-Design and Experience II" 16 thInternational Conference on FBC,Reno, Nevada USA.

2. S. Kavidass.M. Maryamchik,Babcock & Wilcox Barberton, Ohio,U.S.A.,A.K. MandaI KanoriaChemicals & Industries Ltd.Renukoot, U.P.lNDIA ,"B&W's IR-CFB Coal-Fired Boiler OperatingExperiences" Pittsburgh CoalConference, September 14-18, 1998Pittsburgh, Pennsylvania, U.S.A

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