ByS D Prasad, Sr ChemistSandip Mitra, ChemistKajal Acharyya, Chemist&Hiranmoy Misra, Chemist

A Study of Mill Reject Utilization at NTPC-Kahalgaon Using Circulating Fluidized Bed Technology

INDIAN ENERGY SCENARIOCOAL IS THE BACKBONE OF INDIAN ENERGY SECTOR IT WOULD BE HIGHLY UNREALISTIC TO ENVISION A FUTURE WITHOUT INDIA BURNING COAL

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Hydro17.8

Nuclear2.5

oil & Gas20.2

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Need of the hour is any upcoming, state of the art, advanced and environment friendly technology aiming at fuller resource utilization. FUTURE CONCERNSThough India is the 3rd largest coal producer in the world, both quality and availability of coal is a matter of concern.

WASTES GENERATED IN APOWER PLANTMost Ignored

Bowl Mill throws out particle Having density >1.8 g/cc as reject.

Very low grade coal may be thrown out from the system.

Mill reject amounts 1-2% of total coal feed.

Reject amount may vary with unit age and coal quality.

Instant and irregular change in input increases mill rejects both in quantity & quality

Mill Reject Characteristics

Mill Reject CharacteristicsAt KahalgaonAt Kahalgaon Daily Mill Reject Generation ~ 600 MT

PROBLEM WITH THIS REJECT

Do not find any use except as filling material

Problem of disposal.

Pile up in plant premises taking up expensive real estate.

High ash & sulphur leaching damages the vegetation & agro field.

Results fugitive dust emission.

IT IS NOT REJECT BUT BYPRODUCT OF THE SYSTEM

LEAVES NO OPTION EXCEPT TO UTILISE IT

IF UTILISED A potential source of power. Sulphur can be extracted.

Waste to Wealth

throughCirculating Fluidized Bed Technology A Clean Coal Technology Initiative

Circulating Fluidized Bed TechnologyCFB is a fairly known technology today.

Winkler is credited with describing the first fluidized bed in 1921.

Flourished mostly in the decade of 90s.

What is a fluidized bed?

Packed BedU < U mfAFluid Bed , uniform expansionU = U mfBubbling Fluid BedU > U mfBC

Fluidized Bed BehaviourABU< U mfNo flow U > UmfFluid flow

Circulating Fluidized Bed : Combination Of Fluidized Bed and Gas/Solid Separation and Recycle SystemFuelFluidizing GasCyclone separatorFlue exhaustFluidized bed combustor

CHEMICAL ASPECTS OF CFB TECHNOLOGY

EFFICIENT SULPHUR REMOVALLimestone(CaCO3) and Dolomite(CaCO3,MgCO3) used for absorption of SO2 in CFB combustors.

Calcination: CaCO3 CaO + CO2

Sulphation: CaO + SO2 +1/2 O2 = CaSO4

Industrial CFB boilers show 90% S-capture.H

LOW NOx EMISSIONReducing zone contains only 50-60% of the stoichiometric air requirementIn reducing zone fuel N2 does not find O2 to produce NOx.Limited opportunity for the formation of NOx in oxidizing zone at 800-900oCFluidizing GasOxidizing zoneReducing zoneSec.airFuel

Emission Performance of Commercial CFB Units* mg/Nm3 @ 6% O2 dry volume basis

Unit Capacity (MW)SOxNOxCODustGuaranteedMeasuredGuaranteedMeasuredGuaranteedMeasuredGuaranteedMeasured1007751502005920040503030027074150452004020204602004520030200-30

Advantages Of CFB BoilersFuel FlexibilitySimpler Fuel Handling And Feeding SystemHigh Combustion EfficiencyGood Load Following CapabilityHigh Availability

Available Technologies Fuel Sensitivity

PC

VolatilesCFB

O%40%PCAsh Softening TempCFB

11500C13700CPC

Heating ValueCFB

500 Kcal/Kg 5000 KCal/KgPC

HGICFB

40110

SIMPLER FUEL HANDLING AND FEED SYSTEMSingle feeding point & No need of pulverizers.

CFB boilers receive fairly coarse (0-6mm) solid fuels.

Fluidized nature and height of the bed allows high degree of particle mixing.

HIGH COMBUSTION EFFICIENCYSuperior mixing

Large reaction space

Long residence time

Independent of ash properties

GOOD TURN DOWN AND LOAD FOLLOWING CAPABILITYStaged combustion permits a fairly good turn down ratio.

In CFB boiler load can be changed quickly by controlling solid circulation rate.

HIGH AVAILABILITYThe availability records of CFB boilers are more than 90%.

MAJOR DISADVANTAGESErosion of reactor wall

complexities of hydrodynamics

Higher start-up and shut down time

Indian Experience -CFBC18 boilers Installed

Operating maximum capacity 125 MW

Contracted maximum capacity 250 MW firing Neyveli Lignite for M/s Neyveli Lignite Corpn

Fuels used -Gujarat Lignite, NeyveliLignite, Rajasthan Lignite , KutchLignite , Petcoke, Coal , Washery Rejects, Char Source: BHEL

CFB Boiler : An innovative solution of mill reject problem A self sustaining, environment friendly, inexpensive and practical process for mill reject utilisation.

Application of CFB:Very bright future to Indian power plantsCanada -India Boiler Emission upgrade project has build an advanced CFB boiler at Kolaghat Thermal Power Station for research and demonstration.

OPPORTUNITIES AT NTPC KAHALGAON Daily Mill reject Generation: 600 MT (~380Mt from stage-I and ~220MT from Stage-II at full load)

A CFB Boiler may be erected utilizing mill rejects as fuel.

Steam generable in CFBC = 30 MT/hr considering avg GCV of rejects 800 Kcal/Kg = 8 MW (aprx)

SPACE REQUIRED FOR CFB BOILER The space needed for a CFB Boiler (Including TG and ESP) as approx 40M2/MW

For proposed scheme at Kahalgaon it needs only boiler island (Excluding TG and ESP)

Furnace of Bataan CFB project, Philippines, (35 MW), is located only on 50M2 area

Proposed CFB Boiler may be erected adjacent to the mill reject silo area of Unit-I (Stage-I)

CASE STUDY AT NTPC KAHALGAON

This CFBC boiler may be executed with existing ID Fan, ESP & stack.

Stage-I rejects may be transported to the CFB site by the existing reject conveyor system

Generated steam can be used for different auxiliary purpose like PRDS,FOPH, steam ejector, oil burners.

Fluidizing GasFluidized bed combustorESPID FanTurbine & GeneratorBlr DrmMain Stm HeaderHigh Pressure SteamPRVPRDS,FOPH, steam ejector, oil burners and other utilitiesMill rejectA Proposed schematic for generation of power from mill rejects at NTPC-Kahalgaon

Revenue GenerationProjected Investment : ` 10.0 CroreSteam generable in CFBC = 30 MT/hr = 8 MW electricity generation (approx)Considering 80% PLF Annual savings = ` 8x24x365x0.8x2 = ` 11.2 CrProjected Annual O&M Expenses = ` 1.5 Cr (Fuel cost almost free)Depreciation + Interest @14%= ` 1.4 Cr Projected Annual Return= ` 8.3 Cr (PBT)Profit after Tax = ` 7.5 CrProjected Pay back period = 16 Months only

NTPCLightening Every Third Bulb of India NTPC contributes ~ 30% of total power generation with 20% of the total national capacity

In order to combat the forthcoming challenge of competitive bidding,we have to innovate in all directions. NTPCFuture Ahead

NTPC Innovate and LeadInnovative utilization of Mill Reject using CFB Boiler: A step to turn NTPC vision into reality.

References

WASTE TO ENERGY process : paper by Mr.Prabir Basu, DalhousiUniversity, Canada.Website of BHELMr._S.Lakshmanan: CFB- The Technology for IndiaEnergy carbonminus: India and Low Carbon EconomyGreenhouse gas control technologies ; Google BooksCFB Technology & Boiler; Google Books

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