CFBC BOILERSCFBC BOILERSCFBC BOILERSCFBC BOILERS

by

QUEST

Professional Circle16.09.03

by

QUEST

Professional Circle16.09.03

• Minimum Fluidization Velocity:- Critical Velocity at which the pressure drop across the bed equals the weight of particles per unit cross sectional area of bed.

• The bed is called incipiently fluidized at this velocity. • With increase in air velocity the drag force by the fluid on

particles exceeds their weight.• High degree of particle mixing and equilibrium between

gas and particles is rapidly established – Fluidized bed.• Terminal velocity: The velocity at the point where the

pressure drop across the bed starts decreasing.• The bed particles leave the bed and called entrained bed.

DP across the bed becomes zero.

Fluidized Bed Combustion

Variation of Bed Pr. Drop with Superficial Velocity

• Umf- Minimum Fluidizing Velocity

• U-Superficial Velocity

• Ut-Terminal Velocity

A Umf UtD

B C

P B

ed P

r. D

rop

Gas Velocity

Unpacking of bed

Packed bed

Fluidized Regime

between Fixed Grate, Fluidized Bed, and Pulverized Firing

Relationships

Stoker Firing(F ixed Bed )

Fluid ized Bed FiringBFB C FB

G a s

Fue l

A ir A sh

Velo city 8 - 1 0 ft/ sec(2 .3 - 3 .0 m / s)

4 - 1 0 ft/ sec(1 .2 - 3 .0 m / s)

A vera ge BedPa rticle Size

6 ,0 0 0 m

Pulverized Firing(En tra in ed Bed )

G a s

Fue lA ir

A sh

1 5 - 3 3 ft/ sec(4 .6 - 1 0 .0 m / s)

5 0 m

G a s

Fue l &So rben t

A ir A sh

1 ,0 0 0 m 1 0 0 - 3 0 0 m

G a s

Fue l &So rben t

A ir A sh

1 5 - 2 3 ft/ sec(4 .6 - 7 .0 m / s)

A ir

Fluidizing/ Primary Air

Furnace Bed

Bottom Ash Removal

Secondary AirBed FillLignite+Lime stone (Sorbent)

Start-up Burner

Circulating Fluidized Bed Combustion• Bed is divided in to 3 zones

– Lower Zone-Below SA entry.• Fluidized by 40-80% of stoichiometric air for fuel feed Fuel,

sorbent and unburned char from cyclone are received in this zone.• Oxygen deficiency controls Nox emission.• Much denser and serves as an insulated storage of hot solids

providing CFB boiler with a thermal flywheel.• PA/SA increased on increase of Boiler load, transferring greater

amount of hot solids into upper zone of the furnace and increasing solid circulation rate.

– Upper Zone-Above SA entry• Combustion completes with added SA and unburned char to

cyclone for return. More residence time for completing the combustion.

– Hot Gas/ Solids Separator• Cyclone (External)/ U-Beams (internal)

SO2 Capture

CaCO3 --> CaO + CO2

CaO + SO2 + ½ O2 --> Ca SO4

Furnace temperature control is very critical

Limestone consumption varies enormously with

furnace temperature

Optimum temperature :

850 °C

850800 900

SO2Capture efficiency

T (°C)

SO2 Capture achieved by

limestone injection

NOx Emissions

- Combustion temperature

- N2 in fuel

- Excess air and staggering

1 000800 1 200

NOx

T (°C)

NOx Emissions influenced by 3 main parameters :

General Process

Bed temperature

AirAir

Air

Ash

Coal

Flue gas

Optimum temperature :

850 °C

Temperature maintained by heat pick up in exchange surfaces

Either in furnace

Or in FBHE

Types of Fluidized Bed Boilers

• Bubbling Bed Combustion

• Circulating Fluidized Bed Combustion

• Pressurised Fluidized Bed Combustion

CFBC - Advantages

Fuel Flexibility

Low SOX Emission

Simplified Fuel Feeding

CFBC (800-9000C)

Low NoX Emission

High Combustion Efficiency

Operating FlexibilityCompactness

High Availability

AdvantagesFuel Flexibility- • Not sensitive to fuel ash characteristics as firing temperature is

held below the ash softening point.• Wide range of fuels (low grade coals, lignite,bio mass, wastes,

wood chips etc.)with varying ash & sulphur content can be used.

Simplified Fuel Feeding• Fuel pulverization not required and only crushing is sufficient.

Operating Flexibility• Can be designed for cyclic or base load operation. Part loads

down to 25% of MCR and load rates upto 7% per minute are possible.

AdvantagesLow SOx emissions • Sorbent -lime stone reacts with SO2 released from burning

the fuel.• At a Ca/S molar ratio of ~ 2, over 90% of the sulphur in

fuel is converted to gypsum.

Low NOx Emissions• The relatively low combustion temp. (850-900C) helps to

reduce Nox formation .

AdvantagesHigh Combustion Efficiency- • 98-99% carbon burnout can be achieved due to intimate

gas/ solids mixing and long retention / residence time of fuel in the circulating fluidized bed.

Compactness• Small Furnace cross section as high heat release rate per

furnace cross section (5MW/M2)

High Availability• Less Auxs.(pulveriser) more availability. As high

as 98%.

Manufacturers

• Foster Wheeler – USA (> 150 units)

• Lurgi Lentjes babcock Energietechnik GmbH- Germany (42 units)

• M/s Babcock & Wilcox –40 Units

• M/s Alstom –

More than 310 units available world over.

CFBCAKRIMOTA

THERMAL POWER PROJECT

CFBCAKRIMOTA

THERMAL POWER PROJECT

Akrimota, 2 x 125 MWBoiler with CFB

012 217pÄ

Live Steam138 bar538 °C405 t/h

Reheater Steam36 bar

537 °C375 t/h

Feedwater247 °C

FuelHigh Sulphur Lignite

± 0.0 m

+ 50.0 m

Water & Steam Data

Item Mass Flow

T/hr.

Temp.0C

Pressure

Bar

Eco. Inlet 380.268 246.8 152.90

SH5 Outlet (MS) 405 538 138.4

RH1 Inlet (CRH) 374.652 355 37.90

RH2 Outlet (HRH) 378 537 36.03

Furnace– No. of Lignite Feeders - 4 (Rear)– No. of Oil Burners - 8 3(F)+2(L&R)+1(Rear)– Angle of Inclination(Degree) -30– Fuel Oil (HFO) Flow - 4.31Kg./Sec.– Air Flow -60.37 kg./Sec.– Thermal Capacity of Burners -170.9 MW

– Outer Oil Burner Dia.(Throat) - 650mm– Impeller Diameter -450mm– Lance Diameter -70mm

– Bed Fill - Sand / Bottom Ash– Ignitors - High Energy Arc Induction

Air Flow

PA Fan

SA Fan

SCAPH

T A P H

FURNACE

SCAPH

Hot SA

Hot PA

Primary Air– No. of PA Fans - 2– No. of Primary Air Nozzles - 1624(76 W*22 D)– Pitch Between PA Nozzles -165mm– Nozzle Less Due to L- Valve -48– Depth of Grid -3670mm– Width of Grid -12692mm– Thickness of Grid Refractory -80mm

Secondary Air– No of SA Fans -2– No. of Air Nozzles - 16 (F-6, R-2, L&RS-4)– Pitch Between PA Nozzles -165mm– Level of Nozzle above Grid -5000mm– Angle of Inclination -30– Diameter -255mm– Thickness -5mm– Air Flow 100% Load (Lignite)/Nozzle-1.8Kg./s– Max. Air flow/nozzle -2.9Kg./s– Min. Air flow / Nozzle -0.3Kg./S

Air Flow

Fluidizing Air Fan

F A Fan Common Header

CYCLONE SYPHON SEAL

Bottom

Salient Features• Air Distribution. 100% BMCR Design Lignite

Air Mass Flow Flow Rate Rate(Kg./S) %

Kg./S– PA 65 45 57.8-86.7– SA 28.8 19.9 4.2-43.3– Burner Air 28.8 19.9 7.6-53.4– Air to lignite feeding ports 11 7.6– Sealing Air 0.8 0.6– Ash cooler air 4.1 2.8– Air to loop seal 2.2 1.5– Air to lignite feeders 1.3 0.9– Lime stone transport air 1.9 1.3– Flame detrs,ignitrons,oil guns 0.4 0.3– Air for L-valves 0.1 0.1– Total Combustion Air 144.4 100– Air thro’ air pre-heater 133.6 92.5– Coal Air 10.8 7.5

Material Data

• Lignite ConsumptionDesign Coal (T/hr.) Worst Coal (T/Hr.)

– 100% BMCR - 101.704 179.287 – 100% TMCR - 93.841– 60%TMCR - 58.536– 40% TMCR - 41.785

• Lime Stone Consumption– Reqd. Ca/S Ratio - 3.45 3.45– Lime Stone Flow(Kg./S) - 14.54 29.817– Sulphur Content(%) - 3.87 4.55– Ash Content(%) - 21 35– CaO in Ash(%) - 3.98 3.98– CaCO3 in Lime Stone(%) - 80.6 80.6– Desulparisation rate(%) - 97 97

Ash Distribution

Design Lignite (T/hr.) Worst Lignite

– Coal Flow - 28.251 49.802– Lime Stone Flow - 14.54 29.817

• Normal Conditions– Total Ash Flow(Kg./S)- 17.78 41.78– Bottom Ash Flow(kg./S)- 4.81 12.47– Fly Ash Flow(Kg./S) 12.97 29.31– Bottom ash temperature 96 260– Filter Ash temperature 136 157

• Design Conditions– Max. Bottom Ash low(Kg./S) 23– Max. Fly Ash flow 39.84– Max. bottom ash temp. 300– Max. filter ash temp. 200

Cyclone Separator– No. of Cyclone -2 Nos.– Inlet Duct Height -5570– Inlet Duct width at Inlet -

3110– Inlet Duct width at outlet -1990– Diameter -8000– Vortex Finder diameter at I/L -2990– Vortex Finder diameter at O/L -3510– Vortex Finder height -1950– Vortex Finder Eccentric Arrangement -560– Cylinder height -8400– Hopper Height -10160– Total Height -

18560– Outlet Duct diameter Down comer -

1400– Distance I/L duct ceiling to Cyclone Ceiling -880– Slope cyclone Inlet -10– Slope Cyclone Hopper -18– Flue Gas Velocity at Cyclone I/L(max.)(m/s)-27– Flue Gas Velocity at Cyclone I/L(max.)(m/s)-31 – Pressure loss I/L to O/L (mbar) -15

Fuel Air Mix.

Gas Outlet

All dimensions in mm

Unburned particles

Typical Cyclone

056 329p

DownwardInclinedInlet Duct

High PerformanceRefractory for Inlet Area

Eccentric VortexFinder Arrangement

Advanced VortexFinder Shape

Second Pass

Cyclone Collection Efficiency• Ratio of the mass solid particles captured by the cyclone to the

total mass of particles entering the cyclone.

• Collection Efficiency = (M-M1/M)*100 %M- Particles Entering

M1- Particles leaving

• Factors Affecting the collection Efficiency– Collection Efficiency Increases with

• Increase in :a)Particle mass, b) inlet gas velocity, c)Cyclone body length, d) no. of gas revolutions, e)smoothness of cyclone wall.

– Collection Efficiency Decreases with• Increase in : a)Cyclone diameter, b)Gas outlet diameter, c) Gas inlet area.

• Shepherd & Lapple derived Optimum Dimensions for various sections of Cyclone as a function of its body diameter.

Air Data Item Mass Flow

T/hr.

Temp.0C

Pressure

mbar

Primary

TAPH Inlet 233.964 52 182.5

TAPH Outlet 233.964 254 164.5

Secondary

TAPH Inlet 246.96 47 112.5

TAPH Outlet 246.96 254 94.5