FLUIDIZED COMBUSTION - Politechnika...

COMBUSTION AND FUELS

FLUIDIZED COMBUSTION

COMBUSTION AND FUELS

Fluidization phenomenon

gas flow velocity

Fixed bed Bubblingfluidized bed

Circulatingfluidized bed

Pnematictransport

dust

COMBUSTION AND FUELS

What does it mean fluidization

Fluidization is a two-phase process in which dispersed solid material is suspended in a stream of gas flowing upstream through the fluidized grate. The layer of solid body particles suspended in flowing gas forms fluidized bed.

The fluidized bed is in the quasi-equilibrium state only in some range of the velocity of the flowing upstream gas, depending on the size of particles of bed.

The fluidized bed of a boiler contains mainly particles of an inert material, like sand and ash, including particles of SO2 sorbent. The coal content in a fluidized bed is not considerable, it is only from 3 to 5% of the whole mass of the bed.

COMBUSTION AND FUELS

Air flow velocity in boiler furnace vs. combustion pattern

COMBUSTION AND FUELS

Fluidized bed-combustion features

a) direct contact of particles with intensive mass and heat exchange,

b) uniform temperature in the fluidized bed

c) high heat capacity of fluidized bed making it possible to burn fuels of low quality, wet and with high content of ash

d) effectiveness of bed temperature control by supply of fuel, air and heat extraction

COMBUSTION AND FUELS

Major features of fluidized bed boilers

• Higher power of air van

• Larger cross-section of a furnace

• Higher surface loss of heat

• Higher carbon-in-ash level

• Higher erosion rate

• Fuel flexibility

• In situ SO2 removal

• Low NOx emission

• Good system availability

• No slaging

• Low corrosion rate

• Easy sizing of fuel

DisadvantagesAdvantages

COMBUSTION AND FUELS

FLUIDIZED BED COMBUSTION SYSTEMS

COMBUSTION AND FUELS

Types of fluidized bed boilers

Concerning the structure of fluidized bed:

• boilers with bubbling (stationary) fluidized bed (BFB)

• boilers with circulating fluidized bed (CFB)

COMBUSTION AND FUELS

Bubbling fluidized bed BFB

coal

ash

flue gas

water

air

Circulating fluidized bed CFB

steamsuperheater

secondaryair

coal andsorbent

cyclone

steamsuperheaters

waterheater

refractorychamber primary

air

airheater

exhaust

chamber

circ

ulat

ion

duct

COMBUSTION AND FUELS

Bubbling and circulating fluidized bed boilers

Bubbling

Circulating

COMBUSTION AND FUELS

Types of fluidized bed boilers

Concerning the pressure in a furnace:

• atmospheric fluidized bed boilers (pressure approximately atmospheric) (ACFB),

• pressure fluidized bed boilers (pressure much higher that atmospheric) (PCFB).

COMBUSTION AND FUELS

ATMOSPHERIC FLUIDIZED BED BOILERS

COMBUSTION AND FUELS

BUBBLING FLUIDIZED-BED COMBUSTORS

Bubbling fluidized bed furnace(laboratory set)

COMBUSTION AND FUELS

Structure of bubbling fluidized bed

węgiel

III

II

I

splash-zone

złożefluidalne

coal

fluidizedbed

over-bedzone

COMBUSTION AND FUELS

Burning of coal particles in fluidized bed

Mechanism of coal combustion

bed

splash-zone

O2 CO2

CaO

CO

½ O2

CaSO4

CnHm

HO2

CO

fines

CO2

charsorbent

CO2

coalparticle

CO2 CO

CO2

CaOCO

CO2

½O2

CO2

CnHm

CO2HO2

CO2

SO2

½ O2CaO

COMBUSTION AND FUELS

Sources of unburned coal in fluidized bed boilers

Very small particles of coal are blow-up from the bed due to:

�increase of porosity of coal particles due to oxidation

�particles decomposition as a result of thermal thermal tension in particles

�collision of particles in a bed

�friction of particles in a bed

COMBUSTION AND FUELS

Decomposition of coal particle due to inner oxidation and contact with particles of bed

COMBUSTION AND FUELS

CIRCULATING FLUIDIZED BED BOILERS (CFB)

COMBUSTION AND FUELS

Structure of circulating fludized bed

Falling flow

Raising flow

COMBUSTION AND FUELS

Types of boilers with circulating fluidized beds

Circulating fluidized bed boilers: 1 − cyclone separator, 2 − inner separator

1) 2)

fuel

secondary air

primary air

bed

cyclone

COMBUSTION AND FUELS

CFB with cyclone separator

Circulating fluidized bed boiler

Circulating fluidized bed boiler CFB-670 with cyclone separator

flue gas

air

flue gas

air

air to siphon

fluidizing air

seco

ndar

y ai

r

coal

cyclone

coal ash

Bottom of the CFB boiler

Bottom part of the furnaceCFB-670 boiler

solids returnfurnace (lined with refractory )

air nozzles

IR detectors

bedtemp.

bedpress.

COMBUSTION AND FUELS

Closing of the siphon of CFB-670 boiler

COMBUSTION AND FUELS

Bottom of fluidized bed furnace CFB-670 boiler

COMBUSTION AND FUELS

Air nozzles of the fluidizing bed

COMBUSTION AND FUELS

Exploitation parameters of the CFB-670 boiler

1.1-1.2

850-870 °C96-98%

90% (Ca/S = 1.5-2.6)

200-400 mg/m3

Stoichiometric ratio λTemperature of the bed, TzBurnout of coal

Effectiveness of sulfur removal

NOx emission

ValueParameter

COMBUSTION AND FUELS

CFB boilers with inner separator

COMBUSTION AND FUELS

Separation of bed material in inner separator

CFB – 700 boiler

CFB – 700 boiler

with innerseparator

INTREX

COMBUSTION AND FUELS

FUEL HANDLING IN FLUIDIZED BEDS BOILERS

COMBUSTION AND FUELS

Systems of fuel handling

Underbed OverbedIntobed

COMBUSTION AND FUELS

Bubbling bed: pneumatic handling of fuel

COMBUSTION AND FUELS

Bubbling bed - worm-gear coal handling

Circulating bed: overbed handling of fuel

Circulating fluidized bed: silos and coal feeders

coal silos

shorter cell coal feeders(3 pcs.)

coal feeders(4 pcs.)

longcell/rake

coal feeders(2 pcs.)

chutes withair nozzles(10 pcs.)

CFB: gallery of coal handling

COMBUSTION AND FUELS

SELECTED FEATURES OF FLUIDIZED BED BOILERS

COMBUSTION AND FUELS

Effectiveness of combustion in fluidized beds

Lignite

Hard coal

Anthracite

Coal

93 ÷ 9686 ÷ 9378 ÷ 85

20 ÷ 3050 ÷125200 ÷ 300

Degree of burnout, %

Time of burnout, s

COMBUSTION AND FUELS

Advantages of fluidized bed firing over pulverized-bed firing systems

•Simplification of the fuel supply system,

•Possibility of burning low-caloric fuels,

•Possibility of flue gas desulfurization in the bed

•Reduction of NOxemission due to the lower

temperature of burning.

COMBUSTION AND FUELS

Comparison of PF and fluidized bed boilers

Not melted/no

Melted/

yes

Ash/ slaging

3-30

mm

Hard coal, lignite, biomass, wastes

Low:

800-900 oC

CFB

< 0.3

mm

Fuel sizing

Hard coal 31-33

Lignite

High:

1100-1400 oC

PF

FuelTemperatureBoiler

COMBUSTION AND FUELS

Comparison of thermal load of PF and fluidized bed boilers

0.08-0.20.1-0.20.2-0.4

Volume thermalload

qv, MW/m3

3.0-5.51.2-1.51.8-2.5

Surface thermalload:

qA, MW/m2

PCBFBCFB

Type of boiler

Parameter

COMBUSTION AND FUELS

Comparison of PF and CFB ashes

25

26

10.8

9.6

15.4

0.8

0.4

2-12

2-12

40-55

0.5-6

13-30

4-17

1.8-8

0.3-0.7

0.7-15

CaO

SiO2SO3Al2O3Fe2O3MgO

Na2O

LOI

CFBPFCompound

YEAR OF INITIAL OPERATION

UNIT CAPACITY (MWe)

0

50

100

150

200

250

300

350

400

450

500

550

600

1970 1975 1980 1985 1990 1995 2000 2005 2010

Pilot plant Pihlava

LeykamTri-State

Kajaani

KauttuaPilot plant

KuhmoKokkola

Thai Kraft

NPS

Turow 5

FIRST GENERATIONDESIGN

Turow 1

JEA

SECOND GENERATIONDESIGN

Lagisza

Scale-Up and Some Key References

Developing trends

Vaskiluodon Voima

Nova Scotia

Supercritical once-through CFB, PKE ŁAGISZA 460 MWe PLANT

COMBUSTION AND FUELS

Comparison of Turów and Łagisza boilers

COMBUSTION AND FUELS

Boiler’s furnace

INTREX™ SYSTEM

Solids return leg

Air plenums

INTREX™

Superheater

Solids returnchannels

Integrated Heat Exchanger• Heat exchange from solid bed material

to tube surface • High heat transfer rate• Heat exchange rate can be controlled• Less heat transfer surface than in

conventional gas to solid heat exchangers

• Suitable for large-scale unitsSimple Design• No moving parts: no wearResistant to Erosion• Low erosion rate due to low velocitiesResistant to Corrosion• Tube corrosion rate reduced by Cl-free

environment

Intrex

COMBUSTION AND FUELS

Mineral wool

Refractory 25-50 mm

Membrane wall

FEATURES:• Integrated with furnace:

• Panel wall structure

• No / few expansion joints

• Water or steam cooled

• Normal insulation

• Lower radiation loss

• Less refractory

• Shorter start-up time

• Smaller foot-print

FW CFB SOLIDS SEPARATOR

Air preheater

Electrostatic precipitator

COMBUSTION AND FUELS

PRESSURIZED FLUIDIZED BED BOILERS (PFBB)

COMBUSTION AND FUELS

1st generation PFBC

COMBUSTION AND FUELS

Karita, Japan

PFBC - Karita

paste

Technical data PFBC - Karita

P = 1,5 MPa

Diagram of pressurized fluidized bed boiler1st generattion (combined cycle with the PFBC)

COMBUSTION AND FUELS

Cottbus, Germany

PFBC 2st generation Cottbus

Gas turbine, Cottbus in Germany

Cottbus

COMBUSTION AND FUELS

2nd generation PFBB

PCFB 2nd generation

Syngaz

Combustor

Gasturbine

Ceramicfilter

Steamturbine

Flue gas

Gas cooler

Oxidizer PFBC

Partialgasification

Desulfurization CaS

Cyclone

Cyclone

COMBUSTION AND FUELS

Scheme of PCFB - 2nd generation

gasifiercombustor

desulfurizer

(Kita Kyushu City), Electric Power Development Co., Ltd.