COMBUSTION AND FUELS FLUIDIZED COMBUSTION
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.