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Transcript of Combustion Technologies
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Combustion Technologies
Bengt-Johan Skrifvars
Chemistry in Combustion Processes IIntensive course, Åbo Akademi University, March 2010
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Combustion technologies
Principles
- Burning of gaseous-, liquid-, solid fuels
Practice- Various firing techniques;burner firing, grate firing, fluidized beds
- Furnace and flue gas- Boiler and power plant
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Combustion
Heat recovery
Flue gas
Condenser
Steamturbine
Air Fuel
Furnace, flue gas channeland steam cycle
G
Gas cleaningSteam
generator
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Gas
Diffusionflame
Combustion of a gas
O2
CxHy
CO2 +H2O
Air
Gas + air
Premixedflame
CO2 +H2O
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Liquid fuel
Evaporationand
gas combustion
Combustion of a liquid fuel
O2CxHy
CO2 +H2O
Air
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Liquid fuel
Evaporationand
gas combustion
Combustion of a liquid fuel
O2CxHy
CO2 +H2O
Air
(Atomization)
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Char formingliquid fuel
Ash
Pyrolysis/devolatilisation
andgas combustion
Charcombustion
Combustion of a char formingliquid fuel
O2
O2
CxHy
CO2 +H2O
CO2
Air
Atomization
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Solid fuel
Ash
Drying
Pyrolysis/devolatilisation
andgas combustion
Char
combustion
Combustion of a solid fuel
O2
O2
CxHy
CO2 +H2O
CO2
H2O
Air
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Combustion of fuels
Gases; i) gas combustion (diffusion or pre-mixed)Liquids; i) heating and atomization
ii) evaporation and gas combustion
(diffusion flame)Char forming liquids;
i) heating and atomizationii) devolatilization and gas combustion(diffusion flame)
iii) heterogeneous char combustionSolids; i) heating and drying
ii) devolatilization and gas combustion(diffusion flame)
iii) heterogeneous char combustion
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Different amount of time neededto accomplish good combustiondepending on the fuel
Different types of techniques
needed for different types fuels
Combustion of fuels
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Burner firing
Gas, liquid and pulverized solid fuels
High combustion temperatures (> 1000oC)
Short residence times
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Burner firing of a gaseous fuel
- turbulent diffusion flame
Gaseousfuel
Combustion air
Combustion air inswirl like motion
Detached pocket ofburning material
Burn-out pocket ofcombustion products
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Liquidfuel
Pressureatomizing
nozzle
Liquidsheet
FilamentsDroplets
Vaporized fueldroplets.
Behave like
a diffusiongas flame
Burner firing of a liquid fuel
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Burner firing of a solid fuel
Solid fuel
+primary air
Secondary air
Char
combustion
Devolatilization
andgas combustion Ash
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Burner firing concepts I
Burner + furnace
Fuel +Air
Bottom Ash
(solid fuels)
Flue Gas +Fly Ash
Wall-fired Boxer-fired
Tangentially fired
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Coal
+Air
Flue Gas +Fly Ash
Oil
+Air
Flue Gas
Furnace size dependence of fuel
Bottom ash
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Fuel + Air
Flue gas
Burner firing concepts II
Gas turbine
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Burner firing
Gaseous fuels
- turbulent diffusion flamepremixed flame not possible in burnersdue to the back fire risk
- good mixing with air essential (turbulence)laminar diffusion flame not possible in burnersdue to too incomplete combustion (soot)
- practical applications:burner + furnacegas turbine
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Burner firing
Liquid fuels
- atomizing (droplet formation) andgood mixing with air essentialfor good combustion (high burn-out)
- fuel droplet size 10 - 100 μm- char forming liquidsneed longer residence times than
non-char forming ones- practical applications:
burner + furnacegas turbine (non-char forming liquids)
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Burner firing
Solid fuels- fuel milling important for good combustion
(small, even particle size distribution)- good mixing with air essentialfor good combustion (high burn-out)
- fuel particle size 0.1 - 1 mm- longer burn-out times required than forgas or liquid burners (affects the furnace size)
- practical applications; burner + furnacewet bottom: ash removed as liquid slagfrom the furnace bottom
dry bottom: ash removed as solidfrom the flue gases
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Burner firing
+ unlimited size
+ efficient combustion, high burnout+ good load flexibility+ user friendly fly ash (solid fuels)
- high NOx emissions if no reduction device- SOx reduction requires separate flue gas cleaning
- high dust load in flue gas (solid fuels)- slagging and fouling (solid fuels)
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Grate firing
Solid fuels
stationary, moving grates,spreader stokers
Fuel particle size > 1 cm
High furnace temperature (> 1000oC)
Ash removed mainly through the grate (80 %)
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Grate firing
Travelling Grate
Air Air Air
Bottom ash
FuelAir
Drying
Pyrolysis andgas combustion
Char combustion
Ash
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Grate Configurations
Travelling Grate
Air Air Air
Bottom ash
A i r A i r A i r
AirAir
Travelling Grate Spreader StokerFuel
Fuel
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Grate firing
+ suitable for small scale (simple, cheap)+ no pretreatment of fuel
+ ash removed mainly through grate- incomplete combustion
(CO-emissions, rest char in ash)- slow load change rate- some fuels unsuitable
(ash melting on the grate, fuel baking problems)- grate construction heavy- SOx and NOx reduction requires separate flue gas
cleaning devices
Fl idi d b d b ti
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Fluidized bed combustion
Solid fuels- bubbling fluidized bed boilers (BFB)fluidization velocity approx. 1-3 m/s
- circulating fluidized bed boilers (CFBC)fluidization velocity approx. 6-10 m/s
Atmospheric (AFBC) and pressurized (PFBC)
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Bubbling Fluidized Bed Combustion
Air
Air
Fuel
Fuel: Fresh, PartlyPyrolysed, Char
Bed material:
Quartz,Limestone; fresh,calcined, sulphated
Void Space
Ash
Flue Gas + Fly Ash
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Circulating Fluidized Bed Combustion
Air
Air
Fuel
Fuel: Fresh,Partly Pyrolysed,Char
Limestone:Fresh, Calcined,Sulphated
Void Space
Ash
Flue Gas + Fly AshCyclone
Fl idi d b d b i
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Fluidized bed combustion
Fuel particle size 0.1 - 10 mm
Bed material particle size 0.1 - 1 mm
Low furnace temperature (800 - 900oC)
Long residence time of fuel in the furnace
Addition of SOx sorbents to the furnace
Ash removed through bed, rest from the flue gases
Fl idi d b d b il
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Fluidized bed boilers
+ low combustion temperature
+ suitable for a variety of solid fuels+ good combustion efficiency+ no Thermal NOx formation
+ easy SOx reduction possibilities
- ash-bed material interactions may lead
to bed defluidisation- residual ash may be difficult to dispose- high efficiency reduction of all emissions (excl. CO2)
(CO, NOx, SOx, N2O) simultaneously hard to achieve
P i d Fl idi d B d C b ti
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Pressurized Fluidized Bed Combustion
Pressure vessel Pressure vessel
Air
FuelAir
Air
Fuel
Air
BFB modecommercially available
CFB modeat demonstration stage
Air Fuel
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PressurisedCombustion
Heat recovery
Flue gasCondenser
Steamturbine
Pressurised combustion
Gas cleaning
G
G
Gas turbine
Pressurevessel
Pressurized fluidized bed combustion
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Pressurized fluidized bed combustion
Boiler inside a pressure vessel
Working pressures 10-20 bar
BFB mode commercialized, CFB mode at demo stage
+ smaller size than atmospheric versions
+ gas (expansion) turbine gives additional electricity
- new (uncertain) technology
- solids cleaning from the flue gases problematic
Combustion technologies
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gDefinitions
• Fixed bed firing: “The bon fire”• Grate firing: A firing system where a fuel lies on a grid when it is
burned and part of the combustion air is supplied through the
grid• Stoker firing (Stokers): A continuously functioning grate firing
system, i.e. continuous fuel feed in and continuous ash feed out
• Burner firing (suspension firing): A firing system where the fuelis blown as a dust or as droplets into the furnace together withpart of the combustion air and is burned in a gas suspension
(pulverized coal PC, pulverized fuel PF, gas and liquid fuels alsogas turbines)
• Fluidized bed combustion: A firing system where the fuel is fired
in a suspension of an inert solid media and combustion air.circulating (CFBC), bubbling (BFBC), pressurized (PFBC)
Furnace, flue gas channel
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Combustion
Heat recovery
Flue gas
Condenser
Steamturbine
Air Fuel
Furnace, flue gas channeland steam cycle
G
Gas cleaningSteam
generator
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LUVO
Superheatersand reheaters
Boilertubes
Economizer
Burners F
u r
n a c e
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LUVO
Superheatersand reheaters
Boilertubes
Fuelfeed
F u r
n a c e
Economizers
Steamdrum
Air
feed
C y c
l o n e
Ashhoppers
Furnace, flue gas channel, steam cycle
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, g , yDefinitions
• Furnace: The fireplace where the actual combustion takes place• Burner: The device that feeds in the fuel and part of the air
• Heat exchangers: The device that transfers heat from the flue gasto the media to be heated (here water and/or steam)• Super heater: The heat exchangers that heat up the steam
• Boiler tubes: The heat exchangers boil that boil the water• Economizer: The heat exchangers that heat up the feed water• Air pre-heater (LUVO): The heat exchanger that heats up the
combustion air• Steam drum: The device separating the steam from the water in
the conventional steam boiler
• Condenser: The device condensing the steam exiting the steamturbine at a low as possible temperature
Conventional steam cycle
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Steam
generator
Flue gas
Condenser
Steamturbine
Air Fuel
Conventional steam cycle
G
Power plant
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Power plant
Meri Pori power plant
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Meri-Pori power plant- PC fired- Input: 1300MWth
- Output: 560MWe
- 440 kg/s, 540oC, 240 bar
- Supercritical once-through typewith reheater
- SCR, ESP, wet FGD
Power plants
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• Boiler: Closed pressurized vessel for production of hot water orsteam
• Steam generator: Closed pressurized vessel for production of steam
by vaporizing water with heat from a high temperature source• Power plant: A plant that produces electricity only• CHP plant ( C ombined h eat and p ower): A plant that produces both
both heat and electricity• Combined cycle: A power or CHP plant that consists of both a gas
turbine and a steam turbine
• Once-through type boiler: A steam cycle without a steam drumworking at pressures near or above the critical steam pressure of 221bars
• Supercritical steam data: A steam cycle working at pressures abovethe critical steam pressure of 221 bars
Definitions I
Power plantsD
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• Dust removal: The removing of fly ash• ESP: Electrostatic precipitator, a device in the cold end of the flue
gas channel that removes solid particles from the flue gas by aelectrostatic (magnetic) field
• Bag filter: A device in the cold end of the flue gas channel that
removes solid particles from the flue gas by textile bags• Ash hopper: A fly ash collector, usually at the bottom end of a
flue gas channel part where it turns. Forces the gas to turn more
than what the particles can follow.• Cyclone: A particle/gas or liquid/gas separator where the
separation is based on gravitational and inertial forces
Definitions II
Power plantsD fi i i III
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• Primary air: That part of the combustion air that is fed into the lowerfurnace close to/together with the fuel
• Secondary air: That part of the combustion air that is fed into the
furnace after the fuel feed• Tertiary air: That part of the combustion air that is fed into the upper
part of the furnace to ensure complete combustion
• Fly ash: That part of the uncombustible rest of the fuel, the ash,which is carried away from the furnace with the flue gases into theflue gas channel
• Bottom ash: That part of the ash which exits the furnace from itsbottom part
• Bed: The inert solid particle bed in the furnace of fluidized bed
combustion system into which the fuel is feed and through which partof the combustion air is fed
Definitions III
Power plantsD fi iti IV
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• FGD: Flue gas desulfurization device, usually based on wet orsemi-dry absorption processes
• SCR: Selective catalytic reduction, a flue gas NOx removingsystem, consisting of ammonia injection into the flue gas at some200oC followed by a catalysator
• SNCR: Selective non-catalytic reduction, a flue gas NOxremoving system, consisting of ammonia injection into the fluegas at some 800oC
Definitions IV