Combustion Technologies

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Combustion Technologies

Bengt-Johan Skrifvars

Chemistry in Combustion Processes IIntensive course, Åbo Akademi University, March 2010



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



Combustion

Heat recovery

Flue gas

Condenser

Steamturbine

Air Fuel

Furnace, flue gas channeland steam cycle

G

Gas cleaningSteam

generator



Gas

Diffusionflame

Combustion of a gas

O2

CxHy

CO2 +H2O

Air

Gas + air

Premixedflame

CO2 +H2O



Liquid fuel

Evaporationand

gas combustion

Combustion of a liquid fuel

O2CxHy

CO2 +H2O

Air



Liquid fuel

Evaporationand

gas combustion

Combustion of a liquid fuel

O2CxHy

CO2 +H2O

Air

(Atomization)



Char formingliquid fuel

Ash

Pyrolysis/devolatilisation

andgas combustion

Charcombustion

Combustion of a char formingliquid fuel

O2

O2

CxHy

CO2 +H2O

CO2

Air

Atomization



Solid fuel

Ash

Drying

Pyrolysis/devolatilisation

andgas combustion

Char

combustion

Combustion of a solid fuel

O2

O2

CxHy

CO2 +H2O

CO2

H2O

Air



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



Different amount of time neededto accomplish good combustiondepending on the fuel

Different types of techniques

needed for different types fuels

Combustion of fuels



Burner firing

Gas, liquid and pulverized solid fuels

High combustion temperatures (> 1000oC)

Short residence times



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



Liquidfuel

Pressureatomizing

nozzle

Liquidsheet

FilamentsDroplets

Vaporized fueldroplets.

Behave like

a diffusiongas flame

Burner firing of a liquid fuel



Burner firing of a solid fuel

Solid fuel

+primary air

Secondary air

Char

combustion

Devolatilization

andgas combustion Ash



Burner firing concepts I

Burner + furnace

Fuel +Air

Bottom Ash

(solid fuels)

Flue Gas +Fly Ash

Wall-fired Boxer-fired

Tangentially fired



Coal

+Air

Flue Gas +Fly Ash

Oil

+Air

Flue Gas

Furnace size dependence of fuel

Bottom ash



Fuel + Air

Flue gas

Burner firing concepts II

Gas turbine



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



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)



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



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)



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 %)



Grate firing

Travelling Grate

Air Air Air

Bottom ash

FuelAir

Drying

Pyrolysis andgas combustion

Char combustion

Ash



Grate Configurations

Travelling Grate

Air Air Air

Bottom ash

A i r A i r A i r

AirAir

Travelling Grate Spreader StokerFuel

Fuel



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



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)



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



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



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



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



Pressurized Fluidized Bed Combustion

Pressure vessel Pressure vessel

Air

FuelAir

Air

Fuel

Air

BFB modecommercially available

CFB modeat demonstration stage

Air Fuel



PressurisedCombustion

Heat recovery

Flue gasCondenser

Steamturbine

Pressurised combustion

Gas cleaning

G

G

Gas turbine

Pressurevessel

Pressurized fluidized bed combustion



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



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



Combustion

Heat recovery

Flue gas

Condenser

Steamturbine

Air Fuel

Furnace, flue gas channeland steam cycle

G

Gas cleaningSteam

generator



LUVO

Superheatersand reheaters

Boilertubes

Economizer

Burners F

u r

n a c e



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



, 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



Steam

generator

Flue gas

Condenser

Steamturbine

Air Fuel

Conventional steam cycle

G

Power plant



Power plant

Meri Pori power plant



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



• 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



• 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



• 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



• 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

Combustion Technologies

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