Combustion Technologies

Bengt-Johan Skrifvars

Chemistry in Combustion Processes I

Åbo Akademi University, 2017

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

Steam

turbine

Air Fuel

Furnace, flue gas channel

and steam cycle

G

Gas cleaningSteam

generator

Gas

Diffusion

flame

Combustion of a gas

O2

CxHy

CO2 +H2O

Air

Gas + air

Premixed

flame

CO2 +H2O

Liquid fuel

Evaporation

and

gas combustion

Combustion of a liquid fuel

O2

CxHy

CO2 +H2O

Air

Liquid fuel

Evaporation

and

gas combustion

Combustion of a liquid fuel

O2

CxHy

CO2 +H2O

Air

(Atomization)

Char forming

liquid fuel

Ash

Pyrolysis/

devolatilisation

and

gas combustion

Char

combustion

Combustion of a char forming liquid fuel

O2

O2

CxHy

CO2 +H2O

CO2

Air

Atomization

Solid fuel

Ash

Drying

Pyrolysis/

devolatilisation

and

gas 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 atomization

ii) devolatilization and gas combustion

(diffusion flame)

iii) heterogeneous char combustion

Solids; i) heating and drying

ii) devolatilization and gas combustion

(diffusion flame)

iii) heterogeneous char combustion

Different amount of time needed

to accomplish good combustion

depending 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

Gaseous

fuel

Combustion air

Combustion air in

swirl like motion

Detached pocket of

burning material

Burn-out pocket of

combustion products

Liquid

fuel

Pressure

atomizing

nozzle

Liquid

sheet

Filaments

Droplets

Vaporized fuel

droplets.

Behave like

a diffusion

gas flame

Burner firing of a liquid fuel

Burner firing of a solid fuel

Solid fuel

+

primary air

Secondary air

Char

combustion

Devolatilization

and

gas combustionAsh

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 flame

premixed flame not possible in burners

due to the back fire risk

- good mixing with air essential (turbulence)

laminar diffusion flame not possible in burners

due to too incomplete combustion (soot)

- practical applications:

burner + furnace

gas turbine

Burner firing

Liquid fuels

- atomizing (droplet formation) and

good mixing with air essential

for good combustion (high burn-out)

- fuel droplet size 10 - 100 mm

- char forming liquids

need longer residence times than

non-char forming ones

- practical applications:

burner + furnace

gas turbine (non-char forming liquids)

Burner firing

Solid fuels

- fuel milling important for good combustion

(small, even particle size distribution)

- good mixing with air essential

for good combustion (high burn-out)

- fuel particle size 0.1 - 1 mm

- longer burn-out times required than for

gas or liquid burners (affects the furnace size)

- practical applications; burner + furnace

wet bottom: ash removed as liquid slag

from the furnace bottom

dry bottom: ash removed as solid

from 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 and

gas combustion

Char combustion

Ash

Grate Configurations

Travelling Grate

Air Air Air

Bottom ash

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

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, Partly

Pyrolysed, 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

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

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

Pressurized Fluidized Bed Combustion

Pressure vessel Pressure vessel

Air

Fuel

Air

Air

Fuel

Air

BFB mode

commercially available

CFB mode

at demonstration stage

Pressurised

Combustion

Heat recovery

Flue gasCondenser

Steam

turbine

Air Fuel

Pressurised combustion

Gas cleaning

G

G

Gas turbine

Pressure

vessel

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

Definitions

• 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 fuel

is blown as a dust or as droplets into the furnace together with

part of the combustion air and is burned in a gas suspension

(pulverized coal PC, pulverized fuel PF, gas and liquid fuels also

gas 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)

Combustion

Heat recovery

Flue gas

Condenser

Steam

turbine

Air Fuel

Furnace, flue gas channel

and steam cycle

G

Gas cleaningSteam

generator

LUVO

Superheaters

and reheaters

Boiler

tubes

Economizer

Burners

LUVO

Superheaters

and reheaters

Boiler

tubes

Fuel

feed

Economizers

Steam

drum

Air

feed

Ash

hoppers

Furnace, flue gas channel, steam cycle

Definitions

• 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 gas

to 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 steam

turbine at a low as possible temperature

Steam

generator

Flue gas

Condenser

Steam

turbine

Air Fuel

Conventional steam cycle

G

Power plant

Meri-Pori power plant - PC fired

- Input: 1300MWth

- Output: 560MWe

- 440 kg/s, 540oC, 240 bar

- Supercritical once-through type

with reheater

- SCR, ESP, wet FGD

• Boiler: Closed pressurized vessel for production of hot water or

steam

• 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 (Combined heat and power): 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 drum,

working at pressures near or above the critical steam pressure of 221

bars

• Supercritical steam data: A steam cycle working at pressures above

the critical steam pressure of 221 bars

Power plants

Definitions I

• 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 a

electrostatic (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

Power plants

Definitions II

• Primary air: That part of the combustion air that is fed into the lower

furnace 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 the

flue gas channel

• Bottom ash: That part of the ash which exits the furnace from its

bottom part

• Bed: The inert solid particle bed in the furnace of fluidized bed

combustion system into which the fuel is feed and through which part

of the combustion air is fed

Power plants

Definitions III

• FGD: Flue gas desulfurization device, usually based on wet or

semi-dry absorption processes

• SCR: Selective catalytic reduction, a flue gas NOx removing

system, consisting of ammonia injection into the flue gas at some

200oC followed by a catalysator

• SNCR: Selective non-catalytic reduction, a flue gas NOx

removing system, consisting of ammonia injection into the flue

gas at some 800oC

Power plants

Definitions IV