combustion & fuels

8/11/2019 Combustion & Fuels

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COMBUSTION

&

FUELS


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PROCESS FLOW IN

THERMAL POWER PLANTS

ELECTRICAL ENERGY

MECHANICAL ENERGY

HEAT ENERGY

CHEMICAL ENERGY IN FUEL


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PROCESS FLOW IN

THERMAL POWER PLANTS

CHEMICAL ENERGY IN FUEL

HEAT ENERGY

MECHANICAL ENERGY

ELECTRICAL ENERGY

COMBUSTION PROCESS

HEAT TRANSFER

TO WORKING

FLUID

BOILER

TURBINE

GENERATOR


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COMBUSTION

Rapid chemical combination of oxygenwith the combustible elements of thefuel in the process of which heat is

evolved with light combustion. With reference to the furnace it is also

defined as a series of continuous,controlled explosion of fuel particles

with oxygen in the air causing evolutionof heat with light and formation ofproduct gases.


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COMBUSTION

IgnitionEnergy

REACTANTS PRODUCTS + HEAT


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REACTANTS

COMBUSTIBLES IN FUEL

Carbon,Hydrogen,Sulphur

OXYGEN

IGNITION ENERGY

HEAT

COMBUSTION


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MAJOR SEGMENTS OF COMBUSTIONFOR CONSIDERATION

Combustion Efficiency as indicated byflame stability and complete carbonburn out

Slagging and fouling properties of Ash

Potential for metal Corrosion andErosion characteristics of Fly Ash in

Gas streamAir Pollution control requirements of

the combustion product effluentGases (NOx,SOx)


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FACTORS AFFECTINGPERFORMANCE OF COMBUSTION

SURFACE CONTACT AREA OF FUEL WITHAIR

AIR-FUEL RATIO RETENTION TIME

COMBUSTION CHAMBER TEMPERATURE

TURBULANCE IN COMBUSTIONCHAMBER

REMOVAL OF PRODUCTS OFCOMBUSTION


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SURFACE CONTACT AREA OFFUEL WITH AIR

Larger the particle sizelesser the molecularcontact

Inner core of moleculeswill be blanketed by ash

To improve molecularcontact reduce size

Solid Fuel Pulverisation

Liquid Fuel

Atomisation


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AIR-FUEL RATIO

The quantity of air supplied mustbe sufficient enough to provide thenecessary oxygen to thecombustibles of the fuel to getthem completed oxidized so thatall the chemical energy is

completely converted to heatenergy.


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AIR-FUEL RATIO

INSUFFICIENT AIR

Chemical Under burning

C + 02 CO2 + Heat (33820 kJ/Kg)

2C + 02 2CO + Heat (10200 kJ/Kg)

Mechanical Under burningCarbon in Ash


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RETENTION TIME

Combustion reaction needssufficient time to complete, duringwhich ignition energy must be

available to the fuel-air mixture.As the ignition energy generallywill be available only in thefurnace, sufficient retention timemust be provided to the fuel-airmixture in the furnace to completethe combustion reaction


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COMBUSTION

TIME REQUIRED/AVAILABLEDEPENDS

FUEL TYPE

QUALITY

SIZE

FURNACE SIZE

VELOCITY

DRAUGHT

RETENTION TIME


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TEMPERATURE

EFFECTS THERMAL DIFFUSION OF

REACTING MOLECULES DUE TOINCREASED VELOCITY OF MOLECULESWITH INCREASE IN TEMPERATURE

INFLUENCE THE RATE OF REACTION FACTORS AFFECTING TEMPERATURE

Heat absorbed by furnace

Heat absorbed by reactants to bringthem to ignition temperature

Heat absorbed by nitrogen in air


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TURBULENCE

Mechanical agitation of reactants tobring them into physical contact

Requirement is more at final stage of

combustion Lesser the turbulence more

mechanical under burning

Achieved By Tangential Firing

Supply of tertiary air in wall fired boilers


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Opportunity ForContact Between

InteractingMolecules

Are related toConcentration

andDistribution ofReactants in agiven Volume

TURBULENCE


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PREPARATION OF THE REACTANTSAND MECHANICAL TURBULANCE

Influence the reaction rate

Agitation permits greater opportunityfor molecular contact

Agitation improves both the relativedistribution and energy imparted.

Agitation assumes greater significance

if the relative concentration of thereactants is approaching zero.

Preparation and mechanical turbulanceare the main factors for the reaction

rate.


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GENERATION OF FLAMETURBULANCE

Thermal movement of molecules as aconsequence of high temperature flame.

Turbulence produced artificially.Turbulence require much energy as the

viscosity of hot gases have attenuatingeffect

Generally achieved by injectingcombustion air into the flame with highvelocity.


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FUELS

Combustibles

Carbon

Hydrogen

Sulphur

Non-combustibles

Moisture

Ash


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FUELS

SOLID

COAL

Is a mixture of ORGANIC CHEMICALandMINERAL matters produced by a naturalprocess of GROWTH and DECAY,ACCUMULATION of DEBRIS bothVEGETAL and MINERAL with someSORTING and STRATIFICATION andaccomplished by CHEMICAL, BIOLOGICAL

and METAMORPHICaction The ORGANIC Chemical materials

produce heat when burned: the MINERALmatterremains as residue called ASH


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FUELS

SOLID COAL

Anthracite

Bituminous Sub bituminous

Lignite


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FUELS

SOLID BIOMASS

Bagasse

Peanut shell

Paddy husk Coffee bean

Wood chips

Barks

Wood

Pet coke

Municipal refuse

Fused tires


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FUELS

Liquid Light Diesel Oil (LDO)

Furnace Oil (HFO)

Low Sulphur Heavy Stock(LSHS)

Refinary Process wastes

Black liquor


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FUELS

Gas Producer gas

Blast Furnace gas

Corex gas

Natural gas

LPG

Coal gas


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COAL

High Ranking Coal Classification based on Carbon content in

CoalMeta Anthracite - 98% Carbon

Anthracite - 92% Carbon

Semi Anthracite - 86% Carbon

Low Volatile Bituminous - 78% Carbon

Medium volatile Bituminous -69% Carbon


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COAL

Low Ranking Coal Classification based on the heat content

High Volatile BituminousSub Bituminous

Lignite


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FUEL OIL

DistillateLight Diesel oil

Residual Oil

Heavy Fuel Oil (Furnace oil)

Low Sulphur Heavy stock (LSHS)


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FUEL ANALYSIS

PROXIMATE ANALYSIS Moisture

Surface

InherentVolatile matter

Ash

Fixed Carbon


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FUEL ANALYSIS

ULTIMATE ANALYSIS Carbon

Hydrogen

Sulphur Nitrogen

Ash

Oxygen


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FUEL ANALYSIS

MOI V.M ASH F.C.

SINGRAULI 16.0 21.6 30.0 32.4

RAMAGUNDAM 10.0 21.4

6

32.0 36.54

NEYVELI 53.3 24.3 3.3 19.1

SURAT 24.0 32.52

19.0 24.46

IMPORTED 8.3 24.7 13.6 53.3

PROXIMATE


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HEATING VALUE

Number of heatunits liberatedwhen an unitmass of fuel is

burnt at constantvolume in oxygensaturated withwater vapour, the

original and finalproducts beingkept at constanttemperature


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HEATING VALUE

Sl.No ELEMENT HEAT

CONTENT

KJ/Kg

1) Carbon 33820

2) Hydrogen 1,44,000

3) Sulphur 9304


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HEATING VALUE

HIGHER HEATING VALUE

= 33820C + 144000(H-O/8) + 9304SkJ/Kg

LOWER HEATING VALUE

= HHV - 2442 (M+9H) kJ/Kg


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STOCHIOMETRIC AIR

Air required to burn a fuel as perthe Stochiometric equations.

Oxygen required

= 8/3C + 8(H-O/8) + S

Stochiometric Air required

= 100/23 [8/3C + 8(H-O/8) + S]Kg/Kg fuel


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STOCHIOMETRIC AIR

Substance St.Reqt./gm Product gms/gms

O2 Air CO2 H2O N2 CO

Carbon 2.67 11.49 3.67 8.82

Carbon 1.33 5.75 4.42 2.3Hydrogen 8.0 34.48 9.0 26.48

Air Requirement Per One Million Kcal. Heat

Input Coal 1360 Kg

Oil 1325 Kg

Gas 1300 Kg


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AIR SUPPLIED IN ADDITION TOSTOCHIOMETRIC AIR FORCOMPLETE COMBUSTION OF FUEL

OPTIMUM EXCESS AIR DEPENDSON

FUEL QUALITY

FIRING SYSTEM DESIGN

EXCESS AIR

EXCESS AIR


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EXCESS AIR MORE THAN OPTIMUM

INCREASES WDANDSO DRY GAS LOSS

LESS THAN OPTIMUMINCREASES CARBONLOSS

OPTIMUM EXCESS AIRIS DETERMINEDTHROUGH FIELDTESTS

OPTIMUM EXCESS AIRCAN BE MAINTAINEDTHROUGH F.G.ANALYSIS


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PROCESS PRECEEDING COMBUSTION

LIQUID

First converted into gaseous state Ready for combustion only when they have

been mixed with 02carrier and heated toignition temperature.

EVAPORATION - SLOWEST LINK Depends on volatility and molecular

composition of oil.

Different hydrocarbons vaporise at different

temperature.


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Due to more rapid evaporation of lighter H2containing fractions molecules aregenerally enriched with carbon leading tosoot formation.

TO AVOID SOOT

Droplets of atomised oil must be brought tohigh temperature as quickly as possible sothat they can burn immediately.

PROCESS PRECEEDING COMBUSTION


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LIQUID FUELS

TO ACCELERATE EVAPORATION

Needs burner which gives fineatomisation and adequate enlargement

of surface area. of fuel (30 to 200micron)

COMBUSTION EFFICIENCY IS GREATLYINFLUENCED BY IGNITION AND

BURNER FLOW PATTERN.

COAL COMBUSTION


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COALCOMBUSTION

HETROGENEOUS SURFACE REACTION

DEVOLATALISATIONP.F. in flame jet is heated by convection as it

entrains and mixes with hot gases

Also heated by radiation.

On heating above 500OC coal starts todecompose and evolves a mixture ofcombustible and non-combustible gases.

Surface area dictates the rate of gasification.

At temp. above 900OC most of the volatiles areevolved.


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When given adequate air this volatilematter mix into the jet and itscombution sustain the ignition of flame

The remaining char residue then burnsslowly in the flame and furnace.

COAL COMBUSTION

COAL COMBUSTION


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THERMAL DECOMPOSITION OF COAL

Coal particles are porous contrast to

homogeneous oil droplets which are subject tosurface tension.

Gasifying medium penetrate into pores andreact with the combustible substance

Internal pore walls are also included in thereactive surface.

These pores widen on heating and as volatilesleave, enlarge the cavities in the particles

Also explode under internal pressure anddisintegrate into several fragments.

COAL COMBUSTION


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STEPS OCCURING

On introduction into furnace p.f. isdried, devolatalised and ignited.

Volatile matter are momentarily trappedinside.

Gases diffuse both away from coalparticle and into porous mass of coal.

COAL COMBUSTION


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REQUIREMENTS

Flame temperature should be aboveignition temperature

To support combustion flame

temperature should not fall below thelimiting value

High temperature allow high burnoutrate of coal thereby avoid the need ofunduly large furnace

Upper limit on high temperature toprevent volatalisation of ash

COAL COMBUSTION

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