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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