Combustion Efficiency

7/28/2019 Combustion Efficiency

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Combustion ProcessHistorical Introduction

Fire exists from the earliest existence of man on earth

Until 1880, man did not achieve a quantitativeunderstanding of the combustion process

In 1697, G.E.Stahl proposed Phlogiston theory-Phlogiston was a hypothetical mysterious substancewhich combined with a body to render it combustible

In 1774, Joseph Priestly discovered the unique power

of oxygen for supporting combustion In 1781, Henry Cavendish demonstrated the compound

nature of water


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Combustion Process

Historical Introduction

About the same time, Lavoisier made the precise measurementsand formulated the volume and weight relationship that underline

the modern theory of combustion

In 1811, Amendeo Avagadro established that the number of

molecules in a unit volume under standard conditions is same forall gases

During the same period, John Dalton enumerated the law of

partial pressures

In 1803, John Daltons study of the physical properties of gasesled to formulation of atomic theory including the law of

combining weight

In 1808, Gay Lussac observed that gases always combine in

volumes that bear simple ratios to each other


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Combustion Efficiency

Combustion efficiencyeffectiveness ofcombustion equipment to convert the internalenergy in fuel to heat energy for use by the process

Any heat loss lowers the efficiency of the process Combustion efficiency = Total energy contained

per unit of fuel - losses (radiation, unburned andflue gas)

Continuous monitoring of Oxygen andcombustibles (CO or H2)-the best way to improvecombustion efficiency


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Combustion Theory

Three essential components of combustionfuel,oxygen and heat

Chemical elements that react with oxygen torelease heat are Carbon and Hydrogen commonly

known as hydro carbons C + O2 CO2 + 14093 btu/lb (stoichiometric air

150 ft3 of air / lb of fuel)

H2 + O2 H2O + 61000 btu/lb (stoichiometric

air 2.38 ft3 per ft3 of fuel) Stoichiometric combustion just right amount of

oxygen and fuel mixture (without any excess)


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Why air instead pure oxygen?

Air contains 21% by volume or 23% by

weight of Oxygen and is readily available

Pure oxygen needs processing, the cost of

which outweighs the benefit on combustion

and heat release


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Why excess air?

Inadequate mixing of air and fuel, fluctuating

operating and ambient conditions, burner

performance and wear and tear To ensure that fuel is burned completely or with

little combustibles, some amount of excess air is

provided

Normal excess air

Gas 5%, Oil 10%, coal 20%


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Excess Air Solid fuels require the greatest and the gaseous

fuels require the least quantity of excess air At design load,

Gas

Natural gas 5 to 10%Refinery gas 8 to 15%

Blast furnace gas15 to 25%

Coke oven gas 5 to 10%

Solids

Pulverised coal15 to 30%

Coke 20 to 40%

Wood 25 to 50%

Bagasse 25 to 45%

Liquids

Oil 3 to 15%


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Negative aspects of high excess air

Increase in auxiliary power (FD & ID fan) Increase in furnace temperature and NOx

formation

Increase in loss of sensible heat carried away by

flue gas

Increase in erosion due to increase in flue gasvelocity

Limitation on boiler load due to exhaustion of IDfan capacity

Shift in heat transfer from furnace to convectionpass resulting in heating up of down stream

components


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Impact of lesser air than

stoichiometric requirement Incomplete combustion leading to

Reduction in energy release

Increase in unburned hydro carbons (Co &

CnHm) in flue gas

Increase in unburned carbon level in fly and

bottom ashesSlagging in boiler furnaces


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Reference curves for Optimum % Oxygen at

Economiser outlet for minimum heat rate


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Curve to estimate % excess air based % Oxygen


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Basis for controlling excess air

By monitoring oxygen and combustibles in

flue gas at Eco outlet by installing on-line

analysers

Monitoring unburned carbon level in fly

and bottom ashes


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Method of evaluating air leakage in furnace


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b i ffi i


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C b i Effi i


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C b i Effi i


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F ti l R i t f


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Functional Requirement for

combustion Equipment

Easy ignition and reliable flame scanning Maximum Heat release (at desired rate)

Optimum turn down

Efficient combustion (Minimum un-burned)

Optimum temperature

Minimum Excess air

Minimum emission

Minimum slag formation Desired flame shape

Heat release profile matching furnace heat absorptionneed


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Fuel aspects

Organic aspectsPetrography

Heat release rate

Inorganic aspectsCCSEM, Ash formation, TMA,

Physical aspects

Grindabity, Specific gravity, particle sizedistribution


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Heat Value of Fuels

High Heat Value (HHV)

= 8080C+34500(H2-(O2/8))+2220S Kcal/Kg (Dulongs

Formula)

Where C,H2,O2 and S represent weight in kg of Carbon,

Hydrogen, Oxygen and Sulphur per Kg of fuel

Low Heat Value (LHV)

=HHV-Latent heat of steam formed

The amount of steam formed during combustion=9H2

where H2 is the weight of Hydrogen per kg of fuel Latent heat of 1 Kg of steam at 760 mmHg and 100

DegC is 538.9 Kcals/Kg


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Coal

Coals having FC / VM ratio closer to 1 will have

better flame stability

VM less than 13% is not preferable for PC firing Residence time

110 Mw 1.75 sec, 210 Mw-2.2 sec, 500 Mw-3.5 sec

Crossing point temperature -175 to 250 Deg.C

Flammability temperature400 to 600 Deg.C


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Coal Quality Vs Air

regimes

Low ash, high volatile, high moisture, high

CV (imported coal)Flame propagation affected more by moisturethan by ash

Priority for drying coal. Hence PA can not be

reduced below a particular levelNecessary to incorporate split coal nozzle or

diverters


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Coal Quality Vs Air

regimes

Low moisture, low volatile, high ash

Flame propagation affected by high ashReduce primary air to minimum extent possible

Increase OFA


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Arrangement of Coal and

Air Ports in the Wind Box

of a Typical TangentialFired 500 MW Boiler

Furnace


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Primary Air

P.A / Coal ratio 1.5 to 2.5 (2 for better combustion

efficiency)-lower the P.A better the flame stability

P.A normally 1/4th (20-25%) of total air

Variable P.A control gives better scope to improve burner

performance

Primary air velocity 25 m/sec (to be > 20 m/sec to avoid

settling in coal pipe. To be > 15 m/sec to avoid flash back)

Minimum P.A temperature 57 Deg. C to avoid condensation

Maximum P.A temperature tested 127 Deg.C to avoid millfire and softening & sticking of coal in coal pipe

Normal P.A mix temperature is around 80 Deg.C


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Functions Of Primary Air

To dry the moisture in coal and facilitate

better grinding in the mill

Transport the pulverised coal from the mill

to the furnace at a velocity higher thansettling velocity of pulverised particle and

that of flash back


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Functions Of Fuel Air

Helps to position the flame front (Not too

away with potential for blow off-Not too

close with potential for heating & distorting

nozzle)Considerations

Good Flame Stability

View For Flame Scanner Protection of Nozzle From Distortion


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Impact of Fuel Air

increasing with Feeder Speed

Primary stream need not be uniform in all

the four corners Fuel air increase may further degenerate

flame where PA/Coal ratio is already high


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Fuel Air Vs Feeder Speed

Fuel Air can not be increased with feeder speed

With increase in feeder speed, the primary airwould increase

Since Indian coals have more non combustibles (50%

compared to 10-20% in North American coals). Muchmore primary air is required than required for volatilecombustion

Addition of fuel air can affect the flame stability and

unburned carbon level

Feeder speed will increase if fuel CV goes down as well asboiler load increases


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Secondary Air

75 to 80% of total air distributed at different tiers

Secondary air velocity ~40 m/sec for bettermomentum and mixing

Secondary air temperature ~ 227 deg.c

Air distribution in tiers decide combustionefficiency

Fuel air is provided for the twin purposes ofcooling nozzles and for positioning flame front-

close the damper if flame front is away and open ifflame front is close to nozzle

Other damper openings to be adjusted dependingon the operating tiers


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Functions Of Auxiliary Air

Ensuring completion of combustion

Enough momentum to penetrate intoprimary stream (expanded flame jet

containing the char of the coal particles) andprovide air to the whole cross section ofchar to be burnt

Stage the air for gradual mixing to reduceNOx


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0

0

0

0.5 to 0.8mmR

Aux Air

Fuel Air

Aux Air

PA + Coal

Temperature

Flame Front

Typical Mixing and Flame Front In Corner Firing


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Effect of changing

auxiliary air

Auxiliary air quantum should not be below

the level where the momentum is notadequate to penetrate the primary jet flame

This happens if PA is high and FA is also high

and the total air is limited to 3.5% O2

(Under such conditions the wind box pressure


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Functions Of Over Fire Air

Primarily indented to reduce unburned

carbon in fuel burnt in the top elevations

Since the air in the bottom elevations are

proportionately reduced it also reducesThermal NOx formation in the lower

elevations


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Effect of Over Fire Air

OFA intended for reducing unburned and

NOx By closing OFA combustion completion of

lower elevations is advanced, FOT comes

down, Unburned may go up


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Need for biasing / extent

of biasing AuxAir Biasing is

subjectiveFuel, Mill System, Boiler size

ObjectiveUnburned carbon reduction, Lower/higher

FOT, Reduction of spray, Higher SH steam temperature

Eg. IFFCO/PHULPHUR VU40

Biasing towards bottom reduced unburned levels in

bottom ash NTPC, Ramagundam 500 Mw

Biasing towards top reduced SH Spray


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IMPROVING THE EFFICIENCY OF THE

EXISTING COAL BASED POWER PLANTS

Boiler Operational Improvements

Tuning Combustion Air Regimes

Prevention of air leakage PA/Coal ratio around 2

Flame front 300 to 500 mm away from nozzle tip

Fuel air at minimum opening

Excess air to reduce carbon loss and slagging

Minimize OFA if furnace is slagging


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Combustion Efficiency