3. Fuels & Combustion

8/13/2019 3. Fuels & Combustion

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INDUSTRIAL ENERGY EFFICIENCY

PROJECTTRAINING

Module: Fuels & Combustion


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Industrial Energy Efficiency Project 2

Fuels for Industrial applications

Fuels for

Industrialapplication

s

Liquid fuels like Furnace Oil, IDO, etc

Solid fuels like Coal, Bio mass, etc

Gaseous fuels like Natural gas, LPG, etc.


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Selection of Fuels

Criteria for

Selection of

fuels for

Industrial

applications

Availability

Storage & Handling

Pollution

Cost of Fuel

Fuel properties


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Properties of Fuels

Density

Ratio of mass of fuel to volume of fuel at a reference

temperature (Typically 150

C)Unit of Density: Kg/m3

Specific

Gravity

Density of the fuel relative to that of water.

Unit of Specific gravity: Ratio & hence no units

Examples:

Specific gravity of IDO: 0.85-0.87

Specific gravity of Furnace Oil: 0.89-0.95

Viscosity

A measure of internal resistance to flow.

Measured in terms of Stokes/Centistokes, Engler, Saybolt

or Redwood seconds

Temperature

Viscosity

Influences the degree of pre-heat required for handling,

storage and satisfactory atomisation


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Properties of Fuels

Flash Point

The lowest temperature at which the fuel can be heated

so that the vapour gives off flashes momentarily when an

a open flame is passed over it. Ex. Flash point for furnaceoil is 660c

Pour

Point

Specific

Heat

The lowest temp. at which it will pour or flow when

cooled under prescribed conditions. It is a very rough

indication of the lowest temp. at which fuel oil is readily

pumpable.

It is the amount of kcalsneeded to raise the temperature

of 1 kgof oil by 1oC.

The unit of specific heat is kcal/kgoC.

Varies from 0.22 to 0.28 depending on oil specificgravity.

It helps to quantify how much steam or electrical energy

required for preheating.


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Properties of Fuels

CalorificValue

Calorific value is the measurement of heat or energy

produced and is measured either as gross calorific value

or net calorific value.

Carbon

Hydrogen

Sulphur

Moisture

GCV 10,500 Kcal/kg

Water Vapour

Water Vapour

NCV 9800 Kcal/kg

Water

vapour

The difference being the latent heat of condensation of

the water vapour produced during the combustion

process.

Fuel GCV, Kcals/Kg

Kerosene 11,100

Diesel Oil 10,800

IDO 10,700

Furnace Oil 10,500

Low Sulphur Heavy Stock 10,600

Indian Coal 3,500-6000

Typical Gross Calorific values of some of

commenly used fuels


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Properties of Fuels

SulphurContent

Depends mainly on the source of the crude oil and to a

lesser extent on the refining process. The normal sulphur

content for residual fuel oil (heavy fuel oil) is in the orderof 2-4 %.

Risk of Corrosion

Cold end corrosion in cool parts of the chimney or stack,

air pre-heater and economiser.

Water

vapour

CarbonHydrogen

Sulphur

Moisture

Water Vapour

Water Vapour

SO2

SO2

SO2

SO3

SO3

SO3

SO3

H2SO4

H2SO

4

Fuel % Sulphur

Kerosene 0.05-0.2

Diesel Oil 0.3-1.5

IDO 0.5-1.8

Furnace Oil 2.0-4.0Low Sulphur Heavy Stock



Properties of Fuels

Ash

Content

Ash content depends on the inorganic material in the fuel

oil.

These salts may be compounds of sodium, vanadium,

calcium, magnesium, silicon, iron, aluminum, etc.

Typically ash values are in the range of 0.03 0.07 %.

Excessive ash values in liquid fuels can cause fouling

deposits in the combustion equipment. Ash has erosive

effect on the burner tips, causes damage to therefractories at high temperatures and give rise to high

temperature corrosion and fouling of equipments.

Carbon

Residue

Indicate the tendency of oil to deposit a carbonaceous

solid residue on a hot surface, such as a burner or

injection nozzle, when its vapourisable constituents

evapourate. Residual oil contain carbon residue rangingfrom 1% or more

Distillate

Residue



Properties of Fuels

Water

Content

Water contents of furnace oil when supplied is normally

very low as the product at refinery site is handled hot and

maximum limit of 1% is specified in the standard.Water may be present in free or emulsified form

Water can cause damage to the inside of furnace surfaces

during combustion especially if it contains dissolved salts

It can also cause spluttering of the flame at the burner

tip, possibly extinguishing the flame and reducing theflame temperature or lengthening the flame

Typical

specifications of

fuel

IDO FO125 FO180 FO280

Density, Kg/L at 15 Deg. C 0.915 0.985 0.985 0.985

Flash Point, Deg C 66 66 66 66

Pour Point, Deg C 12 12-15 21-24 21-24

GCV, Kcals/Kg 10718 10287 10287 10287

Sediment, % Wt. Max 0.02 0.15 0.15 0.15

Sulphur, % Wt. Max 1.8 3.5 4 4

Water content, % Vol. Max 0.25 0.75 0.75 0.75

Ash, % Wt. Max 0.02

Carbon Residue, % Wt. Max 0.45

Typical Specifications of Fuel Oils



Hazardou

sTo store furnace oil in barrels.

Storage of Fuel Oil

Stored in Cylindrical tanks - either above or below the ground.

Cleaning

of TanksAnnually for heavy fuels & every two years for light fuels.

Leaks From joints, flanges and pipelines must be attended at theearliest.

LOSS OF EVEN ONE DROP OF OIL EVERY

SECOND CAN COST OVER 4000 LITERS AN

YEAR. Approximately 80,000 KSh/Year



Coarse

Strainer

To prevent contaminants such as cotton waste, loose nuts

or bolts entering coarse strainer of 10 mesh size (not

more than 3 holes per linear inch) is positioned on the

entry pipe to the storage tanks.

Finer

Strainers

To prevent finer contaminants such as dust and dirt,

sludge or free carbon, filters are provided in duplicate to

enable one filter to be cleaned while oil supply is

maintained through the other

Fuel Oil should be free from contaminants

such as dirt, sludge and water before it is fedto the combustion system

Removal of Contaminants


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Heavy

Fuel Oils

Best pumped using positive displacement pumps

Pumping fuel Oil

IDO

Circulation gear pumps are suggested (7000-10000 Hours

service)

Diaphragm pumps have a shorter life, but are easier and

less expensive to repair

Centrifug

al pumps

Generally not recommended for heavy fuels, because as

the oil viscosity increases, the efficiency of the pumpdrops sharply and the horse power required increases

Light

fuelsBest pumped with centrifugal or Turbine pumps

Highpressure

applications

When higher pressures are required, piston or diaphragm

should be used


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Heating of Oil for Pumping

Preheating of Oil in the storage tank is

necessary to make it pumpable

Entire tank may be

preheatedIn this form of bulk heating,

steam coils are placed at the

bottom of the tankAdvisable to insulate the

tank where bulk heating is

usedBulk heating is necessary, if

flow rates are very high.

Oil may be heated as it flows

out with an outflow heater

Outflow heater is essentially

a heat exchanger with steam

or electricity as the heating

medium


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Agro Residues & their Properties

Item

DeOiled

Bran

Paddy

Husk

Saw

Dust

Coconut

Shell

Moisture, % 7.11 10.79 37.98 13.95

Mineral Matter, % 19.77 16.73 1.63 3.52Carbon 36.59 33.95 48.55 44.95

Hydrogen, % 4.15 5.01 6.99 4.99

Nitrogen, % 0.82 0.91 0.8 0.56

Sulphur, % 0.54 0.09 0.1 0.08Oxygen, % 31.02 32.52 41.93 31.94

GCV, Kcals/Kg 3151 3568 4801 4565

Composition of some Agro Residues


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COMBUSTION

A chemical reaction during which a fuel is

oxidised and a large quantity of energy isreleased.

High speed, high temperature chemical

reaction.

Rapid union of an element or compound with

oxygen to liberate heat controlled explosion.

Combustion occurs when elements of fuel such

as carbon and hydrogen combine with oxygen.


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Chemical reaction in Combustion

Stoichiom

etric or

Theoretica

l air

Ideal amount of air required for

burning I Kg of Fuel

Eg: 1 Kg of Fuel Oil requires 14.1 Kg of air

for complete combustion


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of Combustion3Ts

3Ts

TimeAll combustion requiressufficient time which depends

upon type of reaction.

Temperatur

e

Temperature must be more

than ignition temperature

Turbulence

Proper turbulence helps inbringing the fuel and air in

intimate contact and gives

them enough time to

complete reaction


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Types of Combustion

Perfect

Combustio

n

Achieved when all the fuel is burned using only thetheoretical amount of air, but perfect combustion

cannot be achieved in practice.

Good/

Complete

Combustio

n

Achieved when all the fuel is burned using the minimal

amount of air above the theoretical amount of air

needed to burn the fuel.Complete combustion is always our goal.

With complete combustion, the fuel is burned at the

highest combustion efficiency with low pollution and

energy loss.

Incomplete

Combustio

n

Occurs when all the fuel is not burned, which results in

the formation of soot and smoke.


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Combustion of Fuel Oil

Viscosity of 100 Redwood secs I at burners

Atomising air 1

3 kg/cm2(about 2% of total airrequirement)14 kg of air/kg fuel is required for complete combustion.

Optimum efficiency with 10% excess air)

Flue gas should be analysed for CO2or O2

Sulphur dew point at 160 0C. Corrosion max. at

300C below dew point.Slightest damage to burner tip may increase fuel

consumption by 10 15% and hence worn out tips

should be replaced immediately.

Oil pressure at burner should be 17 20 kg/cm2

Correct flame is normally short. Impingement on

wall, tubes cause carbon formationToo short a flame indicates excess air and air supply to

burners should be adjusted for light haze brown out of

chimney


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Burners

Burners

Convert fuel oil into millions of small droplets process

called atomisation

Surface to

Volume

Ratio

High surface to volume ratio in oil to facilitate evaporation

and combustion

Basic type

of Burners

3 basic types of burners are pressure jet, air or steam blast

burners and rotary cup.

Turn DownRatio TURNDOWN ratio is the relationship between the max. andmin. fuel input without affecting the excess air level.

For example, a burner whose max. input is

250, 000 Kcals and min. rate 50, 000 Kcals,

has a Turn-down Ratio of 5 to 1


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Pressure Jet Burner

Simple, inexpensive and widely

used.

Oil pumped at pressure through

a nozzle

Good efficiencies at lower loads.

Low turndown ratio of 2 : 1

High oil pre-heat required for

atomisation.

Prone to clogging due to dirt in

oil requires fine filtration.


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Spray at 10 psi pressureSpray at 100-psi pressure

Spray at 300-psi pressure

Pressure Jet Burner


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Air or Steam Blast Burner

High Turndown ratio of 4 :1.

Good control of combustion

over wide range

Good combustion of heavier fuel

oil

Additional Energy required as

steam or compressed air for

atomisation


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Burner Controls

ON-OFF Burner firing at either full rate or OFF

High/Low/

Off

Burner operates at slow firing rate and full firing

rate as per load

Modulating

TypeFiring rate matches the boiler load.


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Combustion Inefficiencies

Fuel

N2

O2

N2

CO2

Soot

Unburnt Fuel

CO

H2H2O

+ =

Deficiency of Air

Air

Flue gas


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Fuel

N2

Excess O2

N2

CO2

Excess O2

H2O

+ =

Too much of Air

Air

Flue gas

O2


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Fuel

N2

O2

N2

CO2

H2O

+ =

Stoichiometric Air

Air Flue


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DecreaseIncrease

Operating in this

Zone results inwasted fuel

Zone of max.

CombustionEfficiency

Operating in this

Zone results inExcess heat loss up

the stack

Unburnt Fuel

LossExcess Air

Loss


Air AirAir


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CO2& Excess Air

0

10

20

30

40

50

60

70

80

90

100

8.4 9 10 11 12 13 14

Carbon dioxide %

Exces

sair%


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Residual O2& Excess Air

Relation between residual oxygen and excess air

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Oxygen (%)

Excessa

ir(%)


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Effect of Excess air on CO2

Carbon di oxide in flue gas (%) when excess air is (%)

Fuel 0 10 20 40 100

Natural gas 12.0 10.7 9.8 8.3 5.7

Distillate oil 15.2 13.8 12.5 10.7 7.4

Residual oil 15.8 14.1 12.9 11.0 7.6

Anthracite

coal

19.8 18.0 16.5 14.1 10.0


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I d t i l E Effi i P j t 32

DraftDraft

Function of draft is to exhaust the products of combustion into

the atmosphere.

Natural Draft : It is the draft

produced by a chimney alone. It

is caused by the difference in

weight between the column of

hot gas inside the chimney andcolumn of outside air of the

same height and cross section

Mechanical Draft: It is the draft

artificially produced by fans.

(Three basic types)

Balanced Draft:Forced-draft

fan pushes air into the furnaceand an induced-draft fan draws

gases into the chimney thereby

providing draft to remove the

gases from the boiler. (0.05 to

0.10 inch of water gauge below

atmospheric pressure)

Induced Draft:Fan provides

enough draft for flow into thefurnace, causing the products

of combustion to discharge to

atmosphere. Furnace is kept

at a slight negative pressure

below the atmospheric

pressure

Forced Draft: Thissystem uses a fan to

deliver the air to the

furnace, forcing

combustion products to

flow through the unit

and up the stack

Documents

3. Fuels & Combustion