Fuels &combustion part1

1. 1. Fuels and Combustion
Dr.M.Vivekanandan M.E.,PhD
Adjunct Faculty, Kongunadu College of Engineering
and Technology,
CEO, TryCAE Industrial Engineering Pvt Ltd.,

2. Syllabus
• Introduction to Fuels
• Properties of Fuel oil, Coal and Gas
• Storage, handling and preparation of fuels
• Principle of Combustion
• Combustion of Oil, Coal, and Gas

3. Introduction to Fuels
• Different type of fuels such as liquid, solid and
gas are available for firing in boilers and
furnaces.
• What is the criteria for selecting the fuel type?
– Availability
– Storage & handling
– Pollution
– Cost of fuel
– Fuel properties

4. Properties of Liquid Fuels and its impacts
• Density: mass of the fuel
volume of the fuel
At a reference temperature typically 15°C. (kg/m3.)
• Specific gravity: weight of a given volume of oil
weight of the same volume of water
at a given temperature.
– If specific gravity is more, heating value is also more.
e.g Light Oil =0.85-0.87, Furnace oil=0.89-0.95, L.S.H.S=0.88-0.98

5. Viscosity
• Internal resistance to flow of fluid
• Viscosity influences the degree of pre-heat required
for handling, storage and satisfactory atomization.

6. Specific Heat
• Specific heat is the amount of kcals needed
to raise the temperature of 1 kg of oil by 1oC.
The unit of specific heat is kcal/kgoC. It
varies from 0.22 to 0.28 depending on the oil
specific gravity.
It helps to quantify how much steam or
electrical energy required for preheating.

7. Flash Point & Pour Point
• Flash Point: The lowest temperature at which the
fuel can be heated so that the vapour gives off
flashes momentarily when an open flame is passed
over it. Ex.Flash point for furnace oil is 66oC.
• Pour Point: The lowest temperature at which it
will pour or flow when cooled under prescribed
conditions. It is a very rough indication of the
lowest temperature at which fuel oil is readily
pumpable.

8. Calorific Value
• The calorific value is the measurement of heat or energy
produced, and is measured either as gross calorific value or net
calorific value.
• The difference being the latent heat of condensation of the
water vapour produced during the combustion process.
Water
vapour
Carbon
Hydrogen
Sulphur
Moisture
GCV – 10,500 Kcal/kg
Water Vapour
Water Vapour
NCV – 9800 Kcal/kg

9. Typical calorific values of fuels
The calorific value of coal varies considerably depending on the ash, moisture
content and the type of coal while calorific value of fuel oils are much more
consistent.
Fuel Oil Calorific Value (Kcal/Kg)
Kerosene - 11,100
Diesel Oil - 10,800
L.D.O - 10,700
Furnace Oil - 10,500
LSHS - 10,600
Indian coal - 4000 to 6000

10. Sulphur Content
Depends mainly on the source of the crude oil and to a lesser
extent on the refining process. The normal sulfur content for
the residual fuel oil (heavy fuel oil) is in the order of 2-4 %.
Fuel oil Percentage of Sulphur
Kerosene 0.05—0.2
Diesel Oil 0.3 – 1.5
L.D.O 0.5 – 1.8
Heavy Fuel Oil 2.0 – 4.0
LSHS < 0.5

11. Disadvantage of sulphur
risk of corrosion
Cold end corrosion in cool parts of the chimney or
stack, air pre heater and economiser.
Sulphur SO2
SO3
H2SO4
H2O

12. 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, nickel, etc.
• Typically ash values are in the range 0.03-0.07 %.
• Excessive ash in liquid fuels can cause fouling
deposits in the combustion equipment. Ash has
erosive effect on the burner tips, causes damage to
the refractories at high temperatures and gives rise to
high temperature corrosion and fouling of
equipments.

13. Carbon Residue
Carbon residue indicates the tendency of oil to
deposit a carbonaceous solid residue on a hot
surface, such as a burner or injection nozzle, when
its vaporisable constituents evaporate. Residual oil
contain carbon residue ranging from 1 % or more.
.
Distillate
Residue

14. Water Content
• Water content 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 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 the flame temperature or lengthening the
flame.

15. Important properties of fuel oil
Typical Specification of Fuel Oils
Properties Fuel Oils
Furnace
Oil
LS.H.S. L.D.O.
Density (Approx. kg/m3
at 150
C) 0.89-0.95 0.88-0.98 0.85-0.87
Flash Point (0
C) 66 93 66
Pour Point (0
C) 20 72 18
G.C.V. (Kcal/kg) 10,500 10,600 10,700
Sediment, % Wt. Max. 0.25 0.25 0.1
Sulphur Total, % Wt. Max. Upto 4.0 Upto 0.5 Upto 1.8
Water Content, % Vol. Max. 1.0 1.0 0.25
Ash % Wt. Max. 0.1 0.1 0.02

16. Storage of Fuel oil
• Hazardous to store furnace oil in barrels.
• Stored in cylindrical tanks
– either above or below the ground.
• Storage capacity
– - at least 10 days of normal consumption.
• Build bund walls around tanks.
• Periodical cleaning of tanks
– annually for heavy fuels and every two years for light fuels.
• leaks
– from joints, flanges and pipelines must be attended at the earliest.
– LOSS OF EVEN ONE DROP OF OIL EVERY SECOND CAN
COST YOU OVER 4000 LITRES A YEAR
• Fuel oil
– should be free from contaminants such as dirt, sludge and water before
it is fed to the combustion system.

17. Removal of Contaminants
• Coarse strainer :
– To prevent contaminants such as cotton waste, loose nuts or
bolts entering the system 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.
Between rail/tank lorry decanting point and
main storage tank
Mesh
10
Between service tank and pre-heater 40
Between pre-heater and burner 100

18. Pumping fuel oil
• Positive displacement pumps - Heavy fuel oils
• Gear pump - LDO
• Diaphragm pumps - a shorter service life, but are
easier and less expensive to repair.
• A centrifugal pump
• Light fuels are best pumped with centrifugal or
turbine pumps. When higher pressures are
required, piston or diaphragm pumps should be
used.

19. Heating of Oil for Pumping
 Pre-heating oil in storage to make it
pumpable
 Heating entire oil tank or outflow heater to
heat oil pumped away
 Outflow heater with steam or electricity
Viscosity
(Centistokes)
Pumping Temperature,
oC
50 7
230 27
900 38
1500 49

20. Heating of Oil for Combustion
 Line heaters to raise oil temperature from
pumping to combustion temperature
 Line heater either by electrical or steam
tracing
Viscosity
(Centistokes)
Burning Temperature, oC
50 60
230 104
900 121

21. Properties of Coal
Coal Classification
• Three main coal classes: anthracite, bituminous, and
lignite(Sub class- semi anthracite, semi bituminous,
and sub bituminous)
• Anthracite-oldest coal,hard coal composed mainly
of carbon with little volatile content and practically
no moisture.
• Lignite -the youngest coal.
• Chemical composition of coal has a strong influence
on its combustibility.

22. Anthracite

23. Bituminous coal

24. Lignite

25. Proximate Analysis
Typical proximate analysis of various coals (in Percentage
by weight)
Parameter Indian Coal Indonesian
Coal
South
African
Coal
Moisture 5.98 9.43 8.5
Ash 38.56 13.99 18
Volatile
matter
20.70 29.79 23.28
Fixed Carbon 34.69 46.79 51.22

26. Significance of Various Parameters
in Proximate Analysis
Fixed carbon:
Fixed carbon gives a rough estimate of heating value of coal
Volatile Matter:
Volatile matters are the methane, hydrocarbons, hydrogen and
carbon monoxide, and incombustible gases like carbon dioxide
and nitrogen found in coal. Thus the volatile matter is an index
of the gaseous fuels present. Typical range of volatile matter is
20 to 35%.
– Proportionately increases flame length, and helps in easier ignition of
coal.
– Sets minimum limit on the furnace height and volume.
Influences secondary air requirement and distribution aspects.

27. Ash Content:
Ash is an impurity that will not burn. Typical range is 0.5 to 40%
– Reduces handling and burning capacity.
– Increases handling costs.
– Affects combustion efficiency and boiler efficiency
– Causes clinkering and slagging.
Moisture Content:
– Moisture decreases the heat content per kg of coal. Typical range is 0.5 to
10%
– Increases heat loss, due to evaporation and superheating
– Helps, to a limit, in binding fines.
Sulphur Content:
– Typical range is 0.5 to 5% normally.
– Affects clinkering and slagging tendencies,Corrodes chimney and other
equipment such as air heaters and economisers,Limits exit flue gas
temperature.

28. Ultimate Analysis:
Typical ultimate analyses of various coals
Parameter Lignite, % Indian Coal, % Indonesian
Coal, %
Moisture(Dry
basis)
50 5.98 9.43
Mineral Matter 10.41 38.63 13.99
Carbon 62.01 41.11 58.96
Hydrogen 6.66 2.76 4.16
Nitrogen 0.60 1.22 1.02
Sulphur 0.59 0.41 0.56
Oxygen 19.73 9.89 11.88
Useful to find the quantity of air required for combustion and the volume and
composition of the combustion gases, calculation of flame temperature and
the flue duct design etc

29. Storage & Handling of Coal
Stocking of coal has its own disadvantages like build-up of
inventory, space constraints, deterioration in quality and potential
fire hazards. Other minor losses associated with the storage of
coal include oxidation, wind and carpet loss.
• Minimise carpet loss and the loss due to spontaneous
combustion.
• The measures to reduce the carpet loses are
– Preparing a hard ground for coal to be stacked upon.
– Preparing standard storage bays out of concrete and brick
• In process Industry, modes of coal handling range from manual to
conveyor systems. It would be advisable to minimise the handling
of coal so that further generation of fines .

30. Preparation of Coal
• Sizing of Coal
– Proper coal sizing, with specific relevance to the type of firing system,
helps towards even burning, reduced ash losses and better combustion
efficiency.
Conditioning of Coal
– Segregation of fines from larger coal pieces can be reduced to a great
extent by conditioning coal with water. Water helps fine particles to stick
to the bigger lumps due to surface tension of the moisture, thus stopping
fines from falling through grate bars or being carried away by the furnace
draft.
– Blending of Coal
– In case of coal lots having excessive fines, it is advisable to blend the
predominantly lumped coal with lots containing excessive fines. Coal
blending may thus help to limit the extent of fines in coal being fired to
not more than 25%. Blending of different qualities of coal may also help
to supply a uniform coal feed to the boiler.

31. Spontaneous Combustion

32. LPG & N.Gas
• LPG is a predominant mixture of propane and Butane with a small
percentage of unsaturates (Propylene and Butylene)
• LPG -gaseous at normal atmospheric pressure, but may be
condensed to the liquid state at normal temperature, by the
application of moderate pressures. Liquid LPG evaporates to
produce about 250 times volume of gas.
• LPG vapour is denser than air
• Natural Gas
• Methane is the main constituent of Natural gas and accounting for
about 95% of the total volume. Other components are: Ethane,
Propane, Butane, Pentane, Nitrogen.sulphur negligible.
• It is lighter than air and disperses into air easily in case

33. What are the agro residues available
and their properties ?
Ultimate analysis of typical agro residues
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 3.52
Carbon 36.59 33.95 48.55 44.95
Hydrogen 4.15 5.01 6.99 4.99
Nitrogen 0.82 0.91 0.80 0.56
Sulphur 0.54 0.09 0.10 0.08
Oxygen 31.02 32.52 41.93 31.94
GCV
(Kcal/kg)
3151 3568 4801 4565

34. Limiting Reagent
200.0 g of iron(III) chloride and 50.00 g
hydrogen sulfide react. The balanced equation is:
2FeCl3 + 3H2S ---> Fe2S3 + 6HCl

35. a) Determine the limiting reagent
Iron(III) chloride
200.0 g / 162.204 g mol-1 = 1.233 mol
1.233 mol / 2 = 0.6165
Hydrogen sulfide
50.00 g / 34.081 g mol-1 = 1.467 mol
1.467 mol / 3 = 0.489
The hydrogen sulfide is the limiting reagent.

36. b) Determine amount of Fe2S3 formed
1) 1.467 mol H2S reacts (use limiting reagent)
2) H2S / Fe2S3 molar ratio is 3/1. The proportion is:
3/1 = 1.467/x
3) Fe2S3 produced is 0.489 mol
4) 0.489 mol x 207.885 g mol-1 = 101.7 g

37. c) Determine excess remaining
1) 1.467 mol H2S reacts (use limiting reagent)
2) FeCl3 / H2S molar ratio is 2/3. The proportion is:
2/3 = x/1.467
3) FeCl3 used is 0.978 mol
4) 1.233 mol - 0.978 mol = 0.255 mol FeCl3 remain
5) 0.255 mol x 162.204 g mol-1 = 41.36 g

38. What is COMBUSTION ?
• 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

39. Chemical reaction in
Combustion
 Stoichiometric or theoretical air is ideal
amount of air required for burning 1 kg of fuel
 Ex:1 kg of fuel oil requires ~14.1 kg of air for
complete combustion
C + O2  CO 2 + 8084 Kcals/kg of Carbon
2C + O2  2 CO + 2430 Kcals/kg of Carbon
2H 2 + O2  2H2O + 28,922 Kcals/kg of Hydrogen
S + O2  SO2 + 2,224 Kcals/kg of Sulphur
Moisture (%)
Mineral matter
(%)
Carbon (%)
Hydrogen
(%)
Nitrogen (%)
Sulphur (%)
Oxygen (%)
GCV
(Kcal/kg)

40. 3 Ts of Combustion
TIME
All combustion requires sufficient Time which depends
upon type of Reaction
TEMPERATURE
Temperature must be more than ignition temperature
TURBULENCE
Proper turbulence helps in bringing the fuel and air in
intimate contact and gives them enough time to
complete reaction.

41. What are the three types of
combustion?
• Perfect Combustion is achieved when all the fuel is burned using only
the theoretical amount of air, but perfect combustion cannot be achieved
in a boiler.
• Good / Complete Combustion is 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.
• Incomplete Combustion occurs when all the fuel is not burned, which
results in the formation of soot and smoke.

42. Fractional Distillation & Fuel oil
Naptha
Petrol
Diesel
Furnace Oil
Bitumen/Asphalt
Crude Oil
Kerosene

43. Distillate Fuel
• Ignite Readily
• Can be stored and handled at ambient
temperatures
• Low sulphur content
• Short flame

44. Residual fuels
• Residues from crude distillation
• Cheaper
• Vary in viscosity. Sulphur and ash content
• Blended with distillates to desired viscosity
range
• Heated for pumpability
• Higher sulphur and contains vanadium

45. Combustion in practice
.
Distillate
Residue

46. Combustion of Fuel oil
• Viscosity of 100 Redwood secs I at burners
• Atomising air 1- 3 Kg/cm2 (about 2 % of total air
requirement)
• 14 Kg of air/kg fuel is required for complete
combustion. Optimum efficiency with 10 %
excess air
• Flue gas should be analysed for CO2 or O2
• Sulphur dewpoint at 160oC. Corrosion max at
30oC below dew point

47. Combustion of Fuel oil (contd.)
• 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. Impingment on
walls, tubes cause carbon formation
• Too short a flame indicates high excess air and air
supply to burners should be adjusted for light haze
brown out of chimney

48. Burners
 Burners convert fuel oil into millions of small
droplets –process called atomization
 High surface to volume ratio in oil to facilitate
evaporation and combustion
 3 basic types of burners are pressure jet, air or
steam blast burners and Rotary Cup
TURNDOWN ratio is the relationship between the maximum
and minimum fuel input without affecting the excess air level
is called ‘Turn-Down Ratio’.
For example, a burner whose maximum input is 250,000
Kcals and minimum rate is 50,000 Kcals, has a ‘Turn-Down
Ratio’ of 5 to 1.

49. 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
atomization
 Prone to clogging due to dirt in
oil –requires fine filtration

50. Spray at 10 psi pressure Spray at 100-psi pressure
Spray at 300-psi pressure

51. 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
atomization

52. 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 BURNER – Firing rate
matches the boiler load

53. Combustion Inefficiencies
Fuel
N2
O2
N2
CO2
Soot
Unburnt Fuel
CO
H2
H2O
+ =
Deficiency of Air
Air
Flue gas

54. Combustion Inefficiencies
Fuel
N2
Excess O2
N2
CO2
Excess O2
H2O
+ =
Too much of Air
Air
Flue gas
O2

55. Combustion Inefficiencies
Fuel
N2
O2
N2
CO2
H2O
+ =
Stochiometric Air
Air Flue

56. Operating in this
Zone results in
wasted fuel
Zone of maximum
Combustion Efficiency
Operating in this
Zone results in
Excess heat loss up the stack
Unburned
Fuel Loss
Excess Air
Loss
Decrease Increase

57. Relation Between CO2 and Excess air
0
10
20
30
40
50
60
70
80
90
100
8.4 9 10 11 12 13 14
Carbon dioxide %
Excess
air
%

58. Relation Between Residual O2 and 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 (%)
Excess
air
(%)

59. Effect of excess air on carbon di oxide
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

60. Combustion Trouble shooting
Starting difficulty
Oil not flowing due to high viscosity/low temperature, choked
burner tips,
Flame splutters
High oil pressure, Broken burner block
Flame Splash back
Too high +ve pressure in combustion chamber
Smoke and Soot
Insufficient draft, poor oil pre-heating, excess oil flow
Excess oil consumption
Poor oil-air mixing, excessive draft, incorrect oil-air pressure,
poor oil pre-heating, poor maintenance

61. Coal Combustion
 1 kg of coal requires 10 -
12 kg of air for complete
combustion
 Primary air is supplied
below the grate and
Secondary air over the
grate
 Supply of PA and SA
regulated with coal bed
thickness
 Secondary air provided to
create good turbulence
 Clinkers formed on
combustion to be removed
immediately