Fuel chemistry

1. FUEL
• Definition,
• Classification
• Calorific value of fuels (gross and net),
• Dulong’s formula (Numericals),
• Determination of calorific value of fuels using
bomb’s calorimeter (Numericals),
• Determination of calorific value of fuels using
Boy’s Gas Calorimeter (Numericals),

2. • Cracking – Thermal & catalytic cracking, Octane
& Cetane numbers with their significance.
• High & Low temperature carbonization,
Manufacture of coke (Otto – Hoffmann oven)
• Proximate and ultimate analysis of Coal
(Numerical)
• Some solid liquid and gaseous fuels: CNG, LPG,
Natural gas, Oil gas, water gas, coal gas,
Producer gas
• Combustion of fuels (Numerical).

3. Fuel
• Any combustible substance, chemical or
reactant containing carbon as the main
constituent which on proper burning
produces heat that can be used
economically for domestic and industrial
purposes and in generation of power.
Fuel + O2 Products + heat

4. Classification : -
• Primary Fuels :-
These include the naturally occurring fuels
found freely on earth’s crust.
Solid Fuels ( Wood )
Liquid Fuels(Petroleum )
Gaseous Fuels (Natural
Gas)

5. • Secondary or Derived Fuels:-
These are artificially manufactured or are
derived from primary fuels.
Solid Fuels(Coke, Charcoal )
Liquid Fuels(Gasoline, Diesel)
Gaseous Fuels (coal gas,
biogas)

6. Characteristics of a good fuel-
• High calorific value
• Low moisture content
• Low noncombustible matter
• Moderate ignition temperature
• Easy to handle ,transport and store.
• Readily available in bulk and at low cost.
• Environment friendly

7. Calorific Value of Fuel-
• The amount of heat obtained by the
complete combustion of a unit mass of
the fuel.
Calorific values are of two types –
1. Gross Calorific Value ( GCV )
2. Net Calorific Value (NCV )

8. GCV or Higher Calorific Value :-
It is the total amount of heat generated
when a unit mass of fuel is completely
burnt and the products of combustion
are cooled down to 60o F, 288o K or 15o C.
NCV or Lower Calorific Value :-
It is the net heat produced when one unit
mass of fuel is completely burnt and the
products are allowed to escape.

9. NCV= GCV – Latent heat of condensation of
water vapor produced
H2 + ½ O2 H2O
2g 18g
1g 9g
One Part by weight of Hydrogen gives
nine parts by weight of water. The latent
heat of steam is 587 cal/g of water vapor
produced.
• NCV = GCV – 9 x H/100 x 587
Or LCV = ( HCV – .09 H x 587 ) cal/gm

10. Units of Calorific Value:-
Solid & Liquid Fuels -
Calorie – The amount of heat required to
increase the temperature of 1 gm of water
through 10C.
• Calories/gm or Kcal/gm
• British thermal unit/pound
Gaseous Fuel-
• Kcal/m3
• BThU/Ft3

11. Determination of Calorific Value:-
Bomb Calorimeter-
• Principle – A known unit mass of the fuel
is burnt and the quantity of the heat
produced is absorbed in water &
measured. Then the quantity of heat
produced by burning a unit mass of the
fuel is calculated.
This apparatus is used to find the
calorific value of solid/liquid fuels.

12. Construction -

13. It consist of the following parts -
1. A strong cylindrical Bomb made up of
stainless steel (chromium nickel-
molybdenum) steel, resistant to
corrosion and capable of withstanding
high pressure up to 50 atm. It is
provided with a lid which can be
screwed firmly on the bomb. The lid in
turn is provided with two electrodes &
an oxygen inlet valve.

14. 2. A copper calorimeter vessel with a known
weight of water in which the bomb stands.
3. The calorimeter is surrounded by an air
jacket and water jacket to prevent the loss
of heat due to radiation.
4. The calorimeter is provided with an
electrical stirrer for stirring water & a
Beckmann thermometer which can read
accurately temperature difference up to .01
of a degree.
5. The crucible used is made up of Ni,
Stainless steel or fused silica.

15. Working -
A weight amount of the fuel is placed in
the silica crucible supported over the ring. A
fine magnesium wire touching the sample
of the fuel is stretched across the
electrodes. About 10 ml of distilled water is
introduced into the bomb to absorb the
vapors of sulphuric acid & nitric acid formed
during the combustion. Oxygen supply is
forced into the bomb to a pressure of 25-30
atm.

16. The bomb is then carefully placed in the
calorimetric vessel containing a known
amount of water. The stirrer is driven and the
initial temperature of water is noted. The
electrodes are then connected to a battery
circuit is completed. The combustion of the
fuel take place with the evolution of heat. The
heat produced by burning is transferred to the
water which is stirred is recorded.
The calorific value of the fuel is
calculated as -

17. Calculations -
Let x = mass of the fuel sample in the
crucible(gm)
W = mass of the water in calorimeter (gm)
w = water eq. of calorimeter, stirrer,
bomb & thermometer
T1 = initial temp. of water in calorimeter
T2 = final temp. of water in calorimeter
L = Higher calorific value (GCV) of fuel
(cal/gm)

18. Heat liberated by the burning of fuel = x L cal
Heat absorbed by water = W x (T2-T1)
Heat absorbedby apparatus (calorimeter) =
w x (T2-T1 )
Hence total heat absorbed/gained by water,
apparatus etc = W x (T2-T1 ) + w x (T2-T1 ) cal
= [(W + w) (T2-T1 )] cal .
Heat liberated by the fuel = Heat gained by
the water & calorimeter

19. Therefore X L = [(W + w) (T2-T1 )]
L = (W + w) (T2-T1 ) cal/gm
X
Corrections –
Fuse Wire Correction ( tf )–
The heat liberated, also
include the heat given out by the ignition of
fuse wire used. Hence it must be subtracted
from the total value.

20. Acid Correction ( ta )-
Fuels containing S and N are
oxidized, under high pressure and temp. of
ignition, to H2SO4 and HNO3 respectively.
S + H2 + 2O2 H2SO4 + Heat
2N + H2 + 3O2 2HNO3 + Heat
Formation of these acids are exothermic
reactions. So, the measured heat also
includes the heat given out during the acid
formation. The amount of these acids may
be measured by the titration.

21. Cooling Correction ( tc )-
Time taken to cool the
water in calorimeter from maximum temp
to room temperature is noted. From the
rate of cooling and the actual time taken for
cooling, The cooling correction is added to
the rise in temp.
Cotton Thread Correction ( tt )–
As the burning of
cotton thread also generates heat.

22. Thus, Higher/Gross Calorific Value,
GCV = (W + w) (T2-T1 + tc) – (ta + tf + tt) cal/gm
X
Numerical Examples –
A sample of coal contains C = 93%, H = 6% & ash = 1%.
Wt of coal burnt = 0.92 gm
Wt of water taken = 2200 gm
Water eq of bomb calorimeter = 550 g
Rise in temp. = 2.42 o C
Fuse wire correction = 10.0 cal
Acid correction = 50 cal
GCV & NCV = ?