1. FUEL AND COMBUSTION
Prof R K Patel
Dept. of Chemistry
Fuels and combustion, Fuels-Classification of fuels, calorific value -
LCV, HCV; measurement of calorific value using bomb calorimeter
(numerical problems). Combustion: Calculation of air qualities
(problems). Solid fuel, proximate and ultimate analysis ( problems).
Carbonization of coal. Liquid Fuels: Knocking and anti-knocking for
petrol and diesel (octane number and cetane number) - diesel index.
Refining of liquid fuels, cracking of petroleum. Gaseous fuels: LPG,
natural gas, CNG: Composition and applications. Biofuels: Biodiesel and
Biogas -composition and applications. Next generation fuels.
2. Fuels
Fuel is a combustible substance which during
combustion gives large amount of heat.
There are chemical fuels, nuclear fuels and fossil
fuels.
Classification of Fuels
These can be classified on the basis of their
occurrence and physical state
On the basis of occurrence they are of two types:
Primary Fuels: Fuels which occur in nature as such
are called primary fuels. E.g., wood, peat, coal,
petroleum, and natural gas.
3. Secondary Fuels: The fuels which are derived from
the primary fuels by further chemical processing are
called secondary fuels. E,g., coke, charcoal,
kerosene, coal gas, producer gas etc.
(ii) On the basis of physical state these may be
classified as:
Solid Fuels
Liquid Fuels
Gaseous Fuels
Calorific value: It is defined as the total quantity of
heat liberated when a unit mass of a fuel is burnt
completely.
4. Units of Calorific value:
System Solid/Liquid Gaseous
Fuels Fuels
CGS Calories/gm Calories/cm3
MKS k cal/kg k cal/m3
B.T.U BTU/lb BTU/ft3
The quantity of heat can be measured in the
following units:
(i) Calorie: It is defined as the amount of heat
required to raise the temperature of 1gm of water by
1oC 1 calorie = 4.184 Joules
5. (ii) Kilo Calorie: 1 k cal = 1000 cal
(iii) British thermal unit: (B. T. U.) It is defined as
the amount of heat required to raise the temperature
of 1 pound of water through 1oF.
1 B.T.U. = 252 Cal = 0.252 k cal
(IV) Centigrade heat unit (C.H.U): It is defined as
the amount of heat required to raise the temperature
of 1 pound of water through 1oC.
1k cal = 3.968 B.T.U.
= 2.2 C.H.U. restore
6. Characteristics of Good Fuel:
(i) Suitability: The fuel selected should be most suitable for
the process. E.g., coke made out of bituminous coal is
most suitable for blast furnace.
(ii) High Calorific value
(iii) Ignition Temperature: A good fuel should have moderate
ignition temperature.
(iv) Moisture content: Should be low
(v) Non combustible matter content
(vi) Velocity of combustion: It should be moderate
(vii) Nature of the products
(viii) Cost of fuel, (ix) Smoke, (x) Control of the process
7. Gross and net calorific Value
Gross Calorific Value: It is the total amount of heat
generated when a unit quantity of fuel is completely
burnt in oxygen and the products of combustion are
cooled down to the room temperature.
As the products of combustion are cooled down to
room temperature, the steam gets condensed into
water and latent heat is evolved. Thus in the
determination of gross calorific value, the latent heat
also gets included in the measured heat. Therefore,
gross calorific value is also called the higher calorific
value.
The calorific value which is determined by Bomb
calorimeter gives the higher calorific value (HCV)
8. Net Calorific Value: It is defined as the net heat
produced when a unit quantity of fuel is completely
burnt and the products of combustion are allowed to
escape.
The water vapour do not condense and escape with
hot combustion gases. Hence, lesser amount than
gross calorific value is available. It is also known as
lower calorific value (LCV).
LCV=HCV-Latent heat of water vapours formed
Since 1 part by weight of hydrogen gives nine parts
by weight of water i.e.
H + 1O → H O
2 2 2 2
9. Therefore,
LCV=HCV-weight of hydrogen x 9 x latent heat of
steam
= HCV-weight of hydrogen x 9 x 587
Determination of Calorific value
1. Determination of calorific value of solid and non
volatile liquid fuels: It is determined by bomb
calorimeter.
Principle: A known amount of the fuel is burnt in
excess of oxygen and heat liberated is transferred to
a known amount of water. The calorific value of the
fuel is then determined by applying the principle of
calorimetery i.e. Heat gained = Heat lost
11. Calculations
Let weight of the fuel sample taken = x g
Weight of water in the calorimeter = W g
Water equivalent of the Calorimeter, stirrer, bomb,
thermometer = w g
Initial temperature of water = t1oC
Final temperature of water = t2oC
Higher or gross calorific value = C cal/g
Heat gained by water = W x ∆t x specific heat of water
= W (t2-t1) x 1 cal
12. Heat gained by Calorimeter = w (t2-t1) cal
Heat liberated by the fuel = x C cal
Heat liberated by the fuel = Heat gained by water and
calorimeter
x C = (W+w) (t2-t1) cal
C=(W+W)(t2-t1) cal/g
x
13. Net Calorific value:
Let percentage of hydrogen in the fuel = H
Weight of water produced from 1 gm of the fuel =
9H/100 gm
Heat liberated during condensation of steam
= 0.09H × 587 cal
Net (Lower calorific value) = GCV-Latent heat of
water formed
= C-0.09H × 587 cal/gm
Corrections: For accurate results the following
corrections are also incorporated:
14. (a)Fuse wire correction: As Mg wire is used for
ignition, the heat generated by burning of Mg wire
is also included in the gross calorific value. Hence
this amount of heat has to be subtracted from the
total value.
(b)Acid Correction: During combustion, sulphur and
nitrogen present in the fuel are oxidized to their
corresponding acids under high pressure and
temperature.
S + O → SO
2 2
2SO + O + 2H O → 2H SO ∆H = -144,000 Cal
2 2 2 2 4
2 N + 5O + 2H O → 4HNO ∆H = -57,160 Cal
2 2 2 3
15. The corrections must be made for the heat liberated
in the bomb by the formation of H2SO4 and HNO3.
The amount of H2SO4 and HNO3 is analyzed by
washings of the calorimeter.
For each ml of 0.1 N H2SO4 formed, 3.6 calories
should be subtracted.
For each ml of 0.01 HNO3 formed, 1.43 calories must
be subtracted.
(C) Cooling correction: As the temperature rises
above the room temperature, the loss of heat does
16. occur due to radiation, and the highest temperature
recorded will be slightly less than that obtained. A
temperature correction is therefore necessary to get
the correct rise in temperature.
If the time taken for the water in the calorimeter to
cool down from the maximum temperature attained, to
the room temperature is x minutes and the rate of
cooling is dt/min, then the cooling correction = x × dt.
This should be added to the observed rise in
temperature.
Therefore, Gross calorific value
C=(W+w)(t2-t1+Cooling correction)-[Acid+ fuse
corrections] / Mass of the fuel.
17. JUNKERS GAS CALORIMETER
AIM :To determine calorific value of gaseous fuel by
Junkers gas calorimeter
APPARATUS: The apparatus mainly consists of a
cylindrical shell with copper coil arranged in two
passage configuration with water inlet and outlet to
circulate through the copper coil, a pressure regulator, a
wet type gas flow meter & a gas Bunsen burner.
DESCRIPTION: Determination of calorific value (heat
value) of combustible gases is essential to assess the
amount of heat given away by the gas while burning a
known amount of gas to heat a known amount of fluid
(water) in a closed chamber.
18.
19. PROCEDURE:
Install the equipment on a flat rigid platform near an
uninterrupted continuous water source of ½” size and a
drain pipe.
Connect the gas source to the pressure regulator, gas
flow meter and the burner respectively in series
Insert the thermometer / temperature sensors, into their
respective places to measure water inlet and outlet
temperatures and a thermometer to measure the flue
gas temperature at the flue gas outlet
Start the water flow through the calorimeter at a study
constant flow rate and allow it to drain through over flow.
Start the gas flow slowly and light the burner out side
the calorimeter
20. Regulate the flow of gas at a steady rate to any
designed flow (Volume)
Insert the burner into the calorimeter and allow the out let water
temperature to attain a steady state
Swing the out let to a 1000 ml jar and start. The stop watch
simultaneously, record the initial gas flow meter reading at the same
time
Note down the time taken to fill 1000ml and at the same time the final
gas flow reading recorded by the gas flow meter
Tabulate all the reading and calculate the calorific valve of the gas under
test
Repeat the experiment by varying the water flow rate or gas flow for
different conditions.
After the experiment is over stop the gas flow, water flow, and drain the
21. Theoretical calculation of Calorific value of a Fuel:
The calorific value of a fuel can be calculated if the
percentages of the constituent elements are known.
Substrate Calorific value
Carbon 8080
Hydrogen 34500
Sulphur 2240
22. If oxygen is also present, it combines with hydrogen to
form H2O. Thus the hydrogen in the combined form is
not available for combustion and is called fixed
hydrogen.
Amount of hydrogen available for combustion = Total
mass of hydrogen-hydrogen combined with oxygen.
1
H 2 + O2 → H 2O
2
1g 8g 9g
Fixed Hydrogen = Mass of oxygen in the fuel
Therefore, mass of hydrogen available for combustion
= Total mass of hydrogen-1/8 mass of oxygen in fuel
=H-O/8
23. Dulong’s formula for calculating the calorific value is
given as:
Gross calorific Value (HCV)
1 O
= [8080C + 34,500( H − ) + 2,240 S ]kcal / kg
100 8
Net Calorific value (LCV)
9H
= [ HCV − × 587]kcal / kg
100
= [ HCV − 0.09 H × 587]kcal / kg
25. Solid Fuels: Primary as well as secondary are widely
used in domestic and industrial purposes.
e.g., wood, coal, charcoal and coke.
Wood: Wood has been used as a fuel from ancient
times. Due to large scale deforestation, wood is no
longer used except in forest areas where wood is
available at a low cost.
26. Wood when freshly cut contains 25-50% moisture.
Normally it is used in air dried condition with 10-15
percent moisture content.
The calorific value of air dried wood is about 3500-
4500 kcal/kg.
When wood burns, the ash content is low but the
oxygen content is very high. This makes even dry
wood a fuel of low calorific value.
Wood charcoal is obtained by destructive distillation of
wood.
The major use of wood charcoal is for producing
activated carbon.
27. Coal: coal is regarded as a fossil fuel produced from
the vegetable debris under conditions of high
temperature and pressure over million of years.
The transformation of the vegetable debris to coal
takes place in two stages:
(a)Biochemical or peat stage: During this stage, the
plant materials were attacked by various micro
organisms.
(b)Chemical stage or metamorphism: In this stage,
the peat deposit buried under sedimentary deposits
lose moisture and volatile components under the
effect of high temperature and pressure.
The peat gets enriched in carbon whereas its oxygen
content decreases.
28. The spongy peat transforms into hard brittle coal
gradually. The time required for the formation of
young brown coal is of the order of 107 years.
Classification of Coal: Coals are mainly classified on
the basis of their degree of coalification from the
parent material, wood. When wood is converted into
coal, there is gradual increase in the concentration of
carbon and decrease in the percentage of oxygen and
nitrogen.
Coal is given a ranking depending upon the carbon
content of the coal from wood to anthracite.
29. Type of Percentage (dry, mineral matter % calorific
coal free basis) moist value
ure
C H O N VM
Wood 45-50 5-6 20-40 0-0.5 - 70-90 4000-
4500
Peat 45-60 3.5-6.5 20-45 0.75-3 45-75 70-90 4125-
5280
6600-
Brown Coal 60-75 4.5-5.5 17-35 0.75-2 45-60 30-50
7100
6600-
Bituminous 75-90 4.0-5.5 20-30 0.75-2 11-50 10-20 8800
coal
8470-
Anthracite 90-95 3-4 2-3 0.5-2 3.8-10 1.5-3.5 8800
30. Analysis of Coal
Coal is analysed in two ways:
1. Proximate analysis
2. Ultimate analysis
The results of analysis are generally reported in the
following ways:
As received basis
Air dried basis
Moisture free basis (oven dried)
Moisture and ash free basis
31. Proximate Analysis
The data varies with the procedure adopted and
hence it is called proximate analysis.
It gives information about the practical utility of coal.
Proximate analysis of coal determines the moisture,
ash, volatile matter and fixed carbon of coal.
1. Moisture Content: Air dried moisture is
determined by heating a known amount of coal to
105-110 oC in an electric hot air oven for about one
hour. After one hour, it is taken out from the oven
and cooled in a dessicator and weighed.
Percentage of moisture= Loss in weight 100
×
Weight of coal taken
32. •Excess of moisture is undesirable in coal.
•Moisture lowers the heating value of coal and takes
away appreciable amount of the liberated heat in the
form of latent heat of vapourisation.
•Excessive surface moisture may cause difficulty in
handling the coal.
•Presence of excessive moisture quenches fire in the
furnace.
2. Volatile Matter: consists of a complex mixture of
gaseous and liquid products resulting from the thermal
decomposition of the coal.
33. It is determined by heating a known weight of
moisture free coal sample in a covered platinum
crucible at 950 ± 20oC for 7 minutes.
Percentage of volatile matter =
Loss of weight due to volatile matter × 100
Weight of coal sample taken
Significance
A high percent of volatile matter indicates that a large
proportion of fuel is burnt as a gas.
The high volatile content gives long flames, high
smoke and relatively low heating values.
34. For efficient use of fuel, the outgoing combustible
gases has to be burnt by supplying secondary air.
High volatile matter content is desirable in coal gas
manufacture because volatile matter in a coal
denotes the proportion of the coal which will be
converted into gas and tar products by heat.
Ash: Coal contains inorganic mineral substances
which are converted into ash by chemical reactions
during the combustion of coal.
Ash usually consists of silica, alumina, iron oxide and
small quantities of lime, magnesia etc.
Ash content is determined by heating the residue left
after the removal of volatile matter at 700 ± 50oC for
½ an hour without covering
35. Weight of the residue left 100
×
Percentage of ash =
Weight of the coal
Ash can be classified as intrinsic ash and extrinsic
ash.
The mineral matter originally present in vegetable
matter from which the coal was formed is called
intrinsic ash. It consists of oxides of Na, K, Mg, Ca and
Si.
The mineral matter like clay, gypsum, dirt which gets
mixed up during mining and handling of coal constitute
the extrinsic ash which remains as a residue after the
combustion. E.g., CaSO4, CaCO3, Fe2O3 etc.
36. The high percentage of ash is undesirable. It
reduces the calorific value of coal.
In furnace grate, the ash may restrict the passage
of air and lower the rate of combustion.
High ash leads to large heat losses and leads to
formation of ash lumps.
The composition of ash and fusion range also
influences the efficiency of coal.
When coal is used in boiler, the fusion temperature
of ash is very significant. Ash having fusion
temperature below 1200oC is called fusible ash and
above 1430oC is called refractory ash.
37. Apart from loss of efficiency of coal, clinker formation
also leads to loss of fuel because some coal particles
also get embedded in the clinkers.
Fixed Carbon: Fixed carbon content increases from
lignite to anthracite. Higher the percentage of fixed
carbon greater is its calorific value and better is the
quality of coal.
The percentage of fixed carbon is given by:
Percentage of fixed carbon = 100-[% of
moisture+volatile matter+ash]
Significance: Higher the percentage of fixed carbon,
greater its calorific value
38. •The percentage of fixed carbon helps in designing
the furnace and shape of the fire-box because it is the
fixed carbon that burns in the solid state.
Ultimate analysis:
It is carried out to ascertain the composition of coal.
Ultimate analysis includes the estimation of carbon,
hydrogen, sulphur, nitrogen and oxygen.
1. Carbon and Hydrogen: A known amount of coal is
taken in a combustion tube and is burnt in excess of
pure oxygen.
C + O → CO
2 2
H + 1O → H O
2 2 2 2
39. Fig 3. Estimation of carbon and hydrogen
2KOH + CO → K CO + H O
2 2 3 2
CaCl + 7 H O → CaCl .7 H O
2 2 2 2
44 g of CO2 contain = 12 g of carbon
Y g of CO2 contain = 12 × y
44
40. Percentage of carbon = 12 × y ×100
44 weight of coal taken
18 g of water contain = 2 g of hydrogen
Z g of water contain = 2 × zg of hydrogen
18
Percentage of hydrogen = 2 × z ×100
18 weight of coal taken
Significance:
Calorific value of a fuel is directly related to its
carbon content.
A higher percentage of carbon reduces the size of
the combustion chamber
41. High percentage of hydrogen also increases the
calorific value of coal. The content of hydrogen in
coals varies between 4.5 to 6.5 percent from peat to
bituminous stage.
2. Nitrogen: Nitrogen present in coal sample can be
estimated by Kjeldahl’s method.
Nitrogen + H SO Heat →( NH ) SO
2 4 42 4
The contents are then transferred to a round bottomed
flask and solution is heated with excess of NaOH.
The ammonia gas thus liberated is absorbed in a
known volume of a standard solution of acid used.
42. Fig 4. Estimation of nitrogen by Kjeldahl’s method
The unused acid is then determined by titrating with
NaOH. From the volume of acid used by NH3
liberated, the percentage of nitrogen can be
calculated.
43. ( NH ) SO 2 NaOH → Na SO + 2 NH + 2H O
42 4 2 4 3 2
NH + H SO → ( NH ) SO
3 2 4 42 4
44. Carbonization of Coal (Manufacture of Coke)
It is the process of heating the coal in absence of air to
a sufficiently high temperature, so that the coal
undergoes decomposition and yields a residue which
is richer in carbon content than the original fuel.
Caking and coking of coals: some coals have a
tendency to soften and swell at higher temperatures,
to form a solid coherent mass with porous structure.
Such coals are called caking coals. The residue
formed is called coke. If the coke is hard, porous and
strong, than the coal, from which it is formed, it is
called coking coal. All coking coals are caking coals
but all caking coals are not coking coals.
This property is found only in bituminous type of coal.
45. Coals with a high percentage of volatile matter are not
fit for coking and are used for gas making. The coals
having 20-30 % volatile matter are good coking coals.
Process of carbonization:
First moisture and occluded gases are driven off.
At about 260-270oC carbon, water, H2S, some low
molecular alkenes and alkanes are evolved.
At about 350oC the decomposition of coal is
accompanied by evolution of gases and elimination of
vapours takes place.
At about 400oC, caking coal becomes soft and plastic.
At about 700oC, hydrogen is evolved
46. Above 800oC, main gaseous products are evolved
Gases evolved from the plastic mass, expand it to give
foam like appearance.
At further high temperatures this foam like mass
solidifies to form a solid mass with porous structure
called coke.
Types of carbonization
(i) Low temperature carbonization
(ii) High temperature carbonization
(i) Low temperature carbonization: When the
destructive distillation of coal is carried out at
temperatures between 500-700oC.
47. It is practiced for the production of semi coke. Which
is also called soft coke.
The yield of coke is about 75-80 %.
The coke thus produced contains 5 to 15 % volatile
matter.
The various products of low temperature
carbonization are semi coke, low temperature tar,
crude low temperature spirit and gas.
LTC plants normally use low rank coals. These low
rank coals produce excessive smoke on burning.
Semi coke from LTC is highly reactive and can be
easily ignited into a smokeless flame
48. The gas which is obtained as a byproduct has higher
calorific value of about 6500-9500 kcal/m 3.
(ii) High temperature carbonization: It is carried out
at 900-1200oC. HTC is used for the production of pure,
hard, strong and porous metallurgical coke containing
1-3 % volatile matter. The yield of the coke is 65-75%.
The byproducts-gas and tar have greater amounts of
aromatic hydrocarbons. The gas which is obtained has
lower calorific value of about 5000-6000 kcal/m 3 than
that produced in LTC; but the yield of the gas is higher.
The coke obtained is very much harder than the coke
obtained from LTC process and hence is called hard
coke.
49. Metallurgical coke: The properties of coke depend
on porosity, reactivity and the amount of volatile
matter retained by coke during carbonization. Coke
is mainly used as a heat source and reducing
agent in metallurgy. A good coke in metallurgical
process should possess the following
characteristics:
(i) Purity: The metallurgical coke should contain
lower percentage of moisture, ash, sulphur and
phosphorous.
(ii) Porosity: The coke should be porous so as to
provide contact between carbon and oxygen.
(iii)Strength: The coke used in metallurgical process
should have high strength so as to withstand the
50. weight of the ore, flux etc. in the furnace.
(iv) size: Metallurgical coke should be of medium
size.
(v) Combustibility: Coke should burn easily. The
combustibility of coke depends on the nature of the
coal, carbonization temperature and reaction
temperature.
(vi) Calorific value: It should be high.
(vii) Reactivity: Reactivity of coke is its ability to react
with CO2, steam, air and oxygen. The reactivity should
not be too high. The reactivity toward CO 2 represent
the reduction of CO2
CO ( g ) + C (s) ⇔ 2CO( g )
2
51. Cost: Coke should be cheap and easily available.
Manufacture of Metallurgical Coke:
(i)
53. Demerits of Beehive ovens: The demerits are
•No recovery of byproducts, which are useful
chemicals and are allowed to escape.
•Lower coke yield due to partial combustion
•Lack of flexibility of operation
(ii) Otto-Hoffmann’s oven or By-product Oven: The
beehive ovens have been replaced by chamber ovens
which works on regenerative principle of heat
economy. All the valuable products are recovered
from the outgoing flue gases.
Construction: It consists of no. of narrow rectangular
chambers made of silica bricks.
54. Fig. 6: A single chamber of Otto Hoffmann’s oven
55. Working: Coal is charged into the chamber.
The coke ovens are heated to 1200oC by burning
gaseous fuels.
The process of carbonization takes place layer by
layer in the coal charge.
As the coal adjacent to the oven walls gets heated, a
plastic zone is formed which moves away from the
walls towards the central zone.
As the coal is converted into coke, there is decrease
in volume. This is because of the removal of volatile
matter in the form of tar and gas at about 500 oC. At
further high temperature, the plastic mass solidifies
into hard and porous mass called coke.
56. Regenerative principle is employed to achieve as
economical heating as possible.
Regenerators are built underneath the ovens.The flue
gases pass their heat to the checker brick work of
regenerators until the temperature rises to 1000 oC.
Regenerators work on the principle of alternate
heating and cooling cycles. This is achieved by
periodically changing the direction of flow of gases
through the vertical flues every 30 min or so.
Carbonization of a charge of coal takes about 11-18
hours. After the process is complete, red hot coke is
pushed outside by means of a ram which is electrically
driven. The coke falls into a quenching car. The yield
is 75 % of coal.
57. Recovery of byproducts: The gases and vapours
evolved on carbonization in coke ovens are not
allowed to mix with the combustion and are collected
separately.The coke oven gas is treated separately
for the recovery of the valuable byproducts.
Fig. 8: Coke-Oven gas treatment plant
58. (i) Recovery of Tar: The gas from the coke ovens is
passed through a tower in which liquor ammonia is
sprayed.Tar and dust get collected in a tank. The
tank is provided with a heating coils to recover
back ammonia.
(ii) Recovery of ammonia: The gases are then
passed through a tower where water is sprayed to
recover ammonia. The ammonia can also be
recovered by dissolving it in H2SO4 to form
(NH4) 2SO4, which is then used as a fertilizer.
(iii) Recovery of Naphthalene: The gases are
passed through a cooling tower, where water at a
low temperature is sprayed. The gas is scrubbed
with water until its temp. reduces.
59. (iv) Recovery of Benzole: The gases are then
introduced into a light oil or benzol scrubber, where
benzene along with its homologue is removed and is
collected at the bottom.
(v) Recovery of H2S and other S compounds: are
removed from the coke oven gas after the light oil has
been separated out.
Fe O + 3H S → Fe S + 3H O
2 3 2 2 3 2
2Fe S +4O → 2FeO + 3SO
2 3 2 2
4FeO + O → 2Fe O
2 2 3
The SO2 obtained can be used for the manufacturing
of sulphuric acid, which can be used to absorb NH 3
60. Liquid Fuels: The importance of liquid fuels is the
fact that almost all combustion engines run on them.
The largest source of liquid fuels is petroleum. The
calorific value of petroleum is about 40000 kJ/kg.
There are other supplements of liquid fuels such as
coal tar, crude benzol, syntheic liquid fuel made from
coal etc.
Petroleum: The term petroleum means rock oil. It is
also called mineral oil.
Petroleum is a complex mixture of paraffinic, olefinic
and aromatic hydrocarbons with small quantities of
organic compounds containing oxygen, nitrogen and
sulphur.
62. The ash of the crude oil is 0.1%.Metals e.g., Silicon,
iron, aluminium, calcium, magnesium, nickel and
sodium.
Crude oil is a mixture of straight chain paraffins and
aromatic hydrocarbons e.g., benzene, toluene,
naphthalenes etc.
Sulphur is present in the form of derivatives of
hydrocarbons such as alkylsulphides, aromatic
sulphides etc. Nitrogen is present in the form of
pyridine, quinoline derivatives, pyrrole etc. Comined
oxygen is present as carboxylic acids, ketones and
phenols.
The objectionable odour of crude petroleum is due to
the presence of sulphur compounds in it.
63. Classification of Crude Petroleum
Residue obtained Name Contents
after distillation
Paraffin wax Paraffin Straight chain
base hydrocarbons and
small amounts
naphthenes and
aromatic hydrocarbons
Aromatic and
Asphalt Asphaltic naphthenic
base hydrocarbons
Paraffin wax and Mixed Paraffins, naphthenes
base and aromatic
asphalt
hydrocarbons
64. Processing of Crude Petroleum:
Petroleum is found deep below the earth crust. The oil
is found floating over salt water or brine. Generally,
accumulation of natural gas occurs above the oil.
Fig. 9: Pumping of oil
65. Refining of Petroleum
Crude oil reaching the surface, generally consists of a
mixture of solid, liquid and gaseous hydrocarbons
containing sand and water.
After the removal of dirt, water and much of the
associated natural gas, the crude oil is separated into
a no of useful fractions by fractional distillation.
The resultant fractions are then subjected to
purification known as refining of petroleum.
Steps involved in refining of petroleum:
(i) Demulsification: The crude oil coming out from
the well, is in the form of stable emulsion of oil and
66. The demulsification is achieved by Cottrell’s process,
in which the water is removed from the oil by electrical
process. The crude oil is subjected to an electrical
field, when droplets of colloidal water coalesce to form
large drops which separate out from the oil.
(ii) Removal of harmful impurities: Excessive salt
content such as NaCl and MgCl2 can corrode the
refining equipment. These are removed by washing
with water.
The objectionable sulphur compound are removed by
treating the oil with copper oxide. The copper sulphide
so formed is separated by filtration.
67. (iii) Fractional Distillation: It is done in tall fractionating
tower or column made up of steel.
In continuous process, the crude oil is preheated to 350-
380 oC in specially designed tubular furnace known as
pipe still.
Fig. 10: Fractional
distillation of crude
petroleum
68. The hot vapours from the crude are passed through a
tall fractionating column, called bubble tower.
Bubble tower consists of horizontal trays provided with
a no of small chimneys, through which vapours rise.
These chimneys are covered with loose caps, known
as bubble caps. These bubble caps help to provide an
intimate contact between the escaping vapours and
down coming liquid.
The temperature in the fractionating tower decreases
gradually on moving upwards.
As the vapours of the crude oil go up, they become
gradually cooler and fractional condensation takes
place at different heights of column.
69. The residue from the bottom of the fractionating tower
is vacuum distilled to recover various fractions
Fig. 11: Vacuum distillation of residual oil
70. There is yet another type of fractional distillation called
Top-flashing.
Fig. 11: Top Flashing
In top flashing, there is better control of product
composition, but requires more pumps and
instruments and hence is an expensive process.
71. Cracking: Gasoline is the most imp fraction of crude
petroleum. The yield of this fraction is only 20% of the
crude oil. The yield of heavier petroleum fraction is
quite high. Therefore, heavier fractions are converted
into more useful fraction, gasoline.
This is achieved by a technique called cracking.
Cracking is the process by which heavier fractions are
converted into lighter fractions by the application of
heat, with or without catalyst. Cracking involves the
rupture of C-C and C-H bonds in the chains of high
molecular weight hydrocarbons.
e.g:
72. C H Cracking→ C H + C H
10 22 5 12 5 10
Decane n - pentane pentene
B.Pt =174ο C B.Pt = 36ο C
C H Cracking→ C H + C H
8 18 5 12 3 6
Nearly 50% of today’s gasoline is obtained by
cracking. The gasoline obtained by cracking is far
more superior than straight run gasoline.
The process of cracking involves the full chemical
changes:
•Higher hydrocarbons are converted to lower
73. hydrocarbons by C-C cleavage. The product obtained
on cracking have low boiling points than initial
reactant.
•Formation of branched chain hydrocarbons takes
place from straight chain alkanes.
•Unsaturated hydrocarbons are obtained from
saturated hydrocarbons.
•Cyclization may takes place.
Cracking can also be used for the production of olefins
from naphthas, oil gas from kerosene. Cracking can be
carried out by two methods
74. Thermal Cracking: When it takes place simply by the
application of heat and pressure, the process is called
thermal cracking. The heavy oils are subjected to high
temperature and pressure, when the bigger
hydrocarbons break down to give smaller molecules of
paraffins, olefins etc. The thermal stability among the
constitutents of petroleum fractions increases as
Paraffins < naphthenes < aromatics
(a) Liquid Phase thermal cracking: The charge is
kept in the liquid form by applying high pressures of
the range 30-100 kg/cm2 at a suitable temperature of
476-530 oC. The cracked products are separated in a
fractionating column.
75. The important fractions are: Cracked gasoline (30-
35%), Cracking gases (10-45%); Cracked fuel oil (50-
55%).
(b) Vapour phase thermal cracking: By this method,
only those oils which vapourize at low temperatures
can be cracked. The petroleum fractions of low boiling
range like kerosene oil, are heated at a temp of 670-
720 oC under low pressure.
Mechanism of thermal cracking: It follows free radical
mechanism.
Initiation
CH (CH ) CH Heat→ CH (CH ) CH + CH (CH ) CH
3 27 3 3 23 2 2 22 3
76. Propagation
The free radical formed are thermally unstable and
undergo fission at the b-position to yield a new radical
and an olefin.
CH 3 − CH 2CH 2 − CH 2 − CH 2 → CH 3 − CH 2 = CH 2
Catalytic cracking: Cracking is brought about in the
presence of a catalyst at much lower temperatures
and pressures. The catalyst used is mainly a mixture
of silica and alumina. Most recent catalyst used is
zeolite. The quality and yield of gasoline is greatly
improved by this method.
77. Advantages of catalytic cracking over thermal
cracking:
•High temp and pressure are not required in the
presence of a catalyst.
•The use of catalyst not only accelerates the cracking
reactions but also introduces new reactions which
considerably modify the yield and the nature of the
products.
•The yield of the gasoline is higher.
•No external fuel is required for cracking.
78. •The process can be better controlled so desired
products can be obtained.
•The product contains a very little amount of
undesirable sulphur because a major portion of it
escapes out as H2S gas, during cracking.
•It yields less coke, less gas and more liquid products.
•The evolution of by-product gas can be further
minimized, thereby increasing t he yield of desired
product.
•Catalysts are selective in action and hence cracking
of only high boiling fractions takes place.
•Coke forming materials are absorbed by the catalysts
as soon as they are formed.
79. Knocking and Anti-knocking
In a spark-ignition petrol engine, a phenomenon that occurs when
unburned fuel-air mixture explodes in the combustion chamber
before being ignited by the spark. The resulting shock waves
produce a metallic knocking sound. Loss of power occurs, which
can be prevented by reducing the compression ratio, re-designing
the geometry of the combustion chamber, or increasing the octane
number of the petrol.(formerly by the use of tetraethyl lead anti-
knock additives, but now increasingly by MTBE – methyl tertiary
butyl ether in unleaded petrol). An antiknock agent is a gasoline
additive used to reduce engine knocking and increase the fuel's
octane rating.
The typical antiknock agents in use are:
Tetra-ethyl lead (phased out)
Methyl cyclo pentadienyl manganese tricarbonyl (MMT)
Ferrocene, Iron pentacarbonyl, Toluene, Isooctane
80. Octane rating of a spark ignition engine fuel is a
measure of the resistance to detonation or knocking
compared to a mixture of iso -octane (2,2,4-tri methyl
pentane, an isomer of octane) and n- heptane. It is a
numerical representation of the antiknock properties of
motor fuel, compared with a standard reference fuel,
such as isooctane, which has an octane number of 100.
Octane rating does not relate to the energy content of
the fuel .It is only a measure of the fuel's tendency to
burn in a controlled manner, rather than exploding in an
uncontrolled manner.
81. Octane number: is defined as the percentage of iso
octane present in a mixture of iso-octane and n-
heptane, which has the same knocking characteristics
as that of fuel under examination, under same set of
conditions.
Thus a gasoline with an octane no of 80, would give
the same knocking as a mixture of iso octane and n-
heptane containing 80% of iso octane by volume.
Greater the octane number, greater is the antiknock
property of the fuel.
Cetane Rating: Fuels required for diesel engine are in
contrast to petrol engine fuels, hence a separate scale
is used to grade the diesel oils as they cannot be
graded on octane number scale.
82. The cetane number of a diesel oil is defined as the
percentage of cetane in a mixture of cetane and a-
methyl naphthalene which will have the same ignition
characteristics as the fuel under test, under same set
of conditions.
Cetane is n-hexadecane
The cetane rating of a fuel depend upon the nature
and composition of hydrocarbon.The straight chain
hydrocarbons ignite quite readily while aromatics do
not ignite easily. Ignition quality order among the
constituents of diesel engine fuels in order of
decreasing cetane no, is as follows:
n-alkanes> naphthenes > alkenes > branched alkanes
> aromatics
83. Aniline Point
This is an approximate measure of the aromatic content of
a hydrocarbon fuel.
It is defined as the lowest temperature at which a fuel oil is
completely miscible with an equal volume of aniline.
Aniline is an aromatic compound and aromatics are more
miscible in aniline than are paraffins.
Hence, the lower the aniline point, the higher the
aromatics content in the fuel oil.
The higher the aromatics content, the lower the
cetane number of the fuel.
The aniline point can thus be used to indicate the
probable ignition behavior of a diesel fuel.
84. Diesel Index
The Diesel Index indicates the ignition quality of the
fuel. It is found to correlate, approximately, to the
cetane number of commercial fuels. It is obtained
by the following equation
Diesel Index =
( ) (
aniline po int o F x Degrees API gravity 60o F )
100
In API (American Petroleum Institute) scale, water
at 600F has a 0API Of 10.
Diesel Index and cetane number are usually about
50. Lower values will result in smoky exhaust
85. Gaseous Fuels
Advantages of gaseous fuels
• Least amount of handling
• Simplest burners systems
• Burner systems require least
maintenance
• Environmental benefits: lowest GHG
and other emissions
86. Gaseous Fuels
Classification of gaseous fuels
(A) Fuels naturally found in nature
-Natural gas
-Methane from coal mines
(B) Fuel gases made from solid fuel
-Gases derived from coal
-Gases derived from waste and biomass
-From other industrial processes
(C) Gases made from petroleum
-Liquefied Petroleum gas (LPG)
-Refinery gases
-Gases from oil gasification
(D) Gases from some fermentation
87. Gaseous Fuels
Calorific value
• Fuel should be compared based on the net
calorific value (NCV), especially natural gas
Typical physical and chemical properties of various gaseous fuels
Fuel Relative Higher Heating Air/Fuel Flame Flame
Gas Density Value kCal/Nm3 ratio m3/m3 Temp oC speed m/s
Natural 0.6 9350 10 1954 0.290
Gas
Propane 1.52 22200 25 1967 0.460
Butane 1.96 28500 32 1973 0.870
88. Type of Fuels
Gaseous Fuels
Liquefied Petroleum Gas (LPG)
• Propane, butane and unsaturates, lighter C2
and heavier C5 fractions
• Hydrocarbons are gaseous at atmospheric
pressure but can be condensed to liquid state
• LPG vapour is denser than air: leaking gases
can flow long distances from the source
90. Type of Fuels
Gaseous Fuels
Natural gas
• Methane: 95%
• Remaing 5%: ethane, propane, butane,
pentane, nitrogen, carbon dioxide, other gases
• High calorific value fuel
• Does not require storage facilities
• No sulphur
• Mixes readily with air without producing smoke or
soot
92. Type of Gaseous Fuels
CNG
Compressed natural gas (CNG) is a fossil fuel substitute for gasoline (petrol), diesel,
or propane/LPG. Although its combustion does produce greenhouse gases, it is a
more environmentally clean alternative to those fuels, and it is much safer than
other fuels in the event of a spill (natural gas is lighter than air, and disperses
quickly when released). CNG may also be mixed with biogas, produced
from landfills or wastewater, which doesn't increase the concentration of carbon in
the atmosphere.
CNG is made by compressing natural gas (which is mainly composed
of methane [CH4]), to less than 1% of the volume it occupies at standard
atmospheric pressure. It is stored and distributed in hard containers at a pressure of
200–248 bar (2900–3600 psi), usually in cylindrical or spherical shapes.
Applications
Cars
Locomotives
93. Liquefied Natural Gas
LNG is natural gas that has been super cooled to
minus 260 degrees F becoming liquid for easier
storage and shipping
LNG is a clear, odorless, colorless, non-corrosive
and non-toxic liquid
LNG takes up 1/600th of the space –simplifying
storage and transportation
95. COMBUSTION
Combustion reactions are exothermic reactions
accompanied by evolution of heat and light and the
temperature rises considerably. The amount of oxygen
or air required for combustion of a given sample of fuel
can be calculated.
Calculation of Air Quantities
To determine the amount of oxygen and hence the amount of air
required for combustion for a unit quantity of fuel, the following
chemical principles are applied.
(1) Substances always combine in definite proportions given by
molecular mass.
C + O2 → Co2
12 32 44
12 g of carbon requires 32 g of oxygen and 44 g of CO2 is formed.
96. (2) 22-4 L of a gas at 0°C and 760 mm pressure has a
mass equal to 1 mol. That is, 22-4 L of oxygen has a
mass of 32 g.
(3) Air contains 21% oxygen by volume and 23% oxygen
by mass. From the amount of oxygen required by the
fuel, the amount of air can be calculated.
1 kg oxygen is supplied by 1 x 100/23 = 4.35 kg of air
1 m3 of oxygen is supplied by 1x100/21= 4.76 m3 of air
(4) The molar mass of air is 28.94 g mol
(5) Minimum oxygen required for combustion is equal to
the theoretical oxygen required minus the oxygen
present in the fuel.
97. certain temperature and pressure by assuming that
the gas behaves ideally.
(PV = nRT)
The total amount of oxygen consumed is given by the
sum of the amount of oxygen required by individual
combustible constituents present in the fuel.
98. Procedure for combustion calculations:
Reaction Weight of oxygen Volume of oxygen
required (g) required (m3)
C + O2 → CO2 A × 32/12 A×1
A gm or m3
H2 + 1/2 O2 → H2O B × 16/2 B × 1/2
B gm or m3
CO + 1/2 O2 → CO2 C × 16/28 C × 1/2
C gm or m3
S + O2 → SO2 D × 1 × 32/32 D×1
D gm or m3
CH4 + 2O2 → CO2 + 2H2O E × 2 × 32/16 E×2
E gm or m3
C2H6 + 3.5O2 → 2CO2 + F × 3.5 × 32/30 F × 3.5
3H2O
F gm or m3
C2H4+3O2 → 2CO2+3H2O G × 3 × 32/28 G×3
G gm or m3
C4H10+6.5O2 → 4CO2+5H2O H × 6.5 × 32/58 H × 6.5
H gm or m3
Total X Y
Less O2 in fuel = - w gm = - w m3
99. Let oxygen required = X – w (g) or Y –w (m3)
Since air has 23% oxygen by weight and 21% oxygen
by volume
Weight of air required = Net oxygen × 100/23 g
Volume of air required = Net oxygen × 100/21 g
Conversion of volume to weight
1 m3 = 1000 L
For air 1 L × (mol/22.4 L) × (28.94/mol)
1 L = 28.94/22.4 gm
100. Composition of Combustion Volume of 02
Fuel gas/m3 Reaction required
H2 = 0.5 m3 H2+ 1/2 O2 = H2O 0.50 x 0.5 = 0.25
m3
C2H6 = 0.06 m3 C2H6 + 3.502 = 0.06 x 3.5 = 0.21
2C02 + 3H20 m3
CH4 = 0.30 m3 CH4 + 2O2 = C02 + 0.30 x 2 = 0.6 m3
2H20
CO = 0.08 m3 CO + 1/2 O2 = CO2 008 x 0.5 = 0.04 m3
Total 1.1 m3
Solution:
Volume of air supplied = 1.1 × 100/21 × 120/100
= 6.6 m3 = 6600 L
Weight of air supplied = 28.94 × 6600/22.4 =
8.5Kg
Editor's Notes
Gas fuels are the most convenient because they require the least amount of handling and are used in the simplest and most maintenance-free burner systems. Gas is delivered "on tap" via a distribution network and so is suited for areas with a high population or industrial density. However, large individual consumers do have gasholders and some produce their own gas.
The following types of gaseous fuels exist: Fuels naturally found in nature: - Natural gas - Methane from coal mines Fuel gases made from solid fuel - Gases derived from coal - Gases derived from waste and biomass - From other industrial processes (blast furnace gas) Gases made from petroleum - Liquefied Petroleum gas (LPG) - Refinery gases - Gases from oil gasification Gases from some fermentation process
Since most gas combustion appliances cannot utlilize the heat content of the water vapour, gross calorific value is of little interest. Fuel should rather be compared based on the net calorific value. This is especially true for natural gas, since increased hydrogen content results in high water formation during combustion. Typical physical and chemical properties of various gaseous fuels are given in this table.
LPG is a predominant mixture of propane and butane with a small percentage of unsaturates and some lighter C2 as well as heavier C5 fractions. LPG may be defined as those hydrocarbons, which are gaseous at normal atmospheric pressure but may be condensed to the liquid state at normal temperature by moderate pressures. Although they are normally used as gases, they are stored and transported as liquids under pressure for convenience and ease of handling. LPG vapour is denser than air. Butane is about twice as heavy as air and propane about one and a half times as heavy as air. Consequently, the vapour may flow along the ground and into drains sinking to the lowest level of the surroundings and be ignited at a considerable distance from the source of leakage.
Methane is the main constituent of natural gas and accounts for about 95% of the total volume. Other components are: ethane, propane, butane, pentane, nitrogen, carbon dioxide, and traces of other gases. As methane is the largest component of natural gas, generally properties of methane are used when comparing the properties of natural gas to other fuels. Natural gas is a high calorific value fuel that doesn’t require any storage facilities. It mixes with air readily and does not produce smoke or soot. It has no sulphur content. It is lighter than air and disperses into air easily in case of leak.
LNG, or liquefied natural gas, is the same natural gas that we use in our homes for heating and cooling - except that, prior to being sent into the nation's pipelines, it is transported and stored in liquid form, rather than as a gas To condense natural gas into a liquid, it must be cooled to approximately 260 degrees Fahrenheit below zero (or minus 162 degrees Centigrade) at atmospheric pressure. When natural gas condenses, it takes up about 1/600th of the volume it did when it was in its gaseous state. LNG is a clear liquid that is odorless, colorless, non-corrosive and non-toxic. The liquefying process removes impurities found in typical pipeline gas resulting in a LNG composition of mostly methane with small amounts of other hydrocarbons and nitrogen.
A typical comparison of carbon contents in oil, coal and gas is given in this table. (Reflection time before continuing)
