chemical energy sources – Fuel, Classification of fuels, Calorific Value

Topics to be covered (chemical energy sources) – Introduction to energy, Fuels-definition, classification, importance of hydrocarbon fuels, Calorific value-definition, Gross and Net calorific value, (SI units).Determination of calorific value of a solid/liquid fuel using Bomb calorimeter. Determination of calorific value of a gaseous fuel using Bouy’s calorimeter Petroleum cracking Fluidized catalytic cracking, Reformation of petrol. Numericals.

FUEL

A fuel is a naturally occurring or artificially manufactured carbonaceous material, which acts as a source of heat and light on combustion with oxygen or air.

CLASSIFICATION OF FUELS

On the basis of occurrence, fuels are classified in two types, they are:

  • Primary fuels occur in nature and are used without processing.
  • Secondary fuels are obtained by chemical processing of primary fuels.
chemical energy sources - Fuel, Classification of fuels, Calorific Value
chemical energy sources

CALORIFIC VALUE

It is defined as the amount of heat liberated when a unit weight or unit volume of a fuel is completely burnt in excess amount of air or oxygen and Unit of calorific value in CGS =cal/g; SI unit =J/Kg; for a gas=J/m^{3}

Fuels generally contain hydrogen in addition to carbon. During combustion, the hydrogen is converted to steam and carbon to carbon dioxide.

GROSS CALORIFIC VALUE (HIGHER CALORIFIC VALUE)

It is defined as the amount of heat liberated when a unit weight or unit volume of a fuel is completely burnt in excess amount of air or oxygen and the products of combustion are cooled to ambient temperature. Therefore the heat liberated includes the sensible heat and the latent heat of condensation of water produced during combustion.
GCV = NCV + latent heat of water condensed.

NET CALORIFIC VALUE (LOWER CALORIFIC VALUE)

It is defined as the amount of heat liberated when a unit weight or unit volume of a fuel is completely burnt in excess amount of air or oxygen and the products of combustion are allowed to escape into the atmosphere. Therefore the heat liberated doesn’t include the latent heat of condensation of water. So GCV is always greater than NCV
NCV = GCV – latent heat of water condensed.

DETERMINATION OF CALORIFIC VALUE OF SOLID AND LIQUID FUELS USING BOMB CALORIMETER

The calorific value of solid and liquid fuels can be determined by burning a known mass of the fuel in O_{2} under high pressure using bomb calorimeter.

Principle: The heat liberated by the fuel = heat absorbed by the H_{2}O surrounding the bomb and water equivalent Copper calorimeter
Construction: The bomb calorimeter consists of a stainless steel airtight cylindrical vessel called Bomb. It has an inlet valve for providing atmospheric O_{2} and an electrical ignition coil for the initiation of combustion of fuel. The bomb is placed in a large well-insulated copper calorimeter containing known mass of the water in it. The copper calorimeter is equipped with a mechanical stirrer for dissipation of heat and a thermometer to read the temperature.

chemical energy sources - Fuel, Classification of fuels, Calorific Value
chemical energy sources

Working

A known mass of the fuel is taken in a stainless steel crucible. The crucible containing the fuel is placed inside the bomb. The bomb is sealed air tight by the lid. The sealed bomb containing the sample is next placed in large well-insulated copper calorimeter it is covered by accurately measured quantity of water. The stirrer keeps the water in constant agitation. Initial temperature of the water is carefully measured, the bomb is filled with O_{2} and the combustion of the fuel is initiated by passing electric current through the ignition coil.
As the sample burns in the bomb heat is liberated and is absorbed by the surrounding water and copper calorimeter. The temperature of water is noted before and after the experiment.

Observations and calculation:

Mass of the fuel: m kg
Weight of H_{2}O taken: W_{1} kg
Water equivalent of cooper calorimeter: W_{2} kg
Initial temperature of water: t_{1} {o}_C
Final temperature of water: t_{2} {o}_C
Difference in temperature: Δt {o}_C
Heat liberated by the fuel = heat gained by the H_{2}O + Copper calorimeter
GCV (Q) = \frac{W_{1}+W_{2}}{m} x Δt {o}_C cal /g
= \frac{W_{1}+W_{2} kg}{m kg} x Δt {o}_C x 4.187 kJ /kg
LCV = GCV- H_{2} % x 0.09 x latent heat of steam (587 kcal/g or 2454 kJ/kg)

DETERMINATION OF CALORIFIC VALUE OF GASEOUS FUELS USING BOUYS CALORIMETER

The calorific value of gaseous fuels can be determined by burning a known volume of the fuel using Bouys calorimeter.

Principle: The heat liberated by the fuel = heat absorbed by the H_{2}O flowing in the Cu coils.

Construction: It consists of a combustion chamber. The outer and inner walls of the chamber are coiled by copper tubing through which the cold water is circulated at constant rate. The water enters the copper tube from the top(as inlet) moves down the bottom of chamber, then goes through the inner coils and finally leaves the combustion chamber as outlet. The water circulated is collected using measuring jar. The gas sample is burnt using burner. Kept inside the combustion chamber. The flow is measured using gas meter. The whole assembly is enclosed in a insulated chamber, the stream produced during combustion is collected as condensed water. The thermometer t_{1} & t_{2} placed at the inlet and outlet to note the initial & final temp respectively recorded at steady temp.

chemical energy sources - Fuel, Classification of fuels, Calorific Value
chemical energy sources

Working

The gas sample is burnt at the burner at constant flow rate of the gas and simultaneously cooling water is circulated at constant rate. The above flow is continued for some time to establish steady conditions. At steady condition following observations are made.

Observations and calculation

Mass of the fuel: V m_{3}
Weight of H_{2}O taken: W kg
Initial temperature of water: t_{1} {o}_C
Final temperature of water: t_{2} {o}_C
Difference in temperature: Δt {o}_C
Mass of condensed water= m kg
Heat liberated by the fuel = heat gained by the H_{2}O
Q= GCV = \frac{W}{V} x Δt {o}_C cal /m_{3}
= \frac{W kg}{V m^{3}} x Δt {o}_C x 4.2 kJ /m_{3}
LCV = GCV- \frac{m}{V} x latent heat of steam (587 kcal/g or 2,454 kJ/kg)

CATALYTIC CRACKING

It is a chemical process where high molecular hydrocarbons (high boiling point) are broken into low molecular hydrocarbons (low boiling point)

C_{10}H_{22} (Decane) \rightarrow C_{8}H_{18} (Octane) + C_{2}H_{4} (ethylene)

FLUIDIZED CATALYTIC CRACKING (MOVING BED CATALYTIC CRACKING)

Conditions
Feed stock: crude oil
Temp: 550 {o}_C
Pr: 10- 25 atm pr
Catalyst: AL_{2}O_{3} + SiO_{2}

chemical energy sources - Fuel, Classification of fuels, Calorific Value
chemical energy sources

In the FCC process, the oil is preheated to 300{o}_C and is sprayed from the bottom into the catalytic chamber, containing the catalyst maintained at 700{o}_C to produce the reaction mixture temperature of 550{o}_C. The oil is allowed to undergo cracking .The cracked vapours are then sent into the fractional column where it gets divided into petrol, diesel etc .The uncracked oil remains at the bottom of fractional column simultaneously, the deactivated catalyst is sent into the regenerating chamber where it get activated by removing the carbon deposit using hot air. The regenerated catalyst is again introduced back into the cracking chamber by a current of air. The uncracked oil is recycled into the cracking chamber to get high yield.

REFORMATION

It is a process of bringing structural modifications in the strait chain hydrocarbons (with lower octane number) to increase the octane number and thereby improving the anti-knocking characteristics of petrol. It’s a process of upgrading the octane number of petrol fraction is called as reformation. By reformation high-octane petrol is obtained. If the reformation is done using a catalyst it is called as catalytic reformation.
The process involves a molecular rearrangement of hydrocarbons with out any change in the no of carbon atoms to form new compounds

Conditions:
Feed stock: Low grade petrol
Temp: 470- 550{o}_C
Pr: 10- 25 atm pr
Catalyst: Pt

Octane number’s defined as the percentage of isooctane present in a standard mixture of isooctane and n-heptane, which knocks at the same compression ratio as the petrol being tested.
Isooctane is the branched chain hydrocarbon has least knocking rate, hence its octane number is arbitrarily fixed as 100. N-heptane a straight chain hydrocarbon has highest tendency to knock hence its octane number’s fixed as zero. Octane number of petrol is 80 means it contains 80% by volume isooctane and 20% by volume n- heptane.

These are some of the topics that you have learnt about chemical energy sources.

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