Fuel is a combustible substance which during combustion gives large amount of heat. There are chemical fuels, nuclear fuels and fossil fuels.

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1 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. 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.

2 (ii) On the basis of physical state these may be classified as: (a) Solid Fuels (b) Liquid Fuels (c) Gaseous Fuels Coal classification Solid Fuels Anthracite: hard and geologically the oldest Bituminous Lignite: soft coal and the youngest Further classification: semi- anthracite, semibituminous, and sub-bituminous

3 Classification Liquid Fuels (a) Diesel (b) Petrol (c) Kerosine (d) Gasoline (e) Ethanol Classification Gaseous Fuels (a) Natural Gas (b) Producer gas (c) LPG (d) Coal Gas

4 Calorific Value Calorific value is defined as the amount of heat liberated when a unit mass of fuel is burnt completely in the presence of air or oxygen. Calorific value is of two types as follows:- 1) Higher calorific value. (HCV) 2) Lower calorific value. (LCV) 1) HCV: - It is the amount of heat liberated when a unit mass of fuels burnt completely in the presence of air or oxygen and the products of combustion are cooled to room temperature. Here it includes the heat liberated during combustion and the latent heat of steam. 2) LCV: - It is amount of heat liberated when a unit mass of fuel is burnt completely in the presence of air or oxygen and the product of combustion are let off completely into air. It does not include the latent heat of steam.

5 Characteristics of Good Fuel: (i) Suitability: (ii) High Calorific value (iii) Ignition Temperature: (iv) Moisture content: (v) Non combustible matter content (vi) Velocity of combustion: (vii) Nature of the products (viii) Cost of fuel, (ix) Smoke, (x) Control of the process

6 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

7 Bomb Calorimeter

8 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 = t 1o C Final temperature of water = t 2o C Higher or gross calorific value = C cal/g Heat gained by water = W x Dt x specific heat of water = W (t 2 -t 1 ) x 1 cal

9 Heat gained by Calorimeter = w (t 2 -t 1 ) cal Heat liberated by the fuel = x C cal Heat liberated by the fuel = Heat gained by water and calorimeter x C = (W+w) (t 2 -t 1 ) cal C=(W+W)(t 2 -t 1 ) cal/g x

10 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:

11 C H Cracking C H C H Decane n-pentane pentene B.Pt 174 ο C B.Pt 36 ο C C H 8 18 Cracking C H 5 12 C H 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

12 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 tetramethyl lead antiknock 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

13 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.

14 n-alkanes> naphthenes > alkenes > branched alkanes > aromatics 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:

15 Type of Fuels Gaseous Fuels Natural gas Methane: 95% Remaing 5%: ethane, propane, butane, nitrogen, carbon dioxide, other gases High calorific value fuel Does not require storage facilities No sulphur pentane, Mixes readily with air without producing smoke or soot

16 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 [CH 4 ]), 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 bar ( psi), usually in cylindrical or spherical shapes.