Fuels and Fossil Fuels

Fuels are substances that undergo combustion to release energy, primarily in the form of heat.

Based on Occurrence

• Primary (Natural) Fuels: Found in nature in their raw form. Examples include wood, coal, crude oil, and natural gas.
• Secondary (Derived) Fuels: Processed or manufactured from primary fuels to improve efficiency. Examples include charcoal, coke, petrol, diesel, and water gas.

Based on Physical State

• Solid Fuels: Wood, coal, peat, and dung cakes. They have lower calorific values and leave significant ash residue.
• Liquid Fuels: Petroleum, diesel, kerosene, and alcohol. They possess higher calorific values than solid fuels and burn without ash.
• Gaseous Fuels: Natural gas, Liquefied Petroleum Gas (LPG), Compressed Natural Gas (CNG), and biogas. They have the highest efficiency and lowest pollutant emission.

Fossil Fuels: Formation and Composition

Fossil fuels are formed via the anaerobic decomposition of buried dead organisms over millions of years under high pressure and temperature conditions (carbonization).

Coal

Coal is a sedimentary rock composed primarily of carbon, hydrogen, and oxygen, along with minor amounts of sulfur and nitrogen. It is classified into four main types based on carbon content and heating value.

Petroleum (Crude Oil)

Petroleum is a naturally occurring, yellowish-black liquid found in geological formations beneath the Earth’s surface. It consists of a complex mixture of hydrocarbons (mostly alkanes, cycloalkanes, and aromatic hydrocarbons) and other organic compounds.

Natural Gas

Natural gas is a gaseous fossil fuel consisting primarily of methane (CH₄, typically 70–90%), along with ethane, propane, butane, and carbon dioxide. It occurs in deep underground rock formations, often alongside petroleum deposits.

Refining of Petroleum and Petrochemicals

Crude oil cannot be used directly in its raw form. It undergoes fractional distillation in a refinery to separate its components based on their varying boiling points.

Fractional Distillation Fractions

Petrochemicals

Petrochemicals are chemical products derived from petroleum or natural gas. They serve as raw materials for producing synthetic fibers (Nylon, Polyester), synthetic rubber, plastics (Polythene, PVC), detergents, dyes, and pharmaceuticals.

Fuel Quality Indicators

The performance, efficiency, and knocking characteristics of internal combustion engines depend directly on the chemical structure of the fuel used.

Octane Number (For Petrol)

• It measures the anti-knock property of petrol (gasoline). Knocking is the pre-ignition of fuel in the cylinder, which reduces engine efficiency and causes damage.
• Iso-octane (C₈H₁₈) is highly resistant to knocking and is assigned an Octane Number of 100.
• n-heptane is highly prone to knocking and is assigned an Octane Number of 0.
• Aviation Fuel: Requires a high octane rating (often greater than 100) to withstand high compression ratios without premature detonation.

Cetane Number (For Diesel)

• It measures the ignition delay period of diesel fuel. A higher cetane number indicates a shorter ignition delay and smoother combustion.
• Hexadecane (Cetane) ignites rapidly under compression and is assigned a Cetane Number of 100.
• Alpha-methylnaphthalene has a long ignition delay and is assigned a Cetane Number of 0.

Gaseous Fuels and Modern Alternatives

Liquefied Petroleum Gas (LPG)

• Composition: Primarily Propane (C₃H₈) and Butane (C₄H₁₀).
• Safety Trivia: LPG is naturally odorless. Ethane thiol (Ethyl Mercaptan, C₂H₅SH) is added during processing to impart a strong, distinct smell for leak detection.

Compressed Natural Gas (CNG)

• Composition: Composed mainly of Methane (CH₄).
• Properties: Compressed at high pressure (200 to 250 bar). It is lighter than air and disperses rapidly in case of a leak, making it safer than LPG in open spaces. It burns more cleanly than petrol or diesel.

Liquefied Natural Gas (LNG)

• Composition: Natural gas cooled to approximately -162°C, changing its state to a liquid for compact storage and long-distance maritime transport where pipelines are non-viable.

Biogas (Gobardhan Gas)

• Composition: Produced by the anaerobic decomposition of organic matter (cattle dung, agricultural waste). It consists of Methane (50–70%), Carbon Dioxide (30–45%), and traces of Hydrogen Sulfide (H₂S).

Coal Bed Methane (CBM)

• Definition: A form of natural gas extracted directly from coal beds. The methane is adsorbed onto the solid matrix of the coal.

Chemistry of Combustion and Calorific Value

Combustion is an exothermic chemical reaction between a fuel and an oxidant (usually atmospheric oxygen) that releases heat and light.

Complete vs. Incomplete Combustion

• Complete Combustion: Occurs in a sufficient supply of oxygen. Hydrocarbons convert entirely into Carbon Dioxide (CO₂) and Water (H₂O). The flame color is blue.
• Incomplete Combustion: Occurs when oxygen supply is limited. It produces Carbon Monoxide (CO), unburnt carbon particles (soot), and less heat. The flame color is yellow.

Calorific Value (CV)

• The amount of heat energy produced by the complete combustion of a unit mass of a fuel. It is expressed in Kilojoules per Gram (kJ/g) or Megajoules per Kilogram (MJ/kg).

Comparative Calorific Values of Common Fuels

Key Facts for UPSC Prelims

• Gasohol: A mixture of 90% petrol and 10% anhydrous ethyl alcohol (ethanol). It acts as an oxygenate to reduce carbon monoxide emissions in automobiles.
• Water Gas: A mixture of Carbon Monoxide (CO) and Hydrogen (H₂), produced by passing steam over incandescent coke.
• Producer Gas: A mixture of Carbon Monoxide (CO) and Nitrogen (N₂), formed by passing air over red-hot coal.
• Syngas (Synthesis Gas): A mixture of Carbon Monoxide, Hydrogen, and Carbon Dioxide, often used as an intermediate to produce synthetic natural gas or ammonia.
• Oil Zapper: A biotechnology solution developed by TERI (The Energy and Resources Institute) using a cocktail of five bacterial strains to degrade crude oil sludge and clean up oil spills.