PRELIMS

Fuels and Combustion

A fuel is a substance that supplies energy through reaction (combustion). Heat produced is measured in Calories.

Calorific Value

The total quantity of heat liberated by complete combustion of a unit mass/volume of fuel in air/oxygen.

• Units: or B.T.U. (British Thermal Unit) per cubic foot.
• Conversion: .

Ideal Fuel Characteristics

1. High calorific value.
2. Low ignition temperature.
3. Cheap, readily available, easy to store/transport.
4. Regulated and controlled combustion.

Gaseous Fuels

Gaseous fuels are generally considered the best fuels (cleaner/higher efficiency).

• Water Gas (Syn Gas): Composition: . Formation / Notes: Steam on red-hot coke ( ). High calorific value ( ).
• Producer Gas: Composition: . Formation / Notes: Burning coke in limited air. Cheapest; low calorific value (due to ).
• Coal Gas: Composition: . Formation / Notes: Destructive distillation of coal ( ).
• Oil Gas: Composition: . Formation / Notes: Thermal cracking of kerosene. Used in laboratories.
• Gobar Gas: Composition: . Formation / Notes: Fermentation of gobar in absence of air. Domestic fuel in villages.
• Natural Gas: Composition: 85% Methane ( ), Ethane, Propane. Formation / Notes: Clean fossil fuel; L.P.G. is mainly Butane and Isobutane.

Octane and Cetane Numbers

• Octane Number: Measure of a petrol/gasoline fuel's resistance to knocking in spark-ignition (petrol) engines. Higher octane = better anti-knock quality.
  • Scale: Iso-octane = 100 (least knocking); n-Heptane = 0 (most knocking).
  • Additives to raise octane number: Tetraethyl lead (TEL, now phased out due to toxicity), MTBE, ethanol blending.
• Cetane Number: Measure of a diesel fuel's ignition quality (how readily it self-ignites under compression) in compression-ignition (diesel) engines. Higher cetane = shorter ignition delay = better quality.
  • Scale: Cetane (n-Hexadecane) = 100; Alpha-methylnaphthalene = 0.
• Note: Octane number and Cetane number trends are roughly inverse — fuels with high octane (good for petrol engines) tend to have low cetane, and vice versa.

Hierarchy of Coal

Classified based on carbon content and calorific value.

• 1.: Type: Peat. % Carbon: 50–60%. Calorific Value: 2500–3500. Notes: Low grade; more smoke and ash.
• 2.: Type: Lignite. % Carbon: 60–70%. Calorific Value: 3500–4500. Notes: High moisture; brown coal.
• 3.: Type: Bituminous. % Carbon: 75–80%. Calorific Value: 7500–8000. Notes: Black, hard; common domestic fuel.
• 4.: Type: Anthracite. % Carbon: 90–95%. Calorific Value: 6700–7500. Notes: Superior quality; hardest form.

UPSC Relevance

Energy is a critical GS-III pillar.

• Fuel Mixtures: Differentiating between Water Gas ( ) and Producer Gas ( ) is a common Prelims check.
• Coal Types: Knowing that Anthracite is the highest quality and Peat the lowest is fundamental for Geography/Science.
• Bio-fuels: The composition of Gobar gas (mostly Methane) links to rural energy schemes.