1. Definition and Classification
A fuel is a substance that releases heat energy on combustion.
Classification:
- Primary/Natural Fuels: Occur naturally (e.g., wood, coal, natural gas, crude oil).
- Secondary/Artificial Fuels: Processed from primary fuels (e.g., coke, kerosene, petrol, diesel, CNG).
State-wise:
- Solid: Wood, Coal, Coke
- Liquid: Petrol, Diesel, Kerosene
- Gaseous: Natural Gas, LPG, CNG, Biogas
2. Calorific Value
The total heat energy liberated by the complete combustion of a unit mass or volume of a fuel.
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- Unit for Solid/Liquid: kcal/kg or kJ/kg
Important Values:
-
- Hydrogen: ~150,000 kJ/kg (Highest)
- Methane (CNG): ~55,000 kJ/kg
- Petrol: ~50,000 kJ/kg
- Diesel: ~45,000 kJ/kg
- Coal: ~25,000-33,000 kJ/kg
- Wood: ~17,000-22,000 kJ/kg
- Biogas: ~35,000-40,000 kJ/m³
3. Ignition Temperature
The minimum temperature at which a fuel catches fire and continues to burn. Kerosene has a higher ignition temperature than petrol, making petrol more flammable.
4. Characteristics of a Good Fuel
- High Calorific Value.
- Moderate Ignition Temperature.
- Low moisture and non-combustible content (ash).
- Easy to transport and store.
- Low cost and readily available.
- Burns without emitting harmful gases (i.e., low N and S content).
5. Important Fuel Concepts
- CNG (Compressed Natural Gas): Mostly Methane (CH₄). Clean burning, less CO₂.
- LPG (Liquefied Petroleum Gas): Mainly Butane (C₄H₁₀) and Propane (C₃H₈).
- Biogas: Produced from anaerobic decomposition of organic matter. ~50-70% Methane (CH₄) and ~30-40% CO₂.
- Coking Coal: Used to make coke, a vital fuel in blast furnaces for iron production.
- Firewood Crisis: Deforestation due to excessive use of wood as a primary fuel.
- Fossil Fuels: Coal, Petroleum, Natural Gas. They are non-renewable.
6. Combustion Equations & Efficiency
The complete combustion of a hydrocarbon fuel produces CO₂ and H₂O.
General Equation:
\[ \ce{C_xH_y + (x + \frac{y}{4}) O2 -> x CO2 + \frac{y}{2} H2O + \text{Heat}} \]
Examples:
\[ \ce{CH4 + 2O2 -> CO2 + 2H2O} \quad \text{(Methane Combustion)} \]
\[ \ce{2C8H18 + 25O2 -> 16CO2 + 18H2O} \quad \text{(Octane/Petrol Combustion)} \]
Incomplete Combustion produces poisonous Carbon Monoxide (CO).
\[ \ce{2C + O2 -> 2CO} \]
Efficiency Calculation:
The efficiency of a fuel is given by:
\[ \eta = \frac{\text{Useful Energy Output}}{\text{Total Energy Input}} \times 100\% \]
7. Environmental Impact
- Carbon Fuels -> CO2 -> Global Warming
- Sulphur -> SO2 -> Acid Rain
- Nitrogen -> NOx -> Smog & Acid Rain
- Incomplete Combustion -> CO (Poisonous) & Soot
8. Important Constants & Values
| Fuel | Calorific Value (approx.) | Key Component |
|---|---|---|
| Hydrogen | 150,000 kJ/kg | H₂ |
| Methane (CNG) | 55,000 kJ/kg | CH₄ |
| LPG | 50,000 kJ/kg | C₃H₈ / C₄H₁₀ |
| Petrol (Gasoline) | 47,000 kJ/kg | C₈H₁₈ |
| Diesel | 45,000 kJ/kg | C₁₂H₂₆ |
| Kerosene | 46,000 kJ/kg | C₁₂H₂₆ – C₁₅H₃₂ |
| Bituminous Coal | 30,000 kJ/kg | Carbon |
| Wood | 20,000 kJ/kg | Cellulose |
| Biogas | 40,000 kJ/m³ | CH₄ (~60%) |
9. Key Differences
- Petrol vs. Diesel: Petrol is more volatile, has lower ignition temp, and is used in SI engines. Diesel is less volatile, has higher ignition temp (compression ignition), and is more efficient.
- CNG vs. LPG: CNG is lighter than air (methane), LPG is heavier than air (propane/butane). CNG is stored under high pressure, LPG is stored as a liquid under moderate pressure.