Last reviewed 16 Sept 2026 · 12 min read
Fuels
A fuel is a combustible substance that releases heat energy on burning.
Classification
| Origin | Solid | Liquid | Gaseous |
|---|---|---|---|
| Primary (natural) | Wood, peat, lignite, coal | Crude petroleum | Natural gas |
| Secondary (derived) | Charcoal, coke, briquettes | Petrol, diesel, kerosene, fuel oil, biodiesel, ethanol | Producer gas, water gas, coal gas, LPG, biogas, hydrogen |
Characteristics of a good fuel
High calorific value; moderate ignition temperature; low moisture and ash; low sulphur and non-polluting combustion products; controllable combustion; easy storage, transport and handling; low cost and availability.
Calorific value
Calorific value (CV) — heat liberated by complete combustion of unit mass (or volume) of fuel.
| Term | Meaning |
|---|---|
| Gross (higher) calorific value, HCV/GCV | Heat released when combustion products are cooled to room temperature — water vapour condensed, latent heat recovered |
| Net (lower) calorific value, LCV/NCV | Heat actually available when water vapour escapes — HCV minus latent heat of steam formed |
Dulong's formula (HCV of coal from ultimate analysis, percentages):
Net calorific value:
(H in %; 587 kcal/kg ≈ latent heat of steam; 9 kg of water forms per kg of hydrogen.)
Bomb calorimeter (solids and liquids): (with corrections), where W = water mass, w = water equivalent of calorimeter, x = mass of fuel. Junkers gas calorimeter — for gaseous fuels.
1 kcal ≈ 4.187 kJ.
Solid fuels — coal
Formation and rank
Plant matter → peat → lignite (brown coal) → sub-bituminous → bituminous → anthracite (coalification with increasing pressure, temperature and time).
With increasing rank: carbon content and calorific value increase; moisture and volatile matter decrease; ignition becomes more difficult (anthracite burns with little smoke).
Analysis of coal
| Analysis | Determines | Significance |
|---|---|---|
| Proximate analysis | Moisture (at |
Practical assessment: high moisture and ash reduce CV; high volatile matter → long smoky flame; high fixed carbon → higher CV |
| Ultimate analysis | C, H, N, S, O (elemental) | Calorific value calculation (Dulong), combustion air, pollution (S) |
Carbonisation
Heating coal in absence of air:
- Low-temperature carbonisation (~500–700 °C) — semi-coke, domestic fuel.
- High-temperature carbonisation (~900–1200 °C) — metallurgical coke (strong, porous, low sulphur) for blast furnaces; by-products coal gas, tar, ammonia (Otto–Hoffman by-product ovens).
Liquid fuels
Petroleum refining
Crude oil is separated by fractional distillation into fractions (approximate boiling ranges): LPG gases → petrol (gasoline) → naphtha → kerosene → diesel → fuel oils → lubricating oils, waxes → bitumen (residue — used in road construction). Cracking (thermal/catalytic) breaks heavy fractions into lighter, more valuable ones; reforming improves octane.
Knocking, octane and cetane numbers
| Concept | Petrol (SI) engines | Diesel (CI) engines |
|---|---|---|
| Knocking | Premature auto-ignition of unburnt end gas → metallic knock, power loss, damage | Long ignition delay → accumulated fuel burns suddenly (diesel knock) |
| Rating | Octane number — % of iso-octane (ON = 100) in a mixture with n-heptane (ON = 0) that knocks like the fuel | Cetane number — % of cetane (n-hexadecane) (CN = 100) in mixture with α-methylnaphthalene (CN = 0) |
| Good fuel | High octane (branched chains, aromatics) | High cetane (straight-chain paraffins) |
| Additives | Anti-knock agents (earlier tetraethyl lead — phased out; now MTBE, ethanol) | Cetane improvers |
- Straight-chain hydrocarbons have low octane but high cetane — so a good petrol is a poor diesel and vice versa.
Alternative liquid fuels
- Biodiesel — methyl esters from vegetable oils/animal fats by transesterification; renewable, biodegradable, lower sulphur.
- Ethanol — from sugarcane molasses, grains; blended with petrol (ethanol blending programmes increase blend ratios over time).
- Power alcohol, methanol.
Gaseous fuels
| Fuel | Main constituents (approx.) | Features / uses |
|---|---|---|
| Natural gas | Mainly methane | High CV; piped gas, power plants, fertilisers |
| CNG | Compressed natural gas (methane) at high pressure | Cleaner vehicle fuel |
| LPG | Propane + butane (liquefied under pressure) | Domestic cooking, industry; heavier than air (leaks settle low); odorant added |
| Biogas | Methane (roughly 55–65%) + CO₂ | Anaerobic digestion of cattle dung, sewage sludge, organic waste; manure by-product |
| Producer gas | CO + N₂ (+ H₂) | Air + steam over hot coke; low CV; furnaces |
| Water gas | CO + H₂ | Steam over red-hot coke; higher CV |
| Coal gas | H₂, CH₄, CO | Carbonisation by-product |
| Hydrogen | H₂ | Highest CV per unit mass, clean (water only); storage/safety challenges; green hydrogen from electrolysis using renewable power |
Advantages of gaseous fuels: clean combustion, easy control, little excess air, no ash; disadvantages: storage and leakage hazards.
Combustion calculations
- C + O₂ → CO₂ : 12 kg C needs 32 kg O₂ (→ 44 kg CO₂)
- 2H₂ + O₂ → 2H₂O : 4 kg H₂ needs 32 kg O₂ (1 kg H needs 8 kg O₂; forms 9 kg H₂O)
- S + O₂ → SO₂ : 32 kg S needs 32 kg O₂
Minimum O₂ per kg fuel (C, H, O, S as mass fractions) Air contains about 23% O₂ by mass (≈ 21% by volume):
Actual air includes excess air for complete combustion.