Unit 4 (Part A) - Chemistry

1) Varieties (ranks) of coal

Peat: Partly decayed plant matter (precursor).
Lignite: Brown coal; high moisture; low CV.
Sub-bituminous: Between lignite and bituminous.
Bituminous: Common grade; coking/non-coking.
Anthracite: Highest rank; high C; clean, hard, shiny.

2) Cetane number (diesel)

Measures ignition quality (ease of auto-ignition) of diesel fuel. Higher cetane ⇒ shorter ignition delay, smooth combustion, less noise.

Cetane no. ~ % reference cetane (n-hexadecane) in blend giving same delay

3) Ways to reduce carbon footprint

Energy efficiency (insulation, LED, efficient motors).
Switch to renewables (solar, wind) & green tariffs.
Transport: public transit, EVs, car-pooling, cycling.
Reduce–reuse–recycle; cut high-impact foods (beef).
Process changes, waste heat recovery, carbon capture.

4) Knocking (SI engine)

Uncontrolled, rapid auto-ignition of the end-gas ahead of the spark-initiated flame → pressure waves → metallic “ping”. Causes power loss, overheating, damage. Reduced by high octane fuels, correct timing, cooling.

5) Calorific value (CV)

Total heat released by complete combustion of unit mass/volume of fuel with products cooled to reference temperature.

Units: kJ·kg⁻¹, MJ·m⁻³, kcal·kg⁻¹, BTU·lb⁻¹.

6) Disadvantage of sulphur in coal

On burning → SO₂ / SO₃ ⇒ acid rain, corrosion, respiratory issues; also causes fouling and catalyst poisoning.

7) What is a calorie? Units of CV

1 calorie = heat to raise 1 g water by 1°C (≈4.186 J).

CV units: J·kg⁻¹ (kJ·kg⁻¹), cal·g⁻¹ (kcal·kg⁻¹), BTU·lb⁻¹, for gases MJ·m⁻³.

8) Classification of coal

By rank (degree of coalification): lignite → sub-bituminous → bituminous → anthracite (increasing fixed carbon, decreasing moisture/volatiles; higher CV).

9) Power alcohol & biodiesel

Power alcohol: Ethanol fuel (from fermentation of sugars/starch) blended with petrol (e.g., E10/E20) to raise octane & cut CO/HC.
Biodiesel: Fatty acid methyl esters (FAME) made by transesterification of vegetable oils/animal fats; used neat (B100) or blends (B5–B20).

10) Spontaneous (auto) ignition temperature

Lowest temperature at which a substance self-ignites in air without external spark/flame. Depends on composition, pressure, vessel size, and turbulence.

11) Octane vs Cetane number

Parameter	Octane (SI – petrol)	Cetane (CI – diesel)
Meaning	Resistance to knock	Ease of auto-ignition
Higher value implies	Less knocking, can use higher compression	Short delay, smoother running
Reference	Iso-octane (100), n-heptane (0)	n-cetane (100), α-methylnaphthalene (0)

12) Advantages of diesel over petrol

Higher thermal efficiency & better mileage.
Greater low-end torque; good for heavy loads.
Engines usually more durable; fuel has higher volumetric energy density.

13) Ignition temperature (definition)

Minimum temperature to which a fuel must be heated (with air/oxygen) to start self-sustained combustion, independent of the external heat source.

14) Classification of fuels

By physical state: Solid (coal, coke, wood), Liquid (petrol, diesel, ethanol), Gas (LPG, NG, H₂, producer gas).
By occurrence: Primary (natural: coal, petroleum, NG), Secondary (derived: coke, gasoline, biogas, producer gas).
By source: Fossil, biofuels, synthetic fuels.

15) Gross calorific value (GCV) vs Net (NCV)

GCV/HCV: Heat released when products are cooled to initial temperature and condensed water’s latent heat is recovered.

NCV/LCV: Excludes latent heat of water vapor. Hence GCV = NCV + latent heat of vaporization.

🔥 Unit 4 – Part A (2-Mark Q&A)

Part A: Fuels & Combustion