Combustion and Fuels — Study Notes

Overview

Combustion and fuels is a practical chemistry topic tested in RRB Group D General Science with 2–3 direct questions expected. Understanding combustion types, calorific values, and fuel properties is crucial since these concepts connect chemistry with everyday applications like cooking, vehicle engines, and energy production. Questions typically ask about ignition temperature, conditions for combustion, comparing calorific values of fuels, or identifying combustion types from scenarios.

Mastery requires knowing the three essential conditions for combustion, differentiating between rapid, spontaneous, and slow combustion, understanding what makes a good fuel, and memorizing calorific values of common fuels like LPG, CNG, coal, and wood. The exam focuses on practical knowledge—recognizing which fuel is most efficient, why certain substances catch fire easily, and how combustion affects daily life and the environment.

This topic overlaps with environmental awareness (carbon emissions, clean fuels) and physics (energy conversion), making it valuable for holistic exam preparation. Focus on definitions, comparative data, and real-world applications rather than complex chemical equations.

Key Concepts

• **Combustion** is a chemical process where a substance reacts with oxygen, releasing heat and light. It is an exothermic oxidation reaction. Common examples include burning of wood, coal, petrol, and natural gas.

• **Three conditions required for combustion**: (1) presence of a combustible substance (fuel), (2) presence of a supporter of combustion (oxygen/air), and (3) heating to ignition temperature. Remove any one condition and combustion stops—this principle is used in fire extinguishers.

• **Ignition temperature** is the minimum temperature at which a substance catches fire and starts burning. Different substances have different ignition temperatures: phosphorus has very low ignition temperature (about 35°C), while coal has high ignition temperature (around 600°C).

• **Combustible vs non-combustible substances**: Combustible substances can burn (wood, paper, petrol, LPG, coal). Non-combustible substances cannot burn (stone, glass, iron, water). Some substances like sodium and phosphorus are highly combustible and can catch fire at room temperature.

• **Calorific value** is the amount of heat energy produced when 1 kilogram of fuel is completely burnt. It is measured in kilojoules per kilogram (kJ/kg). Higher calorific value means better fuel efficiency—more energy per unit mass.

• **Ideal fuel characteristics**: high calorific value, moderate ignition temperature, burns without smoke or harmful gases, easy to store and transport, economical, leaves minimal residue (ash), and readily available. No single fuel meets all criteria perfectly.

• **Flame** is the visible gaseous part of fire. Substances that vaporize during burning produce flame (candles, LPG, kerosene). Substances that don't vaporize don't produce flame (coal, charcoal). Flames have different zones with varying temperatures and combustion states.

• **Clean fuels vs traditional fuels**: CNG, LPG, and biogas produce less pollution compared to coal and wood. Government promotes CNG in vehicles and LPG for cooking to reduce air pollution and improve public health.

Formulas / Key Facts

**Calorific Values of Common Fuels (kJ/kg):**

• Hydrogen: 150,000 (highest among common fuels)
• LPG (Liquefied Petroleum Gas): 55,000
• CNG (Compressed Natural Gas): 50,000
• Petrol: 45,000
• Kerosene: 45,000
• Diesel: 45,000
• Coal: 25,000–35,000 (varies with quality)
• Wood: 17,000–22,000
• Cow dung cake: 6,000–8,000 (lowest efficiency)

**Ignition Temperatures:**

• White phosphorus: 35°C (catches fire at room temperature)
• Petrol: 246°C
• Kerosene: 295°C
• Wood: 300°C
• Coal: 600°C

**Fire Triangle:** Fuel + Oxygen + Heat = Fire (remove any one element to extinguish fire)

**Combustion Equation (general):** Fuel + Oxygen → Carbon dioxide + Water + Heat + Light

**Fossil fuels:** Coal, petroleum, natural gas (formed from dead organisms over millions of years, non-renewable)

**Greenhouse gas from combustion:** Carbon dioxide (CO₂) contributes to global warming

Worked Examples

**Example 1: Which is the better fuel — wood or LPG? Why?**

**Solution:** LPG is the better fuel.

• **Calorific value:** LPG has 55,000 kJ/kg while wood has only 17,000–22,000 kJ/kg. LPG produces nearly 3 times more heat per kg.
• **Smoke and pollution:** LPG burns cleanly with minimal smoke; wood produces significant smoke and harmful particulates.
• **Convenience:** LPG ignites easily, has controllable flame; wood requires preparation and produces ash.
• **Storage:** LPG is stored in compact cylinders; wood requires large storage space.

**Example 2: A matchstick lights when struck but not when simply kept in air. Explain using combustion principles.**

**Solution:**

• Matchstick head contains phosphorus (combustible substance) and surrounding air has oxygen (supporter of combustion).
• However, it doesn't burn at room temperature because it hasn't reached its **ignition temperature**.
• When struck against the rough surface, friction generates heat that raises the temperature above ignition point.
• Once ignition temperature is reached, combustion begins and the matchstick lights up.
• This demonstrates that all three conditions—fuel, oxygen, and sufficient heat—must be present simultaneously.

**Example 3: Why do fire extinguishers use CO₂ or foam to put out fires?**

**Solution:**

• Fire extinguishers work by removing one or more conditions from the fire triangle.
• **CO₂ extinguisher:** Carbon dioxide is heavier than air and forms a blanket over burning material, cutting off oxygen supply. It also cools the burning substance, bringing temperature below ignition point.
• **Foam extinguisher:** Foam covers the fire, preventing oxygen from reaching the fuel. It's especially effective for liquid fires (petrol, oil) as water would spread such fires.
• **Water extinguisher:** Cools burning material below ignition temperature and converts to steam which displaces oxygen. Not suitable for electrical or oil fires.

Common Mistakes

**Mistake 1:** Thinking substances with low ignition temperature are always better fuels. **Correction:** Low ignition temperature substances (like phosphorus) are dangerous for storage and transport. Good fuels need moderate ignition temperature—high enough for safe handling but low enough for easy ignition when needed. Petrol's ignition temperature (246°C) is safer than phosphorus (35°C).

**Mistake 2:** Assuming all combustion is rapid and visible. **Correction:** Combustion can be slow (rusting of iron, human respiration, decomposition of organic matter), spontaneous (phosphorus at room temperature, forest fires in summer), or rapid (burning of LPG, matchstick). Slow combustion doesn't produce visible flame but still releases heat over time.

**Mistake 3:** Confusing flame and fire or thinking all burning produces flame. **Correction:** Flame is specifically the glowing gaseous zone of combustion. Coal and charcoal burn with fire but without flame because they don't vaporize. Candle wax vaporizes, so produces flame. This is why "flameless combustion" exists.

**Mistake 4:** Believing higher calorific value always means better practical fuel. **Correction:** While hydrogen has the highest calorific value (150,000 kJ/kg), it's not widely used as fuel because it's highly explosive, difficult to store safely, and expensive to produce. Practical fuel choice depends on safety, cost, availability, and environmental impact—not just calorific value.

**Mistake 5:** Thinking water can extinguish all types of fires. **Correction:** Water should NOT be used on electrical fires (risk of electrocution), oil/petrol fires (water is denser, oil floats and spreads), or metal fires (some metals react violently with water). Different fire types require specific extinguishing methods.

Quick Reference

• **Combustion = Fuel + Oxygen + Heat → Energy + Products** (always exothermic)
• **Best common fuel for cooking:** LPG (55,000 kJ/kg, clean burning, convenient)
• **Three types of combustion:** Rapid (matchstick), Spontaneous (phosphorus at room temp), Slow (rusting, respiration)
• **Ignition temperature:** Minimum temperature needed for combustion to begin
• **CNG and LPG are cleanest fossil fuels** — less CO₂ and no sulfur/lead emissions
• **Flame zones:** Outer (complete combustion, hottest), Middle (partial combustion), Inner (unburnt vapors, coldest)
• **Fire extinguisher principle:** Remove fuel, oxygen, or heat from the fire triangle
• **Incomplete combustion produces carbon monoxide (CO)** — poisonous gas, causes suffocation
• **Calorific value order:** Hydrogen > LPG > CNG > Petrol/Kerosene > Coal > Wood > Dung cake