Combustion and Fuels

Why it matters

Combustion and fuels are central to energy production, which powers industries, vehicles, and homes. Understanding combustion processes helps in designing efficient engines and reducing emissions, which is crucial for sustainable development.

Key ideas

• Combustion: A chemical process where a fuel reacts with an oxidizer, releasing energy in the form of heat and light. It is typically an exothermic reaction.
• Fuels: Substances that release energy upon combustion. They can be solid (coal), liquid (petrol, diesel), or gaseous (natural gas).
• Stoichiometry: The calculation of reactants and products in chemical reactions. In combustion, it involves balancing the fuel and oxidizer to ensure complete combustion.
• Calorific Value: The amount of energy released when a fuel is completely combusted. It is measured in kJ/kg for solids and liquids, and kJ/m³ for gases.
• Air-Fuel Ratio: The ratio of air to fuel in a combustion process. It is crucial for achieving efficient combustion and minimizing pollutants.
• Incomplete Combustion: Can occur with insufficient oxygen, poor mixing, quenching or inadequate reaction time, leading to the formation of carbon monoxide and other pollutants.

Formulas

• Q = m·CV
  • Q: Heat energy released (kJ)
  • m: Mass of fuel (kg)
  • CV: Calorific value of the fuel (kJ/kg)
• AFR = (mass of air) / (mass of fuel)
  • AFR: Air-Fuel Ratio (dimensionless)
• Stoichiometric AFR = (mass of oxygen required) / [(oxygen mass fraction in air) × (mass of fuel)]
  • Stoichiometric AFR: Stoichiometric Air-Fuel Ratio (dimensionless)

Do not confuse oxygen–fuel ratio with air–fuel ratio. For methane, CH₄ + 2O₂ → CO₂ + 2H₂O requires 64 kg O₂ per 16 kg CH₄ using rounded molar masses. With a specified air oxygen mass fraction 0.232, stoichiometric AFR = 4/0.232 ≈ 17.24 kg air/kg fuel. Excess-air ratio λ = AFR_actual/AFR_stoich and equivalence ratio φ = 1/λ.

Higher heating value includes recovery associated with condensing product water; lower heating value keeps that water as vapor. State the reference conditions, especially for a volumetric gaseous-fuel value. Heat released on a calorific-value basis is not automatically useful heat delivered to a load.

Worked example

Problem: Calculate the heat energy released when 2 kg of a fuel with a calorific value of 45,000 kJ/kg is completely combusted.

Given:

• Mass of fuel, m = 2 kg
• Calorific value, CV = 45,000 kJ/kg

Steps:

1. Use the formula for heat energy released: Q = m·CV
2. Substitute the given values: Q = 2 kg · 45,000 kJ/kg
3. Calculate: Q = 90,000 kJ

Final Answer: 90,000 kJ

Common mistakes

• Confusing the units of calorific value for different states of fuel (solid, liquid, gas).
• Incorrectly balancing the combustion equation, leading to errors in stoichiometric calculations.
• Ignoring the effects of incomplete combustion on energy calculations and emissions.

For GATE ME

Questions often involve calculating the heat energy released, determining the air-fuel ratio, and analyzing the efficiency of combustion processes. Practice balancing combustion equations and understanding the implications of different air-fuel ratios.

Quick check

1. What is the primary product of complete combustion of hydrocarbons?
2. Define calorific value.
3. What happens if the air-fuel ratio is too low?

Answers: 1. Carbon dioxide and water. 2. The energy released per unit mass of fuel when completely combusted. 3. Incomplete combustion occurs, producing carbon monoxide and other pollutants.