combustion of fuel

Combustion of Fuel: Principles, Air-Fuel Ratio, and Efficiency

Combustion is the fundamental process that powers boilers, furnaces, and engines. It is the chemical reaction between fuel and oxygen (O₂) that releases heat energy. Understanding how combustion works, and how air-fuel ratios affect efficiency, is essential for safe and economical boiler operation.  

🔍 Composition of Fuels

All fuels consist mainly of:  

- Carbon (C)  

- Hydrogen (H)  

- Oxygen (O)  

- Nitrogen (N)  

- Sulfur (S)  

- Minerals (ash)  

- Water (H₂O)  

These elements determine the calorific value, combustion characteristics, and emissions of the fuel.  

⚙️ Complete vs Incomplete Combustion

- Complete Combustion:  

  - All carbon burns to CO₂.  

  - All hydrogen burns to H₂O.  

  - All sulfur burns to SO₂.  

  - Requires sufficient oxygen supply.  

- Incomplete Combustion:  

  - Occurs when air supply is insufficient.  

  - Produces carbon monoxide (CO), soot, and smoke.  

  - Leads to fouling of heat transfer surfaces, pollution, lower efficiency, and flame instability.  

🌍 Effect of Air Supply

- Too Little Air:  

  - Incomplete combustion, smoke, CO formation, soot deposits.  

  - Risk of explosion due to unburned fuel.  

- Too Much Air:  

  - Surplus air absorbs heat and exits through the chimney.  

  - Increases stack losses and reduces efficiency.  

- Optimum Excess Air:  

  - Each fuel type has an ideal excess air level for complete combustion.  

  - Balances efficiency, safety, and emission control.  

📊 Stoichiometric Ratio and Fuel Mixtures

- Stoichiometric Ratio: The exact amount of air required for complete combustion of a given fuel.  

- Fuel-Lean Mixture: Air content higher than stoichiometric ratio.  

- Fuel-Rich Mixture: Air content lower than stoichiometric ratio.  

🔥 Air Requirement for Different Fuels

- Fuel Oil:  

  - Requires ~14.1 kg of air per kg of fuel oil.  

  - Recommended excess air: 10–20%.  

- Coal:  

  -  ~7–8 kg of air per kg of coal (depending on composition).  

  - Recommended excess air:  

    - Stoker coal: 35–40%  

    - Pulverized coal: 20–25%  

- Natural Gas:  

  - Requires ~9–10 kg of air per kg of gas.  

  - Recommended excess air: 10–20%.  

📐 Excess Air Formula

Excess Air(EA) = Mass of Air Supplied (kg)/Stoichiometric Air Requirement (kg/kg fuel)

This ratio helps operators determine whether combustion is fuel-rich, fuel-lean, or optimal.  

✅ Flue Gas Indicators for Optimum Combustion

- For fuel oil combustion:  

  - CO₂ in flue gas: 14–15.5%  

  - O₂ in flue gas: 2–3.5%  

Maintaining these values ensures maximum efficiency and minimum emissions.  

📌 Conclusion

Efficient combustion requires balancing the air-fuel ratio to achieve complete combustion while minimizing stack losses.  

- Too little air → incomplete combustion, CO, soot, pollution.  

- Too much air → heat loss, reduced efficiency.  

- Optimum excess air levels vary by fuel type and must be carefully monitored using flue gas analysis.  

By understanding combustion principles and controlling air supply, industries can achieve higher efficiency, lower fuel costs, and safer boiler operation.