What about the air-fuel ratio in the combustion chamber? What is an excess air-fuel ratio for natural gas combustion in a gas turbine?

Air-Fuel Ratio in Combustion Chambers and Excess Air for Natural Gas in Gas Turbines

Efficient combustion is the backbone of energy generation in boilers, furnaces, and gas turbines. One of the most critical parameters in combustion engineering is the air-fuel ratio (AFR), which determines how much oxygen is available to react with the fuel. For natural gas-fired gas turbines, the concept of excess air plays a vital role in balancing complete combustion, flame stability, and turbine metallurgy protection.  

🔍 What is the Air-Fuel Ratio?

- The air-fuel ratio (AFR) is the proportion of air supplied to the combustion chamber relative to the amount of fuel.  

- Stoichiometric AFR: The exact amount of air required to completely burn all the carbon (C), hydrogen (H), and sulfur (S) in the fuel.  

- For complete combustion:  

  - 1 g Carbon (C) requires 2.67 g Oxygen (O₂).  

  - 1 g Hydrogen (H) requires 8 g Oxygen (O₂).  

  - 1 g Sulfur (S) requires 1 g Oxygen (O₂).  

This ensures all carbon converts to CO₂, hydrogen to H₂O, and sulfur to SO₂.  

⚙️ Combustion of Natural Gas

Natural gas is primarily methane (CH₄), which has a high hydrogen content.  

- During combustion, hydrogen converts to water vapor (H₂O).  

- This vapor leaves the stack carrying latent heat of vaporization, which can account for up to 10% of the fuel’s energy loss, depending on flue gas temperature.  

- This is why natural gas combustion efficiency is closely tied to air-fuel ratio optimization.  

🌍 Excess Air in Gas Turbines

- Excess Air: The additional air supplied beyond the stoichiometric requirement.  

- In gas turbines, excess air is not only for complete combustion but also for temperature control.  

- Turbine blades and metallurgy can only withstand certain peak temperatures.  

- Large amounts of air are mixed after combustion to cool the gases to safe operating levels.  

- A significant portion of turbine energy is used to move this air through the system.  

📊 Typical Excess Air Ratios

- For natural gas combustion in gas turbines, excess air ratios are typically 2 to 3 times the stoichiometric requirement.  

- This ensures:  

  - Complete combustion of methane.  

  - Controlled flame temperature.  

  - Protection of turbine blades from overheating.  

🔥 Efficiency Considerations

- Complete combustion of coal or natural gas can save 10–15% fuel compared to incomplete combustion.  

- However, excess air must be carefully optimized:  

  - Too little air → incomplete combustion, CO formation, soot deposits.  

  - Too much air → reduced flame temperature, lower efficiency, higher NOx emissions.  

💡 Innovative Concepts

Research continues into recovering energy lost in water vapor:  

- Since hydrogen in natural gas converts to H₂O vapor, splitting this vapor back into hydrogen and oxygen could theoretically improve efficiency.  

- Advanced fuel technologies (e.g., alumina-rich firing concepts) are exploring ways to reclaim this energy, though practical application is still under development.  

📌 Conclusion

The air-fuel ratio is central to combustion efficiency and emission control. In gas turbines burning natural gas:  

- Excess air is required not only for complete combustion but also for temperature moderation.  

- Typical excess air ratios are 2–3 times stoichiometric, balancing efficiency and turbine safety.  

- Optimizing AFR and excess air can significantly reduce fuel consumption, improve performance, and lower emissions.