9.2 Air-Fuel Ratio and Excess Air Calculations

Air-fuel ratio and excess air are crucial concepts in combustion processes. They determine how efficiently fuel burns and impact emissions. Understanding these ratios helps engineers optimize combustion systems for better performance and reduced environmental impact.

Calculating air-fuel ratios and excess air involves balancing chemical equations and comparing actual to theoretical values. These calculations guide combustion system design and operation, influencing factors like flame temperature, pollutant formation, and overall efficiency.

Air-Fuel Ratio

Air-fuel ratio in combustion

Top images from around the web for Air-fuel ratio in combustion

• 

  Reaction Stoichiometry | Boundless Chemistry View original

  Is this image relevant?

• 

  Air–fuel ratio - Wikipedia View original

  Is this image relevant?

• 

  Stoichiometry - Wikipedia View original

  Is this image relevant?

• 

  Reaction Stoichiometry | Boundless Chemistry View original

  Is this image relevant?

• 

  Air–fuel ratio - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Air-fuel ratio in combustion

• 

  Reaction Stoichiometry | Boundless Chemistry View original

  Is this image relevant?

• 

  Air–fuel ratio - Wikipedia View original

  Is this image relevant?

• 

  Stoichiometry - Wikipedia View original

  Is this image relevant?

• 

  Reaction Stoichiometry | Boundless Chemistry View original

  Is this image relevant?

• 

  Air–fuel ratio - Wikipedia View original

  Is this image relevant?

1 of 3

• Air-fuel ratio (AFR) measures mass of air supplied per unit mass of fuel in combustion
• Formula calculates AFR as $AFR = \frac{m_{air}}{m_{fuel}}$
• Stoichiometric AFR represents theoretical air needed for complete combustion based on balanced equation
• Actual AFR typically exceeds stoichiometric value in real combustion processes
• Calculation involves balancing equation, determining molar ratios, converting to mass using molecular weights

Concept of excess air

• Excess air supplied beyond stoichiometric requirement improves combustion efficiency
• Expressed as percentage above stoichiometric air enhances fuel-air mixing
• Controls flame temperature and reduces pollutant formation (CO, soot)
• Balances complete combustion with thermal efficiency and emissions
• Too little excess air leads to incomplete combustion while too much reduces efficiency and increases NOx

Excess Air Calculations and Effects

Calculation of excess air

• Excess air calculated using formula \text{Excess Air (%)} = \frac{AFR_{actual} - AFR_{stoich}}{AFR_{stoich}} \times 100\%
• Process involves:

1. Determine stoichiometric AFR from balanced equation
2. Compare actual AFR to stoichiometric AFR
3. Apply formula to calculate excess air percentage

• AFR > AFR_stoich indicates excess air present
• AFR = AFR_stoich signifies stoichiometric combustion without excess air

Effects of excess air on flue gases

• Flue gas composition changes with increased O2, decreased CO2, reduced CO and unburned hydrocarbons
• Excess air lowers flue gas temperature through dilution
• Increases flue gas volume and mass flow rate altering heat capacity and density
• Impacts system performance by increasing stack losses and changing heat transfer characteristics
• Optimization balances combustion efficiency, thermal efficiency, and emission regulations
• Equipment design and operating constraints influence excess air management