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
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Air-fuel ratio (AFR) measures mass of air supplied per unit mass of fuel in combustion
Formula calculates AFR as A F R = m a i r m f u e l AFR = \frac{m_{air}}{m_{fuel}} A FR = m f u e l m ai r
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:
Determine stoichiometric AFR from balanced equation
Compare actual AFR to stoichiometric AFR
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