Fuel-Air Ratio (FAR) Considerations

• The chemically accurate, or stoichiometric, fuel-air ratio serves as the reference situation. It based on the C-H ratio of the fuel. For hydrocarbons with the common formula C_nH_2n, the stoichiometric fuel-air ratio is 0.0677. The maximum and minimum stoichiometric FAR for hydrocarbons generally found in petroleum fuels are those for benzene: 0.0754, and for methane: 0.0580.

• General expression for the stoichiometric FAR is:

• Though for pure carbon this ratio is 0.0867, for coal this ratio is high since of the presence of a substantial quantity of combined oxygen in the organic matter of coal.

• In common, heat engines and IC engines work with FAR contrary from the stoichiometric value.

• Boiler furnaces generally operate with 10 to 20% surplus air, both to make sure complete combustion, and to decrease corrosion.

• The gas turbines and diesel engines generally operate with a large quantity of excess air; though maintain close-stoichiometric mixtures in the combustion region.

• Gasoline engines frequently operate with a little richer mixture to acquire smoother combustion.

Good combustion efficiency is attainable in practical devices merely at excess air situations, with the optimum value equivalent to the minimum quantity of excess air.

Whereas excess fuel and excess air both contribute to energy loss, the previous has a more harmful effect.

Excess Fuel:

Energy loss and other unfavorable effects:

(i) Combustion Loss:

Due to unfinished combustion, the full heat of combustion is not recognized, and large quantity of kJ will literally go up the stack.

(ii) Increase Corrosion:

Localized decreasing regions in the furnace, caused by bad air or fuel distribution, can lead to slagging and accelerated tube corrosion.

(iii) Increased Air Pollution:

Mainly, CO, smoke and particulates.

(iv) Safety Hazard:

The fuel-rich operating circumstances can cause explosion. 

Excess Air:

Energy loss and other unfavorable effects:

(i) Sensible Enthalpy Loss:

All air in surplus of the need for optimum combustion will leave the combustion procedure with sensible enthalpy. 

(ii) Heat Transfer Reduction:

The extra air contributes to a raise in the volume, and therefore the velocity, of the flue gas in the fire box, reducing the quantity of time the hot products of combustion are in contact with the heat transfer surfaces. This outcome in decreased heat transfer efficiency and higher flue gas stack exit temperatures. Therefore, excess air contributes not only to a high volume of gas, though also to higher stack exit temperatures, resultant in even harsher sensible enthalpy losses.

(iii) Fan Power Loss:

Excess air needs higher electrical power input to the fan.

(iv) Increased Corrosion:

With S-bearing fuels, surplus air leads to the formation of SO_3, and eventually H₂SO₄. This acid can unite with emitted solids to generate acid smuts that can cause cold-end corrosion, air-preheater fouling, and corrosion to surrounding buildings. Surplus air gives higher SO₃ concentrations, that outcome in higher acid dew point temperature of the flue gas. For every 20^oC that is essential to raise the flue gas stack exit temperature to avoid H₂SO₄ condensation in the duct work, around 1% in operating efficiency is lost.

(v) Increased Air Pollution:

Surplus air plays a significant role in the formation of SO₃ and NO₂ emission.

The minimum quantity of surplus air is function of:
• Fuel; and 
• Combustion device; that determines
• Mixture distribution; and 
• Combustor aerodynamics

Some significant points and definitions composition of air are as follows:

Mass: Nitrogen: Oxygen = 73: 27 = 3.34: 1
Molar: Nitrogen: Oxygen = 77: 23 = 3.76: 1

Equivalent Ratio, φ = Actual FAR / Stoichiometric FAR

φ > 1 : rich mixtures
φ = 1 : stoichiometric mixture
φ < 1 : lean mixtures.