Reaction with Excess air (Lean Combustion)

Fuel and air are often introduced separately (without premixing) into a combustor, e.g., boiler, CI engine etc.  Due to the large flow rates and short residence time there is no assurance that each molecule of fuel is surrounded by the appropriate number of oxygen molecules required for the stoichiometric combustion. Therefore, it is customary to supply excess air in order to facilitate better mixing and thereby ensure complete combustion. The excess oxygen remains unburned and appears in the product.

Reaction with Excess Fuel (Rich Mixture)

Incomplete combustion occurs when the air supplied is less than the stoichiometric amount required. For this condition, the products of incomplete oxidation may contain a mixture of CO, CO₂, H₂ and H₂O.

Equivalent Ratio and Stoichiometric Ratio

The equivalent ratio is defined as the actual fuel-air ratio to stoichimetric fuel-air ratio.

(3.1)

The stoichiometric ratio is given by

(3.2)

For example, if Φ = 0.5, for methane-air combustion, this implies that the excess air is supplied for every kmole of fuel that is burned. In such case SR = 2. In otherwords, air supplied is two times as large as the stoichiometric or theoretical air demand of the fuel. Eq. (1.2) can be written under such condition as

(3.3)

Example

Degree of Reaction

Let us suppose that we have a mixture of four substances, A₁, A₂, A₃ and A₄ capable of undergoing a reaction of the type

(3.4)

where the are the stoichimetric coefficients.

Starting with arbitrary amounts of both initial and final constituents, let us imagine that the reaction proceeds completely to the right with the disappearance of at least one of the initial constituents, say, A₁. Then the original number of moles of the initial constituents is given in the form

n1(original) = n0v1

(3.5)

n2(original) = n0v1 + N2

(3.6)