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FUELS AND COMBUSTION 493

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If a further supply of oxygen is available then the combustion can continue to completion,

2CO + O 2 +^7921 N 2 → 2CO 2 +^7921 N 2 ...(11.7)

By mass :

56 kg CO + 32 kg O 2 +

79 28

21

×

kg N 2 → 88 kg CO 2 +

79 28

21

×

kg N 2

or 1 kg CO +

4

7 kg O^2 +

105 3

56

.

kg N 2 →

11

7 kg CO^2 +

105 3

56

kg N 2. ...[11.7 (a)]

11.8. Theoretical Air and Excess Air

The minimum amount of air that supplies sufficient oxygen for the complete combustion of
all the carbon, hydrogen, and any other elements in the fuel that may oxidise is called the
“theoretical air”. When complete combustion is achieved with theoretical air, the products con-
tain no oxygen.
In practice, it is found that complete combustion is not likely to be achieved unless the
amount of air supplied is somewhat greater than the theoretical amount. Thus 150 per cent theo-
retical air means that air actually supplied is 1.5 times the theoretical air.
The complete combustion of methane with minimum amount of theoretical air and 150 per
cent theoretical air respectively is written as :

CH 4 + 2O 2 + 2

79

21

F

HG

I

KJ N^2 → CO^2 + 2H^2 O + 2

79

21

F

HG

I

KJ N^2 ...(11.8)

CH 4 + 2(1.5) O 2 + 2

79

21

F

HG

I

KJ (1.5) N^2 → CO^2 + 2H^2 O + O^2 + 3

29

21

F

HG

I

KJ N^2 ...(11.9)

(with 150 per cent theoretical air)

The amount of air actually supplied may also be expressed in terms of per cent excess air.

The excess air is the amount of air supplied over and above the theoretical air. Thus 150 per cent

theoretical air is equivalent to 50 per cent excess air.

Note. For complete combustion of fuel we need air. As per theoretical basis there is a minimum amount

of air which is required by the fuel to burn completely, but always, air in excess is used because whole of air

supplied for combustion purposes does not come in contact with the fuel completely and as such portion of fuel

may be left unburnt. But if a large quantity of excess air is used it exercises a cooling effect on combustion process

which however can be avoided by preheating the air. The weight of excess air supplied can be determined from

the weight of oxygen which is left unused. The amount of excess air supplied varies with the type of fuel and the

firing conditions. It may approach a value of 100% but modern practice is to use 25% to 50% excess air.

11.9. Stoichiometric Air Fuel (A/F) Ratio

Stoichiometric (or chemically correct) mixture of air and fuel is one that contains just

sufficient oxygen for complete combustion of the fuel.

A weak mixture is one which has an excess of air.

A rich mixture is one which has a deficiency of air.

The percentage of excess air is given as under :

%age excess air =

Actural A/F ratio Stoichiometric A/F ratio

Stoichiometric A/F ratio

−

...(11.10)

(where A and F denote air and fuel respectively)