FUELS AND COMBUSTION 495

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In eqn. [11.4 (b)], it was shown that 1 kg of carbon produces 11/3 kg of CO 2 when completely
burnt. Hence 1 kg of CO 2 will contain 3/11 kg of carbon.
In eqn. [11.6 (a)], it can be seen that 1 kg of carbon produces 7/3 kg of CO, hence 1 kg CO
contains 3/7 kg of carbon.
Therefore, weight of carbon per kg of fuel

=

3

11

3

(^27)
F CO + CO
HG
I
KJ
where CO 2 and CO are the quantities of carbon dioxide and carbon monoxide present in 1 kg of flue
or exhaust gas.

11.14.Weight of Flue Gases per kg of Fuel Burnt

Due to supply of air, the weight of flue gas or exhaust gas is always more than that of fuel

burnt. The actual weight of dry flue gases can be obtained by comparing the weight of carbon

present in the flue gases with the weight of carbon in the fuel, since there is no loss of carbon

during the combustion process. As the analysis of the exhaust gases is volumetric, so this must

first be reduced to weight analysis.

Also, total weight of carbon in one kg of flue gas is

=

3

11

3

(^27)
F CO + CO
HG
I
KJ
∴ The weight of flue gas/kg of fuel burnt

Weight of carbon in one kgof fuel
Weight of carbon in one kgof fluegas

11.15.Analysis of Exhaust and Flue Gas

The combustion products are mainly gaseous. When a sample is taken for analysis it is
usually cooled down to a temperature which is below the saturation temperature of the steam
present. The steam content is therefore not included in the analysis, which is then quoted as the
analysis of the dry products. Since the products are gaseous, it is usual to quote the analysis by
volume. An analysis which includes the steam in the exhaust is called a wet analysis.
Practical analysis of combustion products :
The most common means of analysis of the combustion products is the Orsat apparatus
which is described below :
Construction. An Orsat’s apparatus consists of the following :
(i) A burette
(ii) A gas cleaner
(iii) Four absorption pipettes 1, 2, 3, 4.
The pipettes are interconnected by means of a manifold fitted with cocks S 1 , S 2 , S 3 and S 4
and contain different chemicals to absorb carbon dioxide (CO 2 ), carbonmonoxide (CO) and oxygen
(O 2 ). Each pipette is also fitted with a number of small glass tubes which provide a greater amount
of surface. These tubes are wetted by the absorbing agents and are exposed to the gas under
analysis. The measuring burrette is surrounded by a water jacket to prevent, changes in tempera-
ture and density of the gas. The pipettes 1, 2, 3, 4 contain the following chemicals :
Pipette 1 : Contains ‘KOH’ (caustic soda) to absorb CO 2 (carbon dioxide)
Pipette 2 : Contains an alkaline solution of ‘pyrogallic acid’ to absorb O 2 (oxygen)