_Actual Volume_
_for One Pound Carbon_ _Per Cent_
_Cubic Feet_ _by Volume_
Carbon Dioxide 32 = 10.45 }
Oxygen 32 = 10.45 } = 20.91 per cent
Nitrogen 242 = 79.09
--- ------
306 = 100.00
In each case the volume of oxygen which combines with the carbon is
equal to (cubic feet of air × 20.91 per cent)--32 cubic feet.
It will be seen that no matter what the excess of air supplied, the
actual amount of carbon dioxide per pound of carbon remains the same,
while the percentage by volume decreases as the excess of air increases.
The actual volume of oxygen and the percentage by volume increases with
the excess of air, and the percentage of oxygen is, therefore, an
indication of the amount of excess air. In each case the sum of the
percentages of CO_{2} and O is the same, 20.9. Although the volume of
nitrogen increases with the excess of air, its percentage by volume
remains the same as it undergoes no change while combustion takes place;
its percentage for any amount of air excess, therefore, will be the same
after combustion as before, if cooled to the same temperature. It must
be borne in mind that the above conditions hold only for the perfect
combustion of a pound of pure carbon.
Carbon monoxide (CO) produced by the imperfect combustion of carbon,
will occupy twice the volume of the oxygen entering into its composition
and will increase the volume of the flue gases over that of the air
supplied for combustion in the proportion of
100 + ½ the per cent CO
1 to -----------------------
100
When pure carbon is the fuel, the sum of the percentages by volume of
carbon dioxide, oxygen and one-half of the carbon monoxide, must be in
the same ratio to the nitrogen in the flue gases as is the oxygen to the
nitrogen in the air supplied, that is, 20.91 to 79.09. When burning
coal, however, the percentage of nitrogen is obtained by subtracting the
sum of the percentages by volume of the other gases from 100. Thus if an
analysis shows 12.5 per cent CO_{2}, 6.5 per cent O, and 0.6 per cent
CO, the percentage of nitrogen which ordinarily is the only other
constituent of the gas which need be considered, is found as follows: