The object of a flue gas analysis is the determination of the
completeness of the combustion of the carbon in the fuel, and the amount
and distribution of the heat losses due to incomplete combustion. The
quantities actually determined by an analysis are the relative
proportions by volume, of carbon dioxide (CO_{2}), oxygen (O), and
carbon monoxide (CO), the determinations being made in this order.
The variations of the percentages of these gases in an analysis is best
illustrated in the consideration of the complete combustion of pure
carbon, a pound of which requires 2.67 pounds of oxygen,[28] or 32 cubic
feet at 60 degrees Fahrenheit. The gaseous product of such combustion
will occupy, when cooled, the same volume as the oxygen, namely, 32
cubic feet. The air supplied for the combustion is made up of 20.91 per
cent oxygen and 79.09 per cent nitrogen by volume. The carbon united
with the oxygen in the form of carbon dioxide will have the same volume
as the oxygen in the air originally supplied. The volume of the nitrogen
when cooled will be the same as in the air supplied, as it undergoes no
change. Hence for complete combustion of one pound of carbon, where no
excess of air is supplied, an analysis of the products of combustion
will show the following percentages by volume:
_Actual Volume_
_for One Pound Carbon_ _Per Cent_
_Cubic Feet_ _by Volume_
Carbon Dioxide 32 = 20.91
Oxygen 0 = 0.00
Nitrogen 121 = 79.09
--- ------
Air required for one pound Carbon 153 = 100.00
For 50 per cent excess air the volume will be as follows: