Column Headings:
A: Analysis by Volume Per Cent
B: Molecular Weight
C: Volume times Molecular Weight
D: Analysis by Weight Per Cent
_____________________________________________________________________
| | | | | |
| Gas | A | B | C | D |
|________________________|_______|___________|________|_______________|
| | | | | |
| | | | | |
| | | | | 536.8 |
| Carbon Dioxide CO_{2} | 12.2 | 12+(2×16) | 536.8 | ------ = 17.7 |
| | | | | 3022.8 |
| | | | | |
| | | | | 11.2 |
| Carbon Monoxide CO | .4 | 12+16 | 11.2 | ------ = .4 |
| | | | | 3022.8 |
| | | | | |
| | | | | 220.8 |
| Oxygen O | 6.9 | 2×16 | 220.8 | ------ = 7.3 |
| | | | | 3022.8 |
| | | | | |
| | | | | 2254.0 |
| Nitrogen N | 80.5 | 2×14 | 2254.0 | ------ = 74.6 |
| | | | | 3022.8 |
|________________________|_______|___________|________|_______________|
| | | | | |
| Total | 100.0 | | 3022.8 | 100.0 |
|________________________|_______|___________|________|_______________|
Application of Formulae and Rules--Pocahontas coal is burned in the
furnace, a partial ultimate analysis being:
_Per Cent_
Carbon 82.1
Hydrogen 4.25
Oxygen 2.6
Sulphur 1.6
Ash 6.0
B. t. u., per pound dry 14500
The flue gas analysis shows:
_Per Cent_
CO_{2} 10.7
O 9.0
CO 0.0
N (by difference) 80.3
Determine: The flue gas analysis by weight (see Table 33), the amount of
air required for perfect combustion, the actual weight of air per pound
of fuel, the weight of flue gas per pound of coal, the heat lost in the
chimney gases if the temperature of these is 500 degrees Fahrenheit, and
the ratio of the air supplied to that theoretically required.
Solution: The theoretical weight of air required for perfect combustion,
per pound of fuel, from formula (11) will be,
(.821 .026 .016)
W = 34.56 (---- + (.0425 - ----) + ----) = 10.88 pounds.
( 3 8 8 )
If the amount of carbon which is burned and passes away as flue gas is
80 per cent, which would allow for 2.1 per cent of unburned carbon in
terms of the total weight of dry fuel burned, the weight of dry gas per
pound of carbon burned will be from formula (16):
11 × 10.7 + 8 × 9.0 + 7(0 + 80.3)
W = --------------------------------- = 23.42 pounds
3(10.7 + 0)
and the weight of flue gas per pound of coal burned will be .80 × 23.42
= 18.74 pounds.
The heat lost in the flue gases per pound of coal burned will be from
formula (15) and the value 18.74 just determined.
Loss = .24 × 18.74 × (500 - 60) = 1979 B. t. u.
The percentage of heat lost in the flue gases will be 1979 ÷ 14500 =
13.6 per cent.
The ratio of air supplied per pound of coal to that theoretically
required will be 18.74 ÷ 10.88 = 1.72 per cent.
The ratio of air supplied per pound of combustible to that required will
be from formula (14):
.803
------------------------- = 1.73
.803 - 3.782(.09 - ½ × 0)
The ratio based on combustible will be greater than the ratio based on
fuel if there is unconsumed carbon in the ash.
Unreliability of CO_{2} Readings Taken Alone--It is generally assumed
that high CO_{2} readings are indicative of good combustion and hence of
high efficiency. This is true only in the sense that such high readings
do indicate the small amount of excess air that usually accompanies good
combustion, and for this reason high CO_{2} readings alone are not
considered entirely reliable. Wherever an automatic CO_{2} recorder is
used, it should be checked from time to time and the analysis carried
further with a view to ascertaining whether there is CO present. As the
percentage of CO_{2} in these gases increases, there is a tendency
toward the presence of CO, which, of course, cannot be shown by a CO_{2}
recorder, and which is often difficult to detect with an Orsat
apparatus. The greatest care should be taken in preparing the cuprous
chloride solution in making analyses and it must be known to be fresh
and capable of absorbing CO. In one instance that came to our attention,
in using an Orsat apparatus where the cuprous chloride solution was
believed to be fresh, no CO was indicated in the flue gases but on
passing the same sample into a Hempel apparatus, a considerable
percentage was found. It is not safe, therefore, to assume without
question from a high CO_{2} reading that the combustion is
correspondingly good, and the question of excess air alone should be
distinguished from that of good combustion. The effect of a small
quantity of CO, say one per cent, present in the flue gases will have a
negligible influence on the quantity of excess air, but the presence of
such an amount would mean a loss due to the incomplete combustion of the
carbon in the fuel of possibly 4.5 per cent of the total heat in the
fuel burned. When this is considered, the importance of a complete flue
gas analysis is apparent.
Table 34 gives the densities of various gases together with other data
that will be of service in gas analysis work.