________________________________________________________________
| | | | | | |
| 1 | 2 | 11 | 12 | 13 | 14 |
|_________________|________|______|____|________________|________|
| | | | | | |
| Carbon | C | 1C | 2 | 2CO_{2} | 14.95 |
| Carbon | C | 1C | 1 | 2CO | 14.95 |
| Carbon Monoxide | CO | 2CO | 1 | 2CO_{2} | 12.80 |
| Hydrogen | H | 2H | 1 | 2H_{2}O | 179.32 |
| Methane | CH_{4} | 1C4H | 4 | 1CO_{2} 2H_{2}O| 22.41 |
| Sulphur | S | 1S | 2 | 1SO_{2} | 5.60 |
|_________________|________|______|____|________________|________|
_____________________________________________________________
| | | | | | |
| 1 | 2 | 15 | 16 | 17 | 18 |
|_________________|________|_______|________|________|________|
| | | | | | |
| Carbon | C | 29.89 | 29.89 | 112.98 | 142.87 |
| Carbon | C | 14.95 | 29.89 | 56.49 | 86.38 |
| Carbon Monoxide | CO | 6.40 | 12.80 | 24.20 | 37.00 |
| Hydrogen | H | 89.66 | 179.32 | 339.09 | 518.41 |
| Methane | CH_{4} | 44.83 | 67.34 | 169.55 | 236.89 |
| Sulphur | S | 11.21 | 11.21 | 42.39 | 53.60 |
|_________________|________|_______|________|________|________|
It will be seen from this table that a pound of carbon will unite with
2-2/3 pounds of oxygen to form carbon dioxide, and will evolve 14,600
B. t. u. As an intermediate step, a pound of carbon may unite with 1-1/3
pounds of oxygen to form carbon monoxide and evolve 4450 B. t. u., but
in its further conversion to CO_{2} it would unite with an additional
1-1/3 times its weight of oxygen and evolve the remaining 10,150
B. t. u. When a pound of CO burns to CO_{2}, however, only 4350 B. t. u.
are evolved since the pound of CO contains but 3/7 pound carbon.
Air Required for Combustion--It has already been shown that each
combustible element in fuel will unite with a definite amount of oxygen.
With the ultimate analysis of the fuel known, in connection with Table
31, the theoretical amount of air required for combustion may be readily
calculated.
Let the ultimate analysis be as follows:
_Per Cent_
Carbon 74.79
Hydrogen 4.98
Oxygen 6.42
Nitrogen 1.20
Sulphur 3.24
Water 1.55
Ash 7.82
------
100.00
When complete combustion takes place, as already pointed out, the carbon
in the fuel unites with a definite amount of oxygen to form CO_{2}. The
hydrogen, either in a free or combined state, will unite with oxygen to
form water vapor, H_{2}O. Not all of the hydrogen shown in a fuel
analysis, however, is available for the production of heat, as a portion
of it is already united with the oxygen shown by the analysis in the
form of water, H_{2}O. Since the atomic weights of H and O are
respectively 1 and 16, the weight of the combined hydrogen will be 1/8
of the weight of the oxygen, and the hydrogen available for combustion
will be H - 1/8 O. In complete combustion of the sulphur, sulphur
dioxide SO_{2} is formed, which in solution in water forms sulphuric
acid.
Expressed numerically, the theoretical amount of air for the above
analysis is as follows:
0.7479 C × 2-2/3 = 1.9944 O needed
( 0.0642 )
( 0.0498 - -------) H × 8 = 0.3262 O needed
( 8 )
0.0324 S × 1 = 0.0324 O needed
------
Total 2.3530 O needed
One pound of oxygen is contained in 4.32 pounds of air.
The total air needed per pound of coal, therefore, will be 2.353 × 4.32
= 10.165.
The weight of combustible per pound of fuel is .7479 + .0418[27] + .0324
+ .012 = .83 pounds, and the air theoretically required per pound of
combustible is 10.165 ÷ .83 = 12.2 pounds.
The above is equivalent to computing the theoretical amount of air
required per pound of fuel by the formula:
( O)
Weight per pound = 11.52 C + 34.56 (H - -) + 4.32 S (10)
( 8)
where C, H, O and S are proportional parts by weight of carbon,
hydrogen, oxygen and sulphur by ultimate analysis.
In practice it is impossible to obtain perfect combustion with the
theoretical amount of air, and an excess may be required, amounting to
sometimes double the theoretical supply, depending upon the nature of
the fuel to be burned and the method of burning it. The reason for this
is that it is impossible to bring each particle of oxygen in the air
into intimate contact with the particles in the fuel that are to be
oxidized, due not only to the dilution of the oxygen in the air by
nitrogen, but because of such factors as the irregular thickness of the
fire, the varying resistance to the passage of the air through the fire
in separate parts on account of ash, clinker, etc. Where the
difficulties of drawing air uniformly through a fuel bed are eliminated,
as in the case of burning oil fuel or gas, the air supply may be
materially less than would be required for coal. Experiment has shown
that coal will usually require 50 per cent more than the theoretical net
calculated amount of air, or about 18 pounds per pound of fuel either
under natural or forced draft, though this amount may vary widely with
the type of furnace, the nature of the coal, and the method of firing.
If less than this amount of air is supplied, the carbon burns to
monoxide instead of dioxide and its full heat value is not developed.
An excess of air is also a source of waste, as the products of
combustion will be diluted and carry off an excessive amount of heat in
the chimney gases, or the air will so lower the temperature of the
furnace gases as to delay the combustion to an extent that will cause
carbon monoxide to pass off unburned from the furnace. A sufficient
amount of carbon monoxide in the gases may cause the action known as
secondary combustion, by igniting or mingling with air after leaving the
furnace or in the flues or stack. Such secondary combustion which takes
place either within the setting after leaving the furnace or in the
flues or stack always leads to a loss of efficiency and, in some
instances, leads to overheating of the flues and stack.
Table 32 gives the theoretical amount of air required for various fuels
calculated from formula (10) assuming the analyses of the fuels given in
the table.
The process of combustion of different fuels and the effect of variation
in the air supply for their combustion is treated in detail in the
chapters dealing with the various fuels.