It will be seen from [this table] that a pound of carbon will unite with 22⁄3 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 11⁄3 pounds of oxygen to form carbon monoxide and evolve 4450 B. t. u., but in its further conversion to CO2 it would unite with an additional 11⁄3 times its weight of oxygen and evolve the remaining 10,150 B. t. u. [Pg 152] When a pound of CO burns to CO2, 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 CO2. The hydrogen, either in a free or combined state, will unite with oxygen to form water vapor, H2O. 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, H2O. 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 SO2 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 × 22⁄3 | = | 1.9944 O needed | |||||||||
| = | 0.3262 O needed | |||||||||
| 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.