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weight of each gas, and divide the products by the sum of all the products: the quotients will be the percentages by weight. For most work sufficient accuracy is secured by using the even values of the molecular weights. The even values of the molecular weights are:
Carbon Monoxide Co
A typical fine gas analysis is as foli, ws. Carbon carbon monoxide, 04: oxygen, 5, nagen 85
oxide, 12.2: al. 2000
Inasmuch as perfect combusuoni coal vi give a higher Coy reading than perfect combustion of a possible error may arise among engineers who have been fami r with coal burning in interpreting the C content in fue gas when burning fael of. The possibilty of this error may be demonstrated by the folowing example: Assume a sample of coal having the following wtimate analysis: Carbon. 78 persen. bedragen. 4 percent. Xren. S bercent and the residue ash In each pound of coal a wi be necessary to supple for com les combastics the carb
0.73 X 2% 1.95 pounds of oxygen. The oxygen required for the complete combustion of the hydrogen will be 0.04 X 80.32 pounds. The total oxygen required, therefore, will be 1.95 +0.32 = 2.27 pounds. The coal itself, however, contains 0.08 pounds of occluded oxygen. Subtracting this amount from the total oxygen required leaves 2.19 pounds of oxygen, which must be furnished by the air. The amount of air necessary to supply the re2.19 quired oxygen is 9.53 pounds and this amount of air will 0.23 contain 7.34 pounds of nitrogen. The amount of CO, in the flue gas which will be produced by the 0.73 pounds of carbon in one pound of coal is 0.73 X 3% = 2.68 pounds. Water vapor to the amount of 0.36 pounds will be formed by the combustion of the hydrogen, but the water vapor before reaching the Orsat apparatus will condense, and, therefore, will not appear in the analysis. Hence, flue gas will contain 2.68 pounds of CO, and 7.34 pounds of nitrogen, totalling 10.02 pounds of gas for each pound of coal. 2.68
for N, since the ratio of the weights of N to CO, is 14 to 22. By
volume the percentages will be = 18.8 per cent CO2 and
5.24 6.45 with an average sample of fuel oil. This may be assumed to contain 85 per cent carbon, 12 per cent hydrogen and 3 per cent oxygen. The oxygen required by the carbon of the fuel oil will be 2.27 pounds and combustion will produce 3.12 pounds of CO.,. The oxygen required for the combustion of the hydrogen will be 0.96 pounds of oxygen per pound of oil burned and water vapor will be produced to the amount of 1.08 pounds. The net oxygen requirements will, therefore, be 2.27 +0.96-0.03 3.20 lbs. To provide this amount of oxygen 13.91 pounds of air must be
81.2 per cent N. Follow the same calculation through
introduced and this amount of air carries with it 10.71 pounds of nitrogen. As in the combustion of coal the water vapor will be condensed and the flue gas per pound of oil will be 3.12 + 10.71 = 13.83 pounds, which by weight will have a composition of 22.5 per cent CO2 and 77.5 per cent nitrogen. The percentage of CO2 by volume will be 15.6 and the percentage of N will be 84.4.
It is, of course, understood that these calculations are based on ideal theoretical conditions where there is complete combustion without excess air. In the samples of coal and oil under discussion, the coal might theoretically give an 18.8 per cent CO2 reading whereas the oil could not possibly show a higher percentage than 15.6 because the oil has a greater amount of hydrogen than has the coal and hydrogen requires oxygen for its combustion and the air supplying the oxygen brings with it nitrogen which appears in the flue gas. The water vapor that the hydrogen produces does not appear in the flue gas analysis and the hydrogen, of course, does not produce CO2. It is easily seen that the higher the hydrogen content of the fuel, the lower will be the theoretical CO, percentage in the flue gas.
A factor which is rarely considered in efficiency tests of fuel oil is the humidity of the atmosphere at the time of the test. With a high humidity of the atmosphere some of the oxygen in a given space is displaced by water vapor, and, therefore, for complete combustion of the fuel oil an excess in the volume of air will be required with a consequent loss of heat in the stack. In tests. conducted by the U. S. Naval Liquid Fuel Board, the decision was arrived at that when operating a boiler at a given capacity the efficiency varies inversely with the humidity.
Table 4 gives the physical changes in air brought about by changes in temperature. Relative humidity is expressed as a percentage and is the ratio of the quantity of water vapor which is present in the air at any given temperature and pressure to the quantity of vapor necessary to saturate completely the space occupied by the air.
Since in the charcoal fire at the temperature of the union of the carbon with oxygen the fuel is solid, it can present a large surface upon which the oxygen can act, and an atom of carbon cannot break away from the fuel bed without being first united with at least one atom of oxygen and forming CO. In burning fuel oil the fuel is already on the way to the chimney before it
is even partially burned and is carried along by the current of gases. Therefore, before being cooled, plenty of time must elapse or otherwise it will form soot. If the oil is not properly atomized at the burner the separate oil particles are too large and at the same time are not surrounded with a sufficient number of particles of air to insure their complete combustion. The heavier drops of oil progressively distill and particles of free carbon or soot are deposited. The lighter oils and gases resulting from this distillation consist, like the gases from coal, principally of carbon and hydrogen. In an atmosphere deficient in oxygen the hydro
gen burns first and the carbon is deposited. Naturally when we consider that oil is a liquid originally and not a dense substance like coal, and particularly that it is blown into the furnace by compressed air or steam, the likelihood of its incomplete combustion with consequent deposition of soot is much less than is the case with coal.
An essential for the successful burning of fuel oil is the exposure of the largest possible surface to the action of the oxygen of the air. Bulk oil presents comparatively a small surface. If a tank of fuel oil is ignited, the air is able to reach only the uppermost surface of the liquid and combustion is relatively slow and incomplete, being accompanied by dense clouds of black smoke consisting of unburned carbon. (See fig. 3.) When fuel oil is broken up into fine drops the surface exposed is the sum of the
surface of all the drops. The smaller the drops the more nearly spherical they are. Drops of oil one one-thousandth of an inch. in diameter are known to assume the spherical form with a rigidity comparable to that of a steel ball one inch in diameter. The drop of oil assumes this spherical form through "surface tension," which is a very peculiar property belonging to both solids and liquids. Cohesion of the molecules appears to be greater at the surface than within the body of the globule. Cohesion may be explained as an attractive force between particles of the same material. It appears as though a thin envelope surrounds and holds together the particles composing the drop of oil.
The work necessarily performed by the atomizing agent is simply the work of stretching the surface of the drops. It will easily be seen, therefore, that to properly atomize fuel oil to such a form that it can be burned efficiently under boilers is purely mechanical rather than a chemical problem.