FOOTNOTES
[ [40] ] B. t. u. calculated.
[ [41] ] Average of two samples.
[ [42] ] Assuming bagasse temperature = 80 degrees Fahrenheit and exit gas temperature = 500 degrees Fahrenheit.
[ [43] ] Dr. Henry C. Sherman. Columbia University.
LIQUID FUELS AND THEIR COMBUSTION
Petroleum is practically the only liquid fuel sufficiently abundant and cheap to be used for the generation of steam. It possesses many advantages over coal and is extensively used in many localities.
There are three kinds of petroleum in use, namely those yielding on distillation: 1st, paraffin; 2nd, asphalt; 3rd, olefine. To the first group belong the oils of the Appalachian Range and the Middle West of the United States. These are a dark brown in color with a greenish tinge. Upon their distillation such a variety of valuable light oils are obtained that their use as fuel is prohibitive because of price.
To the second group belong the oils found in Texas and California. These vary in color from a reddish brown to a jet black and are used very largely as fuel.
The third group comprises the oils from Russia, which, like the second, are used largely for fuel purposes.
The light and easily ignited constituents of petroleum, such as naphtha, gasolene and kerosene, are oftentimes driven off by a partial distillation, these products being of greater value for other purposes than for use as fuel. This partial distillation does not decrease the value of petroleum as a fuel; in fact, the residuum known in trade as “fuel oil” has a slightly higher calorific value than petroleum and because of its higher flash point, it may be more safely handled. Statements made with reference to petroleum apply as well to fuel oil.
In general crude oil consists of carbon and hydrogen, though it also contains varying quantities of moisture, sulphur, nitrogen, arsenic, phosphorus and silt. The moisture contained may vary from less than 1 to over 30 per cent, depending upon the care taken to separate the water from the oil in pumping from the well. As in any fuel, this moisture affects the available heat of the oil, and in contracting for the purchase of fuel of this nature it is well to limit the per cent of moisture it may contain. A large portion of any contained moisture can be separated by settling and for this reason sufficient storage capacity should be supplied to provide time for such action.
A method of obtaining approximately the percentage of moisture in crude oil which may be used successfully, particularly with lighter oils, is as follows. A burette graduated into 200 divisions is filled to the 100 mark with gasolene, and the remaining 100 divisions with the oil, which should be slightly warmed before mixing. The two are then shaken together and any shrinkage below the 200 mark filled up with oil. The mixture should then be allowed to stand in a warm place for 24 hours, during which the water and silt will settle to the bottom. Their percentage by volume can then be correctly read on the burette divisions, and the percentage by weight calculated from the specific gravities. This method is exceedingly approximate and where accurate results are required it should not be used. For such work, the distillation method should be used as follows:
Gradually heat 100 cubic centimeters of the oil in a distillation flask to a temperature of 150 degrees centigrade; collect the distillate in a graduated tube and measure the resulting water. Such a method insures complete removal of water and reduces the error arising from the slight solubility of the water in [Pg 213] gasolene. Two samples checked by the two methods for the amount of moisture present gave,
|
Calorific Value—A pound of petroleum usually has a calorific value of from 18,000 to 22,000 B. t. u. If an ultimate analysis of an average sample be, carbon 84 per cent, hydrogen 14 per cent, oxygen 2 per cent, and assuming that the oxygen is [Pg 214] combined with its equivalent of hydrogen as water, the analysis would become, carbon 84 per cent, hydrogen 13.75 per cent, water 2.25 per cent, and the heat value per pound including its contained water would be,
| ||||||||||||||||||||||||||||||||||||
The nitrogen in petroleum varies from 0.008 to 1.0 per cent, while the sulphur varies from 0.07 to 3.0 per cent.
[Table 46], compiled from various sources, gives the composition, calorific value and other data relative to oil from different localities.
The flash point of crude oil is the temperature at which it gives off inflammable gases. While information on the actual flash points of the various oils is meager, it is, nevertheless, a question of importance in determining their availability as fuels. In general it may be stated that the light oils have a low, and the heavy oils a much higher flash point. A division is sometimes made at oils having a specific gravity of 0.85, with a statement that where the specific gravity is below this point the flash point is below 60 degrees Fahrenheit, and where it is above, the flash point is above 60 degrees Fahrenheit. There are, however, many exceptions to this rule. As the flash point is lower the danger of ignition or explosion becomes greater, and the utmost care should be taken in handling the oils with a low flash point to avoid this danger. On the other hand, because the flash point is high is no justification for carelessness in handling those fuels. With proper precautions taken, in general, the use of oil as fuel is practically as safe as the use of coal.
Gravity of Oils—Oils are frequently classified according to their gravity as indicated by the Beaume hydrometer scale. Such a classification is by no means an accurate measure of their relative calorific values.
Petroleum as Compared with Coal—The advantages of the use of oil fuel over coal may be summarized as follows:
1st. The cost of handling is much lower, the oil being fed by simple mechanical means, resulting in,
2nd. A general labor saving throughout the plant in the elimination of stokers, coal passers, ash handlers, etc.
3rd. For equal heat value, oil occupies very much less space than coal. This storage space may be at a distance from the boiler without detriment.
4th. Higher efficiencies and capacities are obtainable with oil than with coal. The combustion is more perfect as the excess air is reduced to a minimum; the furnace temperature may be kept practically constant as the furnace doors need not be opened for cleaning or working fires; smoke may be eliminated with the consequent increased cleanliness of the heating surfaces.
5th. The intensity of the fire can be almost instantaneously regulated to meet load fluctuations.
6th. Oil when stored does not lose in calorific value as does coal, nor are there any difficulties arising from disintegration, such as may be found when coal is stored.
7th. Cleanliness and freedom from dust and ashes in the boiler room with a consequent saving in wear and tear on machinery; little or no damage to surrounding property due to such dust.
The disadvantages of oil are:
1st. The necessity that the oil have a reasonably high flash point to minimize the danger of explosions.
2nd. City or town ordinances may impose burdensome conditions relative to location and isolation of storage tanks, which in the case of a plant situated in a congested portion of the city, might make use of this fuel prohibitive.
3rd. Unless the boilers and furnaces are especially adapted for the use of this fuel, the boiler upkeep cost will be higher than if coal were used. This objection can be entirely obviated, however, if the installation is entrusted to those who have had experience in the work, and the operation of a properly designed plant is placed in the hands of intelligent labor.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Many tables have been published with a view to comparing the two fuels. Such of these as are based solely on the relative calorific values of oil and coal are of limited [Pg 216]
[Pg 217] value, inasmuch as the efficiencies to be obtained with oil are higher than that obtainable with coal. [Table 47] takes into consideration the variation in efficiency with the two fuels, but is based on a constant calorific value for oil and coal. [This table], like others of a similar nature, while useful as a rough guide, cannot be considered as an accurate basis for comparison. This is due to the fact that there are numerous factors entering into the problem which affect the saving possible to a much greater extent than do the relative calorific values of two fuels. Some of the features to be considered in arriving at the true basis for comparison are the labor saving possible, the space available for fuel storage, the facilities for conveying the oil by pipe lines, the hours during which a plant is in operation, the load factor, the quantity of coal required for banking fires, etc., etc. The only exact method of estimating the relative advantages and costs of the two fuels is by considering the operating expenses of the plant with each in turn, including the costs of every item entering into the problem.
City of San Francisco, Cal., Fire Fighting Station. No. 1. 2800 Horse Power of Babcock & Wilcox Boilers, Equipped for Burning Oil Fuel
Burning Oil Fuel—The requirements for burning petroleum are as follows:
1st. Its atomization must be thorough.
2nd. When atomized it must be brought into contact with the requisite quantity of air for its combustion, and this quantity must be at the same time a minimum to obviate loss in stack gases.
3rd. The mixture must be burned in a furnace where a refractory material radiates heat to assist in the combustion, and the furnace must stand up under the high temperatures developed.
4th. The combustion must be completed before the gases come into contact with the heating surfaces or otherwise the flame will be extinguished, possibly to ignite later in the flue connection or in the stack.
5th. There must be no localization of the heat on certain portions of the heating surfaces or trouble will result from overheating and blistering.
The first requirement is met by the selection of a proper burner.
The second requirement is fulfilled by properly introducing the air into the furnace, either through checkerwork under the burners or through openings around them, and by controlling the quantity of air to meet variations in furnace conditions.
The third requirement is provided for by installing a furnace so designed as to give a sufficient area of heated brickwork to radiate the heat required to maintain a proper furnace temperature.
The fourth requirement is provided for by giving ample space for the combustion of the mixture of atomized oil and air, and a gas travel of sufficient length to insure that this combustion be completed before the gases strike the heating surfaces.
The fifth requirement is fulfilled by the adoption of a suitable burner in connection with the furnace meeting the other requirements. A burner must be used from which the flame will not impinge directly on the heating surface and must be located where such action cannot take place. If suitable burners properly located are not used, not only is the heat localized with disastrous results, but the efficiency is lowered by the cooling of the gases before combustion is completed.
Oil Burners—The functions of an oil burner is to atomize or vaporize the fuel so that it may be burned like a gas. All burners may be classified under three general types: 1st, spray burners, in which the oil is atomized by steam or compressed air; 2nd, vapor burners, in which the oil is converted into vapor and then passed into the [Pg 218] furnace; 3rd, mechanical burners, in which the oil is atomized by submitting it to a high pressure and passing it through a small orifice.
Vapor burners have never been in general use and will not be discussed.
Spray burners are almost universally used for land practice and the simplicity of the steam atomizer and the excellent economy of the better types, together with the low oil pressure and temperature required makes this type a favorite for stationary plants, where the loss of fresh water is not a vital consideration. In marine work, or in any case where it is advisable to save feed water that otherwise would have to be added in the form of “make-up”, either compressed air or mechanical means are used for atomization. Spray burners using compressed air as the atomizing agent are in satisfactory operation in some plants, but their use is not general. Where there is no necessity of saving raw feed water, the greater simplicity and economy of the steam spray atomizer is generally the most satisfactory. The air burners require blowers, compressors or other apparatus which occupy space that might be otherwise utilized and require attention that is not necessary where steam is used.
Steam spray burners of the older types had disadvantages in that they were so designed that there was a tendency for the nozzle to clog with sludge or coke formed from the oil by the heat, without means of being readily cleaned. This has been overcome in the more modern types.
Steam spray burners, as now used, may be divided into two classes: 1st, inside mixers; and 2nd, outside mixers. In the former the steam and oil come into contact within the burner and the mixture is atomized in passing through the orifice of the burner nozzle.
