These gentlemen found that the value of K, which depends on the amount of moisture, ash, and sulphur in the coal, is practically constant for the same coal in the same field, regardless of any local variation in the relative proportions of these constituents. The value of K as determined by them for various coals from Ohio, Pennsylvania, and West Virginia is given in the following table. VALUE OF K FOR VARIOUS COALS ILLUSTRATION 1.-Lord and Haas' Method.-Using coal No. 103 from Pocahontas, Va., as the sulphur is separately determined in the usual way, the percentages of ash and moisture must be recalculated, to what they would be if the sulphur were included. This is done by dividing them by (100+S)÷100 =1.0074, as the coal contains .74% of sulphur. From this the adjusted moisture and ash are, respectively, 1.62% and 5.82%. The value of K to be used, 15,829, is taken from the table. Substituting in formula 1: 15,829 X (100-5.82-.74-1.62)+(.74×4,050) B. T. U.= = 100 = 14,564 Mois As the true calorific value is 14,672, the difference is 108 B. T. U., or .736%. Kent's Method.-Using the same coal as before, No. 103, from the Pocahontas field in Virginia, the heating value of which has been determined by calorimeter to be 14,672 B. T. U. per lb., the proximate analysis is: ture, 1.63%; volatile matter, 17.17%; fixed carbon, 75.34%; and ash, 5.86%; the sum of these four constituents is 100%. The coal dry and free from ash is made up of 75.34÷(75.34+17.17-92.51)=81.44% of fixed carbon, and 17.17(75.34+17.17=92.51)=18.56% of volatile matter. By interpolating in the second column of the table, the heating value per pound of combustible of a coal dry and free from ash, and containing 81.44% of fixed carbon, is found to be 15,803 B. T. U. As just shown, the total combustible (the sum of the fixed carbon and volatile matter) in the coal is 92.51. Hence, the calorific value of this coal is 15,803 X.9251=14,619 B. T. U. This agrees very closely with the calorimetric value, the difference being but 48 B. T. U., or .361 %. ILLUSTRATION 2.-Kent's Method.-Using coal No. 78, of the table given on pages 382 to 385, from Henryetta, Okla., the heating value of which is 12,620 B. T. U., and which contains by analysis: Moisture, 3.87%; volatile matter, 35.73%; fixed carbon, 50.05%; and ash, 10.35%. The fixed carbon in the coal dry and free from ash is 58.35% and the volatile matter is 41.65%. By interpolating in the second column of the table giving the approximate heating value of coals, the calorific value of a coal per pound of combustible and containing 58.35% of fixed carbon on a dry and ash-free basis, is 14,463 B. T. U. But the total combustible (fixed carbon+volatile matter) in this coal is 50.05+35.73=85.78%, hence its heating value is 14,463X.8578=12,406 B. T. U. This differs from the true calorific value by 214 B. T. U., or 1.6%. It will be noted, however, that this coal contains less than 60% of fixed carbon, and is, thence, within the group in which the formula does not apply within 4%. Lord and Haas' Method. Taking the coal just used and recalculating the analysis to include the sulphur, the analysis is: Moisture, 3.80%; ash, 10.15%; and sulphur, 1.95%. The recalculated volatile matter is 35.03%, and fixed carbon 49.07%. This coal is not included in the table of those for which the value of K has been determined. It is possible, however, for illustrative purposes only, to assume a value for this constant. In content of volatile matter this coal is not unlike No. 82 from the Pittsburg seam, at Ellsworth, Pa., 35.73% as against 34.83%. Using the value of K, 15,183, for the Pittsburg seam, and substituting in formula 1, gives for the calorific value of this coal 12,848 B. T. U. per lb. This is 228 B. T. U., or 1.80%, greater than the true value, 12,620 B. T. U. In the amount of fixed carbon, 49.07%, this coal is not unlike No. 75, a Hocking coal from Dixie, Ohio, carrying 46.08% of fixed carbon. Using the value of K, 14,265, for Hocking coal, and substituting in the formula, gives the calorific value of coal No. 78 to be 12,076 B. T. U. This is 544 B. T. U., or 4.31% less than the true value. The illustrations show that while this method gives excellent results with those coals for which the value of K has been experimentally determined, it cannot be relied on to give good results where K is unknown. Nor do Messrs. Lord and Haas make any such claim for it. DETERMINATION OF HEATING VALUE OF COAL FROM AN ULTILIATE ANALYSIS Dulong's Formula.-The available method of determining the heating value of coal from an ultimate analysis is based on the formula devised by Dulong and known by his name. The amount of heat obtained in the burning of 1 lb. of coal under theoretically perfect conditions is expressed as follows: Lb. Cal.=8,080C+34,462 (H−)+2,250S in which C, H, O, and S are the percentages of the elements the symbols represent and the coefficients are the calories_evolved in burning 1 lb. of carbon, hydrogen, and sulphur, respectively. The figures within the parenthesis (H-) represent the available hydrogen, i. e., the amount of that element over and above that required to combine with the oxygen to form water. American engineers commonly employ the British thermal unit in place of the poundcalorie. However, if calculations are made in pound-calories, they may readily be reduced to British thermal units by multiplying by 1.8. In terms of the British thermal unit, the formula becomes, B. T. U. 14,544C+62,032H+4,050S In this formula C, H, O, and S are the percentages of carbon, hydrogen, oxygen, and sulphur, respectively, as determined by an ultimate analysis, and 14,544, 62,032, and 4,050, are the number of British thermal units evolved in burning 1 lb. of those of the foregoing elements that are combustible. ILLUSTRATIONS.-Using coal No. 103 from Pocahontas, Va., B. T. U.=14,544X.8314+62,032× (.0458-0465)+4,050.0075=14,603 This is within 69 B. T. U., or .470% of the calorimeter value. B. T. U.=14,544X.6985+62,028× (.0514 which is 73 B. T. U., or .58%, too low. .1138 +4,050.0199= 12,547 Dulong's formula may be relied on to give results within 2% of the true calorimetrically determined value, and the agreement is generally much closer, as has been shown. It should be remembered that if the more accurate values for the calorific powers of the combustible elements are substituted in the formula, the results obtained are lower than those had through the use of the earlier and approximate ones. A comparison of the results obtained in calculating the heating values of coals No. 103 and No. 78, by the different methods available is here given in tabular form. PETROLEUM AS FUEL 395 Next to natural gas, petroleum is the ideal fuel, 1 lb. of it having a heating value about 50% greater than 1 lb. of average coal. Of the 209,556,048 bbl. produced in the United States in 1910, 24,586,108 bbl. were used for fuel by the railroads along the Pacific coast and in the Southwest, displacing, say, some 7,000,000 T. of coal. If to the consumption of the railroads is added that of steamships, central-station power plants, and other large industrial concerns, it is probable that the amount of oil used as fuel in the regions named is equivalent to about 20,000,000 T. of coal. Petroleum is a dark greenish-black to light-brown oil produced by the decomposition of organic matter contained in the rocks, but whether the organic matter is of vegetable or animal origin is undetermined. The oil con sists of a series of hydrocarbons that may be distilled off in a series of gradually increasing density as the temperature is increased. The residue, consisting of the least volatile portion, which commonly remains as a solid known as the base, affords a means of classifying oils into three groups: paraffin oils, asphalt oils, and olefin oils. The oils in the first group are those produced in the eastern and middle western states and being limited in production and high in yield of very valuable light oils (gasoline, kerosene, etc.) are too high in cost to be used as fuel. In the second group come the oils of California and Texas, produced in large quantities and at low cost, and furnishing the vast bulk of the fuel oil used in the West and Southwest. In the third group are the oils of Baku, Russia, on the Caspian Sea, and, except in so far as they possibly displace American coal in foreign markets, are of no especial interest to the mining engineer. Composition of Crude Petroleum.-Petroleum in the fo rm in which it issues from the earth is known as crude oil. It usually contains from 83 to 87% of carbon; from 10 to 16% of hydrogen; and small amounts of oxygen, nitrogen, and sulphur. Crude oil contains from less than 1% to over 30% of water. The amount of water depends largely on the care with which the oil is pumped from the well. The oil from old producing wells commonly contains more water than that from wells newly drilled. In fact, in many districts, the percentage of water gradually increases during the life of the well, eventually the entire output being salt water. As the amount of water in the crude oil is uncertain and variable and as it separates out if left undisturbed, allowance must be made therefor in providing storage or, as more commonly called, tankage. The accompanying table gives the ultimate analyses of oils from various sources. As stated, the various hydrocarbons composing crude petroleum may be separated by distillation at different temperatures; thus, gasoline is driven off by heating from 140° to 158° F.; a light benzine or naphtha at from 158° to 248° F.; heavier benzines at 248° to 347° F.; kerosene, or ordinary illuminating oil, at 338° F. and upwards; lubricating oils at 482° F. and above; paraffin wax at a higher temperature; leaving a tarry residuum that may be further distilled until nothing but a small quantity of coke remains in the still. If the distillation is stopped after the kerosene has been driven off, the residue may be used for fuel oil. Flash Point and Firing Point.-If a sample of fuel oil or of crude oil is placed in an open cup and heat is applied, the oil will begin to vaporize and inflammable gases will be driven off. If, while the heating proceeds, a lighted match is passed at intervals over the surface of the oil and about in. from it, a point will be reached at which the vapor rising from the oil will ignite and burn with a flicker of blue flame. The temperature of the oil when this flame first becomes apparent is termed the flash point of the oil. If the heating of the sample is continued, the vapors will be given off more rapidly and eventually they will ignite and burn continuously at the surface of the oil when the lighted match is brought near. The temperature of the oil when the burning becomes continuous is termed the firing point of the oil. The flash point and the firing point of an oil depend on the composition, specific gravity, and source of the oil. As a general rule, the heavier oils have a much higher flash point than the lighter ones and an attempt has been made to use this as a basis for classifying them. A specific gravity of .85 is taken as the basis, oils heavier than this having a flash point above 60° F., and oils lighter than this having a flash point lower than 60° F., although this is very far from always being true. It is obvious that a high flash point is very desirable in a fuel oil in order to avoid danger of explosion. 396 Calorific Value of Fuel Oil.-The combustible elements in oil are the same as those in coal, namely, carbon and hydrogen, and usually some sulphur. The calorific value per pound may be determined by means of Dulong's formula, which is applied exactly as in the case of an ultimate analysis of coal. This formula was used to calculate the greater number of the calorific powers given in the accompanying table. From the results of available tests, it is found that the heat of combustion per pound of fuel oil varies from 17,000 to 21,000 B. T. U., California oil averaging about 18,600 B. T. U., and Texas oil some 1,000 B. T. U. higher. At 18,600 B. T. U. per lb. and assuming an average specific gravity of .885, 1 gal. of oil weighs 7.37 lb., and will yield 137,082 B. T. U., and a barrel of 42 gal. will weigh 310 lb. and will yield 5,766,000 B. T. U. Using the same specific gravity and a calorific value of 19,600 B. T. U. per lb., 1 gal. of oil will develop 144,452 B. T. U., and a barrel 6,076,000 B. T. U. The theoretical comparative fuel values of coals of different heating powers 1 Includes nitrogen. 2 Oxygen, by difference. Calculated by means of Dulong's formula. and of fuel oil yielding, respectively, 18,600 and 19,600 B. T. U., per lb. are given in the following table. This table, however, is of more theoretical than practical interest, as it is based on the assumption that combustion is perfect whether oil or coal is used, and that, in consequence, the efficiency of an oil-burning and of a coal-burning boiler is the same. This is far from the case, as the efficiency of a properly designed oil-burning boiler is the greater. To this must be added the advantages outlined, so that only an actual test of the two fuels will determine which is the more economical under a given set of conditions. Advantages and Disadvantages of Oil Fuel.-Babcock and Wilcox summarize the advantages of fuel oil as follows: 1. The cost of handling is much lower, the oil being fed by simple mechanical means, resulting in 2. A general labor saving throughout the plant in the elimination of stokers, coal passers, ash handlers, etc. 3. For equal heat value, oil occupies very much less space than coal. This storage space may be at any distance from the boiler without detriment. 4. 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. 5. The intensity of the fire can be almost instantaneously regulated to meet load fluctuations. 6. 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 where' coal is stored. 7. 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: 1. The necessity that the oil have a reasonably high flash point to minimize the danger of explosions. 2. 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 the use of this fuel prohibitive. 3. Unless the boilers and furnaces are especially adapted for the use of this fuel, the boiler upkeep cost will be higher than if coal is used. The relative cost of the two fuels per unit of power produced is, of course, the deciding factor in the premises, and this varies so greatly in such short intervals of time, even from day to day, that current quotations of the delivered cost of both fuels must always be used in making calculations of the savings possible when substituting oil for coal, and vice versa. |