COMPARISON OF COAL AND FUEL OIL The term "Coal" as applied to fuel is very loosely used. The word is applied to a variety of substances ranging from turf through peat, lignite, semi-bituminous and bituminous coals to anthracite. It is obvious that no comparison can be drawn between coal and any other fuel unless the specifications of the coal are stated. The value of the chemical analysis of a sample of a given coal to an engineer, power-plant superintendent, or coal dealer, is a matter that has given rise to much discussion. The general weight of opinion seems to be that an analysis is often of the highest value, and that the time and labor involved in making it are 'well spent. .However, it is clear that analyses are of greater value to some engineers or users of coal than to others; and that, at the present time, they cannot entirely supplant in all cases the information to be obtained from carefully conducted tests in boiler furnaces but should supplement such information, when the latter is obtainable.
In the testing of coals in Government service the chief difficulties in the way of accepting or rejecting untried coals on the basis of chemical analyses alone have proved to be as follows : (1) An ordinary analysis of a coal shows the percentage of ash, but does not indicate the extent to which this ash may fuse or slag on the grate bars of the furnace, and thus seriously inter fere with the rate and completeness of the Though progress has been made toward the determination of the liability to clinker, through a study of the composition of the ash, the results obtained are not as yet altogether satisfactory.
(2) There seems to be a variability, in the heating value of the volatile matter in the coal, which is not clearly indicated by the percentage of the volatile matter, as determined either by the usual methods, or by the ordinary calorimetric determinations.
(3) The caking.of the surface coal in the fire box appears to interfere with the draft, and hence, with the rate arid complete ness of the combustion, and, therefore, impairs the fuel value of the coal to a degree that is not ordinarily indicated by chemical analyses.
For all practical purposes the coal produced in the United States may be divided into three classes, anthracite, bituminous and lignite. The great bulk of the country's coal supply, however, is bituminous or soft coal. Table lf shows the production of coal in recent years in the United States'.
Bituminous is the chief steam coal and when comparisons are made between coal and fuel oil, bituminous coal is used as a basis. Bituminous coal deposits are almost always underlain by fire clay and almost always are overlain by a stratum of shale. The
fire clay is the residuum of the original soil in which grew the luxuriant vegetation that supplied the material for the coal When the swamps i4 which this vegetation grew subsided and when the water covering them grew deeper, a fine silt was de posited and this silt through pressure became the shale of today. In addition to the impurities such as bands of clay, shale or pyrites which, as shown in fig. 11, are found in the coal itself as it lies in the seam, the method of mining employed in the United States is responsible for the addition to the coal of fire clay from the floor and shale from the roof. A sample of coal taken at the face of a mine is only roughly indicative of the coal loaded in railroad cars at that mine. Although theoretically all pieces of fire clay, shale and pyrites or iron sulphide, are hand picked by the miner and thrown to one side, in practice great quantities of these impurities are loaded out by the miner and appear at the tipple on the surface. Inasmuch as these impurities are usually of small size, a greater percentage of impurity will be found in the small sizes of coal and the screenings or slack coal will contain a very high percentage of impurities. This is well illustrated in fig. 12, which shows die size elements of commercial 2-inch screenings and the size elements of 14-inch screenings. Of the 2-inch screenings, 66.8 per cent passed through a 1-inch screen, 41.1 per cent through a 4-inch, and 26.9 through a Of the 1%-inch screenings, 95.5 per cent passed through a 1-inch screen, 57.6 through a 4-inch, and 37.6 through a 4-inch. (See fig. 13.)a The sizes larger than screenings are used for 'domestic and special purposes. The screenings or slack coal are used for steam purposes, inasmuch as sized coal is much too expensive to be burned under industrial boilers. Slack coal which contains as low as 12 per cent ash is of extremely good quality.and in practice many slack coals are burned which carry as high as 25 per cent ash, Illinois is a representative industrial state. The varieties of fuel used in Illinois power plants are central bituminous coal, as 'represented by those of the coal fields of Illinois, Western Ken tucky and Indiana and eastern bituminous and semi-bituminous or soft coals from the Penhsylvania, West Virginia and Eastern Kentucky fields. All of these coals are composed, of the following materials in varying proportions : (1) Solid or fixed carbon which burns with a glow and flame.