Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/845

FUEL 809 The manner of estimating the heating power of coal has already been considered (vol. vi. p. 80). The heating effect of fuels obtained in practice is always considerably less than that indicated by theory, as the latter supposes complete combustion, a result which cannot be attained in the ordinary system of burning upon a grate of bars with spaces between them for the admission of air, as a certain proportion of unconsumed particles when suffi ciently reduced in size to pass through the grate bars fall through with the ashes, forming cinders which represent so much of the useful fuel lost, at any rate for the time. This proportion varies very considerably with the state of the fuel and its proportion of ash. A summary of the dif ferent observations upon this point made by llartig, Play- fair, Johnson, and Brix gives the total loss in ash. taid cinders observed in the coal trials of various countries as follows : American coals 5 to 18 5 percent. English ,, 2 9 ,, 277 ,, Prussian ,, 1 5 ,, 11 6 ,, Saxon ,, 7 4 ,, 63 4 ,, In one of the latest researches upon tho heating power and other properties of coal for naval use, carried out by the German Admiralty, the following results were obtained with coals from different localities. SlaK left in grate per cent. Ashes in ash-pit per cent. Soot in flues per cent. Water eva porated by 1 tt) of coal. 0-33-6-42 2-83- 6-53 0-32-0-46 6-60-745 tb Do. bituminous coals 0-98-9-10 1-93-.V70 1-97- 9-G3 4-37-10-63 0-24-0-88 0-24-0-48 7-30-8-66 703-8-51 0-92-1-30 3-15- 3-50 0-24-0-30 G-73-7-10 Welsh steam coals 1-20-407 4-07 0-32 8-41 1 92 2-57 C 35 7-28 The evaporative power in these experiments is referred to water at the freezing point, while in the results given in article COAL, vol. vi. p. 81, it is computed from the boiling point. The latter quantities therefore require to be re duced by about one-seventh to bring them into com parison. In many cases, however, the evaporative factor found by practical experiment in a steam boiler is from a third to nearly a half less than that indicated by theory, the differ ences covering waste by imperfection of combustion and losses by radiation, etc., in the furnace and flues. Of the other natural fuels the most important is so- called vegetable refuse, such as cotton stalks, brushwood, straw, and the woody residue of sugar cane after the ex traction of the saccharine juice known as megasse or cane trash. These are extensively used in countries where wood and coal are scarce, usually for providing steam in the manufactures where they arise, e.g., straw for thrashing, cotton stalks for ploughing, irrigating, or working presses, and cane trash for boiling down sugar or driving the cane mill. According to Mr J. Head (Proc. lust, of Civil En gineers, vol. xlviii. p. 75), the evaporative values of 1 lb of these different articles when burnt in a tubular boiler are coal, 8 lb; dry peat, 4 lb; dry wood, 3 58-3 52 lb; cotton stalks or megasse, 3 "2-2 7 lb; straw, 2 46-2 -30 lb. In burning straw it is found most convenient to use a pair of toothed rollers, which pass it continuously into the fire box in a thin layer. Owing to the siliceous nature of the ash, it is also desirable to have a means of clearing the grate bars from slags and clinkers at short intervals, and to use a steam jet to clear the tubes from similar de posits. The common fuel of India and Egypt is derived from the dung of camels and oxen, moulded into thin cakes, and dried in the sun. As might be imagined it has a very low heating power, and in burning gives off acrid ammoniacal smoke and vapour. Somewhat similar to these are the tan cakes made from spent tanners bark, which are used to some extent in eastern France and in Germany. They are made by moulding the spent bark into circular cakes, which are then slowly dried by exposure to the air. Their effect is about equivalent to 80 and 30 per cent, of equal weights of wood and coal re spectively. The same class of fuel made from exhausted dye-wood is considered to be equal to two-thirds of its weight of coal. Liquid fuel in the form of natural petroleum, and tho heavy or so-called dead or creosote oil obtained in coal- tar distilleries, have recently been used to some extent both for heating steam boilers and welding iron. In England tho former cannot be used from its high price, apart from the danger caused by the irregular volatility of its constituents; the latter, however, is perfectly man ageable when blown into a heated combustion chamber as a fine spray by means of steam jets, where it is imme diately volatilized and takes fire. The heating power is very great, one ton of creosote oil being equal to 2 or 2 tons of coal in raising steam. Natural gases, consisting principally of light hydro carbons, have at different times been used as fuel, but the examples of their application are necessarily rare. Tho most conspicuous example at the present time is afforded by the Iron City and Siberia Iron Works, near Pittsburg, in Pennsylvania, where puddling and welding furnaces, as well as steam boilers, are entirely fired by the gas from a well bored for oil, 1200 feet deep, which is brought to the works through a pipe several miles in length, and arrives with a pressure of two atmospheres. Ordinary coal gas, such as is used for illuminating, can also be applied for heat ing purposes, but it is, in spite of its very high calorific power, too expensive for general use. A cheaper material obtained by the distillation of lignite at a high tempera- tura has been tried to some extent in Berlin. The average composition of this is hydrogen, 42 36; carbonic oxide, 40; marsh gas, 11 37; nitrogen, 3 17; carbonic acid, 2 01; and condensable hydrocarbons, 1 09 percent. Ac cording to Ziurek, a thousand cubic feet of such gas cor responds in heating power to 30 or 33 ft) of coal. Sulphur, phosphorus, and silicon, the other principal com bustible elements, are only of limited application as fuels. The first is used in the liquation of sulphur-bearing rocks. The ore is piled into large heaps, which are ignited at the bottom, a certain proportion, from one-fourth to one-third of the sulphur contents, being sacrificed, in order to raise the mass to a sufficient temperature to allow the remainder to melt, and run down to the collecting basin. Phosphorus, which is of value from its low igniting point, receives its only application in the manufacture of lucifer matches, the heat generated by friction against a roughened surface being suffi cient to start the flame, which is ultimately communicated to the dry wood, by means of a somewhat less inflammable sub stance, such as sulphur or paraffin. The high temperature produced by burning phosphorus is due to the product of combustion (phosphoric acid) being solid, and therefore there is less heat absorbed than would be the case with a gaseous product. The same effect is observed in a still more striking manner with silicon, which in the only special case of its application to the production of heat, namely, in the Bessemer process of steel-making, gives rise to an enormous in crease of temperature in the metal, sufficient indeed to keep the softest iron melted. The absolute calorific value of silicon is rather less than that of carbon, but the product of combustion (silicic acid) being fixed at all furnace tem peratures, the whole of the heat developed is available for heating the molten iron, instead of a considerable part being consumed in the work of volatilization, as is the case with carbonic acid. ( H - B -) IX. 102