The corresponding values for the principal carbon com- Dounds are Heat units. Units of water evaporated. Marsh fias, C 2 H 4 Olefiant gas, C 4 H 4 Crude petroleum Oil of turpentine Wax 1 Burnt to carbonic acid,C0 2 , and water, ILO ) Burnt to C0 2 and S0 2 .... Burnt to CO.. f 14,675 11,849 10,190 10,852 10,496 9,027 9,000 7,183 I 5,307 3,400 2,403 26-68 21-55 18-53 1973 19-04 16-41 16-37 13-06 9-65 6-18 4-37 Ether Tallow Alcohol Methyl aleohol (wood spirit)... Bisulphide of carbon, CS. 2 .... Carbonic oxide . . . The fuels of the highest calorific value are, therefore, those containing the largest amount of disposable hydrogen. Such substances are, however, only of special application as being either gases, volatile liquids, or easily fusible solids; they require special contrivances for their combustion in order to avoid an undue production of smoke, or the for mation of vapours liable to become explosive when mixed with air. The ordinary solid vegetable and mineral fuels, wood, peat, coal, &c., are, therefore, of more general in terest economically considered. Wood may be considered as having the following average composition when in the air-dried state : Carbon, 39 6; hydrogen, 4-8; oxygen, 34-8; ash, 1-0; water, 20 per cent. When it is freshly felled, the water may be from 18 to 50 per cent. Air-dried or even green wood ignites readily when a considerable surface is exposed to the kindling flame, but in large masses with regular or smooth surfaces it is often difficult to get it to burn. When pre viously torrefied or scorched by heating to about 200, at which part incipient charring is set up, it is exceedingly inflammable. Th.3 ends of imperfectly charred boughs from the charcoal heaps in this condition are used in Paris and other large towns in France for kindling purposes, under the name of fumerons. The inflammability, however, varies with the density, the so-called hard woods, oak, beech, and maple taking fire less readily than the softer, and more especially the coniferous varieties rich in resin. The calorific power of absolutely dry woods may as an average be taken at about 4000 units, and when air-dried, i.e., con taining 25 per cent, of water, at 2800 to 3000 units, and their evaporative value as 3 -68 and 4 4 4 times their own weight respectively. Wood being essentially a flaming fuel is admirably adapted for use with heat-receiving surfaces of large extent, such as locomotive and marine boilers, and is also very cleanly in use. The absence of all cohesion in the cinders or unburnt carbonized residua causes a large amount of ignited particles to be projected from the chimney, when a rapid draught is used, unless special spark-catchers of wire gauze or some analogous contrivance are used. When burnt in open fire places the volatile products given off in the apart ment on the first heating have an acrid penetrating odour, which is, however, very generally considered to be agree able. Owing to the large amount of water present, no very high temperatures can be obtained by the direct combustion of wood, and to produce these for metallurgical purposes it is necessary to convert it previously either into charcoal, or into inflammable gas in a so-called gazogene or gas- producer. See CHARCOAL and CARBON. Peat includes a great number of substances of very unequal fuel value, the most recently formed spongy light brown kind approximating in composition to wood, while the dense pitchy brown compact substance, obtained from the bottom of bogs of ancient formation, may be compared with lignite, or even in some instances with coal. Unlike wood, however, it contains incombustible matter invariable but large quantity, from 5 to 15 per cent., or even more. Much of this, when the amount is large, is often due to sand mechanically intermixed; when air-dried, the proportion of water is from 8 to 20 per cent. When these constituents are deducted, the average composition may be stated to be carbon, 52 to 66 ; hydrogen, 47 to 7 4; oxygen, 28 to 39; and nitrogen, 1 5 to 3 per cent. Average air -dried peat may be taken as having a calorific value of 3000-3500 units, and when freed from water by a heat of 100 degrees, and with a minimum of ash (4 to 5 per cent), at about 5200 units, or from a quarter to one third more than that of an equal weight of wood. The lighter and more spongy varieties of peat when air-dried are exceedingly inflammable, firing at a temperature of 200 C. : the denser pulpy kinds ignite less readily when in the natural state, and often require a still higher tem perature when prepared by pulping and compression or partial carbonization. Most kinds burn with a red smoky flame, developing a very strong odour, which, however, has its admirers in the same way that wood smoke has. This arises from the destructive distillation of imperfectly carbonized organic matter. The ash, like that of wood, is light and powdery, except when much sand is present, when it is of a denser character. Peat is principally found in high latitudes, on exposed high table-lands and treeless areas in more temperate climates, and in the valleys of slow-flowing rivers, as in Ireland, the west of Scotland, the table-land of Bavaria, the North-German plain, and parts of the valleys of the Somme, Oise, and a few other rivers in northern France. In the last-named country it is dredged from the bottom of ponds, and in the summer time moulded into bricks, which are dried by exposure to the sun. A principal objection to its use is its extreme bulk, which for equal evaporative effect is from 8 to 1 8 times that of coal. On the railways in Bavaria and Oldenburg, where peat is burned, the tenders, in order to have the necessary fuel capacity, are made of equal dimensions with the largest goods waggons, and the water reservoir is placed below the axles, nearly down to the level of the rails. Various methods have been proposed, and adopted more or less successfully, for the purpose of increasing the density of raw peat by com pression, either wither with out pulping; the latter process gives the heaviest products, but the improvement is scarcely sufficient to compensate for the cost. Lignite or brown coal is of intermediate character between peat and coal proper. The best kinds are undis- tinguishable in quality from free-burning coals, and the lowest earthy kinds are not equal to average peat. When freshly raised, the proportion of water may be from 45 to 50 per cent, and even more, which is reduced from 28 to 20 per cent, by exposure to dry air. Most varieties, how ever, when fully dried, break up into powder, which con siderably diminishes their utility as fuel, as they cannot be consolidated by coking. Lignite dust may, however, be compacted into serviceable blocks for burning, by pressure in machines similar to those used for brick-making, either in the wet state as raised from the mines, or when kiln-dried at 200 C. This method, adopted to a very large extent in Prussian Saxony, is noticed in Ure s Dictionary, vol. iv. p. 530, and described in detail in Zeitschr. fur erg-, Hiitten-, u. Salinen-ivesen, xxiv. p. 234. The calorific value, as far as it can be expressed by averages, varies between 3500 and 5000 units, and the evaporative factor from 2 -1C when freshly raised to 5 84 for the best kinds of lignite when perfectly dried.
Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/844
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