combustion coal wood carbon water heat heating calorific dried peat
FUEL. This term includes all substances that may be usefully employed for the production of heat by combustion with atmospheric air or oxygen. Any element or combination of elements susceptible of oxidation, i.e., any substance electropositive to oxygen, may under particular conditions be made to burn; but only those that ignite by a moderate preliminary heating, and burn with comparative rapidity, and, what is practically of equal importance, are obtainable in quantity and at a moderate price, come fairly within the category of fuels. Among elementary substances only hydrogen, sulphur, carbon, silicon, and phosphorus can be so classed, and of these the last two are only of special application. More important than the elements are, however, the carbohydrates, or compounds of carbon, hydrogen, and oxygen, which form the bulk of the natural fuels, wood, peat, and coal, as well as of their liquid and gaseous derivatives, coal gas, coal tar, pitch, oil, .17c., which are possessed of great fuel value. Carbon in the elementary form has its nearest representatives in charcoal and coke.
In the determination of the value of fuel two principal factors are involved, namely, the calorific power, or the total amount of heat obtainable from the perfect combustion of its constituents, and the calorific intensity, or pyrometric effect which is the temperature attained by the gaseous products of the combustion. The first of these is constant for any particular composition, and does not vary with the method of combustion, the quantity of heat developed by the combustion of a unit of carbon or hydrogen being the seine whether it be burnt with oxygen, air, or a metallic oxide. The calorific intensity, on the other hand, being inversely proportional to the volume of gases produced, it is obvious that if the combustion is effected with pure oxygen the resulting carbonic acid (in the case of carbon) may be very much hotter than when air is used, as the duty of heating up an additional quantity of nitrogen rather more than three times the weight of the oxygen is in the latter case imposed upon a similar weight of carbon.
Theoretically 1 unit of carbon combines with 2.67 units of oxygen to form 3.67 units of carbonic acid, whose specific heat is 0.216. The resulting maximum temperature of the gases produced therefore cannot exceed but when a similar weight of carbon is burnt with air, the gases are diluted with 8.88 units of nitrogen, whose specific heat is 0.244. The highest temperature possible in the products of combustion in this case does not exceed , The calorific value of a fuel may be determined by direct experiment, either by complete combustion on the small scale in a calorimeter, or by practical experiment on a working scale, by ascertaining the effect of a weighed quantity in performing a particular kind of work, such as evaporating water in a steam boiler, the result being expressed in the number of pounds of water converted into steam per pound of fuel burnt. It may also be computed from an ultimate chemical analysis, - the carbon and so much of the hydrogen as remains disposable for burning, after deducting sufficient to form water with the oxygen present, being credited with the full heating power derivable from their complete oxidation, according to the results found for these elements by the calorimeter.
The pyrometric effect, on the other hand, cannot be either computed or determined experimentally with complete accuracy, partly because the total combustion of a quantity of fuel in a given time at one operation is practically impossible, but more particularly from the fact that dissociation of the gaseous compounds produced in burning takes place to a greater or less extent at temperatures far below those indicated as possible by calculation based upon comparisons of the weight of the products of combustion and their specific heat with the calorific value of the substance as found by experiment. According to Bunsen, Deville, Dewar, and others who have specially considered this subject, a temperature of about 3000' C. will be the maximum attainable from any fuel by any ordinary process of combustion. The calorific powers of the principal elementary substances susceptible of use as fuels are given in the following table ; they are expressed in calories or heat units, signifying the weight of water raised, in temperature 1° C., by the combustion of one unit of the different substances, and the corresponding weight of water converted into steam from a temperature of 100° C.
The corresponding values for the principal carbon compounds are 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 formation of vapours liable to become explosive when mixed with air. The ordinary solid vegetable and mineral fuels, - wood, peat, coal, &e., - are, therefore, of more general interest 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 previously torrefied or scorched by heating to about 200°, at which part incipient charring is set up, it is exceedingly inflammable. The 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, tinder 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., containing 25 per cent. of water, at 2800 to 3000 units, and their evaporative value as 3'68 and 4.41 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 apartment on the first heating have an acrid penetrating odour, which is, however, very generally considered to be agreeable. 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 i It is necessary to convert it previously either into charcoal, or into inflammable gas in a so-called gazogene or gas-producer. See CIIARCOAL 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 sonic instances with coal. Unlike wood, however, it contains incombustible matter in variable 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 S to 20 per cent. When these constituents are deducted, the average composition may be stated to be - carbon, 52 to 60 ; hydrogen, 4.7 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 spongyvarieties 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 temperature 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.G'erinan 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 aro 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 18 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 compression, either with or without 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 'indistinguishable 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, however, when fully dried, break up into powder, which considerably 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 Bery-,Illitten-, se. Salinen-teesen, 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 2.16 when freshly raised to 5.84 for the best kinds of lignite when perfectly dried.
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 sufficiently 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 different observations upon this point made by Ilartig, Play-fair, Johnson, and 13rix gives the total loss in ash Laid cinders observed in the coal trials of various countries as follows :- In one of the latest researches upon the 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.
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, &c., 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 can d e mill. According to Mr J. Head (Proc. In. 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 • The common fuel of India and Egypt is derived from the dung of camels and oxen, moulded into thin cakes, aik dried in the sun. As might be imagined it has a very low heating power, and in burning gives off acrid ammoniaea; smoke and vapour.
Somewhat similar to these are the tan cakes made from spent tanners' bark, which are used to seine 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 respectively. 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 the 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 the 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. The 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 beat- 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; ture has been tried to some extent in Berlin. The average composition of this is - hydrogen, 42.36 ; carbonic oxide, 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 lueifer matches, - the heat generated by friction against a roughened surface being suili- 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 theBessemer process of steel-making, gives rise to an enormous increase 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 s of combustion (silicie acid) being fixed at all furnace tern.
peratures, the whole of the heat developed is available for r heating the molten iron, instead of a considerable part being l consumed in the work of volatilization, as is the case with carbonic acid. (11. B.) TX,
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