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 same 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

— = 10187° C.;

3'67 × 0*216 ’ 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 , θθθθ ≡= i >731° C

3*67 × 0∙216÷8*88×0*24

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 l o 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. [table]

The corresponding values for the principal carbon compounds are— [table]

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, &c.,—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, IO; 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, 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., containing 25 per cent, of water, at 2800 to 3000 units, and their evaporative value as 3 ’ 68 and 4’44 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 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 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 8 to 20 per cent. When these constituents are deducted, the average composition may be stated to be— carbon, 52 to 66; 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 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 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-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 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 undistinguishable 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 ∖ xs Zeit sehr. für Berg-, Hütten-, u. Salinen-wesen, 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 Hartig, Playfair, Johnson, and Brix gives the total loss in ash und cinders observed in the coal trials of various countries as follows :— [table]

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. [table]

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 reduced by about one-seventh to bring them into comparison.

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 differences covering waste by imperfection of combustion and losses by radiation, <tc., 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 extraction 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. Inst, of Civil Engineers, 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 ìb; dry wood, 3∙58-3 ’ 52 lb; cotton stalks or megasse, 3 ’ 2-2 ’ 7 ïb; 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 deposits.

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 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 manageable when blown into a heated combustion chamber as a fine spray by means of steam jets, where it is immediately volatilized and takes fire. Tho heating power is very great, one ton of creosote oil being equal to 2 or 2¾ tons of coal in raising steam.

Natural gases, consisting principally of light hydrocarbons, 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 heating 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 temperature 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, l , 09 per cent According to Ziurek, a thousand cubic feet of such gas corresponds in heating power to 30 or 33 íb of coal.

Sulphur, phosphorus, and silicon, the other principal combustible 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 sufficient to start the flame, which is ultimately communicated to the dry wood, by means of a somewhat less inflammable substance, 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 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 of combustion (silicic acid) being fixed at all furnace temperatures, 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. (π ∙ B∙)

Dung is used as fuel in the east only when wood cannot be had; for the latter, and even any other combustible substance, is preferred when it can be obtained. The inhabitants of Aleppo, according to Russel, use thorns and fuel of a similar kind for those culinary purposes which require haste, particularly for boiling, which seems to be the reason that Solomon mentions the “crackling of thorns under a pot,” rather than in any other way. The same allusion to the use of thorns for boiling occurs in other parts of the sacred volume: thus, the Psalmist speaks of the wicked, “Before your pots can feel the thorns, he shall take them away as with a whirlwind, both living, and in his wrath.” The Jews are sometimes compared in the prophets to “a brand plucked out of the burning,” Amos iv, 11; Zech. iii, 2; a figure which Chardin considers as referring to vine twigs, and other brushwood which the orientals frequently use for fuel, and which, in a few minutes, must be consumed if they are not snatched out of the fire; and not to those battens, or large branches, which will lie a long time in the fire before they are reduced to ashes. If this idea be correct, it displays in a stronger and more lively manner the seasonable interposition of God’s mercy, than is furnished by any other view of the phrase. The same remark applies to the figure by which the Prophet Isaiah describes the sudden and complete destruction of Rezin, and the son of Remaliah; only in this passage, the firebrands are supposed to be smoking; that is, in the opinion of Harmer, having the steam issuing with force from one end, in consequence of the fire burning violently at the other. The words of the prophet are: “Take heed and be quiet; fear not, neither be faint-hearted, for the two tails of these smoking firebrands, for the fierce anger of Rezin with Syria, and of the son of Remaliah,” Isaiah vii, 4. It is not easy to conceive an image more striking than this; the remains of two small twigs burning with violence at one end, as appears by the steaming of the other, are soon reduced to ashes; so shall the kingdoms of Syria and Israel sink into ruin and disappear.

The scarcity of fuel in the east obliges the inhabitants to use, by turns, every kind of combustible matter. The withered stalks of herbs and flowers, the tendrils of the vine, the small branches of myrtle, rosemary, and other plants, are all used in heating their ovens and bagnios. We can easily recognise this practice in these words of our Lord: “Consider the lilies of the field, how they grow; they toil not, neither do they spin: and yet I say unto you, that Solomon, in all his glory, was not arrayed like one of these. Wherefore, if God so clothe the grass of the field, which today is, and to-morrow is cast into the oven, shall he not much more clothe you, O ye of little faith?” Matt. vi, 28–30. The grass of the field, in this passage, evidently includes the lilies of which our Lord had just been speaking, and, by consequence, herbs in general; and in this extensive sense the word χόρτος is not unfrequently taken. These beautiful productions of nature, so richly arrayed, and so exquisitely perfumed, that the splendour even of Solomon is not to be compared with theirs, shall soon wither and decay, and be used as fuel to heat the oven and the bagnio. Has God so adorned these flowers and plants of the field, which retain their beauty and vigour but for a few days, and are then applied to some of the meanest purposes of life; and will he not much more clothe you who are the disciples of his own Son, who are capable of immortality, and destined to the enjoyment of eternal happiness?