Fuel.

Fuel. The chief mode of artificially producing that condition of matter which is called heat is by burning certain substances in air. These substances contain carbon and hydrogen, which during the chemical change implied by burning unite with the atmospheric oxygen, and as the temperature rises emit light as well as heat. Since these two elements are very widely distributed in nature, the classification of all the compounds which may be termed fuels is somewhat difficult. After using wood for long ages men at last laid the mineral kingdom under requisition, but the fuels thence derived were soon recognised to be undoubtedly of vegetable origin. Some writers include all these under the term natural, and distinguish such derivatives as coke, charcoal, and combustible gases as artificial. Popularly, fuels are a large class of compounds, all of vegetable origin except the animal oils and fats, which produce heat and light when raised to 'kindling temperature.' Thus, besides coal and coke, wood and charcoal, and peat or turf, we must reckon tallow, wax, alcohol, coal and other gases, petroleum, creasote or 'dead-oil,' and others as fuels. To be exhaustive, we should further refer to a sub-class called 'patent' fuels.

The ordinary solid fuels fall under two heads: those containing water in a large proportion—e.g. wood, turf, and most coals—and therefore producing, when burned, hydrogen as well as carbon; and secondly, those which are purely carbonaceous—coke, charcoal, and anthracite. In recent times, since metallurgy has assumed such proportions in all countries, and especially since the application of steam-power, the coking of coal has been more and more perfected, in order to concentrate the carbon and present a fuel capable of producing a higher temperature. Wood as a fuel is either light and soft, as deal, or heavy and hard, as oak; but neither kind is now applied in metal-working, unless in the concentrated form of charcoal. Wood contains so large a proportion of water as to reduce its heat-giving quality both in quantity and intensity, and contains less than half its weight of carbon (see table).

Charcoal is formed by condensing the carbon of wood and expelling the hydrogen and oxygen, just as coke is a concentration of coal by an analogous process. When the wood has been packed and so closed in as to prevent access of air, by raising the whole to a temperature of about 300°, the watery and gaseous particles are entirely expelled, and a mass of almost pure carbon remains. Similarly from coal we have coke, prepared by 'dry distillation' or imperfect combustion, so as to retain the carbonaceous part in a concentrated state and set free the volatile ingredients and part of the sulphur. A special property of coke for metallurgy, as compared with coal, is that, when exposed to high temperatures as in iron-blast furnaces, it does not become pasty.

Turf or peat is an agglomeration of decayed vegetable matter, such as is frequently found on the sites of ancient forests. It is remarked that no instance of its formation occurs within the tropics; though Lyell describes the Great Dismal Swamp between Virginia and North Carolina to be a mass of black peat-like matter, 15 feet deep. Some peaty sediment has also been noted in a Cashmere lake. From holding so small a percentage of carbon, turf is of little use in the arts; but in Bavaria it has been utilised for locomotive engines after being compressed into bricks, and in some districts it has been converted into a species of charcoal.

Superior to the peat fuels, though still inferior in carbon to coal proper, are the lignites or brown coal, which occur in geological deposits of more recent formation than the true coal-measures. The lignites contain a larger proportion of water than coals properly so termed; and are of so many varieties as gradually to pass into the bituminous class, which are known by their smoky flame and derive their name, not from any bitumen in their composition, but from the well-known tars which they produce. With the bituminous must be reckoned the 'coking coal' and the 'cannel' (i.e. candle) coal.' The last-mentioned variety, moreover, includes the Edinburgh 'parrot coal' (so named from its crackling) and the 'horn coal' of South Wales, which is characterised by a smell like that of burnt horn. At the head of this class of fuels is the anthracite coal, holding over 90 per cent. of carbon, and therefore of special value for some purposes in metallurgy and otherwise. Anthracite is very compact, somewhat brittle, and does not stain the fingers like ordinary coal.

For comparing as fuels some leading types of coals the following table—which is an abstract from various returns—will be of use, presenting the percentage of carbon, of hydrogen, and the ash left after combustion :

In primitive times the scarcity of wood in some parts of Egypt and India suggested the use as fuel of sun-dried cakes of the dung of camels and oxen. A similar practice exists to-day in the trackless steppes of Central Asia; and so, too, in various countries of Europe much refuse, especially of a vegetable nature, is utilised which in coal or wood producing districts is rejected as absolutely worthless. In eastern France, for example, and Germany all the spent bark from tanneries is formed into cakes for fuel, and estimated as worth about three-fourths the same weight of wood. Where coal is not found or cannot profitably be conveyed, the preservation of forests is of manifest importance; and in certain parts of Europe, for example, trees are systematically planted in hedgerows and otherwise to provide fuel. For the same reason pollarding is resorted to, the branches being regularly cut, and the trunk left to grow fresh fuel. The scientific world, with as good a reason as the primitive races, have recently found means to largely supplement the natural supply of vegetable and mineral fuels by fluid or gaseous substances. Thus, in smelting iron, for example, the carbonic oxide, which formerly was carried off in the smoke from the blast-furnace, is now sometimes collected and conveyed in pipes to be utilised as fuel under steam-boilers. Natural gas has also been used to good purpose, notably in Pennsylvania, United States, where in several instances it has been transferred for several miles for heating furnaces. In the same district petroleum is a recognised liquid fuel, as well as naphtha, its derivative. Another liquid fuel is creasote-oil, derived from coal-tar, which is reported to possess, weight for weight, at least twice the power of coal for raising steam. The United States chemists and metallurgists are agreed that not only is a 'higher, steadier, and more even heat' produced by liquid fuel, but that, for heating iron more especially, a smaller quantity and shorter time suffice to obtain the same results. Baku petroleum is used as fuel for locomotives and steamers in South-east Russia. See GAS.

Under this head we subjoin some figures from a report of a Royal Commission drawn up in 1871 by Professor Rankine. The first column (A) shows the quantity of heat units generated by the fuel; the second (B) the pounds of water heated from 60° to 212°, and then, of course, converted into steam; and the third column (C) gives the comparative temperature of the fire or flame :

The three points noted in testing a fuel chemically are the intensity of the heat, the quantity of heat developed in combustion, and the luminosity. The last of these, however, affords but an imperfect measure of the temperature, because it is mainly due to the presence of solid particles. Instead of the second some writers use the term 'caloric power.' In ordinary coal combustion there are two steps of the process: (1) the carbon is separated from the hydrogen in light particles, which, unless burned, appear as soot or smoke; (2) the hydrogen becoming ignited heats up the carbon particles, which therefore appear as flame. For the complete combustion, therefore, of a typical hydrocarbon we require not only air in sufficient quantity, but also intensity of heat above the fuel. In a good furnace the supply of coal should by mechanical contrivance be rendered as regular and uniform as that of air; and the body of the furnace should be so protected from the boiler surface and other cooling agents as to steadily maintain a temperature sufficient for thorough ignition of the flame.

What are called 'patent fuels' arise mainly from the desire to utilise the refuse arising from the production or wasteful use of coal. Such artificial fuel, however, is by no means an entirely modern device, since the Chinese have for ages been accustomed to mix coal-dust with clay and bitumen, so much so as to constitute a large branch of industry. The most common form of 'patent fuel' is a mixture of the small coal which accumulates at the pit mouths with sand, marl, or clay, or of some bituminous or resinous substance with sawdust. A second kind has dried and compressed peat as its basis, and is sold in the form of a dense brown solid. Another is an attempt to utilise small coke and the refuse 'breeze,' which is well known in charcoal burning. The 'charbon de Paris' is a combination of the dust of anthracite charcoal and similar refuse with coal-tar, so as to form a paste and be moulded into small cylinders of about 4 inches in length. Briquettes (q.v.) are compounds of waste coal-dust and pitch.

See Report of Royal Commission on the Coal of the United Kingdom (1871); Report on the Coals suited to the Steam Navy (1848); Rumford's Works, vols. ii., iii.; Williams, Fuel: its Combustion and Economy (3d ed. 1886); Phillips, Fuels: their Analysis and Valuation (1890).