FUELS

are materials which can be _ burnt economically for the generation of heat to be applied to some useful purpose, such as domestic heating, steam raising, firing of furnaces, . They may be solid, liquid, or gaseous, and may be used in the natural state or after being prepared by some suitable proce . Solid fuels contain combustible matter consisting largely of carbon and hydrogen either combined or in the free state, and incombustible matter in the form of ash. Liquid and gaseous fuels consist mainly of combustible matter, and generally contain little ash. The combustible matter in solid fuels is present in two forms, viz. volatile matter, which is driven off when the substance is submitted to heat out of contact with air, the residue left behind being charcoal when wood is heated, and coke when coal is heated, each of these mnaterials consisting of fixed carbon together with the ash present in the original material. Natural fuels. The natural fuels are wood, peat, coal, natural oils, and natural gas. Wood may be considered as the natural fuel of man; it was the first and for many ages the only fuel with which he was acquainted. It is still used to some extent for domestic purposes, and in certain parts of the world for some industrial purposes also. It cannot be considered a satisfactory fuel, owing to the large amount of water it retains. In the growing condition, the cells and ve els are filled with sap, and _ freshly-felled wood contains about 50 per cent. of water, the amount varying with the nature of the wood, the part of the tree from which it is taken, the age of the tree, and the season of felling. A tree which has had its bark stripped off, and has been exposed to the air, loses moisture, and the remaining air dried wood contains 15 to 25 per cent of water. During the burning of the wood much heat is used up in the evaporation of the water remaining. Peat consists of partially-decayed vegetable matter occurring in beds of varying thickne from a few feet upwards. In this country the peat is composed almost entirely of the remains of mo es. The top layers of the peat-bogs contain tangled roots and stems of plants only slightly deeayed, are light-brown in colour, and of low specific gravity. Lower down the material becomes darker and denser, and at the greatest depth it may have pa ed into a nearly black compact ma in which all trace of the original matter is lost. When freshly got, peat contains up to 80 per cent. of water, and after air-drying, contains 10 to 15 per cent. _ Peat, even in the air-dried condition, is not a good fuel, as it contains too much water, and may in addition contain a considerable amount of ash. There is a tendency, however, towards the increased use of peat as a fuel, and there is every probability that peat and peat products will become much more important as fuels in the future than they have been in the past. It is estimated that there are over 9,000 sq. miles of peat deposits in Great Britain. Coal is by far the most important natural fuel. In its strictly correct usage the name comprises only the compacted carbonaceous materia such as lignite, bituminous coals, an anthracite. Lignite or brown coal differs from ordinary coal in being the result of partial mineralization of vegetable ti ues. It is used for domestic, manufacturing, and locomotive purposes in districts remote from ordinary coal-fields, Bituminous coals include all the most important cla es of carboniferous fuel, and are divided by Sir Richard Redmayne into the following cla es: (1) gas, (2) coking, (3) house, (4) manufacturing or iron smelting, and (5) steam, which grade one into the other. The names given to the different cla es signify the purposes for which they are most suitable. Anthracite represents a stage of mineralization beyond coal. It contains up to 98 per cent of carbon and very little volatile matter. Owing to its hard and compact structure and difficulty of ignition, it is only useful for steam raising where forced draught is used. It is largely used for malting purposes and for drying hops, where a steady heat is required; for domestic purposes and hot-house heating it is burnt in specially-designed stoves, and is also used to some extent in iron smelting and in the manufacture of producer-gas. The importance of coal as a fuel may be realized from the fact that the estimated world’s production of coal in 1913 was about 1,321,000,000 tons, and in 1931 about 1,035,318,000 tons. Natural oils or petroleums are found in many parts of the world, and are largely used as fuel for steam raising and general industrial purposes. Before use, they are generally submitted to distillation, and are thus to an extent prepared fuels, and will be considered below. N: Natural gas occurs in certain localities in very large quantities, and in. Pennsylvania has been used as a fuel. 599