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production of power by the direct combustion of fuel in internal combustion engines has taken an increasingly important place (see INTERNAL COMBUSTION ENGINES). In 1900 great hopes were entertained that gas-engine units of large size would be used for the generation of electricity at central stations. Great difficulties have however been experienced in maintaining gas- engine cylinders of large size, and the tendency for some years previous to 1920 was to keep down the size of the individual cylinders and to multiply the number of cylinders running on one shaft. Under these conditions the size of the unit engine is necessarily limited to 1,000-2,000 horse-power. Even with units of this size the cost of maintenance may be high, and considerable stand-by plant has to be kept in reserve. Sir Dugald Clerk has estimated that, in Great Britain, more than half a million B.H.P. per annum was derived in 1920-1 from gas- engines combined with suction and other gas producers. The Diesel type of oil-engine also made great progress during 1910-20 on land as well as on sea.

The internal combustion engine made the most remarkable developments after 1910 in its application to motor vehicles and to aeroplanes. The fuel required for this purpose must conform to certain definite requirements, the most fundamental of which is that it must be an inflammable liquid which can be depended on to vaporize on mixture with air at a sufficiently low temperature to ensure that the mixture can be fired in the cylinders of the engine by an electric spark. Petrol or gasoline is the most widely used fuel for this purpose. It is a mixture of the more volatile hydrocarbons which are obtained in the fractional distillation of natural petroleum. It is also obtained from nat- ural gas by compression or cooling, or by oil-stripping.

The enormously increased demands during recent years led to the adoption of cracking processes, by which during dis- tillation the heavier and less volatile fractions of the crude oil are partially broken up into hydrocarbons of a volatility which brings them within the range covered by the motor-spirit require- ments. It is estimated that the development of cracking methods in the United States has added 10% to the yield of petrol ob- tained from the crude oil; while other improvements in collection and refining have added a further 5 to 6%. In 1909 the yield was 10-7%, while in 1918 it had risen to 26-1%. Thus fully one- fourth of the crude oil refined in the United States is being put on the market as petrol. The petrol imported into Great Britain in 1920 was about 250 million gal., or 830,000 tons.

The only sources of motor fuel in Great Britain are shale oil- works, gas-works and coke-ovens. From the shale oil-works about four million gallons of petrol per annum might be obtained, and from gas- and coke-works about 20 million gallons of benzol, though in 1921 the output was much less. Benzol is an excellent motor fuel for land purposes, alone or mixed with petrol, i

CARBONIZATION AND GASIFICATION

In connexion with the fuel problems of coal in their wider aspects, the operations of carbonization and gasification can be most conveniently considered as processes for the sorting out of the constituents of coal into fuels of various degrees of availability and usefulness. Though the hydrocarbons and their derivatives which occur in, or are derived from, coal by destructive dis- tillation must continue to have a deep interest and an economic significance from the chemical point of view, they are relatively insignificant when the use and disposal of hundreds of millions of tons as fuel are being considered. While this is the only safe attitude for the fuel expert to take, it should be clearly under- stood that this in no way excludes the due consideration of chemical and by-products questions when these arise as a necessary part of the fuel problem.

Coke-Ovens. So long as the blast furnace remains the instru- ment for the conversion of iron ore into pig iron, the coke-oven must continue to supply the necessary fuel in the form of hard coke. The " sorting-out process " at the coke-ovens is necessarily coloured by the fact that its primary object is the production of the right kind of coke. So much is this the case that the beehive oven, in which coke is the only product obtained, has only been

partially displaced by the recovery oven, in which the by-products, tar, benzol and ammonia, are saved. In the iron and steel industry to-day the most advanced opinion is in favour of the concentration of coke-ovens, blast furnaces, steel furnaces and rolling-mills on one site, so that the whole of the potential heat of the coal may be pooled and used in a closed cycle for the production of heat and power. Mr. Talbot has estimated that in this way the fuel required for the production of one ton of finished steel would be reduced from 45 to 35 cwt. As any general replacement of existing works, under the financial conditions prevalent in 1921, was likely to involve a prohibitive capital cost, a more general use of coke-oven gas for the purposes of town supply was not to be hoped for.

Carbonization in Gas-Works. In gas-works the sorting-out process is influenced by the fact that the primary purpose is to supply potential light, heat and power in the form of gas. In the British gas industry the fuel problems of the future acquired a new interest after the publication of the report of the Fuel Research Board on the results of their inquiry into the subject of gas standards. The results of this inquiry led to the adoption by the Board of Trade of a new method of charging the consumer for the gas which passes through his meter. The volume of this gas was still measured and recorded, but the consumer no longer paid on thousands of cub. ft. but on the product of the multiplication of the number of cub. ft. passed by the standard calorific value of the gas per cub. foot. The unit of charge was made the " therm," the name adopted for 100,000 British thermal units. Under this system it is now possible to give to the gas under- takings a wide latitude in the selection of the standard of calorific value which they adopt, and therefore a much wider choice of the methods by which gas is manufactured. In the report it was stated that the great gain for the gas undertakings under the new system would be that no undue legislative restrictions would limit them in their development of the most economical pro- duction of thermal units in the form of gas. It was pointed out that there was still great scope for this development; as, according to present practice, only from 21 to 24% of the total potential thermal units of the coal was being sold in the form of gas.

To increase this percentage two known methods are available, both depending on the production of water-gas by one or other of the reactions between steam and carbon at a high temperature. The first of these methods is the old-established one in which a portion of the coke produced in the retorts is transferred to a separate producer, in which it is raised to bright incandescence by an air-blast and then subjected to the action of a current of steam. The thermal efficiency of this operation ranges from 45 to 55% according to the method of blowing-up and steaming adopted. The second method has recently been developed in con- nexion with vertical retorts. In this case the water-gas reactions are carried out in the lower part of the column of red-hot coke in the retort itself, by passing through it a current of steam. The volume of gas produced is much increased, though its calorific value is reduced by the addition of water gas to the hydrocarbon gas resulting from the carbonization of coal.

During 1919-21 continuous experiments were carried out at H.M. Fuel Research Station on the use of steam in vertical retorts with various types of coal. It was proved that, by the use of a moderate percentage of steam, a much larger proportion of the thermal value of the coal can be converted into the fuels of higher availability and value, gas and tar. In the case of a S. Yorkshire coal of good quality the following results were obtained : At a working temperature of I26C. and with 21 % of steam, tha gains per ton of coal were 22 therms in the form of gas, 34 Ib. of tar, and 6 Ib. of ammonium sulphate. While without steam only 23 % of the potential heat of the coal was obtained in the form of gas, with steam 33 % was obtained. The extra heat which had to be supplied to the retorts in order to produce these results was ten therms per ton of coal carbonized, or 3-3 % of the thermal value of the coal. The gas obtained amounted to 22,580 cub. ft. per ton, with a calorific value of 460 B.Th.U. per cub. foot. Both thermally and economically these results are superior to those which would have resulted from the production of an equivalent