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raising practice the disadvantage due to the non-homogeneity of raw coal as a fuel has been met by the design and working of the boilers, while by the use of automatic stoking and ash removal, the boiler house charges under these heads have been greatly reduced. In comparing the best practice on these lines with the most recent experiences in connexion with pulverized fuel in America it is still doubtful whether the latter can be justified on the score of expense.

The valuable papers of Mr. D. Brownlie * throw a much needed light upon the use of coal for steam raising. His analysis of the statistics which he has collected shows that the amount of coal used for steam generation in Great Britain for heat and power production is from 75 to 100 million tons per annum, or about one-half of the whole coal consumption. His conclusions as to the comparative efficiency of the numerous boiler plants he personally examined during seven or eight years, and the extension of these conclusions to cover the whole steam-raising practice of the United Kingdom, supply material on which some broad generali- zations may be based. He divides the boiler installations of the United Kingdom into three classes bad, average and highly efficient. Of the total number he classes: 10% as bad, 85% as average and 5 % as highly efficient. As regards the efficiencies of each class, with water-tube boilers the bad give 61 %, the average give 69%, and the highly efficient give 82%, while with Lan- cashire boilers the bad give 49%, the average give 60% and the highly efficient give 79%.

If we take the minimum figure of 75 million tons as the amount of coal annually used for steam raising in Great Britain, it is clear that the scope for economy is enormous; for even a moderate increase of efficiency of 10% over all would result in a saving of 75 million tons per annum. Mr. Brownlie's own experience of the savings to be effected by a reorganization of plants leads him to take a much higher saving as a possibility. In the case of the colliery steam boiler plants, the average efficiency of which he places at 51 %, he estimates that the coal bill for all the British colliery plants is 185 million tons, and that the efficiency might easily be raised by 10 to 15%, while by the systematic use of colliery waste a further saving of salable coal would be realized. These facts and figures are well worth careful study of all who are seriously interested in fuel economy. They show the enormous possibilities existing for fuel economy, apart from any new revolutionary discoveries.

Before we leave the subject of steam raising, the use of gas, oil and coke for this purpose may be referred to.

Gas, A considerable amount of experience has been accumulated on the use of gas for steam raising. This experience covers a wide range of gases from blast-furnace gas of about too B.Th.U. per cub. ft. to coke-oven gas of over 500 B.Th.U. With the lowest grade gas the thermal efficiency in ordinary practice has generally been of a low order, but with proper care in boiler setting and firing there is no reason why a thermal efficiency of 80% should not be reached, even with low grade gas. The evaporative efficiency per sq. ft. of heating surface however is low, and in ordinary blast-furnace practice it is found that when coal-firing is replaced by gas, a larger number of boilers is required for the evaporation of the same amount of water.

With coke-oven gas there is no reason why the highest thermal efficiency as well as a high evaporation efficiency per sq. ft. of heating surface should not be obtained. From an economic point of view, however, the use of high-grade gas for steam raising can only be justified when it is a waste product for which there is no other outlet. As fuel for steam raising, the availability of the therms in coke-oven gas is only from 10 to 15% higher than that of the therms in the form of raw coal, or, with coal at 253. per ton, about i-2d. per therm; but for distribution as town gas its value is from 2-d. to 3d. per therm, while for use in internal combus- tion engines its value would be at least as high. For a possible gain in thermal efficiency of from 10 to 15%, it will obviously not pay to produce gas as a fuel for steam raising, except under very special conditions.

Oil. As fuel for land boilers, oil is definitely superior to coal in many respects. Chief among these are the ease with which it can be transported, stored and handled, its flexibility as a fuel, and the high efficiency with which it can be burned. These advantages would probably justify a price of 50 to 100% higher than that of coal. As fuel for the ships of the navy, all the above advantages

1 Engineering, July 12 and 19 1918; July 25 and Aug. I 1919; Dec. 10 and 17 1920.

over coal are emphasized, and in addition to these are the greatly enlarged range of action and the possibility of oil bunkering while at sea. As fuel for the ships of the mercantile marine its advantages are now so fully recognized that the only limits to its extended use are the uncertainty as to future supplies and as to its price. In 1914 there were on Lloyd's Register 364 steamers of 1,310,000 tons fitted for burning oil fuel, whereas in 1921 the total was 2,536 vessels of 12,797,000 tons. The following comparison shows the division of motor-power in the two years:

Percentage of total

gross tonnage 1914 1921

Sail power only 7-95 5-05

Oil etc. in internal combustion engines. . 0-47 2-00

Oil fuel for boilers 2-62 20-65

Coal 88-96 72-30

100-00 100-00

It will be seen that only 72% of the tonnage of the British mer- chantmarine in 1921 required coal, while in 1914 the figure was 89 %.

Coke. Much useful work has in recent years been done by the London Coke Committee on the use of coke and coke breeze for steam raising. This has led to the development of the " Sandwich " system of firing with a mixture of coke and bituminous slack. This system, which is in operation in London, Manchester and elsewhere, consists of feeding from a divided hopper on to the chain-grate stoker, coal slack and coke in superimposed layers, the coal being uppermost. With a natural draught of only -25 in. the coke layer may be from 5 to 6 in. in thickness. This layer prevents the percola- tion and consequent loss of coal dust through the grate. The coke layer being relatively porous permits the passage of air required for the combustion of the coal under favourable conditions, so that little or no smoke is produced. When coke alone is used on a chain grate it is difficult to maintain a sufficiently hia;h temperature to ensure its ignition near the front of the grate. Under the Sandwich system the ignition temperature is maintained well to the front of the grate by the flame produced from the layer of slack. Each fuel therefore helps the more efficient combustion of the other. When coke is used by itself for steam raising, special provision has to be made to secure that its ignition takes place as near the front of the grate as possible. If this is secure, advantage can be taken of the high radiating effi- ciency of the bed of incandescent coke by the provision of ample water-cooled surfaces for the direct absorption of the radiant heat.

Direct combustion of coal is likely to maintain a leading place in steam raising for many years to come; and there is no direction in which the scope for increased economy and efficiency is so obvious and so extensive. By the closer association of steam electric-generating stations with gas-works and coke-ovens the use of the products of carbonization, gas, oils, tar and coke, may supplement the use of raw coal to some extent and may lead to higher efficiency and economy of fuel, but this form of association must be carefully thought out in each particular case. Certain general principles which affect this form of association can be laid down, but the purely local and individual condition must always determine the application of these general principles. Their merely superficial adoption will only lead to disappointment and loss. This aspect of fuel economy is referred to below in connexion with carbonization and gasification as a means of sorting out the elements of raw coal into fuels of higher availa- bility and convenience, but it may be said at once that up to 1921 no case had been made out for the general replacement, by fuels of higher availability, of raw coal used for steam raising. There is every reason therefore for the concentration of skill and enterprise on the general application of the well-established principles which govern the most efficient use of raw coal for steam raising. In Great Britain alone it is certain that tens of millions of tons of coal per annum might be saved in this way.

Apart from steam raising the direct combustion of raw coal in industry does not bulk very large in the general fuel bill. In the metallurgical industries coke and gas are the more important fuels, though considerable quantities of raw coal are still used in steel-making. In pottery and brick-making raw coal is still the chief fuel, but movements have been set on foot which may lead to the more extensive use of gas. In the Portland cement in- dustry raw coal is likely to remain the fuel, as it can be used in pulverized form in rotary cement kilns with high efficiency.

POWER PRODUCTION BY INTERNAL COMBUSTION ENGINES While by far the larger proportion of the power requirements of the world is at present supplied by steam boilers and engines, the