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Million (2.) HEAT for industrial purposes. Tons
Iron, steel and other metals 32
Bricks, pottery, glass, cements ..._... 6
Paper, textiles, food-stuffs, fertilizers, chemicals, as steam 20
58 (3.) POWER for transport and industrial purposes.
Railways and coasting steamers 17
Mines 18
Factories 5
87 Coal used in the raw state.
Domestic heating . . . . .
Steam raising for heat
Steam raising for power
Transport, railways, steamers . . . .
Brickworks, potteries, cement, glass, chemicals,
soap, etc
Coal carbonized and gasified.
Gas undertakings
Iron, steel and other metals
Million Tons 35
20
60 17
18 3
189
These figures show that 141 million tons, or three-fourths of the coal used in the United Kingdom, was burned in the raw state; that 35 million tons, or nearly one-fifth of the total con- sumption was used in the raw state for domestic heating; and that 97 milh'on tons, or one-half of the total consumption was used in the raw state for steam raising. Before considering the technical and economic problems which are involved in the re- placement of raw coal as a fuel by the products of its carboniza- tion, gas, petrol, oils and coke, we shall review the position of these great outlets for raw coal, domestic heating and steam raising.
Domestic Heating. The domestic use of coal in the raw state affects the widest range of consumers in most civilized communi- ties. In Great Britain the consumption per head of the popula- tion is in the neighbourhood of one ton per annum. In Ireland peat is still the chief domestic' fuel, about six million tons of air- dried peat being consumed per annum. In no other country but Great Britain does the consumption of raw coal for domestic purposes reach the high figure of one ton per head per annum. The ample supplies and the low price of bituminous coal for centuries prior to the World War have established the open room fire and the kitchen range on a popular foundation in Great Britain from which it is difficult to displace them. The British climate has had much to do with the popularity of the open room fire, the radiation from which can be so readily modified to meet the rapid changes of humidity and temperature which are liable to occur almost from day to day during the year. Only during exceptional winters, when really low temperatures have con- tinued for weeks at a time, has the open fire broken down as a means of maintaining English homes at a habitable temperature. The work of Dr. Margaret Fishenden on open fires has definitely shown that, under reasonable conditions of firing, 20% of the total potential heat of the raw coal is radiated into the room, and that a further 20 to 30% is given up to the fabric of the building before the waste products of combustion leave the chimney. Smoke and soot are, however, an unduly heavy price to pay for the transient cheerfulness of the flaming coals in a well-stoked fire, especially when we remember that over long periods the ordinary fire is only smouldering and dreary-looking. The coal- fired kitchen range, unlike the open room fire, has few if any sentimental associations and its replacement by gas-cookers and coke-fired water-heaters is only a matter of time.
In northern and central Europe, and in the United States and Canada, where really low winter temperatures prevail, close stoves and central heating systems are universally used, and the smoky combustion of bituminous coals has never gained a footing. In the United States and Canada anthracitic coals for domestic purposes are regarded as a necessity, and the Governments of these countries give every encouragement to schemes for the con- version of bituminous coals into smokeless fuel so as to avoid the transport from great distances of anthracitic coals. On the social side of civih'zation it is no exaggeration to say that the cheap and
plentiful supplies of bituminous coal in Great Britain have not been an unmixed blessing. Even on the industrial side this is true, for it has led to the formation of habits of reckless extravagance in the use of fuel, which are so deeply rooted among workmen and manufacturers that it will take many years of high fuel prices to eradicate them. The gas undertakings of the United Kingdom have, however, done much to popularize the use of gas and coke for the replacement of raw coal for domestic use; and there is every prospect that a considerable proportion of the raw coal burned for domestic use will be displaced by the developments in the production of town gas on newer and more economical lines, and by the increased use of gas-works coke for domestic heating.
Steam Raising. The fact that one-half of the coal used in Great Britain is consumed in raising steam for heating purposes and for power production, places the problems of fuel efficiency in this connexion in the forefront from an economic point of view. On the theoretical side these problems lend themselves to simple and direct treatment.
Steam-boiler efficiency depends first on the perfect combustion of the fuel, second on the utilization of the radiant heat of com- bustion, and third on the utilization of the sensible heat of the gaseous products of combustion before they are dismissed to the chimney. The heat for the conversion of water into steam has to pass through steel plates or tubes, and the rate at which this transference takes place is determined by the different temper- ature of the two sides of the plate or tube. The lower the temper- ature on the water side and the higher the temperature on the furnace side, the greater will be the amount of heat which is passed into the water, and the higher will be the evaporative efficiency of that portion of the boiler. Direct radiation from the burning fuel is by far the most effective means of maintaining the temperature on the furnace side of the plates and therefore of obtaining the highest evaporative efficiency per sq. ft. of metal surface. On the water side of the plate or tube the temperature can be kept down only by the maintenance of a very rapid circula- tion of the water over the metal surface. With adequate water circulation sufficient heat for the evaporation of 60 to 80 Ib. of water per sq. ft. per hour can be safely passed through the metal. With inadequate circulation the metal may be raised to a destructive temperature, and the boiler may be ruined. In the ideal boiler the maximum proportion of the radiant heat of com- bustion ought to be absorbed by metal surfaces provided with ample water circulation on their inner side. The utilization of the sensible heat of the products of combustion involves the transfer of the heat of the gases to the metal by convection; the molecules of gas must actually come in contact with the metal surface. Rapid circulation is required in order to obtain this, and high velocity of the gases must be maintained. The work of Nicholson on this subject has received considerable attention during recent years and has to some extent been applied to boiler design. The importance of the direct absorption of the radiant heat of combustion is not as yet so generally recognized, but is likely to lead to important results in boiler design. The theoretical knowl- edge as to the utilization of the heat of combustion in boilers is still somewhat in advance of even the best engineering practice in steam-boiler construction. Unfortunately average practice still lags far behind the best knowledge on the subject.
Coal as ordinarily burned suffers from the disadvantage that it is not a homogeneous fuel like gas, oil or coke, but is in effect a mixture of these three forms of fuel. The only way in which coal can be made to approximate to a homogeneous fuel is by pulverizing it so that its particles are so fine that, when mixed with air, they at once ignite and burn like a jet of gas or a spray of oil. The degree of fineness required to produce this effect involves grinding till 80% of the coal will pass through a screen of 200 meshes to the square inch. For metallurgical and other high temperature purposes the advantages which result from pulveri- zation may more than compensate for the cost of grinding and for the heavy initial cost of the grinding and distribution plant, but for steam raising it is still an open question whether the gain in the efficiency of combustion is sufficient to compensate for the greatly increased cost which is involved. In the best steam-
