Spent tan-bark is commonly used in the furnaces of tanneries and yields about 4,500 British thermal units per pound. Straw is sometimes burned as fuel under boilers and yields about 5,500 British thermal units per pound.
Charcoal is made by evaporating the vola tile constituents of wood, either by partial com bustion or by heating in retorts. By partial combustion as in a common charcoal kiln, 100 pounds of wood will yield 20 pounds of char coal, and by heating in a retort about 30 pounds. A cord of wood generally yields about 50 bushels of charcoal. Since the non-combustible constituents of the wood are driven off in the process of making charcoal, the' latter has a high calorific value, ranging from 11,000 to 13,500 British thermal units per pound.
On account of the large per cent of moisture in peat, wood, sawdust and tan-bark, these sub stances as well as coke and charcoal, are rarely used for steaming or power-producing purposes, but all of them are available as fuel in gas producers, by the use of which the moisture is readily removed from the constituent gases, which can then be used for power-producing purposes by means of internal combustion engines, or be burned directly in furnaces where high temperatures are required, such as those of steel works, rolling mills, smelting furnaces, glass works and chemical works.
Liquid Fuels. Among liquid fuels the oils derived from petroleum occupy practically the whole division. In application they are used in two ways: in external combustion and much more extensively in internal combustion en gines. While the so-called heavy oils are gen erally devoted to the external oil-firing and the lighter oils to the internal combustion type of motor, there is no distinct division as to availability. if there is a suitable adaptation of burners. Closely allied to the lighter petro leum oils is benzol, a by-product of the manu facture of coke. Tars derived from gas works, coke ovens and blast furnaces are much used in engines of the Diesel type. The only other liquid fuel which approaches industrial im portance is alcohol.
Fuel The various kinds of fuel oils are all derived from petroleum, varying in com position as follows: Carbon, 82 to 87 per cent; hydrogen, 11 to 15 per cent; oxygen, to 6 per cent. The theoretical average calori
fic value is about 20,860 B. T. U. per pound, corresponding to a theoretical evaporation of 21 pounds of water from and at Fahr.
American crude petroleum carries more of the lighter oils than the European, Mexican or Peruvian, from which a residuum or fuel oil, consisting largely of the heavy oils, is obtained by distillation. To be effective fuel oils must be vaporized and thoroughly mixed with the proper proportion of air. In this work steam atomizers give better results than the air spray, the steam required to atomize being about 4 per cent of the water evaporated.
°Astatici,* °Mazoot,"distillate,I) ((petroleum °reduced etc., are some of the commercial terms used to designate the various kinds of fuel oil.
Approximately, one pound of fuel oil is equal to 1.45 pounds of coal. A test at the Minneapolis waterworks showed that for the same duty 224 gallons of oil, weighing 6.875 pounds per gallon, equaled one ton (2,240 lbs.) of Youghiogheny coal.
Fuel oils have both advantages and disad vantages. The heating value of a fuel oil is nearly twice its weight of coal. Its use is not as injurious to the furnace as the use of coal, on account of the smaller percentage of sul phur. With oil, the fire can be controlled by means of a single valve and when once regu lated to produce a certain heat, it can be main tained at that point with very little trouble. Where is a much lower temperature in the fire room, particularly noticeable on shipboard. The fire can be started easily and extinguished instantly. The combustion is complete and smokeless, thus making economy and cleanliness important advantages in its use. And there is a very considerable saving in labor, one man with oil taking the place of eight men with coal.
The most important disadvantages are the comparatively high price, danger from explo sion, loss by evaporation and unpleasant odor.