Page:The American Cyclopædia (1879) Volume VII.djvu/530

 518 FUEL pose of retorts. The former mode of drying is employed for mixtures of charcoal dust, tan, and similar substances, with tar or pitch, and the latter when refuse bituminous coal is used with about a quarter of its weight of pitch. Unless this distillation is conducted at a heat of from 400 to 600 F., so as to dispel the volatile in- gredients, there is danger of subsequent spon- taneous combustion. At Blanzy in France the coal is separated from the slaty and py- ritous particles, and is then crushed and in- troduced into a circular metallic basin, which revolves horizontally in a reverberatory fur- nace, the flame of which passes under it. Hot tar or pitch is gradually let in upon the coal from a reservoir over the fire to the amount of V or 8 per cent., and the mixture is stirred by stationary rakes attached to rods let down through the arched cover. When sufficiently mixed, the materials are made to drop through the bottom into a receptacle, whence they are removed while plastic to the moulds and there pressed by the hydraulic machine. The process of Mr. Bessemer appears to be most highly ap- proved. It is applied only to fine bituminous coal without mixture, the object being to render this plastic by heat and mould it by heavy pressure into convenient shapes. In the softening pro- cess the coal may be exposed to the heat long enough for a portion of its volatile elements to be expelled, by which the product is rendered more dense and of the nature of coke ; or it may be softened so quickly as to be but slightly altered in its chemical composition. It is then formed into blocks by machinery working un- der great pressure. There are vast quanti- ties of coal dust lying as waste material at the various extensive coal mines in this country, which might be utilized by mixing with proper proportions of the coal tar of gas works and compressed into bricks by machinery similar to that employed by Bessemer. There can be no doubt that fuel could be furnished in this way at an economical price. The composition of fuels is commonly expressed by stating the proportions of coke or charcoal, volatile mat- ters, moisture, and ash. The ultimate analysis reduces the whole to its elements, and expresses the proportions of carbon, hydrogen, oxygen, nitrogen, and the ingredients of the ash. In order to ascertain the fitness of fuel for making gas and producing the fatty products, the pro- portion of volatile ingredients must first be ascertained, and then the nature of these, as the proportion of the inflammable gases to the liquid products. For other purposes the simple form of analysis is commonly sufficient. The ash is obtained by thorough combustion in an open platinum crucible, continued till nothing is left but the gray or brown ash. The difference of weight of the crucible and its con- tents before and after the operation, deducted from the weight of the fuel employed, gives that of the ash. Another weighed sample sub- jected in a similar way to a heat of about 300 will give by loss of weight the amount of moist- ure ; the crucible containing it is then closely covered to exclude the air, and is set in a Hes- sian crucible also closed with a cover, and con- taining calcined magnesia. This supports the platinum crucible, and keeps it from contact with the outer one. The whole is now exposed to a red heat for an hour. The volatile mat- ters are thus driven off, and the difference of weight of crucible and contents before and after the operation gives their proportions. The charcoal or coke is the difference between the crucible with the residuum it contains and that of the crucible alone less the weight of the ash. This may be again obtained by consuming the carbonaceous residue exposed to a current of air. The intense degree of heat evolved in the use of the condensed fuels adds largely to the capacity of heat of the aqueous vapor, and hence further lessens the value of hydrogen in fuels intended for the uses to which they are applied. But for other objects, requiring a quick heat and at the same time diffused over considerable space, the more inflammable fuels are found more efficient ; and according to the mode in which their heating power is estimated they may even be classed as producing a greater amount of heat than the more carbonaceous varieties. Whenever the heat from the com- bustion of hydrogen can be concentrated, as in the oxyhydrogen blowpipe, a more intense de- gree is obtained than by the use of any other fuel. Other considerations, therefore, besides the chemical composition of fuels, affect their value. For practical purposes a mere change in the mechanical structure may give an en- tirely different character to them, while their real calorific power is not altered. This is ap- parent in the coals, which are rendered almost worthless when reduced to dust, until in the patent fuels they are reconverted into solid form. Wood possesses very different values in solid sticks, in shavings, and in sawdust. In ordinary use other circumstances are to be taken into account, as the arrangements for utilizing the heat produced, so that there shall be the least amount lost ; also the provisions for insuring perfect combustion of the fuel. The loss of heat resulting from imperfect ar- rangements in these respects alone has been estimated at full one half of all that generated. The chimney necessarily carries off a consider- able portion, as there will be no draught, and consequently no continued supply of air to sup- port the combustion, unless the column floating upward by its rarity produces a partial vacuum to be filled with fresh air passing through the fire. The quantity of this admitted should be limited to a proper excess only of that absolutely required for the thorough combustion of the fuel, and this can be determined for each variety of fuel only by the experience and good judg- ment of the operator, the object in view being a uniform rate of combustion, more or less rapid- ly conducted, according to the fuel employed and the special purpose to which it is applied. The quantities necessary for complete coinbus-