CALCULATION OF CALORIFIC INTENSITY OF FUEL.

431

will scarcely be capable of melting copper, and in the latter it will melt. steel. The difference resides in the temperature or calorific intensity of the two fires; in the wind-furnace, through which a rapid draught of air is maintained by a chimney, a much greater weight of atmospheric oxygen is brought into contact with the fuel in a given time, so that, in that time, a greater weight of fuel will be consumed and more heat will be produced; hence the fire will have a higher temperature, for the temperature represents, not the quantity of heat present in a given mass of matter, but the intensity, or extent to which that heat is accumulated at any particular point. In the case of the wind-furnace here cited, a further advantage is gained from the circumstance, that the rapid draught of air allows a given weight of fuel to be consumed in a smaller space, and, of course, the smaller the area over which a given quantity of heat is distributed, the higher the temperature within that area (as exemplified in the use of the common burning-glass). In some of the practical applications of fuel, such as heating steam-boilers and warming buildings, it is the calorific value of the fuel which chiefly concerns us, but the case is different where metals are to be melted, or chemical changes to be brought about by the application of a very high temperature, for it is then the calorific intensity, or actual temperature of the burning mass, which has to be considered. No trustworthy method has yet been devised for determining by direct experiment the calorific intensity of fuel, and it is therefore ascertained by calculation from the calorific value.

Let it be required to calculate the calorific intensity, or actual temperature, of carbon burning in pure oxygen gas.

6 grains (1 eq.) of carbon combine with 16 grains (2 eqs.) of oxygen, producing 22 grains of carbonic acid; hence 1 grain of carbon combines with 2.67 grains of oxygen, producing 3.67 grains of carbonic acid. It has been seen above that 1 grain of carbon evolves 8080 units of heat, or is capable of raising 8080 grains of water from 0° to 1 ̊C., or, on the supposition that the water would bear such an elevation of temperature, the 1 grain of carbon would raise 1 grain of water from 0° to 8080° C. If the specific heat (or heat required to raise 1 grain through 1o, see p. 426) of carbonic acid were the same as that of water, 8080° divided by 3.67 would represent the temperature to which the 3.67 grains of carbonic acid would be raised, and therefore the temperature to which the solid carbon producing it would be raised in the act of combustion. But the specific heat of carbonic acid gas is only 0.2163, so that a given amount of heat would raise 1 grain of carbonic acid to nearly five times as high a temperature as that to which it would raise 1 grain of water.

Dividing the 8080 units of heat (available for raising the temperature of the carbonic acid) by 0.2163, the quantity of heat required to raise 1 grain of carbonic acid 1°, we obtain 37,355 for the number of degrees through which 1 grain of carbonic acid might be raised by the combustion of 1 grain of carbon. But there are 3.67 grains of carbonic acid formed in the combustion, so that the above number of degrees must be divided by 3.67 in order to obtain the actual temperature of the carbonic acid at the instant of its production, that is, the temperature of the burning mass. The calorific intensity of carbon burning in pure oxygen is, therefore, (37,355° C. 367 =) 10,178° C. or 18,352° F. But if the carbon be burnt in air, the temperature will be far lower, because the nitrogen of the air will absorb a part of the heat, to which it contributes nothing. The 2.67 grains of oxygen required to burn 1 grain of carbon would be

432

CALCULATION OF CALORIFIC INTENSITY OF FUEL.

mixed, in air, with 8.93 grains of nitrogen, so that the 8080 units of heat would be distributed over 3.67 grains of carbonic acid and 8-93 grains of nitrogen. Since the specific heat of carbonic acid is 0.2163, the product of 3.67 × 0.2163 (or 0.794) represents the quantity of heat required to raise the 3.67 grains of carbonic acid from 0° to 1° C.

The specific heat of nitrogen is 0.2438; hence 8.93 x 0.2438 (or 2.177) represents the quantity of heat required to raise the 8-93 grains of atmospheric nitrogen from 0° to 1° C.

Adding together these products, we find that 0·794 + 2·177 = 2·971 represents the quantity of heat required to raise both the nitrogen and carbonic acid from 0° to 1° C.

Dividing the 8080° by 2.971, we obtain 2720° C. (4928° F.) for the number of degrees through which these gases would be raised in the combustion, i.e., for the calorific intensity of carbon burning in air.

By heating the air before it enters the furnace (as in the hot blast iron furnace), of course the calorific intensity would be increased; thus if the air be introduced into the furnace at a temperature of 600° F., it might be stated, without serious error, that the temperature producible in the furnace would be 5528° F. (4928° + 600°). The temperature might be further increased by diminishing the area of combustion, as by employing very compact fuel and increasing the pressure of the blast.

In calculating the calorific intensity of hydrogen burning in air, from its calorific value, it must be remembered that in the experimental determination of the latter number the steam produced in the combustion was condensed to the liquid form, so that its latent heat was added to the number representing the calorific value of the hydrogen; but the latent heat of the steam must be deducted in calculating the calorific intensity, because the steam goes off from the burning mass and carries its latent heat with it.

1 grain of hydrogen, burning in air, combines with 8 grains of oxygen, producing 9 grains of steam, leaving 26.77 grains of atmospheric nitrogen, and evolving 34,400 units of heat.

It has been experimentally determined that the latent heat of steam is 537° C., that is, 1 grain of water, in becoming steam, absorbs 537 units of heat (or as much heat as would raise 537 grains of water from 0° to 1° C.) without rising in temperature as indicated by the thermometer. The 9 grains of water produced by the combustion of 1 grain of hydrogen will absorb, or render latent, 537 x 9 = 4833 units of heat. Deducting this quantity from the 34,400 units evolved in the combustion of 1 grain of hydrogen, there remain 29,567 units of heat available for raising the temperature of the 9 grains of steam and 26·77 grains of atmospheric nitrogen. The specific heat of steam being 0-480, the number (0.480 × 9 =) 4·32 represents the quantity of heat required to raise the 9 grains of steam. through 1o C.; and the specific heat of nitrogen (0-2438) multiplied by its weight (26-77 grains), gives 6:53 units of heat required to raise the 26-77 grains of nitrogen through 1° C. By dividing the available heat (29,567 units) by the joint quantities required to raise the steam and nitrogen through 1o C. (4·32 + 6·53 10.85), we obtain the number 2725° C. (4937° F.) for the calorific intensity of hydrogen burning in air.

=

The method of calculating the calorific intensity of a fuel composed of carbon, hydrogen, and oxygen, will now be easily followed.

Let c and h respectively represent the weights of carbon and hydrogen in 1 gr.

WASTE OF HEAT IN FURNACES.

433

Q 8

of fuel, and o that of the oxygen. Then weight of hydrogen required to convert

=

represents the hydrogen which is available for the

production of heat.

8080 c + 84,400 (h

g) represents the

4300 o.

calorific value in °C., 8080 c + 34,400 h

2.67 catmospheric oxygen consumed by the carbon;

as fuel.

or 8 h

-o atmospheric oxygen consumed by the hydrogen available

3-34 (2·67 c + 8 h o) atmospheric nitrogen =8-92 c + 26·72 h — 3·34 0. Multiplying this by the specific heat of nitrogen 0.2438, we obtain— 2-17c+6.51 h 0.81 o for the heat required to raise the nitrogen through 1o C. 0.794 c represents the quantity of heat required to raise the carbonic acid through 1o C., and 4.32 h is the heat required to raise the steam through 1o. Accordingly, the available heat, 8080 c + 34,400 h 4300 o, must be divided by 0-794 c + 4·32 h + (2·17 c + 6·51 h 0·81 o), or 2.96 c + 10·83 h 0.81 o in order to obtain the calorific intensity.

Hence, the calorific intensity, in Centigrade degrees, of a fuel composed of carbon, hydrogen, and oxygen, is represented by the formula

The actual calorific intensity of the fuel is not so high as it should be according to theory, because a part of the carbon and hydrogen is converted into gas by destructive distillation of the fuel, and this gas is not actually burnt in the fire, so that its calorific intensity is not added to that of the burning solid mass. Again, a portion of the carbon is converted into carbonic oxide (CO), especially if the supply of air be imperfect, and much less heat is produced than if the carbon were converted into carbonic acid; although it is true that this carbonic oxide may be consumed above the fire by supplying air to it, the heat thus produced does not increase the calorific intensity or temperature of the fire itself.

One grain of carbon furnishes 2.33 grains of carbonic oxide. These 2.33 grains of carbonic oxide evolve, in their combustion, 5599 units of heat. But if the 1 grain of carbon had been converted at once into carbonic acid, it would have evolved 8080 units of heat, so that 8080 - 5599, or 2481, represents the heat evolved during the conversion of 1 grain of carbon into carbonic oxide, showing that a considerable loss of heat in the fire is caused by an imperfect supply of air. It has been already pointed out, in the section relating to Coal, that the formation of carbonic oxide is sometimes encouraged with a view to the production of a flame from nonflaming coal, such as anthracite.

The table (p. 434) exhibits the average percentage composition of the principal varieties of fuel (exclusive of ash), together with their calculated calorific values and intensities.

In all ordinary fires and furnaces, a large amount of heat is wasted in the current of heated products of combustion escaping from the chimney. Of course, a portion of this heat is necessary in order to produce the draught of the chimney. In boiler furnaces it is found that, for this purpose, the temperature of the air escaping from the chimney must not be lower than from 500° to 600° F. If the fuel could be consumed by sup

434

COMPOSITION AND VALUE OF FUELS.

plying only so much air as contains the requisite quantity of oxygen, a great saving might be effected, but in practice, about twice the calculated quantity of air must be supplied, in order to effect the removal of the products of combustion with sufficient rapidity.

Much economy of fuel may be expected from the use of furnaces constructed on the principle of Siemens' regenerative furnace, in which the waste heat of the products of combustion is absorbed by a quantity of fire-bricks, and employed to heat the air before it enters the furnace, two chambers of fire-bricks doing duty alternately, for absorbing the heat from the issuing gas, and for imparting heat to the entering air, the current being reversed by a valve as soon as the fire-bricks are strongly heated.

(For the principles of smoke prevention, and other particulars of the

chemistry of fuel, see Coal.)

ORGANIC CHEMISTRY.

324. Although it is impossible to propose a definition of the term organic substance which shall not be applicable to some of the substances commonly regarded as inorganic, it is found advantageous for the purposes of study to treat organic chemistry as a separate division of the science, dealing especially with those substances which are usually obtained, either directly or indirectly, from animals and vegetables.

One very important distinction between organic and inorganic substances is, that the former are for the most part composed of carbon, hydrogen, nitrogen, and oxygen, in different proportions and in various modes of arrangement, and that they are, therefore, much more frequently convertible into each other by metamorphosis, without extraneous addition of matter, than inorganic substances are.

It has been already pointed out (p. 83) that the chemist is gradually learning to produce, though by somewhat clumsy and circuitous processes, many of the substances which were formerly believed incapable of being formed, except through the intervention of life; but no substance possessing an organised structure, such as woody fibre or muscular fibre, and no absolutely indispensable constituent of animal or vegetable frames, if we except water, has yet been artificially procured.

It will not escape notice that the four elements which compose the greater number of organic substances, viz., hydrogen, oxygen, nitrogen, and carbon, are, respectively, monatomic, diatomic, triatomic, and tetratomic elements (p. 151), and are, therefore, capable of forming a greater variety of compounds than would be the case if they were elements of equal atomicities.

In the following pages, no strictly scientific classification of organic substances has been adopted, since it would often render it necessary to describe, in separate sections, substances which are, in nature, closely connected with each other, but an empirical arrangement has been followed, so that the reader may find his memory assisted and the interest of the subject sustained, by being enabled to bring the facts and explanations into immediate connection with familiar processes of ordinary life.*

One of the most conspicuous substances standing upon the boundary between organic and inorganic chemistry is the compound of carbon and nitrogen known as cyanogen, which is intimately connected with inorganic substances through some of the processes for its production, and through its similarity to the chlorine group of elements, whilst the origin and

The number of organic substances known to the chemist is so great that a mere list of them would occupy a volume. In the present work a selection has been made of those which are interesting for their practical applications or instructive from theoretical considerations.