We know that when a layer of fuel, not more than 6 or 8 inches thick, is burnt on a gridiron, the air passing across this incandescent layer burns it completely, turning it into carbonic acid by means of the oxygen contained in it, so that the products of combustion consist mainly of nitrogen and carbonic acid, and, in smaller quantities, of water-vapour, oxygen, etc. All these gases are inert.
If the thickness of the fuel be increased, and the quantity of air assisting combustion be reduced, the result is different. The carbonic acid formed in the lower strata of the coal is reduced while passing through "the red-hot upper part, and gives up oxygen; an extremely inflammable gas is thus formed: carbonic oxide. This is the reaction which takes place in high furnaces. At the same time, small quantities of hydrogen and carburet of hydrogen, also inflammable, are produced. The gases given out by this slow combustion of the coal consist especially of nitrogen and carbonic oxide, and, in smaller quantities, of hydrogen, carburets, water-vapour, etc.; this mixture has received the name of generator gas. Another more economical gas called water gas is produced as follows. The water is formed by the combination of oxygen and hydrogen; at a very high temperature, about 1000° C, this combination is destroyed, and the two elements separated. This reaction takes place more especially when water is thrown on an incandescent furnace; the oxygen burns the coal, and the hydrogen is set free. If the operation takes place in presence of an excess of coal, the carbonic acid produced will be reduced to carbonic oxide; the gases of combustion will contain carbonic oxide and hydrogen, both inflammable gases, and, in small quantities, carbonic acid and nitrogen. This result is obtained in practice by passing a current of water-vapour over the incandescent grate of a generator. The decomposition takes place, but as it is endothermic, that is to say, that it absorbs heat, the coal gets cooler, and the reaction would cease if the entrance of vapour were not stopped to give place to an injection of air. During this second period generator gas is produced. When combustion has recommenced the air is stopped and vapour admitted, and the process continues. The duration of these alternate reactions varies according to the conditions of the installation; for instance, water gas will be made for four minutes, and then for eleven minutes heat will be raised again and generator gas produced.
Water gas has a much greater calorific power than generator gas, as their average compositions show -
Composition of 1 cubic metre.
Combustion value in calories.
Carbonic oxide ....
Carburets of hydrogen .
Carbonic acid ....
One kilog. of coal, the heating power of which is about 8000 calories, gives from 4.5 to 4.9 cubic metres of generator gas, which, when cooled to 20° C, has as calorific value: (4.5x1053+4.9x1053)/2 =4949 calories, that is to say only 62 per cent, of the calorific value of the coal used in its preparation.
In water gas we recover 80 to 85 per cent, of the heating power of the fuel, half in the form of generator gas, and half in the form of water gas.
The preceding figures arc the result of experiments made in the Krupp factories at Essen.
It is advantageous, then, to use water gas for the firing of bricks, as these cheap products require as low a cost of production as possible. The apparatus for the production of gas is the same in both cases, and so is the kiln, as we shall see below.
As in the case of kilns with solid fuel, there are a certain number of gas kilns recommended by different makers (Schwan-dorf, Marie, Simon, etc.), which only differ from one another in details.