The usual one-stage combustion with plenty of air:
1. C (carbon) + 2O (oxygen) burns to CO2 (carbon dioxide). Non-poisonous.
The two-stage combustion with insufficient air:
2. C + O burns to CO (carbon monoxide). Poisonous.
3. CO + O burns to CO2. Non-poisonous.
Carbon monoxide asphyxiates by forming a chemical compound with the hæmoglobin of the blood, which therefore is prevented from supplying the body with the oxygen that is required for the sustenance of life.
Carbon dioxide is no such poisonous product, as may be inferred when we remember that it is the gas with which our carbonated waters are charged and which is so commonly served with ice cream in ice cream soda.
Now in a gas producer, by maintaining a sufficiently thick bed of glowing coal and admitting only such amounts of air as will produce mainly carbon monoxide gas, a product of high burning value is obtained. A kilogram (2.2 pounds) of carbon in burning from C to CO generates only 2450 calories or heat units, whereas its complete burning to CO2 would give 8080 calories. So by conducting the carbon monoxide gas—the product of the first stage of the combustion—through brick-lined pipes to the furnace, and in the latter by addition of air allowing it to burn to CO2, the greater amount of heat (i. e., 8080 minus 2450 or 5630 calories) is evolved in the furnace. Of course, some of this theoretical two-thirds which is in this way made available at the furnace is lost because a little CO2 is formed, and always the nitrogen of the air used greatly dilutes the gas. But there are gains, notably the great heat which is carried over by the hot gas from the glowing bed of coal and that from the water-gas which is formed from steam used in the producer. So, all in all, the gas generated in a “battery” of gas producers, all of which discharge into one large main or header to maintain gas of average composition, is quite a satisfactory fuel.
CHAPTER X
CAST IRON
From the preceding chapters we now know pretty well the place which cast iron occupies in the iron family. In the chapters which have succeeded the one in which we discussed the blast furnace and pig iron, every one of the products except crucible steel has been produced through some “refining” operation which greatly changed the composition, structure and properties of the product. Cast iron is not the result of a refining operation in this sense of the word. It is produced through simple mixing of pig irons of various compositions, usually with some admixture of iron castings of similar composition which have outlived their usefulness in the industrial world and have been returned as scrap to be remelted.