Coke
Charging Coal into the Ovens
As charcoal is completely charred wood, so coke for analogy’s sake may be said to be completely charred coal, practically always of the bituminous type. By “baking” bituminous coal at a cherry-red heat, its volatile constituents are driven off as the well-known “coal-gas” of almost every small town, and a strong, brittle and porous material or coke residue is left. If the baking is done without any admission of air to the retort, practically none of the coal burns and the “cake” or coke which is left contains the ash of the original coal and what is known as the “fixed carbon,” i.e., carbon which cannot be distilled or driven off by heat alone, though it would burn were air admitted.
The gases or volatile constituents which are given off consist mainly of moisture and a mixture of gaseous chemical compounds, which are known as “hydro-carbons.” These contain that part of the carbon of the original coal which does not remain as “fixed carbon” in the coke.
Quenching after Coal Has Been Coked
Just why some coals will coke while others of apparently the same composition as shown by the chemist’s analyses, will not, but instead of the hard brittle mass will leave a heap of brown or black powder, is not as yet definitely known. It is easy enough for chemists to determine with accuracy the amounts of hydrogen, nitrogen, oxygen, carbon, sulphur, and other elements; but it is a difficult and perhaps an impossible matter to determine just how these elements are “hitched up” in the very complex mineral, coal,—one of the most complex substances which we know.
Various theories have been advanced in the attempt to explain the coking quality. A bulletin of the United States Geological Survey claims that the relative percentages of hydrogen and oxygen in the coal determines it; others have held that it depends upon the compounds of a tarry or asphaltic nature present. The fact remains that some coals coke without trouble, while others do not coke at all. As yet the only real way to tell whether a new variety of coal will or will not coke is to try it.
Since 1713, when Abraham Darby in England succeeded in introducing it as a substitute for the fast disappearing charcoal for use in blast furnaces, coke has become the standard fuel. It is very strong and will bear up under the great weight of iron ore and limestone with which the furnace is charged. So furnaces for use with coke may be built much larger than those in which charcoal is to be the fuel. The porous nature of coke allows it to burn rapidly with intense heat, so that the output of an iron works is greatly increased through its use—a very desirable thing in these days of big things. It has its disadvantages, of course, mainly high sulphur, a deleterious substance for which molten iron, unfortunately, has a voracious appetite, and a rather high percentage of ash which must be fluxed out. But all in all, it is a very desirable fuel for blast furnace and other metallurgical purposes, as is shown by the fact that it is used in the production of about ninety-nine per cent of all iron and steel now made.