If these peat beds are buried beneath a layer or layers of sediment, especially clay, the peat is sealed up and oxidation stops almost entirely. With the pressure of the superincumbent beds, the peat becomes more and more compact, and changes to a dark-brown or black color. It is then known as lignite. If this lignite is buried still deeper, with consequently more pressure and more time, it changes into the still denser black bituminous coal. This is as far as it will go unless some new agent is added to the forces already working.
The next step in the series of changes forming coal is associated with mountain making. In case the layers of rock containing beds of coal are folded, and that presupposes at least a moderate increase in heat, the bituminous coal is altered to anthracite, which is still denser, and so hard that it breaks with a conchoidal fracture. Alteration may be carried a step still farther, in case the rocks between which lie beds of coal are effected by such high temperatures as accompany metamorphism. Then all the associated hydrogen, oxygen and moisture are driven off, and only the carbon remains, which is then known as graphite. All steps between the stages especially designated occur. The following represent steps only in the series of changes.
[Peat]
Peat is a mass of unconsolidated vegetable matter, which has accumulated under water, and in which the original plant remains are still, at least in part, discernible. It contains a large amount of water, so that before it can be used as a fuel, it is cut out in blocks, which are piled up and left for a time to dry before using. It burns with a long flame and considerable smoke. This country is so well supplied with other fuels, that so far peat has been but little used.
[Lignite]
brown coal
Lignite is more compact than peat, and is found buried to some depth under layers of clay or sandstone. It is dark brown to black in color, and still retains pretty clear traces of the plants from which it was derived. It also usually contains a considerable amount of moisture, and when this is dried out, it tends to crumble badly, so that it is undesirable to handle it much, or to ship it far, before using. It has a fair fuel value and is fairly widely used; but it is very desirable that some method be found, by which lignite could be treated to obtain its by-products, and at the same time make it more compact, so it would not crumble with the handling incident to using it in furnaces. There are extensive lignite deposits in this country in North and South Dakota, Montana, Wyoming, Colorado, New Mexico, Texas, Louisiana, and Mississippi.
[Bituminous Coal]
soft coal
This type of coal is compact, black in color, and breaks readily, but does not crumble as badly as lignite. It contains considerable water, and still has some hydrogen and oxygen compounds in it. Bituminous coal is the product of plant remains which have been preserved for long periods, (millions of years), sealed from the air by the overlying beds of rock. The pressure has made it compact, and nearly all traces of the original plants have disappeared.
Bituminous coal is our most abundant fuel, occurring the world over in seams from less than an inch in thickness to some over fifteen feet thick. The United States is peculiarly fortunate in the abundant and easily accessible deposits of this type of coal, in Pennsylvania, West Virginia, Ohio, Kentucky, Tennessee, Indiana, Illinois, Michigan, Iowa, Missouri, Kansas, Nebraska, Texas, Utah, and Colorado.
The volatile constituents, hydrogen and oxygen compounds, of bituminous coal may be driven off by heating the coal in closed ovens, and the residual mass is known as coke, almost pure carbon. This is distillation, and the ovens in which this is done, without trying to save the volatile products, are called bee-hive ovens, while the more modern ovens which save the by-products are called by-products ovens. A ton of bituminous coal treated in the typical by-products oven, will yield on the average 1410 lb. of coke, 7.1 gallons of tar, 18.9 pounds of ammonia sulphate, etc., 2.4 gallons of light oils, 10440 cubic feet of illuminating gas, about half of this last being used to furnish the heat for the distillation. The coal-tar dye industry is built on the tar thus produced. Toluol, benzol, etc., come from the light oils; and half the gas produced is available for household illumination, etc. Coke is demanded, as it is a superior fuel for melting iron ores, iron and steel, and is made regardless of whether the by-products are used. The coke thus produced is hard, clean, and vesicular; but for some reason as yet unknown, by no means all bituminous coal will produce a coke which has this porous structure. These latter are known as “non-coking,” and are of little use to the steel industry.