The By-product Process
Top of Ovens with Charging Bin and Lorry at Far End
By this system of coking a greater yield of coke is obtained and most of the by-products are saved. The value of the latter depends largely, of course, upon local conditions, such as transportation, costs of the material, cost of labor, and available market for the coke oven gas. They are usually figured as having a value of $1.50 per ton of coal coked, equivalent to a total of $71,000,000 per year for the coal coked in the United States.
Lorry for Charging Coal into Ovens
The ovens and apparatus required are considerably more expensive, but, since this industry has developed in this country during the last twenty-two years to a point where one-quarter of all of the coke manufactured is made by the by-product process, there can be no doubt that it is a profitable proposition and that eventually the wasteful beehive ovens will be a thing of the past.
Practically all of the types of by-product coke ovens in use have been developed in Germany or Belgium, where circumstances forced earlier conservation of resources than in this country. The three best known types are the Semet-Solvay, the Otto Hoffman, and the Koppers—the latter a recent arrival. They differ mainly in details of construction and operation.
Machine for Pushing Coke from Ovens
In a general way a “battery” of coke ovens consists of from 40 to 80 long narrow brick-walled chambers placed closely side by side with heating flues or “checker-work” between them. The fire for the baking process is in these flues, which are interconnected, and the heat developed is sufficient to drive off the moisture and volatile substances of the coal in the narrow chambers just on the other side of the brick walls. Charging is done by a “lorry” as in the beehive process. After from seventeen to twenty-four hours at a red heat, the coke is “pushed” from the ovens, one after another, by an electric ram which enters at one end. The 30 × 7 × 1½ foot block of glowing coke emerges from the other end, where, breaking under its own weight into good-sized pieces, it falls into a steel car on a track just beneath. A spray of water quenches it and it is taken to the storage bins to be sorted.
Rich coal-gas is the main by-product. That which comes off during the first seven hours is the richest and has the greatest illuminating or “candle” power. After washing free from dust, tar, ammonia, etc., the gas is usually run into holders or tanks from which it is distributed for use for illuminating or for heating purposes. That which comes off during the latter part of the coking period has much less of those constituents which give illuminating value. It has good heat value, however, and as fuel is required for keeping the ovens up to the coking temperature, this poorer gas from the coking chambers is switched into and burns in the flues between the coking chambers as mentioned.
Thus the larger part of the gas is sold to customers, usually in the city near which the ovens have been located, and the poorer part is utilized in heating the ovens and the steam boilers which run the plant.
Quenching Car Awaiting Its Load
The coal tar, which the German chemists have made so famous through its manufacture into the almost endless variety of beautiful dyes, is another of the by-products which is recovered by this, but burned or lost in the beehive oven process. From a long main over the tops of the ovens which connects the gas pipes, the tar flows along with the gas to the scrubbing and gas cleaning plant, where by rather intricate operations it is freed from other substances.
In this country much of the tar is used for building purposes, etc., and some as fuel, but not much has been made into the chemical products for which Germany is so famous. For a long time a few dyes and other chemical compounds have been made here from coal tar. Since the early days of the war in Europe and the cessation of imports of such materials on this account, there has come about considerable expansion in their manufacture here; but it is doubtful if the time is yet ripe for a wholesale entry into the manufacture of these coal tar “derivatives,” especially the very extensive variety of dyestuffs.
Naphthalene and benzol from which many other chemical compounds as well as munitions of war can be made, are among the by-products.
Quenching the Coke
Most of the ammonia which the corner drug store sells, comes from the by-product manufacture of coke. The largest part of the ammonia which is produced in the process, however, is manufactured into sulphate of ammonia, a well-known fertilizer.