As the coking process advances most slowly from the top to the bottom, only one layer is consumed at a time; while the surface is covered with red-hot cinders, ready to consume any particles of carburetted or sulphuretted hydrogen gases which may escape from below. The greatest mass cannot emit more gases than the smallest heap.
The coke being perfectly freed from all smoky and volatile matters, by a calcination of forty hours, is cooled down to a moderate ignition by sliding in the dampers and opening the doors, which had been partly closed during the latter part of the operation.
The coal is now converted into a clean, crystalline, porous, columnar mass, of a steel-gray color, and so hard as to cut glass. This is broken up and taken out—coke. It is sometimes extinguished by a watering-pot. This is wrong, it ought not to be wet, and even more, ought to be immediately shut up in fire-proof boxes and bins. Even left to itself in the air, it absorbs moisture rapidly, which must be burned off in the boiler; it should by all means be kept in a dry place. Mr. Woods (England) observes, that coke may absorb as much as eight per cent. of water in going from the oven to the storehouse. The amount of absorption depends upon the nature of the coke. D. K. Clark records the following, the coke being immersed in water.
No. 1. Close-grained and good, absorbed 14.5 per cent. of water.
No. 2. Porous and ordinary, absorbed 21 per cent.
No. 3. Very close-grained and good, 9 per cent.
The time of coking may be stated generally as fifty hours, though it is somewhat improved by being allowed forty hours more; this gives time for a better consolidation, and gives a firmer, brighter, and more crystalline mass.
Mr. Gooch, of the Great Western (England) Railroad, experimented upon the time of coking with the following results.
In oven. Yield per ton of Water evaporated Result. coal. per lb. of coke. 48 hours 12.71 cwt. 7.1 lbs. 902. 72 hours 12.00 cwt. 7.7 lbs. 924.
Thus, though the yield per ton is decreased by a greater time, the value of the coke per pound is augmented, and the increase overbalances the decrease.
Firstrate coal gives from seventy-five to eighty per cent. by weight, of compact glistening coke, weighing about 14 cwt. per chaldron, (thirty-six bushels). The bulk is increased from ten to fifty per cent.