By-product coking is classified as a true distillation process
because all the components of the coal are recovered. One of the
chief differences between by-product ovens and the beehive ovens
is the source of heat. Instead of burning part of the coking coal,
as in the beehive oven, the by-product oven is heated by an external
fire. Several types of coking ovens are used in this process,but
the basic design is similar for all types, and they differ chiefly
in the method of heat application. The three best known types are
the Semet-Solvay, the Otto-Hoffman, and the Koppers.
The coking chambers (ovens) into which the coal is charged are
rectangular and range from 30 to 42 feet in length, from 6 to 14
feet in height, and from 12 to 22 inches in width. These chambers
are constructed with silica fire-brick to withstand the extreme
temperatures of the coking process. Between each oven is a series
of heating flues which are arranged so as uniformly to heat the
entire side of the oven.
The individual by-product coke oven operates alternately, but
each oven is started and stopped at different times, so that the
operation of the entire block continually produces gas of good average
composition. A charge of finely crushed coal is dropped from a larry
car through charging holes (usually four) in the top and into the
oven, where the walls are at approximately 1100 C.
The surface of the coal in the oven is levelled and the charging
holes are covered. At the top and at the ends of the coking chambers
are openings for collection of the volatile matter as it evolves
during the heating cycle. These openings are fitted to off-take
pipes that carry the gases to chemical recovery units where the
by-products of coking are obtained. The coking coal, which entered
the retort (oven) as individual particles, begins to soften at about
400 C, and as the temperature rises the coal fuses into a plastic
mass. Depending on the volatile matter content, the coal expands
or contracts during the plastic range and gas evolution period until
it resolidifies at about 500 C.
At the end of the coking cycle (approximately 17 hours), both end
doors of the chamber are opened, and a "pusher" mechanism
shoves the block of incandescent coke into a quenching car. The
car then advances to a spray system that rapidly cools the coke
to prevent combustion. After quenching, the coke is crushed and
screened for proper sizing to meet specific applications.