hydrogen and marsh
gas is returned to the ovens as fuel.

In the Otto-Hoffmann oven, one of the most generally used forms,
vertical side flues like those of Coppee are adopted. The returned gas
enters by a horizontal flue along the bottom of the coking chamber,
divided into two parts by a mid-feather wall, and is fired by heated
air from a Siemens regenerator on the substructure at one end, and the
flame rising through one half of the side flues to a parallel
collector at the top returns downwards through the flues of the other
half and passes out to the chimney through a similar regenerator at
the other end. The course of the gases is reversed at intervals of
about an hour, as in the ordinary Siemens furnace, each end of the
oven having its own gas supply. In the later modification known as the
Otto-Hilgenstock, the regenerators are abandoned, but provision is
made for more perfect distribution of the heat by a line of sixteen
Bunsen burners in each wall; each of these serves two flues, the
course of the flame being continuously upwards without reversal. In
the newest Otto ovens the same system of burners is combined with
regenerators. In the Bauer system, another vertical flue oven, each
flue has its own burner, which is of a simplified construction.

In the Carves oven, the earliest of the by-product ovens, the heating
flues are arranged horizontally in parallel series along the entire
length of the side walls, the gas being introduced from both ends but
at different levels. This system was further developed by H. Simon of
Manchester, who added a continuous air "recuperator" heated by the
spent flame; this Simon-Carves system has been extensively adopted in
Great Britain. Another horizontal flue oven, the Semet-Solvay, is
distinguished by the structure of the flues, which are independent of
the dividing walls of the ovens, so that the latter can be made with
thinner sides than those of the earlier systems, and are more readily
repaired. In the horizontal ovens it is sometimes difficult to
maintain the heat when the flues are continuous along the whole length
of the wall, especially when the heating value of the gas is reduced
by the removal of the heavy hydrocarbons. This difficulty is met by
dividing the flues in the middle so as to shorten the length of travel
of the flame, and working each end independently. The Hussener and
Koppers systems