Coke is the infusible, solid residue remaining after the distillation of certain bituminous coal or as a by-product of petroleum distillation. Coke may also be obtained from petroleum residue, pitch, and other materials representing the residue of destructive distillation.
Coke will ignite more quickly than anthracite but less readily than bituminous coal. It burns rapidly with little draft. As a result, all openings or leaks into the ash pit must be closed tightly when coke is being burned.
Since less coke is burned per hour per square foot of grate than coal, a larger grate is required and a deep firepot is necessary to accommodate the thick bed of coal. Since coke contains very little hydrogen, the quick-flaming combustion that characterizes coal is not produced, but the fire is nearer even and regular.
The best size of coke recommended for general use, for small firepots where the fuel depth is not over 20 in, is that which passes over a 1-in screen and through a l^-in screen. For large firepots where the fuel can be fired over 20 in deep, coke that passes over a 1-in screen and through a 3-in screen can be used, but a coke of uniform size is always more satisfactory.