The kiln is a cylindrical vessel, inclined slightly from the horizontal, which is rotated slowly about its longitudinal axis.  The process feedstock is fed into the upper end of the cylinder.  As the kiln rotates, material gradually moves down toward the lower end, and may undergo a certain amount of stirring and mixing.  Hot gases pass along the kiln, sometimes in the same direction as the process material (co-current), but usually in the opposite direction (counter-current).  The hot gases may be generated in an external furnace, or may be generated by a flame inside the kiln.  Such a flame is projected from a burner-pipe (or "firing pipe") which acts like a large bunsen burner.  The fuel for this may be gas, oil, pulverized petroleum coke or pulverized coal.