The rotary kiln is a cylinder, inclined at a slight angle, which rotates slowly on its axis. The limestone to be processed is fed into the upper end of the cylinder. As the kiln rotates, material slowly moves down to the lower end. 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. The fuel for this may be gas, oil, pulverized petroleum coke or pulverized coal.