The rotary cement kiln process is intrinsically unstable, there are long time delays and large perturbations acting on it. The control problem consists of maintaining a given temperature profile along the kiln plus obtaining good burning conditions.
Further, the control strategy needs to achieve that at the lowest energy consumption possible, which means riding along constraints such as amount of air in the exhaust gasses. The problem exhibits relatively long time delays related to the slow transport of the raw meal along a series of heat exchangers, cyclones and then the kiln.
Actuators are kiln speed, energy input (fuels), air and feed, while the process parameters to be controlled are temperature in the kiln front (or specially built soft sensor thereof), temperature at the kiln inlet, and oxygen in the gasses traveling through the system. Additional complexity might also come from the usage of alternative fuels, where the control strategy needs to calculate the optimal fuel mix for the given conditions.