Material (G) is converted by a combustion process in a plant (1) while air (L) is supplied. The state of the system in the plant (1) is described by state variables (x, y) and is regulated at least by one control loop (3, 5, 7, 9). Groups of states (Z) are defined for at least one pair of correlated state variables (x, y), with the groups being comparable as regards changes (dx/dt, dy/dt) of the correlated state variables (x, y). Each group of comparable states (Z) is characterized, as regards their transition functions, by parameters (Kp, Tn, Tv) of a standard controller. In the event of changes in the state of the system in the plant (1), the closest groups of comparable states (Z) are selected, and their transition functions, characterized by the parameters (Kp, Tn, Tv), are used for the purposes of regulation the system.