This paper proposes a fractional-order PI controller for controlling the steam pressure in the steam drum of a bagasse fired boiler. The dynamics behavior of this process was experimentally identified. This identification procedure yielded an equivalent third order plus time delay model, and showed wide process static gain variations. We therefore propose a new methodology for the design of fractional-order robust controllers for this class of processes. It was shown that the attained controllers significantly outperformed the robustness achieved with current PI controllers.