The context of this paper is energy recovery in an incineration plant. A dynamic model for the description of a drum unit is presented. In the first part, the physical system in question is presented. This description is performed by following the cycle of water within the system. The different devices of the network are presented, as well as the control valves and their roles. A complete list of the state variables used to represent the process is given. Secondly, we focus on the mathematical description of this system. Indeed, the set of equations representative of the process is rendered. This is a differential algebraic system composed of 6 ordinary differential equations and 113 algebraic ones. A predictor-corrector method (Gear’s algorithm) is used to solve the system. The results of the model are shown in case of the turning off of the plant, when the turbine needs to be bypassed. Finally, an explanation of the observed yields of the model is given.