Dynamic behaviour of a hybrid steam generator equipped with a gate valve

Cogeneration is an often viable and efficient solution to cover combined demands of electric and thermal power, especially when the demand of the latter remains fairly constant over time. The efficient use of energy and the economic advantages that are associate to it have made cogeneration the solution of choice in many industrial applications, among which paper mills represent a significant share. The heat recovery steam generator often has an auxiliary burner to ensure full coverage of the thermal load in all operating conditions. In order to reduce installation and conduction costs, the combustion chamber may lack an extraction fan, which has the drawback that pressure varies according to changes in the loads, and this affects the stability of the flame adversely. In order to ensure constant pressure in the combustion chamber, application of a gate valve downstream of the heat recovery steam generator is proposed. A lumped-parameter model is set up in Simulink® and the feedback loop tested for to sudden variations in the thermal power required by the paper mill, mainly due to variations in the number of operating machines, and to failure of the internal combustion engine. It is demonstrated that the gate valve is necessary and that the use of a PID feedback control system ensures a constant pressure in the combustion chamber.