Control Properties of Bottom Fired Marine Boilers

This paper focuses on model analysis of a dynamic model of a bottom fired one-pass smoke tube boiler. Linearized versions of the model are analyzed and show large variations in system gains at steady state as function of load whereas gain variations near the desired bandwidth are small. An analysis of the potential benefit from using a multivariable control strategy in favor of the current strategy based on single loop theory is carried out and proves that the interactions in the system are not negligible and a subsequent controller design should take this into account. A design using dynamical decoupling showed substantial improvement compared to a decentralized scheme based on sequential loop closing. Similar or better result is expected to be obtainable using a full Multiple input Multiple output scheme. Furthermore closed loop simulations, applying a linear controller to the nonlinear plant model, prove that the model does not call for nonlinear control. However, the results indicate that input constraints will become active when the controller responds to transient behavior from the steam flow disturbance. For this reason an MPC (model predictive control) strategy capable of handling constraints on states and control signals should be considered.