Simulating MPC Controlled Lime Injection for the Flue-gas Treatment at Fortum's Thermal Power Plant

Abstract: The aim of this thesis is to investigate the possibility to improve the separation of HCl and SO2 in the dry ue-gas treatment for boiler #3 at Fortum's thermal power plant in Hogdalen; by using a model predictive control instead of a PID controller to govern the slaked lime injection. To achieve this an ARMAX model was derived using MATLAB's System Identi cation Toolbox and measurements of the incoming and outgoing levels of HCl, SO2 and the speed of the injection motor. The ARMAX model was then converted to a state space model which will be used as the internal model for the MPC predictions. The cost function was a quadratic problem which included the error between the output and the set points, the change rate of the input and the inputs deviation from a default value. The MPC uses both a feedforward and a feedback loop to estimate the error over the prediction horizon. The controller also utilizes the ability to set constraints and tuning of the cost function weights. In conclusion, the thesis shows that a MPC controlled lime injection is possible and would o er some unique possibilities such as: natural constraints handling, more intuitive live tuning for the operator and prioritized input control. However the dry scrubber still struggles to suppress high amounts of incoming SO2 and since the project lacked a measuring unit for incoming HCl concentration the results showing an improvements in pollution separation was not conclusive.