Model-based control of selective catalytic reduction of nitric oxides

This paper presents the design and implementation of a model based control system of selective catalytic reduction of nitric oxides from flue gases. Reduction is obtained by means of addition of ammonia which reacts with nitric oxides within a catalytic converter to form harmless nitrogen and water. Ammonia flow rate has to be carefully controlled. The control system, composed of feedforward and closed loops, guarantees good dynamic behaviour of selective catalytic reduction. The price for feedforward control is an increased set of measurement equipment, that provokes further technical problems and leads to additional costs. In this paper, it is shown that measurement equipment demanded by feedforward control can be substituted by introduction of a model based feedforward scheme. Model based feedforward control employs two theoretical mathematical models that predict flue gas flow rate and nitric oxide concentration. Prediction is made according to the measurement of excess oxygen concentration and fuel flow rate which are already available as a part of combustion process control system. Advantages of the proposed theoretical models are reflected in their simple structure and small parameter set.