Department of Systems and Control

Internal research project

1999 - 2000

Design and implementation of a control system for the process of the selective catalytic reduction (SCR) of nitrogen oxides (NOx) from flue gases after combustion.

Combination of feedback and feedforward control of ammonia flow rate, with the aim to maintain a minimal NOx concentration in the flue gases at the output from the SCR reactor. Feedforward control predicts the required ammonia flow rate on the basis of two signals: input NOx concentration and flue-gas flow rate. These two signals are not measured, rather they are estimated using software sensors based on the measured fuel flow rate and excess oxygen concentration in the combustion process. Thus, a significant part of the expensive measuring equipment is omitted. The prediction error of the feedforward control is corrected by the feedback control, which is based on the measured NOx concentration at the output of the SCR reactor.

Software sensors contain a mathematical model of the combustion. They estimate the concentration of NOx in the flue gases during combustion and the flow rate of the flue gases. The estimation is based on the existing measurements of the excess oxygen concentration and fuel flow rate, which are included in standard combustion control systems.

Efficient control of the output NOx concentration in both stationary and dynamic conditions, omitting the expensive measurement equipment that is normally required to achieve a comparable control performance.

gregor.dolanc@ijs.si