Control of NOx Using Modified Response Surface Methodology (MRSM)

Today, stringent regulatory limits demand new techniques for better controlling NOx. Large furnace volumes and extractive sampling processes inherently delay NOx response to process changes. This frustrates and destabilizes conventional feedback control. To maintain stability one must decrease controller gains. This results in sluggish control with frequent excursions above regulated limits. Feedforward control offers an attractive solution. It can compensate for NOx before the process departs from ideal. However, feedforward control requires an accurate predictive emissions model. This paper presents such a model, describes the important process variables, and gives effective NOx control algorithms now in successful use. One may easily incorporate the technique, known as modified response surface methodology (MRSM), into programmable loop controller (PLC) platforms or distributed control systems (DCS).