This article discusses the fact that lower NOx emission limits are creating demand for Selective Catalytic Reduction (SCR) and Selective Non Catalytic Reduction (SNCR) systems. These systems utilize ammonia injection for reaction with the source NOx and catalytic bed thereby significantly reducing the NOx emissions. However, until recently there has been no accurate or reliable on-line analytical method for monitoring the un-reacted NH3 (known as 'Ammonia Slip'). This paper discusses the technical merits of measuring the ammonia slip directly in the outlet duct of an SCR/SNCR with a Tunable Diode Laser (TDL) analyzer. Significant benefits exist with an in-situ TDL, especially the elimination of the traditionally problematic extractive sample conditioning systems.
