Advanced capillary sampling technique for spatially resolved analysis of complex gas mixtures

To investigate the spatial distribution of gas-phase species within structured catalysts under conditions representative of real exhaust gas environments, an invasive capillary-based sampling method was adapted for use in a hot gas test rig. A Fourier-transform infrared (FTIR) spectrometer was employed for analysis of gas mixtures characteristic for the selective catalytic reduction (SCR) of NO$_x$. To meet the sample volume requirements of the FTIR, a coil-based sampling extension was introduced and its influence was systematically quantified. A linear correlation between gas composition measurements with and without the coil enabled the development of a correction factor, which was generalized using a residence time model, thereby eliminating the need for extensive recalibration under different operating conditions. In addition, the impact of the capillary on local conversion behavior was assessed using an analytical model, accounting for uncertainties in capillary positioning and reduced gas velocities. These findings facilitated reliable spatially resolved species measurements for NH$_3$-SCR reactions. Extension of the methodology to a SCR configuration including the injection of urea water solution revealed significant challenges for a precise quantification of the gas composition due to transient phenomena such as film evaporation and deposit formation.