P4.2 - Tunable Diode Laser Spectrometry (TDLS) - Challenges in NH3 Measurements in Hot and Humid Exhaust Gas

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
P4 - Gas
A. Seifert, M. Studer, T. Hessler - Leister Process Technologies - Axetris Division, Kägiswil (Switzerland)
755 - 755


In power generation, as well as in automotive emission control, NOx can be reduced by 99% with Selective Catalytic Reduction (SCR), where ammonia (NH3) or urea is injected into the exhaust gas and reacts over a catalyst with NOx o nitrogen and water. This process needs to be controlled well in order to reduce “ammonia slip”, which is the target of increasingly stringent emission control requirements. Due to the residual content of ammonia in the exhaust gas, NH3 monitoring is either necessary during operation or for emission control system design and testing. NH3 measurement in the hot, waterladen exhaust gas is a real challenge for detection technologies.

Tuneable Diode Laser Spectroscopy (TDLS) is well suited for this measurement due to its ability to perform contact-less hot-gas measurements, which allows decoupling the sensing part from the hot medium. In our experiments, a 190°C heated flow-through gas cell for extractive measurements is used that is tightly closed by two windows. To perform the measurement, the laser beam emitted from the diode passes through the sample cell to a mirror on the opposite side of the cell, before returning through the cell to the electro-optical module that contains both, the laser diode and the photo diode.
Compared to its ambient temperature counterpart, the hot gas measurement presents some interesting challenges. The most obvious one is the thermal behaviour of the system, which is critical for compact systems. Additionally, the absorption coefficients of the NH3 absorption bands are temperature dependent and their relative strength decreases with increasing gas temperature. This issue has to be taken into account for calibration and leads to performance that is inferior compared to a room temperature measurement.
Lastly, hot exhaust gas contains important quantities of water vapour. At this level, interference between the low ppm-level ammonia and the high water content must be taken into account. We will discus this application and its challenges, as well as results related to the hotgas ammonia measurements at 190°C.