Staff

Motivation

Limitations of polluting exhaust emissions are being tightened worldwide. The Euro-6 standard, coming into effect on 01.09.2014, decreases the maximum NO_x emission allowed for diesel driven passenger cars by 55 % compared to Euro-5 to 80 mg/km. For passenger cars with gasoline engines a limit of 60 mg/km is set ¹. California, where the fiercest emission legislation worldwide applies, has a limit of 0,05 g/mi = 31 mg/km since 2007 ². To comply with these standards a decrease of raw emissions of engines or reduction by means of exhaust after-treatment is required.

An efficient way of reducing nitrogen oxides is to deoxidize them with ammonia (NH₃) in a Selective Catalytic Reduction (SCR) device. Ammonia is added to the hot exhaust gas (e.g. as an aqueous urea solution (NH₂)₂CO), which deoxidizes NO and NO₂ to nitrogen and water in a special catalytic converter. Correct proportioning is essential to maintain highly efficient NO_x-conversion and prevent ammonia slip by overdosing. In cooperation with an industry partner a TDLAS (tunable diode laser absorption spectroscopy) based measurement system is designed, which can detect ammonia concentration accurate to 100 ppm spatially and time resolved directly in the exhaust gas at temperatures up to 900 K.

Method

TDLAS is a calibration free, non-invasive, optical measurement technique where a narrowband diode laser (DFB type) is tuned around a distinctive absorption line of the species to detect. By means of the extended Lambert-Beer law the molecule concentration of the absorbing species can be gathered. However, this method can only provide an integrated value for the whole beam path. The use of multiple beams and appropriate tomographic algorithms makes spatially resolved measurements possible.

1 Directive EG No. 715/2007

2 The California Low-Emission Vehicle Regulations