Laser absorption tomography for ammonia measurement in diesel engine exhaust
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Laser absorption tomography for ammonia measurement in diesel engine exhaust Ehson F. Nasir1 · Scott T. Sanders1 Received: 11 February 2020 / Accepted: 29 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Tomographic measurements of ammonia concentration in diesel engine exhaust dosed with diesel exhaust fluid are demonstrated through spatial scanning of a 10.4 μm quantum cascade laser beam. Optical access through the exhaust pipe was facilitated by a polyimide film. The laser beam was translated in 0.5 cm increments and rotated in 6 degree angular increments, resulting in 2130 unique laser beam positions per experiment. The data was then used to reconstruct ammonia concentration images through an inverse Radon transform. The laser was driven by a current ramp at 200 Hz and phase-locked with the doser. Data were collected for a duration of 500 ms at each laser beam position. Each laser current ramp was used to create one image, resulting in a total of 100 images per experiment. The images were compiled into a 500 ms-duration video showing the evolution of the phase-averaged ammonia concentration field, assuming a repeatable periodic concentration field with respect to each doser pulse. The experiments were performed for the exhaust temperature range of 453–574 K, nominal pipe diameters of 28 cm and 33 cm, and for different mixers to observe the ammonia concentration distribution upstream of an SCR catalyst. The spatially averaged ammonia concentration was within 10% of average values simultaneously obtained from an FTIR system.
1 Introduction Selective catalytic reduction (SCR) is an effective technique used in many commercial diesel engines to reduce NOx emissions and meet air quality standards [1]. The technique relies on the reaction of ammonia with NOx to form nitrogen and water vapor in the presence of a catalyst. The ammonia is normally introduced into the exhaust stream via an aqueous urea solution known commonly as diesel exhaust fluid (DEF). For an SCR system to perform effectively, the injection and mixing of DEF into the exhaust stream should be configured in such a manner that ammonia is spatially distributed uniformly across the cross-section of the exhaust pipe at the appropriate concentration levels. Excessive ammonia concentration may result in ammonia
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00340-020-07530-4) contains supplementary material, which is available to authorized users. * Ehson F. Nasir [email protected] 1
Engine Research Center, University of Wisconsin – Madison, 1500 Engineering Dr, Madison 53706, WI, USA
being expelled into the atmosphere from the exhaust pipe, commonly referred to as “ammonia-slip” [2], while insufficient ammonia concentration may result in unacceptably high levels of NOx emissions. It is therefore important to verify the ammonia concentration field produced by a given DEF injection and mixing configuration for an SCR system. Previous methods for measuri
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