Study on Improvement of NOx Reduction Performance at Low Temperature Using Urea Reforming Technology in Urea SCR System 2019-01-0317
After-treatment system for low NOx emission combined with engine combustion modification has been developed. Recently exhaust gas temperature decreases greatly with further reduction in fuel consumption of engine, it’s urgent to create a high NOx reduction technology at low temperature.
Since NOx reacts with ammonia on SCR catalyst, it’s necessary to decompose the urea aqueous solution into ammonia using a heat of exhaust gas and supply ammonia sufficiently to SCR catalyst. However, both decomposition and hydrolyzation reactions of urea hardly make progress at low temperature because of insufficient exhaust heat.
In order to solve this issue, the urea aqueous solution was forcibly thermal decomposed into ammonia (urea reforming) regardless of exhaust gas temperature. An improvement of NOx reduction performance at low temperature was investigated in this study.
In the case of the first urea reformer, it was found that deviation flow of urea aqueous solution provided loose contact with a part of the glow plug by fluid analysis inside the reformer. It was considered the decomposition into ammonia from urea became insufficient because heat of the glow plug could not be efficiently transferred to the urea aqueous solution.
Therefore, to increase the contact area of the urea aqueous solution with the glow plug, flow paths in the reformer were connected in series and made into a swirling flow around the glow plug to bring into reliable contact urea with six glow plugs.
As a result, within 4 g/min of the urea aqueous solution amount, ammonia production efficiency of the second reformer rose up over 90%. Furthermore, the second reformer was mounted on urea SCR system, and NOx reduction performance in actual engine was measured when it was operated less than 180deg.C. By the second reformer, NOx reduction rate of 60% was obtained at 160deg.C where urea could not be added.