Experimental Investigation on the Distribution Uniformity, Injection Control and Spray Optimization of Reductant for DeNOx Catalyst Systems

Abstract

Nitrogen oxides (NOx) are one of the exhaust emissions from diesel vehicles that utilize fossil fuels, it is well known that NOx create photochemical smog and penetrate the somatic cells of a human body and causes respiratory diseases. As this issue becomes a serious social problem, the regulations of exhaust emissions have been tightened. Accordingly, various combustion technologies for fuel have been developed and improved in order to properly cope with such regulations. However, such efforts have reached their limitations for responding to ever tightening regulations and, therefore, it is now unavoidable to adopt an after-treatment technology. The denitrification (DeNOx) after-treatment technology such as the hydrocarbon-lean NOx trap (HC-LNT) or the urea-selective catalytic reduction (urea-SCR) catalyst used to reduce the NOx takes primarily advantage of hydrocarbon (HC) and urea solution as the reductants and the catalytic converter system of the secondary injection type that sprays it directly on the exhaust gas downstream of the combustion chamber is commonly adopted. The secondary injection system has benefits: it can be controlled independently without disturbing engine control, it can be adapted to various layouts for exhaust systems, it has no oil dilution problems, compared to post injection in the combustion chamber or other supplemental reductnat injections. In this study, the concentration and amount of reductant can be controlled through control of the secondary injection. The secondary injection method is highly dependent upon the type of injector, injection pressure, atomization, spray technology and etc. Therefore, it is necessary to establish injection conditions, and must be well-understood the spray characteristics, such as the spray penetration, sauter mean diameter (SMD), spray angle, injection quantity and etc., and uniform distribution of the reductant corresponding to the maximum NOx reduction in the DeNOx catalyst systems. With this goal in mind, the spray characteristics and the behavior of the secondary injectors were analyzed using visualization and digital image processing techniques. In addition, the typically high temperature (250 – 350°C) in the exhaust manifold affects the spray behavior of the secondary injector. Moreover, the high temperature makes it particularly difficult to achieve uniform distribution of the reducing agent in the manifold. Thus, it is necessary to use a mixing unit (mixer or impingement plate) to improve the fuel distribution in the exhaust manifold. In this study, the effects of various mixer types or impingement plate types on the spatial distribution of the reductant were investigated to improve the performance of the DeNOx catalyst systems. While the reductant was injected directly toward the mixing units in a transparent manifold, spray images were collected using a high-speed camera, and the spatial distribution of the spray was analyzed using image processing techniques. The analysis on the spatial distribution of the spray represented that mixing units were beneficial to achieve uniform distribution of the reductant in exhaust pipe and improve the performance of the DeNOx catalyst systems. On the other hand, the uniformity index (UI) of the reductant is of great importance and referred to as the basis for system optimization because the catalyst reaction efficiency is directly affected by the distribution of the injected reductant. The UI of the reductant may be regarded as an indicator of the NOx reduction efficiency (NRE) and it can reduce the necessary capacity of the catalyst and the high price noble metals. Accordingly, two new methods for the reductant UI measuring were developed in this study. The first method is to acquire the reductant distribution images from exhaust pipe sections upstream of the catalytic converter. The droplet uniformity index (DUI) from the acquired images using image processing to clarify the distribution of the spray droplets was calculated. Using image processing analysis of the quantitative characteristics and spatial distribution of the spray revealed that application of mixers resulted in more uniform distribution of the reductant in the exhaust pipe and improved the performance of the DeNOx catalyst systems. And, the second new method is to calculate the temperature uniformity index (TUI) which was developed to measure the distribution of the exhaust gas temperature upon the exothermic reaction as the reductant passes through the catalytic converter of the SCR catalyst system by means of thermocouples downstream of the SCR catalyst. The system is utilized to measure the TUI in real-time, applied to the actual SCR system, and the results are compared in various engine operating conditions and mixer types in terms of NRE. From these studies, measurement and control method were developed to obtain uniformity, and the DeNOx systems with secondary injection were optimized.