A Study on Urea-Water Solution Spray-Wall Impingement Process and Solid Deposit Formation in Urea-SCR de-NOx System

G.M Hasan Shahariar,Ock Taeck Lim

doi:10.3390/en12010125

Abstract

Selective catalytic reduction (SCR) has been exhibited as a promising method of NOx abatement from diesel engine emissions. Long-term durability is one of the key requirements for the automotive SCR system. A high NOx conversion, droplet distribution and mixing, and fluid film and solid deposit formation are the major challenges to the successful implementation of the SCR system. The current study is therefore three-fold. Firstly, high-speed images disclose detailed information of the spray impingement on the heated impingement surface. The spray impingement investigation took place in a specially-designed optically-accessible visualization chamber where the Z-type shadowgraph technique was used to capture the high-speed images. Wall temperature has a great influence on the film formation and wall wetting. A higher wall temperature can significantly increase the droplet evaporation, and consequently, wall wetting decreases. The numerical analysis was performed based on the Eulerian-Lagrangian approach using STAR CCM+ CFD code. Secondly, the resultant phenomena due to spray-wall impingement such as fluid film generation and transport, solid deposit formation, and thermal decomposition were recorded using a high-speed camera operating at a low frame rate. Infrared thermal imaging was used to observe the spray cooling effect after impingement. Spray impingement caused local cooling, which led to wall film formation, which introduced urea crystallization. Finally, solid deposits were analyzed and characterized using Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). FTIR analysis revealed that urea decomposition products vary based on the temperature, and undecomposed urea, biuret, cyanuric acid, ammeline, and melamine can be formed at different temperatures. TGA analysis showed that accumulated deposits were hard to remove. Moreover, complete thermal decomposition of deposits is not possible at the regular exhaust temperature, as it requires a comparatively long time span.

Highlights

Energy is omnipresent with respect to life, and the quest to obtain energy without interrupting the environment is growing at an exponential rate
As a result of better thermal efficiency, which can effectively lead to a better fuel economy, an increased interest can be seen in diesel engines
Diesel engine gaseous exhaust emissions contain a significant percentage of NOx, a combustion by-product that is generated in the combustion chamber, due to the high temperatures produced by compression in the engine cylinders because of diesel fuel ignition

Summary

Introduction

Energy is omnipresent with respect to life, and the quest to obtain energy without interrupting the environment is growing at an exponential rate. Over the last few years, the SCR system has been a proven technology for NOx emission reduction for the transportation sector, which can achieve over 90% NOx conversion to satisfy the existing emission standards [5] This method uses an injector to inject the urea water solution (UWS) (the commercial name is AdBlue) in the exhaust gas flow before the catalyst. After the injection into the hot exhaust stream, urea decomposes into NH3 , which chemically reacts with the NOx molecules and subsequently produces N2 [6] This urea decomposition process is completed as follows: (i) liquid droplet evaporation, (ii) ammonia (NH3 ) and isocyanic acid formation through urea thermolysis, and (iii) hydrolysis of isocyanic acid [7,8,9]. The conventional pressure-driven urea-SCR injector works at a low injection pressure; no secondary droplet breakup occurs, and relatively large-sized droplets are produced, which leads to spray cooling, as well as wall film generation, which is the predecessor to urea deposit generation [19]

Methods

Results

Conclusion