Environmental safety is one of the hottest research areas nowadays due to increased public awareness. To achieve this goal, efforts are continuously made to reduce pollution and develop green processes. Exhaust emissions from marine Diesel engine are responsible for producing severe environmental pollution, especially nitrogen oxide (NO
x) emissions [
1,
2]. The automobile Diesel engine produces exhaust emissions species such as carbon monoxide, carbon dioxide, and hydrocarbon in abundance compared to NO
x. On the other hand, marine Diesel engine produces more detrimental NO
x emissions [
3]. Hence, in order to reduce exhaust emissions from ships, many national and international organizations have promulgated regulations and also enforced strict requirements on NO
x emissions in Emission Control Areas [
4]. In 2016, International Maritime organization (IMO) Tier III regulations on NO
x emissions have already been enforced in North America emission control areas, including the East and West Coast of USA and Caribbean. As reported, it will also be enforced in North Sea and Baltic Sea in future [
5]. Both high pressure fuel injection and exhaust gas recirculation systems have potential to reduce NO
x emissions, but due to poor engine performance results and continuous increase in engine emission legislations, more primitive and improved processes are needed to overcome the said issues [
6]. The most convenient and easy option is the treatment of exhaust gas. Selective Catalyst Reduction (SCR) is technically mature, and is the most prominent after-treatment technology used to meet the latest NO
x emission regulations due to its high NO
x removal efficiency, cost-effectiveness, and good fuel economy [
7]. However, challenges related with SCR system include improper mixture of urea water solution (UWS) with exhaust gas and ammonia leakage [
8]. UWS (32.5% urea) is injected into exhaust gas [
9,
10]. Urea decomposition occurs in three steps: firstly, water is evaporated from urea water solution; secondly, the urea pyrolysis reaction occurs, which results in the decomposition of urea into isocyanic acid (HNCO) and ammonia (NH
3); and lastly, hydrolysis of HNCO occurs which produces NH
3 and CO
2 [
11,
12]. Ammonia is used as a reducing agent and cannot be used directly due to poisonous nature, storing and handling difficulties [
13]. It has been proved that NO
x is mainly 90% composition of NO at the exhaust of the marine Diesel engine. The key reactions involved in SCR system are described as follows [
14,
15,
16,
17].
CO(NH2)2→ NH3 + HNCO | (Urea pyrolysis reaction) |
HNCO + H2O →NH3+ CO2 | (HNCO hydrolysis) |
4NH3 + 4NO + O2→ 4N2 + 6 H2O | (Standard SCR reaction) |
4NH3 + 2NO + 2NO2→4N2 + 6 H2O | (Fast-speed SCR reaction) |
8NH3 + 6NO2→7N2 + 12H2O | (Slow SCR Reaction) |
A static mixer is commonly adopted to generate uniform distribution of ammonia at the inlet of SCR catalyst [
18]. CFD code (Fire 8.3, 2004) has been used to optimize the design of SCR system. The authors have done many studies about the decomposition and evaporation of urea water solution without using static mixer. Birkhold [
19] studied the urea droplet’s evaporation and decomposition at different temperatures of exhaust gas. Strom [
20] studied the effects of turbulent velocity and different forces on distribution and movements of urea water droplet in the exhaust species. In addition, other authors have established various studies to evaluate the effect of static mixer on the SCR performance. Sivanandi Rajadurai [
21] studied the distribution of ammonia by using wire mesh mixer. Zhang [
22] investigated the uniformity index of ammonia by adopting delta wing mixer. Shazam Williams and Ming Chen studied the velocity and ammonia distribution in the straight pipe together with static mixer having two rows of tabs [
23]. Azael and Ibarra investigated the interaction of fluid with a double vortex mixer to improve the performance of SCR. The evaporation effect and droplet crushing was significantly improved. The distribution of NH
3 was also studied, revealing a smooth and uniform distribution using a double vortex mixer; however, the velocity uniformity distribution, reaction temperature, and droplet distribution time was not studied [
24]. One of the disadvantages of the SCR system is that it occupies the additional space on vessels. Mostly, authors are working to simplify the SCR system according to the space requirements. One of the authors used an 18 mm distance between the two mixers without considering the impact of velocity on SCR system [
18]. Park et al. reported that if the continuous uneven impact of velocity occurs, which causes excessive aerodynamic velocity and temperature, it will lead to the thermal fatigue failure, which reduces the service life [
25]. The first mixer can create turbulence intensity and sudden impact on velocity, which directly affects the second mixer. By considering both of the references, it can be perceived that the optimum condition which can satisfy both of the conclusions from different studies can lie within limits. It therefore looked suitable to trial for 0.2 m distance. One of the study used series of only swirl mixer (SM) near the injection point and in line [
18]. However, use of a line mixer (LM) can result in increasing the mixing flow (exhaust gas plus urea droplets) in the blind corner near the pipe wall, while SM results in increasing the mixing flow in the center [
25]. Some studies investigated LM with different blade angles [
25,
26], and others used SM with different blade angles [
18,
27]. In this study LM was used in front of SM. Initially, LM and SM were used separately and then analyzed for the combined effect of both (LSM). In this work, the blade angle was 45° for both mixers because it has been proven that static mixers consisting of bitched blades with an angle of inclination angle of 45° can generate higher turbulence intensity and a swirling flow in the pipe [
28]. Furthermore, many authors have used a greater number of blades than this study. One author used 36 blades in only LM [
25]. In this study, 18 (LM:12, SM:6) blades were used and distributed in two difference places in the pipe. In addition, if a single mixer has many blades at a certain location, it tends to decrease the wall temperature, which ultimately results in deposit formation [
29]. Hence it is necessary to distribute number of blades in two mixers with different locations to increase the mixing performance and prevents deposit formation. Generally, SCR performance depends upon the velocity uniformity and ammonia uniformity near the inlet of the SCR catalyst. Therefore, a suitable design of mixers for increasing the efficiency of SCR is needed. Furthermore, a numerical model has been developed, which describes the impact of mixer on the ammonia and velocity uniformity, droplet residence time, and temperature distribution in the pipe. In addition, it is also necessary to consider reaction temperature and wall temperature, as it affects the catalyst performance and deposit formation [
1]. In this paper, two different types of mixers—line (LM) and swirl (SM) type mixers alone and in combination (LSM)—were used to indicate the effects on the performance of SCR. The main objective of this investigation is to create the optimum SCR design, to achieve higher uniformity index for velocity and ammonia distribution, better evaporation rate, and droplet distribution by consideration of the reaction temperature and wall temperature distribution based on CFD. Furthermore, for the verification of the simulated results of ISO 8178 a standard marine Diesel engine test cycle E3 was used [
30].