# Flow Analysis of PM/NOX Aftertreatment System for Emergency Generator
## 1 Introduction  
The emergency generator installed in the building uses a diesel engine, and to ensure start ability and stable power generation performance in case of an emergency such as a power outage, the operation inspection should be performed once a week for at least 30 min under no-load condition. During this test operation, many harmful emissions such as NOx and particulate matter (PM) are generated. Emergency generators basically reduce harmful emissions through the same type of aftertreatment system installed on road diesel engines. Exhaust gas is largely reduced through diesel oxidation catalyst (DOC), diesel particulate filter (DPF), and selective catalyst reduction (SCR). Since emergency generators are less limited in space than vehicles during installation, integrated aftertreatment systems developed for vehicle installation, such as catalyzed DPF (CDPF), which combines the functions of DOC and DPF, are not considered. US Environmental Protection Agency regulates the emission of emergency power generators through the EPA emission regulation, which is a non-road engine emission regulation [1]. To meet this, the PM/NOX reduction efficiency and back pressure of the aftertreatment system must be optimized, so the system shape must be optimally designed so that flow uniformity of the reducing agent is sufficiently secured in front of the SCR. Basic configurations of the aftertreatment solutions for emergency generator include a DOC, DPF. SCR, and similar. However, for large aftertreatment systems exceeding 500 kW, rectangle substrates more easily respond to exhaust capacity and are relatively easier to manufacture than circular types. Hence, rectangle types are mainly applied, so optimal design is possible considering changes in internal flow characteristics according to differences in cross-sectional shape. In the urea-SCR system, as the urea-water mixture sprayed from the urea injector installed in the reactor part is thermally decomposed by the exhaust temperature, HNCO is produced, and a hydrolysis reaction is generated in the reactor (mixed region) to produce an ammonia (NH\({}_{3}\))-reducing agent. Exhaust gas containing ammonia then flows into the SCR through the diffuser, and ammonia causes an SCR reaction in the catalyst to convert NO\({}_{\text{X}}\) into harmless H\({}_{2}\)O and N\({}_{2}\)[2]. Therefore, the flow uniformity of ammonia distributed at the front end of the SCR becomes an index capable of determining the reduction efficiency of the system. In addition, by calculating the pressure at the front end and the rear end of the system, the effect of the system on the output power characteristics of the engine can be determined in advance. However, these flow characteristics have limitations when analyzing them in an experimental method. Using the CFD analysis method can greatly reduce trial and error required for optimal design, so analytical research on the aftertreatment system has been conducted using it. CFD studies on the urea-SCR system began with the NOx reduction efficiency study of the SCR system according to the presence or absence of mixer installation, and studies were conducted to analyze internal flow characteristics according to engine load rate and SCR device shape [3; 4; 5; 6; 7; 8]. Recently, an analytical study was conducted to deal with the blade specifications of the urea mixer and the back pressure and flow characteristics of SCR according to the length of the reactor [9]. In this study, ANSYS Fluent, a commercial CFD software, was used as an analysis tool. CFD can be performed on a computer alone without complex separate experimental and analytic devices, which relatively reduces costs and minimizes trial and error required for optimal design by reflecting analysis results at the initial design stage. It has the advantage of being able to interpret actual physical phenomena that are difficult to experimentally analyze, such as complex internal fluid behavior accompanied by turbulent flow.