Date Published: July 12, 2019
Publisher: Public Library of Science
Author(s): Qihua Ma, Dongjian Zhang, Xuehui Gan, Vijay Kumar.
It is difficult to simulate both the flow field and the chemical reaction using, respectively, the flow state and kinetics calculations and actually reflect the influence of the gas flow state on the chemical change in a selective catalytic reduction (SCR) system. In this study, the flow field and the chemical reaction were therefore coupled to simulate a full Cu-Zeolite SCR system and the boundary conditions of the simulation were set by a relevant diesel engine bench test which included the exhaust temperature, the mass flow, and the exhaust pressure. Then, the influence of the gas flow state on the NOx conversion efficiency was investigated. Specifically, an orthogonal experimental design was used to study the influence of the injection parameters (position, angle, and speed) on the NH3 distribution by establishing the NH3 uniformity coefficient γ at the SCR catalyst capture surface in the flow field simulation. Then, the velocity capture surface of the SCR catalyst front section was sliced into coupled data transfer interfaces to study the effects of exhaust temperature, ammonia to NOx ratio (ANR), and the NO2/NOx on the NOx conversion efficiency. This was used as guidelines to optimize the SCR system control strategy. The results showed that a 1150 mm injection position, a 45°injection angle, and a 23 m/s injection velocity provided the most uniform NH3 distribution on the SCR catalyst capture surface. For constant injection parameters, the NOx conversion efficiency was the highest when the exhaust temperature was 200°C—400°C, the ANR was 1.1, and NO2/NOx was 0.5.
Growing concerns about the environment have caused regulations concerning nitric oxides (NOx) to become increasingly stringent . For instance, the NOx emission limits of the European heavy-duty emission regulations have been reduced from 8.0 to 0.46 g/KWh . Therefore, selective catalytic reduction (SCR) has become an essential after-treatment technique to ensure compliance with current stringent emission standards . In this process, a urea-water solution (UWS) is injected into an exhaust pipe and reacts with NOx to form nitrogen (N2) and water vapor (H2O). The use of a catalyst allows this reaction to proceed at a relatively low temperature. Under normal conditions, the UWS contains 32.5% urea and is injected into the exhaust pipe upstream of the SCR catalyst so that it interacts with hot exhaust gas to form gaseous urea. Then, the gaseous urea decomposes into ammonia (NH3) via thermolysis and hydrolysis, and the produced NH3 subsequently adsorbs to the SCR catalyst surface, and NO and NO2 directly react with the adsorbed NH3, via the Eley–Rideal mechanism .
To use the coupling of the flow field and the chemical reaction to simulate an SCR system, each sub-process must be represented by a mathematical model. The accuracy of the simulation results depends on the accuracy of the mathematical model and the boundary conditions. The boundary conditions are measured via a bench test and the mathematical models adopted in this study for the sub-processes also have undergone maturation. This section contains a detailed description of the mathematical models.
NH3 is mixed with the exhaust gas and enters the SCR catalytic reactor to undergo a catalytic reduction reaction. Selective catalytic reduction (SCR) of NOx with NH3 mainly undergoes four chemical reactions:
A method that couples the flow field and the chemical reaction to simulate a full Cu-Zeolite SCR system is proposed in this report. In addition to studying the quality of the injection parameters by establishing the NH3 uniformity coefficient γ at the SCR catalyst capture surface in a flow field simulation using an orthogonal experimental design, the velocity capture surface at the SCR catalyst front section was sliced at the data transfer interfaces. Then, to provide guidelines for optimization of SCR system control strategy, the effects of the exhaust temperature, ANR, and NO2/NOx ratio on the NOx conversion efficiency were investigated, which allowed the following main conclusions to be drawn: