Liquid urea based SCR has been used in the market to reduce NOx in the exhaust emission of the diesel engine vehicle. This system has several problems at low temperature, which are freezing below ?12 ℃, solid deposit formation in the exhaust, and difficulties in dosing system at exhaust temperature below 200 ℃. Also, it is required complicated exhaust packaging equipment and mixer due to supply uniform ammonia concentration. In order to solve these issues, ammonia-transporting materials for solid SCR were studied in sequence.
First, basic research on reaction rate of solid urea, ammonium carbonate, and ammonium carbamate as ammonia sources for the application of solid SCR was performed. TGA(Thermogravimetric Analysis) and DTA(Differential Thermal Analysis) tests for these materials are carried out for various heating conditions. From the results, chemical kinetic parameters such as activation energy and frequency factor are obtained from the Arrhenius plot. Additionally, from test results of DSC(Differential Scanning Calorimeter) for these materials, chemical kinetic parameters using the Kissinger method are calculated. Activation energies of solid SCR from this experiment are compared with proper data of literature study, then obtained data of this experiment are used for the design of reactor and dosing system for candidate vehicle.
Second, analytical study of re-solidification materials from ammonium carbonate intermediate was carried out. Since the decomposition temperature of ammonium carbonate is around 60 ℃, ammonium carbonate produce ammonia gas through sublimation process by proper heating. After simple reactor which has heater control and temperature control of cooling water with visible window was designed and fabricated, pressures and temperatures are measured. From these data, saturation pressure curve as a function of temperature is obtained and will govern the design of dosing system. In order to simulate environment of vehicle operation, several cycles of heating and cooling were settled, then re-solidification materials inside of reactor were extracted for the analysis of XRD(X-Ray Diffraction) and FT-IR(Fourier Transform Infrared Spectroscopy). From analytical results, byproduct and re-solidification materials from heating and cooling cycles are very similar to ammonium carbonate.
Third, CFD(computational fluid dynamics) model is developed to simulate direct injection of ammonia gas from ammonia transporting materials into the SCR catalyst in the exhaust pipe of the engine. Configurations of one-hole and four-hole nozzle, and the effect of mixer which commonly used in liquid injection of AdBlue are considered for complex geometries. Mal-distribution index related to concentration of ammonia gas, flow uniformity index related to velocity distribution, and pressure drop related to flow resistance are compared for different configurations of complex geometries at the front section of SCR catalyst. These results are used to design the injection system for target vehicle.
Fourth, ammonium carbonate which is one of candidates as alternative ammonia sources was used for the application of NOx reduction in engine test. According to test results, NOx reduction performance of ammonium carbonate is the same level of NOx reduction performance of liquid urea solution.