Urea Selective Catalytic Reduction (SCR) has been proven to significantly reduce NOx emissions from diesel engines. The thermal decomposition of urea, which forms the ammonia as the reactant, has a crucial effect on the performance and durability of the NOx-SCR system. The incomplete thermal decomposition of urea not only reduces the NOx conversion ratio and increases the ammonia slip, but also leads to deposit formation on the catalyst surface, which will block the pore and the active sites of the catalyst and then decreases the durability of the SCR systems.
In this paper, the urea thermolysis was measured using the Thermal Gravimetric Analysis (TGA) and Fourier Transform Infrared Spectroscopy (FTIR). Then, the performance of the SCR systems under different injection parameters of the Urea-water solution was investigated on a diesel engine test bench. Finally, the deposits on the catalyst were also analyzed using TGA and FTIR.
The results show that urea thermolysis exhibits three decomposition stages. Each stage started at 193 °C, 250 °C, and 400 °C, respectively. A low-quality spray of the UWS leads to the deposit formation on the surface of the inlet pipe and the catalyst. The TGA results show that the deposit had a similar decomposition process to the urea, but 14.5% of the deposit mass was left even under temperature higher than 1000 °C, which indicates that the deposit was a mixture of urea and a macromolecule complex derived from urea. A high-quality spray of the UWS eliminates the formation of the deposit.